Design and synthesis of NiCo2O4–reduced graphene oxide composites for high performance supercapacitors

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Abstract
In the present work, we used charge-bearing nanosheets as building blocks to construct a binary composite composed of NiCo2O4 and reduced graphene oxide (RGO). Co–Ni hydroxides intercalated by p-aminobenzoate (PABA) ion and graphite oxide (GO) were exfoliated into positively charged hydroxide nanosheets and negatively charged graphene oxide nanosheets in water, respectively, and then these oppositely charged nanosheets were assembled to form heterostructured nanohybrids through electrostatic interactions. The subsequent thermal treatment led to the transformation of the hydroxide nanosheets into spinel NiCo2O4 and also to the reduction of graphene oxide. The as-obtained NiCo2O4–RGO composite exhibits an initial specific capacitance of 835 F g−1 at a specific current of 1 A g−1 and 615 F g−1 at 20 A g−1. More interestingly, the specific capacitance of the composite increases with cycling numbers, reaches 1050 F g−1 at 450 cycles and remains at 908 F g−1 (higher than the initial value) after 4000 cycles. The high specific capacitance, remarkable rate capability and excellent cycling ability of the composites mean that they show promise for application in supercapacitors. Comparison with the capacitive behavior of pure NiCo2O4 and NiCo2O4 mechanically mixed with RGO displays the importance of the self-assembly of the nanosheets in making a wide range of graphene-based composite materials for applications in electrochemical energy storage.

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