Steric and hydrophilic/hydrophobic effects of organic cations on electrochemical CO reduction

Domain: chemistry
Published: 2025-12-23
Editorial outcome: major_revision

Abstract
The identity of cations plays a critical role in electrochemical CO and CO2 reduction reactions. While most research has focused on elucidating the effects of alkali metal cations, the influence of organic cations, which are widely employed in practical anion exchange membrane (AEM) electrolyzers, remains underexplored. Here, we systematically investigate the effects of a series of organic cations with varying sizes and hydrophilic/hydrophobic properties on the CO reduction reaction (CORR). By combining electrochemical online mass spectrometry for kinetic analysis, in-situ Raman spectroscopy for interfacial species characterization, and ab initio molecular dynamics (AIMD) simulations to probe interfacial structures, we uncover that the structure of organic cations significantly influences the accessibility of interfacial water to the electrode surface, and thus the potential-dependent behavior of surface *H coverage, ultimately modulating the CORR selectivity. These findings provide new insights into the mechanisms governing CORR and can guide the development of efficient AEM electrolyzers for CO conversion.

Introduction
Electrochemical CO reduction to valuable fuels and chemicals is a promising approach for mitigating atmospheric CO2 concentration, thus attracting widespread research interest1,2. However, the conversion of CO2 into deeply reduced products like methane (CH4), ethylene (C2H4), ethanol (EtOH), acetic acid (AcOH), and others still faces challenges such as sluggish kinetics and poor selectivity. CO is widely recognized as a key intermediate in the deep reduction of CO2. While the CO2 to CO conversion mechanism has been thoroughly elucidated3–5, the subsequent reduction of CO remains less clear, with various controversial mechanisms proposed for the conversion of CO to different deeply reduced products6–9. Therefore, unveiling the detailed mechanism of CO reduction reaction (CORR) is crucial, as this understanding will facilitate the design of efficient electrocatalytic interfaces for the selective production of high-value products.

Many efforts have been made to clarify the CORR mechanism using various methods. Tafel analysis, a convenient yet effective electrokinetic study method, serves as a valuable tool for dissecting reaction pathways and discerning the rate-determining step (RDS)10–12. For example, the CO conversion toward C2+ products, such as C2H4, EtOH, and AcOH, exhibits a Tafel slope of approximately 118 mV dec−1 over copper electrodes13–16, indicating that the first electrochemical step involving one-electron transfer is the RDS for C2+ production. Early studies suggested that the RDS is the dimerization of two adsorbed *CO with one electron transfer (*CO + *CO + e− → *OCCO−)17–19. Similarly, Deng et al. also claim that the CO-CO coupling (*CO + *CO + δe− → *OCCOδ-) is the RDS of C2+ formation based on a non-linear fit of the reaction order curve in a recent study20. Other studies indicated that the reaction order of C2+ product formation falls between 0 and 1, which is inconsistent with the CO-CO coupling mechanism13,16,18. Additionally, some experimental results obtained in different pH electrolytes revealed that the C2+ production rate is comparable on the standard hydrogen electrode (SHE) scale, indicating the RDS is independent of proton activity14,21. Based on previous findings, Xu et al. proposed that the hydrogenation of *CO with adsorbed water (*H2O) is the RDS for C2+ production (*CO + *H2O + e− → *CO(H) + OH−)22,23, supported by precise kinetic analysis and in-situ surface-enhanced infrared absorption spectroscopy (SEIRAS). In contrast, CH4 formation depends on the proton activity. It exhibited a Tafel slope of 118 mV dec−1 at a pH below 11.3, but it decreased to 59 mV dec−1 when the electrolyte became more alkaline. The shift suggests a transition of RDS from the initial proton-coupled electron transfer (PCET) process of adsorbed *CO and the later chemical hydrogenation step (with adsorbed *H or *H2O) in different pH environments and applied potentials22,23. These studies provide valuable insights into the further CO conversion mechanism, but primarily in liquid electrolytes with alkali cations (Fig. 1 left).

Replacing liquid electrolytes with polyelectrolytes that contain only organic cations represents a notable advancement in modern electrochemical technologies24–26. Specifically, for CO or CO2 reduction, anion exchange membrane (AEM) electrolyzers with pure water anolyte cannot only mitigate risks such as leakage, corrosion associated with liquid electrolytes, but also prevents salt precipitation in gas diffusion electrodes (GDE)27,28. While alkali cations in liquid electrolytes were once considered essential for CO or CO2 reduction29, recent studies have demonstrated that these reductions are feasible in coordination-inhibited cation solutions or at electrode/polyelectrolyte interfaces containing only organic cations30–33. Additionally, pure water-fed electrolyzers for converting CO2 to CO or C2H4 have already been realized34,35. Our previous investigation found that different organic cations exert a significant influence on the product distribution of CO reduction, primarily due to their varying capability to synergistically activate CO236. However, the extent to which organic cations can influence the further *CO conversion remains unclear. Studies, such as that by Li et al.37, have shown that large quaternary alkylammonium cations inhibit the conversion of CO to C2H4, which is attributed to the absence of hydrogen bonding interaction between interfacial water and *CO-*CO intermediate. These investigations were mostly conducted on fully-immersed electrodes, that is, the working electrode is entirely immersed in liquid electrolytes with dissolved CO2 or CO serving as the reactant. As a result, the current densities were far lower than those of GDE-based electrolyzers (>100 mA cm−2). Previous studies on the alkali cation effect have indicated that when the GDE is employed to enhance mass transfer, with an abundant supply of CO reactant at the triple-phase interface, the CORR behavior differs significantly from that observed with fully-immersed electrodes38. Therefore, there is an urgent need for in-depth investigations into the organic cation effect on CORR, especially under the practical high-current-density conditions provided by GDE-based setups (Fig. 1 right).

In this study, we systematically investigated the structure-function relationship of organic cations in CORR using a series of aqueous quaternary ammonium hydroxide (R4NOH) electrolytes. This allowed us to independently examine the influence of organic cations (R4N+) with varying sizes and hydrophilic/hydrophobic properties. A custom-designed GDE-based flow cell was employed for electrochemical tests, ensuring an abundant supply of CO and facilitating systematic investigations into the organic cation effect under high current densities without mass-transfer limitations. Additionally, electrochemical in-situ mass spectrometry (EC-MS) was utilized to track the continuous evolution of gaseous products as a function of applied potential, providing more precise values for Tafel slope analysis. By combining electrokinetic studies, in-situ Raman measurements, and AIMD simulations, we revealed that the molecular structure of organic cations (R4N+) significantly affects the accessibility and configuration of interfacial water, which further influences the generation and consumption of adsorbed hydrogen (*H) on the electrode surface, ultimately altering the CORR selectivity.

Results
Size effect of organic cations on CO reduction performance
A homemade flow cell was employed for CORR (Supplementary Fig. 1). The working electrode used was a Cu gas diffusion electrode, fabricated through magnetron sputtering of Cu onto hydrophobic carbon paper, followed by spray coating with a Nafion binder to mitigate the electrolyte flooding (denoted as MsCu GDE). The cations in the R4NOH electrolytes can permeate the thin Nafion film due to the attraction of the negatively-charged sulfonate groups. Therefore, the cation-Nafion hybrid serves as a platform to study the catalytic characteristics of different organic cations without mass transfer limitations, similar to those at the electrode/polyelectrolyte interface in AEM electrolyzers. Scanning electron microscopy (SEM) image revealed that the deposited Cu layer exhibited a continuous aggregation of nanoparticles (Supplementary Figs. 2 and 3). CORR was first conducted in a typical KOH electrolyte, yielding a Faradaic efficiency (FE) of 8–19% for H2 and 30–40% for C2H4 within the potential range of −0.5~−0.8 V vs. RHE (Supplementary Fig. 4), which aligns with literature reports on the product distribution for Cu-catalyzed CORR39,40. Additionally, it sustained a high current density of up to 1 A cm−2 at −0.8 V, due to sufficient CO supply. Therefore, this cell setup is well-suited for investigating CORR behavior under high-current-density conditions.

To investigate the organic cation effect, a series of quaternary ammonium hydroxide (R4NOH) solutions of varying cation sizes were used as electrolytes for CORR, including symmetric tetraalkylammonium cations (TMA, TEA, TPA, TBA, containing four methyl, ethyl, propyl, and butyl groups, respectively) and an asymmetric cation, diethyl-dimethyl ammonium (DEDMA), Fig. 2a–e present detailed FE of different products and the current densities (Details in Supplementary Tables 1–5). Overall, CORR performance in electrolytes containing organic cations was inferior to that with alkali metal cations (K+), particularly regarding the selectivity for C2+ products like C2H4. Among the organic cations tested, the TMA (Fig. 2a) and DEDMA (Fig. 2b), which have smaller sizes, exhibited superior C2+ selectivity. The FE for C2+ products exceeded 70% at −0.7 V, with the FE for C2H4 reaching 21%. When the cation size exceeded that of TEA (Fig. 2c–e), C2H4 production dropped significantly, consistent with previous research findings37. Additionally, we observed that the production of C1 product CH4 and the competitive product H2 increased rapidly as the cation size increased. To account for the flexibility of the alkyl chain segments, we used their gyration radius as a descriptor of cation size41,42. Obviously, the FE ratios of C2H4 to CH4 descended monotonically with the increase in the gyration radius of these cations (Fig. 2f).

Tafel analysis to explore the size effect of organic cations
Tafel analysis was conducted to elucidate the distinct CORR behaviors in the presence of different R4N+ cations. Conventional Tafel analysis in CORR studies often relies on the linear fitting of several discrete data points43,44, primarily due to the time-consuming process required for product quantification by gas chromatography (GC), which typically takes at least several minutes. Consequently, this method provides only time-averaged information at a few specific potentials, failing to capture real-time changes in different products as the polarization potential varies. We thus constructed an in-situ electrochemical mass spectrometry (EC-MS) method for Tafel analysis (Fig. 3a), by incorporating a capillary sampling branch at the cathode outlet for MS detection. Owing to the rapid response and high sensitivity of MS analysis, this setup can record the ion currents of CORR gaseous products, such as H2, CH4, and C2H4, in real-time during linear sweeping voltammetry (LSV) tests. Since the MS ion currents are proportional to the concentration of detected products, the Tafel slope for each product generation can be determined from the logarithmic MS ion current (log im/z) – potential (E) polarization curve. A slow LSV scanning rate of 1 mV s−1 was set to achieve a steady state and reduce the influence of double-layer charging10,45. In contrast to the distinct polarization behaviors observed for C2H4 generation in these different electrolytes (Supplementary Fig. 5), H2 and CH4 production appear to be somewhat interrelated (Fig. 3b and Supplementary Figs. 6–7). Their production rates all exhibit a notable turning point at around −0.75 V, where the Tafel slope for CH4 production significantly increases at the higher overpotential range. In contrast, the Tafel slope for H2 production markedly decreases. This is likely due to the competitive consumption of surface-adsorbed *H species required for both CH4 and H2 formation, resulting in a competitive reaction dynamic. Moreover, we observed that reducing the CO partial pressure (PCO) from 1 to 0.067 atm to increase the surface *H coverage, and subsequently restoring PCO back to 1 atm, resulted in a significantly higher CH4 ion current compared to the initial level, while the C2H4 ion current returned to its original value (Supplementary Fig. 8). This indicates that CH4 production is strongly dependent on the surface *H coverage, whereas C2H4 production is not.

