Highly permselective tadpole-type ionic anion exchange membranes for electrodialysis desalination

Journal Pre-proof Highly permselective tadpole-type ionic anion exchange membranes for electrodialysis desalination Biaowen Wei, Jun Feng, Caidi Chen, Shixi Zhong, Shijun Liao, Yigang Yu, Xiuhua Li PII:

S0376-7388(19)32669-9

DOI:

https://doi.org/10.1016/j.memsci.2020.117861

Reference:

MEMSCI 117861

To appear in:

Journal of Membrane Science

Received Date: 27 August 2019 Revised Date:

1 December 2019

Accepted Date: 17 January 2020

Please cite this article as: B. Wei, J. Feng, C. Chen, S. Zhong, S. Liao, Y. Yu, X. Li, Highly permselective tadpole-type ionic anion exchange membranes for electrodialysis desalination, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/j.memsci.2020.117861. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier B.V.

Author Contribution Statement

Biaowen Wei: Conceptualization, Methodology, Data curation, Writing- Original draft preparation, Writing- Reviewing and Editing, Formal analysis, Investigation, Resources. Jun Feng: Data curation, Writing- Original draft preparation, Formal analysis. Caidi Chen: Data curation, Validation, Investigation. Shixi Zhong: Data curation, Validation, Investigation. Shijun Liao: Supervision. Yigang Yu: Supervision, Funding acquisition, Project administration. Xiuhua Li: Supervision, Writing- Reviewing and Editing, Funding acquisition, Project administration.

1

Graphical Abstract

1

Highly permselective tadpole-type ionic anion exchange

2

membranes for electrodialysis desalination

3

Biaowen Wei a,b, Jun Feng a,b, Caidi Chen a,b, Shixi Zhong a,b, Shijun Liao a,b,

4

Yigang Yu c*, Xiuhua Li a,b,*

5

a

6

Guangzhou 510641, P.R. China

7

b

8

University of Technology, Guangzhou 510641, P.R. China

9

c

10

School of Chemistry & Chemical Engineering, South China University of Technology,

The Key Laboratory of Fuel Cell Technology of Guangdong Province, South China

School of Food Science & Engineering, South China University of Technology,

Guangzhou 510641, P.R. China

11

12

13

14

15

16

* Corresponding author:

17

E-mail address: [email protected] (Yigang Yu)

18

[email protected] (Xiuhua Li)

19

1

20

Abstract

21

A series of tadpole-type ionic ionomers PPO-DMODAs with long alkyl side

22

chains designed for electrodialysis (ED) desalination were synthesized successfully

23

from

24

(2,6-dimethyl-1,4-phenylene oxide) (BPPO). The series of AEMs display stably low

25

water uptakes ranging from 6.2% to 15.1% and high dimensional stabilities with

26

swelling rations of 1.9% to 4.8% at temperatures of 25 oC to 60 oC. The nano-phase

27

separated tadpole-type ionic structures ensure that the PPO-DMODAs membranes

28

show excellent permselectivities with values above 94.6% and outstanding ED

29

performances in the mostly reported desalination system of NaCl aqueous solutions.

30

Especially, PPO-DMODA-3 surpasses commercial membrane TWEDA1 in ED

31

competitions and ED-stability matches with current efficiency (η) of 88.7%, salt flux

32

(J) of 80.61 mg m-2 s-1, energy consumption (EC) of per kg of NaCl of 2.56 kWh kg-1

33

and η retentions of the repeated runs higher than 97% of that of the origin run.

34

Moreover, PPO-DMODA-3 membrane obtained from the ethanol solution exhibits

35

stable aggregate structure and ED performances comparable to those of the

36

PPO-DMODA-3 membranes cast with NMP and TCE solutions. Good solubility of

37

PPO-DMODAs in ethanol offers an environmentally friendly green method to

38

fabricate ED AEMs. In consideration of the advantages of low cost, simple synthesis

39

process and green membrane casting technology, PPO-DMODA-3 membrane has a

40

strong potential for ED-desalination application.

41

Keywords: tadpole-type ionic anion exchange membrane; high permselectivity;

42

electrodialysis; desalination; low energy consumption

dimethyloctadecylamine

(DMODA)

43 2

and

brominated

poly

44

1.

Introduction

45

Compared with the other separation technologies, electrodialysis (ED) has

46

appeared to be the prime choice for producing clean water or recycling ions because

47

of the inherent advantages of low energy consumption, low cost, high-efficiency

48

separation, environmental friendliness as well as easy operation, and has attracted

49

world-wide attentions[1, 2]. Generally, ED process is selective for the removal of

50

ionic species from one compartment (the diluted compartment) to another

51

compartment (the concentrated compartment) through ion exchange membranes

52

(IEMs) using potential gradient as a driving force, which has been proven to be a

53

robust, efficient and versatile method for such applications[3-7]. Apart from

54

satisfactory mechanical, chemical and dimensional stabilities, conductivities and

55

permselectivities of the cation and anion exchange membranes (CEMs and AEMs,

56

respectively) for ED devices are also key parameters to control the ionic currents

57

efficiencies across the membranes along with counter-ions transportation and co-ions

58

exclusion, and energy consumptions of the certain ED processes. Many reported

59

works have focused on the fabrications of IEMs with high ion conductivities to

60

achieve improved ED performances[8-10]. For commercially available IEMs, CEMs

61

with high performances can be prepared in a reliable way, and AEMs exhibit

62

relatively lower permselectivities (≤93%) compared with that of CEMs (≥97%)[11].

63

Therefore, it is highly desirable to prepare an AEM with both high anion conductivity

64

and excellent permselectivity for ED processes.

3

65

In order to find suitable AEMs with high selectivity and conductivity, many

66

different kinds of material structures including both ionomer backbones and the

67

attached ionic groups for AEMs have been developed. The published ionomers

68

backbones include polystyrene[12], polyepichlorohydrin[13], polysulfone[14, 15],

69

brominated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO)[16, 17] poly(arylene

70

ethers)[18]. The reported inherent cations in ED AEMs contain quaternary

71

ammonium (QA)[19], imidazolium[20, 21], and pyridinium[22]. In the last few years,

72

the researchers have shifted to the diverse approaches like chemical crosslinking[23],

73

inorganic nanoparticles doping[24, 25], organic polymers blending[25, 26] to modify

74

the physical and electrochemical properties of the designed AEMs. However, these

75

methods still have some weaknesses such as complicated preparations, high costs and

76

limited improvements. Recently, Xu and co-workers changed the basicity of the

77

quaternary ammonium groups to improve the anion permeabilities, which included

78

using dimethylethanolamine (DMEA) and N-methylmorpholine (NMM) with higher

79

basicity (pkb = 4.77 and pkb = 6.62 respectively) as positive charge centers to offer

80

AEMs higher permselectivities and lower membrane resistances at low ion exchange

81

capacities (IECs). Both groups had the ED candidates with higher desalting

82

efficiencies than that of a commercial membrane Neosepta AMX at the same ED

83

conditions[27,

84

poly(2,6-dimethyl-1,4-phenylene oxide) as the backbone to give the best candidate

85

AEM-3 with excellent permselectivity of 98% and good conductivity of 7.67 × 10−2 S

86

cm−1, which gave current efficiency (η) of 83.4% and salt flux (J) of 36.5 mg m-2 s-1 at

28].

