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Superelastic graphene nanocomposite for high cycle-stability water capture-release under sunlight Bo Chen, Xue Zhao, and Ya Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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ACS Applied Materials & Interfaces

Superelastic graphene nanocomposite for high cycle-stability water capture-release under sunlight Bo Chen†,‡, Xue Zhao†,‡ and Ya Yang†,‡,⊥* †CAS

Center for Excellence in Nanoscience, Beijing Key Laboratory of Micro-nano Energy and

Sensor, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, 100083, P. R. China. ‡School

of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing

100049, China. ⊥Center

on Nanoenergy Research, School of Physical Science and Technology, Guangxi University,

Naning, Guangxi 530004, P. R. China *To whom correspondence should be addressed: Email: [email protected]. ABSTRACT: The shortage of water resources is an enormous challenge for human society due to the increasing demand caused by the growing industry and population. Atmospheric water is an abundant and nonnegligible fresh water resource, which can be developed as a convenient approach in some water-deficient circumstances. Herein, a graphene nanocomposite foam is designed and demonstrated for harvesting water from air by the use of solar energy. The as-fabricated foam possesses a water harvesting capability of 0.23 g g-1 in a typical 2 h capture – 2 h release cycle at 30% relative humidity, while 1.15 g g-1 at 90% relative humidity. The nanocomposite foam presents a stable water harvesting performance after ten capture – release cycles. Endowed with low density and superelasticity, the graphene nanocomposite could be compressed and portable, which takes water harvesting system one step further to practical application and commercial production. KEYWORDS: atmospheric water capture, solar steam, superelastic, graphene oxide, polyimide.

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1. INTRODUCTION Worldwide, about 2.5% water reserves is fresh water, and only a small quantity of fresh water can be directly utilized due to most of that is stored as deep groundwater and glaciers.1 Nowadays, two-thirds of the world’s population suffers from freshwater scarcity which is becoming a global risk threatening human’s subsistence.2 Desalination methods, such as electrodialysis,3 reverse osmosis,4 ion concentration polarization,5 and capacitive deionization,6 are employed to transform sea water into fresh water to satisfy the urge of drinking in some coastal regions. However, these desalination systems are subject to high energy consumption and high fabricating cost, which limit their industrial application and commercial exploitation. Recently, solar vapour generation, a certain kind of solar-to-thermal technology, is considered as an efficient way to harvest solar energy and to purify water by converting heat to facilitate water evaporation.7-10 Nevertheless, a pivotal environmental condition, that is a certain amount of liquid water either seawater or contaminated fresh water, is essential to carry out the evaporation process. While, in some situations, in a vast grassland or desert, for example, it is hardly to find a liquid water source within a certain area or a short time. An abundant of water equivalent to about 10% of the total fresh water in lakes exists in the earth atmosphere, which can be a nonnegligible fresh water resource to fight against the water shortage.11-13 People make efforts to develop moisture capture systems inspired by some natural plants and animals, such as desert cacti and Namib desert beetles, that can survive in arid area by scavenging atmospheric water14-15. Even though water capture can be realized by dewing, an enormous amount of energy is required to release water for further utilization. The difficulty to establish a high efficiency water capture-release cycle system is to exploit an uptake and release water functional material with low energy consumption. In last few years, metal-organic frameworks 2 ACS Paragon Plus Environment

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endowed with 3D porous structures and solar-to-thermal conversion ability, were introduced to break down the technical barriers.11,

13, 16-17

Besides, concerning the practicability and durability,

such materials are expired to be light, portable, low-cost and well reusable18-21. Herein, we demonstrate a graphene nanocomposite foam based water harvesting system to harvest water from the air. The foam realizes the water harvesting through a capture-release cycle: (1) the capture process is composed of moisture adsorption from air by lithium chloride (LiCl) and water preservation by polyvinyl alcohol (PVA); (2) the release relies on the solar-to-thermal transformer, reduced graphene oxide (rGO), to facilitate the evaporation. In addition, polyimide (PI) is employed as a substrate material for the purpose of 3D porous structure formation and mechanical properties enhancement. The as-fabricated foam can adsorb water up to 2.87 g g-1 in 24 h at a relative humidity (RH) of 90% and a temperature of 30 oC, and release almost all the uptake water when it is exposure under a flux of 1 sun (1000 W m-2, equal to the light intensity of natural sunlight) for 3 h. To be specific, for a capture (2 h, 30% RH) - release (2 h, 50% RH) cycle, approximate 0.23 g water can be collected by 1 g nanocomposite foam at room temperature, while the water harvesting ability could reach to 1.15 g g-1 when the foam is exposed at 90% RH during water capture. At the same time, the functional foam shows superelasticity, light weight and remarkable reusability, thus revealing its possibility to practical use.

