Internal cross-linked anion exchange membranes with improved dimensional stability for electrodialysis

Author’s Accepted Manuscript Internal cross-linked anion exchange membranes with improved dimensional stability for electrodialysis Xu Chen, Yuliang Jiang, Shanshan Yang, Jiefeng Pan, Rongjun Yan, Bart Van der Bruggen, Arcadio Sotto, Congjie Gao, Jiangnan Shen www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(17)31699-X http://dx.doi.org/10.1016/j.memsci.2017.08.026 MEMSCI15492

To appear in: Journal of Membrane Science Received date: 14 June 2017 Revised date: 28 July 2017 Accepted date: 10 August 2017 Cite this article as: Xu Chen, Yuliang Jiang, Shanshan Yang, Jiefeng Pan, Rongjun Yan, Bart Van der Bruggen, Arcadio Sotto, Congjie Gao and Jiangnan Shen, Internal cross-linked anion exchange membranes with improved dimensional stability for electrodialysis, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2017.08.026 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 galley proof before it is published in its final citable 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.

Internal cross-linked anion exchange membranes with improved dimensional stability for electrodialysis Xu Chena, Yuliang Jianga, Shanshan Yanga, Jiefeng Pana, Rongjun Yana, Bart Van der Bruggenb, Arcadio Sottoc, Congjie Gaoa, Jiangnan Shena,* a

Center for Membrane Separation and Water Science & Technology, Ocean College, Zhejiang University of Technology, Hangzhou 310014, P. R. China

b c

Department of Chemical Engineering, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium

School of Experimental Science and Technology, ESCET, Rey Juan Carlos University, Móstoles, Madrid, Spain

Abstract: Anion exchange membranes (AEMs) with a high ion exchange capacity, striking water uptake and excellent dimensional stability were prepared via an internal crosslinking networks strategy. Internal crosslinking networks were formed by reacting 4,4’-bipyridine with brominated poly (2,6-dimethyl-1,4-phenylene oxide) (BPPO). 4,4’-bipyridine not only provides a functional group but also comprises a cross-linking agent without requirements of post-functionalization. The variation of the 4,4’-bipyridine amount into the casting polymer solution was explored to regulate the performance of the anion exchange membranes, and the membrane properties were evaluated by AFM, ion exchange capacity (IEC), water uptake, the linear expansion ratio, tensile strength, thermal stability, membrane area resistance and electrodialysis experiments, etc. The results showed that the cross-linked membrane with the IEC of 1.98 mmol/g has much more outstanding dimensional stability (water uptake: 11.68%; swelling ratio：3.8%) than non-cross-linked BPPO-Tri membrane (water uptake: 53.26%; swelling ratio: 7.71%) and commercial Neosepta AMX membrane (water uptake: 60.29%; swelling ratio: 5.08%), at the high temperature (50 ℃). When being applied in ED application, the cross-linked BPPO-20 membrane (NaCl remove: 59.7%; energy consumption: 5.97 kWh/kg NaCl) exhibits slightly higher desalination efficiency and lower energy consumption than commercial Neosepta AMX membrane (NaCl remove: 58.3%; energy consumption: 6.51 kWh/kg NaCl), suggesting its promising application in ED. Keywords: Anion exchange membranes (AEMs); Cross-linking networks; 4,4’-bipyridine; poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO).

