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Superhydrophobic Anodized Fe Surface Modified with Fluoroalkylsilane for Application in LiBr–Water Absorption Refrigeration Process Jian He, Ming Mao, Yuting Lu, Wei Jiang, and Bin Liang Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 23 Dec 2016 Downloaded from http://pubs.acs.org on December 26, 2016

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Industrial & Engineering Chemistry Research

Superhydrophobic Anodized Iron Surface for LiBr Refrigeration 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

Superhydrophobic Anodized Fe Surface Modified with Fluoroalkylsilane for Application in LiBr–Water Absorption Refrigeration Process

Jian Hea,b, Ming Maoa, Yuting Lua, Wei Jianga,*, Bin Lianga

a

Multi-phases Mass Transfer and Reaction Engineering Laboratory, School of Chemical Engineering, Sichuan University, Chengdu, 610065, China b

State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology, Xuzhou, Jiangsu, 221116, China.

Abstract LiBr refrigerating systems are frequently used in industry, but the pipelines are easily corroded or blocked by the LiBr solution with high flow resistance. Here, a superhydrophobic Fe surface was proposed and tested for applicability. After constructing a rough Fe2O3 nanotube array on a Fe surface by the anodization process, a superhydrophobic Fe surface was obtained by silane modification. The as-prepared superhydrophobic surface exhibited excellent repulsion to LiBr solutions. The modified Fe foil showed a 3.35% decrease in thermal conductivity but a 99.2% improvement of anti-corrosion protection efficiency. LiBr crystals deposited on this surface were easily detached. The flow resistance along the superhydrophobic surface was reduced to 50% of that along a pure Fe surface. The operation temperature of the system was broadened *

Corresponding author. Tel.: +86-28-85990133; fax: +86-28-85460556. E-mail address: [email protected]

1 Environment ACS Paragon Plus



due to low blockage risk. The excellent thermal conductivity, anti-corrosivity, drag reduction, and anti-fouling performance of the superhydrophobic Fe surface exhibits promise for industrial application.

Keywords: Superhydrophobic surface; Lithium bromide refrigeration; Anti-corrosion; Anti-scaling; Drag reduction; Anodization


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1. Introduction Since the 1997 discovery by Neinhuis and Barthlott of the water-repelling properties of lotus leaves 1 , characterized by the so-called “lotus effect,” many efforts have been devoted to mimicking this natural property by preparing artificial superhydrophobic materials with water contact angles (CA) exceeding 150°

2-4

. Various methods have

been developed to construct different superhydrophobic surfaces5,6. The surface structure and surface free energy were identified as crucial factors in controlling surface wettability and thereby obtaining superhydrophobic surfaces. Therefore, two strategies to prepare superhydrophobic surfaces were established 7. In the first, a rough structure is built on the surface of a hydrophobic material. In the other, a rough surface is modified with hydrophobic chemicals. For most engineering metals with high surface energies, it is typical to modify their hydrophilic surfaces with low-surface-energy reagents, such as fluoroalkylsilanes 8, fatty acid 9, and mercaptan

10

, to obtain superhydrophobicity.

Therefore, the construction of rough structures on metal surfaces is crucial to prepare super-hydrophobic surfaces since the modification step is relatively simple. Typical methods for achieving adequately rough surfaces on metal surfaces include templating, sol-gel processing, chemical vapor deposition, laser and plasma treatment, evaporationinduced phase separation, electrospinning, anodization, etching, and electrochemical processing

11-14

. The preparation of superhydrophobic surfaces on metals could permit

broad applications, such as self-cleaning 15, oil/water separation 16, antifogging 17, anticorrosion 18, anti-scaling 19, drag reduction 20, and heat transfer improvement 21. Fe is an important engineering metal for both industrial and domestic applications. In industrial refrigeration processes, LiBr refrigeration systems are common for waste heat recovery and refrigerating. Fe is often used as the material for the pipes conducting the



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flow and heat exchange of the LiBr solution. However, Fe usage has three drawbacks: the corrosion by aqueous Br− of Fe 22, pipe blockage by LiBr crystal deposition at low temperatures solutions

23

, and high flow resistance to highly concentrated, high-viscosity LiBr

24

. Some preventative measures such as anti-corrosion additives

crystallization inhibitors

25

22

or

have been adopted, but these methods may increase

operating costs and environmental risk. Since superhydrophobic surfaces can effectively realize anti-corrosivity, drag reduction, and scaling resistance simultaneously, the application of such a functionalized surface to Fe could solve these three problems simultaneously. Thus, it is necessary to prepare a superhydrophobic surface on Fe and investigate its performance for anti-corrosivity, drag reduction, and scaling resistance with the LiBr system, determining the feasibility of implementation in an industrial LiBr refrigerating system. Building a rough texture is the necessary premise to obtain a superhydrophobic Fe surface. Anodization is a common method for creating rough structures on metals because of its facile processing, low cost, and morphological control

26.

