Kinetic Enhancement of Hydrogen Cycling in NaAlH$_{4 }$ by Melt Infusion into Nanoporous Carbon Aerogel

Enhanced kinetic performance and reversibility have been achieved with uncatalyzed NaAlH$_{4}$ by incorporation into nanoporous carbon aerogel. Aerogel with a pore size distribution peaked at 13 nm and a pore volume of 0.8 cm$^{3}$/g was filled with NaAlH$_{4}$ to 94{\%} capacity by melt-infusion at 189 C under 183 bar H$_{2}$ gas overpressure. Dehydrogenation to NaH + Al with reasonable kinetics was accomplished at 150 C, well below the NaAlH$_{4}$ melting temperature (183 C), compared to hydrogen release above 230 C for bulk uncatalyzed NaAlH$_{4}$. Uncatalyzed bulk samples did not rehydrogenate under laboratory conditions, whereas NaAlH$_{4}$ in a carbon aerogel host was readily rehydrogenated at $\sim $160 C and 100 bar H$_{2}$ to $\sim $85{\%} of its initial capacity. Ball-milled NaAlH$_{4}$ catalyzed with 4 mol{\%} TiCl$_{3}$ showed somewhat better kinetics compared to the infused aerogel; nevertheless, the large kinetic enhancement obtained by incorporation into carbon aerogel, even in the absence of a catalyst, demonstrates the substantial benefit of confining the NaAlH$_{4}$ to nanoscale dimensions.