Switching off Competing Hydrogen Formation in CO2 Electroreduction via Substrate Defect Engineering

doi:10.21203/rs.3.rs-4862097/v1

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Switching off Competing Hydrogen Formation in CO2 Electroreduction via Substrate Defect Engineering

https://doi.org/10.21203/rs.3.rs-4862097/v1

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Carbon nanotubes (CNTs) have emerged as effective substrates for immobilizing molecular catalysts towards many electrocatalytic reactions, such as CO₂ reduction (CO₂R). However, despite the prevailing notion of strong π-π stackings between the molecular catalyst and CNTs, our understanding of their interactions remains inadequate. Here, we employ functionalized nickel phthalocyanines (NiPc), established CO₂R catalysts, immobilized on CNTs as a model system to investigate the catalyst/substrate interactions. Firstly, we find that NiPc-catalysts preferentially anchor on the defects on CNTs rather than adhering via π-π interaction with the ideal graphene-like CNT surface, a finding further validated by theoretical simulations. Consequently, we observe the least uniform NiPc-catalysts distributions on CNTs when the defect-content is the lowest. Notably, this combination exhibits the highest CO₂R selectivity and activity despite the non-uniform catalyst distributions. Through operando X-ray adsorption spectroscopy and theoretical simulations, we reveal that high CNT defect-contents tend to induce substantial D_4h symmetry breaking of the NiPc plane under cathodic potential, consequently resulting in reduced CO₂R selectivity and activity. Therefore, maintaining a low to moderate defect level on CNTs is critical. Guided by this understanding, we fine-tune the defect-level of CNTs through graphitization, achieving an unprecedently high selectivity for CO₂ to CO conversion (CO to H₂ molar ratio exceeding 16100:1, a remarkable suppression of hydrogen evolution by three orders of magnitude) and improved intrinsic-activity (turnover frequency of 1072 s^− 1 at − 0.60 V vs. reversible hydrogen electrode) on an optimized Ni-Pc/CNTs composite. Furthermore, we achieved practical relevant CO production in a zero-gap electrolyzer (electrode size of 100 cm⁻²), reaching high current (up to 50 A), with high CO selectivity (> 95%) and reasonably low cell voltage (approximately 3.5 V), substantially outperforming the state-of-the-art silver catalyst. Moreover, we extend this knowledge to a Co-based molecular catalyst, achieving a high Faradaic efficiency (over 50%) towards methanol production with a high partial current density over 150 mA cm^− 2. Overall, our findings underscore the significance of tuning defect levels on CNT substrates for achieving desired performance for immobilized molecular catalysts.

Physical sciences/Chemistry/Catalysis/Electrocatalysis

Physical sciences/Materials science/Materials for energy and catalysis/Electrocatalysis

Molecular catalysts hold significant promise in various of energy-related electrocatalytic applications due to their precise and tunable coordination environments^1–6, which facilitates effective investigation of reaction mechanisms and optimization of the catalytic performance^7–9. Metal phthalocyanines (MPc), particularly nickel phthalocyanine (NiPc), exhibit high selectivity and appreciable activity in electrocatalytic CO₂ reduction (CO₂R), which has been recognized as a promising pathway for the sustainable production of chemicals and fuels when coupled with renewable electricity^10–14. Several studies have shown that immobilization of MPc on carbonaceous supports can, sometimes significantly, enhance their activity and selectivity towards CO₂R^15–21. Among these supports, multi-walled carbon nanotubes (CNTs) have been identified as one of the most effective substrates^22–24. Despite the diverse factors affecting the adsorption of small molecules on CNTs, such as molecule morphology, functional groups, pH, ion strength, etc.^25–28, the improved CO₂R activity/selectivity of MPc on CNTs is generally ascribed to the strong π-π interaction between the catalyst and substrates^29,30.

However, it remains unclear how the MPc adsorbs onto CNTs via π-π stacking, what configuration it adopts on the CNTs surface (i.e., sandwiched, T-shaped, or parallel-displaced), and how strong these π-π interactions are. Hence, it is a bit arbitrary to simply attribute the enhanced selectivity of NiPc immobilized on CNTs, in comparison to other substrates such as carbon black or graphene, to the higher graphitization degree (more ideal sp2 carbon surface) of CNT. Besides, π-π interactions are believed to be weak van der Waals forces³¹, as evidenced by the relatively easy exfoliation of graphene layers despite their extended sp2 carbon planes. Overall, there is currently insufficient experimental evidence to support the commonly referred to strong π-π interaction effect due to challenges associated with directly measuring the catalyst/substrate interactions, especially under the working conditions. Hence, there is a need to gain a better understanding of the nature of the catalyst/CNT interactions to uncover the origin of the promotion effect of CNT substrate on molecular-catalyst-based catalytic processes.

Separately, the prevalent computational model for the MPc/CNTs systems assumes the carbon nanotube support to be a perfect graphene layer^32,33. However, CNT surfaces are usually not ideal and can consist of various types of defects that can drastically modify their electronic properties³⁴. Nevertheless, the effects of CNT defects on CO₂R and other relevant reactions are rarely, if not none, investigated for the hybrid catalyst of MPc/CNTs.

To understand the MPc/CNT interactions, we employed three CNTs with distinct graphitization degrees to immobilize nickel tetraaminophthalocyanine (NiTAPc). We discovered that NiTAPc preferentially adsorbs onto the defect domains rather than stacking on the perfect graphene-type CNTs surface via π-π interactions, resulting in a distinct dispersibility of NiTAPc on the CNTs with high graphitization degree (GCNT-H) and non-graphitized CNT (nGCNT). Additionally, NiTAPc on GCNT-H exhibits significantly higher CO selectivity (i.e., enhanced CO/H₂ ratios by over two orders of magnitude) during CO₂R, along with a substantially lower overpotential by 300–500 mV, compared to NiTAPc on nGCNT. To further investigate the catalyst/substrate interactions, we designed and executed in-situ X-ray absorption spectroscopy (XAS) by using the same electrochemical cell used for the CO₂R experiments. Notably, we recorded the XAS at practical relevant current densities (up to ~ 200 mA cm^− 2). The obtained results indicate that potential-induced breaking of the Ni-N₄ symmetry occurred on both NiTAPc/GCNT-H and NiTAPc/nGCNT, while more significant changes were observed on the latter. Taken together, we determine that the NiTAPc catalysts primarily interact with the CNT substrates through electrostatic interactions with defects, and higher defect density will lead to more pronounced Pc plane distortions during the electrocatalysis (under cathodic bias). Comparison experiments of nickel tetracarboxylphthalocyanine (NiTCPc) in in-situ XAS present more diminished D_4h symmetry compared with the NiTAPc counterparts, suggesting that NiPc with electron-withdrawing substituent has a stronger interaction with defects on CNTs.

