Elucidating the rate-limiting step of CO_2 electroreduction on metal phthalocyanines

Domain: biology
Published: 2026-03-10
Editorial outcome: major_revision

Abstract
Immobilized molecular catalysts, especially metal phthalocyanines, have garnered substantial interest for the electrochemical CO reduction reaction (CO RR) due to their well-defined active sites and promising performance. Yet, the reaction mechanism, particularly the rate-limiting step, remains debated. Here, using electrochemical analysis and kinetic isotope effect measurements, we identify the rate-limiting step of CO RR to CO on immobilized metal phthalocyanines, with Au as a reference. Notably, cobalt phthalocyanine (CoPc) exhibits dispersion-dependent kinetics: protonation of adsorbed *CO is rate-limiting on molecularly dispersed CoPc supported on carbon nanotubes (CoPc/CNTs), whereas CO adsorption becomes rate-limiting on aggregated CoPc due to a weakened interfacial electric field at the Co active sites. This mechanistic distinction further elucidates the role of electrolyte anions: HCO −, largely a spectator on Au, promotes CO RR on CoPc/CNTs by serving as a proton donor in the rate-limiting protonation step. These findings provide mechanistic insights into CO RR on metal phthalocyanines and guide the rational design of molecular electrocatalysts.

Introduction
Electrochemical CO reduction reaction (CO RR) powered by renewable electricity offers a promising pathway toward carbon neutrality by transforming abundant carbon emissions into value-added chemicals.1−3 To achieve the techno-economic viability of this process, it is essential to develop highly active and selective catalysts.4 Metallic materials have been widely employed to catalyze CO RR, resulting in a range of products including CO,5−8 formate,9,10 and multi-carbon hydrocarbons and oxygenates such as ethylene, ethanol, and propanol.11−14 Although substantial progress has been made in improving the CO RR performance of metallic catalysts, the inherent multiplicity of active sites hinders a clear mechanistic understanding and rational catalyst design. Another challenge for metallic catalysts is to achieve high CO RR selectivity due to the competing hydrogen evolution reaction (HER),15,16 especially in acidic media. In contrast, molecular catalysts with well-defined metal centers have demonstrated notable activity for CO RR and exhibit relative inertness toward HER, providing a promising alternative to metallic catalysts.17

Molecular catalysts with a variety of transition metal centers and macrocyclic structures have been investigated for CO RR.18−21 They are typically immobilized on carbon substrates and exhibit promising catalytic performance for the CO RR to CO. For instance, Faradaic efficiencies for CO production exceeding 90% at relatively low overpotentials have been reported for the CO RR on cobalt phthalocyanines (CoPc),22−24 cobalt porphyrins,25,26 cobalt quaterpyridine complexes,27 nickel phthalocyanines (NiPc),28 and functionalized iron porphyrins.29,30 Among these molecules, metal phthalocyanines have been the most extensively studied due to their facile availability, high CO RR activity and capability of producing more reduced products such as methanol.31−35 Despite the advances in catalytic performance, the reaction mechanism, particularly the rate-limiting step, remains elusive for CO RR on metal phthalocyanines.36−39

For CO RR to CO on metallic catalysts, CO adsorption is generally considered to be the rate-limiting step, as schematically illustrated in Fig. 1a. However, on metal phthalocyanines such as CoPc, both CO adsorption and subsequent protonation of adsorbed *CO have been proposed as rate-limiting steps (Fig. 1b). McCrory and co-workers evaluated the kinetic isotope effect (KIE) for the CO RR on CoPc immobilized on a graphite disk electrode.37 They found that the CO RR activity remained nearly identical in aprotic and deuterated phosphate electrolytes, indicative of a rate-limiting step without proton involvement, and consequently, identified the CO adsorption step as rate-determining. A similar result was reported for CO RR on CoPc supported on carbon nanotubes (CNTs).32 In contrast, Chan and co-workers suggested that CO RR on CNT-supported CoPc is limited by the protonation of adsorbed *CO through a combination of computations and pH-dependent measurements.38 This conclusion was supported by the work of Feng et al., in which the KIE studies showed a significantly higher CO RR activity in H SO compared to that in D SO , indicating that the protonation of *CO is the rate-limiting step.39 Given these controversial results, it is crucial to elucidate the rate-limiting step of CO RR on metal phthalocyanines.

In this work, we investigate and elucidate the rate-limiting step of the CO RR to CO on metal phthalocyanines, with Au catalyst as a reference. By combining electrochemical measurements and KIE analysis, we show that the CO RR on molecularly dispersed CoPc and NiPc supported on CNTs (CoPc/CNTs and NiPc/CNTs) is limited by the protonation of adsorbed *CO . Using CoPc as a model system, we further demonstrate that the rate-limiting step of CO RR depends on the catalyst’s dispersion state, shifting to CO adsorption on aggregated CoPc due to a weakened interfacial electric field at the Co active sites. Considering the rate-limiting protonation step on CoPc/CNTs, we further examine the often-overlooked role of electrolyte anions in CO RR and reveal a more pronounced effect of HCO − compared to Na+, in contrast to observations on typical metallic catalysts. The promotional effect of HCO − is attributed to its role as a proton donor in the protonation of *CO . Our work clarifies the reaction mechanism and rate-limiting step of the CO RR on metal phthalocyanines, thereby laying the groundwork for the rational design of efficient molecular electrocatalysts.

Results
Characterization of CoPc/CNTs and NiPc/CNTs catalysts
Molecular catalysts are typically integrated into electrodes by pre-immobilization or direct deposition onto carbon nanomaterials to achieve molecular dispersion.18 When anchored onto CNT supports, the molecular catalysts can exhibit high activity and selectivity for CO RR, which is attributed to the increased exposure of active sites and enhanced electron transfer.18 Therefore, we prepared CoPc/CNTs and NiPc/CNTs samples using a well-established anchoring strategy (see Methods for details).31 To contextualize our findings within the broader CO RR field, we selected Au as a benchmark catalyst. Therefore, Au nanoparticles supported on carbon black (Au/C) were also prepared to serve as a reference due to its well-understood catalytic mechanism for CO RR. The three samples were characterized for their morphology, structure, and chemical state. As shown in Fig. 2a,d, scanning transmission electron microscopy (STEM) images and corresponding energy-dispersive X-ray spectroscopy (EDS) mapping indicate a homogeneous dispersion of CoPc and NiPc on CNTs in the CoPc/CNTs and NiPc/CNTs samples, respectively. High-resolution X-ray photoelectron spectra (XPS) of Co 2p and N 1s regions (Fig. 2b,c) as well as Ni 2p and N 1s regions (Fig. 2e,f) confirm the presence of CoPc and NiPc molecules in the samples. Moreover, both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images indicate that the morphology of CoPc/CNTs and NiPc/CNTs resembled that of the pristine CNTs, showing no visible molecular aggregates (Supplementary Fig. 1, 2). Their X-ray diffraction (XRD) patterns further support the absence of crystalline aggregates of CoPc and NiPc as their characteristic diffraction peaks were not observed (Supplementary Fig. 1, 2). Collectively, these results confirm a molecular dispersion state of CoPc and NiPc on CNTs. For the Au/C nanopowder, TEM, XRD, and XPS characterizations indicated an average particle size of around 5 nm and a metallic state of the Au nanoparticles, as shown in Supplementary Fig. 3. Each of the three samples was deposited onto an AvCarb GDS2230 carbon substrate to form the corresponding electrode.

Identification of the rate-limiting step
To identify the rate-limiting step of CO RR, we employ the KIE method, as it can determine proton involvement in the key step, thereby distinguishing between the steps of CO adsorption and protonation of adsorbed *CO . The magnitude of KIE is defined by j /j , where j is the partial current density for a specific product in a protic electrolyte and j is that in a deuterated electrolyte.37 If the rate-limiting step is a protonation process, a KIE value >1 will be observed due to the sluggish dissociation of deuterated water and deuteron transfer process. In contrast, a rate-limiting step without proton involvement should result in a KIE value of around 1. To evaluate the KIE, CO RR bulk electrolysis was performed in a two-compartment electrochemical cell (H-cell, Supplementary Fig. 4) with 1 M NaHCO in H O and 1 M NaDCO in D O electrolytes, respectively. All potentials were 100% iR-compensated using the current-interrupt method (unless otherwise specified) and are reported on the reversible hydrogen electrode (RHE) scale throughout this work. Gas-phase products, including CO and H , were quantified using gas chromatography (GC), and liquid-phase products were analyzed by 1H nuclear magnetic resonance (NMR) spectroscopy.

As shown in Fig. 3a, in both NaHCO /H O and NaDCO /D O electrolytes, the partial current density for CO production on the Au/C electrode gradually increased from 3.8 to 10.9 mA cm−2 as the potential shifted from −0.45 to −0.60 V versus RHE, resulting in KIE values of around 1 (Fig. 3b). Meanwhile, the CO Faradaic efficiency was 86%−89% in the NaHCO /H O electrolyte, but increased to 95%−97% in NaDCO /D O, as presented in Fig. 3c. This is because the D partial current densities in NaDCO /D O were significantly lower than H in NaHCO /H O, as shown in Supplementary Fig. 5, leading to a higher CO Faradaic efficiency in NaDCO /D O. This confirms the reliability of the KIE method for determining proton involvement in the rate-limiting step. The result indicates that the CO RR on Au nanoparticles is limited by the CO adsorption step without proton involvement, in consistent with previous studies.38,40 In contrast, the CoPc/CNTs electrode exhibited consistently higher CO partial current densities in NaHCO /H O than in NaDCO /D O over the potential range of −0.55 to −0.70 V versus RHE (Fig. 3d), leading to KIE values of 1.4~1.5 (Fig. 3e). The CO Faradaic efficiencies on CoPc/CNTs in NaDCO /D O were higher than those in NaHCO /H O (Fig. 3f), which is similar to the case of Au catalyst, due to the lower D production rates in NaDCO /D O. These results suggest that the protonation of adsorbed *CO is the rate-limiting step of CO RR on CoPc/CNTs. Similarly, on the NiPc/CNTs electrode, CO production rate was generally higher in NaHCO /H O than in NaDCO /D O, yielding KIE values of 1.3~1.5, as shown in Fig. 3g,h. Meanwhile, NiPc/CNTs exhibited even high selectivity for CO RR, with negligible H detected (Fig. 3i). Therefore, the rate-limiting step on NiPc/CNTs is the protonation of adsorbed *CO , consistent with previous studies showing a high energy barrier for this step at the Ni−N sites.20,38 In addition, the KIE was evaluated for the concurrent HER during the CO electrolysis. As shown in Supplementary Fig. 5, the H partial current densities in NaHCO /H O were consistently higher than the corresponding D partial current densities in NaDCO /D O for all catalysts, demonstrating the reliability of the KIE method in probing proton involvement in the rate-limiting step.

The above studies were performed in near-neutral electrolytes, while electrolyte pH may also influence the rate-limiting step of CO RR on metal phthalocyanines. Therefore, we further examined the rate-limiting step of CO RR on CoPc/CNTs in acidic media (1 M NaClO /H O and 1 M NaClO /D O, pH/pD = 2) using KIE analysis. As exhibited in Supplementary Fig. 6, the partial current densities for CO production in NaClO /H O were consistently higher than those in NaClO /D O, yielding KIE values of 1.3~1.7. This indicates that the rate-limiting step of CO RR on metal phthalocyanines is not altered by pH variation. In addition, methanol can be formed as a liquid product during CO RR on CoPc/CENTs,31 which was detected in 1 M NaHCO electrolyte at more negative potentials (< −0.80 V versus RHE), as shown in Supplementary Fig. 7. However, methanol formation occurs only at high overpotentials with relatively low selectivity compared to CO production. The possible divergence between our methanol production performance and prior reports likely arises from the high sensitivity of this pathway to experimental parameters, such as CoPc-to-CNT mass ratio, catalyst loading, and CO partial pressure.31,33−35 Given that the present work is centered on CO RR to CO on metal phthalocyanines and aims to elucidate the fundamental rate-limiting steps governing this primary reaction pathway, the following studies will focus on the CO -to-CO conversion.

Effect of molecular dispersion state on the rate-limiting step
As previously discussed, the rate-limiting step of CO RR on CoPc catalyst remains controversial, with prior studies also identifying CO adsorption as the rate-limiting step using the KIE method.37 This discrepancy could stem from variations in CoPc immobilization methods, which lead to different dispersion states that have been reported to influence CO RR activity and selectivity.18,33 For instance, the commonly used method of directly depositing CoPc dissolved in N,N-Dimethylformamide (DMF) onto substrates may cause aggregation of CoPc molecules due to the low surface area of substrates. To test this hypothesis, we prepared an additional CoPc electrode by directly depositing a DMF solution of CoPc onto an AvCarb GDS2230 carbon substrate. SEM characterization was then performed to examine its morphology. As shown in Supplementary Fig. 8, the electrode exhibited newly formed nanoparticles and nanoflakes, ranging from submicrometers to a few micrometers, on the substrate, suggesting the formation of CoPc aggregates. The aggregated CoPc electrode was then evaluated for CO RR, which showed a lower activity compared to the above-tested CoPc/CNTs electrode, as exhibited in Fig. 4a. More importantly, the CO partial current densities in NaHCO /H O and NaDCO /D O were nearly identical, resulting in KIE values of around 1, as shown in Fig. 4b. The CO Faradaic efficiencies were all greater than 90% (Fig. 4c), suggesting that CO RR was the dominant reaction. In contrast, the concurrent HER activities in the two electrolytes still showed a notable difference, yielding KIE values greater than 3 and indicating reliable KIE measurements (Supplementary Fig. 9). The results confirm that the rate-limiting step of CO RR on the aggregated CoPc electrode does not involve protons and should be the CO 2 adsorption step.

To further examine the possible influence of CNTs on the above-observed difference in the rate-limiting step, we prepared a control sample by mixing CoPc powders and CNTs, which will hereafter be referred to as the “CoPc/CNTs mixture” sample. As shown in Supplementary Fig. 10, SEM imaging revealed the formation of CoPc aggregates of a few micrometers in size, with their crystalline structure confirmed by the XRD pattern. The CoPc/CNTs mixture sample was then deposited onto an AvCarb GDS2230 substrate to form an electrode and evaluated for the KIE in CO RR. As exhibited in Fig. 4d−f, the sample showed comparable CO RR performance, including the partial current density and Faradaic efficiency for CO production, and the similar KIE values (~1) indicate that the CO RR has the same rate-limiting step — CO adsorption. Meanwhile, the concurrent HER showed a typical KIE on the CoPc/CNTs mixture electrode (Supplementary Fig. 11). Collectively, the differing rate-limiting steps of CO RR on CoPc molecules can be attributed to their distinct dispersion states, which may account for discrepancies among previous studies. This correlation likely arises from variations in the electric field around the Co centers in CoPc molecules, as will be further discussed below.

