Magnetic Photothermal Nanofluids with Excellent Reusability for Direct

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Magnetic Photothermal Nanofluilds with Excellent Reusability for Direct Absorption Solar Collectors Debing Wang, Lingling Wang, Guihua Zhu, Wei Yu, Jia Zeng, Xiaoxiao Yu, Huaqing Xie, Guangwei Xian, and Qiang Li ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00623 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 12, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Energy Materials

Magnetic Photothermal Nanofluids with Excellent Reusability for Direct Absorption Solar Collectors

Debing Wang a, b, &, Lingling Wang

a, b, &

, Guihua Zhu

a, b

, Wei Yu

a, b,

*, Jia Zeng c,

Xiaoxiao Yu d, Huaqing Xie a, b, Guangwei Xian a, b, Qiang Li c

a

School of Environmental and Materials Engineering, College of Engineering,

Shanghai Polytechnic University, Shanghai, 201209, China b

Research Center of Resource Recycling Science and Engineering, Shanghai

Polytechnic University, Shanghai, 201209, China c

School of Energy and Power Engineering, Nanjing University of Science and

Technology, Nanjing, 210094, China d

School of Energy and Power Engineering, Nanjing University of Aeronautics and

Astronautics, Nanjing, 210016, China

1

ACS Paragon Plus Environment

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Solar thermal collector and heat exchanger are two indispensable components in the practical application of nanofluids in the photothermal technology. There is inevitable heat loss produced by the heat exchanger. Here, we propose a new concept “magnetic photothermal nanofluids”, which not only can avoid the heat transfer process of heat exchanger, but also can use the base liquid of nanofluids directly. The magnetic cobalt nanoparticles (NPs) embedded in nanoporous carbon (Co@NC) are prepared via carbonization of zeolitic imidazolate framework-67 (ZIF-67). The obtained Co@NC retains the morphology of original ZIF-67 and the magnetic Co nanoparticles distribute evenly in the nanoporous carbon. Co@NC has an excellent magnetic property and broad absorption in the visible and infrared region. The maximum photothermal conversion efficiency has been achieved by 100ppm Co@NC/EG nanofluids at carbonization temperature of 900℃, which is 99.6% at 60s. This magnetic photothermal nanofluid can be reused at least 60 successive cycles without significant loss of photothermal conversion efficiency. This study paves a new avenue for direct use the base liquid of nanofluids in the solar thermal conversion technique.

KEYWORDS: Magnetic photothermal nanofluids, Photothermal conversion efficiency, Reusability, Solar energy, Direct absorption solar collectors

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1. INTRODUCTION Solar energy, as a clean and inexhaustible renewable energy, attracts great attention due to its expanding applications in photovoltaic, photochemical and photothermal technologies.1 The photothermal technology has made a great progress over the past few decades. However, two inherent challenges still exist for the practical application of photothermal technology. The first involves the poor optical absorption efficiency of solar thermal collectors and low thermal conductivity of working fluids.2-3 The second involves the high heat loss when the absorbed energy of working fluids transfer to target fluids through the heat exchanger.4 Solar thermal collectors are widely used for conversion of solar energy into thermal energy. The conventional solar thermal collector is the flat-plate collector,5 most of the solar energy is absorbed by the plate-tube and then transfers to working fluids in tubes via conduction and convection in the collector.6 The plate-tube is the hottest component of the system, leading to large amount of heat loss.7-8 Therefore, the flat-plate collector suffers from relatively low outlet temperature and efficiency. In order to improve the efficiency of solar thermal collector, the directly absorption solar collector (DASC) is proposed by Minardi and Chuang in the 1970s.1,9 In DASC, the solar radiation is directly absorbed by the working fluids. Compared with the conventional flat-plate collector, DASC has less heat loss at the absorption surface.5 However, the photothermal conversion efficiency is limited by the conventional working fluids such as water, ethylene glycol with poor absorption properties and low thermal conductivities.5,10 Therefore, seeking for efficient working fluids is significant for capturing solar irradiation. Nanofluids, 3



