Modification of cation exchange membranes with conductive polyaniline for electrodialysis applications

Accepted Manuscript Modification of cation exchange membranes with conductive polyaniline for electrodialysis applications Jinli Zhao, Luqin Sun, Qingbai Chen, Huixia Lu, Jianyou Wang PII:

S0376-7388(18)32898-9

DOI:

https://doi.org/10.1016/j.memsci.2019.03.043

Reference:

MEMSCI 16944

To appear in:

Journal of Membrane Science

Received Date: 16 October 2018 Revised Date:

12 March 2019

Accepted Date: 15 March 2019

Please cite this article as: J. Zhao, L. Sun, Q. Chen, H. Lu, J. Wang, Modification of cation exchange membranes with conductive polyaniline for electrodialysis applications, Journal of Membrane Science (2019), doi: https://doi.org/10.1016/j.memsci.2019.03.043. 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.

ACCEPTED MANUSCRIPT

Modification of cation exchange membranes with conductive polyaniline for electrodialysis applications Jinli Zhao, Luqin Sun, Qingbai Chen, Huixia Lu, Jianyou Wang* Tianjin Key Laboratory of Environmental Technology for Complex Trans-Media Pollution, College

RI PT

of Environmental Science and Engineering, Nankai University, Tongyan Road 38, Tianjin, 300071, PR China ABSTRACT

SC

In this study, composite cation exchange membranes (C-CEMs) based on polyvinyl pyrrolidone (PVP) and mainchain sulfonated polyethersulfone (SPES) were prepared by adding conducting

M AN U

polyaniline (PANi). The electrodialysis (ED) properties of these C-CEMs were characterized to determine their feasibility for practical applications. C-CEMs exhibited smooth surfaces and dense cross-sections when PANi was homogeneously dispersed in the matrix material. An increase in C-CEM mechanical stability was attributed to the formation of a crosslinked structure. The experimental results showed that C-CEM with 0.6% PANi possessed the highest ionic conductivity.

TE D

ED process results showed that C-CEM with 0.6% PANi exhibited better desalination properties than those of other C-CEMs and commercial CEM. On the whole, an appropriate PANi content improved C-CEM desalination properties. This study detailed a new method for the preparation of novel,

EP

low-cost C-CEMs with improved properties. It was found that a 0.6% PANi content was optimal for improving membrane performance.

AC C

Keywords: Polyaniline; Sulfonated polyether sulfone; Cation exchange membrane; Desalination

Abbreviations: CEM, Cation exchange membrane; AEM, Anion exchange membrane; ED Electrodialysis PVP, Polyvinyl pyrrolidone; SPES, Sulfonated polyethersulfone; PES, Polyethersulfone; PANi, Polyaniline; DS, Degree of sulfonation; NMP, N-Methyl pyrrolidone; SPVDF, Sulfonated polyvinylidene fluoride; FTIR, Fourier transform infrared spectrometer; SR, Swelling rate ; IEC, Ion exchange capacity; CA, Contact angle ; TGA, Thermogravimetric analysis; CSA, Camphorsulfonic acid; XRD, X-ray diffractometer; FE-SEM, Field-emission scanning electron microscopy; AFM, Atomic Force Microscope; CD, Current density; CE, Current efficiency *

Corresponding author.

E-mail address: [email protected] (J. Wang)

1

ACCEPTED MANUSCRIPT 1. Introduction Cation exchange membranes (CEMs) are widely used in electromembrane processes, such as diffusion dialysis, electrodialysis, electrolysis, and electrodeionization, because of their high separation capacity, high selectivity, and relatively low cost [1–3]. With regard to these applications,

RI PT

CEMs can be expected to offer good separation properties, including good ionic conductivity and permselectivity, and should be highly stable in terms of mechanical, chemical, and thermal stability. Also, CEM development has been focused on producing lower cost membranes through simpler procedures [4–5].

SC

In terms of CEM structure, commercial CEMs are generally divided into two groups, homogeneous and heterogeneous. The homogeneous CEMs have good ionic conductivity and

M AN U

permselectivity. However, complex preparation processes result in more expensive commercial CEMs, such as TingRun, Nafion, Neosepta AFX, and ASTOM [6–9]. Consequently, attention has been paid to other sulfonated materials, such as sulfonated polyetherketone, sulfonated polysulfone, sulfonated

poly(phthalazinone

ether

sulfone

ketone),

sulfonated

polyimides,

sulfonated

poly(2,6-dimethyl-1,4-phenyleneoxide), and sulfonated polyether ether ketone [10–16]. These

TE D

sulfonated materials have a high sulfonation degree (DS), belong to the family of sidechain sulfonated polymers, and can fulfill the required properties of CEMs. However, most of these sidechain sulfonated polymers must be synthesized by anterior-sulfonation [17]. This rigorous

EP

preparation process adds to the high cost of these alternative sidechain sulfonated materials. Over the past decades, mainchain sulfonated materials that could be prepared by post-sulfonation have been seldom used to prepare CEMs due to their low DS. Steric effects incurred during the

AC C

synthetic process were the main cause of low DS in mainchain sulfonated materials [18–19]. However, post-sulfonation has the outstanding advantage of being an excellent and simple preparation process. The sulfonated material preparation process plays a very important role in CEM costs. When the entire process for preparing membranes is considered, mainchain sulfonated materials have been found to be attractive. Poor CEM hydrophilicity with low DS can be ameliorated by the addition of a hydrophilic additive to improve the membrane’s hydrophilicity. In addition, T. Xu et al. have shown that CEM ionic conductivity is not solely dependent on the ion exchange capacity (IEC) of the matrix material [20–21]. Inspired by this work, highly hydrophilic CEMs with a low DS have been easily prepared, as reported in our previous work [22]. Compared with 2

ACCEPTED MANUSCRIPT traditional CEMs prepared with sidechain sulfonated materials, the preparation process of mainchain sulfonated polyethersulfone (SPES) is very simple. Thus, the cost of CEM matrix materials was low in this study, the price of hydrophilic blending materials also low, and the overall CEM preparation cost ideal. These low-cost C-CEMs exhibited good ionic conductivity and ideal stability. To further

RI PT

improve the properties of these C-CEMs, additional work remains to be done. As is well known, polyaniline (PANi) is an interesting electrically conductive polymer [23–26]. Fortunately, N-methyl pyrrolidone (NMP) has proven to be a good solvent for both sulfonated polymers and PANi [27]. However, PANi’s good ionic conductivity can only be obtained with the

SC

help of protonic acids due to its selective conductive mechanism. After doping with a protonic acid, hydrogen ions (H+) enter into the PANi main chain, which converts PANi from an insulator into a

M AN U

conductor [28–29]. Surface polymerization by PANi onto a CEM has been examined as a means for obtaining good permselectivity and ion removal [30]. Here, the resulting composite membrane was doped with p-toluene sulphonic acid (pTSA), which exhibited very high selectivity and removal rate for monovalent ions. However, this membrane had poor chemical and mechanical stability, and ionic conductivity could not be improved because of PANi surface doping. H. Farrokhzad et al. have

TE D

employed a sulfonated polyvinylidine fluoride (SPVDF)/PVDF/PANi composite membrane with different PANi molecular weights to obtain a membrane with good water-softening performance [31]. The highest bivalent selectivity and best removal rate for Ca2+ and Mg2+ have been achieved with the

EP

proper PANi content. SEM observations of these membranes have shown that their surfaces are not smooth and dense, which meant that the mechanical stability of these composite membranes was less than ideal. This might have resulted from poor compatibility of the two polymers, PANi and

AC C

camphorsulfonic acid (CSA). Although NMP is a good solvent for individual polymers SPVDF, PVDF, and PANi, the compatibility of these materials decreased after they were blended in the same casting solution system. An increase in the compatibility of the blended polymers would be helpful for increasing the regularity of the resulting internal structures. Studies of CEMs with added conductive PANi have been reported in recent years, but the effects of internal structural regularity on CEM properties have rarely been reported in the literature. If PANi conductivity effects can help protonic acids and the resulting CEM still possesses good internal structure, it would be beneficial for improving membrane properties. However, the PANi content effects on membranes needed to be investigated. 3

ACCEPTED MANUSCRIPT In our previous study, a series of SPES/PVP C-CEMs was prepared by a simple process, producing matrixes of SPES. SPES supplied hydrogen ions to PANi such that an additional protonic acid was not required in the blended polymer system. As the number of materials in the blended system was decreased, compatibility among these materials improved, which might have been good

