Metastable Phase in Molybdate XMoO4 (X = Ni, Co, Cu) for Oxygen Evolution Reaction: Structural Phase Dependence

Abstract Piezochromic molybdates (XMoO 4 , X = Ni, Co, Cu) can reversibly modulate their color via pressure- or temperature-induced polymorphic phase transitions. Beyond chromic behavior, the structural flexibility of the molybdate sublattice, switching between [MoO 4 ] tetrahedral and [MoO 6 ] octahedral coordination, implies tunable charge-transfer characteristics that are highly relevant to electrochemical applications, particularly when these compounds are regarded as metastable materials with intrinsically active frameworks. Herein, we demonstrate that microscale XMoO 4 powders exhibit distinct oxygen evolution reaction (OER) activities governed by structural instability and electronic structure. Single-phase XMoO 4 micropowders show composition-dependent OER performance (151 mA cm -2 for NiMoO 4 , 218 mA cm -2 for CoMoO 4 , and 206 mA cm -2 for CuMoO 4 in 1 M KOH). Moreover, the electrochemical oxidation behavior of XMoO 4 during water electrolysis varies markedly with the structural phase described (α and β), particularly near an applied potential of ~ 1 V. These results highlight the catalytic potential of metastable ternary metal oxide powders without relying on noble or rare-earth elements and suggest that metastability can serve as an effective design parameter for advanced electrocatalysts. Graphical Abstract