Rapid Cycling and Exceptional Yield in a Metal-Organic Framework Water Harvester

Abstract

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 kg_MOF ^-1 day^-1 in an indoor arid environment (32% relative humidity, 27 °C) and 0.7 L kg_MOF ^-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.

Figure 1

Water sorption properties of MOF-303. (a) Water sorption isotherm at 30 °C. (Inset: structure of MOF-303 [Al(OH)(PZDC), PZDC = 1H-pyrazole-3,5-dicarboxylate]. Hydrogen atoms are omitted for clarity. Color code: Al, blue; C, gray; O, red; N, green.) (b) Dynamic vapor sorption properties of a thin MOF layer: adsorption at 30 °C and 20–40% relative humidity (RH) and subsequent desorption at 85 °C and 0% RH.

Figure 2

Water sorption isotherms at 30 °C of the tested materials. P, partial water vapor pressure; P_sat, saturation water vapor pressure at 30 °C. STP: standard temperature and pressure.

Figure 3

Experimental data is represented by solid lines. The line color indicates the type of sorbent, as specified in the inset. (a) Water adsorption at 30 °C and 20–40% relative humidity (RH). Prior to adsorption, the sorbent materials were fully dehydrated. The initial 95% filling is approximated with a monoexponential fit (dashed lines in the respective color). The curve for Al-fumarate at 20% RH was not fitted due to insufficient water uptake at this RH. (b) Water desorption at 65, 85, and 120 °C, and 0% RH. Prior to desorption, the sorbent materials were saturated at 30 °C and 40% RH.

Figure 4

Water harvesting cycle (WHC) as operated by the atmospheric water harvester. (a) Illustration of the WHC. During the desorption step, the MOF bed is heated to release water vapor, which is immediately transferred to the condenser under convective air flow. During the adsorption step, ambient air is propelled, through forced convection across the exchanger, to facilitate mass transfer to the MOF. (b) Evolution of temperature and relative humidity (RH) during a typical WHC, as measured by sensors at different locations in the harvester. Measured desorption (c) and adsorption (d) kinetics, in terms of mass of water per mass of sorbent against time, measured for MOF-303 under different ambient RH. The ambient temperature was measured to be 25 ± 2 °C for all experiments.

Figure 5

Practical atmospheric water harvesting in the Mojave Desert. (a) Diagram displaying the water harvesting productivity after each water harvesting cycle (WHC), the ambient relative humidity and temperature, and the corresponding dew point. The productivity of the first WHC (*) is higher because the experiment started with a fully saturated MOF bed. All data was measured over the course of three continuous days of operation in the Mojave Desert. The dotted, red line indicates the 5 °C dew point, representing the lower limit for the operating range of direct condensation technologies. (b) Photograph of the water harvester with labeled parts. Close-up views of the MOF exchanger (c) and the water collected under continuous operation (d).