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, Nb6Co7 and Mo6Co7. It was realized that the overpotential at a current density of 10 mA/cm2 (η10) for Nb6Co7 (η10 = 62 mV) and Mo6Co7 (η10 = 143 mV) are both much lower than that of using a single Co metal (η10 = 253 mV) in alkaline electrolytes. The enhancement of electrocatalytic HER activity of Nb6Co7 and Mo6Co7 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 (EF), which induces the synergistic effect to influence the adsorption energy of the H atom (ΔGH) on the surface of Nb6Co7 and Mo6Co7. 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 EF. 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 Nb6Co7 to that of commercial Pt/C (η10 = 67 mV) in alkaline electrolytes.