Role of Fe–O–M Bond in Controlling the Electroactive Species Generation from the FeMO₄ (M: Mo and W) Electro(pre)catalyst during OER

While bulk or surface modification of transition-metal-based pre-catalysts is the most obvious reason of their superior activity during alkaline oxygen evolution reaction (OER), identification of electroactive species and accurately establishing the in situ evolution pathway of such species remain challenging, albeit fundamentally important to correlate the electronic structure with their inherent activity. Given that, a detailed electrochemical OER study with two bimetallic FeIIMVIO4 (M = Mo and W) type electrocatalysts has been performed, and the influence of M in the electronic structure at the molecular level to control the energetics of the electroactive species formation has been studied. FeMoO4 turns out to be better compared to its isotypic FeWO4 which is due to facile electrokinetics to evolve the electroactive species. Post-chronoamperometric characterization and time-dependent quasi in situ Raman analyses reveal a potential-driven hydrolytic dissolution of [MoO4]2- from FeMoO4 to form α-FeO(OH) as the electroactive species. Poor lattice stability (formation enthalpy; ΔHf), weak Fe–O–Mo bonding, and low decomposition enthalpy (ΔHD) of the FeMoO4 lattice favor a facile electrocatalytic decomposition to evolve the highly reactive FeO(OH) surface in which the peroxo (O–O) bond formation is rate limiting with a minimum potential barrier of 38 kJ mol–1 obtained from the variable temperature OER study. Under a very similar electrochemical condition, FeWO4 is relatively robust, with negligible [WO4]2– leaching due to a high ΔHD value and less ionic character of the Fe–O–W bonds. This combined experimental and theoretical study establishes how the material’s electronic structure, that is, the bonding between the anionic counterpart and the active metal, can play an intriguing role to influence the OER activity which is so far relatively less well-explored.