Prior to investigating the underlying mechanism, it is essential to establish that the observed differences in CO RR activity between NaHCO /H O and NaDCO /D O do not arise from extrinsic factors, such as differences in CO solubility or the pK values of H O versus D O. First, the molar solubilities of CO in H O and D O at 25 ℃ and 1 atm are both ~33.7 mmol L⁻1,41 which cannot account for the pronounced CO RR activity differences. Moreover, in 1 M NaHCO /H O and 1 M NaDCO /D O electrolytes, the equilibrium between dissolved CO and bicarbonate species (CO + H O ⇌ H⁺ + HCO ⁻, CO + D O ⇌ D+ + DCO ⁻) can continuously buffer and replenish CO near the cathode, ensuring a sustained and comparable CO supply for the CO RR. Consequently, CO solubility or availability can be excluded as the origin of the observed activity differences in the KIE studies.

Second, H O and D O exhibit a modest difference in their acid–base properties (pK = 14.00 for H O and 14.86 for D O), resulting in bulk pH/pD values of 7.4 and 7.8 for CO -saturated 1 M NaHCO and NaDCO electrolytes, respectively. In the above measurements, all potentials were converted to the RHE scale using the corresponding bulk pH, thereby accounting for pH-dependent thermodynamic effects in the comparison of CO RR activities. Nevertheless, local pH may change during reactions and influence reaction kinetics. To isolate genuine isotope effects from possible pH-induced artifacts, we employed in situ scanning electrochemical microscopy (SECM) to probe local pH under operando conditions. As shown in Supplementary Fig. 12, a customized SECM setup,42 equipped with an Au nanoelectrode tip whose exposed apex was functionalized with 4-hydroxylaminothiophenol (4-HATP)/4-nitrosothiophenol (4-NSTP), was used to measure the local pH near the cathode.43,44 A correlation between the mid-peak potential of the 4-HATP/4-NSTP anodic peak and the electrolyte pH was established as a calibration curve (Supplementary Fig. 12). Using this approach, we quantified the local pH near the CoPc/CNTs electrode under various CO RR current densities, as presented in Supplementary Fig. 13. While a slight increase in local pH was observed during CO RR in both electrolytes, the relative difference between them remained ~0.4 pH units, consistent with the initial bulk pH difference (7.4 versus 7.8). Because this initial difference was already accounted for in the RHE-referenced potentials, the minor local pH shifts do not exert any additional influence on the KIE analysis.

Mechanisms underlying distinct rate-limiting steps
The above electrochemical measurements and KIE analysis indicate that the rate-limiting step for CO RR to CO on CoPc molecules depends on their dispersion state. For molecularly dispersed CoPc on CNTs, the reaction is limited by the protonation of *CO , whereas for aggregated CoPc, CO adsorption becomes the rate-limiting step. We hypothesize that the shift of the rate-limiting step on aggregated CoPc arises from a weakened interfacial electric field at the Co active sites. Bulk CoPc, as an organic semiconductor, exhibits low conductivity and pronounced dielectric behavior,45,46 which limits penetration of the applied electric field into thick CoPc aggregates.33 Consequently, the Co active sites in aggregated CoPc experience a weaker electric field than those in molecularly dispersed CoPc, reducing the driving force for initial CO activation and shifting the rate-limiting step. The change in the electric field at Co sites upon CoPc aggregation was revealed by Stark tuning, based on the potential-dependent frequency shifts of reaction intermediates in sum frequency generation spectra.33 Here we perform further mechanistic investigations to verify the hypothesis.

First, we compared the Co(II)/Co(I) redox responses of the CoPc/CNTs electrode and the CoPc/CNTs mixture electrode to examine the potential-screening effect due to CoPc aggregation. Because CNTs can introduce substantial capacitive currents and parasitic Faradaic contributions, we employed square-wave voltammetry (SWV), which can suppress the charging currents while providing high sensitivity.47,48 A representative SWV waveform is illustrated in Fig. 5a. As shown in Fig. 5b, molecularly dispersed CoPc on CNTs experiences a strong local electric field, resulting in a pronounced and well-defined Co(II)/Co(I) redox peak at around 0 V vs RHE, consistent with previous reports.34 In contrast, despite identical CoPc loadings, the redox peak of the aggregated CoPc is significantly attenuated compared to that of the molecularly dispersed CoPc, indicating a suppression of the Co(II)/Co(I) redox response due to aggregation. When a potential is applied to the electrode, the electric field does not fully penetrate the aggregates, limiting effective potential transmission to the Co active sites and resulting in a substantially attenuated redox signal.

To further examine whether attenuation of the electric field shifts the rate-limiting step to CO adsorption, we intentionally reduced the local electric field around molecularly dispersed CoPc by chelating Na⁺ in the electrolyte using the crown ether 18-crown-6.49 Chelation of Na⁺ effectively increases the apparent cation size and expands the thickness of the outer Helmholtz layer,50,51 thereby reducing the potential gradient and interfacial electric field, as schematically illustrated in Fig. 5c. KIE studies were then performed on the CoPc/CNTs electrode in 1 M NaHCO /H O + 18-crown-6 and in 1 M NaDCO /D O + 18-crown-6 electrolytes. As shown in Fig. 5d, the partial current densities for CO production greatly decreased in the NaHCO + 18-crown-6 electrolyte compared to those in NaHCO (Fig. 3d), indicating reduced activity due to the weakened electric field. Importantly, the CO RR activities in crown-ether-containing H O and D O electrolytes were nearly identical, giving KIE values of ~1 (Fig. 5e). These results indicate that upon weakening the electric field, the rate-limiting step shifts to CO adsorption even for molecularly dispersed CoPc. In contrast, KIE values of ~3 were observed for the concurrent HER (Supplementary Fig. 14), confirming the reliability of KIE analysis in the presence of crown ether. Overall, these findings demonstrate that the Co active sites in aggregated CoPc experience a weakened electric field so that CO adsorption becomes rate-limiting. This provides a clear mechanistic explanation for the dependence of the CO RR rate-limiting step on the dispersion state of CoPc.

Effect of electrolyte anions on the CO RR with CoPc/CNTs
The above insights into the rate-limiting step of CO RR on CoPc catalysts motivated an investigation of electrolyte anion effects, which have been relatively overlooked compared with cations. It is well-established that on typical metallic catalysts, such as Au and Ag, CO RR relies on metal cations to facilitate the rate-limiting CO adsorption by stabilizing adsorbed *CO via electrostatic interaction, resulting in an apparent dependence on the cation identity and concentration.52−54 In contrast, anions, including the widely used HCO −, act more as a spectator without involvement in the rate-limiting step, as evidenced by the zero reaction order measured for CO RR on Au.55,56 Given the distinct rate-limiting step on CoPc/CNTs, the effects of cations and anions on CO RR are expected to differ from those observed on metal catalysts. We thus evaluated the dependence of CO RR activity on the concentrations of Na+ and HCO −, respectively. To ensure a rigorous comparison, the concentration of HCO − was held constant while varying Na+, and vice versa. Electrolyte electroneutrality was maintained by adding ClO − as necessary.

As shown in Fig. 6a, the CO partial current density on CoPc/CNTs at −0.65 V versus RHE increased from 3.1 to 4.7 mA cm−2 as the Na+ concentration increased from 0.1 to 1.0 M, yielding a reaction order of 0.18 (Fig. 6b). This cation effect likely arise from the promoted hydrolysis of the Na+ hydration shell,32 which can facilitate the rate-limiting protonation step of adsorbed *CO . Interestingly, the CO RR activity on CoPc/CNTs also increased with the HCO − concentration, even more pronounced than that for Na+, resulting in a higher reaction order of 0.39. These results indicate that the CO RR on CoPc/CNTs exhibits a stronger dependence on HCO − anion than on Na+ cation, in sharp contrast to the behavior observed for metal catalysts.55,56 This should be attributed to the promotional effect of HCO − on the rate-limiting protonation step of adsorbed *CO on CoPc/CNTs, as HCO − can act as a proton donor in both CO RR and HER, as reported in previous studies.57−59

To further examine this effect, we selected anions with different proton-donating abilities, including ClO − (non-proton-donating), HCO − (pK ~ 10.3), and H PO −/HPO 2− (pK ~ 7.2), thus to compare their influence on the CO RR with CoPc/CNTs. The comparative study was performed in electrolytes containing 1 M Na+ paired with different anions. As shown in Fig. 6c, the CO RR activity within the potential range of −0.55 to −0.90 versus RHE follows the trend: ClO − < H PO −/HPO 2− ~ HCO −, reflecting differences in proton-donating ability, though not strictly mirroring their pK values. This can be explained by including the concurrent HER activity during the electrolysis, which follow the trend: ClO − < HCO − < H PO −/HPO 2− (Fig. 6d). As a result, the H PO −/HPO 2− electrolyte showed the highest total current density (Supplementary Fig. 15), consistent with its stronger proton-donating capacity, although a larger fraction of the available protons was consumed by the HER, leading to lower Faradaic efficiencies for CO production. In addition, the CO RR activity with HCO − became higher than that with H PO −/HPO 2− at more negative potentials (Fig. 6c). This is attributed to the distinct role of HCO − as a CO shuttle, which can increase the local concentration of CO and the CO RR activity,36,60 especially when the mass transport limit is approached at high reaction rates in H-cell. These results indicate that the CO RR activity on CoPc/CNTs increases with the proton donation ability of electrolyte anions, confirming their role as a proton donor in the rate-limiting protonation step of adsorbed *CO .

The proton-donating ability of the anions may not only facilitate protonation steps in CO RR, but also provide buffering capacity to mitigate local pH changes during reactions.61,62 To assess the contribution of such mechanistic factors, we employed the SECM with pH-sensitive probes to monitor the local pH during CO RR in different electrolytes. As shown in Supplementary Fig. 16a, the local pH near the CoPc/CNTs electrode during CO RR in NaClO increased markedly, from an initial bulk pH of 4.3 to a local pH of 8.6 at a current density of 5.0 mA cm−2, reflecting the absence of buffering capacity in NaClO . In contrast, the pH rise in NaHCO was much smaller (Supplementary Fig. 13) due to its buffering nature. To account for pH-dependent thermodynamic effects when comparing CO RR activities, we re-calculated the activities using RHE-referenced potentials corrected by the measured local pH values (Supplementary Fig. 16b). Even after this correction, the CO RR activities in NaHCO remained consistently higher than those in NaClO , indicating that differences in buffering or local pH do not explain the variation in CO RR activity among the anions. Instead, the higher CO RR activity in NaHCO is mainly attributed to enhanced *CO protonation on CoPc/CNTs due to the proton-donating ability of the HCO − anion.

These findings suggest a strategy for enhancing CO electrolysis systems with a rate-limiting protonation step through the use of proton-donating anions.61,62 To further demonstrate this anion strategy and its applicability, we evaluated CO electrolysis in presence of different anions using a flow cell equipped with gas-diffusion electrodes (GDEs), where the CO RR current density can be drastically increased due to enhanced mass transport.63 Indeed, as shown in Supplementary Fig. 17, the NaHCO electrolyte achieved the highest activity for CO RR on CoPc/CNTs, followed by NaH PO /Na HPO and NaClO . Moreover, given the negatively charged cathode during CO RR, we expect electrolyte cations capable of donating protons to be more effective in promoting the rate-limiting protonation step. Such cations can enter and accumulate in the electric double layer, allowing direct interactions with surface-bound intermediates and thereby enabling more efficient proton delivery to facilitate *CO protonation.

Methods
Chemicals. Ethanol (99.5%, anhydrous, 200 proof), 2-propanol (HPLC), dimethylformamide (DMF, Certified ACS), hydrochloric acid (HCl, 34−37%, trace-metal grade), sulfuric acid (H SO , 98%), cobalt (Ⅱ) phthalocyanine (CoPc, C H N Co), tetrachloroauric acid trihydrate (HAuCl ·3H O, Certified ACS), sodium citrate (C H Na O , 99+%), tannic acid (C H O , Certified ACS), hydrogen peroxide (H O , 30%, Certified ACS), sodium carbonate (Na CO , 99.95%), sodium bicarbonate (NaHCO , 99.7+%), sodium perchlorate (NaClO , ACS Grade), sodium phosphate monobasic monohydrate (NaH PO ·H O, 99+%), sodium phosphate dibasic heptahydrate (Na HPO ·7H O, 99+%), deuterium oxide (D O, 99.8 atom% D), titanium gauze (80 mesh), iridium(IV) chloride (IrCl ·xH O), 18-Crown-6 (99%), and Au wires (0.5 mm diameter, 99.95%) were purchased from Fisher Scientific. Nickel (Ⅱ) phthalocyanine (NiPc, C H N Ni), 4-nitrothiophenol (4-NTPR), and Nafion perfluorinated resin solution (5 wt%) were purchased from Sigma Aldrich. Multi-Walled carbon nanotubes (US4403, purity: > 99.9 wt%, outside diameter: 10−20 nm) were purchased from US Research Nanomaterials, Inc. Nafion 115 and 117 membranes, Vulcan XC-72 carbon black, and AvCarb GDS2230 carbon substrates were purchased from Fuel Cell Store. Ag/AgCl reference electrodes (MF-2056, 3M KCl) were purchased from BASi Research Products. Leak-free Ag/AgCl reference electrodes (LF-1-100) were purchased from Innovative Instruments. Ar (99.999%), H (99.999%), CO (99.995%), and CO (99.9%) gases were purchased from Airgas. Deionized water with a specific resistance of 18.2 MΩ·cm was used in all experiments.

