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Reactive and Functional Polymers

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Stable cycloaliphatic quaternary ammonium-tethered anion exchangemembranes for electrodialysis

Yuliang Jianga,1, Junbin Liaoa,1, Shanshan Yanga, Jian Lic, Yanqing Xua, Huimin Ruana,Arcadio Sottob, Bart Van der Bruggenc, Jiangnan Shena,⁎

a Center for Membrane Separation and Water Science & Technology, Ocean College, Zhejiang University of Technology, Hangzhou 310014, Chinab School of Experimental Science and Technology, ESCET, Rey Juan Carlos University, Móstoles, Madrid, Spainc Department of Chemical Engineering, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium

A R T I C L E I N F O

Keywords:Anion exchange membranesBrominated PPON-methylpiperidineAlkaline stabilityElectrodialysis

A B S T R A C T

In this work, we have investigated a series of anion exchange membranes (AEMs) based on brominated poly(2,6-dimethyl-1,6-phenylene oxide) (BPPO) tethered with three saturated heterocyclic quaternary ammonium groups(QAs) of 1-methylpyrrolidine (MPY), N-methylpiperidine (MPRD), and 4-methylmorpholine (MMPH) for elec-trodialysis (ED) applications, respectively, along with BPPO with trimethylamine (TMA) and Neosepta AMXmade for comparison. Our investigations demonstrate that the optimized BPPO-MPRD, having an ion exchangecapacity of 1.67mmol g−1, is highly stable in aqueous KOH (1mol L−1) with ion exchange capacity retentionratio of 85.1% and hydroxide conductivity retention ratio of 80.3% at 60 °C for over 15 days, relative to otherheterocyclic amine decorated AEMs. In ED application process, BPPO-MPRD shows the NaCl removel ratio of98.8% and energy consumption of 12.58 kWh kg−1, outperforming the Neosepta AMX (97.4% &15.76 kWh kg−1). The results demonstrate that the as-prepared BPPO-MPRD AEM can be applied in ED.

1. Introduction

Among many available separation techniques, both reverse osmosis(RO) and electrodialysis (ED) have been considered as the commonly-used desalination processes in the field of water treatment [1, 2]. Re-lative to RO filtration with high pressure requirements, ED processdepending on the ions transport through selective separators, can op-erate in an electrical field at the reduced energy consumption [3].Owing to the various advantages of low cost fabrication, eco-friendlydesign, easy as well as low-energy consuming operation etc., the de-velopment of ED has already shown the vast application prospect formaking safe drinking/process water from the brackish and seawater,separation of the hazardous chemicals from waste water streams, andrecovery of the useful materials from salt production and effluents etc.[4–6]. Also, ED has been widely applied in food, chemical and phar-maceutical industries, to demineralize valuable solutions for fluidpurification [7, 8].

Anion exchange membranes (AEMs) have been widely investigatedto accomplish the selective permeation of anions in the fields of ED,diffusion dialysis (DD), redox flow battery (RFB), fuel cell (FC), electro-membrane reactor and chlor-alkali process [9, 10] etc. In particular,

AEMs, as the key component of ED, allow the effective anion trans-portation via an electrostatic interaction but significantly suppressingthe passing of cations [1–3]. Thus, high-performance AEMs with high-conductivity, desired selectivity, good (chemical, thermal, oxidative)stability are urgently required for the practical applications above-mentioned [1]. So far, numerous types of polymers have been employedfor AEM fabrication, for example, polystyrene [11, 12], polyether imide[13] polysulfone [14], poly(2,6-dimethyl-1,4-phenylene oxide) (PPO)[15], poly(arylene ether) [16], poly(phthalazinone ether ketone)(PPEK) [17], and poly(phthalazinone ether sulfone ketone) (PPESK)[18] etc. However, most of the efficient AEMs are fabricated via highlycarcinogenic and hazardous chloromethylation (CME) of aromatic ringsattached to polymer chains, followed by quarter of amination reaction[8]. In addition, the preparation process is usually time-consuming andsophisticated. Therefore, it is highly desirable to prepare cost-effectiveAEMs through safer routes without using the hazardous materials.

Apart from the concerns, the insufficient stability of most AEMs alsolimits their practical applications under severe condition (e.g., strongalkaline conditions). The long-term alkaline stability is crucial for long-life AEMs [19, 20]. Thus, stable AEMs are highly desirable. It is long-know that trimethylamine (TMA) group-based AEMs have been widely

https://doi.org/10.1016/j.reactfunctpolym.2018.05.014Received 29 March 2018; Received in revised form 20 May 2018; Accepted 31 May 2018

⁎ Corresponding author.

