Superhydrophilic and mechanically robust phenolic resin as double layered photothermal materials for efficient solar steam generation

Materials Today Energy 16 (2020) 100375

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Superhydrophilic and mechanically robust phenolic resin as double layered photothermal materials for efficient solar steam generation Fang Liu a, b, Weidong Liang a, **, Chengjun Wang a, Chaohu Xiao a, Jingxian He a, Guohu Zhao b, Zhaoqi Zhu a, Hanxue Sun a, An Li a, * a b

College of Petrochemical Technology, Lanzhou University of Technology, Langongping Road 287, Lanzhou, 730050, PR China School of Chemistry and Chemical Engineering, Lanzhou City University, Jiefang Road 11, Lanzhou, 730070, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 October 2019 Received in revised form 3 December 2019 Accepted 10 December 2019 Available online xxx

Solar steam generation has been emerged as one of efficient and green technologies for harvesting solar energy for seawater desalination and wastewater treatment. Herein, we demonstrate a facile approach for scalable fabrication of phenolic aldehyde foams (PAFs), which were synthesized by polymerization of formaldehyde with phenol and hydroquinone through one-step hydrothermal method, as double layered photothermal materials for efficient solar steam generation. The resulting PAFs show high porosity of higher than 93.6%, low apparent density (0.152e0.167 g cm
3), low thermal conductivity (0.034 e0.054 W m
1 k
1) and a good mechanical strength (0.07 MPa under 30% strain). The presence of closepacked open channels, by combination with their surface superhydrophilicity (water contact angle ~ 0 ), ensure the PAFs a rapid transportation of water molecules. After facile coating a thin layer of carbon soot on PAFs, a double layered photothermal materials with enhanced light absorption (~90%) was fabricated (named as CPAFs). Under 1 sun irradiation, the CPAFs possess a high water evaporation rate of 1.4922 kg m
2 h
1 and a solar conversion efficiency of 87.86%, making it promising candidate as solar generators for efficient solar desalination. By combining with their simple, scalable and cost-efficient manufacture process, it is suggested that such CPAFs may have great potentials for real applications. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Solar steam generation Phenolic aldehyde foam Photothermal material

1. Introduction The severe fresh water shortage has long been considered as one of bottleneck issues which restricted the sustainable development of modern society [1e3]. Though various conventional technologies have been employed for desalination till now, unfortunately, however, the huge energy consumption usually need by these technologies, e.g. nanofiltration [4], microfiltration [5], etc., on the other hand results in additional environmental pollution issues. Recently, photothermal materials assisted solar steam generation has been exploited as one of emerging technologies with great potentials for desalination to this end, owing to its high evaporation rate as well as only using green solar irradiation as energy input [6e8]. Compared with traditional water evaporation usually suffering the drawback of low energy conversion efficiency caused by its bulk vaporization manner, solar steam generation is a kind of

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (W. Liang), [email protected] (A. Li). https://doi.org/10.1016/j.mtener.2019.100375 2468-6069/© 2019 Elsevier Ltd. All rights reserved.

interfacial vaporization capable of localizing and transferring the solar irradiation into thermal energy (steam) through photothermal materials [9,10], which shows high energy conversion efficiency originated from its unique “thin-film vaporization” in terms of steam generation. The photothermal material is essential and plays crucial role because it supply a platform for both the solar-thermal conversion and vapor generation taking place in solar steam generation system. Generally, the criteria of photothermal materials for solar steam generation lie in excellent wettability, interpenetrated channels for rapid water molecules transportation, broad light absorption and excellent thermal insulation. In recent years, the massive efforts have been made to obtain photothermal materials for efficient solar steam generation, including metallic nanoparticles [11e13], composite materials [14,15] carbon-based materials [16e18], biomass-based materials [19e21], and polymer based materials [22e24], etc. For instance, Mu et al. [25] fabricated the solar steam generator based on conjugated microporous polymer (CMP) aerogel via Sonogashira-Hagihara cross-coupling reaction, achieving high energy conversion efficiency of up to 81% under 1

