Electrochemical nitrate reduction to high-value ammonia on two-dimensional molybdenum carbide nanosheets for nitrate-containing wastewater upcycling

Electrochemical conversion of nitrate wastewater into high-value ammonia fertilizer has attracted extensive attention in wastewater treatment and resource recovery, but presents great challenges due to complicated reaction pathways and competing side reactions. Herein, we report a feasible method for the successful fabrication of Mo2C nanosheets (Mo2C NSs) as electrocatalyst for the electroreduction of nitrate to ammonia. Compared to Mo2C nanoparticles, the Mo2C NSs exhibited superior activity and selectivity in NH3 electrosynthesis with an NH3 yield rate of 25.2 mg·h−1·mg−1cat. at −0.4 V and a Faradaic efficiency of 81.4 % at −0.3 V versus reversible hydrogen electrode. The X-ray diffraction and transmission electron microscopy characterization verifted the controllable conversion of 2D MoO2 NSs into 2D Mo2C NSs. In situ spectroscopic studies and on-line differential electrochemical mass spectrometry revealed the proposed reaction pathway of NO3− to NH3 conversion, *NO3− → *NO2− → *NO→*NOH → *NH2OH → *NH3. Density functional theory calculations further verified the effective N-end NOH pathway with the conversion of *NH2OH to *NH2 as the rate-determining step requiring a low energy barrier of 0.58 eV. Importantly, the key hydrogenation of *NO to form *NOH species underwent a lower energy barrier of 0.39 eV compared with the formation of *ONH species (1.06 eV).