RI PT

for CEM structural regularity. Although PANi is a hydrophilic material, SPES/PVP composite membranes have good hydrophilicity such that the hydrophilicity is decreased. The conjecture here is that SPES, PVP, and PANi might play synergistic effects on C-CEM properties. In the present study, novel SPES/PVP/PANi C-CEMs with the same ratio of SPES/PVP and different PANi proportions

SC

were prepared and the effects of varied PANi content on C-CEM properties investigated. Analysis of these membranes included scanning electron microscopy (SEM), thermogravimetric analysis (TGA),

M AN U

X-ray diffractometry (XRD), and X-ray photoelectron spectroscopy (XPS) to characterize the C-CEM structure and properties. The thermal and mechanical properties and hydrophilicity were also measured and the possibility of actual C-CEM applications also explored by testing ED performance and comparison with commercial CEM. 2. Experimental

TE D

2.1. Materials

Polyether Sulfone (PES) was purchased from BASF SE (Ludwigshafen, Germany). PANi, PVP, and concentrated sulfuric acid were obtained from the Tianjin Chemical Reagent Co., Ltd. (Tianjin,

EP

China). The solvent NMP was purchased from Sigma-Aldrich, Inc. (St. Louis, MO, USA) and used as received. Commercial CEM (TingRun) and anion exchange membranes (AEM, TingRun) was purchased from Beijing Tingrun Membrane Technology Development Co., Ltd. (Beijing, China).

AC C

Deionized (DI) water was produced in-house. All chemicals and solvents were used without further purification.

2.2. Preparation of SPES

SPES was synthesized by an electrophilic substitution as shown in Fig. S1. First, 80 g of concentrated sulfuric acid was added to a 250 mL three-neck flask equipped with a mechanical stirrer and nitrogen inlet. Next, 20 g PES were added gradually to the three-neck flask at 80°C. After mechanically stirring the mixture for 12 h, the reaction was terminated and the polymer solution slowly decanted into cold DI water. The precipitate was repeatedly washed with DI water until the 4

ACCEPTED MANUSCRIPT wash was neutral pH and the SPES dried [22]. (3)

(2)

(6)

(5)

Fig. 1. Images of S/P/PANi C-CEMs.

M AN U

SC

(4)

RI PT

(1)

S/P/PANi-0, 0.3, 0.6, 0.9, 1.2, and 1.5 (1–6, respectively); SPES, PVP, and PANi dissolved in NMP; mass fraction

TE D

materials in solution 35%; all C-CEMs dried at 90°C for 9 h; SPES/PVP ratio in all C-CEMs 1/1 (by wt); and C-CEM thicknesses at 160–180 um.

2.3. Preparation of C-CEMs

EP

Two types of C-CEMs were prepared in this study. One type was a control membrane that contained no PANi while the second type was a group of samples containing various proportions of

AC C

PANi. Both C-CEM types were prepared by the process shown in Fig. S2. First, a certain amount of SPES was added into NMP at 35°C with electromagnetic stirring. Next, a certain amount of PVP was added and blended to the solution, specific quantities of PANi added, and the system stirred at 40°C for 36 h. The resulting solutions was then filtered and sonicated for 90 min. The resulting membrane solutions with the same SPES/PVP ratio and different PANi proportions had a mass fraction of materials (SPES, PVP, and PANi) at 35%. Each casting solution was poured onto a clean glass plate to form a membrane that was then placed in an oven at 90°C for 9 h. After drying, the glass plate with a prepared C-CEM was cooled, placed in DI water, and C-CEMs easily removed from the plate; C-CEM that contained no PANi was prepared in the same manner. C-CEMs were labeled 5

ACCEPTED MANUSCRIPT S/P/PANi-X C-CEMs, where S represents SPES and P represents PVP, with the SPES/PVP ratio at 1/1 (by wt). The term X represents the PANi content in the solution system (e.g., S/P/PANi-0 was the SPES/PVP C-CEM containing no PANi and S/P/PANi-0.3 meant 0.3 wt% PANi content). The C-CEM thicknesses were in the range of 160–180 um (Table 2).

RI PT

2.4. Characterization of SPES membranes

FTIR and DS Analysis. The chemical structures of polymers in the membranes were identified using a Fourier transform infrared spectrometer (FTIR; IRPrestige-21, Shimadzu Corp., Kyoto, Japan) [22]. Samples were scanned 40 times with 4 cm–1 resolution in the range of 400–4000 cm-1.

0.232 M × V × 100 W − 0.08 M × V

M AN U

DS (%) =

SC

The DS of SPES was obtained using the acid-base titration method and calculated using the equation

where, V is the volume of NaOH at the titration end point (mL), M the NaOH concentration (M), and W the SPES weight (g) [32]. 2.5. CEM characterization

Water uptake(%) =

TE D

Water uptake. Water uptake of CEMs was estimated according to the nether equation [33].

Wwet − Wdry ×100 Wdry

EP

Water uptake was obtained from CEM dry and wet weights. Typically, CEMs were immersed in DI water at different temperatures for 24 h, the surfaces blotted dry using filter paper, and CEM

AC C

weights determined using a precision analytical scale. The CEMs were then further dried and the weight of each membrane again determined and labeled Wdry (g). Swelling rate (SR). The CEM swelling ratios were calculated using the formula

SR =

Lwet − Ldry ×100% Lwet

where Lwet is the fully hydrated membrane length and Ldry the dry membrane length. IEC measurements. The IEC of the CEMs was obtained using acid base titration [22, 34]. Each CEM was dried and then immersed in 2 M NaCl solution to exchange H+ for Na+. The solution was then titrated with 0.01 M NaOH solution and the IEC of each CEM obtained using the nether 6

ACCEPTED MANUSCRIPT formula

IEC =

VNaOH × CNaOH Wdry

RI PT

where VNaOH is the NaOH volume at the equivalence point(L), CNaOH the NaOH concentration (M), and Wdry the dry membrane weight (g).

Contact angle (CA). The contact angle of each CEM was determined using a contact angle measurement system (Dataphysics OCA20, DataPhysics Instruments GmbH, Filderstadt, Germany)

SC

[22, 33]. To accomplish this, each membrane sample was fixed onto a glass microscope slide, dried, and 5 µL of water placed on the membrane surface at room temperature. The contact angle of each

different locations on a membrane surface.

M AN U

membrane sample was determined as the average value of at least three measurements of drops at

Ionic conductivity. The ionic conductivity (σ) of each CEM was obtained by an AC impedance technique employing an electrochemical workstation (PAR2273, AMT, USA) [22, 34]. The impedance spectra of each CEM were recorded in the frequency range of 2×106–5×10–2 Hz. A

TE D

two-probe conductivity cell was used to hold CEM samples for measurement, with the cell immersed in water, and the AC impedance spectroscopic measurements of fully hydrated membranes obtained. The membrane impedance was determined from the intercept of the impedance curve with the real

the formula

L R×S

AC C

σ=

EP

axis at the high-frequency end of the spectrum. The membrane ionic conductivity was obtained using

where R is the membrane resistance (ῼ), L the distance between the two electrodes (cm), and S the membrane cross-sectional area (cm2). TG analyses. TGA was used to characterize CEM thermal stability using a TGA Q500 thermal analyzer system (TA Instruments, New Castle, DE, USA) [35]. Before tests, membranes were dried, each CEM sample weighted to ~10 mg, and each subjected to heating from 40 to 700°C at a rate of 20°C /min under a nitrogen atmosphere. Tensile strength and elongation at break. CEM mechanical properties were calculated employing 7

ACCEPTED MANUSCRIPT an electronic strength tester (TH-810, Tabo Equipment Co., Ltd., Jiangsu, China). CEM samples were prepared into 3×1 cm rectangles and immersed in DI water to fully equilibrate. The tensile strength and elongation at breaking of a CEM sample was calculated by the average value of four measurements from each CEM [22, 33].

RI PT

XRD analysis. The crystal structures of polymers in the membrane samples were examined using an XR diffractometer (Ultima IV, Rigaku Corp., Tokyo, Japan). XRD patterns were obtained at ambient temperature in the scanning range of 2θ = 5–90 using Cu-Ka radiation [11].

Oxidative stability. Oxidative stability was calculated by the CEM quality changes when treated in

(3% H2O2 containing 2 ppm FeSO4) at 60°C for 10 h [35].

