In-situ grafting of cobalt phthalocyanine on gas diffusion electrodes enables ampere-level CO_2 reduction

Domain: physics
Published: 2025-12-08
Editorial outcome: major_revision

Abstract
Immobilizing molecular catalysts onto gas diffusion electrodes (GDEs) through covalent bonds provides a strategy to circumvent their issues of aggregation, detachment and poor conductivity during CO2 electrolysis. However, this approach has been limited to catalysts equipped with specially designed functional groups, and directly covalent grafting of pristine molecular catalysts onto GDEs remains a formidable challenge. Herein, using pristine cobalt phthalocyanine (CoPc) as a model catalyst, we propose a polypyrrole (ppy) mediated electro-polymerization strategy that creates robust C–C bonds between GDEs and pristine CoPc. In this scheme, ppy acts as both the conductive linker and scaffold for pristine CoPc, and its electron donation effects further enhances the CO2 electrolysis activity of CoPc centres. Here we show that the assembled CO2 electrolyzer using CoPc/ppy/GDE electrode achieves stable operation for 120 h at 500 mA cm−2 and 50 h at 1 A cm−2 in alkaline media. When coupled with a triple-junction solar cell, the resulting photovoltaic-electrolysis system attains a solar-to-CO efficiency of 19.2%.

Introduction
Electrocatalytic conversion of CO2 into value-added chemicals represents a strategy to mitigate the climate crisis caused by greenhouse gases while also facilitating the storage of renewable energy1–3. Among various catalytic systems, molecular catalysts, particularly metal phthalocyanines and metal porphyrins, have emerged as promising candidates due to their well-defined atomic active sites and tunable electronic structures4–6. However, under operating conditions, especially at industrially relevant current densities (>200 mA cm−2), molecular catalysts often suffer from leaching, agglomeration, or structural degradation, which hinders their practical applications7–12. Recently, metal phthalocyanine-based molecular catalysts have demonstrated highly selective CO2 electrolysis at high current densities in flow reactors13,14. Typically, these molecular catalysts are generally coated onto the carbon-based gas diffusion electrodes (GDEs) to address issues of aggregation and poor conductivity. The majority of the previous works have relied on noncovalent anchoring strategies, which often require binder such as Nafion and inccorporate scaffolds like carbon nanotube (CNT) and graphene to enhance the dispersion and adhesion of the molecular catalysts through π–π stacking interaction15–17.

Considering the possible loss of molecular catalysts during operation, particularly at high current densities, anchoring them directly onto carbon-based GDEs through strong covalent bonds presents a desirable strategy18–20. Currently, the direct covalent grafting of metal phthalocyanine/porphyrin-based molecular catalysts onto carbon-based GDEs requires the use of materials that are specially designed functional groups (such as amino groups) or polymerizable groups (such as carbazole groups). Until now, due to the inherent chemical inertness of pristine metal phthalocyanines/porphyrins, direct electrochemical polymerization or covalent grafting of them onto carbon-based GDEs has been challenging. Furthermore, among the common covalent bonds that can form between metal phthalocyanines/porphyrins (or their derivatives) and carbon-based GDEs, C–C bond offers more inert and robust for extreme reaction conditions, when comparing to those hydrolysable amide and ester bonds19.

Herein, using pristine CoPc as the example, we report an in situ electro-polymerization strategy for the direct covalent grafting of pristine metal phthalocyanines onto GDEs. This method occurs at room temperature and leverages the conductive polymer ppy as a molecular “glue”, forming robust C–C bonds with pristine CoPc. Meanwhile, besides acting as the scaffold, the poly-based matrix also shows electron donation effects that enhance the activity of active Co+ sites. To bridge the gap between material innovation and industrial application, we further engineered a membrane electrode assembly (MEA) electrolyzer incorporating the CoPc/ppy/GDE cathode. This device achieves stable CO production for 50 h at ampere-level current densities (1 A cm−2) in alkaline media. The proposed strategy thus addresses the challenge of direct covalent grafting of pristine metal phthalocyanines onto GDEs for efficient and durable CO2 reduction reaction (CORR) under industrially relevant current densities (>200 mA cm−2).

Results
Synthesis of CoPc/ppy/GDE
Figure 1a illustrates the traditional strategies for immobilizing molecular catalysts onto supports (exemplified by CoPc-based catalysts), including physical adsorption, introducing π‒π stacking interaction (with CNT or graphene), and forming covalent bonds. For the physically adsorbed or CNT-(or graphene-) supported molecular catalysts, they are still prone to aggregation or detachment, due to their relatively weak interactions with the supports. Although covalently bonded molecular catalysts are robust against detachment and aggregation, this approach generally requires the presence of specially designed functional groups (such as amino and carbazole groups). To functionalize molecular catalysts with these groups, sophisticated synthesis protocols are needed, which prohibit the large-scale application of this covalent bonding strategy in molecular catalysts. In contrast, in our strategy of ppy-mediated electro-polymerization, the oxidative polymerization of pyrrole monomers initiates the C–C coupling processes between GDEs and ppy, ppy and pristine metal phthalocyanines, thus connecting them together through robust C–C bonds21. The yielded CoPc/ppy/GDE electrode combines the advantages of robust C–C bonds, binder-free interface, highly conductive ppy linker, appropriate dispersion of Co single-atom sites (Fig. 1b). Moreover, no pretreatment of carbon-based GDEs is required, which minimizes the amount of oxygen-containing functional groups (such as carboxyl and hydroxyl groups), thereby guaranteeing sufficient hydrophobicity for efficient CO2 gas penetration.

We conducted a series of electrochemical analyses to understand the growth process of CoPc/ppy/GDE electrode. The cyclic voltammetry (CV) profiles of CoPc/ppy/GDE and control samples were recoreded in the potential window of −1.2 to 1.6 V versus Ag/AgCl at various scan rates. As shown in Fig. 1c, ppy/GDE obtained from electro-polymerization of pyrrole exhibits only broad and featureless oxidation and reduction waves between −1.0 and 1.0 V versus Ag/AgCl. Comparatively, three redox couples could be resolved for CoPc-modified GDE (CoPc/GDE), with their locations identified in Fig. 1d. These redox couples are probably associated with the redox transitions of Co centres in CoPc (Co2+ ↔ Co+ ↔ Co0). Notably, CoPc/ppy/GDE electrode produced in the presence of both pyrrole and CoPc shows similar redox characteristics to that of CoPc/GDE, with however negatively shifted peak positions and largely enhanced current response in the cathodic scan (Fig. 1d and Supplementary Fig. 1).

Next, we utilized in situ Raman spectroscopy to probe the structural evolution of CoPc/ppy/GDE electrode during the electro-polymerization process. The Raman spectra show a gradual increase in the intensities of peaks that correspond to the characteristic vibrational modes of CoPc. Specifically, the macrocyclic deformation mode at 680 cm−1 exhibits an increasing trend with the number of CV electro-polymerization cycles, quantitatively reflecting the progressive deposition of CoPc22,23. A shift in Co–N vibration at ~755 cm−1 was also observed, which maybe attributed to interfacial strain-induced changes of Co–N bonds in CoPc during electro-polymerization (Supplementary Fig. 2). Collectively, the progressively increased intensities of CoPc-related peaks with the number of CV cycles provide evidence for the in situ growth and integration of CoPc onto the GDE.

Third, we collected FTIR spectrum of CoPc/ppy/GDE and compared it with those of CoPc and ppy/GDE. To eliminate potential interference of free CoPc (i.e., CoPc adsorbed on GDE or ppy), CoPc/ppy/GDE was thoroughly cleaned by ultrasonication and washed with copious amounts of acetonitrile. As shown in Supplementary Fig. 3, the FTIR spectrum of CoPc/ppy/GDE exhibits combined features of both CoPc and ppy/GDE, confirming that pristine CoPc has been successfully incorporated into ppy/GDE24.

Based on these observations (and the exceptional operation stability of CoPc/ppy/GDE as discussed below), we infer that during pyrrole electro-polymerization, pristine CoPc is integrated into ppy, with the configuration of CoPc terminated at the ppy chain; electron donation occurs from ppy to CoPc, thus yielding negatively shifted redox peaks relative to bare CoPc. Such inferences are further supported by density functional theory (DFT) calculations, in which strong chemical bonds form in ppy/GDE and CoPc/ppy/GDE; under this condition, ppy serves as an effective linker enabling robust covalent grafting of pristine CoPc onto GDE (Supplementary Figs. 4–6).

Characterization and properties of CoPc/ppy/GDE
The electrochemical CO2RR performance of CoPc/ppy/GDE electrodes was systematically optimized by adjusting two key parameters, including the number of CV scans and the feed ratio of pyrrole to CoPc (Supplementary Figs. 7–9). Structural characterization using aberration-corrected transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy, and TEM revealed a hybrid morphology of CoPc/ppy/GDE, which is composed of single-atom Co sites and randomly aggregated spherical structures (Fig. 2a–c, Supplementary Fig. 10). Inductively coupled plasma optical emission spectrometer (ICP-OES) quantification confirmed a higher Co content in CoPc/ppy/GDE (0.0837 wt%) compared to CoPc/GDE (0.0046 wt%), demonstrating the critical role of ppy in immobilizing pristine CoPc (Supplementary Table 1). To probe the chemical states, electronic structures, and coordination environments of the CoPc/ppy/GDE electrode, X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) were performed. X-ray absorption near-edge structure (XANES) spectrum of CoPc/ppy/GDE at the Co K-edge indicates an average Co oxidation state between metallic Co(0) and CoO(+2); comparing to pristine CoPc of D4h symmetry, its red-shifted absorption edge and weakened 1s→4pz transition at ~7710 eV suggest reduced symmetry of the Co–N coordination (Fig. 2d)25. Fourier-transform extended X-ray absorption fine structure analyses identify a dominant Co–N coordination peak at ~1.5 Å for both CoPc/ppy/GDE and pristine CoPc, indicating Co2+–N as the primary species in them (Fig. 2e). Wavelet-transforms of EXAFS spectra further confirm this conclusion by revealing prominent intensity maxima at ~5.9 Å−1; meanwhile, a minor intensity maximum is exclusively resolved at ~8.2 Å−1 over CoPc/ppy/GDE, which is inferred to arise from the Co+–N species (Fig. 2f, g, Supplementary Fig. 11)26,27. The XPS measurements further corroborate the modulated electronic structure of CoPc/ppy/GDE, revealing lower binding energies for Co 2p peaks than those of commercial CoPc (780.8 vs. 781.4 eV for Co 2p3/2; 795.9 vs. 796.5 eV for Co 2p1/2)28. These phenomena, as mentioned above, are consistent with the electron donation effects from ppy to the CoPc domain (Supplementary Fig. 12). Collectively, through the ppy-mediated covalent grafting strategy, improved CoPc dispersion, asymmetric coordination of the Co–N centre, and enriched electron density at Co sites are achieved in CoPc/ppy/GDE, which would enhance the CO2RR performance and selectivity27,29.

The CO2RR performance was evaluated in CO2-saturated 0.1 M KHCO3 aqueous solutions (pH=6.8±0.2) using a standard three-electrode H-type cell configuration. As shown by the linear scanning voltammetry (LSV) curves in Fig. 2h and Supplementary Fig. 13, CoPc/ppy/GDE reaches a cathodic current density of up to 40 mA cm−2 at −0.78 V versus reversible hydrogen electrode (RHE), higher than that of ppy/GDE (7.5 mA cm−2), CoPc/GDE (2.1 mA cm−2), and CoPc/CNT-GDE (9.5 mA cm−2). Subsequently, the Faradic efficiency of gaseous and liquid products was analysed via online gas chromatography and nuclear magnetic resonance (NMR) spectroscopy, respectively; only gas-phase CO and H2 were detected, with no liquid products observed over a broad potential range (Supplementary Figs. 14 and 15)30. Notably, CoPc/ppy/GDE achieves a CO Faradaic efficiency (FECO) of 96% at −0.8 V vs. RHE, significantly outperforming the control samples of CoPc/GDE (85%), ppy/GDE (10%), and CoPc/CNT-GDE (93%) under identical conditions (Supplementary Fig. 16). To highlight the excellent performance of CoPc/ppy/GDE electrode, a detailed comparison to other systems is provided in Supplementary Table 2.

To clarify the intrinsic activity of these samples, their cobalt loadings were measured by ICP-OES (Supplementary Table 1). Based on these values, the Co loading-normalized specific activity of these samples has been derived (measured at a scan rate of 1 mV s−1 to minimize the capacitive contributions), which follows the order of CoPc/ppy/GDE > CoPc/CNT-GDE > CoPc/GDE, demonstrating the unique advantages of ppy-mediated covalent grafting (including binder-free interface, highly conductive ppy linker, electron donation effect of ppy, appropriate dispersion and spatial configuration of Co single-atom sites) in CoPc/ppy/GDE for CO2RR (Supplementary Fig. 17). Furthermore, the measurements of electrochemical impedance spectroscopy (EIS) revealed a lower charge-transfer resistance (and correspondingly improved reaction kinetics) of CoPc/ppy/GDE than the control sample of CoPc/GDE (Supplementary Fig. 18 and Supplementary Table 3).

Complementary to electrocatalytic performance, long-term stability is a critical criterion for the practical application of catalysts. As an accelerated degradation test, thousands of CV cycles were performed on CoPc/ppy/GDE at a scan rate of 100 mV s−1 and negligible performance degradation was observed. Chronoamperometry measurements conducted at −0.75 V vs. RHE further reveal that CoPc/ppy/GDE delivers a stable operating current for 200 h, outperforming that of CoPc/CNT-GDE (Supplementary Fig. 19). Subsequently, the post-reacted CoPc/ppy/GDE was subjected to HAADF-STEM, XAS and XPS analyses. HAADF-STEM measurements confirmed that the aggregated sphere-like morphology of CoPc/ppy/GDE catalyst is retained, with well-preserved atomically dispersed Co sites (Supplementary Fig. 20). Meanwhile, no obvious changes in the valence state and coordination environment of Co could be resolved for post-reacted CoPc/ppy/GDE (Supplementary Fig. 21). In addition, XPS analysis reveals that although the intensities of Co 2p peaks in post-reacted CoPc/ppy/GDE have decreased (possibly due to the contamination from electrolytes), their binding energies remain largely unaltered (Supplementary Fig. 22). All these post-reaction analyses together demonstrate that our CoPc/ppy/GDE electrode with the covalent grafting feature is highly resistant to structural collapse/changes under prolonged electrolysis. The negligible demetallation, reduction and aggregation of Co active sites in CoPc/ppy/GDE guarantee its stability under highly reductive and high current density conditions.

While the aqueous-phase tests convincingly demonstrate the high CO2RR activity of CoPc/ppy/GDE, the inherent low solubility of CO2 in aqueous electrolytes (~34 mM) imposes severe mass transport constraints. Thus, a flow cell was constructed to facilitate reactant transport and distribution, in which CoPc/ppy/GDE served as the cathode. The LSV curve in Supplementary Fig. 23 clearly shows that CoPc/ppy/GDE requires only −0.68 V vs. RHE to deliver a current density of 300 mA cm−2, and the associated FECO of 97% and turnover frequency (TOF) of ~10 s−1 further demonstrate the practical application potential of CoPc/ppy/GDE for CO2RR (Supplementary Figs. 24 and 25).

Motivated by the CO2RR performance of CoPc/ppy/GDE, NiPc/ppy/GDE, CuPc/ppy/GDE, and FePc/ppy/GDE were further fabricated to extend the applicability of our covalent grafting strategy (Supplementary Fig. 26). Among them, NiPc/ppy/GDE exhibits a CO selectivity of 92% with a current density of 14.3 mA cm−2 at −0.7 V versus RHE, which are slightly inferior to that of CoPc/ppy/GDE; while FePc/ppy/GDE and CuPc/ppy/GDE demonstrate CO selectivity approaching 90%, accompanied with current densities of 9.3 and 9.8 mA cm−2, respectively. These results validate the compatibility of our covalent grafting approach with diverse phthalocyanine-based molecular catalysts.

Probe of active species and mechanistic insights into CO2-to-CO transformation
Previous studies have established that Co+ species in CoPc formed by one-electron reduction of Co2+ are the active site for CO2RR31,32. Through the diagnostic Raman signatures of Co+ (at ca. 1110 cm−1) and Co2+ species (at ca. 1140 cm−1), the identification and quantification of Co+ active species have been achieved33–35. Following these reports, the evolution of CoPc/ppy/GDE and the distribution of Co+ and Co2+ species (i.e., the ratio of Co+ to Co2+) under operating potentials were monitored through in situ surface-enhanced Raman spectroscopy (SERS) in Supplementary Fig. 27. As shown in Fig. 3a, b, characteristic peaks could be resolved at 1113 and 1146 cm−1 at −0.4 V versus RHE for both electrodes of CoPc/ppy/GDE and CoPc/CNT-GDE, which are reported to be associated with the Co+ and Co2+ species, respectively35. The signal intensity of Co+ species (at 1113 cm−1) over CoPc/ppy/GDE becomes prominent at ca. −0.7 V versus RHE, consistent with the redox behavior of Co2+/Co+ in CoPc; beyond this potential, the spectrum reaches a relatively steady state. In comparison, the formed Co+ species over CoPc/CNT-GDE is significantly lower in amount (Supplementary Figs. 28 and 29). Using the Raman peak area, the ratios of Co+ to Co2+ in CoPc/ppy/GDE and CoPc/CNT-GDE at various applied potentials were quantified. The results in Fig. 3c show that the formed Co+ species over CoPc/ppy/GDE are always higher than that of CoPc/CNT-GDE, which aligns with its better catalytic activity for CO generation36. It is inferred that the electron donation effects from ppy to CoPc are responsible. Furthermore, the stability of Co+ species during CORR was probed by in situ SERS. As shown in Supplementary Fig. 30, with the continuous application of a negative potential of −1.4 V versus RHE, no obvious degradation in the peak intensity of Co+ species could be detected on CoPc/ppy/GDE over a reaction period of 30 min, while for CoPc/CNT-GDE, a gradual decrease of Co+ species appears, possibly due to the progressive loss of CoPc from CoPc/CNT-GDE electrode.

Having determined that the electron-donating ppy promotes active Co+ formation, we employed in situ attenuated total reflection surface enhanced infrared absorption spectroscopy (in situ ATR–SEIRAS) to track the reaction intermediates and gain a molecular-level understanding of CO2-to-CO conversion over CoPc/ppy/GDE (Supplementary Fig. 31). For the CoPc/ppy/GDE electrode (Fig. 3d), upon the application of negative potentials, surface-bonded CO (CO*) appears and grows in intensity with increasing negative potential, indicating the gradual transformation of CO2 to CO37. Under the same condition, negligible CO* signals were observed on ppy/GDE and CoPc/GDE, while CoPc/CNT-GDE prepared according to the previous report showed a delayed onset potential of CO* formation along with a weaker signal intensity (Supplementary Figs. 32–34). Meanwhile, an inverse absorption band appears at 1230 cm−1 and grew in prominence as the applied bias goes more reductive; this phenomenon can be ascribed to the rapid depletion of HCO3− at large overpotentials. It is noted that under identical bias, CoPc/ppy/GDE exhibits the largest absorption bands for CO* and HCO3− among tested samples. These observations suggest the improved activity of CoPc/ppy/GDE for CO2RR than the control samples of ppy/GDE, CoPc/GDE and CoPc/CNT-GDE.

To further assess the evolutionary nature of the atomically dispersed Co active sites, ex situ XAS measurements were carried out. Co K-edge XANES spectra of CoPc/ppy/GDE were acquired after reacting in a CO2-saturated 0.1 M KHCO3 solution for different times. Under this condition, oxygen-containing species (OH−, H2O and reaction intermediates such as HCO3−, CO* and COOH*) may adsorb on CoPc/ppy/GDE, and upon removal of the applied potential valence-state increase of the Co centre would occur (Supplementary Fig. 35)38,39. Further evidences from the EXAFS analysis reveals a slight elongation of the Co–N bond (from 1.98 to 2.05 Å)40,41 and importantly the emergence of a Co–O coordination peak at ~1.2 Å (Fig. 3e)42. These phenomena are consistent with the coordination of oxygen-containing reactants/intermediates (e.g., OH−, HCO3− or COOH* species) to the Co centre and the correspondingly relaxed structure (elongated Co–N bond) in CoPc/ppy/GDE. In comparison, the blue-shift of absorption edge in the XANES curves and the Co–O coordination in the EXAFS spectra for CoPc/CNT-GDE are not as prominent as those of CoPc/ppy/GDE (Supplementary Fig. 36), indicating weaker adsorption of OH−, CO2, H2O reactants and less accumulation of oxygen-containing intermediates over CoPc/CNT-GDE than CoPc/ppy/GDE.

To gain more comprehensive mechanistic insights into ppy-enhanced electrocatalytic activity, free energy diagrams for the CO2-to-CO conversion on both CoPc/ppy/GDE and CoPc/GDE were constructed using DFT calculations. As shown in Fig. 3f, the formation of carboxyl intermediate (COOH*) through proton-coupled electron transfer (CO2 + H+ + e− → COOH*) is identified as the rate-determining step (RDS) for both electrocatalysts. Crucially, the free energy change for the RDS of CoPc/ppy/GDE (1.11 eV) is decreased by 0.2 eV compared to CoPc/GDE (1.31 eV), indicating the covalent grafting of ppy promotes the formation of COOH* on CoPc. Charge density difference analysis (Supplementary Figs. 37 and 38) reveal enhanced electron accumulation around COOH* on CoPc/ppy/GDE, attributed to the electron-donating characteristics of ppy. Thus, this electronic modulation reduces the activation energy of the RDS. Raw simulation data are provided as Supplementary Data 1. Collectively, experimental and theoretical results demonstrate that the incorporated ppy exerts an electron-donating effect on the CoPc centre, which not only facilitates the formation of active Co+ species but also accelerates the reaction kinetics via reducing activation energy for RDS, ultimately boosting the CO2-to-CO conversion activity of CoPc/ppy/GDE.

CO2RR performance of CoPc/ppy/GDE in an electrolyzer
The direct covalent grafting of pristine molecular catalysts onto GDEs not only maintains the structural advantages of the molecular catalyst but also simplifies the following electrolyzer manufacturing process. To integrate CoPc/ppy/GDE into practical electrochemical devices, an electrolyzer was assembled by using CoPc/ppy/GDE as the cathode and a recently reported CoFe-C@GQDs/NF as the anode43. As depicted by the LSV curve in Fig. 4a, CoPc/ppy/GDE-based MEA (CoFe-C@GQDs/NF||CoPc/ppy/GDE) demonstrates excellent activity for CO2-to-CO conversion, achieving a current density of up to 500 mA cm−2 at a cell voltage of ~2.8 V along with a high FECO of 99%. Importantly, the FECO values remain above 97% across a wide range of cell voltages. In addition, CoPc/ppy/GDE-based MEA displays operation stability under stringent conditions. As illustrated in Fig. 4b and Supplementary Fig. 39, when operated in 1.0 M KOH, the CoFe-C@GQDs/NF||CoPc/ppy/GDE electrolyzer achieves device stability of 50 h at 1 A cm−2 and 120 h at 500 mA cm−2, corroborating the resistance of CoPc/ppy/GDE electrode against structural collapse/changes under industrially relevant current densities. A comparative analysis with other reported systems is presented in Supplementary Table 4 and Fig. 4c17,26,44–52, which highlight the unique advantages of CoPc/ppy/GDE for CO2RR. To further assess the industrial applicability of CoPc/ppy/GDE electrode, scalability was executed through electrode area expansion (~9 cm2). As displayed in Supplementary Fig. 40, the scaled-up electrolyzer operated stably at 1 A for 20 h, with a cell voltage of ~2.5 V. Furthermore, a tandem device at the hectowatt scale (180 W) was assembled, which operated at a current density of 1 A cm−2 for 300 min with FECO ranging from 95 to 80% (Supplementary Fig. 41).

The CoFe-C@GQDs/NF||CoPc/ppy/GDE electrolyzer was integrated with a triple-junction solar cell (GaInP2/InGaAs/Ge) to assemble an unbiased, solar-driven CO2RR system (Fig. 4d). To synchronize the maximum outputs of solar cell and electrolyzer, the light intensity was incrementally adjusted (Fig. 4e). At the light intensity equivalent to 5 suns, this solar-driven CO2RR device achieves an impressive solar-to-fuel efficiency of 19.2%, with the electrolyzer’s operating point (2.23 V and 67 mA cm−2) close to solar cell’s maximum output (2.31 V and 73 mA cm−2). Furthermore, under 5-sunlight intensity, this solar-driven device shows essentially stable CO2RR for 50 h, demonstrating the effectiveness and robustness of covalent grafting-derived CoPc/ppy/GDE (Fig. 4f).

