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Nat Commun
Nat Commun
Nature Communications
2041-1723
Nature Publishing Group UK London

39231997
52168
10.1038/s41467-024-52168-x
Article
Potential-driven structural distortion in cobalt phthalocyanine for electrocatalytic CO2/CO reduction towards methanol
http://orcid.org/0000-0002-8426-0541
Yang Haozhou 1
Guo Na 23
Xi Shibo 4
Wu Yao 5
Yao Bingqing 5
http://orcid.org/0000-0003-4891-3581
He Qian 5
http://orcid.org/0000-0002-1581-5806
Zhang Chun phyzc@nus.edu.sg

36
http://orcid.org/0000-0002-1931-7767
Wang Lei wanglei8@nus.edu.sg

17
1 https://ror.org/01tgyzw49 grid.4280.e 0000 0001 2180 6431 Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore, Singapore
2 https://ror.org/01tgyzw49 grid.4280.e 0000 0001 2180 6431 National University of Singapore (Chongqing) Research Institute, Building 4, Internet Industrial Park Phase 2, Chongqing Liang Jiang New Area, Chongqing, China
3 https://ror.org/01tgyzw49 grid.4280.e 0000 0001 2180 6431 Department of Physics, National University of Singapore, Singapore, Singapore
4 grid.185448.4 0000 0004 0637 0221 Institute of Sustainability for Chemical, Energy and Environment (ISCE2), Agency for Science, Technology and Research (A*STAR), Singapore, Singapore
5 https://ror.org/01tgyzw49 grid.4280.e 0000 0001 2180 6431 Department of Material Science Engineering, National University of Singapore, Singapore, Singapore
6 https://ror.org/01tgyzw49 grid.4280.e 0000 0001 2180 6431 Department of Chemistry, National University of Singapore, Singapore, Singapore
7 https://ror.org/01tgyzw49 grid.4280.e 0000 0001 2180 6431 Centre for Hydrogen Innovations, National University of Singapore, Singapore, Singapore
4 9 2024
4 9 2024
2024
15 770316 4 2024
28 8 2024
© The Author(s) 2024
2024
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Cobalt phthalocyanine immobilized on carbon nanotube has demonstrated appreciable selectivity and activity for methanol synthesis in electrocatalytic CO2/CO reduction. However, discrepancies in methanol production selectivity and activity between CO2 and CO reduction have been observed, leading to inconclusive mechanisms for methanol production in this system. Here, we discover that the interaction between cobalt phthalocyanine molecules and defects on carbon nanotube substrate plays a key role in methanol production during CO2/CO electroreduction. Through detailed operando X-ray absorption and infrared spectroscopies, we find that upon application of cathodic potential, this interaction induces the transformation of the planar CoN4 center in cobalt phthalocyanine to an out-of-plane distorted configuration. Consequently, this potential induced structural change promotes the transformation of linearly bonded *CO at the CoN4 center to bridge *CO, thereby facilitating methanol production. Overall, these comprehensive mechanistic investigations and the outstanding performance (methanol partial current density over 150 mA cm−2) provide valuable insights in guiding the activity and selectivity of immobilized cobalt phthalocyanine for methanol production in CO2/CO reduction.

Immobilized cobalt phthalocyanine shows promise in CO2 electroreduction to methanol. However, the reaction mechanism remains unclear. This study reveals that cathodic potential critically influences methanol production by altering the symmetry and binding properties at the cobalt active center.

Subject terms

Electrocatalysis
Electrocatalysis
https://doi.org/10.13039/501100001348 Agency for Science, Technology and Research (A*STAR) U2102d2002 Wang Lei https://doi.org/10.13039/501100001381 National Research Foundation Singapore (National Research Foundation-Prime Minister&apos;s office, Republic of Singapore) NRF-NRFF13-2021-0007 Wang Lei https://doi.org/10.13039/501100001352 National University of Singapore (NUS) CHI-P2022-06 Wang Lei issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

Molecular catalysts have demonstrated effectiveness in various energy-related electrocatalytic applications, owing to their precise and highly tunable coordination environments1–6. These attributes offer opportunities for investigating the detailed reaction mechanisms and consequently guiding future catalyst design with improved performance7–9. Notably, metal phthalocyanines (MPc), specifically, exhibit appreciable selectivity and activity in electrocatalytic CO2 reduction (CO2R) or CO reduction (COR), which have been recognized as a promising approach for the sustainable production of chemicals and fuels when coupled with renewable electricity10–14. In particular, cobalt phthalocyanine (CoPc) has drawn significant attention due to its unique electrocatalytic CO2R/COR selectivity towards methanol (CH3OH), a product often challenging to be produced through CO2R or COR compared with other products15–17.

Recent advancements have demonstrated that CoPc immobilized on carbon nanotubes exhibits high selectivity and activity towards CH3OH in electrocatalytic CO2R/COR reduction17–26. For instance, a recent study has found that single-walled CNTs supported CoPc shows enhanced Faradaic Efficiency towards CH3OH (FECH3OH) in CO2R, which was attributed to the increased *CO binding strength on curved CoPc plane induced by strong catalyst-substrate interactions20. In another study, the authors found that higher electron density on the Co-site leads to stronger *CO binding strength in COR, facilitating CH3OH production21. Despite these advancements, limited current densities achieved in these electrocatalytic assessments and the neglect of CNT substrate in DFT simulations hinder the comprehensive understanding of this system. Separately, it is generally acknowledged that *CO is the key intermediate in CH3OH production, and the catalyst binding strength of *CO plays a crucial role in CH3OH selectivity22,23. However, discrepancies in CH3OH production selectivity and activity between CO2R and COR have been observed, as CO is typically the only CO2R product reported in most literatures27–32, leading to inconclusive mechanisms for CH3OH production on CoPc-based catalysts.

To uncover the interplay between catalyst structure, reaction mechanism, and experimental observations of the CoPc/CNT system for CH3OH production, we take into account several important aspects in this study. Firstly, a systematic comparison between COR and CO2R across a broad potential window is essential. The limiting potential of the reduction of CO2 to CO is lower than that of further reduction of *CO, therefore a wider potential window extending to a more cathodic potential is required for relevant comparison21. Secondly, the investigation of the catalyst structure evolution, particularly the CoN4 configuration, under cathodic bias during CO2R/COR is also crucial. Given that CNT as substrate affords superior CH3OH production activity compared to others18, its interaction with CoPc must be critical. Hence, achieving a clearer understanding of the interaction between the CoPc molecule and CNT substrate would be pivotal. However, few mechanistic investigations, such as density functional theory (DFT) simulations on intermediate energetics, have accounted for the interaction between catalyst and CNT substrate. This underscores the need for a precise model for theoretical guidance, supported by spectroscopic evidence. Lastly, revealing the *CO adsorptions and their dynamic on the Co-center will be beneficial for investigating the difference between COR and CO2R.

