Carbon nanotubes decorated hollow metal–organic frameworks for efficient solar-driven atmospheric water harvesting

https://doi.org/10.1016/j.cej.2021.133086Get rights and content

Highlights

  • Hollow MIL-101(Cr) is formed by etching for fast water capture and release.

  • Carbon nanotubes decorated hollow MIL-101 promotes photothermal performance.

  • The hierarchical porous structure enhances the water affinity under low humidity.

  • The water uptake and release rates are the highest among those reported.

Abstract

High equilibrium uptake capacity and rapid adsorption/desorption kinetics are essential to achieve efficient atmospheric water harvesting. In this work, carbon nanotubes decorated hollow MIL-101(Cr) particles (HMC-2) are prepared and utilized for solar-driven atmospheric water harvesting with nice water adsorption capacity and fast kinetics. The hierarchical porous hollow MIL-101(Cr) particles formed by post-etching show highly active defect sites and low mass transfer resistance, which promotes the adsorption and diffusion of water molecules. The resulted HMC-2 demonstrates a maximum water uptake of 1.074 g/g at 25 °C, 90% RH. Notably, the water adsorption capacity of HMC-2 in low humidity (40% RH) are increased by 113% and 202.6% compared to pure MIL-101(Cr) and carbon nanotubes decorated MIL-101(Cr). Due to the synergistic effect of CNTs and hollow MIL-101(Cr), the HMC-2 composites generate sufficient solar thermal heat and improve the desorption kinetics. The maximum adsorbed water of HMC-2 can be completely desorbed within 90 min under one sunlight. A single cycle of sorption–desorption of HMC-2 can be achieved within 210 min. This means that if the water harvesting cycle is performed three times daily, it is possible to harvest 3.22 L of water per kilogram of adsorbent at the area with high humidity. The water uptake and release rates of HMC-2 are competitive with even superior to most those reported. This may demonstrate a promising potential for atmospheric water generators.

Introduction

Metal-organic frameworks (MOFs) are a class of highly crystalline porous materials with unique topological types, assembled by the bonding of metal ions or clusters with diverse organic linkers to form infinite one, two or three dimensional frameworks[1], [2], [3]. MOFs typically exhibit high surface area and porosity for various applications, such as electromagnetic wave absorption[4], biomedicine[5], gas adsorption or removal of heavy metal ions[6], [7], [8], photocatalytic degradation of toxic dyes[9] and so on[10].

As one of the most important materials for adsorption-based water collection system, the water adsorption capacity and desorption rate of MOFs determine the efficiency of water collection[11]. The pore environment (porosity, specific surface area, pore size and distribution) and the affinity of MOFs for water dominate water adsorption capacity [12]. In recent years, introducing hydrophilic functional groups, post-synthetic modification, combined polymers/salts with MOFs have been carried out to improve the adsorption capacity and transport performance for atmospheric water harvesting[3], [13], [14]. For the desorption process, it is necessary to consider the desorption temperature and the interaction between MOFs and water molecules. Unlike zeolite and deliquescent salt with strong affinity to water molecules, above 160 °C is requested to achieve efficient water desorption[15], [16]. On the contrary, the interaction between MOFs and water molecules is relatively weak. It has shown that the unsaturated metal sites of MOFs can act as new binding sites to attract more water molecules by hydrogen bonds [17]. However, the regenerated temperatures of most MOFs are still higher than 80 °C [14].

Recently, solar-driven water release has aroused great interest[2], [18]. Unfortunately, most MOFs are white or light-colored with poor light absorption ability and low photothermal performance[19], [20], [21], resulting in limited sunlight utilization efficiency. In order to improve the desorption rate of water, electric heating strips, solar cells or coated black light-absorbing layers on the surface of water collection equipment have been utilized to promote the release of water. These greatly increase the complexity and operating cost [22], [23], [24]. Such as Nikita Hanikel etc.[25] applied a solar module to heat the MOFs layer in atmospheric water generators (AWG). However, a typical AWG was estimated to consume ∼ 100 W, and the conversion efficiency of solar PV was very low, about 20–25%[26], [27], [28]; Farhad Fathieh etc.[23] mixed graphite with MOF-801 to enhance light absorption. However, the graphite significantly declined the water adsorption capacity, and releasing only 39% and 76% water under low and high solar fluxes, respectively. How to improve the photothermal performance of MOFs materials without adversely affecting the water capture capacity has become a challenge.

It has shown that hollow MOFs can increase the concentration of the local reactants and promote the reaction kinetics, facilitate the multi-level reflection light path to enhance light absorption, improve the catalysis and separation performance [29], [30], [31]. Hollow MOFs can fabricated by etching[32], layer-by-layer growth[31], Ostwald ripening[33] and template processes [34]. Among them, etching is a simple and efficient way. The hierarchical porous MOFs formed by etching will favorable for substrate transportation. In addition, more defects and exposed active sites benefit for water adsorption by forming chemical interaction or hydrogen bonds with water molecules[35].

As aforementioned, in order to achieve high water capture ability and fast solar-driven desorption kinetics of MOFs-based atmospheric water harvesting device, in this work, we synthesized highly water-adsorbed photothermal composite consisted of hollow MIL-101(Cr) MOFs and photo-thermal agents (carbon nanotubes, CNTs). Due to the high structural stability and water adsorption capacity of hollow MIL-101(Cr) [14], [36], [37] and nice thermal conductivity, photothermal conversion ability and structure stability of CNTs[38], [39], [40], the resulted hollow MIL-101(Cr) MOFs/carbon nanotubes (HMC-2) composite demonstrates nice water adsorption capacity and fast adsorption and desorption. The desorption of water is realized under simulated sunlight quickly. This strategy sheds light on the development of photothermal adsorbents for water capture with easy regeneration.