2. EXPERIMENTAL SECTION Preparation of GO. GO was prepared from the expandable graphite powder by a modified Hummers method. First, 360 mL sulfuric acid (H2SO4, 98%, Beijing Chemical Works) and 40 mL phosphoric acid (H3PO4, 85%, Beijing Chemical Works) were mixed under magnetic stirring in a three-neck flask. Next, 18 g potassium permanganate (KMnO4, 99.3%, Liangfeng Co., Ltd.) and 3 g 3 ACS Paragon Plus Environment


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natural graphite powder (300 mesh, 99.9%, Xfnano Co., Ltd.) were slowly added into the mixture under magnetic stirring. Then the flask was immersed in a 40 oC oil bath under stirring for 12 h. The mixture was subsequently cooled to room temperature and then gradually poured into a beaker containing 1200 mL ice water. 20 mL hydrogen peroxide solution (H2O2, 30%, Beijing Chemical Works) was slowly introduced into the mixture to reduce the excess oxidizing regent. The resulting mixture was further dispersed by an ultrasonic homogenizer and then centrifuged for 5 minutes at 8000 rpm. The precipitation was ordinally washed with deionized water, hydrochloric acid (HCl, 30%, Beijing Chemical Works), and anhydrous ethanol (Sinopharm Chemical Reagent Co., Ltd.) at least three times until the pH of the rinsed water reached neutral. Finally, the dispersion of GO was dried in oven at 40 oC under vacuum for further use. Synthesis of PI materials. Synthetic scheme for PI was shown in Figure S1. The 4,4’-oxydianiline (ODA, 98%, 4.31g, Adamas Reagent Co., Ltd.) was dissolved in 51 g N,N-dimethylacetamide (DMAC, 99%, Shanghai Macklin Biochemical Co., Ltd.) under continuously mechanical stirring to obtain a homogeneous solution in a beaker which was sunk in an ice-water bath. Then, pyromellitic dianhydride (98%, 4.69 g, Adamas Reagent Co., Ltd.) was added into the solution under stirring for 5 h to obtain poly(amic acid) (PAA, PI precursor) solution. Subsequently, triethylamine (TEA, 99%, 2.18 g, TCI Shanghai Co., Ltd.) was dropwise introduced into the PAA solution, and the resulting mixture was kept in 0 oC ice water bath for another 5 h to attain a yellow sticky TEA-capped PAA solution with a solid content of 15 wt%. At last, the mixture was then washed by deionized water for three times to produce yellow precipitate, which was treated by a freeze-drying process in a freeze-dryer (SCIENTZ-10N, Scientz Co., Ltd.) for 48 h to achieve the yellow PI solid product. Preparation of graphene nanocomposite foam and device fabrication. The rGO/PI foams were 4 ACS Paragon Plus Environment

