Composition Tuning of Ultrafine Cobalt-Based Spinel Nanoparticles for Efficient Oxygen Evolution

The kinetically sluggish oxygen evolution reaction has been considered as the bottleneck of energetic overall water splitting for hydrogen generation. Herein, a series of binary CoV and ternary CoFeV spinel catalysts were synthesized via a solvothermal process. The particle size gradually decreased followed by the increase in metal species. As the result of ultrafine size (∼4 nm) and composition optimization, CoFeV spinel nanoparticles with Co:Fe:V molar ratio of 1.8:1.1:1 showed a higher performance than other congeneric Co-based spinels and benchmark RuO₂ electrocatalyst in electrocatalytic oxygen evolution. The needed overpotential to reach 10 mA cm^–2 is only 248 mV with a lower Tafel slope of 52.8 mV dec^–1 and an excellent durability of over 18 h. The high activity of ternary CoFeV spinel nanoparticles was further made evident through a solar-to-hydrogen system for the overall water splitting, achieving the current density as 224 mA cm^–2 at 2.5 V with a high Faradaic efficiency of 97.7% for the hydrogen product. This work further heralds a great potential of developing nanostructures with carefully designed composition for electrochemistry-related applications.