Hydrogen is just about the cleanest fuel around; burn it and the only exhaust is water. Now researchers have developed the most efficient method yet for generating hydrogen: a new photocatalyst that uses light to break apart water molecules.
One of the simplest ways of obtaining hydrogen is electrolysis: an electric current splits water molecules into their constituent hydrogen and oxygen atoms. Previous work had suggested the tantalizing possibility of driving electrolysis with the excited electrons generated in solar cells. Unfortunately, these materials didn't create enough juice under visible light.
A team led by Zhigang Zou of the National Institute of Advanced Industrial Science and Technology in Tsukuba, Japan, has improved solar cells by tweaking a photocatalyst to get what Zou calls "the right band gap." The band gap is a measure of the energy differential between two electron states. With the proper band gap, negatively charged electrons falling from the higher to lower state can provide enough energy needed to split the hydrogen out of the water. At the same time, positive charge carriers, known as electron vacancies or holes, oxidize water to oxygen. This completes the circuit and allows the reaction to proceed.
Zou's group started with an oxide of indium-tantalum, a material that had been shown to have some photocatalytic activity but was inefficient under visible light because its band gap isn't sufficient. They doped the compound with nickel and found that this produced hydrogen and oxygen in quantities significantly greater than the undoped samples when suspended in water and exposed to visible light, the team reports in the 6 December issue of Nature. Zou attributes the improvement to a subtle change in the crystal structure that produces the desired electron band gap.
"It's a breakthrough," says Akihiko Kudo, an applied chemist at the Science University of Tokyo who has also worked on the problem. "There is still a lot of work to be done, but this is likely to affect the course of future research," he adds. Zou admits that the rate of the reaction is still far too low for practical applications. He says, however, that if the catalyst can be improved, it will one day be used to turn water and sunshine into clean-burning fuel.