Nanoconfined NaAlH: prolific effects from increased surface area and pore volume

Nanoconfinement is a promising technique to improve the properties of nanomaterials such as the kinetics for hydrogen release and uptake and the stability during cycling. Here we present a systematic study of nanoconfined NaAlH 4 in nanoporous scaffolds with increasing surface area and pore volume and almost constant pore sizes in the range of 8 to 11 nm. A resorcinol formaldehyde carbon aerogel was CO 2 -activated under different conditions and provided aerogels with BET surface areas of 704, 1267 and 2246 m 2 g −1 and total pore volumes of 0.91, 1.30 and 2.21 mL g −1 , respectively. Nanoconfinement of NaAlH 4 was achieved by melt infiltration and 27 Al MAS NMR reveals that the respective scaffolds incorporate 68, 82 and 91 wt% NaAlH 4 , for the above-mentioned samples, while the remaining fraction decomposes to metallic Al indicating that increasing CO 2 -activation tends to facilitate the infiltration process. The frequencies for the 23 Na and 27 Al MAS NMR centerband resonances from NaAlH 4 vary systematically for the infiltrated samples and are shifted towards higher frequency and become more narrow with increasing degree of CO 2 activation of the scaffolds. This new effect is attributed to increasing interactions with conduction electrons from increasingly graphite-/graphene-like scaffolds. The bulk versus nanoconfined ratio of NaAlH 4 was investigated using Rietveld refinement, revealing that the majority of added NaAlH 4 is confined inside the nanopores. The hydrogen desorption kinetics decreased with increasing surface area and the hydrogen storage capacity is more stable and decreases less during continuous hydrogen release and uptake cycles. In fact, the available amount of hydrogen (2.7 wt% H 2 ) was more than doubled compared to the nanoconfinement in the non-activated carbon aerogel (1.3 wt% H 2 ). Furthermore, it was demonstrated that Ti-functionalization of the CO 2 -activated aerogels combines the high storage capacity with fast hydrogen release kinetics from NaAlH 4 which fully decomposes into Na 3 AlH 6 at T ≤ 100 °C. Scaffold activation provides increasing surface area, pore volume and facilitates melt infiltration and increasing interactions with graphite-/graphene-like conduction electrons.