Herein, we conducted a comprehensive study of the Co phthalocyanine/CNT system and elucidated the origin of the difference in CH3OH selectivity between CO2R/COR. Given the inherent poor stability of CoPc, we employed amino group-substituted cobalt tetraaminophthalocyanine (CoTAPc, Supplementary Fig. 1) as a model catalyst to ensure robust activity evaluation under elevated current densities18,33. Inspired by our previous understandings34, we introduced CoTAPc-CNT defect interaction to the catalyst model to explore the potential configurations of the Co-N4 center on CNT. Consequently, two energetically stable CoTAPc configurations were obtained: quasi-flat and out-of-plane distorted configurations, with the latter proving to be energetically more stable. Then, we examined the distortion of the Co-N4 plane under cathodic potential by incorporating dipole moments. We revealed the crucial role of cathodic potential in inducing the Co-N4 plane distortion, subsequently influencing the Co coordination environment and further the corresponding catalytic performance. Additionally, we conducted in-situ X-ray absorption fine structure (XAFS) measurements for both CO2R and COR on CoTAPc/GCNT. The obtained results further support the existence of the out-of-plane distorted configurations of the Co-N4 plane under the cathodic potentials. Furthermore, we systematically studied the electrocatalytic CO2R/COR performance of CoTAPc/GCNT in electrolytes with different pH values. We found significantly higher FECH3OH in COR under all pH conditions, however, this was accompanied by more negative overpotential applied compared to CO2R at identical current densities. Despite this, we observed very similar CH3OH production rates for CO2R and COR at the same overpotentials, suggesting that cathodic potential is the predominant variable influencing CH3OH production when comparing CoTAPc-based CO2R and COR. Moreover, in-situ attenuated total reflectance-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) characterizations revealed two prominent *CO bands at ~2110-2090 cm−1 and ~1940-1900 cm−1 under catalytic conditions. Based on the corresponding electrochemical behaviors of these two peaks, along with density-functional theory calculations, we attribute these two peaks to the linear *CO (*COL) on Co and bridge *CO (*COB) on Co-N, N domain, respectively. Notably, the concurrent disappearance of *COL and strengthening of *COB under a more negative potential indicate altered *CO binding sites, likely induced by the potential-driven distortion of the Co-N4 plane. Finally, our findings also underscore the challenge of achieving comparable CH3OH production selectivity for CO2R compared with COR, especially at high overpotentials and large current densities (i.e., > 500 mA cm−2). This is associated with the fact that the electrodes are prone to flooding under these conditions, leading to failures in achieving sufficiently high overpotential in the case of CO2R. Overall, our findings elucidated that the origin of the high selectivity of CoPc-based catalysts for CH3OH production electrochemically is the cathodic potential-induced Co-N4 plane distortion. The distorted Co-N4 active center exhibits desirable CO binding strength and configuration for CH3OH formation. This potential-determined behavior also caused the commonly observed difference in CH3OH selectivity between CO2R and COR on CoPc-based systems.

Results

Modeling CoTAPc on CNT

Building on our previous work, graphitized CNT (GCNT) was employed as the catalyst substrate to reduce the substrate defect level and suppress undesirable hydrogen evolution reaction (HER)34. As shown in Fig. 1a, the highly crystalline graphene layers, as revealed in the high-resolution transmission electron microscope (HRTEM) image of CoTAPc immobilized on GCNT (CoTAPc/GCNT), clearly indicate its higher graphitization degree and reduced level of defects. This observation is corroborated by the Raman spectra of the GCNT before and after loading with the CoTAPc. As shown in Fig. 1b, compared to the non-graphitized CNT (nGCNT) as purchased and purified, the much lower D band intensity of the GCNT manifests its significantly reduced defect level. More physical characterizations in Supplementary Fig. 2 further validate that carbon-based defects are the primary source of defects on CNTs. Negligible influence on the defect level of GCNT is observed upon CoTAPc immobilization. Notably, the high-angle annular dark-field (HAADF) scanning transmission electron microscope (STEM) from aberration-corrected (AC)-HRTEM images (Fig. 1c and Supplementary Fig. 3) reveal that CoTAPc tends to adsorb on the defect-rich domain (i.e., intersections) rather than the highly graphitized surface, suggesting that CoTAPc prefers to adsorb on the CNT via catalyst/defect interactions rather than the generally believed π-π interactions20,35–37. A similar observation was made when decreasing the loading of CoTAPc on GCNT (Supplementary Fig. 4), confirming that highly graphitized CNT surfaces exhibit reduced adsorption of CoTAPc molecules due to the reduced defect level. In contrast, CoTAPc immobilized on nGCNT (CoTAPc/nGCNT), as shown in the HAADF-STEM image in Supplementary Fig. 5, exhibits much more uniform dispersion of CoTAPc single sites on nGCNT. This is attributed to the high degree of defects present on nGCNT (Fig. 1b). Such dispersions resemble to those reported in other literature18,21,37, further confirming that CoTAPc tends to adsorb on defect-rich domain, akin to our previous observations with NiTAPc34. Previously, we also found that dipole interaction between the metal center in Ni-Pc and single vacancy is substantially stronger than conventional π-π interaction, providing a reasonable description of the Pc molecule adsorption on the CNT surface. Inspired by this, we established a catalyst model consisting of CoTAPc absorption on a single vacancy on graphene (1V-Gr) to investigate the Co-N4 plane configurations. Intriguingly, two distinct CoTAPc@1V-Gr configurations were identified. As shown in Fig. 1d, the quasi-flat Co-N4 configuration (CoTAPc@1V-Gr-flat, Supplementary Data 1) resembles those reported in the literature using perfect graphene as the substrate37,38, where the Co center is slightly out of the Pc plane, with a N−Co−N angle of ~164°. Besides, a distorted Co-N4 configuration (CoTAPc@1V-Gr-dis, Fig. 1e and Supplementary Data 2) was also observed, wherein the Co center exhibits significant out-of-plane distortion. Notably, the energy of the distorted configuration Co-N4 is 0.35 eV lower than that of the quasi-flat configuration (Fig. 1f), indicating its favored thermodynamic stability. We then compared the adsorption energy of these two CoTAPc configurations on 1V-Gr with that of CoTAPc on pristine graphene (CoTAPc@Gr, Supplementary Fig. 6). As shown in Supplementary Fig. 6b, the formation of CoTAPc@1V-Gr-flat is substantially favored (~0.58 eV) than CoTAPc@Gr. This result further validates the stronger interaction between CoTAPc and defects on CNT rather than π-π interactions. Similar observations were made for the pristine CoPc/CNTs (Supplementary Fig. 7), indicating that this CoPc/defect interaction is likely applicable to the adsorption of other planner molecules on CNTs.Fig. 1 Characterization and modeling of CoTAPc on GCNT.

a HRTEM image of CoTAPc/GCNT, b Raman spectra of CoTAPc/GCNT, GCNT, and nGCNT, the intensity of G band is normalized for comparison; c HAADF-STEM image of CoTAPc/GCNT. d,e top-view and side-view of CoTAPc@1V-Gr-flat and CoTAPc@1V-Gr-dis, respectively; f differences in energies of CoTAPc@1V-Gr-flat (0 eV) and CoTAPc@1V-Gr-dis (−0.35 eV); g dipole moment of CoTAPc@1V-Gr-dis and CoPc@1V-Gr-dis; h,i top-view and side-view of adsorbed CO configurations on CoTAPc@1V-Gr-flat and CoTAPc@1V-Gr-dis, respectively; j comparison of CO and CO2 adsorption energies on two configurations, as well as corresponding C − O bond lengths of the adsorbed *CO. The calculated configurations and energies are provided in Source Data file.

To explore the dynamics of the Co-N4 center on CoTAPc/GCNT under cathodic potentials, dipole moments of CoTAPc@1V-Gr-dis and CoPc@1V-Gr-dis were calculated to be 0.224 and 1.512 Debye towards the Pc plane, respectively (Fig. 1g). Molecules with larger dipole moment are in general have stronger interaction with electric field due to electrostatic interaction. With this result, we anticipate that the external electric field (i.e., cathodic potential) would pull the Co center towards the CNT substrate, leading to out-of-plane distortion, especially under more negative potentials. Therefore, we believe that potential-driven distortion occurs during electrocatalytic CO2R/COR, with the degree of distortion associated with the cathodic potential applied. This is confirmed by our in-situ XAFS measurements, vide infra. Furthermore, the much larger dipole moment of CoPc@1V-Gr-dis compared to that of CoTAPc@1V-Gr-dis suggests that CoPc@GCNT is likely prone to more severe out-of-plane distortion under cathodic overpotentials. This would compromise its stability, in good accordance with our experimental findings (Supplementary Fig. 8) and reports elsewhere18.