*Corresponding author. E-mail address: [email protected] (J. Shen). 1

1. Introduction Electrodialysis (ED) and reverse osmosis (RO) are two membrane separation technologies commonly used for desalination of brackish water and energy conversion processes with environmentally-friendly process, simple in operation and low energy consumption. Generally, in ED, concentrate cells and dilute cells are separated by cation exchange membranes (CEMs) and anion exchange membranes (AEMs) between two electrodes. Thus, ion exchange membranes (IEMs) have attracted much attention from academic and industrial players [1-3]. For commercially available IEMs, the most commonly utilized material is aromatic copolymer, from which cation exchange membranes (CEMs) can be prepared in a reliable way. However, anion exchange membranes (AEMs) still face technical problems in improving their performance, preparation and large-scale separation process. Traditionally, the preparation of AEMs is usually based on the post-modification of pristine polymers, following by quaternization via immersing the membranes into a trimethylamine aqueous solution [4]. However, this method has several disadvantages. For example, the quality of the membrane is not enough stable and membranes usually exhibit a loose and rough morphology due to the erosion of trimethylamine (TMA), which affects the membrane performance [5, 6]. Moreover, as the major component of ED, anion exchange membranes (AEMs) also need several essential qualities like low swelling ratio, prolonged alkaline stability , high ion exchange capacity and long-term chemical stability, etc. [7]. However, the application of AEMs has been limited due to the trade-off between ion exchange capacity and dimensional stability. Ion exchange capacity (IEC) of these membranes reflects a quantitative measurement of charged sites number in the membranes, it must be high enough to ensure ion transport [8-10]. Unfortunately, a high IEC usually relates to a high water uptake, which leads to a poor dimensional stability. Ion exchange capacity and dimensional stability are ones of crucial parameters for AEMs operation, which determines the performance of the ED device [11]. Li-cheng Jheng et al [12] worked with quaternized polybenzimidazoles with imidazolium cation moieties for anion exchange membrane fuel cells; the prepared membrane showed suitable ion exchange capacity, but water uptake was high (IEC: 1.49 mmol/g; water uptake: 39.1%). Cross-linking has been widely used for preparing AEMs with improved dimensional stability. The overall strategy focusing on connecting polymer matrix has been usually implemented using 2

cross-linkers such as diols, dihalides, diamine and hydrazoate [13-18]. Thermal Friedel–Crafts electrophilic substitution-based cross-linking has also been reported for swelling control [19]; and a series of end-group cross-linked membranes were reported by click reaction. However, this method was based on high thermal treatment, which resulted in a complex procedure [17, 20, 21]. Ao Nan Lai et al [22] enhanced dimensional stability of AEMs via cross-linking of ion cluster regions.

Two

steps

were

selected

to

synthesis

the

bromination

of

heptamethyl

phenolphthalein-containing poly (arylene ether sulfone)s, DIM and TMHDA were used as cross-linker agents. The experimental procedure was rather complicated and needed much time, which limited its potential for industrial application. Jing Pan et al [23] reported an mechanically tough and chemically stable AEMs via rigid-flexible semi-interpenetrating networks. The prepared membrane showed better tensile strength (36.4 MPa) and flexibility, but the water uptake was high (96.5%) with medium IEC (1.43 mmol/g) obstructed its further application in ED. All of those methods are based in at least two steps to prepare AEMs. Furthermore, the cross-linker may not be compatible with the polymer chains, which can cause a poor quality of the membrane and the degree of cross-linking has to be enough low to avoid undesired gelation and insolubilization of cross-linker in solvent [24]. Considering these facts, this work is aimed to develop high performance AEMs with improved dimensional stability, high ion exchange capacity and uncomplicated experimental procedure, which are required to be used for industrialization application. An AEM was prepared through internal cross-linking networks in a polymer matrix without heating via a nucleophilic substitution reaction of bromide groups and quaternary ammonium groups. Commercial brominated poly (2,6-dimethyl-1,4-phenyleneoxide) (BPPO) was chosen as the polymer matrix, because of its chemical, mechanical and thermal stabilities. Furthermore, bromination of benzylmethyl has been proved to be a promising method for avoidance of toxic chloromethylation reagents with a safer and more controllable process [4, 25-28]. 4,4’-bipyridine was chosen as quaternization and cross-linking reagent due to the symmetrical structure of two bipyridine rings. Because the pyridine groups have low hydrophilic–lipophilic balance (HLB) value, which is lower than trimethylamine (TMA) [29]; furthermore, it exhibited good hydrazine and alkaline stability due to the high basicity of the delocalized cations pyridine in conjugated pyridinium rings [30, 31]. Therefore, the effect of crosslinking on properties such as IEC, water uptake, swelling ratio, 3

mechanical strength, thermal stability and desalination efficiency were studied via controlling the 4,4’-bipyridine content in the casting solution. Compared to BPPO-Tri membrane without cross-linking, the cross-linked BPPO-20 membrane exhibits much lower water uptake and dimensional stability with high cationic functional groups, all of which could enhance the application of AEMs in ED.

