Regulating the Hydrophilicity of Hyper-Cross-Linked Polymers via Thermal Oxidation for Atmospheric Water Harvesting

Abstract

We explore the thermal oxidation of hyper-cross-linked polymers to enhance their hydrophilicity and efficacy in atmospheric water harvesting. Comprehensive chemical and physical characterizations are used to confirm the successful incorporation of polar oxygen moieties and the preservation of porosity upon thermal treatment. Newly introduced oxygen-based functional groups significantly improve water sorption properties, increasing total water uptake capacities by up to 400% and shifting water uptake onsets to significantly lower relative humidity. We also investigate the regeneration of oxidized hyper-cross-linked polymers after water sorption to probe their potential for multiple water harvesting cycles and reuse. Our findings outline a simple and cost-effective postsynthetic modification route for optimizing porous organic polymers for more sustainable and efficient atmospheric water harvesting.

Keywords: adsorption; atmospheric water harvesting; direct air capture; hydrophilicity; hyper-cross-linked polymers; porous organic polymers; thermal oxidation.

Figure 1

(a) Suggested representative structure for an OHCP. (b) FTIR spectra of BP-HCP and all OHCPs. (c) CP/MAS ssNMR spectra of BP-HCP and all OHCPs, * indicate spinning side bands. (d) X-ray photoelectron C 1s spectrum of OHCP-60. (e) X-ray photoelectron O 1s spectrum of OHCP-60. (f) N₂ isotherms of BP-HCP and all OHCPs measured at −196 °C, filled and empty circles represent adsorption and desorption, respectively.

Figure 2

Water sorption–desorption properties of BP-HCP and OHCPs. (a) Water isotherms at 25 °C. Closed spheres represent uptake and open spheres represent desorption. (b) Desorption of water from OHCP-60 over time at various temperatures, measured using TGA. (c) Water sorption–desorption of OHCP-60 at 10%, 20%, 30%, and 90% RH.

Figure 3

(a) Water sorption–desorption isotherms of OHCP-60 recorded at 25, 35, and 45 °C. (b) Long-term stability of OHCP-60 over 100 adsorption–desorption cycles consisting of a humidity-swing process between 0% and 40% RH.

Figure 4

Comparison of OHCP-60 (brown) and OHCP-60 after 10 sorption–desorption cycles (orange), in which desorption was driven by heating at 90 °C for 1 h per cycle. (a) Water isotherms measured at 25 °C, (b) N₂ isotherms measured at −196 °C. Filled and empty circles represent adsorption and desorption, respectively, and (c) CP/MAS ssNMR spectra, where * indicate spinning side bands.