Macroporous Double-Network Hydrogel for High-Efficiency Solar

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Energy, Environmental, and Catalysis Applications

Macroporous double-network hydrogel for high-efficiency solar steam generation under one-sun illumination Xiangyu Yin, Yue Zhang, Qiuquan Guo, Xiaobing Cai, Junfeng Xiao, Zhifeng Ding, and Jun Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01629 • Publication Date (Web): 13 Mar 2018 Downloaded from http://pubs.acs.org on March 14, 2018

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ACS Applied Materials & Interfaces

Macroporous double-network hydrogel for high-efficiency solar steam generation under one-sun illumination Xiangyu Yin†,#, Yue Zhang †,#, Qiuquan Guo†, Xiaobing Cai†, Junfeng Xiao†, Zhifeng Ding‡, Jun Yang*,†

† Department of Mechanical and Materials Engineering, The University of Western Ontario, London, Ontario, N6A 5B9, Canada ‡

Department of Chemistry, The University of Western Ontario, London, Ontario, N6A 5B7, Canada

ABSTRACT Solar steam generation is one of the most promising solar-energy-harvesting technologies to addressing the issue of water shortage. Despite intensive efforts to develop high-efficiency solar steam generation devices, challenges remain in terms of the relatively low solar thermal efficiency, complicated fabrications, high cost, and difficulty in scaling up. Herein, a double-network hydrogel with a porous structure (p-PEGDA-PANi) is demonstrated for the first time as a flexible, recyclable and efficient photothermal platform for low-cost and scalable solar steam generation. As a novel photothermal platform, the p-PEGDA-PANi involves all necessary properties of efficient broadband solar absorption, exceptional hydrophilicity, low heat conductivity and porous structure for high-efficiency solar steam generation. As a result, the hydrogel-based solar steam generator exhibits a maximum solar thermal efficiency of 91.5% with an evaporation rate of 1.40 kg m−2 h−1 under one-sun illumination, comparable to state-of-the-art solar steam generation devices. Furthermore, the good durability and environmental stability of p-PEGDA-PANi hydrogel enables a convenient recycling and reusing process toward real life applications. The present research not only provides a novel photothermal platform for solar energy harvest, but also opens a new avenue for application of the hydrogel materials in solar steam generation.

KEYWORDS: macroporous structure, double-network hydrogel, PANi nanowires, solar steam generation, high efficiency

INTRODUCTION Solar-driven steam generation is emerging as one of the most promising solar-energy-harvesting technologies for its potential applications in water purification, liquid−liquid phase separaƟon and sterilization.1-3 However, its low solar thermal efficiency requires high intensity of illumination, which limits its utilization in real environments with natural light. Undoubtedly, developing broadband and efficient solar absorption materials, localized heat management, rapid water replenish and unimpeded vapor escape channels would be effective approaches to achieving efficient steam generation.4-8 Porous structure will undoubtedly endow photothermal materials with better light and water absorbing abilities,

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and at the same time it will provide a quick escape channel for the generated steam.9-15 Therefore, many photothermal materials with porous structures have been fabricated and applied to solar steam generation. Owing to the broadband absorption nature and excellent chemical stability, carbon-based materials including graphene,16 carbon nanotube17,

18

and graphite19,

20

stand out as photothermal

materials in this research field. However, the fragility and high thermal conductivity of pure carbon materials profoundly limits their uses.10, 21 Noble metal nanoparticles have also been intensively investigated, due to their localized surface plasmon resonance effect22-25 which can help metallic structures harvest light and then rapidly heat up the surroundings. However, to increase the conversion efficiency, these metallic nanoparticles usually need cumbrous morphology regulation to overcome their inherently narrow absorption bandwidth. Besides, pure noble metal nanoparticles are expensive and the aggregation as well as dispersion stability of nanoparticles still need to be investigated. In addition, black inorganic semiconductors, including MXene Ti3C2,13 Cu7S4 nanocrystal,26 TiOx Nanoparticles,12 etc. have been explored as a new kind of solar-thermal absorbers due to low cost and low cytotoxicity.13, 27 However, the challenge for fabrication of large-scale samples limits their practical application. Low cost, simple and scalable fabrication, biocompatibility, environmental friendliness, and good durability are crucial for the practical application of these materials. Therefore, it is necessary to finger out a cost-effective and scalable solution to fabricate high-performance photothermal materials for solar steam generation. Recently, low-cost and easy-to-produce polymeric photothermal materials, such as polypyrrole (PPy)28 and polydopamine (PDA)29, 30, have been found well suited for solar steam generation application and have aroused increasing attention. Among photothermal polymers, polyaniline (PANi) is the first one reported as a photothermal therapeutic agent for cancer therapy,31 and recent studies have shown that it and its composites are promising photothermal platforms for biomedical applications.32,

