A new kind of solar panel, developed at the University of Michigan,
has achieved 9% efficiency in converting water into hydrogen and oxygen—mimicking a crucial
step in natural photosynthesis. Outdoors, it represents a major leap in the technology,
nearly 10 times more efficient than solar water-splitting experiments of its kind.
But the biggest benefit is driving down the cost of sustainable hydrogen. This is enabled
by shrinking the semiconductor, typically the most expensive part of the device. The
team’s self-healing semiconductor withstands concentrated light equivalent to 160 suns.
Currently, humans produce hydrogen from the fossil fuel methane, using a great deal of
fossil energy in the process. However, plants harvest hydrogen atoms from water
using sunlight. As humanity tries to reduce its carbon emissions, hydrogen is attractive
as both a standalone fuel and as a component in sustainable fuels made with recycled carbon
dioxide. Likewise, it is needed for many chemical processes, producing fertilizers for instance.
In the end, the researchers believe that artificial photosynthesis devices will be much
more efficient than natural photosynthesis, which will provide a path toward carbon neutrality.
The study is reported in the journal Nature.
The outstanding result comes from two advances. The first is the ability to
concentrate the sunlight without destroying the semiconductor that harnesses the light.
The researchers reduced the size of the semiconductor by more than 100
times compared to some semiconductors only working at low light intensity.
The researcher says that Hydrogen produced by their technology could be very cheap.
And the second is using both the higher energy part of the solar spectrum to split water and
the lower part of the spectrum to provide heat that encourages the reaction. The magic is
enabled by a semiconductor catalyst that improves itself with use, resisting the degradation that
such catalysts usually experience when they harness sunlight to drive chemical reactions.
In addition to handling high light intensities, it can thrive in high temperatures that are
punishing to computer semiconductors. Higher temperatures speed up the water splitting process,
and the extra heat also encourages the hydrogen and oxygen to remain separate rather than renewing
their bonds and forming water once more. Both of these helped the team to harvest more hydrogen.
For the outdoor experiment, the team set up a lens about the size of a house window to focus sunlight
onto an experimental panel just a few inches across. Within that panel, the semiconductor
catalyst was covered in a layer of water, bubbling with the hydrogen and oxygen gasses it separated.
The catalyst is made of indium gallium nitride nanostructures,
grown onto a silicon surface. That semiconductor wafer captures the light, converting it into free
electrons and holes—positively charged gaps left behind when electrons are
liberated by the light. The nanostructures are peppered with nanoscale balls of metal,
1/2000th of a millimeter across, that use those electrons and holes to help direct the reaction.
A simple insulating layer atop the panel keeps the temperature at a toasty 75 degrees Celsius,
or 167 degrees Fahrenheit, warm enough to help encourage the reaction while also being cool
enough for the semiconductor catalyst to perform well. The outdoor version of the experiment,
with less reliable sunlight and temperature, achieved 6.1% efficiency at turning the energy
from the sun into hydrogen fuel. However, indoors, the system achieved 9% efficiency.
The next challenges the team intends to tackle are to further improve the efficiency and to
achieve ultrahigh purity hydrogen that can be directly fed into fuel cells.