Electric-field-driven CO_2 polarization and bioinspired proton blocking unlock CO_2 reduction in strong acid without metal cations

Domain: physics
Published: 2026-01-19
Editorial outcome: minor_revision

Abstract
Metal-cation-free CO2 electroreduction (CO2R) in strong acidic media mitigates CO2 reactant losses, eliminates the risk of metal salt precipitation, and broadens device tolerance compared to acidic, neutral, or alkaline system using metal cations. However, such an acidic environment still poses challenges due to the inert and nonpolar nature of CO2 and intensely competitive hydrogen evolution reaction. Inspired by aquaporins in acidophiles, we engineer sharp-triangle Au nanostructures capped with a hexadecyltrimethylammonium chloride (CTAC) layer enriched with cationic sites. The intense local electric fields generated by the high-curvature tips of Au nanocatalyst polarize CO2 molecules, increasing their dipole moment to facilitate adsorption and activation. Meanwhile, the CTAC layer acts as a proton barrier, suppressing HER by mimicking the proton-blocking mechanism of aquaporins. This dual-function design enables continuous CO2R for 100 hours in a flow electrolyzer at pH 1.0, achieving an energy efficiency of 60% and near-unity Faradaic efficiency for CO production. This bioinspired strategy represents a significant advancement in CO2R technology by integrating rational catalyst design principles.

Introduction
Electrochemical reduction of CO2 in ambient conditions is a promising technique for storing renewable electricity, fixing CO2, and producing value-added fuels and chemicals1,2. Among the various approaches, performing CO2R in an acidic medium is particularly attractive for efficient CO2 utilization due to its potential to avoid the formation of carbonates3,4 that typically occur in alkaline5,6 and near-neutral3,7,8 electrolytes. However, efficient and selective CO2R in acidic conditions remains a challenge due to the competitive hydrogen evolution reaction (HER) and the inert nature of the CO2 molecule9–18. Introducing high concentrations of alkali cations into the electrolyte, even utilizing the tip effect to enrich alkali metal ions on the catalyst surface, has been demonstrated to suppress the undesired HER while promoting CO2 activation9,19,20. Despite the progress, alkali cations induce bicarbonate crystallization on the catalyst and gas diffusion electrode during CO2R in the acidic electrolyte, which is still an issue that limits the cell’s lifetime5,10,11,21–23 and hinders the sustainability of CO2 reduction19,21.

Acidic electrolytes without metal cations eliminate carbonate formation, enabling high CO2 utilization efficiency9,11,13. However, the kinetics of COR in such environments inevitably confronts two formidable hindrances: (i) the inherent difficulty in activating the chemically inert and nonpolar CO2 molecule and (ii) the severe competition from HER at low pH9,24–26, often necessitating high overpotentials and resulting in suboptimal energy efficiency. Interestingly, biological systems like aquaporins in acidophiles can selectively block protons due to the presence of specific positively charged residues of amino acid27,28, suggesting that mimicking such biological mechanisms of the proton-blocking process could provide a means to suppress HER in COR (Fig. 1a, b). Simultaneously, engineering strategies to polarize and activate CO2 molecules by breaking their nonpolar ground state are essential for improving CO2 reduction kinetics29,30. Notably, the application of intense local electric fields offers a promising approach to achieve such polarization, providing a viable pathway to overcome the intrinsic challenges of CO2 activation in metal-cation-free acidic media.

Here, we present that a bioinspired sharp-triangle Au nanocatalyst with the cationic surfactant layer enables efficient and durable CO2 reduction in a metal-cation-free strongly acidic medium by promoting CO2 polarization while suppressing competitive HER. For CO2 activation, simulations reveal tenfold higher local electric fields associated with metallic tips of Au nanotriangles (AuNTs) compared to quasi-spherical electrode regions. The field-induced polarization effect not only enables the CO2 adsorption process to be thermodynamically spontaneous, but also increases the dipole moment of nonpolar CO2 (Fig. 1c, d), which in turn elongates the bond length and weakens the bond, lowering the barrier of the rate-determining step (RDS). For inhibition of HER, the positively charged hexadecyltrimethylammonium chloride (CTAC) layer on the Au surface (Fig. 1e) builds on the proton-blocking mechanism of aquaporins (Fig. 1b), allowing electrically neutral CO2 to pass through while preventing proton transfer. This extended charge distribution on the CTAC layer enhances the proton exclusion effect, further suppressing HER. As a result, the AuNTs catalysts achieve about 60% energy efficiency and a near-unity Faradaic efficiency over 100 h of electrolysis in acidic electrolytes (pH = 1.0), making the CTAC-capped AuNTs one of the most efficient catalysts for acidic CO2R to date. This work underscores the potential of bioinspired design principles in advancing CO2R technologies, offering a sustainable pathway for CO2 utilization in acidic conditions.

Results
Theoretical predictions
Since the positive and negative charge centers of the free CO2 molecule with linear structure coincide and the polarity of the bonds cancel each other out, it is a nonpolar molecule with a dipole moment of 0 D. We firstly focus on the CO2 molecule approaching the catalyst surface, before any chemical interaction with Au occurs. Under these conditions, CO2 retains its linear structure, as there is no charge transfer or bonding with the surface to induce bending31–33. Through theoretical calculation simulation (Fig. 2a−c), we discovered that the external electric field could break the nonpolar state of the CO2 molecule, shift and deform its electron cloud, separate positive and negative charge centers, and produce an induced dipole. A dipole is defined as a pair of opposite charges “+q” and “−q” separated by a distance “d” (Fig. 2a). The direction of the dipole moment (μ) in space is from negative charge “−q” to positive charge “+q”34. The CO2 dipole moment, bond length, and bond energy of C=O was highly dependent on the electric field intensity. Specifically, one of the C=O bond energy is declined, bond lengths is increased gradually from 1.169 to 1.182Å (Supplementary Fig. 1) with the corresponding dipole moment of CO2 molecules elevated from 0 to 1.25 D (Fig. 2b) when the electric field intensities are raised from 0 to 1.2 × 108 V m−1, respectively. That is, the higher the electric field strength, the larger the CO2 dipole moment and the weaker the C=O bond. According to the electrostatic potential (ESP) diagram for CO2 molecules under different electric fields in Fig. 2c, higher polarizability of CO2 molecules under a stronger electric field could be ascribed to a higher separation degree of positive and negative charge centers as well as increased charge under stronger electric fields.

As high-curvature structures are known to concentrate electric fields that can affect the spatial distribution of charged or polarizable species35. Under an applied potential, metallic electrodes tend to accumulate free electrons at surface regions with pronounced curvature, leading to locally intensified electrostatic fields. This field enhancement arises from curvature-dependent electron redistribution inherent to charged metal surfaces36. We used COMSOL Multiphysics simulations with a finite-element numerical method37–39 to explore the prospects of tip-enhanced field intensification (Fig. 2d) and the associated field distribution (Supplementary Fig. 2) on Au surface with various acutance. As the tip geometry sharpens from round to triangular, the simulated electric field intensity (Fig. 2d) near the surface increases by roughly one order of magnitude, reaching up to 1.21 × 108 V m−1 at the CO2/CO equilibrium potential (−0.109 V versus standard hydrogen electrode). Under such intensified fields, a polarized CO2 molecule is expected to orient such that its electropositive end faces the high-field region (Supplementary Fig. 2). These results illustrate the effective ability of sharp angles to enhance the electric field, which may promote near-surface polarization and retention of CO2 molecules.

We employed density functional theory (DFT) calculations (Supplementary Data 1) to predict further the adsorption and electrocatalytic conversion of CO2 on typical Au facets—flat Au(111) plane and the stepped Au(211) facet—with/without the enhanced electric field (EEF). The surface electronic structures of the Au nanocrystals would be significantly altered by the applied EEF40. The d-band centers of the Au(111) and Au(211) planes under the EEF of 1.21 × 108 V m−1 were clearly shifted upward by 0.37 and 0.10 eV, respectively, compared with those without the EEF (Fig. 2e). Based on the basic concepts of surface and coordination chemistry, an upward movement of the outermost d-band center in transition metals raises a greater fraction of anti-bonding orbitals above the Fermi energy. This reduces the occupation of anti-bonding states, thereby reinforcing the interaction between the metal surface and adsorbed species, such as CO2 and its intermediates41. In the case of CO2R, the Au surface atoms under the EEF would bond with the adsorbates (e.g., *CO2, *COOH) much more strongly than those without the EEF due to their upshifted 5d bands41. Enhancing adsorption on Au surfaces would theoretically promote CO2R since Au showed relatively weak adsorption to CO2 species42–44 and key intermediates (such as *COOH) and was located on the right side of the volcano plot for CO2R1,45. While these results highlight the role of the d-band center shift in modulating CO2 adsorption, it is also crucial to consider the dipole-field interaction between CO2 and the electric field. As suggested by the dipole-field model46,47, the strong dipole moment of CO2 interacts with the interfacial electric field, further stabilizing the adsorbed *CO2. Our findings are consistent with this model, as we observe significant C–O bond elongation (from 1.182 to 1.354Å on Au(111) and to 1.361Å on Au(211)) and O–C–O bond bending (from 179.6° to 124.3° on Au(111) and 122.7° on Au(211)), both of which are indicative of strong electrostatic interactions. Bader charge analysis further revealed electron transfer from C to O (−0.218 e on Au(111) and −0.302 e on Au(211)), indicating that CO2 activation occurs through both geometric and electronic structural changes. These structural changes suggest that the observed enhancement in CO2 adsorption is not solely attributed to the d-band shift, but also to the polarization of CO2 under the EEF, in agreement with the dipole-field model.

Further evidence was provided by the DFT-calculated Gibbs free energy (ΔG) diagrams for CO2R on different Au facets models at 298 K and 1 atm (Fig. 2f and Supplementary Figs. 3 and 4). Notably, the Au surfaces with the EEF exhibited significantly strengthened CO2 adsorption, which is the first critical step for subsequent catalytic reactions1,41. The free energy of CO2 adsorption on the (111) and (211) facets was 0.44 and 0.26 eV, respectively. But, under the EEF, the CO2 adsorption on the Au surface transforms into a thermodynamic spontaneous process, and its free energy changes decrease to −0.25 and −0.21 eV, respectively. In addition, the EEF modulates the CO2 reduction process and lowers the thermodynamic energy barrier of CO2R for all typical Au facets. On the Au(111) plane, the EEF stabilizes the rate-determining *COOH and *CO intermediates38,45 by 0.14 eV and 0.29 eV, respectively, similar to the under-coordinated Au(211) facet (Fig. 2f). These findings emphasize that the combination of enhanced CO2 adsorption and intermediate stabilization under EEF conditions potentially contributes to the improved catalytic performance of Au nanostructures for CO2R. Especially, the Au facets under the EEF would share a similar potential-determining step (PDS) for CO2R (i.e., *CO2 + H+ + e− → *COOH) to that not under the electric field, but with much lower energy barriers of 0.39 and 0.36 eV, respectively. This reduction in energy barriers corresponds to a significant decrease in the overpotential by 580 and 440 mV for Au(111) and Au(211) facets, respectively. Such a decrease demonstrates the effectiveness of the EEF in facilitating the rate-determining step and enhancing the overall catalytic performance of CO2 reduction. Therefore, the much lower free energy requirement would suggest the boosted CO2 adsorption and reduction on the surface under the EEF. These findings emphasize that CO2 activation under the EEF is governed by multiple factors, and its underlying mechanisms can be described using different descriptors. Notably, in our DFT models, the adsorbed *CO2 species represents a field-stabilized, weakly chemisorbed precursor state prior to the first electron transfer, rather than a fully reduced *CO2− radical.

The pronounced geometric bending and charge redistribution observed under the applied electric field indicate partial electron transfer and polarization, which effectively mimic the early stage of CO2 activation (CO2 → CO2δ−) at the electrode interface. In addition to the d-band center shift, which alters the electronic structure of the catalyst, the dipole-field interaction plays a crucial role in stabilizing adsorbed CO2 via electrostatic polarization. Moreover, the adsorption energy of CO2 serves as a fundamental descriptor for catalytic activity, as stronger CO2 binding often correlates with enhanced reactivity. Finally, the local electric field effects, which modulate the adsorption and reaction energetics of key intermediates, further influence the catalytic performance. These descriptors collectively highlight the considerable potential of Au nano-angle with EEF in boosting CO2 adsorption and activation48. DFT calculations provide qualitative insight, though real catalytic conditions may involve additional complexities beyond the present model. Collectively, these findings establish a dual electric field effect—that is, a two-stage manifestation of the same electric field, where the external field first polarizes and enriches approaching CO2 molecules in the pre-adsorption regime, while the interfacial field subsequently stabilizes the adsorbed intermediates and lowers reaction barriers in the post-adsorption regime, forming a continuous field-mediated activation pathway.

Synthesis and characterizations of Au nanostructures
To probe experimentally the predictions, we used growth-purification process as a convenient and scalable means of preparing desired Au-catalysts of nanoparticle with obtuse-angle surface and high-curvature nanotriangle with acute-angle surface.

The experimental approach for synthesizing AuNTs involves two steps (see “Materials and Methods”): the nanocrystal growth process49 and the product purification50. In a typical experiment, CTAC as the surfactant was first mixed with potassium iodide, followed by the addition of sodium tetrachloroaurate solution. When ascorbic acid is introduced, Au(III) species are promptly reduced to Au(I), which can be visually confirmed by the fading of the solution’s yellow hue to nearly colorless. Subsequently, the swift addition of NaOH triggers the further reduction of Au(I), resulting in the nucleation and growth of AuNTs49. To isolate Au nanotriangles free from quasi-spherical impurities, we employed a purification route based on controlled aggregation and precipitation. The shape-selective separation occurs through depletion-induced attractions among particles in the CTAC micellar medium. These forces emerge when the interparticle gap becomes smaller than the effective size of the micelles acting as the depleting species50. This approach presents two crucial benefits: the stability of the AuNTs is not compromised at high CTAC concentrations, and centrifugation is not necessary due to the nature of the CTAC stock solution. In addition, when cetyltrimethylammonium halide is used, a surfactant bilayer on gold nanostructures has been evidenced51. The removal of residual isotropic particles was visually evident from the gradual color transition of the colloid from deep blue to indigo. Spectroscopically, this purification is reflected by a marked decrease in the 520–530 nm plasmon band associated with isotropic species50, while the characteristic dipolar resonance of AuNTs remained dominant (Supplementary Fig. 5). Transmission electron microscopy (TEM) analysis confirmed the separation of NTs: the supernatant mainly comprised different quasi-isotropic particles (Supplementary Fig. 6), while the high-purity level of NTs was in the precipitate (Supplementary Fig. 7 and Fig. 3a–d).

The blue colloidal solution was obtained by redisperse precipitation with water (inset of Fig. 3e). As displayed in Fig. 3e, the resulting Au nanoplates exhibit a pronounced plasmonic resonance peak centered near 655 nm and a minor shoulder around 570 nm, corresponding to the in-plane and out-of-plane dipolar resonance, respectively49. The absence of a characteristic resonance around 520–530 nm indicates a predominance of anisotropic nanostructures, consistent with the TEM observations. The edge length distribution of the monodispersed AuNTs with sharp tips was 67.30 ± 3.29 nm (Supplementary Fig. 7). High-resolution TEM (HRTEM) images and X-ray diffraction (XRD) patterns suggested that the planar facet was the (111) facet (Fig. 3c and f). The corresponding fast Fourier transform (FFT) pattern taken from the middle area of a nanostructure in the inset of Fig. 3c confirms that the as-prepared AuNTs were single crystalline49.

More detailed information was provided by the aberration-corrected high-angle annular dark-field scanning TEM (AC-HAADF-STEM) images showing the precise surface structures of AuNTs. As shown in Fig. 3d, the edges of the AuNTs were mainly enclosed by the basal (111) planes and the stepped (211) facets, revealing the surface steps on AuNTs, which could thereby create catalytically active sites due to their under-coordinated chemical states.

Gold nanodisks (AuNDs) capped by CTAC were synthesized via tip-selective oxidative etching of AuNTs with HAuCl4, yielding uniform circular disks with consistent (111) facets (Supplementary Fig. 8)52. Besides, Au obtuse-angle nanoparticles (AuNPs) were synthesized using a seed-mediated method in aqueous hexadecyltrimethylammonium bromide (CTAB) solution53. The AuNPs exhibited the plasmon resonance peak of 534 nm with narrow full width at half maximum (Supplementary Fig. 9). TEM imaging (Supplementary Fig. 10) shows that their sizes and shapes are relatively uniform, with an average edge length of 35.98 ± 5.45 nm. In addition, Au nanoparticles in commercial gold carbons (Au commercial particles, AuCPs) exhibit a similar near-spherical morphology with a diameter distribution of 8.48 ± 3.00 nm (Supplementary Figs. 11 and 12). As expected, XRD (Supplementary Figs. 13 and 14), X-ray absorption near-edge structure spectroscopy (XANES, Supplementary Fig. 15) and X-ray photoemissions spectroscopy (XPS, Supplementary Fig. 16) studies suggested that all four samples (i.e., AuNTs, AuNDs, AuNPs, and AuCPs) comprised the same pure Au0 phase.

CO2 electroreduction in metal-cation-free acidic media
To validate the predicted enhancement of CO2R by EEF, we evaluated the catalytic behavior of Au-catalysts (i.e., AuNTs, AuNDs, AuNPs, and AuCPs) in acidic conditions (CO2-saturated 0.07 M H2SO4, pH=1.0, Supplementary Fig. 17 and Supplementary Table 1). A flow cell with gas diffusion electrodes (GDE) was used to promote the mass transfer of reactants (Supplementary Figs. 18 and 19), increase the local concentration of CO2. The GDE was made by applying the catalyst ink on the carbon-based gas-diffusion layer (GDL) substrate by vacuum filtration. In typical CO2R tests, high-purity (99.999%) pre-humidified CO2 gas was delivered to the cathode with a constant flow rate of 20 standard cubic centimeters per minute (sccm). The liquid products were analyzed by 1H nucleic magnetic resonance (NMR) and the gas products were measured by online gas chromatography (GC).

Product analysis with NMR showed that no liquid-phase products could be detected from the electrolysis reactions in electrolytes (Supplementary Fig. 20). Quantitative experiments (Fig. 4a) demonstrated that AuNTs exhibited a high effective current density (jCO) of 122 mA cm–2 at −1.0 V vs. RHE, exceeding 3.1, 4.1, and 8.3 times that of AuNDs (39.4 mA cm–2), AuNPs (29.6 mA cm–2), and AuCPs (14.6 mA cm–2), respectively. As shown in Fig. 4b, c, AuNTs exhibited higher FE_CO and half-cell cathodic energy efficiency (CEE, see “Materials and Methods”) in a wider range of applied potentials (−0.7 to −1.1 V vs. RHE). At the applied potential of −1.0 V vs. RHE, AuNTs could exhibit a near-unity FE_CO with a CEE of about 59% (Fig. 4b, c). Notably, the AuNTs exhibited a stable total current density of ~120 mA cm−2 with an average FE_CO of ~97% at –1.0 V vs. RHE during the long-term (100 h) electrolysis (Fig. 4d). Moreover, the jCO could be further elevated to nearly 200 mA cm–2 at –1.1 V vs. RHE with an FE of ~90% and an EE of ~52% (Fig. 4a–c), providing the opportunity to promote CO production by simply applying more negative cathodic potentials while maintaining high selectivity.

In short, the AuNTs exhibit high activity, selectivity, durability, and energy efficiency for CO evolution in metal cation-free acidic solutions, representing one of the most efficient CO2R electrocatalysts with high overall energy efficiency and CO2 utilization (Supplementary Table 2).

Morphology effect
To elucidate the intrinsic effects of nanostructure morphology on CO2R in metal-cation-free acidic media, we systematically compared the catalytic performance of spherical AuCPs and two-dimensional circular AuNDs. These nanostructures exhibit distinct dimensionality while maintaining similar curvature, allowing us to isolate the influence of dimensionality on CO2R activity. Electrochemical measurements reveal that AuNDs, as two-dimensional nanostructures, exhibit significantly enhanced catalytic performance compared to spherical AuCPs. The jCO of AuNDs is approximately 2.7 times higher than that of AuCPs at −1.0 V vs. RHE (Fig. 4a). This enhancement can be attributed to the increased electrochemically active surface area (ECSA, Fig. 4e and Supplementary Table 3), which provides a greater density of catalytically active sites for CO2 adsorption and activation.

To further isolate the role of morphology from potential contributing factors of surfactant, we removed the cationic surfactant CTAC from AuNDs to obtain bare AuNDs (Supplementary Fig. 21). The bare AuNDs exhibited higher jCO than AuCPs, confirming that the 2D structure contributes to increased CO2 reduction activity due to enhanced surface accessibility and active site exposure (Supplementary Fig. 22). However, their Faradaic efficiency (FE_CO) remained comparable to that of AuCPs, suggesting that the 2D morphology alone does not significantly improve catalytic selectivity in the absence of a cationic surfactant. This highlight the critical role of the cationic CTAC layer in modulating selectivity.

Despite their structural similarity to AuNTs in lateral dimensions, AuNDs exhibit significantly lower catalytic activity than AuNTs across all tested potentials (Fig. 4a). The jCO of AuNDs is ~3.1 times lower than that of AuNTs at −1.0 V vs. RHE, which indicate that morphological effects alone cannot fully account for the enhanced catalytic performance observed in AuNTs. Given that both AuNDs and AuNTs share a two-dimensional framework, the additional enhancement observed in AuNTs suggests the presence of another key factor beyond general morphological contributions.

This comparative analysis establishes that while 2D nanostructures enhance CO2R activity through increased surface area and active site density, the enhanced catalytic performance of AuNTs cannot be solely attributed to morphology. Instead, the presence of high-curvature tip geometries in AuNTs likely plays a crucial role by inducing strong local electric fields, which will be further explored in the next section.

Tip-enhanced electric field effects
Given that both AuNTs and AuNDs exhibit similar two-dimensional structures, lateral dimensions, and ECSA, we hypothesize that the high-curvature tips of AuNTs induce strong local electric fields, thereby facilitating CO2 activation and enhancing reactivity.

To experimentally probe this hypothesis, we performed in-situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) to analyze CO2 adsorption and activation dynamics on AuNTs at varying potentials (Supplementary Fig. 23). The intensification of adsorbed CO peaks suggests an increase in the quantity of adsorbed intermediates. To further elucidate the tip effect on the local electric field, we analyzed the Stark tuning shifts of adsorbed CO species on AuNTs under varying potentials. The Stark shift, which reflects the interaction between the localized electric field and the dipole moment of adsorbed species, is significantly larger for AuNTs compared to AuNDs (Supplementary Fig. 24). This pronounced shift provides quantitatively direct spectroscopic evidence that the sharp tips of AuNTs generate a stronger localized electric field. Kelvin probe force microscopy (KPFM) measurements further corroborate this localized electric field enhancement. Surface potential mapping reveals a pronounced increase in potential at the sharp tips of AuNTs, whereas AuNDs exhibit a more uniform potential distribution (Supplementary Fig. 25). This tip-induced potential contrast provides direct experimental evidence of enhanced charge accumulation at the apexes of AuNTs, in agreement with finite element simulations (Fig. 2d).

To isolate the effect of tip-enhanced electric fields from morphological contributions, we synthesized passivated AuNTs, in which the sharp tips were selectively rounded to suppress local electric field effects while maintaining the overall triangular morphology (Supplementary Fig. 26). Electrochemical testing revealed that compared to AuNTs, passivated AuNTs exhibited a substantial reduction in jCO and FE_CO (Supplementary Fig. 27), closely resembling the performance of AuNDs (Fig. 4a−c). Given that both passivated AuNTs and AuNDs share similar 2D structures, this result confirms that the higher performance of AuNTs originates from their sharp tips rather than their basal two-dimensional framework alone.

Taken together, these experimental and computational results conclusively demonstrate that the enhanced catalytic performance of AuNTs arises from the synergy between their 2D morphology and tip-enhanced local electric fields. The sharp tips of AuNTs concentrate charge, amplify local electric fields, and facilitate CO2 activation, which in turn accelerates reaction kinetics. This understanding forms the basis for the next section, where we analyze the synergistic effects of morphology and electric field enhancement in achieving optimal CO2R performance.

Synergistic effects of nanostructure and local electric field
Tafel analysis elucidated the accelerated kinetics for CO evolution on AuNTs. Previous studies suggest that during two-electron CO2R, the first one-electron step of CO2 to COOH* or CO2δ− intermediates determines the rate for the combined process38 and hence the Tafel slope54. As shown in Supplementary Fig. 28, the Tafel slope measured for the AuNDs, AuNPs and AuCPs of 117, 121, and 123 mV dec−1 agrees well with prior reports55, whereas the much lower Tafel slope of 43 mV dec−1 obtained for the AuNTs indicates a faster first-electron transfer step38,55 and confirms the intrinsic advantage of AuNTs in CO2 reduction56,57. Electrochemical impedance spectroscopy (EIS) measurements also corroborated that AuNTs exhibit the smallest semi-circle diameter of the Nyquist plot, reflecting an acceleration of the charge transfer process at electrode/solution interfaces (Supplementary Fig. 29).

Intrinsic performances can be further compared by normalizing jCO to the ECSA. The AuNTs still exhibited the highest ECSA-normalized jCO at −1.0 V vs. RHE (2.21 mA cm−2), significantly surpassing AuNDs (0.81 mA cm–2), AuNPs (0.54 mA cm–2), and AuCPs (0.30 mA cm–2) (Supplementary Fig. 30), indicating higher intrinsic CO2R activities due to the existence of the tip-enhanced electric field. This finding confirms that the enhanced catalytic activity of AuNTs is not solely attributed to their larger surface area but is an inherent result of tip-enhanced local electric fields, which facilitate CO2 adsorption and electron transfer. Therefore, by combining an increased number of active sites with higher average intrinsic activity per site, AuNTs achieve the highest overall CO2R performance among the four catalysts.