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prepared by a two-step process. In brief, 1g PI solid and 0.48 g TEA were dissolved in 18.5 mL deionized water to obtain a 5 wt% PI solution. The PI solution was mixed with a desirable amount of GO suspension (5 mg mL-1) and transferred into a Teflon mold. Then, the mixture was frozen in a -80 oC refrigerator for 2 h followed by lyophilizing in a freeze-dryer for 48 h to form a bulk foam. Subsequently, the foam was heated to reduce GO in a tube furnace (OTF-1200X-S, Kjmti Co., Ltd.) at 400 oC in nitrogen atmosphere for 3 h at the heating rate is 5 oC min-1. Pure PI foams and various composition rGO/PI foams were prepared by the same procedure. The third step, hydrophilic treatment, was employed to enhance the rGO/PI foams’ hydrophilicity and moisture adsorption performance. Typically, 5 wt% PVA solution was prepared by dissolving PVA (MW ∼ 105000 g mol−1, Sinopharm Chemical Reagent Co., Ltd.) into deionized water at 90 oC. A certain amount LiCl, deionized water and PVA solution were mixed together under magnetic stirring. Then, the glutaraldehyde aqueous solution (50%, Shanghai Macklin Biochemical Co., Ltd.) was added into the PVA solution obtaining a homogenous solution with a mass ratio of PVA to glutaraldehyde as 10:1, followed by adjusting the pH of the solution to 4~5 by 2 wt% sulfuric acid to achieve an PVA/LiCl solution. Next, the as-fabricated rGO/PI foams were immersed into the PVA/LiCl solution following by squeezed several times to fully absorb the solution, and then transferred into a 65 oC oven to heat and crosslink for 6 h. Ultimately, elastic hydrophilic graphene nanocomposite foams (φ 20 × 4 mm) were prepared and sealed in a vacuum desiccator for further use. Abbreviated name was adopted to clearly present the ingredient of the fabricated foam samples. Sample PG71-2A-6L, for instance, stands for a PI/rGO composite foam modified by PVA and LiCl. Specifically, the letter P, G, A, L represent PI, rGO, PVA and LiCl, respectively. The number 71 means the mass ratio between PI and rGO is 7:1. The number 2 and 6 are defined as the 5 ACS Paragon Plus Environment


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mass concentration of PVA and LiCl (i.e. 2 wt% and 6 wt%) in the hydrophilic treatment solution. An acrylic double-layer tube (outer diameter 30 mm, inner diameter 20 mm, wall thickness 2 mm, a 3 mm thickness circular ring space between two tubes) was cut at one end to form a 30° slope cut which was covered by a quartz glass (a diameter of 30 mm and a height of 2 mm). Simply assembling the graphene composite foam into the inner tube, the device can harvest water from the air atmosphere under sunlight. Characterization and Measurements. Foam densities were calculated by measuring the volume and weight of each foam. Scanning electron microscopy (SEM) images of the foam were taken utilizing a field-emission scanning electron microscope (SU8020, Hitachi Co., Ltd., Japan). The SEM samples were prepared by gold sputtering before test. Thermal gravimetric analysis was performed by a thermogravimetric analyzer (TGA, star system, Mettler Toledo International Inc., USA) over a temperature range of 30 to 800 oC at a heating rate of 10 oC min-1 under Argon atmosphere protection. All TGA samples were pre-dried at 50 oC for 2 h under vacuum before TGA test. A contact angle goniometer (SCI3000F, Beijing Huan Qiu Heng Da Technology Co., Ltd., China) was used to achieve the contact angle analysis at room temperature. The volume of the water droplet was about 4.0 μL. A Fourier transform infrared spectrometer (FT-IR, Vertex80v, Bruker, Inc., Germany) was employed to measure FT-IR spectrum of the foams. An infrared thermal image instrument (PI 400, Optris Infrared Sensing, LLC, USA) was used to monitor the temperature variation of foams under irradiation. Compression testing was conducted using an universal testing machine (CMT 5105, MTS, USA). The compression strain rate was set at 20% min−1 for the tests. Cylindrical foams ((φ 20 × 40 mm) were used for the compression testing. A solar simulator (Sirius300P, Zolix Insruments Co., Ltd., China) was employed to offer a stable intensity of 6 ACS Paragon Plus Environment

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irradiation. The Raman spectra were obtained with a highresolution Raman spectrometer system (LabRAM HR Evolution, Horiba, Inc., France) using a He–Ne laser (λex = 532 nm). 1H NMR spectrum was recorded with a Bruker Avance spectrometer (400 MHz) using DMSO-d6 as the solvent. For the water vapour capture experiment, a constant temperature humidity test chamber (Dongguan Sailham Equipment Co., Ltd., China) was employed to offer a constant environment. The water release experiment was performed in the room condition (~25 oC, ~50% RH).

