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].