SC

Fenton's reagent. Square pieces of CEM samples (20×30 mm) were immersed in Fenton's reagent

M AN U

FE-SEM examination. CEM morphologies were observed using a field-emission SEM (FE-SEM) at an acceleration voltage of 25.0 kV using a Hitachi S-4160 (Hitachi Ltd., Tokyo, Japan). Cross-sections were obtained from CEMs that were soaked in liquid nitrogen and then fractured [36]. Atomic force microscopy (AFM). Surface roughness analysis of membranes was measured using an AFM (Dimension Edge, Bruker Corp., Billerica, MA, USA). The surface roughness was described in

TE D

terms of the root mean square roughness (RMS) [33].

Transport number and permselectivity. The transport number (tm) was calculated from the CEM potential (Em) [22, 37]. For this, a cell containing two Perspex compartments was prepared and the

EP

test CEM placed between the two Perspex compartments. One compartment was filled with 0.05 M KCl solution and the other with 0.01 M KCl solution. Thus, a cross-membrane potential difference was created and recorded by a multimeter (UT139C , Shanghai Longsai Electronic Technology Co.,

AC C

Ltd., Shanghai, China). The tm was checked using the equation

Em = (2tm − 1)

RT a1 ln F a2

and then the permselectivity calculated using

Ps =

tm − ti 1 − ti

where tm represents the transport number, Em, R, F, and T are the membrane potential, universal gas constant, Faraday constant, and absolute temperature in Kelvin, respectively, a1 and a2 the NaCl concentrations (M), tm the counter-ion transport number of the solution, ti the transport number of K+ 8

ACCEPTED MANUSCRIPT in 25°C 0.15 M KCl solution, and Ps the permselectivity. Diffusion coefficient. The diffusion coefficient was obtained using a two-compartment Donnan dialyzer that was prepared by polytetrafluoroethylene. A CEM to be tested was placed between the two compartments, creating an effective CEM area of 15 cm2. One dialysis compartment was filled

RI PT

with 1 M NaCl and the other with the same volume of DI water. The electrical conductivity of the

coefficient calculated using [22, 38]

D = KLd

SC

system was recorded (Starter 3100C, Kunshan Jiheli Instrument Co., Ltd., China) and the diffusional

where D is diffusional coefficient, K an instrument constant (K = 2.73×10−6), d the wet membrane

M AN U

thickness, and L the slope of the conductivity vs. time curve.

ED experiments. A representation of the ED arrangement is shown in Fig. 2, in which the whole arrangement comprised of five independent cell compartments and contained two sheets each of CEMs and AEMs; the key parameters are shown in Table 1. The membrane area between two compartments was 15 cm2 and the ED unit had three streams, operated in recirculation mode. The

TE D

volume of the filling solution was 250 mL for each cell compartment and the recirculation rate 100 mL/min. The rinse solution was 0.3 M Na2SO4 and the initial NaCl concentration solution of the dilute stream and concentrate stream were both 0.1 M. The potential between the two electrodes was 30 V according to a similar method and ED arrangement [39, 40]. The electrical conductivity and

EP

current were then recorded separately and the current density (CD), desalination rate (Rd), current

P=

∫

t =t t =0

CE =

U × I × dt

∆N × Mw

CD = Rd =

AC C

efficiency (CE), and energy consumption (P) calculated using the formulas

I S

φ1 − φ 2 × 100% φ1 F∆N

∫

t =t t =0

Idt

9

ACCEPTED MANUSCRIPT J=

∆N St

Where φ 1 and φ 2 represent the initial conductivity and desalination conductivity of dilute solution, respectively (µS/cm), ∆N, F, I, S, V, Mw, and t are the mole difference (mol), Faraday constant,

(A) Fig. 2. Schematic diagram and ED arrangement.

M AN U

SC

RI PT

current (A), membrane area (cm2), voltage (V), molecular weight of NaCl, and time (s), respectively.

(B)

TE D

The schematic diagram for ED experiments in this study is shown in Fig. 2-A and the actual ED arrangement in Fig. 2-B. The whole arrangement was comprised of three streams: electrode rinse, concentrated, and dilute solutions. The membrane (AEMs and CEMs) areas between two compartments were 15 cm2. C-1 and C-2 were commercial CEM and prepared C-CEMs, separately, and A-1 and A-2 the commercial AEM. The arrangement was operated in recirculation mode at 100 mL/min and the potential between the two electrodes 30 V. All parameters were

EP

measured by changes in current and conductivity. Table 1. Key properties of commercial AEM and CEM used in this study CEM

AEM

Type

Homogeneous

Homogeneous

Selectivity

Cation Exchange

Anion Exchange

Thickness (um)

164

172

*IEC (meq/g)

1.6

2.0

*Breaking strength (cN)

640.2

564.3

*SR (%)

2.2

2.8

tm

0.91

0.92

AC C

Membranes

*IEC, breaking strength, and swelling ratio were all measured at 30°C.

3. Results and Discussion 3.1. FTIR analysis of polymer SPES and composite membrane 10

ACCEPTED MANUSCRIPT FTIR analysis was used to characterize functional group formation on the polymer chain (Fig. 3). Comparison of the results for PES with SPES showed a new peak was observed at ~1026 cm-1 [41] in the SPES spectrum, which was attributed to symmetric stretching vibrations of sulfonate groups, and indicated that sulfonate groups were grafted onto PES polymer chains. Coincidentally, another

~1026 cm-1 should overlap the sulfonate group peak of PANi. PES

SC

SPES S/P/PANi-0.6 S/P/PANi-1.2

1026cm

800

1200 1600 2000 Wavenumber(cm-1)

2400

2800

M AN U

PANi -1

400

RI PT

peak appeared at ~1028 cm-1 in the PANi FTIR spectrum. Therefore, the peak for C-CEMs present at

Fig. 3. FTIR of PES, SPES, PANi, and S/P/PANi C-CEMs.

PES, SPES, PANi, and S/P/PANi C-CEMs separately pulverized to a powder state, scanned 40 times at 4 cm–1

3.2 XPS

TE D

resolution over 400–4000 cm-1, with a range of 400–2800 cm-1 used to characterize -SO3H introduction (Fig. 3).

The surface compositions of the prepared C-CEMS were further investigated using XPS, which provided useful information regarding the different states of nitrogenous compounds. All spectra

EP

consisted of at least four principal peaks that were attributed to carbon, oxygen, nitrogen, and sulfur, based on their binding energy signatures (Fig. 4-A) [42]. The characteristic peaks for C1s and O1s

AC C

with binding energies of 284.5 and 531.7eV, respectively, in the XPS spectra were strong, which corresponded to the CEM high carbon contents. Based on the PES composition, sulfur was the main component of PES. After sulfonation, sulfonate groups were grafted to the polymer chain and no new elements introduced into SPES. Thus, the sulfur of SPES was present in two types of functional groups, sulfone (-SO2-) and sulfonate groups (-SO3H). Hence, the peaks for S1s were studied by XPS-peak-differentiation-imitating analysis. The characteristic peaks for the S1s of sulfone and sulfonate groups, with a binding energies of 168.46 and 170.12 eV, respectively, were present (Fig. 4-B), while no similar characteristic peak for sulfonate groups appeared in pure PES/PVP membranes (Fig. 4-C). This indicated that sulfonate groups were successfully grafted to the polymer 11

ACCEPTED MANUSCRIPT chain. In addition, characteristic peak for N1s appeared in all C-CEM curves. The nitrogen in S/P/PANi-0 membranes without PANi originated from PVP. The functional group in PVP and PANi that contained nitrogen was the same amidogen group. Therefore, the characteristic peak for N1s for both polymers appeared at the same binding energy of 396.4 eV. The atomic concentrations of

RI PT

nitrogen were 3.87, 5.02, 5.43, 6.76, 6.47, and 5.85% for S/P/PANi-0, 0.3, 0.6, 0.9, 1.2, and 1.5, respectively. These results showed that the strongest peak intensity for nitrogen appeared in the curve from S/P/PANi-0.9 C-CEM, which corresponded to the actual atomic concentration on membrane surfaces. This implied that PANi dispersities in S/P/PANi-1.2 and S/P/PANi-1.5 were not as good as

S2p

N1S

C1S

O1S

(A)

M AN U

S/P/PANi-0

SC

in C-CEMs with less PANi.

