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 do without power, such as aircraft.
Hydrogen can already be produced sustainably, for example by using power from solar panels to drive a so-called electrolyser that splits liquid water into hydrogen and oxygen. “But that 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 is why we are investigating techniques that could be less expensive.”
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 appeared on Wednesday in the magazine Advanced Materials.
This was previously not possible because the required electrodes were not transparent enough to allow sunlight to pass through for the photoelectrochemical reaction.
“The advantage of our system is that we do not use liquid water,” says Sivula. In systems that use liquid water, you cannot simply throw water out of a lake or ocean. You have to purify it first. Sivula: “We don’t have that problem because we let water condense out of the air. That immediately produces clean water.”
Electrochemical sponge
The basis for the transparent electrodes is a kind of glass wool that the researchers compress and heat to make a compact, spongy structure. They then apply a material that is transparent and electrically conductive – a tricky combination. Then they cover it with a light-absorbing semiconductor. They add a catalyst that allows the hydrogen-producing reaction to take place.
This whole is porous, so that there is enough surface on which water vapor can settle. And it is transparent enough to let the sunlight through for the photoelectrochemical reaction. Tests with a small-scale prototype show that this system does indeed produce hydrogen gas.
The researchers have not yet looked at the minimum humidity required. “A higher humidity, such as in the tropics or over the sea, probably gives 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 intended 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 to produce hydrogen during the day when the sun is shining.
Efficiency still too low
A future in which these devices produce hydrogen on a large scale and cheaply is still a long way off, admits Sivula. “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 for converting sunlight into hydrogen. That must 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 a lot to optimize in terms of materials and construction. Future experiments will have to show whether researchers can boost efficiency without spiraling out of control.