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].