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 Li2B12H12, Na2B12H12, and K2B12H12 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 M2B12H12 salts form the corresponding metal borohydrides MBH4 (M = Li, Na, K) in almost quantitative yield at 100 MPa H-2 and 500 degrees C. In addition, stoichiometric mixtures of Li2B12H12 and MgH2 were found to form MgB2 at 500 degrees 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.