Counts/s

S/P/PANi-0.3 S/P/PANi-0.6

S/P/PANi-0.9 S/P/PANi-1.2

S/P/PANi-1.5

0

200

400

600

800

1000

1200

1400

TE D

Binding Energy (eV) (B)

11000

(C)

6000

168.46eV

10000

-SO2-

6000 5000 4000 174

170.12eV

-SO3H

172

170

168

166 164

162

Counts/s

EP

7000

-SO2-

5000

8000

AC C

Counts/s

9000

168.46eV

5500

4500 4000 3500

160

158

174

172

170

168

166

164

162

160

158

Binding Energy (eV)

Binding Energy (eV)

Fig. 4. XPS curves of C-CEMs.

XPS to quantify membrane surface elemental compositions, spectra obtained with 45° electron mission angle with a 10 mm sampling depth, full C-CEM spectra with different PANi content (4-A) and S spectrum of S/P/PANi-0.9 C-CEM and PES/PVP membranes (4-B and 4-C, respectively), and binding energies of characteristic peaks for S1s of sulfone and sulfonate groups at 168.46 and 170.12 eV, respectively.

3.3 CEM Morphology Surface and cross-section images of all C-CEMs and commercial TingRun CEM showed that all 12

ACCEPTED MANUSCRIPT C-CEMs appeared to be flat, continuous, and free of holes (Fig. 5). As the PANi content in C-CEMs increased from 0.3 to 1.2%, no obvious agglomeration appeared on C-CEM surfaces. As is well known, NMP is a good solvent for dissolving PANi, SPES, and PVP, but the relative solubility of these polymers can have adverse effects when these materials are combined in NMP. However, here,

RI PT

based on SEM of membrane surfaces, it appeared that SPES, PVP, and PANi were compatible when the PANi content was <1.2 wt%. When the PANi content was increased to 1.5%, small particles appeared on membrane surfaces. When compared to TingRun CEM, which appeared uneven and rough by SEM, a smooth membrane surface was observed, which would be beneficial for use in

SC

removing insoluble pollutants. All C-CEM cross-sections were flat, dense, homogeneous, and free of holes (Figs. 5a–5g), except for S/P/PANi-1.5 CEM, on which some particles appeared in membrane

M AN U

cross-sections. This result was consistent with the membrane surface structure and an indication that PANi dispersion in the blended matrix was decreased when PANi content was 1.5%. However, on the whole, C-CEMs exhibited good surfaces and cross-sectional structures when the PANi content was <1.2 wt%. (B)

TE D

(C)

(F)

(a)

(D)

(G)

(H)

(c)

(d)

EP

(E)

AC C

(A)

(e)

(b)

(f)

(g)

13

(h)

ACCEPTED MANUSCRIPT Fig. 5. SEM images of C-CEMs with different PANi content and a TingRun CEM. Surface images magnified 10k-fold of S/P/PANi-0, 0.3, 0.6, 0.9, 1.2, and 1.5, respectively, and TingRun CEM and AEM (A–H, respectively). Cross-sectional images magnified 500-fold of S/P/PANi-0, 0.3, 0.6, 0.9, 1.2, and 1.5, respectively, and TingRun CEM and AEM (5a–5h, respectively). Black circles show smooth structure and red circles agglomeration on S/P/PANi-1.5 C-CEMs. (B)

(a)

(b)

(C)

(c)

TE D

M AN U

SC

RI PT

(A)

Fig. 6. AFM images of S/P/PANi C-CEMs with different PANi content. Phase images of S/P/PANi-0.6, 0.9, and 1.2 CEM recorded from 5×5 µm (A–C) and 200×200 nm areas (a–c,

EP

respectively under ambient conditions.

The hydrophilic/hydrophobic phase separation in these C-CEMs was investigated by obtaining

AC C

phase images of membrane surfaces using an AFM in tapping mode. In these images, brighter sections were related to hydrophobic regions, which were formed by polymer hydrophobic mainchains, while darker sections corresponded to hydrophilic regions that consisted of water and sulfonate group ionic clusters [4]. The size of hydrophilic regions in CEM decreased with increasing PANi content if 0.6, 0.9, and 1.2 wt% (Fig. 6), which might have been due to PANi hydrophobicity. However, all three C-CEMs exhibited a well-defined phase separation. The surface roughness of the these C-CEMs was also examined in triplicate images from 5×5 um areas randomly selected from each image to measure the surface roughness (Ra) and the average roughness of the whole image obtained. The random Ra of S/P/PANi-0.6 CEM was measured to be 1.01, 1.06, and 1.25 nm, with 14

ACCEPTED MANUSCRIPT an average roughness of 1.13 nm. The random Ra of the S/P/PANi-0.9 was 1.78, 1.60, and 1.59 nm, with an average roughness of 1.71 nm. The random Ra of S/P/PANi-1.2 was found to be 6.65, 4.41, and 3.87 nm, with an average roughness of 4.45 nm. The increasing average roughness from 1.13 to 4.45 nm for the three C-CEMs demonstrated that increased PANi content resulted in more uneven

RI PT

membrane surfaces, which corresponded well with the SEM images (Fig. 5). Membrane surfaces appeared uneven and roughness appeared to increase with increased C-CEM PANi content. SEM and AFM images suggested that increased PANi content resulted in decreased matrix material compatibility in this system.

70

100

M AN U

95

60

。

Water uptake (%)

90 Contact angle (。 )

SC

3.4 Hydrophilicity

85 80 75 70 65

50 40

P/S/PANi-0 P/S/PANi-0.6 P/S/PANi-1.2 TingRun

30

P/S/PANi-0.3 P/S/PANi-0.9 P/S/PANi-1.5

20

60

(A)

TE D

0 0.3 0.6 0.9 1.2 1.5 TingRun TingRun and C-CEMs with different content of PANi (%)

30

40 50 60 Temperature（ ℃） (B)

70

EP

Fig. 7. The hydrophilicity of TingRun CEM and S/P/PANi C-CEMs. Image of each contact angle located at the top of corresponding bar graph (A) and water uptake (B), and all CEMs measured in the range of 30–70°C.

The hydrophilicity of an ion exchange membrane plays an important role in its relative ionic conductivity. C-CEM hydrophilicity was characterized by measuring their contact angle and water

AC C

uptake. The contact angles of C-CEMs containing various PANi content and TingRun CEM showed that the C-CEM angle without PANi was 68.2 , which was lower than that of TingRun CEM (64.2 ,º Fig. 7-A). PANi is a hydrophobic polymer, such that as the PANi content in CEMs increased from 0.3 to 1.5%, the C-CEM contact angle increased from 68.2 ºto 87.1 . These results indicated that C-CEM hydrophilicity decreased as PANi content increased. This was illustrated by comparison of S/P/PANi-0.9 with S/P/PANi-1.2 C-CEM, which showed that the contact angle rapidly increased. C-CEM surface roughness might have been another reason for the high contact angle for S/P/PANi-1.2 C-CEM. It has been reported that a membrane’s contact angle will increase with increased surface roughness when the contact angle is <90 15

, but the contact angle will decrease as

ACCEPTED MANUSCRIPT surface roughness increases when the contact angle is >90 [36]. The surface roughness of S/P/PANi-1.2 CEM appeared to increase compared to the other C-CEMs with low PANi content (Fig. 6). The water uptake of C-CEMs at different temperatures showed that C-CEM water uptake increased as temperature increased (Fig. 7-B). However, it appeared that C-CEM water uptake decreased with increased PANi at a set temperature, which was consistent with C-CEM contact angle

RI PT

measurements. PANi hydrophobicity was apparently responsible for the decreased hydrophilicity in C-CEMs as PANi content increased. 3.5 XRD S/P/PANi-0.3

S/P/PANi-0.6

S/P/PANi-0.9

S/P/PANi-1.2

S/P/PANi-1.5

S/P/PANi-0

SPES

PANi

0

10

20

30

40

50

60

70

80

90

2-Theta

M AN U

SC

PES

TE D

Fig. 8. XRD curves of S/P/PANi C-CEMs, PES, PANi, and SPES. X-ray diffraction patterns were obtained at ambient temperature with 2θ = 5–90º using Cu-Ka radiation. Red circle indicates homogeneous amorphous peak of all prepared C-CEMs, while black circle showed obvious crystalline peaks at different PANi locations.