Combining in-situ XAS and electrochemical CO₂R results, we found that minor to moderate distortion of the Ni-N₄ plane benefits CO₂R, while drastic distortion leads to structure degradation and facilitates the side reaction of hydrogen evolution. Besides, attenuated total reflectance-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) further confirmed the improved activity of NiTXPc on GCNT, as indicated by the lower onset potentials and intensified CO₂R intermediate (*COOH) signal. Furthermore, our density functional theory (DFT) calculations revealed the nature of the interactions between NiPc-catalysts and the defects on CNTs, the mechanisms of the cathodic potential-induced structural distortion of the NiPc-catalysts, and the interplays between the structural distortions and the catalytic performance of the NiPc-catalysts in terms of activity, selectivity, and stability. Building upon these new insights, we further optimized the NiTAPc/GCNT system and achieved an unprecedented CO₂R selectivity with a CO/H₂ molar ratio reaching as high as 16000:1 at practical relevant current densities, i.e., 500 mA cm^− 2. In comparison to the control system of NiTAPc/carbon black, the hydrogen evolution has been suppressed by nearly three orders of magnitude using NiTAPc/GCNT. Furthermore, we successfully applied this strategy to a Co-based molecular catalyst, achieving remarkable selectivity towards methanol (up to 53%) at a high current density exceeding 200 mA cm^− 2 in CO reduction. Overall, we believe that our findings hold substantial significance in elucidating the intricate molecular catalyst/substrate interactions and offer valuable insights for employing molecular catalysts for industrial-relevant applications.

Defect-Driven Adsorption of NiPc-based Catalyst on CNTs

We chose three CNTs for our investigations, GCNT with a high graphitized degree (GCNT-H), GCNT with a low graphitized degree (GCNT-L), and nGCNT. TEM and scanning electron microscope (SEM) images (Supplementary Fig. 2) revealed that all the CNTs exhibit a similar physical nature, with negligible differences in the CNT morphology, diameter, and size. Besides, the morphology of nGCNT and GCNTs remained after the immobilization of NiTAPc (Supplementary Fig. 3). The successful immobilization of NiTAPc was confirmed by elemental energy dispersion spectroscopy (EDS, Supplementary Fig. 4). Notably, high-angle annular dark-field (HAADF) scanning transmission electron microscope (STEM) from aberration-corrected (AC)-HRTEM images revealed a notable finding regarding the dispersion of NiTAPc on different CNTs. As shown in Fig. 1a, GCNT-H exhibits the best-graphitized surface among the three CNTs, which is expected to be the most optimized surface for π-π interactions. However, we observed that NiTAPc preferentially absorbed at the less perfect graphitized domains of GCNT-H (i.e., intersections), contradicting the commonly held perception that strong π-π interaction promotes the dispersion of NiPc catalysts. A similar phenomenon was also observed for NiTCPc and CoTAPc on GCNT-H (Supplementary Fig. 5). Instead, much more uniform dispersions of NiTAPc were observed on GCNT-L and nGCNT (Fig. 1b-c). Note, we observed comparable dispersion of metal Pc on nGCNT with relevant literature work^24,32,33. Besides, the loadings of Ni on GCNT-H, GCNT-L, and nGCNT were determined to be 0.22%, 0.22%, and 0.21% in weight percentage, respectively, using inductively coupled plasma optical emission spectrometer (ICP-OES, Supplementary Table 1). These comparable Ni loadings exclude the possibility of that the observed difference in absorption behaviors is due to the differences in catalyst loading. Thus, we propose that the defects on the CNT surface are primarily responsible for the absorption of molecular catalysts such as NiPc.

Raman spectra were recorded to investigate the intrinsic properties of the different CNTs. As shown in Fig. 1d, the GCNT-H exhibits the smallest I_D/I_G ratio, validating that the process of graphitization has significantly reduced the defect level of CNTs^35,36. After the immobilization of NiTAPc, the representative D and G bands in Raman spectra remained unchanged (Supplementary Fig. 6), indicating no chemical changes occurred to the CNTs during the loading of molecular catalysts. Besides, the results of X-ray photoelectron spectroscopy (XPS) (Supplementary Fig. 7) indicate the same valence state for the Ni atoms in these three NiTAPc/CNTs and free NiTAPc. Overall, we can conclude that NiPc-based catalysts predominate interact with the CNT via defects rather than π-π stacking, and these interactions are likely moderate based on our simulations.

Effects of CNT Defects on Electrochemical CO₂R

We first examined the CO₂R performance of these NiTAPc/CNTs catalysts using a conventional H-cell. Since CO and H₂ were the only two detectable products in our systems, we introduced the CO/H₂ molar ratio as an indicator for the selectivity of CO₂R. We believe this is a more effective method to reveal the intrinsic CO₂R selectivity of a given catalyst, as compared to the conventionally reported Faradaic efficiency (FE). For instance, a 99% FE of CO does not warrant a low HER FE due to the experimental error-induced uncertainties that could cause total FE to exceed 100%. Notably, NiTAPc/GCNT-H outperformed the other two catalysts significantly (Supplementary Fig. 8), both in terms of selectivity and activity towards CO. NiTAPc/GCNT-H reached a superior CO/H₂ molar ratio of nearly 1900:1, and exhibited substantially higher CO partial current densities at identical cathodic potentials compared to the NiTAPc/nGCNT electrode. Next, we assessed the CO₂R performance of these catalysts in a gas diffusion electrode (GDE) based flow cell (Supplementary Fig. 9) under industrially relevant current densities. Note, we collected gaseous products from both the gas and cathode chambers to avoid underestimating the H₂ evolution reaction (HER), especially in cases of minor flooding^37,38. Nevertheless, we have also reported all the measured FEs in our study (Supplementary Table 5). Additionally, we conducted meticulous calibration of the gas chromatograph (GC) to ensure the accuracy of the measurements towards a trace amount of H₂ (Supplementary Fig. 10–11). As shown in Fig. 1e, NiTAPc/GCNT-H exhibits a remarkable 6000:1 CO/H₂ molar ratio and an overpotential nearly 400 mV lower than that of NiTAPc/nGCNT (with a CO/H₂ molar ratio of only ~ 580) at 300 mA cm^− 2, under typical CO₂R conditions. NiTAPc/GCNT-H retained its exceptionally high selectivity towards CO even at higher current densities of 400 and 500 mA cm^− 2 (Supplementary Fig. 12). In contrast, significant HER took place on NiTAPc/nGCNT at current densities of 400 mA cm^− 2 or higher, consistent with the above H-cell result and previous literature reports^24,39,40. Meanwhile, NiTAPc/GCNT-L shows moderate CO₂R selectivity and overpotential, further confirming this strong correlation between CO formation selectivity/activity and the defect level of the CNTs supports. We further assessed our catalyst compositions in a membrane-electrode-assembly (MEA) reactor at ampere-level current. As shown in Fig .1e, NiTAPc/GCNT-H again exhibits superior CO selectivity and reduced cell voltage. In contrast, the NiTAPc/nGCNT counter sample exhibits much lower CO selectivity and larger cell voltage, especially at elevated current density of 400 mA cm^− 2 (Supplementary Fig. 12d).

Metal residues and amorphous carbon species are generally used as catalysts in preparing nGCNT. To exclude their influence on our electrochemical testing, the pristine nGCNT was purified before use (see Method section). We believe that most metal residues were removed by acid washing, and any remaining metal impurities are encapsulated in CNTs and thus inaccessible during electrocatalysis (Supplementary Table 3). Supplementary Fig. 13a-b show that the HER kinetics of purified nGCNT resemble those of GCNTs but are in sharp contrast to that of pristine nGCNT, indicating that no obvious electrochemically exposed metal exists in purified nGCNT. The introduction of heteroatoms via acid washing may also influence the catalyst/substrate interactions³³. Supplementary Fig. 13c-e and the corresponding note indicate that our treatment has a negligible effect on pristine CNT nature, further excluding the possibility that different behavior of NiTAPc on nGCNT and GCNT-H corresponds to oxygen-group interaction.