Metals such as

Cu 27, Ti 28, Al 29, Nb 30, and alloys 31 have been used to prepare rough superhydrophilic surfaces via anodization, which can construct fine micro- and nanoscale structures such as nanotubes, nanoneedles, and nanowires on a metallic surface. Such hierarchical nanostructures can assist in creating excellent superhydrophobicity when combined with low-surface-energy modifiers

32, 33

. The anodization of Fe can be performed to

construct a Fe2O3 layer on the surface; this layer can be used as the anode of Li-ion batteries 34, 35 or as a photocatalyst 36 . However, the preparation of a superhydrophobic surface on Fe by the modification of anodized Fe has not been reported before.


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In this research, a superhydrophobic Fe foil was prepared with a two-step anodization process of Fe in an NH4F/glycol solution and treatment with a typical low-free-energy modifier, 1H,1H,2H,2H-perfluorooctyltriethoxysilane (FAS-17). The anodization of Fe to form a nanotube array was investigated in detail. The effect of the preparation parameters, including the temperature and voltage, on the superhydrophobicity of the as-prepared Fe products is discussed. The applicability of superhydrophobic Fe in LiBr refrigeration systems was studied by determining the anti-corrosivity, scaling resistance, thermal conductivity, and drag reduction of the treated Fe in LiBr solutions.




2. Experimental 2.1 Materials Commercially available Fe foils (purity ≥ 99.99%) were purchased from Dongguan Mingjue Metal Materials Co., Ltd. (China). 1H,1H,2H,2H-perfluorooctyltriethoxysilane (FAS-17) was purchased from Sicongprotect Chemicals Corp. (China). All other chemical reagents, including ethylene glycol (EG), NH4F, ethanol, acetone, and LiBr, were analytic-grade and obtained from Kelong Chemicals Corp. (Chengdu, China). All reagents were used without further treatment unless otherwise specified. 2.2 Synthesis of Fe2O3 nanotube films The Fe foil was mechanically polished with 1000 and 1200 grit metallographic sandpapers in succession, and then ultrasonically cleansed with acetone and ethanol for 10 min each. The foil was dried by flowing N2 before anodization. The anodization process was initiated in 200 g aqueous EG solution containing 0.1 mol/L NH4F and 3 vol% deionized water in EG. The Fe foil served as anode while Pt was used as the cathode. The distance between the two electrodes was maintained at 3 cm. A two-step electrochemical anodization method was used to fabricate the Fe2O3 nanotube films. The Fe foil was first anodized at 50 V for 5 min, and the resulting oxidized layer was removed by ultrasonic vibration in ethanol for 10 min. The pretreated Fe was used again as the anode for the second-step anodization at 50 V for 15 min. Aligned nanotube arrays were formed on the foil, which was then ultrasonically cleaned with ethanol for 10 s. The prepared sample was crystallized by annealing in a muffle furnace (TMF5, Thomas) under an O2 atmosphere at 400 °C for 1 h. 2.3 Preparation of superhydrophobic surface


