Self-supported IrMnFeCoNiO_x high entropy spinel as acid resistant and active oxygen evolution catalyst

IrMnFeCoNiO_x and IrO_x oxides self-supported on carbon fibres were synthesised through a facile and rapid thermal decomposition in air (300–500 °C, 1 h). The effect of temperature on the material's properties and their performance towards the acidic oxygen evolution reaction (OER) were assessed. For IrMnFeCoNiO_x catalysts, the higher the annealing temperature, the better the catalytic activity and stability towards the acidic OER, which was assigned to the crystallisation to a spinel structure (cubic Fd3m). The IrMnFeCoNiO_x annealed at 500 °C reached 10 mA cm⁻² at a low overpotential of 283 mV with no evident signs of deactivation for 70 h operating at 10 mA cm⁻². On the contrary, for IrO_x crystallisation towards rutile IrO₂ with increasing temperature resulted in an activity decay, with the overpotential to reach 10 mA cm⁻² progressively worsening from 304 mV to ca. 400 mV. At comparable iridium loading (0.4 mg_Ir cm⁻²), the best performing IrMnFeCoNiO_x catalyst surpasses in terms of activity and stability the best performing IrO_x catalysts. This methodology that required only metal chloride precursors, water and heating enables the synthesis of metal oxide spinel catalysts, including high entropy spinel, through a fast, simple and scalable route. In addition, this route can aid in the design of low Ir-loading catalysts for the production of green hydrogen.