Specifically, the HER Tafel slopes are all much higher than 118 mV dec−1 in the lower overpotential range (E > ca. −0.75 V, Fig. 3b), indicating that the RDS is a non-electrochemical process12. In contrast, at higher overpotentials (E < ca. −0.75 V), the HER Tafel slopes decrease with increasing R4N+ cation size (Fig. 3c). The TMA electrolyte shows a Tafel slope of 175 mV dec−1, suggesting that the RDS remains a non-electrochemical process. However, the Tafel slope drops to 118 mV dec−1 in the TEA electrolyte, which approaches the theoretical value of 118 mV dec−1 for the first electron transfer RDS, indicating the Volmer step of water dissociation (H2O + e− → *H + OH−) becomes the RDS. Further decreases in the Tafel slope are observed in the TPA (104 mV dec−1) and TBA (84 mV dec−1) electrolytes. It is probable that the RDS further shifts from the Volmer step to the subsequent *H consumption steps. Given that the *H has a low coverage on Cu surface22, the Tafel step (2*H → H2) is less likely to occur, and the Heyrovsky step (*H + H2O + e− → H2 + OH−) is more plausible for consuming *H to produce H2. The regular change in HER Tafel slopes implies that large R4N+ cations are more conducive to interfacial water dissociation to generate *H. This is likely due to the relative hydrophobicity of organic R4N+ cations, which are not hydrated as intensely as alkali cations46,47. Under cathode potentials, the interfacial electrical double layer (EDL) formed by these R4N+ cations hinders water access to the electrode surface. The larger R4N+ cations form a looser EDL, allowing easier water molecule access and dissociation on the electrode surface. This finding is also consistent with a recent study indicating that, compared with smaller TMA and alkali cations, the large TPA cation can enhance the hydrogen evolution/oxidation reaction kinetics by facilitating interfacial proton shuttling48.

For CH4 production, in the lower overpotential range (E > ca. −0.75 V), the Tafel slope is 67 mV dec−1 for TMA (Fig. 3c). It is close to the theoretical value of 59 mV dec−1 when the chemical step after a single electron transfer reaction serves as the RDS49. Although the coverage of reactants (e.g., *CO and *CO(H)) can affect Tafel slopes, recent studies have shown a low coverage of *CO on the Cu surface at 1 atm23,50, and thus coverage effects will not significantly influence Tafel slope analysis. Therefore, the first charge-transfer step (*CO + H2O + e− → *CO(H) + OH−) is unlikely to be the RDS for CH4 formation. Instead, it is probable that the subsequent chemical hydrogenation between *CO(H) and *H is the RDS, which is consistent with the views in the literature22. Notably, this is based on the assumption that the *H coverage remains constant and is independent of potential change (Case 1 in Supplementary Note 1). However, the Tafel slope could decrease further if the *H coverage varies with the potential (Case 2 in Supplementary Note 1). Theoretically, when the *H generation from water dissociation (Volmer step: H2O + e− → *H + OH−) is regarded as a pseudo-equilibrium step, the resulting Tafel slope could be lower and approach a minimum value of 30 mV dec−1 (Eq. S10, neglecting reactant coverage influence). The obtained Tafel slope indeed becomes lower for these larger cations and drops to 41 mV dec−1 in the TPA electrolyte (Fig. 3c), which corresponds well to Case 2. This is because a looser EDL, formed by large cations, improves water accessibility to the electrode surface (i.e., sufficient water supply), thus making the subsequent water dissociation more sensitive to potential changes. Regarding the largest TBA cation, there is an anomalous slight increase in the Tafel slope. It can be attributed to the fact that the further enhanced water accessibility allows more water adsorption/dissociation on the electrode surface and thus suppresses *CO adsorption, as evidenced by in-situ Raman spectra showing that the *CO band vanished completely after −0.7 V (Supplementary Fig. 9).

Regarding C2H4 production, although the selectivity is relatively high in the TMA and TEA electrolyte (Fig. 2a, c), the C2H4 Tafel slope is much higher than 118 mV dec−1 (Supplementary Fig. 5), suggesting that the RDS is a non-electrochemical process under such circumstances. In contrast, the C2H4 selectivity becomes very low in the larger-sized TPA and TBA electrolytes, but the C2H4 Tafel slope is relatively close to 118 mV dec−1. This indicates that the first electron transfer process is the RDS. For instance, as proposed in the literature, it could be the *CO(H) formation reaction (*CO + H2O + e− → *CO(H) + OH−), or *CO coupling step involving proton-coupled electron transfer (2*CO + H2O + e− → *COCO(H) + OH−)49.

Hydrophilic/hydrophobic effect of organic cations on CO reduction
The above results indicate that the R4N+ cations with hydrophobic alkyl side chains significantly influence water accessibility and thus the proton supply on the electrode surface, leading to variations in the CORR pathway. We further examined the impact of introducing hydrophilic hydroxyl (-OH) groups into organic cations and their effect on CORR. Figure 4a, b present the CORR performance in the presence of choline and bis(2-hydroxylethyl) dimethylammonium hydroxide (DHDMA), respectively (Details in Supplementary Tables 6 and 7). Upon the introduction of the hydrophilic -OH group, HER was exacerbated, and CH4 production was inhibited, compared to TMA and DEDMA of similar size (Supplementary Figs. 10 and 11). Meanwhile, the C2H4 production was enhanced to some extent, and its FE remained almost unchanged with the negative shift of potential (Supplementary Fig. 12). Consequently, the C2H4/CH4 FE ratio exhibited a gradual increasing trend with the increasing number of -OH groups introduced into the cations, as shown in Fig. 4c.

To clarify how the hydrophilic -OH group affects CORR kinetics, we also utilized the EC-MS method (Fig. 3a) to perform Tafel analyses in mixed electrolytes with varying ratios of hydrophilic DHDMA and hydrophobic TEA cations. As the proportion of hydrophilic DHDMA cations rises, the HER Tafel slope in the lower overpotential range (E > ca. −0.75 V) significantly decreases, and the inflection point around −0.75 V in the polarization curve gradually becomes less distinct (Supplementary Fig. 13). This observation can be attributed to the hydrophilic -OH groups forming hydrogen bonding interactions with interfacial water, as reflected by an increased proportion of strongly hydrogen-bonded water (sh-H2O) under different potentials. Consequently, water accessibility at the electrode surface is enhanced, promoting HER at the lower overpotentials. The HER rapidly consumes surface *H via the Heyrovsky reaction (*H + H2O + e− → H2 + OH−). Given that *H is a crucial reactant in the RDS for CH4 formation, CH4 production is gradually inhibited, and its Tafel slope climbs from 52 to 213 mV dec−1 (Fig. 4d). Moreover, the blurring of the inflection point in the HER polarization curve (Supplementary Fig. 13) may be because, after introducing -OH groups, the interfacial water composition remains largely unchanged within the tested potential range. This behavior differs significantly from that with hydrophobic R4N+ cations (Fig. 5g and Supplementary Figs. 24, 25, vide infra).

In-situ Raman measurements and AIMD simulations
To validate the inferences drawn from the above kinetic analysis, we selected three representative organic cations with different sizes and hydrophilic/hydrophobic properties, including TMA, TPA, and DHDMA, and conducted electrochemical in-situ Raman measurements using a home-designed GDE-type spectroelectrochemical cell (Supplementary Fig. 14). Figure 5a shows the full Raman spectra acquired at −0.8 V across the three electrolytes, while Raman spectra at different potentials are provided in Supplementary Fig. 15-19. The peaks around 280 and 360–380 cm−1 are assigned to frustrated rotation and Cu-CO stretching vibrations51–53, respectively. The peak at 490–530 cm−1, corresponding to the Cu-OH stretching vibration54,55, gradually enhances as the potential becomes more negative. The peaks at 750 and 943 cm−1, attributed to the symmetry and asymmetry N-C stretching vibrations, are notably intense in the TMA electrolyte56, likely due to its smallest ionic size, which allows it to approach closer to the electrode surface. The two dashed lines indicate the D and G bands of the carbon paper substrates (Supplementary Fig. 20). The peaks at 1380 cm−1 and 1450 cm−1 are assigned to the rocking vibration of CH2 and CH3 (δ(C-H)), respectively. The band at 1800–2100 cm−1 is identified as the stretching vibration of surface adsorbed *CO intermediate. The band at 2750–3030 cm−1 is attributed to the stretching vibrations of C-H bonds (ν(C-H)), primarily arising from the contribution of organic cation aggregation at the interface rather than from CORR intermediates, from the comparison with the very weak peak at the corresponding position in the KOH electrolyte (Supplementary Fig. 21). The board band spanning from 3100 to 3700 cm−1 is assigned to the ν(O-H) vibration of interfacial water. These observations provide crucial insights into the interfacial processes during CORR.

Figure 5b–d compares the ν(*CO) band presented in heatmap forms across the three electrolytes57, and Supplementary Fig. 22 separately presents the potential-dependent plots of the peak positions for the *CO (upper) and Cu-CO (lower) bands. Both bands exhibit the greatest shift as the potential becomes more negative, i.e., the largest Stark slope, in the TMA electrolyte, whereas the least shift is observed in the TPA electrolyte. These results suggest that the interfacial electric field is strongest in the presence of TMA cations, corroborating their ability to approach the electrode surface more closely and pack more closely to form a compact double-layer structure due to their relatively smaller size. In contrast, the largest-sized TPA cations form a more loosened interfacial double-layer structure. Previous studies have found that the interfacial electric field significantly affects electrocatalytic CO2 (CO) reduction58,59. In this study, choline and TMA cations, which have similar sizes and the same R-NMe3+ end group, approach the electrode with comparable proximity58. As a result, the intensities of the interfacial electric fields they form are similar, as reflected by the comparable ν(CO) Stark slope values (Supplementary Fig. 23)3,37,60. However, their CORR performances vary greatly (Figs. 2a and 4a), indicating that the interfacial electric field is not the main influencing factor. Instead, as speculated by the kinetic analysis above, the influence of different organic cation properties on interfacial water is the fundamental reason for the variations in CORR performance. In the presence of large organic cations, water molecules are more likely to overcome the barrier posed by the hydrophobic cation layer at the interface and reach the electrode surface when the overpotential increases, as indicated by a higher ν(O-H)/ν(C-H) band intensity ratio in the TPA electrolyte, compared to that in the TMA electrolyte (Fig. 5e). This observation supports the kinetic analysis speculation indicating an enhanced water accessibility in the presence of large-sized organic cations. For the DHDMA cation, the ν(O-H)/ν(C-H) band intensity ratio is even higher, confirming the hydrophilic-enhanced interfacial water supply mentioned earlier.

Comparative analysis shows that the ν(O-H) bands of DHDMA are at relatively lower wavenumbers, indicating a more coherent hydrogen bonding network (Supplementary Fig. 24). The ν(O-H) band was further deconvoluted into three components using Gaussian functions (Supplementary Fig. 25), including isolated water (i-H2O, ~3600 cm−1), weakly hydrogen-bonded water (wh-H2O, ~3450 cm−1), and strongly hydrogen-bonded water (sh-H2O, ~3260 cm−1)61,62. Figure 5f (right) provides a comparative example of the deconvoluted peaks around −0.8 V. For a quantitative perspective, the ratio of sh-H2O component is shown in Fig. 5g. The sh-H2O ratio for the hydrophilic DHDMA is consistently higher than that for the two hydrophobic cations and remains nearly constant, owing to the hydrogen bonding interaction between the -OH group and interfacial water. For both the TMA and TPA, the sh-H2O ratio shows a downward trend as the overpotential increases. This is because the hydrophobic alkyl chains in the EDL disrupt the interfacial hydrogen bonding network, and longer alkyl chains cause more significant disruption. Besides, there appears to be a correlation between the ν(Cu-OH) peak position and the integrity of the interfacial hydrogen bonding network: a higher proportion of sh-H2O tends to shift the ν(Cu-OH) peak towards higher Raman shifts (Fig. 5f left). This suggests the possibility that an interaction occurs between interfacial sh-H2O and surface -OH species, which may weaken the O-H bond and concurrently strengthen the Cu-O bond in the Cu-OH species.

