Bridging of Ultrathin NiCo2O4 Nanosheets and Graphene with Polyaniline: A Theoretical and Experimental Study

Ultrathin inorganic nanosheets that enable fast electrochemical reaction kinetics are highly required in many energy-related applications. Herein, we report a simple strategy for in situ assembly of ultrathin NiCo2O4 nanosheets with enriched surface active sites on graphene surface in a vertical orientation way by employing polyaniline (PANI) as the structure coupling bridge between the two components (denoted by NiCo2O4-P-G). The as-made ultrathin NiCo2O4 nanosheets are rich in metal ions in high valence state and oxygen defective sites, and feature 3D open frameworks with hierarchical pore structure. It has been found that the nitrogen species derived from PANI building blocks as bridging sites tend to bond with metal ions, which effectively tune the electronic structural states and result in strong coupling effects with the NiCo2O4 nanosheets. Benefiting from these structural characteristics, the as-made NiCo2O4-P-G hybrids, when used as pseudocapacitive electrode materials, can deliver a high specific capacitance of 966 F g(-1) (based on the mass of the active NiCo2O4 component) and an excellent rate capability of ca. 84% even the current density increased by 100 times and long-term stability. As the precious metal-free electrocatalyst for the oxygen evolution (OER) reaction, the NiCo2O4-P-G hybrids are also able to deliver a low overpotential of 0.32 V at a current density of 10 mA cm(-2) in 0.1 M KOH aqueous electrolyte (only 70% iR compensation), holding promise for high performance yet cheap electrocatalysts for the OER reaction.