Solid waste and graphite derived solar steam generator for highly-efficient and cost-effective water purification

Applied Energy 261 (2020) 114410

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Solid waste and graphite derived solar steam generator for highly-efficient and cost-effective water purification

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Feng Gonga, ,1, Wenbin Wanga,1, Hao Lib, Dawei (David) Xiac, Qingwen Daib, Xinlin Wub, ⁎ Mingzhou Wangb, Jian Lib, Dimitrios V. Papavassilioud, Rui Xiaoa, a

Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 211189, China School of Materials and Energy, School of Communication and Information Engineering, University of Electronic Science and Technology of China, Chengdu 611731, China c Department of NanoEngineering, University of California San Diego, La Jolla, CA 92093-0448, USA d School of Chemical, Biological, and Materials Engineering, University of Oklahoma, Norman, OK 73019, USA b

H I GH L IG H T S

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solar evaporator was devel• Efficient oped from solid wastes and graphite. evaporation rate as high as • Solar 1.61 kg/m h was achieved under 1 2

• • •

sun. Quantitative studies suggest the design of highly-efficient solar steam generator. High efficiency and cycling stability were proved for purifying various water. Prototype can produce 8–13 L of clean water daily with 1 m2 of material.

A R T I C LE I N FO

A B S T R A C T

Keywords: Solid waste Solar steam generator Ball-milling graphite Cellulose fiber Composite aerogel Scalable clean water production

Utilizing the clean and renewable solar energy to generate steam for wastewater purification or seawater desalination is a promising solution to the worldwide scarcity of fresh water. Herein, we report a highly-efficient, eco-friendly and cost-effective solar steam generator based on ball-milling graphite and cellulose fiber from waste paper. Ball-milling graphite/cellulose fiber composite aerogels are facilely developed via a freeze castingdrying method, endowing a featured vertically aligned porous structure. The fabrication of composite aerogels and the construction of evaporation systems take full advantages of recycled materials, contributing to the low cost and solid waste reclamation. High porosity, strong solar absorption, hydrophilicity and low thermal conductivity of the composite aerogels collectively contribute to a splendid solar evaporation rate of water as high as 1.61 kg m−2 h−1, with photothermal efficiency of ~90% under one sun illumination (1 kW m−2). Quantitative studies reveal the effects of ball-milling graphite/cellulose fiber ratio, ball-milling graphite concentration and thickness of composite aerogels on the solar evaporation rates, suggesting the optimal design of composite aerogels. Furthermore, the cycling stability and the capability for seawater desalination as well as polluted water purification manifest the potential of the composite aerogels to purify diverse water. Outdoor tests show 8–13 L of fresh water could be produced daily by 1 m2 of composite aerogel.

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Corresponding authors. E-mail addresses: [email protected] (F. Gong), [email protected] (R. Xiao). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.apenergy.2019.114410 Received 22 August 2019; Received in revised form 15 December 2019; Accepted 16 December 2019 0306-2619/ © 2019 Elsevier Ltd. All rights reserved.

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1. Introduction

efficient and low-cost photothermal conversion materials for solar steam generation. Both BG and CF were obtained from facile ball-milling process. The BG/CF composite aerogels (BGCAs) were attained by freezing and drying BG/CF solutions in a designed mold (freezecasting). BGCAs achieved solar evaporation rates of water as high as 1.61 kg m−2 h−1 with a photothermal conversion efficiency of 90% under one sun illumination. The outstanding solar evaporation performance is attributed to the superior solar absorption, high porosity, low thermal conductivity and decent hydrophilicity of BGCAs. In addition, factors like the BG/CF ratio, concentration of BG, and thickness BGCAs were systematically and quantitatively studied for correlating their effects to the solar evaporation performance of BGCAs, suggesting the optimal design of BGCAs for better solar evaporation performance. Moreover, BGCAs maintained stable evaporation rates for ~20 wettingdrying usages when purifying different water, displaying superb recycling stability. A facile device was designed to condense steam and collect purified water from seawater and polluted water. The quality of the purified water indicated by the measured ohimic resistance proves that BGCAs are promising for seawater desalination and polluted water purification [29]. In the end, a prototype was constructed to characterize the outdoor clean water production performance of BGCAs. It was revealed that 1 m2 of BGCAs ($11.3 m−2 cost) was able to produce 8–13 L of fresh water daily. The cycling ability, the desalination/purification capacity, and the field tests demonstrate the feasibility of BGCAs for practical and scalable water purification.