Ab initio molecular dynamics (AIMD) simulation results elucidate how distinct organic cations influence interfacial properties. As illustrated in Fig. 5h (atom coordinates in Supplementary Data 1–3), all three cations exhibit a strong affinity for the Cu(111) surface under negative polarization, effectively repelling H2O due to their respective alkyl side chains. Notably, TPA induces a distinctive H2O-repellent cavity. Conversely, DHDMA can form hydrogen bonding interaction with H2O through its -OH group, as highlighted in Fig. 5h left. The statistical analysis of H2O molecules (represented by the density of O atoms, which excludes the O atoms on the DHDMA cation) perpendicular to the Cu surface substantiates the differences among these three organic cations (Supplementary Fig. 26). Despite the presence of an H2O-gap layer at 3.6–5.0 Å from the surface, a region occupied by the cations, the hydrophilic DHDMA exhibits a higher O atom density (i.e., higher H2O content) from 5.3 to 6.0 Å, compared to the hydrophobic TMA and TPA. Further statistics reveal that the -OH groups on DHDMA distribute at approximately 3.5 Å and 4.8 Å (Supplementary Fig. 27), thus it is reasonable to attribute the increased O atom density to the hydrophilic effect of -OH groups. The number of hydrogen bonds was also statistically analyzed (Supplementary Fig. 28). Similar to the aforementioned H2O-gap layer, for all the organic cations, the number of hydrogen bonds decreases significantly at a distance of 4–6 Å from the surface. The largest TPA results in a smaller number of hydrogen bonds at the surface than its smaller counterpart, TMA. However, the hydrogen bond numbers of DHDMA are even higher than those of the small TMA cation. In other words, the -OH groups of DHDMA alleviate the H2O-repellent cavity originating from the steric effect of organic cations. Furthermore, we calculated the phonon density of states (PDOS) for the interfaces with different cations to provide an in-depth statistical view63,64 (details in Supplementary Note 2). Figure 5h shows the PDOS curves within 2800–4000 cm−1, which represent the ν(O-H) stretching vibrations. Notably, the v(O-H) curves shift towards lower wavenumbers in the order TPA > TMA > DHDMA, as further evidenced by their central values marked in Fig. 5h. These simulation results align well with the experimental trends observed in the Raman spectra of the ν(O-H) vibration (Fig. 5f, g), providing a reasonable verification at the theoretical level. These findings corroborate the above hypothesis that hydrophilic cations increase interfacial water content and enhance the hydrogen bonding network at the electrode/electrolyte interface, thereby impacting CORR selectivity.

Methods
Chemicals and pre-treatment
Tetramethylammonium hydroxide pentahydrate (TMAOH‧5H2O, 98%), and deuterium oxide (D2O, 99.8% D atom) were purchased from Energy Chemical. Silver oxide (Ag2O), tetraethylammonium hydroxide (TEAOH, 25 wt% in water), and tetrabutylammonium hydroxide (TBAOH, 40 wt% in water) were purchased from Aladdin. Dimethylsulfoxide (DMSO) and potassium hydroxide (KOH) were purchased from the Sinopharm Chemical Reagent Co., PR China. Choline hydroxide (50 wt% in water), tetrapropylammonium hydroxide (TPAOH, 2.0 mol/L in water), and bis(2-hydroxyethyl)dimethylammonium chloride (DHDMA-Cl, 98%) were purchased from Macklin. Diethyl-dimethyl ammonium hydroxide (DEDMA, 25 wt% in water) was purchased from Adamas. Unless otherwise specified, all chemicals were used without further purification.

Bis(2-hydroxyethyl)dimethylammonium hydroxide (DHDMA-OH) was prepared via anion-exchange method. Specifically, the DHDMA-Cl salt (4.24 g, 25 mmol) was dissolved in deionized water, and then silver oxide (5.79 g, 25 mmol) was added to the solution. The mixture was continuously stirred for 18 h at 40 °C. After centrifugation, the liquid supernatant was collected and diluted to prepare a 0.5 mol/L solution for electrochemical testing. The counter ion was converted to the hydroxide form, and the resulting solution had a pH value of 13.

Electrode fabrication
Hydrophobic carbon paper (AvCarb GDS3250) was used as the GDE substrate. The Cu catalytic layer was prepared by magnetron sputtering deposition of a Cu target (99.999%) onto the carbon paper for 45 min. The sputtering current was 100 mA, and the Ar pressure was 1.7 Pa. A 2.5 mL solution of 0.05 wt% Nafion in ethanol was sprayed onto the Cu surface, covering an area of 20 cm2, to form a hydrophobic layer that prevented electrolyte flooding. The apparent area of the work electrodes is 0.78 cm2 for CORR performance tests. The morphologies of the MsCu electrodes were characterized by scanning electron microscopy (SEM, Zeiss Merlin Compact).

CORR performance tests
All the CORR performance and electrokinetic tests were carried out in a domestic flow cell at room temperature (25 °C). The chronoamperometry method was applied for performance tests over a potential range of −0.65 to −0.90 V vs. RHE, with a scanning step of 50 mV. A porous Ti plate coated with Ir was used as the counter electrode, and an Hg/HgO electrode was used as the reference electrode. The applied potential was converted to the RHE scale using the following equation:

ERHE = ΔE + Eapplied (1)

Here, the ΔE represents the potential difference between the Hg/HgO electrode and the RHE, which was calibrated manually to be 0.92 V (pH=13.7). Eapplied stands for the applied potential. The pH values of different R4NOH electrolytes without hydroxyl groups are around 13.7. The pH values of cations with hydroxyl groups are slightly lower (Choline: 13.3 and DHDMA: 13.0).

CO was introduced to the backside of the GDE at a flow rate of 20 sccm. The electrolyte solution was circulated with a peristaltic pump at a flow rate of 8 mL min−1. Gaseous products were analyzed in-line using gas chromatography (MICRO GC FUSION®, INFICON), and the Faradaic efficiency (FE) is calculated from the corresponding peak area as the following equation:

FE = (nie * Ni * F) / I (2)

Where nie is the electron transfer number of product i, Ni is the amount of substance of product i per unit time quantified by the gas chromatography, I represents the overall current, and F is the Faradaic constant. Liquid products were quantified by the 1H NMR (AVANCE NEO 400 MHz Digital NMR Spectrometer), and the FE is also calculated from Eq. 2.

For the electrokinetic study, the LSV test was conducted with a sweeping rate of 1 mV s−1. The sampling capillary of mass spectroscopy (PM-DEMS, Protech®) was introduced into the outlet pneumatic hose to collect the product mixture. As a result, changes in the signal of a specific m/z over time reflected changes in the production rate with potential. The electrochemical measurements were conducted using the Autolab PGSTAT302N workstation, and the mass spectrum was acquired using the PM-QMS mass spectrometer (Pro-tech).

Electrochemical in-situ Raman measurements
The working electrode was the same as that used in electrochemical performance tests. A roughened Ag wire was used as the quasi-reference electrode, and the potential was calibrated using the HgO/Hg electrode after each test. The potentials are 100% iR-corrected by subtracting the iR drop from the applied potentials after each test. The electrode surface area is 0.07 cm2. In-situ Raman tests were conducted using the HORIBA (XploRA™ PLUS) Raman spectrometry. The spectra subtracting baseline are then processed using a Savitzky-Golay smoothing filter (window size=10, order=2) to achieve a higher signal-to-noise (S/N) ratio, and the excitation laser wavelength was 638 nm. Raman spectra were collected starting from the open circuit potential, decreasing in 100 mV intervals in the negative direction until exceeding the CORR performance testing potential range (−0.9 V vs. RHE).

