Unraveling the Reactivity and Selectivity of Atomically Isolated Metal–Nitrogen Sites Anchored on Porphyrinic Triazine Frameworks for Electroreduction of CO ₂

Open AccessCCS ChemistryRESEARCH ARTICLE1 Oct 2019Unraveling the Reactivity and Selectivity of Atomically Isolated Metal–Nitrogen Sites Anchored on Porphyrinic Triazine Frameworks for Electroreduction of CO2 Ying Hou, Yuan-Biao Huang, Yu-Lin Liang, Guo-Liang Chai, Jun-Dong Yi, Teng Zhang, Ke-Tao Zang, Jun Luo, Rui Xu, Hua Lin, Su-Yuan Zhang, Hui-Min Wang and Rong Cao Ying Hou State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 (China) College of Chemistry and Materials Science, Fujian Normal University, Fuzhou 350007 (China) , Yuan-Biao Huang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 (China) University of the Chinese Academy of Sciences, Beijing 100049 (China) , Yu-Lin Liang University of the Chinese Academy of Sciences, Beijing 100049 (China) Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800 (China) , Guo-Liang Chai State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 (China) University of the Chinese Academy of Sciences, Beijing 100049 (China) , Jun-Dong Yi State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 (China) University of the Chinese Academy of Sciences, Beijing 100049 (China) , Teng Zhang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 (China) University of the Chinese Academy of Sciences, Beijing 100049 (China) , Ke-Tao Zang Center for Electron Microscopy, Institute for New Energy Materials and Low-Carbon Technologies, School of Materials, Tianjin University of Technology, Tianjin, 300384 (China) , Jun Luo Center for Electron Microscopy, Institute for New Energy Materials and Low-Carbon Technologies, School of Materials, Tianjin University of Technology, Tianjin, 300384 (China) , Rui Xu State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 (China) University of the Chinese Academy of Sciences, Beijing 100049 (China) , Hua Lin State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 (China) University of the Chinese Academy of Sciences, Beijing 100049 (China) , Su-Yuan Zhang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 (China) University of the Chinese Academy of Sciences, Beijing 100049 (China) , Hui-Min Wang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 (China) University of the Chinese Academy of Sciences, Beijing 100049 (China) and Rong Cao *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 (China) University of the Chinese Academy of Sciences, Beijing 100049 (China) https://doi.org/10.31635/ccschem.019.20190011 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Electroreduction of CO2 (CO2RR) to value-added chemicals offers a promising approach to balance the global carbon emission, but still remains a significant challenge due to high overpotential, low faradaic efficiency, and poor selectivity of electrocatalysts systems. Thus the key point is to develop low-cost, highly efficient, and durable electrocatalysts for CO2RR. To benefit from their exposed active sites and to maximize atomic efficiency, single-metal atom catalysts that usually show high activities are required. Herein, we unravel the trends in the reactivity and selectivity of atomically isolated M–N4 (M = Ni, Cu, Fe, and Co) sites within porous porphyrinic triazine framework (metal single atoms/PTF) for the electroreduction of CO2 to CO. We found that NiSAs/PTF exhibited the highest faradaic efficiency (98%) at a mild potential of −0.8 V versus reversible hydrogen electrode and the highest turnover frequency of 13,462 h−1 for the production CO at an applied potential of −1.2 V. The relations of catalytic performance of CO2 to CO over the different active M–N4 sites were unraveled by the combination of density functional theory calculations and experiments. This work gives an extensive mechanistic understanding of the selectivity of CO2 to CO from the M–N4 sites at an atomic scale, thus it will bring new inspiration for the design of highly efficient CO2RR. Download figure Download PowerPoint Introduction The large-scale use of fossil fuels has led to a substantial increase in