33

But

unfortunately, it has so far not attracted the attention of scholars in the field of solar steam generation. In this work, polyaniline nanowires (PANi NWs) are rationally chosen as the photothermal material in view of their large specific surface area, broad absorption in the solar spectrum, high photothermal conversion efficiency, and facile, scalable fabrication.31, 34 Besides, the environmental friendliness and stability make PANi NWs promising functional candidates for water treatment.35, 36 PANi is a rigid polymer; when crosslinking it into a soft hydrophilic polymer, the resulting material will be as sturdy and flexible as animal dermis,37 which provides a guarantee for its long-term application in actual solar steam generation. Hydrogel is one of the most common soft hydrophilic materials that can reversibly absorb or release water. Besides, many hydrogels are biocompatible. These properties offer it enormous potential in various applications, such as drug delivery,38 oil separation,39 and water purification.40 In order to improve the water-absorbing rate, macroporous hydrogels are prepared, and the formation of a porous structure enables the rapid uptake of water through capillarity.41, 42 Especially, crosslinked hydrogels are mesh-like hydrophilic structures with high water content and voluminous space in which another polymer can readily crosslink,43 thus providing the possibility to design novel solar steam generation materials by crosslinking photothermal PANi into the hydrogel network. Furthermore, when serving as a photothermal platform, the porous surface of double-network hydrogel can produce multi-scattering of incident solar light, allowing for more incident light to be converted into heat at the


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evaporating surface.21, 44 Hence, in this work, we report a state-of-the-art photothermal, macroporous double-network hydrogel of poly(ethylene glycol) diacrylate (PEGDA) and PANi for high-efficiency steam generation under one-sun illumination. The double-network hydrogel is named p-PEGDA-PANi hydrogel which was prepared via a facile solvent casting/particulate leaching process to obtain porous PEGDA (p-PEGDA) hydrogel, followed by crosslinking PANi NWs, a polymeric photothermal material, into the p-PEGDA hydrogel to form a second PANi network. This fabrication progress is facile, time-saving and cost-efficient. And, the obtained p-PEGDA-PANi hydrogel has been successfully used as bioactive scaffolds in tissue engineering due to the good biocompatibility as well as its diverse abilities to electrically stimulate cells and modulate their functions.45 Here, we utilize the high optical absorption, low thermal conductivity, rapid water supply and unimpeded steam escape of the p-PEGDA-PANi hydrogel, and combine it with a 2D water supply component to achieve highly efficient solar-driven steam generation under one sun illumination.

RESULTS AND DISCUSSION

Figure 1. Schematic illustration of the solar steam generation device. (a) The device consists of expanded polyethylene foam, cellulose coating, and p-PEGDA-PANi hydrogel on top surface. Schematic illustrations of the local light-to heat conversion (b) and the 2D water compensation (c). By integrating the versatile p-PEGDA-PANi hydrogel with the 2D water supply design, high solar thermal efficiency can be realized under one-sun illumination.

Figure 1a shows the structure and working principle of our solar steam generation device in which the p-PEGDA-PANi hydrogel serves as the light absorbing layer, and the cellulose-wrapped expanded polyethylene (EPE) foam serves as both the thermal insulation and the 2D water supply component. When the steam generation device is illuminated by sunlight, the upper surface of the photothermal hydrogel will efficiently convert the strongly absorbed light into localized heat (Figure 1b). After a hot area



is formed, the surface water will evaporate and rapidly escape into the air via the porous structure. As the upper water continuously evaporates, water will be compensated in a timely manner through the 2D water supply path (Figure 1c). Specifically, the first step in the process of water compensation is the water uptake and pump up by the capillary force of the cellulose. Then, the water within the cellulose will be wicked up by its upper hydrogel at the contact interface, and diffuses inside the hydrogel. The whole bottom-up water transport progress ensures the continuous replenishment of water for evaporation.16 Moreover, the good thermal insulation of the porous hydrogel and the EPE foam further confines the photothermal-converted heat at the evaporating surface and consequently improves the steam generation efficiency. Studies have shown that this design can increase heat concentration at the evaporation surface and thus improve solar efficiency from the typical ~50% to higher than 80% under one-sun illumination.11, 16 Therefore, by integrating the versatile p-PEGDA-PANi hydrogel with the 2D water supply design, high solar thermal efficiency is expected from the p-PEGDA-PANi hydrogel-based solar steam generation device.

