Preparation of acid block anion exchange membrane with quaternary ammonium groups by homogeneous amination for electrodialysis-based acid enrichment

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Preparation of acid block anion exchange membrane with quaternary ammonium groups by homogeneous amination for electrodialysis-based acid enrichment ⁎

Ming-yan Cong, Yu-xiang Jia , Hong Wang, Meng Wang

⁎

Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, China College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Acid enrichment Electrodialysis Anion exchange membrane Proton leakage Acid block performance

Proton leakage defect of anion exchange membrane (AEM) seriously restricts the applications of electrodialysis (ED) in acid enrichment. Different from conventional scheme in which weak base groups are usually introduced, strong base AEMs are endowed with acid block property by the specific design of membrane microstructure in this work. Firstly, as confirmed by 1H NMR and FTIR, a series of quaternary ammonium polysulfone containing long alkyl side chains pendant are synthesized for AEM preparation. Small Angle X-ray Scattering experiments show that ionic domains form a distinct and well-separated phase. The introduced hydrophobic side chains strengthen the hydrophobic matrix and then restrict AEM water uptake. Besides, some interesting confinement effects are observed, for examples, the reinforcement of Donnan exclusion for H+ and facilitated transport of counter-ions. Furthermore, influences of pendant carbon chain length and ion exchange capacity are investigated. Notably, ED measurements show as-prepared AEMs indeed display excellent acid block performance, a current efficiency of almost 100% under certain condition, which substantially exceeds that of conventional AEM fabricated by heterogeneous aminaiton. This observation demonstrates an unprecedented result that novel strong base AEM with both outstanding acid block performance and membrane conductivity can be achieved by constructing phase separation microstructure.

1. Introduction It is well known that some inorganic acids such as HCl, H2SO4 and HNO3 are extensively applied in metal finishing processes, steel production industry, hydrometallurgical fields, and so on [1,2]. Large amounts of waste streams generated correspondingly has aroused widespread concern due to the presence of salts, acids and even some organic compounds such as solvents which render them highly corrosive and polluting [3]. In the last few decades, the unremitting efforts have been made at the separation of acids and salts so as to lower the environmental pollution and form a closed loop process in which the resources such as water, metals and metals acid can be recycled [4]. As far as we know, several schemes such as thermal decomposition [5], acid retardation [6], membrane separation [7–9], rectification [10], and solvent extraction [11] have been developed to recover acid from industrial effluent. However, the concentration of the recovered acid is usually too low to put into use directly. Hence, an additional concentrating step, especially some energy intensive processes such as

⁎

evaporation and crystallization, has to be needed. Obviously, this casts a shadow over their economic rationality. As a result, the relevant concentrating technologies which embrace economic sustainability and technical feasibility at the same time are paid much attention. For example, membrane distillation in which the waste heat or solar thermal energy can be utilized more conveniently is used to concentrate the acidic effluents at low temperature with high efficiency and low energy consumption before the conventional solvent extraction process for acid recovery [12,13]. Theoretically, electrodialysis (ED) in which ions migrate directionally driven by electric field, could have played a vital part in concentrating acid as well [14,15]. However, the proton leakage defect of AEM seriously degrades the work performance of ED-based acid enrichment process, for examples, fail to obtain highly concentrated acid and often display a relatively low current efficiency. It has been widely accepted that the above undesirable proton migration through AEM should be attributed to the special transport mechanisms of protons, namely the Grotthuss mechanism and the vehicle mechanism, which all

Corresponding authors at: Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, China. E-mail addresses: [email protected] (Y.-x. Jia), [email protected] (M. Wang).

https://doi.org/10.1016/j.seppur.2019.116396 Received 4 December 2019; Accepted 4 December 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Ming-yan Cong, et al., Separation and Purification Technology, https://doi.org/10.1016/j.seppur.2019.116396

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different content of chloromethyl groups were synthesized by controlling the reaction time along with a continuous mechanical stirring at 45 ℃. Finally, the white CMPS powder can be obtained after a thorough washing with ethanol and drying in a vacuum at 50 ℃ for later use. In this work, the content of chloromethyl groups in CMPS can be calculated after determining the halides content according to the well-known Volhard’s method.

