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Cation Exchange Membranes with Controlled Porosity in Electrodialysis Application jian li, Junyong Zhu, Shushan Yuan, Jiuyang Lin, Jiangnan Shen, and Bart Van der Bruggen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01951 • Publication Date (Web): 28 Jun 2017 Downloaded from http://pubs.acs.org on June 29, 2017

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Industrial & Engineering Chemistry Research

Cation Exchange Membranes with Controlled Porosity in Electrodialysis Application Jian Lia, Junyong Zhua, Shushan Yuana, Jiuyang Linb*, Jiangnan Shenc, Bart Van der Bruggena,d* a

Department of Chemical Engineering, KU Leuven, Celestijnenlaan 200F,

B-3001Leuven, Belgium b

School of Environment and Resources, Qi Shan Campus, Fuzhou University, No.2

Xueyuan Road, University Town, 350116 Fuzhou, Fujian, China c

Center for Membrane Separation and Water Science & Technology, Zhejiang

University of Technology, Hangzhou, 310014, China d

Faculty of Engineering and the Built Environment, Tshwane University of

Technology, Private Bag X680, Pretoria 0001, South Africa

*

Corresponding author. E-mail address: [email protected] (J. Lin) [email protected] (B. Van der Bruggen)

ACS Paragon Plus Environment


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Abstract: Numerous attempts have been made to develop ion exchange membranes with low resistance for various applications like electrodialysis and fuel cells. In this study, strategies of immersion precipitation and dry-casting were combined, to control the membrane porosity with the purpose of improving the physical and electrochemical properties of ion exchange membranes. The porosity was tuned by the time of membrane exposure to an elevated temperature environment. In addition to controlling the porosity to balance the membrane electrical resistance with the diffusion caused by the concentration gradient, it was experimentally shown that the porosity can influence the IEC and water uptake of the membrane, and thus further affect the resistance. Furthermore, the surface hydrophilicity was characterized by water contact angle measurements; the results revealed that the porous membranes were more hydrophilic than dense membranes. As evinced by experimental data for desalination by electrodialysis, it was found that a membrane dried at 60 °C for 1 h has the highest desalination efficiency. This is mainly because porous membranes facilitate the transport of ions. Compared to a membrane with higher porosity, a membrane with 1 h aging time has more steric hindrance, which can decrease the diffusion of ions, so that a superior desalination efficiency can be obtained. To further investigate the impact of the density of -SO32- functional groups on the electrodialysis process, membranes with various weight ratios of SPES and PES were prepared. With increasing content of SPES, the physical and electrochemical properties of the newly developed porous membranes were changed. A membrane with higher density of functional groups was found to have a higher desalination efficiency, due to the


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electrostatic effect of the membrane. These results were consistent with the current efficiency. At optimal membrane preparation conditions, the membrane has a high IEC (1.75 mmol/g) and water uptake (168%). The desalination efficiency reached 95% and the current efficiency reached 100%. It was concluded that the performance of a porous membrane with controllable porosity can enhance the ED process with respect to energy efficiency and desalination efficiency. New routes such as immersion precipitation and dry-casting to fabricate membrane with pores are thought potential ways to decrease the electrical resistance. Keywords: porous ion exchange membrane; SPES; electrodialysis; dry-wet phase inversion



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1. Introduction Water is one of the most precious and important resources in our daily lives, and its safety and availability are inextricably linked with public health, energy production and economic development. However, the direct discharge of industrial wastewater potentially threatens the receiving water body when no proper treatment is applied. Membrane technology has been widely applied in industrial applications as a promising technology for wastewater treatment 1. Compared to other membrane processes, electrodialysis (ED) has been extensively used for the production of potable water from brackish water sources, recovery of valuable metals from the effluents of metal-plating industry, production of high quality industrial process water, and treatment of certain industrial effluents, due to its high efficiency, environmental benignity and operational simplicity 2. As shown in Fig. 1, five parts are essential for a electrodialysis process, including a direct current supply, electrodes, ion exchange membranes, solvents and electrolytes. As the key component of the ED technology, membranes are bound on one end by an anolyte compartment and an anode while on the other end by a catholyte compartment and cathode 3. Advances in membrane technology are related to the development of high performance membranes with high permselectivity, low electrical resistance, strong mechanical strength and high chemical stability 4. In this case, surface modification was found to be a good method to fine-tune physical and chemical properties of the membranes. Methods including plasma treatment immobilization

