Photochemical
cell: Light creates free charge carriers, oxygen (blue)
is pumped through
a membrane
(February 22, 2016) A
photo-electrochemical cell has been developed at TU Wien (Vienna). It can
chemically store the energy of ultraviolet light even at high temperatures.
Nature shows us how it is done: Plants can absorb sunlight
and store its energy chemically. Imitating this on large industrial scale,
however, is 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.
Scientists at TU Wien (Vienna) have now developed a new
concept: By combining highly specialised
new materials, they have managed to combine 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, this method could also be used to split water into
hydrogen and oxygen.
Special Materials for
High Temperatures
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”, says Brunauer. Common photovoltaic cells, however, only work well up to 100°C.
In a solar concentrator plant, much higher temperatures would be reached.
Heated reactor (TU
Wien)
While working on his doctoral thesis, Brunauer managed to
put his ideas into practice. 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.
Creating Voltage and
Pumping Ions
“Our cell consists of two different parts – a photoelectric
part on top and an electrochemical part below”, says Georg 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 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”,
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 920 millivolts at a
temperature of 400°C.