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Vertically aligned Juncus effusus fibril composites for omnidirectional solar evaporation
Carbon 156 (2020) 225e233
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Vertically aligned Juncus effusus fibril composites for omnidirectional solar evaporation Qian Zhang a, b, Lipei Ren b, Xingfang Xiao b, c, Yali Chen b, Liangjun Xia b, Guomeng Zhao b, Hongjun Yang b, Xianbao Wang a, **, Weilin Xu b, * a Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Key Laboratory for the Green Preparation and Application of Functional Materials, Ministry of Education, Hubei Key Laboratory of Polymer Materials, School of Materials Science and Engineering, Hubei University, Wuhan, 430062, PR China b State Key Laboratory of New Textile Materials & Advanced Processing Technologies and Hubei Key Laboratory of Advanced Textile Materials & Application, Wuhan Textile University, Wuhan, 430200, PR China c School of Textile Science and Engineering, Wuhan Textile University, Wuhan, 430200, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 July 2019 Received in revised form 13 September 2019 Accepted 22 September 2019 Available online 23 September 2019
Despite tremendous efforts to improve solar-driven steam generation, most solar evaporators are limited to unidirectional evaporation due to changes in the direction of solar irradiation under real-world conditions. Hence, the development of non-unidirectional evaporators, ideally allowing omnidirectional evaporation, remains a significant challenge. In this work, the first-ever vertically aligned activated carbonejuncus effusus (AC-JE) evaporator (VACJE) for omnidirectional evaporation is demonstrated. ACJEs are produced by uniformly decorating activated carbon powder (a cost-lost material with broad efficient solar absorption) on the fibril skeletons of JE with interconnected network architectures. The surprising photothermal performance (the evaporation rate of 2.23 kg m2 h1 under 1 sun illumination) of the VACJE is attributed to the unique inbuilt interconnected network architecture of JE. This structural feature multiplies the available evaporation area to promote omnidirectional solar evaporation, while also providing excellent light absorption thermal management and salt-resistant. This unexpected finding reveals the hidden potential of JE as a low-cost biomass material and provides inspiration for the future design and development of high-performance solar-driven steam generators. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Solar-driven interfacial steam generation is one of the most promising techniques for desalination and wastewater treatment because of its low cost, high efficiency, safety, inexhaustible green energy source, and environmental friendliness [1e9]. Over the past few years, a broad consensus has been reached that a practical solar-driven interfacial steam generator depends on four main characteristics: broadband absorption, rational thermal management, excellent water supplement channels, and open pores for vapor flow [10]. By combining these essential characteristics, various solar-driven steam generators have been developed to
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Wang), [email protected] (W. Xu). https://doi.org/10.1016/j.carbon.2019.09.067 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
improve the performance of solar-driven steam generation. For example, three-dimensional integral structures were designed to facilitate multiple reflections of the incident light within the captivity of a cup [11,12] or cone [13] structure to enhance light absorption. Vertically aligned graphene sheet membranes were utilized to provide high light-absorption capacity for excellent photothermal transduction and vapor release [14]. Evaporators based on hierarchically nanostructured gels revealed high water evaporation rates of 3.2 and 2.5 kg m2 h1 [15,16], much higher than the conventional theoretical upper limit (~1.65 kg m2 h1) [17] under 1 sun illumination, owing to the penetration of solar absorbers (polypyrrole and reduced graphene oxide, rGO) into a polymeric gel polyvinyl alcohol network. Through rational design, environmental energy-enhanced solar evaporators provide a new route to water evaporation, in which enhanced evaporation rate can be achieved by collecting energy directly from the environment [17,18]. In our previous works, tremendous efforts were also made to design various rGO-based evaporators (rGO-polyurethane
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nanocomposite foams [19], rGO films [20e23], and graphene aerogels [24,25]) for highly efficient photothermic evaporation. However, the above solar evaporators are limited to unidirectional evaporation, whereas in reality the direction of solar irradiation changes throughout the day [26], which is significantly different from simulated sunlight in the laboratory. Therefore, absorbing sunlight from different directions to realize omnidirectional evaporation is a major requirement for efficient solar steam generation. Juncus effusus (JE), known as “Dengxincao” in China, is the most abundant herb belonging to the Juncaceae family, which have been widely used as medicinal herbs in China (Fig. 1a and b) [27e29]. Interestingly, JE possesses a favorable inbuilt non-aggregated porous structure, with fibers that are seamlessly interconnected to form an open fibril network architecture. Here, we report on the development of the first-ever vertically aligned activated carbon-JE (AC-JE) evaporator (VACJE; 30 mm in thickness, 20 mm in wall height, 33 mm in diameter), which mainly comprises a bunch of vertical AC-JEs wrapped with hollow expandable polyethylene (EPE; 30 mm in height and 5 mm in thickness; Fig. 1c). AC-JEs were fabricated by directly decorating the fibril skeletons of the JE matrix with a range of activated carbon particles for solar steam generation. Activated carbon material has been attracted a lot of attention due to its low-cost and broad efficient solar absorption in solardriven interfacial steam generation system. Contrary to previous works that were limited to unidirectional evaporation, our newly proposed VACJE can realize the omnidirectional absorption of sunlight in an outdoor environment (Fig. 1d). The three-dimensional open network is beneficial because it allows the incident light to enter the interior of the AC-JE and provides strong scattering and internal reflection, which broadens the band of light absorption. The fibril skeletons forming micro open cells in the JE endow the evaporator with capillary water transport channels that can store water for solar steam generation. By doing so, photothermal conversion is not confined to the top surface of the evaporator; water in the fibril skeleton is also heated, forming vapor that can escape to the environment through the fibril open-cell pores. Moreover, the abundance of macro-porous open cells blocks the transport of certain dissolved and particulate matter, which endows VACJE with anti-fouling capabilities that are especially useful when performing solar steam generation using turbid water sources [30,31]. The inherent porosity and low thermal conductivity of JE provides microscopic insulation that prevents heat loss to the bulk water source. Since these characteristics of AC-JE match the solar steam criteria very well, the VACJE
developed here (30 mm in thickness, 20 mm in wall height, and 33 mm in diameter) exhibits extremely high photothermal performance under 1 sun (2.23 kg m2 h1), which exceeds that previously reported for other biomass-based materials (woods [32e39], mushrooms [40], lotus seedpods [41], arched bamboo [42], pristine cottons [43]) and is capable of purifying wastewater in an outdoor environment. 2. Experimental 2.1. Materials Juncus effusus (JE) was supplied by Wuhan Textile University. Activated carbon powder (400 mesh) was purchased from Shanghai Macklin Biochemical Co., Ltd. NaCl and acetic acid were obtained from Sinopharm Chemical Reagent Co., Ltd., China. All chemicals were used without any further purification. 2.2. Preparation of activated carbon powder-JE (AC-JE) Activated carbon powder (4 g) was sonicated in aqueous acetic acid (prepared by mixing 16 mL of acetic acid with 800 mL of deionized water). Before the activated carbon treatment, Pristine JE (P-JE) was subjected to a hydrophilic treatment comprising immersion in an aqueous detergent solution (4 mg/L) at 100 C for 0.5 h (the hydrophilic JE was named H-JE). The as-prepared H-JE was dipped in the above activated carbon powder dispersion and then dried at 80 C. To obtain the ideal dark color, this procedure was repeated three to four times. The JE was then rinsed with water to remove excess activated carbon powder, leaving a small amount of residual activated carbon powder coating the H-JE skeleton (ACJE). 2.3. Characterization Sample morphologies were observed using a field-emission scanning electron microscope (FE-SEM; Sigma 500, Zeiss, Germany). Samples were characterized by Fourier-transform infrared (FT-IR) spectroscopy (iS50, Nicolet, USA), X-ray photoelectron spectroscopy (XPS; ESCALAB 250Xi, Thermo Fisher Scientific, USA), and ultravioletevisible absorption spectroscopy using a spectrophotometer equipped with an integrating sphere (Lambda 950, PerkinElmer, USA). The integrating sphere was used to collect the scattered light for accurate measurements.