We then investigated the corresponding *CO adsorption on the two configurations. As shown in Fig. 1h–i, *CO adsorbs on CoTAPc@1V-Gr-flat linearly (linear *CO, denoted as *COL, Supplementary Data 3) on top of the Co-site, such *CO binding configuration are often used in recent literature, however without considering the catalyst/substrate interactions20–22. In contrast, with the CoTAPc@1V-Gr-dis, we found *CO tends to bridge (bridge *CO, denoted as *COB, Supplementary Data 4) on the two adjacent pyrrole N atoms within the Co-N4 center. The *CO adsorption energies of these two binding modes are −0.52 eV and −1.80 eV, respectively. Additionally, the C − O bond length in *CO increases from 1.15 Å (*COL) to 1.21 Å (*COB) (Fig. 1j), indicating higher reactivity towards further hydrogenation steps for methanol production. In contrast, the adsorption energies of CO2 on these two catalyst configurations exhibit little difference and are much lower than that of CO (Fig. 1j), indicating that the low FECH3OH during CO2R is less likely resulted from competition of CO2 adsorption with CO. We also assessed the two *CO adsorption configurations on CoPc@1V-Gr (Supplementary Fig. 7d, e). Likewise, we observed four-folders of enhancement in *CO adsorption energy on CoPc@1V-Gr-dis compared to CoPc@1V-Gr-flat, together with prolonged C − O bond length. This result suggests that this behavior is likely universal among different Co-Pc/CNT systems. Furthermore, we calculated the vibration frequency of the C − O bond in these two configurations of CoTAPc. The calculated frequencies for the *COL and * COB are 2075 cm−1 and 1743 cm−1, respectively. Note that, despite the thermodynamically favored energetics of CoTAPc@1V-Gr-dis, the actual configuration of CoTAPc under cathodic potential is likely determined by the equilibrium between quasi-flat and distortion states. This equilibrium is influenced by the energy barrier associated with the distortion of the CoTAPc plane, which may be significant due to the large planar nature of the Pc molecule. Therefore, we believe that the above two *CO binding states may coexist during CO2R/COR, and the prevalence of linear or bridged *CO is dictated by the applied cathodic potential. Overall, our DFT results suggest that the strong interaction between the Co center and CNT defects will likely lead to the potential-driven distortion of the Co-N4 center. This distortion strengthens the adsorption of *CO and weakens the C − O bond, facilitating its hydrogenation and ultimately promoting the production of CH3OH.

Electrochemical CO2R/COR on CoTAPc/GCNT

The CO2R/COR performances on CoTAPc/GCNT were assessed using a gas diffusion electrode-based flow cell (Supplementary Fig. 9). We first compared the COR performance of CoTAPc/GCNT and CoTAPc/nGCNT. As shown in Supplementary Fig. 10, we observed substantially higher selectivity and activity of CH3OH production with the former, along with suppressed HER, further validating the significance of optimizing the CNT defect level for catalysis. We then conducted both CO2R and COR in three electrolytes with different bulk pH: 1 M KOH (pH 14), 1 M KHCO3 (pH 8.4), and 0.5 M K2SO4 (pH 2, adjusted by H2SO4). As shown in Fig. 2a–c, Supplementary Fig. 11 and Supplementary Table 1, 2, within a broad current density window, FECH3OH in COR substantially outperforms those in CO2R in all three electrolytes. Notably, the maximum FECH3OH in COR at 200 mA cm−2 in alkaline, neutral, and acidic electrolyte reached 47%, 53%, and 45%, respectively. In contrast, the maximum FECH3OH in CO2R attained at 800 mA cm−2 in alkaline, neutral and acidic electrolyte are only 12.8%, 10.5%, and 12.1%, respectively. However, the CH3OH production rates (i.e., partial current density) in the aforementioned scenarios are relatively similar. What merits more attention is that the cathodic potential applied (at the absolute potential scale, i.e., standard hydrogen electrode, SHE) for COR and CO2R to achieve these CH3OH production rates are also close, validating the critical role of electrode potential in CH3OH production for both COR and CO2R. In other words, this discrepancy in FECH3OH between COR and CO2R at identical overall current density is a result of different applied potential (Supplementary Fig. 12 and Supplementary Table 3). A larger cathodic potential affords a higher FECH3OH. To elucidate this correlation further, we plotted the CH3OH production rate for CO2R/COR at different current densities on the function of the applied cathodic potential on the SHE scale. As illustrated in Fig. 2d, the CH3OH production rates of CO2R and COR at similar potentials are, as anticipated, similar in both alkaline and neutral electrolytes. This further confirms the previously mentioned potential-determined CH3OH production in both COR and CO2R. Note, that similar trends are less apparent in the acidic electrolyte. We believe that COR would be further facilitated than CO2R due to the relatively lower local pH39.Fig. 2 Electrocatalytic CO2R/COR performance of CoTAPc/GCNT conducted at room temperature.

a–c FECH3OH and cathodic potential of CO2R/COR at different current densities in electrolytes with various of pH: 1 M KOH (pH = 14), 1 M KHCO3 (pH = 8.4) and 0.5 M K2SO4 (pH = 2, adjusted by H2SO4), respectively; d correlations between CH3OH production rate and cathodic potential in different electrolytes; e applied cathodic potential and f resulted 13CH3OH/12CH3OH distributions for the 13CO2/12CO co-feeding electrolysis, in different electrolytes. Solid and dash columns indicate 12CH3OH and 13CH3OH, respectively; g production rates of CH3OH and H2 in CO2/CO co-feeding electrolysis in different electrolytes. Solid and dash columns denote CH3OH and H2, respectively. Note: All co-feeding experiments were executed at 500 mA cm−2. h Summary of FECH3OH and jCH3OH of CO2R/COR for CoTAPc/GCNT. Catalyst loading amount is 1 mg cm−2. 100% iR compensation was applied for all potentials. Typical solution resistance for 1 M KOH, 1 M KHCO3 and 0.5 M K2SO4 were measured to be 1.1 ± 0.25 Ω, 1.5 ± 0.35 Ω and 1.4 ± 0.30 Ω, respectively. The error bars represent standard deviations from three independent measurements. Relevant source data are provided in the Source Data file.

Modulating the density of surface-active sites on the electrode can significantly influence the operating potential in electrocatalytic CO2R/COR, and subsequently impact the CH3OH selectivity and productivity. To explore this effect, we reduced the CoTAPc loading on GCNT by adjusting the precursors weight ratio (CoTAPc: GCNT) from the previous 1:10 to 1:20 and 1:40. We confirmed that the decreased density of the surface-active sites aligns closely with the CoTAPc loading (Supplementary Fig. 13) by inductively coupled plasma optical emission spectrometry (ICP-OES, Supplementary Table 4). Subsequently, the CO2R/COR performance of CoTAPc/GCNT-1:20 and 1:40 was assessed under the same conditions (Supplementary Fig. 14). As anticipated, the overpotentials for both CO2R and COR increased for the CoTAPc/GCNT with reduced catalyst loadings. Regarding the CoTAPc/GCNT-1:40 sample, the electrocatalysis even failed at high current densities due to the large overpotential applied. Nevertheless, similar potential dependent trends on CH3OH production were observed with CoTAPc/GCNT-1:20 (Supplementary Fig. 15). Taken together, we can conclude that the intrinsic activity of CoTAPc in COR/CO2R for CH3OH production is comparable under the same applied potential. We believe that sufficiently high overpotentials are likely required for further reducing CO to form CH3OH, owning the strong binding of CO on the distorted CoTAPc (Fig. 1j). However, lower overpotential is often applied for CO2R to achieve the same current density compared to COR, due to relatively easy CO2 to CO conversion step. Consequently, the lower overpotential of CO2R will lead to lower selectivity towards CH3OH compared to that of COR. As suggested by the DFT calculations above, we attribute the origin of the potential-dependent effect to the potential-induced structural evolution of the Co-N4 center, specifically the distortion of the Co-N4 plane.