Moreover, we tested the CO₂R performance of CoTAPc/GCNT-H and CoTAPc/nGCNT and compared them with NiTAPc counterparts (Supplementary Fig. 14). The much lower CO/H₂ molar ratio of CoTAPc compared to the NiTAPc, supporting our hypothesis that HER primarily arises from the metal center in the molecular catalysts rather than the CNTs substrates, at least under the CO₂R conditions. Finally, we conducted Ar-feed electrolysis on the NiTAPc/CNTs. Supplementary Fig. 15 shows all three catalysts exhibit higher overpotential for HER than for CO₂R, with the potential trend being opposite to that of CO₂R, and NiTAPc/GCNT-H shows the largest overpotential increase. The well-matched overpotential trend and CO₂R selectivity reveal the high intrinsic activity of NiTAPc for CO₂R over HER, resulting in a much lower overpotential for CO₂R than that of HER at the same current densities. The lowest HER overpotential of NiTAPc/nGCNT further indicates its highest HER activity, which is also congruent with its poorest CO selectivity. In all, through these careful control experiments, we confirm that the above remarkable CO formation selectivity/activity originates from the defect engineering of the CNT supports rather than the presence of other impurities.

Previously, it has been reported that electron-donating groups functionalized NiPc/CoPc can enhance CO₂R selectivity and stability. In contrast, electron-withdrawing groups functionalized NiPc/CoPc can reduce the overpotential of CO₂R, however, at the expense of the catalyst stability, especially under large current densities (> 200 mA cm^− 2).^24,41. On the contrary, some other studies suggested that the latter can improve both the activity and selectivity of CO₂R^41,42. Based on the above findings, we hypothesize that the strength of the interaction between the Ni-Pc-based catalysts and CNT substrates must also play a critical role, which has been considered previously. Hence, we also assessed NiTCPc/CNTs, which involves an electron-withdrawing group functionalized NiPc, for CO₂R under identical conditions to validate our hypothesis. Notably, we observed that, compared to NiTCPc/nGCNT, both NiTCPc/GCNT-H and NiTCPc/GCNT-L are capable of achieving high CO/H₂ molar ratios (> 2000) and significantly lower overpotentials even at high current densities of 400–500 mA cm^− 2 (Supplementary Fig. 16 and Supplementary Table 5), further demonstrating the crucial role of the defect level on CNTs as catalyst supports. Taken together, we believe that the interactions between Ni-Pc-based catalysts and CNTs do not primarily result from the π-π interactions. Instead, the proper defect level on CNTs plays a crucial role in tuning the catalytic performance, and these defects might have stronger interaction with the molecular catalysts and influence their structures and catalytic performance during electrocatalysis.

Exploring the effects of Catalyst/CNTs Interactions during CO₂R

We first employed the technique of cyclic voltammetry (CV) to investigate the differences in redox behaviors of NiPc-based catalysts on GCNTs and nGCNT. As shown in Fig. 2a, in N₂ saturated electrolyte, the two reduction peaks at 0.75 V and 0.17 V vs. RHE could be assigned to the one-electron transfer process of NiTAPc⁻ and NiTAPc²⁻, respectively^44,45. Notably, there is a diminishing trend in the intensity of the redox peaks in the CVs, ranging from GCNT-H to nGCNT. Additionally, this trend is consistent with that of NiTCPc (Fig. 2b), when switching the substrates from GCNT-H to nGCNT. The substantially reduced redox peak intensity of NiPc molecules on nGCNT suggests that the intrinsic redox properties of the NiPc molecules are diminished upon immobilization on the defects of CNT. This difference might be associated with enhanced interactions between the molecular catalysts and the CNT substrate, as demonstrated in recent literature⁴⁶.

We employed X-ray absorption spectroscopy (XAS) to investigate the coordination environment of the Ni active center in the phthalocyanine plane on CNTs, especially during the catalysis⁴⁷. Take NiTAPc as an example, its ex-situ X-ray absorption near-edge structure (XANES) and R-space extended X-ray fine structure (EXAFS) spectra on different CNTs are presented in Fig. 2c and Supplementary Fig. 20. Clearly, the NiN₄ active-centers in the three catalyst-composites exhibit similar coordination states. The arrow-labeled region, positioned at approximately 8351 eV and likely associated with the 1s→4p_{x, y} transition, shifts to lower energy as we move from NiTAPc/GCNT-H to NiTAPc/nGCNT. This shift confirms the deviation from perfect planar structure^48–50. NiTCPc counterparts also exhibit similar trend (lower panel, Fig. 2c). To monitor the structural changes/evolutions of the NiN₄ centers under real CO₂R conditions, we employed the same flow cell used for assessing the CO₂R performance to conduct in-situ electrochemical XAS (Supplementary Fig. 21). Encouragingly, we managed to obtain robust XAFS data under practical relevant current densities (i.e., 70 ~ 200 mA cm^− 2). Note, due to the significantly low loading and the single-atom nature of the Ni-catalysts (Supplementary Table 1), we believe that the XAFS can accurately represent the authentic state of the active sites during real CO₂R electrolysis.

Specifically, we focus on analyzing three moieties of the XANES of these NiTXPc/CNTs (Fig. 2d, 2g). The moieties of a (~ 8333 eV) and b (~ 8339 eV) correspond to the pre-edge associated with quadrupole-allowed 1s → 3d − 4p and dipole-allowed 1s → 4p_z shakeup transition, respectively. These characteristics are the fingerprints of a typical square-planar NiN₄ with D_4h symmetry^51–56. Moiety c (~ 8351 eV) within the white line region corresponds to the 1s→4p_{x, y} transition as mentioned above. As shown in Fig. 2e, the intensity of a, b, and position of c of the XANES spectra of the NiTAPc/GCNT-H during CO₂R only exhibit subtle deviations. This indicates that the initial planar symmetry of the NiN₄ is mostly retained during CO₂R under a small current density (~ 145 mA cm^− 2). When the current density was increased to 230 mA cm^− 2 under elevated cathodic potential, the increased intensities of pre-edge a and peak c in the XANES spectra revealed a more diminished D_4h symmetry of the NiN₄ moiety. Considering our previous hypothesis that the interaction between molecule and defect is predominant, we can conclude that the breaking of D_4h symmetry arises from the axial distortion of the Ni-N₄ out-of-plane towards the substrate.

On the other hand, much more pronounced changes in the D_4h symmetry of Ni-N₄ were observed for NiTAPc/nGCNT under the same conditions (lower panel, Fig. 2d), as evidenced by the significantly increased intensity of pre-edge peak a, diminished intensity of peak b and deviated energy of moiety c (Fig. 2e). Additionally, the oxidation state of Ni slightly decreases, as evidenced by a red shift of its absorption edge energy. These changes suggest a more drastic distorted NiN₄ plane under negative potential.

Thermal treatment has been observed to induce distortion of the M-N₄ plane in M-Pc molecules on carbon substrates^53,57. Therefore, we conducted a similar calcination treatment on NiTAPc/GCNT-H to verify our hypothesis regarding the assignment of symmetry breaking to the distortion of the Ni-N₄ plane. As shown in Supplementary Fig. 22, after one-hour calcination at 330℃ in argon, NiTAPc/GCNT-H exhibits a XANES curve more closely resembles that of NiTAPc/nGCNT instead of the as-prepared NiTAPc/GCNT-H. This result suggests that the observed symmetry breakings in our systems likely resulted from the out-of-plane distortion of the Ni-N₄ center.