XRD was performed on these C-CEMs, including SPES, PANi, and C-CEMs with different PANi

EP

content, to evaluate thematerials’ relative chain ordering and crystallinity (Fig. 8). XRD patterns of PES and SPES were quite similar, with no crystalline peak or homogeneous amorphous peak

AC C

observed in PES and SPES XRD patterns. However, the XRD pattern of PANi exhibited obvious crystalline peaks at different locations, which was an indication of PANi’s crystalline nature. XRD patterns of all C-CEMs exhibited no crystalline peaks, but one homogeneous amorphous peak was present. The major peak of broad scattering maxima for all C-CEMs was located at ~19°. Although PANi had been blended with SPES and PVP, no crystalline peak similar to the PANi XRD pattern was observed. There were two reasons for this result. On one hand, the C-CEM PANi content was low, such that it had little effect on XRD patterns of the blended membranes. On the other hand, these results were a clear indication that PANi was homogeneously dispersed in these blended systems, which agreed with SEM and AFM results. As the C-CEM PANi content increased, it 16

ACCEPTED MANUSCRIPT appeared in SEM images that particles were present on membrane surfaces and surface roughness increased, but based on AFM images, no clear large particle agglomeration appeared to have occurred. This was evidence of good compatibility between PANi and the other matrix materials. 3.6 Ionic conductivity

RI PT

The ionic conductivity of a membrane is a very important parameter for assessing its ability to transport ions [18]. The ionic conductivity of the present C-CEMs was measured at different temperatures and the ionic conductivity found to increase with increasing temperature (Fig. 9). The ionic conductivity of C-CEMs doped with PANi was higher than that of TingRun CEM. However,

SC

the ionic conductivity of all S/P/PANi C-CEMs did not always increase with increased PANi content. The ionic conductivity appeared to increase to an optimal level with increased PANi content and

M AN U

then decreased as additional PANi was added beyond the optimal proportion. A 0.6% PANi content was found to produce the optimal ionic conductivity in these S/P/PANi C-CEMs. In general, the C-CEM ionic conductivity was improved by adding PANi, because C-CEM resistance decreased with the addition of electronically conductive PANi. Hydrophilicity results indicated that S/P/PANi C-CEM hydrophilicity decreased with increased membrane PANi content. SEM and AFM images of

TE D

the modified membranes showed that the compatibility of SPES, PVP, and PANi gradually decreased with increased PANi content. The irregular membrane structure degraded the synergistic effects between the sulfonate groups and PANi backbone thus improving membrane ionic

EP

conductivity. The sulfonate groups acted as protonic acids and the H+ of sulfonate groups entered the PANi mainchain, causing PANi to change from an insulator to a conductor. Therefore, taking into account synergistic effects, hydrophilicity, electrical conductivity, and structural regularity,

AC C

S/P/PANi-0.6 C-CEM possessed the highest ionic conductivity, implying a good internal structure and low resistance. When the PANi content was >0.6%, the C-CEM ionic conductivity gradually decreased. It was concluded, here, that there were two reasons for this effect. First, membrane hydrophilicity decreased with increased PANi content, which adversely affected ion transfer through ionic channels in the polymer. Second, S/P/PANi C-CEM electronic conductivity decreased with increased PANi content, because the PANi did not evenly disperse in these C-CEM when the PANi content was >0.6%. Therefore, based on membrane ionic conductivities, the optimal PANi proportion in these S/P/PANi C-CEMs was 0.6% PANi. 17

ACCEPTED MANUSCRIPT 100 S/P/PANi-0 S/P/PANi-0.3 S/P/PANi-0.6

80

S/P/PANi-0.9 S/P/PANi-1.2

70

S/P/PANi-1.5 TingRun

60 50 40

RI PT

Ionic conductivity (mS/cm)

90

30 20 30

40

50

60

70

Temperature ( )

SC

Fig. 9. Ionic conductivity of C-CEMs and TingRun CEM. Ionic conductivity of each CEM obtained by AC impedance technique employing an electrochemical workstation, impedance spectra recorded from 2×106 to 5×10–2 Hz, and all CEMs measured in the range of 30–70°C.

3.7 Stability and mechanical properties

M AN U

The mechanical properties of these C-CEMs were measured by determining their elongation at breaking and the breaking strength. Mechanical stability appeared to increase with increased PANi content, from 0.3 to 0.9% (Table 2). However, the mechanical stability appeared to decrease quickly when PANi content increased from 0.9 to 1.5%. As described above, PANi compatibility with other matrix materials in these C-CEMs decreased as PANi content increased. This increase in mechanical

TE D

stability was an indication that a crosslinked structure was formed after addition of an appropriate proportion of PANi, thus improving mechanical stability. However, a decrease in mechanical stability was noted with high PANi content, which was scribed to structural reasons, in that the degree of

EP

crosslinking decreased when PANi content increased to 1.5%. The swelling ratio of the present C-CEMs was also characterized and compared with TingRun CEM.

AC C

The swelling ratios of all C-CEMs were observed to decrease with increased PANi content (Table 2), which might have been from crosslinking in the internal structure that decreased the hydrophilicity. However, the swelling rate of all C-CEMs was higher than that of TingRun CEM, which might have reflected PVP effects. Although C-CEM hydrophilicity decreased with the addition of hydrophobic PANi, the C-CEM hydrophilicity was mainly determined by hydrophilic PVP. As the potential applications of the proposed CEMs are electrodialysis, electrolysis, and electrodeionization, the chemical resistance of membranes to acids and alkalis is very important for possible practical applications. Therefore, the chemical stabilities of these C-CEMs and TingRun CEM were determined by subjecting them to an acid, base, and Fenton's reagent (Table 2). After 18

ACCEPTED MANUSCRIPT treatment with acid and base, no weight loss was observed in all C-CEMs and commercial CEM. When treated with Fenton’s reagent, C-CEMs showed a slight weight loss when the PANi content ranged from 0.3 to 0.9%, which indicated that these C-CEMs exhibited good oxidative stability toward chemical reagents when the PANi content was not >0.9%. The oxidative stability was nearly

RI PT

the same as the change in mechanical properties. A decrease in oxidative stability at high PANi content further indicated decreased crosslinking when the PANi content was 1.5%. Table 2. Properties of C-CEMs and TingRun CEM DS of

Thickness

Oxidative

*Breaking

SPES (%)

(um)

stability (%)

strength (cN)

P/S/PANi-0

15

168

91

P/S/PANi-0.3

15

172

93

P/S/PANi-0.6

15

176

94

P/S/PANi-0.9

15

169

P/S/PANi-1.2

15

164

P/S/PANi-1.5

15

178

TingRun

-

164

*Elongation

*SR(%)

at break (%)

SC

CEMs

12.3

3.7

570.5

12.5

3.3

571.3

13.5

3.1

M AN U

560.3

93

590.2

14.6

2.9

84

420.2

9.6

2.8

76

310.3

5.6

2.5

96

640.2

15.1

2.2

*breaking strength, elongation at break, and swelling ratio, all measured at 30°C.

TE D

3.8 TGA

The thermal stability of these C-CEMs was characterized using TGA analysis, in which the C-CEMs and PANi were heated from 40 to 700°C under nitrogen (Fig. 10). PANi exhibited two main

EP

degradation stages, while all C-CEMs appeared to have three stages of weight loss. In addition, the degradation temperatures were different for PANi and C-CEMs. On the whole, the results showed

AC C

that the weight loss for PANi was lower than all C-CEMs. TGA curves of C-CEMs were all similar, but the weight losses for the various C-CEMs were not identical. The first stage of weight loss for PANi and C-CEMs appeared at the same temperature at ~80°C, which was considered to represent the loss of water and residual solvent [34]. The second stage of weight loss for PANi appeared at ~210°C, which was attributed to mainchain degradation. The second stage of weight loss for all C-CEMs appeared at ~300°C, which was attributed to sulfonate group degradation. The last stage of weight loss for all C-CEMs occurred at ~400°C, which was attributed to the main polymer chain degradation in other CEM matrix materials. The degradation temperatures of mainchains in PANi and C-CEMs were different, but the TGA curves among C-CEMs were similar (Fig. 10). This was an 19

ACCEPTED MANUSCRIPT indication of the good compatibility of PANi in these C-CEMs. The weight losses of C-CEMs in the third stage were not equivalent, which was a reflection of the relative compatibility between PANi and other matrix materials in various mixtures. In general, the stability of C-CEMs with PANi was sufficient for practical applications. 100

RI PT

90 80

60 S/P/PANi-0 S/P/PANi-0.3 S/P/PANi-0.6 S/P/PANi-0.9 S/P/PANi-1.2 S/P/PANi-1.5 PANi

50 40 30 20 0

100

200

300

400

500

600

700

800

M AN U

Temperature ( )

SC

Weight (%)

70

Fig. 10. TGA curves of C-CEMs and PANi. Each CEM sample at ~10 mg and subjected to heating in test apparatus from 40 to 700°C at 20°C /min under nitrogen atmosphere.