We conducted electrolysis by co-feeding isotopically labeled CO2/CO to further validate our conclusion (Supplementary Fig. 16). As shown in Fig. 2e, f we present the cathodic potential and the corresponding CH3OH production rates obtained from the isotopically labeled 12CO/13CO2 co-feeding electrolysis at 500 mA cm−2. As anticipated, the overpotential decreased noticeably with an increase in CO2 partial pressure (pCO2), leading to a decrease in CH3OH production. At a low current density, such as 200 mA cm−2, the corresponding small overpotential is likely not sufficient for activating *CO on the active sites resulting in diminished CH3OH production at low pCO (Supplementary Fig. 17). These observations are consistent with the discussions above. Notably, we also found that the majority of the CH3OH was originated from CO rather than CO2 in the co-feeding electrolysis, even at a low CO to CO2 ratio of 1:3 (Fig. 2f). This observation agrees well with the above DFT calculations, where we found that CO exhibited substantially stronger adsorption energy compared to CO2 on CoTAPc (Fig. 1j). Furthermore, as depicted in Fig. 2g, the production rates of CH3OH and H2 surprisingly track each other in both COR to CO2R, indicating that these two pathways are associated, at least to some extent. Future work in decoupling these two pathways is crucial for achieving improved selectivity towards CH3OH production. The Ar/CO co-feeding electrolysis was also carried out. As shown in Supplementary Fig. 18, FECH3OH at 100 mA cm−2 remains relatively stable when decreasing the pCO to 50%, indicating the strong binding of the *CO on the active-sites. However, further reduction in CO partial pressure or increase in current density leads to a noticeable decrease in FECH3OH, likely due to the insufficient mass diffusion of CO. Finally, we summarized the FECH3OH and partial current densities (jCH3OH) for CH3OH production on CoTAPc/GCNT in Fig. 2h. We have achieved high jCH3OH in both CO2R and COR, along with high FECH3OH, validating our design strategies elucidated above. Overall, the results obtained from these co-feeding measurements further elucidated the effect of cathodic potential on CH3OH production in both CO2R/COR, which likely resulted from the potential-induced distortion of the Co-N4 plane.

Exploring the Coordination Environment of CoTAPc during CO2R/COR

We utilized X-ray absorption spectroscopy (XAS) technique to investigate the coordination environment of CoTAPc/GCNT, particularly under catalytic conditions40. Figure 3a shows the ex-situ X-ray absorption near-edge structure (XANES) spectra of CoTAPc and CoTAPc/GCNT in CO atmosphere and vacuum, respectively. Note, the spectral features labeled as a and b can be attributed to 1 s→4px, y transition, and dipole-allowed 1 s→4pz shakeup transition, respectively41–45. In the case of CoTAPc, the feature a shifts to higher energy, and the intensity of feature b increases when switching the testing environment from CO to vacuum, indicating the transformation towards planar D4h structure46. In contrast, CoPc exhibits characteristic moieties of a planar D4h structure in both CO atmosphere and vacuum (Supplementary Fig. 19a). This result suggests that CoTAPc tends to bind CO stronger compared to CoPc under ambient conditions, aligning with our DFT calculation (Fig. 1j and Supplementary Fig. 7). Simulation of XANES based on the corresponding DFT models were carried out to further verify our hypothesis. As shown in Fig. 3b, the simulated XANES of CoTAPc-flat exhibits diminished peak b but similar structure a at the white line region. However, *CO-CoTAPc-flat shows a very different white line structure compared with the former but similar to what we observed of CoTAPc in CO. Therefore, it is reasonable to believe that the change in white line feature a resulted from the axial CO ligand. Moreover, the simulated XANES also aligns with the proposed configurations of the immobilized CoTAPc shown in Fig. 1. As shown in Fig. 3c, both CoTAPc-flat and CoTAPc-dis exhibit an increase in the intensity of the pre-edge c at approximately 7710 eV, although the changes in CoTAPc-dis is substantially larger. Since this pre-edge is associated with the quadrupole-allowed 1 s → 3d − 4p transition, a characteristic descriptor of square-planar D4h symmetry46–48, we can conclude that D4h symmetry of the CoTAPc-dis is diminished more significantly compared to that of CoTAPc-flat, pinpointing the strong interactions between the CoTAPc plane and the defects on CNT substrate.Fig. 3 XAFS simulations and characterizations of the CoTAPc and CoPc composites.

a Ex-situ XANES of CoTAPc and CoTAPc/GCNT in CO atmosphere and vacuum, b simulated XANES of CoTAPc, CoTAPc@1V-Gr-flat (denoted as CoTAPc-flat) and *CO-CoTAPc@1V-Gr-flat (denoted as *CO-CoTAPc-flat), c simulated XANES of CoTAPc, CoTAPc-flat, and CoTAPc@1V-Gr-dis (denoted as CoTAPc-dis); d in-situ XANES of CoTAPc/GCNT in CO2R under different potentials, e in-situ EXAFS of CoTAPc/GCNT at −0.36 V in R-space, f in-situ XANES of CoPc/GCNT in CO2R under different potentials; g–i in-situ XANES of CoTAPc/GCNT in COR under different potentials in 1 M KOH, 1 M KHCO3 and 0.5 M K2SO4 (pH=2), respectively. For CO2R, 1 M KOH is used as electrolyte. Catalyst loading amount is ~2.3 mg cm−2. 100% iR compensation was applied for all potentials. Typical solution resistance for 1 M KOH, 1 M KHCO3 and 0.5 M K2SO4 were measured to be 2.5 Ω, 3.5 Ω and 3.3 Ω, respectively. The larger solution resistance results from different cell setup and catalyst loading. Source data for XANES and EXAFS are provided in the Source Data file.

We believe the CO adsorption on solid CoTAPc is a slow process under ambient conditions, as evident in the XANES and EXAFS spectra of CoTAPc after exposure to a CO atmosphere (Supplementary Fig. 20), which closely resembles that in vacuum. Upon immobilization of CoTAPc on GCNT, the intensity of shoulder peak b and the magnitude of peak a both decrease, indicating a further diminishment of the D4h planar configuration of the Pc ring. This observation is consistent with the simulated CoTAPc-flat. Besides, the XANES of CoTAPc/GCNT in a vacuum resembles that in CO and does not fully recover the flat configuration as the pristine CoPc, further confirming the interactions between CoTAPc and GCNT. The extended X-ray absorption fine structure (EXAFS) in R-space, shown in Supplementary Fig. 19b, reveals that both CoTAPc and CoTAPc/GCNT exhibit reduced coordination numbers of Co−N compared with their CoPc counterparts, suggesting the possible existence of adsorbed CO as an axial ligand. The EXAFS fitting results (Supplementary Fig. 19c) further verify our hypothesis. Furthermore, samples with lower CoTAPc loadings on CNT (CoTAPc/GCNT-1:20 and 1:40) exhibit similar XANES spectra (Supplementary Fig. 21), further validating the similar CoTAPc adsorption on defect-rich domains on CNT.

