Scientists develop material that could harvest water from desert air

The rubbery material is made from hydrogel and can swell to absorb water vapour even in desert-like conditions, where there is only 30 per cent relative humidity in the air. 

Once absorbed by the gel, the water could be heated, condensed and collected as ultrapure water. 

The MIT researchers were able to achieve “record-breaking” vapour absorption by infusing the hydrogel with larger amounts of lithium chloride — a type of salt that is known to be a powerful desiccant.

This new material can be developed at speed and scale, and could potentially be used as a source of drinking water in very dry climates. Moreover, the material could also be fitted into air conditioning units as an energy-saving, dehumidifying element. 

“We’ve been application-agnostic, in the sense that we mostly focus on the fundamental properties of the material,” said Carlos Díaz-Marin, a member of the Device Research Lab at MIT. “But now we are exploring widely different problems like how to make air conditioning more efficient and how you can harvest water. This material, because of its low cost and high performance, has so much potential.”

Hydrogels have been used for years as absorbent material in nappies because they can swell and soak up a large amount of water when it comes in contact with the material.

“Our question was, how can we make this work just as well to absorb vapour from the air?” Díaz-Marin said. 

To achieve this, the MIT team looked at past experiments in which scientists had mixed hydrogels with various salts that are very efficient at absorbing moisture, including water vapour.

Lithium chloride, in particular, is capable of absorbing over 10 times its own mass in moisture. This was therefore the salt chosen to mix with the new hydrogels — producing a material that could both hold in moisture and swell to accommodate more water. 

“It’s the best of both worlds,” said Gustav Graeber, a principal investigator at Humboldt University in Berlin. “The hydrogel can store a lot of water, and the salt can capture a lot of vapour. So it’s intuitive that you’d want to combine the two.”

In previous experiments, researchers had soaked hydrogels in salty water and waited for the salt to infuse into the gels. However, most experiments ended after 24 to 48 hours, as researchers found the process was too slow.

In the new study, the MIT team found that, when given more time to soak, the hydrogels took up more salt. After soaking in a salty solution for 30 days, hydrogels incorporated up to 24 grams, versus the previous record of 6 grams of salt per gram of polymer. Most notably, the team reports that at very dry conditions of 30 per cent relative humidity, the gels captured a “record-breaking” 1.79 grams of water per gram of material. 

“Any desert during the night would have that low relative humidity, so conceivably, this material could generate water in the desert,” said Díaz-Marin.

“The big, unexpected surprise was that, with such a simple approach, we were able to get the highest vapour uptake reported to date,” Graeber says. “Now, the main focus will be kinetics and how quickly we can get the material to uptake water. That will allow you to cycle this material very quickly so that instead of recovering water once a day, you could harvest water maybe 24 times a day.” 

The study’s results have been published in a paper appearing in Advanced Materials.