Thermodynamic and spectroscopic analysis of the ammonia absorption mechanism of a borohydride system

Abstract

Borohydrides are promising ammonia (NH₃) storage materials for controlling the storage pressure of NH₃, where the reaction mechanism between borohydrides and NH₃ is key to designing suitable properties to meet demands. However, the origin of the characteristics of 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 a mixture in the low NH₃ absorption region was dominated by an independent reaction of LiBH₄, a decrease in plateau pressure by synergistic effects was revealed from 1.5 mol mol⁻¹. In this reaction process, NH₃-induced liquefaction of the mixture 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 the transformation conditions to the liquid phase of NaBH₄ at low temperature were changed by stabilization of the solid NH₃ coordinated phases. In addition, theoretical simulation based on molecular models suggested a limit to the NH₃ absorption number for forming a stable NH₃-coordinated phase. The above-obtained results are recognized as important knowledge for establishing guidelines for material design of NH₃ absorption materials.