depend on water molecules [16,17]. Therefore, the relevant schemes have been developed to impart AEM with acid block property by moderately reducing its water uptake. Among them, the weakly dissociated anion exchange groups instead of the conventional quaternary ammonium groups is usually adopted to prepare AEMs for the endowment of the acid block performances. For example, Zheng et al. fabricated acid blockage AEMs from the copolymers synthesized by ethylenically unsaturated aliphatic or aromatic tertiary amine monomer with vinylbenzyl chloride and other crosslinking monomer [18]. Similarly, Wang et al. successfully prepared AEMs from the poly (2,6-dimethyl phenylene oxide) with some tertiary ammonium pendants for H2SO4 recovery [19]. However, all the above results showed that the acid block performance is usually obtained at the cost of sacrificing membrane conductivity. In view of the trade-off between proton blockage and membrane conductivity, a novel membrane-preparing scheme has been put forward from the perspective of membrane structure design [20]. That is, the adoption of a side-chain polymer in which weak base groups were positioned on side-chains grafted onto the polymer backbones contributes to forming a phase segregated microstructure resulting from the polarity difference among the hydrophilic and hydrophobic segments. The confinement effect of as-formed ion-nanochannels not only facilitate the transport of counter-ions (i.e. increasing membrane conductivity) but also reinforce the Donnan exclusion for co-ions (i.e. blocking protons) simultaneously. On the other hand, plenty of investigations on polymer electrolyte membranes (PEMs) for fuel cell have shown that the formed phase separation microstructure also contributes to controlling the membrane swelling [21]. That is to say, the unique membrane structure itself can effectively restrain the water uptake of membrane. That means the conventional schemes for refraining water uptake, for an example, the adoption of weakly dissociated ion-exchange groups, to prepare acid block AEM seem to be unnecessary. Accordingly, a series of quaternary ammonium polysulfone containing long alkyl side chains pendant were synthesized and directly used for the preparation of AEM in this study as shown in Fig. 1. Herein, the emphasis was placed on the relation between the acid block performances of as-prepared AEMs and the formed membrane structure characterized with microphase separation. Moreover, it is necessary to investigate the influences of side chain lengths and ion exchange capacity. Especially, the performances of asprepared AEMs were also compared with that of conventional AEM fabricated by the heterogeneous amination, namely the post-quaternization of the chloromethylated polysulfone membrane with trimethylamine.

2.2.2. Preparation of quaternary ammonium AEM 2.2.2.1. Preparation of AEM by post-quaternization of the CMPS membrane. The synthesized CMPS powder with a fixed content of chloromethyl group (1.5 mmol/g) was completely dissolved in DMF to form a 15 wt% homogeneous casting solution. After defoaming, a film with 200 μm in thickness was cast on a flat glass and then put into the vacuum to thoroughly remove the solvent at 50 ℃. Subsequently, the obtained CMPS membrane was immersed in excessive 33 wt% trimethylamine aqueous solution at room temperature for more than 48 h to perform a complete heterogeneous quaternization. Hereafter, the AEM was named AEM-1C. 2.2.2.2. Preparation of AEM directly from quaternary ammonium PS. The side-chain type PS containing quaternary ammonium groups was prepared by the alkylation substitution reaction between CMPS and excessive tertiary amine, as shown in Fig. 2. Subsequently, the corresponding AEMs was fabricated as follows. Above all, 1 g CMPS with a fixed content of chloromethyl group (1.5 mmol/g) was completely dissolved in DMF to obtain a 15 wt% homogeneous casting solution. Afterwards, a series of tertiary amines with different carbon chain length, including butyldimethylamine, octyldimethylamine and dodecyldimethylamine, were added into the above polymer solution, respectively, at a ratio of 3.5 mmol amino groups to 1.0 g CMPS. After three-hour amination reaction at 25 °C, a polymer film was formed on a clean and flat glass plate by the classical casting technology and then placed into a vacuum to evaporate the solvent at 50 °C for more than 24 h. In order to facilitate understanding, the as-prepared AEMs were labeled as AEM-4C, AEM-8C, and AEM-12C, respectively, according to the corresponding side chain length. Moreover, with respect to the same carbon chain length, the influence of ion-exchange capacity (IEC) was also investigated by changing the content of chloromethyl group in CMPS. Herein, a series of CMPS with different chloromethyl group content were reacted with excessive butyldimethylamine for the preparation of side-chain type PS AEMs with different IECs. According to the increasing order of IECs, the corresponding as-prepared AEMs were named AEM-4C1, AEM-4C2, AEM-4C3 and AEM-4C4, respectively.

