Pore Filling of a Carbon Matrix by Melt-Impregnated LiBH4

A carbon matrix with pores of distinct diameters was gradually filled with LiBH4, and the corresponding nitrogen adsorption isotherms are presented. Four resins were prepared and characterized individually; micropores and mesopores of defined sizes (6, 10, 15, and 25 nm) were observed. Then, the four resins were mixed to obtain a material composed of micropores and four populations of mesopores of comparable volumes (0.18 cm3 for micropores and 0.16 cm3 for each mesopore, 0.82 cm3 in total). This mixture was impregnated with LiBH4 by melt impregnation at 10, 30, 50, 70, and 90 vol %, to determine how LiBH4 fills a carbon matrix, especially how pores of distinct sizes compete with one another. We observed that all pores are filled concomitantly, but smaller pores are filled faster. After the formation of a thin film, the mesopores follow an axial filling (radially, the pores are filled or not) as no sensible modification of the pore diameter was observed during filling, while the pore volume decreased. Calorimetric and volumetric studies were performed for each material filled with LiBH4. Afterward, we determined how hydrogen release affected pore distribution and observed that inversely to LiBH4 filling, LiH liberation affected pore diameters. Finally, we proposed a two-step pore filling protocol: the resins were filled with 10 vol % LiBH4 and dehydrogenated before filling with 20, 40, and 60 vol % LiBH4 to determine if the matrix can be efficiently doped with boron to improve its filling. This protocol also illustrates the inherent difficulties of the system that hinder its reversibility.