Green hydrogen, made from water electrolysis, has great potential to store and transport renewable energy. But the process to make it requires water, and many of the places with the highest potential for solar energy are very dry.
“In Australia, we have abundant solar energy in the central desert areas, but there’s no water at all – no groundwater, no fresh water. The only water we have is the water from the air,” says Dr Gang Kevin Li, a senior lecturer in the Department of Chemical Engineering at the University of Melbourne, and senior author on a paper describing the electrolyser, published in Nature Communications.
“Air is everywhere, and air contains a lot of a lot of water – 12.9 trillion tonnes at any moment.”
The device, called the Direct Air Electrolysis (DAE) module, is made from a porous, sponge-like substance with electrodes at either end. The sponge is “hygroscopic”: it absorbs moisture from the ambient air.
Li compares the substance to the silica gel you often see in little packets, designed to absorb moisture.
“It’s like that, but the material we use is actually much stronger in hygroscopic properties.”
Why isn’t this material already in use for drinking water in deserts? It doesn’t render the water very tasty – a key ingredient is sulphuric acid, for instance.
“The water is readily available for electrolysis, but not drinkable,” says Li.
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The material also contains an electrolyte meaning the H2O collected can be turned into H2 and O2, bubbling out at either electrode.
“It’s actually a completely new concept, because previous electrolysers for hydrogen production from water have needed to be a closed system,” says Li.
“Our system is different: we have a semi-open electrolyser, so we only have the top and bottom electrode closed, but the four sides of that electrolyser are open to the air.
“It means the moisture from the air can transfer into the hygroscopic electrolyte automatically, or spontaneously, without consuming energy. So we don’t need extra energy to extract this water.”
When connected to a power supply, the researchers were able to run the DAE for 12 days without a drop in performance.
They could also run it in extremely dry conditions.
“We can work it down all the way down as low as 4% relative humidity – much lower than an average desert humidity,” says Li.
(The Sahel Desert’s in north Africa average relative humidity is around 20%, and Uluru records an average of about 21%.)
Their modelling showed that on a warm, sunny day, the A5-sized prototype could generate 3,700 litres of hydrogen, per day, per square metre of their tower.
The researchers have received funding to continue developing their prototype.
“We’re going to scale up to one meter squared by the end of this year, and then 10 square metres by the end of next year,” says Li.
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But he wonders if the device could one day end up in space: after all, it can produce both hydrogen and oxygen.
“There is a small, tiny amount of water in the atmosphere of Mars,” he says.
“I imagine that if this device could be stored on extra-terrestrial planets […] it could become a fuel generator and produce oxygen continuously.”
Ellen Phiddian is a science journalist at Cosmos. She has a BSc (Honours) in chemistry and science communication, and an MSc in science communication, both from the Australian National University.
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