2. Experimental 2.1. Materials Brominated poly (2,6-dimethyl-1,4-phenylene oxide) (BPPO) with aryl substitution degree of 0.42 and benzyl substitution degree of 0.55 was supplied by Tianwei Membrane Co. Ltd. Shandong, China. Commercial anion exchange membrane Neosepta AEM and cation exchange membrane Neosepta CEM were purchased from FUJIFLM Corp. Japan. The properties of commercial membranes are shown in Table 1. 1-methyl-2-pyrrolidone (NMP, 99.5%), trimethylamine (TMA), sodium chloride, sodium sulfate, 4,4’-bipyridine (98%) were purchased from Aladdin Reagent Co. Ltd. Shanghai, China, and used without further purification. Distilled water was used throughout. Table1. Characteristics of the commercial CEMs and AEMs used in this study. Thickness (μm)

Membrane area resistance (Ω·cm2)

Neosepta AEM

115

2.52

Neosepta CEM

116

2.70

Membrane type

2.2. Preparation of cross-linked membranes The anion exchange membranes reported in this study were prepared by the solution-casting method [32, 33]. BPPO (1.25 g) was dissolved in NMP (7 mL) in a round-bottom flask at room temperature to form a transparent solution. Then, a certain amount of 4,4’-bipyridine was added. The cross-linking degree and the content of anion conductive pyridine groups were tuned via changing the content of 4,4′-bipyridine (The degree of cross-linking of prepared membranes was shown in Table S1; photographs of pristine BPPO membrane and cross-linked membranes soaked in different solvents were shown in Figure S1). The mixture was stirred for 10 min and then, it was left in repose for 3 min to remove the bubbles from the solution. The solution was cast on a clean glass plate and thermally treated at 50 ℃ during 8 h to ensure the simultaneous 4

cross-linking and quaternization reaction. The prepared anion exchange membranes were eventually obtained, as shown in Figure 1. They are denoted as BPPO-X, where X is the ratio percentage of the mass 4,4′-bipyridine to BPPO.

Figure 1. Synthesis of the crosslinked membranes. 2.3. Preparation of AEMs BPPO was dissolved in NMP to form a transparent solution, and was then left unstirred for 3 min to remove the bubbles from the solution. The solution was cast on a clean glass plate and thermally dried at 60 ℃ for 24 h. The dried membranes were immersed for 24 h into a 1 moL/L trimethylamine (TMA) solution to finish the process of quaternization, the prepared AEMs was obtained, as shown in Figure 2. Then the membranes were washed with distilled water, (denoted as BPPO-Tri).

Figure 2. Synthesis of BPPO-Tri membranes. 2.4. Membrane characterization 2.4.1. FTIR Spectra and morphology Fourier Transform Infrared (FTIR) spectra of dried membranes were recorded by using attenuated total reflectance (ATR) with Vector 22, Bruker, Chorley Lancashire, UK, having a resolution of 2 cm-1 and a total spectral range of 4000–400 cm-1. The surface morphology of the prepared membranes was observed using scanning electron microscopy (SEM, HITACHI, S4700 5

A). The membranes were dried in a vacuum oven at 60 ℃ for 24 h before being tested. In addition, AFM (Dimension Icon, Bruker, Germany) was also used to characterize the surface roughness of the membranes. 2.4.2. Thermal stability The thermal stability of the prepared membranes was evaluated using a Shimadzu TGA-50H analyzer (Shimadzu Corporation, Kyoto, Japan), with temperature ranges from 24 ℃ to 800 ℃ under nitrogen atmosphere and a heating rate of 10 ℃/min. The samples were vacuum dried at 60 ℃ for 24 h before testing. 2.4.3. Water uptake and dimensional stability After being dried at 60 ℃ under vacuum for 24 h, the weight and length of dry membranes were denoted as Wdry and Ldry. The prepared membranes, 4 cm in length and 2 cm in width, were immersed in deionized water at different temperatures for 48 h; subsequently, the membranes were taken out, the excess liquid was carefully removed from the membrane surface by using filter paper, and the weight and length of the membrane were recorded (denoted as Wwet and Lwet). Swelling ratio (SR) was obtained by linear expansion ratio (LER) of the membranes in our work. The water uptake (WU) and swelling ratio (SR) were calculated according to the following equation (Eqs 1 and 2): 𝑊𝑈 = 𝑆𝑅 =