Preparation of catalysts and electrodes. The CoPc/CNTs sample was prepared using a method reported in previous studies.22,31 Commercial CNTs were purified by heat treatment in air for 5 h, followed by sonication in a 5 wt% HCl solution for 30 min. The purified CNTs were collected by filtration, washed with deionized water, and dried at 60 °C. 50 mg of the purified CNTs and 1.5 mg of CoPc were each dispersed in 20 mL DMF and sonicated for 1 h. The two dispersions were then mixed, sonicated for 30 min, and stirred at room temperature (25 °C) for 20 h. The resulting solid was collected by centrifugation, washed with DMF and ethanol, and lyophilized to obtain the CoPc/CNTs sample. The CoPc content in the sample was determined to be 0.96 wt% using inductively coupled plasma optical emission spectroscopy (ICP-OES). The NiPc/CNTs sample was prepared using the same procedure as for CoPc/CNTs, with NiPc substituted for CoPc. The NiPc content in the NiPc/CNTs sample was determined to be 0.84 wt% using ICP-OES. To prepare the CoPc/CNTs mixture sample, 49.5 mg of purified CNTs and 0.5 mg CoPc were grinded together in a motor for 30 min. Catalyst inks were prepared by mixing 5.6 mg of each sample (CoPc/CNTs, NiPc/CNTs, and the CoPc/CNTs mixture) with 1.6 mL of 2-propanol and 45 µL of Nafion solution (5 wt%), followed by sonication for 1 h. The inks were then drop-cast onto an AvCarb GDS2230 carbon substrate (0.71 × 0.71 cm2; same dimensions for all electrodes used in the H-cell studies) until a loading of 0.5 mg cm−2 (geometric area basis) was reached, resulting in the final electrodes. The aggregated CoPc electrode was prepared by drop-casting a DMF solution of CoPc with a concentration of 0.1 mg mL−1 onto an AvCarb GDS2230 carbon substrate until a loading of 0.005 mg cm−2 was reached. The CoPc loading on the CoPc/CNTs, CoPc/CNTs mixture, and aggregated CoPc electrodes was kept nearly identical for a rigorous comparison of their catalytic performances.

The Au/C nanopowder was prepared using a two-step method, including the synthesis of Au nanoparticles and subsequent loading onto carbon support. The Au nanoparticles were synthesized following the procedure described in a previous study.64 1 mL of tetrachloroauric acid (HAuCl , 25 mM) was injected into a mixed solution containing 150 mL of sodium citrate (2.2 mM), 0.1 mL of tannic acid (2.5 mM), and 1 mL of Na CO (150 mM) at 50 °C. The resulting solid was collected by filtration, washed by deionized water, and freeze-dried to obtain the Au nanoparticles. 45 mg of Vulcan X-72 carbon black and 5 mg of the prepared Au nanoparticles were dispersed in 20 mL of deionized water and sonicated for 1 h. The dispersion was filtered, and the precipitate was washed with deionized water and dried at 60 °C overnight to obtain the Au/C sample. To prepare Au/C electrodes, 5.6 mg of the Au/C sample, 1.6 mL of 2-propanol, and 45 µL of Nafion solution (5 wt%) were mixed and sonicated for 1 h to from a catalyst ink. The ink was then drop-cast onto an AvCarb GDS2230 carbon substrate until a loading of 1.0 mg cm−2 was reached.

Materials characterization. SEM images were acquired using a ZEISS Ultra-55 FEG scanning electron microscope. TEM images, STEM images and corresponding EDS mapping were acquired using a FEI Tecnai F30 transmission electron microscope with a field emission gun operated at 200 kV. XRD patterns were collected using a PANalytical Empyrean diffractometer with a 1.8 KW copper X-ray tube. XPS spectra were acquired using a Thermo Scientific ESCALAB XI+ X-ray photoelectron spectrometer with an Al Kα X-ray source (1486.67 eV).

Electrochemical measurements. All electrochemical measurements were performed using a Gamry Interface 1000 potentiostat with a gas-tight two-compartment electrochemical cell (H-cell, Supplementary Fig. 4) at room temperature (25 °C). The above prepared electrodes were used as cathodes, and an Ag/AgCl electrode and a piece of Pt gauze were used as the reference and counter electrodes, respectively. A Nafion 115 membrane (thickness: 127 µm) was used to separate the cathode and anode chambers. Prior to use, Nafion 115 membranes were sequentially heat-treated at 80 °C in 5% H O , 0.5 M H SO , and deionized water for 1 h each, then thoroughly rinsed and stored immersed in deionized water until use. The catholyte and anolyte each contained 15 mL of electrolyte. Four electrolyte compositions were used: 1 M NaHCO , 1 M NaDCO in D O (KIE experiments), 1 M NaClO , and a phosphate buffer (0.3 M NaH PO + 0.35 M Na HPO ). Electrolytes were prepared in H O unless otherwise noted. The NaDCO /D O electrolyte was prepared by bubbling CO gas through a Na CO solution in D O. Prior to CO RR testing, the catholyte was purged with CO for 30 min and then continuously purged at a rate of 5 sccm during bulk electrolysis while stirring at 600 rpm; the CO gas flow rate was controlled and monitored using an Alicat Scientific mass flow controller. The GDE-cell studies were carried out using a home-built GDE flow cell,63 which consists of a Ti current collector with interdigitated gas-diffusion channels, a cathodic GDE with the catalyst layer deposited on AvCarb GDS2230 substrate, a 3D-printed chamber with ports for electrolyte flow and reference electrode, and an IrO /Ti gauze anode inserted in a pocket of another Ti current collector. A leak-free Ag/AgCl electrode was used as the reference electrode. The catholyte and anolyte (15 mL each) were circulated using peristaltic pumps at 2.0 mL min−1. All potentials were 100% iR-compensated using the current-interrupt method (unless otherwise specified) and are reported versus RHE. Potentials were measured versus an Ag/AgCl (3 M KCl) reference electrode and converted to the RHE scale using E = E + 0.210 V + 0.0591 × pH. The reference electrode was calibrated using a standard hydrogen electrode before measurements. The reported current densities were normalized to the geometric surface areas of the electrodes. During CO electrolysis, gas-phase products were quantified by a gas chromatograph (SRI Multiple Gas Analyzer #5) equipped with molecular sieve 5A and HayeSep D columns with Ar carrier gas. Liquid-phase products in the catholyte were analyzed by 1H NMR spectroscopy (Bruker Avance III 500 MHz) after electrolysis.

Calculation of CO RR activity and Faradaic efficiencies. The partial current densities for CO and H production were calculated from the GC peak area as follows:
j = peak area × (gas flow rate) × nFp × (electrode area)−1 (1)
partial α RT
where α is a gas-dependent conversion factor determined by GC calibration, n is the number of electrons transferred for the product, F is the Faraday constant, R is the ideal gas constant, p and T are the gas pressure and temperature, and the gas flow rate is the CO feed rate delivered to the electrochemical cell (H-cell or GDE flow cell). The Faradaic efficiencies for CO and H production were calculated by dividing the corresponding j by the total current density.
partial

Probing local pH using SECM. Local pH measurements were performed using a CH Instruments 920D SECM instrument equipped with three-directional stepper motor and piezoelectric actuators for coarse control and fine positioning of the SECM probe, respectively. A bipotentiostat was used to control the potentials applied to the substrate working electrode (CoPc/CNTs on glassy carbon) and the SECM probe. The Au nanoelectrode SECM probe was fabricated by electrochemical etching of an Au wire to form a nanoscale apex radius, followed by electropolymerization coating to insulate the shaft while leaving the Au apex exposed.42 The exposed Au apex was functionalized with a self-assembled monolayer of 4-nitrothiophenol (4-NTP), which was then electrochemically converted into the 4-HATP/4-NSTP redox couple.43,44 The Au nanoelectrode design ensures that only the tip apex serves as the active sensing area, enabling local pH measurements with minimal perturbation to the reaction environment. To maintain consistent probe–substrate separation across measurements, the Au nanoelectrode probe was first brought to electrical contact with the substrate using the piezoelectric fine-approach routine and then retracted by 1 μm and held at that position during pH measurements under CO RR conditions. Local pH was quantified by recording cyclic voltammograms at the Au nanoelectrode probe and monitoring the Nernstian shift of the mid-peak potential of the surface-bound 4-HATP/4-NSTP redox couple (Supplementary Fig. 12).

