Spinel Ferrites as Electrode Materials for Supercapacitors: Structure–Property Relationships, Synthesis Strategies, Composite Design and Prospects for Sustainable Energy Storage

Abstract Supercapacitors fill the gap between conventional capacitors and rechargeable batteries by delivering high power and long cycle life, yet their energy density still limits wider adoption. Spinel ferrites (MFe2O4, M = Mn, Fe, Co, Ni, Cu, Zn) have attracted interest as electrode materials because they are built from abundant elements, can be made by inexpensive wet-chemical routes, and offer several accessible oxidation states. This review summarises how the spinel crystal structure, particularly the distribution of cations between tetrahedral and octahedral sites, relates to charge-storage behaviour. It compares the main synthesis routes, examines the electrochemical behaviour of individual and mixed-metal ferrites, and evaluates the role of carbon-based composites in overcoming the intrinsically low electronic conductivity of oxides. Representative device-level results are collected and discussed critically. Differences in testing conditions and device configurations make cross-laboratory comparison of ferrite electrodes difficult. To address this, the review recommends standardised reporting and identifies research directions toward low-cost, sustainable energy storage.