Sorbent-assisted
water harvesting from air represents an attractive
way to address water scarcity in arid climates. Hitherto, sorbents
developed for this technology have exclusively been designed to perform
one water harvesting cycle (WHC) per day, but the productivities attained
with this approach cannot reasonably meet the rising demand for drinking
water. This work shows that a microporous aluminum-based metal-organic
framework, MOF-303, can perform an adsorption–desorption cycle
within minutes under a mild temperature swing, which opens the way
for high-productivity water harvesting through rapid, continuous WHCs.
Additionally, the favorable dynamic water sorption properties of MOF-303
allow it to outperform other commercial sorbents displaying excellent
steady-state characteristics under similar experimental conditions.
Finally, these findings are implemented in a new water harvester capable
of generating 1.3 L kgMOF–1 day–1 in an indoor arid environment (32% relative humidity, 27 °C)
and 0.7 L kgMOF–1 day–1 in the Mojave Desert (in conditions as extreme as 10% RH, 27 °C),
representing an improvement by 1 order of magnitude over previously
reported devices. This study demonstrates that creating sorbents capable
of rapid water sorption dynamics, rather than merely focusing on high
water capacities, is crucial to reach water production on a scale
matching human consumption.
Designing water uptake
Although the locations of water molecules in some porous materials have been determined with diffraction techniques, determining the filling sequence of water sites has been challenging. Hanikel
et al
. used single-crystal x-ray diffraction to locate all of the water molecules in pores of the metal-organic framework MOF-303 at different water loadings (see the Perspective by Öhrström and Amombo Noa). They used this information on the water molecule adsorption sequence to modify the linkers of this MOF and control the water-harvesting properties from humid air for different temperature regimes. —PDS
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