Figure 2. Schematic of the fabrication process and characterization of the photothermal p-PEGDA-PANi hydrogel. (a) The p-PEGDA-PANi hydrogel was obtained through a sequence of salt leaching, aniline monomer immersing and polymerizing. FE-SEM images of (b-d) p-PEGDA and (e-g) p-PEGDA-PANi hydrogels with different magnifications.


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The fabrication strategy of the photothermal p-PEGDA-PANi hydrogel is schematically shown in Figure 2a, which can be summarized by the following three steps: (I) fabricating the porous polyethylene glycol diacrylate (p-PEGDA) hydrogel via a solvent casting/salt leaching technology;43 (II) immersing the obtained p-PEGDA hydrogel in aniline (An) monomer dissolved 1 M HCl to achieve migration and self-assembly of An within the hydrogel network; (III) immersing the as-prepared p-PEGDA-An in ammonium persulphate (APS) solution for in-situ polymerization of aniline in the first PEGDA network to form a second PANi network. This fabrication progress is facile, time-saving and cost-efficient. More importantly, through this fabrication process, large p-PEGDA-PANi samples of different shapes can be obtained as shown in Figure S1. And this scalable fabrication is an important guarantee of practical application. Figure 2b-d shows the FE-SEM images of the p-PEGDA prepared after dissolution of the salt. Interconnected macroporous structures are displayed, with the pore diameters ranging from 270-325 μm, in good agreement with the diameter of salt particles. Optical images in Figure S2a indicate a loss of transparency in the p-PEGDA hydrogel compared to the non-porous substrate (in other words, solid PEGDA or s-PEGDA), which could be attributed to the enhanced total absorption performance of the porous structure due to reduced surface reflection and increased internal light scattering (Figure S2b and 2c).46-48 The higher magnification FE-SEM images (Figure 2c and 2d) show that the pore wall of p-PEGDA is composed of many wrinkles, which is caused by the rapid water loss during drying at room temperature. During the chemical polymerizing of aniline, PANi NWs (~150 nm in diameter) grow and fill the space of hydrogel network (Figure 2e-g). The porosity of the obtained p-PEGDA-PANi double-network hydrogel (46.0%) is not much different from that of the single-network p-PEGDA (47.1%), which demonstrates that the crosslinking of PANi does not change the dimensions or distribution of the hydrogel porous structure. These indicate that the inclusion of PANi NWs in the p-PEGDA can endow the porous hydrogel with good photothermal properties without weakening its role in enhancing light absorption. The Fourier-transform infrared (FTIR) spectra of p-PEGDA and p-PEGDA-PANi hydrogels are presented in Figure 3a. For both samples, the peaks at 1093 and 1725 cm−1 are assigned to –C–O– symmetric stretching and –C=O stretching of the acetate group of PEGDA. For the p-PEGDA-PANi sample, the peaks at 802, 1172, 1286, 1486 and 1563 cm−1 indicate the successful polymerization of the hydrochloric acid doped PANi in PEGDA hydrogel.49 The peak at 802 cm−1 corresponds to the bending vibrations of C–H bonds within the 1,4-di-substituted aromatic ring. The characteristic absorption around 1172 cm−1 is assigned to the doped state of PANi.50 The band at 1286 cm−1 can be assigned to C-N aromatic stretching vibrations. The characteristic peaks at 1563 and 1488 cm-1 can be assigned to C=C stretching vibrations of the quinoid ring and benzenoid ring, respectively, in the doped PANi.51 Moreover, by comparing the EDX spectra between p-PEGDA-PANi and p-PEGDA hydrogels (Figure S3), we can see clearly that the sulfur (S) and chlorine (Cl) elements appear and uniformly distribute in the EDX mapping images for the p-PEGDA-PANi hydrogel, which further confirms the successful formation of PANi second network in the former p-PEGDA network. Excellent durability is a prerequisite for real-life applications, and thus the mechanical property of the p-PEGDA-PANi hydrogel was evaluated by cyclic stress–strain tests (Figure 3b). Owing to its high porosity and structural flexibility, the hydrogel is able to be compressed to a large strain (ε = 50%) at low



