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poly (vinylidene fluoride) composites on polyurethane sponges for efficient solar water purification
Solar Energy Materials and Solar Cells 203 (2019) 110127
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Self-floating Bi2S3/poly (vinylidene fluoride) composites on polyurethane sponges for efficient solar water purification
T
Haiyan Cheng, Xinghang Liu, Lixing Zhang, Baofei Hou, Fang Yu, ZhuoXun Shi, Xianbao Wang∗ Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry-of-Education Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei Key Laboratory of Polymer Materials (Hubei University), School of Materials Science and Engineering, Hubei University, Wuhan, 430062, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Solar energy Solar thermal conversion Water purification Bi2S3/poly (vinylidene fluoride) composites Multiple internal reflections
Solar water purification (SWP) has been considered as one of the most sustainable techniques to address clean water shortage problem. Although significant strategies have been proposed to explore portable SWP device, many challenges, such as complicated fabrication, high costs and poor practicability, still remain to be solved. Here, a facilely and integrally designed Bi2S3/poly (vinylidene fluoride) (BP) composites based on polyurethane (PU) Sponges (PBP composites) is reported. The PBP composites consist of integrated structures, in which multiple internal reflections guarantee sufficient harvest of incident light and tip thermal effects of chrysanthemum-like Bi2S3 ensure efficient solar thermal conversion, and porous BP endows it with adequate water supply, thus exhibiting a high water evaporation rate of 1.66 kg m−2 h−1 and a solar thermal efficiency of 92.9% under one sun irradiation. This bionic optical design and heat management design may provide a new inspiration to utilize solar energy in water purification and other related applications.
1. Introduction Accompany with the energy crisis and environmental pollution, the utilization of solar energy has drawn great attention for its sustainability and abundant reserves. Up to now, several techniques relating to the utilization of solar energy have been exploited, involving solar driven power generation [1,2], photo-bioenergy conversion [3,4], photo chemical conversion [5,6] and solar thermal conversion [7–10]. Among them, solar thermal conversion is widely acknowledged as the most efficient one, which can also be applied to water purification and address clean water resource shortage issue. Over the years, to enhance the evaporation efficiency of solar water purification (SWP) device, various absorbers, such as polymers [11], metallic plasmonic materials [12,13] and carbon-based materials [14–18] have been investigated in suspending systems [19,20] or floating systems [7], [21–24]. Recently, semiconductors have attracted people's attention with respect to the application in solar thermal conversion, due to their low cost, low cytotoxicity [25] and their noticeable solar thermal performance stemmed from their free carrierinduced localized surface plasmon resonance (LSPR) effect [26–30]. Among them, Bi2S3, as a narrow band gap semiconductor, has been applied to the field of solar thermal therapy [31], achieving the therapeutic effects via the heat generated by electron-hole nonradiative
∗
recombination under light irradiation [32–34], showing a great application prospect. Besides, the structure design of SWP device is also vital for the promotion of evaporation efficiency [35–39]. Many attempts have been made to enhance water evaporation by optimizing the structure design [40,41], involving the designs of membrane absorbers[ [13], [16–18], [42], graphene aerogels [43,44] and various hydrogels [45–47], but many challenges, such as complicated fabrication approaches, high cost and poor practicability, still remain to be solved. In this work, we design a self-floating all-in-one Bi2S3/poly (vinylidene fluoride) (BP) composites based on polyurethane Sponges (PBP composites). As illustrated in Fig. 1, the upper layer is BP composites, as a light harvester and a water evaporator. The synergistic effect of tip thermal effects (The physical phenomenon in which heat is concentrated in a separate object toward the tip (the tip of the geometry)) and multiple internal reflections [36] of chrysanthemum-like Bi2S3 guarantees efficient utilization of incident light of upper layer. While the bottom layer is pristine PU as a support and heat insulator. Low density and low thermal conductivity of pristine PU ensure it a good heat insulating support. As a result, the PBP-R1.5 composites possess a high evaporation performance up to 1.66 kg m−2 h−1 under one sun illumination and excellent mechanical stability, showing a great potential in the practical application of SWP.
Corresponding author. E-mail address: [email protected] (X. Wang).
https://doi.org/10.1016/j.solmat.2019.110127 Received 24 April 2019; Received in revised form 11 August 2019; Accepted 13 August 2019 0927-0248/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Graphical illustration of PBP composites for solar water purification.
2. Experimental
for at least 3 h, and washed with deionized water for further use.
2.1. Reagents and materials
2.5. Bi2S3/PVDF (BP) composites based on PU sponges (PBP-Rx)
The polyurethane (PU) Sponges was purchased from Guangzhou Sheng Li Foam Rubber Products co., Ltd. The specific property parameters are shown in Table S1. Bismuth nitrate pentahydrate [Bi (NO3)3·5H2O] was purchased from Shanghai Macklin Biochemical Co., Ltd. poly(vinylidene fluoride) (PVDF) was obtained from arkema (France), Thioacetamide (C2H5NS), polyvinylpyrrolidone K30 [(C6H9NO)n] (PVP), urea (H2NCONH2), N,N-dimethylformamidel (C3H7NO, DMF), glacial acetic acid (C2H4O2), hydrochloric acid (HCl), potassium hydroxide (KOH) and ethanol (C2H5O) were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were analytical reagent and were used without further purification.
The nonsolvent-induced phase-inversion approach was applied to fabricate PBP-Rx composites (Rx was the as-obtained Bi2S3 with different molar ratio of S/Bi). The as-prepared Rx was respectively dispersed in DMF with ultrasound, forming uniform Rx suspension with a concentration of 5 mg mL−1 (The details for determination of the optimum amount of Bi2S3 is shown in Fig. S4). Subsequently, PVDF was added, and the solution was magnetically stirred to form a homogeneous slurry with a mass percent of 8 wt%. After that the PVDF-Rx slurry was painted on the prewashed PU (with a thickness of 10 mm) by dropper. Then, the PU painted with PVDF-Rx slurry was dipped in ethanol for at least 3 h, and washed with deionized water for further use (Fig. S1 Route 2).
2.2. Chrysanthemum-like Bi2S3 2.6. Characterization Chrysanthemum-like Bi2S3 samples were synthesized using a facile one-step solvent-thermal method. Bi (NO3)3·5H2O (0.485 g) was dissolved in an appropriate amount of acetic acid, then the solution was diluted to 80 mL. Subsequently, C2H5NS of various corresponding molar mass, H2NCONH2 (0.6 g) and PVP (0.8 g) were added, and the solution was magnetically stirred to form a homogeneous solution. After that, the obtained solution was transferred to a Teflon-lined stainless steel autoclave and treated at 160 °C for 24 h. The precipitates were collected by centrifugation and washed with ethanol, deionized water, and dried in vacuum at 60 °C. Here, we denote Rx as the corresponding products, x refers to the molar ratio of S/Bi, i.e., R0.5, R1, R1.5, R2, R4, respectively.