Section snippets

Chemicals and materials

Chromium nitrate (Cr(NO3)3·9H2O, 99%, AR), terephthalic acid (H2BDC) (C8H6O4, 99%, AR) were obtained from Aladdin. Ethanol (C2H6O, 99%, AR), glacial acetic acid (CH3COOH, AR) and N, N-dimethylformamide (DMF, AR) were obtained from Sinopharm Chemical Reagent Co., Ltd. The carboxylic single-walled carbon nanotubes (CNTs, 1.5 mg mL−1) aqueous dispersion was obtained from Nanjing XFNANO Materials Tech Co., Ltd. Deionized (DI) water (18.2 MΩ, from Milli-Q system) was used throughout the experiments.

Structure and morphology characterization

The chemical structure design and fabrication process of porous hollow MIL-101/CNTs (HMC) composites are illustrated in Scheme 1. Briefly, HMC composites can be obtained by hydrothermal reaction, acid etching, and mixing with CNTs. Non-etched MIL-101/CNTs composites were also synthesized for comparison (See details in Experiment Section). Figure S1a and S2a show that MIL-101 has an octahedral structure. While the etched MIL-101 hold an irregular morphology with obvious cavities (Figure S1d and

Conclusions

In summary, a highly water-adsorbed photothermal composite was designed for atmospheric water harvesting by combining photothermal CNTs with MIL-101 hollow particles. The hierarchical porous hollow structure of HMC-2 not only has abundant unsaturated metal sites and defects to endow them with significant water capture ability under low humidity, but also facilitates water transport and storage. The maximum water uptake of HMC at 90%RH is 1.074 g/g. The water adsorption capacity of HMC-2 in low

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by Major R & D plan of Zhejiang Natural Science Foundation (LD18E020001), the Key program of National Natural Science and Foundation (51632008), and the National Natural Science Foundations of China (21875212).

References (56)

  • M. Anbia et al.

    Development of MWCNT@MIL-101 hybrid composite with enhanced adsorption capacity for carbon dioxide

    Chem. Eng. J.

    (2012)
  • J. Yang et al.

    Synthesis of metal-organic framework MIL-101 in TMAOH-Cr(NO3)3–H2BDC-H2O and its hydrogen-storage behavior

    Micropor. Mesopor. Mat.

    (2010)
  • N.A.A. Qasem et al.

    Synthesis, characterization, and CO2 breakthrough adsorption of a novel MWCNT/MIL-101(Cr) composite

    J. CO2 Util.

    (2017)
  • Y. Chen et al.

    A novel CNT encapsulated phase change material with enhanced thermal conductivity and photo-thermal conversion performance

    Sol. Energy Mater Sol. Cells

    (2018)
  • H. Kou et al.

    Recyclable CNT-coupled cotton fabrics for low-cost and efficient desalination of seawater under sunlight

    Desalination

    (2019)
  • C.M. Granadeiro et al.

    Monovacant polyoxometalates incorporated into MIL-101(Cr): novel heterogeneous catalysts for liquid phase oxidation

    Appl. Catal. A: Gen.

    (2013)
  • S. Zhao et al.

    Research of mercury removal from sintering flue gas of iron and steel by the open metal site of MIL-101(Cr)

    J. Hazard Mater.

    (2018)
  • J. Xu et al.

    Efficient solar-driven water harvesting from arid air with metal-organic frameworks modified by hygroscopic salt

    Angew. Chem. Int. Ed.

    (2020)
  • H. Kim et al.

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

    Science

    (2017)
  • N. Ko et al.

    Tailoring the water adsorption properties of MIL-101 metal-organic frameworks by partial functionalization

    J. Mater. Chem. A

    (2015)
  • B. Davaji et al.

    Microscale direct measurement of localized photothermal heating in tissue-mimetic hydrogels

    Sci. Rep.

    (2019)
  • N.A. Ramsahye et al.

    Impact of the flexible character of MIL-88 iron(III) dicarboxylates on the adsorption of n-alkanes

    Chem. Mater.

    (2013)
  • T. Yan et al.

    Ultrahigh-energy-density sorption thermal battery enabled by graphene aerogel-based composite sorbents for thermal energy harvesting from air

    ACS Energy Lett.

    (2021)
  • N.C. Burtch et al.

    Water stability and adsorption in metal-organic frameworks

    Chem. Rev.

    (2014)
  • J. Canivet et al.

    Water adsorption in MOFs: fundamentals and applications

    Chem. Soc. Rev.

    (2014)
  • A. Karmakar et al.

    Thermo-responsive MOF/polymer composites for temperature-mediated water capture and release

    Angew. Chem. Int. Ed.

    (2020)
  • R. Li et al.

    Hybrid hydrogel with high water vapor harvesting capacity for deployable solar-driven atmospheric water generator

    Environ. Sci. Technol.

    (2018)
  • M.J. Kalmutzki et al.

    Metal-organic frameworks for water harvesting from air

    Adv. Mater.

    (2018)
  • Cited by (42)

    View all citing articles on Scopus
    View full text