Fig. 1. Optical images of (a) JE plants, (b) JEs extracted from the stems, and (c) VACJE. (d) Schematic drawing of the designed VACJE for solar evaporation. (A colour version of this figure can be viewed online.)
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2.4. Solar-powered steam generation The solar-driven steam generation experiments were conducted in the lab without convective regime. The room temperature was ~22 C and the ambient humidity was ~60%. A solar simulator (a xenon lamp, CELHXF300, Education Au-light Co Beijing, China) was used to supply simulated sunlight, and a calibrated power meter (CEL-NP2000, Education Au-light Co Beijing, China) was used to measure the incoming radiative flux. In this work, the incoming radiative flux was measured at five distributed locations and averaged. A solar simulator irradiated all samples with a light intensity of 1 kW m2 (1 sun) for 60 min under steady-state conditions. During irradiation, the mass change of water in the beaker was recorded using an electronic balance (Sartorius, SQP, QUINTIX224-1CN) with an accuracy of 0.0001 g. Moreover, the beaker was wrapped in polyurethane foam (thickness: 26 mm) as a heat-insulating wall to reduce thermal losses. 3. Results and discussion 3.1. Fabrication and characterization of AC-JE To fabricate the VACJE, we first prepared the AC-JE by a dipcoating method. A schematic diagram of the synthetic process is shown in Fig. 2a. Due to the remarkable hydrophobicity of pristine JE (P-JE) (Fig. S1a), the activated carbon powder did not adhere as easily to the fiber skeleton of P-JE compared with the JE after hydrophilic treatment (H-JE), as shown in Fig. S2. Hence, in a typical experiment, P-JE was immersed in an aqueous detergent solution at 100 C for 0.5 h for hydrophilic treatment, and the resultant H-JE, which exhibited outstanding hydrophilicity (Fig. S1b), was used for further experiments. Notably, in the spectrum of H-JE, the peaks at 1250, 1512, 1633, 1732, and 2905 cm1 occurred at lower intensity compared with those of P-JE, signifying that hemicellulose and lignin were partially removed after treatment in aqueous detergent solution (Fig. S3) [29]. The P-JE changed from white to brownish-yellow after the hydrophilic treatment (insets of Fig. 2a), and the aqueous detergent solution also changed from colorless translucent to brilliant yellow after the treatment (Fig. S4). Fig. S5a shows that water is barely absorbed into P-JE by capillary forces due to its hydrophobicity. However, H-JE was effective at absorbing water after the hydrophilic treatment, as indicated by the bottom of the H-JE turning red after being placed into a Congo Red dye solution (Fig. S5b). These processes were recorded and can be seen in Video S1. As shown in Fig. S6, both P-JE and H-JE possess a favorable, inbuilt, threedimensional continuous network with fibers that are seamlessly interconnected to form fibril network open cells, and the surface of the fibril skeleton is very smooth, which indicating that the hydrophilic treatment process did not alter the original architecture. Fig. S7 and Table S1 show the stressestrain curves and mechanical properties, respectively, of the two samples. Interestingly, the tensile strength of JE did not drop off after hydrophilic treatment. Supplementary data related to this article can be found at https://doi.org/10.1016/j.carbon.2019.09.067. The as-prepared H-JE was then dipped into an activated carbon powder dispersion before drying at 80 C, and the AC-JE was obtained after rinsing with water to remove excess activated carbon powder. The color of JEs changed from brownish-yellow to black after being coated by activated carbon powder (inset of Fig. 2a). Due to the interconnected fibril network architecture of JE, the activated carbon powder dispersed in the solution can penetrate into the interior of JE (Fig. S8 shows that the inside of the AC-JEs is also black). The morphology and structure of the as-prepared AC-JE at different sections (Fig. S9) were investigated using field-emission scanning electron microscopy. The as-prepared AC-JE inherits a
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similar fibril interconnected network architecture from H-JE (Fig. 2b1, b2, and b3); however, the surface of the skeleton exhibits a rough texture owing to the coating of activated carbon powder, which contrasts with the smooth skeleton of H-JE. This difference in texture indicates that activated carbon powder assembled around the JE fibril skeleton. The porous features of AC-JE (pore size and distribution) were measured using a Hg intrusion porosimeter (AutoPore Iv9510, USA). Notably. The pore size distributions in H-JE and AC-JE were unimodal (Fig. S10) with pore diameters of 50e100 mm, indicating that JE has excellent porous features. To evaluate the absorption capability of AC-JE, tailored AC-JEs were laid parallel on the surface of a piece of cotton fabric (13 cm 13 cm; Fig. S11). As expected, AC-JE in the wet state exhibits extremely high optical solar absorption (97e98%), which spans the entire visible and near infrared range (Fig. 2c). This outstanding solar absorption is the result of the strong optical absorption of the activated carbon powder [5,7,9] combined with the light scattering originating from the interconnected network, which increases the optical path length within the AC-JE. The continuous replenishment of the water supply in the solar steam generator is another important parameter for ensuring uninterrupted photothermic vaporization. Once water is dropped onto a piece of AC-JE, the water rapidly diffuses into the sample (inset of Fig. 2d) because it is not completely graphitized and contains O (Fig. 2d) as well as C. Acting as a superior capillary (Fig. 2e and Video S2), AC-JE efficiently wicks the bulk water to the top surface by capillary action. As is evident from Fig. 2f and Video S3, a white piece of tissue paper placed on the top of VCAES can be fully wetted by the wicking of water from the underlayer within 20 s, indicating fast fluid transport to the hot region. Furthermore, Figs. S12 and S13 demonstrate that a continuous supply of water is readily achieved for photothermic vaporization over a long time period when the VACJE is exposed to solar illumination. Another unique property of AC-JE is that it can maintain its integrity (Fig. S14) after washing in water for a long time (2 h), which is beneficial for practical application. Supplementary data related to this article can be found at https://doi.org/10.1016/j.carbon.2019.09.067. 3.2. Photothermic evaporation performance of VACJE Taking advantage of its uninterrupted fluid transport and excellent optical absorption, the as-prepared AC-JE should be a promising material for photothermic evaporation. Obviously, the hollow EPE enables the whole evaporator to float on the bulk water source (Fig. S15). To confirm this, the evaporation performance of the AC-JE-based VACJE under 1 kW m2 (1 sun) illumination was systematically investigated using a lab-made, real-time measurement system. As shown in Fig. 3a, the surface temperature of the pure water and the VACJE is captured by an infrared camera (FLIR E8) under 1 sun illumination. The pure water and VACJE have nearly the same initial surface temperature before illumination, which is close to the ambient temperature. The surface temperature of the VACJE rapidly increases and exhibits a uniform distribution within 200 s of illumination, whereas that of the pure water remains close to the initial temperature even after 600 s of illumination. Fig. 3b shows the water mass loss (pure water, vertically aligned H-JE evaporator (VAHJE) and VACJE) over time in the dark environment and under 1 sun illumination. The average water evaporation rates for the five tested samples (Fig. S16) were calculated to be 0.072, 0.26 and 0.31 kg m2 h1 for the pure water, VAHJE and VACJE in the dark environment, respectively. Under 1 sun illumination, the average water evaporation rate for the VACJE is 2.23 kg m2 h1, which is six times higher than that of the pure water without the VACJE (0.35 kg m2 h1). Interestingly, the calculated evaporation
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Fig. 2. (a) Preparation procedure of AC-JE. The insets show optical images of P-JE, H-JE, and AC-JE. Scale bars: 5 mm. Microscopic structure of the cross-section (b1), surface (b2), and (b3) longitudinal section of AC-JE. The insets show the corresponding magnified SEM images. (c) Absorption spectra of H-JE and AC-JE in the wet state over the wavelength range of 200e2500 nm. The inset shows the corresponding reflectance spectra. (d) XPS spectrum of AC-JE and contact angle test (inset). (e) Capillary effect for AC-JE. Scale bars: 10 mm. (f) Photographs showing that within 20 s, tissue paper placed on top of the VACJE can be wetted by the bulk water. Scale bars: 10 mm. (A colour version of this figure can be viewed online.)
