Ammonia electro-synthesis from nitrogen and water using a cyclic lithium-mediated method

Abstract

Ammonia (NH3) synthesis is vital to feed a growing world population. However, its primary synthesizing method, the Haber-Bosch process, is fossil fuels-powered and unsatiable. The lithium-mediated pathway has shown to be a promising alternative to produce ammonia with renewable feedstocks and electric energy. However, its continuous operation is still hampered by limited methods to plate Li stably from its aqueous solution because of its exceptionally high reactivity. In this study, a novel lithium metal battery (LMB)-mimic strategy, consisting of Li+ recovery and Li plating steps, is adapted to electro-plate lithium metals from its aqueous solution. Li+ in the solution is extracted spontaneously by a Li-deficient electrode in the Li recovery step and then electro-plated into metallic Li in a Li-friendly system by the Li plating steps. The obtained Li deposits could be utilized for subsequent nitridation and ammonia synthesis. The LMB-mimic system gave a Li plating efficiency exceeding 95% when the capacity of Li deposits was over 12.65 mAh cm-2. This result was similar to the Faradic efficiency (FE) towards ammonia. After the plating capacity was increased to over 15 mAh cm-2, the FE could reach more than 92 %, which is among the best-reported values. From the experimental results and characterization, the solid-electrolyte interphase (SEI) layer was found to play an essential role in high-efficiency Li plating. During Li plating, a limited amount of Li would be consumed upon its first contact with the electrolyte, forming a protective layer to avoid further consumption. Furthermore, the LMB-mimic system can be operated stably because of the highly reversible phase transition of LiMn2O4 during the Li recovery and plating process. This research provides a stable and efficient method for sustainable ammonia synthesis under ambient pressure.