Highly Selective Electrochemical Reduction of Dinitrogen to Ammonia at Ambient Temperature and Pressure over Iron Oxide Catalysts

The catalytic conversion of dinitrogen (N₂) into ammonia under ambient conditions represents one of the Holy Grails in sustainable chemistry. As a potential alternative to the Haber–Bosch process, the electrochemical reduction of N₂ to NH₃ is attractive owing to its renewability and flexibility, as well as its sustainability for producing and storing value-added chemicals from the abundant feedstock of water and nitrogen on earth. However, owing to the kinetically complex and energetically challenging N₂ reduction reaction (NRR) process, NRR electrocatalysts with high catalytic activity and high selectivity are rare. In this contribution, as a proof-of-concept, we demonstrate that both the NH₃ yield and faradaic efficiency (FE) under ambient conditions can be improved by modification of the hematite nanostructure surface. Introducing more oxygen vacancies to the hematite surface renders an improved performance in NRR, which leads to an average NH₃ production rate of 0.46 μg h⁻¹ cm⁻² and an NH₃ FE of 6.04 % at −0.9 V vs. Ag/AgCl in 0.10 m KOH electrolyte. The durability of the electrochemical system was also investigated. A surprisingly high average NH₃ production rate of 1.45 μg h⁻¹ cm⁻² and a NH₃ FE of 8.28 % were achieved after the first 1 h chronoamperometry test. This is among the highest FEs reported so far for non-precious-metal catalysts that use a polymer-electrolyte-membrane cell and is much higher than the FE of precious-metal catalysts (e.g., Ru/C) under comparable reaction conditions. However, the NH₃ yield and the FE dropped to 0.29 μg h⁻¹ cm⁻² and 2.74 %, respectively, after 16 h of chronoamperometry tests, which indicates poor durability of the system. Our results demonstrate the important role that the surface states of transition-metal oxides have in promoting electrocatalytic NRR under ambient conditions. This work may spur interest towards the rational design of electrocatalysts as well as electrochemical systems for NRR, with emphasis on the issue of stability.