The stability of the electrode materials is a critical requirement
for practical application in supercapacitors. The cycling stability of
the Co0.85Se electrodes is evaluated by repeated charge and
discharge measurements at a current density of 10 A g1 for 5000
cycles, as shown in Fig. 4e. The specific capacitance of Co0.85Se
electrode was 888 F g-1 for the first cycle. With increasing cycle
number, a specific capacitance value of 818 F g-1 was observed
after 5000 cycles, indicating a capacitance retention of 92% of its
initial capacitance, which demonstrates the excellent cycling stability
of the as-synthesized Co0.85Se. Moreover, the conductibility
of the as-synthesized Co0.85Se is studied by EIS measurement, as
demonstrated by the Nyquist plot in Fig. 4f. The measurement was
conducted at the open circuit potential with an amplitude of 5 mV
between 102 and 105 Hz and the equivalent circuit of the Nyquist
plot is shown in the bottom inset of Fig. 4f, where Re is the internal
resistance (sum of the contact resistance at the interface between
electrolyte and electrode, ionic resistance of electrolyte, and
inherent resistance of the electroactive material),