Using solar cells to split water 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 after dark 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. But metal oxide solar cells were also known to be less efficient at converting photons to electrons than silicon solar cells.
A discovery reported in the journal Energy & Environmental Science makes metal oxide solar cells a better candidate for energy storage. The Stanford team showed that as metal oxide solar cells grow hotter, they convert photons into electrons more efficiently. The exact opposite is true with silicon solar cells, which lose efficiency as they heat up.
"We've shown that inexpensive, abundant and readily processed metal oxides could become better producers of electricity than was previously supposed," explained William Chueh, an assistant professor of materials science and engineering.
"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," said Chueh.
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," said Chueh.
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 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."