To monitor the evolution of the CoN4 center in CoTAPc/GCNT during CO2R/COR, we employed the identical flow cell used for assessing CO2R/COR performance with minor modifications for conducting the in-situ electrochemical XAS measurements (Supplementary Fig. 22). As a result, reliable data were obtained at practical relevant current densities, such as up to 260 mA cm−2. Additionally, given the significantly low catalyst loadings and the single-atom nature of the catalysts (Supplementary Table 4), we believe that the XAFS results precisely represent the authentic state of the Co-N4 active center during bulk CO2R/COR electrolysis. Likewise, we also focus on analyzing the characteristic moieties of the planar D4h symmetry: a, b, and c mentioned above. As shown in Fig. 3d, within the in-situ XANES of CoTAPc/GCNT, the intensity of peak a in the white-line region gradually decreases as the overpotential increases. Meanwhile, the intensity of shoulder peak b decreases, and the intensity of pre-edge peak c increases. These changes indicate that the CoN4 structure has deviated from its initial D4h symmetry, likely due to axial interaction with other atoms. Since we have excluded the possibility of gas molecule adsorption, the out-of-plane distorted configuration towards substrate defects under catalytic conditions can be inferred, which aligns well with the simulated XANES. Furthermore, the reduced coordination number of Co−N and the increased Co−N length derived from the EXAFS in R-space (Fig. 3e and Supplementary Fig. 23) provide additional evidence for the above-mentioned structural changes. However, due to the relatively small changes in the bond lengths of Co−N and Co−C from flat to distorted configurations (Supplementary Table 5), the changes in the interatomic distance in EXAFS are not as significant as the symmetry changes observed in XANES. Moreover, we observed similar XANES trends for CO2R on CoTAPc/GCNT in acidic electrolyte (Supplementary Fig. 24), indicating the same potential-driven structure evolution during catalysis. Reducing the CoTAPc loading on GCNT accentuates the spectroscopic changes mentioned above (see Supplementary Fig. 25), owing to more pronounced interactions between fewer CoTAPc molecules and the substrate defects. In-situ EXAFS spectra of post-electrolyzed catalyst, as shown in Supplementary Fig. 26, reveal that three characteristic moieties a, b, and c present partial recovery back to those of initial CoTAPc/GCNT OCV. This result suggests that the recovery of the Co center back to the initial planar state is a relatively slow process after the bulk electrolysis in absence of external electric field.

We observed similar trends in Co-N4 structural evolution in the CoPc/GCNT system. As shown in Fig. 3f, while the deviation from square-planar symmetry is small at low overpotential (−0.36 V), noticeable changes occur in moieties a, b, and c when a more cathodic potential is applied (−0.5 V), and these changes align well with those observed in CoTAPc/GCNT. Furthermore, in-situ XANES spectra of COR in 1 M KOH, 1 M KHCO3, and 0.5 M K2SO4 (pH 2) are shown in Fig. 3g–i, respectively. Under all conditions, the spectroscopic tendencies are similar to those observed in CO2R, with the edge position shifting to lower energy, suggesting a more reduced state of Co-center under more cathodic potentials. Taken together, all these in-situ XANES spectra correspond well with our DFT simulations and strongly support our hypothesis that the out-of-plane distortion of CoN4 would occur under cathodic potential.

Revealing the *CO binding configurations on CoTAPc/GCNT during CO2R/COR

Having demonstrated the potential-driven distortion of the Co center, we systematically conducted in-situ Fourier-Transform Infrared (FT-IR) studies using attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) to track important reaction intermediates in COR and CO2R. In a typical measurement, ATR-SEIRAS over the full wavenumber range exhibits no noticeable signals for intermediates other than *CO, such as C − H, *CHO, *CH2O, etc. (Supplementary Fig. 27), suggesting that the hydrogenation of *CO is likely the rate-determining step for CH3OH formation21. As depicted in Fig. 4a, b, two *CO bands in the IR spectra of CoTAPc/GCNT during COR and CO2R in 0.2 M KHCO3 appear at ~2110 − 2090 cm−1 and ~1950 − 1900 cm−1, labeled as I and II, respectively. It is evident that the band II exhibits stronger intensity with a noticeable tail towards the lower frequency region, suggesting the possible existence of a third species. As a comparison, ATR-SEIRAS of the pristine GCNT under COR conditions does not show any peaks (Supplementary Fig. 28) related to the catalysis, confirming that the above bands are associated with CoTAPc.Fig. 4 In-situ FT-IR analysis of CoTAPc/GCNT during CO2R/COR.

ATR-SEIRA spectra of a COR and b CO2R in 0.2 M KHCO3 electrolyte, c comparison of frequencies and normalized intensities of the corresponding *CO bands in KHCO3; ATR-SEIRA spectra of d COR and e CO2R in 0.2 M KDCO3 electrolyte, f comparison of frequencies and normalized intensities of corresponding *CO bands in KDCO3. ATR-SEIRA spectra of g COR and h CO2R in 0.2 M phosphate buffer saline (PBS) electrolyte (pH = 8), i comparison of frequencies and normalized intensities of corresponding *CO bands in PBS. All potentials are versus SHE. 100% iR compensation was applied for all potentials. Typical solution resistance for 0.2 M KHCO3, 0.2 M KDCO3 and 0.2 M PBS were measured to be 25.8 Ω, 42.9 Ω and 29.7 Ω, respectively. 100% iR compensation was applied for all potentials. Source data for ATR-SEIRAS are provided in the Source Data file.

Notably, band I in both COR and CO2R disappear around −0.95 ~ −1.0 V vs. SHE (Fig. 4a, b), indicating that a more cathodic potential drives the configuration evolution of the *CO. Considering that −1.0 V vs. SHE is too positive for *CO to be protonated to form *CHO, the disappearance of band I is likely not a result of *CO hydrogenation49. Additionally, we found that the disappearance of band I also occurs at a similar potential, ~ −1.0 V vs. SHE, in KDCO3/D2O (Fig. 4d, e). As we observed substantially hindered CH3OH production in COR in both KDCO3 and KOD (Supplementary Fig. 29), we can reasonably exclude the possibility of *CO hydrogenation driving the disappearance of band I. According to the above XAFS result (Fig. 3d), a noticeable out-of-plane distortion has already occurred at potentials lower than −1.1 V vs. SHE. Taken together, we attribute the disappearance of band I to the evolution of CoN4 coordination occurring around −1 V vs. SHE, leading to the altering of the *CO adsorption configuration. Taking into account that this structural transformation occurs at a relatively low overpotential region, and the wavenumber of this *CO band is close to that of the *COL we previously calculated via DFT (2075 cm−1), we attribute this *CO band to the *COL on the quasi-flat CoN4 center. As we discussed earlier, the *CO adsorption on CoN4 is likely determined by the equilibrium between the quasi-flat (*COL) and distorted (*COB) configurations of the CoN4 plane. Therefore, depending on the applied cathodic potential, one could anticipate the bridge *CO lying between the two predicted wavenumbers (2075 ~ 1743 cm−1), aligning with the wavenumber of band II. Hence, we attribute band II to the bridged *CO on the CoN4 center (Fig. 1i).

Regarding the tail of the band II (at a very low wavenumber of 1880 ~ 1800 cm−1), we believe it is likely associated with relatively inert *CO species, which are difficult to both desorb and undergo subsequent hydrogenation. To validate this hypothesis, we conducted similar in-situ IR measurements on FePc/GCNT (Supplementary Fig. 30). As a result, *CO species was observed at a similar wavenumber, validating the above hypothesis since Fe-N4 is known to bind CO strongly50,51. Note, its intensity gradually increases with an increase in overpotential, likely due to the more severe distortion occurring to the catalyst center. Besides, the ATR-SEIRAS, collected during the second cycle after recovering the catalyst from the first run via anodic scan, reveals a slightly more pronounced low-frequency band in both KHCO3 and KDCO3 compared to the first scan at the same potential (Supplementary Fig. 31, denoted as band II’ here). We attribute this to the ineffective recovery of the Pc plane distortion during the cycling (Supplementary Fig. 26), causing distorted CoTAPc molecules to accumulate during the second cathodic scan, thus leading to more *CO adsorption. Besides, compared with transition metal surfaces, where *COL and *COB are typically located at ~2100 cm−1 and ~1900-1800 cm−1, respectively52–57, we tentatively suggest that the band around 1950 ~ 1900 cm−1 is attributed to the bridge *CO instead of atop *CO on the Co-Pc center.