2. Experimental 2.3. Characterization of the as-synthesized materials and the corresponding AEMs

2.1. Materials

Their chemical composition and molecular structure were determined by the high-resolution NMR spectrometer (1H NMR, Bruker AVANCE III 600) and Fourier transform infrared spectra (FTIR, Nicolet iN10 IR Microscope). The 1H NMR spectra were recorded at 600 MHz using deuterium chloroform (CDCl3) or deuterium dimethyl sulfoxide (DMSO) as solvents. The FTIR was recorded in the total spectral range from 500 cm−1 to 4000 cm−1. Sixteen scans were taken for each spectrum at a nominal resolution of 4 cm−1. In addition, Small angle Xray scattering (SAXS) measurements was performed on the wet state membranes by a SAXSess-MC2 X-ray diffractometer (Anton Paar, Austria) with the generator current and voltage of 50 mA and 40 kV, respectively. And, all the data were recorded at room temperature. IEC refers to the molar number of the dissociated ion-exchange groups per gram of membrane. The measuring detail is described as follows. Firstly, the membrane samples which have been kept in 0.1 M HCl solution were fully equilibrated in 1 M KNO3 solution. After removing carefully the free NO3− and water droplets, the NO3−-type

Chloromethyl methyl ether was acquired from Xiya Chemical Reagent Co. Ltd. All the amine reagents, such as trimethylamine, butyldimethylamine, octyldimethylamine and dodecyldimethylamine were purchased from the Aladdin Reagent Company. Polysulfone (PS, P-3500) is a Solvay product. Anhydrous zinc chloride (ZnCl2) was thoroughly dried in a vacuum at 50 °C before use. Other reagents, such as trichloromethane, N, N-dimethylformamide (DMF), ethanol, NaCl, NaOH, HCl,H2SO4, KNO3 and HNO3, were used directly. 2.2. Preparation of membrane material and the corresponding AEM 2.2.1. Synthesis of chloromethylated polysulfone (CMPS) The chloromethylated polysulfone was prepared according to our previous work [20]. That is, specified amount of PS was added into chloroform to obtain a homogeneous solution by continuous stirring. After adding anhydrous zinc chloride and chloromethyl ether as catalyst and chloromethylation reagent, respectively, a series of CMPS with 2

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Fig. 1. Schematic diagram of the side-chain type Polysulfone synthesis and the corresponding AEM with phase separation microstructure.

of the half compartments was kept constant, 0.1 M, whereas the acid concentration of another compartment, such as 0.5 M, 1 M, 1.5 M, and 2 M, were set to explore the influences of concentration gradients on the AEM permselectivity. All the measurements were carried out at room temperature.

membrane was thoroughly immersed in 0.2 M NaCl solution for a complete conversion into Cl− -type. Finally, the content of the exchanged NO3− was determined to calculate the IEC by the spectrophotometer (UV-1600PC, Mapada Instruments, Shanghai, China) at the wavelength of 210 nm. The parallel measurements were made three times and the average values were reported at last. Subsequently, the electrochemical impedance spectra of the membrane samples were recorded using an Electrochemical Workstation (CHI660E, Shanghai Chen-hua Device, China) at a frequency ranging from 1 Hz to 100 kHz and an oscillating voltage of 10 mV. The device is shown in Fig. 3. It can be seen that the measurements were performed in a four-electrode mode, namely two platinum-coated titanium plates which were used as working electrodes and counter electrodes and two calomel electrodes which were inserted in Luggin capillaries and used as reference electrodes. 0.1 M HCl solution and 0.2 M Na2SO4 were employed as testing solution and electrode rinsing solution, respectively. The corresponding impedance which was closest to zero phase angle in the Bode diagram was recorded. Furthermore, the area resistance can be determined based on the following equations:

Rm = Znet × Am

Em=(2tm-1)

RT r1c1 ln F r2c2

logr A ̂ ± =-A |z+z-|

I=

1 2

∑ ci z2i tm

I

(2) (3) (4)

where is the transport number of counter ion in the investigated AEM; C1 and C2 are the HCl concentration; Em is membrane potential; r1 and r2 are the activity coefficients; A, a constant, equals 0.509 at 25 ℃; Z is the charge of the counter ion; I is the ionic strength; F, R, and T represent the Faraday constant, universal gas constant and temperature, respectively. At last, the changes of water uptake and acid adsorption of the asprepared AEMs with the ambient acid concentration were also examined. Before the measurements, the as-prepared AEMs must be completely equilibrated with HCl solution with a specified concentration, such as 4 M, 3 M, 2 M, 1 M, 0.5 M and 0.1 M. Moreover, the AEM samples were immersed into NaOH solutions with the specified concentration and volume again after removing the free acid at the membrane surface. Subsequently, the reduced amount of NaOH was determined by back titration to calculate the acid adsorption. Meanwhile, the water uptakes of the as-prepared AEMs can also be gained on the basis of the mass differences between dry and wet membranes. The parallel measurements were carried out three times and the corresponding average values were recorded eventually.

(1)

Here, Rm is membrane area resistance; Znet is the difference between impedance values with and without membrane; Am is effective area of membrane (0.785 cm2). The apparent transport number of the counter ion in the as-prepared AEM was calculated from the corresponding membrane potential generated by the HCl concentration difference across the investigated AEM according to Eq. (2). The schematic diagram of testing device can be found in Fig. 4. The investigated AEM was sandwiched between two half-compartments into which the HCl solutions of different concentrations were fed, respectively. That is, the acid concentration in one 3

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Fig. 2. Synthesis of side-chain type PS with quaternary ammonium groups.