These results demonstrate that the enhanced CO2R activity of AuNTs is primarily governed by the synergy between their 2D morphology and tip-enhanced local electric fields. The 2D morphology increases the density of electrochemically active sites, while the sharp tips generate localized electric fields that polarize CO2 molecules, reduce activation barriers, and facilitate electron transfer. While these structural features play a decisive role in improving catalytic activity, additional factors may further influence CO2R selectivity. In particular, the biomimetic proton rejection mechanism induced by the cationic surfactant layer will be discussed in the next section.

Improving CO2R selectivity by bionic design
Generally, where water can pass, (hydrated) protons can traverse. However, aquaporin, a specialized protein channel in the cell membrane, selectively permits the passage of electrically neutral water molecules while effectively repelling positively charged protons. This selective function is attributed to the positively charged guanidinium group on an arginine residue located near a constricting histidine within the channel (Fig. 1b). Previous reports have shown that the addition of quaternary ammonium salts to the electrolyte can be expected to act as alkali metal ions58–62. In our system, the cationic surfactant layer of CTAC on the Au surface mimics the function of aquaporin. This layer carries positive charges, enabling the selective passage of electrically neutral CO2 molecules while obstructing proton transport. Moreover, beyond electrostatic proton exclusion, such positively charged quaternary ammonium groups (CTA+) may also contribute to a second-sphere electrostatic environment that influences CO2 activation, conceptually analogous to the “electrostatic stabilization” mechanism proposed for ammonium-functionalized molecular catalysts63. While we do not claim direct stabilization of reaction intermediates here, this conceptual framework provides a plausible context for interpreting the improved CO2R selectivity observed for cationic surfactant–modified catalysts discussed below.

It should be emphasized that this analogy operates at the molecular-functional level rather than structural mimicry—the CTAC bilayer reproduces the electrostatic principle of proton exclusion found in aquaporins, rather than imitating their channel architecture. In addition, the interdigitated alkyl chains within the CTAC layer create a hydrophobic interfacial region of reduced water activity and hydrogen-bond density, analogous to the hydrophobic confinement discussed in recent reports64,65. This hydrophobic microenvironment complements the electrostatic proton-blocking effect, collectively stabilizing the local reaction interface and improving CO2R selectivity under strongly acidic conditions.

The FE_CO of AuNTs, AuNDs, and AuNPs, each capped with hexadecyltrimethylammonium halide surfactants, significantly surpasses that of surfactant-free AuCPs (Fig. 4b). This suggests that the CTAC surfactant on the AuNTs catalyst (Supplementary Fig. 31) may function analogously to aquaporin in electrolytes. The adsorption behavior of CTAC on AuNTs during electrocatalytic reactions was analyzed using in-situ ATR-SEIRAS spectroscopy. The results reveal that, under constant potential (−0.7, −1.0, −1.1 V vs. RHE), CTAC remains stably adsorbed on the gold surface, as indicated by the no obviously changed ν_CH peak intensities at 2864 and 2960 cm−1 in the in-situ ATR-SEIRAS spectra (Fig. 5a and Supplementary Fig. 32). Furthermore, variable potential experiments further explored CTAC adsorption behavior under different applied voltages (Supplementary Fig. 23). Upon applying a positive voltage, the upward peaks at 2860 and 2960 cm−1 reflected a decrease in CTAC coverage relative to the open circuit potential. Conversely, as the voltage became increasingly negative, these peaks inverted and intensified, indicating progressive CTAC adsorption. These observations demonstrate that CTAC primarily exists as CTA+ on the Au surface, with its adsorption behavior influenced by the applied voltage. Background spectra, collected before and after the in-situ ATR-SEIRAS measurements under both constant and variable potentials (Supplementary Fig. 33), showed no significant changes, further confirming that CTAC remains stably adsorbed on the Au surface throughout the electrocatalytic process.

To understand the effect of the cationic surfactant on CO2 reduction in acidic electrolytes devoid of alkali cation, we performed experiments using the following control samples (Supplementary Fig. 21): CTAC-capped AuNTs and bare AuNTs (CTAC removed), CTAC-capped AuNDs and bare AuNDs (CTAC removed). The CTAC-capped AuNTs achieved a FE_CO of ~100%, while bare AuNTs showed a reduced FE_CO of 61.9% (Supplementary Fig. 34). The CTAC-capped AuNDs achieved a FE_CO of 92.5%, while bare AuNDs showed a much lower FE_CO of 54.3%. These results demonstrate that the CTAC capping layer plays a crucial role in enhancing the CO2 reduction selectivity by suppressing competing HER, particularly in acidic conditions. Without the CTAC layer, the FE_CO drops significantly, even for AuNTs, which inherently possess a high-curvature structure that promotes CO2 activation.

In addition, we conducted experiments with AuNTs capped with different surfactants—cationic surfactant CTAC, nonionic surfactant polyvinyl pyrrolidone (PVP), and anionic surfactant sodium dodecyl sulfate (SDS)—and simulated the process with molecular dynamics (MD) and Poisson-Nernst-Planck (PNP) modeling. The zeta potential measurements revealed that CTAC-capped, PVP-capped, and SDS-capped AuNTs exhibited positive, neutral, and negative charges, respectively (Fig. 5b). Correspondingly, the FE_CO values were nearly 100%, 62.2%, and 53.3% (Fig. 5c). The MD simulations (Supplementary Data 2) on three surface models of Au/CTAC, Au/PVP, and Au/SDS reveal that the CTAC-capped surface exhibits the strongest proton-repelling ability (CTAC>PVP>SDS), which provides an effective shield to proton transport and reduces proton availability near the surface (Supplementary Fig. 35). These results suggest that cationic surfactants can inhibit the HER, due to electrostatic repulsion between the positively charged surfactant and protons. In contrast, nonionic surfactants exhibit minimal impact on proton behavior, while anionic surfactants could promote HER, through electrostatic attraction of protons. This indicates that the charge characteristics of surfactants are pivotal in modulating FE_CO of CO2R.

Finite element method simulations based on a Possion-Nernst-Plank model66 were subsequently performed to elucidate the role of the cationic surfactant layer in inhibiting H+ mass transport (Supplementary Fig. 36). The Stern layer, located between the cathode and the outer Helmholtz plane (OHP), is devoid of charged species. A surfactant layer with varying charge properties, positioned outside the OHP, exhibited a charge density of immobilized ionic sites (ρ) that was determined to be +20014 C cm−3, as detailed in the “Materials and Methods” section (Supplementary Figs. 37 and 38). According to the experimental loading of CTAC-capped AuNTs on the electrode, the thickness of the surfactant layer was set to 22.1 μm in the simulation (Supplementary Figs. 36 and 38). Beyond this surfactant layer, the simulation environment included movable H+, OH−, and SO4 2− ions. The efficiency of H+ reduction is constrained by the mass transport of H+. At −1.0 V vs. RHE, the migration rate of H+ outside the surfactant layer decreases as ρ increases, indicating that the cationic surfactant layer can screen the electric field and thereby reduce the migration rate of H+, while the anionic surfactant layer exerts the opposite effect (Fig. 5d).

The charge of the surfactant layer also affects the local pH near the cathode surface. Figure 5e shows the pH profiles near the cathode at −1.0 V vs. RHE for different surfactant charges. A positively charged surfactant layer leads to a higher pH near the electrode surface. Figure 5f further illustrates the pH at the outer Helmholtz plane (OHP) for different potentials on AuNTs catalysts covered by surfactant layers with different charge density (unit: C·cm−3) in H2SO4 (pH 1.0). The presence of a positively charged surfactant layer increases the local pH at the OHP.

When the surfactant layer is uncharged, the pH within the region 2 nm to 22.1 μm from the OHP is higher than the bulk pH due to concentration polarization. However, within 2 nm vicinity of the OHP, the pH is lower than the bulk pH, as protons are electrostatically attracted to the cathode. Figure 5f further illustrates the pH at the OHP under varying electrode potentials. When the surfactant is either neutral or negatively charged, a negative shift in the electrode potential does not substantially increase the pH at the OHP. In contrast, with a surfactant layer carrying positive charge density greater than +1000 C·cm−3, the pH within the surfactant layer rises sharply (Fig. 5e and Supplementary Fig. 39), and the local pH at the OHP increases as the electrode potential shifts negatively beyond −0.4 V vs. RHE (Fig. 5f). Notably, when the charge density exceeds +10000 C·cm−3, the pH at the OHP surpasses the bulk pH at the potential region below −0.4 V vs. RHE. Therefore, a local pH increases at the OHP driven by H+ reduction is observed only when the cathode is capped by a cationic surfactant layer. During CO2 reduction, OH− anions generated from CO2 reduction process can neutralize H+ and further elevate local pH10. Therefore, the local pH under the CO2 reduction condition is expected to be higher than the values shown in Fig. 5f.

These findings highlight the crucial role of the surfactant layer in modulating the local concentration of H+, thereby impacting the kinetics of hydrogen evolution in acidic electrolytes. Specifically, the presence of a cationic surfactant layer can effectively impede H+ transport through Donnan exclusion, with this inhibition becoming progressively pronounced as the positive charge density intensifies, leading to a concomitant decrease in the diffusion rate of H+.

Methods
Materials
Hexadecyltrimethylammonium chloride (CTAC, 25wt% in water), gold(III) chloride trihydrate (HAuCl4·3H2O, ≥49.0% Au basis), polyvinylpyrrolidone (PVP, average molecular weight: 10000), L-ascorbic acid (AA assay: 99.7~100.5%), potassium iodide (KI, ≥99.5%), sodium iodide (NaI, ≥99.5%), sodium borohydride (NaBH4, 99%), sodium hydroxide (NaOH, ≥99%), ethanol (>99.9%, absolute grade), sulfuric acid (H2SO4, 96%), nitric acid (HNO3, 69%), and hydrochloric acid (HCl, 36%) were purchased from Sigma-Aldrich. Carbon black (Vulcan XC-72) was obtained from Carbot Co. (USA). Nafion PFSA polymer dispersions (D520, 5% w/w in water, 1-propanol, and ethanol) were purchased from DuPont (USA). The carbon-based gas-diffusion layer (GDL) substrate was purchased from Freudenberg (Germany). All chemicals were used as received without further treatment. Ultrapure water (resistivity 18.2 MΩ cm at 25 °C) obtained by the Milli-Q system was used in all experiments. All glassware was cleaned with aqua regia (V_HCl:V_HNO3 = 3:1) before use.

Synthesis of the AuNTs capped by CTAC
A CTAC-mediated reduction protocol was employed to prepare AuNTs49,50. In a typical synthesis, an aqueous CTAC solution (0.1 M, 1.6 mL) was diluted with 8 mL of ultrapure water, followed by addition of KI (0.01 M, 75 µL). Sequential introduction of equimolar HAuCl4 (25.4 mM, 80 µL) and NaOH (0.1 M, 20.3 µL) generated a pale-yellow mixture. Upon injection of ascorbic acid (0.064 M, 80 µL) under moderate stirring (~700 rpm), the solution gradually became colorless, indicative of the Au(III) to Au(I) reduction. Subsequent injection of NaOH (0.1 M, 10 µL) with rapid shaking triggered nucleation and anisotropic growth, yielding a blue colloidal dispersion.

Purification of the AuNTs capped by CTAC
The crude AuNT dispersion was purified using a CTAC-induced flocculation method adapted from Scarabelli et al.50,67. A concentrated CTAC stock (0.2 M) was added to the as-synthesized solution to promote selective aggregation of nanotriangles via depletion interactions. After standing overnight, the supernatant was discarded, and the precipitate was redispersed in 500 µL of 40 mM CTAC. The gold content (≈17.5 mM Au0) was estimated from the 400 nm absorbance following standard calibration procedures68,69.

Synthesis of AuNDs
AuNDs were derived from controlled oxidative reshaping of preformed AuNTs following a modified Mirkin-type protocol52. Briefly, HAuCl4 (10 mM) was mixed with an AuNT–CTAC dispersion (23 pM AuNTs, 50 mM CTAC) to reach 8 µM HAuCl4. The reaction was maintained at 28 °C for 4 h, after which the mixture was centrifuged (9400 rcf, 10 min) to remove Au(I) species and redispersed in 50 mM CTAC to prevent secondary deposition.

Synthesis of AuNPs
AuNPs were synthesized through a classical seed-mediated growth route53. In brief, small Au seeds were generated by quickly injecting an ice-cold, freshly prepared NaBH4 solution (0.01 M, 0.3 mL) into an aqueous mixture containing HAuCl4 (0.01 M, 0.125 mL) and CTAB (0.1 M, 3.75 mL) under vigorous inversion. The reaction mixture was aged for 1 h at room temperature to ensure complete reduction of Au(III) and stabilization of the seeds. For particle growth, a separate solution was prepared by sequential dissolving CTAB (0.1 M, 6.4 mL), HAuCl4 (0.01 M, 0.8 mL), and ascorbic acid (0.1 M, 3.8 mL) in 32 mL of deionized water. The aged seed suspension was diluted tenfold with water and then added all at once to the growth solution, followed by gentle inversion for approximately 10 s. The dispersion was then kept undisturbed overnight, yielding well-defined AuNPs.

PVP or SDS coating of AuNTs
For polymer or surfactant exchange50, AuNTs@CTAC were centrifuged (1800 × g, 15 min, 30 °C), the supernatant removed, and the pellet redispersed in 0.1 M CTAC (50 µL). After brief sonication and dilution to 0.5 mM Au0, a 10 g L–1 PVP or SDS solution (prepared by 10 min sonication) was added under vigorous stirring to reach a final PVP or SDS concentration of 1 g L–1. The mixture was stirred overnight at ambient temperature to complete ligand exchange.

CTAC removal of AuNTs and AuNDs
The CTAC removal process was carried out as follows: First, 5 mg of carbon black was dispersed into a mixture of 965 μL of catalyst dispersion (3.45 g L–1 in water) and 3 mL of ethanol. The resulting solution was sonicated for 40 min to ensure uniform dispersion. Subsequently, the solution was dissolved in 100 mL of a hydrochloric acid-ethanol solution (HCl:ethanol=1:9, v/v) and refluxed at 78 °C for 12 h. After refluxing, the product was collected via centrifugation, followed by alternating washing with ethanol and deionized water to remove the CTAC. The washing process was repeated several times to ensure the complete removal of the surfactant. The final catalyst was dried under a vacuum at 50 °C to obtain the CTAC-free (bare) AuNTs and AuNDs catalyst.

Electrode preparation
Catalyst (AuNTs, AuNDs, and AuNPs) inks were prepared by dispersing 5 mg of carbon black in the mixture of 80 μL of Nafion, 965 μL of catalyst dispersions (3.45 g L–1 in water), and 3 mL of isopropanol under sonication for 40 min. Catalyst (AuCPs) inks were prepared by dispersing 8.33 mg of AuCPs (40% gold supported on Vulcan XC-72) in the mixture of 80 μL of Nafion, 965 μL of water, and 3 mL of isopropanol under sonication for 40 min. The GDE was made by seriatim spraying about 2.43 mL of the catalyst ink on the carbon-based gas-diffusion layer (GDL) substrate with an effective electrode geometric area of 2 × 1 cm2 and a metal loading of about 1.0 mg cm–2.

Electrocatalytic CO2R tests in the flow cell
The CO2R was performed in a three-electrode flow cell, which was composed of a catalyst-deposited GDE as the working electrode, an Ag/AgCl reference electrode, and a platinum plate anode. The cathodic and anodic compartments were separated by a Nafion 115 proton exchange membrane. The membrane was pretreated in 5% H2O2 solution (80 °C, 1 h), soaked in 0.5 M H2SO4 solution (80 °C, 1 h), immersed in ultrapure water (80 °C, 1 h), and then washed with ultrapure water several times before use. A high-purity (99.999%) pre-humidified CO2 gas flow with a constant flow rate of 20 sccm monitored by a mass flow controller was delivered behind the GDE to diffuse into the catalyst layer presented at the catalytic area (2 × 1 cm2). The 0.07 M H2SO4 (pH=1.0, Supplementary Fig. 17) solution24 was employed as the electrolyte and circulated (5 mL min–1, 30 mL each for the catholyte and anolyte) through the electrochemical cell with a peristaltic pump. The reactions were run for 1 h before the products were collected for analysis. The outlet gas flow, which passed directly through gas chromatography, was measured by a portable electronic flowmeter (Restek ProFLOW 6000) to evaluate the FE_CO accurately. An online gas chromatograph (GC, Shimadzu GC-2014C) equipped with a packed column (MS-13X), a thermal conductivity detector (TCD), and a flame ionization detector (FID) was used to quantify the gaseous products. And the possible liquid products were analyzed by 1H nuclear magnetic resonance (NMR). Typically, 500 μL of electrolyte was extracted from the cathodic chamber after electrolysis and was mixed with 100 μL of D2O. Then, 0.05 μL of dimethyl sulfoxide (DMSO) was added as an internal standard. The one-dimensional 1H spectrum was measured with water suppression using a solvent pre-saturation technique.

In-situ ATR-SEIRAS measurements
Attenuated total-reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) was employed to monitor the reaction intermediates under electrochemical CO2 reduction conditions. Measurements were carried out on a Thermo Nicolet 8700 spectrometer equipped with a liquid-nitrogen-cooled mercury–cadmium–telluride (MCT) detector. A gold film (~60 nm thick) was chemically deposited on a silicon prism via a two-step wet-chemical procedure. Prior to Au deposition, the reflective surface of the Si prism was mechanically polished with diamond suspension, sonicated in deionized water, and treated in piranha solution (H2SO4:H2O2 = 7:3 v/v) for 2 h to remove organic residues.

After film preparation, ~30 μL of catalyst ink was drop-cast onto the Au-coated surface and dried in air. The modified prism was then assembled into a custom spectroelectrochemical cell, serving as the working electrode. A Pt mesh (1 × 1 cm) and an Ag/AgCl electrode positioned through a Luggin capillary were used as counter and reference electrodes, respectively. Prior to data collection, CO2 gas was bubbled through the cell for 30 min to replace the atmosphere, followed by a 20 min equilibration to ensure complete dissolution.

Spectra were recorded under potentiostatic or potential-step conditions using a CHI electrochemical workstation. The differential spectra were processed as ΔR/R = (E_S − E_R)/E_R, where E_S and E_R represent sample and reference spectra, respectively. Unless otherwise noted, the spectral resolution was 4 cm−1.

Characterizations
Powder X-ray diffraction (PXRD) patterns were acquired on a Rigaku MiniFlex 600 diffractometer with Cu Kα radiation (λ = 0.154056 nm) operated at 40 kV and 50 mA, with a scan rate of 1° min−1. UV–vis absorption spectra were collected on a Shimadzu UV-2600 spectrophotometer (slit width = 2.0 nm; resolution = 0.5 nm). 1H NMR spectra were recorded on a Bruker Ascend 400 MHz spectrometer using D2O as solvent; chemical shifts are referenced to the residual solvent signal or TMS (0.00 ppm). Elemental compositions were quantified by inductively coupled plasma optical emission spectroscopy (ICP-OES, Agilent 720) under the following conditions: RF power 1.2 kW, plasma gas flow 15.0 L min–1, auxiliary gas flow 1.5 L min–1, nebulizer gas flow 0.75 L min–1, injection delay 15 s, stabilization delay 15 s. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Scientific ESCALAB 250Xi system equipped with a monochromatic Al Kα source (hν = 1486.6 eV). Spectra were calibrated against the C 1s peak at 284.8 eV, and measurements were performed at a base pressure of ≈1 × 10–9 mbar (analysis area ≈ 500 µm, sampling depth ≈ 10 nm). Au L-edge X-ray absorption fine structure (XAFS) spectra were collected at beamline BL01C1 of the National Synchrotron Radiation Research Center (NSRRC) in fluorescence mode using a solid-state detector. Gold foil was used as the reference standard. Data reduction and fitting were performed with the Athena and Artemis codes. For the X-ray absorption near-edge structure (XANES) region, μ(E) was background-subtracted and normalized, and the results are presented as normalized absorption for all samples and standards. Transmission electron microscopy (TEM) was performed on JEOL JEM-2100 (200 kV) and FEI Tecnai G2 F30 (300 kV) microscopes. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging and elemental mapping were acquired on the FEI Tecnai G2 F30 operated at 300 kV, while atomic-resolution HAADF-STEM measurements were carried out using an FEI Themis Z with double aberration correctors.

Measurements of Au and CTAC ratios using XPS analysis
We conducted XPS analysis to determine the content of gold (Au) and cetyltrimethylammonium chloride (CTAC) on the surface of AuNTs. The following steps were taken: First, integration of peak areas: The areas under the identified peaks were integrated. The peak areas obtained were: Au 4f: −7805, N 1s: −132380. Second, sensitivity factor correction: The peak areas were corrected using the relative sensitivity factors (RSF) for Au 4f and N 1s, which are 9.58 and 1.8, respectively. The corrected peak areas were: Corrected Au 4f area: −814.72, Corrected N 1s area: −73544.44. Last, atomic ratio calculation: The atomic ratio of N to Au was calculated as follows:

Atomic ratio = (Corrected N 1s area / N 1s RSF) / (Corrected Au 4f area / Au 4f RSF) (1)

The high atomic ratio of N to Au is 90.27, indicating a significant presence of CTAC on the surface of the AuNTs.

Determination of charge density of charge layer
To quantify the number of Au atoms on the electrode, we begin by determining the molar amount of Au using the relation:

Number of Au atoms = n_Au × NA = (m_Au / M_Au) × NA (2)

where the total amount of Au deposited on the electrode is 2 mg, the molar mass of Au is 197 g mol–1, and Avogadro’s number is 6.022 × 1023 mol−1. This yields a total of approximately 6.11 × 1018 Au atoms.

Next, the number of CTAC molecules on the electrode is estimated. Given an atomic ratio of Au to N of 1:90.27, the total number of nitrogen atoms is calculated as:

Number of N atoms = 6.11 × 1018 atoms × 90.27 = 5.52 × 1020 atoms (3)

Since each CTAC molecule contains one nitrogen atom, the total number of CTAC molecules is equal to the number of nitrogen atoms, resulting in 5.52 × 1020 CTAC molecules.

To compute the total charge Q, we recognize that each CTAC molecule carries one positive charge, with an electron charge of 1.602 × 10−19 C. The total charge is therefore:

Q = 5.52 × 1020 molecules × 1.602 × 10−19 C molecule−1 = 88.46 C (4)

The volume of the electrode surface layer V is determined using the surface layer thickness d of 22.1 micrometers and the electrode area A of 2 cm2:

V = A × d = 2 cm2 × 22.1 × 10−4 cm = 4.42 × 10−3 cm3 (5)

The volume charge density ρv is calculated as:

ρv = Q / V = 88.46 C / 4.42 × 10−3 cm3 = 20013.57 C cm−3 (6)

Notably, the surface charge density ρs can also be estimated using the electrode area:

ρs = Q / A = 88.46 C / 2 cm2 = 44.23 C cm−2 (7)

Calculation of Faradaic efficiency (FE)
The FE for CO evolution was measured based on the definition of FE:

FE_CO = (Q_CO / Q_total) × 100% (8)

Q_total and Q_CO can be obtained from the following equations:

Q_total = ∫_0^t I(t)dt (9)

Q_CO = 2 × n_CO × F (10)

where n_CO is the molar amount of the produced carbon monoxide measured by gas chromatograph, F is the Faradaic constant (96484 C mol–1), and t is the total reaction time for reducing carbon dioxide.

Calculation of partial current density of formate (jCO)

j_CO = j_total × FE_CO (11)

where j_total is the total current density obtained from the chronoamperometry tests.

Calculation of half-cell cathodic energy efficiency (CEE)
Half-cell cathodic energy efficiency (CEE) quantifies the ratio of the energy effectively used for CO2 reduction to the total energy input in the electrolysis process. The calculation of half-cell CEE assumed a zero overpotential of the oxygen evolution reaction at the anode side, which is shown in the equation below70:

CEE = [(1.23 + E_CO) × FE_CO] / [1.23 + (E_cathode)] (12)

where E_CO = –0.109 V is the standard thermodynamic potential of CO2/CO (vs. RHE), FE_CO is the Faradaic efficiency of CO production, and E_cathode is the applied cathode potential vs. RHE.

ECSA measurement
The ECSA was determined via cyclic voltammetry in 50 mM H2SO4, utilizing the well-established hydrogen underpotential deposition method. Specifically, we integrated the charge of the hydrogen adsorption peak obtained from cyclic voltammetry scans. Cyclic voltammograms were recorded in a potential range of 0.05 to 1.45 V (versus SCE) at a scan rate of 50 mV s–1 and repeated until stable traces were obtained. The ECSA was calculated using the equation:

ECSA = Q_H / (Q_ref × C_Au) (13)

where Q_H is the integrated charge of the hydrogen adsorption peak obtained from CV scans, Q_ref = 448 μC cm−2 is the reference charge density for a monolayer of hydrogen adsorption on Au, and C_Au is the metal loading on the electrode.

DFT calculations for dipole moment and electrostatic potential
Quantum chemical studies are performed using DFT implemented in the GAUSSIAN 16 package71. Geometry optimization and frequency analysis are calculated at B3LYP hybrid functional72 with GD3BJ dispersion correlation at 6-31G(d) basis sets. The structures are also optimized under different magnitudes of the external electric field73 along the X axis. The electrostatic potential surfaces (ESP) diagram and the dipole moment were performed by Multiwfn 3.874 and VMD v 1.9.375 molecular visualization software. The bonding energy of C=O is calculated as follows: E_b = E(C=O) + E(O) − E(CO2).