Shahi’s

group

4

used

chloromethylated

87

energy consumption (EC) value of per kg of NaCl of 6.53 kWh kg-1 [29]. Szekelys’

88

work reported a series of polybenzimidazolium-based AEMs doping with graphene

89

oxide and the highest permselectivity reached 98% in the case of M2-1, which offered

90

current efficiency of 100% and energy consumption of 10.1 kWh kg-1 [30]. Until now,

91

there is still lack of an AEM to give high comprehensive ED performances including

92

high η and J, and low EC. It is crucial for scientific research workers to find an AEM

93

with both high anion conductivity and permselectivity.

94

Constructing block copolymer architectures[3, 31], appending comb-shaped ionic

95

groups with long and flexible side chains of the same sizes [32, 33] , introducing ion

96

clusters[34, 35] and series-connected multi-cation side chains[36, 37] have been

97

published as effective methods to achieve the nano-phase separated nano-channels in

98

the tailor-made alkaline AEMs, which can transport hydroxyl anions quickly.

99

However, none of the nano-phase separated AEMs has extended to ED desalination

100

applications owing to the difficult synthesis processes or expensive costs. Presently,

101

tadpole-type (denoted as the special comb-shaped ionic groups with long hydrophobic

102

tails of the same sizes, which append to the backbones by a methylene group[33])

103

alkaline AEMs PPO–DMHDA-x derived with N, N-dimethyl-1-hexadecylamine

104

(DMHDA) for fuel cell to afford enhanced OH- conductivity has been published[38].

105

To the best of our knowledge, tadpole-type AEMs have not yet applied in ED

106

processes. In addition, our previous work has disclosed the influences of the positions

107

of quaternary ammonium functional groups in polyether sulfone backbones on the ED 5

108

performances and cleared that the main-chain-ionic AEMs have much better ED

109

performances than that of the side-chain-ionic ED AEMs[39]. The tadpole-type

110

AEMs are a kind of main-chain-ionic AEMs with long hydrophobic alkyl side chains

111

in nature, which afford them nano-phase separated nano-channels. Compared with the

112

newly reported ED AEMs[9, 21, 22], the challenge of the tadpole-type AEMs lies on

113

the relatively higher water uptake (WU) (WU = 20.4% ) of the best OH- conductor

114

PPO–DMHDA-35. Lengthening the alkyl side chains potentially enhances the

115

hydrophobicity and nano-phase separations of the type of AEMs to afford higher

116

permselectivity and conductivity. Herein the low cost commercial starting materials,

117

poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) and N, N-dimethyloctadecylamine

118

(DMODA) were used to fabricate the tadpole-type AEMs in Cl- form to offer

119

enhanced ED performances. The tadpole-type AEMs with different IECs were

120

characterized in terms of structure, thermal stability, mechanical properties,

121

morphology, IEC, WU, swelling ratio (SR), membrane area resistance (Rm),

122

permselectivity (P) and limiting current density (LCD). Moreover, their applications

123

in ED removal of NaCl from aqueous solutions have also been investigated and

124

compared with that of a commercial membrane TWEDA1.

125

2.

126

127 128

Experimental 2.1. Materials

Poly

(2,6-dimethyl-1,4-phenylene

oxide)

(PPO)

was

purchased

from

Sigma-Aldrich Chemicals. N, N-dimethyloctadecylamine (DMODA) was purchased 6

129

from TCI, Shanghai, China. Petroleum ether with a boiling-range of 30-60 oC,

130

azodiisobutyronitrile (AIBN) and N-bromosuccinimide (NBS) were purchased from

131

Aladdin Reagent, Shanghai, China. Sodium chloride, 1-methyl-2-pyrrolidone (NMP),

132

chlorobenzene and other chemical reagents were obtained from commercial sources

133

and used without further treatments. Ti mesh 1.0 electrodes of 4.5 cm × 4.5 cm coated

134

with Ru were purchased from Shaanxi Elade New Material Technology Co. Ltd.,

135

Xi’an, China. The reference electrodes Ag/AgCl were bought from Shanghai INESA

136

Scientific Instrument Co. Ltd., China.

137

2.2. Preparation of brominated poly (2,6-dimethyl-1,4-phenylene oxide) (BPPO)

138

BPPO was prepared according to the published literature[38]. PPO (9 g, 150

139

mmol –CH3) dissolved in 100 mL of chlorobenzene with magnetic stirring, then NBS

140

(9.34 g, 52.5 mmol) and AIBN (0.57 g, 3.5 mmol) were added and stirred at reflux

141

conditions (135 oC) for 3 h. After cooling down to room temperature, the reaction

142

mixture was poured into 1000 mL of methanol to give the crude polymer. The

143

polymer was filtered and washed with methanol to remove residual chlorobenzene.

144

The resulting crude product subsequently dissolved in 50 mL of chloroform, and

145

reverse precipitated into 200 mL acetone to give the pure product as light yellow

146

powder. The product was collected and dried under vacuum for 24 h to offer BPPO

147

with mono-bromination ratio of the repeat unit of 57% and a yield of 92%.

148

2.3. Synthesis of tadpole-type ionic PPO-DMODAs in bromine ion form

149

DMODA (0.6258 g, 2.1 mmol) and BPPO with mono-bromination ratio of the

150

repeat unit of 57% (1 g, 3.5 mmol CH2Br) were dissolved in 22 mL of 7

151

1,1,2,2-tetrachloroethane (TCE) to form a transparent solution. The solution was

152

stirred for 48 h at room temperature and the viscosity of the solution greatly increased

153

with the prolonging reaction time. The viscous mixture was poured into 150 mL of

154

petroleum ether to precipitate the crude product. The resulting polymer was filtered

155

and washed with petroleum ether. After collecting and drying under vacuum for 24 h,

156

gave a tadpole-type ionomer PPO-DMODA-1 with a good yield of 98%. Changing

157

DMODA feedings to 2.7 mmol and 3.5 mmol respectively at the same feeding of

158

BPPO to run the Menshutkin reactions and working up the reaction mixtures similar

159

to the procedure of PPO-DMODA-1 gave the other two ionomers, PPO-DMODA-2

160

and PPO-DMODA-3, with yields of 96% and 95% individually.