9

5

, layer-by-layer assembly

6

, electrodeposition

7,

8

, and

represent a “generating after” strategy, which can be applied to


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improve the antifouling properties and strengthen the mechanical stability, among others

10

. However, such surface modification has a complicated preparation route,

increased resistance and has a risk of detachment of functional materials; for these reasons, their further practical application in water treatment was limited so far 11. Fig. 1 In general, a homogeneous or heterogeneous ion exchange membrane is a dense membrane, which defines the transport of ions. Although many strategies were developed to accelerate ion transport, these were economically not competitive or had an inferior electrochemical performance

12

. Recently, ultrafiltration membranes were

used in the electrodialysis process to facilitate the migration of molecules according to their charge and molecular size. It has been proved that UF membranes have a lower resistance than typical membranes used in electrodialysis

13

. However, during the

application of ultrafiltration membranes in electrodialysis, due to the relatively large pore size of these membranes, separation of smaller ionic species is still a challenge. Pore-filled membranes composed of an ultrafiltration membrane as substrate and polymer with ion-exchange groups that can provide both a high ion conductivity and excellent mechanical properties

14

. The application of pore-filled ion exchange

membranes has been explored for use in fuel cells conversion

14, 16

, vanadium redox flow batteries

recovery by diffusion dialysis

18

15

, electrochemical energy

16

, reverse electrodialysis

and pharmaceutical preparations

17

, acid

19

. In spite of this,

pore filling may have some restrictions in practice for the following reasons. It is accepted that introduction of ion exchange groups to the membrane is the most



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practically effective for the preparation of ion exchange membrane, neverless, the existence of the uncharged porous substrate would hinder the ions transport through membranes. Inspired by methods of electrodialysis with ultrafiltration membranes and preparing pore-filled membranes, porous membranes prepared by charged polymers with functional groups can be another option. A porous ion exchange membrane can be defined as a charged membrane with functional groups and + Such membrane with porous structure may facilitate the migration of ions by reducing the physical steric hindrance of the surface layer. Thus, the migration resistance can be further diminished while the functional groups of the membrane matrix can block the diffusion of ions caused by the concentration gradient. However, the fabrication of porous ion exchange membranes is still a major challenge limiting the optimization of the membrane performance referring to pore size, porosity and resistance. In this study, a dry-wet phase inversion strategy to prepare a porous sulfonated polyethersulfone (SPES) membrane is reported. The objectives of this study are (1) preparation and characterization of a lab-made porous ion exchange membrane, (2) evaluate the desalination efficiency of this porous ion exchange membrane compared to dense SPES membrane for desalination.

2. Experimental

2.1 Materials and preparation process

Polyethersulfone (PES Ultrason E6020P with Mw=58,000 g/mol) was purchased from BASF (Belgium). Sulfonated polyethersulfone (SPES) was supplied by Zhejiang


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University of Technology, China. Dimethyl sulphoxide (DMSO) of analytical grade was purchased from Sigma-Aldrich (Belgium). Distilled water was used throughout this study. In this work, methods of immersion precipitation and dry-casting were combined in dry-wet phase inversion. Briefly, 3 g of SPES was dissolved in 17 g of DMSO at room temperature and stirred until it was fully dissolved. Afterwards, the polymer solution was degassed by sonication. The degassed polymer solution was then casted on a glass by a casting knife with initial thickness of 250 µm (K4340 Automatic Film Applicator, Elcometer). The solution on the plate after casting was dried in an oven at 60 °C with different aging times before precipitation in deionized water. The formed membranes were peeled off from the plate and stored in deionized water for further use. Membranes with different charge densities were prepared by mixing SPES with various amount of PES. The preparation method is the same as that used above. The compositions of the prepared membranes and fabrication conditions are summarized in Table 1. Table 1