3.9 Transport number and permselectivity

The transport number and permselectivity of these CEMs are primarily affected by the functional

TE D

group density, hydrophilicity, and continuity of internal ion transfer channels. An increased density of functional groups can apppear to enhance the coherence of functional regions and facilitated transfer of counter-ions (Na+) through the membrane. In this study, the transport number and

EP

permselectivity of all C-CEMs initially increased and then decreased as PANi content increased in membrane formulations. The ratios of SPES and PVP in these C-CEMs were held equal, which

AC C

meant that the functional group densities were nearly the same, such the transport number and permselectivity of the membranes were mainly affected by PANi content. A proper PANi content helped to enhance the coherence of functional phase regions in these C-CEMs and improved counter-ion transfer. When the PANi content was <0.9 %, the internal C-CEM structure was regular, such that increasing the membrane PANi content contributed to an increased transport number and permselectivity. The transport number and permselectivity of the C-CEMs began to decrease when the PANi content was >0.9% (Table 3). These results indicated that PANi’s hydrophobic nature caused decreased coherence of the internal functional phase regions of these C-CEMs. Here, C-CEM hydrophilicity decreased with PANi addition to >0.9%, which was disruptive to the synergistic 20

ACCEPTED MANUSCRIPT effects of sulfonate groups and PANi polymer for forming good ion transfer channels. Therefore, this resulted in an initial increase in transport number and permselectivity of C-CEMs followed by a subsequent decrease as PANi content increased. Table 3. ED properties of C-CEMs and TingRun CEM ×10-9 cm2·s-1

Transport number

Permselectivity

State2

State1

State2

State1

CEMs State1 2.34

2.32

0.90

0.89

0.81

P/S/PANi-0.3

3.83

3.79

0.91

0.90

0.83

P/S/PANi-0.6

5.21

5.26

0.93

0.94

0.86

P/S/PANi-0.9

4.47

4.41

0.94

0.92

0.87

P/S/PANi-1.2

3.04

2.98

0.92

0.92

P/S/PANi-1.5

2.37

2.34

0.91

0.90

TingRun

1.45

1.48

0.91

0.92

State2

State1

State2

0.82

0.43

0.44

0.83

0.45

0.44

0.85

0.47

0.46

0.87

0.45

0.46

SC

P/S/PANi-0

*IEC (meq·g-1)

RI PT

D

0.81

0.46

0.45

0.81

0.79

0.42

0.44

0.83

0.82

-

-

M AN U

0.84

State 1: Membrane parameters before ED testing; State 2: Membrane parameters after ED testing *IEC, measured at 30°C.

3.10 Diffusion coefficient (D)

The diffusion coefficient (D) of a CEM reflects the synergistic effects of co-ions and counter-ions

TE D

in C-CEMs and is related to several factors [40]. First is the distribution of active sites, such that a uniform distribution of active sites contributes to the formation of regular functional phase regions. Second is the density of functional groups, in that increasing functional group density in functional

EP

phase regions can attract more counter-ions, which will accelerate diffusion. Third is the regularity of membrane internal structure, such that increasing regularity of C-CEM internal structure is

AC C

conducive to increased diffusion coefficient. The diffusion coefficient of the present C-CEMs increased and then decreased as the PANi content increased (Table 3). As SPES contents in all C-CEMs were nearly equal, the sulfonate group density was essentially the same in all C-CEMs. Consequently, the C-CEM diffusion coefficient was mainly dictated by the active point distribution and membrane regularity. The addition of an appropriate PANi content was observed to help increase the active site continuity, but the compatibility of these various materials decreased with increased membrane PANi content. When the PANi content was higher, the poor PANi dispersion in these C-CEMs began to yield an apparently undesirable effect, in which the continuity of active sites declined. The diffusion coefficient of S/P/PANi-0.6 C-CEM was the highest and the diffusion 21

ACCEPTED MANUSCRIPT coefficient of S/P/PANi-1.5 C-CEM the lowest. 3.11 Desalination of NaCl by ED 3.11.1 Current density and current efficiency 0

50

Time (min) 150 200

100

250

300

350

400

0

14

S/P/PANi-0.6 S/P/PANi-1.2 TingRun

400

4.73

60 50 S/P/PANi-0.3 S/P/PANi-1.2 S/P/PANi-0

40 30 5.15

4.67

4.47

4

4.47

4.33

CE (%)

2 CD (mA/cm ) )

4

350

3 2 1 0 0 0.3 0.6 0.9 1.2 1.5 TingRun TingRun and C-CEMs with different content of PANi (%)

80 70 60 50 40 30 20 10 0

69.9

72.3

70.7

RI PT

CE (%)

S/P/PANi-0.3 S/P/PANi-0.9 S/P/PANi-1.5 S/P/PANi-0

6 5

300

S/P/PANi-0.6 S/P/PANi-1.5

67.6

69.3

S/P/PANi-0.9 TingRun

74.6

64.1

SC

CD (mA/cm2)

4

250

70

10

6

Time (min) 150 200

100

80

12

8

50

0 0.3 0.6 0.9 1.2 1.5 TingRun TingRun and C-CEMs with different content of PANi (%)

(B)

(A)

M AN U

Fig. 11. Current density and efficiency of ED devices as a function of time.

Current density (A), current efficiency (B), every ED device operated for 6 h, final current density and efficiency of ED devices equipped with different CEMs shown below main diagram, and 30 V potential between the two electrodes.

The fabricated CEMs were used in an ED process to demonstrate their desalinization capability and for comparison with TingRun CEM. The current density of all CEMs initially increased and then decreased (Fig. 11-A). Membrane current density is principally influenced by the membrane

TE D

resistance in the ED process as well as the electrolyte solution. Initially, the concentration of the test salt solution in the concentrated cell was the same as that in the dilution cell. As the ED process proceeded, the salt concentration in the dilute cell decreased and resistance increased, while the salt

EP

concentration in the concentrated cell increased and resistance decreased. At the beginning of the ED process, the resistance of the whole ED was dictated by the salt concentration in the concentrated

AC C

solution. After process initiation, the dilute and concentrated solution resustances gradually achieved a balance. After reaching a maximum value, the current density was primarily determined by the decreasing resistance of the dilute solution. Thus, the current density decreased with increasing dilute solution resistance. At the same time, the C-CEM current densities were higher than that of TingRun CEM at the beginning of the process, which indicated that the resistance of the prepared C-CEMs was lowered by the presence of conductive PANi (Fig. 11-A). However, at the end of the process, the current density of the prepared C-CEMs was lower than that of TingRun CEM, which was caused by the low salt concentration in the dilute cell. The current efficiency of the process first increased and then slightly decreased as the ED process proceeded (Fig. 11-B). The inflection point of the current 22

ACCEPTED MANUSCRIPT efficiency was observed to be different from that of current density and the inflection point of current efficiency appeared later. This indicated that the effect of concentration polarization was greater than the effect of current efficiency. On the whole, the current density and efficiency of these C-CEMs were both improved by adding an optimal proportion of PANi, which enhanced the synergistic

RI PT

effects of sulfonate groups and PANi. An irregular structure adversely affected C-CEM conductivity and it appeared that the current efficiency of S/P/PANi-1.5 C-CEM decreased at the end of the ED process, which was caused by an uneven internal structure. Thus, it was concluded that C-CEM internal structural regularity was very important for efficient ED properties.

