Researchers at the University of Windsor (Ontario, Canada) have developed a lightweight, non-metal compound (a phosphonium borate) that releases hydrogen when heated above 100° C and reabsorbs it at room temperature.
The compound would support just a low-density storage system at this point: only 0.25 wt%, compared to the Department of Energy’s target of 6.5 wt% by 2010 for storage density for hydrogen fuel-cell vehicles. Although the researchers are exploring ways to increase its density, the material offers more immediate promise as a catalyst, as metal-based hydrogenation catalysts can be expensive as well as environmentally unfriendly.
Other nonmetal compounds such as ammonia boranes have higher storage densities, but are difficult to convert back once they have lost all their hydrogen. By contrast, the phosphonium borate system readily goes in either direction (release or absorb) but doesn’t hold very much.
Chemistry professor Douglas W. Stephan, graduate student Gregory C. Welch, and colleagues at the University of Windsor, in Ontario, published their work in the 17 November issue of Science.
When heated, the phosphonium borate [(C6H2Me3)2PH(C6F4)BH(C6F5)2 (Me, methyl)] readily yields H2 to form a dehydrogenated phosphine borane [(C6H2Me3)2P(C6F4)B(C6F5)2]. The borane, which is stable, can react with H2 at 25° C and 1 atm to regenerate the borate.
The researchers have a theory, but are not definite, on the mechanism.
Regardless of the mechanism, the discovery is important because of the reversible nature of the hydrogen activation.—Gregory Kubas, Los Alamos National Laboratory
“Reversible, Metal-Free Hydrogen Activation”; Gregory C. Welch, Ronan R. San Juan, Jason D. Masuda, Douglas W. Stephan; Science 17 November 2006: Vol. 314. no. 5802, pp. 1124 - 1126 DOI: 10.1126/science.1134230
“Breaking the H2 Marriage and Reuniting the Couple”; Gregory J. Kubas; Science 17 November 2006: Vol. 314. no. 5802, pp. 1096 - 1097 DOI: 10.1126/science.1135430