NiCo2S4/nitrogen and sulfur dual-doped three-dimensional holey-reduced graphene oxide composite architectures as high-rate battery-type cathode materials for hybrid supercapacitors

Abstract

Battery-type electrode materials have recently been considered as a new class of high-capacity cathode materials for construction of hybrid supercapacitors, however, their low electrochemical kinetics prevent the practical application. Here, the NiCo2S4/nitrogen and sulfur dual-doped three-dimensional (3D) holey-reduced graphene oxide (NiCo2S4/N, S-HRGO) composite architectures were synthesized via a two-step hydrothermal approach. Thanks to the introduction of 3D interconnected and highly conductive holey-reduced graphene oxide with mesopore-rich structure providing fast electron/ion transport, the as-resulted NiCo2S4/N, S-HRGO composite achieves enhanced supercapacitive performances, especially exceptional rate capability compared to the pure NiCo2S4 and NiCo2S4/nitrogen and sulfur dual-doped reduced graphene oxide (NiCo2S4/N, S-RGO). The NiCo2S4/N, S-HRGO composite shows a specific capacity of 184.2 mAh g−1 at 1 A g−1, and still maintains a high specific capacity of 119.5 mAh g−1 even at 50 A g−1. The excellent rate capability of the composite can be illustrated by in-depth kinetic analysis based on the calculated diffusion coefficient and charge contribution ratio. Furthermore, a hybrid supercapacitor utilizing NiCo2S4/N, S-HRGO and N, S, P tri-doped holey-reduced graphene oxide (N, S, P-HHGO) respectively as the cathode and anode is further constructed, showing a satisfactory energy density (35.4 Wh kg−1), high power density (15.0 kW kg−1) and good cycle durability.