3. RESULTS AND DISCUSSION To realize the water harvesting from air, two crucial procedures, water capture and water release, are necessary to be well designed and performed. Much efforts have been underway to collect atmospheric water. Porous materials, such as organic polymer, hydrogel, cotton fabric and metal-organic framework, have been utilized to absorb moisture from air over a wide range of humidity values.11, 14, 22-23 Porous structure can offer a larger contact area between air and adsorption materials compared to solid materials, which would increase the water adsorption speed. As shown in Figure 1a, a black graphene nanocomposite foam is prepared via a three-step procedure (i.e. freeze-drying, thermal annealing, and hydrophilic treatment) in this work, in which PI molecular and rGO are constructed as the 3D porous supporting structure. The 1H NMR spectrum was performed and the molecular characteristics of the synthetic polyimide is shown in Figure S2. Additionally, Figure S3 presents the Raman spectra of rGO and rGO/PI foams. Two characteristic peaks at 1340 and 1600 cm−1 are shown which are consistent to the D-band and G-band of graphite carbon, respectively. Deliquescent behaviour of soluble salts that is adsorbing water vapour from surrounding air gives rise to a phase transformation from solid to liquid.24 Revealing notably deliquescence in air, LiCl was adopted in our experiment to accelerate water vapour adsorption 7 ACS Paragon Plus Environment


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procedure. As shown in Figure 2a and S4a, increasing the concentration of LiCl solution (from 0 to 10 wt%) used in the hydrophilic treatment, 24 h water uptake of the resulting foams first rises and then descends, which can be ascribed to the redundant LiCl would block pores and channels in foams. Without LiCl, the nanocomposite foam shows an inferior water uptake capability (0.107 g g-1 after 24 h water adsorption at RH 90%). When the concentration of LiCl solution is 6 wt%, a highest 24 h water uptake and a highest water vapour adsorption speed in the first 10 min can be observed (Figure S4b and S4c). To investigate the influence of LiCl on the water release performance, several samples with different LiCl content were performed under solar irradiation with the same initial water uptake of 2 g g-1. As shown in Figure 2b, with increasing the content of LiCl in foams, the water release speed slightly decreased. These results would be attributed to the outstanding deliquescent behavior of LiCl. Even though LiCl possesses a remarkable water vapour capture ability, it is still a challenge to keep plenty of water. A liquid LiCl solution was seen in the water vapour capture experiment, especially in a relative high RH range. PVA is well known as an inexpensive wet storage agent because of abundant hydroxyl groups on its molecular backbones.25-26 Thus, PVA is introduced into the composite foam to preserve overmuch water and immobilize LiCl. A highest 24 h water uptake and a highest water vapour adsorption speed in the first 10 min can be achieved as the concentration of PVA solution is 2 wt% in the hydrophilic treatment (Figure 2c and Figure S5). Similarly, an excess of PVA (4 and 6 wt%) would decrease the porosity of foam and impede water vapour adsorption procedure. However, less PVA (0 and 1 wt%) cannot preserve enough water captured by LiCl, thus resulting in liquid LiCl solution loss.


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The schematic diagram of the as-prepared graphene nanocomposite foam is illustrated in Figure 1b. A porous structure is established by rGO and PI whereby a freeze-drying and thermal annealing process, and the morphology of the foam is shown in Figure 1c. A rGO/PI nanosheet, the supporting structural unit, is found as a smooth slice (Figure 1d). Modified by LiCl and PVA solution, bumped nanostructures appear on the basic rGO/PI nanosheet, which are considered as LiCl hydrates (Figure 1e and 1f). The hydrophilic performance of the foam is reversed through the thermal annealing and hydrophilic treatment (Figure 3a). The contact angle (CA) enhances from 21.8° to 101.8° which could attribute to the reduction of GO during the thermal treatment. Furthermore, the water droplet was quickly adsorbed into the foam modified by LiCl and PVA. The hydrophilic treatment is further confirmed by Fourier transforming infrared (FT-IR) spectrum as shown in Figure 3b. Broad bands at 3000~3700 cm-1 in all samples’ spectra are attributed to the stretching vibration of the hydroxyl (−OH) group. A typical peak at 2947 cm-1 are associated with the -CH stretching that can only be seen in PVA and PVA modified foam spectra, which verifies PVA has been successfully covered on rGO/PI foam.27-28 To evaluate the mechanical properties of the nanocomposite foam, a compression test has been applied on the foam with the density of 0.13 g cm-3 to demonstrate its durability and recyclable compressibility. As displayed in Figure 3c, the compressive stress-strain curves of the nanocomposite foam subjected to different compressive strains (10%-50%) have been measured. The foam can completely recover the original shape after being compressed to 50% strain. Similar to other porous structures materials reported before, the stress-strain curves present a typical three stages shape during the external loading process.29-30 A linear elastic deformation region up to 8% strain with a compressive modulus (~97.6 kPa) is shown first that could be attributed to the elastic 9 ACS Paragon Plus Environment


