Boron-Coated Crystalline Cobalt Boride Nanosheets as Highly Efficient and Durable Electrocatalysts for Acidic Hydrogen Evolution

The development of highly active and durable non-noble metal electrocatalysts to replace platinum for the hydrogen evolution reaction (HER) is essential for scalable and efficient hydrogen production. Crystalline metal borides have recently emerged as promising HER electrode materials. Here, we demonstrate for the first time that crystalline cobalt boride (CoB) nanosheets─previously considered active primarily in alkaline electrolytes─exhibit excellent HER performance, stability, and durability in acidic media when coated with a thin amorphous boron layer. We further enhanced the catalytic activity by densifying the agglomerated nanosheets, which effectively cover the electrode surface and significantly improve electrical conductivity. The resulting densified CoB electrode requires an overpotential of only 59 mV (with a Tafel slope of 68.1 mV dec –1 ) to achieve a current density of 10 mA cm –2 . Notably, it maintains impressive activity even at high current densities up to 1000 mA cm –2 . These experimental results are corroborated by density functional theory (DFT) calculations, which reveal abundant active sites on the (101) and (111) surfaces of CoB, with mixed Co/B hollow sites identified as the most favorable. Moreover, at high hydrogen coverage (corresponding to high current densities), a synergistic mechanism is proposed to account for the enhanced performance. This work establishes B-coated crystalline CoB nanosheets as a competitive class of high-performing, nonprecious-metal HER catalysts in acidic electrolytes.