Confirming High-Valent Iron as Highly Active Species of Water Oxidation on the Fe, V-Coupled Bimetallic Electrocatalyst: In Situ Analysis of X-ray Absorption and Mo''ssbauer Spectroscopy

Iron (Fe)-based bimetallic oxides/hydroxides have beenwidely investigatedfor promising alkaline electrochemical oxygen evolution reactions(OERs), but it still remains argumentative whether Fe3+ or Fe4+ intermediates are highly active for efficientOER. Here, we rationally designed and prepared one Fe, V-based bimetalliccomposite nanosheet by employing the OER-inert V element as a promoterto completely avoid the argument of real active metals and using ourrecently developed one-dimensional conductive nickel phosphide (NP)as a support. The as-obtained hierarchical nanocomposite (denotedas FeVO x /NP) was evaluated as a modelcatalyst to gain insight into the iron-based species as highly activeOER sites by performing in situ X-ray absorption spectroscopy and Fe-57 Mo''ssbauer spectroscopy measurements. It was foundthat the high-valent Fe4+ species can only be detectedduring the OER process of the FeVO x /NPnanocomposite instead of the iron counterpart itself. Together withthe fact that the OER activities of both the vanadium and iron counterpartsare by far worse than that of the FeVO x /NP composite, we can confirm that the high-valent Fe4+ formed are the highly active species for efficient OER. As demonstratedby density functional theory simulations, the composite of Fe andV metals is proposed to cause a decreased Gibbs free energy as wellas theoretical overpotential of water oxidation with respect to itscounterparts, as is responsible for its excellent OER performancewith extremely low OER overpotential (290 mV at 500 mA cm(-2)) and extraordinary stability (1000 h at 100 mA cm(-2)).