atmospheric carbon dioxide concentration, hence causing environmental problems, including global warming and sea-level rise.1–3 Electrochemical reduction of CO2 (CO2RR) to value-added chemicals and fuels using renewable electricity is a promising strategy to balance the global carbon emission.4–6 Among the electroreduction products, CO is considered one of the most valuable industrial feedstocks, which can be used for the catalytic Fischer–Tropsch synthesis to produce hydrocarbon liquid fuels.7,8 Fortunately, compared with other CO2RR processes, the reduction of CO2 to CO [CO2 + 2H++ 2e− → CO + H2O, −0.11 V vs reversible hydrogen electrode (RHE)] needs only two electron/proton transfers,9 which makes it an essentially less-hindered pathway.10 Nevertheless, due to the low energetic efficiency, sluggish electron transfer kinetic, poor selectivity, and low stability of electrode materials significant challenges remain that impede the practical use and technological commercialization result from the inert chemical bond of CO2 (C = O, 806 kJ·mol−1)11 and the accompanied competing hydrogen evolution reaction (HER).12,13 Furthermore, the high cost and scarcity of noble metal catalysts, such as Au- and Ag-based catalysts, limit their large-scale applications.14,15 Therefore, there is an urgent need to develop highly efficient earth-abundant metal-based electrocatalysts specifically for CO2 to CO conversion with long durability to meet the practical application. Single-atom catalysts (SACs) with atomically isolated metal atoms anchored on supports usually show high activity and selectivity toward various catalytic performances because of their maximum atom efficiency, exposed active sites, and tunable coordination microenvironment.16–18 Thus, SACs anchored on conductive materials such as porous carbons and graphene have displayed great potential to enhance their performance in CO2RR.19–22 However, the preparation of stable and highly efficient SACs remains a great challenge because of the high surface energy of their single atoms and easy formation of inactive aggregates.23,24 Moreover, mechanistic understanding of the relationships between the CO2RR reactivity and the structures of SACs is still elusive, which impedes further designing of highly efficient SACs for the CO2RR.25–27 Porous covalent triazine frameworks constructed by 1,3,5-triazine linkers have attracted much attention in gas adsorption and heterogeneous catalysis because of their high specific surface area, high nitrogen contents, and remarkable acid/base stability.28–30 Recently, our group reported that the porphyrin architectures stabilized Fe/Co single atoms within porphyrinic triazine frameworks (PTF), which can behave as accessible active sites for highly efficient electrocatalysis for oxygen reduction reaction and HER.31,32 Herein, a series of atomically isolated M–N4 sites anchored on highly porous porphyrinic triazine framework [metal single atoms (MSAs)/PTF, M = Ni, Cu, Fe, and Co] were prepared and employed as electrocatalysts for CO2RR. Among these single-atom metal elements, Ni material, NiSAs/PTF, was found to have a very high faradaic efficiency (FE), between 94% and 98% for CO generation over a wide range of applied potential (−0.6 to −1.2 V) versus RHE (RHE scale was used as a reference for all applied potential scales). Impressively, NiSAs/PTF reached a high turnover frequency (TOF) of 13,462 h−1 at a mild applied potential of −1.2 V. Density functional theory (DFT) calculations revealed that NiSAs/PTF and FeSAs/PTF have more positive estimated electrochemical limiting potentials than those of CoSAs/PTF and CuSAs/PTF. The rate determining step for NiSAs/PTF, CoSAs/PTF, and CuSAs/PTF is the *COOH intermediate formation, whereas, with FeSAs/PTF, it is the last step of CO desorption that seemed to be rate determining. Furthermore, the CO2RR occurs easier than the HER over the coordinately unsaturated Ni–N4 site in NiSAs/PTF. Results and Discussion The atomically isolated Ni–N4 and Cu–N4 sites anchored by porphyrin moieties within porous porphyrinic triazine framework (denoted as NiSAs/PTF and CuSAs/PTF, respectively) were