Data availability. The data that support the findings of this study are available in the article and its Supplementary Information file. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This article by Ren et al. presents a mechanistic understanding on the rate-limiting step of CO2 reduction over MPc catalysts. The dispersion state of MPc and electrolyte anions are considered to be important factors for the rate-limiting steps. KIE studies were used as the main method to determine whether the RDS involves a hydrogenation step. Yet, I feel that the manuscript is now too preliminary to be published. And, there are more issues regarding to the key points of MPc catalysts used in CO2 electroreduction. The novelty and quality of this manuscript could not guarantee its publication in Nature Communications.\nAdditional comments below may help the authors to improve the manuscript.\n1. The interesting point of MPc such as CoPc is that methanol can be produced as a major product when being loaded onto carbon nanotubes, rather than CO (refs.31-35). But it is not the case when being loaded on carbon black. I think revealing the underlying mechanism should make more sense.\n2. The CoPc catalyst for both CO and methanol production has been well studied previously, even for the initial CO2 to COOH step experimentally and theoretically, in spite of less KIE method. It is not surprised that the rate-limiting step over MPc is different from that over Au electrode.\n3. With KIE experiments, the authors just demonstrate the possible RDS, but they failed to explain well the mechanism shift especially the role of dispersion on carbon support. The role of local electric field should be revealed with direct evidences, rather than discussing based on literature.\n4. The role of anions including bicarbonate ions has been well documented in the literature. Further, it seems that the authors’ logic just lies in the proton donating ability for a given hydrogenation step.\n5. When conducting KIE studies, how did the authors exclude the effect of mass transport?", "reviewer_2": "This study provides valuable mechanistic insights into the dispersion-dependent rate-limiting steps of CO2RR on CoPc catalysts, supported by electrochemical and KIE analyses. The identification of HCO3-1s proton donor role in molecularly dispersed systems is particularly noteworthy. However, several critical issues require careful addressing to fully substantiate the conclusions. With these revisions, the work could offer a more robust foundation for molecular catalyst design. Consideration for publication would be appropriate after major revision.\nSpecific comments:\n1. The KIE experiments conducted in NaHCO3 and NaDCO3 systems may be affected by the significant pKa difference between D2O and H2O, leading to pH shifts that influence reaction kinetics. The authors should clarify how pH was controlled (e.g., buffer capacity adjustment or in situ monitoring) to isolate genuine isotope effects from pH-induced artifacts.\n2. While SEM/XRD data characterize CoPc aggregation, claims about \"electronic state changes\" and \"positioning farther from OHP\" lack direct evidence. For example, XANES/EXAFS (electronic structure) and TOF-SIMS (interface distribution) should be added to support the proposed dispersion-dependent RLS mechanism.\n3. The \"proton donor\" function of anions near the electrode interface may be constrained by electric double-layer structure (e.g., specific adsorption competition). A Gouy-Chapman-Stern model analysis is needed to distinguish intrinsic proton-donating capacity from interfacial delivery efficiency.\n4. The HCO3- proton donor mechanism depends not only on pKa but also on local pH, buffer capacity, and applied potential. In situ techniques (such as SEIRAS, pH-sensitive probes) should be employed to decouple these intertwined factors.\n5. The difference of CO2 solubility in D2O versus H₂O may artificially reduce current density in isotope experiments. Normalizing jCO values is essential for reliable KIE conclusions.\n6. Anion comparisons in H-cells may not reflect practical electrolyzer conditions (flow cells, high current densities). Additional experiments are recommended to strengthen the applicability of the findings.\n7. The focus on Co/Ni phthalocyanines lacks perspective on generalizability to other metal centers (Fe, Mn) or ligand frameworks (porphyrins, terpyridines). Expanding the discussion on design principles for broader applicability would enhance impact.\n8. All potentials were reported against the SHE. It is intriguing to know how the experiment setup was constructed and how different pH impact was introduced in to the performance reporting.\n9. Why was no liquid product detected from CoPc system?\n10. Were the CoPc and NiPc molecules molecularly dispersed or forming aggregates on the CNT surface? Detailed characterization results should be supplied.", "reviewer_3": "This manuscript examines the reaction mechanism, specifically the rate-limiting step, of the electrochemical conversion of CO2 to CO using metal phthalocyanine catalysts. The rate-limiting step is identified as the protonation of adsorbed *CO2 on dispersed CoPc-supported CNTs. In contrast, on aggregated CoPc, CO2 adsorption becomes the rate-limiting step. The authors investigated how the dispersion rate and electrolyte components (anions and cations) affect the CO2RR to CO on CoPc/CNTs, thereby guiding the rational design of molecular electrocatalysts. Overall, this manuscript offers sufficient evidence to clarify the rate-determining step of CO2RR to CO on metal phthalocyanine catalysts. Major Revision. Some specific comments are provided below.\n1. I would recommend that authors add in-situ microscopy evidence to monitor the reaction intermediates to support the mechanism study further.\n2. Except for the effects of anions and cations, will the pH variation affect the rate-limiting step on metal phthalocyanine?", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "Response to Referees\nReviewer #1:\nThis article by Ren et al. presents a mechanistic understanding on the rate-limiting step of CO\n2\nreduction over MPc catalysts. The dispersion state of MPc and electrolyte anions are considered\nto be important factors for the rate-limiting steps. KIE studies were used as the main method to\ndetermine whether the RDS involves a hydrogenation step. Yet, I feel that the manuscript is now\ntoo preliminary to be published. And, there are more issues regarding to the key points of MPc\ncatalysts used in CO electroreduction. The novelty and quality of this manuscript could not\n2\nguarantee its publication in Nature Communications.\nResponse: We thank the reviewer for the constructive comments and suggestions, which have\nhelped us improve the manuscript. In response, we have performed many new experiments with\nnew data and discussions to deepen the mechanistic understanding and strengthen the manuscript.\nOur major revisions include:\n(1) We have gained a deeper understanding of the mechanism and attributed the shift in the rate-\nlimiting step on aggregated CoPc to a weakened interfacial electric field at the Co active sites,\nas supported by solid evidence presented in Figure 5 of the revised manuscript.\n(2) We have employed in situ scanning electrochemical microscopy (SECM) to probe the local\npH under operando conditions and have directly addressed all comments related to local pH,\nas demonstrated by the results in Supplementary Figs. 12, 13, and 16.\n(3) We have addressed your other comments by adding new results and discussions in the revised\nmanuscript.\nWith the substantial improvements, our study now elucidates the previously debated rate-limiting\nstep of CO RR on metal phthalocyanines, provides new mechanistic insights into the interfacial\n2\nelectric field and the local pH during CO RR, establishes molecular dispersion as a key structural\n2\nparameter, and offers guidance for the rational design of efficient molecular electrocatalysts. These\nadvances provide strong evidence for the novelty and quality of our revised manuscript.\nOur point-to-point responses to the comments and corresponding revisions are described below.\nA copy of the revised manuscript with all changes highlighted using “Track Changes” function is\nprovided to facilitate review.\nAdditional comments below may help the authors to improve the manuscript.\n1. The interesting point of MPc such as CoPc is that methanol can be produced as a major product\nwhen being loaded onto carbon nanotubes, rather than CO (refs. 31-35). But it is not the case when\nbeing loaded on carbon black. I think revealing the underlying mechanism should make more sense.\nResponse: We thank the reviewer for this comment. First, we agree that methanol (CH OH)\n3\nformation on CoPc is intriguing, as CH OH is a more reduced and valuable product than CO and\n3\nis rarely obtained with high selectivity on other electrocatalysts. However, upon surveying the\nliterature, we found pronounced variability in the reported CH OH production performance on\ndensities for CH OH vary widely (from negligible to ~10 mA cm−2), and the corresponding\n1\n7, 1000−1009; ACS Catal. 2024, 14, 366−372; ACS Appl. Energy Mater. 2024, 7, 3091−3098; J.\nMater. Chem. A 2024, 12, 31547–31556). Such large discrepancies should arise from the high\nsensitivity of CH OH formation to operational parameters, including mass ratio of CoPc to CNTs,\n3\ntotal catalyst loading, CO partial pressure, and even the amount of Nafion solution used in the\n2\ncatalyst ink (J. Am. Chem. Soc. 2024, 146, 16348−16354). These reports indicate that CH OH is\n3\nnot stably produced as a major product on CoPc/CNTs, especially when compared to CO.\nMechanistically, CO -to-CH OH on CoPc has been proposed to follow a relay pathway (CO\n2 3 2\n→ CO → CH OH). In this context, the initially generated CO is expected to play a crucial role in\n3\nenabling subsequent CH OH formation. Yet, the primary CO -to-CO step, and particularly its rate-\n3 2\nlimiting step on CoPc, remains debated. This knowledge gap has fundamentally limited the rational\ndesign of CoPc-based electrocatalytic system for efficient CH OH production. Therefore, our work\n3\nthat elucidates the rate-limiting step of CO -to-CO conversion on CoPc as well as other metal\n2\nphthalocyanines is also essential for understanding and guiding CH OH production.\n2\nwe evaluated CO RR in 1 M NaHCO at more negative potentials to address the concern regarding\n2 3\nCH OH production on CoPc/CNTs, as shown in the figure below.\n3\nwindow (<−0.80 V vs RHE), and with much lower Faradaic efficiencies compared to CO. Under\nour experimental conditions, which were not specifically optimized for CH OH formation, CH OH\n3 3\nis not a major product for the CO RR on CoPc. We have added the above figure as Supplementary\nbe formed as a liquid product during CO RR on CoPc/CNTs,31 which was detected in 1 M NaHCO\n2 3\nelectrolyte at more negative potentials (< −0.80 V versus RHE), as shown in Supplementary Fig.\n7. However, methanol formation occurs only at high overpotentials with relatively low selectivity\ncompared to CO production. Given that the present work is centered on CO RR to CO on metal\n2\nphthalocyanines and aims to elucidate the fundamental rate-limiting steps governing this primary\nreaction pathway, the following studies will focus on the CO -to-CO conversion.”\ntested and compared the CH OH production with CoPc supported on CNTs (CoPc/CNTs) and on\n3\ncarbon black (CoPc/CB) at representative potentials, as shown in the figures below.\n3\nrelative to CoPc/CNTs. Thus, under our conditions, the support effect of CoPc molecules on\nmethanol production is not apparent. The possible divergence from other reports can be attributed\nto the extreme sensitivity of CH OH production on CoPc to subtle variations in their experimental\n3\nconditions. While this intriguing topic requires more dedicated and systematic investigation, such\nexploration is clearly beyond the scope of our present study.\n2. The CoPc catalyst for both CO and methanol production has been well studied previously, even\nfor the initial CO to COOH step experimentally and theoretically, in spite of less KIE method. It\n2\nis not surprised that the rate-limiting step over MPc is different from that over Au electrode.\nResponse: We thank the reviewer for this comment. We agree that CO RR on CoPc has been\n2\nextensively investigated and that the rate-limiting step for CO production has been discussed in\nthe literature. However, as discussed in the original manuscript, this crucial step remains under\ndebate, with both CO adsorption and *CO protonation to *COOH proposed as possible rate-\n2 2\nlimiting steps in recent literature (Nat. Commun. 2019, 10, 1683; Nat. Catal. 2024, 7, 1000−1009;\nNat. Catal. 2021, 4, 1024−1031; Angew. Chem. Int. Ed. 2024, 63, e202317942). Clarifying this\nambiguity through reliable KIE analysis is therefore essential for defining a clear mechanistic basis\nfor CO RR on CoPc. Moreover, as the initially produced CO serves as the reactant for downstream\n2\nmethanol formation, establishing the rate-limiting step for CO production also offers mechanistic\ninsights relevant to the methanol pathway.\nIn our work, Au was employed as a benchmark catalyst and reference to demonstrate the\nvalidity of the KIE approach in identifying the CO RR rate-limiting step, rather than to highlight\n2\ndifferences between metals and metal phthalocyanines. Our focus is on revealing the dispersion-\ndependent nature of the rate-limiting step on metal phthalocyanines, namely, the distinct behavior\nobserved between molecularly dispersed and aggregated CoPc, which to our knowledge is reported\nhere for the first time. Furthermore, the underlying mechanism for this dispersion dependence is\nfurther elucidated through additional mechanistic studies in the revised manuscript (as detailed in\nthe following response).\nFinally, we emphasize that distinguishing whether CO adsorption or *CO protonation is\n2 2\nrate-limiting is of direct relevance for rational design of the system. For instance, a rate-limiting\nadsorption step would motivate strategies to strengthen the interfacial electric field, whereas a rate-\nlimiting protonation step would benefit from the use of electrolytes with stronger proton-donating\n3\nability. Therefore, we believe that the present study provides practical mechanistic guidance for\noptimizing CO RR performance on metal phthalocyanines.\n2\n3. With KIE experiments, the authors just demonstrate the possible RDS, but they failed to explain\nwell the mechanism shift especially the role of dispersion on carbon support. The role of local\nelectric field should be revealed with direct evidences, rather than discussing based on literature.\nResponse: We thank the reviewer for this comment, which prompted us to conduct an in-depth\nmechanistic investigation into the rate-limiting step shift induced by the dispersion state of CoPc.\nFollowing the reviewer’s suggestion, we carefully designed and performed additional experiments\nto explicitly demonstrate that this shift arises from changes in the interfacial electric filed at the\nCo active sites. First, we employed the Co(II)/Co(I) redox response of CoPc to evaluate the local\nelectric field using the highly sensitive square-wave voltammetry. We observed a substantially\nattenuated redox peak for the aggregated CoPc relative to that of the molecularly dispersed CoPc,\nindicating a weakened electric field at Co active sites in aggregated CoPc, as shown in the figure\nbelow. Next, we examined whether attenuation of the electric field shifts the rate-limiting step to\nCO adsorption. We intentionally reduced the local electric field around molecularly dispersed\nsize of Na⁺ using a crown ether. KIE experiments reveal that upon weakening the electric field,\nthe rate-limiting step indeed shifts to CO adsorption, even for molecularly dispersed CoPc. These\n2\nresults provide a clear mechanistic explanation that the dispersion-dependent shift in the CO RR\n2\nrate-limiting step on CoPc arises from changes in the interfacial electric field, supported by direct\nevidences obtained from our carefully designed experiments.