The Co1-SNC catalyst was synthesized by ball-milling a mixture of melamine, L-cysteine and CoCl2, followed by a two-step calcination in argon atmosphere (Figure 1a). Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and aberration-corrected high-angle annular dark-field scanning TEM (AC-HAADF-STEM) were applied to investigate the morphology and structure of Co1-SNC and Co1-NC. As shown in Figure 1b-d, the typical morphology of Co1-SNC is sheet-like thin layer with no obviously observed nanoparticles. In Figure 1e, the bright dots can be assigned to the monodispersed Co atoms[19]. Moreover, the elemental mapping and line scan further reveal the uniform distribution of C, N, S, and Co elements on the Co1-SNC catalyst (Figures 1f and g). The Co content (1.04 wt.% and 0.98 wt.% for Co1-NC and Co1-SNC, respectively) was estimated by the inductively coupled plasma optical emission spectrometry (ICP-OES). More morphological characterizations of the Co1-NC are shown in Figure S1.
Figure 2. (a) XRD patterns of Co1-SNC, Co1-NC and NC. (b) Raman spectra of Co1-NC and Co1-SNC. (c) Co 2p XPS spectra of Co1-NC and Co1-SNC. (d) Co k-edge XANES spectra. (e) R space EXAFS spectra of Co1-SNC and Co foil reference. (f) The fitting curve of Co1-SNC.
As shown in Figure 2a, the XRD patterns only show diffraction peaks at 26.2° and 43.6°, which are assigned to (002) and (101) plane of graphitic carbon, respectively. The XRD results further indicate that Co nanoparticles are absent in both Co1-NC and Co1-SNC. Compared to Co1-NC, the XRD peaks for Co1-SNC become broader and weaker, suggesting that S doping results in low crystallinity and defective structure, which is further proved by a higherID/IG ratio of Co1-SNC in the Raman measurement (Co1-NC:1.02; Co1-SNC:1.10) in Figure 2b[20]. Besides, the Brunauer–Emmett–Teller (BET) specific surface area was measured to be 113.12 and 299.66 m2 g-1 for Co1-NC and Co1-SNC, respectively (Figure S2).
Next, we performed XPS measurements to explore more detailed surface structure of Co1-NC and Co1-SNC catalysts. As shown in Figure S3, N 1s XPS spectra can be deconvoluted into pyridinic-N (398.13 eV), pyrrolic-N (399.79 eV), graphitic-N (400.93 eV) and oxidized N species (402.76 eV) for both Co1-NC and Co1-SNC[21]. Besides, the S 2p XPS spectrum for the Co1-SNC catalyst displays a peak at 165.16 eV, which can be attributed to S-Co bond[22]. Figure 2c displays the Co 2p XPS spectra, where the peaks at 780.89 eV and 796.3 eV correspond to the Co 2p3/2 and Co 2p1/2 orbitals of Co2+, respectively. Noteworthily, compared to Co1-NC, the binding energy of Co1-SNC is blue-shifted, suggesting that the incorporation of S can enrich electron density on the Co site, which expects to boost the adsorption of CO2. X-ray absorption near-edge structure (XANES) spectroscopy was performed to examine the coordination structure of Co1-SNC (Figure 2d). The pre-edge of Co1-SNC is observed higher than that of Co foil but lower than that of cobalt phthalocyanine (CoPc), indicating that the cobalt valence state in Co1-SNC is between 0 and +2[23]. Moreover, extended X-ray absorption fine structure (EXAFS) was analyzed to further get information on the local structure of Co1-SNC. As displayed in Figure 2e, the Co1-SNC exhibits the characteristic peaks of the Co-N bond at 1.4 Å and no Co-Co bond (2.1 Å) can be found, which further confirms the existence of atomically dispersed Co. In addition, the fitting result discloses that the coordination number of the Co atom in the first shell is near four with the proposed CoN3S structure (Figure 2f and Table S1).
Figure 3. Electrochemical CO2RR performance of Co1-NC and Co1-SNC. (a) FECO of Co1-NC and Co1-SNC at different potentials. (b) Partial current density of CO (jCO ) over Co1-NC and Co1-SNC. (c) Stability test of Co1-SNC at -0.8 V (vs. RHE) in CO2saturated 0.5 M KHCO3 solution.
The CO2RR performance of the as-prepared catalysts was evaluated in CO2-saturated 0.5 M KHCO3electrolyte (details can be found in the Supporting Information). All potentials here are relative to the reversible hydrogen electrode (RHE) scale unless stated otherwise. According to the linear sweep voltammetry (LSV) curves (Figure S4), Co1-SNC shows comparable hydrogen evolution activity to Co1-NC in Ar-saturated electrolyte. It is worth noting that Co1-SNC exhibits a much higher current density and smaller onset potential compared to Co1-NC in the CO2-saturated electrolyte, indicating a superior CO2RR performance[24]. Furthermore, according to Figures 3a and b, Co1−NSC exhibits a much higher FECO and larger j CO than those of Co1−NC across all testing potentials. The maximum FECO of Co1−SNC reaches 75.6 ± 2% at -0.8 V. Besides, no liquid products can be detected (Figure S5). Furthermore, the durability of Co1−SNC for CO2RR was assessed at a constant applied potential of -0.8 V (Figure 3c). The Co1-SNC catalyst can maintain a stable cathodic current density at 17.5 mA cm-2 and an average FECO of 76%, suggesting excellent catalytic stability of Co1-SNC in the CO2RR.
To further understand the electrochemical properties of electrocatalysts, the electrochemically active surface area (ECSA) was investigated. The electrochemical double-layer capacitance (Cdl) of Co1-SNC (0.75 mF cm−2) is higher than that of Co1-NC (0.25 mF cm−2), indicating the more active sites for CO2RR on Co1-SNC catalyst (Figure S6). However, the CO partial current densities normalization by ECSA of Co1-SNC and Co1-NC are −15.28 and −13.92 mA cm−2, respectively, demonstrating that the intrinsic activity for CO production of the Co1-SNC electrocatalyst is higher than Co1-NC (Figure S7).
In situ ATR-SEIRAS was performed to investigate the mechanism of CO2RR on Co1-NC and Co1-SNC catalysts. The IR spectra were collected from 0 to -1.0 V (vs. RHE) with a step of 0.1 V in a CO2saturated 0.5 M KHCO3 solution. As shown in Figures 4a and b, the peak at 3400 cm-2 and 1950 cm-2 can be assigned to the O-H stretching band of surface water and adsorbed *CO intermediate, respectively[25, 26]. The frequency and intensity of the vibrations are strongly dependent on the electrode potential, indicating that the obtained ATR-SEIRAS signals are mainly from the electrode from the electrode interface[27]. Compared to Co1-NC, the O-H stretching band shifts to lower wavenumbers, suggesting a stronger affinity between water molecules and Co1-SNC (Figure 4c)[28].