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The FAS-17 solution was prepared by mixing 1 g of FAS-17 and 99 g of ethanol for 3 h under continuous stirring at the rate of 100 revolutions per minute. The as-prepared anodized Fe foil was immersed in the FAS-17 solution at ambient temperature for 6 h. The treated Fe foil was then washed with ethanol and thermally treated in a muffle furnace at 400 °C for 1 h. 2.4 Characterization of Fe foil The static CAs of the foil with water were measured with a CA meter (Powereach, JC2000C1) at room temperature. Water droplets of approximately 5 µL were dropped gently onto the surface of the sample. Three points on each sample were tested and the average value of the three measured CAs was reported as the static CA. The surface morphology of the Fe foil was observed with a scanning electron microscope (SEM, JEOL JSM-5900LV) and stereo microscope (SZM45, Changzhou Dedu Precision Instruments Co., LTD. China). The crystal structure was studied with an X-ray diffractometer (XRD, DX2700). The existence of FAS-17 on the treated film was identified by judging the absorption spectra obtained from Fourier-transform infrared spectroscopy (FTIR, Spectrum Two, L1600300). The thermal conductivity of the samples was determined by a hot plate method with a coefficient of thermal conductivity meter (DRH-III, Xiangyi Instrument Co., LTD, China). 2.5 Anti-scaling experiment The scaling resistance performance of the superhydrophobic Fe surface to LiBr precipitation was tested by dropping 5 µL droplets of a 50 wt% aqueous LiBr solution onto the heated Fe surface in an oven with a 5 mL syringe. The temperature was maintained at 100 °C for ~2 h to vaporize the water, forming a LiBr crystal. The foil was removed from the chamber and the rolling angle of the crystal was determined by




lifting the foil. The crystal structure grown on the surface was observed with stereo microscope and SEM. 2.6 Anti-corrosion experiment The anti-corrosion performance was evaluated by electrochemical testing using a conventional three-electrode cell with a capacity of 100 mL. A Pt dish measuring 1 × 1 cm served as the counter electrode, Ag/AgCl/KCl (3 mol·L-1) was the reference electrode, and Fe foil with an exposed area of 1 cm2 was the work electrode. In all cases, the corrosion experiments were performed in a 50 wt% aqueous LiBr solution. The measurements of the open circuit potential (OCP) and Tafel plot spectra were performed on an electrochemical workstation (Shanghai, China. CHI660E). The potential dynamic Tafel plots were obtained at a scan rate of 1 mV·s-1 in a range of ±250 mV versus the OCP. 2.7 Drag reduction experiment The drag reduction effect was demonstrated by testing the flow resistance to a LiBr solution through a home-made microchannel system constructed of the as-prepared superhydrophobic Fe surface. Various liquids ( pure water and LiBr solutions of different concentrations) were pumped into the microchannels by a double-plunger micrometering pump (Beijing Xingda Sci. Dev. Co., Ltd., China. 2ZB-1L10). The pressure drop of liquids flowing through the microchannels was measured with a U-type differential gauge.


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3. Results and Discussion 3.1 The formation mechanism of Fe2O3 nanotubes In this research, a two-step electrochemical anodization method was adopted to prepare Fe2O3 nanotubes (FeNTs). The voltage is maintained at 50 V. The change in current with time is shown in Figure1-A. The current drops dramatically during the first 60 seconds of the first anodization step, which is attributed to the formation of the initial Fe2O3 layer. Later, the current increases slowly because of the competition between the dissolution and successive formation of the oxide layer. After 5 min of anodization, the Fe foil was removed and the formed oxidation layer is wiped off with ultrasonic cleansing. In the second anodization step, the current also decreases swiftly in the first 30 s. However, the lowest current value is somewhat higher than the lowest value in first stage, before it begins to increase gradually. Almost 10 min later, the current begins to stabilize. This can be ascribed to the achievement of competitive equilibrium between the formation and dissolution of the Fe2O3 layer on the surface of the Fe foil. The firststep anodization can be considered as pretreatment for the formation of a well-ordered nanoporous template for the growth of nanotubes in the second anodization stage. The morphology of the surface, marked with a circle, at different stages is observed with SEM. As shown in Figure 1-B, the surface in scenario (i) is smooth without obvious scratches after the mechanical polishing. This assists the formation of uniform nanotubes without counting the effect of the Fe foil surface smoothness. In Figure 1-C, some pits, cracks, and crystals appear on the surface in scenario (ii). This damaged surface can be attributed to the dissolution of the electrochemical oxidation products of Fe. The crystals deposited on the surface may have originated from the accumulation of oxidants because of the relatively slow dissolution rate compared to the high oxide




generation rate. Figure 1-D of scenario (iii), at the end of the first anodization step, shows the top view of the Fe foil surface. More pits and cracks are formed, and no residual crystals are observed. Rudimentary nanotubes can be seen, but the shape, density, and arrangement of the tubes are irregular, with some large nonporous materials covered. These dense oxide covers could hinder the further formation of nanotubes in the second step of the anodization process. After the removal of these covers, the irregular porous structure under the covers can serve as a template for the second anodization step. The cross-section of the oxide layer in inserted SEM graph confirms the embryonic form of nanotubes with shallow depth of ca. 50 nm. Figure 1-E depicts the Fe surface at the turning point in the second step, marked as scenario (vi) in part 1-a. The disordered preliminary morphology of the nanotubes can be observed. After 15 min anodization, the grown nanotubes shown in Figure 1-F have become ordered with clear shapes and edges. The cross-section view of obtained sample in inserted SEM graph shows the array architecture with length of about 600 nm.