2.2 Membrane characterization

2.2.1 Structure of the membranes and water flux

The morphology and structure of the membranes were observed using scanning electron microscopy (SEM) on a JEOL 6300 electron microscope. The cross-section



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surface of the samples was cut in liquid nitrogen. The prepared membranes were dried in a vacuum oven and then coated with gold before the measurement. Fourier transform infrared spectroscopy (FTIR) measurements were performed through an ATR-FTIR spectrometer (Bruker, Germany), which is equipped with a platinum diamond for single reflection. A dead-end filtration setup was used to study the permeability of the resulting membranes. The effective area of the filtration cell was 12.64 cm2. The water flux was tested at a pressure of 4 bar. The water flux F, used to express the rate at which the water permeates the membrane, is typically defined as a volume per area per unit of time.

F=

V A⋅ t

(1)

with V the volume of pure water permeation, t the time and A the area of the membrane.

2.2.2 Ion exchange capacity (IEC) and water uptake

The ion exchange capacity (IEC) of the prepared cation exchange membranes was determined by a titration method. Firstly, the membrane sample was soaked in 1 mol/L HCl solution for 24 h. Then, it was taken out of the solution and carefully wiped with absorbent paper. Finally, the membrane was immersed in 100 mL of 1 M NaCl solution for 24 h to liberate the H+ ions. Then the solution with released H+ ions was titrated against 0.01 mol/L NaOH using phenolphthalein as indicator. The IEC of the prepared cation exchange membrane was calculated by the following equation:


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IEC =

(Vs − Vb ) × CNaOH Wdry

(2)

where Vs and Vb are the consumed volumes of NaOH solution for the membrane and the blank sample, respectively. CNaOH is the concentration of the NaOH solution, and Wdry is the mass of the dried membrane sample. For ion exchange membranes, the water uptake has a profound effect on the ion conductivity and mechanical properties. The water uptake of this experiment was measured at room temperature based on the water retention inside the membrane. The membranes were dried at 60 °C in an oven and then weighed accurately followed by immersing in deionized water for at least 24 h to ensure complete equilibrium and then reweighed after mopping the surface moisture with filter papers. The water uptake was calculated as follows:

WU =

Wwet − Wdry Wdry

(3)

where Wwet is the weight of the membrane samples under wet condition.

2.2.3 Contact angle measurements

The water contact angle of the membrane surface was measured according to the sessile-drop method on an optical contact angle meter with 5 µL water drops at ambient temperature (Data-physics, OCA-20, Germany). The average value of three measurements made at different positions on the sample surface is reported.



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2.2.4

Membrane

resistance,

transport

number

and

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current–voltage

measurements

A two-compartment cell is applied for the current–voltage curve measurement, as shown in Fig. 2. Each compartment has a volume of 200 mL, and was separated by the membrane. The effective membrane area was 13.84 cm2. A titanium electrode coated by ruthenium was used as anode, and a stainless-steel electrode was used as cathode. The current through the electrodes was supplied by direct current power supply and the electrical current was recorded by an ATTEN APS3005Dm potentiostat/galvanostat. The potential difference between the two sides of the membrane was measured by a multimeter (LINI-T UT39A, China) equipped with two platinum filaments located at 1.0 mm approximately in front of each membrane surface. Mechanical stirrers were placed in each compartment. Fig. 2 Membrane transport number was determined by measuring membrane potential. A two-cell apparatus equilibrated membrane with unequal concentrations (C1 = 0.05 M / C2 = 0.01 M) of NaCl solution at both sides were used. The developed potential across the membrane was measured by multimeter with Ag/AgCl electrodes. During the experiment, both sections were stirred vigorously to minimize the effect of boundary layers. The transport number “tm” was then calculated using the following equation: Em =

a RT (2t m − 1) ln 1 F a2

(4)


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where R is universal gas constant (8.314 J/K·mol), F is the Faraday constant (96,487 C/mol), T is the absolute temperature (K), a1 and a2 are the mean activities of electrolyte solutions. The area resistance of a membrane was measured via a conductivity cell. The cell was filled with 1 M NaCl in each compartment separated by a membrane with an effective area of 0.79 cm2. The membrane resistance was measured with a Solartron Electrochemical System by electrochemical impedance spectroscopy (EIS) over a frequency range from 1 kHz to 1 MHz.