1 Contributed Equally to This WorkE-mail address: shenjn@zjut.edu.cn (J. Shen).

Reactive and Functional Polymers 130 (2018) 61–69

Available online 06 June 20181381-5148/ © 2018 Elsevier B.V. All rights reserved.

T

applied, however, the alkaline stability of this positively charged TMAgroups greatly hinders the further applications in strong alkaline con-ditions, due to the degradations resulted from polymer backbone chainscission, Hoffmann elimination and SN2 substitution of cationic sidechains [21]. So far, many AEMs with QAs of imidazolium, guanidiniumand phosphonium groups have been developed for alkaline fuel cell(AFC) [22–24]. The results illustrate that the alkaline resistance prop-erties can be enhanced, however, it is still limited. For example, it isreported that, in 1–3M KOH/NaOH aqueous basic solutions at 60 °C,imidazolium-AEMs are not as stable as expected [25]. Recently, Danget al. [26] have revealed that AEMs with the cycloaliphatic QAs like N-methylpiperidine (MPRD) on pentyl spacer of AEMs show the desirablealkaline stability in 1M NaOH at 90 °C within 16 d. This excellent al-kaline stability can be due to the β-protons with the CeC bond of thesix-membered ring of MPRD rotationally restricted by the its geometryon the elimination transition state.

The accessible advantages of cycloaliphatic QAs have inspired us toinvestigate the potential application in ED. Also, considering accessiblestable BPPO having abundant -CH2Br functional groups, a series ofAEMs has been fabricated herein, by using BPPO tethered with threedifferent saturated-heterocyclic QAs (-MPY, -MPRD and -MMPH) for EDapplication. Though the alkaline stability of MPY, MPRD and MMPHhas been recently revealed by Dang et al. [26], the relationship betweenstructure, properties and ED performance of as-prepared AEMs has notbeen investigated yet. Herein, the physico-chemical properties (e.g., ionexchange capacity, water uptake, linear expansion ratio, thermal sta-bility, mechanical stability, alkaline stability, area resistance andtransport number) of as-prepared AEMs have been systematically in-vestigated for ED process, along with the commercial Neosepta AMXmade for comparison.

2. Experimental

2.1. Materials

All chemicals, including trimethylamine (TMA), 1-methylpyrroli-dine (MPY, 97%), 1-methyl-2-pyrrolidone (NMP, 99.5%), N-methylpi-peridine (MPRD, 99%), 4-methylmorpholine (MMPH, 99.5%), sodiumchloride (NaCl) and sodium sulfate (Na2SO4) were purchased fromAladdin Reagent Co. Ltd., ShangHai, China, and used without furtherpurification. Brominated poly(2,6-dimethyl-1,4-phenylene oxide)(BPPO) having aryl substitution degree of 0.42 and benzyl substitutiondegree of 0.55 was provided by Tianwei Membrane Co. Ltd., Shandongof China. Deionized (DI) water was used throughout the experiments.Commercial AEM Neosepta AMX and CEM Neosepta CMX were pur-chased from FUJIFLM Corp. Japan.

2.2. Membranes preparation

BPPO-based AEMs having trimethylamine (TMA) and tethered withdifferent heterocyclic QA groups were fabricated by using solutioncasting method [27]. The as-obtained AEMs (designated BPPO-X, whereX represents the different amines used for quaternisation, see Scheme 1)were denoted as BPPO-MPY, BPPO-MPRD, BPPO-MMPH, BPPO-TMA,respectively. Herein, we take the fabrication of BPPO-MPRD AEM as anexample. 2.0 g BPPO was dissolved in 10mL NMP solvent to form ahomogeneous solution, followed by adding N-methylpiperidine (MPRD)to the BPPO/NMP solution in a mole ratio of 1:1 and stirring for 24 h atroom temperature. The resultant solution was casted onto a clean glassplate and thermally treated at 60 °C for 8 h, rendering a polymer filmpeeled off from the glass. Subsequently, the as-prepared AEM wasthoroughly washed with deionized water and further dried at 80 °C for24 h.

