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).