Recent progress and strategies on the design of catalysts for electrochemical ammonia synthesis from nitrate reduction

Ammonia (NH3) is an essential raw material in the production of fertilizers and a promising carbon-free energy carrier, however, its synthesis still depends on the energy- and capital-intensive Haber-Bosch process. Recently, the electrochemical N-2 reduction reaction has attracted significant interest as an emerging method for NH3 synthesis under ambient conditions. However, the limited solubility of N-2 in aqueous electrolyte and the strong N=N bonds result in a low NH3 yield rate, inferior faradaic efficiency and unsatisfactory selectivity, impeding its further practical application. Considering the high water solubility of nitrate (NO3-), the electrochemical NO3- reduction reaction (NO3-RR) has become a fascinating route for achieving sustainable production of NH3, and enormous progress has been made in this field. As a consequence, this review discusses the reaction mechanism of the electrochemical reduction of NO3- and systematically summarizes the recent development of electrocatalysts for the NO3-RR, including noble-metal-based materials, single-atom metal catalysts, and transition-metal-based catalysts. Diverse design strategies of the catalysts to boost the NO3-RR performance, such as defect engineering, rational structure design, strain engineering and constructing heterostructures, are discussed. This is followed by an illustration of how a robust understanding of the optimization strategies affords fundamental insights into the NH3 yield rate, faradaic efficiency, and selectivity of the electrocatalysts. Finally, we conclude with future perspectives on the critical issues, challenges and research directions in the design of high-efficiency electrocatalysts for selective reduction of NO3- to NH3.