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sun illumination. Yue et al. [26] reported surface-carbonized of sugarcane as efficient solar steam generators and high evaporation efficiencies of 87.4% are achieved under 1 sun illumination. To date, great progress in this direction has been achieved, however, challenges remain with the reported photothermal materials usually have their own shortcomings such as complexity or high cost fabrication process, which are big obstacles hindering their industrial applications. Besides, a high cost further bloated by the utilization of elaborate technologies (e.g., high-temperature, freezedrying and inert gas shielding, etc.) [27,28]. Consequently, there is an urgent demand for seeking cost-efficient, simple and scalable manufacture process for development of high-performance photothermal materials. Among these existing photothermal materials, polymer-based ones have apparent advantages for their abundant commercial available raw materials, scalable manufacture process and, more importantly, processability. Furthermore, the rational varying the building blocks of polymers could easily tailor their physicochemical properties and thus makes it possible to meet the criteria for construction of an ideal photothermal material as mentioned above, thus showing great designable flexibility. Several kinds of polymeric photothermal materials have been created so far [25,29], however, the need of either complicated synthesis or expensive monomers of these studies seriously restricts their large-scale commercialization. In this study, we demonstrate the design and synthesis of the phenolic aldehyde foams (PAFs) by a simple onestep hydrothermal polymerization method as efficient photothermal materials. PAFs has long been synthesized as engineering resin for various applications, however, preparation of PAFs with porous and flexibly compressible features has rarely been reported. In this case, the as-prepared PAFs show great advantages of low apparent density, excellent thermal insulation, abundant pore structure, flexibility and superhydrophilicity, makes it ideal candidate as photothermal materials. In addition, the surface modification of carbon soot coating (named CPAFs) guarantees a high light absorption. Due to these unique characteristics, the CPAFs obtained the high energy conversion efficiency of 87.86% for CPAF-1 and 87.29% for CPAF-2 under 1 kW m
2 illumination, showing an excellent solar steam generation performance of CPAFs. More interestingly, the hydrophilic/hydrophobic double layer structure of CPAFs displaying a favorable salt-resistant property in saline solution. In combination with their high solar steam generation performance, low-cost raw chemicals, simple and scalable manufacturing processes, the CPAFs should be a kind of ideal solar generators for solar steam generation. 2. Materials and methods 2.1. Materials Phenol and sodium chloride were purchased from Tianjin Damao Chemical Reagent Co., Ltd. Hydroquinone was obtained from Shanghai Zhongqin Chemical Reagent Co., Ltd. Formaldehyde solution (37 wt %), ethanol and hydrochloric acid (36 wt %) were all obtained from Sinopharm Chemical Reagent Co., Ltd. Polyvinyl alcohol (PVA) was purchased from Tianjin Guangfu fine chemical industry research institute, all reagents were used directly without additional purification. 2.2. Synthesis of PAF-1 PAF-1 was prepared by the polymerization of phenol and formaldehyde, with hydrochloric acid as catalyzer under hydrothermal condition. Phenol 1.60 g (0.017 mol) and formaldehyde solution 5.0 mL were dissolved in 15 mL ethanol with 1.5 mL