\nbeen added to the revised manuscript as follows:\n“Mechanisms underlying distinct rate-limiting steps. The above electrochemical\nmeasurements and KIE analysis indicate that the rate-limiting step for CO RR to CO on CoPc\n2\nmolecules depends on their dispersion state. For molecularly dispersed CoPc on CNTs, the\nreaction is limited by the protonation of *CO , whereas for aggregated CoPc, CO adsorption\n2 2\nbecomes the rate-limiting step. We hypothesize that the shift of the rate-limiting step on aggregated\nCoPc arises from a weakened interfacial electric field at the Co active sites. Bulk CoPc, as an\norganic semiconductor, exhibits low conductivity and pronounced dielectric behavior,45,46 which\nlimits penetration of the applied electric field into thick CoPc aggregates.33 Consequently, the Co\nactive sites in aggregated CoPc experience a weaker electric field than those in molecularly\ndispersed CoPc, reducing the driving force for initial CO activation and shifting the rate-limiting\n2\nstep. The change in the electric field at Co sites upon CoPc aggregation was revealed by Stark\ntuning, based on the potential-dependent frequency shifts of reaction intermediates in sum\nfrequency generation spectra.33 Here we perform further mechanistic investigations to verify the\nhypothesis.\nFirst, we compared the Co(II)/Co(I) redox responses of the CoPc/CNTs electrode and the\nCoPc/CNTs mixture electrode to examine the potential-screening effect due to CoPc aggregation.\nBecause CNTs can introduce substantial capacitive currents and parasitic Faradaic contributions,\nwe employed square-wave voltammetry (SWV), which can suppress the charging currents while\nproviding high sensitivity.47,48 A representative SWV waveform is illustrated in Fig. 5a. As shown\nin Fig. 5b, molecularly dispersed CoPc on CNTs experiences a strong local electric field, resulting\nin a pronounced and well-defined Co(II)/Co(I) redox peak at around 0 V vs RHE, consistent with\nprevious reports.34 In contrast, despite identical CoPc loadings, the redox peak of the aggregated\nCoPc is significantly attenuated compared to that of the molecularly dispersed CoPc, indicating\na suppression of the Co(II)/Co(I) redox response due to aggregation. When a potential is applied\nto the electrode, the electric field does not fully penetrate the aggregates, limiting effective\npotential transmission to the Co active sites and resulting in a substantially attenuated redox\nsignal.\nTo further examine whether attenuation of the electric field shifts the rate-limiting step to CO\n2\nadsorption, we intentionally reduced the local electric field around molecularly dispersed CoPc\nby chelating Na⁺ in the electrolyte using the crown ether 18-crown-6.49 Chelation of Na⁺ effectively\nincreases the apparent cation size and expands the thickness of the outer Helmholtz layer,50,51\nthereby reducing the potential gradient and interfacial electric field, as schematically illustrated\nin Fig. 5c. KIE studies were then performed on the CoPc/CNTs electrode in 1 M NaHCO /H O +\n3 2\n18-crown-6 and in 1 M NaDCO /D O + 18-crown-6 electrolytes. As shown in Fig. 5d, the partial\n3 2\ncurrent densities for CO production greatly decreased in the NaHCO + 18-crown-6 electrolyte\n3\ncompared to those in NaHCO (Fig. 3d), indicating reduced activity due to the weakened electric\n3\nfield. Importantly, the CO RR activities in crown-ether-containing H O and D O electrolytes were\n2 2 2\nnearly identical, giving KIE values of ~1 (Fig. 5e). These results indicate that upon weakening the\nelectric field, the rate-limiting step shifts to CO adsorption even for molecularly dispersed CoPc.\nconfirming the reliability of KIE analysis in the presence of crown ether. Overall, these findings\ndemonstrate that the Co active sites in aggregated CoPc experience a weakened electric field so\n5\nthat CO adsorption becomes rate-limiting. This provides a clear mechanistic explanation for the\n2\ndependence of the CO RR rate-limiting step on the dispersion state of CoPc.”\n2\n4. The role of anions including bicarbonate ions has been well documented in the literature. Further,\nit seems that the authors’ logic just lies in the proton donating ability for a given hydrogenation\nstep.\nResponse: We thank the reviewer for this comment. We agree that the role of anions, particularly\nbicarbonate (HCO −), in CO RR has been well documented. However, most previous studies\n3 2\nfocused on metal catalysts (e.g., Au, Cu) and have identified two primary roles for HCO −:\n3\n(i) a spectator role within kinetically controlled regimes (J. Am. Chem. Soc. 2017, 139, 17109−17113;\nAcc. Chem. Res. 2022, 55, 1900−1911), and (ii) a CO -shuttle role when mass-transport limitation\n2\nis approached (ACS Energy Lett. 2018, 3, 722−723; J. Am. Chem. Soc. 2015, 137, 4701−4708).\nThe spectator role is consistent with a rate-limiting CO adsorption step on metal catalysts, which\n2\nis largely unaffected by HCO −, and the CO -shuttle behavior reflects the capability of HCO −\n3 2 3\nreplenishing CO near the electrode under high reaction rates via the HCO −/CO equilibrium.\n2 3 2\nBy contrast, the role of HCO − as a proton donor that participates in a rate-limiting protonation\n3\nstep of CO RR has been rarely reported. In our work we measured a reaction order of ~0.39 with\n2\nrespect to HCO − for CO RR activity on CoPc/CNTs, which indicates a substantial promotional\n3 2\neffect of HCO −. In addition, we further observed significantly higher CO RR activity in HCO −\n3 2 3\nor H PO −/HPO 2− electrolytes compared to ClO −. These results attribute the promotional effect\n2 4 4 4\nof HCO − to enhanced proton donation in the rate-limiting protonation of *CO on CoPc/CNTs.\n3 2\nElucidating the distinct roles of HCO − in CO RR on electrocatalysts with different rate-limiting\n3 2\nsteps is critical, as it reveals the non-negligible impact of electrolyte ions on reaction kinetics and\nguides the rational design of efficient CO electrolysis systems.\n2 2\nCoPc/CNTs in CO -saturated 1 M NaHCO and in 1 M NaClO . To probe local pH under operando\n2 3 4\nconditions, we employed in situ scanning electrochemical microscopy (SECM), as illustrated in\nfigure below. A customized SECM setup developed in our own group (Ph.D. Dissertation, 2023,\nUniversity of Central Florida, Orlando, FL, https://stars.library.ucf.edu/etd2020/1631/) with an Au\nnanoelectrode tip functionalized with 4-hydroxylaminothiophenol (4-HATP)/4-nitrosothiophenol\n(4-NSTP) (Anal. Chem. 2020, 92, 2237−2243; JACS Au 2021, 1, 1915−1924) was used to measure\nthe local pH in the vicinity of the cathode. A correlation between the mid-peak potential of the 4-\nHATP/4-NSTP anodic peak and the electrolyte pH was established as a calibration curve.\n2\nin the figure below, the local pH during CO RR in NaClO increased substantially, from an initial\n2 4\nbulk pH of 4.3 to 8.6 at a current density of 5.0 mA cm−2, reflecting the absence of buffering\ncapacity in NaClO . In contrast, the pH change in NaHCO was much smaller (~0.4 pH unit) due\n4 3\nto its buffering nature.\nMore importantly, to examine whether this pH-buffering capability could explain the variation\nin CO RR activity among the anions, we compared the CO RR activities in the two electrolytes\n2 2\nunder RHE-referenced potentials corrected by the measured local pH values, thereby eliminating\npH-dependent thermodynamic contributions. Even after this correction, the CO RR activity in\n3 4\nThis remaining enhancement implies that the effect of HCO − is not solely a buffering phenomenon,\n3\nbut includes an intrinsic proton-donating contribution that facilitates the rate-limiting protonation\nof *CO on CoPc/CNTs.\n2\ndiscussion has been added: “The proton-donating ability of the anions may not only facilitate\nprotonation steps in CO RR, but also provide buffering capacity to mitigate local pH changes\n2\nduring reactions.61,62 To assess the contribution of such mechanistic factors, we employed the\nSECM with pH-sensitive probes to monitor the local pH during CO RR in different electrolytes.\n2\nin NaClO increased markedly, from an initial bulk pH of 4.3 to a local pH of 8.6 at a current\n4\ndensity of 5.0 mA cm−2, reflecting the absence of buffering capacity in NaClO . In contrast, the pH\n4\nrise in NaHCO was much smaller (Supplementary Fig. 13) due to its buffering nature. To account\n3\nfor pH-dependent thermodynamic effects when comparing CO RR activities, we re-calculated the\n2\nactivities using RHE-referenced potentials corrected by the measured local pH values\n(Supplementary Fig. 16b). Even after this correction, the CO RR activities in NaHCO remained\n2 3\nconsistently higher than those in NaClO , indicating that differences in buffering or local pH do\n4\nnot explain the variation in CO RR activity among the anions. Instead, the higher CO RR activity\n2 2\nin NaHCO is mainly attributed to enhanced *CO protonation on CoPc/CNTs due to the proton-\n3 2\ndonating ability of the HCO − anion.”\n3\n5. When conducting KIE studies, how did the authors exclude the effect of mass transport?\nResponse: We thank the reviewer for this valuable comment. We agree that excluding the\ninfluence of CO mass transport is essential for a reliable interpretation of the KIE results and the\n2\nassociated rate-limiting step of CO RR. In our study, the influence of mass transport is minimized\n2\nin the KIE studies for three reasons. First, CO has essentially identical molar solubility in H O\n2 2\nand D O, and the solubility of CO at 25 °C and 1 atm is ~0.0337 mol L⁻1 in both H O and D O\n2 2 2 2\n(J. Am. Chem. Soc. 1938, 60, 2771−2773). Some literature reports CO solubility on a mol kg⁻1\n2\nbasis, which introduces a slight difference between H O and D O, but this difference arises solely\n2 2\nfrom the higher density of D O. When expressed on a mol L⁻1 basis, which is the relevant metric\n2\nfor mass-transport considerations, the CO solubility is essentially identical in the two solvents.\nthe rapid equilibrium between dissolved CO and bicarbonate species (CO + H O ⇌ H⁺ + HCO ⁻,\n2 2 2 3\nCO + D O ⇌ D+ + DCO ⁻) continuously buffers and replenishes CO near the electrode. This\n2 2 3 2\nequilibrium ensures a sustained and comparable CO supply during CO RR in both H O and D O\n2 2 2 2\nelectrolytes. Finally, the KIE measurements were conducted at relatively low current densities and\nlow overpotentials, conditions under which reaction rates are dominated by intrinsic kinetics rather\nthan mass transport.\nFurthermore, the negligible influence of CO mass transport to the KIE analysis is supported\n2\nby the similar CO RR activities observed on Au in NaHCO /H O and NaDCO /D O (as shown in\n2 3 2 3 2\nthe figure below, from Figure 3 of our manuscript). Since CO adsorption is the rate-limiting step\n2\non Au and thus highly sensitive to local CO concentration, the comparable activities confirm that\n2\nmass transport does not account for the observed activity differences in our KIE studies on metal\nphthalocyanines. Collectively, these results demonstrate that CO mass transport can be excluded\n2\nas the origin of the observed activity differences in the KIE studies.\nthe underlying mechanism, it is essential to establish that the observed differences in CO RR\n2\nactivity between NaHCO /H O and NaDCO /D O do not arise from extrinsic factors, such as\n3 2 3 2\ndifferences in CO solubility or the pK values of H O versus D O. First, the molar solubilities of\n2 a 2 2\nCO in H O and D O at 25 ℃ and 1 atm are both ~33.7 mmol L⁻1,41 which cannot account for the\n2 2 2\npronounced CO RR activity differences. Moreover, in 1 M NaHCO /H O and 1 M NaDCO /D O\n2 3 2 3 2\nelectrolytes, the equilibrium between dissolved CO and bicarbonate species (CO + H O ⇌ H⁺ +\n2 2 2\nHCO ⁻, CO + D O ⇌ D+ + DCO ⁻) can continuously buffer and replenish CO near the cathode,\n3 2 2 3 2\nensuring a sustained and comparable CO supply for the CO RR. Consequently, CO solubility or\n2 2 2\navailability can be excluded as the origin of the observed activity differences in the KIE studies.”\nThis study provides valuable mechanistic insights into the dispersion-dependent rate-limiting steps\nof CO RR on CoPc catalysts, supported by electrochemical and KIE analyses. The identification\n2\nof HCO − is proton donor role in molecularly dispersed systems is particularly noteworthy.\nWith these revisions, the work could offer a more robust foundation for molecular catalyst design.\nConsideration for publication would be appropriate after major revision.\nResponse: We thank the reviewer for the positive evaluation of our work and we deeply appreciate\nthe constructive comments and suggestions, which have helped us greatly improve the manuscript.\nIn response, we have performed many new experiments with new data and discussions to deepen\nthe mechanistic understanding and strengthen the manuscript. Our major revisions include:\n(1) We have gained a deeper understanding of the mechanism and attributed the shift in the rate-\nlimiting step on aggregated CoPc to a weakened interfacial electric field at the Co active sites,\nas supported by solid evidence presented in Figure 5 of the revised manuscript.\n(2) We have employed in situ scanning electrochemical microscopy (SECM) to probe the local\npH under operando conditions and have directly addressed all comments related to local pH,\nas demonstrated by the results in Supplementary Figs. 12, 13, and 16.\n(3) We have revised the CO RR performance reporting from the SHE scale to RHE-referenced\n2\npotentials in the revised manuscript, thus accounting for pH-dependent thermodynamic effects\nin the comparison of CO RR activities.\n2\n(4) We conducted experiments using a GDE flow cell to compare the effects of different anions\nunder practical electrolyzer conditions (Supplementary Fig. 17) to show the applicability.\n(5) We have addressed other remaining minor comments by adding new results and discussions in\nthe revised manuscript.\nOur point-to-point responses to the comments and corresponding revisions are described below.\nWe believe that our manuscript has been greatly improved after the revisions. A copy of the revised\nmanuscript with all changes highlighted using “Track Changes” is provided to facilitate review.\nSpecific comments:\n1. The KIE experiments conducted in NaHCO and NaDCO systems may be affected by the\n3 3\nsignificant pK difference between D O and H O, leading to pH shifts that influence reaction\na 2 2\nkinetics. The authors should clarify how pH was controlled (e.g., buffer capacity adjustment or in\nsitu monitoring) to isolate genuine isotope effects from pH-induced artifacts.\nResponse: We thank the reviewer for this constructive comment. We agree that isolating intrinsic\nisotope effects from pH-induced artifacts is essential for a reliable interpretation of the KIE results\nand the associated rate-limiting step of CO RR. Following the reviewer’s suggestion, we have\n2\nminimized pH-dependent thermodynamic contributions by: (i) reporting the CO RR activities in\n3 2 3 2\nquantifying local pH shifts under operando conditions in both electrolytes using in situ scanning\nelectrochemical microscopy (SECM).\nFirst, we re-compared the CO RR performance in NaHCO /H O and NaDCO /D O on the\n2 3 2 3 2\nRHE scale, rather than on the originally used SHE scale, to eliminate the initial thermodynamic\n10\noffset arising from the bulk pH difference between the two electrolytes. Accordingly, we repeated\nCO RR measurements on all catalysts (Au/C, CoPc/CNTs, NiPc/CNTs, the aggregated CoPc, and\n2\nthe CoPc/CNTs mixture) in CO -saturated 1 M NaDCO /D O while applying a negative potential\n2 3 2\nshift of 24 mV relative to the NaHCO /H O condition. This correction corresponds to −0.0591 ×\n3 2\nΔpH (D O − H O), where ΔpH = 0.4 based on the bulk pH values of 7.4 and 7.8 for CO -saturated\n2 2 2\n3 2 3 2\nthe corresponding figures have been updated in the revised manuscript.\nNew Figure 3 in the revised manuscript:\nAs presented above, the observed differences in CO RR activity between NaHCO /H O and\n2 3 2\nNaDCO /D O for molecularly dispersed CoPc/CNTs and NiPc/CNTs remain after applying the\n3 2\npH-corrected potential shifts, although the magnitude of the KIE values is slightly changed. This\npersistence indicates that the measured KIE primarily arises from an intrinsic isotope effect rather\nthan from pH-induced thermodynamic artifacts, thereby confirming a rate-limiting protonation of\n*CO on the CoPc/CNTs and NiPc/CNTs catalysts.\n2\nadditional thermodynamic shifts that affect kinetics, we further used SECM to probe the local pH\nunder operando conditions in the NaHCO /H O and NaDCO /D O electrolytes, as shown in the\n3 2 3 2\nfigure below. A customized SECM setup developed in our own group (Ph.D. Dissertation, 2023,\nUniversity of Central Florida, Orlando, FL, https://stars.library.ucf.edu/etd2020/1631/) with an Au\nnanoelectrode tip functionalized with 4-hydroxylaminothiophenol (4-HATP)/4-nitrosothiophenol\n(4-NSTP) was used to measure the local pH in the vicinity of the cathode (Anal. Chem. 2020, 92,\n2237−2243; JACS Au 2021, 1, 1915−1924). A correlation between the mid-peak potential of the\n4-HATP/4-NSTP anodic peak and the electrolyte pH was established as a calibration curve.