Dual Function of Hypo-d-electronic Transition Metals in the Brewer Intermetallic Phase for the Highly Efficient Electrocatalytic Hydrogen Evolution Reaction in Alkaline Electrolytes.

Reported are the synthesis, material characterization, and electrocatalytic hydrogen evolution reaction (HER) in acid and alkaline electrolytes for the Brewer intermetallic phase, Nb ₆ Co ₇ and Mo ₆ Co ₇ . It was realized that the overpotential at a current density of 10 mA/cm ² (η ₁₀ ) for Nb ₆ Co ₇ (η ₁₀ = 62 mV) and Mo ₆ Co ₇ (η ₁₀ = 143 mV) are both much lower than that of using a single Co metal (η ₁₀ = 253 mV) in alkaline electrolytes. The enhancement of electrocatalytic HER activity of Nb ₆ Co ₇ and Mo ₆ Co ₇ can be attributed to the hypo-hyper-d-electronic interaction between Nb/Mo and Co elements. Based on the result of density functional theory calculation, alloying between Nb/Mo and Co elements will increase the antibonding state population of the Co-Co bond near the Fermi level ( E _F ), which induces the synergistic effect to influence the adsorption energy of the H atom (Δ G _H ) on the surface of Nb ₆ Co ₇ and Mo ₆ Co ₇ . Moreover, the role of the Nb element is not only a simple electron donor but is also an anchor position for the OH molecule (i.e., dual function) due to the bonding character of the Nb-Co bond near E _F . It can reduce the OH position effect as well as the activation energy for water dissociation, which rationalizes the high and robust HER performance of Nb ₆ Co ₇ to that of commercial Pt/C (η ₁₀ = 67 mV) in alkaline electrolytes.