Hygroscopic hydrogels are emerging as scalable and low-cost sorbents for atmospheric water harvesting, dehumidification, passive cooling, and thermal energy storage.
Polyacrylamide hydrogel disks. Image credit: Gustav Graeber / Carlos D. Díaz-Marín.
“Humanity faces important challenges related to the global supply of energy and water,” MIT researcher Carlos Díaz-Marin and colleagues wrote in their paper.
“Efforts in energy and water have to be in line with the worldwide push toward net-zero emission strategies as well as adapt to constantly evolving environmental conditions in a rapidly changing climate.”
“Research toward innovative, functional materials is a powerful means to address these challenges,” they added.
“One promising class of materials are sorbents. Sorption of water is commonplace in nature and widely used in a variety of technical processes that address water scarcity and enhance energy efficiency.”
“These include freshwater production through atmospheric water harvesting, passive thermal management, thermal energy storage, and space conditioning.”
“For sorbents to be good candidates for these applications, they need to be low-cost, scalable, and sustainable, as well as provide high water vapor uptake (i.e., high hygroscopicity), good sorption kinetics, low desorption enthalpies, and long-term cyclability. Finally, the sorbents must be easy to be integrated into the respective devices.”
Díaz-Marin and co-authors synthesized a superabsorbent material that can soak up a record amount of moisture from the air, even in desert-like conditions.
As the material absorbs water vapor, it can swell to make room for more moisture.
Even in very dry conditions, with 30% relative humidity, the material can pull vapor from the air and hold in the moisture without leaking.
The water could then be heated and condensed, then collected as ultrapure water.
The researchers enhanced the hydrogel’s absorbency by infusing it with lithium chloride — a type of salt that is known to be a powerful dessicant.
They found they could infuse the hydrogel with more salt than was possible in previous studies.
As a result, they observed that the salt-loaded gel absorbed and retained an unprecedented amount of moisture, across a range of humidity levels, including very dry conditions that have limited other material designs.
If it can be made quickly, and at large scale, the superabsorbent gel could be used as a passive water harvester, particularly in the desert and drought-prone regions, where the material could continuously absorb vapor, that could then be condensed into drinking water.
The scientists also envision that the material could be fit onto 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,” Díaz-Marin said.
“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.”
“The big, unexpected surprise was that, with such a simple approach, we were able to get the highest vapor uptake reported to date,” said MIT researcher Gustav Graeber.
“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 team’s work was published in the journal Advanced Materials.
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Gustav Graeber et al. Extreme Water Uptake of Hygroscopic Hydrogels through Maximized Swelling-Induced Salt Loading. Advanced Materials, published online May 18, 2023; doi: 10.1002/adma.202211783
