Fabrication of bilayered attapulgite for solar steam generation with high conversion efficiency

Accepted Manuscript Fabrication of bilayered attapulgite for solar steam generation with high conversion efficiency Juan Jia, Weidong Liang, Hanxue Sun, Zhaoqi Zhu, Chengjun Wang, An Li PII: DOI: Reference:

S1385-8947(18)32643-3 https://doi.org/10.1016/j.cej.2018.12.157 CEJ 20697

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

10 September 2018 26 December 2018 27 December 2018

Please cite this article as: J. Jia, W. Liang, H. Sun, Z. Zhu, C. Wang, A. Li, Fabrication of bilayered attapulgite for solar steam generation with high conversion efficiency, Chemical Engineering Journal (2018), doi: https://doi.org/ 10.1016/j.cej.2018.12.157

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Fabrication of bilayered attapulgite for solar steam generation with high conversion efficiency Juan Jia, Weidong Liang*, Hanxue Sun, Zhaoqi Zhu, Chengjun Wang, An Li*

College of Petrochemical Technology, Lanzhou University of Technology, Langongping Road 287, Lanzhou 730050, P. R. China

*Corresponding author: Prof. Dr. Weidong Liang, Prof. Dr. An Li, Tel.: +86-931-7823125. Fax: +86-931-7823125. E-mail address: [email protected] (W. Liang), [email protected] (A. Li)

Abstract Solar steam generation has been attracting wide attention for boosting the evolution of solar-energy-harvesting technology. Here, we demonstrate the fabrication of a novel bilayer photothermal material based on attapulgite/poly acrylamide composite (APAC), which was prepared by the solution polymerization of acrylamide (AM) in the presence of attapulgite using N, N’ - Methylene bisacrylamide (MBA) as a crosslinker, for efficient solar steam generation. The APAC shows better thermal stability with a decomposition temperature of 250 oC, good mechanical property with a compress strength of up to 125 KPa at 75% strain and a low apparent density (0.0191 g·cm-3) with abundant porosity accompanying with a low thermal conductivity (0.07 W m-1 K-1). To enhance the light absorption of APAC, a thin carbon layer was created on the surface of APAC via a facile flame treatment. Under our conditions, the bilayered APAC shows a high vapor rate of 1.2 kg m-2 h-1 under 1 sun illumination, equal to 85% solar-to-vapor efficiency. With the merits of cost-efficient, scalable manufacture, high solar energy conversion efficiency, the APAC may hold the great potential as high-performance photothermal materials for solar energy generation.

Keywords Solar steam generation; bilayer; attapulgite; conversion efficiency

1 Introduction The development of clean, renewable, and sustainable energy resource and techniques is of great importance in alleviating energy crisis of fossil fuels as well as addressing environmental issues such as emerging ecological concerns and global warming [1], etc. So far, traditional renewable energy resources such as hydro energy, wind energy and biomass energy, in addition to nuclear energy, have been widely used to meet the huge energy consumption demand of modern society. Compared with those energy mentioned above, solar energy has undoubtedly been considered as a kind of safety, inexhaustible green energy source [2]. Different from diverse manners for utilization of solar energy such as production of hydrogen [3], solar power plants [4], photovoltaic cells [5], photocatalysis [6], and water desalination [7], the photothermal materials based solar water evaporation has been proven to be one of the most promising approaches for harvesting solar energy due to its high solar-to-vapor conversion efficiency, which in trun have great potentials for a wide range of applications such as power generation, seawater desalination and wastewater treatment [8-10]. Compared with the traditional water evaporation by solar radiation as heat source which suffers from the drawback of low solar energy conversion efficiency due to the fact that the part solar energy is converted to heat bulk water or is lost to the external environment [11], the high solar energy conversion efficiency of solar steam generation system lies in the fact that the solar radiation is only harvested and located at the water–air interface to heat thin air-water surface layer that can effectively

