Graphene- and Phosphorene-like Boron Layers with Contrasting Activities in Highly Active Mo₂B₄ for Hydrogen Evolution

Two different boron layers, flat (graphene-like) and puckered (phosphorene-like), found in the crystal structure of Mo₂B₄ show drastically different activities for hydrogen evolution, according to Gibbs free energy calculations of H-adsorption on Mo₂B₄. The graphene-like B layer is highly active, whereas the phosphorene-like B layer performs very poorly for hydrogen evolution. A new Sn-flux synthesis permits the rapid single-phase synthesis of Mo₂B₄, and electrochemical analyses show that it is one of the best hydrogen evolution reaction active bulk materials with good long-term cycle stability under acidic conditions. Mo₂B₄ compensates its smaller density of active sites if compared with highly active bulk MoB₂ (which contains only the more active graphene-like boron layers) by a 5-times increase of its surface area.