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Synthesis and Characterisation of A Ultrahigh Ion-Exchange Capacity Polymeric Membranes Weifeng Zhao, Chao He, Chuanxiong Nie, Shudong Sun, and Changsheng Zhao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02770 • Publication Date (Web): 26 Aug 2016 Downloaded from http://pubs.acs.org on August 31, 2016

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Industrial & Engineering Chemistry Research

Synthesis and Characterisation of A Ultrahigh Ion-Exchange Capacity Polymeric Membranes Weifeng Zhao,a,b Chao He,a Chuanxiong Nie,a Shudong Sun*a and Changsheng Zhao**a

a

College of Polymer Science and Engineering, State Key Laboratory of Polymer

Materials Engineering, Sichuan University, Chengdu 610065, People’s Republic of China b

Fiber and Polymer Technology, School of Chemical Science and Engineering, Royal

Institute of Technology (KTH), Teknikringen 56-58, Stockholm, SE-10044, Sweden. *Corresponding author. E-mail: [email protected](*), [email protected] (**) Tel.: +86-28-85400453; Fax: +86-28-85405402.

Keywords: mold casting, membrane, high ion exchange capacity, chemical stability

Abstract: A universal mold casting approach for the preparation of cation exchange membranes (CEMs) and anion exchange membranes (AEMs) with ultrahigh ion-exchange capacities (IECs) is developed based on in-situ cross-linking polymerization of acrylic acid (AA) and 2-vinyl pyridine (2VP), respectively. This new method produced ultrahigh IECs of 7.88 mequiv g-1 for CEM and 6.27 mequiv g-1 for AEM, which are 8.8 and 7.0 times that (0.89 mequiv g-1) of Nafion 117, respectively. Also, the prepared membranes demonstrate excellent thermal and chemical stability, and acceptable conductivity. As a consequence, the membranes show relatively high performance for ion-exchange application and methanol barrier, exhibiting the ions permeabilities of 2.06×10-7 cm2 sec-1 for Na+, 2.57×10-7 cm2 sec-1 for Ca2+, 1.45×10-7 cm2 sec-1 for Cu2+ regarding CEMs and 7.72 ×10−7 cm2 sec-1 for methanol regarding AEMs. These results indicate that the CEMs and AEMs fabricated 1

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from the universal mold casting approach are promising candidates for targeting ultrahigh ion exchange capacity membranes.

1. Introduction Ion-exchange membranes (IEMs) have been widely used in various industries because of their superior selectivities for specific ions and wide applicability in fuel cells, purification of seawater, wastewater treatment, electrochemical dialysis, production of ultra-pure water and other purposes.1-5 There is scientific and technical interest in the development of polymeric ion-exchange membranes with high performance, for instance, high ion-exchange capacity (IEC) is strongly desired. The state-of-the-art perfluorosulfonic acid ionomer membranes, i.e. Nafion® membranes (a commercial product of DuPont Co.) in the late 1960s,6 possess good chemical and electro-chemical stabilities and high proton conductivity.7,

8

In general, most

sulfonated polymers do not possess all the required properties for an excellent CEM, because of the trade-off between IEC improvement and enhanced swelling of membranes after sulfonation.9, 10 Herein, a universal mold casting approach towards fabricating IEMs with ultra-high IECs but low swelling behavior is reported. Over the past decades, the growing interest is to improve the IEC of IEMs to meet specific applications. At the beginning, cross-linked Daramic (W.R. Grace, USA) and poly (sodium 4-styrene-sulfonate) composite membranes exhibit an IEC of 1.03 mequiv g-1.11 Shahi et al. develop urethane acrylate composite ion-exchange membranes and their IECs can reach up to 1.701 mequiv g-1.12 Recently, chloromethylation and quaternization reactions of the precursor copolymers 2


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composed of perfluoroalkylene and phenylene groups produce the IEMs with IECs ranging from 0.79 to 1.74 mequiv g-1.13

More recently, the composite membrane of

uncharged polyphenylsulfone (PPSU) and functionalized poly(phenylene oxide) polyelectrolyte yields an effective IEC of 2.8 mequiv g-1.14 Apparently, the blending approach for improving IECs is far from people`s satisfactory. Thereafter, the research turns the direction towards grafting method. The grafting approach can reach the same level of IECs of copolymers.