proposed by Choi in 1995,11 are especially attractive due to their good thermo-physical and optical absorption properties.12-13 The efficiency of DASC using nanofluid as a working medium is higher than that of flat-plate collector, which is first investigated by Tyagi et al.5 Wen et al. found that the photothermal conversion efficiency was enhanced significantly by gold nanofluids.14 Various other nanofluids including metals,15-16 metal oxide,17-18 carbon nanomaterials19-20 and hybrid nanocomposites21 are also investigated. All the results indicate that adding nanoparticles into base fluids can remarkably improve the photothermal conversion efficiency in DASC. Therefore, DASC with nanofluids offer opportunities to improve the utilization efficiency of solar energy. However, for nanofluids, the base fluid is polluted and cannot be used directly due to the presence of nanoparticles. The base fluid can only be used as a working medium. The typical direct solar thermal absorption with a closed-loop circulating nanofluid is shown in Figure 1a.22 The absorbed energy of nanofluids must be transferred to target fluids through the heat exchanger in the practical application. To obtain higher heat transfer efficiency, various kinds of heat exchangers and nanofluids with high conductivities are attractive.22-23 But the corrosion and fouling of the tube bank of the heat exchanger still cannot be avoided, leading to inevitable heat loss and reduced heat exchange efficiency.24 Therefore, if the assumption, which not only can avoid the heat transfer process of heat exchanger but also can direct use the base liquid of nanofluids, can be realized, it is intriguing. The schematic diagram of direct utilization of the base liquid of nanofluids for DASC is 4


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shown in Figure 1b.25 The key of the assumption is exploring a nanomaterial which is easily separated from the fluids by an external force. Zeolitic imidazolate framework-67 (ZIF-67) is a new coordination polymer formed by self-assembly of metal ions and organic ligands.26 Due to excellent properties with high specific surface area and high porosity, it is widely used in the field of adsorption separation,27 heterogeneous catalysis,28 solar cells29 and environment remediation30. The fantastic property of the carbonated ZIF-67 is the outstanding magnetic controllability,31-32 which paves a new avenue for direct use the base liquid of nanofluids by magnetic separation technique. In the present study, the magnetic cobalt nanoparticles embedded in nanoporous carbon (Co@NC) are prepared via one-step carbonization of ZIF-67 with nitrogen. Then the “magnetic photothermal nanofluids” are prepared by dispersing Co@NC into ethylene glycol (EG) under sonication. The optical property, photothermal performance and the reusability of magnetic nanofluids are investigated in detail.

2. EXPERIMENTAL SECTION

2.1. Materials The main chemicals used in this study are cobalt(II) acetate tetrahydrate (Co(Ac)2.4H2O) and 2-methylimidazole (H-MIM). Both of them are available from Sinopharm Chemical Reagent Co. Ltd., Shanghai, China and used without further purification. 5



2.2. Synthesis of ZIF-67 Crystal In order to synthesize the Co@NC materials, the ZIF-67 crystal is firstly prepared according to the literature,33 which is shown in Figure 2. Typically, 7.7 g of (Co(Ac)2.4H2O) and 110 g of H-MIM are dissolved in 60 mL and 400 mL deionized water (DI) respectively, named solutions A and B. Then the solution A and B are mixed and stirred for 6 h at room temperature. The purple solid is obtained by centrifuging and washing 3 times with water and methanol subsequently, and finally vacuum-dried at 80℃ for 24 h.

2.3. Synthesis of Magnetic Photothermal Nanofluids The obtained ZIF-67 powders are carbonized directly under nitrogen atmosphere at temperatures from 600-900℃. Typically, the ZIF-67 powders are evenly dispersed in the ceramic boat and placed them into the tube furnace, and then exposed to a flow of nitrogen at room temperature for 1 hour. Then the furnace heats to the target temperature (600℃, 700℃, 800℃ and 900℃) with a rate of 10 ℃ min-1 and keeps at the constant temperature for 2 hours. The obtained materials after carbonation at different temperature are denoted as Co@NC-600, Co@NC-700, Co@NC-800 and Co@NC-900, respectively. The nanofluids containing Co@NC are prepared by ultrasonic treatment of the Co@NC powders in the base liquid of EG. Then the stable and well dispersed nanofluids are obtained.

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2.4. Characterization The morphology is observed with a scanning electron microscopy (SEM, Hitachi S4800). The detailed structures are further measured by Transmission electron microscope (TEM, JEOL-JEM-1005). Powder X-ray diffraction (XRD) patterns are recorded using a X-ray diffractometer (XRD, D8-Advance) with monochromatized Cu radiation (λ=1.54 Å ). The magnetic properties of the sample are characterized by a vibrating sample magnetometer (VSM, Lake Shore 7307) apparatus at room temperature. The porous structure and surface area are characterized by nitrogen adsorption-desorption

technique

(TRISTARII

3020)

at

77

K.

The

Ultraviolet-visible-near infrared spectrophotometer (UV-Vis-NIR, Cary 5000) was used to measure the optical property of Co@NC.