With an increase in overpotential, the frequencies of all *CO bands exhibit a red shift, owning to the Stark tuning effect58,59. Notably, as depicted in Fig. 4c, the frequencies of band I and II in both CO2R and COR are very close, indicating that the *CO binding strength is predominately influenced by the applied potential. Separately, the formation and disappearance of band I for both CO2R and COR track each other, however, band II in COR typically exhibits earlier onset potential compared to those in CO2R, suggesting that the formation of *COB during CO2R is not favored due to the relatively small cathodic potentials applied. Similar trends were observed for CO2R/COR in KDCO3 (Fig. 4f), while CO2R shows a slightly more negative potential to reach the optimal intensity of *COB, which might correspond to the slower CO2R rates. In summary, the evolution of *COL and *COB can be divided into three stages based on overpotential, with the predominant *COL to *COB transformation occurs at −0.7 ~ −1.05 V vs. SHE.

Previous literature has indicated that bicarbonate anion could influence the *CO signal in IR measurements (the equilibrium of HCO3− ↔ CO2 + OH− causing low local CO2 concentration)51. To avoid this effect, we also conducted similar in-situ ATR-SEIRAS measurements in 0.2 M phosphate buffer saline (PBS) at ~ pH 8 to exclude the influence of HCO3−. As shown in Fig. 4g, h, similar, if not identical, *CO bands were observed, confirming that the related IR peaks correspond to the *CO intermediates during CO2R/COR. Moreover, the potential dependent trends on the corresponding band frequencies and normalized intensities (Fig. 4i) also resemble those in CO2R/COR in KHCO3 and KDCO3. Finally, we also conducted similar in-situ ATR-SEIRAS measurements in 0.2 M KOH, where we found that band II appears to be broader (Supplementary Fig. 32), especially at negative potentials, and band I appears at more positive potential. This indicates that local pH would also influence the *CO binding configurations. Nevertheless, the potential dependent trends of both *CO bands remain the same. As anticipated, similar trends were observed in ATR-SEIRAS measurements during COR on CoPc/GCNT in different electrolytes (Supplementary Fig. 33).

We further investigated the reversibility of these two *CO intermediates. As shown in Fig. 5a, b, when sweeping the electrode potential back to 0 V after the cathodic scan, the *COL band reappears, and the *COB band exhibits an obvious blue shift. This observation indicates that these two types of *CO are reversible depending on the electrode potential. Compared with the initial IR spectra recorded at 0 V, the existence of both *CO bands suggests that the distorted Pc plane is not fully recovered after the cathodic scan. Moreover, the N2 purging experiments further substantiated that the *COL is weakly bonded and can be easily removed (Fig. 5c, d), while bridge *COB binds stronger and remains stable under negative potentials, being stripped off only during anodic scans (Supplementary Fig. 34). As anticipated, similar observations were made in CO2R/COR in electrolytes of KDCO3 and PBS (Supplementary Fig. 35, 36), further validating our conclusions above. Overall, the results obtained from these in-situ ATR-SEIRAS experiments further validate our hypothesis regarding the potential-dependent structure evolution of the CoN4 plane (from quasi-flat to distorted configurations). This evolution, in turn, leads to distinct binding models of *CO on the active sites, as predicted by our DFT simulations. When comparing the onset potential of these different *CO species to the onset potentials for CH3OH production, we believe *COB (band II) is responsible for the CH3OH production in both CO2R and COR. As illustrated in Fig. 5e, the potential-driven distortion of the CoN4 center in CoTAPc/GCNT induces a shift in the binding mode of *CO. This change in binding mode simultaneously extends the C − O bond length, facilitating the subsequent CO hydrogenation and ultimately promoting CH3OH production.Fig. 5 *CO stability characterizations.

Comparison of ATR-SEIRA spectra of a COR and b CO2R in 0.2 M KHCO3 after cycle back to 0 V from −1.15 V. ATR-SEIRA spectra of N2 purging experiments at fixed potential of c COR and d CO2R in 0.2 M KHCO3. Both CO or CO2 stream and saturated electrolyte were purged and saturated by N2. e Schematic illustration of two CoTAPc@1V-Gr models, their corresponding *CO binding configurations, and the predominant products. All potentials are versus SHE. 100% iR compensation was applied for all potentials. Typical solution resistance for 0.2 M KHCO3 was measured to be 25.8 Ω. Source data for ATR-SEIRAS are provided in the Source Data file.

Discussion

In conclusion, we have elucidated a potential-driven structural evolution of the CoN4 catalytic center in CoTAPc/GCNT during CO2R/COR. This structure evolution induces changes in the binding mode of *CO on CoTAPc, consequently leading to the production of CH3OH. We first developed a theoretical model to explore the interactions between CoTAPc and CNT-defect and found two possible configurations of the CoN4 center (quasi-flat and out-of-plane distorted configurations). Based on the dipole moment analysis, we anticipate that the cathodic potential would induce further distortion in the planner CoN4 configuration. Consequently, two *CO configurations were identified on these two CoN4 configurations, where *CO tends to linearly bind to the Co-center and bridge on the N, N domain on the quasi-flat and distorted CoN4, respectively. Our simulation suggests that the latter *CO binding mode exhibits higher adsorption energy by nearly three-folders compared to the linear *CO, indicating that CoN4 distortion enhances the *CO affinity. Subsequently, we systematically assessed the CO2R/COR performance of CoTAPc/GCNT in a flow cell at practically relevant current densities. The results unveiled that the applied potential overwhelmingly determines methanol productivity in both CO2R and COR. Notably, the CH3OH productivities in CO2R and COR are comparable at the same applied potential (in the SHE scale). Consequently, the distinct selectivity observed in CH3OH production between CO2R and COR is a result of the difference in applied potential. Moreover, detailed in-situ XAFS measurements validate the potential-driven distortion of the CoN4 center during catalysis under relevant conditions. Finally, in-situ ATR-SEIRAS measurements disclose two potential-dependent *CO IR bands, which distinctly match with the DFT predicted linear *CO and bridges *CO, pinpointing our hypothesis discussed above. Overall, our work demonstrates the critical role of substrate defects and electrode potential in electrocatalysis based on molecular catalysts. The insights gained and strategies established in our study offer valuable contributions to future mechanistic studies and the development of new systems with improved performance based on immobilized molecular catalysts.

Methods

Chemicals

Both the nGCNT and GCNT (>99.9%) were purchased from the chemical materials vender of XFNano Inc., China. Other chemicals, such as cobalt phthalocyanine (CoPc), monopotassium phosphate (KH2PO4), N, N-dimethylformamide (DMF), concentrated sulfuric acid (H2SO4, 98%), cobalt chloride hexahydrate (CoCl2·6H2O), ammonium dimolybdate and urea were purchased from Sigma-Aldrich Singapore, and were used without further treatment. Potassium hydroxide (KOH, 99.99% metal basis) and potassium bicarbonate (KHCO3, 99.99% metal basis) were purchased from Aladdin Chemical Shanghai. Potassium deuterium oxide (40 wt.% KOD solution in deuterated water (D2O)) and D2O (D, 99.9%) were purchased from Sigma-Aldrich Singapore and Cambridge Isotope Laboratories, respectively. Ultrapure water (18.2 MΩ) was used in all the experiments in this work.