Methods
Reagents
Iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O, analytical reagent), potassium hydroxide (KOH, analytical reagent) and pyrrole were purchased from Aladdin. 1-butyl-3-methylimidazolium tetrafluoroborate was obtained from TCI. Cobalt nitrate hexahydrate (Co(NO3)2·6H2O) and cobalt phthalocyanine (CoPc) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals were used as received without further purification. A graphene oxide quantum dot (GQD) solution (1 mg mL−1) was purchased from Nanjing Xianfeng Nano Co., Ltd. Nickel foam (approximately 1 mm × 1 cm × 2 cm) was carefully cleaned by a specific HCl solution (1 M), ethanol and deionized water under ultrasonic conditions for 15 min and then dried under vacuum. The gas diffusion electrode (Sigracet 28BC, area: 1 cm × 1 cm) and anion exchange membrane (Sustainion x37-50 Grade RT) were purchased from eChemstore.

Characterization
Scanning electron microscopy images were obtained with a ZEISS ULTRA55 instrument (accelerating voltage of 20 kV). TEM and high-resolution transmission electron microscopy (HRTEM) images were obtained on an FEI Tecnai G2 F30. High-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) images were obtained with a JEOL JEM-ARM200F microscope equipped with a spherical aberration correction system for STEM. XPS measurements were performed using a PHI 5000 VersaProbe (Japan ULVAC-PHI). The binding energies for all the spectra were calibrated with respect to the C 1s line at 284.6 eV. A LabRAM ARAMIS Raman spectrometer (HORIBA Scientific) was used to measure the composition of the material and the stress state of the chemical bonds. A NEXUS 870 Fourier transform infrared (FTIR) spectrometer was used to perform surface chemical analysis in the range of 600‒4000 cm−1. ICP-OES was performed using an ICP Agilent 7850 (MS). The electrochemical tests were performed with a CS310X instrument (CORRTEST, Wuhan, China) and a M8873 instrument (Maynuo Electronics, Nanjing, China). X-ray absorption spectroscopy was performed via easyXAFS 300+ (easyXAFS, USA). The solution-phase products were analysed via Bruker Advance III HD 500 NMR. The gas phase products were analysed by a GC-9720 plus (FuLi, Zhejiang, China).

Synthesis of CoPc/CNT
To prepare CoPc/CNT, 20 mg of CNT were dispersed in 5 mL of pentanol with the assistance of >30 min of sonication. Subsequently, 20 mg of CoPc was added to the homogeneous suspension. The dispersion was subsequently transferred to an autoclave and heated at 180 °C for 2.5 h. The resulting product was collected by centrifugation at 8000 rpm, sequentially washed with ethanol, chloroform, 5% HCl solution, ethanol, and water, and finally vacuum-dried.

Preparation of the working electrodes of CoPc/ppy/GDE, CoPc/GDE and ppy/GDE
Preparations of CoPc/ppy/GDE: A homogeneous electrolyte was prepared by dissolving 0.010 mol of 1-butyl-3-methylimidazolium tetrafluoroborate in 50 mL of acetonitrile, followed by the addition of pyrrole and CoPc at designated molar ratios. Prior to electro-polymerization, the gas diffusion electrode was pretreated via plasma oxidation under vacuum with controlled oxygen flow using an atomic layer deposition system. Electro-polymerization was carried out in a three-electrode configuration: the pretreated catalyst as the working electrode, a Pt foil counter electrode, and an Ag/AgCl reference electrode (3 M KCl). Cyclic voltammetry was performed between −1.2 V and 1.6 V (versus Ag/AgCl) for 40 cycles at a scan rate of 100 mV s⁻¹.
The resultant CoPc/ppy/GDE was rinsed thoroughly with acetonitrile and dried under ambient conditions. The active material loading was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) based on cobalt content. Control samples (CoPc/GDE and ppy/GDE) were synthesized under identical electrochemical parameters.

Preparation of the working electrodes of CoPc/CNT-GDE
A uniform catalyst ink was formulated by dispersing 5 mg CoPc/CNT with 30 µL of 5 wt% Nafion solution in 2 mL of ethanol and vigorously sonicated for 30 min. Then, 200 µL ink was drop cast onto 1 × 1 cm2 carbon paper. The loading amount was calculated according to the cobalt content from ICP results.

Preparation of the working electrodes of CoFe-C@GQDs/NF
Co(NO3)2·6H2O (2 mmol), Fe(NO3)3·9H2O (2 mmol), Na3C6H5O7·2H2O (0.5 mmol) and urea (5 mmol) were dissolved in 60 mL of deionized water and stirred to form a clear solution. The aqueous solution was transferred to a 100 mL Teflon-lined stainless steel autoclave with NF, maintained at 120 °C for 10 h, and then allowed to cool to room temperature naturally. The samples were rinsed with distilled water and ethanol and then dried at 60 °C under vacuum. The mass loading of the catalyst was approximately 1 mg cm−2. One milliliter of GQD solution was diluted with deionized water to 20 mL, transferred to a 50 mL reactor with dried CoFe-C/NF, and maintained at 150 °C for 3 h. Afterwards, the obtained CoFe-C@GQDs/NF was washed and dried.

CO2RR measurements
The CO2RR performances of the CoPc/ppy/GDE electrodes were evaluated and compared to those of the CoPc/CNT-GDE, CoPc/GDE, and ppy/GDE electrodes. The catalytic performances of the electrocatalysts were first evaluated in a customized H-type cell on an electrochemical workstation (CS310M instrument) with two compartments separated by a Nafion 117 proton exchange membrane. The Nafion membrane underwent an immersion treatment in a 5% hydrogen peroxide solution at 80 °C for 30 min, followed by rinsing in deionized water for 10 min, then treatment in 5% sulfuric acid at 80 °C for an hour, and a final deionized water rinse for 30 min. Each compartment contained 10 mL of 0.1 M KHCO3 electrolyte, and the compartment holding the working electrode was prepared to quantify the gaseous products. The electrolyte was purged with CO2 at a flow rate of 5 sccm for 30 min before and during each measurement while stirring.
No special storage conditions were needed for the electrolyte beyond room temperature. Ag/AgCl and platinum (Pt) meshes were used as the reference and counter electrodes, respectively. All electrochemical measurements were performed at room temperature (23 ± 2 °C). The flow rate verification was conducted using a bubble flow meter at the cathode chamber’s outlet.
An 85% iR compensation of the solution resistance was applied automatically via the potentiostat. The measured potential was converted to an RHE scale by V (vs. RHE) = V (vs. Ag/AgCl) + 0.197 V + 0.0591 × pH – iR. Where pH ~ 6.8 ± 0.2 in 0.1 M CO2-saturated KHCO3 solution. The potential of the Ag/AgCl reference electrode was calibrated against a reversible hydrogen electrode (RHE, Gaoss Union, Inc.). Potentials include an 85% iR compensation, with the uncompensated resistance (RΩ) determined by extrapolating high-frequency impedance data, averaging around 10 ± 2 Ω in 0.1 M CO2-saturated KHCO3.
During constant potential electrolysis, we ran the reaction for at least 30 min before sampling the gaseous products. For the electrochemical measurements in a liquid flow cell, the anolyte and catholyte chambers were both filled with 1.0 M KOH and separated by an anion exchange membrane (Sustainion x37-50 Grade RT) on an electrochemical workstation (CS310M instrument). The Sustainion x37-50 Grade RT was soaked in 1.0 M KOH at room temperature for 24 h to fully convert it to the hydroxide form. CO2 at a constant flow rate of 20 sccm was introduced into the system during each measurement. The gaseous products were quantified via an online GC. Similarly, for MEA testing, the anolyte chamber was filled with 1.0 M KOH electrolyte, while the catholyte chamber was fed with humidified CO2 at a constant flow rate of 20 sccm. The chambers were separated by an anion exchange membrane (Sustainion x37-50 Grade RT) on an electrochemical workstation (CS310M instrument). The MEA testing with 9 cm2 was filled with 1.0 M KOH electrolyte, while the catholyte chamber was fed with humidified CO2 at a constant flow rate of 40 sccm a DC Power Supply (M8873 Maynuo). Besides, the tandem device at the hectowatt scale (180 W) was filled with 1.0 M KOH electrolyte, while the catholyte chamber was fed with humidified CO2 at a constant flow rate of 100 sccm on a DC Power Supply (M8873 Maynuo).

Product quantification
The Faradic efficiency (FE) was calculated via the following equation:
FE = z p G F V_CO / J R T (1)
Where V_CO was the volume concentration of CO, J was the steady-state total current, G was the CO2 flow rate. The constants were shown as follows: z = 2, p = 1.013 × 10^5 Pa, T = 298.15 K, F = 96485 C mol−1, R = 8.3145 J mol−1 K−1.
The TOF value was calculated via the following equation:
TOF = j / n F N_Co (2)
where j was the current density (A cm−2), A was the geometric electrode area (1 cm2), n represented the electron transfer number for CO2-to-CO conversion (n = 2), F was Faraday’s constant (96,485 C mol−1), and N_Co was the total number of Co sites quantified by ICP-OES. Actual Co loading of the optimized CoPc/ppy/GDE catalyst (obtained with 40 CV cycles) was measured to be 0.0837 wt% with ICP-OES.

The solar-to-fuel efficiency (STF) was calculated via the following equation:
STF = ΔE × j × FE% / P (3)
where ΔE is the equilibrium potential (1.34 V) for CO2-CO, j is the current density, and P is the incident solar power. A triple junction gallium arsenide battery was purchased from Hasunopto, and the effective area of the chip was 1 cm2.

Electrochemical characterization
In situ attenuated total reflectance-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) measurements.
In situ ATR-SEIRAS measurements were determined on a Thermo Scientific Nicolet iS50 instrument (USA) equipped with a liquid nitrogen-cooled HgCdTe (MCT) detector and an in situ attenuated total reflection-FTIR device (Hefei Insitu Technology Co., Ltd.). A silicon (Si) prism coated with gold particles (particle size <100 nm) and a catalyst was fixed in a commercial three-electrode H-type cell with an Ag/AgCl reference electrode, a Pt-wire counter electrode, and CO2-saturated 0.1 M KHCO3. The electrolyte was 0.1 M KHCO3 with continuous CO2 bubbling. All electrochemical tests were controlled using a CS310M instrument. There is no iR correction for voltages. All IR spectra were obtained via averaging 32 scans at a spectral resolution of 8 cm−1.

Operando surface-enhanced Raman spectroscopy (SERS) measurements
A LabRAM ARAMIS Raman spectrometer (HORIBA Scientific) equipped with an excitation of 532 nm laser was used to measure the composition of the material and the stress state of the chemical bonds. Operando measurements were conducted in a three-electrode cell, assembled with a catalyst-loaded working electrode, a Pt wire counter electrode, an Ag/AgCl reference electrode, and CO2-saturated 0.1 M KHCO3. Before the tests, all working electrodes were sputtered with a layer of nano-gold particles (particle size <100 nm). All electrochemical tests were controlled using a CS310M instrument. There is no iR correction for voltages. The Raman frequencies were calibrated by a Si wafer.

Computational methods
All the theoretical calculations are performed with a plane-wave basis set as implemented in the Vienna ab initio simulation package (VASP)53. The general gradient approximation with Perdew−Burke−Ernzerhof54 was employed to describe the exchange-correlation potential55. DFT+U was chosen for this study, in which the empirical value of U was set to 3.32 eV. A 30 Å vacuum layer was constructed to prevent interactions between layers56,57. The cut-off energy for the plane wave was set to 500 eV, and the Brillouin zone used a 1 × 1 × 1 mesh of special k points58.
The Gibbs free energy (G) of all the reaction steps was calculated via the following equation:
G = E_DFT + E_ZPE + ∫ C_p dT − TS (4)
where E_DFT, E_ZPE, ∫ C_p dT, and TS are the free energies calculated via DFT, zero-point energy, enthalpic temperature correction, and entropic temperature correction, respectively.