Furthermore, R-space extended X-ray absorption fine structure (EXAFS) spectra of NiTAPc/GCNT-H and NiTAPc/nGCNT are analyzed. As shown in Fig. 2f, Supplementary Fig. 23, and Supplementary Table 3, both Ni − N lengths and coordination numbers (CNs) of NiTAPc/GCNT-H and NiTAPc/nGCNT exhibit a slight increase under cathodic potential, indicating that the negative potential will promote the out-of-plane distortion of Ni. In the case of NiTAPc/GCNT-L (Supplementary Fig. 24a), we observed intermediate changes in peaks a, b, and c, indicating a moderate distortion of Ni-N₄ during CO₂R. A similar trend is observed in the case of NiTAPc/GCNT-L (Supplementary Fig. 24b). Overall, a clear correlation between the degree of NiN₄ distortion and the defects on CNTs was determined: NiTAPc/GCNT-H < NiTAPc/GCNT-L < NiTAPc/nGCNT, which corresponds well with their selectivity and activity towards CO₂R.

To further consolidate the above observations, we recorded the XANES of NiTCPc on different CNTs during CO₂R. In contrast to the case of NiTAPc/GCNT-H, slightly more profound changes are observed in the XANES curves of NiTCPc/GCNT-H when obtained at cathodic CO₂R potentials compared to those at OCV (Fig. 2g). The pre-edge peaks a and b both deviate from the original positions (Fig. 2h), indicating a more diminished D_4h symmetry. Besides, the red shift of moiety c and absorption edge position suggests the reduced oxidation state of Ni during CO₂R. As anticipated, NiTCPc/nGCNT exhibits much more drastic changes in the above characteristic regions compared to that of NiTAPc/nGCNT under identical conditions (Fig. 2h). Aside from the similar red shift of the absorption edge position, the characteristic fingerprints of square planar D_4h symmetry at a, b and c almost disappeared, suggesting a drastic structural change of the Ni catalyst during CO₂R. As a result, we can conclude that a much stronger interaction between NiTCPc and CNT substrate defects leads to substantial structural changes in NiTCPc under cathodic bias, likely due to its electron-withdrawing functionalization. Nonetheless, we believe that, when subjected to cathodic potentials, the electric field will induce detrimental distortions to the NiN₄ plane. This, in turn, is expected to result in CO₂R failure, especially under larger current densities, consistent with the aforementioned instability of the NiPc catalyst substituted with electro-withdrawing groups during CO₂R²⁴.

Furthermore, we compared the XANES data of NiTAPc/CNTs with that of NiTCPc/CNTs. As shown in Fig. 2i and 2j, it is evident that both NiTAPc/GCNT-H and NiTAPc/nGCNT exhibit a great degree of preservation in their characteristic square planar structure compared to their NiTCPc counterparts under similar, if not identical, CO₂R conditions. Despite the distinct functional groups, the absorption edge energy of Ni for NiTAPc/nGCNT and NiTCPc/nGCNT are very close, with an estimated average Ni oxidation state between 0 and + 2, suggesting a predominant interactions between Ni active-center and the CNT substrates^58,59. Nevertheless, the more profound decrease in the valence state of the Ni active-centers during CO₂R on NiTXPc on nGCNT is associated with their more distorted Ni-N₄ plane. This observation unveils a stronger interaction between NiTXPc and nGCNT. Taken together, the XAFS analysis supports our hypothesis that NiTXPc molecules have higher binding affinity to defects rather than perfect graphene-like CNT surface, while Ni-Pc catalyst with electron-withdrawing groups presents stronger interaction with defects.

We also recorded XANES for the post-electrolysis sample of NiTCPc/GCNT-H. As shown in Supplementary Fig. 26, while the energy of the absorption edge almost returns to its initial state, the coordination environment of the Ni active center cannot fully recover. This result indicates that either electric field induced structural distortion or the partial reduction of ligands on the Pc ring may not be entirely reversible. However, we believe the former one has more pronounced effects on the Ni-N₄ symmetry and could be the predominant reason for the observed changes. In conclusion, the in-situ XANES experiments validate our hypothesis on the correlation between interactions of the Ni-Pc catalyst/CNTs and CNTs defects. Specifically, defects on CNTs induce the distortion of the planar NiN₄ coordination environment under cathodic potentials, which promotes the undesired HER and compromises the catalyst’s stability towards CO₂R. Additionally, electron withdrawing group functionalized Ni-Pc catalysts exhibit a stronger affinity for defects, resulting in more pronounced structural distortions, and thereby leading to increased degradation and reduced stability when high cathodic potential is applied to afford high current densities.

We also carried out in-situ IR studies to track the reaction intermediates on Ni-Pc catalyst on different CNTs during CO₂R using ATR-SEIRAS. As shown in Fig. 3a-b, both NiTAPc/GCNT-H and NiTAPc/nGCNT exhibit in-situ IR signals for the reaction intermediate of *COOH (C − O stretch signal located at a wavenumber of 1380 cm^− 1, symmetric stretch of *COO⁻ located at 1410 cm^− 1)^60,61, indicating the same reaction mechanism for CO₂R. Note, the prominent peaks at approximately 1640 cm^− 1 are H − O−H bend vibration signals from H₂O^60,62. To better interpret the trend of *COOH, we plot the normalized *COOH peak intensity against the cathodic potential applied. As shown in Fig. 3c, the *COOH signals for NiTAPc/GCNT-H emerge at a lower overpotential compared to that of NiTAPc/nGCNT. Additionally, they exhibit an accelerated augmentation rate, particularly at more cathodic potentials. These findings align well with previous observations, indicating that NiTAPc supported on GCNT-H displays a lower overpotential for CO₂R. As anticipated, the in-situ FT-IR spectra of NiTCPc/GCNT-H and NiTCPc/nGCNT (Fig. 3d-e) exhibit a similar trend to that of the NiTAPc analogue. Notably, the much more accelerated *COOH peak intensity (Fig. 3f) provides compelling evidence that aligns with the above CO₂R performance data. To exclude the interference from carbonate or bicarbonate, which also feature similar O-C-O bonds, we conducted analogous in-situ IR studies in phosphate buffer saline (PBS) with pH = 8. As shown in Supplementary Fig. 28, all four catalysts exhibit similar spectra to those recorded in KHCO₃, validating that the corresponding peaks indeed originate from CO₂R intermediates. Furthermore, to investigate the possible peaks covered by the strong H₂O signals, we conducted deuterium isotopic in-situ IR. The spectra of NiTAPc/GCNT-H and NiTCPc/GCNT-H, presented in Supplementary Fig. 29, indicate that both catalysts show similar spectra to those observed in the protium isotopic solution in the 1370 ~ 1410 cm^− 1 region. The additional appeared to signal at around 1580 cm^− 1 could be indexed to the asymmetric COO⁻ species⁶⁰, which is in good agreement with protium isotopic ATR-SEIRAS.