Solar energy, along with the booming nanotechnology, is playing an increasingly vital role in industrial manufacturing and daily life as clean and renewable natural resource [1]. Simultaneously, the whole world is facing a scarcity of clean water, due to the increasingly severe water pollution and world population explosion [2,3]. To solve this problem, developing novel photothermal conversion technologies has become the unstoppable trend in light of the abundant and powerful solar energy [4]. As solar steam generation can be free of hazardous chemicals and artificial energy consumption, e.g. fuel, electricity, it is considered as one of the most promising methods for water purification and desalination [5,6]. The core photothermal conversion materials facilitate water evaporation by directly converting solar radiation to heat for steam generation, in which efficient “heat” and “transfer” guide the material design for enhanced photothermal conversion. In terms of “heat”, strong solar absorption and low thermal conductivity are essential to absorb solar energy, convert it to heat and preserve the converted heat in materials [7]. When it comes to “transfer”, photothermal conversion materials should have desirable hydrophilicity and porosity for fast water and steam transport [8]. The design of photothermal conversion materials is the main challenge to realize the sustainable and practical application of solar evaporation for water purification and desalination. Recently, plasmonic absorbers [9] and noble metal nanoparticles [10] have been investigated as photothermal conversion materials. Wang et al. [11] fabricated plasmonic membranes using Au nanoparticles for interfacial solar steam generation. The Au membranes achieved a photothermal conversion efficiency of ~85% under 10 sun illumination (10 kW m−2). However, the costly gold nanoparticles and the high optical concentration of poor scalability, hinder the scalable application of such plasmonic membranes under natural condition. In addition, other photothermal conversion materials based on carbon materials [12], polymer-derived materials [13], carbonized materials [14], and bio-inspired materials [15,16] have attracted attention lately. These materials are usually fabricated in the form of sheets or membranes, able to float on water. Then solar energy could be localized to heat the air-water interface for steam generation. Wang et al. [17] reported a reduced graphene oxide/multi-walled carbon nanotubes composite film, which achieved a photothermal conversion efficiency of 80.4% under one sun. However, when sheets or membranous photothermal conversion materials float on water, a considerable fraction of heat will be lost to the water, inducing undesirably low photothermal conversion efficiency. Three-dimensional (3D) porous photothermal conversion materials, including polymer hydrogels [18], sponges [19,20], and carbon aerogels [21,22], have also drawn attention due to their favorable properties for solar steam generation, such as low bulk density, high porosity, and low thermal conductivity. Graphene aerogels are up-and-coming as one type of 3D materials for solar steam generation [23,24]. However, in previous studies, graphene sheets used for fabricating graphene aerogels were commonly derived from graphene oxide, which requires a series of sophisticated preparation steps involving expensive and toxic chemicals [25], as well as significant energy consumption [26]. The preparation process holds a poor perspective of resource utilization, cost/energy savings and scalability. Moreover, the developed porous photothermal conversion materials, such as graphene aerogels, suffer from low solar evaporation rate (< 1.4 kg m−2 h−1) [27,28]. In all, toxic/expensive materials, complicated methods, and undesirable solar evaporation rate obstruct the practical application of photothermal conversion materials for fresh water generation. Therefore, we intend to employ solid waste and low-cost materials with facile methods to develop efficient photothermal conversion materials and prototypes for feasible water treatment. In this work, composite aerogels based on ball-milling graphite (BG) and cellulose fiber (CF) from waste paper are demonstrated as highly-

2. Experimental section 2.1. Materials The natural graphite powder was purchased from Tianrun Electronic Materials Co. Ltd., China. N-methyl-2-pyrrolidone (NMP), and thiocarbamide was purchased from Sinopharm Chemical Reagent Co. Ltd., Shanghai, China. The polytetrafluoroethylene (PTFE) filtration films were purchased from Jinteng Laboratory Equipment Co. Ltd., Tianjin, China. Sulfuric acid (H2SO4) was purchased from Chengdu Kelong Chemicals Co. Ltd., Chengdu, China. Sodium hydroxide (NaOH) was obtained from Aladdin Industrial Co. Ltd., Shanghai, China. Ethanol was purchased from Sigma Aldrich Co. Ltd. 2.2. Materials preparation 2.2.1. Synthesis of ball-milling graphite (BG) Ball-milling graphite (BG) was first fabricated from natural graphite powder with ball-milling, and then treated with sulfuric acid (Fig. 1a). Typically, 0.6 g of graphite powder was dispersed into 20 mL of NMP (30 mg mL−1, NMP was selected because of its close surface energy with graphene: 40.7 mN m−1 for NMP and 54.8 mN m−1 for graphene) [30]. The dispersion and ZrO2 balls (150 g, 2 mm and 0.2 mm in diameter, weight ratio of 1:1) were added into a ZrO2 (50 mL) mill jar, followed with 6 h ball milling (QM-3SP04, Nanjing Laibu Tech. Co. Ltd., China). Afterwards, the obtained dispersion was filtered to separate BG from NMP with a PTFE film (0.45 μm pore size). Then BG was treated with sulfuric acid at 60 °C for 2 h. Finally, after washing with deionized (DI) water and freeze drying (Lab-1A-50, Beijing Biocool Laboratory Instrument Co. Ltd., China), the BG powers were obtained. 2.2.2. Synthesis of cellulose fibers (CF) Cellulose fiber (CF) was prepared from office waste paper (Fig. 1a). 1.5 g of waste paper scraps were added into a solution of deionized water (100 mL), NaOH (2 g) and thiocarbamide (10 g). The mixture was stirred for 3 h. For further dissolution, the mixture was placed in the refrigerator (BCD-118TMPA, Haier Electric Ltd., China) for 24 h [31], followed with ball milling process for 6 h (QM-3SP04, Nanjing Laibu Tech. Co. Ltd., China). Then the solution was centrifuged at 3000 rpm for 10 min (4100g, TD6, Pingfan Co., Changsha, China) and the 2

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Fig. 1. Schematic diagram of the fabrication process of the ball-milling graphite/cellulose aerogels (BGCAs). (a) The ball milling graphite (BG) was synthesized by ball milling with ZrO2 balls in different size and followed with sulfuric acid treatment. Cellulose fiber (CF) was obtained from daily waste paper through ball milling. BGCAs were prepared via freezing the BG/CF mixture in homemade molds using liquid nitrogen (freeze casting), followed by freeze drying. (b) SEM image of BG sheets. (c–d) TEM images of BG sheets with different magnification. (e) XRD patterns of the pristine graphite and BG sheets. (f) Raman spectra of the pristine graphite and BG sheets.

2.3. Materials characterization

sediment was washed repeatedly using DI water until the pH was between 8 and 9. Finally, CF was obtained after freeze drying (Lab-1A-50, Beijing Boicool Laboratory Instrument Co. Ltd., China).

