Ultraviolet light can be directly used to pump oxygen ions through a solid oxide electrolyte. The energy of the UV light is stored chemically. In the future, this method could also be used to split water into hydrogen and oxygen.
The feasibility of such a system depends on whether it is able to work at high temperatures. "This would allow us to concentrate sunlight with mirrors and build large-scale plants with a high rate of efficiency", explained Georg Brunauer, lead scientist on the project. Common photovoltaic cells, however, only work well up to 100°C. In a solar concentrator plant, much higher temperatures would be reached.
Instead of the ordinary silicon based photovoltaics, special metal oxides - perovskites - were used. By combining several different metal oxides, Brunauer assembled a cell which combines photovoltaics and electrochemistry.
"Our cell consists of two different parts - a photoelectric part on top and an electrochemical part below", said Brunauer. "In the upper layer, ultraviolet light creates free charge carriers, just like in a standard solar cell." The electrons in this layer are immediately removed and travel to the bottom layer of the electrochemical cell. Once there, these electrons are used to ionise oxygen to negative oxygen ions, which can then travel through a membrane in the electrochemical part of the cell.
"This is the crucial photoelectrochemical step, which we hope will lead to the possibility of splitting water and producing hydrogen", says Brunauer. In its first evolution step, the cell works as a UV-light driven oxygen pump. It yields an open-current voltage of up to 920mV at a temperature of 400°C.
The research project continues. "We want to understand the origin of these effects by carrying out a few more experiments, and we hope that we will be able to improve our materials even further", said Brunauer.
If the electrical power can be increased, the cell will be able to split water into oxygen and hydrogen. The concept is not only useful for the production of hydrogen, as it could also split carbon dioxide into carbon monoxide. The produced energy carried in the form of hydrogen and carbon monoxide can be used to synthesise fuels.