2.3. Membrane characterization

2.3.1. 1H NMR and FTIR spectra1H NMR spectra were recorded on a NMR spectrometer (Bruker

AVANCE III 500MHz) at room temperature, using dimethyl sulfoxide(DMSO‑d6) and chloroform D (CDCl3) as solvents, respectively. FTIRspectra of dried AEMs were recorded by employing Nicolet 6700spectrometer. The structure was further analyzed by using X-ray pho-toelectron spectroscopy (XPS, Kratos AXIS Ultra DLD), respectively. Themembrane samples were dried under vacuum for 24 h (80 °C) beforecharacterization.

2.3.2. Thermal stabilityThe thermal stability of as-prepared AEMs was undertaken on

Netzsch (STA 449C, Germany) thermal analyzer under N2 flow with aheating rate of 10 °Cmin−1 from 30 to 700 °C.

2.3.3. Ion exchange capacity (IEC)The three pieces of dry membrane samples was accurately weighed

after being dried at 60 °C under a vacuum for 24 h, respectively. Thenthe samples were converted to the Cl− form by soaking them in 0.10MNaCl for two days. Subsequently, samples were rinsed carefully with DIwater, to remove the excess NaCl, followed by immersing in 0.05MNa2SO4 for 24 h. The amount of chloride ions liberated was measuredby titration with 0.1 M AgNO3 and K2CrO4 as indicator. The IEC valueswere calculated using Eq. (1):

=×

IECC V

WAgNO AgNO

dry

3 3

(1)

where CAgNO3 represents the concentration of the AgNO3 solution (M),VAgNO3 represents the volume of the AgNO3 solution (mL), and Wdry

represents the mass of the dried sample (g).

Scheme 1. Preparation of BPPO-based AEMs with different QA groups (BPPO-MPY, BPPO-MPRD, BPPO-MMPH, BPPO-TMA).

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2.3.4. Water uptake and dimensional stabilityThe weight of the dry membrane samples was accurately measured

after being dried at 60 °C under a vacuum for 24 h. Subsequently,samples were immersed in water for 24 h at 25 °C and the weight ofsamples were tested after removal of water on membrane surface withtissue paper. Water uptake (WU) was calculated using Eq. (2):

=−

×WUW W

W100%wet dry

dry (2)

where Wdry and Wwet represent the weight of dry and wet membranesamples, respectively.

The linear expansion ratio (LER) of prepared AEMs was calculatedusing following eq. (3):

=−

×LERL L

L100%wet dry

dry (3)

where Lwet and Ldry represent the lengths of wet and dry membranes,respectively.

2.3.5. Tensile strength and elongation at breakThe tensile strength of the wet membrane samples (2×10 cm2) was

tested on an Instron universal tester (Model 1185, US) at room tem-perature. The samples were fixed between the flat-faced grips of thetester. The tensile strength and elongation at break point were recordedwith speed of testing of 5mmmin−1.

2.3.6. Surface area resistanceSurface area resistance was measured by commercial cell-assembly

(MEIEMP-I, Hefei Chemjoy Polymer Material Co., Ltd., Hefei, China)under a constant current mode [28]. The equipment is shown in Fig. 1.Before the test, the membrane samples were soaked into the 0.5 M NaClsolution for 30min. During the measurement, a 0.50M Na2SO4 solutionwas pumped into the electrode chamber, the intermediate compart-ments were fed by 0.50M NaCl. The potential between the membranesamples was measured by Ag/AgCl electrodes, recorded by a multi-meter (DMM6000, Zhiyuan Electronics Co., Ltd.). Surface area re-sistance (R, Ω·cm2) was calculated as follows:

= − ×R U UI

S0(4)

where U represents the trans-membrane voltage and U0 represents thevoltage of the blank (V), I represents the constant current (fixed at0.004 A) through the AEMs (A), S represents the effective area ofmembrane (7.065 cm2).

2.3.7. Transport numberTransport number of AEMs was determined by a homemade-setup

composing of intermediate chamber II and III in Fig. 1 (without ap-plying the electric current), which is similar to that for surface arearesistance test. Before the measurement, the AEMs were soaked into a0.15M KCl solution for 30min. During testing, continuous flows of KCl(0.1M) and KCl (0.2M) solution were separately pumped into inter-mediate chambers. The membrane potential (Em) between the elec-trodes was recorded by using a multimeter, with Ag/AgCl referenceelectrodes. The transport number ti′ was calculated according to Eq. (5):

⎜ ⎟= ′ − ⎛⎝

⎞⎠

E t RTnF

CC

(2 1) lnm i1

2 (5)

where R represents the universal gas constant (8.314 J K−1·mol−1); Frepresents the Faraday constant (96,485C mol−1); T represents thetesting temperature (K); C1 and C2 represent the concentrations of therespective KCl solutions; n represents the electrovalence (n=1).