The X-ray diffraction (XRD) analysis of all samples was performed on a Bruker-D8 Advanced X-ray Diffractometer (Cu Kα radiation: λ = 1.5415 Å) at a scanning rate of 2° min−1. The Raman spectra measurements were conducted on a confocal micro-Raman spectrometer (Renishaw inVia). The morphologies of BG and BGCAs were characterized using field emission scanning electron microscopy (FESEM, JSM-5900LV, JEOL) and transmission electron microscope (TEM, JEM-2100F, JOEL). The thermal conductivity of BGCAs was measured using a thermal conductivity meter (TC3100, Xixiatech, Xi’an, China). The Ohmic resistance of purified water was measured by using a multimeter (UN71A, Uni-Trend).

2.2.3. Synthesis of ball-milling graphite/cellulose fiber aerogel (BGCA) Circular molds (32 mm in diameter, 40 mm in depth, Fig. 1a) made of polypropylene (PP) wall and aluminum (Al) bottom were applied in the preparation of BGCAs. Typically, 0.36 g of BG and 1 mL of ethanol (as antifreeze) were added into 30 mL of DI water, followed with ultrasonic processing for 30 min (LC-Ⅱ D, Ningbo Licheng Instrument Co. Ltd.). Then 0.36 g of CF were added into the mixture. After, the mixture was stirred to get uniform BG/CF dispersion. The obtained dispersion was poured into the molds and frozen by immersing the bottom of the molds into liquid nitrogen. In this way, the BG/CF mixture can be frozen from the bottom to the top. Then, the ball-milling graphite/ cellulose fiber aerogel (BGCA) was acquired by freeze drying (Lab-1A50, Beijing Boicool Laboratory Instrument Co. Ltd., China). For simplification, BGCAs with different BG/CF ratio were symbolized: BGCAs with BG/CF ratios of 1:1, 2:1, 4:1, 6:1 and 8:1 were represented as BGCA1, BGCA2, BGCA4, BGCA6 and BGCA8, respectively.

2.4. Evaluation of the solar evaporation rate of BGCAs A system based on a beaker (with a diameter of 4.6 cm and a depth of 6 cm) wrapped with polyethylene (PE) foam for thermal insulation was applied for the test of solar steam generation. A circular piece of PE foam was put on the top of beaker to support BGCAs and to avoid direct 3

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The SEM images of BGCAs with BG/CF ratio of 6:1 (BGCA6) are shown in Fig. 2a-d. The uniform porous structure of BGCAs in the cross section is exhibited in Fig. 2a-b. The CF distributes among BG sheets, forming an interconnected framework, which improves the mechanical strength of BGCAs. In the vertical section of BGCAs (Fig. 2c–d), the vertical orientation of BG sheets is consistent with the ice formation direction. The utilization of the mold with PP wall and Al bottom accelerated the bottom-up ice formation owing to the much higher thermal conductivity of Al than PP. The bottom-up ice formation forces BG to vertically arrange, ending up with an oriented structure [34]. Compared with BGCAs with BG/CF ratio of 2:1 (Figure S2), BGCA6 is of more obvious ‘cliff’-like vertical structure and larger pore size in the cross section, ascribed to the fewer CF present to connect BG sheets. The vertical pores between BG sheets provide efficient transport channels for both water and steam in solar evaporation process (Fig. 2e). Besides, the molds can be readily tuned to fabricate BGCAs with preferred shape and size (Figure S3a). As shown in Fig. 2f, BGCAs exhibits an intensive peak at 26.5° in the XRD pattern, corresponding to the (0 0 2) peak of BG (Fig. 1e). BGCAs possess low bulk density, which increases from 12.56 kg m−3 of BGCA8 to 15 kg m−3 of BGCA1 (Fig. 2g). Meanwhile, BGCAs display outstanding mechanical strength due to CF interconnected framework: they can achieve a mass loading ratio above 1: 6000 (a BGCA weighing 0.084 g can support a 500 g weight without fracture, Figure S3b). As shown in Figure S3c, a piece of BGCA quickly transports ink from the bottom to humidify the air-laid paper on the top within only 3 s, manifesting the excellent hydrophilicity and effective water transport of BGCAs. The hydrophilicity and water transport capacity of BGCAs are prerequisite to the efficient and continuous solar steam generation [35]. Besides the low density and superb hydrophilicity, BGCAs also attain ultrahigh porosity (> 99%, Figure S4), which can be estimated from their bulk density (Calculation S1). The extraordinary porosity of BGCAs contributes to the ultralow thermal conductivity [36,37]. The thermal conductivity of BGCAs varies from 0.0361 W m−1 K−1 to 0.0427 W m−1 K−1 with the BG/CF ratio from 1:1 to 8:1 (Fig. 2g). In addition, owing to the intrinsic solar absorption of graphene [23] and the highly-porous structure of BGCAs, BGCA6 achieves strong absorption with negligible transmittance to solar irradiation in the wavelength range of 200–2500 nm (Figure S5). The excellent porosity, hydrophilicity, thermal conductivity and solar absorption of BGCAs are favorable for localizing heat and transferring water/steam, thus