hydrochloric acid. The mixed solution was transferred into a hydrothermal reactor and treatment at 120  C for 12 h after stirring for an hour in a 100 mL beaker. When the system was naturally cooled to room temperature, the crude product was washed with ethanol and water several times to remove unreacted monomers and catalyst, then dried at 75  C for 48 h, and named the resulting product as PAF-1. 2.3. Synthesis of PAF-2 For PAF-2, hydroquinone (1.87 g, 0.017 mol), formaldehyde solution (5.0 mL) and hydrochloric acid (1.5 mL) were dissolved in 10 mL ethanol with 5.0 mL saturated sodium chloride solution (as pore-forming agent). The mixture was stirred for an hour then poured into a hydrothermal reactor and reacted at 120  C for 12 h. The product was cooled and washed by ethanol and water, finally, the PAF-2 was obtained by freeze-drying. 2.4. Fabrication of CPAFs The PAFs were held above the flame of a kerosene lamp, deposition of a soot layer makes the surface of samples turn into black, it is worth noting that before this process the 5 wt% polyvinyl alcohol (PVA) aqueous solution coating can as a “glue” to integrate carbon soot with the PAFs and prevent the carbon particles from dispersing into the water. The samples which modification by PVA and carbon soot were named as CPAFs. 3. Results and discussion The phenolic aldehyde foams (PAFs) were synthesized by condensation polymerization of formaldehyde with phenol and hydroquinone using HCl as catalysts, ethanol and sodium chloride solution as solvents (Fig. 1a). During the reaction, ethanol and sodium chloride solution were applied to solvent and template, because the porous network architecture was formed by removing ethanol and sodium chloride solution [30]. After reaction, the PAF-1 was prepared as yellow cylindrical monolithic material with 32 mm in diameter (Fig. 1b), the PAF-2 shows brownish red cylindrical monolithic (Fig. 1d). And different sizes of PAFs were shown in Figure S1 in the Supporting Information. The scanning electron microscope (SEM) images of PAFs are described in Fig. 1c and e, respectively, it can be clearly seen that both PAF-1 and PAF-2 samples are consisted of polymer spheres of different diameters (2e3 mm). The apparent density of PAFs was calculated by dividing the weight by the volume and the results show that 0.152 g cm
3 for PAF-1 and 0.167 g cm
3 for PAF-2, the low apparent density results in PAF-1 and PAF-2 stand on a China rose (shown in Fig. 2a and b). Besides, the PAFs can withstand 500 g of weight and maintain initial shapes, demonstrating the high mechanical properties of PAFs (shown in Fig. 2c and d). To further confirm the mechanical property, the strain-stress experiments were tested under a compressive strain of 30% for PAF-1 and 20% for PAF-2, and the results are illustrated in Fig. 2e and f, the maximum compression stress at 30% strain was measured to be about 0.07 MPa for PAF-1, and the maximum compressive stress at 20% strain was 0.04 MPa for PAF-2, the both PAF-1 and PAF-2 samples maintain about 84% of the maximum stress and without any collapse after 10 cycles of compression. These results indicate their good mechanical and flexibility, thus is beneficial to transportation and more suitable for practical use [31]. X-ray diffraction (XRD) patterns (Fig. 3a) of PAF-1 and PAF-2 show broad and low intensity diffraction peaks located at 21.4 , indicating PAFs possess the lower crystallinities and the amorphous

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Fig. 1. (a) Synthesis schematic diagram of PAFs. (b) Camera photo of PAF-1. (c) SEM images of PAF-1. (d) Camera photo of PAF-2. (e) SEM images of PAF-2.

Fig. 2. (a) Camera photo of PAF-1 on a China rose. (b) Camera photo of PAF-2 on a China rose. (c) Camera photo of PAF-1 under 500 g weight. (d) Camera photo of PAF-2 under 500 g weight. (e) Compressive stress-strain curves of PAF-1 for the first to tenth test cycles. (f) Compressive stress-strain curves of PAF-2 for the first to tenth test cycles.

character [32]. The surface chemical composition of the PAFs was investigated by Fourier transform infrared (FTIR) spectroscopy. As shown in Fig. 3b, the broad peaks centered at 3415 cm
1 are attributed to eOH stretching vibration from phenol and hydroxymethyl, the absorption bands around 2960 cm
1 are the contribution of aliphatic CeH stretching vibration, the absorption signals at 1640 and 1436 cm
1 can be classified as C]C and CeC stretching vibration regarding aromatic rings, the peaks around 1040 cm
1 corresponding to the hydroxymethyl CeO stretching, the absorption peaks at 1120-1080 cm
1 can be ascribed to CeOeC stretching vibration [33,34]. X-ray photoelectron spectroscopy (XPS) further

confirms the chemical composition of PAF-1 and PAF-2. As depicted in Fig. 3c and e, two peaks centered at 286 eV and 533 eV are observed in both of PAF-1 and PAF-2 samples corresponding to C 1s and O 1s respectively. Fig. 3d and f shows the high-resolution spectrum for the C 1s peak of PAF-1 and PAF-2, respectively, the C 1s spectra of PAFs are mainly composed of three types of carbon bonds, including carbon in aromatic CeC (284.4 eV), aliphatic CeC (285.0 eV) and hydroxyl carbon CeO (286.1 eV) [35e37], and these results conform to the infrared spectra. The porous feature of PAF-1 and PAF-2 were evaluated by mercury porosimetry measurements and nitrogen adsorption