The morphologies of samples were observed by field-emission scanning electron microscope (FE-SEM) (Sigma 500, Zeiss, Germany) and scanning electron microscope (SEM) (JSM-6510LV, Japan, Jeol Ltd.). The X-ray diffraction (XRD) patterns were obtained from an X-ray diffractometer (D8-advance, Bruker, Germany). The thermogravimetric analysis (TGA) data was implemented by a thermal analyzer (TGA1, Mettler-Toledo, Switzerland). The optical absorption, reflectivity and transmission spectra were obtained by a UV–vis–NIR spectrophotometer (UV-3600, Shimadu, Japan), with fine BaSO4 powder as reference. An integrated heating platform (JF-956S, Dongguan) was used to measure thermal conductivity and the temperature was measured by a thermocouple thermometers (HT-9815, Dongguan Xintai Instrument Co., Ltd.). The temperature distributions and surface temperatures were recorded by an IR camera (Flir-E4, Flir, Sweden) (The calibration of the temperature measurement is displayed in Fig. S5). The solar illumination was simulated by an xenon lamp (CEL-HXF300, Beijing) with an AM1.5G optical filter. The illumination intensity of the samples surfaces were measured by a solar power meter (SM206, Shenzhen). The real-time mass of water over the entire evaporation was tracked by a computer controlled electronic balance (JJ223BC, Changshu) with accuracy of 1 mg. Double column universal material testing machine (Instron3360, American Instron) was used to investigate tensile properties of samples.
2.3. Modified PU (MP) For the preparation of MP, a piece of PU with a diameter of 38 mm and a thickness of 10 mm was dipped with 3 mL DMF in a petri dish for several minutes until the DMF was uniformly absorbed on one of the surfaces of PU. Subsequently, DMF was extracted with water. Then the obtained PU was immersed in water for at least 24 h for further use (Fig. S1 Route 1). 2.4. PVDF-PU composites (PP) The nonsolvent-induced phase-inversion approach [26] was applied to fabricate PP composites. PVDF was dissolved in DMF, and the solution was magnetically stirred to form a homogeneous slurry with a mass percent of 8 wt% (The details for determination of the optimum dosage of PVDF is shown in Fig. S2). After that the PVDF slurry was painted on the prewashed PU (with a thickness of 10 mm, the details for determination of the optimum thickness of PU is shown in Fig. S3) by a dropper. Then, the PU painted with PVDF slurry was dipped in ethanol
2.7. Solar water purification To explore the solar thermal conversion ability of samples, evaporation performances were investigated under the simulated intensity of 1 kW m−2 and lower irradiation intensity over 60 min. All samples with a diameter of 38 mm were floated on the surface of water in the beaker wrapped with polystyrene foam, serving as a thermal barrier to 2
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means that the phase of R0.5 is mainly bismutite and prefers to form into orthorhombic Bi2S3. And characteristic peaks of both bismutite and orthorhombic Bi2S3 can be observed from XRD pattern of R1. The diffraction peaks observed at 28.6°, 25.0° are stronger than that of R0.5, while a weaker diffraction peak at 30.2°, revealing the formation of orthorhombic Bi2S3 in R1. Comparing the patterns of all samples, one can find that the diffraction intensity of R1.5 of the orthorhombic Bi2S3 phase becomes stronger as the molar ratio of S/Bi increases. But there no obvious differences at the diffraction intensity are observed along with further increasing of molar ratio of S/Bi. These results show that with the increase of molar ratio of S/Bi, bismuthite is firstly formed in the product, and then converted into orthorhombic Bi2S3. When the ratio is higher than 1.5, there is no new phase be formed. As illustrated in Fig. 3a, pristine PU consists of microporous structures, leading to a low density and providing channels for steam escape. And the low density of PU makes it a self-floating device, reducing direct contact with water and heat loss to bulk water[ [14–16,46]. Consequently, pristine PU exhibits good heat insulation, and can concentrate heat on the air-water interface which can be confirmed in Figs. S8a–b, thus making it possible for the application of PU in SWP device. While the skeleton surface of pristine PU was too smooth (Fig. 3b–c)) to satisfy the requirement of timely water supply in the process of SWP. Therefore, some modifications were made to optimize it (Fig. S1). Fig. 4 shows the structures evolutions of samples during the chemical treating process. After optimization, MP failed to maintain its pristine shape and floating state, while PBP can free-float on the surface of water (Figs. S9a–c) and form an air-water interface in favour of heat localization (Fig. S8c). Besides, only a few micropores can be observed at the smooth skeleton surface of MP, while the skeleton of PBP was covered with microporous structures (Fig. 3 d-l). According to previous researches, open porous structures generally facilitate water diffusion and steam escape [9], [36,39,42], [49]. Additionally, microporous structures of PBP can divide bulk water into numerous capillary areas [24], thus promoting the water diffusion through capillarity. And one can find that PBP device can well acclimatize itself to the undulating water (Video S1), almost exhibited no impacts on its floating state. Supplementary video related to this article can be found at https:// doi.org/10.1016/j.solmat.2019.110127 Fig. 2b shows the TGA patterns of PU, PVDF, R1.5 and PBP-R1.5. At the heating phase from 100 to 230 °C, all samples show a slight weight loss, resulting from the evaporation of physically adsorbed water molecules and the removal of small molecule substance from their surface. The following phase of weight loss for PU starts from 230 °C to 410 °C, while 410 °C–450 °C for R1.5 and 450 °C–600 °C for PVDF. In conclusion, the weight loss of PBP-R1.5 could be attributed to the decomposition of PU (from 230 °C to 410 °C), R1.5 (from 410 °C to 450 °C) and PVDF (from 450 °C to 600 °C), revealing the coexistence of PVDF, R1.5
reduce heat loss to beaker wall. All samples were pre-wetted before floated on the surface of water. A xenon lamp with an AM1.5G optical filter was utilized to simulate the solar illumination, while the illumination intensity of samples’ surface was measured by a solar power meter. A computer controlled electronic balance was utilized to track the real-time mass change of samples under irradiation. And maximum asurface temperatures were measured by an IR camera. To further investigate the durability of samples, the SWP process was cycled for 20 times under the illumination intensity of 1 kW m−2. After each cycle test, the SWP system was shut down for at least 20 min to ensure a consistent initial state of the system. 3. Results and discussion As illustrated in Fig. S6a, the fabrication of chrysanthemum-like Bi2S3 is based on the PVP assisted self-assembly of Bi2S3 nanoribbons in water. During the heating of the precursor, S2− species released through the hydrolysis of thioacetamide, combined with Bi3+ species in a certain proportion and condensed into randomly distributed Bi2S3 nanoribbons via an oriental attachment mechanism. Then the randomly distributed Bi2S3 nanoribbons further self-assembled into chrysanthemum-like architecture. In this fabrication process, PVP served as proton scavenger by consuming excessive acid and facilitated the selfassembly of Bi2S3 nanoribbons by controlling the electrostatic interaction [48] among nanoribbons. At the end of hydrothermal reaction, the chrysanthemum-like Bi2S3 consisting of nanoribbons was obtained (Fig. S6a). The morphology of the as-obtained Rx was characterized by FESEM. As shown in Fig. S7, R0.5 consists of irregular nanoplates with the thickness of ~20 nm. As the molar ratio of S/Bi increases, the size of nanoplates observed in R1 are much smaller than that of R0.5, and nanoribbons begin to be formed. When the ratio increases to 1.5, the FE-SEM images display an obvious chrysanthemum-like morphology, consisting of much smaller nanoribbons. Compared with other samples, chrysanthemum-like morphology of R1.5 is conducive to minimizing the reflection and transmission of incident light and tip thermal effects, stemming from the morphology of nanoribbons, contribute to efficient solar heat conversion (Figs. S6b–c). With further increase of molar ratio of S/Bi, chrysanthemum-like Bi2S3 was gradually converted into amorphous blocks, until the whole nanoribbons were converted into amorphous Bi2S3. Only when the ratio was 1.5, the obtained products exhibited obvious chrysanthemum-like morphology. The crystallization nature of Rx was investigated by XRD. All Rx samples are crystalline with strong diffraction peaks. As presented in Fig. 2a, the diffraction peaks of R0.5 can be well indexed to the standard profile of bismutite (JCPDS No. 41–1488) with the major peaks, inspite of the diffraction peak observed at 28.6° (indexed to the standard profile of orthorhombic phase Bi2S3, JCPDS No. 17–0320), which
Fig. 2. (a) XRD patterns of Rx; (b) The TGA of PU, PVDF, R1.5 and PBP-R1.5. 3
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Fig. 3. The SEM images for (a–c) pristine PU (d–f) MP (g–i) upper surface of PBP-R1.5 (j–l) cross section of PBP-R1.5.
