Heterogeneous, mesoporous NiCo2O4–MnO2/graphene foam for asymmetric supercapacitors with ultrahigh specific energies

Abstract

A major challenge of state-of-the-art supercapacitors as promising energy storage devices lies in their relatively low capacitances and low specific energies. Herein, we report a strategic assembly of two excellent pseudocapacitive materials. Heterogeneous NiCo₂O₄–MnO₂ arrays consisting of a mesoporous NiCo₂O₄ nanowire core and a cross-linked MnO₂ nanosheet shell are grown on a freestanding graphene foam (GF) for ultrahigh-performance supercapacitors. The electrode exhibits a remarkable gravimetric specific capacitance of 2577 F g⁻¹ at 1 A g⁻¹ and areal capacitance of 5.15 F cm⁻² at 2 mA cm⁻², as well as exceptional capacitance retention of 94.3% after 5000 cycles. An asymmetric supercapacitor assembled with NiCo₂O₄–MnO₂/GF and CNT/GF composites as the positive and negative electrodes, respectively, delivers a maximum specific energy of 55.1 W h kg⁻¹ at a specific power of 187.5 W kg⁻¹. The core/shell strategy adopted here to deposit two active materials on a 3D conductive matrix offers a new insight into assembling ternary hybrids for high-performance electrodes in real-world applications.