2.3.8. Hydroxide conductivityThe impedance (R) of AEM samples on Autolab PGSTAT 30 within

frequencies of 1–106 Hz was undertaken by using four probe technique.All membranes were cut into 1 cm×4 cm slices, the membranes weretreated with 1.0 M KOH aqueous solution for 24 h. Then were removedfrom the KOH aqueous solution, washed with deionized water and wereall stored in deionized water for at least 24 h before the measurement.Using eq. (6), the σ values of the membrane samples were determined.

=σ LRWd (6)

where R represents the resistance of the AEM samples; L represents thedistance potential sensing electrodes (1 cm); W and d represent thewidth and thickness of AEMs, respectively.

2.3.9. Alkaline stabilityTo investigate the alkaline stability of the AEMs in a basic en-

vironment, the AEMs were immersed in 1.0M KOH and 6.0M KOH at60 °C for 15 days. Decreases in IEC and hydroxide conductivity due tothe alkaline treatment were carried out, with a comparison made tocommercial Neosepta AMX.

2.3.10. Current-voltage curveCurrent-voltage (IeV) curve of the AEM was undertaken at room

temperature using the same device as that for surface area resistance.0.5 M NaCl solution was employed as the intermediate solution and0.3M Na2SO4 solution was used for the electrode solution. The trans-membrane voltage was measured by Ag/AgCl electrodes, with a mul-timeter (Zhi Yuan, DMM6000).

2.3.11. Electrodialysis testFig. 2 shows the electrodialysis stack in this work, which consists of

four compartments (effective membrane area, 19.625 cm2). Two elec-trode cells were fed by 0.3M Na2SO4 solution (200mL) to prevent pHchange, respectively, while the concentrate cell (CC) and diluate cell(DC) were fed with 0.5M NaCl solutions (90mL). Before testing, eachcell was circulated for 30min to eliminate the visible bubbles andsubsequently operated under constant current density of 28mA cm−2.The conductivity change of NaCl solution in the CC, DC and potentialover the stack were recorded every 10min.

The performances of ED fabricated with as-prepared AEMs and thecommercial Neosepta AMX were evaluated in terms of NaCl removalratio, current efficiency and energy consumption, which were calcu-lated by Eqs. (7–9):

= −R δ δδW

t0

0 (7)Fig. 1. Schematic diagram of surface area resistance measurement of AEMs: (1)the Ag/AgCl electrodes; (2) the Neosepta CEM; (3) the tested AEM.

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= − ×η Z C C V FNIt

( ) 100%t t0(8)

∫=−

E UIC V C V M

dt( )

t

t t b0 0 0 (9)

where E represents the energy consumption of NaCl (kW h kg−1); RW

represents desalination rate (%); δ0 represents the conductivity of NaClsolution before test; δt represents the conductivity of NaCl solution afterthe at time t=2.5 h, where η represents the current efficiency; Ct andC0 represent concentrations of Na+ in the CC at time t and 0 during theED process, respectively; Zrepresents the absolute valence of Na+; Vt

and V0 represents the volume of solution in CC at time t and 0; F re-presents the Faraday constant (96,485C mol−1); I represents the current(I=0.55 A); N represents the number of repeating units (N=1); Urepresents the voltage of the ED stack; Mb represents the molecularweight of NaCl.

3. Results and discussion

3.1. 1H NMR and FTIR spectra

Fig. 3 shows the FTIR spectra of pristine BPPO and BPPO tetheredwith four different QA groups. The band at 1608 cm−1 is attributable toC]C stretching vibration on phenyl groups; the peak at 1190 cm−1 isthe characteristic of C–O–C stretching [24]. After the quaternization,the characteristic band at 1260 cm−1 can be assigned to the stretchingvibration of CeN absent in pristine BPPO, indicating the successfulquaternization reaction between BPPO and MPY, MPRD, MMPH andTMA, respectively [29]. In addition, a wide broad characteristic band ataround 3375 cm−1 is probably related to the stretching vibration of the–OH groups from H2O, which was not observed in FTIR spectra ofpristine BPPO [30]. These results confirm the introduction of MPY,MPRD, MMPH and TMA head groups onto the BPPO structure.