2.2.5 Diffusion dialysis measurement

Conductivity–time curves were used to determine the concentration of ions through diffusion. These curves reflect the ion concentration directly. Here, diffusion dialysis experiments with a 1 M NaCl diluted compartment cell and an initial concentrated compartment cell of deionized water were carried out. The concentration gradient drives ions across the membrane, with an effective area of 13.84 cm2. The dialysis coefficient of the membrane for each component is defined as the amount of the component transported per unit active membrane area, per unit time and per unit concentration difference of the component, and was calculated from the concentration of species by the following equation:

U=

M At ∆C

(5)

where M is the amount of the component transported in moles, A is the effective area (m²); t is the time (h), and ∆C is the logarithmic average concentration between



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the two chambers in mole per cubic meter as in the equation below:

∆C =

C 0f − (C tf − Cdt ) ln

C 0f − Cdt

(6)

C tf

where C 0f and C tf are the feed concentrations at time 0 and t, respectively, and Cdt is the dialysate concentration at time t.

2.2.6 Continuous-mode electrodialysis

The selectivity of the membranes can be evaluated by electrodialysis. In this experiment, a series of electrodialysis experiments was conducted to measure the selectivity of the membrane samples. Electrodialysis is generally operated under direct current, which provides the driving force for transport of ions through the membrane. There were three streams in the ED stack: the diluate, concentrate and the electrode rising solution. A schematic diagram of the continuous-mode ED stack is illustrated in Fig. 3. The flow compartment contained two anion exchange membrane (Fumasep® FAB, FuMA-Tech GmbH, Germany) and a porous cation exchange membrane. Typically, both the concentrate and diluate compartments were filled with 1 L 2 g/L NaCl solution while the electrode rinsing solution was 2 L 20 g/L Na2SO4. The current applied in the membrane stack is 0.3 A. Each compartment had an active membrane area of 64 cm2. Fig. 3 Current efficiency is an important parameter for assessing the suitability of any electrochemical process for practical applications. The overall current efficiency (η)


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was defined as:

η=

z (C0V0 − CtVt ) F ×100% (7) NIt

where C0 and Ct are the concentration of the brine solution at time 0 and t, z is the valence of the ion, V0 and Vt is the circulated volume of brine solution at time 0 and t. F is Faraday’s constant (96 500C mol-1), I is the constant current (0.3 A), N is the number of repeating units (1), and t is the time allowed for the electrochemical process. The desalination efficiency was evaluated by the conductivity change of the diluate compartment during the electrodialysis operation. The desalination efficiency (DE) can be calculated using the following equation: DE(%)=1-

σt σ0

(8)

where σ0 is the primary conductivity of the diluate compartment and σt is the conductivity of the diluate compartment at time t.

3. Results and discussion

3.1 SEM, FTIR results and water flux Phase separation of polymer solutions can be used in various methods of membrane synthesis: thermally induced phase separation (TIPS), vapor-induced phase separation; dry-casting of a polymer solution and immersion precipitation 20, 21. In this work, immersion precipitation and dry-casting were combined, which results in a dry-wet phase inversion method. In this process, the dope solution is exposed at high



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temperature for different time intervals prior to immersion into a coagulation bath. The properties of the membrane, such as IEC, water uptake, resistance and mechanical stability, can be modified by varying preparation conditions. Fig. 4 shows the morphologies of top surface and cross section of membranes prepared with different ageing time. No obvious differences can be seen on the membrane surface. However, the difference in the cross section is more expressed. With increasing ageing time, membrane surfaces were found to become denser. A membrane prepared by solvent evaporation generally has a dense surface structure without any pores, as in Fig. 4(a1). On the other hand, the membranes prepared by phase inversion in DI water without curing have a relatively large pore size, as shown in Fig. 4(a4). From the images of samples Fig. 4(a2 and a3), it can be seen that membranes fabricated with the dry-wet phase inversion method have three layers: two dense layers with a loose layer in between. The only difference is that the membrane with 1 h ageing time has a comparatively dense inner layer, which is layered in shape, while the membrane with 0.5 h aging time has an irregular porous inner layer. This is because the polymer concentration in the surface layer increases as the solvent evaporates, which thus leads to a dense surface layer. This layer on membrane surface would inhibit the exchange solvent and nonsolvent during the immersion process and facilitates the formation of a thick skin layer with smaller pores

22, 23

. The FTIR spectra of the

membrane prepared with different aging time are presented in Figure S1. No obvious change can be seen, which is due to the same polymer casting solution used during the preparation process.