rate of VAHJE was 1.41 kg m2 h1, which is close to that of most of interfacial solar steam generators in previous research. The stability of the VACJE during evaporation, which is important for reusability and recycling without obvious deterioration of its water evaporation ability, is also shown in Fig. S17. The value is much higher than those reported previously for other biomass-based evaporators under 1 sun illumination (Fig. 3c) [32e43]. Desalination is an important practical application of solar-driven steam generation using VACJE. Here taking simulated seawater (3.5% NaCl solution) as an example, it is expected that VACJE can circumvent evaporation-induced salt crystallization fouling for long-term (8 h) photothermic desalination (Fig. 3d). The inner pore size of JE is the root cause of salt-resistant. The crystal particles will fall back to the brine freely under the action of gravity during desalination. As shown in Fig. 3d, the evaporation rate remained stable over a long time period in simulated seawater (3.5% NaCl solution) compared with pure water. On the basis of the above analysis, it can be concluded that seawater has no influence on the evaporation performance of VACJE, which further demonstrates the stability of VACJE as a solar steam generator. A scalable and low-cost solar steam system is highly desirable for practical applications. The elemental VACJE can be assembled into a large one for practical applications (Fig. S18).
3.3. Evaporation mechanism of VACJE The superior photothermic evaporation performance of the VACJE mentioned above indicates that it can be used as a promising system for solar steam generation. Taking a single AC-JE as an example, the evaporation mechanism of VACJE is analyzed in Fig. 4a. Top- and side-view SEM images of the VACJE (Fig. 4b) reveal macropores (macro free spaces) between adjacent AC-JEs and micropores (micro free spaces) inherited from H-JE. These pores in VACJE are filled with this huge amount of gas, which can effectively reduce the heat loss to the bulk water during steam generation due to the low thermal conductivity of the gas. Contrary to previous interfacial solar steam generator (Fig. S19), these free spaces possessed by VACJE simultaneously resolves several crucial design challenges, including (i) multiplying the available evaporation area, (ii) promoting the full directional release of vapor, (iii) increasing the optical path length, and (iv) serving as thermal barriers because of the low thermal conductivity of gas in the pores (~0.025 W m1 K1) [44,45]. As depicted in Fig. 4c, a 10 mm-thick VACJE fully soaked in water (in the wet state) displays a low thermal conductivity of ~0.52 W m1 K1 in the range of 20e80 C (Hotdisk, TPS 2500, Sweden Hot Disk Corporation). This value is lower than that of water (0.61 W m1 K1) at ambient temperature
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Fig. 3. Infrared thermal images showing the surface temperature distribution of (a) pure water and a VACJE after 600 s and 200 s solar illumination, respectively. (b) Mass changes of water (pure water, VAHJE and VACJE) over time in the dark and under 1 sun illumination, respectively. (c) Comparison of the evaporation rate among different biomass evaporators under 1 sun illumination. (d) Mass loss of simulated seawater (3.5% NaCl) and pure water through VACJE under 1 sun illumination as a function of time (8 h). The optical images of VACJE in simulated seawater (3.5% NaCl) during evaporation. (A colour version of this figure can be viewed online.)
[46]. To further minimize heat loss, we carefully examined the thickness (10, 20, and 30 mm) dependence of the insulating properties of VACJE (Fig. 4d). VACJE of different thicknesses were placed on a heating stage set to a temperature of 70 C. Thermal imaging was then performed to investigate the thermal insulation properties of the VACJE [47,48]. The top surface of the 10 mm-thick VACJE reached a high temperature after being placed on the heating stage for 10 min. In contrast, the top surface temperature of the 30 mmthick VACJE remained relatively low over the same time period. The thermal images shown in Fig. 4e show the temperature difference between the bottom and the top of a VACJE (30 mm in thickness and 20 mm in height) after being placed on the heating stage for 5 min and 60 min. Hence, thermal insulation design with appropriate thickness is critical to localize heat at the air/water evaporative interface. To investigate the effect of wall height, we fabricated three VACJEs with wall heights of 0, 10, and 20 mm and measured their water evaporation rates in the dark and under 1 sun illumination (Fig. S20). The average water evaporation rates in the dark
continuously increased from 0.18 through 0.25e0.31 kg m2 h1 for VACJE with wall heights of 0, 10, and 20 mm, respectively, and under 1 sun illumination the corresponding evaporation rates were 1.63, 2.03 and 2.23 kg m2 h1 (Fig. 4f). The enhanced evaporation rate for the 20 mm wall height compared with the 0 mm and 10 mm wall heights is due to the increased available evaporation area under identical conditions [18,49]. Similar outdoor experiments on VACJEs with wall heights of 0, 10, and 20 mm were also carried out on the roof of the laboratory building, which further demonstrated the influence of the wall height on VACJEs. A smaller beaker containing water and VACJEs for evaporation was placed on a Petri dish and covered by a larger upturned beaker for condensation. Vapor produced by the VACJE (30 mm in thickness and 20 mm in wall height) under illumination rapidly condensed and obvious water droplets were observed on the inner sides of the beaker within 5 s. After being illuminated with the solar simulator for 60 s, large numbers of water droplets were observed inside the beaker. In contrast, when the height of the VACJE was 0 mm, water droplets were not observed until 40 s. These experiments were
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Fig. 4. (a) Illustration of the evaporation mechanism of a VACJE during photothermic evaporation. (b) Top-view and side-view SEM images showing the micro-architecture of a VACJE and the corresponding high-magnification images. (c) Thermal conductivity of a 10 mm-thick VACJE. The inset shows the corresponding optical image. (d) Optical images of VACJE of different thicknesses and the corresponding infrared images of VACJE after heating for 10 min on a 70 C heating stage. (e) Infrared images of VACJE 30 mm in thickness and 10 mm in height on a 70 C heating stage after 5 min and 60 min. (f) Average evaporation rates of 30 mm-thick VACJE with different wall heights recorded in the dark and under 1 sun illumination. (g) Comparison of water droplets on the bottom of beaker between 0 mm and 20 mm-high VACJE. (h) Temperature changes of a VACJE 30 mm in thickness and 20 mm in height caused by switching 1 sun illumination on and off. (i) Temperature changes of a VACJE 30 mm in thickness and 20 mm in height in an outdoor environment. (A colour version of this figure can be viewed online.)