provide numerous effective evaporating areas, thus reducing radiation and convection heat losses at the evaporator surface, resulting in an efficient heat management. Additionally, as illustrated in Fig. S10, the reflectivity of PBP-R1.5 is ~1.5% in the solar spectrum (200–2000 nm), while the transmission of PBP-R1.5 is ~1.5% in the solar spectrum (200–2000 nm), both of which are much lower than those of the PU and R1.5. It can be estimated that ~97% of the irradiated solar power can be absorbed by PBP-R1.5 composites (Note S) [50], indicating that the structure design of PBP composites can improve the light harvest, leading to a higher utilization of incident light. To verify the evaporation performance of PU, MP and PP, we investigated their behaviors by utilizing xenon lamp with an AM1.5G optical filter to simulate solar illumination and tracing the weight loss of water in the process of evaporation. Fig. 7a shows the mass change of pristine PU, MP and PBP under the illumination intensity of 1 kW m−2. At the beginning, a large number of heat generated via solar thermal conversion is lost to the surroundings, resulting in lower surface temperatures and evaporation rate. During the constant irradiation, the heat at evaporation surface continuously accumulates, leading to much higher surface temperatures of samples,
in the PU matrix of PBP-R1.5. Heat management plays an essential role in the process of SWP. Here the maximum surface temperatures of samples under 1 sun illumination were recorded by a thermocouple thermometers. As illustrated in Fig. 5a, surface temperatures of all samples show rapid increases within initial 5 min and then slowly reach the maximum values within 50 min and keep constant. IR camera was utilized to display the surface temperatures distributions after 60 min irradiation (Fig. 5b). Surface temperatures of PU, MP and PP were much higher than that of water. While the modified one exhibited higher surface temperatures, and PP possessed the highest (The slightly higher initial temperature of MP is mainly ascribed to the higher light harvest of activated carbon within pristine PU.), revealing the best heat localization of PP. The efficient heat management of PP mainly stems from the intrinsic low thermal conductivity. Thermal conductivity of pristine PU was measured to be 1.339 W m−1 K−1, while that of PBP-R1.5 was 0.861 W m−1 K−1 (Fig. 6a–b). Compared with pristine PU, the micropores covered on the PBP skeleton surface can efficiently suppress heat loss to the surroundings, leading to a lower thermal conductivity corresponding to the results of Fig. 5. Additionally, these micropores 4
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Fig. 4. (a–b) Graphical illustration of the structure evolutions during the chemical treating process of ⅰ) MP; ⅱ) pristine PU; ⅲ) PBP.
and materials reported earlier, Table S2), much higher than that of PBPR1 (1.45 kg m−2). While the mass change for the PBP-R2 is about 1.61 kg m−2, which is slightly lower than that of PBP-R1.5 (1.66 kg m−2). PBP-R1.5 exhibits the highest mass change, revealing the optimal evaporative capacity. In practical terms, the intensity of sunlight is usually lower than 1 kW m−2. To verify the practicability of PBP-R1.5, we have also operate the SWP process under a lower irradiation intensity. Fig. 7d shows the evaporation rates and efficiencies of PBP-R1.5 under different illumination intensities. The evaporation rates of PBP-R1.5 under 0.4, 0.6, 0.8 and 1 sun are 0.36, 0.75, 1.19 and 1.66 kg m−2 h−1, respectively. With the decreases of the illumination intensity, the evaporation rate of PBP-R1.5 (1.19 kg m−2 h−1) under 0.8 sun still remains much higher than that of water (0.18 kg m−2 h−1) under 1 sun. The evaporation efficiency can be calculated based on the following equation [35]:Where ϑ represents the evaporation rate excluding natural evaporation (0.18 kg m−2 h−1); hlv is the total enthalpy of liquid-gas phase change, including the two parts of sensible heat and phase-change enthalpy, that's 2256 kJ kg−1; qi is the optical concentration. Then the evaporation efficiency of PBP-R1.5 can be calculated to be 28.1%, 59.4%, 78.9% and 92.9% (excluding natural evaporation) under 0.4, 0.6, 0.8 and 1 sun, respectively (Fig. 7d), revealing a great promise in practical application.