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Fig. 4 The water flux is an important parameter in the practical application of polymeric porous membranes. It is mainly influenced by structural parameters like pore size and porosity. By increasing the pore size and porosity, the water flux is enhanced. Fig. 5 shows the water flux of porous ion exchange membranes prepared at different ageing times. The results may reflect the pore size and porosity to some extent. As can be seen in Fig. 5, the water flux was negligible for dense membranes. This can be explained by the fact that a dense membrane is not capable of providing pathways for water transport. For the membrane prepared by the dry-wet phase inversion method, the water flux was enhanced by the porous interlayer. The structure of this interlayer explains the differences in water flux for membranes prepared at ageing times of 0.5 and 1 h. For the membranes prepared by phase inversion, the highest water flux (of about 43.0 L/h·m2) was obtained, which is due to the increased pore size and porosity throughout the membrane matrix. These water fluxes are consistent with SEM images of membranes prepared at different ageing times. Fig. 5

3.2 IEC and Water uptake

The content of charged functional groups in an ion exchange membrane plays a key role in providing a hydrophilic and electro-static environment for ion transport. The ion-exchange capacity, an important factor related to the conductivity and transport properties, is used to identify the charge density of the membranes. The IEC



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values of the as-prepared membranes are shown in Fig. 6a. Compared to the membrane directly immersed in water, membranes prepared by dry-wet phase inversion have a smaller IEC. This is mainly attributed to the following two reasons. Firstly, the porosity of the membranes improved the accessibility of the functional groups, which enhances the IEC. Secondly, the high pore volume and free volume caused by porosity enhanced the mobility of ions. Incorporating SPES to the porous membrane is excepted to improve the IEC and water uptake. Water uptake is known to have a profound effect on membrane conductivity and flux. The water uptake is significantly improved after the addition of charged SPES (see Fig. 6b). The elevated water uptake is ascribed to the increase of the SPES content in the PES membrane matrix. The sulfonate groups of SPES are hydrophilic, and the increase of the IEC in SPES can result in a higher water content. Likely, more SPES in the composite membranes yields more hydrophilic sulfonate groups, thereby enhancing the water uptake of SPES/PES blended membranes. Fig. 6

3.3 Contact angle measurements

The IEC and water uptake were reduced with the increase of ageing time due to the decreased porosity in the polymer matrix. This is consistent with the results of contact angle, as it can be seen in Fig. 7, a membrane with high porosity tends to have a higher surface hydrophilicity. Besides, the component of membrane matrix would also have an impact on surface hydrophilicity. By incorporating SPES to the


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membrane matrix, contact angle reduced from 65° to 53°, indicating that higher surface hydrophilicity was obtained. Fig. 7

3.4 Current–voltage curves and membrane resistance

In a typical current–voltage curve, three regions can be distinguished. The first region is the Ohmic region. A linear relationship can be observed between current and voltage drop across the membrane. With the increase of the current density, concentration polarization becomes more pronounced, and a deviation of the linear behavior is induced by the increased resistance. A current plateau is observed after reaching the limiting current density

24

. The limiting current density is the current

necessary to transfer all the available ions. The following region is the overlimiting current region, in which the slope of the current voltage curve increases again and eventually reaches an asymptotic value

25

. Multiple phenomena occur like water

splitting and electroconvection may happen to destabilize the boundary layer. However, the overlimiting current region is not sufficiently well understood referring to the plateau length and the onset of the overlimiting current. The current-voltage curves of the prepared membranes with different porosity are shown in Fig. 8. Different slopes usually refer to a different resistance and the limiting current density depends on the transport number: the higher the transport number (Table 2), the smaller the limiting current density. Compared to a porous membrane, dense membranes result in a higher limiting current density. On the