161

2.4. Membrane casting and ion exchange

162

The PPO-DMODAs polymers in bromide form dissolved in a mono solvent of

163

TCE, NMP and ethanol to form 8 wt% casting solutions. The solutions were cast on

164

flat glass plates for 48 h at room temperature and for 24 h at 80 oC to let the solvents

165

volatilize completely. Then the transparent membranes were peeled off and immersed

166

in 0.5 M NaCl at room temperature for 48 h to give the AEMs in chloride form. After

167

washing with deionized water several times, the PPO-DMODAs membranes in

168

chloride form were stored in deionized water before analysis.

169

2.5. Characterizations and measurements

170

2.5.1. 1H NMR and FTIR

8

171

1

H NMR spectra were recorded on a Bruker Avance 400S using CDCl3 and

172

tetramethylsilane (TMS) as the solvent and the standard respectively. The degrees of

173

bromomethylation (DBMs) are the bromomethyl group numbers per repeating unit in

174

the polymers can be obtained by the 1H NMR spectra, which were calculated by the

175

equation (1).

176

DBM =

177

where k is the number of methyl groups in repeating unit of PPO. Ha and Ha’ are

178

specified as the peaks integral areas of the protons in methyl and bromomethyl groups.

179

Fourier transform infrared (FTIR) spectra were recorded on a Bruker Tensor 27

180

instrument with a scanning range of 3700–600 cm-1.

3kH a′ 3 H a′ + 2 H a

(1)

181

2.5.2. TGA and mechanical properties

182

Thermogravimetric analyses (TGA) ran under N2 atmosphere on a TAINC SDT

183

Q600 thermogravimetric analyzer with a temperature range of 35 to 650 °C at a

184

heating rate of 10 °C per minute. The samples for TGA determination were dried at

185

80 °C for 24 h before the tests. The mechanical properties of the membranes were

186

measured on an Instron M3300 at a test speed of 5 mm min-1 at room temperature and

187

100% RH. The samples sizes were fixed at 40 mm in length and 5 mm in width.

188 189

2.5.3. Scanning electron microscopy (SEM), small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM)

190

The SEM cross-section images were taken on a Hitachi S-3700N with an

191

accelerating voltage of 15 kV. SAXS spectra of the wet PPO-DMODAs membranes 9

192

were recorded on an Anton Paar SAXSess instrument at room temperature with Cu Kα

193

radiation, which had wavelength of 0.154 nm. The effective scattering vectors (q)

194

were calculated by Bragg's Law of q = 4 sin (2 )⁄

195

angle, n = 1,

196

microscope (JEOL, Japan) using an accelerating voltage of 200 kV. The ionomers in

197

WO4- form were dissolved in TCE, NMP and ethanol (1 mg mL−1), dropped on

198

copper grids, and dried at 80 °C for 12 h under vacuum to give the testing samples.

, where 2

is the scattering

is the incident wavelength. TEM images were gotten on a JEM-2100

199

2.5.4. IEC, WU, SR and hydration number (λ)

200

The IECs of the PPO-DMODAs membranes were measured by Mohr method.

201

Membrane samples in Cl - form with the sizes of 3 cm in width and 3 cm in length

202

were dried at 60 oC in a vacuum oven for 24 h and weighed to give the masses of the

203

dry membranes. Then the samples were immersed in 50 mL of 1 M NaNO3 aqueous

204

solutions at room temperature for 48 h to convert the origin counter ions Cl- into NO3-

205

completely. A standardized AgNO3 solution with K2CrO4 as indicator was used to

206

titrate the Cl- ions in the solutions. The IEC (mmol g-1) values were calculated by the

207

equation (2).

208

IEC =

VAgNO3 × CAgNO3

(2)

Wdry

209

where CAgNO3 (M) and VAgNO3 (mL) are the concentration and consumed volume of the

210

titrating AgNO3 solution, Wdry (g) is the mass of a dried sample of PPO-DMODAs

211

membranes in Cl- form.

212

The WUs, SRs and λ (denoted as the number of the bonded water per ammonium

213

group) of the PPO-DMODAs membranes were determined simultaneously by the 10

214

following steps. After measuring the lengths and weights of the samples films (0.5 × 5

215

cm2) in dry form (Ldry and Wdry, respectively), the samples were immersed in

216

deionized water at 25 °C or 60 °C for 24 h to ensure complete water uptake. The

217

water on the surfaces of the wet samples was wiped off promptly using tissue paper.

218

The lengths and weights of the wet samples (Lwet and Wwet, respectively) were

219

determined quickly and recorded. The WUs, SRs and λ of the membranes were

220

calculated using equation (3), (4) and (5).

221

WU =

222

SR =

Wwet − Wdry Wdry Lwet − Ldry Ldry

×100%

×100%

WU ×1000 IEC × M H 2O

(3)

(4)

223

λ=

224

where MH2O is the molecular weight of water.

(5)

225

2.5.5. Rm and permselectivity

226

The Rm values of the PPO-DMODAs membranes were determined by the

227

reported laboratory methods in the literatures[40, 41]. The tests ran in the testing cell

228

consisted of four compartments (Fig. S1). Two electrode chambers at both ends of the

229

cell were filled with 0.25 M Na2SO4 solution as the electrode solution. Another two

230

chambers were filled with 0.5 M NaCl as the intermediate solution. The Nafion-115®

231

(DuPont) membranes were placed next to the Ru coated titanium electrodes and

232

separated the NaCl solution from the rinsing electrode solution. The cell was

233

connected to an IviumStat frequency response analyzer via the Ru coated titanium 11

234

electrodes and the reference electrodes (Ag/AgCl). The test was carried out at a

235

current density of 5 mA cm-2. The voltages between the two testing chambers with

236

and without the testing membrane were measured by Ag/AgCl electrodes for the

237

applied current densities. Rm values were calculated using equation (6).

238

Rm = Rcell − Rsol

239

where Rsol is the electrolyte resistance in the testing chambers without the testing

240

membrane, Rcell is the resistance of the testing chambers with the testing membrane.

(6)

241

The permselectivities of the PPO-DMODAs membranes were acquired by

242

measuring electrochemical potential difference between a dilute solution and a

243

concentrated solution. A membrane sample with the sizes of 5 cm × 5 cm was

244

sandwiched between a lab-made testing cell with two reservoirs. One filled with 0.5

245

M NaCl aqueous solution as a concentrated solution and another filled with 0.1 M

246

NaCl aqueous solution as a dilute solution. The concentration polarization was

247

eliminated by continuous stirring. Two Ag/AgCl reference electrodes were set on both

248

sides of the membrane. The electrochemical potential difference (Emeasured) was

249

obtained by using a multimeter to measure the reference electrodes at room

250

temperature. The permselectivity (P) was calculated using the equation (7).

