From cluster halides to catalysts: nanostructured molybdenum carbides for efficient hydrogen evolution reaction

Abstract

An original synthesis route for tailoring the hydrogen evolution reaction (HER) activity of α-Mo₂C-based catalysts is reported using soluble and air-stable halide precursors built from nanosized [Mo₆Cl₁₄]²⁻ cluster units, namely (H₃O)₂[Mo₆Cl₁₄]·7H₂O and ((n-C₄H₉)₄N)₂[Mo₆Cl₁₄], combined with sucrose as a biosourced carbon source. The resulting catalysts consist of nanosized α-Mo₂C crystallites embedded in residual carbon, with molybdenum preserved in the +2 oxidation state from the halide precursor to the final material. The chemical nature of the precursor strongly influences the phase composition, homogeneity, specific surface area, and morphology of the resulting α-Mo₂C powders. Among the synthesized materials, the catalyst derived from ((n-C₄H₉)₄N)₂[Mo₆Cl₁₄] exhibits the highest HER performance in alkaline media, characterized by the lowest overpotential at 10 mA cm⁻², the largest electrochemical surface area, and the smallest Tafel slope. Long-term electrochemical testing also reveals surface activation during operation, enhancing both activity and stability. These findings indicate that the catalytic behavior of α-Mo₂C cannot be attributed solely to nanostructuring effects but arises from a subtle interplay between crystallite size, porosity, and surface chemistry. The use of Mo₆ cluster-based precursors thus provides an effective and versatile approach to control the composition and molybdenum oxidation state of molybdenum carbides, enabling the design of advanced powder materials with optimized surface and electrocatalytic properties.