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contrary, membranes prepared by immersion and precipitation have the largest pore size and porosity, thereby giving rise to a higher transport number (0.99) with a smaller limiting current density. For membranes with different ageing time, the order of the porosity is as follows: 0 h > 0.5 h > 1 h > 5 h. In this case, a higher porosity with higher transport number leads to a smaller limiting current density, which follows the order: 0 h < 0.5 h < 1 h < 5 h. The resistance of the membranes followed the order of 0 h < 0.5 h < 1 h < 5 h, which conformed that membrane with higher porosity tend to have smaller resistance. Fig. 8 Table 2

3.5 Diffusion dialysis

In diffusion dialysis, solutes pass through an ion exchange membrane from the high to the low concentration side. Fig. 9 shows that the conductivity in the concentrated cell increases with time during the diffusion process; this is caused by sodium diffusion from the concentrate chamber (1 M NaCl) to the dilute chamber (distilled water). However, the migration rate is quite different with different ageing times. Only few ions migrated though the dense membrane, while the migration through the porous membranes was evident. Dialysis coefficients were calculated based on changes in concentration in both compartments of the dialysis cell. The dialysis coefficient of membrane directly immersed in water is 0.67 m h-1 at room temperature. With the increase of ageing time, the dialysis coefficient decreased. The


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dialysis coefficient of a dense membrane reached as low as 0.02 m h-1, which is much smaller than that of the membrane directly immersed in water. It can be concluded from the slope of the conductivity–time curves that a porous membrane with smaller pore size and comparatively dense surface tends to have a lower diffusion of NaCl. This is due to the sterical hindrance effect of the dense surface. The dialysis coefficients of the different membranes were 0.02, 0.23, 0.44, 0.67 for dry, 1 h ageing time, 0.5 h ageing time, and 0 h ageing time membrane, which is consistent with diffusion dialysis results. Fig. 9

3.6 Conductivity change during electrodialysis

Statistical analysis of the conductivity of the diluate compartment revealed that porous ion exchange membranes have a significant effect on the evolution of the conductivity. As presented in Fig. 10, the conductivity of the brine in the diluate compartment decreases during the experiments, but the demineralization rate was different. For the membrane dried directly, the conductivity change is much faster than the porous ion exchange membrane at the first stage. An increasing ageing time led to a membrane with lower porosity and a dense structure, so that salt diffusion from the concentrate to the diluate compartment driven by the concentration gradient was mitigated, and therefore the conductivity change was enhanced. For an ageing time of 1 h, the conductivity change during the first 100 min was similar to that of the membrane dried directly. However, after that, it is found that the demineralization rate



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of the dense membrane was surpassed by the membrane prepared with 1 h ageing time. In this case, the increased demineralization rate can be explained by the comparatively high transport number for ions. For the membrane prepared with 0.5 h and 0 h ageing time, membranes have a larger porosity. In this condition, the effect of diffusion by the salt gradient plays a much more important role, thus the conductivity change is smaller. The trend of the conductivity-time curves of the membrane prepared with 0.5 h and 0 h ageing time was similar with the membranes prepared with 1 h and 5 h ageing time. The conductivity change was similar when the conductivity is above 1.6 ms/cm. After that, the desalination efficiency of the membrane with 0.5 h ageing time is higher. This can be explained by the fact that the membrane with 0.5 h ageing time has a smaller porosity; as a result, sterical hindrance plays an integral part in hindering the salt diffusion. Fig. 10 The SPES content has a great influence on the overall properties of the porous ion exchange membrane. For the membrane with different SPES content, the conductivity changes of SPES/PES composite membranes are shown in Fig. 11. It can be concluded that the desalination efficiency was enhanced with the increase of the SPES content. This can be explained by the fact that an increase of the SPES content enhances the IEC and water content. As a result, ion transport is facilitated and the conductivity reduction of diluate compartment becomes much more apparent. Fig. 11