251

P=

252

where Etheoretical is the calculated by the Nernst equation (8).

253

Etheoretical =

Emeasured ×100% Etheoretical

RT γ conCcon ln nF γ dil Cdil

(7)

(8)

12

254

where n is the valence of the ion species, R and F represent universal gas constant

255

(8.31 J mol−1 K−1) and Faraday constant (96485 C mol-1), T is temperature (K), γcon

256

(0.683) and γdil (0.789) are the activity coefficients of the concentrated solution and

257

dilute solution, and Ccon and Cdil are the concentrations of the concentrated solution

258

and dilute solution.

259

2.5.6. Current-voltage (I-U) characteristic

260

The limiting current densities (LCDs) of the PPO-DMODAs membranes were

261

obtained by I-U characteristic curves recorded on an IviumStat frequency response

262

analyzer in the galvanostatic mode, which was connected to the same device for Rm

263

determinations and ran at room temperature. Before the measurements, the electrode

264

chambers and middle chambers were filled with the electrode solution of 0.3 M

265

Na2SO4 and the intermediate solution of 0.1 M NaCl respectively. Experiments were

266

conducted in a recirculation mode, in which each stream was circulated at 10 mL

267

min-1 flow rate. The I-U characteristic curves were recorded by stepwise current

268

method with a current density range of 0 to 100 mA cm-2 at a rate of 0.1 mA cm-2 s-1.

269

2.5.7. Electrodialysis tests

270

Electrodialysis tests ran in a home-made ED device (Fig. S2). The device was

271

separated into four compartments by two Nafion-115® CEMs and a testing AEM,

272

including a concentrate chamber, a dilute chamber and two electrode chambers at the

273

end of the stack. The concentrate chamber and dilute chamber were pumped

274

cyclically with NaCl aqueous solutions (0.1 M, 338 mL) at a flow rate of 10 mL min-1

275

separately. While two electrode chambers were pumped cyclically with Na2SO4 13

276

aqueous solution (0.3 M, 250 mL) and were linked to keep constant pH. The effective

277

membrane area was 20.25 cm2 and the inter membrane distances were 3 mm. The

278

device was connected to the electrochemical work station (IviumStat) via the Ru

279

coated

280

electrochemical work station offered a constant current of 15 mA cm-2 and recorded

281

the potentials over the stack every 0.2 s during the tests. The conductivities of the

282

NaCl aqueous solution in dilute chamber were recorded every 30 min by a

283

conductivity meter (Lei-ci DDS-307).

titanium

electrodes

and

the

reference

electrodes

(Ag/AgCl).

The

284

The ED performances of the PPO-DMODAs membranes were compared with

285

that of the commercial membrane TWEDA1 in terms of current efficiency (η), energy

286

consumption (EC) and salt flux (J). η, EC and J values were calculated using the

287

equations (9-11).

288

η=

289

where Z is the absolute valence of the Cl-, Ct and C0 are the concentrations of NaCl

290

solution in dilute chamber at time t and 0 during the processes of desalination, F and

291

Vt represent Faraday constant (96485 C mol-1) and the volume of NaCl solution in

292

dilute chamber. N is the number of repeated units (N = 1), I is the actual current using

293

in measurement (I = 303.75 mA).

294

EC = ∫

Z (C0 - Ct )Vt F ×100% NIt

t

0

(9)

UI dt (C0V0 - CtVt ) M NaCl

(10)

14

295

where U is the real-time voltage of the ED stack, MNaCl is the molecular weight of

296

NaCl (MNaCl = 58.44 g mol-1), V0 and Vt are the volumes of the recycling NaCl

297

solution in the dilute chamber at time 0 and t.

298

J=

299

where A is the effective membrane area (20.25 cm2).

C0V0 - CtVt At

(11)

300 301

Scheme 1. Synthesis of PPO-DMODAs. 15

302

303

3.

Results and discussion 3.1. Synthesis and characterizations of the tadpole-type ionic PPO-DMODAs

304 305

Fig. 1. 1H NMR spectra of BPPO and PPO-DMODA-3: (a) BPPO and (b)

306

PPO-DMODA-3.

307 308

Fig. 2. FTIR spectra of PPO, BPPO and PPO-DMODAs membranes: (a) PPO, (b)

309

BPPO, (c) PPO-DMODA-1, (d) PPO-DMODA-2 and (e) PPO-DMODA-3.

16

310

The synthetic route of the tadpole-type ionic PPO-DMODAs with long flexible

311

hydrophobic tails is shown in Scheme 1. PPO-DMODAs membranes with various ion

312

exchange capacities were fabricated by reacting 2.1 mmol, 2.7 mmol and 3.5 mmol of

313

DMODAs with 1 g BPPO. 1H NMR results of BPPO and PPO-DMODAs are shown

314

in Fig. 1. As shown in Fig. 1(a), the methylene (4) and methyl (5 and 6) protons of

315

BPPO resonate at 4.3 ppm and 2.1 ppm. The DBM of BPPO is about 0.57, which was

316

calculated using integral areas of the corresponding methylene protons peak (4) and

317

methyl protons peaks (5 and 6). This confirms that the synthesized BPPO has enough

318

active sites for the subsequent quaternizations. Fig. 1(b) shows four new peaks with

319

an integral area ratio of 3:30:2:8 at the range of 0.5-4 ppm, which are assigned to

320

methyl (7’) and methylene (8’) protons connecting with N+ centers, methylene (9’)

321

protons, continuous 15 methylene groups protons (10’) and the end methyl protons

322

(11’) according to the decreasing electron withdraw effects along the alkyl chains

323

resulting from DMODA molecules. The bromo benzyl peak at 4.3 ppm (Fig. 1(a))

324

disappears and new signal of benzyl protons connecting with N+ groups occurs at 4.75

325

ppm resulted from the synergistic action of the strong electron withdrawing effect of

326

the quaternary ammonium groups and weak electron withdrawing effect of benzene

327

ring. These results combining the following agreements of the theoretical IECs (IECt)

328

and experimental IECs (IECm) confirm that the quaternizations have run successfully.

