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)H-2, 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. (C) 2011 Elsevier B. V. All rights reserved.