Understanding and Mitigating the Effects of Stable Dodecahydro-closo-dodecaborate Intermediates on Hydrogen-Storage Reactions

Alkali metal borohydrides can reversibly store hydrogen; however, the materials display poor cyclability, oftentimes linked to the occurrence of stable closo -polyborate intermediate species. In an effort to understand the role of such intermediates on the hydrogen storage properties of metal borohydrides, several alkali metal dodecahydro- closo -dodecaborate salts were isolated in anhydrous form and characterized by diffraction and spectroscopic techniques. Mixtures of Li 2 B 12 H 12, Na 2 B 12 H 12, and K 2 B 12 H 12 with the corresponding alkali metal hydrides were subjected to hydrogenation conditions known to favor partial or full reversibility in metal borohydrides. The stoichiometric mixtures of MH and M 2 B 12 H 12 salts form the corresponding metal borohydrides MBH 4 (M = Li, Na, K) in almost quantitative yield at 100 MPa H 2 and 500 °C. In addition, stoichiometric mixtures of Li 2 B 12 H 12 and MgH 2 were found to form MgB 2 at 500 °C and above upon desorption in vacuum. The two destabilization strategies outlined above suggest that metal polyhydro- closo -polyborate species can be converted into the corresponding metal borohydrides or borides, albeit under rather harsh conditions of hydrogen pressure and temperature.