Design and synthesis of NiCo2O4/NiCoO2/graphene hybrid nanoarrays with enhanced capacitive performance

Abstract NiCo2O4/NiCoO2/graphene hybrid nanoarrays on Ni foam have been designed and synthesized through a hydrothermal method and post-annealing treatment. Highly conductive graphene sheets were embedded into or coated onto the NiCo2O4/NiCoO2 arrays, which strongly affect influence the morphology and electrochemical performance of hybrid nanoarrays. Under the effect of graphene, the architecture of the NiCo2O4/NiCoO2 consists of cluster-like arrays that are self-assembled from numerous nanoneedles and provides more electroactive sites for the redox reaction. However, without the assistance of graphene, the pure NiCo2O4/NiCoO2 exhibits the morphology of flake-like arrays on the Ni foam. The NiCo2O4/NiCoO2/graphene arrays show an ultrahigh capacity of 1439 C g-1 at a current density of 1 mA cm-2, which is far larger than that of the pure NiCo2O4/NiCoO2 flake-arrays (695 C g-1). Furthermore, even at a high current density of 60 mA cm-2, the NiCo2O4/NiCoO2/graphene arrays maintain a high gravimetric capacity of 1172 C g-1 (capacity retention: 81.4%), which indicates an excellent rate capability. Further, the hybrid capacitor shows a maximum energy density of 34.3 Wh kg-1. The present study suggests that the NiCo2O4/NiCoO2/graphene hybrid arrays have great application potential as a positive electrode for hybrid supercapacitors.