Ab initio molecular dynamics (AIMD) simulations
This work utilizes the Vienna Ab initio simulation package (VASP), employing the plane-wave pseudopotential method, to perform ab initio molecular dynamics (AIMD) simulations65,66. The PBE functional within the GGA framework is chosen to describe electron exchange-correlation interactions, while Grimme’s D3 method enhances accuracy by incorporating dispersion corrections in all simulations67,68. A plane-wave energy cutoff of 400 eV ensures adequate computational accuracy, and the first-order Methfessel-Paxton scheme with a smearing width of 0.1 eV is adopted for electronic structure calculations. All simulations are performed without spin polarization, focusing solely on the gamma point of the Brillouin zone and disregarding any symmetry considerations. To simulate realistic system behavior, a timestep of 1.0 fs is used in conjunction with a Nose-Hoover thermostat to maintain constant temperature (NVT) conditions at 298 K. Before data analysis, each modified interface system undergoes a 17 ps simulation to ensure systems are well equilibrated. Statistical analyses are then conducted using snapshots extracted from the final 12 ps of these trajectories. The simulated Cu/water interface consists of a four-layer 6×6 orthogonal Cu(111) slab, upon which 110 water molecules were overlaid. This arrangement results in an approximate water density of 1.0 g/cm3. A vacuum layer with a thickness of ~2.4 nm separates the system from its periodic images, preventing spurious interactions. A single molecule is strategically positioned near the Cu surface, allowing it to interact with both the metallic substrate and the surrounding water molecules.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_2": "This article investigated the effect of quaternary ammonium cations on the performance of CORR in GDE-based flow cell. Cations with different size and hydrophobicity were studied. CORR under conditions free of alkali cations is of great industrial interest, while the studies on the effects of organic cations are rare and highly demanded. Therefore, the research topic of this article is of great importance and may attract broad interest in this field. The mechanism of CORR is complicated and cations show diverse effects on the local water concentration, hydrogen bond network, absorption of H and CO, and the electric field at the interface. This study employed a variety of technical methods and generated a substantial amount of data. However, further in-depth analysis is still required to process and summarize these data, in order to construct a more quantitative model and arrive at clearer and more definitive conclusions. Therefore, major revision is necessary before publication.\n1. While reading the main text, the abundance of fragmented information makes it difficult for readers to identify a clear narrative thread or form a coherent mental framework. Since several multi-step competing reactions including HER, CH4 formation and C2H4 formation are involved, the direct interpretation of Tafel analysis is difficult. Direct comparison of the experimental Tafel slope with some typical specific number like 118, 59 etc. may not give a reliable conclusion. In the main text, is it possible to build a comprehensive microkinetic model that can be fitted based on the Tafel analysis curves? Then, the influence of organic cation on certain kinetic parameter can be extracted, providing a clear picture of the organic cation effects.\n2.MS was used for real-time analysis of the products generated by the catalyst during LSV testing. However, the inlet of MS was positioned after the electrochemical cell, rather than directly behind the GDE. Considering the volumes of gas flow field and pipe between the cell and the MS inlet, as well as the carrier gas flow rate, can this setup achieve accurate real-time detection of transient processes?\n3. H2O is electroneutral molecule. Why high polarization potential can drive the mass transfer of H2O through the hydrophobic region generated by the organic cations?\n4.C2H4 is a more prominent and desirable product in CORR, yet this study's kinetic analysis focused primarily on the formation pathways of H₂ and CH4 rather than C2H4. Why?\n5. How the cation identity affects the local electric field was characterized through vibrational Stark effect analysis, but the effects of local electric field on the kinetics of different reactions were not sufficiently investigated.", "reviewer_3": "This manuscript investigates the effects of organic cations on electrochemical CO reduction reactions (CORR). Through a comprehensive approach combining electrochemical measurements, electrochemical online mass spectrometry, in-situ Raman spectroscopy, and molecular dynamics simulations, the authors reveal how cation structure influences interfacial water dynamics and surface hydrogen behavior at high current densities. The work provides valuable mechanistic insights for the development of practical anion exchange membrane electrolyzers. I recommend this manuscript be accepted after addressing the following concerns:\n1. The goal of this manuscript is to improve the performance of anion exchange membrane solid-state electrolyzers, but performance testing in an actual anion exchange membrane (AEM) electrolyzer system is missing. This data should be supplemented, and the authors should explain which organic cation results best correlate with the AEM electrolyzer test results.\n2. The polymer cations on anion exchange membranes and their hydrophilic/hydrophobic properties may affect water diffusion dynamics at the catalyst surface, which may be different with small-molecule organic cations. This requires further discussion.\n3. Regarding Fig. 2f, you use the radius of gyration as a descriptor for organic cations. Please explain how this parameter was calculated and why it was chosen over other structural descriptors. Furthermore, please specify which potential was used to calculate the C2H4/CH4 ratio. Why was this particular potential chosen for comparison instead of others within your testing range?\n4. You mention that organic cations have lower hydration numbers than alkali metal cations, indicating their hydrophobicity. Since carbon paper substrates are typically hydrophobic (PTFE-treated) with known affinity for organic species, how does the accumulation of organic cations on the catalyst surface under electric fields affect the overall hydrophobicity of your gas diffusion electrode? This consideration seems important for interpreting your results.\n5. Your results show that hydroxyl-containing cations (choline and DHDMA) promote hydrogen evolution while suppressing CH4 production, despite both processes requiring hydrogenation steps. This seems contradictory - please provide a clearer mechanistic explanation for this interesting observation.\n6. The deconvolution of the ν(O-H) band in Fig. 5f shows inconsistent peak positions across the three samples (TMA, TPA, DHDMA). What criteria did you use to assign the components for isolated water, weakly hydrogen-bonded water, and strongly hydrogen-bonded water? Similar inconsistencies appear in Supplementary Fig. 23. Please justify your peak fitting approach.\n7. In several performance tests shown in Fig. 2 and 4, the total FE is significantly below 100%. What accounts for this gap? Are there undetected products or side reactions?\n8. You attribute differences in catalytic selectivity to the hydrophobicity of organic cations affecting *H coverage by hindering water transport. Have you considered that direct adsorption of these cations on the electrode surface might alter the electronic structure of catalytic sites? How did you distinguish between these potential mechanisms?"}, "author_response": "Response to Referees’ comments\nReviewer 1:\nThe authors present a study investigating the influence of organic electrolyte cations on electrochemical CO reduction rates in a GDE-based flow cell. They find that COR activity varies systematically with cation size. They attribute observed changes in activity to the steric and hydrophobic/hydrophilic properties of the organic cations. They argue that hydrophobic cations in the double layer hinder mass transfer and adsorption of water onto the catalyst surface. This impacts both hydrogen and methane generation rates as their rate limiting steps involve proton transfer. In contrast, ethylene formation is less affected as its rate determining step does not involve proton transfer. Changing cation size changes the packing of ions in the double layer. While this topic is of interest to the field, the results reported here have already been well documented and well explained in other studies (see below). The novelty seems to be that the measurements are done in a GDE, but this does not fundamentally change the role of the cations and may introduce uncertainty related to mass transport (see below). There are furthermore significant methodological flaws in the kinetic analysis and the associated discussion could be stronger. For these reasons, I do not believe Nature Communications is an appropriate venue for this work. The points below further justify this position.\nReply: We deeply appreciate the reviewer for spending time reviewing our manuscript and providing valuable comments. We’ve carefully considered these comments and made revisions (highlighted in the revised manuscript) to address the concerns.\nAs the reviewer pointed out, there have indeed been many studies on the cation effect recently, as more and more studies recognize its significant impact on electrocatalytic reactions (e.g., CO and CO reduction). However, most studies have focused on inorganic alkali cations. Research on organic cations (commonly used in practical membrane electrode assembly (MEA) devices) is limited, and their influence on electrocatalytic reactions is still poorly understood. Moreover, as the reviewer noted, using GDE measurements is a unique aspect of this study. It can solve the problem of insufficient CO mass transfer, enabling us to study catalytic behavior under practical high-current density conditions. The catalytic mechanism under such conditions may differ greatly from that of fully-immersed electrodes in liquid electrolytes (i.e., the electrodes are completely immersed in the solution, using dissolved gases as reactants). Indeed, a recent study on the inorganic cation effect of the CO reduction reaction has shown the notable difference between the GDE system and the fully-immersed method (Ref: J. Am. Chem. Soc. 2025, 147, 9990).\nIn summary, considering the research object and method, as well as the scientific findings (please see the Reply to the first comment below), this study differs significantly from previous reports.\nMajor comments\n- The observed changes in rates with cation size have been observed in prior studies (e.g. Proc. Natl. Acad. Sci. 2019, 116, 9220-9229) and have been clearly explained previously. Changes in cation size for quaternary ammonium cations change the density of cations in the double layer, resulting in changes in field strength experienced by adsorbates (e.g. Nat. Catal. 2025, 8, 79–91). Increased electric field strength promotes C-C coupling while minimally affecting HER and methane formation rates (e.g. Energy Environ. Sci. 2019, 12, 3001-3014; J. Am. Chem. Soc. 2017, 139, 11277-11287). The authors do not seem to discuss these or other models that have made similar observations and described these effects.\nReply: As the reviewer pointed out, many previous studies observed that the cation size has a significant impact on the rate/selectivity of electrocatalytic CO or CO reduction. These effects were attributed to factors such as electric field strength. However, this study differs from previous studies in the following three distinct aspects: (1) Testing methods. In this study, we employed a gas-diffusion electrode (GDE) setup to explore the effect of cations on CO reduction. This approach effectively circumvents the limitations of CO mass transfer, enabling us to investigate the cation effect at higher current densities (>100 mA/cm2) and overpotentials. In contrast, previous reports, including those cited by the reviewer, uniformly utilized fully-immersed electrodes, where the electrodes were completely submerged in liquid electrolytes. To avoid the influence of mass transfer limitation, these studies could only be conducted at low current densities (usually below a few dozen mA/cm2) and overpotentials. (2) Research Objects. The first two papers mentioned by the reviewer focused on the effects of quaternary ammonium cations of varying sizes, while the latter two primarily examined the effects of inorganic alkali cations of different sizes (a well-studied area with numerous existing investigations). The second paper (Nat. Catal. 2025, 8, 79) was carried out in a non-aqueous system. In contrast, our study not only investigated the effects of organic cations with different sizes but also systematically examined the effects of introducing hydrophilic -OH groups (e.g., Choline, DHDMA cations) into organic cations. This aspect has not been covered in previous literature. (3) Scientific Findings. Since our research was conducted at higher overpotentials, we found that in systems with hydrophobic organic cations (TMA, DEDMA, TEA, TPA, TBA), as the overpotential rises, there is a notable inverse relationship in the production of H and CH , because their rate-determining steps (RDS) are both related to the surface *H coverage. This reversal phenomenon can be ascribed to the enhanced water accessibility on the electrode surface under more negative potentials (Fig. 5e in the main text). Additionally, we found that introducing hydrophilic -OH groups into cations can repair the poor connectivity of the hydrogen-bonding network of interfacial water. As a result, it suppresses the CH generation and, to some extent, facilitates the C H production. These two novel insights have not been previously reported. In summary, the motivation behind this study is to align more closely with the actual conditions of anion exchange membrane (AEM) electrolyzers. This includes focusing on the effect of organic cations and conducting research at high current densities, with the aim of better guiding the technological development of practical CO AEM electrolyzers.