Fig. 3. Schematic diagram of testing device for the measurement of membrane impedance. Fig. 4. Schematic diagram of testing device for the measurement of counter ion transport number.

2.4. Work performance of as-prepared AEM in ED-based acid enrichment As shown in the Fig. 5, the ED stack composed of a piece of asprepared AEM and three pieces of CEMs (the effective area, 4 cm × 4 cm) was constructed to perform the acid enrichment experiments. The electrode compartments were equipped with a couple of ruthenium-coated titanium electrodes and filled with 0.1 M sulfuric acid as electrode rinsing solution. Initially, 0.2 M HCl solutions were fed into all the compartments and circulated at a flow rate of 300 mL/min for one minute by a Peristaltic pump (WT600-2J, Longer Pump, China) before switching on the power. Subsequently, the current density, taking 20 mA/cm2 as an example, was exerted to the membrane stack for ED experiments. During this period, the changes in cell voltage,

solution volume and concentration were recorded accurately. It was worth pointing out that the volume of HCl feed for dilution was so large (5 L) that its concentration change can be neglected during the whole ED experiment, while a small volume of HCl feed for enrichment (only 200 mL) can help us to monitor the corresponding concentration changes more accurately. In the end, the current efficiency (η %) can be determined according to Eq. (5) to evaluate the ED work performance.

η=∫ 4

Ci + 1 Vi + 1 − Ci Vi F IAt

(5)

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d, e and f, such as the peak of about 3.1 ppm which should be attributed to the protons of N-CH3 and the peaks appeared at about 2.9 ppm, 1.3 ppm, 1.2 ppm and 0.8 ppm should be assigned to the protons of NCH2-R groups. Furthermore, the evolution of chemical composition from PS to the as-prepared AEMs was also recorded by FTIR and demonstrated in Fig. 7. Above all, some typical absorption peaks of PS were observed in the spectrum. For examples, the peaks at 1488 cm−1 and 1585 cm−1 should be due to the stretching vibration of the conjugated double bonds (C]C) in the benzene ring, while the stretching vibration absorption peaks of sulfone groups (O]S]O) appeared at 1149 cm−1 and 1241 cm−1. After the chloromethylation, it is noticed that a new peak at 735 cm−1 appeared, which should be related to the CeCl stretching vibration [23]. Compared with those of PS and CMPS, some new peaks were also found in the spectra of the synthesized AEMs. For example, the peaks at 1033 cm−1 should result from the stretching vibration of the CeN groups, which indicated that the quaternary ammonium group has been grafted successfully [24]. Besides, the peaks at 2849 cm−1 and 2931 cm−1 should be related to the vibration of eCH2e and eCH3 groups [25], which implied the successful grating of side alkyl groups. Obviously, all these indicated that the quaternary ammonium type AEMs were obtained.

Fig. 5. Schematic diagram of electrodialysis stack for the acid enrichment.

where I is current density, 20 mA/cm2; A is membrane area, 16 cm2; t is the time taken for the concentration change from Ci to Ci+1; Vi and Vi+1 denote the corresponding solution volume. F is Faraday constant, 96,500C/mol; 3. Results and discussion 3.1. Influences of amination methods