2.5. Evaluation of the Photothermal Conversion Properties Graphic description of the photothermal conversion experimental system and the detailed process were depicted in our previous paper.16 In a typical test program, a cuvette filled ( full and sealed ) with 3.5 mL nanofluids is placed in the photothermal conversion system at room temperature of 25℃, then irradiated under 720W m-2 for 1200s. The photothermal conversion efficiency is calculated according to the following formula:

η=

mCp (Tf-Ta) (1)

AG∆t

where m is the mass of nanofluids (3.8g in the current work), Cp is the specific heat of ethylene glycol (EG) and nanofluids. The concentration of Co@NC-based-EG 7



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nanofluids is very low, and the difference of temperature during the whole experiments is not very sharp in our work, so the specific heat of water at 25℃ (2.35×103J/(kg×℃)

can

be

calculated

for

the

specific

heat

of

all

nanofluids approximately. Ta is the initial temperature (25℃), Tf is the instantaneous temperature, A is the exposure area (4 cm2 in the current work), G is the incident solar heat flux (720 W·m-2 in the current work), ∆t is the time exposed to solar radiation (20 min in the current work). The tests were repeated for three times to get the average values and the measurement error ranged from 0.2%~1%.

2.6. The Reusability of Magnetic Photothermal Nanofluids A 1548-Gs NdFeB magnet with a size of 30 mm×10 mm×2 mm is used as an external magnetic field in the magnetic separation experiment. The reusability experimentals are as follow: firstly, the irradiated nanofluids are cooled to room temperature with ice. Secondly, the Co@NC materials are separated from the nanofluids by magnet and then redispersed in EG by ultrasonic. The photothermal performances of the magnetic nanofluids are repeated for 60 times.

3. RESULTS AND DISCUSSION 3.1.Physical Characterizations Figure 3 shows the photographs of the Co@NC-900/EG nanofluids with different concentrations. The colors of the nanofluids are getting darker and darker with the increase of the concentrations. No precipitations are observed for all the nanofluids 8


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with different concentrations, even after being stored for two weeks shown in Figure 3b. The results indicate Co@NC possess good dispersion stability in EG. Figure 4a and Figure 4b-e show SEM images of ZIF-67 and Co@NC obtained at different carbonization temperatures, respectively. ZIF-67 exhibits the regular dodecahedral shapes and high symmetry with smooth surfaces, which is also observed by Guo et al.34 The average sizes of ZIF-67 crystals are about 400 nm. After carbonation of ZIF-67 in N2, Co@NC with highly rough surface is formed. More and more wrinkles on the surface of Co@NC are clearly observed with the increase of temperature. The size of Co@NC is smaller than ZIF-67. All these phenomena can be attributed to the collapse of the structure of ZIF-67 crystals at high temperatures. Fortunately, all Co@NC retain the morphology of original ZIF-67. The clear structure of Co@NC is further observed by TEM as shown in Figure 4f-g and Figure 4h. A number of carbon nanotubes are embedded in Co@NC, which can be observed from Figure 4f-g. The formation of carbon nanotubes can be due to the catalytic effect of Co nanoparticles,34 which origin from the parent ZIF-67 and distribute evenly on the surface of Co@NC. The distinct lattice fringes of Co nanoparticles with a spacing of 0.20 nm have good agreement with the (111) lattice plane of the metallic Co (Figure 4h).35 The size of Co nano-particles becomes larger with the increase of carbonization temperature (Figure 4b-e). Figure 5a shows the XRD patterns of ZIF-67. The characteristic peaks of XRD patterns of ZIF-67 are consistent with the previous report36: 7.31o (011), 10.36o (002), 12.74o (112), 14.40o (022), 16.45o (013), 18.04o (222), 22.15o (114), 24.53o (233), 9



