Chemical Behavior of Mo₂TMB₂ (TM = Fe, Co, Ni) upon the Oxygen Evolution Reaction (OER)

The (electro)chemical behavior of intermetallic compounds Mo2TMB2 (TM = Fe, Co, Ni) under OER conditions has been investigated using electrochemical data combined with extensive bulk- and surface-sensitive material characterization. In situ formation of TM-rich amorphous layers, composed of oxides and hydroxides, accompanied by partial dissolution of molybdenum and boron, was observed for all three compounds. The degree of molybdenum and boron dissolution also influences the electronic state of TMs in their oxides/hydroxides formed on the surface of Mo2TMB2. The in situ-formed Fe2O3 and Ni(OH)2 on the surface of Mo2FeB2 and Mo2NiB2, respectively, are the origin of surface passivation and their OER inactivity. At the same time, the simultaneous presence of Co3O4 and Co(OH)2 on the surface of an OER-exposed Mo2CoB2 electrode allows for the start of OER at a lower overpotential (ca. 290 mV) compared to elemental Co (ca. 370 mV), revealing better electrocatalytic activity. Extensive characterization of these materials as well as variation of the experimental conditions extends our understanding of the chemical properties of intermetallic compounds, which are of clear importance for their possible application as efficient electrocatalysts. © 2025 The Authors. Published by American Chemical Society.