3.2 Characterization of as-prepared FeNTs XRD investigation was performed in order to obtain more insight on the crystal structure and chemical composition of the porous anodized layers. The patterns shown in Figure 2-A confirm that only the characteristic peaks of pure Fe are observed in the freshly anodized Fe sample. However, after annealing at 400 °C in O2 for 1 h, the characteristic peaks of pure rhombohedral Fe2O3 (space group: R3c (167), a = 0.5035, b = 0.5035, c = 1.3748 nm; JCPDS card No. 33-0664) are observed. This confirms that the as-prepared nanotubes are amorphous in structure, but are converted to the crystalline hematite structure by annealing. No other peaks of impurity phases, such as


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g-Fe2O3 or Fe3O4, are observed. The diffraction peaks of the FeNTs are strong and sharp, indicating good crystallization. Therefore, the formation mechanism of the FeNTs can be constructed. In the beginning of the first anodization step, an oxidized hematite layer with a higher electrical resistance than that of pure Fe is formed, causing the sudden drop in the anodic current. The following reaction is suggested as a possibility for the formation of the oxidation layer (reaction 1) 35 . 2Fe + 3H2O → Fe2O3 + 3H2

(1)

About 30 s later, the oxidation layer begins dissolving, possibly following the reaction expressed as follows: Fe2O3 +2F- +6H+ → 2[FeF6]3- + 3H2O

(2)

Most of the dissolved Fe ions leave the surface and enter the solution, but some return to the surface to form crystals because of the high concentration 37 . The possible reactions are expressed as follows: Fe3+ + F-1→ FeF2+

(pK1 = 5.28)

(3)

FeF2+ + F-1→FeF2+

(pK2 = 4.02)

(4)

FeF2+ + F-1→FeF3

(pK3 = 2.76)

(5)

Afterward, with increasing current caused by the thinning of the hematite layer, increasingly more Fe2O3 is dissolved. A balance between the formation and dissolution of the amorphous hematite layer is established. Nanostructures with cracks or holes can form during this balancing process. However, this nanostructure is irregular, because some crystals remain on the surface and affect the balance. Therefore, after removing these crystal barriers from the surface and exposing the fresh surface, the second anodization step can promote the formation of a regular nanostructure, such as the




nanotubes observed in Figure 2-B, because the exposed irregular structures can be utilized as the nucleation point of the second anodization process. The diameter of the nanotubes in the final product is ~50 nm, with a tube wall thickness of ~20 nm. With the two-step anodization, the distribution of the FeNTs became more tightly packed with greater regularity. However, the voltage and temperature of the anodization process affect the final FeNT products, as shown in Supporting Information Figure S1 and S2. The water static CA of the as-prepared anodized Fe surface is 2 ± 1°, as shown in the inset graph in Figure 2-B. Such superhydrophilicity confirms that a sufficiently rough anodic Fe surface assists in converting a hydrophilic hematite surface to superhydrophilic Fe. This can be easily transformed into superhydrophobicity after modification. This transformation is attributed to the anodized Fe foil with well-aligned nanotubes, which has a large specific surface area because of its rough structure.

3.3 Preparation of superhydrophobic surface The superhydrophobic modification of the anodized Fe surface was performed by treatment with FAS-17, a low-surface-energy substance with a typical fluoroalkylsilane structure commonly used to prepare hydrophobic surfaces. Figure 3-A exhibits the morphology of the modified FeNTs. No significant changes are detected except some thickened nanotube openings, attributed to the polymeric growth of FAS-17. The FT-IR result shown in Figure 3-B also confirms the successful loading of FAS-17

38

. The dominant IR bands at 1200 cm-1 and 715 cm-1 represent

different vibration modes of C-F2 groups. The peaks at 1070 cm-1, 810 cm-1 correspond to Si-O-Si asymmetric and symmetric stretching vibration absorptions. The presence of