solar illumination on water. To transport water up to BGCAs, water transfer channels based on air-laid paper were utilized. The whole system was placed under a solar simulator (Surius SS150A, Zolix, Beijing, China), supplying a solar irradiation of 1 kW m−2. The average solar evaporation rate was calculated based on the weight loss of water in 2 separate one-hour tests, which was recorded by an electronic balance. An infrared (IR) camera (FLIR-E64501, Tallinn, Estonia) was used to measure the temperature of BGCAs under solar illumination. 3. Results and discussion 3.1. Structure characterization and physical properties of BGCAs The preparation process of BGCAs is illustrated in Fig. 1a, in which the cellulose fiber (CF) was fabricated from waste paper, and the ballmilling graphite with acid treatment (BG) was synthesized from graphite powder. As seen in Fig. 1b, the SEM image of BG shows clear stratified structure, validating the effect of ball milling and acid treatment on the fabrication of BG sheets from graphite. The TEM images in Fig. 1c–d exhibit the architecture of BG sheets: it is composed of approximately ~10 monolayers of graphene. The morphology of BG is consistent with those in the previous studies [32]. The XRD spectra (Fig. 1e) display that the peak intensity of BG at 26.5° decreases while the half-height width increases, indicating the lowered graphitization degree of BG compared with that of the pristine graphite. Fig. 1f presents the Raman spectroscopy of the pristine graphite and BG. The relative intensity of D band and G band can be used to evaluate the defects of the exfoliated graphene. The intensity ratio of D and G band, ID/ IG, is estimated to be 0.10 and 0.23 for the pristine graphite and BG, respectively, which means that BG acquires defects during the ball milling process [30]. Unfortunately, graphene is known to be hydrophobic, unfavorable for solar steam application. When dispersed in water, BG precipitated in 5 min because of the hydrophobicity of BG sheets (Figure S1a, b). Therefore, we creatively introduce CF to ameliorate the affinity with water. After adding CF into the dispersion, followed with continuous stirring, a uniform mixture was obtained owing to the solubility of CF in water, which could be maintained for over 10 days (Figure S1c, d). The Al bottom and polypropylene (PP) wall of our home-made molds (Fig. 1a) enabled the solution to freeze from the bottom to the top when the Al bottom was immersed into liquid nitrogen (freeze casting) [33].

Fig. 2. Morphology and physical properties of BGCAs. SEM images with different magnification of BGCAs with BG/CF ratio of 6:1 (a–d). (a, b) Cross-section surface. (c, d) Vertical-section surface. (e) Schematic diagram of the water and steam transfer in BGCAs. (f) XRD pattern of BGCAs. (g) Bulk density and thermal conductivity of BGCAs with various BG/CF ratio. 4

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Fig. 3. Solar evaporation performance of BGCAs. (a) Schematic plot of the solar evaporation testing system. PE foams are utilized to support BGCAs and insulate the system from the surrounding environment. Air-laid paper is used to transfer water from the beaker up to BGCAs by capillary effect. The solar evaporation rate is calculated by the weight loss of water in stable span of 2 h. (b) Photograph of the testing equipment under 1 sun irradiation. (c) IR image during the solar evaporation test. (d) Mass change of the system of pure water and BGCA6 in 1 h under one sun irradiation. (e) The surface temperature profiles of pure water and BGCA6 in 1 h under one sun irradiation. (f) The cycling evaporation performance of BGCA6 under 1 sun irradiation on different water.

efficiently captured and converted to thermal energy in BGCAs. Additionally, BGCAs can localize heat inside because of the low thermal conductivity. Then water transported via CF can be heated to generate steam inside BGCAs, which could diffuse through pores outwards. Last but not the least, the total cost to fabricate 1 m2 of BGCA6 with 7 mmthickness was estimated to be only 11.3 dollars (Calculation S2). In a nutshell, the splendid solar evaporation performance, the outstanding cycling ability and the cost-effective fabrication of BGCAs are favorable for the practical application of BGCAs.

facilitating the solar evaporation process. 3.2. Performance and mechanism of solar steam generation The solar evaporation system (Fig. 3a–b) was constructed using a beaker, air-laid paper and waste polyethylene (PE) foam. Waste reclamation not only reduces cost, but also benefits the environment. Because of the capillary effect, air-laid paper can efficiently transfer water from the beaker up to BGCAs: ink can be delivered 8 cm high in 60 s (Figure S6), manifesting its decent water transport ability. The water transfer channel provided by air-laid paper keeps BGCAs from contacting bulk water directly, which helps avoid the heat loss from BGCAs to bulk water. The PE foams are elaborately chosen as thermal insulator between water and BGCAs due to their ultralow thermal conductivity. PE foam can easily support BGCAs to separate them from contacting bulk water. The BGCAs were placed under 1 sun illumination (1 kW m−2) supplied by a solar simulator, and the temperature was measured and recorded using an IR camera (Fig. 3c). At first, the solar evaporation rate of the pure water without BGCAs was tested to be 0.28 kg m−2 h−1 at ambient temperature of 23 °C, which is in consistence with the previous reported data [38,39]. After introducing BGCAs, the solar evaporation rate was enormously enhanced. As seen in Fig. 3d, the water evaporation rate of BGCA6 is 1.54 kg m−2 h−1, which is 5.5 times that of the pure water, demonstrating the excellent ability of BGCAs to accelerate the solar steam generation. The rate is superior to that of most reported materials todate, especially that of graphene aerogels derived from graphene oxide (Figure S7). BGCA6 also shows much higher surface temperature than that of pure water (45.1 °C vs. 30.1 °C), manifesting the strong solar absorption of BGCAs (Fig. 3e). Furthermore, the cycling performance of BGCAs was investigated. After every solar evaporation test, BGCAs were freeze dried for the next test. As shown in Fig. 3f, after ~20 times wetting-drying cycles, BGCA6 still delivers solar evaporation rates of above 1.5 kg m−2 h−1 for pure water and ~1.45 kg m−2 h−1 for seawater, displaying the outstanding cycling stability of BGCAs for steam generation on different types of water [40,41]. Owing to the massive radiation absorption of BGCAs, solar energy could be

