Complex metal hydrides, containing up to 18 wt% H2, are attractive candidates for on-board hydrogen storage. However, only limited reversibility of H2 desorption is achieved under mild conditions, especially in the absence of catalysts. Nanoconfining the materials in porous matrixes facilitates rehydrogenation, but still full reversibility has been rarely achieved. We reveal the factors that limit the reversibility using NaAlH4 in a porous carbon matrix as a model system. Relatively large Al crystallites (>100 nm) are formed after desorption, migrating out of the mesopores of the matrix. However, their formation does not fundamentally limit the reversibility, as these crystallites react with Na(H) and H2 reforming nanoconfined NaAlH4 under relatively mild conditions. We show for the first time that the main limiting factor for the decayed cycling capacity is the loss of active alkali metal species. Evaporation losses are minor, even when dehydrogenating at 325 °C in vacuum. Significant losses (30–40%) occur upon the first hydrogen desorption run, and are attributed to the reaction of Na species with impurities in the carbon matrix. A one-time addition of extra Na compensates for this loss, leading to close to full reversibility (>90%) at 150 °C under 55 bar H2 pressure. A similar effect is found when adding extra Li species to nanoconfined LiBH4. For nanoconfined complex metal hydrides irreversible loss of the reactive alkali metal species due to reaction with impurities can act as a major loss mechanism. However, the one-time addition of extra alkali metal species is very effective in resolving this issue, leading to close to full cycling reversibility under relatively mild conditions even in the absence of catalysts.

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