Finite element analysis
Secondary currents and species diffusion on the electrode surface were established based on COMSOL to compare the effect of different electrode shapes76. COMSOL Multiphysics 6.2 described the species concentration migration diffusion, potential distribution, and current transport. A geometrical model was created with an electrolyte region of 200 nm in length, width, and height, an equilateral triangle with 70 nm side lengths in the region, a truncated octahedron with size of 36 nm, and a sphere with a radius of 9 nm to characterize the electrode morphology. Since the tip effect of the triangle is larger than that of the obtuse-angle and spherical particles, the electric field and current within the electrolyte converge toward the tip of the triangle, while the species diffusion is subjected to the electric field, migrates toward the bottom, and is guided by the electric field to converge in the tip region of the particles. The main boundary conditions of the model: the top loading current density is 103 A m–2, and the electrolyte on the upper surface maintains a 102 mol m−3 ion concentration to equivalently characterize the overall electrolyte. The exchange current density at the electrode surface conformed to the Butler–Volmer equation. The secondary current can be obtained from the following equations:

∇·i_l = Q_l (14)

i_l = −σ_l ∇φ_l (15)

where i_l is the electrolyte current, φ_l is the electrolyte potential, σ_l is the electrolyte conductivity (0.055 S m−1). Due to the charged surface modifiers, ρ_s (44.23 C cm−2) was introduced on the electrode surface.

The electrochemical module in COMSOL was used to obtain the CO2 to CO reaction current density using the Butler–Volmer equation:

i_i = i_0 [exp(α_a Fη / RT) − exp(−α_c Fη / RT)] (16)

where α_a and α_c are the dimensionless anodic and cathodic charge transfer coefficients, respectively, F is the Faraday constant, R is the universal gas constant, and T is temperature, taken to be 298.15 K. The exchange current density i_0 obeys the Arrhenius law:

i_0 = i_0 / exp(−E_a / k_B T) (17)

where k_B is the Boltzmann constant, and E_a is the activation energy of the CO2 to CO reaction, which was experimentally obtained to be 0.59 eV.

Poisson-Nernst-Planck simulation
A one-dimensional stationary simulation was done with COMSOL Multiphysics. Nernst Planck equation and the Poisson equation were employed for the simulation. The simulation model was established referring to66 and the geometry and other settings of the model are specified below. The geometry is one-dimensional and consists of 2 domains, the surfactant layer and the solution layer. According to our experiment, the thickness of the surfactant layer is 22.1 μm. Supplementary Fig. 36 shows the governing equations and boundary conditions, as well a list of all parameters and coefficients used in the simulations. The governing equation for transportation is:

∇·J_i = R_i (18)

where the ∇ operator stands for d/dx, J_i is the flux and R_i is the production rate of the species. J_i was given by:

J_i = −D_i ∇C_i − z_i F C_i D_i ∇φ / RT (19)

In this equation, D, C, and z are the diffusion coefficient, concentration, and charge of ion species, respectively. φ is the potential, F is the Faradaic constant, and u is the velocity of the solvent. The diffusion coefficients in the surfactant layer were set to 1/10 of the values in water (the solution layer) for each species.

The electric charge distribution was assumed to be uniform in the surfactant layer and the value was set −100 C cm−3 to +20014 C cm−3 according to the estimate.

The reversible reaction between H+ and OH− was considered in the simulation:

H2O ↔ H+ + OH− (20)

where kw1 is the rate constant of water dissociation, kw2 is the rate constant of neutralization reaction. Thus, the R_SO4 2− = 0 (21)

R_H+ = R_OH− = kw1 − kw2 C_H+ C_OH− (22)

OHP is defined as the origin of x axis. The left boundary condition (x=0) of the Nernst Planck equation is the flux J_i, which is built in COMSOL using the Flux/Source boundary. J_i is given by:

J_SO4 2−,x=0 = J_OH−,x=0 (23)

For specie H+, J_i is given by:

J_H+ = −Aγ_H+ βC_H+ exp(−Fβ(φ_M − φ_x=0) / RT) (24)

β is the transfer coefficient. γ_H+ is the activity coefficient of H+.

The right boundary (x=L_tot) condition is the concentrations of species in bulk solution, which is given by:

C_H+,x=Ltot = 0.1 M (25)

C_OH−,x=Ltot = 10−13 (26)

C_SO4 2−,x=Ltot = 0.05 M (27)

The Poisson equation is utilized to model the electric field generated from the charged species. The Poisson equation is given by:

ε_r ε_0 ∇^2φ = ρ_v + F Σ z_i C_i (28)

In the above equation, ρ_v is the charge density carried by the surfactant layer, which is 0 outside the layer. ε_0 is the permittivity of vacuum and ε_r is the relative permittivity. The left boundary (x=0) condition of Poisson equation is the surface excess charge density of the cathode, which is built in COMSOL using the Flux/Source boundary. The surface charge density of the cathode σ is given by:

σ = −ε_r ε_0 (φ_x=0 − φ_M) / d (29)

where φ_x=0 is the potential at OHP, φ_M is the potential at electrode, d is the thickness of Stern layer, which was set to 0.4 nm.

The right boundary (x=L_tot) is the potential of bulk solution, which is given by:

Φ_x=Ltot = 0 V (30)

All potential values used in the simulation is on the point of zero charge (PZC) scale.

Potential conversion from gold PZC to RHE
The potential values used in the simulations are referenced to the PZC of gold. In acidic solutions, the PZC of gold is approximately 0.20 V versus the standard hydrogen electrode (SHE). To convert these potential values from the PZC reference to the SHE scale, we apply the following relation: φ_SHE = φ_PZC − 0.20 V. Subsequently, the potentials are converted from the SHE to the RHE scale using the established relationship in acidic media: φ_RHE = φ_SHE + 0.0591 × pH.

DFT calculations for valence band and thermodynamic energy barrier
DFT simulations were performed using the Vienna Ab initio Simulation Package (VASP) within a plane-wave basis framework and the projector augmented wave (PAW) method. The exchange–correlation energy was described by the Perdew–Burke–Ernzerhof (PBE) functional under the generalized gradient approximation (GGA) scheme. A kinetic energy cutoff of 520 eV was applied for the plane-wave expansion. The Brillouin zone was sampled with Γ-centered k-point meshes generated automatically by VASP, using a grid density of 3×3×1 for the Au(111) slab and 2×2×1 for the stepped Au(211) surface. All atomic positions were optimized until the maximum residual force on each atom was below 0.01 eV Å–1, and the total energy convergence criterion was set to 10–5 eV. Electronic occupations were treated with Gaussian smearing of 0.05 eV, and dispersion interactions were corrected using the DFT-D3 scheme of Grimme. For calculations involving an external electric field, a field strength was applied along the surface normal to simulate the interfacial electrochemical environment. The Gibbs free energy change (ΔG) of each elementary step was evaluated using the computational hydrogen electrode model:

ΔG = ΔE_DFT + ΔE_ZPE − TΔS + ΔG_U (31)

where ΔE_DFT, ΔE_ZPE, and ΔS represent the total electronic energy, zero-point energy, and entropy change, respectively. ΔG_U = −n_e U accounts for the effect of the applied potential U relative to the standard hydrogen electrode (SHE), with n_e being the number of transferred electrons.

Overpotential reduction calculation
The reduction in overpotential (ΔE) was calculated from the corresponding free energy change (ΔG) using the thermodynamic relationship:

ΔE = ΔG / nF (32)

where n is the number of electrons transferred in the rate-determining step (n=1) and F is the Faraday constant (96485 C mol−1).

Molecular dynamics simulations
We have investigated the diffusion process of proton and CO2 towards an Au surface that is coated with adsorption layers of CTAC, PVP, and SDS, respectively, using molecular dynamics simulations. Three models, Au/CTAC, Au/PVP, and Au/SDS were built. In each model, the Au surface was in contact with the modification layer and the solution. The Au surface model (1152 Au atoms) was constructed by cleaving a bulk Au crystal along (111) plane to create a 6-layer slab with XY interface area of 34.61 × 39.96 Å2. The CTAC and SDS layers were prepared by placing 48 molecules on the Au slab, respectively, while PVP layer was represented by 24 10-unit PVP oligomers. Due to the inability of non-reactive force fields to capture the dynamic nature of proton hopping, we have chosen to represent protons explicitly as H+ ions in our simulations. For Solution/PVP/Au model, the solution included 100 H+, 50 SO4 2−, 50 CO2, and 4000 H2O molecules. For Solution/CTAC/Au and Solution/SDS/Au, 48 Cl− and 48 Na+ counterions were also added to the solution maintain electrical neutrality. Each model had a Z direction length of 200 Å, leaving a vacuum layer exceeding 50 Å to minimize periodic image interactions.

In each case, at the beginning of the simulation, energy of the model system was minimized to eliminate the steric overlap and correct any inappropriate geometries. This was followed by a 7.0 ns NVT molecular dynamics simulation at a constant temperature of 298 K to track system dynamics. The two bottom most Au layers were held fixed during all simulations. An electric field of −0.012 V/Å, oriented along the Z direction, was applied.

LAMMPS77 and PCFF-INTERFACE force field78 were used to perform the molecular simulations. A cutoff of 1.2 nm was employed to capture van der Waals and short-range electrostatic interactions in the systems, and the PPPM method with a relative precision of 10−5 was used to account for the long-range electrostatic interactions79. During the molecular dynamics simulations, a timestep of 1.0 fs was used, and the temperature was controlled by the Nosé-Hoover thermostat80. Equations of motions were integrated using the Verlet velocity method.