\nRegarding the influence of interfacial electric field strength mentioned by the reviewer, previous studies have indicated that the influence of the interfacial electric field is not a dominant factor in organic cation systems (Ref: Proc. Natl. Acad. Sci. 2019, 116, 9220). A similar observation was also made in our previous research. When using organic cations, the electric field at the electrode/polyelectrolyte interface is lower than that at the electrode/liquid electrolyte interface. However, it is more favorable for the CO reduction reaction, mainly due to the difference in the interfacial water structure (Ref: J. Am. Chem. Soc. 2024, 146, 17377; Electrochimica Acta 2023, 458, 142509). Therefore, this study further explores the influence of organic cations on the interfacial water and their impact on CO reduction.\n- The Tafel slopes are overinterpreted in the current work. It is unjustified to immediately assign changes in Tafel slopes observed with cation size to changes in rate determining steps. As has been discussed in many prior works, changes in Tafel slope can reflect changes in adsorbate coverage or transfer coefficient for a fixed rate determining step (e.g. J. Electrochem. Soc. 2012, 159, 864) and a number of mechanisms can be consistent with a measured Tafel slope (Sci. Rep. 2015, 5, 13801). Furthermore, there is significant uncertainty in Tafel slope determination, with bias towards analysis that yields expected cardinal values (Nat. Commun. 2021, 12, 703). Finally, the authors do not seem to consider the influence of transport limitations on measured Tafel slopes.\nReply: We agree with the reviewer that Tafel analysis can be affected by multiple factors, and the identical Tafel slope might be associated with different reaction mechanisms. We have revised the relevant narrative and deleted some over-interpreted content.\nIn fact, the Tafel slope is influenced by three factors (Ref: Acta Phys. Chim. Sin. 2023, 39, 4, 2210025; J. Electrochem. Soc., 2012, 159, 11, H864), which are as follows: (1) the transfer coefficient; (2) the term determined by the position of the rate-determining step (RDS) and the number of electron transfers; (3) the coverage of the adsorbate. For the Tafel slope analysis of CH production (Supplementary Note 1), based on previous literature studies, we make the following two premises: (1) The RDS of CH formation is a non-electrochemical step (*CO(H) + *H → *CO(H) + *) that follows an initial single-electron transfer reaction (Ref: Nat. Commun. 2021, 12, 3264). Therefore, the transfer coefficient in the prior electrochemical step has little effect on the Tafel slope values. (2) The coverages of *CO (< 0.15 ML according to the Refs: Angew. Chem. Int. Ed. 2022, 61, e202111167; J. Am. Chem. Soc. 2022, 144, 22202) and *H on the Cu electrode surface are both low (since Cu is a typical metal with high hydrogen evolution overpotential). Thus, it is reasonable to speculate that the *CO(H) coverage (θ ) is also low. Indeed, as the reviewer mentioned, many studies reveal that the adsorbate coverage would affect the Tafel slope analysis (Ref: Sci. Rep. 2015, 5, 13801; J. Electrochem. Soc., 2012, 159, 11, H864). We also considered the influence of adsorbate coverage when deriving the Tafel slope formula. Based on the assumption that the coverages of adsorbates are low, we can determine that the Tafel slopes under two different conditions are approximately 59 mV dec−1 and 30 mV dec−1, respectively, according to the final expressions (Eq.S1-7, Eq.S2-3, as shown below):\nCase 1: When the coverage of *H (θ ) is low enough and insensitive to the applied potential.\n𝜕𝜂 −1\n− = 𝑓−1(1− [𝐿]`−1𝜃 ) ≈ 2.303×𝑓−1 (59 mV dec−1) (𝐸𝑞.𝑆1−7)\n𝐶𝑂(𝐻)\n𝜕𝑙𝑜𝑔𝑖\n𝐶𝐻4\nCase 2: When the coverage of *H (θ ) cannot be regarded as a constant (i.e., it changes exponentially with the potential):\n𝜕𝜂 −1\n− = 0.5×𝑓−1(1− [𝐿]−1(𝜃 +𝜃 )) ≈ 2.303×0.5 𝑓−1 (30 mV dec−1)(𝐸𝑞.𝑆2−3)\n𝐶𝑂(𝐻) 𝐻\n𝜕𝑙𝑜𝑔𝑖\n𝐶𝐻4\nOverall, our Tafel analysis is based on the *CO(H) hydrogenation step (as the RDS) under different θ responses to the polarization potential change, and no other mechanisms are included. We have clearly defined this in the revised manuscript.\nThe interpretation of the Tafel slope, which leans towards the preference for the central value, is a topic of widespread discussion. Numerous studies have offered novel algorithms for the nonlinear Tafel curve (Ref: Nat. Commun. 2021, 12, 703; J. Phys. Chem. C 2019, 123, 30252; Joule 2023, 7, 1289). The nonideal bias of Tafel slopes primarily results from mass transport limitations. Here, we also carried out the Bayesian fitting described in the previous paper (Ref: Nat. Commun. 2021, 12, 703). The peak of the probability distribution is marked in Figure R1. The regularity agrees well with that derived from the linear fit in our manuscript. The only flaw of this result is the exceptionally high Tafel slope of TMA (86.1 mV dec-1). It is because the limiting current of CH in TMA electrolytes does not exist in the potential range of the LSV test. Therefore, the non-linear fitting results deviated from the values obtained by linear fitting. In conclusion, the results we obtained from linear fitting are reliable due to the good linearity of experimental data and the sufficient supply of reactant on the GDE electrode.\nFigure R1. Probability distribution function of the non-linear Bayesian fitting result of the LSV-MS signal (m/z=15).\nFor the last concern, the reason why we adopted the gas diffusion electrode (GDE) setup was to eliminate the influence of transport limitations. It can also be seen from the results of in-situ Raman spectra (Supplementary Fig. 15) that in the potential range (from −0.65 V to −0.8 V) for Tafel analysis of CO reduction, the peak of surface-adsorbed *CO is very obvious, which also confirms to some extent that mass transfer limitations do not affect our kinetic analysis.\n- The kinetic discussion for relating reaction orders to reaction rate constants are only true at fixed overpotential. However, the way that the experiments are done is at fixed applied potential. The Nernst equation shows that changing the pressure of CO changes the equilibrium potential for the COR reaction. Therefore, at fixed applied potential, the overpotential is changing. This makes kinetic parameters extracted this way inaccurate for kinetic analysis.\nReply: Thanks for the reviewer’s correction. We did overlook the change of equilibrium potential under different CO partial pressures. Although the potential change is often ignored when determining the reaction order in many literature studies, fundamentally speaking, the equilibrium potential will indeed change with the partial pressure. For CO reduction reaction to CH (CO + 5H O + 6e- → CH + 6OH-), the equilibrium potential can be expressed as:\n2.303RT 𝑃(𝐶𝐻 )[OH-]6\nE0` = E0 - log 4\n6F 𝑃(𝐶𝑂)\nFrom the above formula, we can see that the potential will change with the partial pressures of CH and CO. It is difficult to obtain the accurate equilibrium potential, though the absolute change would not be large. To be more rigorous, we have deleted the relevant content in our revised manuscript.\n- The authors do not seem to consider the possibility of alkali metal impurities present in the system, which have been shown to have an important influence (J. Am. Chem. Soc. 2023, 145, 16787-16795).\nReply: We have quantitatively tested the alkali metal impurities (including Na+ and K+) using atomic absorption spectroscopy (AAS, Table S1). Almost all the concentrations of these cations are less than 0.1 mmol/L. The concentration of Na+ in the TPAOH solution is slightly higher, but still less than 1 mmol/L. The level of these impurities is far lower than that of organic cations. Moreover, the experimental catalytic patterns do not correlate with the content of alkali metal impurities. Therefore, we believe that these trace amounts of alkali metal impurities are not the main influencing factors.\nTable S1. Concentrations of Na+ and K+ impurities in the electrolytes used in this study.\nElectrolytes c(K+) (mmol/L) c(Na+) (mmol/L)\nTMA-OH 0.0128 0.058\nTEA-OH 0.0148 -\nTPA-OH 0.0445 0.799\nTBA-OH 0.0286 0.063\nDHDMA-OH 0.0564 0.097\n- In their discussion, the authors conflate processes related to mass transport with those related to surface reaction kinetics. For example, they conduct reaction order and Tafel analysis to identify kinetically relevant steps but then argue that the “nature of organic cations significantly influences interfacial H2O mass transfer and consequently the proton supply processes”. The authors do not seem to account for the possibility of mass transport limitations in any of their kinetic analysis. Mass transport and surface kinetics are fundamentally distinct phenomena that should be clearly delineated.\nReply: As we mentioned above, we used a gas diffusion electrode (GDE) setup to eliminate the impact of mass transfer. The influence of mass transfer limitation on the Tafel slope analysis lies in the fact that the corresponding reactant in the rate-determining step (RDS) cannot be described by its former quasi-equilibrium equation, which is not consistent with the present study.\nOf course, as the reviewer pointed out, the initial description of “interfacial H O mass transfer” was not very accurate, which is likely to cause misunderstandings. A more rigorous description would be “the accessibility of water molecules to the electrode surface under polarization potential”. In other words, it refers to the ability of water molecules to penetrate the interfacial double layer between the organic cations and the electrode (i.e., the outer Helmholtz plane, OHP) when the potential shifts negatively. The small-sized TMA+ cations, which are adsorbed tightly on the electrode, suppress the penetration of water molecules as the potential shifts negatively. Therefore, the *H coverage (θ ) can be considered constant. In contrast, the large-sized TPA+ cations are adsorbed loosely on the electrode, and their long and flexible alkyl chains are more likely to move. As a result, the penetration of water molecules and thus the *H coverage (θ ) are more easily affected by the change in potential. In summary, the organic cations primarily influence the responsiveness of interfacial water penetration through the outer Helmholtz plane (OHP) to potential variations. This subsequently affects the interfacial *H coverage (θ ), thereby influencing the kinetics of hydrogen evolution and CH production, where *H directly participates in the rate-determining step (RDS). The organic cations do not impact the apparent mass transfer of the CO or H O reactants and only influence the coverage change with potential shifts.\n- The total Faradaic efficiencies are relatively low in many cases. Charge balances should be close to 100% in all cases.\nReply: In the systems with TMA and DEDMA cations, the total Faradaic efficiencies are all close to 100%. Those for other organic cations deviate from 100% to some extent because the CO reduction liquid products in these systems are not accounted for. The overlapping 1H NMR signals of these organic cations and the liquid products (Figure R2, take TPA as an example) make it difficult to quantify the latter. Nevertheless, the proportion of unknown liquid products in these systems is low, so it will not affect the study’s conclusion.\nFigure R2. 1H NMR spectrum of the 0.5 M TPA solution.\n- The authors could highlight more appropriate citations. When summarizing the accepted mechanisms in the field they exclude more comprehensive discussions on this topic (e.g. Nat. Energy 2019, 4, 732-745; Chem. Rev. 2019, 119, 7610-7672). They furthermore cite work (J. Am. Chem. Soc. 2018, 140, 9337-9340) which may contain systematic artifacts in its analysis (J. Am. Chem. Soc. 2019, 141, 4191-4193).\nReply: Thank you for your prudent suggestion regarding our reference issue. We’ve added these two previous studies (Nat. Energy 2019, 4, 732-745; Chem. Rev. 2019, 119, 7610-7672) in the introduction section of our revised manuscript.\nRegarding the isotopic scrambling problem and other misunderstandings about the cited work (J. Am. Chem. Soc. 2018, 140, 9337-9340), it’s an important aspect that requires further attention and has sparked recent controversies (Ref: Nat. Catal. 2025, 8, 513). Nonetheless, the OH− attack on ketone species mentioned in this reference has been recognized in numerous papers (Ref: Energy Environ. Sci. 2022, 15, 3978; Nat. Catal. 2019, 2, 423). To avoid potential misinterpretation, we have replaced this reference with other related works.\nReviewer #2 (Remarks to the Author):\nThis article investigated the effect of quaternary ammonium cations on the performance of CORR in GDE-based flow cell. Cations with different size and hydrophobicity were studied. Cations with different size and hydrophobicity were studied. CORR under conditions free of alkali cations is of great industrial interest, while the studies on the effects of organic cations are rare and highly demanded. Therefore, the research topic of this article is of great importance and may attract broad interest in this field. The mechanism of CORR is complicated and cations show diverse effects on the local water concentration, hydrogen bond network, absorption of H and CO, and the electric field at the interface. This study employed a variety of technical methods and generated a substantial amount of data. However, further in-depth analysis is still required to process and summarize these data, in order to construct a more quantitative model and arrive at clearer and more definitive conclusions. Therefore, major revision is necessary before publication.\nReply: We sincerely appreciate the reviewer’s positive feedback and the valuable suggestions for revision. We have made every effort to carry out a more in-depth analysis and meticulously refine this manuscript in line with these suggestions.\n1. While reading the main text, the abundance of fragmented information makes it difficult for readers to identify a clear narrative thread or form a coherent mental framework. Since several multi-step competing reactions including HER, CH4 formation and C2H4 formation are involved, the direct interpretation of Tafel analysis is difficult. Direct comparison of the experimental Tafel slope with some typical specific number like 118, 59 etc. may not give a reliable conclusion. In the main text, is it possible to build a comprehensive microkinetic model that can be fitted based on the Tafel analysis curves? Then, the influence of organic cation on certain kinetic parameter can be extracted, providing a clear picture of the organic cation effects.\nReply: Thanks for the reviewer’s comments. We have made our best effort to revise the manuscript (highlighted in yellow background) to clarify the expression.\nTafel analysis is a mathematical tool for mechanism discussion, and it is only applicable to systems where the elementary reaction pathways and the corresponding rate-determining step (RDS) are known. As pointed out by the reviewer, multi-step competing reactions, including HER, CH formation, and C H formation, are involved in this study. The reaction pathways of these product formations and the corresponding RDS are based on the widely accepted knowledge in the literature (Red: Nat. Commun. 