minimizes the heat loss [8]. In the past years, a variety of materials such as carbon materials [12-16], gold nanoparticles [17-20], and bilayered biofoams [8, 21-24] have been used as photothermal materials to enhance solar energy harvesting performance. However, there is still the challenge that remains in connection to the material cost or complicated fabrications [25, 26]. Therefore, the creation of new photothermal materials, which can solve all the above issues (e.g. cost-efficient, simple and scalable manufacture) meanwhile having broadband sunlight absorbability, low thermal conductivity, open porosity for rapid water molecules transportation and high energy conversion efficiency, is of great significance for construction of efficient solar steam generation systems. In this direction, especially, a number of natural biomass-based materials such as woods [25, 27, 28], mushrooms [29], etc., have been exploited as photothermal materials for solar steam generation. Compared with these synthetic materials, these materials have obvious advantages

for their abundance,

biodegradability, cost-effective and, in particular, high solar energy conversion efficiency. It is amazing that a solar steam generation efficiency beyond 100% was obtained for the wood-PDA system [27]. In this work, we report an approach for creation of new photothermal materials by using the natural clay as raw materials for solar steam generation. The clay of attapulgite (ATP) as a kind of hydrated magnesium aluminium silicate with a layered chain-like structure has attracted considerable attention owing to its natural nanochannel structure, large specific surface, abundant, inexpensive [30, 31]. Based on these unique characteristics, it would be assumed that the ATP is a candidate for

high-performance solar steam generation. Here, we used ATP to prepare solar thermal conversion materials of APAC based on the reaction, which was prepared by the solution polymerization of AM in the presence of ATP using MBA as a crosslinker. And the APAC shows better thermal stability, exceptional mechanical strength, a low apparent density and thermal conductivity with excellent processability, and three-dimensional structure with sheets and layers. Sample of the bilayered APAC shows a high evaporation rate of 1.2 kg m-2 h-1 and solar energy conversion efficiency of up 85% under 1 sun illumination. All of these results demonstrate that the APAC hold promising potentials in high-performance solar steam generation.

2 Methods 2.1 Fabrication of APAC 1 g of AM and 4g of ATP powder were dissolved in 25 ml of distilled water and treated by ultrasound for 10 min. 30 mg of MBA as the crosslinker was added to the above AM–ATP mixture solution and stirred at room temperature for 10 min. Then, the mixture was transferred into a 250 ml three-neck flask and heated at 30 oC for 1h under N2 protection. After adding 30mg of radical initiator (ammonium persulfate), the mixture solution was heated to 80 oC for 1h under vigorous stirring. After that, 5 ml sodium hydroxide solution (NaOH) (5 mol·l -1) was added into the mixture and reacted at 80 oC for 1h. Finally, the resulting product was washed with distilled water for several times until pH = 7 [32]. Water was fully absorbed by the APAC, after 12 h, standby application. 2.2 Modification of the APAC

Firstly, 2.6 g analytical polyvinyl alcohol (PVA) was dispersed distilled water and named as saturated solution A. Then the same amount of polyvinyl pyrrolidone (PVP) was dispersed distilled water and named as saturated solution B. Finally, saturated solution A and B were added to the APAC and heated slowly to 70 oC under vigorous stirring for 1 h. 2.3 Preparation of the APAC-based composites The modified APAC was put in a moulds then put in refrigerator to freeze for 10 h at -18 oC, followed by freeze-drying by using freezer dryer for 3 day under -50 oC . 2.4 Preparation of bilayered APAC (BAPAC) solar steam generator The APAC-based composite placed on the top of the butane lighter for a simple flame treatment [33] to obtain a double ATP-carbon based composites, which is called BAPAC and the top of the butane lighter for a simple flame treatment is the F-APAC. 2.4 Material characterizations The morphology of the samples were examined by scanning electron microscopy (SEM, JSM-6701F, JEOL, Ltd.) and transmission electron microscopy (TEM Tecnai G2TF20 ), all samples used for SEM testing were fixed to the copper column with double-sided tape. And the hydrophobic angle Contact angle (CA) of the samples by the contact angle meter (DSA100, Kruss). The structures of the products were performed by an X-ray diffraction (XRD) on a Japanese Neo Confucianism (Rigaku, D/Max-2400) diffractometer at 2° to 80°. The mechanical property of APAC was investigated by an electrical universal material testing machine with a 500 N load cell (EZ-Test, SHIMADZU) at a stress rate of 5 mm·min-1. The thermal stability of the

