Engineers at MIT have created a novel material with "record-breaking" vapor
absorption.
Engineers at MIT have created a superabsorbent substance that, even under
desert-like circumstances, can absorb a record quantity of moisture from the
atmosphere.
The material may expand to accommodate additional moisture as it takes up
water vapor. With a relative humidity of 30%, the material can draw vapor
from the air and retain moisture without leaking even in extremely dry
situations. After being heated and condensed, the water might then be
collected as ultrapure water.
The translucent rubber-like substance is formed of hydrogel, a naturally
absorbent substance often utilized in disposable diapers. The scientists
made the hydrogel more absorbent by adding lithium chloride, a salt that is
well-known for being an effective dessicant.
More salt could be infused into the hydrogel than was previously
conceivable, according to the researchers. They discovered that the
salt-loaded gel absorbed and held onto a record amount of moisture in a
variety of humidity levels, including extremely dry circumstances that have
previously restricted the development of other material designs.
The superabsorbent gel may be utilized as a passive water harvester if it
can be produced fast and in large quantities, especially in desert and
drought-prone areas where the substance might continually collect vapor and
subsequently condense it into drinkable water. The material might also be
attached to air conditioners as a dehumidifying, energy-saving component,
according to the researchers.
"We've been application-agnostic, in the sense that we mostly focus on the
fundamental properties of the material," says Carlos Dáz-Marin, a doctoral
student in mechanical engineering and part of the Device Research Lab at
MIT. But right now, we're looking at quite different issues, including how
to capture water and improve the efficiency of air conditioning. This
material has so much promise because of its inexpensive cost and good
performance.
In an article that will be published in the journal Advanced Materials
today, Dáz-Marin and his colleagues have published their findings. Yang
Zhong, Bachir El Fil, Xinyue Liu, Evelyn Wang, Gustav Graeber, Leon Gaugler,
and others from MIT are the study's other co-authors.
The "best of both worlds"
Researchers at MIT's Device Research Lab are creating new materials to
address the world's energy and water problems. The scientists focused on
hydrogels, which are slick, stretchy gels consisting primarily of water and
a little amount of cross-linked polymer, when searching for materials that
can aid in the extraction of water from the atmosphere. Since they can
expand and absorb a lot of water when it comes into touch with them,
hydrogels have been utilized for years as an absorbent material in
diapers.
How can we make this operate just as effectively to capture vapor from the
air was our main concern. Dáz-Marin claims.
He and his colleagues looked through the literature and discovered that
others had tried combining different salts with hydrogels. Certain salts are
highly effective in absorbing moisture, including water vapor, such as the
rock salt used to melt ice. The finest of them is lithium chloride, a salt
that can take up more moisture than ten times its own mass. Lithium
chloride, if left in a pile by itself, might draw vapor from the air, but
the moisture would just collect around the salt and there would be no way to
store the absorbed water.
In order to create a polymer that could both hold in moisture and expand to
contain additional water, researchers have tried to infuse salt into
hydrogel.
"It's the best of both worlds," Graeber, who is currently a lead
investigator at Berlin's Humboldt University, explains. "The salt can absorb
a lot of vapor, and the hydrogel can hold a lot of water. Therefore, it
seems sense that you would want to mix the two.
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However, the MIT team discovered that some researchers had to stop adding
more salt to their gels. The most effective examples to yet were hydrogels
that had 4 to 6 grams of salt per gram of polymer injected into them. In dry
settings with a 30% relative humidity, these samples absorbed around 1.5
grams of vapor per gram of material.
The majority of the investigations used samples that were created by
soaking hydrogels in salty water and waiting for the salt to dissolve into
the gels. Most trials came to an end after 24 to 48 hours because scientists
discovered the process to be extremely sluggish and that the gels only
contained little amounts of salt. The samples absorbed very little water
vapor when the material's capacity to absorb it was tested since there
wasn't enough salt in them to begin with to absorb the moisture.
What would happen if the process of creating the materials was permitted to
continue for a few days or even weeks? If given enough time, might a
hydrogel absorb even more salt? The MIT researchers experimented with
lithium chloride, a superabsorbent salt, and polyacrylamide, a popular
hydrogel, to find a solution. Hydrogel tubes were created by the researchers
using conventional mixing techniques. The tubes were then cut into thin
disks, and each disk was deposited into a solution of lithium chloride with
a variable salt content. Each day, they weighed the disks to establish how
much salt had absorbed into the gels, then put them back in their solutions
after weighing.
In the end, they discovered that hydrogels did actually absorb more salt
when given additional time. Compared to the previous record of 6 grams of
salt per gram of polymer, hydrogels integrated up to 24 after 30 days of
soaking in a salty solution.
The scientists next conducted absorption experiments on numerous samples of
the salt-filled gels under a variety of humidity levels. They discovered
that the samples could expand and take in more moisture without leaking at
all humidity levels. The gels caught a "record-breaking" 1.79 grams of water
per gram of material in extremely dry circumstances of 30 percent relative
humidity, according to the research team.
According to Daz-Marin, who is currently working on ways to accelerate up
the material's superabsorbent capabilities, "Any desert during the night
would have that low relative humidity, so theoretically, this material could
generate water in the desert."
"The big, unexpected surprise was that, with such a simple approach, we
were able to get the highest vapor uptake reported to date," adds Graeber.
"At this point, kinetics—how rapidly we can get the material to absorb
water—will be the key emphasis. That will enable you to cycle this material
fast, allowing you to perhaps capture water up to 24 times each day rather
of just once.
The Swiss National Science Foundation and the American Office of Energy
Efficiency and Renewable Energy both provided some funding for this
study.