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3.7 Current efficiency

In order to evaluate the performance of membranes with different ageing time for brine recovery, the current efficiency of the ED stack was calculated for the case when the conductivity of the diluate compartment reaches 1.0 ms/cm, as shown in Fig. 12 (right figure). The current efficiency increased first from 90% to 100% due to the formation of pores inside the membrane that facilitated the transport of ions; however, with reduced ageing time, the porosity further increased, and the current efficiency reduced again. This can be explained by the fact that a further increase of the porosity would enhance the diffusion resulting from the gradient of salt concentration. As a result, the current efficiency was reduced. In order to investigate the influence of the SPES content, the current efficiency was calculated when the desalination process was conducted for 180 min. It was found that the SPES content has a profound effect on the current efficiency. With the reduction of SPES, the IEC and water uptake were reduced. The resistance of the membrane increased, thus the current efficiency was reduced. Fig. 12

4. Conclusions

Combination of immersion precipitation and dry-casting to control the membrane porosity is an effective way to improve the membrane physical and electrochemical properties. The membrane resistance can be largely reduced by introducing the pores in the membrane matrix. The modified membrane shows an



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improved IEC and water uptake and a decreased contact angle. The current-voltage curve and diffusion experiments confirm that the resistance of membrane with higher porosity is reduced, and thus diffusion of ions through membranes is also enhanced. A compromise between the membrane resistance and diffusion should be made to optimize the mechanical stability and separation. During desalination by ED, the conductivity change was obvious for the membrane with 1 h ageing time and for the dense membrane. However, for the membrane directly immersed in water and the 0.5 h ageing time membrane, the desalination efficiency was smaller; this can be explained by the enhanced diffusion of membrane with high porosity. Comparing with the dense membrane, the membrane with 1 h ageing time has a higher desalination efficiency. This trend was similar to the trend of the current efficiency. The desalination efficiency reached 95% and the current efficiency is over 100% at optimized conditions. This study provides new insights into how to develop porous ion exchange membranes and lays a foundation for further research on low resistance membranes.

Supporting Information FTIR spectra of the SPES membranes with different aging time were given in Supporting Information.


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Electrochim. Acta 2016, 222, 212-220. (15) Lee, J.-H.; Lee, J.-Y.; Kim, J.-H.; Joo, J.; Maurya, S.; Choun, M.; Lee, J.; Moon, S.-H. SPPO Pore-Filled Composite Membranes with Electrically Aligned Ion Channels via A Lab-Scale Continuous Caster for Fuel Cells: An Optimal DC Electric Field Strength-Iec Relationship. J. Membr. Sci. 2016, 501, 15-23. (16) Lee, M.; Kang, H.; Jeon, J.; Choi, Y.; Yoon, Y. A Novel Amphoteric Ion-Exchange Membrane Prepared by the Pore-Filling Technique for Vanadium Redox Flow Batteries. RSC Adv. 2016, 6, 63023-63029. (17) Lee, M. S.; Kim, H. K.; Kim, C. S.; Suh, H. Y.; Nahm, K. S.; Choi, Y. W. Thin Pore‐Filled Ion Exchange Membranes for High Power Density in Reverse Electrodialysis: Effects of Structure on Resistance, Stability, and Ion Selectivity. ChemistrySelect 2017, 2, 1974-1978. (18) Lin, X.; Kim, S.; Zhu, D. M.; Shamsaei, E.; Xu, T.; Fang, X.; Wang, H. Preparation of Porous Diffusion Dialysis Membranes by Functionalization of Polysulfone for Acid Recovery. J. Membr. Sci. 2017, 524, 557-564. (19) Åkerman, S.; Viinikka, P.; Svarfvar, B.; Järvinen, K.; Kontturi, K.; Näsman, J.; Urtti, A.; Paronen, P. Transport of drugs across porous ion exchange membranes. J. Controlled Release 1998, 50, 153-166. (20) Van de Witte, P.; Dijkstra, P.; Van den Berg, J.; Feijen, J. Phase Separation Processes in Polymer Solutions in Relation to Membrane Formation. J. Membr. Sci. 1996, 117, 1-31. (21) Gao, L.; Tang, B.; Wu, P. An Experimental Investigation of Evaporation Time and the Relative Humidity on A Novel Positively Charged Ultrafiltration Membrane Via Dry–Wet Phase Inversion. J. Membr. Sci. 2009, 326, 168-177. (22) Jansen, J. C.; Macchione, M.; Oliviero, C.; Mendichi, R.; Ranieri, G. A.; Drioli, E. Rheological Evaluation of the Influence of Polymer Concentration and Molar Mass Distribution on the Formation and Performance of Asymmetric Gas Separation Membranes Prepared by Dry Phase Inversion. Polym. 2005, 46, 11366-11379. (23) Buonomenna, M.; Macchi, P.; Davoli, M.; Drioli, E. Poly (vinylidene fluoride) Membranes by Phase Inversion: the Role the Casting and Coagulation Conditions Play in their Morphology, Crystalline Structure and Properties. Eur. Polym. J. 2007, 43, 1557-1572. (24) Krol, J.; Wessling, M.; Strathmann, H. Concentration Polarization with Monopolar Ion Exchange Membranes: Current–Voltage Curves and Water Dissociation. J. Membr. Sci. 1999, 162, 145-154. (25) Choi, J.-H.; Lee, H.-J.; Moon, S.-H. Effects of Electrolytes on the Transport Phenomena in A Cation-Exchange Membrane. J. Colloid Interface Sci. 2001, 238, 188-195.