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deformation of the rGO/PI supporting structure. Afterwards, the curve reveals a plateau region between 8% and 40% strain with slowly enhancing compressive stress because of the plastic yielding of the rGO/PI nanosheets. At last, a densification region at high strain (40%-50%) appears with a sharply increased slope. In addition, the nanocomposite foam was performed to a fatigue cyclic compression test with 50 loading – unloading cycles and a maximum strain of 50% (Figure 3d). There is no obvious shape of size changes after the cyclic compression test. These mechanical tests results demonstrate that the as-prepared nanocomposite foam possesses an excellent superelasticity and durability, showing a wide range of potential application conditions. Overall, the foam can realize the water capture – release cycle by use of sunlight (Figure 1g). LiCl and PVA are responsible for the water capture from the air and water uptake storage in the air, respectively. The adsorbed water is stored as crystallized water in LiCl hydrates and the free water molecular restrained by hydroxyl groups on PVA through the hydrogen bond, which leads to the transformation of the nanosheet from dry status to wet status. However, rGO is employed to realize the solar-to-thermal transformation process that facilitates water release under solar irradiation. Aiming to investigate how environmental conditions impacts on the water capture capacity, various temperature and humidity environment have been performed in the experiments. Increasing the temperature from 20 oC to 35 oC at 90% RH, a slight increment of 24 h water uptake could be noted (Figure 2d and Figure S6a). In terms of water capture speed, small difference can also be observed, which is a higher temperature leading to a higher capture speed in the first 20 min (Figure S6b and S6c). Meanwhile, humidity shows a significate influence on the water capture performance of the nanocomposite foam. Adjusting the humidity ranging from 30% to 90% RH at 30 oC, the 24 h water uptake at 90% RH (2.87 g g-1) is almost 8 times higher than that of at 30 % (0.31 g g-1) (Figure 10 ACS Paragon Plus Environment

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2e and Figure S7a). At low humidity (30%, 50%, 70% RH), the foam can reach a saturated state in about 3 h, which means a dynamic equilibrium has been achieved between the water vapour capture and water evaporation. Compared to the water uptake capability of reported metal-organic frameworks (MOF-801, ~0.7 g g-1, 90% RH)11, the rGO/PI foam presents a better water harvesting potential. Revealing a wide range of working humidity, the nanocomposite foam could be used in relative dry environment. Apparent water capture speed difference at different RH can be seen from the curves in Figure S7b and S7c. Another key process for water harvesting cycle is water release process which could be performed by exposing the foam under one sun (1000 W m-2). Endowed with remarkable water dispersibility, excellent mechanical properties, and plenty of functional groups, GO is one of the most popular functional materials to prepare porous materials and an important precursor to obtain graphene.31-32 To enhance the energy utilization efficiency, rGO is involved as the solar-to-thermal transformer in the foams. In addition, rGO also contributes to the enhancement of the foam water uptake after a 24 h water capture process (Figure 2g), and no obvious effect on the water vapour adsorption speed can be seen by increasing the content of rGO (Figure S8a and S8b). However, the content of rGO largely affects the water release speed (Figure S8c). The water loss rate increases first before 30 min which could be attributed to the rapid temperature increase of the foam. And then, with the continuous temperature rising, the water loss rate declines till almost zero at 3 h due to the reduction of the total amount of remaining water. The largest water loss rate for the sample PG71-2A-6L that can be up to 0.028 g g-1 min-1 is shown around at 30 min exposed under solar irradiation. For the sample PG71-2A-6L, 43% water uptake can be attained in 2 h compared to the 24 h total water uptake. Undergoing a 3 h water release process, most of the adsorbed water could 11 ACS Paragon Plus Environment