Data availability
The data that support the findings of this study are available within the article and its Supplementary Information and from the corresponding author upon reasonable request. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript reports an in situ growth method for immobilizing molecular catalysts on gas diffusion electrodes (GDE) through electrochemical covalent grafting at room temperature, with the goal of enhancing the stability and catalytic performance of cobalt phthalocyanine (CoPc). This approach demonstrates promising CO2-to-CO conversion efficiency and claims a record solar-to-fuel efficiency when integrated with a photovoltaic-electrolysis device. While this work suggests a potentially valuable contribution to CO2 reduction technology, it lacks sufficient structural characterization and fails to convincingly establish the novelty required for publication in Nature Communications. The main concerns are as follows:\n1.The manuscript lacks direct evidence supporting the formation of covalent bonds between CoPc and the polypyrrole (ppy) network. The FTIR spectra provided merely confirm the presence of CoPc and ppy/GDE individually but do not substantiate the claimed covalent interactions between CoPc molecules and the ppy matrix. To address this, the authors should incorporate additional spectroscopic techniques to directly confirm and visualize the proposed covalent bonding. Without such evidence, the structural integrity and uniqueness of the CoPc/ppy@GDE system remain speculative.\n2.The authors suggest that the lower frequency of the CO* peak on CoPc/ppy@GDE, compared to CoPc/CNT, reflects stronger CO* adsorption and greater CO coverage. However, it is unclear why the Co site would not undergo reduction to a zero-valence state under electrochemical reduction conditions. This assumption appears inconsistent with the Co K-edge spectra, which indicate an increase in the Co valence state during the reaction. The manuscript would benefit from a more detailed mechanistic explanation or additional experimental data to reconcile this apparent discrepancy in the behavior of the Co(I) site during CO2RR.\n3.Although the study reports promising catalytic stability at high current densities, further structural analysis of the CoPc/ppy@GDE catalyst before and after extended catalytic testing is essential to validate its durability.\n4.The electropolymerization approach for anchoring ppy on carbon supports is well-established and widely applied in CO2 reduction reactions. The manuscript does not sufficiently explain what differentiates CoPc/ppy@GDE from similar electropolymerized systems reported in the literature. The authors are encouraged to provide a clear comparative analysis to highlight any unique structural characteristics or performance advantages of CoPc/ppy@GDE.", "reviewer_2": "The authors address the immobilization limitations of phthalocyanine catalysts on electrode towards industrial-scale CO2 reduction, and graft CoPc onto gas diffusion electrode through polypyrrole, which was applied in membrane electrode assembly electrolyser for CO2 to CO. The present work was well organized with large characterization including XAS, in situ technology,and DFT calculation. However, there are still problems need further revision.\n1. How to know the molecular weight? Whether it influence the performance of CoPc/ppy@GDE catalyst? And how to ascertain the best polypyrrole?\n2. Line 136-139. Since the electron density of Co is enhanced, the binding energy should blueshift, while not redshift. This part needs reconsideration. The result is not consistent with XAS analysis.\n3. The fig.2a, it should be labelled for different color for better understanding. The Fig. 2f.g caption was not correct.\n4. Line 153, the pH needs further tested, it cannot be 8.3.\n5. How TOF was calculated according to the all Co sites? i.e. how to ascertain the Co sites?\n6. Line 241, from the calculation, we can only know that ppy alters the catalytic thermodynamics while not kinetic.\n7. Fig. 4f in present is not quite convincing.\n8. Since former work has published with CoPc cross-linked ppy( ChemSusChem 2022,15(23):e202201455), CoPc incorporated with ppy( ELECTROCHEMICAL AND SOLID STATE LETTERS 2009,12(8):E17-E19), what’s the novelty of present work?", "reviewer_3": "The authors presented a study on molecular catalyst immobilization through direct covalent grafting, demonstrating that CoPc anchored via pyrrole linkers (CoPc/ppy@GDE) exhibits enhanced stability and catalytic performance. The evidence from XAFS and XPS provides interesting insights into the electronic effects of pyrrole grafting on the Co center, while in-situ SERS and SEIRAS offer valuable mechanistic information about Co oxidation states and their role in the reaction pathway.\nI believe the fundamental observation of enhanced stability through pyrrole-based grafting is well-supported; However, I have a number of significant scientific and technical concerns that should be addressed in a major revision. I think the work could be suitable for publication after the authors strengthen their mechanistic understanding and provide more comprehensive evidence. Here are some detailed comments:\n1. The manuscript lacks a direct comparison of electrochemical impedance between CoPc@GDE and CoPc/ppy@GDE systems. To better understand the electron transfer characteristics of these catalysts, please provide Nyquist plots along with the corresponding serial resistance values for both systems.\n2. In lines 130-133, the notation system using Co(I)-N and Co(II)-N creates confusion in interpreting the oxidation states of cobalt. For example, Co(I)-N in CoPc@GDE can be interpreted specifically refers to Co(+1) coordinated with nitrogen. Please provide clearer terminology to distinguish the different cobalt species.\n3. There appears to be a typographical error in line 135 where \"XAENS\" should be corrected to \"XANES\"\n4. While Figure 2h demonstrates performance comparisons between CoPc/ppy@GDE, CoPc-GDE, and CoPc-CNT, the data appears to be collected using different cobalt loadings. Additionally, there is a discrepancy in the Co content values reported in the main text versus Supplementary Table 1. To make a fair assessment of catalyst performance, please provide comparisons with normalized cobalt content across all catalyst systems.\n5. The manuscript attributes the redshifted and broadened CO* bands observed on CoPc/ppy@GDE to surface coverage effects. However, two important questions need to be addressed: First, could dynamic dipole coupling of CO* produce similar spectral changes in single-atom catalysts? Second, the cited reference 37 indicates that high surface coverage typically results in blue-shifted CO* bands, which contradicts the current explanation. Please clarify these apparent discrepancies.\n6. The time-dependent blue shift in absorption edges shown in Figure 3d indicates a gradual increase in cobalt valence state during the reaction. This observation seems to contradict the proposed mechanism where catalyst stability and activity are attributed to maintained Co+ active species. To resolve this inconsistency, please provide time-resolved in-situ SERS data of CoPc/ppy@GDE at fixed potential.\n7. The authors showed the changes of CoPc/ppy@GDE at the fixed potential over the time and discuss the changes of the XANES and EXAFS spectra. However, control sample CoPc@GDE is also required to be measured over the same period of time to investigate whether the changes of the Co oxidation states are associated with ppy introduction.\n8, The author mentioned that “The absorption edges blueshifted with reaction time, indicating that the valence state of Co gradually increased during the reaction.” Is it reversible? If so, after the reaction, (post-analysis) showed the similar states of Co or does it go back to the initial states. If not, the authors want to discuss how the ppy helps the charge transfer. The changes of the oxidation states can be an indication of the degradation of the CoPc. Show the post-analysis, (TEM, EXAFS, XANES, XPS) can provide the information whether the catalyst maintained the initial CoPc/ppy structure.\n9. Can it be applicable to other Pc such as NiPc and FePc for electorchemical CO2RR?\n10. How the current density affected by the loading amount of the catalyst? That can be the useful information.", "reviewer_4": "This manuscript reported the synthesis of CoPc for the efficient conversion of CO2 to CO product. A photovoltaic-electrolysis device is also constructed and achieved a record solar-to-fuel efficiency of 19.2%. Despite this work provides a complete study, the novelty of the electrocatalytic CO2 to CO is not enough. Moreover, the electrolysis in alkaline electrolytes under high current density do not provide new insight. Whether this catalyst is suitable for MEA testing under pure water conditions. Thus, the novelty and depth of academic significance does not reach the level of Nature Communications."}, "author_response": "Responses to Editor and Reviewers' Comments\n(manuscript ID: NCOMMS-24-65311-T)\nGeneral response:\nWe express sincerely thank for the editor, editorial staff and all reviewers for your valuable\nfeedbacks that we have based on to further enhance the quality and completeness of our manuscript.\nThe manuscript has been modified point-by-point after addressing all the suggestions as listed below.\n(Our response is given in blue and the corrections in the revised manuscript are highlighted with a\nyellow background).\nResponse to the Editor Office:\nWe agree with the importance of validating our findings under practical conditions and will conduct\nadditional activity and stability tests in a scaled-up MEA with an electrode area of 9 cm2 (Figure\nR1). These new experiments will be analyzed, and integrated into the revised manuscript to\nstrengthen the applicability of our conclusions.\nFigure R1. a, LSV and Faradaic efficiency of CO -to-CO conversion in the CoFe-\ni\nStability test at 1 A with corresponding Faradaic efficiency of the electrolyzer in 1.0 M KOH.\nReviewer #1:\nComment: The manuscript reports an in situ growth method for immobilizing molecular catalysts\non gas diffusion electrodes (GDE) through electrochemical covalent grafting at room temperature,\nwith the goal of enhancing the stability and catalytic performance of cobalt phthalocyanine (CoPc).\nThis approach demonstrates promising CO -to-CO conversion efficiency and claims a record solar-\n2\nto-fuel efficiency when integrated with a photovoltaic-electrolysis device. While this work suggests\na potentially valuable contribution to CO2 reduction technology, it lacks sufficient structural\ncharacterization and fails to convincingly establish the novelty required for publication in Nature\nCommunications. The main concerns are as follows:\nComment 1: The manuscript lacks direct evidence supporting the formation of covalent bonds\nbetween CoPc and the polypyrrole (ppy) network. The FTIR spectra provided merely confirm the\npresence of CoPc and ppy/GDE individually but do not substantiate the claimed covalent\ninteractions between CoPc molecules and the ppy matrix. To address this, the authors should\nincorporate additional spectroscopic techniques to directly confirm and visualize the proposed\ncovalent bonding. Without such evidence, the structural integrity and uniqueness of the\nCoPc/ppy@GDE system remain speculative.\nReply to comment 1: Thank you for your valuable comments on our manuscript. We understand\nyour concerns regarding the direct evidence for the formation of covalent bonds between CoPc and\nthe ppy network. We have taken your suggestions seriously and have conducted additional\nexperiments and analyses to address this issue.\nFirst, we utilized in situ Raman spectroscopy to monitor the structural evolution of CoPc during\nthe electro-polymerization on GDE. As shown in Figure R2, the Raman spectra reveal gradually\nincreased peaks that correspond to the characteristic vibrational modes of CoPc. Specifically, the\nmacrocyclic deformation mode at 680 cm−1 exhibits an increase trend with the number of CV\npolymerization cycles, quantitatively correlating with the progressive deposition of CoPc. We also\nobserved a shift of Co–N vibration at ca. 755 cm−1, which is possibly associated with the interfacial\nstrain-induced changes of Co–N bonds in CoPc during polymerization1. Collectively, the\nprogressively increased intensities of CoPc-related peaks with the number of CV cycles2, provide\nevidence for the in situ growth and integration of CoPc onto the GDE.\nSecond, to provide further evidence for the covalent integration of CoPc with ppy on the GDE,\nCV cycles were systematically performed within under controlled potential windows. As shown in\nFigure R3, ppy generated from electrochemical polymerization of pyrrole shows only broad and\nfeatureless oxidation and reduction waves in the potential range of –1.0 to 1.0 V vs. Ag/AgCl.\nComparatively, three redox couples could be resolved for CoPc-modified GDE (CoPc/GDE), with\ntheir locations presented in Table R1. These redox couples are probably associated with the redox\nbehaviours of Co centres of CoPc (Co2+↔Co+↔Co0). Interestingly, CoPc/ppy/GDE electrode\nproduced in the presence of both pyrrole and CoPc shows similar redox characteristics to that of\nCoPc/GDE, with however negatively shifted peak positions and largely enhanced current response\nin the cathodic scan. Based on this observation, we infer that during pyrrole polymerization CoPc\nis integrated into ppy, with the configuration of CoPc terminated at the ppy chain; electron donation\noccurs from ppy to CoPc, thus yielding negatively shifted redox peaks relative to bare CoPc.\nFigure R2. In situ Raman spectra of CoPc/ppy/GDE collected at different CV cycles in 0.1 M\n[BMIM]BF with 4 mM pyrole and 1 mM CoPc.\nof CoPc/ppy, which would exclude the influence of carbon-based substrates. However, the weak\nbonding between FTO and CoPc/ppy led to easy detachment of product. We also tried to monitor\nthe deposition process of CoPc/ppy through the microcrystal quartz balance, but no available signals\ncould be collected. After these attempts, we collected FTIR spectrum of CoPc/ppy/GDE and\ncompared to those of CoPc and ppy/GDE. To avoid the possible contamination of free CoPc (i.e.,\nCoPc adsorbed on GDE or ppy), CoPc/ppy/GDE was thoroughly cleaned by ultrasonication and\nwashed by copious amounts of acetonitrile. As shown in Figure R4, the FTIR spectrum of\nCoPc/ppy/GDE shows a combined feature of CoPc and ppy/GDE, which indicats that CoPc had\nbeen successfully incorporated into ppy/GDE.\nFourth, CoPc/ppy/GDE electrode exhibits an exceptional 200 hours stability during\nchronoamperometry measurements at −0.78 V versus RHE, while the control sample of CoPc/CNT-\nGDE shows only a considerably shorter operational lifespan of 12 hours. After 200 hours stability\ntest, no obvious valence state, structure and coordination environment changes were detected over\nCoPc/ppy/GDE by XPS, HAADF-STEM and XAS. Furthermore, the as-assembled CoFe-\nCi@GQDs/NF||CoPc/ppy/GDE electrolyzer achieves record device stability of 50 hours at 1000\nmA cm−2 and 120 hours at 500 mA cm−2.\nAll these experimental observations point to the formation of covalent bonds between CoPc\nand the ppy network, based on which improved activity (due to electron transfer from ppy to CoPc)\nand robust operational stability are simultaneously achieved.\nFigure R3. Comparative CV patterns of ppy/GDE in 0.01 M [BMIM]BF with 4 mM pyrole,\n4 4\nmM pyrole and 1 mM CoPc at 100 mV s−1.\nFigure R4. FTIR spectra of CoPc/GDE, CoPc/ppy/GDE and ppy/GDE.\nTable R1. The location of the CV peaks\nCatalysts Reduction 1 Reduction 2 Reduction 3\nCoPc/ppy/GDE 0.52 -0.04 -0.98\nCoPc/GDE 0.61 0.03 -0.88\nOxidation 1 Oxidation 2 Oxidation 3\nCoPc/ppy/GDE -0.27 0.36 1.18\nCoPc/GDE -0.15 0.48 1.32\nWe have incorporated these findings into the revised manuscript and have provided detailed\nexplanations and references to support our conclusions. We hope that these additional analyses\nwould address your concerns and strengthen the scientific validity of our work.\n“We performed following experiments and analyses to understand the growth process of\nCoPc/ppy/GDE electrode. The cyclic voltammetry (CV) characteristics of CoPc/ppy/GDE and\ncontrol samples were monitored in the potential window of −1.2 to 1.6 V versus Ag/AgCl at various\nscan rates. As shown in Fig. 1c, ppy/GDE generated from electro-polymerization of pyrrole exhibits\nonly broad and featureless oxidation and reduction waves in the potential range of −1.0 to 1.0 V\nversus Ag/AgCl. Comparatively, three redox couples could be resolved for CoPc-modified GDE\n(CoPc/GDE), with their locations presented in Fig. 1d. These redox couples are probably associated\nwith the redox behaviours of Co centres in CoPc (Co2+↔Co+↔Co0). Interestingly, CoPc/ppy/GDE\nelectrode produced in the presence of both pyrrole and CoPc shows similar redox characteristics to\nthat of CoPc/GDE, with however negatively shifted peak positions and largely enhanced current\nresponse in the cathodic scan (Fig. 1d and Supplementary Fig. 1).\nNext, we utilized in situ Raman spectroscopy to monitor the structural evolution of\nCoPc/ppy/GDE during the electro-polymerization process. The Raman spectra reveal gradually\nincreased peaks that correspond to the characteristic vibrational modes of CoPc. Specifically, the\nmacrocyclic deformation mode at 680 cm−1 exhibits an increase trend with the number of CV\nelectro-polymerization cycles, quantitatively correlating with the progressive deposition of\nCoPc22,23. We also observed a shift of Co–N vibration at ~ 755 cm−1, which is possibly associated\nwith the interfacial strain-induced changes of Co–N bonds in CoPc during electro-polymerization\n(Supplementary Fig. 2). Collectively, the progressively increased intensities of CoPc-related peaks\nwith the number of CV cycles, provide evidences for the in situ growth and integration of CoPc onto\nthe GDE.\nThird, we collected FTIR spectrum of CoPc/ppy/GDE and compared to those of CoPc and\nppy/GDE. To avoid the possible influence of free CoPc (i.e., CoPc adsorbed on GDE or ppy),\nCoPc/ppy/GDE was thoroughly cleaned by ultrasonication and washed by copious amounts of\nacetonitrile. As shown in Supplementary Fig. 3, the FTIR spectrum of CoPc/ppy/GDE possesses a\ncombined feature of CoPc and ppy/GDE, which indicates that pristine CoPc has been successfully\nincorporated into ppy/GDE24.\nBased on these observations (and the exceptional operation stability of CoPc/ppy/GDE as\ndiscussed below), we infer that during pyrrole electro-polymerization pristine CoPc is integrated\ninto ppy, with the configuration of CoPc terminated at the ppy chain; electron donation occurs from\nppy to CoPc, thus yielding negatively shifted redox peaks relative to bare CoPc. Such inferences are\nfurther supported by density functional theory (DFT) calculations, in which strong chemical bonds\nform in ppy/GDE and CoPc/ppy/GDE; under this condition, ppy serves as an effective linker to\nrealize robust covalent grafting of pristine CoPc onto GDE (Supplementary Figs. 4–6).”\nReferences\n[1] Szybowicz, M. et al. Micro-Raman spectroscopic investigations of cobaltphthalocyanine thin\nfilms deposited on quartz and diamondsubstrates. Cryst. Res. Technol. 45, 1265–1271 (2010).\n[2] Chen, Y. T. et al. Charge transfer and electromagnetic enhancement processes revealed in the\nSERS and TERS of a CoPc thin film. Nanophotonics, 8, 1533–1546 (2019).\nComment 2: The authors suggest that the lower frequency of the CO* peak on CoPc/ppy@GDE,\ncompared to CoPc/CNT, reflects stronger CO* adsorption and greater CO coverage. However, it is\nunclear why the Co site would not undergo reduction to a zero-valence state under electrochemical\nreduction conditions. This assumption appears inconsistent with the Co K-edge spectra, which\nindicate an increase in the Co valence state during the reaction. The manuscript would benefit from\na more detailed mechanistic explanation or additional experimental data to reconcile this apparent\ndiscrepancy in the behavior of the Co(I) site during CO RR.\nbehaviour and the valence state of Co in CoPc/ppy/GDE during CO RR. According to your\n2\nsuggestions, we have conducted additional experiments and analyses to provide a more\ncomprehensive understanding of the underlying mechanisms.\nWe employed in situ ATR-SEIRAS to monitor the CO evolution pathway over CoPc/ppy/GDE.\nappears and grows in intensity with increasing negative potential, indicating the gradual\ntransformation of CO to CO. Under the same condition, negligible CO* signals were observed on\n2\nppy/GDE and CoPc/GDE, while CoPc/CNT-GDE prepared according to the previous report showed\na delayed onset potential of CO* formation along with a weaker signal intensity. These observations\nsuggest the improved activity of CoPc/ppy/GDE for CO RR than the control samples of ppy/GDE,\nthan those of control samples, we have deleted these descriptions, because after carful checking the\ndata, we found that no direct relationship could be established between the shift degree of CO*\nfrequency and its formation rate.\nFigure R5. In situ ATR-SEIRAS spectra of CoPc/ppy/GDE collected in CO -saturated 0.1 M\ncollected in CO -saturated 0.1 M KHCO with an H-type cell, the potential was applied from −0.4\n2 3\nto −0.8 V versus RHE.\nThen, according to your suggestion, we tried to understand the unusual increase of Co valence\nstate during reaction. Indeed, under highly reductive conditions, Co2+ sites are expected to undergo\na reduction event, leading to the formation of Co+ or even Co0. After revisiting the measurement\nprocedures, we rationalize the absence of Co0 centre and unusual increase of Co valence state during\nreaction to the following reasons.\nFirst, we should make it clear that our XAS measurements were performed under ex situ\nconditions on an easyXAFS setup (Figure R7), due to the low intensity of collected signals with a\nconventional electrochemical cell; the CoPc/ppy/GDE electrode was polarized at −0.8 V versus\nRHE, and at different reaction times the CoPc/ppy/GDE electrode was taken out from the\nelectrochemical cell and subjected to XAS measurements. Under this condition, oxygen-containing\nspecies (OH−, H O and reaction intermediates such as HCO − and COOH*) may adsorb on\n2 3\nCoPc/ppy/GDE, and upon removal of the applied potential valence-state increase of the Co centre\nwould occur (Figure R8a)1. More evidences come from the EXAFS analysis, which reveals a slight\nelongation of the Co–N bond (from 1.98 to 2.05 Å) and importantly the emergence of a Co–O\ncoordination peak at ~1.2 Å (Figure R8b).\nThese phenomena are consistent with the coordination of oxygen-containing\nreactants/intermediates (e.g., OH−, HCO − or HCOO* species) to the Co centre and the\n3\ncorrespondingly relaxed structure (elongated Co–N bond) in CoPc/ppy/GDE. In comparison, the\nblue-shift of absorption edge in the XANES curves and the Co–O coordination in the EXAFS\nspectra for CoPc/CNT-GDE are not as prominent as those of CoPc/ppy/GDE (Figure R9), indicating\nweaker adsorption of OH−, CO , H O reactants and less accumulation of oxygen-containing\n2 2\nintermediates over CoPc/CNT-GDE than CoPc/ppy/GDE. DFT calculations provide additional\nevidences, which reveal that Co sites in CoPc/ppy/GDE exhibit a strong interaction with\nintermediates, such a feature is crucial for accelerating CO conversion (Figure R10). We attribute\n2\nthe enhanced stabilization of reaction intermediates on CoPc/ppy/GDE than CoPc/CNT-GDE to the\nelectron donation effects from ppy to CoPc.\nFigure R7. Picture of the easyXAFS/XES setup.\nFigure R8. Co K-edge (a) XANES and (b) EXAFS spectra of CoPc/ppy/GDE polarized at −0.8 V\nversus RHE for different reaction periods; commercial CoPc was used as the reference. Note that\nthese spectra were collected under ex situ conditions, during the CO -to-CO conversion the\nsubjected to measurements.\nFigure R9. Co K-edge (a) XANES and (b) EXAFS spectra of CoPc/CNT-GDE polarized at −0.8 V\nversus RHE for different reaction periods; commercial CoPc was used as the reference. Note that\nthese spectra were collected under ex situ conditions, during the CO -to-CO conversion the\nsubjected to measurements.\nFigure R10. Schematic diagram of differential charge density of CoPc/ppy and Bader charge\nanalysis. The cyan (yellow) region shows electron loss (gain).\nSecond, researchers have claimed that Co+ species in CoPc formed by one-electron reduction of\nCo2+, is the active site for CO RR2,3. Through the diagnostic Raman signatures of Co+ (at ca.\n2\n1110 cm−1) and Co2+ species (at ca. 1140 cm−1), the identification and quantification of Co+ active\nspecies have been achieved4. Following these reports, we have also monitored the evolution of\nCoPc/ppy/GDE and the distribution of Co+ and Co2+ species (i.e. the ratio of Co+ to Co2+) with\napplied potentials through in situ surface-enhanced Raman spectroscopy (SERS). As shown in\nFigure R11, at −0.4 V versus RHE, characteristic peaks could be resolved at 1110 and 1140 cm−1 on\nboth electrodes of CoPc/ppy/GDE and CoPc/CNT-GDE, which are reported to be associated with\nthe Co+ and Co2+ species, respectively5,6. The signal intensity of Co+ species (at 1110 cm−1) over\nCoPc/ppy/GDE becomes prominent at ca. −0.7 V versus RHE, consistent with the redox behaviour\nof Co2+/Co+ in CoPc; beyond this potential, the spectrum reaches a relatively steady state. In\ncomparison, the formed Co⁺ species over CoPc/CNT-GDE is significantly lower in amount.\nUsing\nthe Raman peak area, we calculated the ratios of Co+ to Co2+ in CoPc/ppy/GDE and CoPc/CNT-\nGDE at various applied potentials. The results in Figure R12 show that the formed Co⁺ species over\nCoPc/ppy/GDE are always higher than that of CoPc/CNT-GDE, for which we infer that the\nelectron donation effects from ppy to CoPc is responsible7.\nFigure R11. a, b, In situ SERS of the CoPc/ppy/GDE and CoPc/CNT-GDE during CO RR under\n2\napplied bias ranging from −0.4 to −1.4 V versus RHE.\nFigure R12. The ratio of Co+: Co2+ during CO RR under applied bias ranging from −0.4 to −1.4 V\n2\nversus RHE in CO -saturated 0.1 M KHCO .