3.3. Effects of the BG/CF ratio, concentration of BG, thickness of BGCAs on the solar evaporation performance of BGCAs 3.3.1. Effect of the BG/CF ratio on solar water evaporation of BGCAs The effect of the BG/CF ratio on the solar evaporation performance of BGCAs is presented in Fig. 4a. Obviously, with the increase of the BG/CF ratio, the solar evaporation rate of BGCAs also enhances. BGCAs with the BG/CF ratio of 1:1, 2:1, 4:1, 6:1 and 8:1 achieved average solar evaporation rates of 1.43, 1.44, 1.49, 1.54 and 1.61 kg m−2 h−1, which correspond to the photothermal conversion efficiency of 78%, 79%, 82%, 85% and 90%, respectively (Fig. 4c). The calculation of photothermal conversion efficiency is demonstrated in Calculation S3. Not only the solar evaporation rate, but also the surface temperature of BGCAs is elevated with the BG/CF ratio (Fig. 4b). It is reasonably concluded that higher BG/CF ratio means more BG sheets, which act as local heating agents and convert more solar energy to heat. With the adequate water supply, the increased heat can produce more steam, which finally leads to higher evaporation rates. Moreover, since BG has lower specific heat capacity (~700 J kg−1 °C−1) [42,43] than that of cellulose (~2000 J kg−1 °C−1) [44], BGCAs with the higher BG/CF ratio will possess lower specific heat capacity. This is another reason why the surface temperature of BGCAs is proportional to the BG/CF ratio when placed under same solar illumination. 3.3.2. Effect of the BG concentration on solar evaporation of BGCAs During the synthesis of BGCAs, BG and CF were dispersed in water to form uniform mixtures (Figure S1c, d). It was revealed that the 5

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Fig. 4. Effects of the BG/CF ratio, concentration of BG and thickness of BGCAs on the solar evaporation performance and surface temperature of BGCAs under 1 sun illumination. (a) Mass change, (b) surface temperature and (c) average solar evaporation rates/photothermal conversion efficiency (with errors) of BGCAs with the BG/CF ratio varies from 1:1 to 8:1 (BG concentration of 12 mg mL−1, thickness of 7 mm). (d) Mass change, (e) surface temperature and (f) average solar evaporation rates/photothermal conversion efficiency (with errors) of BGCAs while BG concentration changes from 6 to 21 mg mL−1 (BG/CF ratio of 6:1, thickness of 7 mm). (g) Mass change, (h) surface temperature and (i) average solar evaporation rates/photothermal conversion efficiency (with errors) of BGCAs with thickness from 5 mm to 10 mm (BG/CF ratio of 6:1, BG concentration of 12 mg mL−1). Error bars in (c), (f) and (i) are the standard deviations of two separate tests (Calculation S4).

3.3.3. Effect of the thickness on solar evaporation of BGCAs During the solar steam generation, the thickness of BGCAs is also a significant factor. To study the thickness effect on the solar evaporation performance, BGCA6s with thickness of 5–10 mm were prepared. As shown in Fig. 4g, i, the average solar evaporation rate increases from 1.43 kg m−2 h−1 to 1.54 kg m−2 h−1 when the thickness of BGCA6s increases from 5 to 7 mm. The corresponding photothermal conversion efficiency improves from 78% to 85% accordingly. However, the evaporation rate decreases when the BGCA6s are thicker than 7 mm, from 1.54 kg m−2 h−1 to 1.39 kg m−2 h−1 when the thickness increases from 7 mm to 10 mm. Meanwhile, the corresponding photothermal conversion decreases from 85% to 75%. The change of solar evaporation rate with BGCA thickness is ascribed to the following reasons. When the thickness of BGCA6 is lower than 7 mm (i.e., smaller volume), the thermal energy converted from solar irradiation will heat less water, leading to higher temperature (Fig. 4h). Meanwhile, thinner BGCAs are not efficient to localize heat as the thicker ones, so that some heat may be lost to the surroundings, causing the comparatively lower solar evaporation rate. When BGCAs become thicker, the temperature of BGCAs decrease (Fig. 4h), inducing lower solar evaporation rate. Moreover, BGCAs with larger thickness (i.e., with bigger pore volume) contains more water. Then under the same solar irradiation, thermal energy converted from solar illumination needs to heat more water, which will lead to lower solar evaporation rate.

concentration of BG has considerable effect on BGCAs formation. A solution with low BG concentration is not able to induce the 3D structure for the limited BG content [45]. Then BGCA6 with BG concentration of 6, 9, 12, 15, 18, and 21 mg mL−1 was utilized to study its effect on solar evaporation performance of BGCA6. As presented in Fig. 4d, f, when BG concentration increases from 6 to 12 mg mL−1, the average solar evaporation rate also enhances from 1.40 kg m−2 h−1 to 1.54 kg m−2 h−1, corresponding to the improved photothermal conversion efficiency from 76% to 85%. However, the solar evaporation rate of BGCA6 decreases from 1.54 kg m−2 h−1 to 1.41 kg m−2 h−1, even with the BG concentration rising from 12 mg mL−1 to 21 mg mL−1. Furthermore, the temperature of BGCA6s under solar illumination continuously decreases with the rising BG concentration (Fig. 4e). The interesting phenomenon appears that BGCA6 synthesized with BG concentration under 12 mg mL−1 has lower solar evaporation rate but higher surface temperature. This seemingly inconsistent finding is however reasonable from a structural perspective. BGCA6 (BG concentration < 12 mg mL−1) has lower density, suggesting that its interior structure is relatively loose, which cannot provide an efficient water transfer network, but poor evaporation rates. For BGCA6 fabricated with BG concentration above 12 mg mL−1, the evaporation rates decrease for two main reasons. On one hand, the relatively low temperature will retard the generation of steam. On the other hand, the higher BG concentration will lead to higher density of BGCA6, which will further result in shrunken or blocked pores, inhibiting the diffusion of steam to the ambient.

