Accepted Manuscript Title: Membrane-Spacer Assembly for Flow-Electrode Capacitive Deionization Authors: Ki Sook Lee, Younghyun Cho, Ko Yeon Choo, SeungCheol Yang, Moon Hee Han, Dong Kook Kim PII: DOI: Reference:
S0169-4332(17)32944-6 https://doi.org/10.1016/j.apsusc.2017.10.021 APSUSC 37364
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Please cite this article as: Ki Sook Lee, Younghyun Cho, Ko Yeon Choo, SeungCheol Yang, Moon Hee Han, Dong Kook Kim, Membrane-Spacer Assembly for Flow-Electrode Capacitive Deionization, Applied Surface Science https://doi.org/10.1016/j.apsusc.2017.10.021 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Membrane-Spacer Assembly for Flow-Electrode Capacitive Deionization Ki Sook Lee,a,b,1 Younghyun Cho,a,1 Ko Yeon Choo,a,b SeungCheol Yang,c Moon Hee Han,b,* Dong Kook Kima,* a
Separation and Conversion Materials Laboratory, Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea. bGraduate
School of Energy Science and Technology, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 304-764, Republic of Korea. cMarine
Energy Convergence and Integration Laboratory, Korea Institute of Energy Research, 200, Haemajihaean-ro, Gujwa-eup, Jeju-si, Jeju-do 63357, Republic of Korea.
Author information Corresponding Author a,
*Tel: +82 42 860 3152. FAX: +82 42 860 3133. E-mail: [email protected]
*Tel: +82 42 821 8600. FAX: +82 42 822 3334. E-mail: [email protected]
1 Co-first author. b,
Highlights We combined the membrane and spacer into a single unit, by coating the ion-exchange membra ne on a porous ceramic structure. In order to prevent the penetration of ion-exchange membrane into a porous ceramic structure, S iO2 particles and polymer multilayer films were coated. Ion-exchange membrane could be coated only onto the top surface of porous structures, while th e internal pores was maintained.
ABSTRACT: Flow-electrode capacitive deionization (FCDI) is a desalination process designed to overcome the limited desalination capacity of conventional CDI systems due to their fixed electrodes. Such a FCDI cell system is comprised of a current collector, freestanding ion-exchange membrane (IEM), gasket, and spacer for flowing saline water. To simplify the cell system, in this study we combined the membrane and spacer into a single unit, by coating the IEM on a porous ceramic structure that acts as the spacer. The combination of membrane with the porous structure avoids the use of costly freestanding IEM. Furthermore, the FCDI system can be readily scaled up by simply inserting the IEM-coated porous structures in between the channels for flow electrodes. However, coating the IEM on such porous ceramic structures can cause a sudden drop in the treatment capacity, if the coated IEM penetrates the ceramic pores and prevents these pores from acting as saline flow channels. To address this issue, we filled the larger microscale pores on the outer surface with SiO2. To address this issue, we blocked the larger microscale pores on the outer surface with SiO2 and polymeric multilayers. Thus, the IEM is coated only onto the top surface of the porous structure, while the internal pores remain empty to function as water channels.