Chemical structures of as-prepared AEMs were further confirmed by1H NMR, as shown in Fig. 4. In particular, The chemical shift at 2.7 ppmcan be assigned to the solvent N-Methyl pyrrolidone (NMP) residual inAEMs, which has been reported by other literatures [31, 32]. Regardingto BPPO, the characteristic peak centered at 2.0 ppm is attributed tomethyl groups (Fig. 4 (a)). The peak at 4.3 ppm is assigned to the

methylene group in benzyl bromide and the peaks at approximately6.5 ppm correspond to the aromatic portions [33]. Some newly-formedpeaks are observable after quaternization reaction, as shown in Fig. 4(b–e). We take BPPO-MPY as an example. The characteristic protonbands at 2.1 ppm and 3.4 ppm in the spectrum are assigned to themethylene protons in the 1-methylpyrrolidine cations. Additionally, thechemical shift of the quaternized methyl groups can be observed atapproximately 3.0 ppm, indicating the formation of 1-methylpyrroli-dine cations in the BPPO [34]. For other fabricated AEMs, similar re-sults can be observed and the characteristic peaks can be seen. We havedone the test of XPS and the results are shown in Fig. S1 and Table S1.From Table S1 in Supporting Information, we can see that BPPO-basedAEMs with different QA groups (BPPO-MPY, BPPO-MPRD, BPPO-MMPH, BPPO-TMA) show the similar C1s, O1s, N1s and Br3d chemicalcomposition, respectively. In particular, the atomic percent ratios ofC:Br of AEMs from theoretical values are in accord with that from ex-perimental ones. This solid evidence confirms that the average numbersof MPRD, MMPH, MPY and TMA groups in molecular chain are similar,though there is deviation regarding to N1S atomic percent. The resultsare suggestive of the successful synthesis of MPY, MPRD, MMPH andTMA head groups onto the BPPO structure.

3.2. Thermal stability and mechanical property

Thermal stabilities of as-prepared BPPO, BPPO-MPY, BPPO-MPRD,BPPO-MMPH and BPPO-TMA have been investigated by thermogravi-metric analysis (TGA). From Fig. 5, three main degradation steps areobserved, arising from the processes of thermal desorption of water,thermal deamination, and thermal oxidation of the polymer matrix,respectively. During the first stage, all the AEMs exhibited a weight lossin 30–160 °C interval, owing to the loss of residual H2O from the AEMmatrix. The second weight loss was noticed around 200 °C, owning tothe degradation of the quaternary ammonium groups [35]. The finalweight loss occurred above 350 °C corresponding to the degradation ofmain polymer chain. Notably, the TGA results suggest that all of pre-pared AEMs exhibited an adequate thermal stability below 200 °C. Ingeneral, the usual working temperature for ED application is below100 °C. The BPPO tethered with four saturated heterocyclic quaternaryammonium groups exhibit a slightly weight loss within 30–100 °C. Theresult suggests that as-prepared AEMs meet the requirements of ED(< 100 °C).

Further, the tensile strength and elongation at break (Eb) of as-prepared AEMs have been investigated at room temperature under wetcondition. As shown in Table 1, the as-prepared BPPO-MPRD has alowest tensile and a highest elongation at break, revealing the highestflexibility, while BPPO-TMA shows a highest tensile strength. The high

Fig. 2. Schematic diagram of test cell for electrodialysis (ED).

Fig. 3. FTIR spectra of (a) BPPO; (b) BPPO-MPY; (c) BPPO-MPRD; (d) BPPO-MMPH; (e) BPPO-TMA.

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water uptake usually leads to a poor dimensional stability and me-chanical strength. One can see that BPPO-TMA has a relatively low/compared water uptake and dimensional stability (in term of linearexpansion ratio) compared to the others, which will be confirmed in thefollowing section (see Table 2). Thus, to some extent, a relatively hightensile strength of BPPO-TMA is reasonable. In additional, the com-mercial Neosepta AMX possessed a tensile strength value around35.07MPa and an elongation at break about 21.31%. The relativelyhigh values are attributable to the substrate contained therein. How-ever, the tensile strength (TS) values of as-prepared AEMs can be foundin range of 25.65–30.11MPa, which are higher than IEMs previouslyreported [36], indicating that the as-prepared AEMs have desired

mechanical stability for ED applications.

3.3. Physico-chemical properties of various AEMs

IEC is an important parameter of AEMs, because it determines theamount of ion transport site of AEMs [37]. Herein, IECs from theore-tically calculation and experiment are shown in Table 2. TheoreticalIECs were calculated based on the composition of as-prepared AEMswith the assumption that all four QA groups completely reacted withthe BPPO, whereas experimental values of IEC were obtained by thetitration method. Though four AEMs with thickness of 70–78 μm showthe lower IECs (~1.70mmol g−1) relative to Neosepta AMX

Fig. 4. 1H NMR spectra of (a) BPPO, (b) BPPO-MPY, (c) BPPO-MPRD, (d) BPPO-MMPH and (e) BPPO-TMA.

