Researchers in Canada have developed a new solid material that can store and release hydrogen near room temperature without involving a transition metal. The discovery could lead to the development of low-cost and lightweight materials for the onboard storage of hydrogen fuel in cars (Science 314 1124).
Hydrogen is often touted as an environmentally-friendly fuel for road vehicles of the future. When consumed in a fuel-cell powered electric car, it produces nothing more than pure water as a by-product. However, many technological challenges remain before it can be used commercially. In particular, hydrogen has a low energy density compared to conventional fuels and therefore it must be stored as a liquid or an extremely high-pressure gas to ensure that reasonable distances can be travelled before refuelling.
These storage methods are both expensive and cumbersome and some researchers believe that it would be better to store hydrogen within solid materials that can absorb large quantities of the gas. In such materials a chemical reaction splits the hydrogen molecule into two hydrogen atoms at the surface of the material. The atoms then migrate into the bulk of the material and form a metal-hydride compound. The hydrogen can be released by heating the material.
Current materials that can easily absorb and discharge hydrogen near room temperature contain transition metals and the storage process must be catalysed by expensive precious metals such as platinum. This makes them too heavy and too expensive for commercial use.
But now Douglas Stephan and colleagues at the University of Windsor have developed the first non-metallic material that can absorb and store hydrogen at room temperature, releasing the gas when heated above 100 ºC. The material contains pairs of boron and phosphorous atoms, which are separated by a ring of carbon atoms. This structure has a net neutral charge but the boron and phosphorous atoms carry a positive and negative charge respectively. The researchers believe that this property allows the two atoms to work together to split the gaseous hydrogen molecules into two hydrogen atoms, which are then covalently bound within the material. According to Stephan, this mechanism is known as “heterolytic cleavage” and has only been observed in transition-metal complexes.
Although the metal-free material offers hope of lighter and cheaper storage materials, Stephan admits that there is still a long way to go. Crucially, the material stores less than 0.25% of its weight in hydrogen, which is far off the US Department of Energy’s target of 6% set for 2010 and the 2.5% achieved by some transition metal materials. Stephan describes the DOE target as a “challenging problem” and the researchers are currently exploring alternative molecular structures.