recorded and can be seen in Fig. 4g and Video S4. Fig. S21 shows that the number of water droplets decreased significantly as the wall height decreased after 2 h illumination under the same conditions. After 10 h of operation in outdoor experiments on a sunny day, the mass losses for VACJE 0, 10, and 20 mm in height were 9.4, 14.1, and 17.2 g, respectively. The value for the 20 mm-high VACJE is close to two times higher than that of the 0 mm-high VACJE (Fig. S22). Supplementary data related to this article can be found at https://doi.org/10.1016/j.carbon.2019.09.067. Based on the concept of interfacial evaporation, faster photothermic evaporation should be realized by localized heating at the water surface while minimizing heat loss to the bulk water source. The temperature of a VACJE (30 mm in thickness and 20 mm in wall height) was also measured as a function of time, both in the laboratory and outdoors to verify its outstanding photothermal
properties. It is obvious from the respective position temperatures under 1 sun illumination in the laboratory that the same process occurs on both the top, inner and side surfaces of the VACJE (Fig. 4h), which occurs in three stages. The first stage is a sharp temperature increase under light illumination, which is caused by the enhanced light absorption of the AC-JE. The second stage is the equilibration of the temperature in the top, inner and side surfaces. The top, inner and side surface temperatures rise to ~34 C, ~32 C and ~32 C within 15 min under 1 sun illumination, respectively. As mentioned previously, there are macro and micro free spaces in the VACJE (Fig. 4b) that lead to omnidirectional light absorption. Clearly, the temperature of the inner and side surface of the VACJE are relatively low compared with that of the top surface, which is due to the position of the simulated light. The third stage is a sharp temperature decline at the top and side surfaces of the VACJE when the light is switched off. In contrast, the temperature of the bulk
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water only slightly increased (by ~5 C) throughout the entire illumination process due to its intrinsic thermal insulation. The temperature of a VAHJE (30 mm in thickness and 20 mm in wall height) was also measured as a function of time (Fig. S23), which is similar to that of VACJE. Since the position of the sun varies throughout the day [26], which differs significantly from simulated sunlight in the laboratory, similar experiments were carried out using a VACJE (30 mm thickness and 20 mm) in natural sunlight outdoors for one whole day (recording times of 9:00, 11:00, 13:00, 15:00, and 17:00; 11 November 2018). The inset in Fig. 4i shows that the. East and west sides were named the sun side and night side, respectively. The temperature changes for the outdoor VACJE are summarized in Fig. 4i. It is clear that the temperature of every surface of the VACJE always exceeded the ambient temperature after sunrise, which reveals that the photothermic performance of the VACJE is less dependent on the position of the sun. However, the temperature change of each surface varies significantly with the position of the sun. It is expected that the temperature of the sunny side is higher than that of the top and the night side at 9:00 a.m. This is because the sunlight is approximately normally incident on the sunny side when the sun just begins to rise. After 2 (11:00) and 4 h (13:00), the temperature of the top side increased to become close to and then higher than that of the sunny side, which is due to the position of the sun being directly above the VACJE at noon. As the sun begins to set (13:00 and 15:00), the temperature trend is
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similar to that during sunrise. This outdoor experiment indicates that the VACJE can effectively capture sunlight over a range of angles of incidence under realistic operating conditions. 3.4. Wastewater purification using VACJE in an outdoor environment The VACJE works well for the practical application of wastewater purification using a self-made device (Fig. 5a and Fig. S24) in an outdoor environment. Taking simulated seawater (3.5% NaCl solution) as an example, the resistance values of 3.5% NaCl solution, condensed water after VACJE treatment, and deionized water were 133.70 KU, 5.73 MU, and 6.14 MU, respectively. The value of the condensed water is close to that of deionized water, indicating the successful desalination of seawater into freshwater (Fig. 5b). Actual lake water from Wuhan Textile University was also chosen to evaluate the purification performance of VACJE. After purification, the lake water became more transparent, as can be seen from the optical images shown as insets in Fig. 5c1 and 5d1. Optical microscopy images (Fig. 5c1 and 5d1) show that no obvious impurities were detected in the lake water after purification compared with the original lake water. Furthermore, in bacterial culture experiments, obvious colonies can be seen in Petri dishes incubated with original water samples (Fig. 5c2), whereas no colonies were formed on the agar plate after purification by VACJE (Fig. 5d2), indicating that the bacteria were all detached from the original water after purification.
Fig. 5. Wastewater purification using a VACJE in an outdoor environment. (a) Optical images of the device used for wastewater purification. (b) Evaluation of the desalination performance using a multimeter with a constant distance between the electrodes. Optical microscopy images of lake water before (original; c1) and after purification (d1). The inset shows the corresponding optical images of lake water before (original) and after purification. Images of agar plates inoculated with lake water before (original; c2) and after purification (d2). (e) UVevis spectra of 20 ppm Congo Red solution before (red line) and after condensation of the steam (blue line). The insets show the corresponding optical images and images of the agar plates. (f) Concentrations of heavy metal ions before and after VACJE treatment. The black dashed lines indicate the maximum permissible ion concentrations, as stipulated by emission standards for pollutants. (A colour version of this figure can be viewed online.)
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The effect of dye wastewater treatment in the textile industry (20 ppm Congo Red solution as simulated industrial dye wastewater) was also examined in the same way. As can be seen from Fig. 5e, the condensed water was colorless and the absorption peak of Congo Red, which occurs around 495 nm, almost vanished after purification, which is equivalent to a dye rejection of almost 100%. The outstanding results obtained by VACJE purification include the near 100% rejection rate and sterilization (insets of Fig. 5e), which are better than those obtained using commercially available dye waste purification methods. In further tests, water contaminated with common heavy metal pollutants (Cu2þ, Cd2þ, Pb2þ, and Zn2þ ions) was purified by VACJE. The results of inductively coupled plasmaeoptical emission spectroscopy (Fig. 5f) show that the condensed water contained the metal ions at concentrations (Cu2þ, Cd2þ, Pb2þ, and Zn2þ ion concentrations of 0.020, 0.0060, 0.076, and 0.060 mg/L, respectively) several orders of magnitude lower than the initial concentrations, and far below the limit of emission standards of pollutants for secondary copper, aluminum, lead and zinc industry in China (Cu2þ: 0.2 mg/L, Cd3þ: 0.1 mg/L, Pb2þ: 0.2 mg/L, and Zn2þ: 1.0 mg/L) [50]. 4. Conclusions For the first time, we rationally designed a VACJE based on AC-JE for omnidirectional solar-driven steam generation and wastewater purification. The AC-JE is prepared by homogeneously decorating the fibril skeletons in a H-JE matrix with activated carbon. The highly efficient photothermic performance of AC-JEs is due to their unique inbuilt fibril interconnected network architecture, which has the following five advantages: (1) superior light-absorption properties, (2) excellent thermal management, (3) efficient water supply, (4) enhanced evaporation area, and (5) omnidirectional solar evaporation. Our findings not only reveal the hidden potential of JE as a low-cost biomass material for solar steam generation but also provide inspiration for the future design and development of high-performance solar-driven steam generators. Notes The authors declare no competing financial interests. Acknowledgment This work was financially supported by the National Key R&D Program of China (2016YFA0200200), the National Natural Science Foundation of China (51773158), Wuhan Science and Technology Bureau of China (2018010401011280) and Hubei key laboratory of digital textile equipment (DTL2018018). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.09.067. References [1] P. Zhang, Q. Liao, H. Yao, Y. Huang, H. Cheng, L. Qu, Direct solar steam generation system for clean water production, Energy Storage Mater. 18 (2019) 429e446. [2] L. Zhu, M. Gao, C.K.N. Peh, G.W. Ho, Recent progress in solar-driven interfacial water evaporation: advanced designs and applications, Nano Energy 57 (2019) 507e518. [3] Z. Li, C. Wang, J. Su, S. Ling, W. Wang, An M, fast-growing field of interfacial solar steam generation: evolutional materials, engineered architectures, and synergistic applications, Sol. RRL 0 (0) (2019), 1800206. [4] Q. Zhang, X. Xiao, G. Wang, X. Ming, X. Liu, H. Wang, et al., Silk-based systems for highly efficient photothermal conversion under one sun: portability,