3.1.2. Membrane morphology The nanostructure of side-chain type PS AEMs, including AEM-4C, AEM-8C and AEM-12C, was characterized in their wet state using SAXS at room temperature. Meanwhile, a conventional AEM-1C, prepared directly from CMPS membrane and then being aminated by trimethylamine according to the heterogeneous amination method, was also investigated for comparison. In general, the characteristic membrane microstructure of the ordered microphase separation can be reflected from the appearance of the q peak in SAXS results. As can be seen from Fig. 8, a relatively weak peak with large width emerged in the SAXS profile of the AEM-1C. Similar results which have also been reported by Pan et al. that whether the q peak appeared or not depended on the dry and wet state of the membrane [26]. As a comparison, the much stronger and sharper peaks can be found in the SAXS profiles of AEM4C, AEM-8C and AEM-12C. This indicated that the ionic domains indeed formed a distinct, well-separated phase in as-prepared AEMs. Especially, it was clearly observed that the q values shifted to higher values with the increase of the alkyl chain length. That is, a series of ionomer peaks appeared at 12.89 nm−1 (AEM-4C), 13.25 nm−1 (AEM8C) and 13.36 nm−1 (AEM-12C), corresponding to periodic structure with the d spacing of 0.487 nm (AEM-4C), 0.474 nm (AEM-8C) and 0.470 nm (AEM-12C), respectively, according to Bragg equation (d = 2π/q, where q is the peak maximum). Furthermore, it was noticed that the influences of alkyl side-chain length on the phase separation microstructures reported in literatures were inconsistent, sometimes even opposite [19,26–28]. This suggested the micro-phase separation behaviors were complicated and affected by a combinations of multiple factors, such as IEC, polarity difference between the hydrophobic and hydrophilic segments and the chain flexibility. In addition, it can be seen clearly from Fig. 9 that the transparency of the side-chain type AEMs is obviously inferior to that of the conventional AEM-1C. Especially, the membrane transparency was further reduced with the increase of alkyl side-chain length. This should also be ascribed to the occurrence of phase separation. Just as we know, the transparency of the material is mainly determined by its transmittance in visible light and the degree of light scattering [29]. For example, Zhao et al. [30] adjusted the space between nanofibers of the composite nanofibrous film to reduce the light scattering phenomenon and then improved the transparency of the polymer film. Lee et al. [31] employed branched chain extenders to decrease the crystallinity and achieved transparent thermoplastic polyurethanes. Especially, some researching works showed that the phase separation in the hard segment phase of thermoplastic polyurethanes often cause light scattering

It is well known that the current commercial AEMs, taking widely used Polystyrene series AEM as an example, is mainly produced by means of heterogeneous amination technology. That is, the membranes were firstly cast in their nonionic form and then aminated. It can be imagined that the anion exchange groups must disperse in hydrophobic polymer matrix which may restrain the transport of counter-ions to a certain extent. When the IEC is high enough, the AEM behaves more like a gel, for examples, significant swelling and reduction in permselectivity for co-ions. More recently, some relevant researches have indicated that the homogeneous amination technology by which the AEMs were prepared directly from the polymer in its ionic form generally results in the formation of the phase separation microstructure. That is to say, the continuous ionic aggregation occurs during membrane formation and forms the interconnected nanochannels for ion conduction. For instance, the relationship between the conductivity and the amination types has been explored by Yan and Hickner [22]. Their results suggested that an efficient phase separation microstructure can be constructed by the homogeneous amination technology and the asprepared membranes demonstrated a more outstanding performance than those fabricated by the heterogeneous amination technology. Herein, it is also expected that the enhancement of local charged concentration resulting from the possible confinement effects occurred in the formed nanochannels will contribute to strengthening the Donnan exclusion for co-ions and then blocking the proton transport. Therefore, a series of quaternary ammonium PS with different alkyl side-chains were tried to be synthesized for the fabrication of acid blockage AEM in this work. 3.1.1. Evolution of chemical composition Above all, series of 1H NMR spectra were recorded to confirm the successful synthesis of the corresponding target products. In this work, a series of anion exchange membranes were fabricated from the chloromethylated PS and its further quaternization before or after membrane preparation. As is known to us, the chloromethylation of PS is an electrophilic Fridel Craft substitution reaction, whereas the ether group of PS is an electron donating substituent. Compared with the 1H NMR spectrum of PS, the successful introduction of chloromethyl group was confirmed by the representative chemical shift of about 4.6 ppm as shown in Fig. 6b. Subsequently, the 1H NMR spectra of as-prepared membrane samples were also recorded and compared with those of PS and CMPS. Indeed, some typical chemical shifts were noticed in Fig. 6c, 5

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Fig. 6. 1H NMR spectra of the as-prepared membrane s in comparison with PS and CMPS: (a) PS; (b) CMPS; (c) AME-1C; (d) AME-4C; (e) AME-8C; (f) AME-12C.

expected, the achievement of IEC depended on the amination technologies and the carbon chain length of the used amination reagents at the same time, for examples, the AEM-4C and AEM-12C corresponded to the highest and the lowest IECs, respectively. The IEC results of AEM4C, AEM-8C and AEM-12C showed that the amination of chloromethylated polysulfone became more and more difficult with the increase of carbon chain length of the tertiary amines due to the steric hindrance effect. Moreover, the fact that the IEC of AEM-1C is even lower than that of AEM-4C also confirmed that the homogeneous amination is more beneficial to the achievement of AEM with a high IEC. In general, the membrane conductivity is determined by the content of ion-exchange groups to a large extent. However, it can be noticed that AEM-1C demonstrated a much higher membrane resistance than those of other AEMs although they embraced the similar IECs. This should be attributed to the formation of the phase separation microstructure in the AEMs prepared by the homogeneous amination technology. That is, the polarity difference between the PS backbones and

and result in decreased transparency [32–34]. In addition, Chen’s work also indicated that the transmittance of membrane was also closely related to the surface roughness and thickness of film [35]. In this work, the synthesized anion exchange membranes embraced similar surface roughness and film thickness. However, their micro-phase separation phenomena become more and more obvious with the increase of alkyl side chain lengths. Enlightened by the above experimental results, it is reasonable to think that the micro-phase separation behaviors strengthen the light scattering and then lead to the reduction of the membrane transparency.