𝑤𝑤𝑒𝑡 − 𝑤𝑑𝑟𝑦 × 100% 𝑤𝑑𝑟𝑦 𝐿𝑤𝑒𝑡 − 𝐿𝑑𝑟𝑦 𝐿𝑑𝑟𝑦

(1)

× 100%

(2)

2.4.4. Ion exchange capacity (IEC) The ion exchange capacity (IEC) of the membrane was measured by using Mohr’s method. The mass of dry membranes (Wdry) was measured, the membranes were then immersed in 0.1 mol/L －

NaCl solution for 24 h, so that they were converted to the Cl

form. The absorbed NaCl was

thoroughly washed off with deionized water. The membranes were then immersed in 0.05 mol/L Na2SO4 for 24 h, to convert them to the SO4- form. The solution was titrated using 0.1 M AgNO3 and K2CrO4 as indicator. The IEC was calculated using the following equation: 𝐼𝐸𝐶 =

𝐶𝐴𝑔𝑁𝑂3 ×𝑉𝐴𝑔𝑁𝑂3 𝑊𝑑𝑟𝑦

(3)

where CAgNO3 (M) is the concentration of the AgNO3 solution, and VAgNO3 (mL) is the volume of the AgNO3 solution 6

2.4.5. Tensile strength and elongation at break The tensile strength of the dried membrane was measured using an Instron universal tester (Model 1185, US) at 25 ℃. The membranes were placed between the flat-faced grips of the testing machine. The speed of testing was set at a rate of 5 mm/min. Before characterization, the membrane samples were clipped to a size of 15 mm width and 100 mm length. 2.4.6. Membrane area resistance Laboratory methods for measuring the membrane area resistance have been reported in the literatures[34, 35]. The equipment is shown in Figure 3. The cells used in this study consisted of four compartments. A 0.05 M Na2SO4 solution was used for the electrode solution and 0.1 M NaCl solution was employed as the intermediate solution. Two electrode chambers were at both ends of the cell. The voltage between the membranes was measured by Ag/AgCl electrodes, linked with a multimeter. The applied constant current is 0.04 A, and the area of the membrane was 7.065 cm2. The membranes were immersed into a 0.5 mol/L NaCl solution for 30 min before each experiment. The membrane area resistance was calculated by Eq. 4. 𝑅=

𝑈−𝑈0 𝐼

×𝑆

(4)

where U is the the voltage of the membrane, and U0 is the voltage in a blank experiment.

Figure 3. Schematic diagram of membrane area resistance measurement setup: (1) calomel electrode; (2) Neosepta CEM; (3) tested membrane. 7

2.4.7. Membrane transport number The membrane transport number t- was determined by the same equipment used to measure the membrane area resistance. Solution of 0.1 M KCl and 0.2 M KCl was pumped into the intermediate chambers separately. The voltage (Em) between the membranes was measured by Ag/AgCl electrodes, which were linked with a multimeter. The membranes were immersed into a 0.15 M KCl solution for 30 min before the test. The transport number 𝑡𝑖′ was calculated using the following equation: 𝑅𝑇

𝐶

𝐸𝑚 = (2𝑡𝑖′ − 1) 𝑛𝐹 𝑙𝑛 (𝐶1 ) 2

(5)

where R is the universal gas constant (8.314 J/K·mol), F is the Faraday constant (96,485 C/mol), T is the testing temperature (K), C1 and C2 are the concentrations of the respective KCl solutions. n is the electrovalence (n is 1 in the test). 2.4.8. Current–voltage (i–v) characteristic The limiting current density of membranes was measured at room temperature using the same device as that for membrane area resistance. A 0.3 M Na2SO4 solution was used for the electrode solution and 0.1 M NaCl solution was employed as the intermediate solution. The voltage between the membranes was measured by Ag/AgCl electrodes, linked with a multimeter. 2.4.9. Electrodialysis experiments A schematic diagram of the ED stack is shown in Figure 4. The experiments include four cells, of which the middle two cells are the concentrate cell and the diluate cell, surrounded by two electrode cells at the ends of the stack. The electrode cells were fed with 0.3 M Na2SO4 (200 mL) and were linked to control the pH of the solution. The concentrate cell and diluate cell were fed with 0.5 M NaCl (90 mL). The effective membrane area was 19.625 cm2. The conductivity of the NaCl solution in the concentrate cell, diluate cell and the stack potential between the stack were recorded every 10 min.

