Unraveling the Performance Descriptors for Designing Single‐Atom Catalysts on Defective MXenes for Exclusive Nitrate‐To‐Ammonia Electrocatalytic Upcycling

Electrocatalytic nitrate reduction reaction (NO₃ RR) is a promising approach for converting nitrate into environmentally benign or even value-added products such as ammonia (NH₃ ) using renewable electricity. However, the poor understanding of the catalytic mechanism on metal-based surface catalysts hinders the development of high-performance NO₃ RR catalysts. In this study, the NO₃ RR mechanism of single-atom catalysts (SACs) is systematically explored by constructing single transition metal atoms supported on MXene with oxygen vacancies (O_v -MXene) using density functional theory (DFT) calculations. The results indicate that Ag/O_v -MXene (for precious metal) and Cu/O_v -MXene (for non-precious metal) are highly efficient SACs for NO₃ RR toward NH₃ , with low limiting potentials of -0.24 and -0.34 V, respectively. Furthermore, these catalysts show excellent selectivity toward ammonia due to the high energy barriers associated to the formation of byproducts such as NO₂ , NO, N₂ O, and N₂ on Ag/O_v -MXene and Cu/O_v -MXene, effectively suppressing the competitive hydrogen evolution reaction (HER). The findings not only offer new strategies for promoting NH₃ production by MXene-based SACs electrocatalysts under ambient conditions but also provide insights for the development of next-generation NO₃ RR electrocatalysts.