\nunder various CO RR current densities, as presented in the figure below. While a slight increase\n2\nin local pH was observed during CO RR in both electrolytes, the relative difference between them\n2\nremained ~0.4 pH units, consistent with the initial bulk pH difference (7.4 versus 7.8). Because\nthis initial difference was already accounted for in the RHE-referenced potentials, the minor local\npH shifts do not exert any additional influence on the KIE analysis.\nThe above two figures has been added as Supplementary Figure 12 and Supplementary Figure\n13, respectively. The following discussion has been incorporated into the revised manuscript:\n“Second, H O and D O exhibit a modest difference in their acid–base properties (pK = 14.00 for\n2 2 w\nH O and 14.86 for D O), resulting in bulk pH/pD values of 7.4 and 7.8 for CO -saturated 1 M\n2 2 2\nNaHCO and NaDCO electrolytes, respectively. In the above measurements, all potentials were\n3 3\nconverted to the RHE scale using the corresponding bulk pH, thereby accounting for pH-\ndependent thermodynamic effects in the comparison of CO RR activities. Nevertheless, local pH\n2\nmay change during reactions and influence reaction kinetics. To isolate genuine isotope effects\nfrom possible pH-induced artifacts, we employed in situ scanning electrochemical microscopy\n(SECM) to probe local pH under operando conditions. As shown in Supplementary Fig. 12, a\ncustomized SECM setup,42 equipped with an Au nanoelectrode tip whose exposed apex was\n13\nfunctionalized with 4-hydroxylaminothiophenol (4-HATP)/4-nitrosothiophenol (4-NSTP), was\nused to measure the local pH near the cathode.43,44 A correlation between the mid-peak potential\nof the 4-HATP/4-NSTP anodic peak and the electrolyte pH was established as a calibration curve\n(Supplementary Fig. 12). Using this approach, we quantified the local pH near the CoPc/CNTs\nelectrode under various CO RR current densities, as presented in Supplementary Fig. 13. While a\n2\nslight increase in local pH was observed during CO RR in both electrolytes, the relative difference\n2\nbetween them remained ~0.4 pH units, consistent with the initial bulk pH difference (7.4 versus\n7.8). Because this initial difference was already accounted for in the RHE-referenced potentials,\nthe minor local pH shifts do not exert any additional influence on the KIE analysis.”\n2. While SEM/XRD data characterize CoPc aggregation, claims about \"electronic state changes\"\nand \"positioning farther from OHP\" lack direct evidence. For example, XANES/EXAFS\n(electronic structure) and TOF-SIMS (interface distribution) should be added to support the\nproposed dispersion-dependent RLS mechanism.\nResponse: We thank the reviewer for this comment, which prompted us to conduct an in-depth\nmechanistic investigation into the rate-limiting step shift induced by the dispersion state of CoPc.\nFollowing the reviewer’s suggestion, we designed and performed new experiments that explicitly\ndemonstrate that this shift arises from changes in the interfacial electric filed at the Co active sites.\nNotably, the change in the electric field at Co sites upon CoPc aggregation has been revealed in\nprevious studies using Stark tuning, based on the potential-dependent frequency shifts of reaction\nintermediates in sum frequency generation spectra (Nat. Catal. 2024, 7, 987−999). In this work,\nwe conducted additional mechanistic studies to provide further evidence for this hypothesis.\nFirst, we employed the Co(II)/Co(I) redox response of CoPc to evaluate the local electrical\nfield using the highly sensitive square-wave voltammetry. We observed a substantially attenuated\nredox peak for the aggregated CoPc relative to that of molecularly dispersed CoPc, indicating a\nweakened electric field at Co active sites in aggregated CoPc, as shown in the figure below.\n2\nadsorption. We intentionally reduced the local electric field around molecularly dispersed CoPc\nby expanding the thickness of the outer Helmholtz layer, achieved by increasing the effective size\nof Na⁺ using a crown ether. KIE experiments reveal that upon weakening the electric field,\nthe rate-limiting step indeed shifts to CO adsorption, even for molecularly dispersed CoPc.\nresults provide a clear mechanistic explanation that the CoPc dispersion-dependent shift in the\nCO RR rate-limiting step arises from changes in the interfacial electric field.\nbeen added to the revised manuscript as follows:\n“Mechanisms underlying distinct rate-limiting steps. The above electrochemical\nmeasurements and KIE analysis indicate that the rate-limiting step for CO RR to CO on CoPc\n2\nmolecules depends on their dispersion state. For molecularly dispersed CoPc on CNTs, the\nreaction is limited by the protonation of *CO , whereas for aggregated CoPc, CO adsorption\n2 2\nbecomes the rate-limiting step. We hypothesize that the shift of the rate-limiting step on aggregated\nCoPc arises from a weakened interfacial electric field at the Co active sites. Bulk CoPc, as an\norganic semiconductor, exhibits low conductivity and pronounced dielectric behavior,45,46 which\nlimits penetration of the applied electric field into thick CoPc aggregates.33 Consequently, the Co\nactive sites in aggregated CoPc experience a weaker electric field than those in molecularly\ndispersed CoPc, reducing the driving force for initial CO activation and shifting the rate-limiting\n2\nstep. The change in the electric field at Co sites upon CoPc aggregation was revealed by Stark\ntuning, based on the potential-dependent frequency shifts of reaction intermediates in sum\nfrequency generation spectra.33 Here we perform further mechanistic investigations to verify the\nhypothesis.\nFirst, we compared the Co(II)/Co(I) redox responses of the CoPc/CNTs electrode and the\nCoPc/CNTs mixture electrode to examine the potential-screening effect due to CoPc aggregation.\nBecause CNTs can introduce substantial capacitive currents and parasitic Faradaic contributions,\nwe employed square-wave voltammetry (SWV), which can suppress the charging currents while\nproviding high sensitivity.47,48 A representative SWV waveform is illustrated in Fig. 5a. As shown\nin Fig. 5b, molecularly dispersed CoPc on CNTs experiences a strong local electric field, resulting\nin a pronounced and well-defined Co(II)/Co(I) redox peak at around 0 V vs RHE, consistent with\nprevious reports.34 In contrast, despite identical CoPc loadings, the redox peak of the aggregated\nCoPc is significantly attenuated compared to that of the molecularly dispersed CoPc, indicating\na suppression of the Co(II)/Co(I) redox response due to aggregation. When a potential is applied\nto the electrode, the electric field does not fully penetrate the aggregates, limiting effective\npotential transmission to the Co active sites and resulting in a substantially attenuated redox\nsignal.\nTo further examine whether attenuation of the electric field shifts the rate-limiting step to CO\n2\nadsorption, we intentionally reduced the local electric field around molecularly dispersed CoPc\nby chelating Na⁺ in the electrolyte using the crown ether 18-crown-6.49 Chelation of Na⁺ effectively\nincreases the apparent cation size and expands the thickness of the outer Helmholtz layer,50,51\nthereby reducing the potential gradient and interfacial electric field, as schematically illustrated\nin Fig. 5c. KIE studies were then performed on the CoPc/CNTs electrode in 1 M NaHCO /H O +\n3 2\n18-crown-6 and in 1 M NaDCO /D O + 18-crown-6 electrolytes. As shown in Fig. 5d, the partial\n3 2\ncurrent densities for CO production greatly decreased in the NaHCO + 18-crown-6 electrolyte\n3\ncompared to those in NaHCO (Fig. 3d), indicating reduced activity due to the weakened electric\n3\n15\nfield. Importantly, the CO RR activities in crown-ether-containing H O and D O electrolytes were\n2 2 2\nnearly identical, giving KIE values of ~1 (Fig. 5e). These results indicate that upon weakening the\nelectric field, the rate-limiting step shifts to CO adsorption even for molecularly dispersed CoPc.\nconfirming the reliability of KIE analysis in the presence of crown ether. Overall, these findings\ndemonstrate that the Co active sites in aggregated CoPc experience a weakened electric field so\nthat CO adsorption becomes rate-limiting. This provides a clear mechanistic explanation for the\n2\ndependence of the CO RR rate-limiting step on the dispersion state of CoPc.”\n2\n3. The \"proton donor\" function of anions near the electrode interface may be constrained by electric\ndouble-layer structure (e.g., specific adsorption competition). A Gouy-Chapman-Stern model\nanalysis is needed to distinguish intrinsic proton-donating capacity from interfacial delivery\nefficiency.\nResponse: Yes, the intrinsic proton-donating capacity of the anions may not be very efficient and\nmay be limited by their interfacial delivery efficiency, as they predominantly reside in the diffuse\nlayer due to the electric double-layer structure. In contrast, cations with proton-donating ability\ncan enter and accumulate within the electric double layer, allowing direct interactions with surface-\nbound intermediates and thereby enabling more efficient proton delivery. We have incorporated\nthe following discussion into the revised manuscript:\n“Moreover, given the negatively charged cathode during CO RR, we expect electrolyte cations\n2\ncapable of donating protons to be more effective in promoting the rate-limiting protonation step.\nSuch cations can enter and accumulate in the electric double layer, allowing direct interactions\nwith surface-bound intermediates and thereby enabling more efficient proton delivery to facilitate\n*CO protonation.”\n2\n4. The HCO − proton donor mechanism depends not only on pK but also on local pH, buffer\n3 a\ncapacity, and applied potential. In situ techniques (such as SEIRAS, pH-sensitive probes) should\nbe employed to decouple these intertwined factors.\nResponse: We thank the reviewer for this comment. We agree that decoupling the proton-\ndonation role of HCO − from its pH-buffering behavior is essential for elucidating the origin of its\n3\npromotional effect on CO RR with CoPc/CNTs. First, a comparison of the anion effects at various\n2\npotentials, especially between HCO − and H PO −/HPO 2−, was included in Figure 6 and discussed\n3 2 4 4\nin the manuscript. To evaluate the local pH and buffering capability of HCO −, we employed the\n3\nin situ SECM equipped with a pH-sensitive probe to monitor and compare the local pH near the\nCoPc/CNTs electrode at various CO RR current densities in CO -saturated 1 M NaHCO and 1 M\n2 2 3\nNaClO electrolytes, as shown in newly added Supplementary Figs. 12, 13, and the figure below.\n2 4\nlocal pH of 8.6 at a current density of 5.0 mA cm−2, reflecting the absence of buffering capacity in\nNaClO . In contrast, the pH change in NaHCO was much smaller due to its buffering nature.\n4 3\nThen, to decouple the proton-donation effect of HCO − from its pH-buffering capacity, we\n3\nfurther compared the CO RR activities in NaHCO and NaClO using RHE-referenced potentials\n2 3 4\ncorrected by the measured local pH values, thereby removing pH-dependent thermodynamic\ncontributions. As shown in the figure below (b), even after this correction, the CO RR activity in\n3 4\n16\ndo not explain the variation in CO RR activity among the anions. Instead, the higher CO RR\n2 2\nactivity in NaHCO is mainly attributed to the intrinsic proton-donation effect of HCO −, which\n3 3\nenhances the *CO protonation step on CoPc/CNTs.\nhas been added to the revised manuscript: “The proton-donating ability of the anions may not only\nfacilitate protonation steps in CO RR, but also provide buffering capacity to mitigate local pH\n2\nchanges during reactions.61,62 To assess the contribution of such mechanistic factors, we employed\nthe SECM with pH-sensitive probes to monitor the local pH during CO RR in different electrolytes.\n2\nin NaClO increased markedly, from an initial bulk pH of 4.3 to a local pH of 8.6 at a current\n4\ndensity of 5.0 mA cm−2, reflecting the absence of buffering capacity in NaClO . In contrast, the pH\n4\nrise in NaHCO was much smaller (Supplementary Fig. 13) due to its buffering nature. To account\n3\nfor pH-dependent thermodynamic effects when comparing CO RR activities, we re-calculated the\n2\nactivities using RHE-referenced potentials corrected by the measured local pH values\n(Supplementary Fig. 16b). Even after this correction, the CO RR activities in NaHCO remained\n2 3\nconsistently higher than those in NaClO , indicating that differences in buffering or local pH do\n4\nnot explain the variation in CO RR activity among the anions. Instead, the higher CO RR activity\n2 2\nin NaHCO is mainly attributed to enhanced *CO protonation on CoPc/CNTs due to the proton-\n3 2\ndonating ability of the HCO − anion.”\n3\n5. The difference of CO solubility in D O versus H O may artificially reduce current density in\n2 2 2\nisotope experiments. Normalizing j values is essential for reliable KIE conclusions.\nCO\nResponse: We thank the reviewer for this comment. We agree that excluding the influence of CO\n2\nsolubility or availability is essential for a reliable interpretation of the KIE results and the\nassociated rate-limiting step of CO RR. The solubility of CO at 25 °C and 1 atm is ~0.0337 mol\n2 2\nL⁻1 in both H O and D O (J. Am. Chem. Soc. 1938, 60, 2771−2773). Some literature reports CO\n2 2 2\nsolubility on a mol kg−1 basis, which introduces a slight difference between H O and D O, but this\n2 2\ndifference arises solely from the higher density of D O. When expressed on a mol L−1 basis, which\n2\nis the relevant metric for mass-transport considerations, the CO solubility is essentially identical\n2\nin the two solvents. Furthermore, all KIE measurements in our study were performed using 1 M\nbicarbonate electrolytes, where the equilibrium between dissolved CO and bicarbonate species\n2\n(CO + H O ⇌ H⁺ + HCO ⁻, CO + D O ⇌ D+ + DCO ⁻) continuously buffers and replenishes\n2 2 3 2 2 3\n2 2\nCO RR in both H O and D O electrolytes. Therefore, the CO solubility or availability can be\n2 2 2 2\nexcluded as the origin of the observed activity differences in the KIE studies.\nWe have added the following discussion to the revised manuscript: “Prior to investigating the\nunderlying mechanism, it is essential to establish that the observed differences in CO RR activity\n2\nbetween NaHCO /H O and NaDCO /D O do not arise from extrinsic factors, such as differences\n3 2 3 2\nin CO solubility or the pK values of H O versus D O. First, the molar solubilities of CO in H O\n2 a 2 2 2 2\nand D O at 25 ℃ and 1 atm are both ~33.7 mmol L⁻1,41 which cannot account for the pronounced\n2 3 2 3 2\nthe equilibrium between dissolved CO and bicarbonate species (CO + H O ⇌ H⁺ + HCO ⁻, CO\n2 2 2 3 2\n+ D O ⇌ D+ + DCO ⁻) can continuously buffer and replenish CO near the cathode, ensuring a\n2 3 2\nsustained and comparable CO supply for the CO RR. Consequently, CO solubility or availability\n2 2 2\ncan be excluded as the origin of the observed activity differences in the KIE studies.”\n6. Anion comparisons in H-cells may not reflect practical electrolyzer conditions (flow cells, high\ncurrent densities). Additional experiments are recommended to strengthen the applicability of the\nfindings.\nResponse: We thank the reviewer for this comment. Following the suggestion, we tested CO RR\n2\non CoPc/CNTs in the presence of different anions using a flow cell equipped with gas-diffusion\nelectrodes (GDEs), where CO mass transport is greatly enhanced and high current densities are\n2\naccessible. The CO RR performances, referenced to RHE, are shown in the figure below.\n2\n2 3 2 4 4 4\ntrend observed in H-cell, supporting the applicability of this anion-based strategy at high current\ndensities. The consistently higher activity in HCO − relative to ClO − arises from its proton-\n3 4\ndonation role in the rate-limiting protonation step of CO RR, as well as a CO -shuttle role that\n2 2\nsustains a higher local CO concentration. The reduced performance gap between H PO −/HPO 2−\n2 2 4 4\nand ClO − at higher overpotentials likely arises from the stronger electrostatic repulsion of\n2 4 4\ncontribution.\nThe above figure has been added as Supplementary Figure 17, with the following discussion\nadded to the manuscript: “These findings suggest a strategy for enhancing CO electrolysis systems\n2\nwith a rate-limiting protonation step through the use of proton-donating anions.61,62 To further\ndemonstrate this anion strategy and its applicability, we evaluated CO electrolysis in presence of\n2\ndifferent anions using a flow cell equipped with gas-diffusion electrodes (GDEs), where the\nCO RR current density can be drastically increased due to enhanced mass transport.63 Indeed, as\n2\nshown in Supplementary Fig. 17, the NaHCO electrolyte achieved the highest activity for CO RR\n3 2\non CoPc/CNTs, followed by NaH PO /Na HPO and NaClO .”