corresponding to the results of Fig. 5a. After 60 min illumination, mass change of PU, MP and PBP can reach up to 0.96, 1.19 and 1.28 kg m−2, respectively, much higher than that of water (0.18 kg m−2), sufficiently demonstrating that heat localization can improve the evaporation performance, consistent with previous research results [7–9], [15,46]. In addition, compared with PU and MP, PP exhibited the highest mass change, revealing the best heat localization of PP. To further explore the evaporative capacity of PBP-Rx, evaporation behaviors of PBP-Rx were investigated. Fig. 7b shows the maximum surface temperatures of PBP-Rx under 1 sun illumination. Under irradiation, as the heat accumulates, temperature keeps rising, while the generated steam becomes more and more. Compared with water, surface temperatures of PBP-Rx show rapid increases within the initial 5 min and slowly reach the maximum values within 50 min and then keep stable. Among these samples, PBP-R1.5 displays the highest surface temperatures, ascribing to the tip thermal effects of chrysanthemum-like Bi2S3, corresponding to the results of Figs. S6b–c. Then the mass change of PBP-Rx under 1 sun illumination was further studied. As illustrated in Fig. 7c, the mass change of PBP-R0.5, R1, R1.5, R2 and R4 under 1 sun illumination can reach up to 1.36, 1.45, 1.66, 1.61 and 1.41 kg m−2, respectively. Obviously, the mass change for PBP-R1.5 reaches up to 1.66 kg m−2 (superior to other different systems
Fig. 5. (a) Maximum surface temperatures of water, PU, MP, PP under one sun illumination; (b) Surface temperatures distributions of SWP device (ⅰ) without and (ⅱ) with PU, (ⅲ) with MP (ⅳ) with PP after 1 h illumination were displayed by IR camera. 5
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Fig. 6. Thermal conductivity of (a) PU and (b) PBP-R1.5; The representative image of temperature gradient along the thickness of PU and PBP-R1.5 (inset). η = ϑ⋅hlv / qi
(1)
and evaporation [46], thus promoting the solar water evaporation. Consequently, an efficient SWP device can be obtained. Durability of SWP devices also plays a vital role in practical application. To demonstrate the durability of PBP-R1.5, we designed cycling test and some strength tests. The experiment of solar water purification was cycled for 20 times under the same condition. As presented in Fig. 7e, the results basically kept stable at 1.64 kg m−2 h−1 without damaging its structure nor changing its floating state. And after cycling test, no obvious component shedding was observed from the inset optical images, verifying the excellent reusability and structure stability of PBP-R1.5. The mechanical stability was further investigated via the compressive strength test. Fig. 7f displayed the compressive stress–strain curve of PU and PBP-R1.5. As illustrated, PBP-R1.5 exhibited breaking strength of 181.04 kPa, while that of PU was measured to be
The excellent evaporation performance of PBP-R1.5 mainly stems from the systematic designs of light harvest, heat management, water diffusion, and evaporation. The intrinsic light harvest and tip thermal effects of chrysanthemum-like Bi2S3 and the multiple internal reflections of microporous structures of BP lead to an efficient utilization of incident light and a high level of solar thermal conversion. Owing to the low thermal conductivity of PU matrix and the microporous composites covered on BP skeleton surface, most of the heat generated by solar thermal conversion can be concentrated on the surface of PBP-R1.5, only causing ~1% radiation heat loss, ~1% convection heat loss and ~5% conduction heat loss (The detailed calculation can be found in the section of the analysis of heat loss in the supporting information). On the other hand, numerous capillary areas stemming from micron channels covered on BP skeleton surface can facilitate water diffusion
Fig. 7. (a) Mass change of SWP device with PU, MP and PP under 1 sun illumination, respectively; (b) Maximum surface temperatures of PBP-Rx under 1 sun illumination; Surface temperatures distributions of SWP device with PBP-R1.5 (ⅰ) before and (ⅱ) after 1 h illumination were monitored by IR camera (inset); (c) Mass change of SWP device with PBP-Rx under 1 sun illumination; (d) Evaporation rates and efficiencies of SWP device with PBP-R1.5 at different illumination intensities; (e) Evaporation cycle performance of SWP device with PBP-R1.5 under 1 sun illumination; Optical images of floating PBP-R1.5 before and after cycling test (inset); (f) The compressive stress–strain curve of PU and PBP-R1.5. 6
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181.72 kPa. After the coverage of porous structures upon the skeleton surface of PU, the breaking strength of PBP-R1.5 shows no remarkable decrease, further proving the mechanical stability of PBP-R1.5. In addition, as shown in the optical images of Figs. S11a–b, after bending and compression, PBP-R1.5 didn't exhibit noticeable damages, further confirming the mechanical stability of PBP-R1.5. Additionally, chemical stability was verified via the 24 h immersion in 2 M HCl and 2 M KOH aqueous solution. There no obvious impairment on structures nor floating states were observed (Fig. S11c). Additionally, evaporative capability of PBP-R1.5 in 2 M HCl aqueous solution was studied. One can find that fine droplets condense on the colder surface within 30 s under 6 sun irradiation (Video S2), revealing the chemical stability of PBP-R1.5. The durability, mechanical and chemical stability of PBPR1.5 make it possible to put it into the application on a large scale and portable use in SWP. Supplementary video related to this article can be found at https:// doi.org/10.1016/j.solmat.2019.110127
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4. Conclusion In summary, we have demonstrated a self-floating all-in-one device for SWP utilizing PU via a simple nonsolvent-induced phase-inversion approach. With microporous insulated PU matrix as a support, microporous BP composites covering on the skeleton surface of PU as water diffusion channels and evaporator, PBP-R1.5 composites show a great potential for the practical application in SWP. Owing to multiple internal reflections of microporous BP composites and tip thermal effects of chrysanthemum-like Bi2S3, incident light could be efficiently used to generate heat through solar thermal conversion. The capillarity caused by numerous micropores of BP composites could facilitate water diffuse from bulk water to the surface of evaporator and reduce the heat loss to the surroundings coupled with the low thermal conductivity of porous PU skeleton. In addition, these micropores of BP composites provide numerous evaporating areas, thus accelerating water evaporation. Consequently, PBP composites exhibit a high evaporation rate up to 1.66 kg m−2 h−1 with a solar thermal efficiency of 92.9% (excluding natural evaporation) under 1 sun illumination. The demonstrated approach to enhance solar driven evaporation efficiency is simplified, cost-effective, reusable and scalable, showing a potential to put into the application on a large scale and portable use. This approach can be further investigated to develop a portable device for SWP with high performance and stability, extending the applications of PP composites in other fields. Acknowledgements This work is financially supported by the Ministry of Science and Technology of China (Grant 2016YFA0200200) and Wuhan Science and Technology Bureau of China (2018010401011280). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solmat.2019.110127. References [1] Y. Li, M. Payo, S. Singh, J. Chen, F. Duerinckx, A. Hajjiah, J. Poortmans, Optical and electrical performance of rear side epitaxial emitters for bifacial silicon solar cell application, Sol. Energy Mater. Sol. Cells 195 (2019) 43–48 https://doi.org/10. 1016/j.solmat.2019.01.051. [2] S. Ullah, H. Ullah, F. Bouhjar, M. Mollar, B. Marí, Synthesis of in-gap band CuGaS2: Cr absorbers and numerical assessment of their performance in solar cells, Sol. Energy Mater. Sol. Cells 180 (2018) 322–327 https://doi.org/10.1016/j.solmat. 2017.06.062. [3] K. Sakimoto, A. Wong, P. Yang, Self-photosensitization of nonphotosynthetic bacteria for solar-to-chemical production, Science 351 (2016) 74–77 https://doi.org/ 10.1126/science.aad3317.