SC

3.11.2 NaCl flux

The NaCl flux is also an important parameter for determining membrane suitability for use in ED

M AN U

processes. The NaCl flux of the present C-CEMs initially increased and then decreased (Fig. S3). Before the ED process, the NaCl concentrations in the concentrated and dilute cells were equal. At the beginning of the ED process, the NaCl flux across the membrane increased, because the concentration polarization was small and, as the ED process proceeded, the concentration polarization gradually increased to its maximum. Then, the concentration polarization continued to

TE D

increase, which decreased the NaCl flux. The time to reach the maximum NaCl flux was observed to vary with the CEM, which was related to the CEM internal structure. 3.11.3 Desalination rate and energy consumption

EP

The desalination rate was observed for all CEMs and, as the ED process proceeded, the desalination rate of all CEMs increased, but the desalination rate increased slowly at the end of the process (Fig. 12). The current density and efficiency initially increased and then decreased as a result

AC C

of concentration polarization, while the desalination rate was affected by the current density and efficiency (Fig. 11). Thus, there was a slow increase in the desalination rate at the end of the process. However, the final desalination rates of all C-CEMs were observed to be higher than those of S/P/PANi-0 and TingRun CEMs. The addition of an optimal quantity of PANi to the C-CEMs contributed to the increased desalination rate, because the resident PANi reduced the CEM electrical resistance and ion transfer. S/P/PANi-0.6 CEM exhibited the highest desalination rate, which was higher than those of other prepared C-CEMs and TingRun CEM. This was probably a result of the more regular internal structure and low resistance of S/P/PANi-0.6 CEM. The energy consumption of all C-CEMs initially decreased and then increased slightly as the ED process proceeded (Fig. 13). 23

ACCEPTED MANUSCRIPT Many factors affect energy consumption and, here, the membrane resistance and diffusion coefficient were two very important factors. The increased diffusion coefficient helped decrease the ED energy consumption. The diffusion coefficient of S/P/PANi-0.6 CEM was the highest of the prepared C-CEMs, because PANi addition effectively reduced the membrane electrical resistance. Thus, the

RI PT

energy consumption of S/P/PANi-0.6 was the lowest. Although PANi addition reduced the membrane resistance, a higher PANi content decreased the C-CEM internal compatibility. In particular, the internal regularity of S/P/PANi-1.5 was concluded to be worse than those of other C-CEMs. Considering the reported results from previous studies, the C-CEM desalination rate was estimated

0

50

Time (min) 150 200

100

250

300

350

SC

from the functional group density, hydrophilicity, resistance, and regularity of the membrane. 400

60 S/P/PANi-0.3 S/P/PANi-0.9 S/P/PANi-1.5 S/P/PANi-0

40 20 0

100

93.6

90.6

94.3

94.4

93.4

S/P/PANi-0.6 S/P/PANi-1.2 TingRun

93.2

93.3

R(%)

80 60 40 20 0 0 0.3 0.6 0.9 1.2 1.5 TingRun TingRun and C-CEMs with different content of PANi (%)

TE D

(A)

M AN U

R(%)

80

Fig. 12. Desalination rate of ED devices as a function of time, at 30 V constant potential. Every ED process operated for 6 h, effective membrane (AEMs and CEMs) area in ED experiment 15 cm2.

50

100

Time (min) 150 200

P (Kw·h·Kg-1)

30

20 15

P (Kw·h·Kg-1)

25

10 5

19.7

AC C

25

15

300

S/P/PANi-0.3 S/P/PANi-0.9 S/P/PANi-1.5 S/P/PANi-0

35

20

250

19.3

350

400

EP

0 40

18.6

20.3

19.8

S/P/PANi-0.6 S/P/PANi-1.2 TingRun

21.5 19.1

0 0 0.3 0.6 0.9 1.2 1.5 TingRun TingRun and C-CEMs with different content of PANi (%)

(B)

Fig. 13. Energy consumption of an ED device as a function of time, with 30 V constant potential. ED device operated for 6 h and effective membrane (AEMs and CEMs) area 15 cm2.

4. Conclusions Based on the results of our previous study, a series of C-CEMs containing the conductive polymer 24

ACCEPTED MANUSCRIPT PANi were prepared using solution casting and solvent evaporation. The properties of these C-CEMs were characterized using contact angle measurements, SEM, AFM, and TGA. The results showed that these S/P/PANi C-CEMs exhibited good surface and cross-sectional structures, in which the surface roughness increased with increased PANi content. The mechanical stability of these C-CEMs

RI PT

increased with the addition of an appropriate PANi content, which suggested that a crosslinked structure had been formed that improved membrane stability. The electroconductibility of PANi resulted from sulfonate group hydrogen ions. The hydrophilicity of these C-CEMs was found to decrease with increased PANi content because of PANi hydrophobicity, but the CEM ionic

SC

conductivity initially increased and then decreased. S/P/PANi-0.6 CEM had the highest ionic conductivity. The feasibility of practical applications for these CEMs was determined through ED

M AN U

experiments, which showed that S/P/PANi-0.6 CEM had better ED properties than a commercial CEM and the other prepared C-CEMs. The presence of PANi and sulfonate groups in C-CEMs was concluded to have exerted synergistic effect that improved the membrane properties. This work offered a method for improving C-CEM properties through the addition of appropriate quantities of

Acknowledgements

TE D

sulfonated, hydrophilic, and conducting materials.

This work was supported by the National Key Research and Development Program of China (Nos. 2017YFC0404003 and 2016YFC0400707), Tianjin Special Project of Ecological Environment

EP

Management Science and Technology (18ZXSZSF00050) and the Fundamental Research Funds for the Central Universities, Nankai University (20180017), the State Key Laboratory of Separation

AC C

Membranes and Membrane Processes (Tianjin Polytechnic University), No. M2-201703. References

[1] J. Lee, J. Lee, S. Ryu, S. Yun, S. Moon, Electrically aligned ion channels in cation exchange membranes and their polarized conductivity, J. Membr. Sci. 478 (2015) 19-24. [2] L. Ma, W. Cai, J. Li, K. Fan, Y. Jiang, L. Ma, H. Cheng, A high performance polyamide-based proton exchange membrane fabricated via construction of hierarchical proton conductive channels, J. Power Sources 302 (2016) 189-194. [3] C. Gonzalez, A. Ramos, R. Ibañez, Y. Chen, A. Irabien, Valorization of desalination brines by electrodialysis with bipolar membranes using nanocomposite anion exchange membranes, 25

ACCEPTED MANUSCRIPT Desalination 406 (2017) 16-24. [4] Y. Zhuo, A. Lai, Q. Zhang, A. Zhu, M. Ye, Q. Liu, Enhancement of hydroxide conductivity by grafting flexible pendant imidazolium groups into poly(arylene ether sulfone) as anion exchange membranes, J. Mater. Chem. A. 3 (2015) 18105-18114.

RI PT

[5] F. Gashoul, M. Parnian, S. Rowshanzamir, A new study on improving the physicochemical and electrochemical properties of SPEEK nanocomposite membranes for medium temperature proton exchange membrane fuel cells using different loading of zirconium oxide nanoparticles, Int J Hydrogen Energy 42 (2017) 590-602.

electrodialysis, J. Clean. Prod. 143 (2017) 250-256.

SC

[6] X. Sun, H. Lu, J. Wang, Recovery of citric acid from fermented liquid by bipolar membrane

M AN U

[7] X. Yang, Q Ye, P. Cheng, Matching of water and temperature fields in proton exchange membrane fuel cells with non-uniform distributions, Int. J. Hydrogen Energy 36 (2011) 12524-12537.

[8] D. Garcia-Nieto, V. Barragan, A comparative study of the electro-osmotic behavior of cation and anion exchange membranes in alcohol-water media. Electrochim. Acta 154 (2015) 166-176.

TE D

[9] Y. Wei, L. Shen, Z. Wang, W. Yang , H. Zhu, H. Liu, A novel membrane for DMFC - Na2Ti3O7 nanotubes/Nafioncomposite membrane, Int. J. Hydrogen Energy 36 (2011) 5088-5095. [10] J. Lee, W. Li, A. Manthiram, Poly(arylene ether sulfone)s containing pendant sulfonic acid

EP

groups as membrane materials for direct methanol fuel cells, J. Membr. Sci. 330 (2009) 73-79. [11] J. Pan, B. Wu, L. Wu, Y. He, J. Miao, L. Ge, T. Xu, Proton exchange membrane from tetrazole-based poly (phthalazinone ether sulfone ketone) for high-temperature fuel cells, Int. J.