We conducted spin-polarized DFT-based calculations to investigate the interactions between the Ni-Pc catalysts and their possible structural changes upon adsorption. Specifically, a 10×10 supercell of graphene (Gr) was constructed as the model substrate due to the large diameters of CNTs (over 15 nm) compared to Ni-Pc-based catalysts, with different amounts of carbon vacancies introduced onto the graphene supercell as defects. Figure 4a illustrates the relaxed geometry of the immobilized NiTAPc on perfect Gr (NiTAPc@Gr, Fig. 4b) with a binding energy of − 2.59 eV, including van der Waals correction. Upon introducing a single vacancy on Gr (1V-Gr), the adsorption of flat NiTAPc molecule (NiTAPc@1V-Gr-flat) is favored by approximately 0.11 eV (Fig. 4d). This result aligns with the HRTEM images in Fig. 1, confirming that NiTAPc prefers absorption on the defect-rich domains on CNTs, further ruling out the possibility of π-π stacking. Notably, we discovered a novel configuration of distorted NiTAPc on 1V-Gr (NiTAPc@1V-Gr-dis) as shown in Fig. 4c. The difference in adsorption energy between the flat and distorted structures is as small as 0.03 eV, indicating that these two configurations are energetically degenerated and can interconvert under mild conditions. Hence, we believe the above XAS characterizations record the combination of NiTAPc in two states, with the ratio between them serving as a representation of the prevailing structural composition. Similar configurations were also acquired for NiTCPc (Supplementary Fig. 30). The adsorption energy differences between flat and distorted conditions for both NiTAPc and NiTCPc were summarized in Fig. 4d. As can be seen, while both NiTAPc and NiTCPc exhibit favorable adsorptions on defected CNTs, NiTCPc@1V-Gr-dis shows slightly enhanced adsorption (in terms of adsorption energy) than its flat counterpart, in contrasts to NiTAPc, suggesting a stronger interaction between Ni center and defects on CNTs.

We then investigated the mechanism of the potential-induced catalyst distortion. As shown in Fig. 4e, the charge redistribution analysis of both NiTXPc@1V-Gr-dis shows positive charge accumulation at the Ni site, resulting in a dipole moment of 0.19 Debye and 7.97 Debye for NiTAPc and NiTCPc, respectively, pointing from the substrate to the Pc plane. Therefore, when a cathodic bias is applied, the Ni coordination center tends to move towards the CNTs substrate from the initial Pc plane, facilitating the Pc plane distortions. Additionally, compared to NiTAPc, the significantly larger dipole moment of NiTCPc indicates its stronger interaction with defects on CNTs, resulting in more drastic structural distortions under similar cathodic bias.

Based on these configurations, we simulated the corresponding XANES spectra (Supplementary Fig. 31a-b). Specifically, the intensity of the pre-edge a exhibit a slight increase for both NiTAPc@Gr and NiTAPc@1V-Gr-flat. In contrast, significant increase in the intensity of the pre-edge a was observed for the 1V-Gr-dis configuration (Note 1 and 2 in Supplementary Fig. 31). Notably, similar observations were made for the NiTCPc systems. The simulated XANES further supports that the increased intensity of pre-edge a observed experimentally (Fig. 2d, g) should be ascribed to the interaction of Ni with substrate defects. The fitting of EXAFS in R-space based on the NiTAPc@1V-Gr-dis model (Supplementary Fig. 31c) indicates that the Ni − N and Ni − C have similar coordination number (CN) and bond lengths compared with the Ni − N path fitted using NiTAPc@1V-Gr-flat as the model. The consistency in bond lengths observed in the simulated structures (Supplementary Table 4) also agrees with the above EXAFS fittings.

Moreover, our simulations also reveal that the structural distortion substantially changes the electronic structures of the Ni active center near the Fermi level (E_Fermi). As shown in Fig. 4f-g, the partial density of states (PDOS) of NiTAPc@1V-Gr-dis reveals that the Ni d_z² orbital has split and moved closer towards E_Fermi in comparison to that of NiTAPc@1V-Gr. In general, increased electronic density around E_Fermi benefits the corresponding electrocatalytic activity. We then calculated the CO₂ molecule adsorption energy for three configurations. The results, as shown in Fig. 4h, show that CO₂ binds more strongly on NiTAPc@1V-Gr-dis, with the adsorption enhanced by 0.13 eV and 0.16 eV compared to two flat configurations, respectively. Similarly, NiTCPc assembles the same trend (Fig. 4h). This enhanced CO₂ adsorption would favor the subsequent CO₂R, highlighting the important role of having certain defects on CNTs in improving the CO₂R activity. On the other hand, our calculations revealed that in the case of HER, the Gibbs free energy of *H-NiTAPc@1V-Gr-dis also decreases when distortion occurs (Supplementary Fig. 32), potentially promoting competing HER and compromising CO₂R selectivity. We believe this distortion is attributed to the interaction between the NiPc-plane and substrate defects, which is facilitated by cathodic potentials. Consequently, the defect level can be used as an effective strategy to tune the number of distorted NiPc-planes, which in turn affects the catalyst performance. The central goal of this work is to prove that both CO₂R activity and selectivity can be drastically improved by controlling the substrate defect level. As for the nGCNT substrate, given its high defect level (Fig. 1c), we proposed a possible model shown in Supplementary Fig. 33. Under this circumstance, one of the adjacent NiTAPc molecules becomes substantially curved, leading to poor structural stability under more cathodic bias and promoting HER. Overall, our computational results further confirm and explain the effects of defects on CNTs in both adsorption and catalysis processes.

Enhancing CO₂R Through Catalyst/CNTs Interactions

Future implementation of CO₂R catalysts requires them to demonstrate great stability and reduced voltage under practical relevant operation conditions^63–65. Besides, in a typical CO₂R process in an alkaline electrolyte, losses of CO₂ due to carbonation can lead to decreased CO₂ utilization efficiency. Conducting CO₂R in acidic media, on the other hand, is advantageous for reducing unnecessary losses and enhancing CO₂ utilization^66,67. However, a high concentration of proton in acidic electrolyte kinetically facilitates the HER, underscoring the importance of catalyst selectivity towards desired CO₂R products^68–70. Therefore, we performed CO₂R on NiTAPc/GCNT-H(/L) in the acidic electrolyte (pH = 2). As shown in Fig. 5a, both catalyst-composites exhibit outstanding selectivity towards CO even in acidic media, close to the high selectivity we observed in alkaline electrolytes. Besides, similar trends in CNT defect-level dependent selectivity and activity were observed in acidic CO₂R (Fig. 5a, Supplementary Fig. 34), demonstrating the importance of controlling the defect level of CNT substrates. Previous literature revealed the local pH increase at the cathode-electrolyte interface during electrolysis under large current densities which will lead to an increase of bulk pH^67,69, therefore we used sulfuric acid as anolyte and proton exchange membrane to maintain the bulk pH of the catholyte during CO₂R. Consequently, we carried out a stability assessment in acidic electrolyte for NiTAPc/GCNT-H (Supplementary Fig. 35). Notably, we observe that the catalyst maintains stability at 200 mA cm^− 2 for over 20 hours with negligible changes in CO selectivity. Additionally, the use of acidic electrolytes also enabled us to precisely evaluate the single-pass conversion (SPC) of CO₂ in our system⁷¹ (Supplementary Fig. 36). Encouragingly, a high SPC of approximately was achieved with constantly high CO selectivity (CO/H₂ ratio > 300) at the current density of 300 mA cm^− 2 using NiTAPc/GCNT-H.

