Influence of Mixed-Valence Iron in Rutile-Type Oxy-Hydroxy-Fluorides on Their Performance as Oxygen Evolution Reaction Catalysts in Alkaline Media

Oxygen evolution reaction (OER) catalysts play a pivotal role in the efficiency of electrochemical energy conversion systems, including water splitting. In this study, we report the synthesis of bimetallic Co–Fe oxy-hydroxy-fluorinated catalysts with a rutile-type structure, prepared through a two-step process involving the co-precipitation of hydrated fluoride precursors Co2+1–xFe2+xF2(H2O)4 followed by their thermal decomposition under air. This method allows for control of the Co/Fe ratio and stabilization of the Fe2+ oxidation state in the catalyst, which is still relatively unexplored. A variety of characterization techniques were used to analyze the cationic and anionic compositions, the oxidation states of iron, the crystal structures of the synthesized materials and to evaluate their influence on catalytic performance. Electrocatalytic measurements toward OER in alkaline media revealed that the best composition of the series is Co2+0.38Fe2+0.36Fe3+0.26O0.26(OH)0.15F1.59, exhibiting an overpotential of 325 mV to reach a current density of 10 mA·cm–2 and a low Tafel slope of 34 mV·dec–1 with cobalt identified as the active center. This observed trend seems to indicate that maximum catalytic performance is reached when all cations (i.e., Co2+, Fe2+, and Fe3+) are in nearly stoichiometric proportions, thereby reducing the optimal Co content compared with the purely trivalent Fe compounds. Our study highlights a synergistic effect of Fe/Co cationic mixing and shows that the oxidation states of iron do not appear to improve catalytic performance. In addition, the catalyst exhibited excellent stability over 10 days, slightly better than that of the Fe2+-free analog.