prepared via a one-pot ionothermal trimerization reaction of the corresponding 5,10,15,20-tetrakis(4-cyanophenyl)porphyrinato]-M (M-TPPCN, M = Ni, Cu), catalyzed by the molten zinc chloride as Lewis acid at 600 °C for 40 h (Figure 1). After washing with diluted hydrochloric acid for NiSAs/PTF or nitric acid for CuSAs/PTF, followed by deionized water and ethanol, black powder samples were obtained after oven drying in vacuum at 120 °C for 12 h. The FeSAs/PTF and CoSAs/PTF materials containing single-atom sites of Fe–N4 and Co–N4, were prepared respectively, according to a previously reported method.31,32 The polymerization reaction and formation of polytriazine networks were confirmed by Fourier-transform infrared analysis (). The disappearance of the carbonitrile stretching band at ca. 2225 cm−1 demonstrated that the conversion of the monomers M-TPPCN (M = Ni, Cu) was completed successfully. Meanwhile, a strong absorption band at 1587 cm−1 assigned to triazine rings was observed, indicating the success of the trimerization reaction.33,34 Figure 1 | Schematic illustration of the fabrication of MSAs/PTF (M = Fe, Co, Ni, Cu). Download figure Download PowerPoint The powder X-ray diffraction (PXRD) patterns of NiSAs/PTF and CuSAs/PTF (Figure 2a) are similar to those of Fe- and CoSAs/PTF,31,32 indicating that all the MSAs/PTF have amorphous structures. A prominent wide peak, centered at ca. 25.6° was apparent, which could be assigned to the amorphous (001) reflection of aromatic sheets.35 In addition, a weak peak at ca. 43.0° corresponding to the appearance of (101) reflection of graphitic carbon, and also, indicated that their graphitization occurred at 600 °C. Notably, there was no evidence of an appearance of diffraction peak relative to metal-based particles or carbides. Strikingly, the results obtained from the inductively coupled plasma atomic emission spectroscopy () revealed that the samples NiSAs/PTF, CuSAs/PTF, FeSAs/PTF, and CoSAs/PTF have high corresponding metal contents of 1.11, 0.82, 2.60, and 0.85 wt%, respectively. Thus, we deduced that these metals may possess an atomic-level distribution and that single-atom metals were successfully embedded in the PTFs. The Raman spectra (Figure 2b) also showed no peak of metal nanoparticles (NPs) in NiSAs/PTF and CuSAs/PTF, as they were similar to the Fe- and CoSAs/PTF samples.31,32 Moreover, these samples have high IG/ID ratios (where IG and ID represent the G-band and D-band intensity, respectively) and a broad weak two-dimensional (2D) peak at ca. 2800 cm−1, suggesting that partial graphitization phenomenon had occurred and graphitic layered structures were presented in the catalysts. The Nyquist plots from the electrochemical impedance spectroscopy (EIS) measurements for the MSAs/PTF (M = Ni, Cu, Co, and Fe) further demonstrated that these materials have small semicircles (Figure 2c and ), and that the partial graphitized structures are beneficial for improving the electron transfer rate between the interfaces of the catalyst and the electrolyte, thus enhancing CO2RR activity. Figure 2 | (a) PXRD patterns, (b) Raman spectra of NiSAs/PTF and CuSAs/PTF, (c) Nyquist plots, and (d) CO2 adsorption–desorption isotherms at 273 K for FeSAs/PTF, CoSAs/PTF, NiSAs/PTF, and CuSAs/PTF. Download figure Download PowerPoint The nitrogen adsorption measurements revealed that all the MSAs/PTF (M = Ni, Cu, Fe, and Co) samples have high N2 adsorption uptake and large Brunauer–Emmett–Teller surface areas in the range of 454–873 m2 g−1 (). Moreover, distinct hysteresis loops were observed at the P/Po range of 0.45–1.0, indicating meso- and/or macropores were formed in all the MSAs/PTF materials, attributed to the porogenic agent zinc chloride.36 The pore size distributions based on the nonlocal DFT method () for the MSAs/PTF family further revealed that their pores are in the range of 0.7–9.9 nm. The hierarchically porous structure could imply that the single-atom metal active sites are highly accessible to the reactive species and electrolytes, thus facilitating mass transportation and improving energy conversion efficiency. The CO2 adsorption