Cobalt tetraaminophthalocyanine (CoTAPc) was prepared on the basis of our previously reported procedures (Supplementary Fig. 1)32. In details, 3.8 g of 4-nitrophthalonitrile, 1.2 g of CoCl2·6H2O, 9.6 g of urea and 0.1 g of ammonium dimolybdate were carefully mixed together, followed by thorough grinding. The resulted mixture was then reacted in solid-state at the temperature of 120 °C for 1 h and then increased to 180 °C for another 4.5 h. After the reaction was completed, the mixture was allowed to cool. The crude product was then transferred and boiled sequentially in 300 mL of 1 M HCl and 1 M KOH aqueous solutions for 1 hour each. As a result, the solid residue was filtered and thoroughly washed with large amounts of water and then methanol. Crude cobalt tetranitrophthalocyanine (CoTNPc) was obtained after vacuum drying. The CoTNPc was then dispersed in a mixed solvent of 3 mL water and 27 mL DMF. After adding 9 g of Na₂S·9H₂O into the mixture, it was further reacted for 10 h at 60 °C. The remaining solution was evaporated, and the resulting solid was first washed with ultrapure water, then boiled in 1 M KOH and ethanol. The precipitate was collected by filtration, washed several times with water until the filtrate was neutral, and then washed with methanol. Finally, the synthesized CoTAPc was dried under vacuum.

Catalyst preparation

First, CoTAPc powders (5 mg) were ground carefully using a mortar in DMF (3 mL). The resulting mixture was then transferred to a glass flask, and an additional 37 mL of DMF was added. The solution was ultrasonicated for 10 minutes, after which 50 mg of GCNT was introduced, followed by an additional half an hour of ultrasonication. The solution was then stirred at room temperature for an additional 24 h. After this, the resulting mixture was centrifuged and thoroughly washed with DMF and ethanol several times before being vacuum-dried. The CoPc/GCNT samples were synthesized following the same procedures. For samples with different catalyst/CNT ratios, such as 1:20 or 1:40, the amount of CoTAPc was adjusted to 2.5 mg and 1.25 mg, respectively.

Materials characterizations

Transmission electron microscopy (TEM) was performed on a JEOL JEM − 2100 F at 200 kV. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images were acquired using a FEI Talos F200X and a JEOL JEM-ARM200F, both operated at 200 kV. Scanning electron microscopy (SEM) was conducted using a JEOL JEM-7610M, equipped with an energy-dispersive X-ray spectrometer (EDS), operating at 15 kV. Raman spectra were obtained using a homemade SpectroPro HRS500-based Raman spectrograph. The Co atomic weight percentage was determined using an inductively coupled plasma optical emission spectrometer (ICP-OES, ThermoFisher CCAPG200 DUO). X-ray photoelectron spectroscopy (XPS) was carried out using a scanning X-ray microprobe (Kratos Axis Ultra DLD) operating at 15 kV with monochromated Al Kα radiation (1486.71 eV). The binding energies were calibrated with reference to the sp3 C 1 s peak at 284.8 eV.

Electrochemical CO(2)R assessments

All electrocatalytic assessments were carried out using a BioLogic VMP-3e potentiostat, operated with EC-Lab software (V11.50). All electrochemical data were exported in ASCII format via EC-Lab for further analysis and plotting. A flow cell equipped with a three-electrode configuration (Supplementary Fig. 7, working electrode area of 1 cm2) was used for the CO(2)R assessments. The distance between the cathodic and anodic chambers was set to 3 mm. The reference electrode used was Ag/AgCl (3 M KCl), and the counter electrode was an IrO₂-coated Ti mesh60.

Typical electrolytes, 1 M KOH and 1 M KHCO₃, were prepared by dissolving the definite amounts of KOH or KHCO3 into ultrapure water. The pH of these electrolytes was measured to be 13.9 ± 0.25 and 8.38 ± 0.03, respectively, using a Horiba LAQUA PH1300 bench pH meter. Additionally, the acidic electrolyte, 0.5 M K2SO4 (pH 2), was prepared by adding H₂SO4 to the 1 M KHCO₃ solution until the pH dropped to 2.0. All electrolytes were treated with Chelex-100 resin (Na-form, 50 − 100 mesh, Sigma-Aldrich, converted to K-form using KOH according to literature procedure61) to remove any ionic impurities. For CO(₂)R conducted in both 1 M KOH and 1 M KHCO3, 1 M KOH was used as the anolyte for the counter reaction, with a fresh Fumasep FAA-3-PK-130 anion exchange membrane (130 μm, purchased from Fuel Cell Store, rinsed and stored in ultrapure water before use) separating the cathodic and anodic chambers (shown in Supplementary Fig. 37 and Note 1 in Supplementary Table 2). For CO₂R in the acidic electrolyte of 0.5 M K₂SO₄ (pH 2), 0.5 M H₂SO₄ was used as the anolyte. In this case, a proton exchange membrane of Nafion 117 (185 μm, purchased from Fuel Cell Store, U.S., boiled in 5% H₂SO4 + 3% H2O2 overnight before use) was employed. During CO(2)R, the shuttled protons from the anolyte helped maintain the bulk pH of the catholyte, preventing a rapid increase in pH under large current densities.

To prepare the catalyst layer, 10 mg of the catalyst composite (CoPc catalyst together with CNT substrates) was first dispersed in 1 mL of an ethanol/water (3:1) mixture to form a uniform catalyst ink through thorough ultrasonication. Then, 60 μL of Nafion solution (D520, 5 wt.%, purchased from Fuel Cell Store) was added to the ink, followed by additional ultrasonication for at least 30 min. Subsequently, 260 μL of the resulting catalyst ink was drop-cast onto a gas diffusion electrode (GDE, Freudenberg H23C6, Fuel Cell Store, treated with 12% PTFE after purchase, 2.5 × 1.0 cm2) to achieve a catalyst loading of approximately 1 mg cm−2. Finally, the as-prepared GDE with the loaded catalyst was dried in a vacuum oven at 100 °C for 1 h before use.

During the CO(2)R experiments, gas flow rates for CO₂ (99.995%, Air Liquide) or CO (99.97%, Air Liquide) were regulated using mass flow controllers and set to 20 sccm. Additionally, two peristaltic pumps were employed to circulate the catholyte and anolyte, at the flow rates of 12 mL min⁻¹ and 17 mL min⁻¹, respectively. The CO(2)R experiments were generally conducted using the chronopotentiometric technique controlled by EC-Lab. Consequently, Faradaic efficiencies (FE) for different products were measured and estimated at various current densities. The cathodic potential on the reversible hydrogen electrode (RHE) scale was calculated according to the Nernst equation with appropriate iR compensation, as outlined in the following equations:1 E(RHE)=Emeasured+EΘ(Ag/AgCl)+0.05916×pH−iR

Seperately, the potential at the standard hydrogen electrode (SHE) scale was calculated as:2 E(SHE)=Emeasured+EΘ(Ag/AgCl)−iR

Electrochemical impedance spectroscopy (EIS) was employed to measure the contact and solution resistance during CO(2)R at any specific potential. Note, for all the experiments, the reference electrode was calibrated by a reversible hydrogen electrode (RHE) assembled in our laboratory. In specific, a thin Pt wire and a graphite rod were used as working and counter electrodes, respectively. In this case, 0.5 M H2SO4 (pH~0) was used as electrolyte. Then, the cyclic Voltammetry (CV) technique was employed to acquire the standard potential, with the potential scan rate set to 1.0 mV s−1. The reduction scan and oxidation scan intersect the horizontal axis at two points. Subsequently, the average of these two values was used to determine the actual potential of the reference electrode in reference to RHE. In our case, the Eϴ(Ag/AgCl) was measured to be 0.207 V.

