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

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 N-2, 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 N-2 reduction reaction and electrochemical NOx 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.