Metal−Organic Framework Nanosheet-Derived CuO for Efficient Electrochemical Nitrate Reduction to Ammonia

Electrocatalytic nitrate reduction (NO3RR) offers a “two birds with one stone” strategy by simultaneously removing pollutants and producing value-added ammonia (NH3). However, its multiple electron- and proton-transfer steps require catalysts with both high activity and selectivity. Copper (Cu) is widely regarded as one of the most promising NO3RR electrocatalysts due to its low cost and relatively high activity, yet its ammonia yield rate is usually constrained by sluggish nitrite conversion kinetics. Herein, a series of Cu-based catalysts with distinct oxidation states and crystallinity were derived from Cu-based metal−organic framework nanosheets (Cu-MOF NSs) by adjusting the pyrolysis conditions, and their NO3RR performance in alkaline media was evaluated. The low-crystallinity copper oxide catalyst (CuO-1) achieved the highest NH3 yield rate of 11.5 mg cm−2 h−1 at −0.8 V vs reversible hydrogen electrode (RHE), significantly outperforming the metallic Cu catalyst (Cu@NC), the highly crystalline CuO catalyst (CuO-2), and most previously reported catalysts. Mechanistic investigations suggest that CuO-1 undergoes partial reduction during the reaction to generate Cu0/Cu+ species, which may serve as key active species promoting ammonia production by enhancing active hydrogen generation. Finally, NH4Cl was recovered through acid absorption and rotary evaporation, demonstrating a practical route for converting NO3− pollutants into valuable NH3 products.