8

Figure 4. Schematic principle of the continuous-mode ED stack. 2.4.10. Efficiency assessment The current efficiency and energy consumption are significant for evaluating the performance of ED process. The current efficiency was calculated according to the following Eq. 6.



Z (Ct - C0 )VtF 100% NIt

(6)

where Ct and C0 are concentration of Na+ in the concentrate cell at time t and 0 during the ED process. Z is the absolute valence of Na+, Vt is the volume of solution in concentrate cell at time t, F is the Faraday constant (96,485 C/mol), I is the direct current (I=0.3 A in the test), and N is the number of repeating units (N=1). The integral energy consumption was calculated for dealing with 1 kg of NaCl by Eq. 7.

UI dt 0 CV M t t b

E

t

(7)

where U is the voltage of the ED stack, I is the direct current (I=0.3A in the test), Ct is the concentration of NaCl at the time t, Vt is the volume of solution in concentrate cell at time t, Mb represents the molecular weight of NaCl.

3. Results and discussion 3.1. FT-IR spectra analysis of the cross-linked membranes An FTIR analysis was performed to confirm that the quaternization reaction had taken place. Figure 5 shows the infrared spectra of the pristine BPPO membranes and AEMs with different contents of of 4,4’-bipyridine. The band at 1608 cm-1 is attributed to the C=C stretching vibration in phenyl groups and was found in all of membranes. However, compared with the pristine BPPO membrane, the newly prepared membranes had several new absorption peaks. The sharp 9

absorption peaks around 1634 cm-1 can be assigned to the stretching vibration of C-N, suggesting the successful reaction between BPPO and 4,4’-bipyridine. A broad band at 3382 cm-1 is associated to the stretching vibration of the -OH groups, or may be produced by bound water in the prepared membranes, which was absent in the pristine BPPO membrane. A new peak at around 1556 cm-1 is attributed to the stretching of the pyridine C=N bond of the 4,4’-bipyridine [31, 36]. These results demonstrated that bipyridine groups were successfully incorporated into the membranes structure.

Figure 5. FTIR spectra of pristine BPPO and BPPO-X with different amounts of 4,4’-bipyridine. 3.2. Membrane morphology The morphology of the prepared membranes was explored by SEM, and is shown in Figure 6. All of the AEMs showed identical morphological features, with a homogeneous, dense and compact surface, and without phase separation between BPPO matrix and 4,4’-bipyridine, suggesting a better compatibility between BPPO and 4,4’-bipyridine through the covalent and hydrogen bonds during membrane formation. Some aggregation on the membrane surface was observed for the pristine BPPO membrane, which may be due to some impurities or non-dissolved polymer. In general, no holes and cracks were observed on the membrane surface, which proved the compact and homogeneous structure of the prepared AEMs. Figure 7 clearly demonstrated the surface roughness of cross-linked BPPO-X membrane and non-cross-linked BPPO-Tri membrane. BPPO-Tri membrane (Ra: 9.59 mm) had rougher surface 10

than BPPO-16 membrane (Ra: 3.53 mm), which may be caused by two reasons. The cross-linked BPPO-16 has a denser and compacter structure; and the BPPO-Tri membrane has been corroded in quaternization via immersing the membranes into a trimethylamine aqueous solution, which leaded to a rough surface.

Figure 6. SEM mages of prepared anion exchange membranes. (a) BPPO, (b) BPPO-8, (c) BPPO-16, (d) BPPO-20.

Figure 7. AFM mages of prepared anion exchange membrane and BPPO-Tri membrane. 3.3. Thermal stability The thermal stability of BPPO, BPPO-8 and BPPO-16 was tested by thermogravimetric analysis (Figure 8). The evolution of weight loss can be separated into three stages over the temperature ranging from 30 ℃ to 800 ℃. In the first stage, all membranes showed a typical weight loss around at 50-150 ℃，which is thought related to the evaporation of bound water and solvent from the membrane matrix. The second weight loss stage was around 200 ℃, this is typical for the degradation of quaternary ammonium groups and the decomposition of bipyridine cations [37] . The third step of degradation, which occurred above 330 ℃ is attributed to the 11