stress (σ≈3.5 kPa). During repeated compressions, the porous hydrogel can recover most of its thickness and maintain a similar compressive stress at maximum strain (ε = 50%) in every cycle, which demonstrates the excellent mechanical stability and good elastic recovery of the p-PEGDA-PANi hydrogel. Meanwhile, no segregation phenomenon occurs between the p-PEGDA substrate and the nanostructured PANi, which indicates excellent mechanical stability even under harsh conditions (50 cycle tests). In fact, the p-PEGDA-PANi hydrogel can immediately recover to its original shape with no plastic deformation after the release of pressure (insets in Figure 3b). From the linear elastic regime of the stress-strain curve during the first cycle of compression, the compressive modulus of the p-PEGDA-PANi hydrogel is estimated to be around 8.9 kPa, which is much lower than those reported for carbon nanotube (CNT) sponge52, 53 and graphene sponge54, 55. In comparison, solid PEGDA (s-PEGDA) and solid PEGDA-PANi (s-PEGDA-PANi) hydrogels show a highly fragile nature and less compressibility than p-PEGDA-PANi hydrogel. The compressive stress–strain curves in Figure S4 illustrate the brittleness of the s-PEGDA and s-PEGDA-PANi hydrogels, and their elastic modulus are calculated to be 3.16 and 3.54 MPa, respectively, three orders of magnitude higher than that of the p-PEGDA-PANi hydrogel. From the above comparisons, we conclude that the porous structure endows PEGDA-PANi hydrogel with good flexibility and mechanical stability. The porous structure also grants the hydrogel rapid water absorption, which is vital for efficient water supply to the solar steam generator.5, 6 The water wetting characteristics of different hydrogels were first tested by tracing the time-dependent movement of Rhodamine B (Rh B) stained DI water dipped on the edge of rectangular samples (2 cm × 1 cm × 0.2 cm). Figure 3c shows that the p-PEGDA hydrogel can adsorb water immediately once dipped in Rh B solution, and it takes about 15 s to be fully wetted. Moreover, the water-pumping behavior of porous hydrogel is less likely to be affected by the doping of PANi NWs. The water uptake rate of the p-PEGDA-PANi hydrogel is calculated to be 263.9 kg m-2 h-1, which is much higher than the steam generation rate under one-sun illumination. This uptake rate ensures timely water supply and prevents drying out of the photothermal hydrogel during the evaporation process. In contrast, a pink water mark is observed at the bottom of the s-PEGDA sample after 15 s of contact with Rh B solution (Figure S5), showcasing the relatively slower water absorption of solid hydrogel compared to the porous ones. This comparision illustrates that the driving force for water absorption in the porous hydrogel is not only owing to its hydrophilicity and the dynamic motion of PEG chains,56 but also by the capillary action of the porous structure. In order to confirm the actual water supply performance of the hydrogel-based solar evaporator, the p-PEGDA and p-PEGDA-PANi hydrogels were placed onto the cellulose-wrapped EPE foam, respectively. And once the foam was put on the surface of water, water was pumped by capillary force through the 2D water path and quickly wet the entire porous hydrogels (Video S1 and S2), indicating a timely water supply to the evaporating surface. Moreover, the porous structure also significantly affects the subsequent equilibrium swelling of hydrogels (Table S1 and S2). The weight and volume swelling ratio of p-PEGDA-PANi is about 5.6 times and 2.4 times larger, respectively, than the s-PEGDA-PANi at 20 oC. Although the swelling behavior of porous hydrogels is dependent on the doping of PANi in the crosslinking network and water temperature, the p-PEGDA-PANi


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hydrogel still displays better water retaining capacity than most hydrogels at temperatures as high as 50 o

C.57

Figure 3. Characterization of the double-network hydrogel. (a) FTIR spectra of p-PEGDA and p-PEGDA-PANi hydrogels. Insets are the molecular formulas of PANi and PEGDA. (b) Cyclic stress-strain curves of p-PEGDA-PANi hydrogel at 50% compressive strain. Insets are the optical images of the p-PEGDA-PANi hydrogel in compression-recovery processes. (c) Images showing the water absorbing capacity of the s-PEGDA, p-PEGDA and p-PEGDA-PANi hydrogels (the yellow dashed lines represent the wet parts of p-PEGDA-PANi hydrogel). The dimensions of samples are 2 cm × 1 cm × 0.2 cm.

