Green hydrogen can be used in many ways. In addition to being used as a fuel or feedstock, it also acts as a chemical energy store that stores excess electricity from wind or solar energy. Ideally, the hydrogen used is obtained by electrochemical water splitting (electrolysis) using solar or wind energy. However, this requires liquid water. Researchers have now succeeded in developing an electrolysis system that uses moisture from the ambient air instead of liquid water.
water straight from the air
Most electrolysis systems use pure fresh water. However, there are already systems that have been adapted to use seawater.
If there is a lot of wind and solar energy in a location, but there is a lack of liquid water, then it has been difficult to produce hydrogen through electrolysis. “On most continents, the potential for solar and wind energy is greatest in areas where water is scarce, explains a research team led by Jining Guo from the University of Melbourne. Using liquid water for electrolysis under such conditions would exacerbate the existing water shortage.
But even where water shortages are an issue, there is one water source that has barely been used until now: the ambient air. “Even in the desert of the Sahel, the average relative humidity is around 20 percent, at Uluru in the central desert of Australia it is 21 percent the researchers said. To harness this potential, the team has developed an electrolysis system that can use the humidity in the air to split water. “This Direct Air Electrolysis Module (DAE) can operate even in bone-dry conditions with as little as four percent humidity and produce green hydrogen with minimal environmental impact team explains.
A 12-day field test showed promise
The system consists of two grid-shaped platinum electrodes, each connected to its own gas collector. Between the electrodes is a spongy block of gas wool that is saturated with a liquid electrolyte. This electrolyte attracts water and can also absorb water vapor from the air, providing the liquid needed for water splitting.
In the first experiments, the team used sulfuric acid as the electrolyte. However, it would also be possible to use caustic potash, which has an even better absorbency. However, it also reacts with the carbon dioxide in the air, so it should be filtered out beforehand. The power for the test set-up comes from a solar panel that is connected to the system. However, it can easily be replaced by a wind turbine.
In a field test with a prototype consisting of five stacked test modules, the researchers tested their direct air electrolysis for 12 days. The relative humidity was between 20 and 40 percent. During the test, data such as sunshine duration, solar energy consumption and the amount of hydrogen produced were recorded.
On a day when the sun was shining continuously, the DAE module was able to produce 186 milliliters of hydrogen per hour, giving the researchers 1.49 liters of hydrogen at the end of the day. With a cathode surface of one square meter, this corresponds to 745 liters of hydrogen per day“, according to the team. On a relatively cloudy day, the system was still able to produce 1.2 liters of hydrogen with a significantly lower power supply.
During the 12-day test, the system was able to achieve 95 percent Faraday efficiency for the hydrogen. The purity of the hydrogen produced was over 99 percent.
The system is almost infinitely scalable
“With this we demonstrated a method that can produce very pure hydrogen from air by using a hygroscopic electrolyte in a porous medium as a moisture absorbing agent. the researchers said. The new DAE system is superior to previous methods of steam-based electrolysis or photocatalytic water splitting. The system can also work autonomously. “After the prototype had been outside for eight months without any maintenance, the Faraday efficiency on hydrogen was still around 90 percent team reports.
Direct air analysis opens up new possibilities for green hydrogen production without resorting to liquid water, according to the researchers. The water absorption by sulfuric acid has been tested in climate chambers up to humidity of four percent.
“Such DAE farms have the potential to generate abundant hydrogen even in arid and semi-arid regions – with minimal impact on the environment and regional humidity‘ conclude the researchers. Moreover, the system is almost unlimitedly scalable and can be linked to various sustainable energy sources.