Figure 3 Electrocatalytic measurements. (a) LSV curves of the NiFeCo, NiFeCoCN and IrO2/Ti electrodes in a 0.5 M H2SO4 solution with a scan rate of 5 mV s-1. (b) The iR s-corrected Tafel plots of all as-prepared electrodes derived from the curves of Figure 3a. (c) Nyquist plots of the EIS of all as-prepared electrodes measured at an OER voltage of 1.702 (vs . RHE) over a frequency range of 0.01 Hz-100 kHz in a 0.5 M H2SO4 solution. The insets are the equivalent circuit ofRs (QdlRct ) and the plots in the high frequency region. (d)Cdl of NiFeCo and NiFeCoCN in a 0.5 M H2SO4 solution. (e) Galvanostatic tests of the electrodes at a constant current density of 100 mA cm-2 in a 0.5 M H2SO4solution. HRTEM images of the NiFeCoCN electrode (f) before and (g), (h) after electrochemical ageing. (i) SAED images of the NiFeCoCN bulk after ageing and (j) SAED images of the NiFeCoCN MEA surface after ageing. (k) Schematic diagram of the metal oxide layer reconstruction.
The alkalinous OER performance of the NiFeCoCN was also measured to evaluate the potential prospect of practical application. Figure 4 shows the LSV, Tafel slope, EIS and multicurrent step diagram of NiFeCo, NiFeCoCN and IrO2/Ni foam during OER in a 1 M KOH solution. In Figure 4a, the overpotential of the NiFeCoCN electrode for the alkalinous OER at a current density of 10 mA cm-2 is only 177 mV, which is significantly lower than those of IrO2/Ni foam (215 mV) and NiFeCo (239 mV). In Figure 4b, its Tafel slope reaches a low value of 36.1 mV dec-1, which is also lower than IrO2/Ni foam (68.8 mV dec-1) and NiFeCo (46.2 mV dec-1). In addition, the NiFeCoCN electrode has an extremely low charge transfer resistance (see Figure 4c and Table S7) and the NiFeCoCN electrode also has higher ECSA of 167.5 cm2 and specific activity of 2.2 mA cm-2 than the NiFeCo (80 cm2 and 1.6 mA cm-2) (see Figure 4d and Table S8), which indicates the higher intrinsic activity for the OER compared to other electrodes in a 1 M KOH solution. Figure 4e displays the multicurrent step experiment of all electrodes in a 1 M KOH solution at 10, 50, 100 and 200 mA cm-2. The potential of the NiFeCoCN electrode exhibits almost no fluctuation at different currents, which confirms its excellent stability in alkaline solution. NiFeCoCN electrode also exhibits an excellent Faradaic efficiency of 97.6% in a 1 M KOH solution (Figure S7).