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Self-floating Bi2S3/poly (vinylidene fluoride) composites on polyurethane sponges for efficient solar water purification
T
Haiyan Cheng, Xinghang Liu, Lixing Zhang, Baofei Hou, Fang Yu, ZhuoXun Shi, Xianbao Wang∗ Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry-of-Education Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei Key Laboratory of Polymer Materials (Hubei University), School of Materials Science and Engineering, Hubei University, Wuhan, 430062, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Solar energy Solar thermal conversion Water purification Bi2S3/poly (vinylidene fluoride) composites Multiple internal reflections
Solar water purification (SWP) has been considered as one of the most sustainable techniques to address clean water shortage problem. Although significant strategies have been proposed to explore portable SWP device, many challenges, such as complicated fabrication, high costs and poor practicability, still remain to be solved. Here, a facilely and integrally designed Bi2S3/poly (vinylidene fluoride) (BP) composites based on polyurethane (PU) Sponges (PBP composites) is reported. The PBP composites consist of integrated structures, in which multiple internal reflections guarantee sufficient harvest of incident light and tip thermal effects of chrysanthemum-like Bi2S3 ensure efficient solar thermal conversion, and porous BP endows it with adequate water supply, thus exhibiting a high water evaporation rate of 1.66 kg m−2 h−1 and a solar thermal efficiency of 92.9% under one sun irradiation. This bionic optical design and heat management design may provide a new inspiration to utilize solar energy in water purification and other related applications.
1. Introduction Accompany with the energy crisis and environmental pollution, the utilization of solar energy has drawn great attention for its sustainability and abundant reserves. Up to now, several techniques relating to the utilization of solar energy have been exploited, involving solar driven power generation [1,2], photo-bioenergy conversion [3,4], photo chemical conversion [5,6] and solar thermal conversion [7–10]. Among them, solar thermal conversion is widely acknowledged as the most efficient one, which can also be applied to water purification and address clean water resource shortage issue. Over the years, to enhance the evaporation efficiency of solar water purification (SWP) device, various absorbers, such as polymers [11], metallic plasmonic materials [12,13] and carbon-based materials [14–18] have been investigated in suspending systems [19,20] or floating systems [7], [21–24]. Recently, semiconductors have attracted people's attention with respect to the application in solar thermal conversion, due to their low cost, low cytotoxicity [25] and their noticeable solar thermal performance stemmed from their free carrierinduced localized surface plasmon resonance (LSPR) effect [26–30]. Among them, Bi2S3, as a narrow band gap semiconductor, has been applied to the field of solar thermal therapy [31], achieving the therapeutic effects via the heat generated by electron-hole nonradiative
∗
recombination under light irradiation [32–34], showing a great application prospect. Besides, the structure design of SWP device is also vital for the promotion of evaporation efficiency [35–39]. Many attempts have been made to enhance water evaporation by optimizing the structure design [40,41], involving the designs of membrane absorbers[ [13], [16–18], [42], graphene aerogels [43,44] and various hydrogels [45–47], but many challenges, such as complicated fabrication approaches, high cost and poor practicability, still remain to be solved. In this work, we design a self-floating all-in-one Bi2S3/poly (vinylidene fluoride) (BP) composites based on polyurethane Sponges (PBP composites). As illustrated in Fig. 1, the upper layer is BP composites, as a light harvester and a water evaporator. The synergistic effect of tip thermal effects (The physical phenomenon in which heat is concentrated in a separate object toward the tip (the tip of the geometry)) and multiple internal reflections [36] of chrysanthemum-like Bi2S3 guarantees efficient utilization of incident light of upper layer. While the bottom layer is pristine PU as a support and heat insulator. Low density and low thermal conductivity of pristine PU ensure it a good heat insulating support. As a result, the PBP-R1.5 composites possess a high evaporation performance up to 1.66 kg m−2 h−1 under one sun illumination and excellent mechanical stability, showing a great potential in the practical application of SWP.
Corresponding author. E-mail address: [email protected] (X. Wang).
https://doi.org/10.1016/j.solmat.2019.110127 Received 24 April 2019; Received in revised form 11 August 2019; Accepted 13 August 2019 0927-0248/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Graphical illustration of PBP composites for solar water purification.
2. Experimental
for at least 3 h, and washed with deionized water for further use.
2.1. Reagents and materials
2.5. Bi2S3/PVDF (BP) composites based on PU sponges (PBP-Rx)
The polyurethane (PU) Sponges was purchased from Guangzhou Sheng Li Foam Rubber Products co., Ltd. The specific property parameters are shown in Table S1. Bismuth nitrate pentahydrate [Bi (NO3)3·5H2O] was purchased from Shanghai Macklin Biochemical Co., Ltd. poly(vinylidene fluoride) (PVDF) was obtained from arkema (France), Thioacetamide (C2H5NS), polyvinylpyrrolidone K30 [(C6H9NO)n] (PVP), urea (H2NCONH2), N,N-dimethylformamidel (C3H7NO, DMF), glacial acetic acid (C2H4O2), hydrochloric acid (HCl), potassium hydroxide (KOH) and ethanol (C2H5O) were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were analytical reagent and were used without further purification.
The nonsolvent-induced phase-inversion approach was applied to fabricate PBP-Rx composites (Rx was the as-obtained Bi2S3 with different molar ratio of S/Bi). The as-prepared Rx was respectively dispersed in DMF with ultrasound, forming uniform Rx suspension with a concentration of 5 mg mL−1 (The details for determination of the optimum amount of Bi2S3 is shown in Fig. S4). Subsequently, PVDF was added, and the solution was magnetically stirred to form a homogeneous slurry with a mass percent of 8 wt%. After that the PVDF-Rx slurry was painted on the prewashed PU (with a thickness of 10 mm) by dropper. Then, the PU painted with PVDF-Rx slurry was dipped in ethanol for at least 3 h, and washed with deionized water for further use (Fig. S1 Route 2).
2.2. Chrysanthemum-like Bi2S3 2.6. Characterization Chrysanthemum-like Bi2S3 samples were synthesized using a facile one-step solvent-thermal method. Bi (NO3)3·5H2O (0.485 g) was dissolved in an appropriate amount of acetic acid, then the solution was diluted to 80 mL. Subsequently, C2H5NS of various corresponding molar mass, H2NCONH2 (0.6 g) and PVP (0.8 g) were added, and the solution was magnetically stirred to form a homogeneous solution. After that, the obtained solution was transferred to a Teflon-lined stainless steel autoclave and treated at 160 °C for 24 h. The precipitates were collected by centrifugation and washed with ethanol, deionized water, and dried in vacuum at 60 °C. Here, we denote Rx as the corresponding products, x refers to the molar ratio of S/Bi, i.e., R0.5, R1, R1.5, R2, R4, respectively.
