Canonic-Like HER Activity of Cr_{1- x} Mo_x B₂ Solid Solution: Overpowering Pt/C at High Current Density

Abundant transition metal borides are emerging as substitute electrochemical hydrogen evolution reaction (HER) catalysts for noble metals. Herein, an unusual canonic-like behavior of the c lattice parameter in the AlB₂ -type solid solution Cr_{1- x} Mo_x B₂ (x = 0, 0.25, 0.4, 0.5, 0.6, 0.75, 1) and its direct correlation to the HER activity in 0.5 M H₂ SO₄ solution are reported. The activity increases with increasing x, reaching its maximum at x = 0.6 before decreasing again. At high current densities, Cr_0.4 Mo_0.6 B₂ outperforms Pt/C, as it needs 180 mV less overpotential to drive an 800 mA cm^-2 current density. Cr_0.4 Mo_0.6 B₂ has excellent long-term stability and durability showing no significant activity loss after 5000 cycles and 25 h of operation in acid. First-principles calculations have correctly reproduced the nonlinear dependence of the c lattice parameter and have shown that the mixed metal/B layers, such as (110), promote hydrogen evolution more efficiently for x = 0.6, supporting the experimental results.