Data availability
The authors declare that the data supporting the findings of this study are available within the paper and its Supplementary Information files. Schematic diagrams are provided for Fig. 1 in the paper. Source data for Figs. 2–5 and Supplementary Figs. are included with this paper. Additional data are available from the corresponding author upon request. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this manuscript, the authors reported that they prepared and applied triangle Au nanostructures capped with a CTAC layer for CO2 reduction in claimed “metal-cation-free” H2SO4 electrolytes. They concluded that high FE for CO in acidic conditions was due to the so-called “electric-field-driven” effect and “bioinspired proton blocking” effects.\n1. “electric-field-driven” effect\n(a) No experimental evidence to support the “electric-field-driven” effect claimed by the authors\nTip-enhanced electric field intensification is not a new concept. It was studied and reported for electrocatalysis as early as 2016 (Nature 537, 382–386 (2016)). Since 2016, several experimental methods have been developed to characterize and prove this “tip-enhanced electric field intensification”, for example, conductive atomic force microscopy; Kelvin probe atomic force microscopy; stark tunning shift of adsorbed species (such as CO in this study) by in-situ ATR-SEIRAS spectra and so on. However, there is no experimental data or evidence in this manuscript to support the so-called “Tip-enhanced electric field intensification” claimed by the authors.\nTheoretical calculation simulation is a useful tool to help explain some experimental phenomena and results. However, merely relying on computational simulation but not any experimental evidence or data is unacceptable. The conclusion is not convincing to this reviewer. In addition, most information/statements provided by the simulation results in this study have already been reported and discussed in the literature of the last few years (https://doi.org/10.1016/j.jallcom.2024.175549).\n(b) There is no clear correlation between the good catalytic performance of triangle Au and the “electric-field-driven” effect\nThe authors claimed that the higher intrinsic catalytic activity of AuNTs compared to those of AuNPs and AuCPs is due to the “tip-enhanced electric field”, which is not convincing based on the results in this work. As AuNTs, AuNPs and AnCPs have different sizes and shapes/morphology and each of them exposes different facets and structural edges, all these factors can lead to different activity (normalized by ECSA) and Tafel slope. The authors didn’t discuss these factors, but instead assigned the improved activity to the “electric-field-driven” effect, which didn’t make any sense.\nThe result of ATR-SEIRAS used to show that “the stronger potentials enhance the adsorption of these intermediates” doesn’t make sense! As mentioned in point 1, the local electric field should be characterized by a Stark tunning shift of adsorbed species (such as CO in this study) using in-situ ATR-SEIRAS spectra.\nExcept for the calculation and the authors’ own explanation, no clear or supporting experimental results suggest the correlation between improved activity and the “electric-field-driven” effect.\n2. “bioinspired proton blocking” effects\n(a) What’s the indeed similarity between Aquaporin and the CTAC layer?\n(b) Aquaporin has a water transport channel. What’s the channel structure of the CTAC layer?\n(c) How can the author demonstrate that CTAC forms a double layer shown in Figure 1e? Is there any experimental evidence or data to support this structure? Otherwise, the figure is very misleading.\n(d) Based on the authors’ explanation, the selectivity of CO2R in an acidic solution is due to the presence of the cationic CTAC layer. However, CTAC-capped, PVP-capped, and SDS-capped AuNTs all show >50% FE for CO, which indicates that the cationic layer is unlikely the main reason for the good selectivity of CO2R in acidic conditions.\n3. “metal-cation-free” H2SO4 electrolytes\nThe authors claimed that they conducted CO2R in a “metal-cation-free” solution. However, they didn’t describe how they pretreated the electrolyte to obtain such a “metal-cation-free” solution.\nIs it a real “metal-cation-free” solution?\nDid the authors use ICP-MS to measure the M+ concentrations before and after the experiments to confirm it is a “metal-cation-free” solution?\nIs there any M+ impurity introduced into the solution by the catalysts or materials used in the experiments?\nPlease read the reference Nat. Catal. 2021, 4, 654−662 and study the pretreatment methods in this reference where the “metal-cation-free” condition is strictly created, discussed, and studied for electrocatalysis.", "reviewer_2": "Response Letter to Reviewer Comments\nWe sincerely thank all reviewers for their careful evaluation of our manuscript and their insightful\nfeedback. Their constructive comments have significantly contributed to the improvement of the\nmanuscript. We have addressed all reviewers’ points raised and made corresponding revisions, which\nare detailed below (reviewers’ comments in black; author responses in blue), with the revisions\naccordingly highlighted in the updated manuscript.\nReviewer #1\nIn this manuscript, the authors reported that they prepared and applied triangle Au nanostructures\ncapped with a CTAC layer for CO reduction in claimed “metal-cation-free” H SO electrolytes. They\n2 2 4\nconcluded that high FE for CO in acidic conditions was due to the so-called “electric-field-driven”\neffect and “bioinspired proton blocking” effects.\nReviewer #2:\nReferee report on the manuscript \"Electric-field-driven CO polarization and bioinspired proton\n2\nblocking unlock CO reduction in strong acid without metal cation\" by Chen et al on October 30, 2024.\n2\nThe authors have developed a bio-inspired, proton-transport blocking approach to control the activity\nand selectivity of gold-catalyzed CO reduction reaction (CO RR) by introducing a hydrophobic\n2 2\nhexadecyltrimethylammonium (CTAC) layer on differently-shaped gold nanoparticles (NPs). The\nresults show that the introduction CTAC on triangular AuNPs leads to the highest activity, selectivity,\nand efficiency in CO RR. The improvements are suggested to result from the high electric field at the\n2\nsharp tips of the AuNPs and blocked proton transport through the CTAC-layer. The results are\ninteresting as they demonstrate a practical proton-blocking strategy to improve the CO RR selectivity.\n2\nHowever, I am not fully convinced by the mechanistic picture behind the improved performance and\nmore discussion is needed.", "reviewer_3": "This manuscript presents exciting results in the field of CO2 reduction in acidic media, showing excellent CO current densities and suppression of HER. The authors use two approaches to accomplish this: electric-field enhancement via tuning the catalyst shape, and charge-selective layers to limit local proton concentration. While neither approach is particularly novel on its own, their combination with application to CO2RR in acidic media without alkali metal cations is. These strategies may also be broadly applicable to other areas of electrocatalysis and engineered microenvironments.\nThe authors present compelling, technically sound data. However, I have a number of suggestions and clarification points that should be addressed before it is suitable for publication.\nMy first few comments center around the fact that the authors motivate their paper with the need to suppress HER, accomplished via the cationic surfactant bilayers. However, most of the paper focuses on the field-enhancement effects of the shape-controlled nanoparticles. I am a little confused about the interplay between these two effects, and have the following comments:\n1.The electric field enhancement should be better motivated in the introduction\n2.The paper would be more impactful if it was divided into three sections in which the shape and the electric field were both investigated independently, and then a final section with both strategies combined at once.\n3.In the “electrochemical study of tip effect” and “CO2 electroreduction in metal-cation-free acidic media” sections, the goal is “To validate the predicted enhancement of CO2R by [enhanced electric field]”. However, both the field and the cation effects are convoluted here because two of the shapes have cationic surfactants, while the commercial catalyst particles do not.\nTherefore, while the shape is likely enhancing the electric field, it makes it difficult to ascribe performance benefits solely to the shape when the surfactant is also likely playing a role. This also speaks to my above comment 2 that distinct sections that clearly decouple these effects would be beneficial to the story’s overall impact.\n4.The electric field measured at the catalyst surface will be dependent on electrolyte conditions. What were the simulation conditions used in Figure 2? Additionally, the simulations were conducted for the case (I believe) with no surfactants. The extended and distributed charges present in the surfactant bilayer will undoubtedly influence the electric field, and possibly even screen it. The authors even note this on page 12, line 341. This critically would mean that the advantages of both individual approaches may not sum together, again motivating comment 2. Electric field simulations with the bilayers should be conducted.\nAdditional comments:\n5.What evidence is there that the bilayers form on the NTs? And on the NPs? Could they be monolayers? I’d imagine the shape of the particles will also impact the conformity of the surfactant layer.\n6.For the PNP simulations, the surfactant layer was set at 22 um. That seems very large – how was this number chosen? Especially compared with the thickness of the EDL.\n7.The authors present in situ ATR-SEIRAS measurements to demonstrate that the surfactant remains stably adsorbed to the gold surface. What potential were those measurements conducted at? Is the adsorption stable at all potentials? The authors note that there is potential-dependent adsorption, so I would assume not. Also, the system was monitored for only 45 minutes. What happens beyond that time?\n8.The Tafel slopes were fit using only 3 data points. Tafel slope fitting is prone to extreme error/subjectivity (see for example Nature Communications volume 12, Article number: 703 (2021)) Could the authors comment on the stringency of analysis and perhaps add additional points to the fit?\n9.What carbon black was used in the catalyst inks? Was it also a Vulcan XC-72 as used in the commercial catalysts?", "reviewer_4": "This manuscript introduces an approach to CO₂ electroreduction (CO₂R) in strong acidic media, addressing several critical challenges in the field. The study emphasizes the advantages of a metal-cation-free system, which reduces CO₂ reactant losses, eliminates the risk of metal salt precipitation, and enhances device tolerance compared to systems using metal cations.\nThe researchers developed sharp-triangle gold (Au) nanostructures capped with a hexadecyltrimethylammonium chloride (CTAC) layer enriched with cationic sites. Inspired by aquaporins in acidophiles, this design leverages intense local electric fields generated by the high-curvature tips of the Au nanocatalyst to polarize CO₂ molecules. This polarization increases the dipole moment of CO₂, facilitating its adsorption and activation. Additionally, the CTAC layer acts as a proton barrier, suppressing the hydrogen evolution reaction (HER) by mimicking the proton-blocking mechanism of aquaporins.\nThis dual-function design enabled continuous CO₂R for 100 hours in a flow electrolyzer at pH 1.0, achieving an impressive energy efficiency of 60% and nearly 100% Faradaic efficiency for CO production. This bioinspired strategy represents a significant advancement in CO₂R technology by integrating innovative catalyst design principles, potentially paving the way for more efficient and sustainable CO₂ reduction processes.\nOverall, the paper is well-written and presents highly promising results for achieving electrocatalytic CO₂ reduction under acidic conditions. The outcomes in terms of efficiency and selectivity are remarkable, especially given that the electrolyzer is a flow cell, which allows for process scalability.\nI believe this manuscript meets the standards for consideration by Nature Communications and will significantly contribute to the important field of CO₂ utilization via electrocatalysis. Moreover, the presentation quality of the paper, both in terms of text and figures, is outstanding.\nHowever, I have some remarks for the authors to consider:\n1. Extension to Other Products: What about extending this approach to produce more valuable products (e.g., C₂+ molecules)? The work shows high selectivity and good partial density for CO formation, but this is an already well-solved problem.\n2. Supporting Information Figures: The figures in the supporting information should be named, for example, Fig. S1, to avoid confusion with the figures in the main text.\n3. Clarification on CO₂ Adsorption: Lines 143-145 state, “Enhancing CO₂ adsorption on Au surfaces would theoretically promote CO₂R since Au showed relatively weak adsorption to CO₂ species and was located on the right side of the volcano plot for CO₂R1,43.” This key concept should be expanded. Moreover, the referenced paper (Phys. Chem. Chem. Phys., 2014, 16, 4720) discusses the binding of intermediates (*CO, *COH, *CHO, and *COOH) rather than CO₂ adsorption.\n4. Figure Reference: Lines 146-147 seem to omit a reference. Is it also Figure 2f?\n5. Overpotential Reduction: Lines 153-155 should report the lowering of the overpotential when the external electric field (EEF) is applied, which can be derived from the free energy of the limiting step.\n6. Activation Check: Lines 159-160 suggest that adsorption energy alone is insufficient to check activation, as this could still be a physisorption process. Consider looking at Amira’s paper and investigating C-O elongation, O-C-O bending, and Bader charge analysis for charge transfer from the surface to CO₂.\n7. Synthesis Methodology: How extensible is the synthetic methodology for Au nanostructures to other metals? While Au is excellent for CO production with high selectivity, achieving high selectivity for more valuable chemicals remains a challenge.\n8. Comparison with Literature: How do the partial current density values of CO on the three Au systems compare with other literature values for CO₂-to-CO conversion on Au catalysts?\n9. Definition of CEE: Define the half-cell cathodic energy efficiency (CEE) on page 238."}, "author_response": "We sincerely thank all reviewers for their careful evaluation of our manuscript and their insightful feedback. Their constructive comments have significantly contributed to the improvement of the manuscript. We have addressed all reviewers’ points raised and made corresponding revisions, which are detailed below (reviewers’ comments in black; author responses in blue), with the revisions accordingly highlighted in the updated manuscript.\nReviewer #1\nIn this manuscript, the authors reported that they prepared and applied triangle Au nanostructures capped with a CTAC layer for CO reduction in claimed “metal-cation-free” H SO electrolytes. They concluded that high FE for CO in acidic conditions was due to the so-called “electric-field-driven” effect and “bioinspired proton blocking” effects.\nAuthor Reply:\nThank you for the reviewer’s attention and support of our work. We noted your interest in the “metal-cation-free H SO electrolyte” and the proposed mechanisms, including the “electric-field-driven effect” and “bioinspired proton blocking effects”. To address this, we have provided more rigorous and detailed explanations in the revised manuscript to further validate and support these conclusions. We hope the revised manuscript addresses your concerns and meets your expectations for publication. Thank you again for your valuable feedback.\nR1Q1. “electric-field-driven” effect\n(a) No experimental evidence to support the “electric-field-driven” effect claimed by the authors\nTip-enhanced electric field intensification is not a new concept. It was studied and reported for electrocatalysis as early as 2016 (Nature 537, 382–386 (2016)). Since 2016, several experimental methods have been developed to characterize and prove this “tip-enhanced electric field intensification”, for example, conductive atomic force microscopy; Kelvin probe atomic force microscopy; stark tunning shift of adsorbed species (such as CO in this study) by in-situ ATR-SEIRAS spectra and so on. However, there is no experimental data or evidence in this manuscript to support the so-called “Tip-enhanced electric field intensification” claimed by the authors.\nTheoretical calculation simulation is a useful tool to help explain some experimental phenomena and results. However, merely relying on computational simulation but not any experimental evidence or data is unacceptable. The conclusion is not convincing to this reviewer. In addition, most information/statements provided by the simulation results in this study have already been reported and discussed in the literature of the last few years (https://doi.org/10.1016/j.jallcom.2024.175549).\nAuthor Reply:\nWe sincerely thank the reviewer for the insightful comments and for highlighting the need for a clear experimental evidence and differentiation of the novelty in our study.\n1. Novelty\nWe fully acknowledge the wealth of prior literature on tip-enhanced electric field effects, principally leveraging such fields to attract alkali metal cations and enhance their local concentration on catalyst surfaces (e.g., Nature 2016, 537, 382; Angew. Chem. Int. Ed. 2020, 132, 8784; Nano Lett. 2022, 22, 1963; Angew. Chem. Int. Ed. 2023, 62, e202309351; Adv. Mater. 2023, 35, 2301127; J. Am. Chem. Soc. 2023, 145, 11323; Energy Environ. Sci. 2024, 17, 2897). However, we emphasize that the novelty of our work lies in its focus on a metal-cation-free strongly acidic system, where the tip-enhanced electric field serves a fundamentally different purpose: polarizing and activating nonpolar CO molecules.\nIn our study, the locally intensified electric field at the sharp tips of the Au nanotriangles (AuNTs) catalysts polarizes CO molecules, leading to a shift and deformation of their electron cloud, an increase in their dipole moment, bond length elongation, and bond energy weakening. This mechanism directly enhances CO activation in the absence of alkali metal cations, which distinguishes our approach from the previously reported strategies. By integrating this field effect with a bioinspired proton-blocking mechanism, we achieve efficient CO electroreduction in a strongly acidic medium—a significant step forward for the field. Additionally, we have expanded the discussion (Lines 48–49, 54–56, 62–64, Page 2) to highlight the distinctiveness of our work compared to prior studies.\n2. In-situ ATR-SEIRAS characterization of tip-enhanced electric field intensification\nTo address the reviewer's concerns regarding the experimental evidence for tip-enhanced electric field intensification, we have performed in-situ ATR-SEIRAS spectroscopy to monitor the Stark tuning shifts of adsorbed CO on Au nanostructures under varying applied potentials. To enhance the comparative analysis and ensure a more rigorous evaluation of the electric field effect, we have included Au nanodisks (AuNDs) as a new control sample. The synthesis and characterization of AuNDs have been detailed in our response to R1Q1(b).\nThe newly included data (Fig. R1) provides direct experimental evidence supporting the existence and influence of the locally intensified electric field at the AuNTs tips. Fig. R1a and b present the in-situ ATR-SEIRAS spectra of adsorbed CO on Au nanotriangles (AuNTs) and Au nanodisks (AuNDs), respectively, across a range of applied potentials. A systematic blueshift in the CO stretching frequency is observed with increasing negative potential. The tuning rate of the Stark shift, quantified in Fig. R1c, is significantly larger for AuNTs compared to AuNDs, suggesting a stronger localized electric field effect at the tips of AuNTs. This is attributed to the geometric field enhancement at the nanostructure tips, which concentrates the applied field more effectively compared to less sharp nanostructures.\nStark tuning shifts arise from the interaction between the applied electric field and the dipole moment of adsorbed CO. The significantly larger shifts observed for AuNTs provide compelling evidence of a stronger localized electric field. This field enhancement would facilitate the polarization of CO molecules, thereby lowering the activation energy barrier for their reduction. The comparison between AuNTs and AuNDs underscores the critical role of nanostructure morphology—sharp corners versus smooth surfaces—in modulating the electric field distribution and its catalytic consequences.\nThe direct correlation between Stark tuning shifts of adsorbed CO species and operando electric field effects not only substantiates our hypothesis of tip-enhanced field intensification but also highlights its role in activating CO molecules for efficient reduction.\nFig. R1. Enhanced Stark tuning effects on Au nanotriangles (AuNTs) with sharp corners, demonstrating stronger electric field modulation of adsorbed CO compared to smoother Au nanodisks (AuNDs). (a, b) In-situ ATR-SEIRAS spectra of adsorbed CO species on AuNTs and AuNDs at various applied potentials, showing the evolution of the C≡O stretching vibration peaks with applied potential. The sharp corners of AuNTs enhance the local electric field, resulting in stronger Stark tuning effects, evidenced by the larger blueshift compared to the smoother geometry of AuNDs. (c) Comparison of Stark tuning slopes. A clear Stark tuning shift of adsorbed CO is observed, with a Stark slope of 86.78 cm⁻1/V for AuNTs and 19.28 cm⁻1/V for AuNDs. This difference highlights the significant geometric impact on electric field strength, with AuNTs exhibiting superior modulation due to their sharp corners.\n3. Kelvin probe force microscopy evidence and challenges in conductive AFM\nTo further substantiate the presence of tip-enhanced electric field intensification, we have conducted kelvin probe force microscopy (KPFM) measurements on AuNTs and AuNDs. The results reveal a pronounced enhancement in surface potential at the apex of the triangular tips (Fig. R2a–c), compared to the low-curvature nanostructure (Fig. R2d). Since surface potential is directly related to the local electric field intensity, these findings provide strong experimental evidence that sharp tip geometries indeed enhance the localized electric field, thereby modulating charge distribution at the nanoscale. These results, in combination with our in-situ ATR-SEIRAS measurements, reinforce our hypothesis that the tip-enhanced field facilitates CO polarization and activation.\nHowever, we acknowledge the reviewer’s suggestion to employ conductive atomic force microscopy (c-AFM) and attempted such measurements. Unfortunately, two fundamental factors prevent c-AFM from being an effective characterization tool in our system: (i) Nanostructure mobility constraints: The lateral size of our AuNTs (~70 nm) is significantly smaller than conventional electrocatalytic nanostructures studied via c-AFM. Since c-AFM requires direct physical contact between the probe and the sample, the weak adhesion of free-standing AuNTs on the substrate led to sample displacement during scanning, causing severe drift and preventing stable current mapping (ACS Nano 2019, 13, 8055). (ii) Intrinsic conductivity of gold nanostructures: Unlike semiconductors or oxide materials, metallic nanostructures, such as our AuNTs, exhibit inherently high and relatively uniform electrical conductivity. While localized electric fields may modulate charge distribution at the surface, the intrinsic conductivity of gold is too high for c-AFM to resolve meaningful contrast between different surface regions. Given that c-AFM primarily maps spatial variations in local conductivity rather than directly measuring electric field intensity, this technique is not well-suited to verifying our hypothesis.\nFig. R2. KPFM mapping of AuNTs and AuNDs, revealing localized surface potential variations. (a–c) KPFM surface potential maps of AuNTs, showing enhanced surface potential at the triangular tips compared to the basal facets. The localized potential enhancement at the sharp tips indicates stronger electric field concentration, supporting the hypothesis of tip-enhanced field intensification. (d) KPFM surface potential map of AuNDs, exhibiting relatively uniform surface potential distribution. The absence of significant localized enhancement in AuNDs underscores the role of sharp geometries in modulating the local electric field. Arrows point perpendicularly to the edge of the nanomaterial.\nDespite these constraints, the KPFM data robustly supports our hypothesis by directly visualizing the localized electric field enhancement at the AuNT tips, complementing our in-situ ATR-SEIRAS and theoretical studies. We sincerely hope this explanation clarifies the feasibility and limitations of different characterization techniques.\nWe have also revised the manuscript accordingly (Lines 311–324, Page 11) and inserted Figs. R1 and R2 in the revised SI (Supplementary Figs. 24 and 25). We have inserted the reference mentioned by reviewers (J. Alloy. Compd. 2024, 1002, 175549) into the revised manuscript (Ref. 35) for clarity and completeness. We hope these findings and explanations could demonstrate the existence and functionality of the tip-enhanced electric field, addressing the reviewer’s concerns and reinforcing the mechanistic insights of our study, while highlighting its unique contributions.\nR1Q1 (b)-1 There is no clear correlation between the good catalytic performance of triangle Au and the “electric-field-driven” effect\nThe authors claimed that the higher intrinsic catalytic activity of AuNTs compared to those of AuNPs and AuCPs is due to the “tip-enhanced electric field”, which is not convincing based on the results in this work. As AuNTs, AuNPs and AnCPs have different sizes and shapes/morphology and each of them exposes different facets and structural edges, all these factors can lead to different activity (normalized by ECSA) and Tafel slope. The authors didn’t discuss these factors, but instead assigned the improved activity to the “electric-field-driven” effect, which didn’t make any sense.\nAuthor Reply:\nWe sincerely appreciate the reviewer’s insightful comments regarding the role of the tip-enhanced electric field in the catalytic performance of Au nanotriangles (AuNTs). To clarify the distinct contributions of morphology and electric field effects, we have conducted additional control experiments, including the synthesis and characterization of gold nanodisks (AuNDs) and passivated AuNTs, to systematically decouple these factors.\n1. Influence of 2D morphology: AuNDs vs. AuCPs\nFig. R3. Morphological and structural characterization of AuNDs. (a) TEM image showing their uniform disk-like morphology. (b) HRTEM image highlighting the lattice spacing of 0.235 nm corresponding to the Au(111) planes. (c) HAADF-STEM image and EDX mapping, confirming the uniform distribution of Au, Cl, and N elements. (d) XRD pattern, showing prominent diffraction peaks assigned to Au(111). (e) XPS spectrum of the AuNDs surface, focusing on the N 1s region. (f) UV-Vis spectrum, illustrating their plasmonic resonance characteristics.\nWe first introduce AuNDs as a comparative sample to evaluate the influence of morphology on catalytic activity. AuNDs were synthesized by oxidizing AuNTs under controlled conditions (adapted from Mirkin et al., Nano Lett. 2015, 15, 1012). TEM image (Fig. R3a) confirms their well-defined disk-like morphology, with an average diameter of 68 nm comparable to the edge length of AuNTs. High-resolution TEM (Fig. R3b) and XRD analysis (Fig. R3d) indicate that AuNDs primarily expose (111) facets, similar to AuNTs. HAADF-STEM elemental mapping (Fig. R3c) confirms a uniform distribution of Au, Cl, and N on the surface of AuNDs, consistent with AuNTs, indicating the presence of the hexadecyltrimethylammonium chloride (CTAC) capping layer. This is further supported by XPS analysis (Fig. R3e), which shows a characteristic N 1s peak at 401.9 eV, verifying the presence of CTAC on the AuNDs’ surface.\nFig. R4. Catalytic performance of Au-catalysts for CO R. (a) Current density (j ) versus applied potential (E vs. RHE) for Au nanocatalysts. (b) Faradaic efficiency for CO (FE ) at different applied potentials. (c) Half-cell cathodic energy efficiency (CEE) as a function of applied potential.\nDespite the structural similarity in round curvature between AuNDs and AuCPs, their differing dimensionality leads to distinct catalytic performance. Comparative electrochemical measurements (Fig. R4a) show that AuNDs exhibit significantly higher j than AuCPs, demonstrating the advantage of 2D nanostructures in increasing the electrochemically active surface area (ECSA) and exposing more active sites, thereby enhancing CO reduction efficiency.\n2. Isolating the local electric field effect: AuNDs vs. AuNTs; Passivated AuNTs vs. AuNTs\nAlthough AuNDs share structural similarities with AuNTs in terms of lateral size, two-dimensional morphology, and facet exposure, AuNDs exhibit significantly lower catalytic activity (j ) than AuNTs across all tested potentials, with a j ~3.1 times lower at −1.0 V vs. RHE. These results indicate that morphology effects alone cannot fully explain the superior performance of AuNTs, suggesting the presence of an additional key factor beyond general morphological contributions.\nTo isolate the effect of tip-enhanced local electric fields, we synthesized passivated AuNTs, where the sharp tips were chemically modified into rounded edges while maintaining their triangular shape, 2D morphology, and overall size (Fig. R5a–e), effectively minimizing the localized electric field contribution. Structural characterization using TEM and aberration-corrected HAADF-STEM (Fig. R5a–d) confirms the elimination of high-curvature tips, with the exposed facets primarily corresponding to Au(111). XPS analysis (Fig. R5f) further verifies the presence of the CTAC capping layer, consistent with AuNTs. Despite retaining similar surface area and facet exposure, passivated AuNTs exhibit a substantial decrease in j across all applied potentials (Fig. R6a), demonstrating that the sharp tips play a crucial role in enhancing CO activation and reaction kinetics through localized electric field effects.\nFig. R5. Structural and optical characterization of passivated AuNTs. (a, b) TEM images of passivated AuNTs showing uniform triangular shapes with rounded corners after passivation. (c) High-magnification TEM image of a single passivated AuNT, highlighting the rounded corners. (d) Aberration-corrected high-angle annular dark-field scanning TEM image showing the lattice spacing of 0.235 nm corresponding to the Au (111) plane. The region highlighted by the yellow dashed box in (c) is magnified in (d), providing detailed atomic-scale structural information. (e) UV-Vis extinction spectrum of passivated AuNTs, with plasmonic resonance peaks. (f) XPS spectrum of N 1s for passivated AuNTs, indicating the presence of CTAC.\nFig. R6. Electrocatalytic CO R performance of AuNTs and passivated AuNTs. (a) Partial current densities for CO production (j ) as a function of applied potential (E vs. RHE). (b) FE at various potentials. (c) Half-cell cathodic energy efficiency (CEE) as a function of applied potential.\nBy systematically comparing spherical AuCPs, 2D AuNDs, and sharp AuNTs—along with passivated AuNTs as a further control—we demonstrate that while 2D morphology enhances catalytic performance by increasing ECSA, it does not fully account for the superior activity of AuNTs. The presence of sharp tips uniquely contributes to the catalytic enhancement, as evidenced by the significant performance drop in passivated AuNTs. This highlights the crucial role of tip-enhanced local electric fields in facilitating CO reduction, beyond the general morphological advantages of 2D nanostructures.\n3. Validating the synergistic effect: electrochemical analysis\nTo further validate our conclusions, we performed additional electrochemical analyses to elucidate the interplay between morphology and the tip-enhanced local electric field in CO reduction. The ECSA-normalized j of AuNDs is far below that of AuNTs despite their similar 2D structure (Fig. R7a), reinforcing that while morphology plays a role in catalytic activity, it alone cannot account for the observed enhancement in AuNTs. Tafel slope analysis further supports this assertion. AuNDs exhibit a Tafel slope of 117 mV dec−1, similar to AuNPs (121 mV dec−1) and AuCPs (123 mV dec−1), whereas AuNTs show a significantly lower value of 43 mV dec−1 (Fig. R7b). This suggests that while increased surface area and facet exposure contribute to catalytic activity, they are not sufficient to explain the enhanced kinetics observed in AuNTs. Instead, the notably lower Tafel slope of AuNTs aligns with our proposed electric-field-driven mechanism, in which localized electric fields at the sharp tips facilitate CO activation and accelerate the rate-determining step. Additionally, Nyquist plots (Fig. R7c) reveal distinct charge transfer resistances among the catalysts, with AuNDs exhibiting an intermediate impedance between AuNTs and AuNPs. However, the substantial differences in catalytic activity (Fig. R7a), reaction kinetics (Fig. R7b), and charge transfer resistance (Fig. R7c) indicate that the combination of 2D morphology and tip-enhanced local electric fields uniquely optimizes CO reduction performance. The synergy between these two factors—where 2D morphology increases active site density and the tip-enhanced electric field promotes CO activation—accounts for the exceptional catalytic efficiency of AuNTs, distinguishing them from other nanostructures.\nFig. R7. Comparative electrochemical performance of AuCPs, AuNPs, AuNDs, and AuNTs in CO R. (a) ECSA-normalized partial current densities of CO production (j ) as a function of applied potential (E vs. RHE) for different Au nanostructures. (b) Tafel plots showing the logarithm of j versus overpotential (E vs. RHE) for AuCPs, AuNPs, AuNDs, and AuNTs. The corresponding Tafel slopes are also labeled. (c) Electrochemical impedance spectroscopy (EIS) Nyquist plots for the four Au nanostructures, illustrating their charge transfer resistance differences.\nThese new results collectively reinforce our conclusion that while morphology does play a role in catalytic performance, the primary enhancement in our system is driven by the engineered electric field. We have incorporated this discussion in the revised manuscript (Lines 236–238, 250, 251, 261, Page 9; Lines 271–274, 276, 280–286, Page 10; Lines 287–306, 308–315, Page 11; Lines 316–339, 343, Page 12; Lines 350–364, Page 13), and inserted Figs. R3−R7 in the revised manuscript (Fig. 4a−c and Supplementary Figs. 8, 26–30). We hope this systematic analysis addresses the reviewer’s concerns and provides a more convincing explanation for the observed catalytic behavior.\nR1Q1 (b)-2 The result of ATR-SEIRAS used to show that “the stronger potentials enhance the adsorption of these intermediates” doesn’t make sense! As mentioned in point 1, the local electric field should be characterized by a Stark tunning shift of adsorbed species (such as CO in this study) using in-situ ATR-SEIRAS spectra.\nExcept for the calculation and the authors’ own explanation, no clear or supporting experimental results suggest the correlation between improved activity and the “electric-field-driven” effect.\nAuthor Reply:\nWe appreciate the reviewer’s insightful comments regarding the interpretation of our in-situ ATR-SEIRAS results. We would like to address the concern and provide additional clarification:\nAs shown in the original Supplementary Fig. 22 (now Supplementary Fig. 23), the intensity of the ν(C≡O) peak increases systematically as the applied potential becomes more negative. This trend indicates enhanced surface coverage of CO intermediates on the catalyst, correlating with the applied potential. The stronger peak intensity reflects the greater accumulation of adsorbed intermediates under higher negative potentials.\nWhile our original analysis focused primarily on intensity changes, we acknowledge the reviewer’s suggestion to examine Stark tuning shifts as a direct indicator of the local electric field (please see also our reply to R1Q1(a)). Upon closer inspection of the ν(C≡O) peak position, we observe a blueshift with increasingly negative potentials on the AuNTs (Fig. R1a). This blueshift corresponds to the Stark effect, which arises from the interaction between the local electric field and the dipole moment of the adsorbed CO. The observed shift quantitatively validates the presence and intensification of the local electric field under operando conditions, further supporting the role of field enhancement in catalytic activity.