APAC and the F-APAC were measured by a thermogravimetric analysis (TGA, Perkin Elmer) with a heating rate of 10 oC·min-1 under nitrogen atmosphere. The specific surface area and porosity of both APAC and F-APAC were measured by N2 adsorption and desorption at 77.3 K using a volumetric sorption analyzer (micromeritics ASAP 2020).The light adsorption of the APAC and BAPAC were performed via JASCO Corp., V-570, Rev. 1.00 Fourier transform infrared spectroscopy (FTIR) was recorded from in the range of 4000-400 cm-1 using a Mexus 670 spectra instrument. The thermal conductivity of the APAC was evaluated using Netzsch laser flash apparatus (LFA 457) at 25 degrees Celsius and was calculated (k = α·ρ·Cp). The density of the APAC (ρ) was calculated by the ratio of mass (m) and volume (v) using the equation of ρ

[22].

3 Results and discussion Figure 1a shows the preparation process of APAC. ATP, AM and MBA were firstly dispersed into water and treated by the ultrasound to obtain the uniform mixture solution. The ATP and AM reacted through graft copolymerization reaction by using MBA as a crosslinker and ammonium persulfate (APS) as an initiator at 80 oC under N2 atmosphere to obtain APAC [34]. The resulting hydrogel was then treated by 5 ml sodium hydroxide solution (5 mol·L-1) at 80 oC for 1h and washed with deionized water for several times until the PH=7. To improvement the mechanical property, the as-prepared hydrogel was then treated by the PVP and PVA. The mixture was then put into a mould and put into refrigerator to freeze for 10 h at -18

o

C. After

freezing-drying for 3 day at -50 oC by using a freezer dryer, APAC was obtained.

During the freezing process, the ice crystals grow along the axis of mould and after freezing-drying, the ice crystals was removed by sublimating ice into vapor to produce an oriented porous monolithic structure due to the template effect of the ice crystals during freezing process. As shown in Figure 1b, the as-prepared APAC exhibits lamellar porous monolithic structure with 1 cm in height and 5 cm in diameter which can stand on a dandelion and can support a 100 g of weight without any collapsing of its original structure after removing the load, indicating the low density and excellent mechanical property of the resulting APAC. To improve the light absorption, the APAC was partially carbonized at 500 oC by using a lighter of butane to obtain a BAPAC and F-APAC (Supplementary Figure 1 ). As shown in Figure 1c, after carbonization, the resulting materials show black surface and an original porous monolithic structure which make them become a promising candidate for the solar steam generation by combination of strong light absorption and porous channels for the rapid transportation of water molecules. The morphological structure of the APAC and the F-APAC were characterized by scanning electron microscopy (SEM). As shown in Figure 2a, the APAC exhibits a lamellar porous structure and high resolution SEM image (Figure 2b) show that the APAC is composed by the random distributed polymer-coated ATP nanorods. After carbonization, the structure of the F-APAC remains unchanged and still exhibit lamellar porous structure (as shown in Figure 1c). High resolution SEM image (Figure 1d) show clearly ATP nanorods and accompany with the presence of carbon nanoparticles. Transmission electron microscopy (TEM), as shown in Figure 2e,

