Mesoporous MnCo₂O₄, NiCo₂O₄, and ZnCo₂O₄ Thin-Film Electrodes as Electrocatalysts for the Oxygen Evolution Reaction in Alkaline Solutions

The oxygen evolution reaction (OER) is the bottleneck of the electrochemical water-splitting process, where the use of porous metal oxide electrodes is beneficial. In this work, we introduce a one-pot synthesis method to fabricate a series of mesoporous metal cobaltite (m-MCo₂O₄, M = Mn, Ni, and Zn) electrodes for the OER. The method involves preparation and coating of a homogeneous clear solution of all ingredients (metal salts and surfactants) over a fluorine-doped tin oxide surface as a thin lyotropic liquid crystalline film and calcination (as low as 250 °C) to obtain a 400 nm thick crystalline m-MCo₂O₄ electrode with a spinel structure. Mesophases and m-MCo₂O₄ films are characterized using structural and electrochemical techniques. All electrodes are stable during the electrochemical test in 1 M KOH aqueous solution and perform at as low as 204 mV overpotential at 1 mA/cm² current density; the m-MnCo₂O₄ electrode works at current densities of 1, 10, and 100 mA/cm² at 227, 300, and 383 mV overpotentials after compensating the IR drop, respectively. The Tafel slope is 60 mV/dec for the m-NiCo₂O₄ and m-ZnCo₂O₄ electrodes, but it gradually increases to 85 mV/dec in the m-MnCo₂O₄ electrode by thermal treatment, indicating a change in the OER mechanism.