3.1.3. Electrodialytic transport properties In this work, the CMPS with a specified content of chloromethyl group (1.5 mmol/g) were selected to prepare a series of AEMs by means of heterogeneous and homogeneous amination technologies in excessive tertiary amines with different side chain lengths such as trimethylamine, butyldimethylamine, octyldimethylamine and dodecyldimethylamine. Their IECs were displayed clearly in Fig. 10. As 6

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PS

1585

CMPS

1488 1241

1585 1488

AEM-1C

2931 2849

AEM-4C

735 1149

1035

2931 2849

AEM-8C

1241

1149

1035

2931 2849

1035

2931 2849

1035

AEM-12C

3200

2800

1500

1000

-1

Wavenumber (cm ) Fig. 7. FTIR spectra of the as-prepared membranes in comparison with PS and CMPS. 8.0

1.2

6.4

0.9

4.8

0.6

3.2

0.3

1.6

IEC(mmol/g)

Intensity

2

13.36

IEC(mmol/g) 2 Area resistance(Ω·cm )

Area resistance(Ω·cm )

1.5

AEM-1C AEM-4C AEM-8C AEM-12C

13.25

12.89

0.0

AEM-1C

AEM-4C

AEM-8C

AEM-12C

0.0

Membrane samples 2

4

6

8

10

-1

12

14

16

Fig. 10. IECs and area resistances of as-prepared AEMs from the CMPS with the same substitution degree but different amination schemes.

18

q (nm ) Fig. 8. SAXS profiles of AEM-1C, AEM-4C, AEM-8C and AEM-12C.

the alkyl side-chains drove the ionic clusters to be aggregated and the interconnected ionic channels to be generated which is expected to facilitate the transport of the counter ions. Besides the relatively low membrane resistance, the side-chain type

Fig. 9. The digital photos of the as-prepared AEMs, such as (a) AEM-1C; (b) AEM-4C; (c) AEM-8C and (d) AEM-12C. 7

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24

AEM-1C AEM-4C AEM-8C AEM-12C

Water uptake(%)

18 16 14 12 10 8 6 4 0.1M

0.5M

1M

2M

3M

Current efficiency(%)

20

100

Acid concetrationin concentrated chamber(mol/L)

22

0.36

0.32

80

60

40

20

0

AEM-1C

AEM-4C

AEM-8C

AEM-12C

0.28

AEM-1C AEM-4C AEM-8C AEM-12C

0.24

0.20 0

4M

50

100

150

200

Time(min)

Acid concentration in solution phase(mol/L)

Fig. 13. Concentration evolution with time in enrichment compartment.

Fig. 11. Water uptakes of AEM-1C, AEM-4C, AEM-8C and AEM-12C measured in HCl solutions of different concentrations.

imagined that the charged density in the formed ion nano-channel increased due to the confinement effect therein and then resulted in the enhancement of the Donnan exclusion for protons.

AEMs also exhibited a much lower water uptake than that of the AEM1C prepared by the conventional heterogeneous amination with the similar IEC, as shown in Fig. 11. Obviously, this phenomenon is inconsistent with our conventional understanding on the relation between the water uptake and IEC. Herein, the introduced hydrophobic side chains must contribute to strengthening the hydrophobic matrix in the as-prepared AEMs and then effectively restricts the water uptake behavior of the membranes. Similar results have also been observed during the preparation of fuel cell membrane with phase separation microstructure. At last, acid adsorption behaviors of the AEM samples were also examined in HCl solutions of different concentrations. As shown in Fig. 12, there exists an obvious acid-enriching phenomenon in membrane phase. With increasing ambient acid concentration, it can be observed that acid-enriching behaviors got weaker and even disappeared. Obviously, this should be ascribed to the characteristic affinity of AEMs for acid. Moreover, it was noticed that for the AEMs containing pendant side chains the adsorption saturation of acid seemed much easier than that of the conventional AEM-1C. Especially, for AEM-4C, it can be noticed the acid concentration of membrane phase was even much lower than that of ambient acid. It is reasonable to believe that above different acid adsorption behaviors should also be closely related to their different membrane microstructures. It can be

