Systematic Pore-Size Effects of Nanoconfinement of LiBH4: Elimination of Diborane Release and Tunable Behavior for Hydrogen Storage Applications

The effects of nanoconfinement on the structural phase transition, H2 release and uptake, and the emission of toxic diborane (B2H6) on desorption of LiBH4 have been comprehensively investigated in the presence of various porous hard carbon templates at a variety of pore sizes. Calorimetry signatures of both the structural phase transition and melting of nanoconfined LiBH4 shifted to a lower temperature with respect to the bulk, finally vanishing below a pore size around 4 nm. The desorption of LiBH4 confined in these nanoporous carbons shows a systematic and monotonic decrease in the desorption temperature and concomitantly, mass spectroscopic analysis indicated a gradual reduction of the partial pressure of B2H6 with decreasing pore size, suggesting that formation of stable closoborane salts may be avoided by interrupting the reaction pathway. This represents a major breakthrough in the reversibility of boron-based hydrogen storage systems, where capacity is lost in the formation of stable B−H species on cycling. Different carbon preparation techniques suggest that the confinement size, and not solely surface interactions, may be used to tune the properties of complex hydrides for kinetic and reaction pathway improvements for hydrogen storage applications.