Simple solid-state synthesis of mesoporous NiCo₂O₄/N-doped reduced graphene oxide nanosheet composites with enhanced performance in electrochemical capacitors

A simple solid-state method was developed for large-scale growth of spinel nickel cobaltite (NiCo₂O₄) nanoparticles on the surface of N-doped reduced graphene oxide nanosheet (N-rGO) as the enhanced performance electrode for electrochemical capacitors. The synthesis involves the room-temperature solid state reaction of a bimetallic (Ni, Co) hydroxide precursor and subsequent thermal transformation to NiCo₂O₄ nanoparticles on the N-rGO support. Morphological analyses reveal that, with loading rate 49 wt%, NiCo₂O₄ nanoparticles with an average size of around 12 nm are formed and uniformly dispersed on the surface of N-rGO nanosheets (NiCo₂O₄/N-rGO). Electrochemical measurements of NiCo₂O₄/N-rGO composites display a high specific capacitance of 1135 F g^− 1 (based on NiCo₂O₄) at a current density of 1 A g^− 1, and excellent cycling performance (100% capacitance retention after 2000 cycles) even at a very high current density of 8 A g^− 1, which are significantly higher than that of NiCo₂O₄ nanoparticles prepared without using N-rGO. This much enhanced electrochemical performance is enabled by both the well-dispersed NiCo₂O₄ nanoparticles that offer large accessible surface area and the mesoporous structure of NiCo₂O₄/N-rGO composites that facilitate electrons/ions transport. The facile two-step synthesis process developed in this work is promising to prepare electrode materials for high-performance electrochemical capacitors.