Controlled electrodeposition of iron oxide/nickel oxide@Ni for the investigation of the effects of stoichiometry and particle size on energy storage and water splitting applications

Abstract

Controlled synthesis of nickel/iron multimetal oxides with different stoichiometry and particle sizes was carried out by varying the pH of the reaction medium. Electrodeposited samples grown at different pH values showed a wide range of electrochemical properties such as dissimilar current response and potential window due to the formation of different stoichiometry and surface morphologies. Smaller particle size and higher content of NiO are advantageous due to the creation of a facile diffusion path. Moreover, electrical conductivity as well as series resistance increased for the samples with smaller particle size due to the quantum size effect. The quantum size effect was confirmed from the blue shift of the UV-vis absorbance spectrum. The facile diffusion path lowered the charge transfer resistance and accelerated the reaction rate for water splitting. Furthermore, the quantum size effect shifted the flat-band potential and increased the overpotential in the water splitting reaction. Multimetal oxides exhibited a small overpotential of ∼−0.27 V (corresponding to the current response of 10 mA cm⁻²) and a small Tafel slope of ∼63 mV dec⁻¹. Finally, an asymmetric supercapacitor (ASC) cell was fabricated with electrodeposited samples, which showed a large potential window of ∼1.6 V along with a high energy and power density of ∼91 W h kg⁻¹ and 7200 W kg⁻¹, respectively. Furthermore, the ASC exhibited very low relaxation time constant (∼1.3 ms) and long stability of ∼83% after 10 000 CD cycles, ensuring the effectiveness of electrodeposited multimetal oxides for energy storage as well as water splitting applications.