From scheelite BaMoO₄ to perovskite BaMoO₃: Enhanced electrocatalysis toward the hydrogen evolution in alkaline media

The discovery of new, inexpensive, and efficient electrocatalysts for the hydrogen evolution reaction (HER) in alkaline solutions holds key to the realization of clean hydrogen production through water electrolysis. While molybdenum (Mo)-based inorganic compounds have been extensively investigated as catalyst candidates toward the alkaline HER, Mo-based oxides, in particular, complex oxides, are rarely explored. Here we report a Mo-containing, metallic BaMoO₃ perovskite, which is derived from a scheelite-type, insulating BaMoO₄ oxide through a thermal reduction-induced phase transformation, as a potential electrocatalyst for driving the alkaline HER. The BaMoO₃ perovskite made of interconnected MoO₆ octahedra is found to be more HER-active than the BaMoO₄ scheelite consisting of isolated MoO₄ tetrahedra, showing a significant increase in intrinsic activity by about one order of magnitude, which is due to the stabilized Mo tetravalent state, the increased surface oxygen vacancy concentration, and the improved electrical conductivity of BaMoO₃. Importantly, BaMoO₃ exhibits a smaller overpotential to deliver a geometric current density of −10 mA cm_geo⁻² when compared with many of the bulk-sized perovskite catalysts comprising other transition metals (e.g., Mn, Fe, Co, and Ni) and the intensively studied Mo-based catalysts (e.g., MoS₂), making it highly promising as an alternative electrocatalyst for the alkaline HER.