Nanoconfined NaAlH4: Determination of Distinct Prolific Effects from Pore Size, Crystallite Size, and Surface Interactions

Nanoconfinement is a new method to improve the hydrogen storage properties for metal hydrides. A systematic study of melt-infiltrated NaAlH 4 in resorcinol formaldehyde carbon aerogels with pore sizes of D max = 4, 7, 10, 13, 19, 22, 26, 39 and >100 nm is presented. A linear correlation between pore size and crystalline domain size is observed using PXD. The distinct effects from pore size, crystallite size, and interfacial contact between NaAlH 4 and carbon aerogel on hydrogen release and uptake properties are investigated. In situ synchrotron powder X-ray diffraction shows that formation of crystalline NaAlH 4 nanoparticles only occurs in the nanoporous scaffolds (4 < D max < 100 nm). The hydrogen desorption kinetics are significantly improved by confinement in the macroporous scaffold ( D max > 100 nm) as compared to bulk NaAlH 4, i.e., reduction of the temperature for maximum hydrogen release rate of Δ T max = −90 °C. Additional improvement is induced by reducing the pore sizes in the range 7 ≤ D max ≤ 39 nm and thereby the NaAlH 4 crystallite sizes, but this effect is small (Δ T max = −12 to −16 °C) relative to the catalytic effects induced by the aerogel surface. Sieverts’ measurements reveal similar stability and preserved reversible hydrogen storage capacity after four hydrogen release and uptake cycles for both nano- ( D max = 13 nm) and macroporous scaffolds ( D max > 100 nm) of ∼53% of the original capacity. The results suggests that a significant contribution to the observed improvements of kinetics and reversibility by nanoconfinement of NaAlH 4 in carbon aerogels can be assigned to the catalytic properties of the scaffold surfaces and a minor contribution arises from nanoconfinement and reduction of the pore size in the range of 39–7 nm.