Nano-thin heterogeneous NiCo MOF/NiO nanosheets for high-rate energy storage

Abstract

Nickel-based materials can perform redox reactions in aqueous alkaline electrolytes, promising to assemble low-cost and high-safety electrochemical devices with attractive energy/power density. Nevertheless, the poor intrinsic electronic/ionic conductivity and low redox activity restrict these materials' reversible capacity and rate performance. Herein, amorphous NiO-coated NiCo metallic organic framework nanosheets (NiCo-MOF/NiO) which combine the advantages of heterostructure and two-dimensional structure are fabricated via a simple hydrothermal reaction and atomic layer deposition. In general, the two-dimensional structure accelerates the diffusion of ions along the material surface, resulting in rapid ion diffusion kinetics. In-situ Raman results suggest that constructing heterostructure can improve the redox kinetics of NiCo-MOF and facilitate the oxidation of Ni ions toward high valence state, leading to effectively boosted capacitance. As a result, the NiCo-MOF/NiO electrode with optimized Ni/Co ratio and NiO coating thickness exhibits a high areal capacitance of 3010 mF cm⁻² at 5 mA cm⁻² and remains 62.5 % of initial capacitance over 5000 cycles, much better than that of pure NiCo MOF (2150 mF cm⁻²). In addition, the asymmetric supercapacitor (ASC) constructed with NiCo-MOF as the positive electrode and commercial activated carbon (AC) as the negative electrode exhibits a large operating voltage window of 1.6 V, with an areal capacitance of 34.4 mF cm⁻² at a current density of 11 mA cm⁻², and retains 77.8 % of its initial capacity after 5000 cycles. This work may provide a reference for advanced Ni-based electrode design by constructing heterostructures.