2021, 12, 3264; Angew. Chem. Int. Ed. 2022, 61, e202111167; J. Am. Chem. Soc. 2017, 139, 130; Nat. Commun. 2019, 10, 32). Only on this basis can we use Tafel analysis to interpret the kinetic information in different organic cation systems. Since the pathways and RDS of these reactions vary, it is hard to build a comprehensive microkinetic model for analysis.\nIn addition, Tafel slope analysis demonstrates the extent to which an electrochemical reaction is influenced by the electrochemical driving force. It is generally believed that when the measured Tafel slope value is much higher than 118 mV dec−1, it may be affected by other factors such as mass transfer (Ref: ACS Catal. 2018, 8, 8121). In this case, it is difficult to derive the Tafel slope expression and analyze the kinetics. This is exactly the case for the Tafel slopes of C H production measured in different organic cation systems.\nTherefore, based on established CH generation pathways reported in the literature, this study focuses on a detailed derivation of the Tafel slope expression for CH production (Supplementary Note 1), and examines how the potential-dependent response of surface *H coverage influences the Tafel slope. By combining the mathematical analysis of Tafel slope, experimentally measured Tafel slope values, in-situ Raman spectroscopy, and AIMD simulations, we clarified the interface characteristics in different organic cation systems and their influence on the electrocatalytic CO reduction. Therefore, we did not derive more instinct kinetic parameters based on the Tafel slope results. Further works are needed to provide a deeper insight.\n2.MS was used for real-time analysis of the products generated by the catalyst during LSV testing. However, the inlet of MS was positioned after the electrochemical cell, rather than directly behind the GDE. Considering the volumes of gas flow field and pipe between the cell and the MS inlet, as well as the carrier gas flow rate, can this setup achieve accurate real-time detection of transient processes?\nReply: The homemade flow cell is equipped with a very thin serpentine flow field (the total dead volume is estimated to be only ca. 0.2 ml). During the test, the flow rate of CO gas is 20 ml/min, which can ensure that the products in the gas diffusion electrode are quickly carried out to the MS sampling port. In addition, the inner diameter of the pipe used for MS sampling is also very thin (Φ = 0.5 mm, with a very small dead volume). The mass spectrometer is equipped with a backing pump and a turbomolecular pump, which have strong suction power and can quickly carry the products to the ion source-mass analyzer for analysis. These two aspects ensure that the MS has high real-time responsiveness (~ 4 s). When performing LSV scanning-MS analysis, we have already subtracted the response time.\n3. H2O is electroneutral molecule. Why high polarization potential can drive the mass transfer of H2O through the hydrophobic region generated by the organic cations?\nReply: As the reviewer pointed out, H O is an electroneutral molecule. However, the H O molecule possesses a dipole moment that can be influenced by an electric field. For example, numerous studies have found that the reorientation of H O molecules under different electrode potentials (Ref: Angew. Chem. Int. Ed. 2020, 59, 22397; Nat. Mater. 2019, 18, 697). In this study, H O affects the interfacial reaction due to the differences in the structure of the interfacial double-layer composed of organic cations. We have revised our expression of “mass transfer” of water in the main text. The organic cations primarily influence how water accessibility responds to changes in potential. Under high negative polarization potential, H O is easier to approach the electrode surface and produce more H species in larger cations such as TPA with a less compact arrangement at the interface.\n4.C2H4 is a more prominent and desirable product in CORR, yet this study's kinetic analysis focused primarily on the formation pathways of H₂ and CH4 rather than C2H4. Why?\nReply: C H is indeed a more prominent and desirable product for CORR in typical alkali-cation electrolytes (for example, in KOH electrolyte as shown in Supplementary Fig. 4). However, for the magnetron-sputtered copper electrode used in this study, the CORR selectivity for C H is extremely low in the electrolytes with organic cations, especially those with larger cations like TPA and TBA. Moreover, the Tafel slope for C H is notably higher than 120 mV dec⁻¹ with small cations like TMA and TEA, making it challenging to perform kinetic analysis (as the rate-determining step (RDS) is not related to charge transfer). Further research is needed to increase C H production in the presence of organic cations, so that Tafel analysis can be carried out. In contrast, H and CH are the dominant products in these tests and show distinct regularities. Hence, we can infer the corresponding interfacial properties from the kinetic studies of the H and CH reaction pathways.\n5. How the cation identity affects the local electric field was characterized through vibrational Stark effect analysis, but the effects of local electric field on the kinetics of different reactions were not sufficiently investigated.\nReply: Although the identity of cations indeed affects the local electric field, previous studies have indicated that the influence of the interfacial electric field is not a dominant factor in organic cation systems (Ref: Proc. Natl. Acad. Sci. 2019, 116, 9220). Their results demonstrated that the change in the electric field caused by the nature of the cations is small, so the CO adsorption energy would not change significantly. A similar observation was also made in our previous research. When using organic cations, the electric field at the electrode/polyelectrolyte interface is lower than that at the electrode/solution interface. However, it is more favorable for the CO reduction reaction, mainly due to the difference in the interfacial water structure (Ref: J. Am. Chem. Soc. 2024, 146, 17377; Electrochimica Acta 2023, 458, 142509). Therefore, this study specifically focuses on the influence of organic cations on the interfacial water structure and its impact on CO reduction.\nReviewer #3 (Remarks to the Author):\nThis manuscript investigates the effects of organic cations on electrochemical CO reduction reactions (CORR). Through a comprehensive approach combining electrochemical measurements, electrochemical online mass spectrometry, in-situ Raman spectroscopy, and molecular dynamics simulations, the authors reveal how cation structure influences interfacial water dynamics and surface hydrogen behavior at high current densities. The work provides valuable mechanistic insights for the development of practical anion exchange membrane electrolyzers. I recommend this manuscript be accepted after addressing the following concerns:\nReply: We sincerely appreciate the reviewer’s recommendation.\n1. The goal of this manuscript is to improve the performance of anion exchange membrane solid-state electrolyzers, but performance testing in an actual anion exchange membrane (AEM) electrolyzer system is missing. This data should be supplemented, and the authors should explain which organic cation results best correlate with the AEM electrolyzer test results.\nReply: Thank the reviewer for the comment and suggestion. The goal of this study is indeed to provide guidance for the design of polyelectrolytes (especially ionomers) in anion-exchange membrane solid-state electrolyzers, aiming to improve their performance.\nIt is worth noting that the differences between alkaline polyelectrolytes and common alkaline solution electrolytes (e.g., KOH solution) lie not only in the types of ions (organic cations vs. inorganic cations) but also in the mobility of ions (constrained by polymers vs. freely moving in solutions) (Ref: J. Am. Chem. Soc. 2024, 146, 17377; Electrochimica Acta 2023, 458, 142509). This study currently only focuses on the former, i.e., the difference in ion types, and thus systematically investigates the effect of organic cations on CO reduction. The reason for not considering the influence of ion mobility simultaneously is that there are substantial challenges in introducing these organic cations (especially large-sized ones) onto the polymer backbone. Quaternary ammonium cations are typically grafted onto polymer chains through the Menshutkin reaction. Due to steric effects, it is difficult to graft larger tertiary amines onto the polymer chain. Moreover, the synthesis and comparison of different polyelectrolytes can also be affected by other factors, such as the degree of polymerization, the aggregation of polymer chains, etc.\nTo systematically compare the effects of different organic cations on CO reduction in a parallel manner, we use a simulated interfacial environment in this study. Specifically, we used a Nafion-coated Cu gas diffusion electrode (GDE) for testing and comparison in electrolytes containing various quaternary ammonium-type organic cations. There are two reasons for choosing Nafion modification. Firstly, it mimics the relatively hydrophobic environment in an actual gas diffusion electrode, which facilitates CO gas mass transfer. Secondly, these quaternary ammonium cations can penetrate through Nafion to reach the electrode surface and have electrostatic interactions with the sulfonate anions in Nafion. This can, to some extent, simulate the bonding effect of organic cations in alkaline polyelectrolytes. By doing so, we can avoid the complex synthesis of polyelectrolytes and systematically explore the impact of organic cations on CO reduction.\nIn addition, as suggested by the reviewer, we synthesized two typical alkaline polyelectrolytes for comparison (Figure R3a). We grafted relatively small TMA and choline cations, respectively, onto the same polymer backbone (pTPN-Br, Ref: ACS Macro Lett. 2017, 6, 566). Subsequently, we used these two polyelectrolytes as ionomers in the catalytic layer for the assembly and testing of a CO AEM electrolyzer (Figure R3b- R3c). We found that, similar to the flow cell testing, when the cations contain the hydrophilic -OH group (i.e., choline), the hydrogen evolution side reaction is more severe. However, there are also some different experimental observations. No CH product was detected in the AEM electrolyzers. This may be attributed to the difference in ion mobility, which requires further investigation in the future.\nFigure R3. (a) Two typical alkaline polyelectrolytes featuring different organic cations were synthesized in this study. CO electrolysis performance was evaluated when using pTPN-TMA (b) and pTPN-choline (c) as the cathode ionomers, respectively. The CO AEM electrolyzer was assembled with an ionomer-modified Cu gas diffusion electrode (GDE) as the cathode, an IrO -coated porous Ti electrode as the anode, and a commercial QAPPT membrane. Pure water was fed to the anode side to exclude any possible influence from other free-moving cations.\n2. The polymer cations on anion exchange membranes and their hydrophilic/hydrophobic properties may affect water diffusion dynamics at the catalyst surface, which may be different with small-molecule organic cations. This requires further discussion.\nReply: We agree with the reviewer that organic cations on alkaline polyelectrolytes have different effects on electrocatalytic behavior compared to free-moving small-molecule organic cations, as found in our previous studies (Ref: J. Am. Chem. Soc. 2024, 146, 17377; Electrochimica Acta 2023, 458, 142509). As mentioned in the Reply to the previous question, if polyelectrolytes are directly used to study the organic cation effect, there are significant challenges in synthesizing polymers incorporating these different organic cations, especially large-sized ones. This is the reason why we constructed a simulation system for systematic research. We used a Nafion-modified Cu gas diffusion electrode (GDE) as the working electrode. This approach can ensure consistent apparent hydrophilic/hydrophobic properties and, to some extent, simulate the constrained state of organic cations at the interface (due to the electrostatic interaction between the sulfonate groups in Nafion and these organic cations). This setup facilitates a systematic investigation of the influence of different organic cations on CO reduction. A comparison of the performance of CO AEM electrolyzers using two typical polyelectrolytes (as the cathode ionomers) we synthesized shows some similarities with the flow cell test results (Figure R3). For example, organic cations containing hydrophilic -OH groups can promote the hydrogen evolution side reaction. However, there are also differences. For instance, there is evident CH production in the flow cell tests, but no CH is produced in the AEM electrolyzer, which may be due to the difference in the mobility of organic cations (constrained by polymers vs. freely moving in solutions). Further research is needed in this field in the future.\nFigure R3. (a) Two typical alkaline polyelectrolytes featuring different organic cations were synthesized in this study. CO electrolysis performance was evaluated when using pTPN-TMA (b) and pTPN-choline (c) as the cathode ionomers, respectively. The CO AEM electrolyzer was assembled with an ionomer-modified Cu gas diffusion electrode (GDE) as the cathode, an IrO -coated porous Ti electrode as the anode, and a commercial QAPPT membrane. Pure water was fed to the anode side to exclude any possible influence from other free-moving cations.\n3. Regarding Fig. 2f, you use the radius of gyration as a descriptor for organic cations. Please explain how this parameter was calculated and why it was chosen over other structural descriptors. Furthermore, please specify which potential was used to calculate the C2H4/CH4 ratio. Why was this particular potential chosen for comparison instead of others within your testing range?\nReply: The radius of gyration is commonly used to describe the size of polymers. Because the side-chains of these organic cations are flexible and highly mobile, it might be more suitable to use the mean square radius of gyration as an effective measure of their size. Besides, the DEMDA cation is less symmetric than other tetraalkylammonium structures; the molecule diameter (mainly describes the two farthest atoms with their van der Waals radius) is not suitable to be compared. Using the Multifwn program, we calculate and compare the gyration radius with the van der Waals volume of these organic cations and found that the gyration radius is a good descriptor of the molecule's volume (Figure R4).