Researchers work on metal oxides to store solar energy

A team of researchers at Stanford University has tried to work on metal oxides, like rust, to convert solar energy into electricity and store it by splitting water into hydrogen and oxygen.

The team, led by William Chueh, an assistant professor of materials science and engineering, and Nicholas Melosh, an associate professor in the same department, tested three metal oxides -- bismuth vanadium oxide, titanium oxide and iron oxide, more commonly known as rust.

The work is based on the fact that metal oxides can be fashioned into solar cells capable of splitting water, or H2O, into hydrogen and oxygen.

Splitting H2O by day is a way to store energy for use at night. The photons captured by the cell are converted into the electrons that provide the energy to split water. Recombining hydrogen and oxygen would be a way to reclaim that energy and "dispatch" power back into the electrical grid -- without burning fossil fuels and releasing more carbon into the atmosphere.

The solar power potential of metal oxides was previously known, the researchers said, but metal oxide solar cells were also known to be less efficient at converting photons to electrons than silicon-based solar cells.

Silicon solar cells are good at converting visible and ultraviolet light into electricity. But they waste the infrared light, which bears heat, beating down on them.

"Standard cells utilize a relatively small portion of the spectrum, and the rest is lost as heat," Chueh said.

In comparison, the Stanford team found that as metal oxide solar cells grow hotter, they convert photons into electrons more efficiently.

In laboratory tests, all the three metal oxides "increased production of hydrogen and oxygen at higher temperatures," said Liming Zhang, a postdoctoral scholar in Chueh's lab and co-lead author of a paper published in the journal Energy & Environmental Science. "We realized that the higher temperatures were enhancing the carrier mobility of these cells -- the speed at which electrons can pass through the metal oxides."

The increase in efficiency was remarkable, said Xiaofei Ye, the other co-lead author of the paper. "Our results shows that heating up metal oxides with sunlight can double rate of hydrogen generation."

Bismuth vanadium oxide was the most efficient of the three oxides. And team members believe the heat-enhancing effect may work for many different metal oxides.

"We've shown that inexpensive, abundant and readily processed metal oxides could become better producers of electricity than was previously supposed," Chueh said, adding that the discovery could lead to a revolutionary change in how we produce, store and consume energy.

"By combining heat and light, solar water-splitting cells based on metal oxides become significantly more efficient at storing the inexhaustible power of the sun for use on demand," he said.