further reveals that after carbonization, the F-APAC consist of ATP nanorods and carbon nanoparticles. The BAPAC of F-APAC lay have hydrophobic, with a CA of 140o on the surface of water (Figure f), and the APAC lay have Super hydrophilic (Supplementary video 1), which structure is really conducive to the vapor escape and transmission of water [12, 27, 33, 34]. We can see the the water transport capacity in the vertical direction in Supplementary figure 2, the APAC can be full impregnated by water with rhodamine B in 90 s. The horizontal plane and longitudinal compressive stress-strain tests of APACs were probed by an electrical universal material testing machine and the measurement results are shown in Figure 3a-3b. The horizontal plane proven that have sharp densification stage up to 0.7 strain at the stress is 88 KPa. The longitudinal of APAC evidence three stages of compressive behaviors of primary linear reply before yield point at 0.24 strain, plastic deformation with slower stress increment after the yield point, and the last sharp densification stage up to 0.75 strain at the stress is 125 KPa. This large strain deformation evidences that have good mechanical property [33-36] and does not break apart under compression, so that the APAC can confirms its portability [22]. At the same time, the different compressive stress-strain tests were measured, which shows the resistance of the horizontal plane and longitudinal of the APAC is different (Supplementary Figure 3), which is related to the pore structure formed during pre-freezing. Thermal stability of the APAC and F-APAC was tested by thermogravimetric analysis (TGA) and the results are illustrated in Figure 3c. The mass loss of the APAC was 10% at 100 oC and 75% at 1000 oC. By comparison, the

F-APAC shows relative higher thermal stability with onset of decomposition above 400 oC than that of APAC. The light absorption of the BAPAC from 250 nm-2500 nm is shown in Figure 3d, the BAPAC exhibits a broadband light absorption from Vis to IR and the light absorption exceed 99 %, indicating the excellent light absorption of the BAPAC. The BET specific surface area and pore size distributions were evaluated by the N2-adsorption/desorption measurements at 77.3 K and the results are shown in Figure 3e-f, from the figure 3e we can see that the APAC basically belong to mixture of the type II and IV isotherm which is characteristic of mesoporous and macroporous feature of the materials according to the IUPAC [37]. The BET specific surface area of the APAC was calculated to be 19.73 m2 g-1. The single-point total pore volume of the pores at P/P0=0.9965 was found to be 0.048 g cm-3 for APAC. The pore size distribution calculated from the isotherm by using BJH method is illustrated in the Figure 3f. It is shown that pore sizes of APAC mainly focus on 1.92 nm, 4 nm, 10 nm, 85 nm and 110 nm which are corresponding derived from the ATP and freeze-drying produced pores. X-ray diffraction (XRD) pattern is presented in Figure 3g, the pattern displays peaks at 25.3° which correspond to planes of carbon [38], the APAC is different from nature ATP and similar to the F-APAC, because the peak of the carbon in organic matter covered the peak of the ATP and the carbon content is the same before and after carbonization. The FTIR spectra of natural ATP, the APAC and the F-APAC are shown in Figure 3h. For natural ATP, the absorption band located at 3550 cm

-1

are assigned to the –OH stretching vibration in (Al, Mg)-OH of ATP [39,

40], the peaks at 3402 cm-1 and 1655 cm-1 could be assigned to the bend vibration of

H2O in ATP [41,42]. vibrational bands arising from stretching of ATP groups in 1195 cm-1, 980 cm-1 and 482 cm-1 [42-46] were observed. There are peak appears in 2942 cm-1 and 1292 cm-1 is ascribed to the peak of –CH [47] and –NH [48, 49] stretching vibration of the poly AM. The stretching vibration peak in 3402 cm-1 became stronger that could be related to the –NH band [48]. It is clear that the absorption band of the APAC weakens or disappears after flame treatment, indicating the carbonization of the polymers and only presence of the ATP in its structure. Owing to its porous structure and inherent polymer/clay-based skeleton, the APAC possess desired heat-insulated property. The thermal conductivity of APAC was measured to be 0.07 W m-1 K-1 at room temperature by using flash method. By combinging the strong light absorption of the surface and low thermal conductivity of the bottom, the bilayer design makes APAC is beneficial for the efficient conversion the light to heat and preventing the heat loss during solar steam generation process.

