Figure 1 (a) Preparation process of Nanoporous NiFeCoCN MEAs.
(b) XRD patterns of NiFeCoCN (c, d) FESEM images of the NiFeCoCN at
different magnifications. (e) HRTEM and
SAED images of the NiFeCoCN
before ageing. (f) Part of the HRTEM images in e. (g) IFFT images of f.
(h) HAADF-STEM images of the NiFeCoCN before ageing. (i) Ni, (j) Fe, (k)
Co, (l) C, (m) N and (n) O elemental mapping from EDX in the unaged
NiFeCoCN.
To explore the real intrinsic nature of the acidic OER, the NiFeCoCN
electrode for XPS measurement was subjected to an ageing
of 5000 seconds at a current
density of 100 mA cm-2 in a 0.5 M
H2SO4 solution. Figure 2 displays the
XPS spectra of the NiFeCoCN electrode before and after electrochemical
ageing. The XPS full spectra of NiFeCoCN before and after
electrochemical ageing confirm the Ni, Fe, Co, C, N and O elements exist
on the outer surfaces (Figure S5 and Table S3). The Ni 2p
spectrum of the unaged sample in
Figure 2a displays the two main peaks of Ni 2p3/2 at 852.1 eV and Ni
2p1/2 at 859.3 eV correspond to Ni0+, Ni 2p3/2 at
854.6 eV and Ni 2p1/2 at 873.1 eV correspond to Ni2+.
After electrochemical ageing, the peaks of Ni 2p3/2 at 856.3 eV and Ni
2p1/2 at 874.1 eV correspond to Ni2+, and the peaks of
Ni0+ disappear [43-45]. The Fe 2p spectrum of the
unaged sample in Figure 2b displays the two main peaks of Fe 2p3/2 at
706.7 eV and Fe 2p1/2 at 719.5 eV
correspond to
Fe0+, Fe 2p3/2 at 709.7 eV and Fe 2p1/2 at 724.3 eV
correspond to
Fe2+.
After electrochemical ageing, the
peaks of Fe 2p3/2 at 711.1 eV and Fe 2p1/2 at 724.8 eV
correspond to
Fe3+ and the peaks of Fe0+ disappear
[45-47]. The Co 2p spectrum of the unaged sample in Figure 2c
displays the two main peaks of Co 2p3/2 at 780.3 eV and Co 2p1/2 at
795.5 eV correspond to Co2+, the peak of Co 2p3/2 at
777.5 eV correspond to Co0+. After electrochemical
ageing, the peaks of Co 2p3/2 at
781.8 eV and Co 2p1/2 at 798.2 eV correspond to Co3+,
Co 2p3/2 at 786.2 eV can correspond to Co2+ and the
peaks of Co0+ disappear [48, 49]. In Figure 2d,
the O 2p spectra of the unaged sample located at 532.9 eV, 531.3 eV, and
529.4 eV correspond to adsorbed water, absorbed oxygen and lattice
oxygen, respectively [44, 48, 50]. After electrochemical ageing, the
binding energy of the absorbed oxygen peak is positively shifted by
approximately 0.4 eV. Obviously, the binding energy of metals and
low-valence metal oxides (e.g., Ni, Fe, Co, NiO, FeO, CoO) is positively
transferred to a higher binding energy to form high-valence metal oxides
(e.g., NiO, Fe2O3,
Co3O4), which indicates the NiFeCoCN has
surface reconstruction during the initial OER process in a 0.5 M
H2SO4 solution. The C 1s spectrum of the
unaged sample in Figure 2e shows peaks at 284.6 eV, 286.1 eV, 284.6 eV,
and 282.0 eV, which correspond to C-C, C-N, C-O and
NiCx, respectively [40]. However, the
NiCx peak disappears after electrochemical ageing, which
indicates a reconstruction of surface structure during the initial OER
process. The N 1s spectrum of the unaged sample presents two peaks at
399.9 eV and 404.4 eV in Figure 2f, which belong to pyridinic N and
graphite N, respectively. The pyridinic N in OER electrocatalysts tends
to receive electrons, and graphite N tends to provide electrons, both of
which positively influence the OER activity [51, 52]. To explore the
real intrinsic nature of the alkalinous OER, the NiFeCoCN electrode was
also subjected to an ageing of 5000 seconds at a current density of 100
mA cm-2 in a 1 M KOH solution. Figure S6 displays the
XPS spectra of the NiFeCoCN electrode before and after electrochemical
ageing. Studies have shown that metal catalysts can be reconstructed as
metal oxides in the initial stage of OER under acidic conditions, while
catalysts including transition metals can be reconstructed as metal
(oxy)hydroxides under alkaline conditions, which is consistent with
previous work [33, 36].