Swinging Hydrogen Evolution to Nitrate Reduction Activity in Molybdenum Carbide by Ruthenium Doping

A common challenge for electrochemical ammonia synthesis in an aqueous phase is the consumption of Faradaic charge by the competing hydrogen evolution reaction (HER), which reduces the Faradaic efficiency for the desired conversion, i.e., the nitrate reduction reaction (NO3RR) to ammonium. This problem is particularly severe when a single-phase catalyst is operated at high current limits, thus a cocatalyst system that works synergistically for hydrogen acquisition and deoxygenation is needed to promote NO3RR over HER. Herein, we select a wellknown HER catalyst Mo2C and investigate how metal doping can switch its kinetics from HER-dominated to NO3RR-dominated pathways. At 3.8 wt % Ru doping of Mo2C, a 75% single pass conversion of nitrate (0.1 M) to ammonium in a 16 cm2 flow electrolyzer was achieved, corresponding to an ammonium yield rate of 9.07 mmol h-1 at a full cell voltage of 2 V. As confirmed by DFT calculations and kinetic isotope experiments, ruthenium dopants in the matrix serve as the sink point for adsorbed hydrogen during NO3RR to promote the cooperative deoxygenation of *NO3 and *NO2 on the Ru-Mo cocatalytic site. Our study suggests that optimizing hydrogen acquisition and deoxygenation reactions in cocatalytic systems is an effective strategy for electrochemical synthesis.