\nTo provide a more comprehensive response, we have expanded our analysis to quantify the Stark tuning shifts observed in the ν(C≡O) band. These results, now incorporated into the revised Supplementary Information, provide direct experimental evidence of the modulation of the local electric field by the applied potential. Furthermore, this analysis reinforces our hypothesis that the electric-field-driven effect plays a critical role in enhancing catalytic performance.\nIn summary, the increasing intensity of the ν(C≡O) peak reflects enhanced surface coverage of CO intermediates, while the observed Stark tuning shift provides direct evidence of local electric field modulation. We hope that the expanded analysis and clarified discussion (Lines 311–319, Page 11) in the revised manuscript address the reviewer’s concerns and strengthen the conclusions of our study.\nR1Q2. “bioinspired proton blocking” effects\nR1Q2. (a) What’s the indeed similarity between Aquaporin and the CTAC layer?\nAuthor Reply:\nWe thank the reviewer for the insightful question regarding the similarity between aquaporins and the CTAC layer. We appreciate the opportunity to clarify the bioinspired design principle underlying our system. Below, we detail the functional and mechanistic analogies, supported by experimental evidence, and clarify the differences:\nFunctional analogy: The aquaporins selectively allow neutral water molecules to pass through while effectively repelling proton via positively charged residues (arginine, [HNC(NH ) ]+) near the channel constriction, creating a proton-blocking electrostatic barrier. Similarly, the CTAC layer on the AuNTs surface forms a positively charged environment via quaternary ammonium groups ([N(CH ) ]+). This environment selectively blocks protons, mimicking the proton exclusion function of aquaporins, while allowing neutral CO molecules to approach and interact with the catalyst surface.\nMechanistic similarity: Both systems utilize positively charged sites to reduce local proton concentration. In aquaporins, this mechanism prevents proton translocation through the channel. In our system, the CTAC layer extends this concept by creating a broader proton exclusion zone, which suppresses the HER and promotes selective CO reduction.\nAs depicted in Fig. 5b of the original or now manuscript, the CTAC layer imparts a significant positive charge to the AuNTs surface, resembling the electrostatic proton-blocking sites in aquaporins. Fig. 5c (in original or now manuscript) illustrates the markedly reduced HER and enhanced CO R efficiency on CTAC-capped AuNTs, further supporting the proton-blocking role. Simulations (Fig. 5d−f in original or now manuscript) reveal that the cationic surfactant layer inhibits proton migration by increasing the positive charge density, resulting in a higher local pH near the catalyst surface.\nAquaporins are biological proteins with a highly defined structural organization and chemical specificity, optimized through evolution to facilitate selective water transport while excluding protons. In contrast, the CTAC layer is a synthetic system, lacking the structural complexity of proteins but effectively replicating the functional principles of proton blocking through its positively charged quaternary ammonium groups. This simplified yet effective design is particularly well-suited for the challenging acidic electrochemical conditions.\nIn conclusion, the CTAC layer mimics the key functional aspects of aquaporins in proton blocking, providing an innovative approach to enhancing CO reduction in acidic media.\nR1Q2. (b) Aquaporin has a water transport channel. What’s the channel structure of the CTAC layer?\nAuthor Reply:\nWe appreciate the reviewer’s insightful question about the channel structure in the CTAC layer and its role in CO or proton transport. Below, we address the current understanding and propose additional evidence to further clarify this aspect:\nUnlike aquaporins, the CTAC layer does not feature a well-defined molecular channel. Instead, it forms a positively charged bilayer on the AuNT surface. This bilayer creates an electrostatic barrier that suppresses proton migration while allowing neutral CO molecules to diffuse through. The transport mechanism relies primarily on the surface charge distribution and the adsorption–desorption equilibrium at the catalyst surface, rather than a physical channel.\nNeutral CO molecules can diffuse through the CTAC layer due to their uncharged nature, encountering minimal resistance from the electrostatic barrier. This transport is facilitated by the molecular-level interactions between CO and the CTAC bilayer, as well as the underlying Au surface.\nProtons, being positively charged, are repelled by the electrostatic environment created by the quaternary ammonium groups in the CTAC layer. This prevents their effective migration to the AuNT surface, significantly suppressing the HER. The transport behaviors of CO and protons have been validated through molecular dynamics simulations (please see our reply to R1Q2(d)).\nIn summary, while the CTAC layer lacks a physical channel akin to aquaporins, its electrostatic properties create a functional equivalent by selectively repelling protons and permitting neutral CO transport. We hope our explanation could address the reviewer’s concerns.\nR1Q2. (c) How can the author demonstrate that CTAC forms a double layer shown in Figure 1e? Is there any experimental evidence or data to support this structure? Otherwise, the figure is very misleading.\nAuthor Reply:\nWe thank the reviewer for raising this critical point. Below, we address the concerns with our efforts, supporting literature, and a refined discussion of our findings:\nWe have actively sought direct experimental evidence through small-angle neutron scattering (SANS) to confirm the bilayer structure of CTAC. To this end, we contacted multiple prominent facilities, including the China Spallation Neutron Source (CSNS) and the China Institute of Atomic Energy (CIAE). Unfortunately, due to scheduling constraints, we were unable to obtain access to their instrumentation within the required timeframe.\nThe bilayer structure of cetyltrimethylammonium halide on gold nanomaterials has been extensively studied in the literature. Fabienne Testard and Luis M. Liz-Marzán (Langmuir 2012, 28, 1453, Ref. 46 in our original manuscript) used SANS to determine a bilayer thickness of 32±2 Å. Considering that the fully stretched alkyl chain length in a single layer is 20−22 Å, this suggests that the bilayer structure arises due to partially interdigitated alkyl chains and electrostatic stabilization. This structural arrangement is primarily driven by hydrophobic interactions between the alkyl tails, reducing their exposure to water, minimizes steric hindrance, and maximizing van der Waals interactions between adjacent chains. Simultaneously, electrostatic repulsion between the positively charged ammonium head groups at the bilayer surface ensures colloidal stability, preventing nanoparticle aggregation while preserving the integrity of the surfactant shell. Sara Bals and Luis M. Liz-Marzán et al. (Acc. Chem. Res. 2023, 56, 1204) summarized that bilayer structures are common at certain surfactant concentrations, governed by interdigitation, ionic strength, and solution conditions.\nTo avoid misinterpretation, we have revised the legend for Fig. 1e (Lines 91-93, Page 3) to clarify that it conceptually illustrates the role of CTAC in creating a proton-blocking environment rather than presenting direct structural evidence. We hope these clarifications can address the reviewer’s concerns.\nR1Q2. (d) Based on the authors’ explanation, the selectivity of CO R in an acidic solution is due to the presence of the cationic CTAC layer. However, CTAC-capped, PVP-capped, and SDS-capped AuNTs all show >50% FE for CO, which indicates that the cationic layer is unlikely the main reason for the good selectivity of CO R in acidic conditions.\nAuthor Reply:\nWe thank the reviewer for the insightful comment. To address the concern, we performed additional experiments using the following control samples: (i) bare AuNTs (CTAC removed), (ii) CTAC-capped AuNDs, and (iii) bare AuNDs (CTAC removed).\nThe CTAC removal process was carried out as follows: First, 5 mg of carbon black was dispersed into a mixture of 965 μL of catalyst dispersion (3.45 g L–1 in water) and 3 mL of ethanol. The resulting solution was sonicated for 40 minutes to ensure uniform dispersion. Subsequently, the solution was dissolved in 100 mL of a hydrochloric acid-ethanol solution (HCl:ethanol = 1:9, v/v) and refluxed at 78 °C for 12 hours. After refluxing, the product was collected via centrifugation, followed by alternating washing with ethanol and deionized water to remove the CTAC. The washing process was repeated several times to ensure the complete removal of the surfactant. The final catalyst was dried under a vacuum at 50 °C to obtain the CTAC-free (bare) AuNTs and AuNDs catalyst. XPS analysis confirmed the removal of CTAC from the surface of AuNTs and AuNDs, as evidenced by the disappearance of the N 1s signal after the surfactant removal process (Fig. R8).\nFig. R8. N 1s XPS spectra of Au nanostructures before and after surfactant removal. (a) N 1s XPS spectrum of CTAC-capped AuNTs. (b) N 1s XPS spectrum of AuNTs after the removal of CTAC. (c) N 1s XPS spectrum of CTAC-capped AuNDs. (d) N 1s XPS spectrum of AuNDs after the removal of CTAC. The elimination of the N 1s signal following surfactant removal indicates the successful detachment of CTAC from the surface of both nanostructures.\nThe CTAC-capped AuNTs achieved a Faradaic efficiency for CO production (FE ) of ~100%, while bare AuNTs showed a reduced FE of 61.9% (Fig. R9). The CTAC-capped AuNDs achieved a FE of 92.5%, while bare AuNDs showed a much lower FE of 54.3%. These results demonstrate that the CTAC capping layer could enhance the CO reduction selectivity by suppressing competing HER, particularly in acidic conditions. Without the CTAC layer, the FE drops significantly, even for AuNTs, which inherently possess a high-curvature structure that promotes CO activation.\nWhile we acknowledge that non-cationic capped AuNTs (e.g., PVP and SDS) also exhibit CO Faradaic efficiencies greater than 50%, their values are significantly lower than those of CTAC-capped AuNTs (nearly 100%), which highlights the unique contribution of the cationic layer in combination with the high-curvature structure. Moreover, the CTAC-capped AuNDs and CTAB-capped AuNPs also exhibit high CO selectivity of 92.5% and 88.2%, respectively, further supporting the role of the cationic CTAC layer, independent of the nanostructure's curvature. In conclusion, the cationic CTAC layer is indeed a key factor in achieving excellent CO selectivity in acidic solutions, as it effectively modulates the local microenvironment to favor CO reduction over HER.\nFig. R9. The effect of morphology and surfactant on the FE of gold nanostructures at –1.0 V vs. RHE. CTAC: Au nanostructures capped with CTAC. Bare: Au nanostructures after CTAC removal.\nAdditionally, we have provided further molecular dynamics simulation results to complement our experimental findings (Fig. R10). The simulations explore how different surfactants (CTAC, PVP, and SDS) influence the distribution of protons (H+) and CO molecules near the gold surface. As shown in Fig. R11a, the anionic surfactant SDS attracts protons, resulting in a high proton concentration near the surface. In contrast, the cationic CTAC layer repels protons, leading to a significantly lower proton density near the gold surface. This is reflected in the proton density peak being located furthest from the surface for CTAC-capped Au, indicating a strong proton exclusion effect induced by the cationic CTAC layer. The nonionic PVP surfactant shows an intermediate behavior, with proton density distance between those observed for SDS and CTAC. These results underscore the key role of the cationic CTAC layer in modulating the local microenvironment around the gold surface, effectively reducing proton availability. In addition, the surface CO concentration in the Au/CTAC model exhibits only a marginal increase compared to the Au/PVP and Au/SDS models (Fig. R11b). This trend aligns with the experimentally observed higher Faradaic efficiency (~100%) of CTAC-capped AuNTs, suggesting that CTAC primarily functions to suppress protonation, thereby enhancing selectivity.\nWe have also revised the manuscript accordingly (Lines 403–410, 413, 416–419, Page 15) and updated relevant figures in the revised SI (Supplementary Figs. 21, 34, 35). We hope this additional experimental and theoretical data further clarifies the critical role of the CTAC capping layer in achieving high CO selectivity in acidic conditions.\nR1Q3. “metal-cation-free” H SO electrolytes\nThe authors claimed that they conducted CO R in a “metal-cation-free” solution. However, they didn’t describe how they pretreated the electrolyte to obtain such a “metal-cation-free” solution.\nIs it a real “metal-cation-free” solution?\nDid the authors use ICP-MS to measure the M+ concentrations before and after the experiments to confirm it is a “metal-cation-free” solution?\nIs there any M+ impurity introduced into the solution by the catalysts or materials used in the experiments?\nPlease read the reference Nat. Catal. 2021, 4, 654−662 and study the pretreatment methods in this reference where the “metal-cation-free” condition is strictly created, discussed, and studied for electrocatalysis.\nAuthor Reply:\nThank you for your thoughtful insights, which has helped us improve the rigor of our experimental design. We would like to provide the following detailed response concerning the pretreatment of the electrolyte:\n1. Electrolyte pretreatment method\nTo ensure the electrolyte was free from metal cations, we followed a comprehensive cleaning procedure for all experimental equipment and containers. First, all glassware was soaked in aqua regia, then thoroughly rinsed with ultrapure water. After the rinsing, the glassware was boiled at least five times in ultrapure water to ensure the complete removal of any metal ion residues. Second, the electrolyte used was H SO purchase from Sigma-Aldrich (ultrapur), which is a high-purity reagent that minimizes the presence of metal ions in the solution. The cleaning process is consistent with the need for a metal-cation-free condition as discussed in Nat. Catal. 2021, 4, 654 (Ref. 24 in original manuscript). While their use of potassium permanganate and piranha cleaning is effective, our use of aqua regia followed by boiling in ultrapure water achieves the same rigorous standard for removing metal cations, as verified by ICP-MS measurements.\n2. ICP-MS measurements for the electrolyte\nTo confirm the absence of metal ions in the electrolyte, we performed ICP-MS measurements before and after 100 hours of electrolysis (Table R1). The results indicated that the metal ion concentrations were below the detection limit, confirming the electrolyte was indeed metal-cation-free.\nTable R1. Metal ion concentrations in the electrolyte and AuNTs catalysts as measured by ICP-MS. The concentrations of Li, Na, K, Rb, and Cs in the electrolyte before and after electrolysis, as well as in the AuNTs catalysts prior to electrolysis, were all below the detection limits of the ICP-MS system. Detection limits for the elements are as follows: Li (0.33 μg L–1), Na (6.36 μg L–1), K (4.50 μg L–1), Rb (0.11 μg L–1), and Cs (0.12 μg L–1), ensuring the metal-cation-free condition throughout the experiments. Error bars are omitted as all values are below the quantifiable limits.\n3. Metal ion contamination excluded from catalysts\nTo ensure that no alkali metal ions were introduced by the catalysts, we performed XPS surface analysis on the catalysts and found no evidence of alkali metal contamination (Fig. R12). Additionally, we performed ICP-MS on the catalysts after digestion (Table R1), and the analysis showed that the metal ion concentrations were also below the detection limit, confirming that the catalysts did not release metal ions during the experiments.\nWe have added Fig. R12 to the revised SI (Supplementary Fig. 17), and incorporated Table R1 into the revised SI (Supplementary Table 1) to further clarify the metal-cation-free condition. We hope these explanations can address the reviewer’s concerns.\nReviewer #2:\nReferee report on the manuscript \"Electric-field-driven CO polarization and bioinspired proton blocking unlock CO reduction in strong acid without metal cation\" by Chen et al on October 30, 2024.\nThe authors have developed a bio-inspired, proton-transport blocking approach to control the activity and selectivity of gold-catalyzed CO reduction reaction (CO RR) by introducing a hydrophobic hexadecyltrimethylammonium (CTAC) layer on differently-shaped gold nanoparticles (NPs). The results show that the introduction CTAC on triangular AuNPs leads to the highest activity, selectivity, and efficiency in CO RR. The improvements are suggested to result from the high electric field at the sharp tips of the AuNPs and blocked proton transport through the CTAC-layer. The results are interesting as they demonstrate a practical proton-blocking strategy to improve the CO RR selectivity. However, I am not fully convinced by the mechanistic picture behind the improved performance and more discussion is needed.\nAuthor Reply:\nThanks for the reviewer’s positive comments and support for our work.\nTo further improve the quality of our manuscript and address your concerns, we have made careful revisions according to each comment of the reviewers. We hope the revised manuscript can address the concerns of the reviewers and meet the high standard required for publication in Nat. Commun.\nMain issues\nR2Q1. The picture used for the induced CO dipole appears incorrect. In Figure 2, the CO dipole seems to be forming along the linear O-C-O molecule, which might be relevant if CO adsorbed on Au in a linear manner from an oxygen atom. However, this is not how CO adsorbs on Au; CO adsorbs from the carbon atom and often does not remain linear but adopts V-shaped geometry under relevant electrocatalytic conditions/electrode potentials/electric field (see e.g. https://doi.org/10.1021/acscatal.4c00347 and https://www.nature.com/articles/s41467-019-13777-z) Hence, the induced dipole moment analysis and its discussion needs to be revised.\nAuthor Reply:\nWe appreciate the reviewer’s insightful comment regarding the CO dipole representation in Fig. 2a–c. We fully agree that CO adsorption on Au primarily occurs via the carbon atom rather than oxygen, and that under electrocatalytic conditions and applied electric fields, CO often adopts a bent (V-shaped) geometry rather than remaining linear. In fact, our DFT calculations also confirm that CO interacts with Au through the carbon center in the adsorption process (Fig. R13).\nHowever, we would like to clarify that the specific purpose of Fig. 2 a–c is investigating the non-adsorbed CO approaching the surface, where no chemical bonding or charge transfer with Au has occurred. This allows us to isolate the effect of the external electric field on the molecular dipole. The linear geometry used in Fig. 2a–c is thus appropriate, as CO remains linear in the absence of direct interaction with the Au surface. Importantly, this does not contradict the fact that upon adsorption, CO transitions to a V-shaped configuration. Once CO undergoes significant charge transfer between Au and CO , the system no longer represents an isolated dipole moment but instead a chemisorbed system, making it challenging to decouple the contributions of electronic effects and the applied electric field to the induced dipole moment.\nIn simulations, it is possible to introduce an extra electron to CO , which would naturally adopt a V-shaped geometry. However, under such conditions, an accurate determination of the dipole moment becomes problematic. This is because for charged species, the dipole moment depends on the choice of coordinate origin in simulations, meaning that the same molecule placed at different spatial positions would yield different dipole moment values. In contrast, under charge-neutral conditions, the dipole moment is well-defined and independent of spatial positioning due to rotational invariance. Since lacks this invariance, its dipole moment is inherently ill-defined, making it unsuitable for precise quantitative analysis.\nThis analysis (Fig. 2a–c) highlights how CO molecules undergo polarization (generation of induced dipole moment) upon entering the strong electric field near the catalyst surface, thereby facilitating their subsequent adsorption, and polarization is a crucial precursor step before surface interaction. Moreover, irrespective of whether CO is in a linear or bent configuration, the general trend remains unchanged: an increasing external electric field enhances the induced dipole moment, which facilitates CO interaction with the catalyst surface. This aligns well with the findings of Ringe et al. (Nat. Commun. 2020, 11, 33), where the interfacial electric field was shown to play a critical role in CO adsorption.\nWe have revised the figure caption and discussion to explicitly state that the dipole moment calculation in Fig. 2a–c represents the initial stage of CO approaching the Au surface, rather than the chemisorbed state (Lines 96, 98–100, Page 4; Lines 128–131, Page 5). This distinction is essential for understanding the polarization effects that precede adsorption and does not contradict the fact that CO eventually adopts a bent geometry upon chemisorption. We have also cited the two references (ACS Catal. 2024, 14, 8549; Nat. Commun. 2020, 11, 33) to support our discussion in the revised manuscript (Ref. 31 and 32). We hope this clarification could address the reviewer’s concern.\nR2Q2. Shifts in the d-band center due to the electric field are too small to explain the large changes in the reaction energies leading to CO ∗ and especially COOH∗. Instead, these molecules respond to the electric field strongly due to their large dipole moment; this is the dipole-field model often used the CO RR literature (https://www.nature.com/articles/s41929-021-00705-y). However, even this model presents several theoretical issues (https://doi.org/10.1063/5.0138197). Overall, I wish the authors would discuss the different descriptors used for understanding the CO RR, with a focus on field effects (see e.g. https://www.nature.com/articles/s41467-023-37929-4).\nAuthor Reply:\nWe sincerely appreciate the reviewer’s insightful comments and the valuable references provided. We fully acknowledge the reviewer’s concern regarding the adequacy of d-band center shifts in explaining the significant changes in reaction energies for *CO and *COOH. We also agree that the dipole-field interaction plays a crucial role in CO RR. Below, we provide a detailed discussion addressing these points and incorporating insights from the referenced literature.\n1. On the role of the d-band center shift in CO activation\nWe recognize that the shift in the d-band center under an applied electric field is relatively small. By itself, this shift may not fully account for the substantial changes in reaction energies observed for *CO and *COOH, as the reviewer correctly pointed out. However, we find that the d-band center shift should not be viewed as an isolated factor but rather as part of a broader set of electronic effects that collectively influence CO adsorption and activation.\nIn particular, the d-band center shift modifies the electronic structure of the catalyst, thereby influencing the degree of charge redistribution upon CO adsorption. This, in turn, affects the polarization of *CO , which plays a critical role in its activation. While the direct effect of the d-band shift is modest, its indirect role in modulating dipole-field interactions and local charge distribution remains significant.\n2. Discussion on the dipole-field model and its relevance to our work\nThe dipole-field model, as highlighted in Nat. Catal. 2021, 4, 1024, suggests that the interaction between the *CO dipole moment and the interfacial electric field is a dominant factor in adsorption stability. Our findings align with this perspective, as we also observe that the electric field significantly enhances CO adsorption due to polarization effects.\nTo provide a more comprehensive perspective, we compared the d-band center shift with the dipole-field interaction and found that these two factors work in a complementary manner: the d-band center shift modifies the electronic structure, which indirectly influences how the dipole-field interaction manifests at the interface.\n3. Theoretical limitations of the dipole-field model\nA key limitation of the dipole-field model, as discussed in J. Chem. Phys. 2023, 158, 144701, is that it does not explicitly differentiate between electrostatic interactions and charge transfer effects. This distinction is crucial in electrochemical environments, where both factors can play a role in determining adsorption and reaction energetics.\nAdditionally, the applicability of the dipole-field model varies across different catalyst systems. For example, on Au surfaces, CO adsorption is known to be strongly influenced by local electronic states rather than purely by dipole interactions. Recognizing these limitations, we have clarified under what conditions the dipole-field model provides a valid description and where alternative descriptors may be more appropriate.\n4. Broader discussion on CO RR descriptors with a focus on field effects\nFollowing the reviewer’s suggestion, we have expanded our discussion of different descriptors relevant to CO RR, particularly those related to electric field effects. Instead of focusing solely on charge transfer descriptors and the potential of zero charge, we have provided a broader perspective on key descriptors, including: d-band center shifts (modulating catalyst electronic structure), dipole-field interactions (stabilizing *CO adsorption), CO adsorption energy (as a fundamental activity descriptor), local electric field effects (altering reaction barriers and intermediate stability). This revised discussion ensures that our response fully aligns with the reviewer’s request while remaining grounded in the available data from our study.\nWe have carefully revised our manuscript to incorporate these important points (Lines 152-163, Page 6; Lines 182-190, Page 7), ensuring a balanced and rigorous analysis of field effects in CO RR. We have inserted these two references (Nat. Catal. 2021, 4, 1024; J. Chem. Phys. 2023, 158, 144701; Nat. Commun. 2023, 14, 2598) into the revised manuscript (Ref. 45-47). We hope that these clarifications and additions could address the reviewer’s concerns.\nR2Q3. The authors write that \"the individual active sites on AuNTs exhibited the highest average intrinsic catalytic activity due to the existence of the tip-enhanced electric field, which was also supported by the DFT calculation, Tafel analysis and EIS measurements.\" I do not fully agree with this as there are also different possibilities not fully considered in the paper. In particular, based on the points below and the results in the paper, I would say that changes in CO RR activity and selectivity result from changes in CO mass transfer to the surface due to the CTAC and the electric field together rather than due to in the field-driven changes in CO RR energetics or intrinsic activity. I ask the authors consider this proposition given the following points:\n(a) Related to point 2, the authors seem to implicitly assume that the CTAC can impact CO2RR energetics/thermodynamics only by electric field effects. However, even without the CTAC the electric field at electrochemical interface is very large; for instance, a 1 V potential drop over a 10 Å double layer thickness, relevant to highly concentrated electrolytes, results in field of ∼109 V/m. Rather than attributing changes in CO RR performance to field-induced changes in CO RR energetics, it has been shown that surfactant similar to CTAC dramatically change the double-layer properties (https://doi.org/10.1021/acscatal.9b00449) and local CO RR environment (https://doi.org/10.1021/acscatal.0c02387). The former can change the surface charge at a given potential while the latter can change the CO –cation interactions. These two possibilities can be tested rather easily using DFT by i) studying the impact of surface charge on the CO RR energetics and by ii) including CTAC explicitly in the simulations. These simulations would significantly contribute to mechanistic understanding by separating the field-effects, potential-charge relations (capacitance), and direct CTAC-CO , and I encourage the authors to carry out these simulations.\nAuthor Reply:\nWe thank the reviewer for their constructive feedback regarding the role of the electric field and the CTAC surfactant layer in modulating CO RR energetics and the local reaction microenvironment.\nWe agree that the electric field at the electrochemical interface, even in the absence of CTAC, is inherently strong, and that the surfactants like CTAC could dramatically alter double-layer properties and the local CO RR microenvironment by influencing surface charge properties and altering CO -cation interactions. These effects, as demonstrated in Banerjee et al. (ACS Catal. 2019, 9, 5631; ACS Catal. 2020, 10, 9907), are highly relevant and warrant further investigation. In our study, performed in a metal-cation-free electrolyte (0.07 M H SO , pH = 1.0), we observe that the CTAC surfactant plays a critical role in modulating the surface charge properties and local microenvironment, contributing to enhanced CO RR reactivity. This is supported by several observations: Zeta potential analysis (original or now Fig. 5b) demonstrates that the CTAC-capped AuNTs exhibit a significantly positive surface charge, highlighting the influence of CTAC on the local ionic environment and the double-layer properties. Poisson-Nernst-Planck simulations (original or now Fig. 5d-f) further reveal that the cationic surfactant reduces proton migration rates and increases the local pH near the catalyst surface, thereby suppressing the competitive hydrogen evolution reaction (HER). These effects are consistent with the surfactant’s impact on the interfacial environment. Experimentally, CTAC-capped AuNTs exhibit significantly higher Faradaic efficiencies for CO production and suppressed HER compared to uncapped counterparts, which strongly supports the hypothesis that CTAC contributes directly and indirectly to enhanced CO RR performance.\nWe endorse the reviewer’s suggestion to explicitly include CTAC in our DFT simulations to further dissect its direct impact on reaction energetics. While we attempted this approach, we faced significant computational challenges due to the large size and complexity of CTAC molecules: Modeling a strictly realistic CTAC bilayer on Au facets involves approximately >1,200 atoms in the slab model, which far exceeds the capacity of standard DFT calculations and is computationally prohibitive. To address this limitation, we systematically explored a series of progressively simplified models using smaller analogs (e.g., tetraalkylammonium chloride) to represent CTAC. We first considered a full bilayer adsorption, but found that the excessive molecular flexibility hindered convergence. To mitigate this, we employed an interdigitated bilayer configuration, followed by a further simplification to a monolayer. Even after removing the counterions to minimize additional complexity, none of these models achieved reliable convergence under the applied electric field of 0.012 V Å−1 (Fig. R14). This suggests that the fundamental challenge arises from the inherent difficulty of stabilizing surfactant–electric field interactions within DFT frameworks, rather than from model complexity alone.\nDespite these challenges, the experimental trends observed in our study—higher Faradaic efficiency and suppressed HER for CTAC-capped AuNTs—significantly support the hypothesis that CTAC modulates the local microenvironment in a way that complements the field-induced polarization effects. These findings are in alignment with prior studies referenced by the reviewer (ACS Catal. 2019, 9, 5631; ACS Catal. 2020, 10, 9907), which demonstrate that surfactants like CTAC can alter double-layer properties and CO -cation interactions. In our work, original or now Fig. 2e,f demonstrates field-induced polarization and its impact on CO energetics in the absence of surfactants, establishing a baseline to highlight the intrinsic ability of high-curvature Au tips in enhancing local electric fields and polarizing CO molecules. While we agree with the reviewer that incorporating the CTAC layer in simulations would provide a more comprehensive understanding of the synergistic effects between the surfactant and the electric field, this remains a challenge to be addressed for future studies.\nTo disentangle the individual contributions of electric field effects, double-layer properties, and surfactant interactions, we propose the following directions: Combining molecular dynamics (MD) with DFT explicitly accounts for the dynamic interactions between surfactants, proton, and CO molecules, providing a more realistic representation of the surfactant's role in interfacial properties (please see also our reply to R2Q3(b)). These efforts will provide a more comprehensive understanding of the interplay between electric fields, surfactant layers, and interfacial properties in CO RR.\nWe have inserted these two references (ACS Catal. 2019, 9, 5631; ACS Catal. 2020, 10, 9907) into the revised manuscript (Ref. 56 and 57). We hope that these efforts and explanations, along with the proposed future directions, could address the reviewer’s concerns and provide a more comprehensive perspective on the synergistic effects between CTAC and electric fields.\nR2Q3 (b) Besides changing the CO RR energetics, both field and CTAC can also impact the electrolyte and CO transport to the catalyst surface. Hence, the field and CTAC may increase the CO concentration at the catalyst surface and thereby improve the catalytic performance rather than by impacting the intrinsic activity.\nAuthor Reply:\nWe thank the reviewer for this insightful comment regarding the potential roles of CTAC and the electric field in influencing electrolyte and CO transport. Below, we clarify the individual contributions of CTAC and the electric field.\n1. Role of CTAC in modulating the local microenvironment\nAs highlighted in prior studies (ACS Catal. 2019, 9, 5631; ACS Catal. 2020, 10, 9907), cationic surfactants like CTAC can alter the local electrolyte microenvironment near the catalyst surface, impacting proton exclusion and interfacial charge distributions. To investigate this, we performed molecular dynamics (MD) simulations on three surface models of Au/CTAC, Au/PVP, and Au/SDS (Fig. R15). The results (Fig. R16) reveal the CTAC-capped surface exhibits the strongest proton-repelling ability (CTAC > PVP > SDS), which reduces proton availability near the surface and suppresses the competitive HER. In addition, the surface CO concentrations for the Au/CTAC model shows only a marginal increase over Au/PVP and Au/SDS models, which suggests that primary role of CTAC is modifying the local charge environment rather than significantly increasing CO surface concentration. Thus, while CTAC modulates the local microenvironment, it does not fundamentally alter CO mass transport.\n2. Role of the electric field in modulating CO transport and adsorption\nIn addition to CTAC, the electric field plays an independent and critical role in enhancing CO adsorption and transport to the catalyst surface. Unlike metal-ion-based systems, the electrolyte in our study consists is dilute sulfuric acid (pH = 1). The absence of metal cations simplifies the system by eliminating the complexities arising from interactions between metal cations and CO . Under these conditions, the electric field directly polarizes CO molecules and facilitates their adsorption at the interface, independent of cation-CO interactions that occur in metal-ion-containing systems. Finite element simulations (original or now Supplementary Fig. 2) demonstrate that the intense local electric fields at the sharp tips of AuNTs polarize CO molecules, increasing their dipole moment and facilitating adsorption. This field-driven effect is highly geometry-specific, as obtuse or round surfaces exhibit lower electric field intensities and weaker CO enrichment. Therefore, in our system, the enhanced electric field generated by the sharp-tip effect contributes directly to CO RR performance by enhancing CO adsorption and polarization, independent of the effects of CTAC.\n3. Synergistic interaction between CTAC and the electric field\nCTAC and the electric field independently yet synergistically enhance CO RR performance. On CTAC-modified Au surfaces, the proton-repelling properties of CTAC suppress HER and create a favorable local microenvironment for CO adsorption and activation. Simultaneously, the electric field complements this by directly polarizing CO molecules, increasing their dipole moments, and strengthening adsorption, thereby accelerating reduction kinetics. Together, these effects collectively optimize CO RR performance by selectively promoting CO activation while suppressing HER.