The morphologies of samples were observed by field-emission scanning electron microscope (FE-SEM) (Sigma 500, Zeiss, Germany) and scanning electron microscope (SEM) (JSM-6510LV, Japan, Jeol Ltd.). The X-ray diffraction (XRD) patterns were obtained from an X-ray diffractometer (D8-advance, Bruker, Germany). The thermogravimetric analysis (TGA) data was implemented by a thermal analyzer (TGA1, Mettler-Toledo, Switzerland). The optical absorption, reflectivity and transmission spectra were obtained by a UV–vis–NIR spectrophotometer (UV-3600, Shimadu, Japan), with fine BaSO4 powder as reference. An integrated heating platform (JF-956S, Dongguan) was used to measure thermal conductivity and the temperature was measured by a thermocouple thermometers (HT-9815, Dongguan Xintai Instrument Co., Ltd.). The temperature distributions and surface temperatures were recorded by an IR camera (Flir-E4, Flir, Sweden) (The calibration of the temperature measurement is displayed in Fig. S5). The solar illumination was simulated by an xenon lamp (CEL-HXF300, Beijing) with an AM1.5G optical filter. The illumination intensity of the samples surfaces were measured by a solar power meter (SM206, Shenzhen). The real-time mass of water over the entire evaporation was tracked by a computer controlled electronic balance (JJ223BC, Changshu) with accuracy of 1 mg. Double column universal material testing machine (Instron3360, American Instron) was used to investigate tensile properties of samples.
2.3. Modified PU (MP) For the preparation of MP, a piece of PU with a diameter of 38 mm and a thickness of 10 mm was dipped with 3 mL DMF in a petri dish for several minutes until the DMF was uniformly absorbed on one of the surfaces of PU. Subsequently, DMF was extracted with water. Then the obtained PU was immersed in water for at least 24 h for further use (Fig. S1 Route 1). 2.4. PVDF-PU composites (PP) The nonsolvent-induced phase-inversion approach [26] was applied to fabricate PP composites. PVDF was dissolved in DMF, and the solution was magnetically stirred to form a homogeneous slurry with a mass percent of 8 wt% (The details for determination of the optimum dosage of PVDF is shown in Fig. S2). After that the PVDF slurry was painted on the prewashed PU (with a thickness of 10 mm, the details for determination of the optimum thickness of PU is shown in Fig. S3) by a dropper. Then, the PU painted with PVDF slurry was dipped in ethanol
2.7. Solar water purification To explore the solar thermal conversion ability of samples, evaporation performances were investigated under the simulated intensity of 1 kW m−2 and lower irradiation intensity over 60 min. All samples with a diameter of 38 mm were floated on the surface of water in the beaker wrapped with polystyrene foam, serving as a thermal barrier to 2
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means that the phase of R0.5 is mainly bismutite and prefers to form into orthorhombic Bi2S3. And characteristic peaks of both bismutite and orthorhombic Bi2S3 can be observed from XRD pattern of R1. The diffraction peaks observed at 28.6°, 25.0° are stronger than that of R0.5, while a weaker diffraction peak at 30.2°, revealing the formation of orthorhombic Bi2S3 in R1. Comparing the patterns of all samples, one can find that the diffraction intensity of R1.5 of the orthorhombic Bi2S3 phase becomes stronger as the molar ratio of S/Bi increases. But there no obvious differences at the diffraction intensity are observed along with further increasing of molar ratio of S/Bi. These results show that with the increase of molar ratio of S/Bi, bismuthite is firstly formed in the product, and then converted into orthorhombic Bi2S3. When the ratio is higher than 1.5, there is no new phase be formed. As illustrated in Fig. 3a, pristine PU consists of microporous structures, leading to a low density and providing channels for steam escape. And the low density of PU makes it a self-floating device, reducing direct contact with water and heat loss to bulk water[ [14–16,46]. Consequently, pristine PU exhibits good heat insulation, and can concentrate heat on the air-water interface which can be confirmed in Figs. S8a–b, thus making it possible for the application of PU in SWP device. While the skeleton surface of pristine PU was too smooth (Fig. 3b–c)) to satisfy the requirement of timely water supply in the process of SWP. Therefore, some modifications were made to optimize it (Fig. S1). Fig. 4 shows the structures evolutions of samples during the chemical treating process. After optimization, MP failed to maintain its pristine shape and floating state, while PBP can free-float on the surface of water (Figs. S9a–c) and form an air-water interface in favour of heat localization (Fig. S8c). Besides, only a few micropores can be observed at the smooth skeleton surface of MP, while the skeleton of PBP was covered with microporous structures (Fig. 3 d-l). According to previous researches, open porous structures generally facilitate water diffusion and steam escape [9], [36,39,42], [49]. Additionally, microporous structures of PBP can divide bulk water into numerous capillary areas [24], thus promoting the water diffusion through capillarity. And one can find that PBP device can well acclimatize itself to the undulating water (Video S1), almost exhibited no impacts on its floating state. Supplementary video related to this article can be found at https:// doi.org/10.1016/j.solmat.2019.110127 Fig. 2b shows the TGA patterns of PU, PVDF, R1.5 and PBP-R1.5. At the heating phase from 100 to 230 °C, all samples show a slight weight loss, resulting from the evaporation of physically adsorbed water molecules and the removal of small molecule substance from their surface. The following phase of weight loss for PU starts from 230 °C to 410 °C, while 410 °C–450 °C for R1.5 and 450 °C–600 °C for PVDF. In conclusion, the weight loss of PBP-R1.5 could be attributed to the decomposition of PU (from 230 °C to 410 °C), R1.5 (from 410 °C to 450 °C) and PVDF (from 450 °C to 600 °C), revealing the coexistence of PVDF, R1.5
reduce heat loss to beaker wall. All samples were pre-wetted before floated on the surface of water. A xenon lamp with an AM1.5G optical filter was utilized to simulate the solar illumination, while the illumination intensity of samples’ surface was measured by a solar power meter. A computer controlled electronic balance was utilized to track the real-time mass change of samples under irradiation. And maximum asurface temperatures were measured by an IR camera. To further investigate the durability of samples, the SWP process was cycled for 20 times under the illumination intensity of 1 kW m−2. After each cycle test, the SWP system was shut down for at least 20 min to ensure a consistent initial state of the system. 3. Results and discussion As illustrated in Fig. S6a, the fabrication of chrysanthemum-like Bi2S3 is based on the PVP assisted self-assembly of Bi2S3 nanoribbons in water. During the heating of the precursor, S2− species released through the hydrolysis of thioacetamide, combined with Bi3+ species in a certain proportion and condensed into randomly distributed Bi2S3 nanoribbons via an oriental attachment mechanism. Then the randomly distributed Bi2S3 nanoribbons further self-assembled into chrysanthemum-like architecture. In this fabrication process, PVP served as proton scavenger by consuming excessive acid and facilitated the selfassembly of Bi2S3 nanoribbons by controlling the electrostatic interaction [48] among nanoribbons. At the end of hydrothermal reaction, the chrysanthemum-like Bi2S3 consisting of nanoribbons was obtained (Fig. S6a). The morphology of the as-obtained Rx was characterized by FESEM. As shown in Fig. S7, R0.5 consists of irregular nanoplates with the thickness of ~20 nm. As the molar ratio of S/Bi increases, the size of nanoplates observed in R1 are much smaller than that of R0.5, and nanoribbons begin to be formed. When the ratio increases to 1.5, the FE-SEM images display an obvious chrysanthemum-like morphology, consisting of much smaller nanoribbons. Compared with other samples, chrysanthemum-like morphology of R1.5 is conducive to minimizing the reflection and transmission of incident light and tip thermal effects, stemming from the morphology of nanoribbons, contribute to efficient solar heat conversion (Figs. S6b–c). With further increase of molar ratio of S/Bi, chrysanthemum-like Bi2S3 was gradually converted into amorphous blocks, until the whole nanoribbons were converted into amorphous Bi2S3. Only when the ratio was 1.5, the obtained products exhibited obvious chrysanthemum-like morphology. The crystallization nature of Rx was investigated by XRD. All Rx samples are crystalline with strong diffraction peaks. As presented in Fig. 2a, the diffraction peaks of R0.5 can be well indexed to the standard profile of bismutite (JCPDS No. 41–1488) with the major peaks, inspite of the diffraction peak observed at 28.6° (indexed to the standard profile of orthorhombic phase Bi2S3, JCPDS No. 17–0320), which
Fig. 2. (a) XRD patterns of Rx; (b) The TGA of PU, PVDF, R1.5 and PBP-R1.5. 3
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Fig. 3. The SEM images for (a–c) pristine PU (d–f) MP (g–i) upper surface of PBP-R1.5 (j–l) cross section of PBP-R1.5.