During the CO(₂)R experiments, the generated gas products were analyzed using an in-line Shimadzu system GC − 2014C chromatograph, which was equipped with a thermal conductivity detector (TCD) and two flame ionization detectors (FID), respectively. The Faradaic efficiency (FE) of gas products could be calculated by the following equation:3 FE(%)=n⋅F⋅f⋅xnj

Here, n represents the number of electrons needed for the specific products (n = 2 for both CO and H2, n = 4 for methanol in COR and 6 in CO2R), F is Faraday constant (96485.3 C mol−1), f is the gas actual flow rate at the GC inlet, determined using a separate gas flow bubble meter, xn is the volume fraction of H2 or CO obtained by GC, and j is the current density.

After the CO(2)R experiments, the generated liquid products were detected using NMR spectroscopy (Bruker Ascend 400 MHz), using an internal standard of dimethyl sulfoxide (DMSO). In specific, after CO(2)R experiments, a 500 μL catholyte aliquot was acquired and mixed with 50 μL of D2O for the subsequent products quantification using NMR. When 1 M KOH was used as the electrolyte, the sample was neutralized to approximately pH 7 using 10 M HCl before the NMR analysis.

Regarding the CO2/CO co-feeding electrolysis, CO2 and CO were thoroughly mixed in a gas reservoir (volume of 15 mL) before being introduced into the gas chamber.

Finally, the density of electrocatalyst surface-active sites (Γ0) was estimated by collecting CVs with scan rates ranging from 40 mV/s to 100 mV/s in the CO2-saturated electrolyte of 0.2 M KHCO3 (pH of 6.8). Besides, the surface density of redox-active Co centers can be estimated according to the measured slopes of the plots obtained by plotting the current densities of the reduction wave peak on the function of the scan rate (υ)62:4 slope=n2F2Γ04RT

where n = 1, F is Faraday constant, R is the ideal gas constant (8.3145 J mol−1 K−1), T is room temperature (298.15 K). Hence, Γ0 could be obtained from the slope of the plot.

XAFS characterization

First, the ex-situ X-ray absorption fine spectroscopy (XAFS) of the Co K-edge for both CoPc and CoTAPc was collected at the XAFCA beamline of Singapore Synchrotron Light Source (SSLS) using the transmission mode. The ex-situ XAFS of the catalyst/GCNT composite was also recorded at SSLS using the fluorescence mode. Besides, the in-situ XAFS tests of CO(2)R were performed at the BL14B2 beamline of SPring-8 in the fluorescence mode using a 7-element silicon drift detector, and at the XAFCA beamline of SSLS in fluorescence mode using a conversion electron yield detector. The same flow cell with minor modification (shown in Supplementary Fig. 18) was used for conducting these in-situ measurements aiming to replicate the conditions of the electrochemical performance assessments conducted in our lab. Besides, all these in-situ electrochemical tests were conducted using a portable potentiostat of BioLogic SP-200, with an increased catalyst loading of approximately 2.3 mg cm−2 to achieve enhanced signal quality. All other conditions were kept the same with those used in the lab-based flow cell assessments. Data analysis for X-ray absorption spectroscopy (XAS) were conducted using Athena from the Demeter software package63. XANES simulations were carried out using the FDMNES code, based on configurations derived from our DFT calculated models and structures64.

ATR-SEIRAS measurements

The in-situ experiments based on the attenuated total reflectance-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) were carried out using a custom-designed spectro-electrochemical cell. The target catalyst was loaded onto a Si-ATR crystal (45o) coating with gold thin film, which served as the working electrode. Besides, an Ag/AgCl electrode and a clean graphite rod were used as the reference electrode and counter electrode, respectively. Before a typical measurement, gases CO2, CO, or N2 gas was vigorously purged into the corresponding electrolyte for at least 30 min to reach the steady state. Then, the SEIRAS spectra were collected using a Shimadzu IRTracer-100 equipped with a PIKE VeeMAX III ATR accessory and a mercury cadmium telluride (MCT) detector with liquid-nitrogen-cooling. Additionally, all spectra were recorded with a spectral resolution of 8 cm−1 and plotted in the unit of absorbance. Each SEIRA spectrum shown in this study corresponds to 20 co-added scans, corresponding to approximately 14 s of sampling time. Lastly, the electrolyte of 0.2 M PBS was prepared by dissolving KOH pellets into the solution of 0.1 M KH2PO4 to reach pH=8. The electrolyte of 0.2 M KDCO3/D2O was prepared by purging ultrapure CO2 to 0.2 M KOD/D2O solution overnight.

Computational methods

In this study, all the first-principles calculations were performed using spin-polarized density functional theory (DFT) with the Vienna ab-initio simulation package (VASP)65,66. The investigations associated to CO2R and HER, such as geometry relaxations, total electronic energy calculations and vibrational frequency calculations, all employed the generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerh (PBE) format67, the projector-augmented wave (PAW) pseudopotentials68 and a plane wave basis with the cut-off energy of 450 eV. Besides, Van der Waals force (DFT + D3) is also considered during the reaction simulations69. The convergence criteria for each electronic steps and structural relaxations were set to 10−4 eV and 0.01 eV/Å, respectively.

For the graphene substrate, it is modeled as a 10 × 10 supercell in our calculations, with a vacuum layer of 20 Å in the direction perpendicular to the graphene sheet to prevent artificial interaction between the adjacent images. Gamma-point sampling was used throughout. Systems of single carbon vacancy graphene were modeled by removing one carbon atom from the 10 × 10 graphene supercell.

The Visualization for Electronic and Structural Analysis (VESTA) software70 was used for visualization of the atomic structures.

Supplementary information

Supplementary Information

Peer Review File

Description of Additional Supplementary Files

Supplementary Data 1

Supplementary Data 2

Supplementary Data 3

Supplementary Data 4

Source data

Source Data

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-024-52168-x.

Acknowledgements

We acknowledge the National University of Singapore and Ministry of Education for their financial support, through the grants A-0009176-02-00 and A-0009176-03-00, as well as A*STAR (Agency for Science, Technology and Research) under its LCERFI program (award no. U2102d2002). We would also like to acknowledge the support of the National Research Foundation (NRF) Singapore, under the NRF Fellowship (NRF-NRFF13-2021-0007), as well as the support from the Centre for Hydrogen Innovations at the NUS (CHI-P2022-06; A-8000138-03-00). Y.W., B.Y. and Q.H. would like to acknowledge the support by National Research Foundation (NRF) Singapore (NRF-NRFF11-2019-0002), Singapore Low-Carbon Energy Research Funding Initiative hosted under A*STAR (award no. LCERFI01-0017) and Ministry of Education (MOE), Singapore, Academic Research Fund Tier 2 (MOE-T2EP10123-0001). We also thank Dr. Takeshi Watanabe at SPring-8 for his assistance on XAFS characterization as well as Jiguang Zhang for his help on GC-MS test. Computations were done in NUS high-performance clusters and National Supercomputing Center in Singapore.

Author contributions

L. W. supervised the project. L. W. and H. Y. conceived the idea. H. Y. designed and performed the experiments. N. G. conduced the DFT computations under the supervision of C. Z. H. Y. and S. Xi conducted the XAS experiments. Y. W., B. Y. and Q. H. helped with the TEM images collection. L. W., H. Y., N. G., and C. Z contributed to the data interpretaion and wrote the manuscript. All the authors contribute to the revision of the manuscript.

Peer review

Peer review information

Nature Communications thanks Feng Jiao, and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

Data availability

Source data are provided with this paper and are available from the corresponding authors upon request. Source data are provided with this paper.

Competing interests

The authors declare no competing interests.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

These authors contributed equally: Haozhou Yang, Na Guo, Shibo Xi.
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