The transition to a sustainable energy system requires techniques to convert sustainable energy, such as solar energy, into fuels, such as hydrogen. This is important for energy storage and applications that cannot yet run without power, such as aircraft.
Hydrogen can already be produced sustainably, for example by using electricity from solar panels to drive a so-called electrolyser that splits liquid water into hydrogen and oxygen. “But it is still too expensive to compete with current hydrogen production from natural gas,” says chemical engineer Kevin Sivula of the Technical University of Lausanne in Switzerland. “That’s why we’re investigating techniques that can be cheaper.”
The researchers at the Technical University of Lausanne have now taken an important step in this direction. They have developed electrodes that are porous and transparent so that sunlight and water from the air can reach the semiconductor to initiate a photoelectrochemical reaction that produces hydrogen. The results were published in the journal on Wednesday Advanced materials.
This was previously not possible because the necessary electrodes were not transparent enough to allow sunlight to pass through for the photoelectrochemical reaction.
“The advantage of our system is that we don’t use liquid water,” says Sivula. In systems that use liquid water, you can’t just throw water out of a lake or ocean. You need to clean it first. Sivula: “We don’t have that problem because we let water condense out of the air. It immediately provides clean water.”
The basis for the transparent electrodes is a kind of glass wool, which the researchers compress and heat into a compact, spongy structure. They then use a material that is transparent and electrically conductive – a difficult combination. Then they cover it with a light-absorbing semiconductor. They add a catalyst that allows the hydrogen-producing reaction to take place.
The whole is porous, so there is enough surface on which water vapor can settle. And it is transparent enough to let the sunlight through for the photoelectrochemical reaction. Experiments with a small prototype show that this system actually produces hydrogen gas.
The researchers have not yet looked at the minimum humidity required. “A higher humidity, such as in the tropics or over the ocean, probably produces the best results,” says Sivula. “But even in the desert, there is enough water in the air to produce hydrogen.”
In addition, the device is designed to take advantage of the changing conditions of day and night. At night, when it cools down and the relative humidity rises, the system can absorb water from the air and then use it to produce hydrogen during the day when the sun is shining.
The efficiency is still too low
A future where these units produce hydrogen on a large scale and cheaply is still a long way off, Sivula admits. “We now have a low-cost system that is scalable. But the efficiency is still too low.”
The prototype had an efficiency of about 1 percent in converting sunlight into hydrogen. It has to be at least ten times better before it is commercially interesting.
Joost Reek, professor of chemistry at the University of Amsterdam and not involved in the research, agrees: “It still needs further optimization and development, but it looks promising.”
“Theoretically, it seems possible,” says Sivula. There is still much to optimize in terms of materials and construction. Future experiments will show whether researchers can increase efficiency without getting out of control.