decomposition of the polymer backbones. In general, all the AEMs were found to have a good thermal stability below 200 ℃. The weight loss of BPPO-8 and BPPO-16 is lower than for the pristine BPPO, which proves that the cross-linking network is beneficial for the membrane thermal stability. Additionally, the decomposition temperature around 200 ℃ of bipyridine cations is higher than the typical quaternary ammonium cation degradation temperatures of 120 ℃ [38]. Generally, the working temperature of the membranes for ED is below 100 ℃, cross-linked membranes show a slight weight loss of around 7% under this temperature. TGA results indicate that the thermal stability of BPPO-X membranes is acceptable for electrodialysis.

Figure 8. TGA and DTG curves of pristine BPPO, BPPO-8, BPPO-16 anion exchange membranes. 3.4. Mechanical stability The tensile strength (TS) and elongation break (EB) of prepared anion exchange membranes with different bipyridine contents are shown in Figure 9. TS values are in the range of 52.4-24.7 MPa, while EB values change from 3.9% to 2.7%. With the increase of the bipyridine content, the TS values decrease dramatically, while the EB values decrease only slightly. In general, the strength and flexibility of the membranes decreased, suggesting that the tight cross-linked networks that are formed are disadvantageous to the membrane flexibility. Neosepta AMX membrane has a higher tensile strength than BPPO-20 membrane due to the enhancement of fabrics. However, BPPO-20 with IEC 1.98mmol/g still has a high mechanical strength (24.7 MPa). This is attributed to the formation of a cross-linking network in the membranes, which made the 12

membranes more compact and stiff, leading to an excellent tensile strength and swelling resistance even at a high IEC.

Figure 9. Tensile strength (TS) and elongation break (EB) of anion exchange membranes. 3.5. IEC, water uptake and swelling ratio IEC, water uptake, swelling ratio are among the most significant factors that determine the performance of anion exchange membranes. The IEC is directly proportional to the content of charged functional groups. The IEC, water uptake and swelling ratio values for the studied membranes are summarized in Table 2. The theoretical IEC was calculated from the composition of the membranes with the assumption that the BPPO and bipyridine reacted completely, and experimental values were obtained by titration. As expected, with a mass ratio percentage of 4,4′-bipyridine to BPPO increasing from 8% to 20%, the calculated IEC increases, from 0.89 mmol/g to 2.13 mmol/g, and the experimental values change from 0.43 mmol/g to 1.98 mmol/g, which further increases the anion transport sites. The experimental IEC is lower than the theoretical one, which can be ascribed to the fact that bipyridine does not completely react with BPPO, leading to a decrease in the number of quaternized ion groups. The water uptake and swelling ratio are also important parameters for the performance of AEMs. A suitable water uptake is necessary for maintaining the stability in ED. An excessive water uptake may give rise to a weak swelling resistance and a poor mechanical performance. The water uptake (WU) and swelling ratio (SR) were measured at different temperatures; results are shown 13

in Table 2 and Figure 10. Water uptake and swelling ratio increased gradually to a similar extent for all selected temperatures, which made great progress compared to non-cross-linked BPPO-Tri and commercial Neosepta AEM. Even at high temperature (50℃)，the cross-linked BPPO-20 membrane also keeps excellent dimensional stability (48 h) with the high degrees of functionalization. The interesting swelling behavior and dimensional stability of the membrane may be affected by two reasons. First, a higher bipyridine content makes the cross-linked structure formed by the BPPO strains more compact, so that the prepared membranes have less free space to allow diffusion of the water molecules, which minimizes the increase in WU and SR. Secondly, with the increase of bipyridine groups, the content of hydrophilic imidazolium groups increases the hydrophilic character of the membranes. Compared to the BPPO-Tri membrane and the commercial Neosepta AMX membrane, the prepared BPPO-X AEMs have an excellent swelling resistance, even with a high IEC, which meets the requirements in ED. Comparison of water uptake between cross-linked membrane in this work and some membranes reported in the literatures has been listed in Table 3. Taking IEC and test condition into consideration, cross-linked membrane has the best satisfactory dimensional stability with high IEC, which is fairly profound for the ED application. Table 2. Ion exchange capacity (IEC), water uptake (WU) and swelling ratio (SR) of the composite membranes at different temperatures. Membrane BPPO-8 BPPO-12 BPPO-16 BPPO-20 BPPO-Tri Neosepta AMX

IEC（mmol/g） Cal. Exp.

