Fig. 4. Electrochemical measurements of the Ni₃N thin film electrode. (a) CV curves, (b) calculated areal capacitance and specific capacitance with respect to different scan rates, (c) the cycle performance of the Ni₃N electrode carried out at the scan rate of 100 mV s⁻¹ with calculated areal capacitance up to 2000 cycles (the inset figure represents the CV curves of the 1^st, 1000^th and 2000^th cycles) and (d) charge–discharge profiles taken at different current densities.