Since thermal insulation plays an important role in confining the photothermally converted heat to the evaporative surface, we investigated the thermal conductivity of p-PEGDA-PANi hydrogel in both wet and dry states. Firstly, the thermal conductivity was obtained by measuring the steady-state temperature difference of the p-PEGDA-PANi hydrogel produced by heating the lower surface of the bottom glass slide. Figure 4a shows that the thermal conductivity of dry and wet p-PEGDA-PANi hydrogels are about 0.27 and 0.43 W m−1 K−1, respectively, both of which are lower than that of water (0.61 W m−1 K−1).58 The porous structure enables the trapping of air (0.026 W m−1 K−1 at 300 K59) in the p-PEGDA-PANi hydrogel, which serves as an effective thermal barrier to confine heat to the evaporating surface and lays the foundation for efficient solar steam generation.



To determine its light absorption ability, absorption spectra of the hydrogel samples were studied with a UV–Vis–NIR spectrophotometer equipped with an integrating sphere. In Figure 4b, the averaged absorption (weighted by AM1.5G solar spectrum) across 200–2500 nm is about 75.5% for p-PEGDA hydrogel, while just 32.6% for s-PEGDA. The higher absorption ability of p-PEGDA is dependent on its rough and porous surface structure, which enhances multi-scattering, as confirmed by the difference in light transmittance performance (Figure S2). After crosslinking PANi, the porous hydrogel sample exhibits a broader and stronger absorption (98.5%) than that of the pure PEGDA samples, especially in the visible and near infrared (NIR) regions, verifiying the strong absorption capibility of PANi. The detailed solar absorption in the UV-to-visible and NIR regions of all samples is presented in Table S3.

Figure 4. Photothermal performance of the double-network hydrogel. (a) Thermal conductivity of p-PEGDA-PANi hydrogel in dry- and wet-states. (b) Solar spectrum and UV-Vis-NIR absorption spectra of the s-PEGDA, p-PEGDA and p-PEGDA-PANi hydrogels. (c) Top-view temperature distribution of the pure water, p-PEGDA and p-PEGDA-PANi evaporation devices under one-sun illumination at various time points measured by using an IR camera. (d) Infrared images showing the cross-sectional temperature distribution of the pure water (I), p-PEGDA (II) and p-PEGDA-PANi (III) evaporation devices under one-sun illumination for 15 min. (IV) is the digital photo for p-PEGDA-PANi hydrogel-based evaporator.

Due to the excellent light-absorbing performance and low thermal conductivity of the porous hydrogel-based evaporator, we predicted that light absorbed by the p-PEGDA-PANi hydrogel will be converted into heat energy and localized at the directly irradiated surface. To verify this prediction, an infrared camera was employed to probe the top-view and cross-sectional temperature distribution of the


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p-PEGDA-PANi sample under a solar simulator with an intensity of 1 kW m−2 (1 sun). For comparison, the temperature change of pure water and the p-PEGDA sample was also measured under the same conditions. Figure S6 shows that in the initial irradiation stage, the surface temperature of all samples gradually increases as the irradiation time increased. After ~12 min, the surface temperature of each sample reaches stability, and further irradiation does not make great influence on the surface temperature. Infrared images in Figure 4c show that the surface temperature of pure water slightly increases by about 7 °C within 15 min of solar light irradiation, whereas for the p-PEGDA sample, its surface temperature can reach at ~32 °C (ΔT≈12 °C), slightly higher than that of pure water. This slight increase in temperature can be mainly attributed to the good thermal insulation of the evaporation device, in which the floating EPE foam effectively confines the generated heat within the top light absorber surface and hinders heat dissipation to the underlying bulk water (Figure 4d(I) and Figure 4d(II)). Howevere, the solar energy harvesting abillity of the white p-PEGDA hydrogel is not satisfied, so its temperature rise under 1 sun illumination is not significant. On the other hand, the surface temperature of p-PEGDA-PANi hydrogel rapidly increases from 20 °C to nearly 43 °C within 15 min of solar irradiation, and keeps the temperature plateaus for the remaining time. The cross-sectional infrared image in Figure 4d(III) indicates a sharp temperature gradient in the device, which further verifies the heat barrier role of the EPE foam in suppressing the downward heat conduction from the solar irradiating surface to the underlying water (Figure 4d(IV)).

