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MXenes and doped carbon nanotubes (CNTs) have entered into researcharenas for high-rate energy storage and conversion. Herein, a method of postsynthesis of MXenes in boron, nitrogen codoped CNTs (BNCNTs) is reported with their electrocatalytical hydrogen evolution performance. and h-MoN(001)/BNC. The h-MoN@BNCNT electrocatalyst exhibits a small overpotential of 78 mV at 10 mA cm −2 and Tafel slope of 46 mV per decade, which are dramatically improved over all reported MoN-based materials and doped CNTs. Additionally, it also exhibits outstanding electrochemical stability in environments with various pH values and seawater media from South China Sea.

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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 (NO 3 RR) 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 NO 3 RR over HER. Herein, we select a wellknown HER catalyst Mo 2 C and investigate how metal doping can switch its kinetics from HER-dominated to NO 3 RR-dominated pathways. At 3.8 wt % Ru doping of Mo 2 C, a 75% single pass conversion of nitrate (0.1 M) to ammonium in a 16 cm 2 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 NO 3 RR to promote the cooperative deoxygenation of *NO 3 and *NO 2 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.