329

To clarify the structure changes in the quaternizations, Fig. 2 displays the FTIR

330

spectra of mother polymers PPO and BPPO, and tadpole-type ionomers

331

PPO-DMODAs. The stretching vibrations of C-H bonds of methyl moieties with

332

varying chemical circumstances of PPO appear as multi-peak signals at 2922 to 2856

333

cm-1. The skeletal stretching vibrations of C-C bonds in the aromatic rings of PPO 17

334

occur at 1605 cm-1and 1470 cm-1. The out-of-plane bending vibrations of isolated C-H

335

bonds in the tetra-substituted aromatic moieties benzene rings appear at 856 cm-1,

336

which is overlapping with the characteristic doublet signal of rocking of methyl

337

groups at 831 cm-1 and 856 cm-1. The asymmetrical bending vibrations of methyl

338

groups occur at 1429 cm-1 as shoulder of 1470 cm-1 peak. Their symmetrical bending

339

vibrations arouse a tiny peak at 1379 cm-1. The out-of-plane scissoring or bending of

340

the methyl groups is observed at 1306 cm-1. The signal at 1188 cm-1 is assigned to the

341

asymmetrical stretching vibrations of C-O-C bonds. The symmetrical stretching

342

vibrations of C-O-C bonds offer two separated peaks at 1020 cm-1 and 958 cm-1

343

respectively owing to the asymmetrical connections of C-O-C bonds (Fig. 2(a)). The

344

obvious changes brought by the bromination are caused by the averagely introduced

345

0.57 benzyl bromide groups in every two repeat unit of original PPO. The

346

characteristic band at 590 cm-1 belonging to the stretching vibrations of C-Br bonds is

347

observed in the FTIR spectrum of BPPO. The strong wagging vibrations of C-H

348

bonds in –CH2Br groups appear at 1223 cm-1 as shoulder of the absorption of the

349

symmetrical stretching vibrations of C-O-C at 1188 cm-1. The strong electron

350

withdrawing effect of C-Br bonds splits the signals of the symmetrical stretching

351

vibrations of C-O-C bonds into three peaks from 958 cm-1 to 1020 cm-1. The other

352

absorption signals resulted from unchanged units of PPO remain at the similar

353

wavenumbers (Fig. 2(b)). The successfully quaternizations give the tadpole-type ionic

354

PPO-DMODAs with long hydrophobic tails containing continuous (CH2)17 moieties,

355

which arouse the characteristic absorptions of the rocking vibrations of C-H at 721

356

cm-1 in Fig. 2(c-e). The intensities of the signals increase significantly with the

357

increasing DMODA additions because of the successful quaternizations. The signals

358

around 2922 cm-1 to 2856 cm-1 resulted from the stretching vibrations of C-H show the 18

359

similar tendency owing to the same reason. Moreover, the broad peaks at 3420 cm-1

360

are aroused by the stretching of O-H bonds of the bonding H2O molecules of the N+

361

groups in the resulting tadpole-type ionomers. Besides, the signals at 986 cm-1

362

resulted from the spiting of the symmetrical stretching vibrations of C-O-C bonds

363

caused by the introduction of –CH2Br groups decrease with the stepwise additions of

364

DMODA, which change the –CH2Br groups into the -CH2N+- groups with much

365

stronger polarity. The signals at 958 cm-1 enhance with the stepwise additions of

366

DMODA owing to the same reason (Fig. 2(c-e)).

367

3.2. Thermal stability and mechanical properties

368 369

Fig. 3. TGA and DTG curves of BPPO and PPO-DMODA-3: (a) BPPO, (b)

370

PPO-DMODA-3.

371

The thermal stability tests of BPPO and PPO-DMODAs were evaluated by TGA

372

taking nitrogen as shielding gas. The results were shown in Fig. 3 and Fig. S3. The

373

weight loss process of BPPO separates into two stages over the temperature ranging

374

from 35 oC to 650 oC. In the first stage, weight loss of 24.2% between 214.0 oC and

375

366.4 oC is contributed by the leaving of –Br groups, which is close to the theoretical

376

content of Br in BPPO 27.6%. Weight loss of 32.2% in the second weight loss stage 19

377

with decomposing temperatures higher than 366.4 oC is ascribed to the decomposition

378

of the polymer backbone and leaving of the aliphatic moieties (Fig. 3(a)). Because of

379

the strong hydrophilicity of quaternary ammonium groups in PPO-DMODA-3, there

380

appears another weight loss stage with weight loss of 3.9% below 145.9 oC resulted

381

from water loss (Fig. 3(b)). Moreover, the leaving of long alkyl chains modified

382

tadpole-type ionic groups from the backbone occurs at 145.9 oC to 374.4 oC offering a

383

multi-peak signal in DTG curve owing to their complex bond-breaking positions and

384

producing a weight loss of 54.6% in TG curve (Fig. 3(b)). The last stage starting from

385

374.4 oC is the decomposition of the polymer backbone of PPO-DMODA-3 giving a

386

weight loss of 30.3% (Fig. 3(b)), which is close to the weight loss of the second stage

387

of BPPO (Fig. 3(a)) because of their derivative relationship. The TGA curves of

388

PPO-DMODAs with different IECs are displayed in Fig. S3. The remaining weights

389

of the PPO-DMODAs membranes at the end of the second step decrease with the

390

increased IECs because of the enhancing leaving amounts of long alkyl chains

391

modified tadpole-type ionic groups. These TGA results ensure that the

392

PPO-DMODAs meet the thermal stable requirements for electrodialysis desalination

393

processes, which are usually run at the temperature lower than 100 oC.

394 395

Fig. 4. Ts and Eb values of PPO-DMODAs membranes. 20

396

Tensile strengths (Ts) and elongations at break (Eb) as two parameters to evaluate

397

the mechanical properties of the PPO-DMODAs membranes were measured under

398

wet condition at room temperature and the results have been illustrated in Fig. 4. The

399

PPO-DMODAs membranes possess Ts of 13.8–21.5 MPa and Eb of 5.7–13.3%, which

400

are comparable to that of the IEMs[16] using in ED desalination. All the

401

PPO-DMODAs membranes have survived several times of ED tests runs. The fact

402

strongly supports that the mechanical properties of the prepared membranes satisfy

403

the requirements of ED desalination processes.

404

3.3. Morphology of PPO-DMODAs membranes

405 406

Fig. 5. SAXS profiles of PPO-DMODAs membranes.

407

SAXS profiles of PPO-DMODAs membranes (Fig. 5) clear that the series of

408

tadpole-type ionomers offer obvious nano-phase separation aggregate structures at the

409

investigating IECs range (Fig. 7(a)). It is evident that the peak positions (qmax) of the

410

phase separation signals of the PPO-DMODAs membranes fall into a very narrow

411

range of 1.88-2.00 nm-1 with Bragg spacings (d values (d = 2 π/qmax)) in the range of

412

3.34-3.14 nm, which approximately fit the length of the alkyl side chains[38]. These 21

413

aggregate structures confine the anion transport channels diameters smaller than 3.34

414

nm, which ensure strong sieve effect and Donnan effect to offer high permselectivity.

415

The intensities of the signals enhance with the increasing feedings of DMODA, which

416

are bound to bring forth more tadpole-type ionic groups leading to stronger

417

fluctuations in electron cloud density between the hydrophilic and hydrophobic

418

microphase regions.