provide numerous effective evaporating areas, thus reducing radiation and convection heat losses at the evaporator surface, resulting in an efficient heat management. Additionally, as illustrated in Fig. S10, the reflectivity of PBP-R1.5 is ~1.5% in the solar spectrum (200–2000 nm), while the transmission of PBP-R1.5 is ~1.5% in the solar spectrum (200–2000 nm), both of which are much lower than those of the PU and R1.5. It can be estimated that ~97% of the irradiated solar power can be absorbed by PBP-R1.5 composites (Note S) [50], indicating that the structure design of PBP composites can improve the light harvest, leading to a higher utilization of incident light. To verify the evaporation performance of PU, MP and PP, we investigated their behaviors by utilizing xenon lamp with an AM1.5G optical filter to simulate solar illumination and tracing the weight loss of water in the process of evaporation. Fig. 7a shows the mass change of pristine PU, MP and PBP under the illumination intensity of 1 kW m−2. At the beginning, a large number of heat generated via solar thermal conversion is lost to the surroundings, resulting in lower surface temperatures and evaporation rate. During the constant irradiation, the heat at evaporation surface continuously accumulates, leading to much higher surface temperatures of samples,
in the PU matrix of PBP-R1.5. Heat management plays an essential role in the process of SWP. Here the maximum surface temperatures of samples under 1 sun illumination were recorded by a thermocouple thermometers. As illustrated in Fig. 5a, surface temperatures of all samples show rapid increases within initial 5 min and then slowly reach the maximum values within 50 min and keep constant. IR camera was utilized to display the surface temperatures distributions after 60 min irradiation (Fig. 5b). Surface temperatures of PU, MP and PP were much higher than that of water. While the modified one exhibited higher surface temperatures, and PP possessed the highest (The slightly higher initial temperature of MP is mainly ascribed to the higher light harvest of activated carbon within pristine PU.), revealing the best heat localization of PP. The efficient heat management of PP mainly stems from the intrinsic low thermal conductivity. Thermal conductivity of pristine PU was measured to be 1.339 W m−1 K−1, while that of PBP-R1.5 was 0.861 W m−1 K−1 (Fig. 6a–b). Compared with pristine PU, the micropores covered on the PBP skeleton surface can efficiently suppress heat loss to the surroundings, leading to a lower thermal conductivity corresponding to the results of Fig. 5. Additionally, these micropores 4
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Fig. 4. (a–b) Graphical illustration of the structure evolutions during the chemical treating process of ⅰ) MP; ⅱ) pristine PU; ⅲ) PBP.
and materials reported earlier, Table S2), much higher than that of PBPR1 (1.45 kg m−2). While the mass change for the PBP-R2 is about 1.61 kg m−2, which is slightly lower than that of PBP-R1.5 (1.66 kg m−2). PBP-R1.5 exhibits the highest mass change, revealing the optimal evaporative capacity. In practical terms, the intensity of sunlight is usually lower than 1 kW m−2. To verify the practicability of PBP-R1.5, we have also operate the SWP process under a lower irradiation intensity. Fig. 7d shows the evaporation rates and efficiencies of PBP-R1.5 under different illumination intensities. The evaporation rates of PBP-R1.5 under 0.4, 0.6, 0.8 and 1 sun are 0.36, 0.75, 1.19 and 1.66 kg m−2 h−1, respectively. With the decreases of the illumination intensity, the evaporation rate of PBP-R1.5 (1.19 kg m−2 h−1) under 0.8 sun still remains much higher than that of water (0.18 kg m−2 h−1) under 1 sun. The evaporation efficiency can be calculated based on the following equation [35]:Where ϑ represents the evaporation rate excluding natural evaporation (0.18 kg m−2 h−1); hlv is the total enthalpy of liquid-gas phase change, including the two parts of sensible heat and phase-change enthalpy, that's 2256 kJ kg−1; qi is the optical concentration. Then the evaporation efficiency of PBP-R1.5 can be calculated to be 28.1%, 59.4%, 78.9% and 92.9% (excluding natural evaporation) under 0.4, 0.6, 0.8 and 1 sun, respectively (Fig. 7d), revealing a great promise in practical application.
