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So far it has been impractical to use water-splitting as a way to store the sun's energy. One reason is cost-efficiency. Silicon-based solar cells, such as those used in rooftop solar arrays, are good at converting visible and ultraviolet light into electricity. But silicon cells 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.
Until the recent Stanford experiments, it was believed that metal oxides also became less efficient as they became hotter. And since they were less efficient than silicon to start with, that made them less interesting as a water-splitting technology.
The Stanford experiments change that misconception.
The researchers tested three metal oxides – bismuth vanadium oxide, titanium oxide and iron oxide, more commonly known as rust. They wanted to see how efficient these oxides were at converting photons to electrons – and splitting water into hydrogen and oxygen – at different temperatures.
"In all three cases we observed increased production of hydrogen and oxygen at higher temperatures," said Liming Zhang, a postdoctoral scholar in Chueh's lab and co-lead author of the paper. "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, who recently defended his thesis centered on this project.
"Our results shows that heating up metal oxides with sunlight can double rate of hydrogen generation," Ye said.Bismuth vanadium oxide was the most efficient of the three oxides they tested. But team members believe this heat-enhancing effect may work for many different metal oxides, and they plan to test more materials previously considered impractical as solar cell material.
"We'll also be looking for those temperature sweet spots where performance is optimized," said graduate student and team member Madhur Boloor.
Discovering that heating up metal oxides produces more energy means that relatively simple engineering could be applied to heat these solar cells to enhance their efficiency.
"You don't have to add energy from an outside source," said graduate student and team member Andrey Poletayev. "You can do it for free by concentrating solar radiation, either through a magnifying lens or parabolic mirrors."
Chueh believes that this discovery will refocus attention on developing metal oxides as cost-effective alternatives to silicon solar cells. Quite apart from their potential use in a day-to-night energy storage scenario, he envisions that pure hydrogen gas produced by water-splitting could be used to power vehicles or other machines directly and without pollution.
"We can store these gases, we can transport them through pipelines, and when we burn them we don't release any extra carbon," said Chueh. "It's a carbon-neutral energy cycle."
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