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Fig. 5. Demonstration of the desalination, purification ability, and outdoor clean water production capability of BGCAs. (a) Mass change, (b) surface temperature of BGCAs, (c) average solar evaporation rate/photothermal conversion efficiency on pure water, seawater, and river water under 1 sun illumination. Error bars are the standard deviations of two separate tests. (d–h) The resistances of seawater, purified seawater, river water, purified river water and domestic water validate the excellent desalination and purification ability of BGCAs. (i) The digital photo of BGCA with a diameter of 20 cm. (j) The digital photo of clean water production prototype under natural sunlight. (k) The solar intensity during the field test.

(Fig. 5e, g), which is 38 times that of seawater (58.7 kΩ, Fig. 5d) and 11 times that of river water (0.2 MΩ, Fig. 5f). Moreover, the ohmic resistance of the purified water is ~6 times that of the domestic water supplied in Chengdu, Sichuan Province, China (0.38 MΩ, Fig. 5h), indicating the marvelous water purification capability of BGCAs [13]. The purified water tests corroborate the great potential of BGCAs for practical water desalination and purification. To further prove that BGCAs are ideal photothermal conversion materials for scalable clean water production, a prototype was designed for field tests. Fig. 5i is the digital image of a large BGCA with a diameter of 20 cm, which is prepared and employed for outdoor tests. The prototype (Fig. 5j, S9) was built using cheap and recycled materials (a large piece of PE foam, air-laid paper, a transparent glass cover, a layer of activated carbon, and a plastic bottle). The PE foam is utilized for floating the prototype on water and supporting BGCAs. Air laid-paper acts as the water transfer channel, which supplies adequate water for BGCAs. The transparent glass cover not only enables solar irradiation to BGCAs, but also condenses steam on the inner surface. Activated carbon was for pretreating river water to eliminate contaminants like silt and oil slick, which might pollute air-laid paper and break BGCA. The pretreatment of river water with activated carbon is favorable for the longterm stability of the prototype [4]. The plastic bottle was used for the collection and storage of the condensed water. It is obvious that the preparation of BGCAs and the construction of prototype took full advantage of waste and chemical reclamation. A total cost reduction of $9.77 could be achieved for preparing 1 m2 of BGCA (with BG/CF ratio of 6:1 and thickness of 7 mm) and constructing the matching prototype (see Calculation S5). The prototype was placed in the Qingshui River (Chengdu, Sichuan Province, China) from 9:20 to 17:20 under natural conditions for clean water production. As shown in Fig. 5k, the solar

3.4. The solar seawater desalination, polluted water purification and outdoor clean water production performance of BGCAs The ultimate purpose of developing BGCAs for solar evaporation is to apply them for practical water purification. Therefore, we conducted experiments to demonstrate the solar desalination and purification performance of BGCAs with seawater and polluted river water. Seawater was simulated by adding 3.5 wt% NaCl and a little amount of other salts, like MgCl2 (0.06 wt%), MgSO4 (0.09 wt%), and CaCl2 (0.04 wt%) etc., to DI water. River water was obtained from the Qingshui River (Chengdu, Sichuan Province, China). All the tests were implemented in the system shown in Fig. 3a. Under 1 sun illumination, the average solar evaporation rates of BGCA6 for seawater and river water were measured as 1.44 and 1.43 kg m−2 h−1 (seen in Fig. 5a, c), lower than 1.54 kg m−2 h−1 for pure water. The corresponding photothermal conversion efficiencies are 78.6% and 77.9% (Fig. 5c), respectively. The relatively poorer solar evaporation performance resulted from the salt in seawater and pollutant in river water, which would decelerate the transfer of water to BGCAs. As presented in Fig. 5b, the temperature of BGCA6 on seawater is higher than that on river water, which is consistent with the previous study [4]. To determine the ability of BGCAs for seawater desalination and river water purification, a convenient method was applied to characterize the water quality: measuring its electrical resistance. The steam generated by BGCAs from seawater and river water rose to a condenser (a transparent plastic cover above BGCAs, as illustrated in Figure S8), and the condensate water was collected for further test. As shown in Fig. 5d–g, the purified seawater and river water display much higher resistance than those of the untreated samples. The purified water from seawater and river water shows almost same resistance of ~2.2 MΩ 7

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Zhishan Scholar of Southeast University.

intensity of the sun was recorded every 5 min. After 8 h test, the fresh water production rate of the prototype was calculated to be 1.08 kg m−2 h−1. Then we can speculate that the device is able to produce 8–13 L of fresh water with 1 m2 of BGCA per day, assuming a sunshine duration of 8 to 12 h.

Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apenergy.2019.114410.

4. Conclusion References In conclusion, an efficient photothermal conversion material, ballmilling graphite/cellulose fiber composite aerogel, has been developed from ball-milling graphite and waste paper driven cellulose fibers. The solar steam generator is facilely designed with cheap stuffs (e.g. air-laid paper) and recycled materials (e.g. waste paper, packaging polyethylene foams, N-methyl-2-pyrrolidone), holding a respectable perspective of scalability and environmental protection. With the assistance of freeze casting, the composite aerogels develop a highly porous structure with aligned graphene sheets and interconnected CF, as well as low thermal conductivity. Ball-milling graphite/cellulose fiber composite aerogels reach an excellent solar evaporation rate of 1.61 kg m−2 h−1 with photothermal conversion efficiency of 90% under 1 sun illumination. The composite aerogels also have displayed decent cycling ability (maintain solar evaporation rates of ~1.5 kg m−2 h−1 under 1 sun irradiation after 20 wetting-drying usages). Quantitative studies demonstrate that the solar evaporation performance improves with the growth of ball-milling graphite/cellulose ratio, while, the effects of the ball-milling graphite concentration and thickness of the composite aerogels display a critical point. We have found that the composite aerogels with ball-milling graphite/ cellulose ratio of 6:1, ball-milling graphite concentration of 12 mg mL−1 and the thickness of 7 mm attained the highest solar evaporation rate. Additionally, the composite aerogels exhibit decent evaporation rates, long-term stability, and outstanding water purification capability on both seawater and polluted river water. The purified water is proved to be even better than the domestic water, manifesting the great potential for solar desalination of seawater and purification of polluted water. The field test with the prototype device achieved a fresh water production rate of 1.08 kg m−2 h−1 and 8–13 L of fresh water could be produced with 1 m2 of composite aerogel in one day. In all, this work has proposed a cheap, facile and effective approach to solve the increasingly severe shortage of clean water.

