Phase-Pure High-Entropy Spinel Oxide (Ni,Fe,Mn,Cu,Zn)₃O₄ via Thermal Plasma: A Promising Electrocatalyst for Oxygen Evolution Reaction

High-entropy oxide (HEO) has recently emerged as a potential candidate material for electrochemical water splitting, with excellent activity and catalytic stability, as an alternative to the state-of-the-art oxygen evolution reaction (OER) catalyst. The main characteristics of HEO are its unique structure and the ability to tune physical, chemical, and mechanical properties. On tuning these properties, it exhibits high thermal stability, electrical conductivity, and catalytic activity. In this work, by utilizing a carbonaceous thermal plasma with Ar and CO 2 as the major plasma-forming gases, a single-phase (NiFeMnCuZn) 3 O 4 high-entropy oxide material has been produced at a discharge power of 12 kW for the first time. The as-synthesized powder was annealed at various temperatures (300, 350, and 400 °C) and characterized using various microscopic and spectroscopic tools. The dynamic light scattering study showed that the as-synthesized powder had a single-phase spinel structure with an average particle size of ∼140 nm. The synthesized materials were subjected to electrocatalytic water oxidation reaction studies. The results show that HEO annealed at 350 °C has an excellent OER catalytic activity in 1 M KOH electrolyte, with an overpotential of 308 mV at a current density of 50 mA cm –2 and a Tafel slope of 54 mV dec –1 . Further, Operando -EIS analysis at different potentials evidences that HEO annealed at 350 °C shows less resistance toward the charge-transfer kinetics at the interface. This work provides insight into the rapid synthesis of phase-pure HEO electrocatalysts with excellent performance in energy-related applications.