isotherms further revealed that all the MSAs/PTF materials have large adsorption capacities, ranging from 52–97 cm3 g−1 at 273 K (Figure 2d), illustrating that there was a strong interaction between these nitrogen-rich MSAs/PTF materials and CO2 molecules, which, in turn, enhanced the electrocatalytic activity of CO2RR. Transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and aberration-corrected HAADF-STEM with subangstrom resolution measurements were further used to confirm the atomic levels of the metal sites. As shown in the TEM (Figure 3a and ) and HAADF-STEM (Figure 3d and ) images of the NiSAs/PTF and CuSAs/PTF, no obvious NPs were observed, a finding which is consistent with their PXRD (Figure 2a) and Raman results (Figure 2b). Notably, the TEM element mapping images (Figure 3c and ) show that Ni and Cu elements are uniformly distributed at atomic-level throughout the architecture of NiSAs/PTF and CuSAs/PTF, respectively. In addition, a large number of bright spots related with the heavier Ni and Cu elements with atomic dispersion could be seen in the aberration-corrected HAADF-STEM images for NiSAs/PTF (Figure 3b) and CuSAs/PTF (), respectively.37–42 These results suggested that single-atom metal sites within NiSAs/PTF and CuSAs/PTF have been fabricated successfully. Figure 3 | (a) TEM images of NiSAs/PTF. (b) Aberration-corrected HAADF-STEM images showing the atomic dispersion of Ni in NiSAs/PTF. HAADF image (c) and corresponding C, N, and Ni EDS elemental mapping. Download figure Download PowerPoint In order to confirm the local chemical environment of single-atom metals, X-ray photoelectron spectroscopy (XPS) was first employed. As shown in , the N 1s signal band of all the MSAs/PTF materials split into three-type configurations, originating from triazinic N (398.5 eV), porphyrinic-like metal-coordinated M–Nx (399.7 eV), and graphitic N (401.1 eV) species.43,44 The high-resolution metal 2p XPS spectra of MSAs/PTF (M = Ni, Cu) are displayed in . As shown in the fitted Ni 2p spectrum, two dominated peaks were centered at 855.4 eV (2p3/2) and 872.5 eV (2p1/2), similar to those of Ni2+ in Ni-TPPCN (855.3 and 872.6 eV, ), but higher than the Ni(0) metal (854.3 and 871.9 eV).45 The Fe 2p3/2 and Co 2p3/2 are 709.6 and 780.6 eV, which are near to those of Fe2+ and Co2+, respectively.31,32 As for CuSAs/PTF, the dominating signals at binding energies of 935.8 eV (2p3/2) and 955.9 eV (2p1/2) are originated from the positive Cu–N bond.46,47 These results revealed that all the metals are significantly positive, implying that no metal NPs were detected,48 which are in good agreement with PXRD (Figure 2a) and the TEM results (Figure 3a). Synchrotron-based X-ray absorption spectroscopy was further performed to confirm the coordination environment of these single-atom metals in MSAs/PTF. As shown in the X-ray absorption near edge structure (XANES) of the Ni K-edge in NiSAs/PTF (Figure 4a), both of the pre-edge and absorption edge energy position of NiSAs/PTF are located in between Ni foil and NiO, indicating that the Ni single atoms carried positive charges between metallic Ni(0) and the fully oxidized states Ni(II),49 consistent with the X-ray photoelectron spectroscopy (XPS) () and electron paramagnetic resonance results ().50 Moreover, a weak pre-edge peak (A position) at ca. 8330 eV appeared in NiSAs/PTF, and the precursor Ni-TPPCN was attributed to the dipole-forbidden but quadrupole-allowed transition (1s → 3d), which suggested that hybridization of 3d and 4p orbitals of the Ni central atoms had occurred.9 Additionally, a strong peak at ca. 8336 eV (peak B) was observed in both of NiSAs/PTF and the precursor Ni-TPPCN, which was due to the 1s → 4pz transition. These transitions serve as fingerprints for square-planar configuration with D4h centrosymmetry, and confirm that the NiSAs/PTF contains Ni–N4 porphyrin-like moieties originating from the Ni-TPPCN monomer. Furthermore, the relatively larger intensity ratio for peaks C and D than Ni-TPPCN, in which the two peaks are associated with the 1s → 4px,y transitions and multiple scattering processes, respectively, are expected