Keywords: Capacitive deionization, Ion-Exchange Membrane, SiO2 particle, Polymer multilayer coating, Membranespacer assembly 1. Introduction Water is an absolute necessity for human life, and thus it is very important to have sufficient safe and clean water available[1,2]. As a result, desalination technologies have received much attention. Currently, the most popular desalination technologies include reverse osmosis (RO) and multistage flash distillation (MSF). However, because of their high energy consumption (RO: 2–4 kWhm-3, MSF: 55–80 kWhm-3), attention is being focused on capacitive deionization (CDI) that uses electrochemical methods[3–6]. A CDI cell has two carbon electrodes stacked on each side of a spacer layer that acts as the channel for water flow. When a set voltage is applied between the carbon electrodes, the cation (Na+) and anion (Cl-) in the saline water flowing through the spacer channel migrate towards the carbon electrodes, and become adsorbed on the electric double layer there, thereby eliminating the salt (NaCl)[7–13]. However, because the carbon electrode surface area in CDI is fixed, adsorption of the ions on them is saturated after a certain time, and then no further adsorption occurs. Consequently, CDI must have a process for removing the ions adsorbed on the electrode, by applying a reverse or zero potential. Much effort has been put forth to increase the efficiency and capacity of CDI in order to address this issue. Membrane CDI (MCDI) with assembly of the IEM inside the CDI cell has been developed to increase the desalination efficiency. It also resolved the problem of ions going over to the counter electrode and becoming adsorbed, when a reverse potential is applied during the regeneration process, which increases the driving force for removing ions in the next cycle[14–22]. Moreover, to eliminate the fundamental need for a regeneration process, a problem shared by all CDI systems, the flow-electrode capacitive deionization (FCDI) process was developed by using flow carbon electrodes instead of the fixed ones used in conventional CDI [23–29]. As a result, the FCDI process could carry out continuous desalination without the need for a discharging step. In the FCDI system, IEMs are stacked on both sides of a spacer layer, which acts as a channel for water flow just as in the case of MCDI. However, there is the main difference that the carbon electrode is a flowing electrode, not a fixed one. Consequently, continuous desalination becomes possible since the salt ions are removed as they are adsorbed on the flowable carbon electrodes. This FCDI process overcomes the most fundamental limitation from the fixed electrode that CDI faces. However, it is not sufficiently cost-effective, because the process system is complex and requires an expensive freestanding IEM. Moreover, studies up to now have increased the cell size or stacking the cells to increase the scale of FCDI. Yet doing so also requires more freestanding IEM, which also makes it uneconomical. In the present study, we coated the IEM on the surface of a porous ceramic structure (spacer) to create a cell system in which the IEM and spacer were combined into a single unit for FCDI. By combining the IEM and the spacer, expanding the cell size for scaling up the FCDI no longer requires the use of many expensive film-type IEMs. The scaling-up process also becomes much simpler by inserting the coated ceramic structure in between the current collectors. However, since the ceramic structure includes pores with a diameter of tens of μm, IEM coated on the surface of the porous ceramic structure could block all pores in the structure that act as the channel for saline water flow. Accordingly, there is a sudden drop in the saline water treatment capacity, even completely stopping the desalination process. To resolve these issues, we blocked micro-sized pores on the surface of the porous ceramic structure with spherical particles and polymeric multilayers. Then, IEM was coated on top of them in an effort to prevent the IEM from penetrating the pores. We used a two-step approach to minimize the surface pore size. First, the pores were filled with SiO2 particles hundreds of nm in size to reduce the large micro-sized pores. During this step, the micropores were mostly blocked by hexagonally packed particle arrays. However, since smaller nanopores remained between the particles, it was still possible for the IEM to penetrate these pores. Accordingly, the remaining surface pores (tens of nm in size) were coated with a multilayer polymer film[32–34]. After the second step, even nm-sized pores were completely blocked in an efficient manner. This prevented the IEM from penetrating the pores, and allowed the successful creation of a single combined unit of membrane-spacer system. 2. Experimental 2.1. Materials and experimental method The porous ceramic structure that acts as the spacer was comprised of cordierite (Mg2Al4Si5O18) (902, REPTON, Japan) with thickness of 2 mm and porosity of 44 %. The sample was sonicated in DI water for 1 h using an ultrasonic cleaner (POWERSONIC 603, Hwashin Technology Co., Korea), followed by drying for 1 h in an oven set to 105°C. Then, SiO2 particles with a diameter of 600 nm (SS03N, Bangs Laboratories, Inc., U.S.A.) were used to fill the pores in the structure. Specifically, 5 wt% of SiO2 particles were added to a mixture of DI water and ethanol (1:1 volume ratio). The ceramic
