Recent advances of metal oxide catalysts for electrochemical NH3 production from nitrogen-containing sources

Abstract

World ammonia production reached 235 million tons in 2019, and roughly 88% of NH₃ produced goes into the agriculture sector as fertilizers. Industrial ammonia production relies on the Haber–Bosch process, which is highly energy demanding and results in high CO₂ emission. More than 1% of global energy generation was required to power this Haber–Bosch process. While sustainable development has become a general consensus across the globe, there has been enormous research interest toward the possible modification or replacement of the Haber–Bosch process, aiming to reduce the environmental impact of NH₃ production. With the successful commercialization of various renewable source-powered electricity generation techniques, the electrochemical reduction of nitrogen-containing chemicals (including N₂, NO₃⁻, NO₂⁻ and NO) to produce NH₃ under ambient conditions has emerged as a potential green alternative to the Haber–Bosch process. This technique utilizes renewable electricity to achieve small-scale, on-site and on-demand ammonia production, serving as a critical contribution to the overall carbon neutral economy. The design and synthesis of novel catalysts with high NH₃ production rate and selectivity is the key challenge in determining economic feasibility for this electrochemical NH₃ production. In view of the rapid and fruitful development of metal oxides as promising electrocatalysts toward NH₃ formation, this review summarizes different types of metal oxides used for the electrochemical N₂ reduction reaction and electrochemical NO_x reduction reaction, together with design strategies to enhance their catalytic performance. As a concluding remark, our thoughts are given on the critical challenges in this field, suggesting possible future research directions to accomplish industrialization for the electrosynthesis of NH₃.