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.