[1] Mekhilef S, Saidur R, Safari A. A review on solar energy use in industries. Renew Sustain Energy Rev 2011;15:1777–90. [2] Rijsberman FR. Water scarcity: fact or fiction? Agric Water Manage 2006;80:5–22. [3] Seckler D, Barker R, Amarasinghe U. Water Scarcity in the Twenty-first Century. Int J Water Resour Dev 1999;15:29–42. [4] Gong F, Li H, Wang W, Huang J, Xia D, Liao J, et al. Scalable, eco-friendly and ultrafast solar steam generators based on one-step melamine-derived carbon sponges toward water purification. Nano Energy 2019;58:322–30. [5] Chen C, Zhou L, Yu J, Wang Y, Nie S, Zhu S, et al. Dual functional asymmetric plasmonic structures for solar water purification and pollution detection. Nano Energy 2018;51:451–6. [6] Zhou X, Zhao F, Guo Y, Zhang Y, Yu G. A hydrogel-based antifouling solar evaporator for highly efficient water desalination. Energy Environ Sci 2018;11:1985–92. [7] Liu C, Huang J, Hsiung C-E, Tian Y, Wang J, Han Y, et al. High-performance largescale solar steam generation with nanolayers of reusable biomimetic nanoparticles. Adv Sustain Syst 2017;1:1600013. [8] Wang P. Emerging investigator series: the rise of nano-enabled photothermal materials for water evaporation and clean water production by sunlight. Environ Sci Nano 2018;5:1078–89. [9] Awad FS, Kiriarachchi HD, AbouZeid KM, Özgür Ü, El-Shall MS. Plasmonic graphene polyurethane nanocomposites for efficient solar water desalination. ACS Appl Energy Mater 2018;1:976–85. [10] Amjad M, Raza G, Xin Y, Pervaiz S, Xu J, Du X, et al. Volumetric solar heating and steam generation via gold nanofluids. Appl Energy 2017;206:393–400. [11] Wang X, He Y, Liu X, Cheng G, Zhu J. Solar steam generation through bio-inspired interface heating of broadband-absorbing plasmonic membranes. Appl Energy 2017;195:414–25. [12] Liu X, Wang X, Huang J, Cheng G, He Y. Volumetric solar steam generation enhanced by reduced graphene oxide nanofluid. Appl Energy 2018;220:302–12. [13] Zhao F, Zhou X, Shi Y, Qian X, Alexander M, Zhao X, et al. Highly efficient solar vapour generation via hierarchically nanostructured gels. Nat Nanotechnol 2018;13:489–95. [14] Liu P-F, Miao L, Deng Z, Zhou J, Gu Y, Chen S, et al. Flame-treated and fast-assembled foam system for direct solar steam generation and non-plugging high salinity desalination with self-cleaning effect. Appl Energy 2019;241:652–9. [15] Liu S, Huang C, Luo X, Guo C. Performance optimization of bi-layer solar steam generation system through tuning porosity of bottom layer. Appl Energy 2019;239:504–13. [16] Liu H, Zhang X, Hong Z, Pu Z, Yao Q, Shi J, et al. A bioinspired capillary-driven pump for solar vapor generation. Nano Energy 2017;42:115–21. [17] Wang Y, Wang C, Song X, Megarajan SK, Jiang H. A facile nanocomposite strategy to fabricate a rGO–MWCNT photothermal layer for efficient water evaporation. J Mater Chem A 2018;6:963–71. [18] Yin X, Zhang Y, Guo Q, Cai X, Xiao J, Ding Z, et al. Macroporous double-network hydrogel for high-efficiency solar steam generation under 1 sun illumination. ACS Appl Mater Interfaces 2018;10:10998–1007. [19] Zhang Z, Mu P, He J, Zhu Z, Sun H, Wei H, et al. Facile and scalable fabrication of surface-modified sponge for efficient solar steam generation. ChemSusChem 2019;12:426–33. [20] Liu Y, Liu Z, Huang Q, Liang X, Zhou X, Fu H, et al. A high-absorption and selfdriven salt-resistant black gold nanoparticle-deposited sponge for highly efficient, salt-free, and long-term durable solar desalination. J Mater Chem A 2019;7:2581–8. [21] Qin D-D, Zhu Y-J, Chen F-F, Yang R-L, Xiong Z-C. Self-floating aerogel composed of carbon nanotubes and ultralong hydroxyapatite nanowires for highly efficient solar energy-assisted water purification. Carbon 2019;150:233–43. [22] Mu P, Zhang Z, Bai W, He J, Sun H, Zhu Z, et al. Superwetting monolithic hollowcarbon-nanotubes aerogels with hierarchically nanoporous structure for efficient solar steam generation. Adv Energy Mater 2019;9:1802158. [23] Hu X, Xu W, Zhou L, Tan Y, Wang Y, Zhu S, et al. Tailoring graphene oxide-based aerogels for efficient solar steam generation under one sun. Adv Mater 2017;29:1604031. [24] Zhang P, Li J, Lv L, Zhao Y, Qu L. Vertically aligned graphene sheets membrane for highly efficient solar thermal generation of clean water. ACS Nano 2017;11:5087–93. [25] Xia D, Gong F, Pei X, Wang W, Li H, Zeng W, et al. Molybdenum and tungsten disulfides-based nanocomposite films for energy storage and conversion: a review. Chem Eng J. 2018;348:908–28. [26] Chen C-M, Huang J-Q, Zhang Q, Gong W-Z, Yang Q-H, Wang M-Z, et al. Annealing a graphene oxide film to produce a free standing high conductive graphene film. Carbon 2012;50:659–67. [27] Ma S, Chiu CP, Zhu Y, Tang CY, Long H, Qarony W, et al. Recycled waste black polyurethane sponges for solar vapor generation and distillation. Appl Energy. 2017;206:63–9.