sample was immersed in the SiO2 suspension for 30 min, followed by 30 min of heat treatment in an oven at 70°C. This process was repeated twice. After SiO2 particle coating, multiple polymer coatings were applied using poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrenesulfonate) (PSS) (both from Sigma-Aldrich Co., Korea). For this step, PAH and PSS (1 mg/ml each) were dissolved separately in aqueous solutions with 1 M NaCl. The sample was dipped in the PAH solution for 10 min and then dipped in DI water for 2 min for washing. The washing process was repeated for a total of two times. Subsequently, the sample was dipped in the PSS solution for 10 min, and also washed twice. We repeated this cycle for a total of 10 times (designated as (PAH/PSS)10), and finally the coated sample was dried at room temperature for 24 h. One side of the sample was coated with a cation-exchange membrane (CEM) solution (20 wt%, poly (phenylene oxide) in dimethylacetamide), and the other side with an anion-exchange membrane (AEM) solution (17 wt%, poly (phenylene oxide) in N-methyl-2-pyrrolidone), then dried in an oven set to 70°C. 2.2. Water content measurements We measured the water content in the samples coated with only SiO2 and those coated with (PAH/PSS)10 multilayers after SiO2 coating. To eliminate moisture prior to the measurement, samples were dried thoroughly for over 1 h in a 60°C oven in vacuum. After immersion in DI water for over 1 h, the samples were taken out and their outer surfaces wiped dry. The measurements were taken for 30 min at 105°C using an infrared moisture balance (FD610, KETT, Japan). 2.3. Ion transportation measurements A constant-voltage desalination test was performed, in which the button-shaped samples with different coatings were soaked in saline water (3.5 g/L) for about 1 h; placed between two graphite current collectors; and the same amount of carbon slurry was injected into the graphite part on both sides with an injector (Fusion 100 Touch, REVODIX, Korea). The carbon slurry was prepared by adding 10 wt% of activated carbon to NaCl solution with a concentration of 3.5 g/L, and stirring for 24 h. A voltage of 1.2 V was applied, using a potentiostat (Zive SP2, Wonatech, Korea). The migration patterns of Na+ and Cl-
Fig. 1. Schematic drawings of (a) membrane-spacer assembly system for FCDI and (b) the coating procedure.
were identified from the current data, and the total amount of ion migration was calculated by integrating the current over time.
3. Results and discussion Fig. 1 shows the schematics of the combined single spacer-IEM unit for the FCDI cell (top), and the coating method for reducing the surface pore size on the porous ceramic structure (bottom). We attempted to simplify the cell system by directly coating the IEM on the surface of the porous ceramic structure that acts as the spacer. However, because of the large pores on the ceramic structure (10–30 μm), the coated IEM could possibly penetrate the pores. In that case, the pores designed to act as the spacer are blocked, causing a sudden drop in saline water treatment capacity. To resolve this problem, we applied two other coatings prior the IEM coating. The first coating is SiO2 particles (hundreds of nm in size), followed by a polymer multilayer. Thus, the IEM is prevented from penetrating the pores since the surface pores are effectively blocked. Coating with the SiO2 particles alone left small pores (several to tens of nm in size) between the particles, which the IEM can still penetrate. Therefore, a polymer multilayer on top of the SiO2 coating is necessary to block these small spaces. The parameters for the SiO2 particle coating were varied (Fig. S1) to determine the following optimal conditions: particle size of 600 nm, 2 coatings of 5 wt% of SiO2. When using very small SiO2 particles (390 nm), large pores remained after the coating, whereas very large SiO2 particles (960 and 3200 nm) could not efficiently block the pores. Fig. 2 shows the field emission-scanning electron microscope (FE-SEM) images of various samples. In Fig. 2(a) and (f), the pristine structure had pores with a diameter of 10–30 μm. Therefore, NaCl solution can easily flow through the pores, but the IEM can also easily pass through them to penetrate the pores (Fig. 2(b), (g)). Specifically, when the surface is directly coated with the IEM, the resulting IEM penetration eventually blocks all the pores (Fig. 2(b) and (g)). Since the blocked pores no longer function as channels for the flow of NaCl solution, the cell for desalination could not function. Next, we blocked the larger pores (μm size) on the surface with
Fig. 2. FE-SEM plane images of coated porous ceramic structures: (a) pristine structure, (b) with IEM coating, (c) with SiO2 coating, (d) with SiO2 and polymer coatings, and (e) with SiO2, polymer, and IEM coating. (f)–(j) are the corresponding cross-sectional images.
Fig. 3. Contact angle measurements of the ceramic structure: (a) pristine, (b) with SiO2 coating, and (c) with SiO2 and (PAH/PSS)10 coatings.
