γ-FeOOH and amorphous Ni–Mn hydroxide on carbon nanofoam paper electrodes for hybrid supercapacitors

Abstract

This work reports the preparation of γ-FeOOH and amorphous Ni–Mn hydroxide on carbon nanofoam paper (CNFP) by galvanostatic electrodeposition for application in hybrid FeOOH-CNFP∥Ni–Mn hydroxide-CNFP supercapacitors working in alkaline electrolytes. The γ-FeOOH on CNFP displayed a porous morphology composed of fluffy nanoflakes. It showed an enhanced redox response in the negative working potential range of −1.5–0 V vs. SCE with a storage capacity of 3.48 C cm⁻² at 10 mA cm⁻² and a response rate of 71% when the current densities increased from 10 mA cm⁻² to 50 mA cm⁻². The amorphous Ni–Mn hydroxide on CNFP displayed a porous morphology composed of fine nanoflakes. It showed an enhanced redox response in the positive working potential range of 0–0.5 V vs. SCE with a storage capacity of 1.54 C cm⁻² at 10 mA cm⁻² and a response rate of 73% when the current densities increased from 10 mA cm⁻² to 50 mA cm⁻². The hybrid FeOOH-CNFP∥Ni–Mn hydroxide-CNFP supercapacitor delivered a high energy density of 1515 mW h cm⁻², at a power density of 9 mW cm⁻² at a working voltage of 1.8 V and a long cycle life with a capacity retention of 95% after 10 000 charge–discharge cycles. Self-discharge studies revealed the main contribution of the FeOOH-CNFP electrode to the voltage drop of the supercapacitor cell. Activation-controlled processes governed the self-discharge of the FeOOH-CNFP electrode, while mixed activation-controlled and diffusion-controlled processes governed the self-discharge of the Ni–Mn hydroxide-CNFP electrode.