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the new peaks of C-F2 and Si-O-Si verifies that the FAS-17 molecules cover the surface of the FeNTs. These peaks emerged after modification, thereby confirming the loading of FAS-17. The CA value of the modified FeNT sample, determined in Figure 3-C, increased significantly to 156 ± 2° and the rolling angle shown in Figure 3-D is only 3 ± 1°. The excellent superhydrophobicity and low adhesion of the FeNTs modified with FAS-17 is thereby confirmed. Such superhydrophobicity and adhesion are reaffirmed by the water droplet contact experiment. A water droplet suspended from a needle tip touches the surface lightly and is lifted immediately. As shown in Figure 3-E, the contact area of the water droplet on the surface is very small, corresponding to a large CA and high superhydrophobicity. After lifting, no obvious deformation of the water droplet is observed, and the lifting process occurs without delaying and residue. Figure 3-F confirms the uniformity of the superhydrophobicity, since water droplets form spheres anywhere on the large-area FeNT surface.

3.4 Influence of modification time and temperature on superhydrophobicity Surface roughness and low surface energy are the two decisive factors influencing the superhydrophobicity of a substance. The anodization process supplies the necessary surface roughness, and treatment with FAS-17 provides the proper surface energy. However, the modification time and temperature of the FAS-17 treatment significantly influence the superhydrophobicity and successive application thereof. The effect of the modification time on CA was investigated as shown in Figure 4-A. The water CA increases significantly with increasing reaction time. The determined CA is only 8.3° for modification times of less than 5 min. After 6 h of modification, the CA




value is increased to 100.5°. When the time is prolonged to 18 h, the CA reaches 142.8°. This phenomenon is ascribed to the self-aggregation of FAS-17 monomers on the FeNT surface with its high surface area and energy. From this experiment, the optimal modification time of 12 h is determined, because the calculated CA has leveled off at this time. Although FAS-17 is loaded onto the surface of FeNTs to produce a large CA, this does not necessarily result in superhydrophobicity. A thermal treatment is introduced to decompose the self-aggregated FAS-17 and obtain superhydrophobic FeNTs. Samples loaded with FAS-17 were annealed at temperatures of 100 °C, 200 °C, 300 °C, 400 °C, and 500 °C for 1 h. Their CA values were determined as shown in Figure 4-B. The modified surface reaches superhydrophobic status with a CA value of 152 ± 2° when the temperature is above 300 °C. At this point, the aggregated FAS-17 is decomposed and dispersed evenly onto the surface, thereby inducing superhydrophobicity. For a temperature of 500 °C, the surface returns to superhydrophilicity. Thus, the optimal temperature of the thermal treatment step is chosen as 400 °C because of the determined CA of 156 ± 2°.

3.5 Wettability of LiBr solution on modified surface Since Fe with a high water static CA was successfully prepared, its wettability against LiBr solutions was then tested. A series of LiBr solutions with different concentrations were prepared and measured for their CA values on the modified FeNT surfaces, as shown in Figure 5-A. The results confirm that all determined CA values exceed 150°, regardless of the LiBr concentration. However, the CA value decreases slightly with increase in the LiBr concentration, which can be ascribed to the increase in


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the solution density with increasing salt concentration. Thus, it can be concluded that the superhydrophobic FeNTs surface can repel the LiBr solutions very well, which is beneficial to the proposed application in LiBr refrigeration systems as a repellent surface. The stability of the superhydrophobic FeNT surface with the 50 wt% LiBr solution is evaluated, with determined CA values plotted against contact time in Figure 5-B. The superhydrophobic status is maintained for more than 10 days of immersion in 50 wt% LiBr solution. However, a gradual decrease in the CA is observed. This degradation can be ascribed to the slow corrosion of the Fe2O3 nanotubes by the LiBr solution.

3.6 Electrochemical measurement In order to investigate the corrosion process of the superhydrophobic FeNT surface in detail, confirming its anti-corrosion performance, electrochemical testing including measurements of the OCP and polarization curve were conducted. The performances of a pure Fe foil, anodized Fe surface, and modified Fe surface were compared in a typical aqueous solution of 50 wt% LiBr. The OCP curve in Figure 6-A provides information on the corrosion process and the stability of the coatings without the application of an external current or voltage

39, 40

.