419 420

Fig. 6. The optical photograph, SEM cross-section images and TEM images of

421

PPO-DMODAs membranes: (a) PPO-DMODA-3 (optical), (b) PPO-DMODA-1

422

(SEM,

423

PPO-DMODA-3 (SEM, solvent: TCE), (e) PPO-DMODA-3 (SEM, solvent: NNP), (f)

424

PPO-DMODA-3 (SEM, solvent: Ethanol), (g) PPO-DMODA-3 (TEM, solvent: TCE),

425

(h) PPO-DMODA-3 (TEM, solvent: NNP), (i) PPO-DMODA-3 (TEM, solvent:

426

Ethanol).

solvent:

TCE),

(c)

PPO-DMODA-2

22

(SEM,

solvent:

TCE),

(d)

427

The optical photograph, SEM cross-section images and TEM images of

428

PPO-DMODAs membranes are shown in Fig. 6 to reveal the surface and aggregation

429

morphologies of PPO-DMODAs membranes. The optical photograph clears that the

430

tadpole-type ionic membranes is transparent, pale brown, glossy and has neat

431

appearance without any bubbles, cracks and macro phase separations (Fig. 6(a)). The

432

SEM cross-section images at magnification of 10000 ensure that the PPO-DMODAs

433

membranes cast from various solutions have homogeneous and dense nature without

434

any holes (Fig. 6(b-f)). Excitingly, the aggregation morphologies of PPO-DMODAs

435

membranes are not affected by the casting solvents. All the casting solutions of TCE,

436

NNP and ethanol have the ability to offer homogeneous and dense membranes

437

without observable differences at magnifications of 10000 (Fig. 6(d-f)). The TEM

438

images of the ultrathin PPO-DMODA-3 membranes obtained from various solvents

439

convey that the casting solvents have little effects on the aggregation structures of

440

PPO-DMODA-3 and these membranes show similar sizes of the nano-phase

441

separations (Fig. 6(g-i)), which agree well with the results of SAXS (Fig. 5). All of

442

the facts reveal that using the green solvent of ethanol to cast film possibly not affect

443

the electrodialysis performances of the PPO-DMODAs membranes.

444

3.4. IEC, Rm, permselectivity, λ, WU, SR of PPO-DMODAs membranes

445

The IEC values including theoretical IECs (IECt) and experimental IECs (IECm)

446

of the PPO-DMODAs membranes are illustrated in Fig. 7(a). Apparently, the IECm

447

value coincides well with the theoretical one in the case of PPO-DMODA-1, which

448

supports that the quaternizations of BPPO and DMODA are nearly quantitative at the 23

449

low feedings of DMODA. The consistency of IECm values with IECt values decrease

450

with the increasing feedings of DMODA. The possible reason is that grafting

451

DMODA molecules with long alkyl group onto BPPO greatly increases the molecular

452

weights of the resulting PPO-DMODAs, which heavily augment the viscosities of

453

reaction mixtures. The coaction of the increasing space steric hindrance of the fixed

454

long alkyl side chains and the enhanced viscosity prevents DMODA from attacking

455

the isolated benzyl bromide groups of BPPO at the ends of the quaternizations. Even

456

so, the quaternizations of PPO-DMODAs membranes have completed over 88%.

457 458

Fig. 7. IEC (a), P and Rm (b) of PPO-DMODAs membranes.

459

Rm and permselectivity (P) are two crucial parameters to control the energy

460

consumption in ED desalination. For comparison's sake, we ran Rm determinations of

461

the tadpole-type ionomers AEMs with 0.5 M NaCl aqueous solution, which is the

462

testing condition most popularly used by the current commercial and reported ED

463

AEMs. The results are displayed in the Fig. 7(b). The area resistances of the prepared

464

membranes significantly decrease with the improving ion exchange capacities (Fig.

465

7(b)). P is an important indicator conveying the ability of ion exchange membrane to

466

transfer counter ions and reject common ions, which is controlled by the

467

concentration of the fixed charge groups and aggregation structure of the membrane 24

468

in constant external conditions on the basis of reported electric double layer and

469

Donnan equilibrium theory[42]. The higher concentration of the fixed charge groups

470

inside the ion transport channels, the stronger the forming electric fields, which attract

471

counter ions and exclude common ions. This increases the membrane permselectivity.

472

P values of PPO-DMODAs membranes change in the similar tendency of Rm with the

473

increasing IECs (Fig. 7(b)). PPO-DMODA-3 membrane has the lowest P of 94.6%

474

and the lowest Rm value of 2.78 Ω cm2 among the prepared membranes. This is

475

attributed to an increase of quaternary ammonium groups enhancing the water

476

absorption of PPO-DMODA-3 membrane and increasing the continuity of the anion

477

transport channels inside the membrane. The relative deterioration of permselectivity

478

is caused by the lower density of the fixed charge groups inside the ion transport

479

channels of the swollen membrane. To reveal the Rm and P levels of the designed

480

membranes, Rm and P values of PPO-DMODAs, the counterpart commercial

481

TWEDA1, and some reported commercial and designed ED AEMs have been listed

482

in Table 1. All the listed commercial ED AEMs have excellent P above 94% with

483

various applicable Rms. Among the commercial AEMs, TWEDA1 ranks middle in Rm

484

and top in P, the mostly reported Neosepta AMX has the best Rm and second P

485

value[22, 27, 28]. PPO-DMODA-3 possesses comparable Rm and P to that of

486

Neosepta AMX[22, 27, 28], di-pyridinium-crosslinked BPPO-20[22] and the

487

advanced side-chain-imidazolium functioned PAEK-60-im[43]. Compared with

488

newly reported PPO-based ED AEMs (di-imidazolium-crosslinked BPPO-im 0.3[21],

489

MDPP-phosphonium functioned MDPP-43[44], NMM-QA functioned NMM-18[27],

490

TMA-QA functioned aPPO-27[14], TMA-QA functioned QPAES-c[39] and

491

TMA40-PAES[45], imidazolium modified IMD40-PAES[45] and ABCO-QA

492

functioned ABCO40-PAES[45], PPO-DMODA-3 shows competitive Rm and 25

493

improved P. To dig out the relationship between the properties of Rm and P and the

494

ED AEMs structures, we determined λ values of PPO-DMODAs membranes and

495

listed in Table 1. The λ values of the above-mentioned ED AEMs also have been

496

displayed in the table. λ has been denoted as the number of the bonded water per

497

ammonium group of IEMs, which clear the hydrophilicity of the certain ionic groups.

