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Water harvesting from air with metal-organic frameworks powered by natural sunlight

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Science  28 Apr 2017:
Vol. 356, Issue 6336, pp. 430-434
DOI: 10.1126/science.aam8743
  • Fig. 1 Working principle of water harvesting with MOFs.

    (A) Water-adsorption isotherms of Zr-based MOFs (MOF-801, MOF-841, UiO-66, and PIZOF-2) at 25°C, showing a rapid increase in adsorption capacities (in kilograms of water per kilogram of MOF) with a relatively small change in the relative humidity (RH) (P Psat\–1, vapor pressure over saturation pressure) (10). The background color map shows the minimum difference between the temperatures of the ambient air (Tamb) and the condenser (Tdew) required for dew collection with active cooling. (B) Water-adsorption isotherms of MOF-801, measured at 25° and 65°C, illustrating that the temperature swing can harvest greater than 0.25 kg kg–1 at >0.6 kPa vapor pressure (20% RH at 25°C). (C) A MOF water-harvesting system, composed of a MOF layer and a condenser, undergoing solar-assisted water-harvesting and adsorption processes. During water harvesting (left), the desorbed vapor is condensed at the ambient temperature and delivered through a passive heat sink, requiring no additional energy input. During water capture, the vapor is adsorbed on the MOF layer, transferring the heat to the ambient surroundings (right). Ads. and cond., adsorption and condensation, respectively. (D) Zr6O4(OH)4(-COO)12 secondary building units are linked together with fumarates to form MOF-801. The large yellow, orange, and green spheres are three different pores. Black, C; red, O; blue polyhedra, Zr.

  • Fig. 2 Experimental characterization of harvested water from an adsorption-desorption cycle with MOF-801.

    (A) Image of the MOF-801 layer and condenser. (B) The schematic illustrates the vapor adsorption and desorption experiments carried out under isobaric conditions. Vapor was adsorbed through the sample surface by diffusion. Desorption was achieved by applying an incident solar flux on an absorber with a solar absorptance of 0.91, and the desorbed vapor was condensed simultaneously in the condenser to harvest water. The condensation heat was monitored using a heat flux sensor (HFS) with active cooling through a thermoelectric (TE) cooler. (C) Layer temperature and chamber vapor pressure as functions of time during the water-harvesting cycle. The background color map represents the estimated RH from the chamber pressure and the layer temperature, and the upper abscissa represents the overall water uptake predicted from the theoretical model as a function of time (lower abscissa). (D) Experimentally characterized water-harvesting rate (liters per kilogram per second) and cumulative harvested water (liters per kilogram) during desorption. The shaded region represents the error based on uncertainties of the heat flux and MOF-801 weight measurements. The predicted temperature profile and cumulative water harvested are also included in (C) and (D), respectively, showing good agreement. The activated MOF-801 has a weight of 1.79 g, a layer thickness of 0.41 cm, and a packing porosity of ~0.85. sim and exp, simulated and experimental results, respectively.

  • Fig. 3 Adsorption-desorption dynamics of MOF-801 in ambient air with a flux of 1 sun.

    Predicted adsorption-desorption dynamics with a packing porosity (ε) of 0.7, solar flux of 1 sun (1 kW m–2), and various thicknesses (1 to 5 mm). MOF-801 was initially equilibrated at 20% RH and 25°C, and the partial vapor pressure rapidly increased to 100% RH at 25°C during desorption for vapor condensation. After desorption, the surrounding air-vapor mixture reverted to 20% RH. The durations of solar exposure for thicknesses of 1, 3, and 5 mm were 1, 2.3, and 4.2 hours, respectively. The duration of solar exposure is plotted only for the 5-mm-thick sample (red dashed line) for simplicity. The 1-mm, 3-mm, and 5-mm layers can harvest 0.08, 0.24, and 0.4 liters m–2 per complete water-harvesting cycle, respectively. More than 90% of the initially adsorbed water could be harvested under these conditions. The inset shows a predicted temperature profile of the 5-mm-thick layer during the adsorption-desorption processes.

  • Fig. 4 Proof-of-concept water-harvesting prototype.

    (A) Image of a water-harvesting prototype with activated MOF-801 with a weight of 1.34 g, a packing porosity of ~0.85, and outer dimensions of 7 by 7 by 4.5 cm. (B) Formation and growth of droplets of water as a function of MOF temperatures (TMOF) and local time of day. (C) Representative temperature profiles for the MOF-801 layer (experimental, red solid line; predicted, red dashed line), ambient air (gray line), the condenser (blue line), and the ambient dew point (green line), as well as solar flux (purple line), as functions of time of day (14 September 2016). The background color map represents the estimated RH from the condenser saturation pressure and the layer temperature, and the upper abscissa represents the water uptake predicted from the theoretical model as a function of time (lower abscissa). Because of losses from the absorber solar absorptance (α, 0.91) and the glass plate solar transmittance (τ, 0.92), 84% of the solar flux shown in (C) was used for desorption. The layer temperature and full water-harvesting potential based on complete desorption were predicted using the solar flux and environmental conditions at the end of the experiment (dashed lines). The fluctuations of the solar flux from 10:20 to 11:00 were due to the presence of clouds.

Supplementary Materials

  • Water harvesting from air with metal-organic frameworks powered by natural sunlight

    Hyunho Kim, Sungwoo Yang, Sameer R. Rao, Shankar Narayanan, Eugene A. Kapustin, Hiroyasu Furukawa, Ari S. Umans, Omar M. Yaghi, Evelyn N. Wang

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Materials and Methods
    • Supplementary Text
    • Figs. S1 to S8
    • References
    Correction (24 April 2017): In fig. S6B, the x-axis values were given as proportions, whereas they should have been percentages in accordance with the x-axis label ("P Psat−1 %"). The PDF has been corrected.
    The original version is accessible here.

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