Figure 5. Solar steam generation performance of the p-PEGDA-PANi hydrogel-based evaporator under one-sun illumination. (a) Schematic of the p-PEGDA-PANi hydrogel-based solar-driven interfacial evaporation device. (b)



Evaporation mass loss for three different evaporation devices under one-sun illumination for 1 h. (c) Comparing the solar thermal efficiency of p-PEGDA-PANi hydrogel with other evaporation devices. (d) Evaporation mass loss and solar thermal efficiency of p-PEGDA-PANi hydrogel-based solar-driven interfacial evaporation device under repeated operations.

Based on the above results, we conclude that the p-PEGDA-PANi hydrogel possesses all merits required for solar-driven localized steam generation, including strong photothermal capability, low thermal conductivity, rapid water replenishment and robust mechanical stability. These merits, combined with its simple and scalable fabrication, low-cost and reusability, make the photothermal hydrogel a very attractive candidate for water evaporation purposes. Therefore, we investigated the steam generation performance of the p-PEGDA-PANi hydrogel (3.0 cm2 exposed area) by recording water mass change over time. Specifically, the steam generator was placed on a precision balance and exposed to a solar simulator with an illumination intensity of 1 kW m−2 for 1 h (Figure 5a). Note that in this steam generating device, the thickness of the photothermal hydrogel layer was 3 mm. This is because our hydrogel has a porous structure. If the hydrogel layer is too thin, it cannot provide enough practical contact between the photothermal polymer PANi with the incident solar light; if the hydrogel layer is too thick, too much water would be adsorbed which will cause unnecessary heat conduction inside the hydrogel layer, thus reducing the overall efficiency of the solar steam generation system. Figure 5b shows that the water mass loss gradually increases with time when exposed to one-sun illumination, and that the p-PEGDA-PANi hydrogel evaporation device exhibits a larger mass loss after 1 h of solar radiation (1.32 kg m−2) than that of pure water only (0.27 kg m−2) or the p-PEGDA hydrogel (0.63 kg m−2) evaporation device, thus producing the best solar steam generation performance of the tested interfacial evaporation devices. The solar thermal efficiency (η, %) can be calculated from the mass loss rate of water using a classic equation:

=

×

(4)

where Δm is the water evaporation rate during steady-state evaporation, hLV is the total enthalpy of liquid–vapour phase change (sensible heat + latent heat of evaporation), and I is the power density of light illumination. Under a solar illumination of 1 kW m−2, the solar thermal efficiency of p-PEGDA-PANi hydrogel-based evaporator is as high as 91.5% (see details of the calculation in Supplementary Note and Table S4), about 4.7 times higher than that of pure water (19.6%). For comparison, the solar thermal efficiency of p-PEGDA hydrogel device is only 44.0%, which indicates the important role of PANi in absorbing and converting solar light into heat. More impressively, the solar thermal efficiency of the p-PEGDA-PANi hydrogel device under one-sun illumination is higher than or comparable to that of other materials reported in literature, as shown in Figure 5c. The distinguished solar thermal efficiency of the p-PEGDA-PANi hydrogel-based solar steam generation device can be attributed to the unique properties of the porous hydrogel, which enable comprehensive device optimizations for light absorption, thermal insulation, water replenishment and vapor evacuation. Firstly, the porous structures possess a rough surface that improves the light absorption of polyaniline (98.5%) and efficiently convert it into heat,


