Thermodynamic and Spectroscopic Analysis for Ammonia Absorption Mechanism of Borohydride System

Abstract

Borohydrides are promising ammonia (NH₃) storage materials to control the storage pressure of NH₃, where the reaction mechanism between borohydrides and NH₃ is key to designing suitable properties for demands. However, the origin of characteristics for NH₃ absorption processes, such as the formation of solid complexes and solution phases, is unclear. In this work, the NH₃ absorption properties of the LiBH₄ and NaBH₄ mixed system are investigated by thermodynamic and spectroscopic analyses, and the results are compared with those of single components. Although the NH₃ absorption properties of mixture at the low NH₃ absorption region was dominated by independent reaction of LiBH₄, decrease in plateau pressure by synergistic effects was revealed from 1.5 mol/mol. In this reaction process, NH₃-induced liquefaction of the mixture was occurred due to the dissolution of LiBH₄(NH₃)₃ into a NaBH₄ based solvation phase, NaBH₄(NH₃)₂. The synergistic thermodynamic variation originated in the liquefaction of NaBH₄. In fact, the required NH₃ absorption amount at 243 K was higher than that at 293 K because transformation conditions to liquid phase of NaBH₄ at low temperature was changed by stabilizing the solid NH₃ coordinated phases. In addition, theoretical simulation based on molecular models suggested limit of NH₃ absorption number to form stable NH₃-coordinated phase. The above obtained results are recognized as important knowledge to establish a guideline for material designs of NH₃ absorption materials