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

Abstract

In this study, we investigated the deliquescence of NaBH4 thin films under atmospheric conditions toward their hydrogen supply application. Upon exposed to air, the film deliquesced along with generating hydrogen due to the decomposition of [BH4]- ions, but the decomposition rate was slow enough for a sizable amount of [BH4]- ions remained stable in the deliquesced film. As a result, a reversible deliquescence-recrystallization cycle in the NaBH4 thin film could be achieved through control of humidity. Molecular dynamics simulations suggested that Na+ and [BH4]- ions tended to neighbor each other with almost the same intra-distance as that in the crystal. A kind of such local ordered structures in deliquesced NaBH4 may prevent an excess hydration and subsequent decomposition of [BH4]- ions and is responsible for the stability of [BH4]- ions, enabling the reversible deliquescence-recrystallization cycle. These combined experimental and theoretical insights offer new perspectives on controlling hydrogen release from borohydrides based on humidity conditions in the thin film form.