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enabling an excellent photothermal effect in the solar steam generation device. Secondly, the poor thermal conductivity of the polymer itself plus the air within its porous structure allows the entire photothermal layer to have a lower thermal conductivity, so that it can effectively confine the photothermally converted heat to the evaporating surface to minimize heat loss. Thirdly, the capillary action of the porous structure coupled with the inherent hydrophilic nature of hydrogel contributes towards rapid and timely water recruitment to the evaporative surface for efficient solar steam generation. Finally, the open-cell structure offers the p-PEGDA-PANi hydrogel with vapor escape channels, further improving its steam generation efficiency. As a promising photothermal platform for efficient solar steam generation, performance stability and reusability are of great importance for practical applications. To confirm these features, the same p-PEGDA-PANi hydrogel was used to perform 50 cycles of the solar-driven evaporation experiment under the same conditions. For each cycle, the evaporator was illuminated by a solar simulator with a density of 1 kW m−2. After 5 h, the wetted p-PEGDA-PANi hydrogel was dried with paper towel and arranged for the next repeat experiment. Figure 5d shows that the water evaporation is 2.0±0.06 g during each cycle. Furthermore, the solar thermal efficiency remains stable at 86.8±3.9% under one-sun illumination during 50 recycling tests, indicative of the good mechanical stability of p-PEGDA-PANi hydrogel (Figure 3b).

CONCLUSIONS In summary, we have demonstrated a cost-effective, robust and high-efficiency solar steam generation device that consists of a porous, photothermal hydrogel as the top layer and cellulose-wrapped EPE foam as the floating bottom layer. Our steam generation device exhibits a solar thermal efficiency of 91.5% with an evaporation rate of 1.40 kg m−2 h−1 under one-sun illumination, comparable to the state-of-the-art solar steam generation devices. This high efficiency is attributed to its unique porous structure, exceptional hydrophilicity, broadband solar light absorption, low heat conductivity and high photothermal conversion capability of p-PEGDA-PANi hydrogel. These features enable efficient absorption of sunlight, rapid water replenishment, unimpeded vapor evacuation and suppressed heat loss. Furthermore, it also possesses excellent mechanical properties. The solar thermal efficiency remains stable at 86.8±3.9% under one-sun illumination even after 50 recycling tests. The only drawback of this work is that all the above properties are obtained under laboratory conditions. In the future work, we will carry out more outdoor experiments to promote the practical application of the photothermal hydrogel-based solar steam generation device.

EXPERIMENTAL SECTION Materials. Aniline monomer, ammonium persulfate (APS), poly(ethylene glycol) (PEG, Mw=4000 Da), and

dichloromethane,

trimethylamine,

acryloyl

chloride,

toluene,

hydrochloric

acid

(HCl),

(2,4,6-trimethylbenzoyl)diphenylphosphine oxide (TPO) were purchased from Sigma Aldrich, Canada. All chemicals were of analytical grade and used as received.

Fabrication of p-PEGDA hydrogel. Porous Poly(ethylene glycol) diacrylate (p-PEGDA) hydrogel was produced via an adapted solvent casting/salt leaching methodology based on NaCl crystals as porogen



particle and saturated aqueous NaCl solution as solvent,63 in which PEGDA is dissolved but the NaCl crystals (particle size distribution: 270–325 μm) are insoluble. PEGDA (Mw=4,000) was synthesized as described previously.64 A saturated hydrogel precursor was prepared by dissolving 5 g of PEGDA in 20 mL of saturated salt water. TPO (0.5% W/V) was then added to the precursor as the photo initiator. When NaCl was added to the solution, the salt particles settled naturally overnight. The transparent top layer PEGDA solution and the precipitated salts were collected respectively to be used as the non-porous and porous hydrogel precursor. Both collections were polymerized under UV light (365 nm, 36 W) for 10 min. The distance between the collections and UV light was about 7 cm. The salts were leached out of the hydrogel by incubation in deionized water (DI) water to obtain the porous PEGDA hydrogel.

Fabrication of p-PEGDA-PANi hydrogel. In a typical synthesis, 0.32 M aniline monomer and 0.08 M APS initiator solution were separately dissolved in a 1 M hydrochloric acid solution and labeled as solution A and solution B, respectively. The p-PEGDA hydrogel was then immersed in solution A for 24 h. After washing with DI water, the aniline-monomer-contained p-PEGDA hydrogel was transferred to solution B. After 5 min, the local surface of p-PEGDA assumed a bluish-green color, indicating the beginning of the polymerization reaction. The reaction was left to proceed for 2 hours at room temperature. After washing with DI water, the porous two-network hydrogel of PEGDA and PANi (p-PEGDA-PANi) was obtained.