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evaporate out of the foam PG71-2A-6L and around 89% uptake water can be released in 2 h. The influence of irradiation intensity on water release process has also been investigated to illustrate the practicality of the foam in low light intensity environment such as in cloudy day and non-summer season. As shown in Figure 2f and Figure S9, the nanocomposite foam could work in a wide range of solar intensity (from 200 to 1000 W m-2). Herein, a 2 h capture – 2 h release cycle has been performed by the sample PG71-2A-6L to investigate the reusability of the foam. As displayed in Figure 2h, the nanocomposite foam presents a stable water harvesting performance after ten 2 h capture – 2 h water release cycles. For each cycle, around 1.15 g water can be collected by 1 g foam in one cycle. The morphology of the sample after ten water capture-release cycles has been characterized, as shown in Figure S10. There is no obvious difference between the sample before and after water capture-release cycles indicating its remarkable reusability. The temperatures of different positions (i.e. top center, top edge, bottom edge on the foam and background) have been recorded to further demonstrate the evaporation during the water release process (Figure 4a). The foam shows a higher temperature than background environment (Figure 4b). The temperature curves show two typical stages including a rapid growth stage at the first 5 min and a slowly increase slope after 5 min. The top surface, directly irradiated by the solar light, presents the highest temperature at a certain moment. In the vertical direction, no obvious temperature gradient is perceived between the top edge and bottom edge, indicating that the foam possesses a good thermal conductivity. The heat, transformed from solar energy, can quickly conduct from the top surface to the whole foam which could be ascribed to the excellent thermal conductivity of rGO in the foam framework. The high temperature (~37 oC at 5 min under one sun) is beneficial to facilitating the water evaporation at the top surface of the foam. Then the water nearby replenishes the vacancies on 12 ACS Paragon Plus Environment

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the surface through the interconnected channels and pores due to the capillarity. Additionally, since the captured water stored in other parts of the foam has been preheated in situ, the phase transition from liquid to vapour can be attained with less thermal energy when these water flows to the top surface of the foam, thus accelerating the evaporation. To illustrate the evaporation phenomenon, a stronger solar irradiation (about five sun) has been employed to illuminate the foam resulting in a vapour rise that can be seen by naked eyes (Figure 4d). An analogous temperature variation during the water release process can be observed in a 24 h capture – 3 h release cycle (Figure S11). Integrating a functional foam, an acrylic double-layer tube and a quartz glass together, a prototype of the water harvesting device has been shown in Figure 4e. The round quartz glass is designed as the roof cover because the quartz possesses an excellent transmission of light that can decrease the solar energy loss. The double-layer tube could separate the condensed water and the foam. The condensed water can be collected by the device in a couple of minutes after being exposed in the solar irradiation (Figure 4f and 4g). The water harvesting efficiency (ηharvest) is approximately estimated as a ratio of enthalpy change in the generated water vapour by the total incoming solar energy:

harvest =

m h fg qsolar A

where m stands for the instantaneous mass change of the foam during evaporation, hfg is the latent heat of liquid water to vapour (2.26 MJ kg-1), qsolar represents the solar irradiation per area, and A is the area of the top surface of the foam (3.14 cm2). The water harvesting efficiency of a 2 h release process first increases in the first 25 min and then declines showing a peak value about 89.8% (Figure 4b). These results are in consistence with the water release rate mentioned above. The water 13 ACS Paragon Plus Environment


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harvesting efficiency keeps above 25% in the first 2 h evaporation, illustrating a 2 h water release process is an appropriate strategy to harvest water. The performance of an absorber (Pabsorber) is a parameter that can evaluate the water uptake capacity of the foam, which can be defined as follow:

Pabsorber =

mcapture mabsorber

where mcapture represents the mass of water adsorbed from air by foam, and mabsorber is the mass of the used foam. The Pabsorber of sample PG71-2A-6L is 2.87 g g-1, which adsorbs water vapour at 90% RH and 30 oC for 24 h. The performance of the water release can be estimated by release efficiency (ηR) of various samples:

R =

mrelease mcapture

where mrelease is the mass of released water. For the 2 h capture – 2 h release cycle, the release efficiency of the first cycle in Figure 2h is upto 90.6%, which demonstrates more than 90% adsorbed water can be release in 2 h under one sun. To evaluate the thermal stability of the nanocomposite foam, thermogravimetric test has been performed in argon over a temperature range of 30 to 800 oC. The samples of PVA, PI, the foam with/without hydrophilic treatment are investigated in the experiment (Figure 4c). On the curve of the foam after hydrophilic treatment (blue solid line), a three-step weight loss can be seen. A sharp weight loss from 60 to 170 oC is observed which can be considered as a dehydration step corresponding to the mass of crystallized water molecules in the hydrate LiCl·xH2O (1≤x≤5) lost 14 ACS Paragon Plus Environment

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during the heating.33-35 The weight loss ranging from 240 to 460 oC is consistent with the rapid weight loss of PVA (red solid line) at the same temperature scope which is the decomposition of PVA molecular. At last, the decomposition of PI molecular could explain the weight loss of the foam between 460 and 640 oC.

4. CONCLUSIONS To meet increased demands for harvesting fresh water in some water-deficient circumstances, such as living in arid region, doing outdoor activities and struggling in field survival, a portable, superelastic, and reusable nanocomposite foam is explored for water collection from air. With the purpose of realizing portability, the low density (0.13 g cm-3) foam could be squeezed under vacuum press to reduce volume. The as-fabricated foam possesses a water harvesting capability of 0.23 g g-1 and 1.15 g g-1 in a typical 2 h capture – 2 h release cycle when corresponding RH are 30% and 90% during water capture. The water capture ability and water release ability of the foam play important roles on the water harvesting efficiency in the system. In this work, LiCl and PVA are selected to absorb water vapour and remain the collected water in the foam, respectively. And rGO, as the solar-to-thermal transformer, is embedded in the foam supporting skeleton to facilitate water evaporation. The nanocomposite foam presents a stable water harvesting performance after ten 2 h capture – 2 h water release cycles. Even though the rGO/PI nanocomposite foam can harvest fresh water from air, it is essential to enhance water harvesting efficiency. One simple and convenient way to improve evaporation is enhancing the irradiation intensity by optical concentration system. Concentrated solar light can increase temperature which would help to release the crystallized water molecules in LiCl hydrates. 15 ACS Paragon Plus Environment


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Another smart strategy is involving bionic structures (i.e. some micro- and nanostructures utilized by natural plants and animals) to enable water droplet growing rapidly. Besides, reducing heat loss and light reflection in the water release process are also efficient approaches to increasing the energy utilization efficiency. Considering the solar-to-thermal conversion ability, the nanocomposite foam may offer other potential applications, such as desalination and distillation. Another big challenge impedes the water harvesting system utilization is to explore a more cost-effective way to prepare the products. Though the three-step synthesis method and the composition of the foam have been optimized, it is still necessary to reduce the cost and increase the fabrication efficiency. Meanwhile, environment friendly materials are recommended, which would take the water harvesting system one step further to commercial application and large-scale production.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Dependence of LiCl content in hydrophilic treatment solution on water capture capacity.; Dependence of PVA content in hydrophilic treatment solution on water capture capacity.; Dependence of temperature on water capture capacity.; Dependence of humidity on water capture capacity.; Dependence of GO content on water capture and release capacity.; Dependence of irradiation intensity on water release capacity.; The temperature variation on different positions of the foam.; Reaction scheme of the water-soluble polyimide. (PDF)

AUTHOR INFORMATION 16 ACS Paragon Plus Environment

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Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Key R&D Program of China (Grant No. 2016YFA0202701), the National Natural Science Foundation of China (Grant No. 51472055, Grant No. 61404034), External Cooperation Program of BIC, Chinese Academy of Sciences (Grant No. 121411KYS820150028), the 2015 Annual Beijing Talents Fund (Grant No. 2015000021223ZK32), Qingdao National Laboratory for Marine Science and Technology (No. 2017ASKJ01), the University of Chinese Academy of Sciences (Grant No. Y8540XX2D2), China Postdoctoral Science Foundation Grant (Grant No. 2018M631415), and the "thousands talents" program for the pioneer researcher and his innovation team, China.

