Modulating the Alkalinity of Molten Chloride Salt with Proton Sources for Ammonia Synthesis

Abstract

Ammonia is not only a crucial chemical feedstock for the production of nitrogen fertilizers, but also a promising hydrogen energy carrier. In recent years, molten salts have emerged as promising materials for applications in ammonia synthesis. However, a common issue is the limited release of ammonia from the molten salts due to the absorption by nitride ions (N3–) and hydride ions (H–). Herein, we present a study on the ammonia release from molten LiCl–KCl salt by introducing different proton sources, among which the acidic HCl gas effectively reacts with the alkaline nitride ions and hydride ions and thereby facilitates the ammonia release. Based on that, we have designed and tested a three-step method for ammonia synthesis comprising of molten salt electrolysis, Li nitridation, and HCl addition. A synthetic efficiency of 87.21% has been achieved if considering both ammonia and ammonium chloride as the products after an experiment involving 1-hour molten salt electrolysis at 1.5 A current and HCl as the proton source. This work not only unveils the interplay between the proton sources and the alkaline species in the molten salt, but also presents a promising stepwise method for enhancing ammonia synthesis in molten salt systems.