SiO2 particles prior to the IEM coating. We dipped the samples in SiO2 solution, during which time the SiO2 particles were packaged by a self-assembly process into a hexagonal pattern in the surface pores (Fig. S2). As shown in Fig. 2(c) and (h), this step resulted in a smaller pore size than that of the pristine sample. However, even assuming perfect hexagonal packaging of these SiO2 particles (several hundred nm in size), smaller spaces (tens of nm) remained between the particles or from defects, and these smaller spaces still could allow IEM penetration. Therefore, we applied PAH/PSS multilayer coating after
Fig. 4. (a) Water content of coated structure and (b) water content ratio before and after IEM coating.
the SiO2 coating to block the smaller pores, as shown in Fig. 2(d) and (i). The PAH and PSS polymers were used to form the multilayer by layer-by-layer assembly method. By applying both SiO2 and polymer multilayer coatings, the surface pores were effectively blocked while the internal pores remained, thereby resolving the problem of the IEM diffusing into and blocking the pores. At the same time, a sufficient amount of internal pores remained to channel the flow of NaCl solution (Fig. 2(e), (j)). Energy-dispersive X-ray (EDX) spectroscopy analysis also supports their different coating behavior as shown in Fig. S3. On the pristine porous structure, the carbon signal originating from IEM was uniformly distributed through the cross direction. In contrast, it was only concentrated at the top surface of the sample coated with SiO2 and polymer layers. The same observation could be made with the naked eye, as shown in Fig. S4. Furthermore, we measured the water contact angles of the samples with only SiO2 and both SiO2 and polymer coatings, in order to determine how much water could diffuse through the surface and indirectly measure the ratio of blocked sample surface area. The contact angle was measured for 5 s in 1 s increments after the water droplet touched the sample surface. As shown in Fig. 3(a), the pristine ceramic structure had very large pores; hence, the water droplet diffused into the sample immediately. This suggests that water (and possibly the IEM) could penetrate the very large pores immediately. Fig. 3(b) shows that the sample coated with only SiO2 still allowed water droplets to penetrate, but at a slower rate than the pristine sample since the microscale surface pores had been blocked. Similarly, it was possible for the IEM to penetrate these small-sized pores. In the sample coated with both SiO2 and polymer multilayers, even the small surface pores were blocked, hence water diffusion occurred very slowly for over 5 s (Fig. 3(c)). Based on these results, it can be deduced that samples coated with both SiO2 and polymer multilayers have the smallest surface pores and consequently the lowest amount of IEM penetration during IEM coating. To determine the amount of internal pores in different samples through which water can flow, we also measured the water holding capacity of the coated samples, as shown in Fig. 4. Samples sufficiently soaked in water were placed on the infrared moisture balance, after removing moisture on the surface. The samples were then dried under infrared light for 30 min at 105°C, and the water content was measured as the weight difference before and after drying. As shown in Fig. 4(a), the samples without IEM coating showed similar water contents of 17.1%, 17.3%, and 17.0%. It indicates that the porous ceramic structure coated with either only SiO2 or both SiO2 and polymer multilayers include enough internal pores for
Fig. 5. (a) Schematic of constant-voltage desalination experiment, and (b) change of electric current of membrane-spacer assembly system with time for different coating conditions operated at NaCl concentration of 3.5 g/L and a constant voltage of 1.2 V.