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Contents lists available at ScienceDirect
Carbon journal homepage: www.elsevier.com/locate/carbon
Vertically aligned Juncus effusus fibril composites for omnidirectional solar evaporation Qian Zhang a, b, Lipei Ren b, Xingfang Xiao b, c, Yali Chen b, Liangjun Xia b, Guomeng Zhao b, Hongjun Yang b, Xianbao Wang a, **, Weilin Xu b, * a Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Key Laboratory for the Green Preparation and Application of Functional Materials, Ministry of Education, Hubei Key Laboratory of Polymer Materials, School of Materials Science and Engineering, Hubei University, Wuhan, 430062, PR China b State Key Laboratory of New Textile Materials & Advanced Processing Technologies and Hubei Key Laboratory of Advanced Textile Materials & Application, Wuhan Textile University, Wuhan, 430200, PR China c School of Textile Science and Engineering, Wuhan Textile University, Wuhan, 430200, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 July 2019 Received in revised form 13 September 2019 Accepted 22 September 2019 Available online 23 September 2019
Despite tremendous efforts to improve solar-driven steam generation, most solar evaporators are limited to unidirectional evaporation due to changes in the direction of solar irradiation under real-world conditions. Hence, the development of non-unidirectional evaporators, ideally allowing omnidirectional evaporation, remains a significant challenge. In this work, the first-ever vertically aligned activated carbonejuncus effusus (AC-JE) evaporator (VACJE) for omnidirectional evaporation is demonstrated. ACJEs are produced by uniformly decorating activated carbon powder (a cost-lost material with broad efficient solar absorption) on the fibril skeletons of JE with interconnected network architectures. The surprising photothermal performance (the evaporation rate of 2.23 kg m2 h1 under 1 sun illumination) of the VACJE is attributed to the unique inbuilt interconnected network architecture of JE. This structural feature multiplies the available evaporation area to promote omnidirectional solar evaporation, while also providing excellent light absorption thermal management and salt-resistant. This unexpected finding reveals the hidden potential of JE as a low-cost biomass material and provides inspiration for the future design and development of high-performance solar-driven steam generators. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Solar-driven interfacial steam generation is one of the most promising techniques for desalination and wastewater treatment because of its low cost, high efficiency, safety, inexhaustible green energy source, and environmental friendliness [1e9]. Over the past few years, a broad consensus has been reached that a practical solar-driven interfacial steam generator depends on four main characteristics: broadband absorption, rational thermal management, excellent water supplement channels, and open pores for vapor flow [10]. By combining these essential characteristics, various solar-driven steam generators have been developed to
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Wang), [email protected] (W. Xu). https://doi.org/10.1016/j.carbon.2019.09.067 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
improve the performance of solar-driven steam generation. For example, three-dimensional integral structures were designed to facilitate multiple reflections of the incident light within the captivity of a cup [11,12] or cone [13] structure to enhance light absorption. Vertically aligned graphene sheet membranes were utilized to provide high light-absorption capacity for excellent photothermal transduction and vapor release [14]. Evaporators based on hierarchically nanostructured gels revealed high water evaporation rates of 3.2 and 2.5 kg m2 h1 [15,16], much higher than the conventional theoretical upper limit (~1.65 kg m2 h1) [17] under 1 sun illumination, owing to the penetration of solar absorbers (polypyrrole and reduced graphene oxide, rGO) into a polymeric gel polyvinyl alcohol network. Through rational design, environmental energy-enhanced solar evaporators provide a new route to water evaporation, in which enhanced evaporation rate can be achieved by collecting energy directly from the environment [17,18]. In our previous works, tremendous efforts were also made to design various rGO-based evaporators (rGO-polyurethane
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nanocomposite foams [19], rGO films [20e23], and graphene aerogels [24,25]) for highly efficient photothermic evaporation. However, the above solar evaporators are limited to unidirectional evaporation, whereas in reality the direction of solar irradiation changes throughout the day [26], which is significantly different from simulated sunlight in the laboratory. Therefore, absorbing sunlight from different directions to realize omnidirectional evaporation is a major requirement for efficient solar steam generation. Juncus effusus (JE), known as “Dengxincao” in China, is the most abundant herb belonging to the Juncaceae family, which have been widely used as medicinal herbs in China (Fig. 1a and b) [27e29]. Interestingly, JE possesses a favorable inbuilt non-aggregated porous structure, with fibers that are seamlessly interconnected to form an open fibril network architecture. Here, we report on the development of the first-ever vertically aligned activated carbon-JE (AC-JE) evaporator (VACJE; 30 mm in thickness, 20 mm in wall height, 33 mm in diameter), which mainly comprises a bunch of vertical AC-JEs wrapped with hollow expandable polyethylene (EPE; 30 mm in height and 5 mm in thickness; Fig. 1c). AC-JEs were fabricated by directly decorating the fibril skeletons of the JE matrix with a range of activated carbon particles for solar steam generation. Activated carbon material has been attracted a lot of attention due to its low-cost and broad efficient solar absorption in solardriven interfacial steam generation system. Contrary to previous works that were limited to unidirectional evaporation, our newly proposed VACJE can realize the omnidirectional absorption of sunlight in an outdoor environment (Fig. 1d). The three-dimensional open network is beneficial because it allows the incident light to enter the interior of the AC-JE and provides strong scattering and internal reflection, which broadens the band of light absorption. The fibril skeletons forming micro open cells in the JE endow the evaporator with capillary water transport channels that can store water for solar steam generation. By doing so, photothermal conversion is not confined to the top surface of the evaporator; water in the fibril skeleton is also heated, forming vapor that can escape to the environment through the fibril open-cell pores. Moreover, the abundance of macro-porous open cells blocks the transport of certain dissolved and particulate matter, which endows VACJE with anti-fouling capabilities that are especially useful when performing solar steam generation using turbid water sources [30,31]. The inherent porosity and low thermal conductivity of JE provides microscopic insulation that prevents heat loss to the bulk water source. Since these characteristics of AC-JE match the solar steam criteria very well, the VACJE
developed here (30 mm in thickness, 20 mm in wall height, and 33 mm in diameter) exhibits extremely high photothermal performance under 1 sun (2.23 kg m2 h1), which exceeds that previously reported for other biomass-based materials (woods [32e39], mushrooms [40], lotus seedpods [41], arched bamboo [42], pristine cottons [43]) and is capable of purifying wastewater in an outdoor environment. 2. Experimental 2.1. Materials Juncus effusus (JE) was supplied by Wuhan Textile University. Activated carbon powder (400 mesh) was purchased from Shanghai Macklin Biochemical Co., Ltd. NaCl and acetic acid were obtained from Sinopharm Chemical Reagent Co., Ltd., China. All chemicals were used without any further purification. 2.2. Preparation of activated carbon powder-JE (AC-JE) Activated carbon powder (4 g) was sonicated in aqueous acetic acid (prepared by mixing 16 mL of acetic acid with 800 mL of deionized water). Before the activated carbon treatment, Pristine JE (P-JE) was subjected to a hydrophilic treatment comprising immersion in an aqueous detergent solution (4 mg/L) at 100 C for 0.5 h (the hydrophilic JE was named H-JE). The as-prepared H-JE was dipped in the above activated carbon powder dispersion and then dried at 80 C. To obtain the ideal dark color, this procedure was repeated three to four times. The JE was then rinsed with water to remove excess activated carbon powder, leaving a small amount of residual activated carbon powder coating the H-JE skeleton (ACJE). 2.3. Characterization Sample morphologies were observed using a field-emission scanning electron microscope (FE-SEM; Sigma 500, Zeiss, Germany). Samples were characterized by Fourier-transform infrared (FT-IR) spectroscopy (iS50, Nicolet, USA), X-ray photoelectron spectroscopy (XPS; ESCALAB 250Xi, Thermo Fisher Scientific, USA), and ultravioletevisible absorption spectroscopy using a spectrophotometer equipped with an integrating sphere (Lambda 950, PerkinElmer, USA). The integrating sphere was used to collect the scattered light for accurate measurements.
