Hierarchical Nanostructured Co–Mo–B/CoMoO_4–<i>x</i> Amorphous Composite for the Alkaline Hydrogen Evolution Reaction

Abstract

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-<i>x</i> composite using a hydrothermal method, followed by annealing and NaBH₄ reduction treatments. Our study found that NaBH₄ reduction of CoMoO₄ resulted in the concurrent formation of amorphous Co-Mo-B and an O-vacancy-rich CoMoO_4-<i>x</i> 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 CoMoO₄·<i>n</i>H₂O 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-<i>x</i> substrate imparted the electrocatalyst with good electrical conductivity. As a result, the Co-Mo-B/CoMoO_4-<i>x</i>/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.