3.1.4. Acid block performance Subsequently, the as-prepared AEMs were attempted in a simulated application of concentrating HCl, respectively, to directly compare their work performances. As can be seen in Fig. 13, the concentrations of the acid enrichment compartments increased almost linearly with time. Furtermore, it was observed that the increasing rate of acid concentration in the membrane stack assembled with the side-chain type AEM is much more significant than the case of the conventional AEM1C. For example, the current efficiency of the corresponding ED process reached as high as 94% when AEM-4C was used, which substantially exceeded that of ED stack installed with AEM-1C, 68%. Actually, the outstanding acid block result was bit unexpected but also in there already. For example, it has been found from the above experiments that the as-prepared side-chain type AEM displayed simultaneously the lower water uptake, acid adsorption and even membrane resistance than those of the conventional AEM-1C. Then, taking the typical AEMs, such as AEM-1C and AEM-4C, as examples, the continuous ED-based acid enriching experiments were carried out for more than 12 h. It can be seen clearly from Fig. 14 that the acid concentration increased almost linearly initially, and then the concentration growth decreased gradually with time. In comparison

1.2

AEM-1C AEM-4C AEM-8C AEM-12C

Acid concetration in concetrated chamber (mol/L)

Acid concentration in membrane phase(mmol/g H2O)

1.4

1.0

0.8

0.6

0.4 0.1M

0.5M

1M

2M

3M

0.7

AEM-4C AEM-1C

0.6

0.5

0.4

0.3

0.2 0

4M

2

4

6

8

10

12

Time (h)

Acid concentration in solution phase(mol/L)

Fig. 12. Acid adsorptions of AEM-1C, AEM-4C, AEM-8C and AEM-12C measured in HCl solutions of different concentrations.

Fig. 14. Evolution of HCl concentration in concentrated chamber during ED process. 8

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4.8

1.8

3.6

1.2

2.4

0.6

1.2

AEM-4C1

AEM-4C2

AEM-4C3

AEM-4C4

0.0

IEC/(mmol/g)

Membrane samples

Fig. 16. IECs and area resistances of AEM-4C series.

further development of acid block AEMs. Hopefully, the IEC can be optimized for a simultaneous achievement of outstanding proton block performance and membrane conductivity. On account of the experimental results of the side chain length on the performances of as-prepared AEMs, the AEM-4C series were selected to investigate the effects of IEC in this work. As shown in Fig. 16, the corresponding membrane resistance decreased with the increase of membrane IEC as expected. In view of the important roles taken by water molecules in the proton transport, the water uptakes of the AEM4C series with different IECs were also measured and demonstrated in Fig. 17. Firstly, it can be seen clearly that the water uptakes of membranes decreased with increasing the ambient solution concentration to some extent due to the alteration of osmotic pressure. Moreover, it was noticed that water uptakes of AEMs increased correspondingly with the increase of membrane IEC. Especially, the water uptake took on a significant growth when the membrane IEC is high enough. On the other hand, the acid blocking performances of AEM-4C series was also reflected in terms of the current efficiencies of the ED process. As shown in Fig. 18, the corresponding current efficiency increased firstly and then decreased as the IECs went up. Especially, the current efficiency reached almost 100% when the as-prepared AEM-4C3 whose IEC is 1.401 mmol/g was assembled. Initially, the increase of side-chain density facilitated the formation of ion-nanochannels and made them more interconnected. Moreover, the corresponding increase of positively charged anion-exchange groups also effectively reinforced the Donnan exclusion for co-ions therein, namely H+. For example, when the IEC increased from 0.9 (AEM-4C1) to 1.289 (AEM-4C2), the

As known to all, increasing the IEC of membrane can effectively facilitate the transport of counter-ions and strengthen Donnan exclusion for co-ions at the same time. On the other hand, a higher IEC usually corresponds to a higher water uptake and then must be detrimental to the acid block performance of AEM. Now to examine and explore from different angles into the influence of IEC has practical significance for

20

AEM-1C AEM-4C

AEM-4C1 AEM-4C2 AEM-4C3 AEM-4C4 15

0.8

Water uptake(%)

Transport number

6.0

2.4

0.0

3.2. Influences of IECs

0.9

IEC/(mmol/g) 2 Area resistance(Ω·cm )

