Electrochemically engineered zinc(iron)oxyhydroxide/zinc ferrite heterostructure with interfacial microstructure and hydrophilicity ideal for supercapacitors.

Zinc ferrite@nickel foam (ZF@Nf) is a potential commercial supercapacitor electrode due to its large theoretical capacity, abundant elemental composition, excellent conductivity, and stability. However, deficient active sites limit its specific capacitance (SC). Herein, we demonstrate that engineering ZF's interfacial microstructure and hydrophilicity mitigate this limitation. ZF@Nf is used as the working electrode in a 3-electrode cell and subjected to multiple oxygen evolution reaction cycles in potassium hydroxide. Systematic changes in ZF's porosity, crystallinity, hydrophilicity, and composition after each cycle were characterised using spectroscopy, sorption isotherm, microscopy and photography techniques. During cycling, the edges of ZF partially phase-transform into a dense polycrystalline zinc(iron)oxyhydroxide film via semi-reversible oxidation resulting in zinc(iron)oxyhydroxide/ZF interface formation. The maximum ion-accessible zinc(iron)oxyhydroxide film density is obtained after 1000 cycles. Strong ionic interaction at the interface induces high hydrophilicity, this together with the 3-dimensional diffusion channels of the zinc(iron)oxyhydroxide significantly increase electroactive surface area and decrease ion diffusion resistance. Consequently, the SC, energy density, and rate-capability of the interface compare favourably with state-of-the-art electrodes. The strong interfacial interaction and polycrystallinity also ensure long-term electrochemical stability. This study proves the direct correlation between interfacial microstructure and hydrophilicity, and SC which provides a blueprint for future energy-storage electrode design.