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Fig. 3. (a) XRD pattern of PAF-1 and PAF-2. (b) FTIR spectrum of PAF-1 and PAF-2. (c) XPS spectra of PAF-1. (d) C 1s XPS spectra of PAF-1. (e) XPS spectra of PAF-2. (f) C 1s XPS spectra of PAF-2.

measurements, respectively. According to Figure S2 in the Supporting Information, which indicate that the PAF-1 is basically composed of macropores, and a significant number of pores falls in a diameter of 8 mm [38]. Conversely, the adsorption-desorption isotherms of PAF-2 conform to a mixture of type I and type IV of physisorption isotherms [39], indicating that the presence of micropores and mesoporous in the PAF-2. The BET surface area of PAF2 was calculated to be 528 m2 g
1, and the average pore size was calculated to be 3.4 nm by using the BJH method. These results suggesting that the prepared PAF-1 and PAF-2 have different porous characteristics, which is attributed to the type of monomers and solvents in the solvothermal route. Besides, the porosity was estimated to be 94.9% for PAF-1 and 93.6% for PAF-2 by liquid displacement method [40], the high porosity can be attributed to

their porous network architecture which is composed by irregular spheres of PAFs. As depicted in Fig. 4a and b, clearly, a water permeates completely into PAFs within 0.15 s when the water droplet was dropped on the surface of PAFs, it is suggested that PAFs possess the superhydrophilic characteristic and the ability to absorb water molecules quickly, such hydrophilic property due to the functional groups of eOH and CeO [41,42]. Therefore, the super hydrophilicity bonding with the interconnected pore structure provide the PAFs with excellent capillarity which guarantee the water transport during solar steam generation [43]. Ultra-high light absorption is one of the key properties of photothermal materials during solar steam generation. In order to improve the light absorption and solar energy conversion efficiency of PAFs, we describe a simple way to make black coatings. In our

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Fig. 4. (a) Camera photos of droplet impregnation process on the surface of PAF-1. (b) Camera photos of droplet impregnation process on the surface of PAF-2.

case, the 5 wt % PVA solution as a “glue” firstly apply to the surface of the PAFs by a brush, then the PAFs were held above the flame of a kerosene lamp, deposition of a soot layer turns the surface of samples to black (Fig. 5a), the carbon soot-modified PAFs named as

CPAFs (the camera photo of CPAFs shown in inset of Fig. 5c). The stability of the CPAFs was tested and the results are shown in Figure S3, after immersing of the CPAF-1 and the CPAF-2 in deionized water, 0.1 M HCl or 0.1 M NaOH, there was no obvious black carbon

Fig. 5. (a) Schematic illustration of synthesis of carbon soot coated PAFs for solar steam generation. (b) SEM image of soot coating, inset in (b): SEM image of the soot coating in high magnifications, and the water contact angle of soot coating. (c) UVeviseNIR absorption spectra of the CPAFs, inset in (c): the camera photo of CPAFs. (d) Schematic diagram of the CPAFs as evaporator for solar steam generation.

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Fig. 6. (a) Infrared images of CPAFs under different illuminations and time. (b) Surface temperature change of CPAF-1 under different illuminations and time. (c) Surface temperature change of CPAF-2 under different illuminations. (d) Time-dependent mass change of water due to CPAF-1 under different illuminations. (e) Time-dependent mass change of water due to CPAF-2 under different illuminations. (f) Evaporation rate (pink, left-hand side axis) and solar steam efficiency (blue, right-hand side axis) of CPAF-1 under different illuminations. (g) Evaporation rate (purple, left-hand side axis) and solar steam efficiency (blue, right-hand side axis) of CPAF-2 under different illuminations.