25.62o (224), 26.69o (134), 29.67o (044), 30.62o (334), 31.55o (244) and 32.43o (235). It is proved that the ZIF-67 prepared by the current method has high purity. Figure 5b shows that XRD patterns of Co@NC have broad and less-resolved peaks compared with the sharp peaks of the parent ZIF-67 crystals. Three main peaks at 44.28°, 51.63° and 75.97° are assigned to the indexes of (111), (200) and (220) of Co (PDF#89-4307), suggesting the good crystallization of Co nanoparticles. The peak at 26° is attributed to the (002) diffraction mode of the graphitized carbon structure. Usually cobalt nanoparticles are easily oxidized to cobalt oxide in air. However, no characteristic peaks of CoO are found in the current work, which indicate that Co nanoparticles are uniformly embedded in nanoporous carbon. It is significant to prevent Co nanoparticles from being destroyed by the air, which is the key factor for achieving a complete separation of Co@NC from nanofluids by the magnetic separation technology. N2 adsorption-desorption isotherms are used to evaluate the specific surface area and pore texture of ZIF-67 and Co@NC-900. As shown in Figure 6, ZIF-67 has a type I isotherm which is characteristic of microporous structure. However, Co@NC exhibits a type IV isotherm with a small hysteresis loop in high pressure (P/P0), indicating the presence of mesopore structure. The BET surface area and total pore volume of Co@NC-900 is 272.9 m2g-1 and 0.4 cm3g-1 (Table 1), which is much smaller than ZIF-67 (1721.5 m2g-1 and 0.8 cm3g-1). The pore volume due to mesopore structure becomes larger after carbonization. All these changes indicate the ZIF-67 structure collapses, leading to the formation of nanoporous carbon. The 10


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presence of Co nanoparticles in nanoporous carbon could further reduce the BET surface area of Co@NC-900. Magnetization plays a key role in direct use of the base liquid of nanofluids in the current study, so the room-temperature magnetization curves of Co@NC-600, Co@NC-700, Co@NC-800 and Co@NC-900 are investigated and shown in Figure 7. All the samples show the typical hysteresis loops, indicating Co@NC have ferromagnetic and superparamagnetic properties with permanent magnetic moment.30 The

saturation

magnetization

(Ms)

values

of

Co@NC-600,

Co@NC-700,

Co@NC-800, Co@NC-900 are calculated as 41.2, 48.6, 52.2 and 53.9 emu/g, respectively. The increase of Ms values of Co@NC with the carbonization temperature can be attributed to the better crystallinity or larger size of the magnetic cobalt under high temperature,30 which corresponds with the XRD and SEM results. The separation of Co@NC-900 from the nanofluids by using an external magnetic field is shown in Figure 7 (inserted).

3.2. Optical Properties Figure 8a shows the UV-Vis-NIR spectra of EG and Co@NC nanofluids obtained at different carbonization temperatures. Compared with pure EG, the transmittance of all the Co@NC/EG nanofluids is lower than 40% from 200 nm to 1500 nm, implying that the addition of Co@NC can remarkably enhance optical absorption ability. The optical absorption ability of Co@NC/EG nanofluids increases with the carbonization temperatures, which can ascribe to better graphitized carbon 11



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structure. Figure 8b shows the UV-Vis-NIR spectra of different concentrations of Co@NC-900/EG nanofluids (10, 30, 50, 70 and 100ppm). The optical absorption of the Co@NC-900/EG nanofluids improves reasonably with the increases of concentrations. In order to further confirm the optical properties of these nanofluids, the extinction coefficient ( Ke) was calculated according to the Beer-Lambert law: T(λ) = EXP (-Ked)

(2)

In the formula, T(λ) is transmittance of nanofluids, d represents optical length (1 cm). The extinction coefficient of the Co@NC/EG nanofluids increases with the carbonization temperature (Figure 9). The extinction coefficient of Co@NC/EG nanofluids is 2.1 cm−1 at 900℃, much higher than Co@NC/EG nanofluids obtained at 600℃ (1.1 cm−1). The spectral irradiance increases with the concentrations (Figure 10), which is consistent with the extinction coefficient of samples (Figure 9b). In the spectral irradiance graph, the area under the curve represents the energy absorbed by the nanofluids.37 It can be clearly observed that the areas under curves of Co@NC/EG nanofluids are much bigger than those of pure EG. The areas become larger with the increase of concentration. In the practical application, the spatial distribution of the absorbed energy is important for design the optimal system dimension. As shown in Figure 11a, all the Co@NC/EG nanofluids obtain higher absorbed energy distribution than pure EG. The penetration distance of the absorbed energy distribution becomes shallower with the increase of carbonization temperatures. For example, when the absorbed power 12


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fraction is 0.90, the penetration distance for Co@NC-900-10 and Co@NC-900-100 is 7.0 cm and 1.2 cm, respectively. In other words, to gain the same absorbed power fraction, the penetration distance of Co@NC-900-10/EG nanofluids must be 5.8 cm deeper than that of Co@NC-900-100/EG nanofluids. According to the above analysis, the Co@NC-900-100/EG nanofluid is a good candidate for application in the working fluids for DASC.