\nFigure R4. Gyration radius and their van der Waals volume of organic cations.\nWe selected −0.8 V vs. RHE for comparing the C H /CH ratio. This is because, overall, the CORR Faradaic efficiency at this potential is the highest for these organic cations. Certainly, if other potentials are chosen for comparison, the resulting catalytic patterns won't vary significantly (Figure R5).\nFigure R5. The ratio of CH /C H FE under different potentials with different organic cations\n4. You mention that organic cations have lower hydration numbers than alkali metal cations, indicating their hydrophobicity. Since carbon paper substrates are typically hydrophobic (PTFE-treated) with known affinity for organic species, how does the accumulation of organic cations on the catalyst surface under electric fields affect the overall hydrophobicity of your gas diffusion electrode? This consideration seems important for interpreting your results.\nReply: The direct measurement of the hydrophilicity/hydrophobicity of the gas diffusion electrode under working conditions (i.e., under electric fields) presents a significant challenge. As far as we know, effective methods are still lacking. However, on one hand, we compared the contact angles of these electrolytes containing different quaternary ammonium cations (0.5 M) on the surface of the carbon paper (Avcarb GDS-3250) substrate. The contact angles were quite similar (Figure R6). On the other hand, during the CO reduction test, no obvious water droplets discharged from the cathode flow field were observed. Consequently, it is reasonable to infer that the accumulation of organic cations on the catalyst surface under electric fields does not result in differences in the apparent hydrophilic/hydrophobic properties of the gas diffusion electrode, that is, there is not much difference in the apparent wetting state of the gas diffusion electrodes.\nFigure R6. Contact angles of the electrolytes containing different quaternary ammonium cations on the surface of the carbon paper (Avcarb GDS-3250) substrate.\n5. Your results show that hydroxyl-containing cations (choline and DHDMA) promote hydrogen evolution while suppressing CH4 production, despite both processes requiring hydrogenation steps. This seems contradictory - please provide a clearer mechanistic explanation for this interesting observation.\nReply: We agree with the reviewer that both the hydrogen evolution and CH production require hydrogenation steps, which need surface-adsorbed *H as a reactant. The *H is all produced from the Volmer step of water decomposition (H O + e− → *H + OH−). However, their mechanisms of consuming *H to produce H or CH are quite different (Figure R7).\nFor the hydrogen evolution reaction, *H can be converted to H through either the Heyrovsky (*H + H O + e− → H + OH−) or Tafel (2*H → H ) step. However, since Cu is a metal with a high hydrogen overpotential, the surface *H coverage is low. It is reasonable to deduce that H is mainly produced via the Heyrovsky step. That is, more H O would facilitate the consumption of *H to produce H .\nFor the CH production reaction, the rate-determining step (RDS) is the chemical hydrogenation step (*CO(H) + *H → *CO(H) ). Consequently, the over-consumption of *H is not conducive to CH formation. We pointed out that there is a shift point of the Tafel slopes for hydrogen evolution and CH formation (Fig. 2d in the main text). This is because the Heyrovsky reaction between *H and H O to produce H intensifies at more negative potentials, leading to the suppression of CH formation. The suppression of CH formation caused by hydroxyl-containing cations is similar to what was previously discussed. Hydroxyl groups on these cations help repair the integrity of the hydrogen-bonding network in the interfacial water, which is more favorable for promoting the Heyrovsky reaction between *H and H O to produce H .\nFigure R7. Schematic illustration of the difference in the consumption of *H for hydrogen evolution and CH production on the Cu electrode.\n6. The deconvolution of the ν(O-H) band in Fig. 5f shows inconsistent peak positions across the three samples (TMA, TPA, DHDMA). What criteria did you use to assign the components for isolated water, weakly hydrogen-bonded water, and strongly hydrogen-bonded water? Similar inconsistencies appear in Supplementary Fig. 23. Please justify your peak fitting approach.\nReply: Thank the reviewers for pointing out this issue. We have re-performed a more rigorous peak deconvolution of the ν(O-H) band by more strictly determining the peak positions and fixing the full-width at half-maximum (FWHM) of each component.\nThe peak positions were determined based on previous studies, including isolated water at around 3600 cm-1 (i-H O), weakly hydrogen bonded water at around 3450 cm-1 (wh-H O), and strongly hydrogen bonded water at around 3260 cm-1 (sh-H O, Ref: Angew. Chem. Int. Ed. 2020, 59, 22397; J. Phys. Chem. C 2020, 124, 13240; Electrochimica Acta 2023, 458, 142509). Another criterion is that the peak fitting of the ν(O-H) band should keep the full-width at half-maximum (FWHM) approximately the same. We use a Gaussian function to represent the three typical water species and fit the overall ν(O-H) band (Ref: ACS Catal. 2024, 14, 15665; ACS Appl. Energy Mater. 2024, 7, 11154). The results of the re-peak deconvolution fitting (Figure R8) are consistent with the previous conclusions. That is, the sh-H O component occupies the largest portion in DHDMA cation compared to other hydrophobic cations (TMA and TPA). Additionally, the sh-H O ratio of hydrophobic cations decreases more sharply than that of DHDMA. That means, hydrophilic cations enhance the connectivity of interfacial hydrogen bonds and suppress the change of water configuration under negative potentials. Therefore, the interfacial water structure solidifies our previous deduction from the electrokinetic results.\nFigure R8. Deconvolution of the ν(O-H) stretching vibration band in the TMA (a), DHDMA (b), and TPA (c) electrolytes.\n7. In several performance tests shown in Fig. 2 and 4, the total FE is significantly below 100%. What accounts for this gap? Are there undetected products or side reactions?\nReply: In the systems with TMA and DEDMA cations, the total Faradaic efficiencies are all close to 100%. Those for other organic cations deviate from 100% to some extent because the CO reduction liquid products in these systems are not accounted for. The overlapping 1H NMR signals of these organic cations and the liquid products (Figure R2, take TPA as an example) make it difficult to quantify the latter. Nevertheless, the proportion of unknown liquid products in these systems is low, so it will not affect the study’s conclusion.\nFigure R2. 1H NMR spectrum of the 0.5 M TPA solution.\n8. You attribute differences in catalytic selectivity to the hydrophobicity of organic cations affecting *H coverage by hindering water transport. Have you considered that direct adsorption of these cations on the electrode surface might alter the electronic structure of catalytic sites? How did you distinguish between these potential mechanisms?\nReply: To alter the electronic structure of catalytic sites, these organic cations should form chemical bonding interactions with the electrode surface and undergo electron transfer. However, the organic cations employed primarily consist of electroneutral and saturated alkyl groups. Moreover, the positively-charged nitrogen atom (lacking lone-pair electrons) is encircled by these hydrocarbon groups. As a result, they neither accept nor donate electrons from/to the electrode surface.\nOf course, there are also some studies about the specific adsorption of quaternary ammonium cations, demonstrating that direct interaction may occur between the organic cations and the electrode (Ref: J. Phys. Chem. Lett. 2016, 7, 4464, J. Electrochem. Soc. 2018, 165, F114). A previous study also considered this possibility and found the different nature of organic and inorganic cations (Ref: Nat. Cat. 2022, 5, 624). In this reference, they claimed that adsorbed TMA (the frequency of δ(C-H) is 1482 cm-1, while the frequency of δ(C-H) in hydrated TMA is 1490 cm-1) is substituted by alkali cations. The author claimed that a slight partial charge transfer from the electrode surface to the C-H bond. Through DFT calculation, they found the electrosorption valence of TMA+ is 0.51, which is smaller than all alkali cations (Li+: 0.78, Na+: 0.73, K+: 0.64, and Cs+: 0.61). This means that organic cations interact with the electrode surface more weakly than the commonly used alkali metal cations do."}, {"round": 2, "reviewer_comments": {"reviewer_2": "All my comments have been properly addressed in the revision. I recommend the manuscript to be accepted.", "reviewer_3": "My previous concerns have been satisfactorily addressed in the current version. The manuscript has been significantly improved through the revision process. I recommend this revised version for publication in Nature Communications."}, "author_response": "Reply to Reviewers’ comments\nReviewer 1:\nThe authors have made some efforts to address reviewer comments but the fundamental issues with the paper regarding a lack of novelty and weaknesses in convincing mechanistic interpretations remain unchanged compared to the first version of the paper. The paper neither makes new interesting observations nor advances the fundamental understanding of cation effects in CO2 reduction. Therefore, in my opinion, this paper is far from meeting the bar for publication in Nature Communications. Below I outline the main issues with the paper.\nReply: We’re deeply grateful to Reviewer 1 for re-reviewing our manuscript and offering detailed comments. While we don’t agree with some of them (see the specific responses below), we appreciate the time and effort Reviewer 1 has put in, which reflects a rigorous academic attitude and great attention to this research. We have provided point-to-point responses to “the main issues” in the hope of addressing Reviewer 1’s concerns.\nSpecific comments\n- The main observation of changing organic ammonium cation size leading to differences in reactivity is reported in many studies. This is true in water and organic solvent. For example, see EES Catal., 2025, 3, 1055-1061; Nat. Catal. 2025, 8, 79; Proc. Natl. Acad. Sci. 2019, 116, 9220; ACS Catal. 2024, 14, 12928–12939; Electrochem. Commun. 2015, 52, 58-62; ACS Catal. 2015, 5, 703–707; J. Am. Chem. Soc. 2017, 139, 18552–18557).\nReply: It should be emphasized that when discussing the cation effect, it only makes sense to consider the same reaction within the same electrolyte system. As Reviewer 1 mentioned, indeed, there have been multiple papers studying the influence of the organic ammonium cation size. However, they are quite different from this study for the following reasons:\nFirst, in most of the papers listed above, the majority focus on CO reduction in the non-aqueous electrolyte, with the CO•- as the crucial intermediate and oxalate or CO as the product (Nat. Catal. 2025, 8, 79; ACS Catal. 2024, 14, 12928–12939; Electrochem. Commun. 2015, 52, 58-62; ACS Catal. 2015, 5, 703–707; J. Am. Chem. Soc. 2017, 139, 18552–18557), which is a different topic from our study. Moreover, not all of these papers follow the same regularity. For example, in the MeCN electrolyte, large organic cations like TBA+ promote CO -to-CO conversion compared with smaller cations. The overall reduction current is also higher than that of smaller cations under CO atmosphere (ACS Catal. 2024, 14, 12928–12939).\nSecond, only two papers have studied the organic cation effect in the aqueous electrolyte (EES Catal. 2025, 3, 1055-1061; Proc. Natl. Acad. Sci. 2019, 116, 9220), and the former one is still about CO reduction. In CO reduction, the subsequent CO conversion is affected by the preceding CO activation and conversion steps (i.e., the rate at which CO produces *CO intermediate). Therefore, although the advantage of small cations like TMA in CO reduction has been demonstrated in previous studies, it still fails to clearly explain the effect of organic cations on the further conversion of CO. Moreover, a hot press treatment with PTFE was introduced in their electrode preparation to provide a highly hydrophobic interfacial environment in order to suppress the hydrogen evolution reaction (HER). The onset potential of HER with TMA is 400 mV more negative than that with TEA and TPA cations (Fig.2d in EES Catal. 2025, 3, 1055−1061), which is different from previous studies (Chem. Sci. 2023, 147, 11076−11087). This difference in the onset potential of HER between different studies indicates the complexity of the cation effect. Regarding the latter paper on CO reduction (Proc. Natl. Acad. Sci. 2019, 116, 9220), the CO reduction tests were carried out on the fully-immersed electrode with insufficient CO supply, which leads to a low current density (< 5 mA cm−2) that cannot meet the conditions under practical electrolysis. Meanwhile, the specific potential-dependent CO reduction selectivity was not discussed either. In contrast, we investigated the effects of organic cations at industrial-scale current densities (>100 mA cm−2), considering not only the influence of cation size (as mentioned by Reviewer 1) but also the presence of hydrophilic -OH groups, which have never been studied in previous research.\nThe cation effect is a crucial influencing factor in CO and CO reduction. There have been numerous studies on the effect of alkali cations (Li+ ~ Cs+) (>380 papers according to Web of Science, using “CO reduction” and “alkali cation” as keywords). Nevertheless, relevant studies are still being reported continuously. In contrast, there are still very few studies on the effect of organic cations, and the relevant understanding is still very limited (Representative studies as listed by Reviewer 1). Notably, studying the effect of organic cations is even more important than that of alkali cations because in the alkaline polyelectrolyte (APE), a key material in practical CO membrane electrode assembly (MEA) electrolyzers, only organic cations can be grafted onto the polymer chain. Therefore, we think the importance of our study’s findings shouldn’t be dismissed just because of a few seemingly similar but actually different previous studies.\nIn conclusion, our work provides a more comprehensive and systematic view of not only the steric but also the hydrophilic properties of organic cations on CO reduction, which is of great value for the development of this field.