4 Solar steam generation Taking inherent merits of the BAPAC composites such as unique porous monolithic structure, high chemical and thermal stability, excellent mechanical strength, low thermal conductivity and apparent density, BAPAC should be a promising candidate for the solar steam generator. To illuminate this, the solar steam generation experiment was conducted by using a lab-made, real-time measurement system [30, 50, 51] (shown in Figure 4a) which is consist of a solar simulator, computer, infrared camera and electronic balance. Figure 4b shows the temperature variations of the APAC at the air condition. The surface temperature of the BAPAC rapidly reached up

to 100 oC within initial 210 s and then reached an equilibrium temperature of around 103 oC within 540 s under 1sun irradiation. By comparison, the surface temperature of the APAC only reached up to 42.9 oC within 210 s at the same condition. Thus, the surface carbonization of the BAPAC has positive impact on the solar to heat conversion performance of the APAC, which is benefit to the solar steam generation. Figure 4c and 4d illustrates the infrared camera photograph, from the photograph we can intuitively further obtain the surface temperature change of the BAPAC and APAC under dry condition, respectively. Figure 5a illustrates the surface temperature change curves of the BAPAC and APAC under wet condition monitored by the infrared camera. The surface temperature of the BAPAC can reach 35 oC within 14 min and then reach up to a maximum temperature of 40 oC under 1sun illumination. By contrast, the maximum temperature of the APAC is only 33 oC within 1 h under 1sun illumination. We have prepared APACs of different thicknesses in order to account the effect of thickness on APAC efficiency (Supplementary Figure 4). Furthermore, the as-prepared APACs and BAPACs can float on the surface of the water without any additional support due to their porous structure and low density, the Supplementary Figure 5 show the BAPAC floats on the water. The solar steam generation performance of the APACs and the different BAPAC were evaluated by the time-dependent mass change due to the water evaporation and the results are illustrated in Figure 5b. The evaporation rate calculated from the slope of the time-dependent mass change curves are shown in Figure 5c. The evaporation rate was calculated to be 1.261 kg m-2 h-1 for BAPAC and

0.456 kg m-2 h-1 for APAC under 1 sun illumination. The recyclability is of great importance for a solar steam generator in practical application. To confirm this, the APAC and BAPAC samples after solar steam generation testing were freeze-dried again and then conducted the solar steam generation experiment to evaluate their recyclability performance. The corresponding energy efficiency (η) for solar steam generation of the BAPAC can be calculated by using the following equation [15, 27, 30, ]: Η = ṁhLV/qiCopt where ṁ (kg m-2 h-1) is the rate of total water mass loss in the solar evaporation process, hLV (kJ kg-1) is the total enthalpy (including liquid-vapor phase change and the sensible heat (hLV = 2260+4.2(T2-T1)), T2 is final surface temperature of the APAC and BAPAC, and T1 is the initial temperature of the water), Copt is the optical concentration and qo is the normal solar radiation intensity. Accordingly, the energy efficiencies of the BAPAC (1.3 cm) was calculated to be 80% at the 1sun illumination, representing one of higher values for reported solar generation devices [13], by comparison, the energy efficiency was found to be 32% for APAC-based solar generator, and as can be seen from the Figure 5d, the APAC and BAPAC have good stability. As the Figure 5e shown, the surface temperature change curves of the BAPAC (1.3 cm) under 2 and 3 sun, the surface temperature of the BAPAC can reach 50.9 oC under 2 sun and 56.7 oC under 3sun illumination. The solar steam generation performance of the BAPAC was evaluated under different sun by the time-dependent mass change due to the water evaporation and the results are illustrated in Figure 5f.

The energy efficiency calculated from the slope of the time-dependent mass change curves are shown in Figure 5g. The evaporation rate was calculated to be 1.113 kg m-2 h-1 for BAPAC (1.3 cm) and 0.556 kg m-2 h-1 for APAC (1.3 cm), and 0.376 kg m-2 h-1 for natural water under 1 sun illumination. The energy efficiencies of the BAPAC (1.3 cm) was calculated to be 80% at the 1 sun, 51% at the 2 sun and 47% at 3 sun and 24% for natural water, respectively, which is much less than that of the BAPAC, indicating the excellent solar to heat conversion performance of the BAPAC in Figure 5h. We have performed the long-term photothermal testing and high solar concentration testing. As the Figure 5i shown the energy efficiency of the BAPAC (1.3 cm) under different sun in 2 h and the measured error shown the Supplementary Figure 6. With these values, the efficiency of BAPAC can compete with that of those recently reported porous photothermal materials for efficient solar steam generation[52-57]. In addition, based on this approach, the APAC can be easily one-step fabricated at large scale and without limitation in materials sizes or shapes, which thus shows great potentials for practical applications.

