Solar power, stored chemically

February 16, 2016 // By Christoph Hammerschmidt
Researchers at the Vienna Technical University have devices a novel photochemical cell that stores the energy of ultraviolet light at high temperatures.

Plants can do it: They can catch sunlight and store its energy by chemical reactions. Technology has tried to copy this mechanism but so far has failed to achieve the same efficiency. Photovoltaics converts sunlight directly into electrical power but the efficiency of conventional solar cells is rather modest, in particular at high temperatures. If this electric energy is used to generate hydrogen, the energy can be stored chemically, but again, the efficiency of the process is rather limited.

Scientists at the Vienna Technical University have developed a novel concept: They succeeded in combining high-temperature photovoltaics with an electrochemical element and thus utilise ultraviolet light to pump oxygen ions through a ceramic electrolyte membrane. In other words, they succeeded in storing the energy of the UV light chemically. This could become interesting because it could lead the way to split water into hydrogen and oxygen, just by means of (UV) light.

In his doctoral thesis, Georg Brunauer utilised perovskite mixed metal oxides instead of silicon-based materials to create solar cells. By combining multiple perovskites he succeeded in creating a solar cell that combines high-temperature photovoltaics with electrochemistry. The cell consists of two different layers. In the upper (photoelectrical) layer, illumination generates free charge carriers. However, in contrast to a normal solar cell, the charge carriers are immediately moved to the lower (electrochemical) layer. The result: The oxygen atoms there are negatively charged, enabling them to be moved through the lower layer.

“This is the decisive step”, Brunauer explains. “In the future, we can use this mechanism to split water and generate hydrogen.” A demonstrator is already operational; it generates a no-load voltage up to 920 millivolts at temperatures up to 400°C.

Provided it will be possible to increase the electric power, splitting water into hydrogen and oxygen atoms will become within reach. Transferred to the industrial scale, this mechanism could be used not only to generate hydrogen; it also would