For example, side-chain-type ion exchange

membranes are prepared by grafting poly (ether ether ketone) (PEEK) containing propenyl groups with sodium sulfonic styrene (StSO3Na) and KH570. The IECs of the membranes range from 2.27 to 2.50 mequiv g-1 and the water content range from 107.2 to 126.1%, with both the parameters increased with increasing the StSO3Na grafting degree.15 Hamada et al. find PEEK-based graft-type polymer electrolyte membranes prepared by radiation-induced graft polymerization have the maximum IEC of 3.08 mequiv g-1.16 A series of partially fluorinated, radiation-grafted proton-conducting membranes based on tetrafluoroethylene-g -polystyrene sulfonic acid are fabricated with the IEC of 3.22 mequiv g-1.17 including

poly(vinylidene

Recently, fluoropolymers

fluoride-co-hexafluoropropylene)

and

poly-(ethylene-co-tetra fluoroethylene) are selected as matrix polymers. Either acrylic acid or 4-vinylpyridine is grafted onto fluoro-polymer films, resulting in the preparation of poly(acrylic acid) (PAA)- and poly(4-vinylpyridine) (P4VP)-grafted membranes, respectively. The P4VP-grafted ones, have much larger IEC values compared to those of anion membranes in the literatures (0.19 mequiv g-1 for Aciplex 3



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A-172

18

and 1.23 mequiv g-1 for PFAM 3020

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19

). The PAA-grafted PVDF-co-HFP

membrane has a maximum IEC of 9.53 mequiv g-1 and the P4VP-grafted PE-co-TFE one has the high IEC of 3.31 mequiv g-1.20 However, several drawbacks are behind: 1) the grafting method is restricted by the dimension of polymeric membranes; 2) the fabrication of IEMs needs several steps and tedious purification; 3) high energy of rays has the risk of sacrificing membrane physical/chemical properties. The gap of IECs between laboratory and industry has not been filled. For example, the PFSA-Li polymer with high IEC (1.18 mequiv g-1) is used as the ionomer, and Cai et al. claim that the use of lithiated perfluorinated sulfonic ion-exchange membranes swollen with non-aqueous solvent is a promising approach to the development of future polymer electrolytes.21 Despite previous efforts, a method for fabricating IEMs with both increasing IEC property and facile synthesis processes has not yet been developed. This challenging task might be realized if the negative/positive-charged functional groups are abundantly available to provide high IECs but not with high swelling degree via conventional synthesis methods. Thus, we hypothesize that universal mold casting approach might address the aforementioned challenge. In other words, it would be desirable to use mold casting cross-linking negative/positive-charged monomers in the presence of hydrophobic polymers, thereby, cross-linking monomers provide high IECs while restricting the swelling of these hydrophilic monomers via high cross-linking density and hydrophobic polymers. In this work, we choose AA for fabricating cationic-exchange membranes, 2-vinylpyridine (2VP) for fabricating anionic-exchange membranes, and polyethersulfone (PES) as a hydrophobic polymer 4


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22

to limit the high swelling of cross-linked PAA and P2VP. The primary advantage of

AA and 2VP is that they exhibit high charge per unit with relatively low molecular weight and then contribute to high IECs. The universal mold casting approach towards ultra-high IEC polymeric membranes is schematically illustrated in Scheme 1. In fact, the principle of in-situ cross-linked polymerization has already been employed to prepare various functional membranes in our previous study.23 However, a solution casting onto glass plate after polymerization is indispensable to fabricate the membranes. To simplify the process, mold casting is employed in the current study.

Scheme 1 a) Chemical structures of PAA and P2VP. These two polymers are separately used in the universal mold casting process to yield high IEC polymeric membranes. b) Schematic diagram showing the fabrication process of high IEC composite membranes by a universal mold casting process.

2. Experimental section 2.1 Materials Acrylic acid (AA, 99%, Aladdin), 2-vinylpyridine (2VP, 99%, Aladdin), ethyleneglycol dimethacrylate (EGDMA, 99%, Aladdin), azobisisobutyronitrile 5



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(AIBN, 99%, Aladdin) and N, N-dimethylacetamide (DMAc, 99.8%, Kelong) were used as received. Polyethersulfone (PES, Ultrason E6020P, and CAS No. 25608-63-3) was purchased from BASF chemical Co. (Germany). Ultrapure water was used throughout the study. 2.2 Preparation and characterization 2.2.1 Mold casting towards polymeric membranes PES (2 g) and EGDMA (4 g) were dissolved in DMAc (18 g) to form a transparent solution, after which a certain amount of monomer AA or 2VP was added with 0.8 wt. % of AIBN to monomer. The mixture was stirred for 6 h and then left unstirred for 2 h to remove the bubbles from the solution. The solution was casting into a clean glass mold (Scheme 1) and in situ thermally treated at 70 °C for 12 h to obtain cross-linked membranes. The received membranes were immersed into ultrapure water, resulted in white solid membranes via a phase inversion technique. The solvent and un-reacted reagents could be removed during liquid-liquid phase inversion process. The membranes were immersed in de-ionized water (changing the water frequently) for 2 days. The obtained high IEC polymeric membranes were denoted as CEM-x (AEM-x), where x is the weight of AA (2VP) in casting solution. The thickness of the prepared membranes was controlled by the glass mold, measured using cross-sectional imaging on an optical microscope and confirmed by a micrometer with 0.5 N applied force. 2.2.2 Characterization