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(2.16 mmol g−1), the area resistances are comparable. The lower IEC isassociated to the less hydrophilic regions in the AEM matrix. Theswelling behavior including water uptake and linear expansion ratiosignificantly influences the mechanical properties and the morphologicstability [38]. The water molecules inside membrane matrix can pro-mote the dissociation of the charged functional groups, modifying theions transport across the membrane [39, 40]. The as-prepared AEMshave the desired lower water uptake ranging from 22.3–30.1% andlinear expansion ratio of 7.6–14.3%, which is liable for long timerunning in ED application.

3.4. Alkaline stability test

The strong nucleophilicity of OH– can result in the degradation ofthe cationic groups via nucleophilic substitution at the α‑carbon andHofmann elimination at the β-H [41]. Therefore, this will have sig-nificantly impact on long-term stability of AEMs in alkali environment.Herein, the alkaline stability has been evaluated in term of IEC reten-tion ratios and conductivity retention ratios. Fig. 6 (a) shows the IECretention ratio of BPPO-MPY, BPPO-MPRD and BPPO-MMPH AEMs

exposure to 1M at 60 °C for 15 days and 6M KOH at 60 °C for 3 days,respectively, with BPPO-TMA and commercial Neosepta AMX made forcomparison. In a 1M KOH solution at 60 °C, BPPO-MMPH, BPPO-TMAand Neosepta AMX have the similar IEC retention ratios of< 70% after15 days, whereas both BPPO-MPY and BPPO-MPRD showing the similarbut much higher values (~86%). Meanwhile, BPPO-MPRD performs theslightly increased values, relative to BPPO- MPY.

To further confirm the results, a similar test was also carried out byexposing AEMs in 6M KOH solution at 60 °C for 3 days. Though the IECretention ratios of AEMs reduce sharply in higher concentration of KOHsolution with more hash environment, both groups show the similartendency. We can reason that the IEC retention ratio reduction of as-prepared various AEMs is in order of BPPO-MPRD > BPPO-MPY > > BPPO-TMA > Neosepta AMX > BPPO-MMPH, whichagrees well with the conductivity retention ratios described in Fig. 6(b). The TMA with no heterocyclic QA groups showing much pooreralkaline stability than N, N-dimethylpiperidinium and N,N-di-methylpyrrolidinium at 160 °C in 6M NaOH has been revealed byMarino et al. [42]. Relative to BPPO-MPY with five-member ring, six-membered BPPO-MPRD having the great enhancement of alkaline sta-bility, can be possibly attributable to the six-member rings endow lattertwo with lower ring strain that increases the transition state energy ofboth elimination and substitution reactions [43]. While both BPPO-MPRD and BPPO-MMPH have a six-membered ring, the conductivityretention ratio of BPPO-MPRD is higher than that of BPPO-MMPH. Thismay be explained that the oxygen atom on MMPH draws away elec-trons inductively across the entire bonded ring, rendering the lowerdegree of ion dissociation relative to that MPRD cation [26]. Havingheterocyclic QA groups with six-member ring with no oxygen, BPPO-MPRD is effective for alkaline stability as confirmed herein.

3.5. Transport number and limiting current density

During ED process, transport number can be denoted as the fractionof total current carried by counter ions passing through the membranematrix [44]. A higher counter ion transport number of membrane in-dicates a lower amount of required energy and higher perm-selectivityto counter-ions. The obtained counter ion transport numbers of fabri-cated membranes are tabulated in Table 2. Due to the lower IEC of as-prepared AEMs having relatively less ion transporting passages, thetransport numbers of as-prepared AEMs ranging from 0.95–0.97 areslightly lower than that of commercial Neosepta AMX (0.98). Theseclose values, we reason, can still satisfy the requirements for ED ap-plication.

Limiting current density (Ilim) is an important parameter that in-fluences the ED performance of IEMs. It is long-known that currentdensity applied is below the Ilim, the mass transfer can reach a balancein both boundary layer and membrane matrix. However, when thecurrent density is higher than Ilim, the mass transfer across the IEM is

Fig. 5. TGA curves of BPPO, BPPO-MPY, BPPO-MPRD, BPPO-MMPH and BPPO-TMA.