\nWe have also revised the manuscript accordingly (Lines 413, 416-419, Page 15) and updated relevant figures in the revised SI (Supplementary Fig. 35) to explicitly distinguish between the roles of CTAC and the electric field in modulating transport and adsorption, ensuring a balanced and comprehensive interpretation of our findings. We hope our clarification and revisions could address the reviewer’s concern.\nR2Q3. (c) In my understanding, the charge transfer resistance depends on the total rate, i.e both the surface concentration and the rate constant–not only the rate constant and intrinsic activity. How quantitative or accurate is the determination of intrinsic activity from EIS?\nAuthor Reply:\nWe sincerely thank the reviewer’s insightful question regarding the interpretation of charge transfer resistance (R ) in electrochemical impedance spectroscopy (EIS) and its relation to intrinsic activity.\nWe agree that R is a function of the interfacial reaction rate, which is dictated by the reaction rate constant and the local reactant concentration. While a lower R is generally associated with faster charge transfer kinetics, it could not uniquely determine the intrinsic activity of a catalyst, because local CO concentration and reaction microenvironment also influence the observed values. To determine whether R predominantly reflects intrinsic catalytic activity (the rate constant) rather than mass transport effects (surface reactant concentration), we have conducted a systematic analysis considering the following aspects.\nWe analyzed the Warburg slopes (Z = ΔZ′′/ΔZ′) from the low-frequency region of our impedance spectra (original Supplementary Fig. 20 or now Supplementary Fig. 29). The Warburg slopes for AuNTs, AuNPs, and AuCPs were found to be 1.974, 1.981, and 1.983, respectively. These nearly identical values indicate minimal differences in reactant mass transport across the three catalysts in an H-type cell. If CO diffusion exhibited different influence R for three samples, we would expect to see more substantial differences in Warburg slopes among the catalysts, which was not observed. Therefore, these results support that the mass transfer conditions of the three catalysts are similar, so the difference in R is mainly due to intrinsic catalytic activity. When mass transport effects are negligible, R follows the Butler-Volmer equation:\n𝑅𝑇 1\n𝑅 = ×\nct 𝛼𝐹 𝑗\n0\nwhere j =nFk is the exchange current density, directly related to the intrinsic rate constant k. This equation shows that under kinetic control, R is inversely proportional to k, making it a valid metric for intrinsic activity.\nTo further validate this, we cross-validated our EIS results with independent electrochemical analyses. (i) We compare AuNTs, AuNPs, and AuCPs, all of which exhibit similar electrochemical surface areas (ECSA, original Supplementary Table 1 or now Supplementary Table 3). By normalizing the CO production rate to ECSA, we evaluate intrinsic activity (please see also our reply to R2Q4(d)). (ii) Tafel slopes provide insight into the charge transfer mechanism. The much lower Tafel slope for AuNTs (43 mV dec−1) compared to AuNPs (121 mV dec−1) and AuCPs (123 mV dec−1) indicates an intrinsically faster charge transfer kinetics process on AuNTs, independent of reactant availability. Since the Tafel slope primarily reflects the charge transfer step and is less sensitive to mass transport limitations than EIS, their consistency reinforces the validity of using R to probe intrinsic activity.\nIn summary, while we acknowledge that R inherently reflects both the rate constant and the surface concentration of reactants, our multiple independent analyses demonstrate that our EIS results primarily reflect intrinsic catalytic activity. Additionally, we present R as part of a broader set of analyses, including Tafel slope measurements, ECSA-normalized current densities, and simulations of local reactant distributions. We hope that our discussion addresses these concerns and provides a more balanced perspective.\nR2Q4. The electrochemical analysis could be improved:\n(a) A clear change in Tafel slope and thereby the CO RR mechanism on all considered catalysts is seen in Supplementary Figure S19. What is the Tafel slope between 1-2 mA/cm2 and how does it inform about the mechanism?\n(b) The best CO RR performance is seen around -1.0 V vs RHE and the most relevant potential for the mechanistic understanding from Tafel slopes would be around -1.0 V vs RHE rather at than e.g. -0.7 V for the AuNTs. What are the Tafel slopes around -1.0 V vs RHE and what mechanistic insight does this provide?\nAuthor Reply:\nThank you for your thoughtful question regarding the Tafel slopes and their mechanistic implications.\nTafel slope analysis provides direct mechanistic insight by linking the rate-determining step (RDS) to the overpotential dependence of the reaction rate.\nIn the original Supplementary Fig. 19 (now Supplementary Fig. 28), the x-axis represents the logarithm of the partial current density for CO [log(j /mA cm⁻2)] rather than the linear current density (j/mA cm⁻2). The conventional Tafel region, where kinetic control dominates, is confined to low-overpotential conditions, where current density increases exponentially with overpotential, governed primarily by the electron transfer process. Beyond log(j ) ≈ 1 (~10 mA cm⁻2), the system transitions into a mixed regime influenced by mass transport and concentration polarization, leading to deviations from intrinsic kinetic behavior. Specifically, at high current densities, such as that under –1.0 V vs. RHE, rapid CO consumption near the electrode surface induces concentration polarization, disrupting the exponential current-overpotential relationship. These extrinsic factors dominate the reaction behavior, causing the apparent Tafel slope to deviate from intrinsic kinetic properties and the true RDS. Therefore, the slope variations in the log(j ) > 1 region do not necessarily indicate a change in the rate-determining step (RDS) but rather likely reflect the overall interplay of various factors (Science 2019, 364, 1091).\nOur study focuses on the intrinsic activity of the catalyst; therefore, our original manuscript exclusively analyzes the Tafel region. In our study, Tafel slopes for AuNTs, AuNPs, and AuCPs were measured in the low-overpotential range, ensuring that the derived kinetic insights are governed solely by electron transfer, and unaffected by transport limitations or other extrinsic factors. The low Tafel slope (43 mV dec⁻1) for AuNTs indicates a highly efficient electron transfer process and supports a mechanism in which the RDS involves the first electron transfer to adsorbed CO , forming the CO •− or *COOH intermediate (Nature 2016, 537, 382). This is consistent with theoretical studies emphasizing the role of sharp-tip structures and enhanced electric fields in stabilizing this intermediate. By contrast, the higher Tafel slopes (~121–123 mV dec⁻1) observed for AuNPs and AuCPs reflect slower kinetics and suggest that the RDS involves a more energetically demanding pathway.\nFig. R17. Tafel plots for CO partial current density (log(j )) versus applied potential (E vs. RHE) for AuCPs, AuNPs, and AuNTs. The dashed lines indicate the Tafel slopes in the Tafel region, with AuCPs (123 mV dec⁻1), AuNPs (121 mV dec⁻1), and AuNTs (43 mV dec⁻1). At higher overpotentials (–1.0 V to –1.1 V vs. RHE), additional Tafel slopes were extracted, showing increased values: 754 mV dec⁻1 for AuCPs, 504 mV dec⁻1 for AuNPs, and 271 mV dec⁻1 for AuNTs. The transition from the Tafel region to a mixed kinetic and mass transport regime is observed beyond log(j ) ≈ 1.\nWhile the best CO reduction performance is observed at –1.0 V vs. RHE (i.e., the highest Faradaic efficiency for CO production), this potential lies outside the Tafel region. At this higher overpotential, the reaction behavior is dominated by mass transport and possible surface saturation effects, leading to deviations from the intrinsic kinetics. As a result, any apparent Tafel slope calculated under these conditions would not reflect the intrinsic reaction kinetics or provide reliable mechanistic insight, as the conditions are far beyond the range where electron transfer is the sole limiting factor (ACS Nano Editorial Board, ACS Nano 2018, 12, 9635; Xu B. et al. Acta Phys. -Chim. Sin. 2023, 39, 2210025.). However, our data (Fig. R17) confirm that even under higher overpotentials (–1.0 V to –1.1 V vs. RHE), AuNTs maintain superior kinetic advantages (lower slope, 271 mV dec⁻1) over AuNPs (504 mV dec⁻1) and AuCPs (754 mV dec⁻1), reflecting their enhanced electrocatalytic properties.\nWe hope this explanation could address the reviewer’s concerns and clarify the rationale behind our analysis.\nR2Q4. (c) Impedance spectroscopy at the low frequency region could inform about changes in the CO mass transport/diffusion through the CTAC layer. Can some changes be seen and can they be linked with the CO RR performance of the different catalysts?\nAuthor Reply:\nWe appreciate the reviewer’s insightful question regarding the potential impact of CO mass transport/diffusion through the CTAC layer on the CO RR performance, through the CTAC layer on CO RR performance, particularly as reflected in the low-frequency region of impedance spectra. This is indeed a crucial aspect for understanding the catalytic mechanism.\nTo investigate this, we carefully analyzed the impedance spectra, particularly focusing on the Warburg slopes (Z = ΔZ′′/ΔZ′) at low frequencies, which serve as indicators of mass transport effects. Our results show that the Warburg slopes for bare AuCPs, CTAB-capped AuNPs, and CTAC-capped AuNTs are 1.983, 1.981, and 1.974, respectively. These values are nearly identical, suggesting that CO mass transport conditions are similar across all three catalysts in the H-type cell. If significant differences in CO diffusion existed due to the CTAC layer, we would expect notable deviations in the Warburg slopes among the three catalysts. However, this was not observed, indicating that CO transport is not a dominant factor affecting the observed variations in charge transfer resistance.\nThus, while impedance spectroscopy is indeed a valuable tool for probing diffusion behavior, our data suggest that differences in CO diffusion among the three catalysts could not be observed in the H-type cell. We hope this could clarify our approach and address the reviewer’s concerns.\nR2Q4. (d) How is the ECSA analysis performed in practice and how quantitatively is it with intrinsic activity? This is needed to assess the plausibility of a key conclusion: \"Therefore, AuNTs can provide more surface-active sites (i.e., larger ECSA) with higher intrinsic catalytic activity, exhibiting the highest overall reactivity for CO R among the three catalysts.\"\nAuthor Reply:\nWe sincerely thank the reviewer’s insightful comment.\nThe electrochemical surface area (ECSA) was determined via cyclic voltammetry (CV) in 50 mM H SO , utilizing the well-established hydrogen underpotential deposition (H-UPD) method. Specifically, we integrated the charge of the hydrogen adsorption peak obtained from CV scans. Cyclic voltammograms were recorded in a potential range of 0.05 V to 1.45 V (versus SCE) at a scan rate of 50 mV s⁻1 and repeated until stable traces were obtained. The ECSA was calculated using the equation:\n𝑄\nH\n𝐸𝐶𝑆𝐴 =\n𝑄 ×𝐶\nref Au\nwhere Q is the integrated charge of the hydrogen adsorption peak obtained from CV scans, Q = 448 μC cm⁻2 is the reference charge density for a monolayer of hydrogen adsorption on Au, and C is the metal loading on the electrode. The CV curves, calculated ECSA, and detailed calculation steps have been included in the original or now Fig. 4e, original Supplementary Table 1 (now Supplementary Table 3), and revised Materials and Methods section.\nThe catalyst activity is determined by both its electrochemically active surface area (ECSA), which reflects the number of active sites, and its intrinsic catalytic properties. By normalizing the current density to the ECSA, we can accurately assess the catalyst’s intrinsic activity (Nature 2016, 529, 68). To rigorously link ECSA with intrinsic activity, we normalized the CO partial current density (j ) to the ECSA, obtaining the ECSA-normalized activity (j /ECSA), which represents the turnover frequency per electrochemically active site. As shown in original Supplementary Fig. 21 (now Supplementary Fig. 30), the ECSA-normalized j for AuNTs (2.21 mA cm–2) is significantly higher than that for AuNPs (0.54 mA cm–2), and AuCPs (0.30 mA cm–2), confirming that AuNTs not only possess a larger electrochemically active surface but also exhibit superior intrinsic catalytic activity per site. The higher intrinsic activity of AuNTs can be attributed to their unique high-curvature triangular structure, which generates localized electric fields that enhance CO adsorption and activation, as previously confirmed by our DFT calculations and COMSOL simulations (original or now Fig. 2). This synergistic effect contributes to their superior performance compared to other Au nanostructures.\nTherefore, our conclusion that “AuNTs provide more surface-active sites (i.e., larger ECSA) with higher intrinsic catalytic activity, exhibiting the highest overall reactivity for CO RR among the three catalysts” is quantitatively supported by both ECSA measurements and intrinsic activity normalization, and further corroborated by theoretical simulations.\nWe have clarified this point in the revised manuscript (Lines 350−357, Page 13; Lines 670−679, Page 21). We hope these revisions can address the reviewer’s concerns.\nMinor questions/comments/suggestions\nR2Q5. I don’t understand what the caption of Figure 1c means. In what sense is the CO symmetry broken? Does this refer to charge redistribution or change in the CO geometry? See also point 1 above.\nAuthor Reply:\nWe appreciate the reviewer’s insightful question regarding the caption of Fig. 1c and the description of CO symmetry breaking. To clarify, the symmetry breaking of the CO molecule refers specifically to the redistribution of its electron density under the influence of the external electric field, rather than a change in its molecular geometry. In its natural state, CO is a linear molecule with no net dipole moment because the bond dipoles of the C=O bonds cancel each other. However, when exposed to a sufficiently strong electric field, the electron cloud within the CO molecule becomes polarized, leading to a separation of positive and negative charge centers and the induction of a dipole moment.\nImportantly, this analysis is based on simulations of the electron cloud (charge distribution) changes in free CO molecules within an electric field, prior to their chemical adsorption onto the catalyst surface. These results emphasize the changes occurring to CO molecules in close proximity to the catalyst but not yet chemically bound. As illustrated in original or now Fig. 2c, the electrostatic potential maps provide quantitative evidence of this polarization process, showing an increased separation of positive and negative charge centers with stronger electric fields. Specifically, the dipole moment of CO increases from 0 to 1.25 D as the electric field intensity rises from 0 to 1.2 × 108 V m−1 (original or now Fig. 2b). This change is accompanied by a gradual increase in the C=O bond length (from 1.169 Å to 1.182 Å) and a weakening of the bond energy, as detailed in original or now Supplementary Fig. 1.\nTo improve clarity, we have revised the caption of Fig. 1c (Lines 85-87, Page 3) to explicitly state that the symmetry breaking refers to charge redistribution in free CO molecules under an external electric field, rather than changes in molecular geometry. The updated caption now reads: “Charge redistribution in a free CO molecule under an external electric field, leading to symmetry breaking and dipole induction. The migration of the electron cloud creates an asymmetric charge distribution, making CO more prone to adsorption and activation.” We hope this clarification could address the reviewer’s concerns comprehensively.\nR2Q6. Please change the electrode potential scale in Figure 4e to the RHE scale for easier comparison with other results in the work.\nAuthor Reply:\nThank you for your thoughtful suggestion.\nWe have carefully revised Fig. 4e to convert the electrode potential scale from the SCE reference to the RHE reference, making the data easier to compare with other studies. The conversion was performed using the formula: E =E + 0.242 + 0.059⋅pH, where the pH of 50 mM H SO is approximately 1.21.\nFig. R18. Cyclic voltammograms for AuNTs, AuNPs, and AuCPs in 50 mM H SO , with electrode potentials referenced to the RHE scale. The current densities (j ) are plotted as a function of applied potential (E vs. RHE). The potential conversion from SCE to RHE was performed using the formula E = E + 0.242 + 0.059⋅pH, where the pH of the solution is approximately 1.21.\nWe have also inserted Fig. R18 in the revised manuscript (updated Fig. 4e). We appreciate your feedback, which has improved the clarity and relevance of the presented data.\nR2Q7. Caption of Supplementary Figure S25 should include a description of the used symbols.\nAuthor Reply:\nThank you for pointing out the need for a more comprehensive description of the symbols used in original Supplementary Fig. 25 (now Supplementary Fig. 36). We have carefully revised the caption to address this issue. Specifically, we included a detailed explanation of the symbols and parameters depicted in the figure.\nWe have clarified and enhanced the description of constants and variables: The simulation constant parameters (previously listed in Supplementary Table 3 of the original SI) have been further refined for clarity and completeness, defining key fixed values. In addition, we have added a new section for simulation variables, which provides definitions of dynamic parameters relevant to the simulation. The revised information is as follows:\nSupplementary Fig. 36. Schematic representation of geometry and physical model used in the simulation. The symbols used in the figure are as follows:\n(1) Simulation constant parameters\n(2) Simulation variables\nWe have also inserted two table in the revised manuscript (Supplementary Tables 4 and 5). We hope that these revisions can address the reviewer’s concerns and improve the clarity and completeness of Supplementary Fig. 36. Thank you for your valuable feedback, which has helped enhance the overall quality of the manuscript.\nR2Q8. Please describe the geometry of the adsorbed CO at different surfaces and fields? Is it bent or linear?\nAuthor Reply:\nWe sincerely appreciate the reviewer’s insightful question.\nCO near the surface (not adsorbed): Regardless of the facets type (e.g., Au(111), Au(211)) or the presence of an external electric field, the CO remains in a linear configuration (O–C–O bond angle ~180°) due to the weak van der Waals interactions between CO and the metallic surface in this state. The absence of significant charge transfer prevents substantial geometric distortion.\nAdsorbed CO : Upon adsorption onto the Au surface, CO undergoes geometric transformation and adopts a bent (V-shaped) configuration. This bending occurs due to electron transfer from the metal to the formerly unoccupied π* antibonding orbital of CO , leading to elongation of the C=O bonds and a reduction in the O–C–O bond angle. The effect is further pronounced under strong local electric fields, which enhance charge redistribution and promote the formation of the key *CO ⁻ intermediate, facilitating the subsequent reduction process.\nThese findings are supported by our computational results (Fig. R13) and align with previous studies on CO activation on metal surfaces (J. Am. Chem. Soc. 2023, 145, 14267; Acc. Chem. Res. 2021, 54, 1454). This distinction between linear *CO near the surface and V-shaped *CO upon adsorption provides fundamental mechanism into the initial activation step in CO electroreduction. We hope that these explanations can address the reviewer’s concerns.\nReviewer #3:\nThis manuscript presents exciting results in the field of CO reduction in acidic media, showing excellent CO current densities and suppression of HER. The authors use two approaches to accomplish this: electric-field enhancement via tuning the catalyst shape, and charge-selective layers to limit local proton concentration. While neither approach is particularly novel on its own, their combination with application to CO RR in acidic media without alkali metal cations is. These strategies may also be broadly applicable to other areas of electrocatalysis and engineered microenvironments.\nThe authors present compelling, technically sound data. However, I have a number of suggestions and clarification points that should be addressed before it is suitable for publication.\nMy first few comments center around the fact that the authors motivate their paper with the need to suppress HER, accomplished via the cationic surfactant bilayers. However, most of the paper focuses on the field-enhancement effects of the shape-controlled nanoparticles. I am a little confused about the interplay between these two effects, and have the following comments:\nAuthor Reply:\nThanks for the reviewer’s positive comments on our work.\nTo further improve the quality of our manuscript and address your concerns, we have made careful revisions according to each comment of the reviewers. We hope the modifications will address the reviewers’ concerns and fulfill the rigorous criteria for publication in Nat. Commun.\nR3Q1. The electric field enhancement should be better motivated in the introduction.\nAuthor Reply:\nWe sincerely thank the reviewer for highlighting the need to better motivate the role of electric field enhancement in the introduction. In response, we have revised the first two paragraphs of the introduction to emphasize the importance of the electric field in addressing the challenges of CO activation under acidic conditions.\nSpecifically, while acidic systems provide a significant advantage by eliminating carbonate formation and improving CO utilization efficiency, the inherent stability and nonpolarity of CO molecules complicate their activation on catalyst surfaces. This often necessitates high overpotentials, leading to suboptimal energy efficiency. To address this, we highlight the role of intense local electric fields, which can break the nonpolar ground state of CO molecules, inducing dipole moments that facilitate adsorption and activation. Unlike prior studies that primarily focused on electric field effects to enrich alkali metal ions on the catalyst surface, our work uniquely explores the polarization and activation of CO molecules in a metal-cation-free acidic medium using field-enhanced sharp-tip Au nanostructures. This approach represents a novel and essential step toward improving CO reduction kinetics under these challenging conditions.\nWe have incorporated these points into the introduction (Lines 48–49, 54–56, 62–64, Page 2) to better align the motivation of the study with the findings presented in the manuscript. The revised text now explicitly states the dual challenges of HER suppression and CO activation, while emphasizing the critical role of the electric field enhancement in addressing both issues synergistically. We hope this revision could address the reviewer’s concerns.\nR3Q2. The paper would be more impactful if it was divided into three sections in which the shape and the electric field were both investigated independently, and then a final section with both strategies combined at once.\nAuthor Reply:\nWe sincerely appreciate the reviewer’s insightful suggestion to structure the manuscript into three sections focusing on (1) shape effects, (2) electric-field effects, and (3) the synergistic contributions of both strategies. Following this recommendation, we have restructured the study to provide a more systematic and comprehensive evaluation, while incorporating additional experimental results and analysis. Below is an overview of the updated structure:\n1. Shape effects: establishing the role of 2D morphology\nFig. R19. Morphological and structural characterization of AuNDs. (a) TEM image showing their uniform disk-like morphology. (b) HRTEM image highlighting the lattice spacing of 0.235 nm corresponding to the Au(111) planes. (c) HAADF-STEM image and EDX mapping, confirming the uniform distribution of Au, Cl, and N elements. (d) XRD pattern, showing prominent diffraction peaks assigned to Au(111). (e) XPS spectrum of the AuNDs surface, focusing on the N 1s region. (f) UV-Vis spectrum, illustrating their plasmonic resonance characteristics.\nWe first introduce gold nanodisks (AuNDs) as a comparative sample to evaluate the influence of morphology on catalytic activity. AuNDs were synthesized by oxidizing AuNTs under controlled conditions (adapted from Mirkin et al., Nano Lett. 2015, 15, 1012). TEM image (Fig. R19a) confirms their well-defined disk-like morphology, with an average diameter of 68 nm comparable to the edge length of AuNTs. High-resolution TEM (Fig. R19b) and XRD analysis (Fig. R19d) indicate that AuNDs primarily expose (111) facets, similar to AuNTs. HAADF-STEM elemental mapping (Fig. R19c) confirms a uniform distribution of Au, Cl, and N on the surface of AuNDs, consistent with AuNTs, indicating the presence of the CTAC capping layer. This is further supported by XPS analysis (Fig. R19e), which shows a characteristic N 1s peak at 401.9 eV, verifying the presence of CTAC on the AuNDs’ surface.\nFig. R20. Catalytic performance of Au-catalysts for CO R. (a) Current density (j ) versus applied potential (E vs. RHE) for Au nanocatalysts. (b) Faradaic efficiency for CO (FE ) at different applied potentials. (c) Half-cell cathodic energy efficiency (CEE) as a function of applied potential.\nDespite the structural similarity in round curvature between AuNDs and AuCPs, their differing dimensionality leads to distinct catalytic performance. Comparative electrochemical measurements (Fig. R20a) show that AuNDs exhibit significantly higher j than AuCPs, demonstrating the advantage of 2D nanostructures in increasing the electrochemically active surface area (ECSA) and exposing more active sites, thereby enhancing CO reduction efficiency.\n2. Electric-field effects: validating the contribution of tip-enhanced local fields\nAlthough AuNDs share structural similarities with AuNTs in terms of lateral size, two-dimensional morphology, and facet exposure, AuNDs exhibit significantly lower catalytic activity (j ) than AuNTs across all tested potentials, with a j ~3.1 times lower at −1.0 V vs. RHE. These results indicate that morphology effects alone cannot fully explain the superior performance of AuNTs, suggesting the presence of an additional key factor beyond general morphological contributions.\nTo isolate the effect of tip-enhanced local electric fields, we synthesized passivated AuNTs, where the sharp tips were chemically modified into rounded edges while maintaining their triangular shape, 2D morphology, and overall size (Fig. R21a–c), effectively minimizing the localized electric field contribution. Structural characterization using TEM and aberration-corrected HAADF-STEM (Fig. R21a–d) confirms the elimination of high-curvature tips, with the exposed facets primarily corresponding to Au(111). XPS analysis (Fig. R21f) further verifies the presence of the CTAC capping layer, consistent with AuNTs. Despite retaining similar surface area and facet exposure, passivated AuNTs exhibit a substantial decrease in j across all applied potentials (Fig. R22a), demonstrating that the sharp tips play a crucial role in enhancing CO activation and reaction kinetics through localized electric field effects.\nFig. R21. Structural and optical characterization of passivated AuNTs. (a, b) TEM images of passivated AuNTs showing uniform triangular shapes with rounded corners after passivation. (c) High-magnification TEM image of a single passivated AuNT, highlighting the rounded corners. (d) Aberration-corrected high-angle annular dark-field scanning TEM image showing the lattice spacing of 0.235 nm corresponding to the Au (111) plane. The region highlighted by the yellow dashed box in (c) is magnified in (d), providing detailed atomic-scale structural information. (e) UV-Vis extinction spectrum of passivated AuNTs, with plasmonic resonance peaks. (f) XPS spectrum of N 1s for passivated AuNTs, indicating the presence of CTAC.\nFig. R22. Electrocatalytic CO R performance of AuNTs and passivated AuNTs. (a) Partial current densities for CO production (j ) as a function of applied potential (E vs. RHE). (b) Faradaic efficiencies (FE ) for CO production at various potentials. (c) Half-cell cathodic energy efficiency (CEE) as a function of applied potential.\nBy systematically comparing spherical AuCPs, 2D AuNDs, and sharp AuNTs—along with passivated AuNTs as a further control—we demonstrate that while 2D morphology enhances catalytic performance by increasing ECSA, it does not fully account for the superior activity of AuNTs. The presence of sharp tips uniquely contributes to the catalytic enhancement, as evidenced by the significant performance drop in passivated AuNTs. This highlights the crucial role of tip-enhanced local electric fields in facilitating CO reduction, beyond the general morphological advantages of 2D nanostructures.\n3. Synergistic effects: unifying the contributions of morphology and local electric fields\nWe analyzed how the combination of 2D morphology and electric-field enhancement leads to superior catalytic performance in AuNTs. By integrating insights from Sections 1 and 2, we systematically decoupled the contributions of shape and electric field. AuNTs achieved the highest CO production rates and energy efficiency, outperforming AuNDs and passivated AuNTs. This underscores the unique synergy between their 2D morphology and tip-enhanced electric field. The combination of a high ECSA (from 2D morphology) and a localized electric field (from sharp tips) facilitates CO activation and charge transfer, as supported by Tafel analysis and electrochemical impedance spectroscopy (EIS) (Fig. R23). The synergy between these two factors—where 2D morphology increases active site density and the tip-enhanced electric field promotes CO activation—accounts for the exceptional catalytic efficiency of AuNTs, distinguishing them from other nanostructures.\nFig. R23. Comparative electrochemical performance of AuCPs, AuNPs, AuNDs, and AuNTs in CO R. (a) ECSA-normalized partial current densities of CO production (j ) as a function of applied potential (E vs. RHE) for different Au nanostructures. (b) Tafel plots showing the logarithm of j versus overpotential (E vs. RHE) for AuCPs, AuNPs, AuNDs, and AuNTs. The corresponding Tafel slopes are also labeled. (c) Electrochemical impedance spectroscopy (EIS) Nyquist plots for the four Au nanostructures, illustrating their charge transfer resistance differences.\nWe hope this revised structure provides a clearer and more logical narrative, addressing the reviewer’s suggestion while strengthening the scientific rigor of our study. The division into three sections allows for an independent analysis of shape and electric-field effects, followed by their combined contributions, which collectively explain the superior performance of AuNTs. We have incorporated this revised framework in the manuscript (Lines 236–238, 250, 251, 261, Page 9; Lines 271–274, 276, 280–286, Page 10; Lines 287–306, 308–315, Page 11; Lines 316–339, 343, Page 12; Lines 350–364, Page 13) and updated Figs. R19–R23 in the revised SI (Fig. 4a−c and Supplementary Figs. 8, 26–30).\nR3Q3. In the “electrochemical study of tip effect” and “CO electroreduction in metal-cation-free acidic media” sections, the goal is “To validate the predicted enhancement of CO R by [enhanced electric field]”. However, both the field and the cation effects are convoluted here because two of the shapes have cationic surfactants, while the commercial catalyst particles do not. Therefore, while the shape is likely enhancing the electric field, it makes it difficult to ascribe performance benefits solely to the shape when the surfactant is also likely playing a role. This also speaks to my above comment 2 that distinct sections that clearly decouple these effects would be beneficial to the story’s overall impact.\nAuthor Reply:\nThank you for your thorough review and constructive suggestions. Regarding the difficulty in disentangling the effects of shape and the cationic surfactant, we have performed additional experiments and analyses to decouple these factors more clearly.\nSpecifically, based on the framework described in R3Q2, we removed the cationic surfactant CTAC from AuNDs to obtain bare AuNDs (synthesis details on CTAC removal are provided in the next paragraph). By comparing the catalytic performance of bare AuNDs and commercial gold particles (AuCPs), we observed that bare AuNDs exhibited significantly higher partial current density for CO production due to higher electrochemically active surface area, but their Faradaic efficiency (FE) was comparable to that of AuCPs (Fig. R24). This suggests that, in the absence of a cationic surfactant, the 2D structure alone does not significantly enhance catalytic selectivity, further highlighting the critical role of the cationic surfactant layer.\nFig. R24. Comparison of electrocatalytic performance between two-dimensional AuNDs without CTAC and AuCPs. Partial current density (a) and Faradaic efficiency (b) for CO production as a function of the applied potential versus RHE.\nMoreover, we systematically compared the performance of AuNTs and AuNDs before and after CTAC removal. The CTAC removal process was carried out as follows: First, 5 mg of carbon black was dispersed into a mixture of 965 μL of catalyst dispersion (3.45 g L–1 in water) and 3 mL of ethanol. The resulting solution was sonicated for 40 minutes to ensure uniform dispersion. Subsequently, the solution was dissolved in 100 mL of a hydrochloric acid-ethanol solution (HCl:ethanol = 1:9, v/v) and refluxed at 78 °C for 12 hours. After refluxing, the product was collected via centrifugation, followed by alternating washing with ethanol and deionized water to remove the CTAC. The washing process was repeated several times to ensure the complete removal of the surfactant. The final catalyst was dried under a vacuum at 50 °C to obtain the CTAC-free (bare) AuNTs and AuNDs catalyst. XPS analysis confirmed the removal of CTAC from the surface of AuNTs and AuNDs, as evidenced by the disappearance of the N 1s signal after the surfactant removal process (Fig. R25), leaving the nanostructure surfaces clean for subsequent application.\nFig. R25. N 1s XPS spectra of Au nanostructures before and after surfactant removal. (a) N 1s XPS spectrum of CTAC-capped AuNTs. (b) N 1s XPS spectrum of AuNTs after the removal of CTAC. (c) N 1s XPS spectrum of CTAC-capped AuNDs. (d) N 1s XPS spectrum of AuNDs after the removal of CTAC. The elimination of the N 1s signal following surfactant removal indicates the successful detachment of CTAC from the surface of both nanostructures.\nThe results showed that bare AuNTs and bare AuNDs exhibited significantly lower CO Faradaic efficiency (FE ) compared to their CTAC-capped counterparts (Fig. R26). Specifically, CTAC-capped AuNTs and CTAC-capped AuNDs achieved FE values of ~100% and 92.5%, respectively, while their bare counterparts displayed reduced FE values of 61.9% and 54.3%. These findings further confirm that the cationic CTAC layer plays a pivotal role in enhancing catalytic selectivity, particularly by suppressing HER in acidic conditions. Without the CTAC layer, the FE drops significantly, even for AuNTs, which inherently possess a high-curvature structure that promotes CO activation.\nFig. R26. The effect of morphology and surfactant on the Faradaic efficiency of gold nanostructures at –1.0 V vs. RHE. CTAC: gold nanostructures capped with CTAC. Bare: gold nanostructures after CTAC removal.\nMolecular dynamics simulations on three surface models of Au/CTAC, Au/PVP, and Au/SDS further support this conclusion (Fig. R27). The CTAC-capped surfaces exhibit the strongest proton-repelling ability (CTAC > PVP > SDS), which reduces proton availability near the surface and suppresses the competitive HER (Fig. R28).\nFig. R27. Molecular dynamics simulations of the interaction between gold (Au) surfaces and different surfactants in aqueous solution. (a) CTAC-capped Au surface; (b) PVP-capped Au surface; (c) SDS-capped Au surface. Colors: O, red; C, grey; H, white; Au, golden; N, blue; S, orange; Cl, cyan.\nFig. R28. Density profiles of proton distribution near the gold surface with different surfactant layers.\nWe have incorporated these additional results and analyses into the revised manuscript (Lines 290–296, Page 11; Line 375, Page 13; Lines 403–410, 413, 416–419, Page 15) and updated relevant figures (Supplementary Figs. 21, 22, 34, 35). We hope these new data address your concerns and further strengthen the scientific rigor of our work.