Electrochemical synthesis of ammonia in molten salts

Abstract

Abstract Ammonia is important feedstock for both fertilizer production and carbon-free liquid fuel. Many techniques for ammonia formation have been developed, hoping to replace the industrial energy-intensive Haber–Bosch route. Electrochemical synthesis of ammonia in molten salts is one promising alternative method due to the strong solubility of N3− ions, a wide potential window of molten salt electrolytes and tunable electrode reactions. Generally, electrochemical synthesis of ammonia in molten salts begins with the electro-cleavage of N2/hydrogen sources on electrode surfaces, followed by diffusion of N3−/H+-containing ions towards each other for NH3 formation. Therefore, the hydrogen sources and molten salt composition will greatly affect the reactions on electrodes and ions diffusion in electrolytes, being critical factors determining the faradaic efficiency and formation rate for ammonia synthesis. This report summarizes the selection criteria for hydrogen sources, the reaction characteristics in various molten salt systems, and the preliminary explorations on the scaling-up synthesis of ammonia in molten salt. The formation rate and faradaic efficiency for ammonia synthesis are discussed in detail based on different hydrogen sources, various molten salt systems, changed electrolysis conditions as well as diverse catalysts. Electrochemical synthesis of ammonia might be further enhanced by optimizing the molten salt composition, using electrocatalysts with well-defined composition and microstructure, and innovation of novel reaction mechanism.