\n2 3\nIn the previous study, Co+ species in CoPc is reported to be stable up to the potential of −2.2 V\nversus RHE, as the percentage of Co+ species reaches a steady state starting from ca. −1.0 V versus\nRHE4. Our results under operando conditions in Figure R11 also indicate that no obvious reduction\nof Co+ species occurs up to −1.4 V versus RHE. For these phenomena, we infer that the presence of\noxygen-containing species (OH−, H O and reaction intermediates such as HCO − and HCOO*) may\n2 3\nstabilize the Co centre in the Co+ form under operational conditions. In addition, several recent\nstudies propose that high-valence metal single-atom sites (e.g., Fe3+, Sn4+) with specific\ncoordination environments can be stable and dominate the CO RR activity. For instance, in Fe3+\n2\nsingle atom catalyst, Fe3+ is stabilized on N-doped carbon carriers via pyrrole-nitrogen coordination8.\nIt maintains a +3 oxidation state during CO reduction, with its activity attributed to the optimized\n2\nadsorption of COOH* intermediate at Fe3+. Similarly, Sn4+ single atom catalysts synthesized in a\nCuO matrix could achieve 98% CO selectivity, due to the Sn4+-O -Cu⁺ coordination promoted high-\n3\nvalence state of Sn and correspondingly accelerated CO desorption9. We have also evaluated the\nstability of Co+ species during CO RR using in situ SERS. As shown in Figure R13, with the\n2\ncontinuous application of a negative potential of −1.4 V versus RHE, no obvious degradation in the\npeak intensity of Co+ species could be detected on CoPc/ppy/GDE over a reaction period of 30 min;\nwhile for CoPc/CNT-GDE, a gradual decrease of Co+ species appears, possibly due to the\nprogressive loss of CoPc from CoPc/CNT-GDE electrode.\nFigure R13. Time-dependent in situ SERS of the (a) CoPc/ppy/GDE (b) CoPc/CNT-GDE during\nCO RR from 0 to 30 min at −1.4 V versus RHE in CO -saturated 0.1 M KHCO .\n2 2 3\nThird, we collected the XAS and XPS spectra before and after the stability test in the Co 2p\nregion. As shown in Figures R14 and 15, after 200 hours reaction no sign for the formation of zero-\nvalence Co could be detected.\nFigure R14. a, XAS spectra at Co K-edge of CoPc/ppy/GDE and b, Co K-edge FT EXAFS spectra\nin the R space of CoPc/ppy/GDE of initial state and after 200 hours in CO -saturated 0.1 M KHCO\n2 3\nwith an H-type cell at −0.75 V versus RHE.\nFigure R15. Comparison of the Co 2p XPS spectra of CoPc/ppy/GDE of initial state and after 200\nhours in CO -saturated 0.1 M KHCO with an H-type cell at −0.75 V versus RHE.\n2 3\nCombined all the above experiments and analyses together, we propose that Co+ could be the\nactive species in CoPc/ppy/GDE for CO RR. Co+ species are possibly stabilized by the\n2\nmicroenvironment and ligands, forming a dynamically stable active centre; upon the removal of\napplied potential, oxygen-containing species (OH−, H O and reaction intermediates such as HCO −\n2 3\nand COOH*) may push the oxidation state of Co to a higher value. However, we agree with you that\nzero-valence Co may appear during CO RR, especially at highly negative potentials and high\n2\ncurrent densities, the presence and significance of which would require further investigation in the\nfuture.\nWe have included these additional experimental data and detailed mechanistic explanations in\nthe revised manuscript. We hope that these additional analyses would provide a clearer\nunderstanding on the behaviour of CoPc/ppy/GDE during CO RR. Thank you once again for your\n2\nvaluable feedback.\nReferences\n[1] Wang, S. F. et al. Manipulating C-C coupling pathway in electrochemical CO reduction for\n2\nselective ethylene and ethanol production over single-atom alloy catalyst. Nat. Commun. 15,\n10247 (2024).\n[2] Corbin, N. et al. Heterogeneous molecular catalysts for electrocatalytic CO reduction. Nano\n[3] Hu, X. M. et al. Enhanced catalytic activity of cobalt porphyrin in CO electroreductin upon\n2\nimmobilization on carbon materials. Angew. Chem. Int. Ed. 56, 6468–6472 (2017).\n[4] Ren, S. et al. Catalyst aggregation matters for immobilized molecular CO RR electrocatalysts.\n[5] Jiang, S. et al. Investigation of cobalt phthalocyanine at the solid/liquid interface by\nelectrochemical tip-enhanced raman spectroscopy. J. Phys. Chem. C 123, 9852–9859 (2019).\n[6] Chen, X. et al. Operando observation of molecular-scale manipulation using electrochemical tip-\nenhanced raman spectroscopy. J. Phys. Chem. C 123, 24329–24333 (2018).\n[7] Zhu, M. et al. Elucidating the reactivity and mechanism of CO electroreduction at highly\n2\ndispersed cobalt phthalocyanine. ACS Energy Lett. 3, 1381–1386 (2018).\n[8] Gu, J. et al., Atomically dispersed Fe3+ sites catalyze efficient CO electroreduction to CO.\n[9] Chen, R. R. et al. Operando Mössbauer spectroscopic tracking the metastable state of atomically\ndispersed tin in copper oxide for selective CO electroreduction. J. Am. Chem. Soc. 145, 20683–\n2\n20691 (2023).\n“Previous studies have claimed that Co+ species in CoPc formed by one-electron reduction of\nCo2+, are the active site for CO RR31,32. Through the diagnostic Raman signatures of Co+ (at ca.\n2\n1110 cm−1) and Co2+ species (at ca. 1140 cm−1), the identification and quantification of Co+ active\nspecies have been achieved33–35. Following these reports, the evolution of CoPc/ppy/GDE and the\ndistribution of Co+ and Co2+ species (i.e. the ratio of Co+ to Co2+) with applied potentials were\nmonitored through in situ surface-enhanced Raman spectroscopy (SERS). As shown in Figs. 3a and\nb, characteristic peaks could be resolved at 1113 and 1146 cm−1 at −0.4 V versus RHE for both\nelectrodes of CoPc/ppy/GDE and CoPc/CNT-GDE, which are reported to be associated with the\nCo+ and Co2+ species, respectively35. The signal intensity of Co+ species (at 1113 cm−1) over\nCoPc/ppy/GDE becomes prominent at ca. −0.7 V versus RHE, consistent with the redox behaviour\nof Co2+/Co+ in CoPc; beyond this potential, the spectrum reaches a relatively steady state. In\ncomparison, the formed Co+ species over CoPc/CNT-GDE is significantly lower in amount\n(Supplementary Figs. 25 and 26). Using the Raman peak area, the ratios of Co+ to Co2+ in\nCoPc/ppy/GDE and CoPc/CNT-GDE at various applied potentials were calculated. The results in\nFig. 3c show that the formed Co+ species over CoPc/ppy/GDE are always higher than that of\nCoPc/CNT-GDE, aligned with its better catalytic activity for CO generation36. It is inferred that the\nelectron donation effects from ppy to CoPc is responsible. Furthermore, the stability of Co+ species\nduring CO RR was evaluated by in situ SERS. As shown in Supplementary Fig. 27, with the\n2\ncontinuous application of a negative potential of −1.4 V versus RHE, no obvious degradation in the\npeak intensity of Co+ species could be detected on CoPc/ppy/GDE over a reaction period of 30 min;\nwhile for CoPc/CNT-GDE, a gradual decrease of Co+ species appears, possibly due to the\nprogressive loss of CoPc from CoPc/CNT-GDE electrode.”\n“Having determined that the electron-donating ppy promotes active Co+ formation, we\nemployed in situ attenuated total reflection surface enhanced infrared absorption spectroscopy (in\nsitu ATR–SEIRAS) to track the reaction intermediates and gain a molecular-level understanding of\nCO -to-CO conversion over CoPc/ppy/GDE. For the CoPc/ppy/GDE electrode (Fig. 3d), upon the\n2\napplication of negative potentials, surface-bonded CO (CO*) appears and grows in intensity with\nincreasing negative potential, indicating the gradual transformation of CO to CO37. Under the\n2\nsame condition, negligible CO* signals were observed on ppy/GDE and CoPc/GDE, while\nCoPc/CNT-GDE prepared according to the previous report showed a delayed onset potential of CO*\nformation along with a weaker signal intensity (Supplementary Figs. 28–30). Meanwhile, an inverse\nabsorption band appears at 1230 cm−1 and becomes more prominent as the applied bias goes more\nreductive; this phenomenon can be ascribed to the rapid consumption of HCO – at large\n3\noverpotentials. It is noted that under identical bias, CoPc/ppy/GDE exhibits the largest absorption\nbands for CO* and HCO – among tested samples. These observations suggest the improved activity\n3\nof CoPc/ppy/GDE for CO RR than the control samples of ppy/GDE, CoPc/GDE and CoPc/CNT-\n“To further assess the evolutionary nature of the atomically dispersed Co active sites, ex situ\nXAS measurements were carried out. Co K-edge XANES spectra of CoPc/ppy/GDE were collected\nafter reacting in a CO -saturated 0.1 M KHCO solution for different times. Under this condition,\n2 3\noxygen-containing species (OH−, H O and reaction intermediates such as HCO −, CO* and COOH*)\n2 3\nmay adsorb on CoPc/ppy/GDE, and upon removal of the applied potential valence-state increase\nof the Co centre would occur (Supplementary Fig. 31)38,39. More evidences come from the EXAFS\nanalysis, which reveals a slight elongation of the Co–N bond (from 1.98 to 2.05 Å)40,41 and\nimportantly the emergence of a Co–O coordination peak at ~1.2 Å (Fig. 3e)42. These phenomena\nare consistent with the coordination of oxygen-containing reactants/intermediates (e.g., OH−,\nHCO − or COOH* species) to the Co centre and the correspondingly relaxed structure (elongated\ncurves and the Co–O coordination in the EXAFS spectra for CoPc/CNT-GDE are not as prominent\nas those of CoPc/ppy/GDE (Supplementary Fig. 32), indicating weaker adsorption of OH−, CO ,\n2\nthan CoPc/ppy/GDE.”\nComment 3: Although the study reports promising catalytic stability at high current densities,\nfurther structural analysis of the CoPc/ppy@GDE catalyst before and after extended catalytic testing\nis essential to validate its durability.\nReply to comment 3: We appreciate your suggestion regarding the importance of post reaction\nanalysis of the CoPc/ppy/GDE catalyst to validate its durability. To address this concern, we have\nconducted additional experiments to examine the structural integrity of the CoPc/ppy/GDE catalyst\nbefore and after extended catalytic testing.\nThe CoPc/ppy/GDE electrode after testing at −0.78 V vs. RHE for 200 h was subjected to XAS,\nXPS and HAADF-STEM analyses. As shown in Figure R16, no obvious changes in the valence\nstate and coordination environment of Co could be resolved for post-reacted CoPc/ppy/GDE. XPS\nanalysis reveals that although the intensities of Co 2p peaks in post-reacted CoPc/ppy/GDE have\ndecreased (possibly due to the contamination from electrolyte), their positions remain largely\nunaltered (Figure R17). In addition, HAADF-STEM measurements suggest that the morphology of\npost-reacted CoPc/ppy/GDE catalyst is retained, with well-preserved atomically dispersed Co sites\n(Figure R18). All these post-reaction analyses together demonstrate that our CoPc/ppy/GDE\nelectrode is highly resistant to structural collapse/changes under prolonged electrolysis, the\nnegligible demetallation, reduction and aggregation of Co active sites in CoPc/ppy/GDE guarantee\nits promising stability under highly reductive and high current density conditions.\nFigure R16. a, XAS spectra at Co K-edge of CoPc/ppy/GDE and b, Co K-edge FT EXAFS spectra\nin the R space of CoPc/ppy/GDE of initial state and after 200 hours in CO -saturated 0.1 M KHCO\n2 3\nwith an H-type cell at −0.75 V versus RHE.\nFigure R17. Comparison of the Co 2p XPS spectra of CoPc/ppy/GDE of initial state and after 200\nhours in CO -saturated 0.1 M KHCO with an H-type cell at −0.75 V versus RHE.\n2 3\nFigure R18. a, TEM image of post-reacted CoPc/ppy/GDE after 200 hours. b, HAADF-STEM\nimage of post-reacted CoPc/ppy/GDE after 200 hours.\nIn summary, our structural analyses on post-reacted CoPc/ppy/GDE provide strong evidences\nfor its high durability under prolonged CO RR conditions. We have incorporated these findings into\n2\nthe revised manuscript to further validate the performance and stability of this catalyst.\n“Subsequently, the post-reacted CoPc/ppy/GDE was subjected to HAADF-STEM, XAS and\nXPS analyses. HAADF-STEM measurements suggest that the aggregated sphere-like morphology\nof CoPc/ppy/GDE catalyst is retained, with well-preserved atomically dispersed Co sites\n(Supplementary Fig. 19). Meanwhile, no obvious changes in the valence state and coordination\nenvironment of Co could be resolved for post-reacted CoPc/ppy/GDE (Supplementary Fig. 20). In\naddition, XPS analysis reveals that although the intensities of Co 2p peaks in post-reacted\nCoPc/ppy/GDE have decreased (possibly due to the contamination from electrolytes), their\npositions remain largely unaltered (Supplementary Fig. 21). All these post-reaction analyses\ntogether demonstrate that our CoPc/ppy/GDE electrode with the covalent grafting feature is highly\nresistant to structural collapse/changes under prolonged electrolysis, the negligible demetallation,\nreduction and aggregation of Co active sites in CoPc/ppy/GDE guarantee its promising stability\nunder highly reductive and high current density conditions.”\nComment 4: The electropolymerization approach for anchoring ppy on carbon supports is well-\nestablished and widely applied in CO reduction reactions. The manuscript does not sufficiently\n2\nexplain what differentiates CoPc/ppy@GDE from similar electropolymerized systems reported in\nthe literature. The authors are encouraged to provide a clear comparative analysis to highlight any\nunique structural characteristics or performance advantages of CoPc/ppy@GDE.\nReply to comment 4: We thank the reviewer for raising this critical point, an explanation and a\ncomparative analysis are indeed needed to show the uniqueness and advantages of the\nCoPc/ppy/GDE electrode for CO RR.\nyielded ppy/carbon catalysts are generally less efficient for CO RR due to the lack of metal-based\n2\nactive centres. Incorporating molecular catalysts with atomically defined active sites, tunable\nelectronic structures, into carbon-based gas diffusion electrodes (GDEs) is a popular strategy to\ncircumvent their limitations of aggregation and poor conductivity, through which significant\nprogresses in CO RR activity and product selectivity have been achieved.\nincorporated carbon electrodes have been prepared and applied for CO RR. However, significant\n2\ndifferences exist between these electrodes and our CoPc/ppy@GDE. Specifically, in the paper of\n“Cobalt phthalocyanine cross-linked polypyrrole for efficient electroreduction of low concentration\nCO to CO” (Chemsuschem 2022, 15, e202201455), CoPc modified with four sulfylphenoxy groups\n2\n(defined as CoPcS ) was employed as both the dopant and gelator to crosslink ppy; the chemical\n4\nbonds formed between CoPcS and ppy are ionic bonds instead of C–C covalent bonds in our\n4\ncloth) electrode exhibits much lower CO RR activity and stability than our CoPc/ppy/GDE under\n2\nsimilar conditions (ca. −6.5 mA cm−2 vs. ca. −60 mA cm−2 at −1 V in an H-cell; −2.1 mA cm−2\nRHE\nfor 3500 s vs. −20 mA cm−2 for 200 h). While in the paper of “Electrocatalytic reduction of carbon\ndioxide by cobalt-phthalocyanine-incorporated polypyrrole” (Electrochem. Solid St. 2009, 12, E17),\nCoPc-ppy modified glassy carbon is obtained by electrochemical polymerization of ppy at glassy\ncarbon followed by drop-casting of CoPc solution.\nFigure R19. Synthesis of CoPcS −PPy.\nor CoPc derivatives (Liang yongye, Wang hailiang), CoPc-derived/containing covalent organic\nframeworks or metal organic frameworks (Peng tianyou, Adv. Mater. 2022, 2203139; Zhang liming,\nJ. Am. Chem. Soc. 2022, 144, 21502), with carbon-based gas diffusion electrodes; while ultrasonic\nspray could offer a more uniform catalyst dispersion. For these CoPc-based powder catalysts,\nbinders like Nafion are usually required, scaffolds such as CNT (carbon nanotube) and graphene are\nalso included to enhance their dispersion and adhesion strength through π-π stacking interaction\n(Adv. Mater. 2023, 35, 2303179).\nConsidering the possible loss of CoPc centres during operation (especially at high current\ndensities), anchoring these molecular catalysts directly onto carbon-based GDEs through strong\ncovalent bonds is a desirable strategy. Presently, direct covalent grafting of CoPc (and other metal\nphthalocyanine/porphyrin) based molecular catalysts onto carbon-based GDEs would require the\nuse of CoPc with specially designed functional groups (such as amino group) or polymerizable\ngroups (such as carbazole). Among common covalent bonds that could form between CoPc (or its\nderivatives) and carbon-based GDEs, C–C bond is more inert and robust for extreme reaction\nconditions, when comparing to those hydrolysable amide and ester bonds.\nIn our study, to offer robust C–C bonds between carbon-based GDEs and pristine CoPc, we\nintroduce ppy as the linker which initiates the C–C coupling process between GDEs and ppy, ppy\nand pristine CoPc thereby connecting them together via C–C bonds. During this covalent grafting\nprocess, pristine CoPc without polymerizable groups is applied; meanwhile, no pretreatment of\ncarbon-based GDEs is required, which minimizes the amount of oxygen-containing functional\ngroups (such as carboxyl and hydroxyl groups) thereby guaranteeing sufficient hydrophobicity for\nefficient CO gas penetration. As far as we know, such a convenient and efficient approach has not\n2\nbeen explored for direct grafting of pristine CoPc (rather than CoPc derivatives) on carbon-based\nGDEs, the employed ppy not only acts as the highly conductive and robust linker between CoPc\nand GDEs, but also shows electron donation effects to promote the CO RR activity of CoPc centers\n2\n(negative shift of Co 2p peaks by ~0.6 eV Figure R20). Importantly, this covalent grafting strategy\nallows facile control over catalyst loadings (e.g., through adjusting the polymerization parameters\nof CV number, potential range), and could be well extended to other pristine metal phthalocyanines\nor porphyrins (Figure R21).\nFigure R20. Comparison of the Co 2p XPS spectra of CoPc/ppy/GDE and commercial CoPc.\nFigure R21. a, LSV curves of CoPc/ppy/GDE, FePc/ppy/GDE, CuPc/ppy/GDE and NiPc/ppy/GDE\nduring CO RR in CO -saturated 0.1 M KHCO with an H-type cell. b, Faradaic efficiency and\n2 2 3\ncurrent density of CoPc/ppy/GDE, FePc/ppy/GDE, CuPc/ppy/GDE and NiPc/ppy/GDE during\nCO RR in CO -saturated 0.1 M KHCO with an H-type cell at −0.7 V versus RHE.\n2 2 3\nBased on the combined advantages of robust C–C bonds, binder-free interface, highly\nconductive ppy linker, appropriate catalyst dispersion, and active Co single-atom sites, our in situ\ncovalent grafting derived CoPc/ppy/GDE establishes promising activity and stability for CO RR.\nversus RHE, better than previously reported CoPc-based catalysts (Table R2 and R3); an exceptional\n200 hours stability at −0.78 V versus RHE is demonstrated on CoPc/ppy/GDE, and at the end of\nstability test no obvious valence state, structure and coordination environment changes of Co are\ndetected by XPS, HAADF-STEM and XAS (Figure R22−R24). Furthermore, the as-assembled\nCoFe-Ci@GQDs/NF||CoPc/ppy/GDE electrolyzer achieves a record device stability of 50 hours at\n1000 mA cm−2 and 120 hours at 500 mA cm−2, which strongly demonstrate that our CoPc/ppy/GDE\nelectrode is highly resistant to structural collapse/changes under industrially relevant current\ndensities.\nTable R2. Comparison of the CO -to-CO conversion of different electrocatalysts in H-type cells.\nCO\nCatalysts Electrolyte Ref.\n(%) (V vs. RHE) (mA cm−2) (h)\n0.1(cid:3031)M\nCoPc/ppy/GDE 96 −0.78 −40 200 This work\nKHCO\n3\n0.5(cid:3031)M\nEP-CoP 95 −0.62 −8.5 42 17\nKHCO\n3\n0.5(cid:3031)M\nCoTPP/CNT 91 −0.62 −3.2 4 S8\nKHCO\n3\n0.1(cid:3031)M\nFePGF/CFP 98.7 −0.54 −1.68 10 S9\nKHCO\n3\n0.5(cid:3031)M\nCo-PMOF 98.7 −0.8 −18.3 36 S10\nKHCO\n3\n0.5(cid:3031)M\nSTPyP-Co 96 −0.62 −6.6 48 S11\nKHCO\n3\n0.5(cid:3031)M\nFe-PB 100 −0.63 ~−0.6 24 S12\nKHCO\n3\n0.5(cid:3031)M\nCo-TTCOF 91.3 −0.7 −2.02 40 S13\nKHCO\n2\n93.6 −0.68 ~−6 12 S14\nCo@CNT KHCO\n3\n0.5(cid:3031)M\nCOF-367-Co 91 −0.55 −3.3 24 S15\nKHCO\n3\n0.5(cid:3031)M\nCoPc-TFPN COF 99.8 −0.9 −14.1 60 S16\nKHCO\n3\n0.5(cid:3031)M\nTT-Por(Co)-COF 91.4 −0.6 ~−1.5 10 S17\nKHCO\n3\n0.5(cid:3031)M\nCoPc-PI-COF-1 90 −0.7 −10 40 S18\nKHCO\n88 −0.7 −6 50 S19\nCOF-Co KHCO\n3\n0.1(cid:3031)M\nCoPc/CNT 92 −0.63 −10 10 S20\nKHCO\n3\n0.5(cid:3031)M\nCoTMAPc@CNT 95 −0.62 ~−12 12 S21\nKHCO\n3\n0.5(cid:3031)M\nCoPPc/CNT 90 −0.54 −12 24 S22\nNaHCO\n3\n0.5(cid:3031)M\nCoPc-2H2Por 95 −0.6 −5 70 S23\nKHCO\nCatalysts Electrolyte FE References\nCO\n(V) cm−2) (h)\nCoPc/ppy/GDE 1.0 M KOH 99% ~2.8 500 120 This work\nCoPc/ppy/GDE 1.0 M KOH 98% ~3.2 1000 50 This work\nCo-CNTs MW 1.0 M KOH 98.3% ~2 100 12 44\nCoPc 1.0 M KOH 88% ~2.52 200 6 45\nMWNT/\n2.0 M KOH 85% ~2.25 158 8 46\nPyPBI/Au\nAg/C 1.0 M KOH 83% ~2.75 120 0.417 47\n1.0 M\nAg KOH+0.33\n98% ~2.16 100 10 48\nnanoparticles M\nUrea\nZn P O 1.0 M KOH 93.9% 100 7 49\n2 2 7\nAu 0.1 M KOH 90.0% ~3 100 100 50\n97% ~2.6 50 60 17\nCoPc/MWCNTs KHCO\n3\n0.1 M\nCoPc/Mg(OH) 95% 3.4 100 20 51\n3\nβ-CoPc/CP 1.0 M KOH 92% 2 100 29.3 52\nAG- 0.1 M\n80% 3 50 12 26\nCoPc/MWCNTs KHCO\n80% 3.2 50 40 26\nCoPc/MWCNTs KHCO\nin the R space of CoPc/ppy/GDE of initial state and after 200 hours in CO -saturated 0.1 M KHCO\n2 3\nwith an H-type cell at −0.75 V versus RHE.\nFigure R23. Comparison of the Co 2p XPS spectra of CoPc/ppy/GDE of initial state and after 200\nhours in CO -saturated 0.1 M KHCO with an H-type cell at −0.75 V versus RHE.\n2 3\nFigure R24. a, TEM image of post-reacted CoPc/ppy/GDE after 200 hours. b, HAADF-STEM\nimage of post-reacted CoPc/ppy/GDE after 200 hours.\nIn summary, our in situ electropolymerization yields CoPc/ppy/GDE electrode with robust C–\nC covalent bonds between CoPc and ppy, ppy and GDEs. Performance evaluations including\nactivity and stability tests, as well as the structural and spectroscopic analyses demonstrate the\nunique advantages of CoPc/ppy/GDE for CO RR, which presents a significant improvement in the\n2\ndesign of molecular catalysts for sustainable CO -to-fuel technologies., To highlight the merits of\n2\nour electropolymerization-derived CoPc/ppy/GDE electrode, the introduction section has been\nrevised to offer a more explicit comparison with conventional synthesis methods.\nReference\n[1] Chen, J. M. et al. Cobalt phthalocyanine cross-linked polypyrrole for efficient electroreduction\nof low concentration CO to CO. Chemsuschem 15, e202201455 (2022).\n2\n[2] Zhang, A. J. et al. Electrocatalytic reduction of carbon dioxide by cobalt-phthalocyanine-\nincorporated polypyrrole. Electrochem. Solid St. 12, E17–E19 (2009).\n“Electrocatalytic conversion of CO into value-added chemicals can mitigate the climate crisis\n2\ninduced by greenhouse gases while simultaneously enabling renewable energy storage1–3. Among\nvarious catalytic systems, molecular catalysts, particularly metal phthalocyanines and metal\nporphyrins, have emerged as promising candidates due to their atomically defined active sites and\ntunable electronic structures4–6. However, under operating conditions, especially those with\nindustrially relevant current densities (>200 mA cm−2), molecular catalysts often suffer from\nleaching, agglomeration, or even structural degradation, which hinder their practical\napplications7–12. Recently, metal phthalocyanine-based molecular catalysts have demonstrated\nhighly selective CO electrolysis at high current densities in flow reactors13,14. To this end, these\n2\nmolecular catalysts are generally coated onto the carbon-based GDEs to circumvent their\nlimitations of aggregation and poor conductivity, with the majority of the previous works utilizing\nthe noncovalent anchoring strategies. In these strategies, binders like Nafion are usually required,\nscaffolds such as carbon nanotube (CNT) and graphene are also included to enhance the dispersion\nand adhesion strength of molecular catalysts through π-π stacking interaction15–17.\nConsidering the possible loss of molecular catalysts during operation (especially at high\ncurrent densities), anchoring them directly onto carbon-based GDEs through strong covalent bonds\nis a desirable strategy18–20. Presently, direct covalent grafting of metal phthalocyanine/porphyrin\nbased molecular catalysts onto carbon-based GDEs would require the use of these materials with\nspecially designed functional groups (such as amino group) or polymerizable groups (such as\ncarbazole group). Until now, due to the inherent chemical inertness of pristine metal\nphthalocyanines/porphyrins, direct electrochemical polymerization or covalent grafting of them\nonto the carbon-based GDEs is challenging. On the other hand, among common covalent bonds\nthat could form between metal phthalocyanines/porphyrins (or their derivatives) and carbon-based\nGDEs, C–C bond is more inert and robust for extreme reaction conditions, when comparing to those\nhydrolysable amide and ester bonds19.\nHerein, using pristine CoPc as the example, we report an in situ electro-polymerization\nstrategy for the direct covalent grafting of pristine metal phthalocyanines onto GDEs. This method\noccurs at room temperature and leverages the conductive polymer ppy as a molecular “glue”,\nforming robust C–C bonds with pristine CoPc; meanwhile, besides as the scaffold, ppy-based matrix\nalso shows electron donation effects to promote the activity of active Co+ sites. Finally, to bridge\nthe gap between material innovation and industrial application, we further engineered a membrane\nelectrode assembly (MEA) electrolyzer incorporating the CoPc/ppy/GDE cathode. This device\nachieves 50 hours of stable CO production at ampere-level current densities (1 A cm−2) in alkaline\nmedia. The proposed strategy here effectively addresses the challenge of direct covalent grafting of\npristine metal phthalocyanines onto GDEs for efficient and durable CO reduction reaction (CO RR)\n2 2\nunder industrially relevant current densities (>200 mA cm−2).”\nReviewer #2:\nComment: The authors address the immobilization limitations of phthalocyanine catalysts on\nelectrode towards industrial-scale CO reduction, and graft CoPc onto gas diffusion electrode\n2\nthrough polypyrrole, which was applied in membrane electrode assembly electrolyser for CO to\ntechnology,and DFT calculation. However, there are still problems need further revision.\nComment 1: How to know the molecular weight? Whether it influence the performance of\nCoPc/ppy@GDE catalyst? And how to ascertain the best polypyrrole?\nReply to comment 1: We appreciate the reviewer’s insightful question regarding the molecular\nweight control in the CoPc/ppy/GDE catalyst. Due to the inherent chemical inertness of pristine\nCoPc (without specially designed functional groups or polymerizable groups), direct\nelectrochemical polymerization or grafting of it onto the carbon-based GDEs is challenging. To\naddress this issue, we propose a ppy-mediated grafting strategy to offer robust C–C bonds between\nGDEs and pristine CoPc, in which ppy serves as both the conductive linker and molecular scaffold.