Characterization. Filed emission scanning electron microscopy (FE-SEM) images were obtained on a Hitachi S-4500 field emission SEM with an acceleration voltage of 10 kV. Absorption spectra were collected using a Shimadzu UV-3600 UV−Vis-NIR spectrophotometer. Fourier transform infrared (FTIR) spectra of the samples were recorded on a Thermo Scientific NICOLET 6700 spectrophotometer. The surface temperature distribution of test samples was recorded with an infrared thermal camera (FLIR C2 Infrared Camera).

Compression tests. The compression tests were conducted on an Instron universal testing machine according to ASTM D575. p-PEGDA-PANi hydrogel was cut into cubic samples (2 cm × 2 cm × 2 cm). During testing, the top stage was set to move downward at a constant compressive rate of 2 mm min-1. Load and displacement data were recorded using data acquisition software. The corresponding true stress–true strain curves were computed and then plotted.

Water-adsorbing experiments. The water uptake rate was first determined by tracing the time-dependent movement of water. Specifically, p-PEGDA-PANi hydrogels were cut into rectangular specimens (2 cm × 1 cm × 0.2 cm). During testing, one edge of the sample was immersed in a 2 mM rhodamine B aqueous solution, and the time-dependent movement of stained water was captured with a high-speed camera. The weight swelling ratio (ηw, %) was measured by immersing the preweighed dry hydrogel samples in DI water of various temperatures until they swelled to equilibrium. After excessive surface water was removed with filter paper, the fully swollen samples were weighed. ηw was calculated from the following equation:

=

× 100

(1)

where ms is the weight of the swollen state of the sample at equilibrium and md is the weight of the dry


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state of the sample. The volume swelling ratio (ηV, %) was also determined by measuring sample dimensions before and after equilibrium swelling. ηV was calculated from the following equation:

=

× 100

(2)

where Vs is the volume of the swollen state of the sample at equilibrium and Vd is the volume of the dry state of the sample. The swelling experiments were repeated five times and the average value was reported.

Thermal conductivity measurements. The thermal conductivity of dry and wet p-PEGDA-PANi hydrogel was measured by sandwiching the material between two glass slides. The “sandwich” was placed between a heat source (ceramic plate heater) and a cool source (ice-water bath). The p-PEGDA-PANi hydrogels were cut into cubic blocks (about 1 cm × 1 cm × 1 cm). The upper and lower surface temperature was monitored with a thermocouple. The Fourier equation was used to calculate the thermal conductivity using the formula:

=

∆ ∆

(3)

where ΔT is temperature difference, and Δx is the height of sample. The conductivity of glass is 1.05 W m−1 K−1. The calculation of thermal conductivity was based on the assumptions that the sample and the glass slides were experiencing the same heat flux, and the emissivity coefficient of sample and glass slide were both 0.9.

Solar steam generation experiments. All the solar steam generation experiments were conducted using the same solar simulator (Newport Oriel 69907) with an AM 1.5G filter. The optical intensity used was 1 kW m-2, which was measured with a Newport 91150 V calibrated reference cell and meter. The illuminated sample within a PTFE beaker was placed on an electronic balance (Sartorius® MSU 125P, accuracy: 0.01 mg) to record in real time the weight loss of water, which was used for measuring the water evaporation rate. The exposed area under irradiation was about 3.0 cm2 with a thickness of ~3 mm. The room temperature was 20±2 oC and the ambient humidity was about 60%.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Figures S1−S6, Tables S1-S4 and detailed calculation of solar thermal efficiency (PDF) Movies S1 and S2: The water absorbing behavior of p-PEGDA and p-PEGDA-PANi hydrogels (MP4)

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]

Author Contributions #These two authors contributed equally to this work. The manuscript was written by Y.Z. and X.Y., and the



other authors contributed methods, data acquisition, and processing.

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGEMENTS The authors thank Prof. Paul Charpentier of The University of Western Ontario for the UV-Vis-NIR test. This research was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC: RGPIN-2016-05198) and the Research Accelerator Grant Program of The University of Western Ontario.

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A macroporous double-network hydrogel composed of PEGDA and PANi is introduced for highly efficient solar steam generation. The porous hydrogel exhibits a highly energy-effective steam generation with an energy conversation of ≈91% under one-sun illumination.


Macroporous Double-Network Hydrogel for High-Efficiency Solar

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