3.3. Photothermal Conversion Performance Figure 12a shows the temperature rise of EG and Co@NC nanofluids with different carbonization temperatures. The temperatures of all the samples increase with the irradiation time. The temperature rise of Co@NC/EG nanofluids is more obvious than EG, suggesting that adding a small amount of Co@NC nanoparticles into EG can remarkably improve the solar energy absorption. When the carbonization temperature of Co@NC materials is 900℃, the Co@NC/EG nanofluids exhibit the best optical absorption among all the samples. Furthermore, the temperature rises of different concentrations of Co@NC-900/EG nanofluids are also investigated. The higher concentrations of Co@NC-900/EG nanofluids, the higher temperature rises. Figure 12c-d show that the Co@NC-900-100/EG nanofuids have the best photothermal conversion efficiency among all the samples. The instantaneous photothermal conversion efficiency declines gradually with the irradiation time. The photothermal conversion efficiency is 48% at 1200s, much less than 99.6% at 60s. The temperature in the liquid is low and the heat dissipation is not significant in the 13



beginning. With the increase of temperature in the liquid, the temperature disparity between the liquids and the environment becomes more distinct, leading to more heat dissipation. Therefore, the temperature rise ratio slows down and the photothermal conversion efficiency gradually reduces, which is also observed by Chen et al.38 The reusability of magnetic photothermal nanofluids was further evaluated and shown in Figure 13. The histogram represents photothermal conversion efficiency, the green line indicates the initial temperature and the red line is the terminal temperature after irradiation for 1200s. After the irradiation, the Co@NC nanoparticles are separated from nanofluids under the external magnetic field and cooled to room temperature with ice cubes before the next cycle. It is observed that the photothermal conversion efficiency can remain around 45% in 20 min and there is no obvious decrease after 60 successive cycles of reuse. Meanwhile, the terminal temperatures are all around 40℃ after irradiation for 1200s. All these suggest that Co@NC exhibits the outstanding reusability in the photothermal conversion process and it is ideal for direct utilization of the base liquid in nanofluids.

4. CONCLUSIONS In this work, Co@NC is selected as a magnetic material which can be prepared via one-step carbonization of ZIF-67. The maximal Ms value of Co@NC is 53.9 emu/g and the magnetic Co nanoparticles distribute evenly in the porous carbon lattice, which is favorable to separate Co@NC particles from nanofluids by the external magnetic field. The photothermal conversion efficiencies of all the samples are higher 14


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than pure EG. The Co@NC-900-100/EG nanofluids exhibit the best photothermal conversion efficiency among all the samples, and there is no obvious decrease even after 60 successive cycles of reuse. The outstanding reusability and photothermal conversion performance of the current “magnetic photothermal nanofluids” enable the direct utilization of base liquid in nanofluids realize.

AUTHOR INFORMATION Corresponding Authors *(W. Y.) E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (51476094 and 51590901), Shanghai Municipal Natural Science Foundation (Grant No. 17ZR1411000), the Key Subject of Shanghai Polytechnic University (Material Science and engineering, XXKZD1601 and EGD18YJ0042), and Gaoyuan Discipline of Shanghai-Environmental Science and Engineering (Resource Recycling Science and Engineering).

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Collectors: A Review. Nanomaterials, 2017, 7,131.1-31. [23] Kumar, V.; Tiwari, A. K.; Ghosh, S. K. Effect of Variable Spacing on Performance of Plate Heat Exchanger using Nanofluids. Energy 2016, 114, 1107-1119. [24] Maré, T.; Halelfadl, S.; Sow, O.; Estellé, P.; Duret, S.; Bazantay F. Comparison of the Thermal Performances of Two Nanofluids at Low Temperature in a Plate Heat Exchanger. Exp. Therm. Fluid. Sci. 2011, 35, 1535-1543. [25] Teng, K. H.; Kazi, S. N.; Amiri, A.; Habaliet, A. F.; Bakar, M. A.; Chew, B. T.; Al-Shamma'a, al. A.; Shaw, A.; Solangi, K. H.; Khan, G. Calcium Carbonate Fouling on Double-Pipe Heat Exchanger with Different Heat Exchanging Surfaces. Powder Technol. 2017, 315, 216-226. [26] Dhakshinamoorthy. A.; Alvaro, M.; Garcia, H. Metal-Organic Frameworks as Heterogeneous Catalysts for Oxidation Reactions. Catal. Sci. Technol. 2011, 1, 856-867. [27] Zhang, Z. H.; Zhang, J. L.; Liu, J. M.; Xiong, Z. H.; Chen X. Selective and Competitive Adsorption of Azo Dyes on the Metal-Organic Framework ZIF-67. Water Air Soil Poll. 2016, 227, 471.1-12. [28] Yang, L.; Carreon, M. A. Deoxygenation of Palmitic and Lauric Acids over Pt/ZIF-67 Membrane/Zeolite 5A Bead Catalysts. ACS Appl. Mater. Interfaces 2017, 9, 31993-32000. [29] Jing, H. Y.; Song, X. D.; Ren, S. Z.; Shi, Y. T.; An, Y. L.; Yang, Y.; Feng, M. Q.; Ma, S. B.; Hao, C. ZIF-67 Derived Nanostructures of Co/CoO and Co@N-doped 19



