Surface Reconstruction in Alkaline Electrolyte of Co–Ni–Fe Oxy(hydroxy)fluorides: Formation of Efficient OER Catalysts Involving Lattice Oxygen Mechanisms

Gaining insight into the oxygen evolution reaction (OER) mechanism for the most promising electrocatalysts is crucial for elucidating how key parameters, such as chemical composition, structure, and particle morphology, govern their performance. In this study, we present a comprehensive mechanistic investigation of the catalyst series (Co (1– x )/2 Ni x /2 ) 2+ Fe 3+ 0.5 O 0.5 F 1.5– y (OH) y, with particular attention given to the composition (Co 0.25 Ni 0.25 ) 2+ Fe 3+ 0.5 O 0.5 F 1.3 (OH) 0.2 exhibiting the highest catalytic activity within this series. The combination of in situ and ex situ electrochemical and analytical techniques revealed a surface chemical reconstruction of this oxy(hydroxy)fluoride into a layered double hydroxide-type MOOH phase, which acts as the final active material. OER catalysis would occur in part via a lattice oxygen mechanism (LOM), involving the coupling of an adsorbed intermediate with an oxygen atom from the lattice, demonstrated by an isotope labeling study. For the composition with an equal Co:Ni ratio, both cobalt and nickel are identified as the main active sites, and their synergy is deemed to be the origin of its performance within the series, enabling to obtain the lowest apparent activation energy. Overall, this study highlights the interest of F-based hetero-anionic materials as OER catalysts, which, after a pre-catalytic step, are converted into a high-performing MOOH-type phase. The detailed LOM pathway proposed after the pre-catalytic step would be analogous to that of transition-metal oxyhydroxides, which is still in debate today.