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Fig. 7. (a) Energy efficiency of the recycled CPAFs under one sun illumination for 6 times. (b) Camera images of salt precipitates on the CPAFs at different times. (c) Time-dependent mass change of water in strong brine and pure water due to CPAFs (1 sun). (d) Evaporation efficiency of CPAFs in strong brine (1 sun). (e) and (f) The concentration of four primary ions in simulated seawater before desalination and the concentration of fresh water after obtained by condensing steam, the dashed colored lines refer to the WHO standards for drinkable water.

powder observed in the beaker, indicating the carbon particles are not easy to fall off from the PAFs substrate due to the strong adhesion of the PVA coating. In addition, the SEM images of PVA coating are shown in Figure S4, from the SEM images, it can be clearly seen that the PVA presents a smooth film covering the surface of the polymer spheres of PAFs substrate, and the PVA did not completely covered and closed all the pores in the PAFs substrate. Therefore, the PVA coating could not affect the solar steam generation performance. As depicted in Fig. 5b, it can be seen clearly that the soot is consists of numerous carbon particles with diameter 40e80 nm and formed a loose, fractal-like network [44]. In addition, the water contact angle of soot coating was measured by a contact angle meter and the result was 156 (shown in inset of Fig. 5b), indicating the hydrophobicity of the soot coating. The

optical property of the CPAF-1 and CPAF-2 was provided in Fig. 5c, both samples show high light absorption about 90% in the studied wavelength range, a broadband absorption characteristic is benefit to collect solar energy and convert sunlight to heat, thus increasing efficient solar steam generation. Besides, the thermal conductivity of PAFs were investigated by using the transient plane source method, the results show 0.034 W m
1 K
1 for PAF-1 and 0.054 W m
1 K
1 for PAF-2, which is attributed to the organic polymer properties and highly open porous structure of PAFs, resulting in an excellent thermal insulation [45], thus limits the heat transfer to water and reduces the energy loss. Taking these advantages mentioned above such as superhydrophilicity, high light absorption, excellent mechanical strength, light weight and low thermal conductivity, CPAFs can be regarded as a promising photothermal

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material for solar steam generation. Fig. 5d exhibit the schematic diagram of the CPAFs as evaporator for solar steam generation, the hydrophilic/hydrophobic porous double layer structure is beneficial to solar water evaporation performance, on the one hand, a hydrophobic layer of carbon soot for broad solar harvesting and releasing water vapor, on the other hand, a hydrophilic polymer layer guarantees continuous water supply and prevent heat loss during solar steam generation. In order to investigate the solar steam generation performance of CPAFs, the solar energy conversion efficiency was measured by a solar simulator under different illumination of 1 kW m
2, 2 kW m
2 and 3 kW m
2. The infrared images of CPAFs solar steam generator (Fig. 6a) and the surface temperature change of CPAFs under different time and illuminations (Fig. 6b and c) were recorded by infrared thermal camera. As shown in Fig. 6a, the surface temperature increases of CPAFs have the same trend, for the case of CPAF-1, the surface temperature rose sharply from 21.8  C to 38.8  C under 1 kW m
2 illumination, 48.6  C under 2 kW m
2 illumination, and 55.4  C under 3 kW m
2 illumination within the initial 5 min, then further reached up to 43.8  C, 54.5  C and 61.2  C under 1 sun, 2 sun and 3 sun illumination, indicating the heat localization property of the CPAFs solar steam generator. According to Fig. 6b and c, it can be seen that the surface temperature of CPAFs increased evidently within the initial 500s, and the temperature rose steadily until half an hour later, the maximum surface temperature of the CPAF-1 up to 45.7  C under 1 kW m
2, 55.3  C under 2 kW m
2 and 62.6  C under 3 kW m
2 illumination, respectively. Obviously, under the same illumination, the maximum surface temperature of CPAFs is much higher than that of the pure water (32  C), it is because that CPAFs as solar steam generator may absorb more solar light and converted into thermal energy. The time-dependent mass change of water during steam generator was recorded by the electronic analytical balance. When CPAFs were used as solar steam generator, the evaporation of water is obviously much larger than that of pure water under the same illumination (Fig. 6d and e). To further confirm the excellent solar steam generation performance of CPAFs, the evaporation rates were calculated according to the slope of the fitted time-dependent mass change curves [46]. As shown in Fig. 6f and g, the results show that 1.4922 kg m
2 h
1 under 1 kW m
2, 2.6520 kg m
2 h
1 under 2 kW m
2, and 3.6612 kg m
2 h
1 under 3 kW m
2 for CPAF-1 based solar steam generator. In the case of CPAF-2, the evaporation rates were verified to be 1.4982 kg m
2 h
1 under 1 kW m
2, 2.6076 kg m
2 h
1 under 2 kW m
2, and 3.5766 kg m
2 h
1 under 3 kW m
2, respectively. In addition, the evaporation rate with the dark environment was found to be 0.2230 kg m
2 h
1 for CPAF-1 and 0.2380 kg m
2 h
1 for CPAF-2 (the mass change of water under dark environment are shown in Figure S5). The energy conversion efficiency was calculated according to the previously reported literature [47,48] and the calculation details are depicted in the Supporting Information. As shown in Fig. 6f and g, the energy conversion efficiency was 87.86% for CPAF-1 and 87.29% for CPAF-2 under 1 kW m
2, 84.69% for CPAF-1 and 82.67% for CPAF-2 under 2 kW m
2, 80.31% for CPAF-1 and 78.01% for CPAF-2 under 3 kW m
2, respectively. These values can compete with that of those previous reported solar absorber for steam generation [49,50]. To demonstrate the stability of the system, the cyclic experiments under 1 sun illumination were carried out and the results are illustrated in Fig. 7a, the evaporation efficiency is relatively stable within 6 cycles, the both samples remains the original morphology after the cycle experiment, indicating prepared CPAFs own the excellent stability. To investigate the salt-resistant ability of the CPAFs, the solar evaporation tests under 1 sun illumination were performed in saline solution with 20% NaCl concentration for

