Hydrogen Spillover in Hydrogen Storage Materials.
by Prof. A.D. Lueking

Heterogeneities in Storage Materials. An emerging theme in solid state hydrogen storage materials is that heterogeneities improve hydrogenation kinetics and/or increase overall hydrogen uptake, as evidenced by the following: (1) transition metals improve the hydrogenation/dehydrogenation kinetics of traditional and complex hydrides; (2) the hydrogen uptake of many carbon-based materials can be attributed to catalytic materials; (3) carbon added to traditional hydrides improves the hydrogenation/ dehydrogenation kinetics; and (4) carbon linkages are combined with metal oxide building blocks to form the metal-organic framework.

Carbon-Metal Synergy or Hydrogen Spillover. The term hydrogen spillover was used by Lueking & Yang to describe a synergistic effect between a multi-wall carbon nanotube (MWNT) and a Ni0.4Mg0.6O catalyst left from synthesis: Hydrogen spillover increased the uptake of the MWNT by up to 40% [2] and high-pressure studies with this material showed adsorption and desorption to be 3.7% and 3.6% hydrogen by weight respectively at 69 bar and 300 K [3]. This hydrogen uptake was comparable to similar studies reported concurrently for carbon-based materials with uncharacterized metal content. The composite consisted of both well-dispersed metal in contact with MWNT and regions primarily containing residual catalyst (Figure 1).

At aatmospheric pressure, the hydrogen to total metal ratio of the MWNT/NiMgO composite was 1.4, exceeding that expected for a metal hydride and suggesting a synergistic carbon-catalyst effect. A second low-temperature desorption peak (Figure 2) suggested that hydrogen spilled over to the carbon surface. Integration of these desorption peaks indicates that hydrogen bound carbon exceeds hydrogen bound catalyst by a factor of two at atmospheric pressure. The degree of spillover was dependent upon the carbon-catalyst interface [3], the carbon support, catalyst dispersion and doping, and concentration of acceptor “sites” [14], highlighting variables by which to optimize the carbon-catalyst-hydrogen interactions. Similar hydrogen uptake was observed in previously inert carbon materials if properly doped.

Similar work suggests that the original 5-10% report of hydrogen storage in SWNT was also a result of carbon-metal synergy and theoretical studies of Fe-doped bucky balls suggest that application of pressure to doped nanocarbons may induce defects that lead to additional carbon adsorption sites with a hydrogen binding energy between classic physical and chemical adsorption.