\n2 4 2 4 4\n7. The focus on Co/Ni phthalocyanines lacks perspective on generalizability to other metal centers\n(Fe, Mn) or ligand frameworks (porphyrins, terpyridines). Expanding the discussion on design\nprinciples for broader applicability would enhance impact.\nResponse: We thank the reviewer for this comment. We have added the following discussion in\nthe revised manuscript highlighting the broader applicability of our design principles:\n“The above discussions focus on enhancing CO RR on CoPc with a rate-limiting protonation step\n2\nby leveraging the proton-donating ability of electrolyte ions. More broadly, the methodology\ndemonstrated here, which identifies whether the CO RR rate-limiting step involves proton transfer\n2\nthrough KIE analysis and then optimizes the electrolyte accordingly, is applicable to other metal\nphthalocyanines with different metal centers (Ni, Fe, Cu, Mn) and to related molecular catalysts\nsuch as porphyrins and quaterpyridines. For catalysts with a protonation-limited pathway, CO RR\n2\nperformance can be enhanced by selecting electrolyte ions with appropriate proton-donating\nability. Conversely, for systems where CO adsorption is rate-limiting, CO RR activity can be\n2 2\nimproved by strengthening the interfacial electric field, for example through the use of partially\nor weakly solvated cations.54 Together, these insights provide generalizable guidance for the\nrational design of efficient CO electrolysis systems.”\n2\n8. All potentials were reported against the SHE. It is intriguing to know how the experiment setup\nwas constructed and how different pH impact was introduced into the performance reporting.\nResponse: We thank the reviewer for this comment. In our study, all potentials were recorded\nusing a leak-free Ag/AgCl reference electrode with iR-compensation applied using the dynamic\nCurrent-Interrupt method. The recorded potentials were subsequently converted to the SHE or the\nRHE scales. We agree with the reviewer that reporting CO RR activity on the RHE scale is more\n2\nappropriate when comparing electrolytes with different pH values, because it removes pH-induced\nthermodynamic differences. Accordingly, we have reported the CO RR performance using RHE-\n2\nreferenced potentials in the revised manuscript.\n3 2 3 2 2\neach catalyst (Au/C, CoPc/CNTs, NiPc/CNTs, the aggregated CoPc, and the CoPc/CNTs mixture)\nin CO -saturated 1 M NaDCO /D O while applying a negative potential shift of 24 mV relative to\n2 3 2\nthe NaHCO /H O condition. This correction corresponds to −0.0591 × ΔpH (D O − H O), where\n3 2 2 2\nΔpH = 0.4 based on the bulk pH values of 7.4 and 7.8 for CO -saturated 1 M NaHCO /H O and\n2 3 2\nNaDCO /D O, respectively. We found the previously observed differences in CO RR activity\n3 2 2\nbetween NaHCO /H O and NaDCO /D O for molecularly dispersed CoPc/CNTs and NiPc/CNTs\n3 2 3 2\nremain after applying the pH-corrected potential shift, although the magnitude of the KIE values\nis slightly reduced. This persistence confirms a rate-limiting *CO protonation step on CoPc/CNTs\n2\nand NiPc/CNTs. Furthermore, considering that local pH can shift from bulk pH during CO RR,\n2\npossibly introducing additional thermodynamic shifts, we further used SECM equipped with pH-\nsensitive probes to monitor the local pH near the CoPc/CNTs electrode under operando conditions.\nAs a result, while a slight increase in local pH was observed during CO RR in both NaHCO /H O\n2 3 2\nand NaDCO /D O electrolytes, the relative difference between them remained ~0.4 pH unit, which\n3 2\nis consistent with the initial bulk pH difference (7.4 versus 7.8). Because this initial difference was\nalready accounted for in the RHE-referenced potentials, the minor local pH shifts do not exert any\nadditional influence on the KIE analysis.\nSecond, for the study of electrolyte anion effects, we re-evaluated the CO RR performance of\nthe figures below.\n2\ndepletion is not pronounced, the total current\ndensity on CoPc/CNTs, including both CO RR and HER, follows the trend: ClO − < HCO − <\n2 4 3\nH PO −/HPO 2−, consistent with the relative proton-donation capabilities indicated by their pK\n2 4 4 a\nvalues. This supports the role of HCO − and H PO −/HPO 2− as proton donors in the rate-limiting\n3 2 4 4\nprotonation step. To further distinguish the proton-donation role from the pH-buffering behavior,\nwe monitored and compared the local pH near the CoPc/CNTs electrode at various CO RR current\n2\ndensities in CO -saturated 1 M NaHCO and in 1 M NaClO electrolytes using the SECM equipped\n2 3 4\nwith pH-sensitive probes. The local pH during CO RR in NaClO increased substantially, from an\n2 4\ninitial bulk pH of 4.3 to a local pH of 8.6 at a current density of 5.0 mA cm−2, reflecting the absence\nof buffering capacity in NaClO . In contrast, the pH change in NaHCO was much smaller due to\n4 3\nits buffering nature. Then, to decouple the proton-donation effect of HCO − from its pH-buffering\n3\nability, we compared CO RR activities in NaHCO and NaClO using RHE-referenced potentials\n2 3 4\ncorrected by the measured local pH values, thereby removing pH-dependent thermodynamic\ncontributions. Even after this correction, the CO RR activity in NaHCO remained consistently\n2 3\nhigher than in NaClO , indicating that differences in pH-buffering do not explain the variation in\n2 2 3\nattributed to the intrinsic proton-donation effect of HCO −, which enhances the *CO protonation\n3 2\nstep on CoPc/CNTs.\nThe updated and newly added figures, along with the following discussion, have been\nincorporated into the revised manuscript:\n21\n“Second, H O and D O exhibit a modest difference in their acid–base properties (pK = 14.00\n2 2 w\nfor H O and 14.86 for D O), resulting in bulk pH/pD values of 7.4 and 7.8 for CO -saturated 1 M\n2 2 2\nNaHCO and NaDCO electrolytes, respectively. In the above measurements, all potentials were\n3 3\nconverted to the RHE scale using the corresponding bulk pH, thereby accounting for pH-\ndependent thermodynamic effects in the comparison of CO RR activities. Nevertheless, local pH\n2\nmay change during reactions and influence reaction kinetics. To isolate genuine isotope effects\nfrom possible pH-induced artifacts, we employed in situ scanning electrochemical microscopy\n(SECM) to probe local pH under operando conditions. As shown in Supplementary Fig. 12, a\ncustomized SECM setup,42 equipped with an Au nanoelectrode tip whose exposed apex was\nfunctionalized with 4-hydroxylaminothiophenol (4-HATP)/4-nitrosothiophenol (4-NSTP), was\nused to measure the local pH near the cathode.43,44 A correlation between the mid-peak potential\nof the 4-HATP/4-NSTP anodic peak and the electrolyte pH was established as a calibration curve\n(Supplementary Fig. 12). Using this approach, we quantified the local pH near the CoPc/CNTs\nelectrode under various CO RR current densities, as presented in Supplementary Fig. 13. While a\n2\nslight increase in local pH was observed during CO RR in both electrolytes, the relative difference\n2\nbetween them remained ~0.4 pH units, consistent with the initial bulk pH difference (7.4 versus\n7.8). Because this initial difference was already accounted for in the RHE-referenced potentials,\nthe minor local pH shifts do not exert any additional influence on the KIE analysis.”\n“The proton-donating ability of the anions may not only facilitate protonation steps in CO RR,\n2\nbut also provide buffering capacity to mitigate local pH changes during reactions.61,62 To assess\nthe contribution of such mechanistic factors, we employed the SECM with pH-sensitive probes to\nmonitor the local pH during CO RR in different electrolytes. As shown in Supplementary Fig. 16a,\n2\nthe local pH near the CoPc/CNTs electrode during CO RR in NaClO increased markedly, from\n2 4\nan initial bulk pH of 4.3 to a local pH of 8.6 at a current density of 5.0 mA cm−2, reflecting the\nabsence of buffering capacity in NaClO . In contrast, the pH rise in NaHCO was much smaller\n4 3\n(Supplementary Fig. 13) due to its buffering nature. To account for pH-dependent thermodynamic\neffects when comparing CO RR activities, we re-calculated the activities using RHE-referenced\n2\npotentials corrected by the measured local pH values (Supplementary Fig. 16b). Even after this\ncorrection, the CO RR activities in NaHCO remained consistently higher than those in NaClO ,\n2 3 4\nindicating that differences in buffering or local pH do not explain the variation in CO RR activity\n2\namong the anions. Instead, the higher CO RR activity in NaHCO is mainly attributed to enhanced\n2 3\n*CO protonation on CoPc/CNTs due to the proton-donating ability of the HCO − anion.”\n2 3\n9. Why was no liquid product detected from CoPc system?\nResponse: We thank the reviewer for this comment. To address the concern regarding CH OH\n3\nproduction on CoPc/CNTs, we further performed bulk electrolysis of CO RR in 1 M NaHCO at\n2 3\nmore negative potentials, as shown in the figure below. CoPc/CNTs indeed produces CH OH, but\n3\nonly within a narrow and more negative potential window (<−0.80 V vs RHE) and substantially\nlower Faradaic efficiencies compared to CO. Thus, under our experimental conditions, CH OH is\n3\nnot a major CO RR product on CoPc/CNTs. Interestingly, upon surveying the literature, we found\n2\npronounced variability in the reported CH OH production performance on CoPc. Even when CoPc\n3\nis supported on CNTs, the reported partial current densities for CH OH vary widely (from\n3\nnegligible to ~10 mA cm−2), and the corresponding Faradaic efficiencies span from negligible to\n~40% (Nature 2019, 575, 639−642; Nat. Catal. 2024, 7, 1000−1009; ACS Catal. 2024, 14,\n366−372; ACS Appl. Energy Mater. 2024, 7, 3091−3098; J. Mater. Chem. A 2024, 12, 31547–\n22\n31556). Such discrepancies suggest that CH OH formation on CoPc is highly sensitive to subtle\n3\nvariation in experimental conditions. While CH OH production on CoPc is an intriguing topic, a\n3\ndetailed investigation of this pathway is beyond the scope of our present study, which focuses on\nCO RR to CO on metal phthalocyanines and aims to elucidate the fundamental rate-limiting step.\nthe revised manuscript: “In addition, methanol can be formed as a liquid product during CO RR on\n3\n−0.80 V versus RHE), as shown in Supplementary Fig. 7. However, methanol formation occurs\nonly at high overpotentials with relatively low selectivity compared to CO production. Given that\nthe present work is centered on CO RR to CO on metal phthalocyanines and aims to elucidate the\n2\nfundamental rate-limiting steps governing this primary reaction pathway, the following studies\nwill focus on the CO -to-CO conversion.”\n2\n10. Were the CoPc and NiPc molecules molecularly dispersed or forming aggregates on the CNT\nsurface? Detailed characterization results should be supplied.\nResponse: We thank the reviewer for this comment. Characterization of the dispersion state of\nCoPc and NiPc in the CoPc/CNTs and NiPc/CNTs samples was conducted, and the corresponding\ndiscussion is included in the “Characterization of CoPc/CNTs and NiPc/CNTs Catalysts” section\nof the original manuscript. For clarity and verification, we provide a brief summary of these results\nbelow. The molecular dispersion of CoPc and NiPc in the CoPc/CNTs and NiPc/CNTs samples is\nfurther supported by: (i) comparison of morphology with intentionally aggregated samples, and (ii)\nexamination of their crystalline structures using XRD.\nFirst, CoPc aggregates in the aggregated CoPc sample (prepared by drop-casting a DMF\nsolution of CoPc onto an AvCarb GDS2230 substrate) and in the CoPc/CNTs mixture sample\n(prepared by grinding CoPc powders with CNTs) exhibits a nanoparticle or nanoflake morphology,\nwith feature sizes ranging from sub-micrometers to several micrometers, as shown in the figures\nbelow.\nThe CoPc/CNTs mixture sample (Supplementary Fig. 10):\nIn contrast, no such aggregated domains were observed in the CoPc/CNTs or NiPc/CNTs\nsamples. SEM imaging over large areas, as well as TEM imaging combined with EDS mapping,\nconsistently reveal a uniform dispersion on CNTs without any detectable CoPc or NiPc aggregates\n(see the figures below).\nThe NiPc/CNTs sample (Supplementary Fig. 2):\nThe CoPc/CNTs and NiPc/CNTs sample (Fig. 2):\nSecond, our XRD analysis confirmed the absence of molecular aggregation in the CoPc/CNTs\nsample. No diffraction peaks corresponding to bulk crystalline CoPc were observed, whereas a\npronounced CoPc peak appears in the CoPc/CNTs mixture sample, as shown in the figures above.\nThis structural evidence corroborates that CoPc is molecularly dispersed on the CNT support in\nthe CoPc/CNTs sample.\nThis manuscript examines the reaction mechanism, specifically the rate-limiting step, of the\nelectrochemical conversion of CO to CO using metal phthalocyanine catalysts. The rate-limiting\n2\nstep is identified as the protonation of adsorbed *CO on dispersed CoPc-supported CNTs. In\n2\ncontrast, on aggregated CoPc, CO adsorption becomes the rate-limiting step. The authors\n2\ninvestigated how the dispersion rate and electrolyte components (anions and cations) affect the\nCO RR to CO on CoPc/CNTs, thereby guiding the rational design of molecular electrocatalysts.\n2\nto CO on metal phthalocyanine catalysts. Major Revision. Some specific comments are provided\nbelow.\nResponse: We thank the reviewer for the positive evaluation of our work and we deeply appreciate\nthe constructive comments and suggestions, which have helped us greatly improve the manuscript.\nIn response, we have further performed new experiments with new data and discussions to deepen\nthe mechanistic understanding and strengthen our manuscript. Our major revisions include:\n(1) We have gained a deeper understanding of the mechanism and attributed the shift in the rate-\nlimiting step on aggregated CoPc to a weakened interfacial electric field at the Co active sites,\nas supported by solid evidence presented in Figure 5 of the revised manuscript.\n(2) We have employed in situ scanning electrochemical microscopy (SECM) to probe the local\npH under operando conditions and have directly addressed all comments related to local pH,\nas demonstrated by the results in Supplementary Figs. 12, 13, and 16.\n(3) We have addressed other comments by incorporating new results and discussions in the revised\nmanuscript.\nOur point-to-point responses to the comments and corresponding revisions are described below.\nWe believe that our manuscript has been greatly improved after the revisions. A copy of the revised\nmanuscript with all changes highlighted using “Track Changes” is provided to facilitate review.\n1. I would recommend that authors add in-situ microscopy evidence to monitor the reaction\nintermediates to support the mechanism study further.\nResponse: We thank the reviewer for this valuable comment. Following the reviewer’s suggestion,\nwe performed additional mechanistic studies using in situ scanning electrochemical microscopy\n(SECM). In the revised manuscript, SECM is employed to monitor the local pH evolution near the\ncathode in H O and D O electrolytes, as well as in electrolytes containing different anions. These\n2 2\nmeasurements allow us to directly probe and decouple the possible thermodynamic contributions\narising from local pH shifts during CO RR. Consequently, the operando SECM results confirm a\n2\nrate-limiting *CO protonation step for CO RR on molecularly dispersed metal phthalocyanines,\n2 2\nas well as the proton-donation role of HCO − in facilitating this step. These findings substantially\n3\nstrengthen the mechanistic understanding in the revised manuscript.\nFirst, isolating intrinsic isotope effects from pH-induced artifacts is essential for a reliable\ninterpretation of the KIE results and the associated rate-limiting step of CO RR. We used SECM\n2\nto probe the local pH near the CoPc/CNTs electrode under reaction conditions in the NaHCO /H O\n3 2\nand NaDCO /D O electrolytes, as shown in the figure below. A customized SECM setup that was\n3 2\ndeveloped by our own group (Ph.D. Dissertation, 2023, University of Central Florida, Orlando,\nFL, https://stars.library.ucf.edu/etd2020/1631/) with an Au nanoelectrode tip functionalized with\n26\n4-hydroxylaminothiophenol (4-HATP)/4-nitrosothiophenol (4-NSTP) was used to measure the\nlocal pH near the cathode (Anal. Chem. 2020, 92, 2237−2243; JACS Au 2021, 1, 1915−1924). A\ncorrelation between the mid-peak potential of the 4-HATP/4-NSTP anodic peak and the electrolyte\npH was established as a calibration curve (see the figure below).