CPAFs-based solar receiver. As illustrated in Fig. 7b, we can see clearly that no obvious NaCl crystals appear on the surface of CPAFs within 3 h. As shown in Fig. 7c, the mass change of high concentration brine is a little different from that of pure water, and relatively high evaporation efficiency was received (Fig. 7d) under 1 sun illumination. Such double layered structure with superhydrophobic coating on the top layer would be responsible for their high salt-resistance as its non-wet feature could provide vapor evaporation pathways but effective prevent salt water infiltration [51]. In addition, the experiment of seawater desalination was conducted by using the artificial seawater with salinity of 3.5% and the results are shown in Fig. 7e and f. The concentrations of the four primary ions of Naþ, Mg2þ Kþ, Ca2þ have decreased significantly and the ion removal is higher than 99%, which is far below the WHO (World Health Organization) standard for ion concentration of drinking water [2,52], indicating that the potential application in desalination of the CPAFs. 4. Conclusions In summary, we have presented the method for design and fabrication of CPAFs as efficient photothermal materials for solar steam generation. The prepared PAFs possess porous network architecture, low apparent density, high mechanical property, superhydrophilicity and low thermal conductivity. After coating of carbon soot on the PAFs, the samples show stronger light absorption. Taking advantages of CPAFs displayed above, thus particularly high evaporation efficiency of up to 87.86% for CPAF-1 and 87.29% for CPAF-2 are acquired under 1 kW m
2 illumination. Moreover, the CPAFs also show good performance in salt-resistant. These results of this study may offer a new and promising photothermal materials with cost-efficient and facile fabrication for efficient solar steam generation. Author contributions Fang Liu synthesized the materials and conducted the experiment; Chengjun Wang and Chaohu Xiao performed samples characterization; Jingxian He, Zhaoqi Zhu and Hanxue Sun performed data analysis and theoretical investigation; An Li, Weidong Liang and Guohu Zhao contributed important suggestions and revised the manuscript. All authors wrote the manuscript. Declaration of Competing Interest No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. This work described was original research which has not been published previously, and not under consideration for publication elsewhere. Acknowledgements The authors are grateful to the National Natural Science Foundation of China (Grant No. 21975113, 51962018, 51663012), Project of Collaborative Innovation Team, Gansu Province, China (Grant No. 052005), Support Program for Hongliu Young Teachers of LUT, 2019 Key Talent Project of Gansu, and Innovation and Entrepreneurship Talent Project of Lanzhou (Grant No. 2017-RC-33). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.mtener.2019.100375.

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