498

The λ data clear that the common characteristic of the advanced homogenous

499

ionomers ED AEMs with high P and low Rm is their low λ values (λ < 5.0). The kind

500

of moderate hydrophobicity can be achieved steadily by the methods of

501

side-chain-imidazolium[43] and tadpole-type ionic groups in this paper. The reported

502

methods of di-cation-crosslink structures have offer different hydrophobicity. The

503

di-pyridinium-crosslinked structure possesses strong hydrophobicity with λ of 2.4 at

504

IEC of 2.13 mmol g-1, which offer BPPO-20 higher concentration of the fixed charge

505

groups inside the ion transport channels to offset the negative effect of the stiff

506

di-cation-crosslink structure and show high P of 94.0%[22]. The offsetting effect

507

decreases in di-imidazolium-crosslinked structure with the relatively higher λ of 3.5 at

508

lower IEC of 1.94 mmol g-1, which offer BPPO-im 0.3 relatively lower P of

509

90.0%[21]. The P values of the listed commercial ED AEMs show weak dependence

510

of λ because that the pore sizes of the supported fabric restrict the aggregation

511

structures of the pore-filling ionomers, where the hydrophilicity of the ionic groups

512

are restricted. While the ionic groups in the free ionomers’ layers absorb water

513

molecules as much as they can. The value of λ is the average number of the bonded

514

water per ammonium group of the testing membranes. Moreover, recasting the

515

cross-link membranes and the fabric supported ones is nearly impossible owing to the

516

insolubilities. Good solubility of tadpole-type ionic PPO-DMODAs membranes in

26

517

TCE, NMP and ethanol significantly enhances the competitiveness as ED AEM

518

candidates.

519

Table 1. λ, Rm and P of PPO-DMODAs, TWEDA1 and some reported AEMs. Membrane PPO-DMODA-1

Backbone and Cation IEC Rm P supporting fabric (mmol g-1) (Ω cm2) (%) PPO, none tadpole-type-QA 1.42±0.05 6.92±0.42 96.8±0.4

λ 2.4

PPO-DMODA-2

PPO, none

tadpole-type-QA 1.53±0.03 3.56±0.31 95.6±0.6

3.2

PPO-DMODA-3

PPO, none

tadpole-type-QA 1.71±0.07 2.78±0.34 94.6±0.6

4.4

a

TWEDA1

b

Neosepta AMX[22, 27, 28] c AEM-Type-Ⅱ [21, 43] BPPO-20[22]

co-(DMC-DMAM A-DVB), Nylon 6,6 PS/DVB, N.A

TMA-QA

1.01±0.03 3.88±0.16 98.6±0.2

8.3

TMA-QA

2.16

2.51

96.0

11.4

N.A, N.A

TMA-QA

1.90

4.97

94.0

5.3

PPO, none

2.13

2.45

94.0

2.4

1.94

2.75

90.0

3.5

1.52

2.9

90.0

9.3

1.93

-

89.6

8.8

BPPO-im 0.3[21]

PPO, none

MDPP-43[44]

PPO, none

aPPO-27[14]

PPO, none

di-pyridinium -crosslinked di-imidazolium -crosslinked MDPP-phosphoni um TMA-QA

NMM-18[27]

PPO, none

NMM-QA

1.71

1.50

92.0

8.9

PAEK-60-im[43]

PAEK, none

2.01

2.83

96.0

4.3

QPAES-c[39]

PAES, none

side-chain-imidazo lium TMA-QA

1.51

1.39

86.6

7.5

TMA40-PAES[45 ] IMD40-PAES[45]

PAES, none

TMA-QA

1.45

1.45

91.6

11.5

PAES, none

imidazolium

1.48

1.65

94.4

4.9

ABCO40-PAES[45

PAES, none

ABCO-QA

1.48

1.59

93.5

7.5

]

520 521 522

a

Tianwei Membrane Technology Co. Ltd., Shandong, China. ASTOM, Japan. c FUJIfilm Corp. Japan b

523

The WU and SR are two key parameters to affect the ionic conductivity,

524

counter-ions permselectivity and mechanical properties of AEMs. A suitable WU is

525

essential for ionic conductivity and an excess of WU usually brings excessive SR and

526

weak mechanical properties. WUs and SRs of the PPO-DMODAs membranes were

527

measured at 25 oC and 60 oC respectively because the ED working temperatures are

528

usually lower than 60 oC. The results convey that the tadpole-type ionic membranes 27

529

with long alkyl side chains have lower WUs and SRs in the investigating IECs

530

compared with the report ED AEMs[17, 27, 29], and the WUs and SRs increase

531

slowly with the rising IECs and temperatures. Even at high temperature of 60 oC and

532

with the highest IEC of 1.71 mmol g-1, the tadpole-type ionic membrane

533

PPO-DMODA-3 shows high dimensional stability with SR of 4.8% (Fig. 8). The very

534

low swelling behaviors and high dimensional stabilities of the PPO-DMODAs

535

membranes are attributed to the enhanced hydrophobicity resulted from the attached

536

long alkyl side chains, which ensure the membranes stable Rm and permselectivities.

537 538 539

Fig. 8. WUs and SRs of PPO-DMODAs membranes. 3.6. Limiting current density (LCD)

540 541

Fig. 9. I-U curves of PPO-DMODAs membranes and TWEDA1. 28

542

LCD of IEMs is regarded as another important factor to affect the ED

543

performances. The published works[42, 46] have declared that applying current density

544

not higher than LCD results in extremely low energy consumption because of no

545

existence of water dissociation, which cause extra consumption. Fig. 9 displays the

546

I-U curves of the prepared PPO-DMODAs membranes and TWEDA1, which are

547

acquired at a testing solution of 0.1 M NaCl. The LCDs of PPO-DMODA-1 and

548

PPO-DMODA-2 with lower IECs are slightly lower than that of TWEDA1.

549

PPO-DMODA-3 membrane has a higher LCD value than that of TWEDA1.

550

Apparently, PPO-DMODA-3 possessing the lowest Rm and highest LCD of 14.99 mA

551

cm-2 is considered as a good candidate to offer excellent ED performances in

552

desalination.

553

3.7. ED performances in desalination

554

ED tests in continuous-mode at the applied currents of 15.0 mA cm-2 (which

555

equals the LCD of PPO-DMODA-3) were performed at room temperature on the

556

homemade device to evaluate the desalination performances of the tadpole-type ionic

557

PPO-DMODAs membranes and TWEDA1. The changes in the conductivity of NaCl

558

solution in diluted cell were measured to characterize the changes in the concentration

559

every 30 min and the results are shown in Fig. 10(a). The drops in the conductivities

560

of NaCl solutions in dilute cells with the prolonging ED times follow the sequence of

561

low to high as TWEDA1, PPO-DMODA-1, PPO-DMODA-2, PPO-DMODA-3 (Fig.