Fig. 1. Optical images of (a) JE plants, (b) JEs extracted from the stems, and (c) VACJE. (d) Schematic drawing of the designed VACJE for solar evaporation. (A colour version of this figure can be viewed online.)
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2.4. Solar-powered steam generation The solar-driven steam generation experiments were conducted in the lab without convective regime. The room temperature was ~22 C and the ambient humidity was ~60%. A solar simulator (a xenon lamp, CELHXF300, Education Au-light Co Beijing, China) was used to supply simulated sunlight, and a calibrated power meter (CEL-NP2000, Education Au-light Co Beijing, China) was used to measure the incoming radiative flux. In this work, the incoming radiative flux was measured at five distributed locations and averaged. A solar simulator irradiated all samples with a light intensity of 1 kW m2 (1 sun) for 60 min under steady-state conditions. During irradiation, the mass change of water in the beaker was recorded using an electronic balance (Sartorius, SQP, QUINTIX224-1CN) with an accuracy of 0.0001 g. Moreover, the beaker was wrapped in polyurethane foam (thickness: 26 mm) as a heat-insulating wall to reduce thermal losses. 3. Results and discussion 3.1. Fabrication and characterization of AC-JE To fabricate the VACJE, we first prepared the AC-JE by a dipcoating method. A schematic diagram of the synthetic process is shown in Fig. 2a. Due to the remarkable hydrophobicity of pristine JE (P-JE) (Fig. S1a), the activated carbon powder did not adhere as easily to the fiber skeleton of P-JE compared with the JE after hydrophilic treatment (H-JE), as shown in Fig. S2. Hence, in a typical experiment, P-JE was immersed in an aqueous detergent solution at 100 C for 0.5 h for hydrophilic treatment, and the resultant H-JE, which exhibited outstanding hydrophilicity (Fig. S1b), was used for further experiments. Notably, in the spectrum of H-JE, the peaks at 1250, 1512, 1633, 1732, and 2905 cm1 occurred at lower intensity compared with those of P-JE, signifying that hemicellulose and lignin were partially removed after treatment in aqueous detergent solution (Fig. S3) [29]. The P-JE changed from white to brownish-yellow after the hydrophilic treatment (insets of Fig. 2a), and the aqueous detergent solution also changed from colorless translucent to brilliant yellow after the treatment (Fig. S4). Fig. S5a shows that water is barely absorbed into P-JE by capillary forces due to its hydrophobicity. However, H-JE was effective at absorbing water after the hydrophilic treatment, as indicated by the bottom of the H-JE turning red after being placed into a Congo Red dye solution (Fig. S5b). These processes were recorded and can be seen in Video S1. As shown in Fig. S6, both P-JE and H-JE possess a favorable, inbuilt, threedimensional continuous network with fibers that are seamlessly interconnected to form fibril network open cells, and the surface of the fibril skeleton is very smooth, which indicating that the hydrophilic treatment process did not alter the original architecture. Fig. S7 and Table S1 show the stressestrain curves and mechanical properties, respectively, of the two samples. Interestingly, the tensile strength of JE did not drop off after hydrophilic treatment. Supplementary data related to this article can be found at https://doi.org/10.1016/j.carbon.2019.09.067. The as-prepared H-JE was then dipped into an activated carbon powder dispersion before drying at 80 C, and the AC-JE was obtained after rinsing with water to remove excess activated carbon powder. The color of JEs changed from brownish-yellow to black after being coated by activated carbon powder (inset of Fig. 2a). Due to the interconnected fibril network architecture of JE, the activated carbon powder dispersed in the solution can penetrate into the interior of JE (Fig. S8 shows that the inside of the AC-JEs is also black). The morphology and structure of the as-prepared AC-JE at different sections (Fig. S9) were investigated using field-emission scanning electron microscopy. The as-prepared AC-JE inherits a
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similar fibril interconnected network architecture from H-JE (Fig. 2b1, b2, and b3); however, the surface of the skeleton exhibits a rough texture owing to the coating of activated carbon powder, which contrasts with the smooth skeleton of H-JE. This difference in texture indicates that activated carbon powder assembled around the JE fibril skeleton. The porous features of AC-JE (pore size and distribution) were measured using a Hg intrusion porosimeter (AutoPore Iv9510, USA). Notably. The pore size distributions in H-JE and AC-JE were unimodal (Fig. S10) with pore diameters of 50e100 mm, indicating that JE has excellent porous features. To evaluate the absorption capability of AC-JE, tailored AC-JEs were laid parallel on the surface of a piece of cotton fabric (13 cm 13 cm; Fig. S11). As expected, AC-JE in the wet state exhibits extremely high optical solar absorption (97e98%), which spans the entire visible and near infrared range (Fig. 2c). This outstanding solar absorption is the result of the strong optical absorption of the activated carbon powder [5,7,9] combined with the light scattering originating from the interconnected network, which increases the optical path length within the AC-JE. The continuous replenishment of the water supply in the solar steam generator is another important parameter for ensuring uninterrupted photothermic vaporization. Once water is dropped onto a piece of AC-JE, the water rapidly diffuses into the sample (inset of Fig. 2d) because it is not completely graphitized and contains O (Fig. 2d) as well as C. Acting as a superior capillary (Fig. 2e and Video S2), AC-JE efficiently wicks the bulk water to the top surface by capillary action. As is evident from Fig. 2f and Video S3, a white piece of tissue paper placed on the top of VCAES can be fully wetted by the wicking of water from the underlayer within 20 s, indicating fast fluid transport to the hot region. Furthermore, Figs. S12 and S13 demonstrate that a continuous supply of water is readily achieved for photothermic vaporization over a long time period when the VACJE is exposed to solar illumination. Another unique property of AC-JE is that it can maintain its integrity (Fig. S14) after washing in water for a long time (2 h), which is beneficial for practical application. Supplementary data related to this article can be found at https://doi.org/10.1016/j.carbon.2019.09.067. 3.2. Photothermic evaporation performance of VACJE Taking advantage of its uninterrupted fluid transport and excellent optical absorption, the as-prepared AC-JE should be a promising material for photothermic evaporation. Obviously, the hollow EPE enables the whole evaporator to float on the bulk water source (Fig. S15). To confirm this, the evaporation performance of the AC-JE-based VACJE under 1 kW m2 (1 sun) illumination was systematically investigated using a lab-made, real-time measurement system. As shown in Fig. 3a, the surface temperature of the pure water and the VACJE is captured by an infrared camera (FLIR E8) under 1 sun illumination. The pure water and VACJE have nearly the same initial surface temperature before illumination, which is close to the ambient temperature. The surface temperature of the VACJE rapidly increases and exhibits a uniform distribution within 200 s of illumination, whereas that of the pure water remains close to the initial temperature even after 600 s of illumination. Fig. 3b shows the water mass loss (pure water, vertically aligned H-JE evaporator (VAHJE) and VACJE) over time in the dark environment and under 1 sun illumination. The average water evaporation rates for the five tested samples (Fig. S16) were calculated to be 0.072, 0.26 and 0.31 kg m2 h1 for the pure water, VAHJE and VACJE in the dark environment, respectively. Under 1 sun illumination, the average water evaporation rate for the VACJE is 2.23 kg m2 h1, which is six times higher than that of the pure water without the VACJE (0.35 kg m2 h1). Interestingly, the calculated evaporation
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Fig. 2. (a) Preparation procedure of AC-JE. The insets show optical images of P-JE, H-JE, and AC-JE. Scale bars: 5 mm. Microscopic structure of the cross-section (b1), surface (b2), and (b3) longitudinal section of AC-JE. The insets show the corresponding magnified SEM images. (c) Absorption spectra of H-JE and AC-JE in the wet state over the wavelength range of 200e2500 nm. The inset shows the corresponding reflectance spectra. (d) XPS spectrum of AC-JE and contact angle test (inset). (e) Capillary effect for AC-JE. Scale bars: 10 mm. (f) Photographs showing that within 20 s, tissue paper placed on top of the VACJE can be wetted by the bulk water. Scale bars: 10 mm. (A colour version of this figure can be viewed online.)
