Figure 4. In situ ATR-SEIRAS spectra of Co1-NC (a) and Co1-SNC (b) at different applied potentials. (c) O–H stretching wavenumbers of interfacial water at different applied potentials. (d) Potential-dependent *CO wavenumbers and peak-integrated intensities on Co1-NC and Co1-SNC catalysts, respectively. Solid and dotted lines represent *CO wavenumbers and peak intensities, respectively.
Furthermore, Figure 4d shows that the *CO band frequencies increase initially and then decrease with increasing the cathodic potential over Co1-SNC. Besides, there are no obvious *CO peaks on the surfaces of both catalysts in an Ar-saturated 0.5 M KHCO3 solution (Figure S8). According to previous reports, the Stark effect could cause the frequency shift of *CO band to a lower wavenumber with decreasing cathodic potential, and a higher *CO coverage would induce the frequency shift to a higher wavenumber due to the dipole−dipole coupling effect[29]. The *CO peak-integrated intensities further show a higher *CO coverage over Co1-SNC, explaining the superior CO2RR to CO performance over Co1-SNC[30].
Additionally, as proton-feeding is important for the formation of the crucial intermediate (*COOH) during CO2RR, kinetic isotope effect (KIE) of H/D over Co1-NC and Co1-SNC catalysts was performed to explore the water activation process. As shown in Figure 5a, the KIE of Co1-NC and Co1-SNC are close to 2, indicating that the activation of water was involved in the RDS of CO2RR, which was further demonstrated by the effect of pH of electrolyte that a higher local pH environment favored the formation of CO (Figure 5b)[31]. Besides, compared to Co1-NC (1.98), the decreased KIE value (1.72) indicates accelerated H2O dissociation over Co1-SNC[32]. Furthermore, electrochemical impedance spectroscopy (EIS) was preformed to evaluate the interfacial charge-transfer process, and the Co1-SNC shows a smaller charge transfer resistance, corresponding to a faster charge-transfer process to form reactive intermediates (Figure 5c)[33].
To further understand the superior CO2RR performance of Co1-SNC, we performed density functional theory (DFT) calculations. Typical CoN4 and CoN3S model were constructed (Figure 5d), with match well with the EXAFS fitting results. As shown in Figure 5e, the formation of *COOH intermediate via the proton-electron transfer process is the rate-determining step for CO2RR to CO[34]. Moreover, the reaction free energy of the RDS reduces from 1.88 eV over CoN4 to 1.54 eV over CoN3S. Furthermore, the density of states (DOS) and Bader charge analysis were performed to reveal the electronic structure of CoN4 and CoN3S. As displayed in Figure 5f, the S doping elevates the d-band center of cobalt from -1.02 eV to -0.79 eV, which benefit the CO2activation[35]. Besides, differential charge density distribution analysis shows that the Co1-N3S possesses a stronger electronic interaction with *COOH intermediate, facilitating the CO2 adsorption (Figure 5g, Figure S9). Therefore, S doping can optimize the adsorption and activation of CO2on the Co active site.