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

Abstract

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 N₂ reduction reaction has attracted significant interest as an emerging method for NH₃ synthesis under ambient conditions. However, the limited solubility of N₂ in aqueous electrolyte and the strong NN 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.