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Fig. 1 Scheme of a typical electrodialysis process set-up (1. Direct current supply, 2. Electrodes, 3. Ion exchange membranes, 4. Solvents, 5. Electrolytes)

Fig. 2 The experimental setup for I-V curves: current electrodes, silver electrode, and bipolar membrane



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Fig. 3 Scheme of the continuous-mode electrodialysis

Fig. 4 SEM images of membranes prepared with different aging time (a1 – a4: cross section of membranes prepared by aging time of 5 h, 1 h, 0.5 h and 0 h, respectively; b1 – b4: surface images of membranes prepared by aging time aging time of 5 h, 1 h, 0.5 h and 0 h, respectively)


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50

40

2

Water flux (L/h m )

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30

20

10

0

0.0

0.5

1.0

4.5

5.0

Aging time (h) Fig. 5 Effect of ageing time on water flux for porous ion exchange membranes

Fig. 6 IEC and water uptake change with ageing time and radio of PES/SPES



Fig. 7 Contact angle change with ageing time and ratio of PES/SPES

70

Dry 1h 0.5 h 0h

60 2

Current density (mA/cm )

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50

Overlimiting current

40

Plateau 30 20

Ohmic

10 0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Voltage (V)

Fig. 8 Current voltage curves of membranes with different ageing times


3.5

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6.0 5.5

0h 1h 0.5 h Dry

5.0

Conductivity (ms/cm)

4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 0

10

20

30

40

50

60

T (min)

Fig. 9 Conductivity change of concentrated compartment during diffusion progress for membrane with different aging times 4.5

Dry 1h 0.5 h 0h

4.0 3.5

Conductivity (ms/cm)

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3.0 2.5 2.0 1.5 1.0 0.5 0.0 0

50

100

150

200

250

300

Time (min)

Fig. 10 Conductivity change of diluate compartment during ED process for membrane with various ageing times



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Fig. 11 Conductivity change of diluate compartment during ED process for membrane with different SPES content

Fig. 12 Current efficiency of ED process at different ageing times and SPES content


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Table 1. Compositions of PES/SPES cation exchange membrane and preparation conditions Membrane

PES (g)

SPES (g)

DMSO (g)

Aging time (h)

PS0

0

3

17

5

PS1

0

3

17

1

PS2

0

3

17

0.5

PS01

0

3

17

0

PS12

1

2

17

0

PS11

1.5

1.5

17

0

PS21

2

1

17

0

PS10

3

0

17

0

Table 2. Resistance and transport number of membrane with different aging time 2

Membrane resistance (Ω cm ) Transport number (tm)

PS0

PS1

PS2

PS01

1.92 0.89

1.64 0.92

1.27 0.98

0.97 0.99



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