Photo-electrochemical cell stores UV light energy at industrial scale
Plants can absorb sunlight and store its energy chemically but imitating this process on large industrial scale has proved difficult. Photovoltaics convert sunlight to electricity, but at high temperatures, the efficiency of solar cells decreases. Electrical energy can be used to produce hydrogen, which can then be stored – but the energy efficiency of this process is limited.
The scientists at TU Wien (Vienna) have developed a new concept which combines specialized materials to create high temperature photovoltaics with an electrochemical cell. 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, the method could also be used to split water into hydrogen and oxygen.
As a student at TU Wien, Georg Brunauer started pondering possible combinations of photovoltaics and electrochemical storage. The feasibility of such a system depends crucially 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 Brunauer. Common photovoltaic cells only work well up to 100°C. In a solar concentrator plant, much higher temperatures would be reached.
The key to success was an unusual choice of materials. Instead of the ordinary silicon based photovoltaics, special metal oxides – so-called perovskites – were used. By combining several different metal oxides, Brunauer managed to assemble a cell which combines photovoltaics and electrochemistry. Several research partners at TU Wien contributed to the project. Georg Brunauer is a member of Prof. Karl Ponweiser’s research team at the Institute for Energy Systems and Thermodynamics, Prof. Jürgen Fleig’s group (Chemical Technologies and Analytics) and the Institute for Atomic and Subatomic physics were involved as well.
“Our cell consists of two different parts – a photoelectric part on top and an electrochemical part below,” said Georg Brunauer. “In the upper layer, ultraviolet light creates free charge carriers, just like in a standard solar cell.”
The electrons in the upper layer are immediately removed and travel to the bottom layer of the electrochemical cell. Once there, the electrons are used to ionize 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,” suggested Brunauer. In its first evolution step, the cell works as a UV-light driven oxygen pump yielding an open-current voltage of up to 920 millivolts at a temperature of 400°C.
To make the leap from the university lab to an industrial prototype, Georg Brunauer has founded the startup company NOVAPECC. Together with TU Wien, he has filed several patents.
Reference
Brunauer et al, Advanced Functional Materials 26,1 (2016). UV-Light-Driven Oxygen Pumping in a High-Temperature Solid Oxide Photoelectrochemical Cell. DOI: 10.1002/adfm.201503597
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