WU (%) 25℃ 40℃ 50℃

0.89 1.37 1.77 2.13 3.40 --

2.23 4.01 8.28 9.20 31.74 44.23

0.43 1.07 1.42 1.98 2.08 2.16

2.41 5.67 8.81 9.93 45.2 57.26

3.35 6.34 9.49 11.68 53.26 60.29

25℃

SR (%) 40℃

50℃

0.41 1.23 2.17 3.20 3.68 4.22

0.74 1.54 2.48 3.51 5.80 4.92

1.07 1.75 2.54 3.80 7.71 5.08

14

Figure 10. Swelling ration of anion exchange membranes at different temperatures. Table 3. Comparison of water uptake between cross-linked membrane in this work and some membranes reported in the literature.

Methods Crosslinking of ion cluster regions Menshutkin reaction between BPPO and HLTEI 25 Inter-crosslinking CPPO/BPPO blends Semi-interpenetrating polymer networks Azide-assisted crosslinked quaternized polysulfone Cross-linked quaternary polysulfone Pendant-type cross-linked AEMs BPPO-20 Neosepta AMX

IEC (mmol/g)

Test temperature (℃)

Test time (h)

Water uptake (%)

References

1.98 ± 0.05

30

48

39.2 ± 1.4

[22]

1.67±0.01

25

24

22.39±0.37

[39]

1.99

25

24

83.1

[40]

1.43

25

24

96.5

[23]

1.85

25

48

14

[41]

1.86

25

24

38.74

[42]

0.85

25

24

17.5

[43]

1.98 2.16

25 25

48 48

9.26 44.23

In this work --

3.7. Membrane area resistance, transport number and limiting current density (LCD) 15

In addition to the ion exchange capacity, water uptake, swelling ratio, mechanical stability, the membrane area resistance and transport number are also important factors affecting the desalination efficiency. As shown in Table 4, the membrane area resistance decreases significantly from 91.05 to 2.46 Ω·cm2 with the rise of the bipyridine contents for the increased IEC. This is attributed to the fact that bipyridine groups have ionic transport properties. The area resistance of the BPPO-20 membrane is lower than that of the commercial membrane (Neosepta AMX, 2.51 Ω cm2, measured at the same conditions), which is appropriate for application in desalination. Accordingly, the transport number of cross-linked membranes increases from 0.92 to 0.97. A higher transport number for an anion exchange membrane reflects a higher permselectivity to anions and rejection to cations. Furthermore, the BPPO-20 anion exchange membrane has a higher transport number, which is due to the higher IEC and denser network. Limiting current density (LCD) is another important parameter to determine the ED performance of the membranes. As we known, when the applied current density is lower than LCD, the mass transfer is balanceable in both boundary layer and membrane matrix. However, the mass transfer across the membrane is much faster than that in boundary layer when it is higher than LCD, which results in concentration polarization, low desalination efficiency and high energy consumption. Figure 11 shows the I–V curves acquired for the membrane in a 0.1 M NaCl solution. All curves are divided into three stages: the first stage is approximately ohmic behavior; the second stage shows a plateau, as a result of the concentration polarization near the membrane interface; the third stage is contributed by many phenomena like gravitational convection, water dissociation and electroconvection, etc [32, 35]. As shown in Figure 11, the LCD of cross-linked BPPO-20 membrane is slightly lower than commercial membrane (Neosepta AMX). Apparently, BPPO-20 possessing the low area resistance and modest LCD is expected to achieve an excellent desalination performance. Table 4. Area resistance, transport number and thickness of anion exchange membranes. Membranes Area resistance (Ω·cm2) Transport number Thickness (μm)