\nR3Q4. The electric field measured at the catalyst surface will be dependent on electrolyte conditions. What were the simulation conditions used in Figure 2? Additionally, the simulations were conducted for the case (I believe) with no surfactants. The extended and distributed charges present in the surfactant bilayer will undoubtedly influence the electric field, and possibly even screen it. The authors even note this on page 12, line 341. This critically would mean that the advantages of both individual approaches may not sum together, again motivating comment 2. Electric field simulations with the bilayers should be conducted.\nAuthor Reply:\nWe appreciate the reviewer’s insightful comments regarding the influence of electrolyte conditions and the distributed charges in the surfactant layer on the electric field at the catalyst surface.\nThe electric field simulations in Fig. 2 were performed using COMSOL Multiphysics 6.2 to compare electric field intensities around different electrode geometries, including sharp triangle, truncated octahedron, and sphere. An electrolyte region of 200 nm in length, width, and height was modeled, with the current density of 103 A·m⁻2 at the top boundary and a fixed ion concentration of 102 mol·m⁻3 at the electrolyte surface. A surface charge density of 44.23 C·cm⁻2 was introduced to simulate the effects of surface modifications, the surfactant layer, on the electric field distribution. In this model, we accounted for the influence of sharp geometries, which concentrate the electric field at the tips due to higher curvature. This model provides a comprehensive understanding of the electric field distribution in the presence of surface charge, effectively considering the influence of the cationic surfactant layer on the electrochemical behavior. The simulation conditions for Fig. 2d are detailed in Lines 690–716, Page 22 in the revised manuscript.\nIn addition, we would like to clarify that the distributed charges from the CTAC surfactant were simulated by introducing the surface charge density (ρ=44.23 C·cm⁻2) on the electrode, as shown in Fig. 2d. The value of 44.23 C·cm⁻2 was determined based on calculations detailed in original manuscript (Lines 513–531, Page 16) and is also provided in the revised manuscript (Lines 626–646, Page 20). This approach effectively mimics the extended charge distribution from the surfactant layer and its impact on the electric field. To further illustrate this screening effect, we conducted a comparative simulation of the electric field distribution around AuNTs without the presence of surface charge (ρ=0, i.e., no surfactant layer). The results show a slight enhancement in the electric field to 1.30 × 108 V·m⁻1 (Fig. R29) compared with 1.21 × 108 V·m⁻1 in the presence of surface charges, confirming that the distributed charges in the surfactant layer attenuate the electric field. As noted in the original manuscript (Line 341, Page 12), the extended charges induce a screening effect, which reduces the electric field intensity compared to an unmodified surface.\nFig. R29. COMSOL Multiphysics simulations of the electric field distribution around the Au surface of AuNTs without additional surface charge density.\nWhile the introduction of the surfactant layer reduces the electric field strength, it also introduces critical advantages, such as proton blocking and improved CO selectivity, as discussed in the manuscript. The combination of the sharp-tip geometry and surfactant-induced proton repulsion work synergistically to achieve high CO R performance, balancing electric field modulation with selective reaction pathways.\nWe hope this detailed explanation, along with the additional simulation data, addresses the reviewer’s comments.\nAdditional comments:\nR3Q5. What evidence is there that the bilayers form on the NTs? And on the NPs? Could they be monolayers? I’d imagine the shape of the particles will also impact the conformity of the surfactant layer.\nAuthor Reply:\nWe thank the reviewer for raising this critical point regarding the formation of surfactant layers on nanotubes (NTs) and nanoparticles (NPs), the possibility of monolayer formation, and the impact of particle shape on layer conformation.\nThe structure of surfactant on NTs and NPs is indeed influenced by the concentration of the surfactant. Literature studies have shown that hexadecyltrimethylammonium halide typically forms bilayers or micelles when the concentration exceeds the critical micelle concentration (CMC, 0.96 mM for CTAB; 1.3 mM for CTAC). In our system, the surfactant concentration exceeds the CMC, leading to the formation of a dense surfactant layer, likely resembling the bilayer structures observed in previous small-angle neutron scattering (SANS) studies (Langmuir, 2012, 28, 1453).\nThe shape of the particles indeed plays a crucial role in the organization of the surfactant layer. Literature reports (e.g., Acc. Chem. Res. 2023, 56, 1204; J. Am. Chem. Soc. 2015, 137, 2265) indicate that high-curvature regions, such as the corners of nanoparticles, may result in less ordered or patchy surfactant layers. In contrast, flatter regions, such as the sidewalls of nanorods or spherical nanoparticles, are more likely to support dense and uniform layers. This shape dependence could lead to variations in the local structure of the surfactant layer, which aligns with the reviewer’s observations.\nWe have made efforts to directly validate the surfactant layer structure. Specifically, we contacted multiple facilities, including the China Spallation Neutron Source (CSNS) and the China Institute of Atomic Energy (CIAE), to arrange small-angle neutron scattering measurements. Unfortunately, due to scheduling constraints, we were unable to secure beam time within the required timeframe. Despite this limitation, we remain committed to pursuing such experiments to further verify our findings.\nWe have revised the manuscript to replace the term “bilayers” with “layers” (Line 372, Page 13) to avoid overinterpretation. Additionally, we have updated the legend for Fig. 1e (Lines 91-93, Page 3) to ensure that the conceptual nature of the illustration is clear. We hope these clarifications can address the reviewer’s concerns.\nR3Q6. For the PNP simulations, the surfactant layer was set at 22 um. That seems very large – how was this number chosen? Especially compared with the thickness of the EDL.\nAuthor Reply:\nThank you for your insightful question regarding the 22 μm thickness of the “surfactant layer” in the PNP simulations. We appreciate the opportunity to clarify this point further, which have helped us better articulate the rationale behind our parameter choices.\nThe “surfactant layer” in the manuscript corresponds to the “catalyst layer,” which is composed of gold nanoplates, each coated with the layer of CTAC surfactant, resulting in a continuous surfactant medium within the catalyst layer. The 22 μm thickness was determined through experimental characterization using scanning electron microscopy (SEM), as shown in original Supplementary Fig. 27 (now Supplementary Fig. 38). This measurement reflects the physical thickness of the catalyst coating applied during electrode preparation and ensures that the PNP simulations are based on experimentally validated parameters.\nWhile we acknowledge that the thickness of the catalyst layer is significantly larger than the electrical double layer (EDL), which is typically on the nanometer scale, this discrepancy is not contradictory. The PNP approach is well-suited for simulating the macroscopic electrochemical behavior of the entire system at a larger scale, capturing ion transport and charge distribution throughout the entire layer, rather than focusing exclusively on the fine details of the EDL at the electrode-electrolyte interface, due to the inherent limitations in resolving nanometer-scale phenomena.\nWhile the study of the EDL’s molecular-level features is indeed an important aspect of electrochemistry, it was not the primary focus of our work. For investigations focused on the nanoscale EDL, different modeling approaches may be more appropriate, but these fall outside the scope of this study. We fully appreciate the reviewer’s concern about the scale discrepancy in and would like to clarify that our modeling choice was made to reflect the experimental conditions and to focus on the broader electrochemical behavior of the system.\nWe have revised the figure to replace “surfactant layer” with “catalyst layer” to eliminate any ambiguity. The updated figure is now presented in the revised Supplementary Fig. 36. We hope these clarifications can address the reviewer’s concerns.\nR3Q7. The authors present in situ ATR-SEIRAS measurements to demonstrate that the surfactant remains stably adsorbed to the gold surface. What potential were those measurements conducted at? Is the adsorption stable at all potentials? The authors note that there is potential-dependent adsorption, so I would assume not. Also, the system was monitored for only 45 minutes. What happens beyond that time?\nAuthor Reply:\nWe greatly appreciate the reviewer’s insightful comments. To address the concerns raised, we have conducted additional experiments and provide the following detailed clarifications:\nThe ATR-SEIRAS spectra presented in the original Fig. 5a were obtained at a constant potential of −1.0 V vs. RHE. This potential was chosen because it corresponds to the optimal selectivity conditions for CO electroreduction. To further validate the stable adsorption behavior of the surfactant CTAC on the electrode surface, we have added new constant potential ATR-SEIRAS data at −0.7 V and −1.1 V vs. RHE, representing the two ends of the potential range studied (Fig. R30). The results show that throughout the experimental time (45 minutes in the supplementary experiment), the CTAC adsorption spectra remain consistent, indicating that CTAC exhibits excellent stability across the entire studied potential range.\nFig. R30. Time-dependent adsorption behavior of CTAC and CO under constant potential conditions. In-situ ATR-SEIRAS spectra collected at constant potentials of (a) −1.1 V vs. RHE and (b) −0.7 V vs. RHE, showing the stability of CTAC adsorption (ν , red dashed lines) on the AuNTs surface and the progressive consumption of CO reflected by the decrease in ν intensity (yellow dashed lines) over time.\nWith respect to the potential-dependent adsorption of CTAC, we would like to clarify the following, based on the original data: As shown in original Supplementary Fig. 22 (now Supplementary Fig. 23), the intensity of the ν peaks (around 2860 cm⁻1 and 2960 cm⁻1) varies with applied potential. At positive potentials, these peaks are upward, indicating a decrease in CTAC coverage relative to the open circuit potential. As the applied potential becomes more negative, these peaks invert and intensify, reflecting a progressive increase in CTAC adsorption. This observation confirms the potential-dependent nature of CTAC adsorption, with stronger adsorption occurring at more negative potentials. While there is no significant shift in the ν peaks, the change in peak intensity (and upward/downward) clearly indicates the modulation of CTAC adsorption by the electric field.\nOur electrochemical performance evaluation demonstrates that Au nanotriangles (AuNTs) modified with CTAC maintain a Faradaic efficiency of over 97% throughout 100 hours of continuous operation (Fig. 4d in the original or now manuscript). To further probe the role of CTAC, a control experiment was conducted by removing the surfactant, yielding bare AuNTs, which were subsequently tested for CO reduction. Under identical conditions, the selectivity dropped significantly to approximately 60% (Fig. R26). This pronounced decline highlights the critical role of CTAC in sustaining high selectivity. Moreover, the stable selectivity observed over 100 hours strongly suggests that CTAC remains persistently adsorbed on the catalyst surface, further reinforcing its essential function in promoting efficient CO reduction.\nIt is challenging to monitor extremely long times with ATR-SEIRAS experiments due to the complexity of the experimental setup. Therefore, we selected a representative experimental time scale for characterization. However, the long-term electrochemical testing results provide indirect evidence of the stability of CTAC on the catalyst surface. Therefore, the combined results from ATR-SEIRAS and electrochemical testing conclusively show that CTAC remains stably adsorbed on the Au surface over long reaction times, ensuring the high efficiency and stability of the catalytic system.\nWe have also revised the manuscript accordingly (Lines 379, 380, 382, 393, Page 14) and inserted Fig. R30 in the revised SI (Supplementary Fig. 32). We hope these revisions can address the reviewer’s concerns.\nR3Q8. The Tafel slopes were fit using only 3 data points. Tafel slope fitting is prone to extreme error/subjectivity (see for example Nature Communications volume 12, Article number: 703 (2021)) Could the authors comment on the stringency of analysis and perhaps add additional points to the fit?\nAuthor Reply:\nThank you for raising this important point regarding the Tafel slope fitting and the limited number of data points in our initial analysis.\nTo address this concern, we increased the number of data points used for the fitting, all of which fall within the original Tafel region (Fig. R31). The additional points enhance the data density within this linear region without extending beyond it, thereby improving the reliability and robustness of the fitting. This approach reduces the risk of error associated with insufficient data density and ensures consistency with the underlying physical processes. To further validate the updated fitting, we calculated the coefficient of determination (R2), which now exceeds 0.999 for all samples. This significant improvement demonstrates the high quality of the fit and provides strong statistical confidence in the results.\nFig. R31. Tafel plots of various gold nanostructures (AuCPs, AuNPs, AuNDs, and AuNTs) for CO reduction. The data points represent the electrode potential (E vs. RHE) as a function of the logarithm of the CO current density (logj ), with the corresponding Tafel slopes indicated for each structure. Dashed lines represent the linear fitting of the experimental data for each material.\nWhile we recognize the potential benefits of Bayesian data analysis methods, as discussed in the referenced study (Nat. Commun. 2021, 12, 703), our approach of increasing data density within the Tafel region provides comparable rigor. This method effectively addresses the concerns raised while ensuring a clear and reliable evaluation of the Tafel slope. We plan to explore more advanced methods, including Bayesian approaches, in future studies to further enhance the robustness of our analyses.\nWe have incorporated Fig. R31 into the revised Supplementary Information (Supplementary Fig. 28). Additionally, we have included the reference (Nat. Commun. 2021, 12, 703) in the revised manuscript as Ref. 53. We hope these revisions could address the reviewer’s concerns.\nR3Q9. What carbon black was used in the catalyst inks? Was it also a Vulcan XC-72 as used in the commercial catalysts?\nAuthor Reply:\nThanks for the reviewer’s kind question.\nThe carbon black used in the catalyst inks was Vulcan XC-72 (Carbot Co., Boston, MA, USA), consistent with the commercial catalysts. We have updated the Materials and Methods section accordingly to clarify this point (Line 476, Page 17). We hope these revisions can address the reviewer’s concerns.\nReviewer #4:\nThis manuscript introduces an approach to CO electroreduction (CO R) in strong acidic media, addressing several critical challenges in the field. The study emphasizes the advantages of a metal-cation-free system, which reduces CO reactant losses, eliminates the risk of metal salt precipitation, and enhances device tolerance compared to systems using metal cations.\nThe researchers developed sharp-triangle gold (Au) nanostructures capped with a hexadecyltrimethylammonium chloride (CTAC) layer enriched with cationic sites. Inspired by aquaporins in acidophiles, this design leverages intense local electric fields generated by the high-curvature tips of the Au nanocatalyst to polarize CO molecules. This polarization increases the dipole moment of CO , facilitating its adsorption and activation. Additionally, the CTAC layer acts as a proton barrier, suppressing the hydrogen evolution reaction (HER) by mimicking the proton-blocking mechanism of aquaporins.\nThis dual-function design enabled continuous CO R for 100 hours in a flow electrolyzer at pH 1.0, achieving an impressive energy efficiency of 60% and nearly 100% Faradaic efficiency for CO production. This bioinspired strategy represents a significant advancement in CO R technology by integrating innovative catalyst design principles, potentially paving the way for more efficient and sustainable CO reduction processes.\nOverall, the paper is well-written and presents highly promising results for achieving electrocatalytic CO reduction under acidic conditions. The outcomes in terms of efficiency and selectivity are remarkable, especially given that the electrolyzer is a flow cell, which allows for process scalability.\nI believe this manuscript meets the standards for consideration by Nature Communications and will significantly contribute to the important field of CO utilization via electrocatalysis. Moreover, the presentation quality of the paper, both in terms of text and figures, is outstanding.\nHowever, I have some remarks for the authors to consider:\nAuthor Reply:\nThanks for the reviewer’s positive comments and support for our work.\nBased on the reviewers’ suggestions, we have made careful revisions to our manuscript. We hope that the revised manuscript can address the reviewers’ concerns and can be accepted for publication in Nat. Commun.\nR4Q1. Extension to Other Products: What about extending this approach to produce more valuable products (e.g., C + molecules)? The work shows high selectivity and good partial density for CO formation, but this is an already well-solved problem.\nAuthor Reply:\nThank you for your insightful comment regarding the extension of our approach to produce more valuable C products. In response, we conducted additional experiments using copper nanotriangles (CuNTs) coated with hexadecyltrimethylammonium chloride, as copper is known to favor C + product formation under appropriate conditions. The CuNTs were carefully synthesized and characterized via TEM, XRD, XPS, and UV-Vis spectroscopy to confirm their morphology, crystalline structure, and surface composition (please see also our replies to R4Q7).\nFig. R32. CO reduction performance comparison of CTAB-capped Cu nanotriangles (CuNTs) and Cu nanoparticles (CuNPs) in acidic electrolyte (0.07 M H SO , pH = 1). (a) Total current density (j ) as a function of applied potential (E) versus the reversible hydrogen electrode (RHE), with CuNTs and CuNPs represented by the orange and blue lines, respectively. (b, c) Faradaic efficiencies (FE) of various products at different potentials for both CuNTs and CuNPs. Products include H , CO, HCOOH (formic acid), C H (ethylene), EtOH (ethanol), and PrOH (n-propanol).\nIn acidic CO reduction experiments, CuNTs exhibited superior performance compared to CuNPs. At −1.5 V vs. RHE, CuNTs achieved a Faradaic efficiency (FE) of 69.4% for C + products, including ethylene, ethanol, and n-propanol, with a partial current density of 117 mA cm⁻2 (Fig. R32). In contrast, CuNPs showed a significantly lower FE for C + products, highlighting the enhanced catalytic activity of CuNTs for the production of more valuable multicarbon molecules. This result underscores the versatility of the CTA+-capped nanotriangles strategy, which not only favors CO production but also enables efficient conversion to multicarbon products. The comparison between CuNTs and CuNPs further demonstrates the potential of our approach for sustainable CO conversion, suggesting that this method could be applied to other catalytic systems and electrolytes to expand the scope of CO utilization technologies.\nWe hope such additional discussion could address the reviewer’s concerns.\nR4Q2. Supporting Information Figures: The figures in the supporting information should be named, for example, Fig. S1, to avoid confusion with the figures in the main text.\nAuthor Reply:\nWe sincerely thank the reviewer for their careful reading of our manuscript and for pointing out the potential for confusion in the naming of figures in the Supporting Information. We understand the importance of ensuring clarity and consistency in figure labeling to facilitate easy reference.\nTo clarify, the figures in the Supporting Information are labeled as “Supplementary Fig. X” in accordance with the formatting guidelines provided by Nature Communications. This naming convention aligns with the journal’s style requirements and ensures consistency across submissions to the journal.\nWe appreciate the reviewer’s thoughtful suggestion and assure you that we have carefully followed the journal’s guidelines to avoid any potential confusion. If additional clarification or further modifications are needed, we are more than happy to address them.\nThank you once again for your valuable feedback and constructive suggestions.\nR4Q3. Clarification on CO Adsorption: Lines 143-145 state, “Enhancing CO adsorption on Au surfaces would theoretically promote CO R since Au showed relatively weak adsorption to CO species and was located on the right side of the volcano plot for CO R1,43.” This key concept should be expanded. Moreover, the referenced paper (Phys. Chem. Chem. Phys., 2014, 16, 4720) discusses the binding of intermediates (*CO, *COH, *CHO, and *COOH) rather than CO adsorption.\nAuthor Reply:\nWe appreciate the reviewer’s feedback on the discussion of CO adsorption and the suggestion to refine the referenced content. Upon review, we recognize that the cited paper (Phys. Chem. Chem. Phys. 2014, 16, 4720) focuses primarily on the binding energies of intermediates (*CO, *CHO, *COH, and *COOH) rather than the adsorption of CO itself. Below, we address this issue and elaborate on the role of CO adsorption and intermediate stabilization in our work.\nClarification of CO adsorption and key intermediates: At the molecular scale, CO adsorption on the catalyst surface is the first (initial) critical step for subsequent electrocatalytic reactions. Effective CO adsorption is a prerequisite for interfacial charge transfer, enabling the formation and stabilization of key reaction intermediates, thereby driving the CO R forward. While Au exhibits relatively weak adsorption of CO species and key intermediates due to its electronic structure, our findings demonstrate that the enhanced electric field (EEF) significantly improves the CO adsorption and intermediates stabilization, as discussed in our DFT calculations (Fig. 2e,f in original or now manuscript).\nTo address the reviewer’s comment, we have revised the manuscript on Line 147, Page 5; Lines 151, 166, 167, 172-175, Page 6 to better reflect CO adsorption and intermediate stabilization as key factors influencing CO R performance. The revised text emphasizes the following points: The EEF modifies the surface electronic structure, which not only strengthens CO adsorption but also stabilizes key intermediates, thereby lowering the overall reaction energy barriers.\nThe referenced paper (Phys. Chem. Chem. Phys. 2014, 16, 4720) provides valuable insights into intermediate stabilization but does not directly address CO adsorption. We have clarified this distinction in the revised manuscript and included additional references (Ref. 41-43) that specifically discuss CO adsorption (Chem. Soc. Rev. 2023, 52, 4343; Nat. Commun. 2023, 14, 1724; Joule 2024, 8, 1999) to provide a balanced and accurate perspective. We hope these revisions can address the reviewer’s concerns.\nR4Q4. Figure Reference: Lines 146-147 seem to omit a reference. Is it also Figure 2f?\nAuthor Reply:\nThank you for pointing out this oversight.\nYou are correct that the reference to Lines 146-147 should have explicitly cited Fig. 2f, which presents the DFT-calculated Gibbs free energy diagrams for CO reduction on Au(111) and Au(211) facets. To improve clarity, we have revised the manuscript to explicitly state this reference. The updated sentence now reads: “Further evidence was provided by the DFT-calculated Gibbs free energy (ΔG) diagrams for CO R on different Au facet models at 298 K and 1 atm (Fig. 2f, Supplementary Figs. 3 and 4).”\nWe appreciate your careful review and hope this revision improves the clarity of our manuscript.\nThe modification has been made in the revised manuscript (Line 165, Page 6).\nR4Q5. Overpotential Reduction: Lines 153-155 should report the lowering of the overpotential when the external electric field (EEF) is applied, which can be derived from the free energy of the limiting step.\nAuthor Reply:\nWe sincerely thank the reviewer for highlighting the need to clarify the lowering of the overpotential when the external electric field (EEF) is applied, as derived from the free energy changes in the rate-determining step (RDS). The detailed calculations are as follows:\nFrom the Gibbs free energy diagrams presented in Fig. 2f, the energy barrier for the RDS (*CO + H+ + e⁻ → *COOH) decreases significantly under the influence of the EEF: On Au(111): The free energy barrier decreases from 0.97 eV (without EEF) to 0.39 eV (with EEF), corresponding to a reduction of 0.58 eV. On Au(211): The free energy barrier decreases from 0.80 eV (without EEF) to 0.36 eV (with EEF), corresponding to a reduction of 0.44 eV. To convert this free energy reduction into the overpotential reduction (ΔE), we use the relationship:\n∆G\n∆𝐸 =\nn𝐹\nwhere n = 1 (electron transfer number of RDS) and F = 96485 C mol−1 (Faraday constant). The calculated overpotential reduction is 580 mV and 440 mV for Au(111) and Au(211), respectively.\nWe acknowledge that these theoretical reductions (580 mV and 440 mV) are larger than the experimental overpotential difference observed between Au nanoparticles (spherical) and Au nanotriangles (sharp tips), which is approximately 100 mV. This discrepancy could arise from the averaged effect of different exposed facets in real nanoparticles versus the idealized surfaces considered in the DFT calculations. Additionally, it is important to note that Au nanoparticles (spherical) also generate a certain level of local electric field due to their surface curvature, albeit much weaker than that of the sharp tips in Au nanotriangles. This residual electric field further reduces the experimental overpotential difference compared to the purely “no-field” condition assumed for spherical surfaces in DFT. We are currently investigating these aspects further to better reconcile the theoretical and experimental results.\nThis explanation has been added to the revised manuscript for better clarity (Lines 178–180, Page 6; Line 181, Page 7; Lines 792–797, Page 24). We appreciate the reviewer’s insightful suggestion, which has strengthened the discussion and relevance of our findings.\nR4Q6. Activation Check: Lines 159-160 suggest that adsorption energy alone is insufficient to check activation, as this could still be a physisorption process. Consider looking at Amira’s paper and investigating C–O elongation, O–C–O bending, and Bader charge analysis for charge transfer from the surface to CO .\nAuthor Reply:\nWe greatly appreciate your insightful comment on the necessity of evaluating CO activation beyond adsorption energy, as it could indeed involve a physisorption process. We have investigated the activation mechanisms of CO on Au surfaces under enhanced electric fields, including the investigation of C–O bond elongation, O–C–O bond angle bending, and Bader charge analysis for charge transfer.\nFig. R33. Structural configurations and activation of CO on Au surfaces under enhanced electric fields. (a, b) Initial adsorption configurations of CO on Au(111) and Au(211) surfaces, respectively, showing linear CO geometry before activation. (c, d) Optimized adsorption configurations of CO on Au(111) and Au(211) surfaces under enhanced electric field, illustrating significant activation with elongated C–O bonds and bent O–C–O angles. The red, brown, and gold spheres represent oxygen, carbon, and gold atoms, respectively.\n1. C–O bond elongation: Under the enhanced electric field (EEF), significant elongation of the C–O bond was observed, which indicates the activation of CO on the catalyst surface: On the Au(111) facet, the C–O bond length increased from 1.182 Å (non-adsorbed state) to 1.354 Å. On the Au(211) facet, the C–O bond length increased from 1.182 Å to 1.361 Å.\n2. O–C–O bond angle bending: The O–C–O bond angle showed substantial bending under EEF, suggesting that CO transitioned from its linear configuration to a bent structure upon adsorption: On the Au(111) facet, the bond angle decreased from 179.6° (non-adsorbed state) to 124.3°. On the Au(211) facet, the bond angle decreased from 179.6° to 122.7°.\n3. Bader charge analysis: Bader charge calculations confirmed electron transfer from the Au surface to CO , further validating its activation: On the Au(111) facet, a charge transfer of −0.218 e was observed from C to O atoms. On the Au(211) facet, a more substantial charge transfer of −0.302 e was observed.\nThese results collectively demonstrate that the enhanced electric field not only lowers the energy barrier for CO adsorption but also induces significant geometric and electronic structural changes, thereby facilitating CO activation. The elongation of the C–O bond, the bending of the O–C–O bond angle, and the evident charge transfer strongly indicate that CO undergoes a transition from physisorption to chemisorption, consistent with the activation mechanism.\nWe have included these new findings in the revised manuscript (Lines 155–163, Page 6). We hope these analyses address your concern and further reinforce the robustness of our conclusions.\nR4Q7. Synthesis Methodology: How extensible is the synthetic methodology for Au nanostructures to other metals? While Au is excellent for CO production with high selectivity, achieving high selectivity for more valuable chemicals remains a challenge.\nAuthor Reply:\nThank you for raising this important point regarding the extensibility of our synthetic methodology to other metals and the potential to achieve high selectivity for more valuable chemicals. While the synthetic protocol for Au nanotriangles (AuNTs) involves precise control over reduction kinetics and capping agents (CTAC), the general principle of tailoring morphology and surface charge through surfactant-mediated growth is transferable to other metals.\nTo illustrate this, we have extended our synthesis approach to copper, a metal known for favoring C + product formation under CO reduction conditions. Specifically, we synthesized copper nanotriangles (CuNTs) using a surfactant-assisted hydrothermal method. This approach involves the reduction of Cu(NO ) ·3H O in the presence of hexamethylenetetramine and CTAC, which promotes anisotropic growth. The resulting CuNTs exhibit a well-defined morphology with sharp edges, similar to the high-curvature features of AuNTs, which are critical for enhanced local electric fields and CO activation.\nThe synthesis details are as follows: In a typical synthesis, 50 mg of Cu(NO ) ·3H O and 100 mg of L-ascorbic acid were dissolved in 15 mL of ultrapure water to form a homogeneous solution. Subsequently, 100 mg of CTAC and 100 mg of hexamethylenetetramine were added, followed by stirring for 30 minutes. The solution was then sealed and heated to 80 °C (at a rate of 2 °C min−1) and maintained at this temperature for 3 hours. After the reaction, the product was collected by centrifugation and washed with a mixture of ethanol and ultrapure water.\nCu nanoplates (CuNTs) were synthesized using a surfactant-assisted hydrothermal method, achieving well-defined triangular shapes with sharp edges and highly crystalline structures. The UV-vis absorption spectrum (Fig. R34a) displays a peak in the 663 nm, corresponding to localized surface plasmon resonance (LSPR) effects. X-ray diffraction (XRD, Fig. R34b) confirms the high crystallinity of the CuNTs, with diffraction peaks at 43.3°, 50.4°, and 74.2° indexed to the (111), (200), and (220) planes of metallic copper, matching JCPDS card #70-3039. X-ray photoelectron spectroscopy (XPS, Fig. R34c) further corroborates the metallic nature of the product, as evidenced by Cu 2p and Cu 2p peaks at 932.7 eV and 952.5 eV, respectively, with negligible oxidation states detected. Transmission electron microscopy (TEM, Fig. R34d, e) images highlight the uniform triangular morphology with side lengths of 1.29 μm, while the aberration-corrected high-angle annular dark-field scanning TEM (AC-HAADF-STEM, Fig. R34f) reveals a lattice spacing of 0.209 nm, corresponding to the (111) plane of face-centered cubic copper. The distinct morphology and crystallographic features of CuNTs demonstrate the successful extension of the synthetic approach used for Au nanoplates to other metallic systems. Additionally, it aligns with our efforts to explore materials that can facilitate the formation of C + products, as copper is well-documented for such applications (for performance data, please see our reply to R4Q1).\nFig. R34. Characterization of CuNTs. (a) Extinction spectra of CuNTs. The peaks correspond to in-plane dipole resonances. (b) XRD pattern of CuNTs. (c) Cu 2p core level XPS spectrum. (d, e) TEM image exhibiting the sharp angle of CuNTs. (f) AC-HAADF-STEM image showing the (111) planes of CuNTs.\nWe are continuing to expand this work, including further optimization of synthetic protocols and characterization of the resultant nanostructures, to explore their potential in producing other valuable chemicals. Future studies will aim to validate the scalability and adaptability of this methodology for a broader range of metal nanostructures and catalytic applications.\nR4Q8. Comparison with Literature: How do the partial current density values of CO on the three Au systems compare with other literature values for CO -to-CO conversion on Au catalysts?\nAuthor Reply:\nThank you for this important question.\nWe have added the partial current density values for CO (j ) of the other two Au systems (AuNPs and AuCPs) into Supplementary Table 1 to provide a more complete comparison with literature values. The AuNTs achieve a significantly higher j of ~120 mA cm⁻2 compared to AuNPs (~29.6 mA cm⁻2) and AuCPs (~14.6 mA cm⁻2) at −1.0 V vs. RHE. These values are benchmarked against other state-of-the-art Au catalysts for CO -to-CO conversion as listed in the updated Supplementary Table 2.\nThis addition further emphasizes the superior performance of AuNTs, not only in comparison to our other tested Au systems (AuNPs and AuCPs) but also relative to values reported in the literature. For example, our AuNTs demonstrate higher j under similar experimental conditions than many previously reported Au catalysts, confirming the advantages of our design.\nWe appreciate the reviewer’s insightful comment and hope that the expanded comparison now provides a comprehensive context for evaluating the performance of our catalyst.\nR4Q9. Definition of CEE: Define the half-cell cathodic energy efficiency (CEE) on page 238.\nAuthor Reply:\nThank you for highlighting this point.\nHalf-cell cathodic energy efficiency (CEE) quantifies the ratio of the energy effectively used for CO reduction to the total energy input in the electrolysis process, under the assumption of a zero overpotential for the oxygen evolution reaction at the anode. The calculation is given by:\n[𝐸 +(−𝐸 )]×FE\nOER CO CO\nCEE =\n𝐸 +(−𝐸 )\nOER cathode\nwhere E = +1.23 V is the standard thermodynamic potential for the O /H O couple, representing the energy requirement for the oxygen evolution reaction at the anode. E = –0.109 V is the standard thermodynamic potential of CO /CO (vs. RHE), FE is the Faradaic efficiency of CO production, and E is the applied cathode potential (vs. RHE).\nWe have also revised the manuscript accordingly (Line 263, Page 9; Lines 662–663, Page 21). We hope this addition improves the clarity and completeness of the manuscript.\nReviewer #2 (Remarks to the Author):\nI wish to thank the authors for carefully addressing all raised points - I am now happy to support the publication of this work.\nResponse to Reviewer #2\nWe sincerely thank the reviewer for the positive and encouraging final comment. We are truly grateful for the reviewer’s continued engagement and for the generous support of our revised manuscript.\nThe reviewer’s thoughtful feedback throughout the review process has been invaluable in improving both the scientific depth and clarity of our work. We are delighted that the final version now meets the reviewer’s expectations and are very thankful for the reviewer’s recommendation to support publication in Nature Communications."}]