rate of VAHJE was 1.41 kg m2 h1, which is close to that of most of interfacial solar steam generators in previous research. The stability of the VACJE during evaporation, which is important for reusability and recycling without obvious deterioration of its water evaporation ability, is also shown in Fig. S17. The value is much higher than those reported previously for other biomass-based evaporators under 1 sun illumination (Fig. 3c) [32e43]. Desalination is an important practical application of solar-driven steam generation using VACJE. Here taking simulated seawater (3.5% NaCl solution) as an example, it is expected that VACJE can circumvent evaporation-induced salt crystallization fouling for long-term (8 h) photothermic desalination (Fig. 3d). The inner pore size of JE is the root cause of salt-resistant. The crystal particles will fall back to the brine freely under the action of gravity during desalination. As shown in Fig. 3d, the evaporation rate remained stable over a long time period in simulated seawater (3.5% NaCl solution) compared with pure water. On the basis of the above analysis, it can be concluded that seawater has no influence on the evaporation performance of VACJE, which further demonstrates the stability of VACJE as a solar steam generator. A scalable and low-cost solar steam system is highly desirable for practical applications. The elemental VACJE can be assembled into a large one for practical applications (Fig. S18).
3.3. Evaporation mechanism of VACJE The superior photothermic evaporation performance of the VACJE mentioned above indicates that it can be used as a promising system for solar steam generation. Taking a single AC-JE as an example, the evaporation mechanism of VACJE is analyzed in Fig. 4a. Top- and side-view SEM images of the VACJE (Fig. 4b) reveal macropores (macro free spaces) between adjacent AC-JEs and micropores (micro free spaces) inherited from H-JE. These pores in VACJE are filled with this huge amount of gas, which can effectively reduce the heat loss to the bulk water during steam generation due to the low thermal conductivity of the gas. Contrary to previous interfacial solar steam generator (Fig. S19), these free spaces possessed by VACJE simultaneously resolves several crucial design challenges, including (i) multiplying the available evaporation area, (ii) promoting the full directional release of vapor, (iii) increasing the optical path length, and (iv) serving as thermal barriers because of the low thermal conductivity of gas in the pores (~0.025 W m1 K1) [44,45]. As depicted in Fig. 4c, a 10 mm-thick VACJE fully soaked in water (in the wet state) displays a low thermal conductivity of ~0.52 W m1 K1 in the range of 20e80 C (Hotdisk, TPS 2500, Sweden Hot Disk Corporation). This value is lower than that of water (0.61 W m1 K1) at ambient temperature
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Fig. 3. Infrared thermal images showing the surface temperature distribution of (a) pure water and a VACJE after 600 s and 200 s solar illumination, respectively. (b) Mass changes of water (pure water, VAHJE and VACJE) over time in the dark and under 1 sun illumination, respectively. (c) Comparison of the evaporation rate among different biomass evaporators under 1 sun illumination. (d) Mass loss of simulated seawater (3.5% NaCl) and pure water through VACJE under 1 sun illumination as a function of time (8 h). The optical images of VACJE in simulated seawater (3.5% NaCl) during evaporation. (A colour version of this figure can be viewed online.)
[46]. To further minimize heat loss, we carefully examined the thickness (10, 20, and 30 mm) dependence of the insulating properties of VACJE (Fig. 4d). VACJE of different thicknesses were placed on a heating stage set to a temperature of 70 C. Thermal imaging was then performed to investigate the thermal insulation properties of the VACJE [47,48]. The top surface of the 10 mm-thick VACJE reached a high temperature after being placed on the heating stage for 10 min. In contrast, the top surface temperature of the 30 mmthick VACJE remained relatively low over the same time period. The thermal images shown in Fig. 4e show the temperature difference between the bottom and the top of a VACJE (30 mm in thickness and 20 mm in height) after being placed on the heating stage for 5 min and 60 min. Hence, thermal insulation design with appropriate thickness is critical to localize heat at the air/water evaporative interface. To investigate the effect of wall height, we fabricated three VACJEs with wall heights of 0, 10, and 20 mm and measured their water evaporation rates in the dark and under 1 sun illumination (Fig. S20). The average water evaporation rates in the dark
continuously increased from 0.18 through 0.25e0.31 kg m2 h1 for VACJE with wall heights of 0, 10, and 20 mm, respectively, and under 1 sun illumination the corresponding evaporation rates were 1.63, 2.03 and 2.23 kg m2 h1 (Fig. 4f). The enhanced evaporation rate for the 20 mm wall height compared with the 0 mm and 10 mm wall heights is due to the increased available evaporation area under identical conditions [18,49]. Similar outdoor experiments on VACJEs with wall heights of 0, 10, and 20 mm were also carried out on the roof of the laboratory building, which further demonstrated the influence of the wall height on VACJEs. A smaller beaker containing water and VACJEs for evaporation was placed on a Petri dish and covered by a larger upturned beaker for condensation. Vapor produced by the VACJE (30 mm in thickness and 20 mm in wall height) under illumination rapidly condensed and obvious water droplets were observed on the inner sides of the beaker within 5 s. After being illuminated with the solar simulator for 60 s, large numbers of water droplets were observed inside the beaker. In contrast, when the height of the VACJE was 0 mm, water droplets were not observed until 40 s. These experiments were
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Fig. 4. (a) Illustration of the evaporation mechanism of a VACJE during photothermic evaporation. (b) Top-view and side-view SEM images showing the micro-architecture of a VACJE and the corresponding high-magnification images. (c) Thermal conductivity of a 10 mm-thick VACJE. The inset shows the corresponding optical image. (d) Optical images of VACJE of different thicknesses and the corresponding infrared images of VACJE after heating for 10 min on a 70 C heating stage. (e) Infrared images of VACJE 30 mm in thickness and 10 mm in height on a 70 C heating stage after 5 min and 60 min. (f) Average evaporation rates of 30 mm-thick VACJE with different wall heights recorded in the dark and under 1 sun illumination. (g) Comparison of water droplets on the bottom of beaker between 0 mm and 20 mm-high VACJE. (h) Temperature changes of a VACJE 30 mm in thickness and 20 mm in height caused by switching 1 sun illumination on and off. (i) Temperature changes of a VACJE 30 mm in thickness and 20 mm in height in an outdoor environment. (A colour version of this figure can be viewed online.)
