A new type of solar cell developed at the University of Michigan has achieved 9% efficiency in converting water into hydrogen and oxygen, mimicking a key step in natural photosynthesis. In the open air, this represents a huge technological leap, being about 10 times more efficient than similar solar water splitting efforts.
But the biggest advantage is the low cost of sustainable hydrogen. This is made possible by shrinking the semiconductor, which is usually the most expensive part of the device. The device's self-healing semiconductor can withstand concentrated light equivalent to 160 suns.
Currently, humans produce hydrogen from the fossil fuel methane, consuming large amounts of fossil energy. However, plants remove hydrogen atoms from water with the help of sunlight. As humanity strives to reduce carbon emissions, hydrogen is attractive both as a stand-alone fuel and as a sustainable fuel component made from recycled carbon dioxide. Additionally, it is required for many chemical processes, such as fertilizer production.
"Ultimately, we believe that artificial photosynthesis devices will be much more efficient than natural photosynthesis in providing a path to carbon neutrality," said Zetian Mi, an electrical and computer engineering professor who led the research, which was published in Nature - Who?
Excellent results are obtained due to two accomplishments. The first is the ability to use light to focus sunlight without destroying the semiconductor.
"We reduced the size of the semiconductor by 100 times compared to some semiconductors that only work at low light intensities," said Peng Zhu, UM research associate in electrical and computer engineering and first author of the study. "Hydrogen produced with our technology can be very cheap."
And the second uses both the high energy part of the solar spectrum to split water and the lower part of the spectrum to generate heat by driving reactions. The charm is provided by a semiconductor catalyst that improves with use, preventing the degradation that these catalysts typically experience when using sunlight to carry out chemical reactions.
In addition to handling high light intensities, it can develop at high temperatures that punish computer semiconductors. Higher temperatures speed up the process of water splitting, and excess heat encourages hydrogen and oxygen to dissociate rather than form water again. Both helped the team collect more hydrogen.
For outdoor testing, Zhu installed a lens the size of a house window to focus sunlight onto a few inches of the test panel. Inside this panel, the semiconductor catalyst was covered by a layer of water that was boiling from the released hydrogen and oxygen gases.
The catalyst consists of indium and gallium nitride nanostructures grown on a silicon surface. This semiconductor wafer captures light, converting it into free electrons and holes, positively charged holes that emit light when electrons are emitted. The nanostructures are coated with metal spheres 1/2000th of a millimeter in diameter that use these electrons and holes to drive reactions.
A simple layer of insulation on top of the panel keeps the temperature at 75 degrees Celsius or 167 degrees Fahrenheit, hot enough to promote the reaction and cool enough for the semiconductor catalyst to work well. An outdoor version of the experiment, with less reliable sunlight and temperature, achieved a 6.1% efficiency for converting solar energy into hydrogen fuel. However, internally, the system achieved 9% efficiency.
The next challenges the team wants to tackle are to further improve efficiency and obtain ultra-high-purity hydrogen that can be fed directly into fuel cells.
Some intellectual property associated with this work belongs to NS Nanotech Inc. and NX Fuels Inc., co-founded by Mi, licensed. The University of Michigan and I have financial interests in both companies.
This work was supported by the National Science Foundation, the Department of Defense, the Michigan Innovation Center for Translational Research and Commercialization, the University of Michigan College of Engineering Blue Sky Program, and the Army Research Office.
Video: https://youtu.be/uNQLOU8aATc