Neosepta AMX

BPPO-8

BPPO-12

BPPO-16

BPPO-20

2.51

91.05

16.87

2.97

2.45

0.98 115

0.92 62

0.94 69

0.95 71

0.97 74

16

Figure 11. Current density–voltage curves of crosslinked membranes and Neosepta AMX. 3.8. Desalination performance A continuous-mode ED test was performed to evaluate the desalination performance of the prepared cross-linked membranes (BPPO-16, BPPO-20) and the Neosepta AMX membrane. The diluate chamber and concentrate chamber contained 0.5 M NaCl (90 mL), and the two electrode cells were filled with 0.3 M Na2SO4 (200 mL); a direct current of 0.3 A was applied (corresponding to a current density of 15.3 mA/cm2, which is lower than the limiting current density of membranes). The change in conductivity of the NaCl solution in the diluate cell and concentrate cell, and the potential over the stack during the test were recorded. As shown in Figures 12 and 13, the cross-linked BPPO-20 membrane was found to have a slightly higher desalination performance than the commercial Neosepta AMX for the applied test duration (150 min). Slightly higher desalination efficiency of the cross-linked BPPO-20 membrane may be caused by thinner thickness and self-supporting structure, which leads to a higher ion transmission speed. On the other hand, the BPPO-20 membrane has the highest concentration effect. The changes of the potential over the stack during the ED test are shown in Figure 14. Furthermore, the current efficiency and energy consumption are obtained in Figure 15. The potential over the stack with BPPO-20 is lower than that for the commercial membrane (Neosepta AMX). In addition, it can be seen that the BPPO-20 membranes ED stack (5.97 kWh/kg NaCl) has lower energy consumption than commercial membrane (Neosepta AMX, 6.51 kWh/kg NaCl) with a 17

lowest membrane area resistance, all of which suggest that BPPO-20 has a great potential in ED application.

Figure 12. Electrodialysis test: the change of conductivity in dilute chamber and concentration cell.

Figure 13. Final NaCl removal ratio in ED test after 2.5 h.

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Figure 14. Change of potential over stack during ED test.

Figure 15. Current efficiency and energy consumption in ED test after 2.5 h.

4. Conclusions In this work, a novel bipyridine- functionalized AEMs with internal cross-linking networks has been studied, using 4,4’-bipyridine to provide functional groups and cross-linker simultaneously, which is different from the classical post-quaternization processes. In addition, the prepared membranes have effectively suppressed water uptake by the formation of cross-linking networks with short reaction time, and simultaneously provide high functional groups to provide enough ion 19

exchange capacity. Striking dimensional stability was obtained for the cross-linked BPPO-20 membrane (water uptake: 11.68%) relative to the non-cross-linked BPPO-Tri membrane (water uptake: 53.26%) and commercial membrane (Neosepta AMX, water uptake: 60.29%), at high temperature (50 ℃). Compared to the pristine BPPO membranes, BPPO-X membranes also exhibit a better thermal and mechanical stability. Accordingly, when applied in ED, the optimized BPPO-20 membrane has a slightly higher desalination efficiency and lower energy consumption (5.97 kWh/kg NaCl) than commercial Neosepta AMX membrane (energy consumption: 6.51 kWh/kg NaCl), suggesting its promising application in ED. In addition, taking simple synthesis process and without heating into consideration, it is a considerable potential for industrial applications.

Acknowledgement The research was supported by the Natural Science Foundation of China (NO. 21676249), the Public Welfare Project of the Science and Technology Committee of Zhejiang Province (NO. 2013C31038) and the National High Technology Research and Development Program 863 (No. 2015AA030502).

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Research highlights 1. The anion exchange membranes with improved dimensional stability for electrodialysis has been prepared via internal cross-linking networks 2. 4,4’-bipyridine not only provides a functional group but also comprises a cross-linking agent without requirements of post-functionalization 3. The cross-linked membrane with IEC of 1.98 mmol/g has much more outstanding dimensional stability (water uptake: 11.68%; swelling ratio：3.8%) than non-cross-linked BPPO-Tri membrane (water uptake: 53.26%; swelling ratio: 7.71%) and commercial Neosepta AMX membrane (water uptake: 60.29%; swelling ratio: 5.08%), at the high temperature (50 ℃).

4. When being applied in ED application, the cross-linked BPPO-20 membrane exhibits much higher desalination efficiency and lower energy consumption (NaCl removal ratio: 59.7%; energy consumption: 5.97 kWh/kg NaCl) than commercial Neosepta AMX membrane (NaCl removal ratio: 58.3%; energy consumption: 6.51 kWh/kg NaCl), suggesting its promising application in ED.

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