Graphitic Carbon as Counter Electrode for Highly Efficient Dye-Sensitized Solar Cells. Electrochim Acta 2016, 213, 252-259. [30] Torad, N. L.; Hu, M.; Ishihara, S.; Sukegawa, H.; Belik, A. A.; Imura, M.; Ariga K.; Sakka, Y.; Yamauchi, Y. Direct Synthesis of MOF-Derived Nanoporous Carbon with Magnetic Co Nanoparticles toward Efficient Water Treatment. Small 2014, 10, 2096-2107. [31] Lin, K. Y. A.; Lee, W. D. Self-Assembled Magnetic Graphene Supported ZIF-67 as a Recoverable and Efficient Adsorbent for Benzotriazole. Chem. Eng. J. 2016, 284, 1017-1027. [32] Li, X. L.; Zhang, W.; Liu, Y. S.; Li, R. Palladium Nanoparticles Immobilized on Magnetic Porous Carbon Derived from ZIF-67 as Efficient Catalysts for the Semihydrogenation of Phenylacetylene under Extremely Mild Conditions. ChemCatChem 2016, 8, 1111-1118. [33] Qian, J.; Sun, F.; Qin, L. Hydrothermal Synthesis of Zeolitic Imidazolate Framework-67 (ZIF-67) Nanocrystals. Mater. Lett. 2012, 82, 220-223. [34] Guo, M. X.; Gao, T, Ma, H.; Li, H. B. Weaving ZIF-67 by Employing Carbon Nanotubes to Constitute Hybrid Anode for Lithium Ions Battery. Mater. Lett. 2018, 212, 143-146. [35] Lin, K. Y. A.; Hsu, F. K.; Lee, W. D. Magnetic Cobalt-Graphene Nanocomposite Derived from Self-Assembly of MOFs with Graphene Oxide as an Activator for Peroxymonosulfate. J. Mater. Chem. 2015, 3, 9480-9490. [36] Gross, A. F.; Sherman, E.; Vajo, J. Aqueous Room Temperature Synthesis of 20


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Cobalt and Zinc Sodalite Zeolitic Imidizolate Frameworks. Dalton Trans. 2012, 41, 5458-5460. [37] Meng, Z. G.; Li, Y.; Chen, N.; Wu, D. X.; Zhu, H. T. Broad-Band Absorption and Photo-Thermal Conversion Properties of Zirconium Carbide Aqueous Nanofluids. J. Taiwan Inst. Chem. E. 2017, 80, 286-292. [38] Chen, L. L.; Xu, C.; Liu, J.; Fang, X. M.; Zhang, Z. G. Optical Absorption Property and Photothermal Conversion Performance of Graphene Oxide/Water Nanofluids with Excellent Dispersion Stability. Sol. Energy 2017, 148, 17-24.

21



Page 22 of 36

Nomenclature m

mass of nanofluids (g)

Ta

initial temperature (℃)

A ∆t T(λ) d

Cp specific heat (J/(kg×℃)) Tf

instantaneous temperature (℃)

2

G the irradiation power on the surface (W.m-2) top surface area of the cuvette (cm ) the time exposed to solar irradiation (s) ŋ photothermal conversion efficiency (%) transmittance (%) Ke extinction coefficient (cm-1) optical length (cm)

Table 1 Comparison of textural properties between ZIF-67 and Co@NC-900. SBETa

Vtotalb

DBJHc

Vmicrod

Vmesoe

( m2g-1 )

( cm3g-1)

(nm)

( cm3g-1)

( cm3g-1)

ZIF-67

1721.5

0.80

1.9

0.64

0.16

Co@NC-900

272.9

0.40

5.9

0.05

0.35

Samples

a

SBET calculated using BET equation in P/P0 range 0.05–0.3.

b

Single point pore volume at P/P0 = 0.99.

c

DBJH is pore size calculated from BJH theoretical model from desorption branch.

d

Calculated by using the t plot method.

e

Vmeso = Vtotal - Vmicro.