Aside from the effect of defects, we also investigated the impact of functional groups on CNT during CO₂R. Taking − OH and − COOH functionalized CNTs (both non-graphitized and graphitized) as examples, we immobilized NiTAPc on them via the same methods (denoted as NiTAPc/GCNT-X or NiTAPc/CNT-X, X = − OH or − COOH). Similar diameter and morphology of these CNTs were confirmed by HRTEM (Supplementary Fig. 37). Besides, the introduction of these functional groups has negligible influence on the defect level of these CNTs (Supplementary Fig. 38). As shown in Fig. 5b, both NiTAPc/GCNT-X show higher CO selectivity compared to that of NiTAPc/GCNT-H under the same conditions. Remarkably, a high CO/H₂ molar ratio of 16100:1 was achieved on NiTAPc/GCNT-OH at a high current density of 500 mA cm^− 2. Note, to ensure precise product quantification, we conducted careful GC calibration and optimized the product collection procedures (Supplementary Fig. 39). Hence, we could claim that this catalyst-composite technically switches off the competing HER and achieves stoichiometric CO₂R. Besides, when compared to non-graphitized counterparts (Supplementary Fig. 40), NiTAPc/GCNT-X also demonstrates reduced overpotentials and significantly suppressed HER (on the orders of two magnitudes). Compared with NiTAPc/GCNT-H, we can conclude that the influence of functional groups at the terminal of GCNT on CO₂R is not significant. Therefore, the different defect densities should account for the distinct CO₂R selectivity and activity.

The in-situ IR and XANES spectra of the two NiTAPc/GCNT-X were collected to further explore the CNT-functional-groups effect (Supplementary Fig. 41–42). Both catalyst-composites, NiTAPc/GCNT-OH and NiTAPc/GCNT-COOH, exhibited similar *COOH intermediate IR signals at similar potentials, suggesting similar reaction kinetics with NiTAPc/GCNTs. XANES of NiTAPc/GCNT-X retains the pristine planar characteristics, which is also similar to NiTAPc/GCNT-H. However, by analyzing the water contact angle on each catalyst-composites (Supplementary Fig. 43), we found that NiTAPc/GCNT-X exhibits a higher water contact angle before and after electrolysis. Therefore, we attribute the enhanced performance of CO₂R to the improved hydrophobicity of the electrode, which facilitates CO₂ transport across GDE to the reaction interface, thereby reducing the concentration overpotential for CO₂R.

We also calculated turnover frequency (TOF) for our catalysts firstly based on the weight percentage of Ni assuming all Ni sites are active, and compared them with the TOFs of recently reported CNT-heterogenized molecular catalysts for CO₂R^18,24,40,72. As shown in Fig. 5c, we plotted the TOF_CO together with $\:{\text{FE}}_{{\text{H}}_{\text{2}}}$ to demonstrate the selectivity of a given catalyst under different reaction rates. Clearly, our strategy of engineering the defect level of the CNT-substrates leads to remarkable advantages in both activity and selectivity. Additional TOF comparisons were made using surface active-sites density (Supplementary Fig. 47). In this scenario, NiTAPc/GCNT-COOH exhibits approximately four orders of magnitude inhibition in HER and the highest TOF_CO of 1072 s^− 1, surpassing other state-of-the-art systems, such as NiTAPc/Vulkan XC72R (Supplementary Fig. 46). Overall, we demonstrate the significance of catalyst-substrate interactions, which can significantly alter catalytic performance.

The use of zero-gap MEA electrolyzer for CO₂R shows great promise towards practical applications^73–76. Hence, we also evaluated the performance of NiTAPc/GCNT-H in an MEA reactor with electrode size of 100 cm² operated at ampere-level (Fig. 5d). We also assessed the state-of-the-art Ag catalyst (~ 100 nm nanoparticles) identical conditions for comparison. Notably, an almost 100% FE_CO could be obtained at a total current as high as 40 A using NiTAPc/GCNT-H. Besides, the CO selectivity remains high (> 95%) at an even higher current of ~ 50 A. In contrast, CO₂R selectivity of Ag-based MEA dropped dramatically at these large currents, with $\:{\text{FE}}_{{\text{H}}_{\text{2}}}$ exceeding 20% under the same conditions. Additionally, MEA reactor based on NiTAPc/GCNT-H exhibits consistently lower cell voltage compared to the Ag-based MEA, further highlighting the promise of our catalyst composites. Moreover, we conducted a stability test in the scaled-up MEA electrolyzer and found that NiTAPc/GCNT-H could retain the nearly 100% CO selectivity for over 30 hours of continuous operation at a high current of 20 A (Fig. 5e). The minor selectivity drop at the end of the test was likely due to salt formation in the cathode chamber, a persisted challenge for anion-exchange membrane type MEA-based CO₂R^77–79. Nevertheless, the NiTAPc/GCNT catalyst composite clearly demonstrates significant promise for practical implementations.

Lastly, we extended our strategy to Co-Pc analog, cobalt tetraaminophthalocyanine (CoTAPc), which has demonstrated methanol production activity, to assess its efficacy for CO to methanol conversion^21,80. As shown in Fig. 5f, under identical testing conditions, CoTAPc/GCNT-H exhibits a higher $\:{\text{FE}}_{{\text{CH}}_{\text{3}}\text{OH}}$ and more importantly substantially lower overpotential at all current densities assessed, particularly at high current densities. Therefore, we believe that the catalyst-substrate interactions we have demonstrated here are applicable to other molecular catalysts and likely to other electrochemical applications as well.

In summary, we have elucidated the nature of the interaction between molecular catalysts and CNT substrates. Contrary to the conventionally believed π-π stacking, our findings reveal that the molecular catalyst tends to adsorb on the defects on CNT via electrostatic interactions. Based on this finding, taking NiPc-based catalysts as examples, we established remarkable correlations between CO₂R performance and the defect levels of CNT substrates. Specifically, we have determined that maintaining low to moderate defect levels on CNTs is desirable for achieving selective and active CO₂R. Through the carefully designed in-situ XAFS, we were able to track the structural distortions of the NiN₄ square-planar coordination of NiPc-based catalysts on CNTs with varying defect levels during CO₂R under high current densities. In particular, we observed that Ni-Pc-based catalysts exhibited less structural distortions on graphitized CNTs (low defect density) compared to non-graphitized CNTs (high defect density) during CO₂R. Besides, NiPc-based catalysts functionalized with electron-withdrawing groups exhibit more pronounced distortions than their counterparts containing electron-donating functional groups, suggesting a stronger catalyst/substrate interaction. Additionally, we found that the dipole interaction between the Ni centers and the CNT surface defects plays a significant in promoting these structural distortions. This could be the predominant reason for catalytic failure if the catalyst/interactions are excessively strong and highly negative cathodic potentials are applied. Inspired by these findings, we further optimized the CNTs substrates and achieved an extraordinary CO₂R selectivity (16100:1 CO/H₂ molar ratio) and high TOF of over 1072 s^− 1 at − 0.60 V vs. RHE for CO production in a flow reactor, and practical relevant high current (up to 50 A) in a MEA reactor with high CO selectivity (> 95%) and reasonably low cell voltage (~ 3.5 V). Furthermore, we extended this design to a CoTAPc/GCNT-H system, showcasing a remarkable methanol production through CO reduction, i.e., over 53% selectivity and high partial current density over 150 mA cm^− 2. Overall, our findings uncover the nature of planar molecular catalyst/CNT interactions and underscore a critical correlation between the above interactions and the performance of the catalyst. We anticipate that these insights can serve as a guide for future designs aimed at enhancing performance by modulating the interactions between molecular catalysts and CNTs.