The tested final OCP of the pure Fe is -0.48 V, lower than that of the anodized Fe, -0.41 V. The modified Fe foil initially shows a somewhat lower OCP value of -0.321 V than the anodized Fe value of -0.303 V. However, the OCP value of the modified Fe foil decreases much more slowly than those of the anodized and pure Fe. After 30 min, the determined OCP of the superhydrophobic FeNT is -0.35 V. This relatively high OCP value for the modified Fe foil confirms that it resists corrosion better than the other two



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foils in 50 wt% LiBr solution, which can be ascribed to the loading of the superhydrophobic FAS-17 layer. The SEM image of the tested sample in Figure 6-B shows that the morphology of the superhydrophobic FeNTs is preserved with little change from the original state, although some large particles are found on the top openings of the nanotubes. These enlarged particles are aggregates of the FAS-17, which cause the decrease in the CA value of the modified Fe surface. The polarization curves of the three samples in 50 wt% LiBr solution are determined with a scanning rate of 0.01 V/s and used to evaluate their corrosion rates. The Tafel plots shown in Figure 6-C exhibit the corrosion potential (Ecorr) and corrosion current (icorr). The corrosion rate, v, can be calculated by Tafel extrapolation as follows: v=

M jcorr nF

(6)

Here M is the metal molar mass, n is the atomic valency of the oxidized metal, and F is the Faraday constant. jcorr is the corrosion current density, here equal to the determined icorr because the areas of all samples are 1 cm2. The protection efficiency can be expressed as 41 :

η=

i0 - icorr × 100% i0

(7)

where i0 is the corrosion current of pure Fe and icorr represents the corrosion current of the anodized or modified Fe. The calculated values of the protection efficiency, η, of the anodized and modified Fe are 5.4% and 99.2%, respectively. Based on Figure 6-C, the corrosion rates of pure, anodized, and modified Fe are calculated as 1.02 × 10-6 g·m-2·h-1, 5.18 × 10-8 g·m-2·h-1, and 7.24 × 10-9 g·m-2·h-1, respectively. The calculated protection efficiency of an anodization FeNT layer on Fe is only 5.4%, while modification with the superhydrophobic FAS-17 increases the value of


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η to 99.2%. The SEM image of the measured superhydrophobic FeNT surface in Figure 6-D also confirms the cracking of the loaded FAS-17 layer on the top of the nanotube array. This damage suggests that the superhydrophobicity of the modified Fe foil provides the anti-corrosivity, preventing the further corrosion of Fe in the LiBr solution.

3.7 Deposition and crystallization of LiBr on modified FeNT surface A serious problem in the LiBr refrigeration industry is the potential blockage of fluid channels by LiBr crystals deposited from concentrated LiBr solutions. Here the possibility of LiBr crystallization on the superhydrophobic Fe surface, and the adhesion and detachment of LiBr crystals on it, are investigated. A droplet of 60 wt% LiBr solution is placed on the modified FeNT surface as shown in Figure 7-A. The droplet keeps spherical and clear under room temperature. Then such droplet is cooled and heated sequentially. The droplet shown in Figure 7-B is postcooling at 15 °C. No significant change is observed, except for some white precipitate found at the bottom of the droplet. However, the droplet maintains its initial sphere shape, and can roll freely on the surface. In Figure 7-C, an aspheric solid particle is observed after the superhydrophobic surface is heated with an electric furnace to vaporize water. The LiBr crystal easily rolls away from the surface when the surface is inclined at ~3°. This indicates extremely low adhesion between the solidified LiBr crystal and the superhydrophobic surface, which is conducive to relieving the risk of blockage formation if LiBr were to precipitate from the solution during operation. It is supposed that the LiBr fouling grown on the treated surface could be easily detached with erosion by the flowing liquid. In this way, the




superhydrophobic surface could reduce the accumulation of crystals in the transportation tube. Polarizing microscope photos shown in Figure 7-D exhibit the cooled droplet. A white crystal precipitate at the bottom is observed through the liquid layer. However, in Figure 7-E and Figure 7-F, a semi-solidified ball formed during the heating process and the final solid ball after the termination of heating are observed, respectively. It can be determined that the wettability of the superhydrophobic surface does not affect the formation of LiBr crystals, but the aspheric shape of the droplet on the superhydrophobic surface limits the crystallization process in a confined space, resulting in the final spherical shape of the solidified LiBr crystals. After splitting the solidified LiBr particle in half, a hollow structure is shown in Figure 7-G. It can be deduced that the vaporization of water and crystallization of the LiBr solute occurs from the outside to the inside of the solution droplet. The SEM photograph of the LiBr shell in Figure 7-H exhibits many microchannels, which are passages for the escaping vaporized water inside the crystal. Observing the contact point of the FeNT surface with the LiBr solid, we find no LiBr residue. Therefore, it can be concluded that the modified Fe surface exhibits excellent anti-fouling and self-cleaning performance.

