Sponge-like MOFs harvest water from thin air

Scientists in the US have identified metal-organic frameworks (MOFs) capable of passively adsorbing almost nine litres of water a day per kilo.

Passive water capture has the potential to impact millions of lives around the world, allowing people to extract water directly from the atmosphere using little or no energy. It can work in arid conditions where other sources of water may not be available and could be a powerful tool for vulnerable communities dealing with the consequences of climate change.

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MOFs are an ideal candidate for passive water capture. Their sponge-like crystal structure gives them the highest surface area per gram of any material. Just one gram of some MOFs would have a surface area of more than an acre if laid flat in a single layer. While some research into using MOFs for water capture has been conducted, this latest work involved an extensive exploration of various MOFs in an effort to find the most adsorbent. The study is published in Scientific Reports.

“Initial experiments have proved that the concept can work,” said co-author Zhiyong Xia, from the Research and Exploratory Development Department at Johns Hopkins’ Applied Physics Laboratory (APL).

“But the problem has been capacity. Other research teams have been able to produce as much as about 1.3 litres of water per day per kilogram of sorbent under arid conditions — enough only for one person. To create an optimal water-harvesting device requires a better understanding of the structure property relationship controlling adsorption and delivery.”

The team tested a number of MOFs and the potential impact of temperature, humidity and powder bed thickness on the adsorption-desorption process. Alongside colleagues Matthew Logan and Spencer Langevin, Xia identified a compound called Zr-MOF-808 that significantly outperformed previous MOFs.

“We identified a MOF that could produce 8.66 litres of water per day per kilogram of MOF under ideal conditions, an extraordinary finding,” he said. “This will help us deepen our understanding of these materials and guide the discovery of next-generation water-harvesting methods.”

The Johns Hopkins team is now exploring other MOFs with low relative humidity influx points, high surface areas and polar functional properties to see how they perform in arid environments. It is also researching different configurations of MOFs in pursuit of optimal adsorption levels.