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

Electrocatalytic nitrate reduction reaction (NO3RR) 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 NO3RR catalysts. In this study, the NO3RR mechanism of single-atom catalysts (SACs) is systematically explored by constructing single transition metal atoms supported on MXene with oxygen vacancies (Ov-MXene) using density functional theory (DFT) calculations. The results indicate that Ag/Ov-MXene (for precious metal) and Cu/Ov-MXene (for non-precious metal) are highly efficient SACs for NO3RR 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, N2O, and N2 on Ag/Ov-MXene and Cu/Ov-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 NO3RR electrocatalysts. Single-atom catalysts (SACs) consisting of 3d, 4d, and 5d transition metals on defective MXenes with oxygen vacancies (Ov) are explored computationally for their NO3- reduction performance with Ag and Cu found to be exceptionally active and selective for NH3 production, thereby revealing the underlying descriptors for efficient nitrate-to-ammonia upcycling.image