2

with the case of conventional AEM (AEM-1C), the membrane stack in which the newly developed side-chain type AEM (AEM-4C) was assembled displayed much higher concentration growth rate and final concentration. Obviously, this indicates that the unique micro-phase separated structure of AEM-4C indeed contributes to weakening not only the proton leakage but also the water migration. As far as we know, the series of weakly dissociated anion exchange groups, but not the conventional quaternary ammonium groups, have been usually used to reduce the water uptake of AEM and then weaken its proton leakage. As a result, the acid block property was endowed at the expense of sacrificing membrane conductivity, which has been confirmed by many experiments. However, according to the present work, what’s interesting is the confinement effect in formed ion-nanochannels due to the phase separation can not only contribute to the transport of counter-ions as a result of the improvement in membrane conductivity but also reduce the water uptake of membrane and enhance the Donnan exclusion for co-ions therein for the achievement of acid block property. This observation demonstrates an unprecedented result in the preparation of novel acid block AEM with outstanding membrane conductivity and anti-swelling property at the same time. Besides the effect of applied electric field, another driver for the proton leakage through AEM should come from the concentration difference between the enrichment and the dilution compartments. Especially, the concentration gradient rises relentlessly during a practical ED process and then the acid block task of AEM gets harder and harder. Accordingly, the typical AEM samples, such as AEM-1C and AEM-4C, were selected as studying objects to explore the influences of concentration gradient on the proton leakage behaviors of AEM. As expected, it can be deduced from the changes of their counter-ion transport numbers displayed in Fig. 15 that all their proton leakage phenomena became more and more serious as the concentration ratio increased from 5 to 20. However, it can’t be denied that the changes of transport numbers of AEM-4C were relatively small, which indicated AEM-4C indeed embraced a more outstanding acid block performance than that of AEM-1C. In fact, all these can also be predicted from their distinct acid adsorption and water uptake behaviors.

Area resistance(Ω·cm )

M.-y. Cong, et al.

0.7

10

0.6 5

0.5

5:1

10:1

15:1

20:1

0.1M

Concentration ratio

0.5M

1M

2M

3M

Acid concentration in solution phase(mol/L)

Fig. 15. Transport numbers of typical AEMs under different concentration ratios.

Fig. 17. Water uptakes of AEM-4C series. 9

4M

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Declaration of Competing Interest

80

0.40

Current efficiency(%)

Acid concetrationin concentrated chamber(mol/L)

100

0.45

The authors declared that there is no conflict of interest. 60

Acknowledgements

40

20

This research is supported in part by the National Natural Science Foundation of China (Award Nos. 21276245 and 21576249) and the Key Research and Development Program of Shandong Province (Award Nos. 2015GSF117018 and 2018GSF117043). This is a MCTL Contribution, No. 298.

0.35 0

AEM-4C1

AEM-4C2

AEM-4C3

AEM-4C4

0.30

AEM-4C1 AEM-4C2 AEM-4C3 AEM-4C4

0.25

0.20 0

50

100

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Time(min) Fig. 18. Concentration evolution with time in the enrichment compartment.

membrane resistance decreased from 5.61 to 1.49, whereas the corresponding current efficiency increased from 46.6% to 84.9%. However, it can be observed that the current efficiency was deteriorated with further increasing IEC. For instance, the corresponding current efficiency decreased from 99.4% to 80.7% as the IEC increased from 1.40 (AEM-4C3) to 2.27 (AEM-4C4). That indicated that the increase of water uptake due to the excessive amounts of charged side chains must result in the expansion of the ion-nanochannels and then the reduction of the fixed ion concentration therein. The above findings were consistent with our previous studies [20]. 4. Conclusions As confirmed by 1H NMR and FTIR, a series of quaternary ammonium PS containing long alkyl side chains pendant were synthesized and directly used for the preparation of AEM in this study. For comparison, another strong base AEM was also fabricated by means of the conventional heterogeneous quaternization. SAXS results showed that the ionic domains indeed formed a distinct and well-separated phase in the side chain type membranes. With the increase of side chain lengths, the microphase separation became so obvious that transparency of membrane was diminished. Some interesting confinement effects in the aggregated and the interconnected ionic channels resulted from the microphase separation were also observed, for examples, the facilitated transport of the counter-ions and reinforcement of Donnan exclusion for co-ions due to the increase of the charged density therein. Besides, the introduced hydrophobic side chains also contributed to strengthening the hydrophobic matrix in the as-prepared AEMs and then effectively restricted their water uptake behaviors. Noteworthy, the practical acid enriching experiments by ED showed as-prepared AEMs indeed displayed excellent acid block performance. For example, the corresponding current efficiency reached almost 100% under certain condition when the novel AEM was assembled, which substantially exceeded that of ED stack installed with conventional AEM, 68%. Subsequently, the exploration on the influence of IEC indicated that there exists an optimized IEC for the as-prepared AEM which embraced the outstanding acid block performance and membrane conductivity simultaneously. As anticipated, the higher the IEC was, the lower membrane resistance and the higher water uptake were observed. However, an excess IEC will result in an excess increase of water uptake and then deteriorate acid block performance. Nevertheless, this observation represents an unprecedented result that the novel strong base AEM with both outstanding acid block performance and membrane conductivity can be achieved by means of the design of membrane material and its membrane morphology, the formation of phase separation microstructure. 10

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