Potassium octahydridotriborate: Diverse polymorphism in a potential hydrogen storage material and potassium ion conductor

Octahydridoborate, i.e. [B₃H₈]^- containing compounds, have recently attracted interest for hydrogen storage. In the present study, the structural, hydrogen storage, and ion conductivity properties of KB₃H₈ have been systematically investigated. Two distinct polymorphic transitions are identified for KB₃H₈ from a monoclinic (α) to an orthorhombic (α′) structure at 15 °C via a second-order transition and eventually to a cubic (β) structure at 30 °C by a first-order transition. The β-polymorph of KB₃H₈ displays a high degree of disorder of the [B₃H₈]^- anion, which facilitates increased cation mobility, reaching a K⁺ conductivity of ∼10^-7 S cm^-1 above 100 °C. β-KB₃H₈ starts to release hydrogen at ∼160 °C, simultaneously with the release of B₅H₉ and trace amounts of B₂H₆. KBH₄ and K₃(BH₄)(B₁₂H₁₂) are identified as crystalline decomposition products above 200 °C, and the formation of a KBH₄ deficient structure of K_3-x(BH₄)_1-x(B₁₂H₁₂) is observed at elevated temperature. The hydrogen-uptake properties of a KB₃H₈-2KH composite have been examined under 380 bar H₂, resulting in the formation of KBH₄ at T ≥ 150 °C along with higher metal hydridoborates, i.e. K₂B₉H₉, K₂B₁₀H₁₀, and K₂B₁₂H₁₂.