\nIn this approach, precise control over the molecular weight or chain length of ppy is difficult\nbecause the electropolymerization process is dynamically influenced by various steps including\nmonomer diffusion, radical coupling, and CoPc incorporation. We have tried to use F-doped tin\noxide (FTO) as the conductive substrate to study the grow process and molecular weight of\nCoPc/ppy, however, the weak bonding between FTO and CoPc/ppy has led to easy detachment of\nproduct. We have also tried to monitor the deposition process of CoPc/ppy through the microcrystal\nquartz balance, but no available signals could be collected.\nDespite the lack of precise control over the ppy molecular weight, our CoPc/ppy/GDE catalysts\nshow controllable and reproducible electrochemical behaviors towards CO RR, when the\n2\nelectropolymerization conditions are well organized. Specifically, the electrochemical CO RR\n2\nperformance of CoPc/ppy/GDE electrodes can be systematically and finely optimized through\ntuning two key parameters including the number of CV scans and the feed ratio of pyrrole to CoPc,\nbased on which the chain length of ppy and CoPc loading/incorporating could be controlled. Figure\nR25 illustrates the effects of CV number during electropolymerization, the current density that\ncorresponds to the electrochemical growth of CoPc and ppy on GDE initially increases with cycle\nnumber but exhibits a saturation trend beyond 40 cycles. We infer that mass transport or ppy\nresistance issues gradually dominate and cease the continuous growth of CoPc and ppy on GDE in\nthe applied CV potential range. Correspondingly, the CO RR performance of yielded\ncycles, the gain in current density becomes limited, along with the consideration of synthesis time,\nthe CV number for CoPc/ppy/GDE fabrication is set at 40.\nNext, considering that the CO RR performance of CoPc/ppy/GDE electrodes is closely\n2\ncorrelated with the loading/incorporating amount of CoPc, we optimized the synthetic procedure by\nvarying the pyrrole-to-CoPc feed ratio (1:6 to 6:1). As shown in Figure R27, only small activity\ndifferences could be observed for these CoPc/ppy/GDE electrodes obtained in the presence of both\npyrrole and CoPc, and the best performing one is obtained with a high pyrrole-to-CoPc ratio of 4:1;\nwhen only CoPc is applied, the yielded CoPc/GDE electrode shows largely suppressed CO RR\n2\nactivity. Based on these results, we infer that the electropolymerization step of ppy dominates the\nwhole formation process of CoPc/ppy/GDE electrodes, as pristine CoPc is chemically less active\ntowards electropolymerization; under this condition, ppy initiates the C–C coupling process\nbetween GDEs and ppy, ppy and pristine CoPc, thereby realizing covalent grafting of pristine CoPc\nonto carbon-based GDEs via robust C–C bonds.\nCollectively, through utilizing ppy as the electroactive \"glue\", covalent grafting of pristine\nCoPc onto carbon-based GDEs via robust C–C bonds has been achieved. The optimal conditions\nfor CoPc/ppy/GDE electrode fabrication are determined to be 40 of CV cycles and 4:1 of pyrrole-\nto-CoPc ratio that balance performance and cost efficiency. Although the precise control over the\nppy molecular weight remains a challenge and is worth of future studies, the yielded CoPc/ppy/GDE\nelectrodes show reproducible CO RR activity and promising stability under high current densities,\n2\nsuggesting the effectiveness of our in situ electropolymerization strategy for covalent grafting of\npristine molecular catalysts onto carbon-based GDEs. We have added these optimizations and\ndiscussion in the revised manuscript. Thank the reviewer again!\nFigure R25. a, The different deposition cycles of CoPc/ppy/GDE during the electro-polymerization\nin 0.01 M [BMIM]BF with 4 mM pyrole and 1 mM CoPc at 100 mV s−1. The potentials were\n4\nrecorded with the Ag/AgCl scale. b, The variation of current density with the number of deposition\ncycles.\nFigure R26. a, The LSV of CoPc/ppy/GDE during CO RR in CO -saturated 0.1 M KHCO with an\n2 2 3\nH-type cell.\nFigure R27. a, The CV patterns of the growth of CoPc/ppy on GDE in 0.1 M [BMIM]BF with\n4\ndifferent ratios of pyrole and CoPc. b, The Faradaic efficiency and current density of\nCoPc/ppy/GDE synthesized by different precursors of pyrole and CoPc at −0.8 V versus RHE during\nCO RR in CO -saturated 0.1 M KHCO with an H-type cell.\n2 2 3\nComment 2: Line 136-139. Since the electron density of Co is enhanced, the binding energy should\nblueshift, while not redshift. This part needs reconsideration. The result is not consistent with XAS\nanalysis.\nReply to comment 2: We sincerely appreciate the reviewer’s insightful question regarding the shift\nof Co binding energy in our XPS data. Upon re-examining the data, we found that the charge\ncorrection step had been ignored, which led to an erroneous interpretation of the Co 2p peak shifts.\nWe have recalibrated the XPS spectra, and the revised data in Figure R28 now show reduced binding\nenergies of Co 2p peaks (780.8 and 795.9 eV) in CoPc/ppy/GDE than those of commercial CoPc\n(781.4 and 796.5 eV), consistent with the XAS data (absorption edge of CoPc/ppy/GDE shifts\ntowards the lower energy direction compared to commercial CoPc in Figure R29) and the enhanced\nelectron density at Co sites in CoPc/ppy/GDE due to the electron donation effects of ppy. We\nappreciate your attention to this mistake, thanks again!\nFigure R28. Comparison of the Co 2p XPS spectra of CoPc/ppy/GDE and commercial CoPc.\nFigure R29. a, XAS spectra at Co K-edge of CoPc/ppy/GDE and the reference Co foil and CoPc\nsamples. b, Co K-edge FT EXAFS spectra in the R space of CoPc/ppy/GDE and reference Co foil\nand CoPc samples.\n“The XPS measurements further corroborate the modulated electronic structure of\nCoPc/ppy/GDE, which shows lower binding energies for Co 2p peaks than those of commercial\nCoPc (780.8 vs. 781.4 eV for Co 2p3/2; 795.9 vs. 796.5 eV for Co 2p1/2)28. These phenomena, as\nmentioned above, are attributed to the electron donation effects from ppy to the CoPc domain\n(Supplementary Fig. 12).”\nComment 3: The fig.2a, it should be labelled for different color for better understanding. The Fig.\n2f.g caption was not correct.\nReply to comment 3: Thank you for your suggestions regarding the clarity and accuracy of the\nfigure and figure caption in our manuscript. We have revised these figure and figure caption to\nclearly and correctly show the information we want to express (Figure R30).\nFigure R30. Structural analyses and intrinsic activity of CoPc/ppy/GDE. a, Schematic illustration\nof CoPc/ppy/GDE. b, TEM image of CoPc/ppy/GDE. c, HAADF-STEM image of CoPc/ppy/GDE.\nThe atomically dispersed Co sites are marked by red circles. d, XAS spectra at Co K-edge of\nCoPc/ppy/GDE and the reference Co foil and CoPc samples. e, Co K-edge FT EXAFS spectra in\nthe R space of CoPc/ppy/GDE and reference Co foil and CoPc samples. f, g, WT-EXAFS plots of\nCoPc samples and CoPc/ppy/GDE. h, LSV curves of CoPc/ppy/GDE, CoPc/CNT-GDE, CoPc/GDE\nand ppy/GDE during CO RR in an H-cell with CO -saturated 0.1 M KHCO electrolyte. i, CV curve\n2 2 3\nof CoPc/ppy/GDE during CO RR in an H-cell with CO -saturated 0.1 M KHCO electrolyte.\n2 2 3\nComment 4: Line 153, the pH needs further tested, it cannot be 8.3.\nReply to comment 4: Thank you for your comment regarding the pH value of the 0.1 M KHCO\n3\nsolution. To ensure the accuracy of the pH values reported in our manuscript, we have re-conducted\npH measurements using a calibrated pH meter, following the standard procedures. Our experimental\nresults show that the pH of the 0.1 M KHCO solution is around 8.3 and 0.1 M CO saturated\n3 2\nKHCO solution is around 6.8. We have corrected the error and thank you once again for your\n3\nvaluable comments.\nComment 5: How TOF was calculated according to the all Co sites? i.e. how to ascertain the Co\nsites?\nReply to comment 5: We appreciate the reviewer's question regarding the TOF calculation based\non all cobalt sites.\nTOF estimation methods face limitations when dealing with single-atom systems. In our\nCoPc/ppy/GDE electrode, due to the influence of carbon-based porous GDE, and the lack of\ncapacitance value of atomically dispersed Co-N site, obtaining the amount of Co sites through\n4\nelectrochemical double-layer capacitance is challenging. In-situ spectroscopic quantification (e.g.,\noperando XAS) of active sites, while theoretically ideal, suffers from insufficient sensitivity.\nPrecise\ndetermination of Co amount in our CoPc/ppy/GDE electrode through spectroscopic methods is\nfurther complicated by the ultra-low loading of CoPc in the single atom configuration.\nTherefore, in our study we have adopted the ICP-OES to quantify the Co concentration in\nCoPc/ppy/GDE. The obtained Co amount in CoPc/ppy/GDE is comparable to those of recently\nreported M-N-C catalysts1,2.\nThe TOF value was then determined using the following formula:\njA\nTOF \nnFN\nCo\nwhere j was the current density (A cm−2), A was the geometric electrode area (1 cm2), n represented\nthe electron transfer number for CO -to-CO conversion (n=2), FF was Faraday's constant (96485 C\n2\nmol−1), and N was the total number of Co sites quantified by ICP-OES. Actual Co loading of the\nCo\noptimized CoPc/ppy/GDE catalyst (obtained with 40 CV cycles) was measured to be 0.0837 wt%\nwith ICP-OES. For our CoPc/ppy/GDE catalyst, we assumed all Co sites participate in the CO RR\n2\nreaction1,2. Therefore, the TOF value at 40 mA cm−2 was calculated as follows:\nmA\nj  10−3  A cm2\n cm2\n\n  \nTOF \n2  96485  N\nCo\nmA\n40   10−3 1 cm2\n cm2\n\nTOF   1.46 s−1\nC 0.0837%  0.01 g\n2  96485 \nmol  g \n59\n \nmol\n“The TOF value was determined using the formula:\njA\nTOF \nnFN\nCo\nwhere j was the current density (A cm−2), A was the geometric electrode area (1 cm2), n represented\nthe electron transfer number for CO -to-CO conversion (n=2), FF was Faraday's constant (96485\nCo\nthe optimized CoPc/ppy/GDE catalyst (obtained with 40 CV cycles) was measured to be 0.0837 wt%\nwith ICP-MS. For our CoPc/ppy/GDE catalyst, we assumed all Co sites participate in the CO RR\n2\nreaction1,2. Therefore, the TOF value at 40 mA cm−2 was calculated as follows:\nmA\nj  10−3  A cm2\n cm2\n\n \nTOF \n2  96485  N\nCo\nmA\n40   10−3 1 cm2\n cm2\n\nTOF   1.46 s−1\n”\nC 0.0837%  0.01 g\n2  96485 \nmol  g \n59\n \nmol\nReferences\n[1] Zhou, S. et al. Amphiphilic cobalt phthalocyanine boosts carbon dioxide reduction. Adv. Mater.\n35, 2300923 (2023).\n[2] Zhang, H., et al. Mechanistic insights into CO conversion chemistry of copper bis-(terpyridine)\n2\nmolecular electrocatalyst using accessible operando spectrochemistry. Nat. Commun. 13, 6029\n(2022).\nComment 6: Line 241, from the calculation, we can only know that ppy alters the catalytic\nthermodynamics while not kinetic.\nReply to comment 6: Thank you for your insightful comment regarding the promoting effects of\nppy on the catalytic performance of CoPc/ppy/GDE electrode. We agree with you that our\ntheoretical calculations can only reach a conclusion that ppy could affect the overall reaction\nthermodynamics of CoPc/ppy/GDE electrode through altering the interaction strength of CO and\nrevised manuscript. Thank the reviewer again!\n“To gain more comprehensive mechanistic insights into ppy-enhanced electrocatalytic activity,\nfree energy diagrams considering thermodynamics of CO -to-CO conversion on both\nComment 7: Fig. 4f in present is not quite convincing.\nReply to comment 7: Thank you for your comment regarding the strength of Fig. 4f in supporting\nour conclusion. We have revised Fig. 4f as supplementary Fig. 6 (Figure R31), and added\ncorresponding discussion as follows.\nFigure R31. Schematic diagram of differential charge density of CoPc/ppy and Bader charge\nanalysis. The cyan (yellow) region shows electron loss (gain).\n“Bader charge analysis suggest that ppy has a tendency to transfer electrons to CoPc\n(Supplementary Fig. 6). The ppy acts as a linker to anchor CoPc onto the GDE through covalent\nbonds, while simultaneously modulating the electronic structure of CoPc for improved catalytic\nperformance.” Thank the reviewer again!\nComment 8: Since former work has published with CoPc cross-linked ppy(ChemSusChem\n2022,15(23):e202201455), CoPc incorporated with ppy (ELECTROCHEMICAL AND SOLID\nSTATE LETTERS 2009,12(8):E17-E19), what’s the novelty of present work?\nReply to comment 8: We thank the reviewer for raising this critical point, an explanation and a\ncomparative analysis are indeed needed to show the uniqueness and advantages of the\nCoPc/ppy/GDE electrode for CO RR.\nyielded ppy/carbon catalysts are generally less efficient for CO RR due to the lack of metal-based\n2\nactive centres. Incorporating molecular catalysts with atomically defined active sites, tunable\nelectronic structures, into carbon-based gas diffusion electrodes (GDEs) is a popular strategy to\ncircumvent their limitations of aggregation and poor conductivity, through which significant\nprogresses in CO RR activity and product selectivity have been achieved.\nincorporated carbon electrodes have been prepared and applied for CO RR. However, significant\n2\ndifferences exist between these electrodes and our CoPc/ppy@GDE. Specifically, in the paper of\n“Cobalt phthalocyanine cross-linked polypyrrole for efficient electroreduction of low concentration\nCO to CO” (Chemsuschem 2022, 15, e202201455), CoPc modified with four sulfylphenoxy groups\n2\n(defined as CoPcS ) was employed as both the dopant and gelator to crosslink ppy; the chemical\n4\nbonds formed between CoPcS and ppy are ionic bonds instead of C–C covalent bonds in our\n4\ncloth) electrode exhibits much lower CO RR activity and stability than our CoPc/ppy/GDE under\n2\nsimilar conditions (ca. −6.5 mA cm−2 vs. ca. −60 mA cm−2 at −1 V in an H-cell; −2.1 mA cm−2\nRHE\nfor 3500 s vs. −20 mA cm−2 for 200 h). While in the paper of “Electrocatalytic reduction of carbon\ndioxide by cobalt-phthalocyanine-incorporated polypyrrole” (Electrochem. Solid St. 2009, 12, E17),\nCoPc-ppy modified glassy carbon is obtained by electrochemical polymerization of ppy at glassy\ncarbon followed by drop-casting of CoPc solution.\nFigure R32. Synthesis of CoPcS −PPy.\nor CoPc derivatives (Liang yongye, Wang hailiang), CoPc-derived/containing covalent organic\nframeworks or metal organic frameworks (Peng tianyou, Adv. Mater. 2022, 2203139; Zhang liming,\nJ. Am. Chem. Soc. 2022, 144, 21502), with carbon-based gas diffusion electrodes; while ultrasonic\nspray could offer a more uniform catalyst dispersion. For these CoPc-based powder catalysts,\nbinders like Nafion are usually required, scaffolds such as CNT (carbon nanotube) and graphene are\nalso included to enhance their dispersion and adhesion strength through π-π stacking interaction\n(Adv. Mater. 2023, 35, 2303179).\nConsidering the possible loss of CoPc centres during operation (especially at high current\ndensities), anchoring these molecular catalysts directly onto carbon-based GDEs through strong\ncovalent bonds is a desirable strategy. Presently, direct covalent grafting of CoPc (and other metal\nphthalocyanine/porphyrin) based molecular catalysts onto carbon-based GDEs would require the\nuse of CoPc with specially designed functional groups (such as amino group) or polymerizable\ngroups (such as carbazole). Among common covalent bonds that could form between CoPc (or its\nderivatives) and carbon-based GDEs, C–C bond is more inert and robust for extreme reaction\nconditions, when comparing to those hydrolysable amide and ester bonds.\nIn our study, to offer robust C–C bonds between carbon-based GDEs and pristine CoPc, we\nintroduce ppy as the linker which initiates the C–C coupling process between GDEs and ppy, ppy\nand pristine CoPc thereby connecting them together via C–C bonds. During this covalent grafting\nprocess, pristine CoPc without polymerizable groups is applied; meanwhile, no pretreatment of\ncarbon-based GDEs is required, which minimizes the amount of oxygen-containing functional\ngroups (such as carboxyl and hydroxyl groups) thereby guaranteeing sufficient hydrophobicity for\nefficient CO gas penetration. As far as we know, such a convenient and efficient approach has not\n2\nbeen explored for direct grafting of pristine CoPc (rather than CoPc derivatives) on carbon-based\nGDEs, the employed ppy not only acts as the highly conductive and robust linker between CoPc\nand GDEs, but also shows electron donation effects to promote the CO RR activity of CoPc centers\n2\n(negative shift of Co 2p peaks by ~0.6 eV Figure R33). Importantly, this covalent grafting strategy\nallows facile control over catalyst loadings (e.g., through adjusting the polymerization parameters\nof CV number, potential range), and could be well extended to other pristine metal phthalocyanines\nor porphyrins (Figure R34).\nFigure R33. Comparison of the Co 2p XPS spectra of CoPc/ppy/GDE and commercial CoPc.\nFigure R34. a, LSV curves of CoPc/ppy/GDE, FePc/ppy/GDE, CuPc/ppy/GDE and NiPc/ppy/GDE\nduring CO RR in CO -saturated 0.1 M KHCO with an H-type cell. b, Faradaic efficiency and\n2 2 3\ncurrent density of CoPc/ppy/GDE, FePc/ppy/GDE, CuPc/ppy/GDE and NiPc/ppy/GDE during\nCO RR in CO -saturated 0.1 M KHCO with an H-type cell at −0.7 V versus RHE.\n2 2 3\nBased on the combined advantages of robust C–C bonds, binder-free interface, highly\nconductive ppy linker, appropriate catalyst dispersion, and active Co single-atom sites, our in situ\ncovalent grafting derived CoPc/ppy/GDE establishes promising activity and stability for CO RR.\nversus RHE, better than previously reported CoPc-based catalysts (Table R4 and R5); an exceptional\n200 hours stability at −0.78 V versus RHE is demonstrated on CoPc/ppy/GDE, and at the end of\nstability test no obvious valence state, structure and coordination environment changes of Co are\ndetected by XAS, XPS and HAADF-STEM (Figure R35−R37). Furthermore, the as-assembled\nCoFe-Ci@GQDs/NF||CoPc/ppy/GDE electrolyzer achieves a record device stability of 50 hours at\n1000 mA cm−2 and 120 hours at 500 mA cm−2, which strongly demonstrate that our CoPc/ppy/GDE\nelectrode is highly resistant to structural collapse/changes under industrially relevant current\ndensities.\nTable R4. Comparison of the CO -to-CO conversion of different electrocatalysts in H-type cells.\nCO\nCatalysts Electrolyte Ref.\n(%) (V vs. RHE) (mA cm−2) (h)\nCoPc/ppy/GDE 0.1(cid:3031)M KHCO 96 −0.78 −40 200 This work\n(OMe) - 0.5(cid:3031)M KHCO 93.6 −0.68 ~−6 12 S14\n2 3\nCo@CNT\nCOF-367-Co 0.5(cid:3031)M KHCO 91 −0.55 −3.3 24 S15\n0.5(cid:3031)M KHCO 99.8 −0.9 −14.1 60 S16\nTT-Por(Co)-\n0.5(cid:3031)M KHCO 91.4 −0.6 ~−1.5 10 S17\nCoPc-PI-COF-1 0.5(cid:3031)M KHCO 90 −0.7 −10 40 S18\n0.5(cid:3031)M KHCO 88 −0.7 −6 50 S19\nCoPc/CNT 0.1(cid:3031)M KHCO 92 −0.63 −10 10 S20\n0.5(cid:3031)M KHCO 95 −0.62 ~−12 12 S21\nCoPPc/CNT 0.5(cid:3031)M NaHCO 90 −0.54 −12 24 S22\nCatalysts Electrolyte FE References\nCO\n(V) cm−2) (h)\nCoPc/ppy/GDE 1.0 M KOH 99% ~2.8 500 120 This work\nCoPc/ppy/GDE 1.0 M KOH 98% ~3.2 1000 50 This work\nCo-CNTs MW 1.0 M KOH 98.3% ~2 100 12 44\nCoPc 1.0 M KOH 88% ~2.52 200 6 45\nMWNT/\n2.0 M KOH 85% ~2.25 158 8 46\nPyPBI/Au\nAg/C 1.0 M KOH 83% ~2.75 120 0.417 47\n1.0 M\nAg KOH+0.33\n98% ~2.16 100 10 48\nnanoparticles M\nUrea\nZn P O 1.0 M KOH 93.9% 100 7 49\n2 2 7\nAu 0.1 M KOH 90.0% ~3 100 100 50\n97% ~2.6 50 60 17\nCoPc/MWCNTs KHCO\n3\n0.1 M\nCoPc/Mg(OH) 95% 3.4 100 20 51\n3\nβ-CoPc/CP 1.0 M KOH 92% 2 100 29.3 52\nAG- 0.1 M\n80% 3 50 12 26\nCoPc/MWCNTs KHCO\n80% 3.2 50 40 26\nCoPc/MWCNTs KHCO\nin the R space of CoPc/ppy/GDE of initial state and after 200 hours in CO -saturated 0.1 M KHCO\n2 3\nwith an H-type cell at −0.75 V versus RHE.\nFigure R36. Comparison of the Co 2p XPS spectra of CoPc/ppy/GDE of initial state and after 200\nhours in CO -saturated 0.1 M KHCO with an H-type cell at −0.75 V versus RHE.\n2 3\nFigure R37. a, TEM image of post-reacted CoPc/ppy/GDE after 200 hours. b, HAADF-STEM\nimage of post-reacted CoPc/ppy/GDE after 200 hours.\nIn summary, our in situ electropolymerization yields CoPc/ppy/GDE electrode with robust C–\nC covalent bonds between CoPc and ppy, ppy and GDEs. Performance evaluations including\nactivity and stability tests, as well as the structural and spectroscopic analyses demonstrate the\nunique advantages of CoPc/ppy/GDE for CO RR, which presents a significant improvement in the\n2\ndesign of molecular catalysts for sustainable CO -to-fuel technologies., To highlight the merits of\n2\nour electropolymerization-derived CoPc/ppy/GDE electrode, the introduction section has been\nrevised to offer a more explicit comparison with conventional synthesis methods.\nReference\n[1] Chen, J. M. et al. Cobalt phthalocyanine cross-linked polypyrrole for efficient electroreduction\nof low concentration CO to CO. Chemsuschem 15, e202201455 (2022).\n2\n[2] Zhang, A. J. et al. Electrocatalytic reduction of carbon dioxide by cobalt-phthalocyanine-\nincorporated polypyrrole. Electrochem. Solid St. 12, E17–E19 (2009).\n“Electrocatalytic conversion of CO into value-added chemicals can mitigate the climate crisis\n2\ninduced by greenhouse gases while simultaneously enabling renewable energy storage1–3. Among\nvarious catalytic systems, molecular catalysts, particularly metal phthalocyanines and metal\nporphyrins, have emerged as promising candidates due to their atomically defined active sites and\ntunable electronic structures4–6. However, under operating conditions, especially those with\nindustrially relevant current densities (>200 mA cm−2), molecular catalysts often suffer from\nleaching, agglomeration, or even structural degradation, which hinder their practical\napplications7–12. Recently, metal phthalocyanine-based molecular catalysts have demonstrated\nhighly selective CO electrolysis at high current densities in flow reactors13,14. To this end, these\n2\nmolecular catalysts are generally coated onto the carbon-based GDEs to circumvent their\nlimitations of aggregation and poor conductivity, with the majority of the previous works utilizing\nthe noncovalent anchoring strategies. In these strategies, binders like Nafion are usually required,\nscaffolds such as carbon nanotube (CNT) and graphene are also included to enhance the dispersion\nand adhesion strength of molecular catalysts through π-π stacking interaction15–17.\nConsidering the possible loss of molecular catalysts during operation (especially at high\ncurrent densities), anchoring them directly onto carbon-based GDEs through strong covalent bonds\nis a desirable strategy18–20. Presently, direct covalent grafting of metal phthalocyanine/porphyrin\nbased molecular catalysts onto carbon-based GDEs would require the use of these materials with\nspecially designed functional groups (such as amino group) or polymerizable groups (such as\ncarbazole group). Until now, due to the inherent chemical inertness of pristine metal\nphthalocyanines/porphyrins, direct electrochemical polymerization or covalent grafting of them\nonto the carbon-based GDEs is challenging. On the other hand, among common covalent bonds\nthat could form between metal phthalocyanines/porphyrins (or their derivatives) and carbon-based\nGDEs, C–C bond is more inert and robust for extreme reaction conditions, when comparing to those\nhydrolysable amide and ester bonds19.\nHerein, using pristine CoPc as the example, we report an in situ electro-polymerization\nstrategy for the direct covalent grafting of pristine metal phthalocyanines onto GDEs. This method\noccurs at room temperature and leverages the conductive polymer ppy as a molecular “glue”,\nforming robust C–C bonds with pristine CoPc; meanwhile, besides as the scaffold, ppy-based matrix\nalso shows electron donation effects to promote the activity of active Co+ sites. Finally, to bridge\nthe gap between material innovation and industrial application, we further engineered a membrane\nelectrode assembly (MEA) electrolyzer incorporating the CoPc/ppy/GDE cathode. This device\nachieves 50 hours of stable CO production at ampere-level current densities (1 A cm−2) in alkaline\nmedia. The proposed strategy here effectively addresses the challenge of direct covalent grafting of\npristine metal phthalocyanines onto GDEs for efficient and durable CO reduction reaction (CO RR)\n2 2\nunder industrially relevant current densities (>200 mA cm−2).”\n. \nReviewer #3:\nComment: The authors presented a study on molecular catalyst immobilization through direct\ncovalent grafting, demonstrating that CoPc anchored via pyrrole linkers (CoPc/ppy@GDE) exhibits\nenhanced stability and catalytic performance. The evidence from XAFS and XPS provides\ninteresting insights into the electronic effects of pyrrole grafting on the Co center, while in-situ\nSERS and SEIRAS offer valuable mechanistic information about Co oxidation states and their role\nin the reaction pathway.\nI believe the fundamental observation of enhanced stability through pyrrole-based grafting is\nwell-supported; However, I have a number of significant scientific and technical concerns that\nshould be addressed in a major revision. I think the work could be suitable for publication after the\nauthors strengthen their mechanistic understanding and provide more comprehensive evidence.\nHere are some detailed comments:\nComment 1: The manuscript lacks a direct comparison of electrochemical impedance between\nCoPc@GDE and CoPc/ppy@GDE systems. To better understand the electron transfer\ncharacteristics of these catalysts, please provide Nyquist plots along with the corresponding serial\nresistance values for both systems.\nReply to comment 1: Thank you for your suggestion regarding the comparison of electrochemical\nimpedance between the CoPc/GDE and CoPc/ppy/GDE systems. We agree that this comparison is\nessential for understanding the electron transfer characteristics of these catalysts. To address your\ncomment, we have conducted electrochemical impedance spectroscopy (EIS) measurements at −0.1\nV vs. RHE. As shown in Figure R38, CoPc/ppy/GDE and CoPc/GDE exhibit comparable solution\nresistances R of 10.59 Ω and 9.81 Ω, respectively. Equivalent circuit modeling yields a much lower\n1\nelectrode resistance R of 4.53 Ω for CoPc/ppy/GDE than that of CoPc/GDE (9.68 Ω), as well as an\n2\nalmost halved interfacial charge transfer resistance R (38.47 Ω versus 69.20 Ω; Table R6). The\n3\nmeasurements of EIS further confirm the lower charge-transfer resistance (and correspondingly\nimproved reaction kinetics) of CoPc/ppy/GDE than the control sample of CoPc/GDE.