Chemicals. Nickel tetraaminophthalocyanine (NiTAPc), nickel tetracarboxylphthalocyanine (NiTCPc), and cobalt tetraaminophthalocyanine (CoTAPc) were synthesized according to our previously modified method (Supplementary Fig. 1)⁸¹. All kinds of nGCNT, GCNTs, and GCNT-X were purchased from XFNano Inc., China. nGCNT was ultrasonicated in 5% HCl for 2 h and then stirred overnight. After that, purified nGCNT was washed with quantities of water and ethanol to fully remove acid followed by vacuum drying. Finally, the as-purified nGCNT was annealed in argon under 900 ℃ for 3 h to remove the oxygen residue. Ag powder (~ 100 nm), N, N-dimethylformamide (DMF), concentrated H₂SO₄ (98%) and KH₂PO₄ were purchased from Sigma-Aldrich in ACS analytical grade, KOH (99.99% metal basis), KHCO₃ (99.99% metal basis) and Cs₂CO₃ (99.99% metal basis) were obtained from Aladdin Chemical. KOD (40 wt% solution in D₂O) and D₂O (D, 99.9%) were purchased from Sigma-Aldrich and Cambridge Isotope Laboratories, respectively. Ultrapure water (18.2 MΩ) was used in all experiments.

Preparation of Catalysts. 5 mg of NiTAPc, NiTCPc, or CoTAPc were carefully ground with mortar in 3 mL DMF first and then transferred in a glass bottle and added another 47 mL DMF. After ultrasonication for 10 minutes, 50 mg corresponding CNT, GCNT, or GCNT-X was added, followed by an additional 30 min ultrasonication. The solution was then stirred for 24 hours to finish the preparation. The mixture was centrifuged and washed with DMF and ethanol by several times before vacuum drying. It should be noted that the as-prepared NiTAPc and NiTCPc are highly crystalline so the dissolving to form a uniform solution is difficult.

Materials Characterizations. SEM was performed on JEOL JEM-7610M equipped with an energy dispersive X-ray spectrometer (EDS) and analyzed at 15 kV. TEM was conducted with JEOL JEM − 2100F operating at 200 kV. HAADF-STEM images were obtained on FEI Talos F200X operated at 200 kV. ICP-OES (ThermoFisher CCAPG200 DUO) was used for Ni weight percentage determination. XPS was performed on scanning X-ray microprobe (Kratos Axis Ultra DLD) operated at 15 kV, 1486.71 eV with monochromated Al Kα radiation. Binding energies were corrected by reference to the sp³ C 1s peak at 284.8 eV. Raman spectra were acquired using a homemade SpectroPro HRS500-based Raman spectrograph.

Electrochemical CO ₂ reduction tests. All electrocatalytic tests were conducted using BioLogic VMP-3e potentiostat. A typical three-electrode flow cell (Supplementary Fig. 9, 1 cm² electrode area) was used for all GDE measurements. Ag/AgCl was chosen as the reference electrode and IrO₂ loaded Ti mesh (prepared according to literature⁸²) was used as the counter electrode. Fumasep FAA-3-PK-75 anion exchange membrane (Fuel Cell Store) separated the cathodic and anodic chambers. For catalyst loading, 10 mg NiTAPc/CNT was first dispersed in 1 mL 3:1 ethanol/water mixture to form a uniform ink, then 70 µL 5% Nafion (D520, 5 wt%, Fuel Cell Store) was added and the ink was ultrasonicated for 30 min using an ultrasonic homogenizer (1000 W). A certain amount of ink was drop-cast on GDL (YLS30T, Suzhou Sernos, 12% PTFE treated after purchase) to achieve a loading weight of 1 mg cm^− 2. The GDL was dried in an oven under 100 ℃ then. Chelex-100 resin (Na-form, 50 − 100 mesh, Sigma-Aldrich, converted into K-form according to literature⁸³ before use) treated 1 M KOH was used as electrolyte. Two peristaltic pumps were used for the cycling of catholyte and anolyte. The circulation rate was 13 mL min^− 1. CO₂ (99.995%, Air Liquide) flow rate was controlled by a mass flow controller and set as 25 sccm during electrocatalysis. In a typical experiment, multiple times of chronopotentiometric tests (10 min intervals) were conducted with corresponding injections to GC to determine the gas products for every run. For every test, Reverse hydrogen electrode (RHE) potential was calculated by Nernst equation with iR compensation: E (RHE) = E_measured + E^ϴ(Ag/AgCl) + 0.05916×pH − iR. Contact impedance was determined by electrochemical impedance spectroscopy (EIS) recorded at the corresponding potential.

The gas products were analyzed using an in-line Shimadzu GC − 2014C chromatograph equipped with a thermal conduction detector (TCD) and two flame ionization detectors (FID). To ensure the accuracy of our evaluation, we used only TCD for the determination of H₂ and CO to avoid the inaccuracy of FID from large amount detection. Calibration curves were carefully made for both gases, as shown in Supplementary Fig. 9, with particular attention paid to accurately detecting tiny amounts of H₂. The Faradaic efficiency (FE) of H₂ and CO could be calculated by the following equation:

$$\:\text{FE}\left(\text{\%}\right)=\frac{n\cdot\:\text{F}\cdot\:f\cdot\:{x}_{n}}{j}$$

Where n is the number of transferred electrons (for H₂ and CO, n = 2), F is Faraday constant (96485.3 C mol^− 1), f is the flow rate of outlet stream from the flow cell in molar, x_n is the volume fraction of H₂ or CO acquired by GC. j is the current density. The outlet gas was injected into GC after 7 minutes of electrolysis.

For electrocatalysis in acidic media, 0.5 M K₂SO₄ with pH = 2 was used as an electrolyte. This electrolyte was prepared by adding concentrated H₂SO₄ into 1 M KHCO₃ until pH reached 2.00 (monitored by a Horiba LAQUA PH1300 pH meter). 0.5 M H₂SO₄ was used as anolyte. Nafion 115 (Fuel Cell Store) was used as a membrane. During the electrolysis, the shuttled protons could maintain the bulk pH of the catholyte, otherwise, a rapid increase of bulk pH under large current densities would be observed.

Performance evaluation in zero-gap electrolyzer was carried out using MEA (2×2 cm² and 10×10 cm²). A stability test was carried out using MEA electrolyzer (10×10 cm²). QAPPT and 10% QAPPT ionomer were used as membrane and binder, respectively. IrO₂-loaded Ti mesh was used as the anode. Anolyte is 0.1 M KOH. Preparation of GDE for MEA is similar to that for flow cell but using airbrush instead of drop-cast.