Table 1The tensile strength and elongation at break of as-prepared AEMs and NeoseptaAMX.

Membranes BPPO- MPY BPPO-MPRD

BPPO-MMPH

BPPO-TMA

NeoseptaAMX

Tensile strength(MPa)

26.00 25.66 26.21 30.11 35.07

Elongation atbreak (%)

13.39 15.96 14.26 13.46 21.32

Table 2The physico-chemical properties of as-prepared various AEMs and commercial Neosepta AMX.

AEM IECa IECb Thickness WUc LERd ARe Transport number

mmol g−1 μm % Ω·cm2

BPPO-MPY 2.24 1.74 ± 0.02 75 22.3 ± 2.1 12.3 ± 3.7 2.47 0.97BPPO-MPRD 2.17 1.67 ± 0.01 70 30.1 ± 4.2 11.4 ± 2.4 2.38 0.96BPPO-MMPH 2.16 1.68 ± 0.01 72 25.1 ± 3.9 14.3 ± 3.5 2.67 0.96BPPO-TMA 2.37 1.75 ± 0.03 78 23.3 ± 3.4 7.6 ± 3.2 2.58 0.95Neosepta AMX –– 2.16 ± 0.03 115 44.2 ± 2.4 4.2 ± 2.3 2.52 0.98

a IEC, calculated values based on bromination degree that supposing all eBr on BPPO have been reacted;b IEC, experimental values by titration;c WU, water uptake;d LER, linear expansion ratio;e AR, area resistance.

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much faster than that in boundary layer. This will cause concentrationpolarization, poor ED desalination performance and high energy con-sumption [45]. In a typical current-voltage curve, it shows the differentbehaviors of IEMs over a wide range of currents and displays threedifferent important parameters based on ohmic region, limiting currentregion, and over limiting region. Seen from Fig. 7, the first stage is

approximately ohmic behavior, while the second stage shows a plateau,indicative of the concentration polarization near the membrane inter-face. The following stage is contributed by many phenomena likegravitational convection, water dissociation and electroconvection, etc.[27].

Fig. 7 shows the current-voltage curves of BPPO-MPY, BPPO-MPRD,BPPO-MMPH, BPPO-TMA and commercial Neosepta AMX. Obviously,all curves show the clarified three stages. In the Ohmic region, the as-prepared AEMs and Neosepta AMX have almost the overlapped curves,whereas BPPO-TMA and Neosepta AMX have more severe concentra-tion polarization than those of BPPO-MPY and BPPO-MPRD in thesecond plateau stage. These are in accord with the results of BPPO-MPY,BPPO-MPRD, BPPO-MMPH having the lower area resistance (2.47 and2.38Ω cm2), relative to BPPO-TMA (2.58Ω cm2) and Neosepta AMX(2.52Ω cm2) in Table 2, as well as the IEC retention ratios and con-ductivity ratios described in Fig. 6. In particular, as for BPPO-MPRD,because SO4

2− ions can be significantly suppressed through its AEMmatrix and therefore they have a slighter effect on Ilim, but this willaffect Ilim more for the Neosepta AMX. The similar phenomenon is ac-cordance with in our previous work and other works [33, 34]. Thiscomparison confirms superior performance of BPPO-MPRD in desali-nation application.

3.6. Desalination performance

The suitability of as-prepared AEMs for ED applications has been

Fig. 6. IEC retention ratios (a) and conductivity ratios (b) of as-prepared various AEMs exposure to 1M at 60 °C for 15 days and 6M KOH at 60 °C for 3 days,respectively.

Fig. 7. Current density vs. voltage curves of BPPO-MPY, BPPO-MPRD, BPPO-MMPH, BPPO-TMA and Neosepta AMX.

Fig. 8. (a) Conductivity of NaCl solutions in dilute cell (DC) and concentration cell (CC) of ED with various AEMs; (b) Potential change of various AEMs over stackduring ED test.