AC C

Hydrogen Energy 41 (2016) 12337-12346. [12] A. Badami, O. Lane, H. Lee, A. Roy, J. Mcgrath, Fundamental investigations of the effect of the linkage group on the behavior of hydrophilic-hydrophobic poly(arylene ether sulfone) multiblock copolymers for proton exchange membrane fuel cells, J. Membr. Sci. 333 (2009) 1-11. [13] Z. Zhang, T. Xu, Proton-conductive polyimides consisting of naphthalenediimide and sulfonated units alternately segmented by long aliphatic spacers, J. Mater. Chem. A. 2 (2014) 11583-11585. [14] A. Roy, H. Lee, J. Mcgrath, Hydrophilic-hydrophobic multiblock copolymers based on poly (arylene ether sulfone)s as novel proton exchange membranes-part B, Polymer 49 (2008) 5037-5044. [15] Y. Oh, H. Lee, M. Yoo, H. Kim, J. Han, T. Kim, Synthesis of novel crosslinked sulfonated 26

ACCEPTED MANUSCRIPT poly(ether sulfone)s using bisazide and their properties for fuel cell application, J. Membr. Sci. 323 (2008) 309-315. [16] E. Bakangura, L. Ge, M. Muhammad, J. Pan, L. Wu, T. Xu, Sandwich structure SPPO/BPPO proton exchange membranes for fuel cells: Morphology- electrochemical properties relationship, J.

RI PT

Membr. Sci. 475 (2015) 30-38. [17] W. Harrison, F. Wang, J. Mecham, V. Bhanu, M. Hill, Y. Kim, J. Mcgrath, Influence of the bisphenol structure on the direct synthesis of sulfonated poly(arylene ether) copolymers. I, J. Polym. Sci. Part A. Polym. Chem. 41 (2003) 2264-2276.

SC

[18] M. Cui, Z. Zhang, T. Yuan, H. Yang, L. Wu, E. Bakangura, T. Xu , Proton-conducting membranes based on side-chain-type sulfonated poly(ether ketone/ether benzimidazole)s via one-pot

M AN U

condensation, J. Membr. Sci. 465 (2014) 100-106.

[19] X. Yan, H. Jiang, G. Zhao, L. Zeng, T. Zhao, Preparations of an inorganic-framework proton exchange nanochannel membrane, J. Power Sources 326 (2016) 466-475. [20] Z. Yao, Z. Zhang, L.Wu, T. Xu, Novel sulfonated polyimides proton-exchange membranes via a facile polyacylation approach of imide monomers, J. Membr. Sci. 455 (2014) 1-6.

TE D

[21] Z. Yang, Y. Liu, R. Guo, J. Hou, L. Wu, T. Xu, Highly hydroxide conductive ionomers with fullerene functionalities, Chem Comm. 52 (2016) 2788-2791. [22] Z. Yang, R. Guo, R. Malpass-Evans, M. Carta, N. Mckeown, M. Guiver, L. Wu, T. Xu, Highly

EP

Conductive Anion-Exchange Membranes from Microporous TrçgerQs Base Polymers, Angew. Chem. Int. Ed. 55 (2016) 11499-11502. [23] A. Macdiarmid, J. Chiang, A. Richter, A. Epstein, Polyaniline: a new concept in conducting

AC C

polymers, Synth. Met. 18 (1987) 285-290. [24] C. Wu, T. Bein, Conducting Polyaniline Filaments in a Mesoporous Channel Host, Science 1994 264 (1994) 1757-1759.

[25] S. Xu, Y. Han, Y. Xu, H. Meng , J. Xu, J. Wu, Y. Xu, X. Zhang, Fabrication of polyaniline sensitized grey-TiO2 nanocomposites and enhanced photocatalytic activity, Sep. Purif. Technol. 184 (2017) 248-256 [26] K. Dutta, S. Das , P. Kundu, Highly methanol resistant and selective ternary blend membrane composed of sulfonated PVdF-co-HFP, sulfonated polyaniline and nafion, J. Appl. Polym. Sci. 133 (2016) 43294 (1-10). 27

ACCEPTED MANUSCRIPT [27] H. Lee, S. Yang, Synthesis and Characterization of Polyaniline/Silica Doped with Camphorsulfonic Acid and Dodecylbenzylsulfonic Acid, J. Appl. Polym. Sci. 116 (2010) 934-945. [28] Khoiruddin. D. Ariono, Subagjo, I. Wenten, Surface modification of ion-exchange membranes: Methods, characteristics and performance, J. Appl. Polym. Sci. 48 ( 2017) 45540.

RI PT

[29] F. Amadoa, L. Rodrigues Jr., M. Rodrigues, A. Bernardes, J. Ferreirab, C. Ferreiraa, Development of polyurethane/polyaniline membranes for zinc recovery through electrodialysis, Desalination 186 (2005) 19-206.

[30] H. Farrokhzad, M. Moghbeli, T. Van Gerven, B. Van der Bruggen, Surface modification of

SC

composite ion exchange membranes by polyaniline, React. Funct. Polym. 86 (2015) 161-167. [31] H. Farrokhzad, S. Darvishmanesh, G. Genduso, T. Van Gerven, B. Van der Bruggen,

M AN U

Development of bivalent cation selective ion exchange membranes by varying molecular weight of polyaniline, Electrochim. Acta 158 (2015) 64-72.

[32] C. Li, L. Wang, X. Wang, M. Kong, Q. Zhang, G. Li, Synthesis of PVDF-g-PSSA proton exchange membrane by ozone-induced graft copolymerization and its application in microbial fuel cells, J. Membr. Sci. 527 (2017) 35-42.

TE D

[33] J. Li, M. Zhou, J. Lin,W. Ye,Y. Xu, J. Shen, C. Gao, B. Van der Bruggen, Mono-valent cation selective membranes for electrodialysis by introducing polyquaternium-7 in a commercial cation exchange membrane, J. Membr. Sci. 486 (2015) 89-96.

EP

[34] H. Han, H. Li, M. Liu, L. Xu, J. Xu, S. Wang, H. Ni, Z. Wang, Effect of “bridge” on the performance of organic-inorganic crosslinked hybrid proton exchange membranes via KH550, J. Power Sources 340 (2017) 126-138.

AC C

[35] R. Chen, G. Li, S. Yang, M. Xiong, J. Jin, Sulfonated poly(arylene ether sulfone) polymers containing 3,4-difluoro-phenyl moiety as proton exchange membranes, Solid State Ionics 300 (2017) 157-164.

[36] G. Hurwitz, G. Guillen, E. Hoek, Probing polyamide membrane surface charge, zeta potential, wettability and hydrophilicity with contact angle measurements, J. Membr. Sci. 349 (2010) 349-357 [37] N. Kwak , J. Koo, T. Hwang, E.Choi. Synthesis and electrical properties of NaSS-MAA-MMA cation exchange membranes for membrane capacitive deionization (MCDI), Desalination 285 (2012) 138-146. [38] F. Radmanesh, T. Rijnaarts, A. Moheb, M. Sadeghi, W. de Vos, Enhanced selectivity and 28

ACCEPTED MANUSCRIPT performance of heterogeneous cation exchange membranes through addition of sulfonated and protonated Montmorillonite [J]. J. Colloid Interface Sci. 533 (2019) 658-670. [39] M. Zhou, X. Chen, J. Pan, S. Yang, B. Han, L. Xue, J. Shen, C. Gao, B. Van der Bruggen, A novel UV-crosslinked sulphonated polysulfone cation exchange membrane with improved

RI PT

dimensional stability for electrodialysis, Desalination 415 (2017) 29-39. [40] B. Tong, M. Hossain, Z. Yang, C. Cheng, Y. Wang, C. Jiang, T. Xu, Development of heterogeneous cation exchange membranes using functional polymer powders for desalination applications, J Taiwan Inst. Chem. Eng. 67 (2016) 435-442.

SC

[41] M. Kumar, B. Tripathi, V. Shahi, Electro-Membrane Process for In Situ Ion Substitution and Separation of Salicylic Acid from its Sodium Salt, Ind. Eng. Chem. Res. 48 (2009) 923-930.

M AN U

[42] H. Wang, X. Li, X. Zhuang, B. Cheng, W. Wang, W. Kang, L. Shi, H. Li, Modification of Nafion membrane with biofunctional SiO2 nanofiber for proton exchange membrane fuel cells, J.

AC C

EP

TE D

Power Sources 340 (2017) 201-209.

29

AC C

EP

TE D

M AN U

SC

RI PT

Highlights · A simple and economical preparation method of CEM with ideal properties was provided ACCEPTED MANUSCRIPT · The ED properties of CEM with proper PANi were better than commercial CEM · Polyaniline and sulfonate group play better synergistic effect on membrane property