\nFigure R38. The EIS of CoPc/ppy/GDE and CoPc/GDE in 0.1 M CO -saturated KHCO electrolyte\n2 3\nat −0.1 V versus RHE.\nTable R6. The values derived from EIS Nyquist plot fitting.\nRs R1 R2\nCoPc/ppy/GDE 9.81 4.532 38.47\nCoPc/GDE 10.59 9.678 69.20\n“Besides, the measurements of electrochemical impedance spectroscopy (EIS) further confirm\nthe lower charge-transfer resistance (and correspondingly improved reaction kinetics) of\nCoPc/ppy/GDE than the control sample of CoPc/GDE (Supplementary Fig. 17 and Supplementary\nTable 3).”\nComment 2: In lines 130-133, the notation system using Co(I)-N and Co(II)-N creates confusion\nin interpreting the oxidation states of cobalt. For example, Co(I)-N in CoPc@GDE can be\ninterpreted specifically refers to Co(+1) coordinated with nitrogen. Please provide clearer\nterminology to distinguish the different cobalt species.\nReply to comment 2: Thank you for your insightful suggestions regarding the notation of cobalt\noxidation states in the CoPc/ppy/GDE catalyst. We understand the potential confusion caused by\nthe notation and would like to take the opportunity to clarify this aspect.\nTo avoid confusion and ensure clarity, we have revised the terminology used to describe the\ncobalt species in our catalyst. Specifically, Fourier-transform extended X-ray absorption fine\nstructure (FT-EXAFS) analyses identify a dominant Co–N coordination peak at ~1.5 Å for both\nCoPc/ppy/GDE and pristine CoPc, indicating Co2+–N as the primary species in them (Figure R39e).\nWavelet-transforms of EXAFS spectra further confirm this conclusion by revealing prominent\nintensity maxima at ~5.9 Å−1; meanwhile, a minor intensity maximum is exclusively resolved at\n~8.2 Å−1 over CoPc/ppy/GDE, which is inferred to arise from the Co+–N species (Fig. 39f–g). The\ncorresponding change in the revised manuscript is as follows:\nFigure R39. Structural analyses and intrinsic activity of CoPc/ppy/GDE. a, Schematic illustration\nof CoPc/ppy/GDE. b, TEM image of CoPc/ppy/GDE. c, HAADF-STEM image of CoPc/ppy/GDE.\nThe atomically dispersed Co sites are marked by red circles. d, XAS spectra at Co K-edge of\nCoPc/ppy/GDE and the reference Co foil and CoPc samples. e, Co K-edge FT EXAFS spectra in\nthe R space of CoPc/ppy/GDE and reference Co foil and CoPc samples. f, g, WT-EXAFS plots of\nCoPc samples and CoPc/ppy/GDE. h, LSV curves of CoPc/ppy/GDE, CoPc/CNT-GDE, CoPc/GDE\nand ppy/GDE during CO RR in an H-cell with CO -saturated 0.1 M KHCO electrolyte. i, CV curve\n2 2 3\nof CoPc/ppy/GDE during CO RR in an H-cell with CO -saturated 0.1 M KHCO electrolyte.\n2 2 3\n“Fourier-transform extended X-ray absorption fine structure (FT-EXAFS) analyses identify a\ndominant Co–N coordination peak at ~1.5 Å for both CoPc/ppy/GDE and pristine CoPc, indicating\nCo2+–N as the primary species in them (Fig. 2e). Wavelet-transforms of EXAFS (WT-EXAFS)\nspectra further confirm this conclusion by revealing prominent intensity maxima at ~5.9 Å−1;\nmeanwhile, a minor intensity maximum is exclusively resolved at ~8.2 Å−1 over CoPc/ppy/GDE,\nwhich is inferred to arise from the Co+–N species (Fig. 2f–g, Supplementary Fig. 11)26,27.”\nComment 3: There appears to be a typographical error in line 135 where \"XAENS\" should be\ncorrected to \"XANES\"\nReply to comment 3: Thank you for pointing out the typographical error in line 135. We have\nrevised this error and reviewed the manuscript to make the necessary correction.\nWe appreciate your help in ensuring the accuracy of our manuscript.\nComment 4: While Figure 2h demonstrates performance comparisons between CoPc/ppy@GDE,\nCoPc-GDE, and CoPc-CNT, the data appears to be collected using different cobalt loadings.\nAdditionally, there is a discrepancy in the Co content values reported in the main text versus\nSupplementary Table 1. To make a fair assessment of catalyst performance, please provide\ncomparisons with normalized cobalt content across all catalyst systems.\nReply to comment 4: Thank you for your insightful comments regarding the cobalt loading values\nfor performance comparisons and discrepancies in Figure R40 and Table R7.\nFigure R40. LSV curves normalized by ECSA of CoPc/ppy/GDE, CoPc-GDE, and CoPc/CNT-\nGDE during CO RR in an H-cell with CO -saturated 0.1 M KHCO electrolyte with the scan rate\n2 2 3\nof 1 mV s−1.\nWe have checked the cobalt loadings for various samples, and the Co content values for\nCoPc/ppy/GDE, CoPc/GDE, and CoPc/CNT-GDE samples are measured to be 0.0837 wt%, 0.0046\nwt% and 6.4097 wt%, respectively (Table R7). Based on these values, we have derived the cobalt\nloading-normalized LSV curves eliminating the influence from GDE for CoPc/ppy/GDE,\nCoPc/GDE, and CoPc/CNT-GDE samples. As shown in Figure 40, the intrinsic activity (Co loading-\nnormalized specific activities) follows the order of CoPc/ppy/GDE > CoPc/CNT-GDE > CoPc/GDE,\ndemonstrating the unique advantages of ppy-mediated covalent grafting (including binder-free\ninterface, highly conductive ppy linker, electron donation effect of ppy, appropriate dispersion and\nspatial configuration of Co single-atom sites) in CoPc/ppy/GDE for CO RR. We have corrected the\n2\ncorresponding discussion in the revised manuscript. Thank the reviewer again!\nTable R7. Co content of CoPc/ppy/GDE, CoPc/CNT-GDE and CoPc/GDE\nCatalyst Content (wt. %)\nCoPc/ppy/GDE 0.0837%\nCoPc/CNT-GDE 6.4097%\nCoPc/GDE 0.0046%\n“To clarify the intrinsic activity of these samples, their cobalt loadings were measured by ICP-\nOES (Supplementary Table 1). Based on these values, the Co loading-normalized specific activity\nof these samples has been derived (measured at a scan rate of 1 mV s−1 to minimize the capacitive\ncontributions), which follows the order of CoPc/ppy/GDE > CoPc/CNT-GDE > CoPc/GDE,\ndemonstrating the unique advantages of ppy-mediated covalent grafting (including binder-free\ninterface, highly conductive ppy linker, electron donation effect of ppy, appropriate dispersion and\nspatial configuration of Co single-atom sites) in CoPc/ppy/GDE for CO RR (Supplementary Fig.\n2\n16).”\nComment 5: The manuscript attributes the redshifted and broadened CO* bands observed on\nCoPc/ppy@GDE to surface coverage effects. However, two important questions need to be\naddressed: First, could dynamic dipole coupling of CO* produce similar spectral changes in single-\natom catalysts? Second, the cited reference 37 indicates that high surface coverage typically results\nin blue-shifted CO* bands, which contradicts the current explanation. Please clarify these apparent\ndiscrepancies.\nReply to comment 5: We sincerely appreciate your valuable comments. We would like to offer\nthe following clarifications regarding the points you raised. First, dynamic dipole coupling typically\nresults in a blue shift of the CO* vibrational bands on high-coverage metal surfaces (as observed on\ncopper surfaces in Reference 37), due to enhanced dipole interactions between adsorbed CO\nmolecules. However, in single-atom catalysts such as CoPc/ppy/GDE, the active sites are atomically\ndispersed (e.g., isolated Co single atoms), different from the status of bulk metals or nanoparticle\nsystems. Under this condition, the CO* adsorption sites of Co are spatially isolated, leading to little\nchance for CO*–CO* interactions. Therefore, using the dynamic dipole coupling to explain the red-\nshifted CO* bands over CoPc/ppy/GDE is wrong.\nSecond, as discussed above, minimal dynamic dipole coupling effects occur for\nCoPc/ppy/GDE, as contrary to the metallic copper surfaces with high CO* coverage mentioned in\nReference 37. Regarding the observed red-shifted and broadened CO* bands, we infer that the\nunique electronic structure of CoPc/ppy/GDE, due to the electron donation effects from ppy, may\nbe responsible for this spectral change of CO* (Figure R41 and R42). After carful checking the data,\nwe found that no direct relationship could be established between the shift degree of CO*\nfrequency and its formation rate. Therefore, we have deleted these descriptions to avoid misinterpretation,\nand have made the corresponding revision in the manuscript. Collectively, according to your comments\nand after carefully checking the requirements for dynamic dipole coupling, we have realized that\nthe spatial separation of Co single atoms significantly weakens the dynamic dipole coupling effects\nin CoPc/ppy/GDE, which contribute minimally to the shift of CO* vibrational frequency. We have\nrevised the manuscript to avoid misinterpretation, as shown below. Once again, we thank you for\nyour insightful comments on our manuscript.\nFigure R41. In situ ATR-SEIRAS spectra of the CoPc/ppy/GDE during CO RR under applied bias\n2\nranging from −0.4 to −0.8 V versus RHE in CO -saturated 0.1 M KHCO .\n2 3\nFigure R42. In situ ATR-SEIRAS spectra of (a) CoPc/CNT-GDE (b) CoPc/GDE (c) ppy/GDE\nduring CO RR under applied bias ranging from −0.4 to −0.8 V versus RHE in CO -saturated 0.1 M\n2 2\nKHCO .\n3\n“For the CoPc/ppy/GDE electrode (Fig. 3d), upon the application of negative potentials,\nsurface-bonded CO (CO*) appears and grows in intensity with increasing negative potential,\nindicating the gradual transformation of CO to CO37. Under the same condition, negligible CO*\n2\nsignals were observed on ppy/GDE and CoPc/GDE, while CoPc/CNT-GDE prepared according to\nthe previous report showed a delayed onset potential of CO* formation along with a weaker signal\nintensity (Supplementary Figs. 28–30). Meanwhile, an inverse absorption band appears at 1230\ncm−1 and becomes more prominent as the applied bias goes more reductive; this phenomenon can\nbe ascribed to the rapid consumption of HCO – at large overpotentials. It is noted that under\n3\nidentical bias, CoPc/ppy/GDE exhibits the largest absorption bands for CO* and HCO – among\n3\ntested samples. These observations suggest the improved activity of CoPc/ppy/GDE for CO RR than\n2\nthe control samples of ppy/GDE, CoPc/GDE and CoPc/CNT-GDE.”\nComment 6: The time-dependent blue shift in absorption edges shown in Figure 3d indicates a\ngradual increase in cobalt valence state during the reaction. This observation seems to contradict the\nproposed mechanism where catalyst stability and activity are attributed to maintained Co+ active\nspecies. To resolve this inconsistency, please provide time-resolved in-situ SERS data of\nCoPc/ppy@GDE at fixed potential.\nReply to comment 6: We appreciate your insightful comments regarding the blue shifts of\nabsorption edge in the XANES spectra and the change behaviour of Co valence state in\nCoPc/ppy/GDE during reaction. According to your comments and suggestions, we have conducted\nadditional experiments and analyses to provide a more comprehensive understanding of the\nunderlying mechanisms.\nWe should make it clear that our XAS measurements were performed under ex situ conditions\non an easyXAFS setup (Figure R43), due to the low intensity of collected signals with a\nconventional electrochemical cell; the CoPc/ppy/GDE electrode was polarized at −0.8 V versus\nRHE, and at different reaction times the CoPc/ppy/GDE electrode was taken out from the\nelectrochemical cell and subjected to XAS measurements. Under this condition, oxygen-containing\nspecies (OH−, H O and reaction intermediates such as HCO − and COOH*) may adsorb on\n2 3\nCoPc/ppy/GDE, and upon removal of the applied potential valence-state increase of the Co centre\nwould occur (Figure R44a)1. More evidences come from the EXAFS analysis, which reveals a slight\nelongation of the Co–N bond (from 1.98 to 2.05 Å) and importantly the emergence of a Co–O\ncoordination peak at ~1.2 Å (Figure R44b).\nThese phenomena are consistent with the coordination of oxygen-containing\nreactants/intermediates (e.g., OH−, HCO − or HCOO* species) to the Co centre and the\n3\ncorrespondingly relaxed structure (elongated Co–N bond) in CoPc/ppy/GDE. In comparison, the\nblue-shift of absorption edge in the XANES curves and the Co–O coordination in the EXAFS\nspectra for CoPc/CNT-GDE are not as prominent as those of CoPc/ppy/GDE (Figure R45),\nindicating weaker adsorption of OH−, CO , H O reactants and less accumulation of oxygen-\n2 2\ncontaining intermediates over CoPc/CNT-GDE than CoPc/ppy/GDE. DFT calculations provide\nadditional evidences, which reveal that Co sites in CoPc/ppy/GDE exhibit a strong interaction with\nintermediates; such a feature is crucial for accelerating CO conversion (Figure R46). We attribute\n2\nthe enhanced stabilization of reaction intermediates on CoPc/ppy/GDE than CoPc/CNT-GDE to the\nelectron donation effects from ppy to CoPc. Regarding the change behaviour of Co+ species under\noperando conditions, we fixed the potential at −1.4 V versus RHE and monitored the evolution of\nCo+ and Co2+ species in CoPc/ppy/GDE through in situ surface-enhanced Raman spectroscopy\n(SERS). As shown in Figure R47, with the continuous application of a negative potential of −1.4 V\nversus RHE, no obvious degradation in the peak intensity of Co+ species (at ca. 1110 cm−1) could\nbe detected on CoPc/ppy/GDE over a reaction period of 30 min; while for CoPc/CNT-GDE, a\ngradual decrease of Co+ species appears, possibly due to the progressive loss of CoPc from\nCoPc/CNT-GDE electrode.\nIn terms of further reduction of Co+ species to zero-valence Co, a previous study demonstrated\nthat Co+ species in CoPc is stable up to the potential of −2.2 V versus RHE, as the percentage of\nCo+ species reaches a steady state from ca. −1.0 V versus RHE2. If a further reduction of Co+ species\noccurs, a decreased Co+ percentage is expected. Consistent with the previous study, our results under\nvaried potentials reveal that the signal intensity of Co+ species (at 1110 cm−1) over CoPc/ppy/GDE\nbecomes prominent at ca. −0.7 V versus RHE, and beyond this potential the spectrum reaches a\nrelatively steady state (Figure R48).\nFigure R43. Picture of easyXAFS/XES.\nFigure R44. a, XAFS spectra at the Co K-edge of CoPc/ppy/GDE during CO RR at −0.8 V versus\n2 3\nduring CO RR at −0.8 V versus RHE from 0 to 2 h in CO -saturated 0.1 M KHCO .\n2 2 3\nFigure R45. a, XAFS spectra at the Co K-edge of CoPc/CNT-GDE during CO RR at −0.8 V versus\n2 3\nGDE during CO RR at −0.8 V versus RHE from 0 to 2 h in CO -saturated 0.1 M KHCO .\n2 2 3\nFigure R46. Schematic diagram of differential charge density of CoPc/ppy and Bader charge\nanalysis. The cyan (yellow) region shows electron loss (gain).\nFigure R47. Time-dependent in situ surface-enhanced Raman spectra of the (a) CoPc/ppy/GDE (b)\nCoPc/CNT-GDE during CO RR from 0 to 30 min at −1.4 V versus RHE in CO -saturated 0.1 M\n2 2\nKHCO .\n2\nversus RHE in CO -saturated 0.1 M KHCO .\n2 3\nFinally, the CoPc/ppy/GDE electrode after testing for 200 h was subjected to XAS, XPS and\nHAADF-STEM analyses. As shown in Figure R49, no obvious changes in the valence state and\ncoordination environment of Co could be resolved for post-reacted CoPc/ppy/GDE. XPS analysis\nreveals that although the intensities of Co 2p peaks in post-reacted CoPc/ppy/GDE have decreased\n(possibly due to the contamination from electrolyte), their positions remain largely unaltered (Figure\nR50). In addition, HAADF-STEM measurements suggest that the morphology of post-reacted\nCoPc/ppy/GDE catalyst is retained, with well-preserved atomically dispersed Co sites (Figure R51).\nAll these post-reaction analyses together demonstrate that our CoPc/ppy/GDE electrode is highly\nresistant to structural collapse/changes under prolonged electrolysis, the negligible demetallation,\nreduction and aggregation of Co active sites in CoPc/ppy/GDE guarantee its promising stability\nunder highly reductive and high current density conditions.\nFigure R49. a, XAS spectra at Co K-edge of CoPc/ppy/GDE and b, Co K-edge FT EXAFS spectra\nin the R space of CoPc/ppy/GDE of initial state and after 200 hours in CO -saturated 0.1 M KHCO\n2 3\nwith an H-type cell at −0.75 V versus RHE.\nFigure R50. Comparison of the Co 2p XPS spectra of CoPc/ppy/GDE of initial state and after 200\nhours in CO -saturated 0.1 M KHCO with an H-type cell at −0.75 V versus RHE.\n2 3\nFigure R51. a, TEM image of post-reacted CoPc/ppy/GDE after 200 hours. b, HAADF-STEM\nimage of post-reacted CoPc/ppy/GDE after 200 hours.\nCombined all the above experiments and analyses together, we propose that Co+ could be the\nactive species in CoPc/ppy/GDE for CO RR. Co+ species are possibly stabilized by the\n2\nmicroenvironment and ligands, forming a dynamically stable active centre; upon the removal of\napplied potential, oxygen-containing species (OH−, H O and reaction intermediates such as HCO −\n2 3\nand COOH*) may push the oxidation state of Co to a higher value.\nWe have included these additional experimental data and detailed mechanistic explanations in\nthe revised manuscript. We hope that these additional analyses would provide a clearer\nunderstanding on the behaviour of CoPc/ppy/GDE during CO RR. Thank you once again for your\n2\nvaluable feedback.\nReferences\n[1] Wang, S. F. et al. Manipulating C-C coupling pathway in electrochemical CO reduction for\n2\nselective ethylene and ethanol production over single-atom alloy catalyst. Nat. Commun. 15, 10247\n(2024).\n[2] Ren, S. et al. Catalyst aggregation matters for immobilized molecular CO RR electrocatalysts.\nComment 7: The authors showed the changes of CoPc/ppy@GDE at the fixed potential over the\ntime and discuss the changes of the XANES and EXAFS spectra. However, control sample\nCoPc@GDE is also required to be measured over the same period of time to investigate whether\nthe changes of the Co oxidation states are associated with ppy introduction.\nReply to comment 7: Thank you for your suggestion regarding the measurements of control sample\nto understand the change behaviour of Co in CoPc/ppy/GDE and the impact of introduced ppy.\nAs\nthe loading amount of CoPc in CoPc/GDE is significantly low (below the detection limit of the\ninstrument), CoPc/CNT-GDE (without ppy) was employed as the reference to decouple the\ncontribution of ppy in CoPc/ppy/GDE.\nAs mentioned above, our XAS measurements were performed under ex situ conditions on an\neasyXAFS setup, due to the low intensity of collected signals with a conventional electrochemical\ncell. At different reaction times, the CoPc/ppy/GDE was taken out from the electrochemical cell and\nsubjected to XAS measurements. Under this condition, oxygen-containing species (OH−, H O and\n2\nreaction intermediates such as HCO − and COOH*) may adsorb on CoPc/ppy/GDE, and upon\n3\nremoval of the applied potential valence-state increase of the Co centre would occur (XANES\nspectra in Figure R52a)1. More evidences come from the EXAFS analysis, which reveals a slight\nelongation of the Co–N bond (from 1.98 to 2.05 Å) and importantly the emergence of a Co–O\ncoordination peak at ~1.2 Å (Figure R52b).These phenomena are consistent with the coordination\nof oxygen-containing reactants/intermediates (e.g., OH−, HCO − or HCOO* species) to the Co centre\n3\nand the correspondingly relaxed structure (elongated Co–N bond) in CoPc/ppy/GDE. In comparison,\nthe blue-shift of absorption edge in the XANES curves and the Co–O coordination in the EXAFS\nspectra for CoPc/CNT-GDE are not as prominent as those of CoPc/ppy/GDE (Figure R53),\nindicating weaker adsorption of OH−, CO , H O reactants and less accumulation of oxygen-\n2 2\ncontaining intermediates over CoPc/CNT-GDE than CoPc/ppy/GDE. DFT calculations provide\nadditional evidences, which reveal that Co sites in CoPc/ppy/GDE exhibit a strong interaction with\nintermediates; such a feature is crucial for accelerating CO conversion (Figure R54). We attribute\n2\nthe enhanced stabilization of reaction intermediates on CoPc/ppy/GDE than CoPc/CNT-GDE to the\nelectron donation effects from ppy to CoPc.\nWe have added these data and discussion in the revised manuscript. Thanks again for your\nvaluable feedback.\nFigure R52. a, XAFS spectra at the Co K-edge of CoPc/ppy/GDE during CO RR at −0.8 V versus\n2 3\nduring CO RR at −0.8 V versus RHE from 0 to 2 h in CO -saturated 0.1 M KHCO .\n2 2 3\nFigure R53. a, XAFS spectra at the Co K-edge of CoPc/CNT-GDE during CO RR at −0.8 V versus\n2 3\nGDE during CO RR at −0.8 V versus RHE from 0 to 2 h in CO -saturated 0.1 M KHCO .\n2 2 3\nFigure R54. Schematic diagram of differential charge density of CoPc/ppy and Bader charge\nanalysis. The cyan (yellow) region shows electron loss (gain).\nReference\n[1] Wang, S. F. et al. Manipulating C-C coupling pathway in electrochemical CO reduction for\n2\nselective ethylene and ethanol production over single-atom alloy catalyst. Nat. Commun. 15, 10247\n(2024).\nComment 8: The author mentioned that “The absorption edges blueshifted with reaction time,\nindicating that the valence state of Co gradually increased during the reaction.” Is it reversible?\nIf\nso, after the reaction, (post-analysis) showed the similar states of Co or does it go back to the initial\nstates. If not, the authors want to discuss how the ppy helps the charge transfer. The changes of the\noxidation states can be an indication of the degradation of the CoPc. Show the post-analysis,\n(TEM, EXAFS, XANES, XPS) can provide the information whether the catalyst maintained the initial\nCoPc/ppy structure.\nReply to comment 8: We appreciate your comments regarding the oxidation states of Co in\nCoPc/ppy@GDE catalyst.\nAs mentioned in above replies, our XAS measurements were performed under ex situ\nconditions. Upon the removal of applied potentials, coordination of oxygen-containing\nreactants/intermediates (e.g., OH−, H O and reaction intermediates such as HCO −\n2 3\nand COOH*) occurs and leads to\nvalence-state increase of the Co centre. Therefore, we can observe the blue-shift of absorption edge\nin the XANES curves and the emergence of a Co–O coordination peak at ~1.2 Å in the EXAFS\nspectra for CoPc/CNT-GDE. As long as the negative potentials are applied (or as long as the reaction\nproceeds without stop), this increase of Co valence state in CoPc/CNT-GDE is reversible.\nIn the previous study, Co+ is considered as the active species for CO RR and through the\n2\ndiagnostic Raman signatures of Co+ (at ca. 1110 cm−1) and Co2+ species (at ca. 1140 cm−1), their\nidentification and quantification have been achieved2. To provide a real-time monitor of the Co\nvalence state (change of Co⁺ and Co2+ species) in CoPc/CNT-GDE, we also performed surface-\nenhanced Raman spectroscopy (SERS) measurements under operando conditions. We fixed the\npotential at −1.4 V versus RHE and monitored the evolution of Co+ and Co2+ species in\nCoPc/ppy/GDE. As shown in Figure R55, with the continuous application of a negative potential of\n−1.4 V versus RHE, no obvious degradation in the peak intensity of Co+ species (at ca. 1110 cm−1)\ncould be detected on CoPc/ppy/GDE over a reaction period of 30 min; while for CoPc/CNT-GDE,\na gradual decrease of Co+ species appears, possibly due to the progressive loss of CoPc from\nCoPc/CNT-GDE electrode (Figure R56).\nIn terms of further reduction of Co+ species to zero-valence Co, a previous study demonstrated\nthat Co+ species in CoPc is stable up to the potential of −2.2 V versus RHE, as the percentage of\nCo+ species reaches a steady state from ca. −1.0 V versus RHE2. If a further reduction of Co+ species\noccurs, a decreased Co+ percentage is expected. Consistent with the previous study, our results under\nvaried potentials reveal that the signal intensity of Co+ species (at 1110 cm−1) over CoPc/ppy/GDE\nbecomes prominent at ca. −0.7 V versus RHE, and beyond this potential the spectrum reaches a\nrelatively steady state (Figure R57).\nFigure R55. a, b, In situ SERS of the CoPc/ppy/GDE and CoPc/CNT-GDE under applied bias\nranging from −0.4 to −1.4 V versus RHE.\nFigure R56. Time-dependent in situ surface-enhanced Raman spectra of the (a) CoPc/ppy/GDE (b)\nCoPc/CNT-GDE during CO RR from 0 to 30 min at −1.4 V versus RHE in CO -saturated 0.1 M\n2 2\nKHCO .\n2\nversus RHE in CO -saturated 0.1 M KHCO .\n2 3\nFinally, according to your suggestion, we performed detailed structural characterizations using\nXAS, XPS and HAADF-STEM on the post-reacted CoPc/ppy/GDE catalyst (polarized at −0.75 V\nversus RHE for 200 hours). As shown in Figure R58, no obvious changes in the valence state and\ncoordination environment of Co could be resolved for post-reacted CoPc/ppy/GDE. XPS analysis\nreveals that although the intensities of Co 2p peaks in post-reacted CoPc/ppy/GDE have decreased\n(possibly due to the contamination from electrolyte), their positions remain largely unaltered (Figure\nR59). In addition, HAADF-STEM measurements suggest that the morphology of post-reacted\nCoPc/ppy/GDE catalyst is retained, with well-preserved atomically dispersed Co sites (Figure R60).\nAll these post-reaction analyses together demonstrate that our CoPc/ppy/GDE electrode is highly\nresistant to structural collapse/changes under prolonged electrolysis, the negligible demetallation,\nreduction and aggregation of Co active sites in CoPc/ppy/GDE guarantee its promising stability\nunder highly reductive and high current density conditions.\nFigure R58. a, XAS spectra at Co K-edge of CoPc/ppy/GDE and b, Co K-edge FT EXAFS spectra\nin the R space of CoPc/ppy/GDE of initial state and after 200 hours in CO -saturated 0.1 M KHCO\n2 3\nwith an H-type cell at −0.75 V versus RHE.\nFigure R59. Comparison of the Co 2p XPS spectra of CoPc/ppy/GDE of initial state and after 200\nhours in CO -saturated 0.1 M KHCO with an H-type cell at −0.75 V versus RHE.\n2 3\nFigure R60. a, TEM image of post-reacted CoPc/ppy/GDE after 200 hours. b, HAADF-STEM\nimage of post-reacted CoPc/ppy/GDE after 200 hours.\nCombined all the above experiments and analyses together, we propose that Co+ could be the\nactive species in CoPc/ppy/GDE for CO RR. Co+ species are possibly stabilized by the\n2\nmicroenvironment and ligands, forming a dynamically stable active centre; upon the removal of\napplied potential, oxygen-containing species (OH−, H O and reaction intermediates such as HCO −\n2 3\nand COOH*) may push the oxidation state of Co to a higher value.\nWe have included these additional experimental data and detailed mechanistic explanations in the\nrevised manuscript. We hope that these additional analyses would provide a clearer understanding\non the behaviour of CoPc/ppy/GDE during CO RR. Thank you once again for your valuable\n2\nfeedback.