High-Entropy Spinel Oxide Nanoparticles Derived from Metal Fumarates as Electrocatalyst for the Oxygen Evolution Reaction

Noble-metal-free electrocatalysts are highly sought to overcome kinetically sluggish water oxidation in alkaline media. Transition-metal-based high-entropy spinel oxides (HEOs) have emerged as promising candidates for oxygen evolution due to their compositional flexibility and rich defect chemistry. Herein, we report a facile hydrothermal synthesis of high-entropy fumarate precursors, followed by controlled calcination to obtain spinel-type high-entropy oxide nanoparticles. Variation in calcination temperature significantly affects the porosity, crystallinity, and phase stability of HEO nanoparticles. Calcination at 550 °C yields nanoparticles with an optimal balance of mesoporosity and crystallinity, which promotes efficient charge transport and enhanced OER activity. In contrast, the 350 °C sample exhibits low crystallinity, while the 750 °C sample shows partial phase segregation, whereas HEO-550 displays a well-developed spinel phase with uniform cation distribution and a porous nanoparticle architecture. The optimized HEO (HEO-550) exhibits an abundant oxygen vacancy (O1/O2) area ratio of 1.08, which synergistically accelerates charge-transfer kinetics and boosts catalytic activity. As a result, HEO-550 delivers low overpotentials of 226 and 291 mV at current densities of 10 and 50 mA cm–2, respectively, along with stable operation for 48 h. Post-OER analysis confirms favorable surface chemical evolution and preserved nanoparticle morphology that demonstrates excellent structural and chemical stability. This simple, scalable two-step strategy highlights the potential of high-entropy spinel oxide nanoparticles for efficient and durable energy-conversion applications.