562

10(a)). After similar 150 min ED processes, the conductivities of NaCl solutions

563

treated with PPO-DMODA-1, PPO-DMODA-2, PPO-DMODA-3 and TWEDA1

564

decreased to 3.22 mS cm-1, 3.04 mS cm-1, 2.77 mS cm-1 and 3.35 mS cm-1

565

respectively. All the prepared PPO-DMODAs membranes have higher current 29

566

efficiencies with η values varying from 83.1% to 88.7% compare with that of

567

TWEDA1 (81.9%) (Fig. 10(b)), which possesses the highest P (Table 1). This is

568

because the permselectivities of the membranes tested without applied electric field

569

cannot completely reflect the ion selective transport of the AEMs under ED

570

conditions. In addition, the data of η, EC, and J of PPO-DMODAs membranes,

571

TWEDA1 and other reported AEMs were listed in the Table 2. Obviously,

572

PPO-DMODA-3 with the highest IEC in the family of PPO-DMODAs membranes

573

displays the highest salt flux value of 80.61 mg m-2 s-1 and lowest energy consumption

574

value of 2.56 kWh kg-1 NaCl. It has improved about 8.1% in J and reduced about 13.2%

575

in EC respectively compared with that of commercial TWEDA1 at the same ED test

576

conditions. HGA[46] shows a higher η of 90.6% and lower EC of 0.76 kWh kg-1

577

NaCl at a very small J of 13.19 mg m-2 s-1, which is about 16.4% of that of

578

BPPO-DMODA-3 membrane. J of MPDD-43 (102.76 mg m-2 s-1)[44] has risen to

579

about 27.4% of that of BPPO-DMODA-3 membrane, but its EC (29.52 kWh kg-1

580

NaCl) is nearly 12 folds of that of BPPO-DMODA-3, which is intolerable in ED

581

process. To clear the effects of the minor change in testing current densities on the ED

582

performances, the ED tests were performed at lower current densities of 13.75 mA

583

cm-2, which equaled the LCD of PPO-DMODA-1 (the lowest LCD among the testing

584

membranes in this work) and ensure all of the testing membranes including TWEDA1

585

ran the tests under their LCDs to give the lowest EC and the highest η. The

586

comprehensive ED performances were displayed in Fig. S4. The comprehensive ED

587

performances sequence has remained as that of at the applied currents of 15.0 mA

588

cm-2 (Fig. 10). The improvements in η are negligible within a few thousandths owing

589

to the change in applied current densities of 9.2 percent. The improvements in EC are

590

below 11.9 percent at the cost of decreases in J of 12.0 percent (Fig. S4). In brief, the 30

591

minor change in testing current density has not very different effects on the ED

592

performances.

593 594

Fig. 10. ED performances charts of PPO-DMODAs membranes and TWEDA1 at I of

595

15.0 mA cm-1: (a) the conductivities of NaCl solutions in dilute cell with ED time, (b)

596

η and EC.

597

Table 2. ED performance of PPO-DMODAs membranes and some of the published

598

ED AEMs. Membrane

η

IEC -1

J

EC

(mmol g )

(%)

(kWh kg )

(mg m-2 s-1)

PPO-DMODA-1

1.42±0.05

83.1

3.38

75.59

PPO-DMODA-2

1.53±0.03

84.8

2.88

76.72

PPO-DMODA-3

1.71±0.07

88.7

2.56

80.61

TWEDA1

1.01±0.03

81.9

2.95

74.58

1.01

90.6

0.76

13.19

1.36

59.4

29.52

102.76

[46]

HGA

MPDD-43

[44]

-1

599

In addition, we have run another 4 times of the ED tests with PPO-DMODA-3 at

600

the same ED conditions to characterize the stability of ED performance and the results

601

are described in Fig. 11. The current efficiencies of the repeat runs remain higher than

602

97% of that of the origin run, which are better than that of TWEDA1 (94.3%)

603

reported in the previous work[39]. Moreover, the current efficiency can be enhanced

604

by adequately rinsing the AEMs used in ED tests with deionized water, probably 31

605

owing to the eliminating of physical fouling on the surface of the membranes resulted

606

from the absorbed tiny dust in the ED solutions.

607 608

Fig. 11. The η and η retentions of PPO-DMODA-3 after various ED runs.

609 610

Fig. 12. The η and EC of PPO-DMODA-3 membranes cast with different solvents

611

The ED membranes cast from the PPO-DMODA-3 solutions of TCE, NMP and

612

ethanol were assessed on the same ED device at the same ED conditions, and the

613

results are shown in Fig. 12. Excitedly, the membrane casting solvents have negligible

614

effects on the current efficiencies and energy consumptions with the respective

615

swings of 0.35% and 0.08 kWh kg-1. It confirms the above-mentioned conclusion that

616

using the green solvent of ethanol to cast film possibly does not affect the

617

electrodialysis performances of the PPO-DMODAs membranes, which is based on the 32

618

results of the morphologies of PPO-DMODA-3 membranes cast from various solvents.

619

The easy recasting of ethanol solutions meets the requirements of environment

620

protection and reduces the production costs of the tadpole-type ionic membranes. All

621

the results of ED tests suggest that PPO-DMODA-3 has a great potential in ED

622

desalination.

623

4.

Conclusions

624

A series of tadpole-type ionic ionomers, PPO-DMODAs, were synthesized

625

successfully from a long chain tertiary amine DMODA and BPPO. The nano-phase

626

separated PPO-DMODAs ED membranes with IECs ranging 1.42 to 1.71 mmol g-1

627

were fabricated by solution casting and ion exchanges. The effective suppression of

628

water uptake and swelling provided by the long hydrophobic alkyl side chains offers

629

the PPO-DMODAs membranes excellent permselectivity with values above 94.6%.

630

150 min ED tests at the applied current density of 15.0 mA cm-2 on the same

631

home-made ED device clear that the tadpole-type ionic PPO-DMODA-3 shows much

632

better ED performances ( η: 88.7%, J: 80.61 mg m-2 s-1, EC: 2.56 kWh kg-1) than that

633

of TWEDA1 (η: 81.9%, J: 74.58 mg m-2 s-1, EC: 2.95 kWh kg-1). Furthermore, in

634

consideration of its simple synthesis and green membrane casting technology,

635

PPO-DMODA-3 membrane holds great potential for ED desalination application.

636

Acknowledgments

637

This work was supported by the National Key R&D Program of China (No.

638

2018YFD0400805), R&D Projects in Key Areas of Guangdong Province 33

639

(2019B020212003), Guangzhou Science Technology and Innovation Commission

640

(No.201804010436).

34

641

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Highlights 1. A series of tadpole-type ionomers PPO-DMODAs were successfully synthesized. 2. Good solubility of the ionomers in ethanol offers a green method to give ED AEMs. 3. The nano phase separated structures offer the ED AEMs excellent permselectivity. 4. PPO-DMODA-3 shows much better and more stable ED performances than TWEDA1.

Conflict of Interest The authors declare no conflict interest.