holding water, despite blockage of the surface pores. It allows the coated porous structures to be utilized as water channels. However, pristine samples directly coated with IEM showed the lowest water content of 1.4%, due to the IEM penetrating the pores to block most of the pores that can hold water. However, the water content (11.3%) increased drastically in the sample coated with IEM after SiO2 coating, because the SiO2 coating reduced large surface pores, and significantly reduced the level of penetration by IEM. Moreover, when the IEM was coated after both SiO2 and polymer multilayer coatings, the water content (13.8%) was higher than that coated with only SiO2 and IEM. This is because the additional polymer coating blocked the very small pores. Based on the measurement, water content ratios of the samples were derived using Equation (1):
𝑊𝑎𝑡𝑒𝑟 𝐶𝑜𝑛𝑡𝑒𝑛𝑡 𝑅𝑎𝑡𝑖𝑜 =
Here, W1 represents the water content prior to IEM coating, while W2 represents that after IEM coating. Theobtained water contents and their ratios are shown in Table 1. According to Fig. 4(b), the water content ratio between the pristine ceramic structure and that directly coated with IEM is 0.082, meaning that only about 8.2% of the water holding capacity was retained after the IEM coating. In the sample coated with SiO2, applying the IEM coating reduced the water holding capacity by much less, to about 65.3%. Finally, by applying IEM coating on samples coated with both SiO2 and polymer, as much as 81.2% of the water holding capacity remained. Such drastic increase in water content ratio with the introduction of SiO2 and polymer onto porous structures strongly indicates that the penetration of IEM is efficiently prevented, which provides internal room for the water flow channels. A constant-voltage desalination test was performed using samples coated with SiO2 particles, polymer multilayer, and then IEM, by measuring the electric current under a constant applied voltage. These currents indicate the amount of Na and Cl ions within the saline water that migrated within a set time period, while the total amount of ion migration could be calculated by integrating the electric current over time. Thus, the desalination effect was indirectly derived. Button-shaped cell samples were soaked in 3.5 or 1.75 g/L NaCl solution for 5 min, then a constant voltage of 1.2, 0.9, and 0.6 V was applied, and the changes in the electric current were observed over time. Here, the amount of carbon suspension was fixed. Fig. 5(a) is an illustration of the ion migration when a constant voltage is applied to coated samples containing NaCl, and the experimental results are shown in Fig. 5(b) and Fig. Fig. S5. The initial electric current (I0) of each sample and the integrated current over a 1-h period (Q) are
shown in Table 2. The lowest Q value means the lowest amount of ion migration during a set amount of time. When the pristine ceramic structure was coated with IEM, IEM penetrated the pores and blocked the channel for water flow, and thus the volume of NaCl solution it can hold was extremely low, hence the low values of I0 and Q up to 1 h. In the sample coated with SiO2 followed by IEM, the I0 and Q values up to 1 h did not show significant improvement. This could be attributed to that while the μm-sized pores were blocked by the SiO2 particles, IEM could still penetrate the remaining smaller pores (such as the gaps between SiO2 particles and defects). However, these values increased significantly when IEM coating was applied after both SiO2 and polymer multilayer coatings. We believe the reason is that the polymer coating blocked the remaining small pores on the surface, preventing the coated IEM from penetrating the pores. As a result, the ceramic structure could hold more NaCl solution, allowing a larger amount of ions to migrate under the constant applied voltage and leading to high I0 and Q values. These results correspond well with the measured water contents. 4. Conclusions In conclusion, in an effort to simplify the FCDI cell system, we developed a method to combine the spacer and IEM as a single unit. A porous ceramic structure was used as the spacer, which had very large pores of about 10–30 μm to allow easy flow of NaCl solution. However, when the IEM was coated on top of this porous ceramic structure, IEM penetrated these large pores (i.e., channels that allow the flow of NaCl solution) and blocked them. In order to resolve such problems, we applied two different coating steps to reduce the surface pore size prior to the IEM coating. In the developed layering method, first the micro-sized pores were filled with SiO2 particles, and then coating of PAH and PSS polymer multilayers blocked the remaining smaller pores. Consequently, the sample coated with SiO2–polymer–IEM layers had more internal pores than those with IEM coating directly on the pristine structure or that after SiO2 coating. As a result, the first sample also had more channels that NaCl solution can flow through. Therefore, its water content was higher, and so were the I0 and integrated Q values during desalination tests. We expect that our approach can be applied to design and fabricate FCDI cells to scale-up desalination systems with lower costs and less laborious processes.
Acknowledgements This work was supported by the Korea Institute of Energy Research (KIER) (B7-2461-01)
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Table 1. Water Content and Water Content Ration of Various Ceramic Structure Samples. Water Content (%) Pristine
Water Content Ratio
IEM on Pristine
IEM on SiO2
Polymer Layer IEM on SiO2 & Layer
Table 2. Initial and Integrated Electric Currents in Coated Structures at Various Experimental Conditions.
3.5 g/L NaCl at 1.2 V
IEM on Pristine IEM on SiO2 IEM on SiO2 & Polymer
1.75 g/L NaCl at 1.2 V
3.5 g/L NaCl at 0.9 V
3.5 g/L NaCl at 0.6 V
Q (c) (1 h)
Q (c) (1 h)
Q (c) (1 h)
Q (c) (1 h)