Chemical Behavior of Mo₂<i>TM</i>B₂ (<i>TM</i> = Fe, Co, Ni) upon the Oxygen Evolution Reaction (OER)

The (electro)-chemical behavior of intermetallic compounds Mo₂ <i>TM</i>B₂ (<i>TM</i> = Fe, Co, Ni) under OER conditions has been investigated using electrochemical data combined with extensive bulk- and surface-sensitive material characterization. <i>In situ</i> formation of <i>TM</i>-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 <i>TM</i>s in their oxides/hydroxides formed on the surface of Mo₂ <i>TM</i>B₂. The <i>in situ</i>-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.