Nanoconfined NaAlH4: prolific effects from increased surface area and pore volume

Abstract

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 NaAlH₄ 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 CO₂-activated under different conditions and provided aerogels with BET surface areas of 704, 1267 and 2246 m² g⁻¹ and total pore volumes of 0.91, 1.30 and 2.21 mL g⁻¹, respectively. Nanoconfinement of NaAlH₄ was achieved by melt infiltration and ²⁷Al MAS NMR reveals that the respective scaffolds incorporate 68, 82 and 91 wt% NaAlH₄, for the above-mentioned samples, while the remaining fraction decomposes to metallic Al indicating that increasing CO₂-activation tends to facilitate the infiltration process. The frequencies for the ²³Na and ²⁷Al MAS NMR centerband resonances from NaAlH₄ vary systematically for the infiltrated samples and are shifted towards higher frequency and become more narrow with increasing degree of CO₂ 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 NaAlH₄ was investigated using Rietveld refinement, revealing that the majority of added NaAlH₄ 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% H₂) was more than doubled compared to the nanoconfinement in the non-activated carbon aerogel (1.3 wt% H₂). Furthermore, it was demonstrated that Ti-functionalization of the CO₂-activated aerogels combines the high storage capacity with fast hydrogen release kinetics from NaAlH₄ which fully decomposes into Na₃AlH₆ at T ≤ 100 °C.