3.8 Thermal conductivity and flow pressure drop The thermal conductivity coefficients of pure Fe, anodized Fe, and modified Fe foils are determined as 80.02 W/m·K, 78.83 W/m·K, and 77.34 W/m·K, respectively. Only a slight degradation of 3.35% can be observed with the modifications. This negligible performance decay can be ascribed to the generation of an extremely thin Fe2O3 layer


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and the loading of the FAS-17 layer; both of these materials are poor thermal conductors compared to pure Fe. Combining the superhydrophobic FeNT surface into a device as shown in Figure 8-A, the heat exchange performance and pressure drop are measured. The temperature difference between the inlet and outlet is determined with a high-precision temperature sensor. The temperature of the inlet water and the aqueous 55 wt% LiBr solution was maintained at 20.0 °C. The heating source was an electric heating jacket set to 100 °C. The determined temperature differences of the water and LiBr solution with pure, anodized, and modified Fe foils are plotted in Figure 8-B. Only a slight decrease in the temperature difference with increasing flow rate can be observed, which confirms the negligible loss in heating efficiency of the superhydrophobic Fe surface. The pressure drop of water and 55 wt% LiBr solution flowing through the microchannel is detected with a U-tube differential gauge, as shown in Figure 8-C. The difference in pressure drop for the three samples is significant. The pressure drops of both water and LiBr solution flowing through the microchannel with the superhydrophobic surface are only ~50% of that through the anodized Fe channel, and ~60% of that through the pure Fe channel. This demonstrates the drag reduction effect of the superhydrophobic surface, which decreases flow resistance. The pressure drop of 65 wt% LiBr solution flowing through the microchannel is plotted against the channel temperature, as shown in Figure 8-D. The pressure drop increases

with

decreases

in

the

surrounding

temperature.

The

modified

superhydrophobic Fe exhibits the drag reduction effect under most operation temperatures. For temperatures of 45 °C and below, the pressure drop increases sharply. For pure and anodized Fe, the pressure drop increases to more than 5,000 Pa at 40 °C,




which could cause liquid leakage with further temperature decrease. This is attributed to the blockage of the microchannel by LiBr crystals, since the precipitation temperature of a 65 wt% LiBr solution is 47 °C. However, the increase of the pressure drop in the microchannel prepared with the superhydrophobic Fe foil is moderate. The lowest operation temperature is 30 °C with a pressure drop of ~4,300 Pa. It can be concluded that the employment of a superhydrophobic Fe surface would significantly decrease the risk of channel blockage, broadening the operating temperature range of a LiBr refrigeration system.


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4. Conclusion In this study, structured FeNTs were successfully prepared through a two-step anodization process in an ethylene glycol solution with 0.1 mol/L NH4F and 3 vol% water. The obtained FeNTs were modified with FAS-17 to create a superhydrophobic surface, which was tested for compatibility with a LiBr system to realize simultaneous anti-corrosivity, anti-fouling, and drag reduction. Water and LiBr solutions of different concentrations have contact angles exceeding 150° on the modified FeNT surface, and maintain these values for more than 10 days. A decrease of ~3.35% in the thermal conductivity of the modified Fe foil relative to that of pure Fe was observed. However, the anti-corrosion protection efficiency reached 99.2%, and the LiBr crystals precipitated onto the treated surface were easily detached, demonstrating anti-fouling behavior. The pressure drops of water and LiBr solution flowing through microchannels lined with the as-prepared superhydrophobic FeNT was significantly reduced to ~50% of that in microchannels with pure and anodized superhydrophilic Fe2O3 surface. The risk of channel blockage by precipitated LiBr crystals was lowered, and the operation temperature regime was broadened. This work confirmed the feasibility of the employment of superhydrophobic Fe surfaces in LiBr refrigerating systems, with promise for further investigation and possible application.

