Effect of solvents on the morphology of NiCo2O4/graphene nanostructures for electrochemical pseudocapacitor application

The large internal surface areas and outstanding electrical and mechanical properties of graphene have prompted to blend graphene with NiCo2O4 to fabricate nanostructured NiCo2O4/graphene composites for supercapacitor applications. The use of graphene as blending with NiCo2O4 enhances the specific capacitance and rate capability and improves the cyclic performance when compared to the pristine NiCo2O4 material. Here, we synthesized two different nanostructured morphologies of NiCo2O4 on graphene sheets by solvothermal method. It has been suggested that the morphologies of oxides are greatly influenced by dielectric constant, thermal conductivity, and viscosity of solvents employed during the synthesis. In order to test this concept, we have synthesized nanostructured NiCo2O4 on graphene sheets by facile solvothermal method using N-methyl pyrrolidone and N,N-dimethylformamide solvents with water. We find that mixture of N-methyl pyrrolidone and water solvent favored the formation of nanonet-like NiCo2O4/graphene (NiCoO-net) whereas mixture of N,N-dimethylformamide and water solvent produced microsphere-like NiCo2O4/graphene (NiCoO-sphere). Electrochemical pseudocapacitance behavior of the two NiCo2O4/graphene electrode materials was studied by cyclic voltammetry, chronopotentiometry, and electrochemical impedance spectroscopy techniques. The supercapacitance measurements on NiCoO-net and NiCoO-sphere electrodes showed specific capacitance values of 1060 and 855 F g−1, respectively, at the current density of 1.5 A g−1. The capacitance retention of NiCoO-net electrode is 93 % while that of NiCoO-sphere electrode is 77 % after long-term 5000 charge-discharge cycles at high current density of 10 A g−1.