to show good electrocatalytic performances. In addition, the Ni L-edge XANES spectrum () of NiSAs/PTF displayed a distinctive single peak related to 3d-electrons at the L3-edge, which is similar to Ni rather than Ni-based oxides.51 The N K-edge of the XANES spectrum of NiSAs/PTF () shows a resonance peak of the π*-transition pyrrolic-N that bonded to Ni atoms to form Ni–Nx species. The formation of Ni–Nx was confirmed further by the observation of weak π* peak assigned to sp2-hybridized carbon of C–N–Ni in C K-edge of NiSAs/PTF (). Figure 4 | (a) Normalized Ni K-edge XANES spectra of the Ni-TPPCN, NiSAs/PTFs, NiO, and Ni foil. (b) Normalized Cu K-edge XANES spectra of the CuSAs/PTFs, Cu-TPPCN, Cu2O, CuO, and Cu foil. (c) Fourier-transform EXAFS spectra of Ni-TPPCN, NiSAs/PTFs, and Ni foil. (d) Fourier-transform EXAFS spectra of CuSAs/PTFs, Cu-TPPCN, and Cu foil. The corresponding EXAFS fitting curves of (e) NiSAs/PTF-600 and (f) CuSAs/PTF-600. Download figure Download PowerPoint For the XANES spectrum of CuSAs/PTF, its absorption edge energy was positioned between Cu foil and Cu(II)-TPPCN (Figure 4b), suggesting partial Cu atoms in CuSAs/PTF had been reduced to Cu(I). A similar phenomenon was also found in FeSAs/PTF, in which the Fe atoms have been reduced to Fe(II) from Fe(III) of Fe-TPPCN at high synthesis temperature.31 Moreover, the pre-edge peak at 8985 eV was similar to the monomer Cu-TPPCN, which is associated with 1s → 4pz shakedown transition feature for a square-planar configuration with high D4h symmetry structure. These results indicated that Cu–N4 porphyrin-like structure predominates in CuSAs/PTF, which is similar to that of NiSAs/PTF, FeSAs/PTF, and CoSAs/PTF.31,32 Concurrently, in the Fourier-transform, k3-weighted χ(k) function of the extended X-ray absorption fine structure (EXAFS) spectra for NiSAs/PTF (Figure 4c) exhibited a strong major peak at 1.44 Å associated with Ni–N/O, which is similar to the monomer Ni-TPPCN (1.47 Å), suggesting that the Ni atoms coordinated with N elements. Furthermore, no obvious peak at the positions of Ni foil was observed for NiSAs/PTF sample, indicating that no Ni–Ni bond was presented in NiSAs/PTF. These results suggested that no Ni NP was formed, and all Ni sites were in atomically isolated form, which is also consistent with the PXRD and aberration-corrected HAADF-STEM results. In addition, the fitting results from EXAFS analysis in Figure 4e showed that the coordination number of Ni species in NiSAs/PTF is ca. 4.0 (), suggesting that the Ni porphyrin-like structure was retained, which agreed well with the XANES results (Figure 4a). The same analysis for CuSAs/PTF (Figure 4d) also revealed that Cu NPs were absent, and the single copper atoms adopted identical coordination configuration to NiSAs/PTF with Cu–N4 structure (Figure 4f and ), which was similar to the Fe–N4 and Co–N4 in FeSAs/PTF and CoSAs/PTF.31,32 Inspired by the unique structures of these MSAs/PTF (M = Ni, Cu, Fe, and Co) materials, their activities relative to CO2RR were then evaluated by observing their performance in a gas-tight H-type electrochemical cell, separated by a Nafion-117 proton exchange membrane. The comparison of the CO2RR activities of these materials is summarized in Figure 5. As shown in the linear sweep voltammetry (LSV) curves in Figure 5a, it was apparent that at CO2-saturated (1 atm), 0.5 M KHCO3, all the MSAs/PTF materials exhibited much more positive onset potentials and higher current densities than those in N2-saturated, 0.5 M KHCO3, suggesting that their activities might have originated from CO2 reduction. CoSAs/PTF exhibited a small onset potential of which was much more positive than that of FeSAs/PTF NiSAs/PTF and CuSAs/PTF V) (Figure Figure | (a) in the CO2-saturated and 0.5 M at a rate of (b) CO current density of MSAs/PTF at various (c) at various applied (d) at various applied Download figure Download PowerPoint The reduced and for these catalysts at different potentials within 0.5 h are displayed in Figure Notably, no liquid was found in spectroscopy and the CO and were by gas in the applied potential range between and −1.2 V