recorded and can be seen in Fig. 4g and Video S4. Fig. S21 shows that the number of water droplets decreased significantly as the wall height decreased after 2 h illumination under the same conditions. After 10 h of operation in outdoor experiments on a sunny day, the mass losses for VACJE 0, 10, and 20 mm in height were 9.4, 14.1, and 17.2 g, respectively. The value for the 20 mm-high VACJE is close to two times higher than that of the 0 mm-high VACJE (Fig. S22). Supplementary data related to this article can be found at https://doi.org/10.1016/j.carbon.2019.09.067. Based on the concept of interfacial evaporation, faster photothermic evaporation should be realized by localized heating at the water surface while minimizing heat loss to the bulk water source. The temperature of a VACJE (30 mm in thickness and 20 mm in wall height) was also measured as a function of time, both in the laboratory and outdoors to verify its outstanding photothermal
properties. It is obvious from the respective position temperatures under 1 sun illumination in the laboratory that the same process occurs on both the top, inner and side surfaces of the VACJE (Fig. 4h), which occurs in three stages. The first stage is a sharp temperature increase under light illumination, which is caused by the enhanced light absorption of the AC-JE. The second stage is the equilibration of the temperature in the top, inner and side surfaces. The top, inner and side surface temperatures rise to ~34 C, ~32 C and ~32 C within 15 min under 1 sun illumination, respectively. As mentioned previously, there are macro and micro free spaces in the VACJE (Fig. 4b) that lead to omnidirectional light absorption. Clearly, the temperature of the inner and side surface of the VACJE are relatively low compared with that of the top surface, which is due to the position of the simulated light. The third stage is a sharp temperature decline at the top and side surfaces of the VACJE when the light is switched off. In contrast, the temperature of the bulk
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water only slightly increased (by ~5 C) throughout the entire illumination process due to its intrinsic thermal insulation. The temperature of a VAHJE (30 mm in thickness and 20 mm in wall height) was also measured as a function of time (Fig. S23), which is similar to that of VACJE. Since the position of the sun varies throughout the day [26], which differs significantly from simulated sunlight in the laboratory, similar experiments were carried out using a VACJE (30 mm thickness and 20 mm) in natural sunlight outdoors for one whole day (recording times of 9:00, 11:00, 13:00, 15:00, and 17:00; 11 November 2018). The inset in Fig. 4i shows that the. East and west sides were named the sun side and night side, respectively. The temperature changes for the outdoor VACJE are summarized in Fig. 4i. It is clear that the temperature of every surface of the VACJE always exceeded the ambient temperature after sunrise, which reveals that the photothermic performance of the VACJE is less dependent on the position of the sun. However, the temperature change of each surface varies significantly with the position of the sun. It is expected that the temperature of the sunny side is higher than that of the top and the night side at 9:00 a.m. This is because the sunlight is approximately normally incident on the sunny side when the sun just begins to rise. After 2 (11:00) and 4 h (13:00), the temperature of the top side increased to become close to and then higher than that of the sunny side, which is due to the position of the sun being directly above the VACJE at noon. As the sun begins to set (13:00 and 15:00), the temperature trend is
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similar to that during sunrise. This outdoor experiment indicates that the VACJE can effectively capture sunlight over a range of angles of incidence under realistic operating conditions. 3.4. Wastewater purification using VACJE in an outdoor environment The VACJE works well for the practical application of wastewater purification using a self-made device (Fig. 5a and Fig. S24) in an outdoor environment. Taking simulated seawater (3.5% NaCl solution) as an example, the resistance values of 3.5% NaCl solution, condensed water after VACJE treatment, and deionized water were 133.70 KU, 5.73 MU, and 6.14 MU, respectively. The value of the condensed water is close to that of deionized water, indicating the successful desalination of seawater into freshwater (Fig. 5b). Actual lake water from Wuhan Textile University was also chosen to evaluate the purification performance of VACJE. After purification, the lake water became more transparent, as can be seen from the optical images shown as insets in Fig. 5c1 and 5d1. Optical microscopy images (Fig. 5c1 and 5d1) show that no obvious impurities were detected in the lake water after purification compared with the original lake water. Furthermore, in bacterial culture experiments, obvious colonies can be seen in Petri dishes incubated with original water samples (Fig. 5c2), whereas no colonies were formed on the agar plate after purification by VACJE (Fig. 5d2), indicating that the bacteria were all detached from the original water after purification.
Fig. 5. Wastewater purification using a VACJE in an outdoor environment. (a) Optical images of the device used for wastewater purification. (b) Evaluation of the desalination performance using a multimeter with a constant distance between the electrodes. Optical microscopy images of lake water before (original; c1) and after purification (d1). The inset shows the corresponding optical images of lake water before (original) and after purification. Images of agar plates inoculated with lake water before (original; c2) and after purification (d2). (e) UVevis spectra of 20 ppm Congo Red solution before (red line) and after condensation of the steam (blue line). The insets show the corresponding optical images and images of the agar plates. (f) Concentrations of heavy metal ions before and after VACJE treatment. The black dashed lines indicate the maximum permissible ion concentrations, as stipulated by emission standards for pollutants. (A colour version of this figure can be viewed online.)
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The effect of dye wastewater treatment in the textile industry (20 ppm Congo Red solution as simulated industrial dye wastewater) was also examined in the same way. As can be seen from Fig. 5e, the condensed water was colorless and the absorption peak of Congo Red, which occurs around 495 nm, almost vanished after purification, which is equivalent to a dye rejection of almost 100%. The outstanding results obtained by VACJE purification include the near 100% rejection rate and sterilization (insets of Fig. 5e), which are better than those obtained using commercially available dye waste purification methods. In further tests, water contaminated with common heavy metal pollutants (Cu2þ, Cd2þ, Pb2þ, and Zn2þ ions) was purified by VACJE. The results of inductively coupled plasmaeoptical emission spectroscopy (Fig. 5f) show that the condensed water contained the metal ions at concentrations (Cu2þ, Cd2þ, Pb2þ, and Zn2þ ion concentrations of 0.020, 0.0060, 0.076, and 0.060 mg/L, respectively) several orders of magnitude lower than the initial concentrations, and far below the limit of emission standards of pollutants for secondary copper, aluminum, lead and zinc industry in China (Cu2þ: 0.2 mg/L, Cd3þ: 0.1 mg/L, Pb2þ: 0.2 mg/L, and Zn2þ: 1.0 mg/L) [50]. 4. Conclusions For the first time, we rationally designed a VACJE based on AC-JE for omnidirectional solar-driven steam generation and wastewater purification. The AC-JE is prepared by homogeneously decorating the fibril skeletons in a H-JE matrix with activated carbon. The highly efficient photothermic performance of AC-JEs is due to their unique inbuilt fibril interconnected network architecture, which has the following five advantages: (1) superior light-absorption properties, (2) excellent thermal management, (3) efficient water supply, (4) enhanced evaporation area, and (5) omnidirectional solar evaporation. Our findings not only reveal the hidden potential of JE as a low-cost biomass material for solar steam generation but also provide inspiration for the future design and development of high-performance solar-driven steam generators. Notes The authors declare no competing financial interests. Acknowledgment This work was financially supported by the National Key R&D Program of China (2016YFA0200200), the National Natural Science Foundation of China (51773158), Wuhan Science and Technology Bureau of China (2018010401011280) and Hubei key laboratory of digital textile equipment (DTL2018018). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.09.067. References [1] P. Zhang, Q. Liao, H. Yao, Y. Huang, H. Cheng, L. Qu, Direct solar steam generation system for clean water production, Energy Storage Mater. 18 (2019) 429e446. [2] L. Zhu, M. Gao, C.K.N. Peh, G.W. Ho, Recent progress in solar-driven interfacial water evaporation: advanced designs and applications, Nano Energy 57 (2019) 507e518. [3] Z. Li, C. Wang, J. Su, S. Ling, W. Wang, An M, fast-growing field of interfacial solar steam generation: evolutional materials, engineered architectures, and synergistic applications, Sol. RRL 0 (0) (2019), 1800206. [4] Q. Zhang, X. Xiao, G. Wang, X. Ming, X. Liu, H. Wang, et al., Silk-based systems for highly efficient photothermal conversion under one sun: portability,
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