CRediT authorship contribution statement Feng Gong: Conceptualization, Methodology, Investigation, Writing - original draft. Wenbin Wang: Methodology, Formal analysis, Resources, Investigation. Hao Li: Resources, Investigation, Writing original draft. Dawei (David) Xia: Methodology, Formal analysis, Writing - review & editing. Qingwen Dai: Investigation, Data curation. Xinlin Wu: Investigation, Data curation. Mingzhou Wang: Investigation. Jian Li: Investigation. Dimitrios V. Papavassiliou: Writing - review & editing, Supervision. Rui Xiao: Supervision, Project administration, Funding acquisition. 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. Acknowledgement The authors would like to thank the financial support from the National Natural Science Foundation of China (51602038, 51525601), the Fundamental Research Funds for the Central Universities, China (ZYGX2018J044) and the Ministry of Science and Technology of China (2018YFC1902600). Feng Gong would like to thank the support from 8

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[28] Li T, Liu H, Zhao X, Chen G, Dai J, Pastel G, et al. Scalable and highly efficient mesoporous wood-based solar steam generation device: localized heat, rapid water transport. Adv Funct Mater 2018;28:1707134. [29] Zhou X, Zhao F, Guo Y, Rosenberger B, Yu G. Architecting highly hydratable polymer networks to tune the water state for solar water purification. Sci Adv 2019;5. eaaw5484. [30] Teng C, Xie D, Wang J, Yang Z, Ren G, Zhu Y. Ultrahigh conductive graphene paper based on ball-milling exfoliated graphene. Adv Funct Mater 2017;27. 1700240-n/a. [31] Nguyen ST, Feng J, Ng SK, Wong JPW, Tan VBC, Duong HM. Advanced thermal insulation and absorption properties of recycled cellulose aerogels. Colloids Surf A 2014;445:128–34. [32] Liu Y, Mateti S, Li C, Liu X, Glushenkov AM, Liu D, et al. Synthesis of composite nanosheets of graphene and boron nitride and their lubrication application in oil. Adv Eng Mater 2018;20:1700488. [33] Wang Z, Shen X, Han NM, Liu X, Wu Y, Ye W, et al. Ultralow electrical percolation in graphene aerogel/epoxy composites. Chem Mater 2016;28:6731–41. [34] Gong Feng, Wang Wenbin, Li Hao, Xia Dawei, Papavassiliou Dimitrios V. Predictions of the thermal conductivity of multiphase nanocomposites with complex structures. J Mater Sci 2018;53(17):12157–66. https://doi.org/10.1007/ s10853-018-2486-y. [35] Xu N, Hu X, Xu W, Li X, Zhou L, Zhu S, et al. Mushrooms as efficient solar steamgeneration devices. Adv Mater. 2017;29:1606762. [36] Fan Z, Gong F, Nguyen ST, Duong HM. Advanced multifunctional graphene aerogel – Poly (methyl methacrylate) composites: Experiments and modeling. Carbon 2015;81:396–404. [37] Gong F, Tam YS, Nguyen ST, Duong HM. Prediction of thermal resistances and heat

[38]

[39]

[40] [41]

[42]

[43]

[44]

[45]

9

conduction of carbon nanotube aerogels in various permeated gases. Chem Phys Lett 2015;627:116–20. Wang Y, Zhang L, Wang P. Self-Floating carbon nanotube membrane on macroporous silica substrate for highly efficient solar-driven interfacial water evaporation. ACS Sustainable Chem Eng 2016;4:1223–30. Ito Y, Tanabe Y, Han J, Fujita T, Tanigaki K, Chen M. Multifunctional porous graphene for high-efficiency steam generation by heat localization. Adv Mater 2015;27:4302–7. Zhu M, Li Y, Chen F, Zhu X, Dai J, Li Y, et al. Plasmonic wood for high-efficiency solar steam generation. Adv Energy Mater 2018;8:1701028. Zhu L, Sun L, Zhang H, Yu D, Aslan H, Zhao J, et al. Dual-phase molybdenum nitride nanorambutans for solar steam generation under one sun illumination. Nano Energy 2019;57:842–50. Li X, Xu W, Tang M, Zhou L, Zhu B, Zhu S, et al. Graphene oxide-based efficient and scalable solar desalination under one sun with a confined 2D water path. Proc Natl Acad Sci 2016;113:13953–8. Gong F, Ding Z, Fang Y, Tong C-J, Xia D, Lv Y, et al. Enhanced electrochemical and thermal transport properties of graphene/MoS2 heterostructures for energy storage: insights from multiscale modeling. ACS Appl Mater Interfaces 2018;10:14614–21. Yang W, Zhao Z, Wu K, Huang R, Liu T, Jiang H, et al. Ultrathin flexible reduced graphene oxide/cellulose nanofiber composite films with strongly anisotropic thermal conductivity and efficient electromagnetic interference shielding. J Mater Chem C 2017;5:3748–56. Gong F, Liu X, Yang Y, Xia D, Wang W, Duong H, et al. A facile approach to tune the electrical and thermal properties of graphene aerogels by including bulk MoS2. Nanomaterials. 2017;7:420.