versus a of was found using FeSAs/PTF electrode (). As shown in Figure NiSAs/PTF has the CO current density in a wide potential compared with the other single-atom catalysts. as shown in Figure NiSAs/PTF showed activity for CO2 to CO with high over 94% in the wide potential from to −1.2 V and a maximum of 98% at V with a high CO partial current density of (Figure As for FeSAs/PTF, it exhibited the maximum for CO with a current density of at an applied potential of V. the (Figure but the (Figure with an applied the NiSAs/PTF and FeSAs/PTF catalysts, was the major for CuSAs/PTF and CoSAs/PTF (Figure indicating that HER is the the conversion over the applied potential the presented by the CO2RR performance of the indicated that the of CO is on the of the active metal sites. we CO metal As shown in a potential at −0.8 V was the CO followed the order of NiSAs/PTF FeSAs/PTF Furthermore, the maximum of 13,462 h−1 was over NiSAs/PTF at −1.2 which most of the reported catalysts our CO2RR we that Ni is the most efficient metal in MSAs/PTF for the reduction of CO2 to CO. in 0.5 M KHCO3, no CO and other were for NiSAs/PTF identical reaction suggesting that the CO was from In order to confirm single-atom Ni was the active site for a series of were The catalysts were also fabricated by synthesis from at 600 °C (denoted as or of Ni species from NiSAs/PTF, using acid as as The Ni NPs on were also prepared (), which is as is that the carbon showed high onset and no CO2RR occurred (Figure which that the CO originated from carbon As shown the results in Figure NiSAs/PTF displayed an onset potential of which is much more positive than those of samples and and the of single-atom Ni active sites in the CO2RR. Figure | (a) curves of NiSAs/PTF, and carbon (b) of metal and (c) curves of NiSAs/PTF and after the of M in 0.5 M (d) of NiSAs/PTF at a potential of V versus RHE for h. Download figure Download PowerPoint Moreover, by the NiSAs/PTF of maximum of 98% at −0.8 V and at a large current density of with of and both only exhibited maximum of and (Figure a significant of HER for the atomic isolated active sites. To further the Ni atom a was the atomic isolated Ni sites were by reactive in which a distinct of catalytic activity for NiSAs/PTF was observed (Figure which was by a much more This result can be to that Ni–N4 atoms by in the two thus the active site of the structure. These indicated the high CO2RR activity of NiSAs/PTF originated from Ni–N4 active sites. In addition, the NiSAs/PTF exhibited stability of h of at which no obvious in and current density was (Figure a good electrochemical stability of the Ni–N4 active sites. The TEM image for NiSAs/PTF after catalysis showed no Ni that the Ni–N4 structure (). were employed to the CO2 to CO further to the The of MSAs/PTF (M = Fe, Co, Ni, and Cu) is shown in Figure The metal sites are active sites. The CO2RR energy for MSAs/PTF are shown in Figure and . the CO2RR are formed, and the corresponding electrochemical limiting potentials can be obtained by the energy by as in the and Figure and , we can the electrochemical limiting potentials for NiSAs/PTF, FeSAs/PTF, CoSAs/PTF, and CuSAs/PTF are and V versus respectively. The electrochemical limiting potentials for CoSAs/PTF and CuSAs/PTF are for CO2RR The rate determining step for NiSAs/PTF, CoSAs/PTF, and CuSAs/PTF is the first electron step for *COOH whereas, with to FeSAs/PTF, it is the last step of CO FeSAs/PTF showed a relatively electrochemical limiting potential of the CO desorption was as high as that the HER is only V for FeSAs/PTF, as shown in . Thus the FeSAs/PTF be more for compared with CO2RR. For NiSAs/PTF, the electrochemical limiting potential is and the is 0.85 V for the of CO2 to CO. However, the HER for NiSAs/PTF, is as high as These results that the NiSAs/PTF show high selectivity for CO rather than the electrochemical reduction which was in agreement with the NiSAs/PTF catalyst exhibited high at a mild potential of V. Figure | (a) The atomic structure of MSAs/PTF (M = Fe, Co, Ni, and Cu). The and C, and N respectively. The energy for (b) NiSAs/PTF, (c) FeSAs/PTF, and (d) CoSAs/PTF the CO2RR