Green energy from thin air

Hydrogen as a form of green energy can be produced directly from the air, new research from the University of Melbourne published in Nature Communications has demonstrated.

Tests were conducted by researchers from the University’s Department of Chemical Engineering together with partners from the University of Manchester and the Chinese Academy of Sciences.

The team found that by using a hygroscopic electrolyte solution – meaning one that absorbs and retains moisture – in combination with electrolysis powered by renewable wind or solar energy, they were able to produce hydrogen from moisture in the air, without consuming any liquid water.

The finding is a breakthrough for green hydrogen production for energy and storage, especially in areas where water is a scarce resource.

“This work opens up sustainable, freshwater-free pathways for green hydrogen as the ultimate carrier for the clean energy transition,” says Associate Professor Gang (Kevin) Li.

Read the study

As the study notes, hydrogen is the most abundant element in the universe, and its production through renewable-powered water-splitting is one of the most promising avenues for the switch to green energy and reducing global emissions.

However, the hot, dry parts of the world where renewable energy sources like solar and wind are plentiful are typically the same areas where water is in short supply.

Arid or semi-arid areas make up one-third of the earth’s land surface, and are home to 20% of its population. Freshwater is hard to come by in these areas, with water scarcity compounded by challenges such as pollution, industrial consumption and climate change. Beyond meeting daily needs, there is not a lot to spare for water-based processes of hydrogen production through electrolysis.

Other methods of water-splitting for hydrogen have been explored. In coastal areas, research has looked into desalination of sea water for electrolysis, though this has so far proven to be costly and complex. Splitting of saline water has been attempted, but poses the challenge of a chlorine byproduct. Photocatalytic water-splitting has shown some promise, but low solar-to-hydrogen efficiency. Few studies have considered cutting liquid water out of the equation.

“What’s really different about our method is that it bypasses the need for freshwater input, enabling deployment of electrolysers for hydrogen production to parts of the world that otherwise would not be viable,” says A/Prof Li.

Where water is scarce on the ground, it is still abundant in the air. Even at Uluru in the Central Australian desert, the average daytime relative humidity is 21%. In the Sahel desert, it’s 20%. The study states that at any moment, there are 12.9 trillion tons of water in the the earth’s atmosphere air around us.

The new research taps into this invisible resource, and uses it to produce energy from the air – even in bone-dry conditions with a humidity as low as 4%.

Researchers designed a Direct Air Electrolysis module, made up of a water harvesting unit with electrodes on each side paired with gas collectors, and hooked up to a renewable generator such as a solar panel or wind turbine. Geothermal and tidal sources were also explored.

The module harvests water using types of sponge soaked in a hygroscopic electrolyte solution, which attracts and stores water from the air. The captured water is split in situ by the renewable-powered electrodes, collecting hydrogen and oxygen separately as pure gas.

A prototype was tested over the Australian summer on the University of Melbourne campus, made up of five stacked modules topped with a solar panel. Air temperature varied from 20 to 40 degrees Celsius, with a humidity of 20% to 40%. A unit powered by a wind turbine was also tested, achieving efficient production of high-purity hydrogen.

The solar unit managed to achieve steady hydrogen production, at rates varying depending on the weather conditions, but overall approaching or exceeding the target of 20% solar-to-hydrogen energy efficiency set by the United States Department of Energy – meaning that the technology is a scalable and economically viable replacement for fossil fuels.

“These results are very promising, and we look forward to developing the technology and testing its performance at an even bigger scale,” A/Prof Li says.

Researchers involved in the study alongside A/Prof Li included the Melbourne team, Jining Guo, Yuecheng Zhang, Ali Zavabeti, Kaifei Chen and Yalou Guo, Dr Guoping Hu from the Chinese Academy of Sciences, and Professor Xiaolei Fan from Manchester University.