A New Metal-Decorated Carbon Catalyst for Dehydrogenation of Complex Hydride Hydrogen Storage Materials

In recent years, many new solid-state hydrides with a high hydrogen capacity that meet DOE requirement (> 7.5 wt.% H₂) have been synthesized as promising on-board hydrogen storage materials. In these hydrides, however, only a small fraction of hydrogen can be released at a desired on-board operating temperature (<85°C) due to a slow dehydrogenation kinetics of the hydrides. It has been discovered that the kinetics can be improved by adding transition metal catalysts to form metal-hydride intermediates that facilitate hydrogen transportation and desorption. Nevertheless, identification of such catalyst for various hydrides is a challenging task because the catalytic effect is component-specific. Here we report a new form of catalyst for dehydrogenation of complex hydride, using a well-studied NaAlH₄ as a test case. Dehydrogenation of NaAlH₄ can be greatly facilitated by activated carbon catalysts. The catalytic function can be further enhanced by decorating the carbon with Co, Ni, or Cu nanoparticles. The decomposition temperature was lowered by as much as 100°C using a 3 wt.% Co or Ni-decorated activated carbon, comparable to a Ti-based catalyst, which were the most effective among the metals tested. The catalytic effect is likely due to a combination of hydrogen spillover effect, high contact area between carbon and the hydride, and confinement of the hydride as nano-sized domains in the pores of the carbon matrix. The catalysts were also effective in facilitating rehydrogenation of NaAlH₄ under moderate pressure (75.8 bar H₂) and low temperature (120°C), when no rehydrogenation would occur without the catalyst. The fact that this new catalyst system is not specific to any hydride offers many potential applications.