Structural and Electronic Factors behind the Electrochemical Stability of 3D-Metal Tungstates under Oxygen Evolution Reaction Conditions

Transition metal tungstates (TMTs) possess a wolframite-like lattice structure and preferably form via an electrostatic interaction between a divalent transition metal cation (M^II) and an oxyanion of tungsten ([WO₄]^2-). A unit cell of a TMT is primarily composed of two repeating units, [MO₆]_oh and [WO₆]_oh, which are held together via several M-μ₂-O-W bridging links. The bond character (ionic or covalent) of this bridging unit determines the stability of the lattice and influences the electronic structure of the bulk TMT materials. Recently, TMTs have been successfully employed as an electrode material for various applications, including electrochemical water splitting. Despite the wide electrocatalytic applications of TMTs, the study of the structure-activity correlation and electronic factors responsible for in situ structural evolution to electroactive species during electrochemical reactions is still in its infancy. Herein, a series of TMTs, M^IIW^VIO₄ (M = Mn/Fe/Co/Ni), have been prepared and employed as electrocatalysts to study the oxygen evolution reaction (OER) under alkaline conditions and to scrutinize the role of transition metals in controlling the energetics of the formation of electroactive species. Since the [WO₆]_oh unit is common in the TMTs considered, the variation of the central atom of the corresponding [MO₆]_oh unit plays an intriguing role in controlling the electronic structure and stability of the lattice under anodic potential. Under the OER conditions, a potential-dependent structural transformation of MWO₄ is noticed, where MnWO₄ appears to be the most labile, whereas NiWO₄ is stable up to a high anodic potential of ∼1.68 V (vs RHE). Potential-dependent hydrolytic [WO₄]^2- dissolution to form MO_x active species, traced by in situ Raman and various spectro-/microscopic analyses, can directly be related to the electronic factors of the lattice, viz., crystal field splitting energy (CFSE) of M^II in [MO₆]_oh, formation enthalpy (ΔH_f), decomposition enthalpy (ΔH_d), and Madelung factor associated with the MWO₄ ionic lattice. Additionally, the magnitude of the Löwdin and Bader charges on M of the M-μ₂-O-W bond is directly related to the degree of ionicity or covalency in the MWO₄ lattice, which indirectly influences the electronic structure and activity. The experimental results substantiated by the computational study explain the electrochemical activity of the TMTs with the help of various structural and electronic factors and bonding interactions in the lattice, which has never been realized. Therefore, the study presented here can be taken as a general guideline to correlate the reactivity to the structure of the inorganic materials.