Exploring the capacitance of a novel nickel fluoride hydroxide nanomaterial in aqueous solutions

Abstract

The pursuit of an electrode material that is electrochemically active in aqueous solutions and possesses both high energy density and abundant resources is critical for the development of aqueous hybrid capacitors, which are integral to clean energy storage systems. In this study, we introduce for the first time a novel nickel compound with two anions, displaying a high theoretical energy density—the Ni₄OHF₇ nanomaterial. We provide a comprehensive analysis of its structure and its electrochemical performance in alkaline aqueous solutions. The Ni₄OHF₇ nanomaterial is found to be composed of particles approximately 20 nm in size. The electrochemical behavior of Ni₄OHF₇ in alkaline aqueous solution is mediated by redox reactions occurring between Ni⁴⁺ and Ni³⁺, as well as between Ni³⁺ and Ni²⁺. The electrochemical mechanism is primarily diffusion-controlled faradic intercalation process. The study delves into the impact of charge–discharge regulations. Notably, the voltammetric specific capacitance reaches 84.5 F g ⁻¹ at a scan rate of 0.03 V s ⁻¹, while the discharge specific capacitance is as high as 1128.3 F g ⁻¹ at a current density of 1 A g ⁻¹. The voltammetric specific capacitance initially increases along with cycling before diminishing and ultimately stabilizes at approximately 30 F g ⁻¹ at a scan rate of 0.1 V s ⁻¹. The decline in capacitance is attributed to the progressive increase of the charge transfer resistance. The assembled activated carbon (AC)/Ni₄OHF₇ hybrid capacitor exhibits superior energy and power densities (103.0 Wh kg ⁻¹, 7560.7 W kg ⁻¹).