Supporting Information The effect of anodizing voltages and anodizing temperature on the structure of obtained nanostructure of anodized Fe foil. This material is available free of charge via the Internet at http://pubs.acs.org.




Acknowledgements We appreciate the financial support from the National Natural Science Foundation of China Project (No. 21176157 and 21476146).

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Figure Captions Figure 1 The anodization of iron: (a) Current change in two-step anodization process; morphology of iron surface in the first step at (b) 0 s; (c) 100 s; (d) 300 s; morphology of iron surface in the second step at (e) 30 s; (f) 900 s.

Figure 2 Characterization of fresh anodized iron: (a) XRD analysis; (b) nanotubes array structure and its superhydrophilicity.

Figure 3 Modified FeNT surface and its superhydrophobicity: (a) SEM photograph; (b) FT-IR analysis; (c) water static contact angel; (d) rolling angel; (e) up-and-down motion of a water droplet; (f) uniformity of as-prepared sample.

Figure 4 Wettability change of modified FeNT surface against: (a) modification time; (b) modification temperature.

Figure 5 Wettability of LiBr solution on modified FeNT surface against: (a) different LiBr mass concentration; (b) contact time with 50 wt% LiBr solution.

Figure 6 Anti-corrosion performance of modified FeNT surface: (a) OCP results of pure, anodized, and modified iron in 50wt% LiBr solution; (b) morphology of modified iron sample after OCP testing; (c) Tafel polarization curves of pure, anodized, and modified iron in 50wt% LiBr solution; (d) morphology of modified iron sample after polarization.

Figure 7 Crystallization of LiBr on modified FeNT surface: (a) A droplet of 60 wt%




LiBr solution on surface; (b) post-cooled LiBr solution droplet on surface at 15 °C; (c) LiBr solid particle after heated; (d) polarizing microscope photo of cooled LiBr solution droplet; (e) of semi-solidified ball in heating process; (f) of solidified ball after heating; (g) hollow structure of solidified LiBr particle; (h) microchannels on the wall of ball; (i) SEM result of FeNT after removing LiBr ball.

Figure 8 Heat transfer and fluid flow through the channel constructed with modified FeNT surface: (a) schematic diagram of the testing device; (b) temperature difference between the inlet and outlet; (c) pressure drop between the inlet and outlet against liquid volume flow; (d) pressure drop against surrounding temperature.


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Figures

Figure 1 The anodization of iron: (a) Current change in two-step anodization process; morphology of iron surface in the first step at (b) 0 s; (c) 100 s; (d) 300 s; morphology of iron surface in the second step at (e) 30 s; (f) 900 s.




Figure 2 Characterization of fresh anodized iron: (a) XRD analysis; (b) nanotubes array structure and its superhydrophilicity.


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Figure 3 Modified FeNT surface and its superhydrophobicity: (a) SEM photograph; (b) FT-IR analysis; (c) water static contact angel; (d) rolling angel; (e) up-and-down motion of a water droplet; (f) uniformity of as-prepared sample.




Figure 4 Wettability change of modified FeNT surface against: (a) modification time; (b) modification temperature.


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Figure 5 Wettability of LiBr solution on modified FeNT surface against: (a) different LiBr mass concentration; (b) contact time with 50 wt% LiBr solution.




Figure 6 Anti-corrosion performance of modified FeNT surface: (a) OCP results of pure, anodized, and modified iron in 50wt% LiBr solution; (b) morphology of modified iron sample after OCP testing; (c) Tafel polarization curves of pure, anodized, and modified iron in 50wt% LiBr solution; (d) morphology of modified iron sample after polarization.


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Figure 7 Crystallization of LiBr on modified FeNT surface: (a) A droplet of 60 wt% LiBr solution on surface; (b) post-cooled LiBr solution droplet on surface at 15 °C; (c) LiBr solid particle after heated; (d) polarizing microscope photo of cooled LiBr solution droplet; (e) of semi-solidified ball in heating process; (f) of solidified ball after heating; (g) hollow structure of solidified LiBr particle; (h) microchannels on the wall of ball; (i) SEM result of FeNT after removing LiBr ball.




Figure 8 Heat transfer and fluid flow through the channel constructed with modified FeNT surface: (a) schematic diagram of the testing device; (b) temperature difference between the inlet and outlet; (c) pressure drop between the inlet and outlet against liquid volume flow; (d) pressure drop against surrounding temperature.


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