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evaluated in a continuous-mode ED in terms of current efficiency (CE)and energy consumption. Fig. 8 (a) shows the conductivity change ofNaCl solutions in dilute cell (DC) and concentration cell (CC) of ED withvarious AEMs. One can see that the conductivity of NaCl solution in DCreduces, whereas conductivity of NaCl solution in CC increases as theincrease of the desalination time. According to the conductivity modelemployed by Aguado et al. [46], the conductivity change of the dilutesolution can be due to: (i) the significantly reduced ion concentrationunder the applied electricfield; (ii) the increased rate of ionic diffusionand the increased molar conductivity caused by the reduction of con-centration ions in the DC. As a result, a net decrease of conductivity ofdilute solution can be determined. As seen from Fig. 8 (a), ED fabricatedwith BPPO-MPRD shows the highest conductivity in CC and lowestconductivity in DC within 140min, suggesting the most effective re-moval of NaCl from aqueous solution. This can be attributable to thatCl− ions can migrate rapidly from dilute cell to concentrate cell, re-sulting from the lower area resistance of BPPO-MPRD. The result hasalso been confirmed by the potential change illustrated in Fig. 8 (b). Inparticular, we can observe that the voltage of BPPO-MPRD has beenretarded after 90min, relative to that Neosepta AMX.

As a result, NaCl removal ratio of ED fabricated with BPPO-MPRDwithin 2.5 h shows the highest value of 98.8%, compared with otherAEMs of BPPO-TMA (84.4%), BPPO-MPY (97.1%), BPPO-MMPH(96.6%), and Neosepta AMX (97.1%), as shown in Fig. 9. Accordingly,it can be seen that BPPO-MPRD requires lower energy consumption(12.58 kWh kg−1 NaCl) relative to Neosepta AMX membrane(15.76 kWh kg−1 NaCl). This might be due to that the transport numberdetermines the current efficiency and finally the salt removal efficiencybelow Ilim. In addition, at over Ilim, other negative factors, such as theability to initiate electro convection and water splitting: higher watersplitting causes lower current efficiency and lower degree of desalina-tion. The ED experiment was carried out with batch mode under aconstant current density. The concentration of the dilute solution de-creases, the ratio of the current density to the Ilim increases. When thesalt removal ratio exceeds 90%, therefore, the current density becomesoverlimiting. The voltage over the stack increases from about 12.5 Vto> 30.0 V. Among the AEMs, BPPO-MPRD having the highest IEC,smaller thickness, lower area resistance and highest Ilim, endows BPPO-MPRD desired desalination performance. Hence, under the same con-ditions, the BPPO-MPRD is expected to have the potential in ED ap-plication.

4. Conclusions

In this work, a series of anion exchange membranes (AEMs) basedon BPPO tethered with three saturated heterocyclic amines of N-me-thylpiperidin, 4-methylmorpholine and 1-methylpyrrolidine for

electrodialysis applications has been investigated. The as-preparedBPPO-MPRD AEM is highly stable in aqueous KOH (1M) at 60 °C forover 15 days, outperforming other three and the commercial NeoseptaAMX. In ED application, due to the rapid transport of Cl− ions from DCto CC across the BPPO-MPRD matrix with lower area resistance, itshows the superior desalination performance (NaCl removal, 98.8%)and lower energy consumption (energy consumption, 12.58 kWh kg−1

NaCl), relative to the commercial Neosepta AMX (NaCl removel: 97.4%;energy consumption: 15.76 kWh kg−1 NaCl). This is the first demon-stration of the effects of saturated heterocyclic amines on the ED per-formance.

Acknowledgement

We thank the funding support from the Natural Science Foundationof China (No. 21676249), National key research and development plan(NO. 2017YFC0403700).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.reactfunctpolym.2018.05.014.

Nomenclature

BPPO Brominated poly(2,6-dimethyl-1,6-phenylene oxide)MPY 1-methylpyrrolidineMPRD N-methylpiperidineMMPH 4-methylmorpholineTMA TrimethylamineQAs Quaternary ammonium groupsRO Reverse osmosisDD Diffusion dialysisRFB Redox flow batteryFC Fuel cellPPEK Poly(phthalazinone ether ketone)PPESK Poly(phthalazinone ether sulfone ketone)AEM Anion exchange membraneCEM Cation exchange membraneED ElectrodialysisCME ChloromethylationAFC Alkaline fuel cellDI DeionizedIEC Ion exchange capacityWU Water uptakeLER Linear expansion ratioAR Area resistance

Fig. 9. NaCl removal ratio (a) and current efficiency & energy consumption (b) of ED fabricated with various AEMs within 2.5 h.

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IeV Current-voltageCC Concentrate cellDC Diluate cellTGA Thermogravimetric analysisTS Tensile strengthEb Elongation at breakIlim Limiting current densityCE Current efficiencyR Surface area resistance Ω·cm2

σ The hydroxide conductivity of the membrane samplesRW The desalination rate %η The current efficiencyE The energy consumption of NaCl kWh kg−1

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