Thermal driven structural evolution of high entropy spinel oxide for enhanced oxygen evolution electrocatalysis

The expansion of efficient and durable electrocatalysts for the oxygen evolution reaction (OER) is critical for advancing sustainable hydrogen production via water electrolysis. Herein, we report the synthesis of spinel-type high-entropy oxides (HEOs) composed of Ni, Co, Fe, Cr, and Mn using a simple and scalable co-precipitation method followed by controlled calcination at different temperatures (400, 600, and 800 °C). Structural and morphological analyses confirmed the formation of a single-phase spinel framework with improved crystallinity upon increasing the annealing temperature, while uniform elemental distribution was achieved through entropy stabilization. X-ray photoelectron spectroscopy revealed the co-existence of multivalent transition metal cations and abundant oxygen vacancies, which favoured the enhanced redox activity and charge transport. Electrochemical evaluation demonstrated that the HEO prepared at 600 °C exhibited superior OER performance, delivering a low overpotential of 230 mV, the largest electrochemically active surface area, and the lowest charge-transfer resistance, 0.714 Ω among the investigated samples. Moreover, the optimized catalyst showed an excellent durability with only a minor increase in overpotential after 100 h of continuous operation. This enhanced electrocatalytic activity is attributed to the optimized crystallinity, synergistic interactions among multiple cations, and defect-rich oxygen environments induced by the high-entropy effect. Therefore, this study highlights the potential of spinel high-entropy oxides as a robust and cost-effective OER electrocatalyst for long-term water-splitting applications.