\nReferences\n[1] Wang, S. F. et al. Manipulating C-C coupling pathway in electrochemical CO reduction for\n2\nselective ethylene and ethanol production over single-atom alloy catalyst. Nat. Commun. 15,\n10247 (2024).\n[2] Ren, S. et al. Catalyst aggregation matters for immobilized molecular CO RR electrocatalysts.\n“Previous studies have claimed that Co+ species in CoPc formed by one-electron reduction of\nCo2+, are the active site for CO RR31,32. Through the diagnostic Raman signatures of Co+ (at ca.\n2\n1110 cm−1) and Co2+ species (at ca. 1140 cm−1), the identification and quantification of Co+ active\nspecies have been achieved33–35. Following these reports, the evolution of CoPc/ppy/GDE and the\ndistribution of Co+ and Co2+ species (i.e. the ratio of Co+ to Co2+) with applied potentials were\nmonitored through in situ surface-enhanced Raman spectroscopy (SERS). As shown in Figs. 3a and\nb, characteristic peaks could be resolved at 1113 and 1146 cm−1 at −0.4 V versus RHE for both\nelectrodes of CoPc/ppy/GDE and CoPc/CNT-GDE, which are reported to be associated with the\nCo+ and Co2+ species, respectively35. The signal intensity of Co+ species (at 1113 cm−1) over\nCoPc/ppy/GDE becomes prominent at ca. −0.7 V versus RHE, consistent with the redox behaviour\nof Co2+/Co+ in CoPc; beyond this potential, the spectrum reaches a relatively steady state. In\ncomparison, the formed Co+ species over CoPc/CNT-GDE is significantly lower in amount\n(Supplementary Figs. 25 and 26). Using the Raman peak area, the ratios of Co+ to Co2+ in\nCoPc/ppy/GDE and CoPc/CNT-GDE at various applied potentials were calculated. The results in\nFig. 3c show that the formed Co+ species over CoPc/ppy/GDE are always higher than that of\nCoPc/CNT-GDE, aligned with its better catalytic activity for CO generation36. It is inferred that the\nelectron donation effects from ppy to CoPc is responsible. Furthermore, the stability of Co+ species\nduring CO RR was evaluated by in situ SERS. As shown in Supplementary Fig. 27, with the\n2\ncontinuous application of a negative potential of −1.4 V versus RHE, no obvious degradation in the\npeak intensity of Co+ species could be detected on CoPc/ppy/GDE over a reaction period of 30 min;\nwhile for CoPc/CNT-GDE, a gradual decrease of Co+ species appears, possibly due to the\nprogressive loss of CoPc from CoPc/CNT-GDE electrode.”\n“To further assess the evolutionary nature of the atomically dispersed Co active sites, ex situ\nXAS measurements were carried out. Co K-edge XANES spectra of CoPc/ppy/GDE were collected\nafter reacting in a CO -saturated 0.1 M KHCO solution for different times. Under this condition,\n2 3\noxygen-containing species (OH−, H O and reaction intermediates such as HCO −, CO* and COOH*)\n2 3\nmay adsorb on CoPc/ppy/GDE, and upon removal of the applied potential valence-state increase\nof the Co centre would occur (Supplementary Fig. 31)38,39. More evidences come from the EXAFS\nanalysis, which reveals a slight elongation of the Co–N bond (from 1.98 to 2.05 Å)40,41 and\nimportantly the emergence of a Co–O coordination peak at ~1.2 Å (Fig. 3e)42. These phenomena\nare consistent with the coordination of oxygen-containing reactants/intermediates (e.g., OH−,\nHCO − or COOH* species) to the Co centre and the correspondingly relaxed structure (elongated\ncurves and the Co–O coordination in the EXAFS spectra for CoPc/CNT-GDE are not as prominent\nas those of CoPc/ppy/GDE (Supplementary Fig. 32), indicating weaker adsorption of OH−, CO ,\n2\nthan CoPc/ppy/GDE.”\nComment 9: Can it be applicable to other Pc such as NiPc and FePc for electorchemical CO2RR?\nReply to comment 9: Thank you for your question regarding the applicability of our strategy to\nother Pc-based catalysts, such as NiPc, FePc and CuPc, for the electrochemical CO RR. We have\n2\nconducted these experiments, the results show that FePc, NiPc and CuPc can be successfully\nincorporated into the ppy matrix and act as effective electrocatalysts for CO RR (Figure R61).\ncm−2 at −0.7 V versus RHE, which are slightly inferior to that of CoPc/ppy/GDE. While\nFePc/ppy/GDE and CuPc/ppy/GDE demonstrate CO selectivities approaching 90%, accompanied\nwith current densities of 9.3 and 9.8 mA cm−2, respectively. These results conclusively validate the\ncompatibility of our methodology with diverse phthalocyanine-based catalytic systems. We have\nincluded these additional experimental data and discussion in the revised manuscript.\nFigure R61. a, LSV curves of CoPc/ppy/GDE, FePc/ppy/GDE, CuPc/ppy/GDE and NiPc/ppy/GDE\nduring CO RR in an H-cell with CO -saturated 0.1 M KHCO electrolyte. b, Faradaic efficiency of\n2 2 3\nCoPc/ppy/GDE, FePc/ppy/GDE, CuPc/ppy/GDE and NiPc/ppy/GDE during CO RR at −0.7 V\n2\nversus RHE in an H-cell with CO -saturated 0.1 M KHCO electrolyte.\n2 3\n“Motivated by the excellent CO RR performance of CoPc/ppy/GDE, NiPc/ppy/GDE,\ncovalent grafting strategy (Supplementary Fig. 24). Among them, NiPc/ppy/GDE exhibits a CO\nselectivity of 92% with a current density of 14.3 mA cm−2 at −0.7 V versus RHE, which are slightly\ninferior to that of CoPc/ppy/GDE; while FePc/ppy/GDE and CuPc/ppy/GDE demonstrate CO\nselectivity approaching 90%, accompanied with current densities of 9.3 and 9.8 mA cm−2,\nrespectively. These results validate the compatibility of our covalent grafting strategy with diverse\nphthalocyanine-based molecular catalysts.”\nComment 10: How the current density affected by the loading amount of the catalyst? That can be\nthe useful information.\nReply to comment 10: Thank you for raising this critical question. Despite the lack of precise\ncontrol over the ppy molecular weight, our CoPc/ppy/GDE catalysts show controllable and\nreproducible electrochemical behaviors towards CO RR, when the electropolymerization\n2\nconditions are well organized. Specifically, the electrochemical CO RR performance of\nparameters including the number of CV scans and the feed ratio of pyrrole to CoPc, based on which\nthe chain length of ppy and CoPc loading/incorporating could be controlled.\nFigure R62a illustrates the effects of CV number during electropolymerization, the current\ndensity that corresponds to the electrochemical growth of CoPc and ppy on GDE initially increases\nwith cycle number but exhibits a saturation trend beyond 40 cycles. We infer that mass transport or\nppy resistance issues gradually dominate and cease the continuous growth of CoPc and ppy on GDE\nin the applied CV potential range. Correspondingly, the CO RR performance of yielded\nNext, considering that the CO RR performance of CoPc/ppy/GDE electrodes is closely\n2\ncorrelated with the loading/incorporating amount of CoPc, we optimized the synthetic procedure by\nvarying the pyrrole-to-CoPc feed ratio (1:6 to 6:1). As shown in Figure R63, only small activity\ndifferences could be observed for these CoPc/ppy/GDE electrodes obtained in the presence of both\npyrrole and CoPc, and the best performing one is obtained with a high pyrrole-to-CoPc ratio of 4:1;\nwhen only CoPc is applied, the yielded CoPc/GDE electrode shows largely suppressed CO RR\n2\nactivity.\nBased on these results, we infer that the electropolymerization step of ppy dominates the whole\nformation process of CoPc/ppy/GDE electrodes, as pristine CoPc is chemically less active\ntowards electropolymerization; under this condition, ppy initiates the C–C coupling process\nbetween GDEs and ppy, ppy and pristine CoPc, thereby realizing covalent grafting of pristine CoPc\nonto carbon-based GDEs via robust C–C bonds.\nCollectively, through utilizing ppy as the electroactive \"glue\", covalent grafting of pristine\nCoPc onto carbon-based GDEs via robust C–C bonds has been achieved. The optimal conditions\nfor CoPc/ppy/GDE electrode fabrication are determined to be 40 of CV cycles and 4:1 of pyrrole-\nto-CoPc ratio that balance performance and cost efficiency. Although the precise control over the\nppy molecular weight remains a challenge and is worth of future studies, the yielded CoPc/ppy/GDE\nelectrodes show reproducible CO RR activity and promising stability under high current densities,\n2\nsuggesting the effectiveness of us in situ electropolymerization strategy for covalent grafting of\npristine molecular catalysts onto carbon-based GDEs. We have added these optimizations and\ndiscussion in the revised manuscript. Thank you again!\nFigure R62. a, The different deposition cycles of CoPc/ppy/GDE during the electro-polymerization\nin 0.01 M [BMIM]BF with 4 mM pyrole and 1 mM CoPc at 100 mV s−1. The potentials were\n4\ncalibrated to the Ag/AgCl scale. b, The variation of current density with the number of deposition\ncycles.\nFigure R63. a, The LSV of CoPc/ppy/GDE during the electro-polymerization in an H-cell with\nCO -saturated 0.1 M KHCO electrolyte. The potentials were calibrated to the Ag/AgCl scale. b,\n2 3\nThe variation of current density with the number of deposition cycles.\nIn summary, we have considered the current density affected by loading amount of the catalyst.\nThrough systematic optimization of synthesis conditions and cost-effective, we investigated the\noptimal number of deposition cycles for catalyst fabrication to achieve balanced performance and\neconomic feasibility. We have included detailed information on these optimizations in the revised\nmanuscript.\nThank you again for your valuable feedback. We look forward to your further comments and\nsuggestions.\nReviewer #4:\nComment: This manuscript reported the synthesis of CoPc for the efficient conversion of CO to\nfuel efficiency of 19.2%. Despite this work provides a complete study, the novelty of the\nelectrocatalytic CO to CO is not enough. Moreover, the electrolysis in alkaline electrolytes under\n2\nhigh current density do not provide new insight. Whether this catalyst is suitable for MEA testing\nunder pure water conditions. Thus, the novelty and depth of academic significance does not reach\nthe level of Nature Communications.\nThanks for your comments. The uniqueness and progresses of this work can be summarized as\nfollows.\n(1) The importance of CO electrolysis to CO.\n2\nimportant pathway for achieving carbon neutrality, but also can be combined with thermocatalysis\n(such as Fischer-Tropsch synthesis) or electrolysis to manufacture various high-value chemicals\nsuch as methanol and olefins1-3. Following are some examples.\nProf. Jiao group have constructed a tandem CO electrolyzer for the generation of multicarbon\n2\nproducts at the kilowatt scale; in this tandem electrolysis system, CO is first converted to CO in a\n2 2+\n(Figure R64; Kilowatt-scale tandem CO electrolysis for enhanced acetate and ethylene production,\n[FIGURE REDACTED]\nFigure R64. Two-step CO electrolysis stack for multicarbon chemical production.\n2\nsame reactor, leading to enhanced activity and product selectivity. For example, Prof. Wang group\nhave developed a tandem catalyst consisting of CoPc and Zn-N-C to promote CH production in\n2 4\nproduction rate ratio over the CoPc@Zn-N-C tandem catalyst (Figure R65; Enhancing CO\n2\nelectroreduction to methane with a cobalt phthalocyanine and zinc–nitrogen–carbon tandem catalyst,\nAngew. Chem. Int. Ed., 2020, 59, 22408–22413).\n[FIGURE REDACTED]\nFigure R65. CO electroreduction to methane with a cobalt phthalocyanine and zinc–nitrogen–\n2\ncarbon tandem catalyst.\nProf. Chen group have proposed an electrocatalytic–thermocatalytic tandem strategy for\ncarbon nanofiber (CNF) production, which integrates the co-electrolysis of CO and water into\n2\nsyngas (CO and H ) with a subsequent thermochemical process at relatively mild conditions (370–\n2\n450(cid:3031)°C, 1(cid:3031)atm), yielding CNF at a high production rate (average 2.5(cid:3031)g (cid:3031)g −1(cid:3031)h−1) (Figure R66;\ncarbon metals\nCO fixation into carbon nanofibres using electrochemical–thermochemical tandem catalysis,\n[FIGURE REDACTED]\nFigure R66. Electrochemical–thermochemical tandem process for producing carbon nanofibers and\nrenewable H .\nwhich 52% concentrated CO is produced and is further converted into single layer graphene film\n(Figure R67; Direct electrosynthesis of 52% concentrated CO on silver’s twin boundary, Nature\nCommunications, 2021, 12, 2139).\n[FIGURE REDACTED]\nFigure R67. The schematic diagram of “two-step” synthesis of graphene from CO 6.\n2\n(2) The importance and innovation of covalent grafting of pristine cobalt phthalocyanine\non gas diffusion electrodes (GDEs) for CO RR.\nhave demonstrated highly selective CO electrolysis at high current densities in flow reactors.\nthis end, molecular catalysts are generally coated onto the carbon-based GDEs (to circumvent their\nlimitations of aggregation and poor conductivity), with the majority of the previous works utilizing\nthe noncovalent anchoring strategies. In these strategies, binders like Nafion are usually required,\nscaffolds such as CNT (carbon nanotube) and graphene are also included to enhance the dispersion\nand adhesion strength of molecular catalysts through π-π stacking interaction.\nHowever, considering the possible loss of molecular catalysts during operation (especially at\nhigh current densities), anchoring them directly onto carbon-based GDEs through strong covalent\nbonds is a desirable strategy. Presently, direct covalent grafting of CoPc (and other metal\nphthalocyanine/porphyrin) based molecular catalysts onto carbon-based GDEs would require the\nuse of CoPc with specially designed functional groups (such as amino group) or polymerizable\ngroups (such as carbazole). Until now, due to the inherent chemical inertness of pristine CoPc,\ndirect electrochemical polymerization or covalent grafting of it onto the carbon-based GDEs is challenging.\nOn the other hand, among common covalent bonds that could form between CoPc (or its derivatives)\nand carbon-based GDEs, C–C bond is more inert and robust for extreme reaction conditions, when\ncomparing to those hydrolysable amide and ester bonds.\nIn our study, to offer robust C–C bonds between carbon-based GDEs and pristine CoPc, we\nintroduce ppy as the linker which initiates the C–C coupling process between GDEs and ppy, ppy\nand pristine CoPc thereby connecting them together via C–C bonds. During this covalent grafting\nprocess, pristine CoPc without polymerizable groups is applied; meanwhile, no pretreatment of\ncarbon-based GDEs is required, which minimizes the amount of oxygen-containing functional\ngroups (such as carboxyl and hydroxyl groups) thereby guaranteeing sufficient hydrophobicity for\nefficient CO gas penetration.\nthrough adjusting the polymerization parameters of CV number, potential range), and could be well\nextended to other pristine metal phthalocyanines or porphyrins.\n(3) The advancements of our CoPc/ppy/GDE electrode.\nBased on the combined advantages of robust C–C bonds, binder-free interface, highly\nconductive ppy linker, appropriate catalyst dispersion, electron donation effects from ppy, and active\nCo single-atom sites, our in situ covalent grafting derived CoPc/ppy/GDE establishes promising\nactivity and stability for CO RR. Specifically, CoPc/ppy/GDE electrode exhibits a high current\n2\ndensity of −40 mA cm−2 at −0.78 V versus RHE in H-type cells, far better than previously reported\nCoPc-based catalysts (Table R8); an exceptional 200-hour stability at −0.78 V versus RHE is\ndemonstrated on CoPc/ppy/GDE, and at the end of stability test no obvious valence state, structure\nand coordination environment changes of Co are detected by XPS, HAADF-STEM and XAS.\nFurthermore, the as-assembled CoFe-Ci@GQDs/NF||CoPc/ppy/GDE electrolyzer achieves a record\ndevice stability of 50 hours at 1000 mA cm−2 and 120 hours at 500 mA cm−2, which strongly\ndemonstrate that our CoPc/ppy/GDE electrode is highly resistant to structural collapse/changes\nunder industrially relevant current densities (Table R9 and Figure R68). Based on the CoFe-\nCi@GQDs/NF||CoPc/ppy/GDE electrolyzer, a solar-driven CO RR device is constructed, which\n2\ndelivers a high solar-to-fuel efficiency of 19.2%.\nIn addition, we have also tested the availability of our MEA electrolyzer under pure water\nconditions. As exhibited in Figure R69, the electrolyzer generates severely limited current densities\n(for example 30 mA cm−2 at 3.0 V) using pure water, possibly due to its 10-fold higher system\nresistance compared to that in 1 M KOH (55 Ω versus 0.45 Ω). At present, we tentatively attribute\nthis ohmic loss of our MEA electrolyzer in pure water to the intrinsically low conductivity of pure\nwater and inappropriate MEA architecture. Our future investigations will focus on these issues, and\nmore importantly, exploring the possibility of running our MEA electrolyzer with proton exchange\nmembrane in pure water (aiming to resolve the mass transport limitation induced by salt\nprecipitation and the low carbon utilization efficiency due to carbonate formation).\nThanks again for your comments.\nTable R8. Comparison of the CO -to-CO conversion of different electrocatalysts in H-type cells.\nCO\nCatalysts Electrolyte Ref.\n(%) (V vs. RHE) (mA cm−2) (h)\nCoPc/ppy/GDE 0.1(cid:3031)M KHCO 96 −0.78 −40 200 This work\n(OMe) - 0.5(cid:3031)M KHCO 93.6 −0.68 ~−6 12 S14\n2 3\nCo@CNT\nCOF-367-Co 0.5(cid:3031)M KHCO 91 −0.55 −3.3 24 S15\n0.5(cid:3031)M KHCO 99.8 −0.9 −14.1 60 S16\nTT-Por(Co)-\n0.5(cid:3031)M KHCO 91.4 −0.6 ~−1.5 10 S17\nCoPc-PI-COF-1 0.5(cid:3031)M KHCO 90 −0.7 −10 40 S18\n0.5(cid:3031)M KHCO 88 −0.7 −6 50 S19\nCoPc/CNT 0.1(cid:3031)M KHCO 92 −0.63 −10 10 S20\n0.5(cid:3031)M KHCO 95 −0.62 ~−12 12 S21\nCoPPc/CNT 0.5(cid:3031)M NaHCO 90 −0.54 −12 24 S22\nCatalysts Electrolyte FE References\nCO\n(V) cm−2) (h)\nCoPc/ppy/GDE 1.0 M KOH 99% ~2.8 500 120 This work\nCoPc/ppy/GDE 1.0 M KOH 98% ~3.2 1000 50 This work\nCo-CNTs MW 1.0 M KOH 98.3% ~2 100 12 44\nCoPc 1.0 M KOH 88% ~2.52 200 6 45\nMWNT/\n2.0 M KOH 85% ~2.25 158 8 46\nPyPBI/Au\nAg/C 1.0 M KOH 83% ~2.75 120 0.417 47\n1.0 M\nAg KOH+0.33\n98% ~2.16 100 10 48\nnanoparticles M\nUrea\nZn P O 1.0 M KOH 93.9% 100 7 49\n2 2 7\nAu 0.1 M KOH 90.0% ~3 100 100 50\n97% ~2.6 50 60 17\nCoPc/MWCNTs KHCO\n3\n0.1 M\nCoPc/Mg(OH) 95% 3.4 100 20 51\n3\nβ-CoPc/CP 1.0 M KOH 92% 2 100 29.3 52\nAG- 0.1 M\n80% 3 50 12 26\nCoPc/MWCNTs KHCO\n80% 3.2 50 40 26\nCoPc/MWCNTs KHCO\nReferences\n[1] Kim, Y. et al. Integrated CO capture and conversion to form syngas. Joule 8, 3106–3125 (2025).\n2\n[2] Khoshooei, M. A. et al. An active, stable cubic molybdenum carbide catalyst for the high-\ntemperature reverse water-gas shift reaction. Science 8, 3106–3125 (2025).\n[3] Reaction-induced unsaturated Mo oxycarbides afford highly active CO2 conversion catalysts.\ndoi.org/10.1038/s41929-023-00937-0.\n[4] Hao, S. Y. et al. Improving the operational stability of electrochemical CO reduction reaction\n2\nvia salt precipitation understanding and management. Nat. Energy 10, 266–277 (2025).\n[5] McGregor, J. M. et al. Organic electrolyte cations promote non-aqueous CO reduction by\n2\nmediating interfacial electric fields. Nat. Catal. 8, 79–91 (2025).\n[6] Yu, X. H. et al. Coverage enhancement accelerates acidic CO electrolysis at ampere-level\n2\ncurrent with high energy and carbon efficiencies. Nat. Commun. 15, 1711 (2024).\n[7] Tang, X. H. et al. Direct electrosynthesis of 52% concentrated CO on silver’s twin boundary.\nNat. Commun. 12, 2139 (2021)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "After reviewing the manuscript, I acknowledge the authors' efforts in conducting additional experiments and expanding the discussion. Although similar systems have been reported in the literature, this work presents some performance advantages. Given the sound methodology and the relevance of the results, I recommend the manuscript for publication.", "reviewer_2": "The authors have revised the ms according to the comments and can be accepted. Still, there are some grammar and words needing further revision, e.g. times or time?", "reviewer_4": "Although the authors have addressed some of the previous comments, the overall quality of the manuscript still does not meet the high publication standards of Nature Communications. Therefore, I recommend submitting this work to a more specialized journal. Specific suggestions for improvement include:\n1.While the revised manuscript provides additional data, the fundamental mechanism underlying the enhanced activity and stability of the CoPc/ppy/GDE system remains insufficiently elucidated. For instance: the role of polypyrrole in modulating the electronic structure of CoPc is discussed, but direct experimental evidence to confirm the proposed electron-donation effect is still lacking.\n2.The claim that C–C covalent bonds are formed between CoPc and ppy relies heavily on indirect evidence. More rigorous characterizationis needed to unambiguously verify the bonding configuration.\n3.The manuscript emphasizes the novelty of covalent grafting for pristine CoPc, but the performance metrics do not significantly surpass recent reports using functionalized molecular catalysts. The stability data is commendable but could be further contextualized against industrial benchmarks.\n4.The post-reaction characterization suggests structural retention, but the origin of eventual performance decay is not investigated. Long-term stability tests under intermittent operation would strengthen the practical relevance.\n5.The DFT analysis of CO₂ reduction energetics is oversimplified. Key aspects are overlooked: How does the ppy linker influence the local pH or CO₂ concentration at the catalyst surface? Are the proposed active sites (Co⁺) stable under alkaline conditions, given prior reports of CoPc demetallation at high pH?"}, "author_response": "Responses to Reviewers' Comments\nReviewer #1:\nComment: After reviewing the manuscript, I acknowledge the authors' efforts in conducting\nadditional experiments and expanding the discussion. Although similar systems have been reported\nin the literature, this work presents some performance advantages. Given the sound methodology\nand the relevance of the results, I recommend the manuscript for publication.\nReply to comment: Thanks for your positive feedback! Based on your insightful comments and\nsuggestions, the quality of our manuscript has been significantly improved. Thanks again!\nReviewer #2:\nComment: The authors have revised the ms according to the comments and can be accepted. Still,\nthere are some grammar and words needing further revision, e.g. times or time?\nReply to comment: Thanks for your positive feedback! With your valuable comments and\nsuggestions, the quality of our manuscript has been significantly improved. Based on your\nsuggestion, we have carefully reviewed the entire manuscript to correct the errors associated with\ngrammar, word and phrasing. Thanks again!\nReviewer #2's comments on your response to Reviewer #3's concerns:\nComment: The authors have replied the comment 3 and revised the ms according to the comments.\nThe reply was correct and helpful to the final ms. The present ms can be accepted.\nReply to comment: Thanks for your positive feedback! We are grateful for your time and insightful\ncomments throughout the review process, which have greatly improved the quality of our work.\nThanks again!\nReviewer #4:\nComment: Although the authors have addressed some of the previous comments, the overall quality\nof the manuscript still does not meet the high publication standards of Nature Communications.\nTherefore, I recommend submitting this work to a more specialized journal. Specific suggestions\nfor improvement include:\nComment 1: While the revised manuscript provides additional data, the fundamental mechanism\nunderlying the enhanced activity and stability of the CoPc/ppy/GDE system remains insufficiently\nelucidated. For instance: the role of polypyrrole in modulating the electronic structure of CoPc is\ndiscussed, but direct experimental evidence to confirm the proposed electron-donation effect is still\nlacking.\nReply to comment 1:\nWe sincerely thank the reviewer for raising this important point, which encourages us to find\nthe underlying mechanism using obtained experimental data.\nIn our study, to offer robust C–C bonds between carbon-based GDEs and pristine CoPc, we\nintroduce ppy as the linker which initiates the C–C coupling process between GDEs and ppy, ppy\nand pristine CoPc thereby connecting them together via C–C bonds. During this covalent grafting\nprocess, pristine CoPc without polymerizable groups is applied; meanwhile, no pretreatment of\ncarbon-based GDEs is required, which minimizes the amount of oxygen-containing functional\ngroups (such as carboxyl and hydroxyl groups) thereby guaranteeing sufficient hydrophobicity for\nefficient CO gas penetration. As far as we know, such a convenient and efficient approach has not\n2\nbeen explored for direct grafting of pristine CoPc (rather than CoPc derivatives) on carbon-based\nGDEs, the employed ppy not only acts as the highly conductive and robust linker between CoPc\nand GDEs, but also shows electron donation effects to promote the CO RR activity of CoPc centers.\nprovided. The first evidence is from XPS analysis. Compared to commercial CoPc (781.4 eV and\n796.5 eV), the binding energies in the Co 2p XPS spectra of CoPc/ppy/GDE (780.8 eV and 795.9\neV for Co 2p and Co 2p , respectively) decrease by 0.6 eV, indicating an increased electron\n3/2 1/2\ndensity around the Co centers (Figure R1). The second evidence is from XAS analysis. We\nperformed XAS at the Co K-edge on both pristine CoPc and the CoPc/ppy/GDE composite. The\nresults are presented in the Figure R2. The absorption edge of CoPc/ppy/GDE is redshifted\ncompared to pristine CoPc, with weakened 1s→4p transition at ~7710 eV; this suggests reduced\nz\nsymmetry of the Co–N coordination in CoPc/ppy/GDE, and the average Co oxidation state locates\n4\nbetween metallic Co (0) and CoO (+2). The Co K-edge white line intensity in CoPc/ppy/GDE is\nlower than that in pristine CoPc. This decrease in white line intensity indicates an increase in\nelectron density at the Co metal center, consistent with electron transfer from the electron-rich ppy\nmatrix. Fourier-transform extended X-ray absorption fine structure (FT-EXAFS) analyses identify\na dominant Co–N coordination peak at ~1.5 Å for both CoPc/ppy/GDE and pristine CoPc, indicating\nCo2+–N as the primary species in them. Wavelet-transforms of EXAFS spectra further confirm this\nconclusion by revealing prominent intensity maxima at ~1.5 Å; meanwhile, a minor intensity\nmaximum is exclusively resolved at ~2.7 Å over CoPc/ppy/GDE, which is inferred to arise from the\nCo+–N species (Figure R3). The presence of this Co+–N species along with the negatively shifted\nbinding energies of Co 2p peaks together support the electron-donation effect from ppy to CoPc.