corresponding to the results of Fig. 5a. After 60 min illumination, mass change of PU, MP and PBP can reach up to 0.96, 1.19 and 1.28 kg m−2, respectively, much higher than that of water (0.18 kg m−2), sufficiently demonstrating that heat localization can improve the evaporation performance, consistent with previous research results [7–9], [15,46]. In addition, compared with PU and MP, PP exhibited the highest mass change, revealing the best heat localization of PP. To further explore the evaporative capacity of PBP-Rx, evaporation behaviors of PBP-Rx were investigated. Fig. 7b shows the maximum surface temperatures of PBP-Rx under 1 sun illumination. Under irradiation, as the heat accumulates, temperature keeps rising, while the generated steam becomes more and more. Compared with water, surface temperatures of PBP-Rx show rapid increases within the initial 5 min and slowly reach the maximum values within 50 min and then keep stable. Among these samples, PBP-R1.5 displays the highest surface temperatures, ascribing to the tip thermal effects of chrysanthemum-like Bi2S3, corresponding to the results of Figs. S6b–c. Then the mass change of PBP-Rx under 1 sun illumination was further studied. As illustrated in Fig. 7c, the mass change of PBP-R0.5, R1, R1.5, R2 and R4 under 1 sun illumination can reach up to 1.36, 1.45, 1.66, 1.61 and 1.41 kg m−2, respectively. Obviously, the mass change for PBP-R1.5 reaches up to 1.66 kg m−2 (superior to other different systems
Fig. 5. (a) Maximum surface temperatures of water, PU, MP, PP under one sun illumination; (b) Surface temperatures distributions of SWP device (ⅰ) without and (ⅱ) with PU, (ⅲ) with MP (ⅳ) with PP after 1 h illumination were displayed by IR camera. 5
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Fig. 6. Thermal conductivity of (a) PU and (b) PBP-R1.5; The representative image of temperature gradient along the thickness of PU and PBP-R1.5 (inset). η = ϑ⋅hlv / qi
(1)
and evaporation [46], thus promoting the solar water evaporation. Consequently, an efficient SWP device can be obtained. Durability of SWP devices also plays a vital role in practical application. To demonstrate the durability of PBP-R1.5, we designed cycling test and some strength tests. The experiment of solar water purification was cycled for 20 times under the same condition. As presented in Fig. 7e, the results basically kept stable at 1.64 kg m−2 h−1 without damaging its structure nor changing its floating state. And after cycling test, no obvious component shedding was observed from the inset optical images, verifying the excellent reusability and structure stability of PBP-R1.5. The mechanical stability was further investigated via the compressive strength test. Fig. 7f displayed the compressive stress–strain curve of PU and PBP-R1.5. As illustrated, PBP-R1.5 exhibited breaking strength of 181.04 kPa, while that of PU was measured to be
The excellent evaporation performance of PBP-R1.5 mainly stems from the systematic designs of light harvest, heat management, water diffusion, and evaporation. The intrinsic light harvest and tip thermal effects of chrysanthemum-like Bi2S3 and the multiple internal reflections of microporous structures of BP lead to an efficient utilization of incident light and a high level of solar thermal conversion. Owing to the low thermal conductivity of PU matrix and the microporous composites covered on BP skeleton surface, most of the heat generated by solar thermal conversion can be concentrated on the surface of PBP-R1.5, only causing ~1% radiation heat loss, ~1% convection heat loss and ~5% conduction heat loss (The detailed calculation can be found in the section of the analysis of heat loss in the supporting information). On the other hand, numerous capillary areas stemming from micron channels covered on BP skeleton surface can facilitate water diffusion
Fig. 7. (a) Mass change of SWP device with PU, MP and PP under 1 sun illumination, respectively; (b) Maximum surface temperatures of PBP-Rx under 1 sun illumination; Surface temperatures distributions of SWP device with PBP-R1.5 (ⅰ) before and (ⅱ) after 1 h illumination were monitored by IR camera (inset); (c) Mass change of SWP device with PBP-Rx under 1 sun illumination; (d) Evaporation rates and efficiencies of SWP device with PBP-R1.5 at different illumination intensities; (e) Evaporation cycle performance of SWP device with PBP-R1.5 under 1 sun illumination; Optical images of floating PBP-R1.5 before and after cycling test (inset); (f) The compressive stress–strain curve of PU and PBP-R1.5. 6
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181.72 kPa. After the coverage of porous structures upon the skeleton surface of PU, the breaking strength of PBP-R1.5 shows no remarkable decrease, further proving the mechanical stability of PBP-R1.5. In addition, as shown in the optical images of Figs. S11a–b, after bending and compression, PBP-R1.5 didn't exhibit noticeable damages, further confirming the mechanical stability of PBP-R1.5. Additionally, chemical stability was verified via the 24 h immersion in 2 M HCl and 2 M KOH aqueous solution. There no obvious impairment on structures nor floating states were observed (Fig. S11c). Additionally, evaporative capability of PBP-R1.5 in 2 M HCl aqueous solution was studied. One can find that fine droplets condense on the colder surface within 30 s under 6 sun irradiation (Video S2), revealing the chemical stability of PBP-R1.5. The durability, mechanical and chemical stability of PBPR1.5 make it possible to put it into the application on a large scale and portable use in SWP. Supplementary video related to this article can be found at https:// doi.org/10.1016/j.solmat.2019.110127
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4. Conclusion In summary, we have demonstrated a self-floating all-in-one device for SWP utilizing PU via a simple nonsolvent-induced phase-inversion approach. With microporous insulated PU matrix as a support, microporous BP composites covering on the skeleton surface of PU as water diffusion channels and evaporator, PBP-R1.5 composites show a great potential for the practical application in SWP. Owing to multiple internal reflections of microporous BP composites and tip thermal effects of chrysanthemum-like Bi2S3, incident light could be efficiently used to generate heat through solar thermal conversion. The capillarity caused by numerous micropores of BP composites could facilitate water diffuse from bulk water to the surface of evaporator and reduce the heat loss to the surroundings coupled with the low thermal conductivity of porous PU skeleton. In addition, these micropores of BP composites provide numerous evaporating areas, thus accelerating water evaporation. Consequently, PBP composites exhibit a high evaporation rate up to 1.66 kg m−2 h−1 with a solar thermal efficiency of 92.9% (excluding natural evaporation) under 1 sun illumination. The demonstrated approach to enhance solar driven evaporation efficiency is simplified, cost-effective, reusable and scalable, showing a potential to put into the application on a large scale and portable use. This approach can be further investigated to develop a portable device for SWP with high performance and stability, extending the applications of PP composites in other fields. Acknowledgements This work is financially supported by the Ministry of Science and Technology of China (Grant 2016YFA0200200) and Wuhan Science and Technology Bureau of China (2018010401011280). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solmat.2019.110127. References [1] Y. Li, M. Payo, S. Singh, J. Chen, F. Duerinckx, A. Hajjiah, J. Poortmans, Optical and electrical performance of rear side epitaxial emitters for bifacial silicon solar cell application, Sol. Energy Mater. Sol. Cells 195 (2019) 43–48 https://doi.org/10. 1016/j.solmat.2019.01.051. [2] S. Ullah, H. Ullah, F. Bouhjar, M. Mollar, B. Marí, Synthesis of in-gap band CuGaS2: Cr absorbers and numerical assessment of their performance in solar cells, Sol. Energy Mater. Sol. Cells 180 (2018) 322–327 https://doi.org/10.1016/j.solmat. 2017.06.062. [3] K. Sakimoto, A. Wong, P. Yang, Self-photosensitization of nonphotosynthetic bacteria for solar-to-chemical production, Science 351 (2016) 74–77 https://doi.org/ 10.1126/science.aad3317.
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