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

Octahydridoborate, i.e. [BH] containing compounds, have recently attracted interest for hydrogen storage. In the present study, the structural, hydrogen storage, and ion conductivity properties of KBH have been systematically investigated. Two distinct polymorphic transitions are identified for KBH 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 KBH displays a high degree of disorder of the [BH] anion, which facilitates increased cation mobility, reaching a K conductivity of ∼10 S cm above 100 °C. β-KBH starts to release hydrogen at ∼160 °C, simultaneously with the release of BH and trace amounts of BH. KBH and K(BH)(BH) are identified as crystalline decomposition products above 200 °C, and the formation of a KBH deficient structure of K(BH)(BH) is observed at elevated temperature. The hydrogen-uptake properties of a KBH-2KH composite have been examined under 380 bar H, resulting in the formation of KBH at T≥ 150 °C along with higher metal hydridoborates, i.e. KBH, KBH, and KBH.