To quantify the capacitive performance of various electrodes, the GCD curves were carried out at 1–10 A·g
–1 in Fig.6, S14 and S15 (cf. ESM). Obviously, all the GCD curves had obvious charging and discharging platform in Fig.6(a) and Fig.6(b), S14(a) and S15(a), meaning that the Faradaic redox reactions, which corresponded to CV results. The C-CNFs/NiCo
2O
4 nanoflake arrays showed a long discharge time (Fig.6(b)) and a high specific capacitance of 1010 F·g
–1 at 1 A·g
–1, which was larger than those capacitances of P-CNFs/NiCo
2O
4 nanoflake arrays (976 F·g
–1, Fig.6(c)) and bare NiCo
2O
4 (966.7 F·g
–1, Fig. S15(b)). Meanwhile, even at a high current density of 10 A·g
–1, the specific capacitance of C-CNFs/NiCo
2O
4 nanoflake arrays still maintained 666.7 F·g
–1. Moreover, the electrochemical performance of bare C-CNFs was also illustrated in Fig. S16 (cf. ESM), it showed a small specific capacitance of 174.4 F·g
–1 at 1 A·g
–1, only accounting for 17.3% of the specific capacitance of the C-CNFs/NiCo
2O
4 nanoflake arrays. Consequently, the NiCo
2O
4 contributed most of the specific capacity in composite electrodes instead of C-CNFs as a good conductive carbon matrix. From Fig.6(d), the long-term stability of C-CNFs/NiCo
2O
4 and P-CNFs/NiCo
2O
4 nanoflake arrays was established, and the retention of capacitance was 90.84% and 76.33% at 10 A·g
–1 after 3000 cycles, respectively. However, the capacitance retention of bare NiCo
2O
4 was only 56.4% at 10 A·g
–1 after 1000 cycles in Fig. S15(c). The results revealed that the C-CNFs/NiCo
2O
4 electrodes had best cycling stability in all electrode materials. The reason of results might be that both high conductivity and specific surface area of C-CNFs were larger than that value of P-CNFs. On one hand, compared with P-CNFs, such high capacity was due to a larger mass of NiCo
2O
4 nanoflake arrays anchoring to the C-CNFs; on the other hand, high conductivity of electrodes could accelerate the rapid transfer of electrons and specific surface area of electrodes with multiple mesopores also could promote the permeation and reservoir of the electrolyte ions, which could improve capacity and ensure stable cycles during the fast charging/discharging process [
22].