\nFigure R1. Comparison of the Co 2p XPS spectra of CoPc/ppy/GDE and commercial CoPc.\nFigure R2. a, XAS spectra at Co K-edge of CoPc/ppy/GDE and the reference Co foil and CoPc\nsamples. b, Co K-edge FT EXAFS spectra in the R space of CoPc/ppy/GDE and reference Co foil\nand CoPc samples.\nFigure R3. a, b WT-EXAFS plots of CoPc samples and CoPc/ppy/GDE.\nFurthermore, researchers have claimed that Co+ species in CoPc formed by one-electron\nreduction of Co2+, is the active site for CO RR1,2. Through the diagnostic Raman signatures of Co+\n2\n(at ca. 1110 cm−1) and Co2+ species (at ca. 1140 cm−1), the identification and quantification of Co⁺\nactive species have been achieved3. Following these reports, we have also monitored the evolution\nof CoPc/ppy/GDE and the distribution of Co+ and Co2+ species (i.e. the ratio of Co+ to Co2+) with\napplied potentials through in situ surface-enhanced Raman spectroscopy (SERS). As shown in\nFigure R4, at −0.4 V versus RHE, characteristic peaks could be resolved at 1110 and 1140 cm−1 on\nboth electrodes of CoPc/ppy/GDE and CoPc/CNT-GDE, which are reported to be associated with\nthe Co+ and Co2+ species, respectively4,5. The signal intensity of Co+ species (at 1110 cm−1) over\nCoPc/ppy/GDE becomes prominent at ca. −0.7 V versus RHE, consistent with the redox behaviour\nof Co2+/Co+ in CoPc; beyond this potential, the spectrum reaches a relatively steady state. In\ncomparison, the formed Co⁺ species over CoPc/CNT-GDE is significantly lower in amount. Using\nthe Raman peak area, we calculated the ratios of Co+ to Co2+ in CoPc/ppy/GDE and CoPc/CNT-\nGDE at various applied potentials. The results in Figure R5 show that the formed Co+ species over\nCoPc/ppy/GDE are always higher than that of CoPc/CNT-GDE, for which we infer that the electron\ndonation effects from ppy to CoPc is responsible6.\nFigure R4. a, b, In situ SERS of the CoPc/ppy/GDE and CoPc/CNT-GDE during CO RR under\n2\napplied bias ranging from −0.4 to −1.4 V versus RHE.\nFigure R5. The ratio of Co+: Co2+ during CO RR under applied bias ranging from −0.4 to −1.4 V\n2\nversus RHE in CO -saturated 0.1 M KHCO .\n2 3\nIn the previous study, Co+ species in CoPc is reported to be stable up to the potential of −2.2 V\nversus RHE, as the percentage of Co+ species reaches a steady state starting from ca. −1.0 V versus\nRHE3. Our results under operando conditions in Figure R4 also indicate that no obvious reduction\nof Co+ species occurs up to −1.4 V versus RHE. We have also evaluated the stability of Co+ species\nduring CO RR using in situ SERS. As shown in Figure R6, with the continuous application of a\n2\nnegative potential of −1.4 V versus RHE, no obvious degradation in the peak intensity of Co+ species\ncould be detected on CoPc/ppy/GDE over a reaction period of 30 min; while for CoPc/CNT-GDE,\na gradual decrease of Co+ species appears, possibly due to the progressive loss of CoPc from\nCoPc/CNT-GDE electrode. We attribute the enhanced stabilization of Co+ on CoPc/ppy/GDE than\nCoPc/CNT-GDE to the electron donation effects from ppy to CoPc.\nFigure R6. Time-dependent in situ SERS of the (a) CoPc/ppy/GDE (b) CoPc/CNT-GDE during\nCO RR from 0 to 30 min at −1.4 V versus RHE in CO -saturated 0.1 M KHCO .\n2 2 3\nTo gain more comprehensive mechanistic insights into ppy-enhanced electrocatalytic activity,\nfree energy diagrams considering thermodynamics of CO -to-CO conversion on CoPc/ppy was\n2\nconstructed using DFT calculations. Bader charge analysis suggests that ppy has a tendency to\ntransfer electrons to CoPc (Figure R7). The ppy acts as a linker to anchor CoPc onto the GDE\nthrough covalent bonds, while simultaneously modulating the electronic structure of CoPc for\nimproved catalytic performance.\nFigure R7. Schematic diagram of differential charge density of CoPc/ppy and Bader charge analysis.\nThe cyan (yellow) region shows electron loss (gain).\nCombined all the above experiments and analyses together, we propose that polypyrrole can\nmodulate the electronic structure of CoPc and the Co+ could be the active species in CoPc/ppy/GDE\nfor CO RR. Co+ species are possibly stabilized by the microenvironment and ligands. We hope these\n2\nadditions have decisively addressed the reviewer's concern again.\nReferences\n[1] Wang, S. F. et al. Manipulating C-C coupling pathway in electrochemical CO reduction for\n2\nselective ethylene and ethanol production over single-atom alloy catalyst. Nat. Commun. 15, 10247\n(2024).\n[2] Corbin, N. et al. Heterogeneous molecular catalysts for electrocatalytic CO reduction. Nano\n[3] Hu, X. M. et al. Enhanced catalytic activity of cobalt porphyrin in CO electroreductin upon\n2\nimmobilization on carbon materials. Angew. Chem. Int. Ed. 56, 6468–6472 (2017).\n[4] Ren, S. et al. Catalyst aggregation matters for immobilized molecular CO RR electrocatalysts.\n[5] Jiang, S. et al. Investigation of cobalt phthalocyanine at the solid/liquid interface by\nelectrochemical tip-enhanced raman spectroscopy. J. Phys. Chem. C 123, 9852–9859 (2019).\n[6] Chen, X. et al. Operando observation of molecular-scale manipulation using electrochemical tip-\nenhanced raman spectroscopy. J. Phys. Chem. C 123, 24329–24333 (2018).\nComment 2: The claim that C–C covalent bonds are formed between CoPc and ppy relies heavily\non indirect evidence. More rigorous characterizationis needed to unambiguously verify the bonding\nconfiguration.\nReply to comment 2: Thank you for your valuable comments on our manuscript. We understand\nyour concerns regarding the direct evidence for the formation of covalent bonds between CoPc and\nthe ppy network. We have taken your suggestions seriously and have conducted additional\nexperiments and analyses to address this issue.\nFirst, we utilized in situ Raman spectroscopy to monitor the structural evolution of CoPc during\nthe electro-polymerization on GDE. As shown in Figure R8, the Raman spectra reveal gradually\nincreased peaks that correspond to the characteristic vibrational modes of CoPc. Specifically, the\nmacrocyclic deformation mode at 680 cm−1 exhibits an increase trend with the number of CV\npolymerization cycles, quantitatively correlating with the progressive deposition of CoPc. We also\nobserved a shift of Co–N vibration at ca. 755 cm−1, which is possibly associated with the interfacial\nstrain-induced changes of Co–N bonds in CoPc during polymerization1. Collectively, the\nprogressively increased intensities of CoPc-related peaks with the number of CV cycles2, provide\nevidence for the in situ growth and integration of CoPc onto the GDE.\nFigure R8. In situ Raman spectra of CoPc/ppy/GDE collected at different CV cycles in 0.1 M\n[BMIM]BF with 4 mM pyrole and 1 mM CoPc.\nCV cycles were systematically performed within under controlled potential windows. As shown in\nFigure R9, ppy generated from electrochemical polymerization of pyrrole shows only broad and\nfeatureless oxidation and reduction waves in the potential range of –1.0 to 1.0 V vs. Ag/AgCl.\nComparatively, three redox couples could be resolved for CoPc-modified GDE (CoPc/GDE), with\ntheir locations presented in Table R1. These redox couples are probably associated with the redox\nbehaviours of Co centres of CoPc (Co2+↔Co+↔Co0). Interestingly, CoPc/ppy/GDE electrode\nproduced in the presence of both pyrrole and CoPc shows similar redox characteristics to that of\nCoPc/GDE, with however negatively shifted peak positions and largely enhanced current response\nin the cathodic scan. Based on this observation, we infer that during pyrrole polymerization CoPc\nis integrated into ppy, with the configuration of CoPc terminated at the ppy chain; electron donation\noccurs from ppy to CoPc, thus yielding negatively shifted redox peaks relative to bare CoPc.\nFigure R9. Comparative CV patterns of ppy/GDE in 0.01 M [BMIM]BF with 4 mM pyrole,\n4 4\nmM pyrole and 1 mM CoPc at 100 mV s−1.\nTable R1. The location of the CV peaks\nCatalysts Reduction 1 Reduction 2 Reduction 3\nCoPc/ppy/GDE 0.52 -0.04 -0.98\nCoPc/GDE 0.61 0.03 -0.88\nOxidation 1 Oxidation 2 Oxidation 3\nCoPc/ppy/GDE -0.27 0.36 1.18\nCoPc/GDE -0.15 0.48 1.32\nThird, we used F-doped tin oxide (FTO) as the conductive substrate to study the grow process\nof CoPc/ppy, which would exclude the influence of carbon-based substrates. We collected FTIR\nspectrum of CoPc/ppy/GDE and compared to those of CoPc and ppy/GDE. To avoid the possible\ncontamination of free CoPc (i.e., CoPc adsorbed on GDE or ppy), CoPc/ppy/GDE was thoroughly\ncleaned by ultrasonication and washed by copious amounts of acetonitrile. As shown in Figure R10,\nthe FTIR spectrum of CoPc/ppy/GDE shows a combined feature of CoPc and ppy/GDE, which\nindicates that CoPc had been successfully incorporated into ppy/GDE.\nFigure R10. FTIR spectra of CoPc/GDE, CoPc/ppy/GDE and ppy/GDE.\nFourth, we have also collected solid-state 1H NMR spectra of CoPc/ppy, ppy and pristine CoPc\n(Figure R11). Critically, pristine CoPc exhibits a broad hump in the 6–7 ppm region, while ppy\ndisplays its characteristic aromatic proton peaks in the same range. In contrast, the CoPc/ppy\ncomposite shows distinct, well-resolved peaks between 5–6 ppm, indicating a significant alteration\nin its local electronic environment. Importantly, the peak intensity at ca. 7.5 ppm in CoPc/ppy is\nobviously enhanced than that of ppy, making the signal shape of ppy matrix no longer symmetric.\nWe attribute this phenomenon to protons proximal to a newly formed covalent C–C bond between\nCoPc and the ppy chain, generated via radical coupling during the electrochemical polymerization\nprocess. This covalent grafting changes the chain end structure of ppy, thereby perturbs the electron\ndensity and results in a unique absorption.\nFifth, to probe the molecular structure of CoPc/ppy, matrix-assisted laser desorption/ionization\ntime-of-flight mass spectrometry (MALDI-TOF-MS) was employed (Figure R12). The analysis\nrevealed suppression of the characteristic low-mass oligomer peaks of ppy (e.g., the series around\nm/z 65, 130, and 195). Concurrently, new signals associated with CoPc fragments emerged and\nbecame dominant, most notably an intense peak at m/z 128, which is attributed to phthalonitrile\n(C H N +˙), along with a peak at m/z 59 corresponding to the cobalt ion.\n8 4 2\nFigure R11. Solid-state 1H NMR spectra of CoPc/ppy, CoPc and ppy.\nFigure R12. MALDI-TOF-MS spectra of CoPc/ppy, CoPc and ppy.\nSixth, CoPc/ppy/GDE electrode exhibits an exceptional 200 hours stability during\nchronoamperometry measurements at −0.78 V versus RHE, while the control sample of CoPc/CNT-\nGDE shows only a considerably shorter operational lifespan of 12 hours. After 200 hours stability\ntest, no obvious valence state, structure and coordination environment changes were detected over\nCoPc/ppy/GDE by XPS, HAADF-STEM and XAS. Furthermore, the as-assembled CoFe-\nCi@GQDs/NF||CoPc/ppy/GDE electrolyzer achieves record device stability of 50 hours at 1000\nmA cm−2 and 120 hours at 500 mA cm−2.\nAll these experimental observations point to the formation of covalent bonds between CoPc\nand the ppy network, based on which improved activity (due to electron transfer from ppy to CoPc)\nand robust operational stability are simultaneously achieved. We hope that these additional analyses\nwould address your concerns and strengthen the scientific validity of our work.\nReferences\n[1] Szybowicz, M. et al. Micro-Raman spectroscopic investigations of cobaltphthalocyanine thin\nfilms deposited on quartz and diamondsubstrates. Cryst. Res. Technol. 45, 1265–1271 (2010).\n[2] Chen, Y. T. et al. Charge transfer and electromagnetic enhancement processes revealed in the\nSERS and TERS of a CoPc thin film. Nanophotonics 8, 1533–1546 (2019).\nComment 3: The manuscript emphasizes the novelty of covalent grafting for pristine CoPc, but\nthe performance metrics do not significantly surpass recent reports using functionalized molecular\ncatalysts. The stability data is commendable but could be further contextualized against industrial\nbenchmarks.\nReply to comment 3: We thank the reviewer for this insightful comment, which allows us to better\narticulate the significance and novelty of our work. We agree that the field of molecular CO RR\n2\ncatalysis is advancing rapidly, with many high performing systems being reported. The value of our\nwork demonstrates a highly effective, scalable, and generalizable strategy for integrating molecular\ncatalysts onto electrodes, which addresses critical stability and scalability issues that often plague\nmolecular-based systems.\n(1) Novelty in simplicity and generalizability:\nThe reviewer notes that functionalized molecular catalysts can achieve high performance.\nHowever, the synthesis of such specially designed molecules (e.g., with pyrene or ethynyl anchoring\ngroups) often involves multi-step, low-yield organic synthesis requiring specialized expertise,\nwhich poses a significant barrier to scale-up and cost reduction (Figure 13). In our study, to offer\nrobust C–C bonds between carbon-based GDEs and pristine CoPc, we introduce ppy as the linker\nwhich initiates the C–C coupling process between GDEs and ppy, ppy and pristine CoPc thereby\nconnecting them together via C–C bonds. During this covalent grafting process, pristine CoPc\nwithout polymerizable groups is applied; meanwhile, no pretreatment of carbon-based GDEs is\nrequired, which minimizes the amount of oxygen-containing functional groups (such as carboxyl\nand hydroxyl groups) thereby guaranteeing sufficient hydrophobicity for efficient CO gas\n2\npenetration. As far as we know, such a convenient and efficient approach has not been explored for\ndirect grafting of pristine CoPc (rather than CoPc derivatives) on carbon-based GDEs, the employed\nppy not only acts as the highly conductive and robust linker between CoPc and GDEs, but also\nshows electron donation effects to promote the CO RR activity of CoPc centers.\ncovalent grafting strategy allows facile control over catalyst loadings (e.g., through adjusting the\npolymerization parameters of CV number, potential range), and could be well extended to other\npristine metal phthalocyanines or porphyrins (Figure R14).\nFigure R13. Prices of pristine CoPc and functionalized molecular catalysts.\nFigure R14. a, LSV curves of CoPc/ppy/GDE, FePc/ppy/GDE, CuPc/ppy/GDE and NiPc/ppy/GDE\nduring CO RR in CO -saturated 0.1 M KHCO with an H-type cell. b, Faradaic efficiency and\n2 2 3\ncurrent density of CoPc/ppy/GDE, FePc/ppy/GDE, CuPc/ppy/GDE and NiPc/ppy/GDE during\nCO RR in CO -saturated 0.1 M KHCO with an H-type cell at −0.7 V versus RHE.\n2 2 3\n(2) Contextualizing stability against industrial benchmarks:\nWe thank the reviewer for this positive feedback and valuable suggestion. We fully agree that\ncontextualizing stability performance relative to emerging industrial benchmarks is essential for\nevaluating the practical potential of our technology. The performance of CoPc/ppy/GDE not only\nrival those of advanced pre-functionalized systems but also demonstrate exceptional industrial\nrelevance and scalability. The as-assembled CoFe-C@GQDs/NF||CoPc/ppy/GDE electrolyzer\ni\nachieves a record device stability of 120 hours at 500 mA cm−2 and 50 hours at 1000 mA cm−2\n(Figure R15), which strongly demonstrate that our CoPc/ppy/GDE electrode is highly resistant to\nstructural collapse/changes under industrially relevant current densities. To contextualize this\nperformance, we provide a comprehensive multi-parameter comparison with several recently\nreported high-performing MEA devices based on CoPc or its derivatives, using a radar chart that\nquantitatively evaluates key metrics such as operating current density, stability, CO Faradaic\nefficiency, and cell voltage (Figure R16, Table R2). Crucially, the performance remains consistent\nwhen the device is scaled to a 9 cm2 electrode and operates continuously for 20 hours at a total\ncurrent of 1 A (Figure R17). Furthermore, we demonstrated a hectowatt-level (180 W) stack\noperating at 1 A cm−2, providing concrete evidence of the practical viability and scalability of our\napproach (Figure R18). Coupled with an unbiased solar-to-CO efficiency of 19.2%, these results\nunderscore that our method using pristine catalysts delivers industrially-relevant performance while\noffering superior simplicity and universality.\nFigure R15. Radar chart comparing the performance metrics of different molecular catalysts in a\nMEA.\nTable R2. Comparison of the CO -to-CO conversion of different electrocatalysts in MEA.\nCatalysts FE (%) J (mA cm−2) Stability (h)\nCO (V)\nCoPc/ppy/GDE 99 ~2.8 500 120\nCo-CNTs MW 98.3 ~2 100 12\nCoPc 88 ~2.52 200 6\nMWNT/PyPBI/Au 85 ~2.25 158 8\nTC-CoPc/MWCNTs 97 ~2.6 50 60\nCoPc/Mg(OH) 95 3.4 100 20\n2\nβ-CoPc/CP 92% 2 100 29.3\nAG-CoPc/MWCNTs 80 3 50 12\nMD-\n80 3.2 50 40\nCoPc/MWCNTs\nFigure R16. Long-term stability test at 1 A cm−2 with corresponding Faradaic efficiency of the\nelectrolyzer in 1.0 M KOH.\nFigure R17. a, LSV and Faradaic efficiency of CoFe-C@GQDs/NF||CoPc/ppy/GDE electrolyzers\ni\nwith electrode area of 9 cm2 during CO RR in 1.0 M KOH. b, Stability test at 1 A with corresponding\nFigure R18. Stability and power of the electrolyzer at a current density of 1 A cm−2.\nIn summary, we present a simplified and robust integration strategy that enables a cheap,\npristine catalyst to perform on par with or exceed the durability of more complex, customized\nsystems. We believe this approach, which prioritizes scalability and stability alongside high\nperformance, is a valuable and novel addition to the field.\nComment 4: The post-reaction characterization suggests structural retention, but the origin of\neventual performance decay is not investigated. Long-term stability tests under intermittent\noperation would strengthen the practical relevance.\nReply to comment 4: We are grateful to the reviewer for this exceptionally valuable suggestion.\nInvestigating the degradation mechanisms and testing stability under practical, intermittent\noperating conditions are crucial steps toward assessing the practical potential of any electrocatalyst.\nWe have conducted these recommended experiments, and the results have strengthened its practical\nrelevance.\n(1) Investigating the origin of performance decay.\nTo probe the origin of the eventual activity decay observed in our long-term test, we performed\na detailed post-mortem analysis of the CoPc/ppy/GDE electrode after 50 hours of continuous\noperation at 500 mA cm−2. We have conducted a post-mortem analysis of the cell components and\nexplored the reasons for the gradual performance loss. The post-test images of the GDE and the\nflow field reveal substantial blockage of the critical gas and liquid transport pathways by these\ninsoluble salts (Figure R19). This phenomenon, often referred to as salt precipitation, is a well-\nknown challenge in CO electrolysis operating in aqueous alkaline electrolytes, particularly at high\n2\ncurrent densities and extended durations. The precipitation physically blocks the triple-phase\nboundaries, severely hindering the mass transport of CO to the active sites and leading to increased\n2\npolarization and eventual performance decay.\nFigure R19. Photograph of the electrolysis cell and electrodes.\nThe structural and chemical retention of the CoPc/ppy/GDE catalyst itself, as confirmed by\npre- and post-reaction spectroscopic analyses (XAS, XPS, and HAADF-STEM), indicates that the\neventual performance decay is caused by a system-level issue of salt management, rather than by\nintrinsic catalyst deactivation (Figure R20–22). We thank the reviewer again for prompting this\nimportant investigation, which significantly strengthens the practical relevance of our study.\nFigure R20. a, XAS spectra at Co K-edge of CoPc/ppy/GDE and b, Co K-edge FT EXAFS spectra\nin the R space of CoPc/ppy/GDE of initial state and after 200 hours in CO -saturated 0.1 M KHCO\n2 3\nwith an H-type cell at −0.75 V versus RHE.\nFigure R21. Comparison of the Co 2p XPS spectra of CoPc/ppy/GDE of initial state and after 200\nhours in CO -saturated 0.1 M KHCO with an H-type cell at −0.75 V versus RHE.\n2 3\nFigure R22. a, TEM image of post-reacted CoPc/ppy/GDE after 200 hours. b, HAADF-STEM\nimage of post-reacted CoPc/ppy/GDE after 200 hours.\n(2) Long-term stability under intermittent operation:\nFollowing the reviewer's excellent suggestion, we conducted a new stability test under\nintermittent operation to simulate the intermittent nature of renewable electricity sources. The\nexperiment consisted of 50 cycles. The results are presented in the Figure R23. Remarkably, the\ncatalyst quickly recovers its activity after each idle period, maintaining a stable FE > 80%\nCO\nthroughout all 50 cycles. This demonstrates the outstanding robustness and resilience of the\nCoPc/ppy/GDE catalyst. The covalent linkage prevents detachment or reconstruction during the\nfrequent potential swings and idle times, which is a common failure mode for non-covalently\nattached catalysts.\nFigure R23. Long-term stability tests under intermittent operation of 100 mA cm−2 of the\nelectrolyzer in 1.0 M KOH.\nWe believe these additions, prompted by the reviewer's comment, have transformed a simple\nstability demonstration into a mechanistic investigation of electrode longevity, enhancing the impact\nand significance of our work.\nComment 5: The DFT analysis of CO reduction energetics is oversimplified. Key aspects are\n2\noverlooked: How does the ppy linker influence the local pH or CO concentration at the catalyst\n2\nsurface? Are the proposed active sites (Co+) stable under alkaline conditions, given prior reports of\nCoPc demetallation at high pH?\nReply to comment 5: We thank the reviewer for raising these important questions, which allow us\nto clarify the scope and conclusions of our computational study. In response to the query regarding\nthe potential influence of the ppy linker on the local chemical environment, we modelled the effect\nof varying local proton and hydroxide concentrations by introducing H+ and OH− into the solvation\nsphere of the Co center in both the CoPc and CoPc/ppy models.\nAs shown in Figure R24, we computed the average adsorption energy of H+ for CoPc and\nCoPc/ppy with varying numbers of protons. The results indicate that the trend in adsorption energy\nas a function of H+ number is nearly identical for both systems, with minimal differences observed\nat each specific value. This suggests that the presence of the ppy linker does not significantly alter\nthe local proton affinity around the Co site. Similarly, Figure R25 presents the corresponding\nanalysis for OH−. Figures R25a and R25b show representative optimized structures of CoPc and\nCoPc/ppy with different numbers of OH−, with the resulting adsorption energies. Again, the trends\nare similar between CoPc and CoPc/ppy, and the energy differences are negligible. These results\nindicate that the ppy matrix does not promote the accumulation of OH− near the catalytic center.\nTherefore, the DFT calculations demonstrate that the local pH environment around the Co site\nremains largely unaffected by the presence of ppy. Consequently, the enhancing effect of the ppy\nmodifier is likely electronic and structural in origin, rather than mediated through changes in the\nlocal concentration of protons or CO . Moreover, since each individual CoPc unit possesses only\n2\none CO adsorption site, the ppy matrix is unlikely to influence the local CO concentration at the\n2 2\ncatalytic center.\nFigure R24. Adsorption configurations a, b and (c) corresponding adsorption energies of CoPc and\nCoPc/ppy under different acidities.\nFigure R25. Adsorption configurations a, b and (c) corresponding adsorption energies of CoPc and\nCoPc/ppy under different alkalinities.\nRegarding the stability of the proposed Co+ species under alkaline conditions, we fully\nappreciate the reviewer’s concern based on known demetallation pathways of molecular CoPc. Our\ncomputational stability assessments included evaluation of the coordination strength of the Co\ncenter in the presence of explicit hydroxide ions. Figures R26a and R26b show the anchoring of Co\nin the phthalocyanine macrocycle without OH−, while Figures R26c and R26d illustrate the\nstructures with coordinated OH⁻. The calculated binding energies of Co in the macrocyclic\nframework are exceptionally high, indicating strong stabilization. The presence of OH− cause a\nminor change in the binding energy (on the order of 0.4 eV), leading us to conclude that alkaline\nconditions do not significantly compromise the stability of the Co site.\nFigure R26. The binding energy of Co of CoPc/ppy under alkaline conditions.\nOur approach offers a thermodynamically grounded assessment that the proposed mechanism\nand active site are viable. We hope this addresses the reviewer’s concern regarding the potential\noversimplification of the DFT analysis.\nResponses to Reviewer' Comments\nReviewer #4:\nComment: I thank the authors for their thorough revisions in response to the feedback. They have\naddressed the majority of the concerns raised. I have no further issues and believe the manuscript is\nnow suitable for publication.\nReply to comment: Thanks for your positive feedback! Based on your insightful comments and\nsuggestions, the quality of our manuscript has been significantly improved. Thanks again!"}, {"round": 3, "reviewer_comments": {"reviewer_4": "I thank the authors for their thorough revisions in response to the feedback. They have addressed the majority of the concerns raised. I have no further issues and believe the manuscript is now suitable for publication."}, "author_response": "Responses to Reviewer' Comments\nReviewer #4:\nComment: I thank the authors for their thorough revisions in response to the feedback. They have\naddressed the majority of the concerns raised. I have no further issues and believe the manuscript is\nnow suitable for publication.\nReply to comment: Thanks for your positive feedback! Based on your insightful comments and\nsuggestions, the quality of our manuscript has been significantly improved. Thanks again!"}]

Decision letter