22


Page 23 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a) Nanofluids Inlet

(b)

Heat Exchange Target Fluids Inlet Target Fluids

Circulit Nanofluids Outlet

Solar Collector

TargetFluidsOutlet

Solar Collector

Flow Control Valve

Flow Control Valve

Nanofuids Supply Tank

Regeneration Tank

Separation Tank

CirculatingPump

Circuliting Pump

Figure 1. (a) Typical DASC with a closed-loop circulating nanofluid, (b) Magnetic photothermal nanofluids system for DASC.

23



Page 24 of 36

(Co(Ac)2.4H2O) + CH3OH ZIF-67 A

Ambient temperature Stirred for 6 h

Mixing

Centrifugation/Washed

B

H-MIM + CH3OH Carbonized

Co@NC Pipe furnace N2

N2

Figure 2. Schematic illustration for the synthesis of Co@NC derived from ZIF-67.

24


Page 25 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a

b

Figure 3. Photographs of the Co@NC-900/EG nanofluids with different concentrations (a) fresh prepared and (b) after being stored for two weeks

25



（ a）

Page 26 of 36

（ b）

CoNPs

1µm

1µm

（ c）

（ d） CoNPs CoNPs

1µm

1µm （ f）

（ e） CoNPs

1µm 1µm

100nm 100nm

（ g）

(h) Co(111) 0.20nm

5nm

50nm

Figure 4. SEM images of (a) ZIF-67 crystals, (b-e) Co@NC-600, Co@NC-700, Co@NC-800 and Co@NC-900, (f-g) TEM images of Co@NC-900 and (h) HRTEM image of Co@NC-900.

26


Page 27 of 36

(a)

(b)

( 011 )

ZIF-67

Co@NC-90 0 Co@NC-70 0

C (002)

5

10

15 20 25 2Theta (degree)

Intensity (a .u.)

•

30

C o(11 1) ? Co (200) ?

C o(22 0) ?

( 235 )

( 044 ) ( 334 )

( 244 )

( 114 )

( 233 ) ( 224 ) ( 134 )

013 ) ( 222 ) (

( 022 )

( 002)

( 112 )

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


35

40

20

30

40

50

60

70

80

2Theta (degree)

Figure 5. XRD patterns of (a) as-prepared ZIF-67 and (b) Co@NC materials obtained at different carbonization temperatures.

27



600

Volume adsorbed (cm3g-1 STP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 36

ZIF-67 500

Co@NC-900

400 300 200 100 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (p/p0)

Figure 6. Nitrogen adsorption-desorption isotherms of ZIF-67 and Co@NC-900.

28


Page 29 of 36

60 Magnetization ( emu/g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40 20

Co@NC-600 Co@NC-700 Co@NC-800 Co@NC-900

0 -20 -40 -60 -10000

-5000 0 5000 Magnetic field( Oe )

10000

Figure 7. Magnetization hysteresis curves of Co@NC-600, Co@NC-700, Co@NC-800 and Co@NC-900.

29



100

(a)

EG Co@NC-600-100ppm Co@NC-700-100ppm Co@NC-800-100ppm Co@NC-900-100ppm

80 60 40

Transmittance (%)

100

Transmittance ( % )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

20 0 200

Page 30 of 36

(b)

EG Co@NC-900-10ppm Co@NC-900-30ppm Co@NC-900-50ppm Co@NC-900-70ppm Co@C-900-100ppm

80 60 40 20

400

600

800 1000 1200 Wavelength ( nm )

1400

1600

0 200

400

600 800 1000 1200 Wavelength ( nm )

1400

Figure 8. UV-Vis-NIR spectra of (a) EG and Co@NC nanofluids obtained at different carbonization temperatures and (b) Co@NC-900/EG nanofluids with different concentrations.

30


1600

(a)

Co@NC-900-100ppm Co@NC-800-100ppm Co@NC-700-100ppm Co@NC-600-100ppm EG

4

2

0 200

400

600 800 1000 Wavelength (nm)

1200

Extinction Coefficient (cm-1 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Extinction Coefficient (cm-1 )

Page 31 of 36

1400

(b) Co@NC-900-100ppm Co@NC-900-70ppm Co@NC-900-50ppm Co@NC-900-30ppm Co@C-900-10ppm EG

4

2

0 200

400

600 800 1000 Wavelength (nm)

1200

1400

Figure 9. Extinction coefficients of (a) EG and Co@NC nanofluids obtained at different carbonization temperatures and (b) Co@NC-900/EG nanofluids with different concentrations.