Apparent turnover frequency (TOF) value was calculated based on overall Ni loading weight:

$$\:{\text{TOF}}_{\text{appr}}\left({\text{s}}^{-\text{1}}\right)=\frac{{n}_{\text{C}\text{O}}}{{n}_{\text{N}\text{i}}}=\frac{{j}_{\text{CO}}}{{2\text{F}\bullet\:}_{\frac{w\%\bullet\:{m}_{\text{N}\text{i}}}{{M}_{\text{N}\text{i}}}}}$$

while j_CO is CO partial current density, F is Faraday constant, w% is Ni weight percent, and m_cat is the overall catalyst loading weight on the electrode (1 mg cm^–2). M_Ni is molar mass of Ni (58.69 g mol^–1).

TOF based on concentration of surface-active sites are evaluated according to the following equation:

$$\:\text{TOF}\left({\text{s}}^{-\text{1}}\right)=\frac{{n}_{\text{C}\text{O}}}{{n}_{\text{N}\text{i}}}=\frac{{j}_{\text{CO}}}{{2\text{F}\bullet\:\varGamma\:}_{0}}$$

while j_CO is CO partial current density, F is the Faraday constant, and Γ₀ is the surface concentration (mol cm^–2). Γ₀ can be evaluated by the following procedure:

Cyclic voltammograms from 40 mV/s to 100 mV/s were collected in CO₂-saturated 0.2 M KHCO₃ with a pH of 6.8. The surface concentration of redox-active Ni centers was extracted from the slope of the linear relationship between the current densities of the reduction wave peak and the scan rate (𝜐)⁸⁴:

$$\:\text{slope}=\frac{{n}^{2}{\text{F}}^{2}{\varGamma\:}_{0}}{4\text{R}T}$$

where n = 1, F is the Faraday constant, R is the ideal gas constant (8.3145 J mol^− 1 K^− 1), and T is the temperature (298.15 K). Therefore, Γ₀ could be obtained from the slope. Supplementary Fig. 45–46 present the CV and linear fitting plots of different catalysts.

XAFS characterization. Ex-situ X-ray absorption fine spectroscopy (XAFS) of Ni K-edge was conducted at BL14B2 of SPring-8 in fluorescence mode using a 19-element Ge solid-state detector. In-situ XAFS of Ni K-edge was conducted on XAFCA beamline of Singapore Synchrotron Light Source (SSLS) in fluorescence mode using conversion electron yield detector and BL14B2 of SPring-8 in fluorescence mode using 7-element silicon drift detector. The flow cell mentioned above with minor modification was employed for in-situ study to realize more similar conditions to our electrochemical performance assessment (Supplementary Fig. 21). Electrochemical tests were carried out with BioLogic SP-200 potentiostat. Catalyst composite loading was around 2.3 mg cm^− 2. Other conditions were consistent with the flow cell test. XAS data were processed and analyzed using Athena from Demeter software package⁸⁵. Simulations of XANES were carried out with FDMNES code employing configurations from DFT calculations⁸⁶.

ATR-SEIRAS measurements. In-situ attenuated total reflectance-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) tests were conducted in a home-designed spectro-electrochemical cell with a three-electrode configuration. The catalyst was drop-casted onto the gold film chemically deposited on Si ATR crystal as the working electrode (Supplementary Fig. 28), and a graphite rod and an Ag/AgCl electrode served as the counter and reference electrode, respectively. During the test, CO₂ gas was vigorously bubbled into the 0.2 M KHCO₃ electrolyte. SEIRAS was recorded using Shimadzu IRTracer-100 equipped with a liquid-nitrogen-cooled mercury cadmium telluride (MCT) detector and a PIKE VeeMAX III ATR accessory. All spectra were collected at an 8 cm^− 1 spectral resolution and presented in transmittance, where a negative and positive peak signifies an increase and decrease in the interfacial species, respectively. All SEIRA spectra presented in this work correspond to 20 co-added scans lasting about 12 seconds. 0.2 M KDCO₃/D₂O solution was prepared by purging CO₂ into 0.2 M KOD/D₂O solution overnight.

Computational Method

All the first-principles calculations were conducted with spin-polarized density functional theory (DFT) by utilizing the Vienna ab-initio simulation package (VASP)^87,88. For the studies of CO₂R and HER, including geometry relaxations, total electronic energy calculations, and vibrational frequency calculations, the generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerh (PBE) format⁸⁹, the projector-augmented wave (PAW) pseudopotentials⁹⁰ and a plane wave basis with the cut-off energy of 450 eV are employed. Van der Waals force (DFT + D3) is also included in the reaction simulations⁹¹. The convergence criteria for electronic steps and structural relaxations were set to 10^− 4 eV and 0.01 eV/Å, respectively.

The zero-point energy (ZPE) and entropy (S) of all species except for small gas-phase molecules along the reaction paths are obtained by vibrational frequency calculations using the harmonic approximation. The values of small gas-phase molecules including CO₂, CO, H₂, and H₂O are referenced from the NIST database. 298.15 K is chosen as Temperature (T) in Gibbs free energy correction calculations. The evaluation of the electrocatalytic properties was carried out under the computational hydrogen electrode (CHE) formalism by using the relation of H⁺ + e⁻ = 1/2H₂.

The graphene substrate is modeled as a 10×10 supercell in all calculations with a vacuum layer of 20 Å in the direction perpendicular to the graphene sheet to avoid artificial interaction between the neighboring images. Gamma-point sampling was applied in all calculations. The systems of single carbon vacancy graphene are modeled by removing one carbon atom of a 10×10 supercell of graphene.

The Visualization for Electronic and Structural Analysis (VESTA) software⁹² was used for visualization of the atomic structures.

Data availability

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

Competing Interests

The authors declare no competing interests.

Acknowledgments

We acknowledge the National University of Singapore and the 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). We also acknowledge the National Natural Science Foundation of China (22302204), the China Postdoctoral Science Foundation (2023M743429 and 2023TQ0350), Postdoctoral Fellowship Program of CPSF (GZC20232588). We also thank Dr. Takeshi Watanabe at SPring-8 for his assistance on XAFS characterization, and Dr. Zhang Jia and Dr. Li Yuke for their help on the adsorption energy computations. Computations were done in NUS high-performance clusters and National Supercomputing Center in Singapore.

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Switching off Competing Hydrogen Formation in CO2 Electroreduction via Substrate Defect Engineering

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Abstract

Figures

Introduction

Results

Defect-Driven Adsorption of NiPc-based Catalyst on CNTs

Effects of CNT Defects on Electrochemical CO₂R

Exploring the effects of Catalyst/CNTs Interactions during CO₂R

Enhancing CO₂R Through Catalyst/CNTs Interactions

Discussion

Methods

Computational Method

Declarations

Data availability

Competing Interests

Acknowledgments

References

Additional Declarations

Supplementary Files

Status:

Version 1

Switching off Competing Hydrogen Formation in CO2 Electroreduction via Substrate Defect Engineering

Status:

Version 1

Abstract

Figures

Introduction

Results

Defect-Driven Adsorption of NiPc-based Catalyst on CNTs

Effects of CNT Defects on Electrochemical CO2R

Exploring the effects of Catalyst/CNTs Interactions during CO2R

Enhancing CO2R Through Catalyst/CNTs Interactions

Discussion

Methods

Computational Method

Declarations

Data availability

Competing Interests

Acknowledgments

References

Additional Declarations

Supplementary Files

Status:

Version 1

Effects of CNT Defects on Electrochemical CO₂R

Exploring the effects of Catalyst/CNTs Interactions during CO₂R

Enhancing CO₂R Through Catalyst/CNTs Interactions