\nUsing this approach, we successfully measured the local pH near the CoPc/CNTs electrode\nunder various CO RR current densities, as presented in the figure below. While a slight increase\n2\nin local pH was observed during CO RR in both electrolytes, the relative difference between them\n2\nremained ~0.4 pH units, consistent with the initial bulk pH difference (7.4 versus 7.8). Because\nthis initial difference was already accounted for in the RHE-referenced potentials, the minor local\npH shifts do not exert any additional influence on the KIE analysis. This indicates that the\nmeasured KIE primarily arises from an intrinsic isotope effect rather than from pH-induced\nthermodynamic artifacts, thereby confirming a rate-limiting *CO protonation step on the\n2\nmolecularly dispersed metal phthalocyanines.\n13, respectively. The following discussion has been incorporated into the revised manuscript:\n“Second, H O and D O exhibit a modest difference in their acid–base properties (pK = 14.00\n2 2 w\nfor H O and 14.86 for D O), resulting in bulk pH/pD values of 7.4 and 7.8 for CO -saturated 1 M\n2 2 2\nNaHCO and NaDCO electrolytes, respectively. In the above measurements, all potentials were\n3 3\nconverted to the RHE scale using the corresponding bulk pH, thereby accounting for pH-\ndependent thermodynamic effects in the comparison of CO RR activities. Nevertheless, local pH\n2\nmay change during reactions and influence reaction kinetics. To isolate genuine isotope effects\nfrom possible pH-induced artifacts, we employed in situ scanning electrochemical microscopy\n(SECM) to probe local pH under operando conditions. As shown in Supplementary Fig. 12, a\ncustomized SECM setup,42 equipped with an Au nanoelectrode tip whose exposed apex was\nfunctionalized with 4-hydroxylaminothiophenol (4-HATP)/4-nitrosothiophenol (4-NSTP), was\nused to measure the local pH near the cathode.43,44 A correlation between the mid-peak potential\nof the 4-HATP/4-NSTP anodic peak and the electrolyte pH was established as a calibration curve\n(Supplementary Fig. 12). Using this approach, we quantified the local pH near the CoPc/CNTs\nelectrode under various CO RR current densities, as presented in Supplementary Fig. 13. While a\n2\nslight increase in local pH was observed during CO RR in both electrolytes, the relative difference\n2\nbetween them remained ~0.4 pH units, consistent with the initial bulk pH difference (7.4 versus\n7.8). Because this initial difference was already accounted for in the RHE-referenced potentials,\nthe minor local pH shifts do not exert any additional influence on the KIE analysis.”\nFurthermore, decoupling the proton-donation role of HCO − from its pH-buffering behavior\n3\nis essential for elucidating the origin of its promotional effect on CO RR with CoPc/CNTs. To\n2\nevaluate the buffering capability of HCO −, we monitored and compared the local pH near the\n2 2 3\nNaClO electrolytes using the SECM equipped with pH-sensitive probes, with the results exhibited\n4\nin the figure below.\nThe local pH during CO RR in NaClO increased substantially, from an initial bulk pH of 4.3\n2 4\nto 8.6 at a current density of 5.0 mA cm−2, reflecting the absence of buffering capacity in NaClO .\n3\ndecouple the proton-donation effect of HCO − from its pH-buffering capacity, we compared the\n2 3 4\nmeasured local pH values, thereby removing the pH-dependent thermodynamic contributions. As\nshown in the figure above, even after this correction, the CO RR activity in NaHCO remained\n2 3\n28\nconsistently higher than in NaClO , indicating that differences in pH-buffering do not explain the\n4\nvariation in CO RR activity among the anions. Instead, the higher CO RR activity in NaHCO is\n2 2 3\nprimarily attributed to the intrinsic proton-donation effect of HCO −, which enhances the *CO\n3 2\nprotonation step on CoPc/CNTs.\nThe above figure has been added as Supplementary Figure 16, and the following discussion\nhas been added to the revised manuscript: “The proton-donating ability of the anions may not only\nfacilitate protonation steps in CO RR, but also provide buffering capacity to mitigate local pH\n2\nchanges during reactions.61,62 To assess the contribution of such mechanistic factors, we employed\nthe SECM with pH-sensitive probes to monitor the local pH during CO RR in different electrolytes.\n2\nin NaClO increased markedly, from an initial bulk pH of 4.3 to a local pH of 8.6 at a current\n4\ndensity of 5.0 mA cm−2, reflecting the absence of buffering capacity in NaClO . In contrast, the pH\n4\nrise in NaHCO was much smaller (Supplementary Fig. 13) due to its buffering nature. To account\n3\nfor pH-dependent thermodynamic effects when comparing CO RR activities, we re-calculated the\n2\nactivities using RHE-referenced potentials corrected by the measured local pH values\n(Supplementary Fig. 16b). Even after this correction, the CO RR activities in NaHCO remained\n2 3\nconsistently higher than those in NaClO , indicating that differences in buffering or local pH do\n4\nnot explain the variation in CO RR activity among the anions. Instead, the higher CO RR activity\n2 2\nin NaHCO is mainly attributed to enhanced *CO protonation on CoPc/CNTs due to the proton-\n3 2\ndonating ability of the HCO − anion.”\n3\n2. Except for the effects of anions and cations, will the pH variation affect the rate-limiting step\non metal phthalocyanine?\nResponse: We thank the reviewer for this comment. To evaluate whether electrolyte pH influences\nthe rate-limiting step of CO RR on metal phthalocyanines, we performed additional KIE analysis\n2\non CoPc/CNTs in acidic media. We note that alkaline electrolytes are unsuitable for this analysis\nas the rapid reaction of continuously supplied CO with OH− steadily neutralizes the solution.\n2 2 4 2\n4 2\nbe adjusted to such acidic conditions because of their buffering capacity. As shown in the figure\nbelow, the partial current densities for CO production in NaClO /H O were consistently higher\n4 2\nthan those in NaClO /D O, yielding KIE values of 1.3~1.7. This indicates that the rate-limiting\n4 2\nstep remains the *CO protonation step on CoPc/CNTs in acidic media, and it is not altered by\n2\nchanging the electrolyte pH from near-neutral to acidic conditions.\nthe revised manuscript:\n“The above studies were performed in near-neutral electrolytes, while electrolyte pH may also\ninfluence the rate-limiting step of CO RR on metal phthalocyanines. Therefore, we further\n2\nexamined the rate-limiting step of CO RR on CoPc/CNTs in acidic media (1 M NaClO /H O and\n2 4 2\n4 2\npartial current densities for CO production in NaClO /H O were consistently higher than those in\n4 2\nNaClO /D O, yielding KIE values of 1.3~1.7. This indicates that the rate-limiting step of CO RR\n4 2 2\non metal phthalocyanines is not altered by pH variation.”\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is\npart of the Nature Communications initiative to facilitate training in peer review and to provide\nappropriate recognition for Early Career Researchers who co-review manuscripts.\nResponse: We thank the reviewer for co-reviewing our manuscript and for offering constructive\ncomments that have strengthened the quality of our work.\n31"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have improved the quality of their manuscript by conducting new experiments and clarifying further mechanistic understanding. The local pH measurement by SECM is highly recommended. I am fine with most of their responses. Significant improvement can be seen in the revised manuscript compared to the original one.\nRegarding to the product selectivity of CO versus methanol over CoPc with different supports, desperation and even Nafion additives, I would suggest the authors to provide more robust evidences, as this is an important unaddressed issue in the field. While the reported methanol FE in this work is low even at very negative potentials, higher methanol selectivity at less negative potentials is often reported in literature. Insights into such divergence would make much sense.", "reviewer_2": "The authors have provided comprehensive and thorough responses to the raised questions, supplemented with robust evidence. The manuscript has been substantially improved and is now recommended for publication.", "reviewer_3": "They have done an excellent work for the revision. I do not have extra comments this time. Please make a decision for this manuscript.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "Response to Referees\nReviewer #1:\nThe authors have improved the quality of their manuscript by conducting new experiments and\nclarifying further mechanistic understanding. The local pH measurement by SECM is highly\nrecommended. I am fine with most of their responses. Significant improvement can be seen in the\nrevised manuscript compared to the original one.\nResponse: We sincerely thank the reviewer for the positive evaluation of the revised manuscript.\nWe appreciate your recognition that our new experiments and discussions have strengthened the\nmechanistic understanding and improved the overall quality of the manuscript. We appreciate the\ntime and effort you invested in the review process and your constructive comments, which have\ngreatly improved the quality of our work.\nRegarding to the product selectivity of CO versus methanol over CoPc with different supports,\ndesperation and even Nafion additives, I would suggest the authors to provide more robust\nevidences, as this is an important unaddressed issue in the field. While the reported methanol FE\nin this work is low even at very negative potentials, higher methanol selectivity at less negative\npotentials is often reported in literature. Insights into such divergence would make much sense.\nResponse: We thank the reviewer for this comment and suggestion. We agree that there seems to\nbe a divergence between our results and some prior reports regarding CO RR selectivity toward\n2\nmethanol (CH OH) on CoPc/CNTs. However, we would first like to emphasize that the potential\n3\nwindow and the optimal potential (around −1.0 V vs RHE in near-neutral electrolytes) for CH OH\n3\nproduction observed in our study are consistent with recent reports (Nat. Catal. 2024, 7, 987−999;\nAngew. Chem. Int. Ed. 2024, 63, e202310623; J. Mater. Chem. A 2024, 12, 31547−31556; ACS\nAppl. Energy Mater. 2024, 7, 3091−3098). Actually, CH OH formation at less negative potentials\n3\nhas been primarily reported when CO is used as the initial reactant (Nat. Synth. 2023, 2, 1194−1201;\nACS Catal. 2024, 14, 366−372). This difference is likely associated with the relay pathway on\nCoPc (CO → CO → CH OH), where direct CO electrolysis provides a higher CO availability and\n2 3\neliminates competition from CO adsorption, thereby facilitating CO reduction to CH OH.\n2 3\nTo explain and clarify the performance gap in CO RR to CH OH, we systematically optimized\n2 3\nseveral experimental parameters, including the support for CoPc, the Nafion content in the catalyst\nink, and the catalyst loading, as suggested by the reviewer. Regarding the dispersion of CoPc,\nprior studies show that aggregated CoPc greatly suppresses CH OH formation relative to molecularly\n3\ndispersed CoPc, an effect attributed to weakened cation stabilization of key reaction intermediates\n(Nat. Catal. 2024, 7, 987−999). In our work, CoPc is already molecularly dispersed on CNTs, so\nwe did not further modify the dispersion state of CoPc here.\nWe next compared CH OH production on CoPc supported on CNTs (CoPc/CNTs) and carbon\n3\nblack (CoPc/CB) at representative potentials. As included in our response to the first-round review,\nCoPc/CB exhibits slightly lower activity but comparable selectivity for CH OH production relative\n3\n1\nto CoPc/CNTs, as shown in the figure below (a,b). Therefore, under our experimental conditions,\nthe effect of the carbon support on CH OH production is not apparent.\n3\nproduction. As presented in the figure above (c,d), both the partial current density and Faradaic\nefficiency for CH OH remain largely unchanged as the Nafion amount increases from 2.5 to 15\n3\nµL mg−1 . The results suggest that, within the range examined, the Nafion content does not\nCoPc/CNTs\nplay a critical role in determining CH OH selectivity.\n3\n3\nfigure above (e,f). Notably, increasing the total catalyst loading results in an apparent increase in\nCH OH production. This trend is consistent with prior reports (Angew. Chem. Int. Ed. 2024, 63,\n3\ne202310623; J. Mater. Chem. A 2024, 12, 31547−31556) and can be rationalized by enhanced CO\nadsorption and subsequent reduction when competition from CO is alleviated per Co active site.\n2\nof ~8 mA cm−2 and a Faradaic efficiency >20% for CH OH production under our experimental\n3\nconditions. This performance is comparable to recent reports, such as 7−10 mA cm−2 partial current\ndensity with ~30% Faradaic efficiency for CH OH production by Shao-Horn et al. (Nat. Catal.\n3\n2024, 7, 1000−1009), and ~8 mA cm−2 partial current density with ~30% Faradaic efficiency for\nCH OH production by Wang et al. (Nat. Nanotechnol. 2025, 20, 515−522).\n3\nand cited relevant references to support this discussion, as follows: “In addition, methanol can\nbe formed as a liquid product during CO RR on CoPc/CNTs,31 which was detected in 1 M NaHCO\n2 3\nelectrolyte at more negative potentials (< −0.80 V versus RHE), as shown in Supplementary Fig.\n7. However, methanol formation occurs only at high overpotentials with relatively low selectivity\ncompared to CO production. The possible divergence between our methanol production\nperformance and prior reports likely arises from the high sensitivity of this pathway to\nexperimental parameters, such as CoPc-to-CNT mass ratio, catalyst loading, and CO partial\n2\npressure.31,33−35 Given that the present work is centered on CO RR to CO on metal\n2\nphthalocyanines and aims to elucidate the fundamental rate-limiting steps governing this primary\nreaction pathway, the following studies will focus on the CO -to-CO conversion.”\nthis present work is centered on CO RR to CO on metal phthalocyanines and aims to elucidate the\n2\nfundamental rate-limiting steps governing this primary reaction pathway. Therefore, we prefer not\nto incorporate the additional figure and related CH OH results into the current manuscript, as doing\n3\nso may expand the scope beyond the central mechanistic objectives. We appreciate the reviewer’s\nsuggestion and will address CH OH production on CoPc in our future work.\n3\nThe authors have provided comprehensive and thorough responses to the raised questions,\nsupplemented with robust evidence. The manuscript has been substantially improved and is now\nrecommended for publication.\nResponse: We sincerely thank the reviewer for the positive evaluation of the revised manuscript.\nWe appreciate your recognition of the quality of our responses and supporting evidence. We also\nappreciate the time and effort you invested in the review process and the constructive comments,\nwhich have greatly improved the quality and depth of our work.\nReviewer #3:\nThey have done an excellent work for the revision. I do not have extra comments this time. Please\nmake a decision for this manuscript.\nResponse: We thank the reviewer for the positive evaluation. We appreciate the time and effort\nyou invested in the review process and your constructive comments, which have helped improve\nthe quality of the manuscript.\nReviewer #4:\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is\npart of the Nature Communications initiative to facilitate training in peer review and to provide\nappropriate recognition for Early Career Researchers who co-review manuscripts.\nResponse: We thank the reviewer for co-reviewing the manuscript. We appreciate the time and\neffort you invested in the review process and your constructive comments, which have helped\nimprove the quality of the manuscript.\n4"}, {"round": 3, "reviewer_comments": {"reviewer_1": "The authors have addressed my comments. The current manuscript is now suitable for publication in Nature Communications."}, "author_response": "Response to Referees\nReviewer #1:\nThe authors have addressed my comments. The current manuscript is now suitable for publication\nin Nature Communications.\nResponse: We thank the reviewer for the positive evaluation. We are grateful for your time and\nconstructive input throughout the review process, which have greatly improved the quality of our\nwork.\n1"}]

Decision letter