31



1.6

Spectral Irradiance (W*m -2*nm -1)

1.8 Spectral Irradiance (W*m -2*nm -1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

400

600

800 1000 1200 Wavelength (nm )

1400

1600

Page 32 of 36

1.6 (b)

Co@NC-900-100ppm Co@NC-900-70ppm Co@NC-900-50ppm Co@NC-900-30ppm Co@NC-900-10ppm EG

1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

400

600

800

1000

1200

1400

1600

Wavelength (nm )

Figure 10. Spectral irradiance of (a) EG and Co@NC nanofluids obtained at different carbonization temperatures and (b) Co@NC-900/EG nanofluids with different concentrations.

32


Page 33 of 36

1.0

(a)


0.8

Absorbed Power Fraction

Absorbed Power Fraction

1.0

0.6 0.4 0.2

(b)

0.8

0.6

Co@NC-900-100ppm Co@NC-900-70ppm Co@NC-900-50ppm Co@NC-900-30ppm Co@NC-900-10ppm

0.4

0.2

EG

0.0

0.0

0

2

4 6 Penetration Distance (cm)

10

8

10

0

8 6 4 2

EG

600 700 800 Carbonization Temperature (oC)

4

6

8

10

(c)

0

2

10

Penetration Distance (cm)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8 6 4 2 0

900

(d)

0

20 40 60 80 Concentration of Co@C (ppm)

100

Figure 11. Absorption power fraction of (a) EG and Co@NC nanofluids obtained at different carbonization temperatures, (b) Co@NC-900/EG nanofluids with different concentrations, (c) dependence of penetration distance on carbonization temperatures of Co@NC at absorption power fraction≈0.90 and (d) dependence of penetration distance on concentrations of Co@NC-900/EG nanofluids at absorption power fraction≈0.90.

33



42

Temperature (oC)

36 34 32

38 36 34 32

30

30

28

28

26

0

100

200

400

600 Time (s)

800

1000


(c)

80

60

40

20

28

30

32 34 36 Temperature (oC)

38

26

1200

40

42

(b)


40

Photothermal Conversion Efficiency (%)

Temperature (oC)

38

42

(a)


40

Photothermal Conversion Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 36

0

200

400

600 Time (s)

800

1000

1200

120

Photothermal conversion efficiency at 60 s Photothermal conversion efficiency at 120 s

(d)

100

80

60

40

20

0

EG

10ppm

30ppm

50ppm

70ppm 100ppm

Concentration

Figure 12. Temperature rises of (a) EG and Co@NC nanofluids obtained at different carbonization

temperatures,

(b)

Co@NC-900/EG

nanofluids

with

different

concentrations and (c-d) the instantaneous photothermal conversion efficiencies of Co@NC-900/EG nanofluids with different concentrations.

34


50

50

40

45

30

40

20

35

10

30

0

0

10

20 30 40 Recycling times

50

60

Temperature (oC)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Photothermal conversion efficiency (%)

Page 35 of 36

25

Figure 13. Multiple-cycle photothermal conversion efficiencies and temperature variations of 100ppm Co@NC-900/EG nanofluids. (The green line indicates the initial temperature and the red line is the terminal temperature after irradiation for 1200s).

35



Graphic for Manuscript

Magnetic Photothermal Nanofluids with Excellent Reusability for Direct Absorption Solar Collectors Debing Wang a, b, &, Lingling Wang

a, b, &

, Guihua Zhu

a, b

, Wei Yu

a, b,

*, Jia Zeng c,

Xiaoxiao Yu d, Huaqing Xie a, b, Guangwei Xian a, b, Qiang Li c

The magnetic cobalt nanoparticles embedded in nanoporous carbon (Co@NC) are prepared via carbonization of zeolitic imidazolate framework-67 (ZIF-67). The novel magnetic photothermal nanofluids not only can avoid the heat transfer process of heat exchanger, but also can direct use the base liquid of nanofluids. This magnetic photothermal nanofluids can be reused at least 60 successive cycles without significant loss of photothermal conversion efficiency. This study paves a new avenue

50

50

40

45

30

40

20

35

10

30

0

0

10

20 30 40 Recycling times


50

60

25

Temperature (℃)

for direct use the base liquid of nanofluids in the solar thermal conversion technique.

Photothermal conversion efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 36

Magnetic Photothermal Nanofluids with Excellent Reusability for Direct

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