Comparison of hydrogen cycling kinetics in NaAlH4-carbon aerogel composites synthesized by melt infusion or ball milling

This work compares the initial desorption and hydrogen cycling kinetics of NaAlH₄ melt infused into carbon aerogel with NaAlH₄-carbon aerogel composite synthesized by ball milling. Samples having comparable carbon content (47.4 wt%) prepared by either method yield virtually identical desorption and cycling behavior. Furthermore, the ball milled material can be made with lower carbon content and still maintain only slightly reduced kinetic improvements. Surprisingly, there is no evidence for mixed infused-like and bulk-like behavior even for carbon content as low as 9.1 wt%. Unlike melt infused NaAlH₄, where co-infusion of TiCl₃ catalyst has proven difficult, the ball milled material can easily accommodate co-doping with both carbon and TiCl₃. The inclusion of carbon with TiCl₃ results in a modest but significant improvement in kinetics compared to NaAlH₄ doped with TiCl₃ alone, especially for rehydrogenation. Ball milling with activated carbon produces an improvement very similar to that of carbon aerogel, whereas graphene and graphite have smaller effects, in that order. During cycling of the second stage NaAlH₄ reaction, i.e., Na₃AlH₆ ↔ 3NaH + Al + (3/2)H₂, addition of either activated carbon or carbon aerogel at 23.1 wt%, but without TiCl₃, results in kinetic performance as good as or better than NaAlH₄ doped with TiCl₃.