Stable nature of [BH₄]⁻ ions in deliquescence thin-film NaBH₄ and humidity control of their decomposition toward hydrogen supply and storage application

This study investigates the deliquescence behavior of NaBH₄ films under atmospheric conditions, with a focus on their potential application as hydrogen supply systems [1]. Upon exposure to air, the films underwent deliquescence accompanied by hydrogen generation due to [BH₄]⁻ ion decomposition. However, the decomposition rate was sufficiently slow to allow a substantial quantity of [BH₄]⁻ ions to remain stable within the deliquesced film. As a result, we demonstrated the achievement of a reversible deliquescence-recrystallization cycle in NaBH₄ films through precise humidity control. Molecular dynamics simulations revealed that Na⁺ and [BH₄]⁻ ions exhibit a tendency to maintain proximity to each other, with interatomic distances very close to those observed in the crystalline state. We postulate that these localized ordered structures in deliquesced NaBH₄ may inhibit excessive hydration and subsequent decomposition of [BH₄]⁻ ions, thereby contributing to their stability and enabling the reversible deliquescence-recrystallization cycle.

This integration of experimental and theoretical approaches provides novel insights into the hydrogen release from borohydrides in thin film form, based on the control of ambient humidity conditions. Our findings have implications significant for the development of advanced hydrogen storage and supply systems, potentially contributing toward a sustainable energy society.

[1] T. Sato, K. Kaminaga et al., Int. J. Hydrogen Energy. 74, 232-237 (2024).