Research ArticleAPPLIED SCIENCES AND ENGINEERING

A water lily–inspired hierarchical design for stable and efficient solar evaporation of high-salinity brine

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Science Advances  05 Jul 2019:
Vol. 5, no. 7, eaaw7013
DOI: 10.1126/sciadv.aaw7013
  • Fig. 1 Design concept of the water lily–inspired hierarchical structure.

    (A and B) Water lily and water lily–inspired design for solar vapor generation, respectively. They share several key features: the upper epidermis with hydrophobic surface absorbs the sunlight and provides stomata for water vapor escape, lacunae (air chamber) at the bottom of the leaf keep a water lily afloat on the water, and vascular bundles (water path) provide a confined water supply. (C) Microscale schematic of a confined water layer sandwiched between the hydrophobic top solar absorber and the bottom stand with low thermal conductivity. Evaporation occurs at the water surface below the absorber, and salt/solute is excreted by the water path, avoiding accumulation/crystallization of solute on the absorber. (D) Nanoscale light trapping for the top solar absorber. (E) Molecular-scale surface modification for the hydrophobic surface of the top solar absorber.

  • Fig. 2 Fabrications and characterizations of a WHS.

    (A) Schematics of the fabrication processes of the top solar absorber. From left to right: the original Cu foam, after chemical etching, after Al2O3 coating, and subsequent carbon black (CB) decoration. The insets show optical photographs of the absorber at different fabricating stages. (B) Scanning electron microscopy (SEM) images of the Cu foam with micrometer-sized pores. (C to E) High-resolution SEM images of the absorber at different process stages: surface of the original Cu foam (C), after etching (D), and after Al2O3 coating and CB decoration (E). Inset of (E): contact angle of the absorber. (F) Absorption spectra of the absorber at different fabricating steps. From top to bottom: the original Cu foam, after etching, and after atomic layer deposition (ALD) coating and CB decoration. (G) Photographs of the top, bottom, and cross-sectional views for the WHS. The through-holes of the bottom stand provide the water supply. The diameter of the absorber is 4 cm.

  • Fig. 3 Performance of solar-vapor generation.

    (A) Evaporation rates and energy conversion efficiencies of WHS for DI water, 10 wt % brine, and 30 wt % wastewater. (B) Ion concentrations before and after water purification. Seawater (collected from the Bohai Sea, China, with an average salinity of ∼1 wt %) and wasterwater (with heavy metal ions, Ni2+ and Cd2+) were used as water sources. The dashed blue lines and dashed violet lines show the WHO standard of ion concentrations for drinking water and standard for discharge, respectively. (C) Mass changes and evaporation rates of the WHS and a conventional solar absorber over 8 hours. Brine (10 wt %) was used as the water source. The evaporation rates out of pure water are also listed at 0 hours for comparison. (D) Photographs of the WHS and a conventional solar absorber over time when treating brine with 10 wt % salinity initially. (E) Outdoor solar evaporation performance of the WHS and a conventional solar absorber over 18 days when treating brine with 10 wt % salinity (photo credit: Ning Xu, Nanjing University).

  • Fig. 4 Complete separation of water and solute after stable and efficient solar evaporation.

    (A and B) Evolutions of evaporation rates over time for solar desalination and wastewater treatment, respectively. Insets show the time-dependent photographs of the devices for outdoor solar desalination (A) and wastewater treatment (B). Side views/top views of the device are presented in the first row/second row, respectively (the dashed red lines indicate the bottom of the WHS). Photographs of recycled salt and solute are presented on the bottom right of (A) and (B), respectively (photo credit: Ning Xu, Nanjing University).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/7/eaaw7013/DC1

    Fig. S1. Photographs and IR photographs of the conventional absorber and the WHS device after working for 8 hours.

    Fig. S2. Heat localization in WHS.

    Fig. S3. Thermal properties for the top solar absorber of the WHS.

    Fig. S4. SEM image of the Cu foam after etching and Al2O3 coating.

    Fig. S5. IR emittance spectra of the absorbers after Al2O3 coating and CB decoration.

    Fig. S6. The mechanism of water supply for the WHS device.

    Fig. S7. Vapor generation performances of the WHS treating brine with different salinities.

    Fig. S8. Mass changes of different water sources over time with the WHS device.

    Fig. S9. The schematic of the setup used for collecting purified water.

    Fig. S10. Salinities of different brine before (3.5 wt %, 5 wt %, and 10 wt %) and after purification.

    Fig. S11. The schematic of the conventional solar absorber.

    Fig. S12. Characterizations of the conventional solar absorber.

    Fig. S13. Performances of solar evaporation for the WHS device and conventional solar absorber at the beginning (0 day) and after 18 days of working.

    Fig. S14. Concentrations of ions in the water before and after purification.

    Table S1. Comparison of optical performances of the absorbers after Al2O3 coating and CB decoration.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Photographs and IR photographs of the conventional absorber and the WHS device after working for 8 hours.
    • Fig. S2. Heat localization in WHS.
    • Fig. S3. Thermal properties for the top solar absorber of the WHS.
    • Fig. S4. SEM image of the Cu foam after etching and Al2O3 coating.
    • Fig. S5. IR emittance spectra of the absorbers after Al2O3 coating and CB decoration.
    • Fig. S6. The mechanism of water supply for the WHS device.
    • Fig. S7. Vapor generation performances of the WHS treating brine with different salinities.
    • Fig. S8. Mass changes of different water sources over time with the WHS device.
    • Fig. S9. The schematic of the setup used for collecting purified water.
    • Fig. S10. Salinities of different brine before (3.5 wt %, 5 wt %, and 10 wt %) and after purification.
    • Fig. S11. The schematic of the conventional solar absorber.
    • Fig. S12. Characterizations of the conventional solar absorber.
    • Fig. S13. Performances of solar evaporation for the WHS device and conventional solar absorber at the beginning (0 day) and after 18 days of working.
    • Fig. S14. Concentrations of ions in the water before and after purification.
    • Table S1. Comparison of optical performances of the absorbers after Al2O3 coating and CB decoration.

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