\n- There is no difference in the way that cations affect surface chemistry in a gas diffusion electrode versus a traditional H-cell design. Inside of a gas diffusion electrode, the catalyst surface must be partially wetted to allow for ionic connection with the rest of the cell. Mass transport is facilitated by the fact that the size of this liquid layer is much smaller than the diffusion layer in a bulk electrolyte. However, from the perspective of the catalyst surface, the two situations are indistinguishable. The local (~nm) environment which influences reaction kinetics is the same in both cases (assuming no mass transport limitations). Thus, simply measuring cation effects that are known and expected from H-cell studies in a GDE is not fundamentally new and does not provide new scientific insights.\nReply: It remains an open question whether the electrocatalytic reaction in the gas diffusion electrode (GDE) happens at the solid/liquid (thin electrolyte layer, as shown in the above Scheme) interface or at the gas/solid/liquid triple-phase boundary. Currently, there is still no effective way to clarify it. Personally, I tend to agree with the Reviewer’s view that the reaction occurs at the solid/liquid (thin liquid film) interface. However, the mass transfer at this interface is significantly different from that at the conventional solid/liquid interface (Ref: ACS Catal. 2020, 10, 14093−14106). However, we still cannot deny the possibility that the real reaction sites are at the gas/solid/liquid triple-phase boundary. This is because in the absence of bulk electrolyte, there may be spillover of adsorbed species (Ref: Chem. Rev. 1995, 95, 759–788) and unique assembly/transport mechanisms for ions on the catalyst surface (Ref: Nat. Nanotech. 2024, 19, 479–484). Anyway, the assumption “assuming no mass transport limitations” made by Reviewer 1 is not valid. When using a fully-immersed electrode for CO reduction, the current density is limited by slow CO mass transfer and can only reach a maximum of a few mA cm−2. In contrast, with the GDE setup, we can eliminate the CO mass transfer limitation and investigate the interfacial mechanism at industrial-scale current densities (> 100 mA cm−2).\nBesides, many studies have already revealed that reaction kinetics could diverge with different mass transport conditions. Here we present some examples:\n(1) In this paper (Ref: ACS Energy Lett. 2023, 8, 2185−2192), the authors have revealed that the convection at the electrode surface significantly changed the kinetics of CO to C H by changing the hydrodynamics of the electrolyte via the H-cell design. By enhancing the convection, the Tafel slope for C H decreased from 100 mV dec-1 to 40 mV dec-1. These results demonstrated that the mass transfer issue could significantly change the reaction mechanism.\n(2) In this paper (Ref: J. Am. Chem. Soc. 2025, 147, 9990−10001), the authors contrasted the alkali cation effect in the H-cell and a GDE setup. They found that the promotion effect from Li+ ~ Cs+ is somewhat amplified in conventional H-cell studies. With sufficient CO supply in the GDE setup, there is no significant difference among these cations. The CO reduction performance in CsOH is even poorer than that in LiOH at low concentration. This difference in cation effects indicates that the GDE setup, which provides a more efficient mass-transfer environment, is essential to accurately study the organic cation effect, especially at high current density.\nThe above-mentioned differences in reaction kinetics and cation effects between different electrochemical cells are precisely why we must use the GDE setup to eliminate the influence of CO mass transfer and thereby explore the organic cation effect on CO reduction. Moreover, the introduction of hydroxyl-containing cations directly demonstrates the effect of H* coverage and distinguishes the effect of interfacial water from the electric field. To the best of our knowledge, our work provides new and systematic regularity of organic cation effects in CO reduction performance at industrial-level current densities and deepens the understanding of this field.\n- The authors state that “the interfacial electric field is not a dominant factor in organic cation systems”. This is simply untrue. The paper cited (Proc. Natl. Acad. Sci. 2019, 116, 9220) and others (Nat. Catal. 2025, 8, 79; Langmuir 1992, 8, 1451-1458; J. Electroanal. Chem. Interfac. Electrochem. 1991, 318, 335-347) show systematic differences in field strength with cation size with these cations across both a range of organic solvents and water. These papers show that these differences in field strength do account for differences in reactivity (Nat. Catal. 2025, 8, 79) based on the large precedent for field influences on CO R (e.g. ACS Catal. 2016, 6, 7133-7139; J. Am. Chem. Soc. 2017, 139, 11277-11287; Surf. Sci. 2016, 654, 56-62). The fact that these observations are made across solvents (e.g. the same trends are observed in water and organic solvent and for each different organic solvent) actually highlights that interactions with water are not the main driver of observed cation effects in any solvent, rather than making these studies not relevant to the current paper. The authors cannot simply discard this explanation.\nReply: We acknowledge that numerous studies have reported the significant influence of the interfacial electric field on certain electrocatalytic reactions. We have indeed observed some differences in the interfacial electric field across different organic cation electrolytes, as evidenced by variations in the Stark slope of adsorbed *CO in Raman spectra (Figure 5b-5d). Admittedly, the interfacial electric field exerts a profound impact on driving the electrocatalytic reactions. However, we cannot simply regard it as the decisive factor in any case. This is because, apart from the electric field, other factors also make substantial contributions to the catalytic process. As previously stated, it was found that large cations such as TBA promote the reduction from CO to CO compared with smaller cations. The overall current was also higher than that with smaller cations under CO atmosphere (Ref: ACS Catal. 2024, 14, 12928–12939). Based on these observations, it becomes evident that the interfacial solvent properties caused by different cations also account for the catalytic characteristics, especially in aqueous electrolytes. This is because CO reduction involves a series of proton-coupled electron transfer (PCET) processes in the presence of water.\nIn addition, we chose organic cations with similar sizes for comparison. The gyration radius of choline is 1.98 Å, which is between TMA (1.48 Å) and TEA (2.12 Å). Therefore, the interfacial electric fields they form are also comparable. The asymmetric structure with the positively charged R-NMe+ group makes choline easier to approach the electrode like TMA. Previous studies have shown that organic cations with positively charged head groups have a comparable distance to the surface (Ref: Nat. Catal. 2025, 8, 79). As shown in Figure R1 (Supplementary Fig. 23), the *CO Stark slope of TMA (177 cm-1 V-1) is indeed close to that of choline (183 cm-1 V-1), revealing that they have comparable interfacial electric fields. However, their CO reduction selectivity differs significantly (Figure 2a, 4a). Therefore, the interfacial electric field cannot be the decisive factor. Instead, combined with kinetic analysis, in-situ Raman characterizations, and computational simulations, our experimental results clearly indicate the crucial role of interfacial water in determining the catalytic outcomes.\nFigure R1 (Supplementary Fig. 23). (a) Potential-dependent in-situ Raman spectra (ν(CO)) of the MsCu electrode in 0.5 M choline hydroxide (left) and 0.5 M TMAOH (right). (b) Change in peak position of ν(CO) with potential shifts. The Stark slopes are labeled beside the curves.\n- The mechanism proposed by the authors for the cation effect remains highly speculative. While it is plausible, it is far from demonstrating clear connections between rates and hydrophobicity at the interface. The fact that no alternative explanations are considered makes this even less convincing.\nReply: As previously mentioned, we attribute the change in CO reduction performance resulting from the hydrogenation process mainly to different hydrogen-bond environments (interfacial water) rather than the electric field induced by the volume of organic cations. This conclusion is not a baseless guess but is drawn from solid experimental data, including kinetic analyses, in-situ Raman characterizations, and theoretical simulations. However, the interfacial electric field mentioned by Reviewer 1 cannot account for our experimental phenomena (as stated in Reply 3). The only assumption in our study is that, during the kinetic analysis, we used the elementary reaction pathways for H evolution and CO reduction, which are widely recognized in previous research. Therefore, based on the comprehensive evidence presented, our conclusions are solid and reliable.\nCertainly, the properties of organic cations affect both the interfacial water environment and the interfacial electric field, and our work does not deny the potential promotional effect of the interfacial electric field on CO reduction. Nevertheless, we find that the interfacial water has a more significant effect on the CO reduction selectivity than the interfacial electric field in this work. In conclusion, this study provides a comprehensive explanation of how changes in the interfacial environment regulate the hydrophilic and hydrophobic properties of organic cations to control the selectivity of CO reduction. Furthermore, this work provides guidance for the structural design of alkaline ionomers for CO electrolysis conversion."}, {"round": 3, "reviewer_comments": {"reviewer_1": "The fundamental issues with the paper are still unchanged compared to the last review. The authors' responses did not adequately address the questions brought up and included a number of misconceptions or inaccuracies. In my view, the paper does not make new interesting observations nor advance fundamental understanding of cation effects in CO2 reduction. In my opinion, this paper is far from meeting the bar for publication in Nature Communications. If the other reviewers and editor suggest publication, they should proceed at this point, but I do not believe the paper warrants publication in this journal.", "reviewer_4": "The manuscript by Xue et al. presents a systematic study on the effect of organic cations in CORR. The authors explore both the cation size effect and the influence of different numbers of hydrophilic hydroxyl-group introductions. They find that the difference in CORR behavior mainly result from the impact of organic cations on interfacial water, causing variations in surface hydrogenation processes. This study is highly significant in CORR, deepening the understanding of organic-cation effect. Overall, I think this paper is worthy of publication after addressing the following minor issues:\n1. The right side of Fig. 1 shows only TMA, but the manuscript compared different organic cations. To be more comprehensive, the scheme should include other cations studied.\n2. The presentation of some graphs should be revised to avoid misunderstandings. For example, the ordinates of Fig. 2f and Fig. 4c should be the C2H4/CH4 FE ratios rather than ambiguous C2H4/CH4. The arrows in Fig. 5e and the vertical lines (intensity scale) in Fig. 5f are not explained."}, "author_response": "Reply to Reviewers’ comments\nReviewer 1:\nThe authors have made some efforts to address reviewer comments but the fundamental issues with the paper regarding a lack of novelty and weaknesses in convincing mechanistic interpretations remain unchanged compared to the first version of the paper. The paper neither makes new interesting observations nor advances the fundamental understanding of cation effects in CO2 reduction. Therefore, in my opinion, this paper is far from meeting the bar for publication in Nature Communications. Below I outline the main issues with the paper.\nReply: Anyway, we are very grateful to the reviewer for reviewing our manuscript.\nReviewer #4 (Remarks to the Author):\nThe manuscript by Xue et al. presents a systematic study on the effect of organic cations in CORR. The authors explore both the cation size effect and the influence of different numbers of hydrophilic hydroxyl-group introductions. They find that the difference in CORR behavior mainly result from the impact of organic cations on interfacial water, causing variations in surface hydrogenation processes. This study is highly significant in CORR, deepening the understanding of organic-cation effect. Overall, I think this paper is worthy of publication after addressing the following minor issues:\nReply: We’re deeply grateful to the reviewer for the recommendations and suggestions.\n1. The right side of Fig. 1 shows only TMA, but the manuscript compared different organic cations. To be more comprehensive, the scheme should include other cations studied.\nReply: As suggested by the Reviewer, we have redrawn Fig. 1 (Fig. R1).\nFig. R1 (Fig. 1). Schematic illustration of the cation effect on CORR using the gas diffusion electrode (GDE) and different electrolytes. The left panel highlights the common focus in the literature on alkali cations in liquid electrolytes, while the right panel investigates the effect of organic cations, generally used in practical AEM electrolyzers.\n2. The presentation of some graphs should be revised to avoid misunderstandings. For example, the ordinates of Fig. 2f and Fig. 4c should be the C H /CH FE ratios rather than ambiguous C H /CH . The arrows in Fig. 5e and the vertical lines (intensity scale) in Fig. 5f are not explained.\nReply: Thanks for the reviewer’s comments. We have revised the ambiguous vertical axis in Fig. 2f (Fig. R2a) and Fig. 4c (Fig. R2b) as pointed out by the reviewer. In addition, the arrows in Fig. 5e (Fig. R2c), which previously indicated the potential-dependent data turning-points, have been deleted. The two vertical lines in Fig. 5f represent the Raman signal intensity, which has been clearly marked in the revised Fig. 5f (Fig. R2d).\nFig. R2 (Fig. 2f, Fig. 4c, Fig. 5e, Fig. 5f). (a) Relationship between the cation gyration radius and the C H /CH FE ratio. (b) C H /CH FE ratios under the influence of organic cations containing different amounts of -OH groups. (c) Ratios of the ν(O-H)/ν(C-H) band area under different potentials. The gray-shaded region denotes the potential range for CORR performance testing. (d) Raman spectra of ν(Cu-OH) and ν(O-H) with peak deconvolution at −0.8 V."}, {"round": 4, "reviewer_comments": {"reviewer_4": "I am very satisfied with the revised version of the author's paper and believe it is now ready for publication."}, "author_response": ""}]

Decision letter