Review on Ammonia Absorption Materials: Metal Hydrides, Halides, and Borohydrides

Ammonia is well-known as a hydrogen carrier owing to its high hydrogen capacity (17.8 wt %). However, the toxicity and the high storage pressure limit the application of ammonia. Consequently, storing ammonia in solid state has become the promising method to utilize ammonia for practical applications. In this review, ammonia absorption properties of metal hydrides, halides, and borohydrides to form metal amides and metal ammine complexes with various coordination numbers have been systematically summarized. Through this research, we found the correlation between the reactivity with ammonia and the Pauling electronegativity of neutral atoms according to different systems. Metal hydrides with small electronegativity value of the neutral atom of the cations can react with ammonia to form metal amides, which can be used as hydrogen storage material. For metal halides or borohydrides, the lower plateau pressure of ammonia absorption can be obtained in the material with larger electronegativity value of the neutral atom of cations. This useful tendency can be used in the materials design for the potential applications of ammonia-fed fuel cells.