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FIGURES

Figure 1. Photograph, schematic diagram and SEM images of the graphene nanocomposite foam. (a) A photograph of the graphene nanocomposite foam. (b) Schematic diagram of the graphene nanocomposite foam. The foam was prepared through a three-step process: freeze-drying, thermal annealing, and hydrophilic treatment. The rGO/PI nanosheet, as the basic unit, can achieve the water harvesting capture-release cycle without additional energy input. (c) A SEM image presents porous structure of the rGO/PI foam without hydrophilic treatment. (d) A magnified SEM image of the rGO/PI foam without hydrophilic treatment to show a relatively smooth surface of the nanosheet. (e) A SEM image of the graphene nanocomposite foam after hydrophilic treatment. (f) A magnified SEM image of the hydrophilic rGO/PI foam with bumped nanostructures. (g) The schematic diagram of the water vapour capture-release cycle. LiCl and PVA were responsible for the water capture and water storage, respectively. The adsorbed water was stored as crystallized water in LiCl hydrates and the free water molecular restrained by hydroxyl groups on PVA through the hydrogen bond, which led to the transformation of the nanosheet from dry status to wet status. The opposite procedure, from wet status to dry status, was realized by the rGO conversing the solar energy to thermal energy to facilitate the water evaporation under irradiation.


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Figure 2. Performance of the water harvesting with the foam (φ 20 × 4 mm). (a,b) Experimentally characterized water capture curves (a) and water release curves (b) for the foam samples with different content of LiCl (water capture 30 oC, 90% RH, 24 h and water release 25 oC, 50% RH, 2 h). (c) Experimentally characterized water capture curves for the foam samples with different content of PVA (30 oC, 90% RH, 24 h). (d,e) Experimentally characterized water capture curves for the foam sample PG71-2A-6L at different temperature (d) and different RH (e) (30 oC, 90% RH, 24 h). (f) Experimentally characterized water release curves for the foam sample PG71-2A-6L under different intensity of irradiation (25 oC, 50% RH, 3 h). (g) Experimentally characterized water uptake curves for the foam samples with different content of GO under one sun (water capture 30 oC, 90% RH, 24 h and water release 25 oC, 50% RH, 3 h). (e) The 2 h capture – 2 h water release water harvesting cycle was performed for ten times to show a good reusability of the foam. For each cycle, the water harvesting capacity could be 1.15 g g-1.



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Figure 3. The chemical analysis and characterization of the foam. (a) Water contact angle measurements of the foams at different synthesis procedure. Modified by LiCl and PVA, the surface of the foam successfully conversed to a hydrophilic surface. (b) FT-IR spectra of the foam and its components. (c) The stress-strain curves for the nanocomposite foam at different strains. The foam can recover its original shape when the strain is up to 50%. (d) A fatigue cyclic compression test with 50 loading – unloading cycles with a maximum strain of 50% is performed to illustrate the good durability of the foam.


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Figure 4. Characterized thermal properties and the photograph of the as-fabricated water harvesting device. (a) The infrared thermal images of the foam under solar irradiation at different times (0-120 min). (b) The temperature-time curves for different positions of the foam (red, yellow, green and blue lines) and the instantaneous water release efficiency (purple line) during a 2 h water release process under one sun irradiation at 25 oC at 50% RH. (c) The TGA curves of the foam and its components. (d) A photograph presents the water evaporation of the foam under five sun irradiation. (e) The as-fabricated water harvesting device consists of the functional foam, a round quartz glass roof cover and a double-layer acrylic tube. (f,g) Photographs shows the collected water by the water harvesting device in a couple of minutes after being exposed in one sun irradiation.



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