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

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