5 Conclusions In summary, we have prepared a new bilayer photothermal materials based on ATP/poly composites for efficient solar steam generation. The resulting ATP/poly composites show porous monolithic structure, low thermal conductivity, exceptional mechanical property and strong light absorption. As for a solar steam generator, the ATP/poly composites show a high energy conversion efficiency of 85 % under 1sun illumination. The findings of this work may provide a new approach for design and

fabrication natural-based low cost solar to heat conversion materials for the solar steam generation.

Author information Corresponding Author Prof. Dr. An Li, Tel.: +86-931-7823125. Fax: +86-931-7823125. E-mail address: [email protected] Present Addresses College of Petrochemical Technology, Lanzhou University of Technology, Langongping Road 287, Lanzhou 730050, P. R. China Author Contributions An Li has a holistic grasp of the central thought of the whole article; Juan Jia, Weidong Liang*, Hanxue Sun, Zhaoqi Zhu, Chengjun Wang, An Li* provided technical support. All authors co-wrote the manuscript. Notes There are no conflicts to declare. Acknowledgment The authors are grateful to the National Natural Science Foundation of China (Grant No. 51663012, 51462021), Support Program for Hongliu Young Teachers of LUT, Collaborative Innovation Team, Gansu Province, China (Grant No. 052005) and Innovation and Entrepreneurship Talent Project of Lanzhou (2017-RC-33).

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Figure Captions Fig. 1 a The preparation process of the attapulgite/poly acrylamide-based composites (BAPAC), b Photograph of the APAC standing on top of a dandelion, c The BAPAC standing on top of a dandelion. Fig. 2 a-d a SEM images of APAC, b High resolution SEM image of the APAC, c SEM images of F-APAC, d High resolution SEM image of F-APAC a) 100 μm, b) 1μm, c) 10 μm, d) 1μm., e TEM image of the F-APAC, f Water CA images of F-APAC. Fig. 3 a-b Compressive stress-strain curves of APAC, a Horizontal plane and longitudinal b, c TGA of the based APAC composites, d The light absorption of the BAPAC from 250 nm-2500 nm, e The pore size distribution of APAC calculated from the isotherm by using BJH method, f N2-adsorption/desorption isotherms of the APAC, g XRD patterns about the APAC, F-APAC and nature ATP, h FT-IR spectra of the natural ATP, APAC and F-APAC. Fig. 4 a Schematic of the BAPAC solar-driven interfacial evaporation device, b The surface temperature variation BAPAC and the APAC in dry state under 1 sun irradiation, c and d Infrared images of the APAC and the BAPAC at different time intervals under 1 sun irradiation. Fig. 5 a Surface temperature change of APAC and BAPAC under 1 sun illumination, b Time-dependent mass change curves of the APAC, BAPAC of different thickness under 1 sun, c Time-dependent evaporation rate change of the APAC, BAPAC of different thickness under 1 sun, d Cycle performance of the APAC and BAPAC (1.3

cm), e Surface temperature change of BAPAC under 2 sun illumination and 3 sun illumination, f Time-dependent mass change curves of the BAPAC (1.3 cm) under 1,2 and 3 sun, respectively, and g Time-dependent evaporation rate change of the BAPAC (1.3 cm) under 1,2 and 3 sun, respectively, h The corresponding conversion energy efficiency of the APAC, BAPAC and water in 1 h, and i The corresponding conversion energy efficiency of the BAPAC (1.3 cm) in 2 h.

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Highlights 1. The ATP/polymers composites were first prepared for solar steam generation. 2. It possesses a high evaporation efficiency of 85 % under 1 sun irradiation. 3. It has advantages of cost-efficient, simple and scalable manufacture process