Decision letter
Peer Review File
Electric-field-driven CO2 polarization and bioinspired proton
blocking unlock CO2 reduction in strong acid without metal
cations
Corresponding Author: Professor Jibin Song
This file contains all reviewer reports in order by version, followed by all author rebuttals in order by version.
Attachments originally included by the reviewers as part of their assessment can be found at the end of this file.
Version 0:
Reviewer comments:
Reviewer #1
(Remarks to the Author)
In this manuscript, the authors reported that they prepared and applied triangle Au nanostructures capped with a CTAC layer
for CO2 reduction in claimed “metal-cation-free” H2SO4 electrolytes. They concluded that high FE for CO in acidic
conditions was due to the so-called “electric-field-driven” effect and “bioinspired proton blocking” effects.
1. “electric-field-driven” effect
(a) No experimental evidence to support the “electric-field-driven” effect claimed by the authors
Tip-enhanced electric field intensification is not a new concept. It was studied and reported for electrocatalysis as early as
2016 (Nature 537, 382–386 (2016)). Since 2016, several experimental methods have been developed to characterize and
prove this “tip-enhanced electric field intensification”, for example, conductive atomic force microscopy; Kelvin probe atomic
force microscopy; stark tunning shift of adsorbed species (such as CO in this study) by in-situ ATR-SEIRAS spectra and so
on. However, there is no experimental data or evidence in this manuscript to support the so-called “Tip-enhanced electric field intensification” claimed by the authors.
Theoretical calculation simulation is a useful tool to help explain some experimental phenomena and results. However,
merely relying on computational simulation but not any experimental evidence data is unacceptable. The conclusion is
not convincing to this reviewer. In addition, most information/statements provided by the simulation results in this study have
already been reported and discussed in the literature of the last few years (https://doi.org/10.1016/j.jallcom.2024.175549).
(b) There is no clear correlation between the good catalytic performance of triangle Au and the “electric-field-driven” effect
The authors claimed that the higher intrinsic catalytic activity of AuNTs compared to those of AuNPs and AuCPs is due to
the “tip-enhanced electric field”, which is not convincing based on the results in this work. As AuNTs, AuNPs and AnCPs
have different sizes and shapes/morphology and each of them exposes different facets and structural edges, all these factors
can lead to different activity (normalized by ECSA) and Tafel slope. The authors didn’t discuss these factors, but instead
assigned the improved activity to the “electric-field-driven” effect, which didn’t make any sense.
The result of ATR-SEIRAS used to show that “the stronger potentials enhance the adsorption of these intermediates” doesn’t
make sense! As mentioned in point 1, the local electric field should be characterized by a Stark tunning shift of adsorbed
species (such as CO in this study) using in-situ ATR-SEIRAS spectra.
Except for the calculation and the authors’ own explanation, no clear or supporting experimental results suggest the
correlation between improved activity and the “electric-field-driven” effect.
2. “bioinspired proton blocking” effects
(a) What’s the indeed similarity between Aquaporin and the CTAC layer?
(b) Aquaporin has a water transport channel. What’s the channel structure of the CTAC layer?
(c) How can the author demonstrate that CTAC forms a double layer shown in Figure 1e? Is there any experimental
evidence or data to support this structure? Otherwise, the figure is very misleading.
(d) Based on the authors’ explanation, the selectivity of CO2R in an acidic solution is due to the presence of the cationic
CTAC layer. However, CTAC-capped, PVP-capped, and SDS-capped AuNTs all show >50% FE for CO, which indicates that
the cationic layer is unlikely the main reason for the good selectivity of CO2R in acidic conditions.
3. “metal-cation-free” H2SO4 electrolytes
The authors claimed that they conducted CO2R in a “metal-cation-free” solution. However, they didn’t describe how they
pretreated the electrolyte to obtain such a “metal-cation-free” solution.
Is it a real “metal-cation-free” solution?
Did the authors use ICP-MS to measure the M+ concentrations before and after the experiments to confirm it is a “metal-
cation-free” solution?
Is there any M+ impurity introduced into the solution by the catalysts or materials used in the experiments?
Please read the reference Nat. Catal. 2021, 4, 654−662 and study the pretreatment methods in this reference where the
“metal-cation-free” condition is strictly created, discussed, and studied for electrocatalysis.
Reviewer #2
(Remarks to the Author)
See attachment
Reviewer #3
(Remarks to the Author)
This manuscript presents exciting results in the field of CO2 reduction in acidic media, showing excellent CO current
densities and suppression of HER. The authors use two approaches to accomplish this: electric-field enhancement via
tuning the catalyst shape, and charge-selective layers to limit local proton concentration. While neither approach is
particularly novel on its own, their combination with application to CO2RR in acidic media without alkali metal cations is.
These strategies may also be broadly applicable to other areas of electrocatalysis and engineered microenvironments.
The authors present compelling, technically sound data. However, I have a number of suggestions and clarification points
that should be addressed before it is suitable for publication.
My first few comments center around the fact that the authors motivate their paper with the need to suppress HER,
accomplished via the cationic surfactant bilayers. However, most of the paper focuses on the field-enhancement effects of
the shape-controlled nanoparticles. I am a little confused about the interplay between these two effects, and have the
following comments:
1.The electric field enhancement should be better motivated in the introduction
2.The paper would be more impactful if it was divided into three sections in which the shape and the electric field were both
investigated independently, and then a final section with both strategies combined at once.
3.In the “electrochemical study of tip effect” and “CO2 electroreduction in metal-cation-free acidic media” sections, the goal is
“To validate the predicted enhancement of CO2R by [enhanced electric field]”. However, both the field and the cation effects
are convoluted here because two of the shapes have cationic surfactants, while the commercial catalyst particles do not.
Therefore, while the shape is likely enhancing the electric field, it makes it difficult to ascribe performance benefits solely to
the shape when the surfactant is also likely playing a role. This also speaks to my above comment 2 that distinct sections
that clearly decouple these effects would be beneficial to the story’s overall impact.
4.The electric field measured at the catalyst surface will be dependent on electrolyte conditions. What were the simulation
conditions used in Figure 2? Additionally, the simulations were conducted for the case (I believe) with no surfactants. The
extended and distributed charges present in the surfactant bilayer will undoubtedly influence the electric field, and possibly
even screen it. The authors even note this on page 12, line 341. This critically would mean that the advantages of both
individual approaches may not sum together, again motivating comment 2. Electric field simulations with the bilayers should
be conducted.
Additional comments:
5.What evidence is there that the bilayers form on the NTs? And on the NPs? Could they be monolayers? I’d imagine the
shape of the particles will also impact the conformity of the surfactant layer.
6.For the PNP simulations, the surfactant layer was set at 22 um. That seems very large – how was this number chosen?
Especially compared with the thickness of the EDL.
7.The authors present in situ ATR-SEIRAS measurements to demonstrate that the surfactant remains stably adsorbed to the
gold surface. What potential were those measurements conducted at? Is the adsorption stable at all potentials? The authors
note that there is potential-dependent adsorption, so I would assume not. Also, the system was monitored for only 45
minutes. What happens beyond that time?
8.The Tafel slopes were fit using only 3 data points. Tafel slope fitting is prone to extreme error/subjectivity (see for example
Nature Communications volume 12, Article number: 703 (2021)) Could the authors comment on the stringency of analysis
and perhaps add additional points to the fit?
9.What carbon black was used in the catalyst inks? Was it also a Vulcan XC-72 as used in the commercial catalysts?
Reviewer #4
(Remarks to the Author)
This manuscript introduces an approach to CO₂ electroreduction (CO₂R) in strong acidic media, addressing several critical
challenges in the field. The study emphasizes the advantages of a metal-cation-free system, which reduces CO₂ reactant
losses, eliminates the risk of metal salt precipitation, and enhances device tolerance compared to systems using metal
cations.
The researchers developed sharp-triangle gold (Au) nanostructures capped with a hexadecyltrimethylammonium chloride
(CTAC) layer enriched with cationic sites. Inspired by aquaporins in acidophiles, this design leverages intense local electric
fields generated by the high-curvature tips of the Au nanocatalyst to polarize CO₂ molecules. This polarization increases the
dipole moment of CO₂, facilitating its adsorption and activation. Additionally, the CTAC layer acts as a proton barrier,
suppressing the hydrogen evolution reaction (HER) by mimicking the proton-blocking mechanism of aquaporins.
This dual-function design enabled continuous CO₂R for 100 hours in a flow electrolyzer at pH 1.0, achieving an impressive
energy efficiency of 60% and nearly 100% Faradaic efficiency for CO production. This bioinspired strategy represents a
significant advancement in CO₂R technology by integrating innovative catalyst design principles, potentially paving the way
for more efficient and sustainable CO₂ reduction processes.
Overall, the paper is well-written and presents highly promising results for achieving electrocatalytic CO₂ reduction under
acidic conditions. The outcomes in terms of efficiency and selectivity are remarkable, especially given that the electrolyzer is
a flow cell, which allows for process scalability.
I believe this manuscript meets the standards for consideration by Nature Communications and will significantly contribute to
the important field of CO₂ utilization via electrocatalysis. Moreover, the presentation quality of the paper, both in terms of text
and figures, is outstanding.
However, I have some remarks for the authors to consider:
1. Extension to Other Products: What about extending this approach to produce more valuable products (e.g., C₂+
molecules)? The work shows high selectivity and good partial density for CO formation, but this is an already well-solved
problem.
2. Supporting Information Figures: The figures in the supporting information should be named, for example, Fig. S1, to avoid
confusion with the figures in the main text.
3. Clarification on CO₂ Adsorption: Lines 143-145 state, “Enhancing CO₂ adsorption on Au surfaces would theoretically
promote CO₂R since Au showed relatively weak adsorption to CO₂ species and was located on the right side of the volcano
plot for CO₂R1,43.” This key concept should be expanded. Moreover, the referenced paper (Phys. Chem. Chem. Phys.,
2014, 16, 4720) discusses the binding of intermediates (*CO, *COH, *CHO, and *COOH) rather than CO₂ adsorption.
4. Figure Reference: Lines 146-147 seem to omit a reference. Is it also Figure 2f?
5. Overpotential Reduction: Lines 153-155 should report the lowering of the overpotential when the external electric field (EEF)
is applied, which can be derived from the free energy of the limiting step.
6. Activation Check: Lines 159-160 suggest that adsorption energy alone is insufficient to check activation, as this could still
be a physisorption process. Consider looking at Amira’s paper and investigating C-O elongation, O-C-O bending, and Bader
charge analysis for charge transfer from the surface to CO₂.
7. Synthesis Methodology: How extensible is the synthetic methodology for Au nanostructures to other metals? While Au is
excellent for CO production with high selectivity, achieving high selectivity for more valuable chemicals remains a challenge.
8. Comparison with Literature: How do the partial current density values of CO on the three Au systems compare with other
literature values for CO₂-to-CO conversion on Au catalysts?
9. Definition of CEE: Define the half-cell cathodic energy efficiency (CEE) on page 238.
Version 1:
Reviewer comments:
Reviewer #2
(Remarks to the Author)
I wish to thank the authors for a very detailed response and carrying out several additional analyses, which greatly support
the main conclusions of the work. I consider the work to be of very high significance and to offer new, well-supported results
to understand factors controlling CO2 reduction. At this point, I have only minor suggestions and comments, which may
further strengthen the mechanistic understanding.
1) I was still doubtful about direction of CO2 polarisability (along the O-C-O axis or perpendicular to it) but the high-level
calculations in https://doi.org/10.1007/s00214-021-02797-y have convinced me that the polarisation (at least in gas-phase)
does indeed take place along the O-C-O axis as the authors have indicated. I believe adding this reference in the paper will
convince also other critical readers.
2) The proton-blocking effect CTAC appears to be closely related to the hydrophobic effect that has been identified as an
effective way to improve the CO2 reduction (e.g. https://doi.org/10.1002/chem.202302461 and
https://doi.org/10.1021/jacs.3c13602). Discussing the impact of CTAC from the hydrophobicity perspective could improve the
generality and transferability of the mechanistic understanding from the present work.
3) The Tafel slope analysis suggests that the rate determining step on the catalysts with slopes ~120mV/dec (AuNDs,
AuNPs and AuCPs) is the first electron transfer step while the AuNTs' Tafel slope of 43mV/dec suggest the COOH formation
to be the rate-determining step (https://doi.org/10.1038/s41467-021-20924-y). This is not fully supported by the
thermodynamic analysis from DFT, which shows the COOH formation to have the highest thermodynamic barrier on all
studied surfaces. Some discussion on this issue is needed and I hope the authors can discuss the kinetics of the CO2
reduction reaction, in particular for the first and second reaction steps (e.g. ET through outer- or inner-electron transfer
https://doi.org/10.1038/s41467-023-43300-4).
4) The authors describe two different kinds of electric field effects: (i) one for the stage where CO2 is approaching the surface
and (ii) another one where the CO2 is already adsorbed on the surface. While it appears that both are important, the reasons
seem different. (i) appears to be important for CO2 transport and initial binding while (ii) seems to impact the reaction
thermodynamics of surface reactions. I hope the authors can enforce the discussion on such a "dual electric field effect",
which can potentially be a useful concept to enhance CO2 reduction and other electrocatalytic reactions.
5) It is unfortunate but understandable that the DFT calculations of the surfactant cation effects were not successful.
However, I believe this cation effect to be a key factor in the high CO2 reduction activity as has been discussion for
homogeneous catalysts (https://doi.org/10.1002/anie.202014255). Some general discussion on the role on organic or
ammonium cations on the CO2 reduction reaction would make the mechanistic discussion more comprehesive.
Reviewer #3
(Remarks to the Author)
The authors have addressed all of Reviewer 1's concerns, and I believe the manuscript is now suitable for publication.
Reviewer #4
(Remarks to the Author)
The authros have addressed rfeally well all my comments.
I have only a minor suggestion:
Fig. R32. CAption. "(b, c) Faradaic efficiencies (FE) of various products at different potentials for both CuNTs and CuNPs.
Products include H2, CO, HCOOH (formic acid), C2H4 (ethylene), EtOH (ethanol), and PrOH (n-propanol)." CLearly state
what (b) and (c) refers to. Is (b) CuNTs and (c) CuNPs?
Reviewer #5
(Remarks to the Author)
I have reviewed both the manuscript and the response letter. The authors have made significant efforts to address the
reviewers’ previous concerns. Although I find the so-called bioinspired proton-blocking strategy somewhat far-fetched and
the manuscript is not yet in perfect form, I believe it merits consideration for publication in this journal.
Version 2:
Reviewer comments:
Reviewer #2
(Remarks to the Author)
I wish to thank the authors for carefully addressing all raised points - I am now happy to support the publication of this work.