Hierarchical Nanostructured Co–Mo–B/CoMoO4–x Amorphous Composite for the Alkaline Hydrogen Evolution Reaction

Transition metal borides (TMBs) are a class of important but less well-explored electrocatalytic materials for water splitting. The lack of an advanced methodology to synthesize complex nanostructured TMBs with tunable surface properties is a major obstacle to the exploration of the full potential of TMBs for electrocatalytic applications. Here, we report the facile fabrication of a cobalt foam (CF)-supported hierarchical nanostructured Co–Mo–B/CoMoO 4– x composite using a hydrothermal method, followed by annealing and NaBH 4 reduction treatments. Our study found that NaBH 4 reduction of CoMoO 4 resulted in the concurrent formation of amorphous Co–Mo–B and an O-vacancy-rich CoMoO 4– x substrate, which cooperatively catalyzed the hydrogen evolution reaction (HER) in an alkaline electrolyte. The hierarchical nanoporous structure derived from the dehydration and partial reduction reactions of the CoMoO 4 · n H 2 O precursor could offer ample accessible active sites, as well as interconnected channels for rapid mass transfer. In addition, the in situ growth of electrically conductive Co–Mo–B nanoparticles on the defective structured CoMoO 4– x substrate imparted the electrocatalyst with good electrical conductivity. As a result, the Co–Mo–B/CoMoO 4– x /CF catalyst showed impressively high activity and outstanding stability for the alkaline HER, outperforming most reported TMB electrocatalysts. For instance, it required an overpotential of 55 mV to afford 10 mA·cm –2 and showed a fluctuation of only ±8 mV in a 100 h constant-current test at 100 mA·cm –2 .