6


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Attenuated total reflection Fourier transform infrared spectroscopy: ATR-FTIR analysis was performed on a Nicolet 560 instrument, equipped with a single-bounce attenuated total reflection cell, a detector, and a ZnSe single crystal. All IR spectra were collected at 4 cm-1 resolution, with 32 scans, and within a range between 4000 and 600 cm-1. Thermogravimetric analysis:

Prior to thermal analysis, polymeric membranes

were pre-dried at 30 °C for at least 24 h in a vacuum oven. TGA of specimens was performed on a TG209F1 (Netzsch Co., Germany).

At first, the samples were heated

from ambient temperature to 700 °C at a rate of 10 °C min-1, protected by dry nitrogen under atmospheric pressure. Then the resulting data were subsequently compiled to generate TG and derivative thermogravimetric (DTG) curves. Dynamic mechanical analysis: For the mechanical property test, the obtained membranes were tested by using a dynamic mechanical analyzer (DMA, Q800 USA). The membrane samples (4 mm × 50 mm, width×length) were measured under a ramp rate of 1 N min-1 in the tensile mode at 25 °C. Scanning electron microscopy: The microstructure and chemical composition of the IEMs were characterized by SEM (Quanta-250, FEI, USA) and EDX attached to the SEM. SEM images were obtained after coating the composite membranes with 5 nm of gold layer. Cross-sectional images were obtained by cryo-fracturing composite membranes in liquid nitrogen. Solid Content (%) means the portion of reserved materials after cross-linking reaction, which indirectly reflecting the amount of functional materials in case the 7



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amount of PES is fixed. The Volume Change (%) was calculated using the formula of

△V= (LsWsHs-LdWdHd)/LdWdHd×100, where Ls,Ws and Hs stand for the length, width and height of the IEMs in swollen state, respectively; Ld, Wd and Hd stand for the length, width and height of the IEMs in dried state, respectively. Water uptake: The water uptake of specimens was determined by the weight difference between the wet membrane (Wwet) and the dry membrane (Wdry). Initially, the dry weight of the membranes was determined, and then the samples were immersed in de-ionized water at room temperature for at least 24 h. Subsequently, the wet weight was measured after the water was rapidly wiped off from the specimen surface with filter papers. The water uptake can be calculated using the following equation:

Water uptake wt. % =

× 100%

(1)

The swelling ratio (SR) of the hydrogels was determined by immersing dry membranes in PBS at room temperature. The weights of the samples in the swollen state (ms,t) at equilibrium points were measured after gently removing excess water with filter paper. The SR was calculated using Equation (2), where md denotes the weight of the samples in the dry state 24, 25:

% =

,! − # × 100 2 #

Conductivities: Conductivities (σ) of the polymeric membranes at 25 °C were performed on a CS350 (Wuhan CorrTest Instrument Corporation, Wuhan, China) electrochemical workstation coupled with a computer. The conductivity cell consisted of two stainless steel electrodes with the area of 0.63 cm2. Prior to performing the 8


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electrical resistance measurement, the ion-exchange membranes were immersed in a 0.5 M NaCl solution overnight. Afterwards, the membranes were removed from the flask and sandwiched between the stainless steel electrodes at 25 °C and 100 % RH. The impedance spectra of the polymeric membranes were recorded from 100 kHz to 0.1 Hz. The x-intercept of each impedance plot was taken to be the ionic resistance of the electrolyte. The conductivity was calculated using following equation 26:

σ=

&

(3)

'(

where σ, L, R, and A represent the conductivity, the thickness, the measured resistance, and the cross-sectional area of the membrane perpendicular to the current flow, respectively. IEC: The membranes (about 0.02-0.05 g) were conditioned by repeated alternating treatments with 1 M HCl and 1 M NaOH, and then washed in-between with water. In the final step, the membranes were converted into free base form by a treatment with 1 M NaOH followed by a thorough washing with deionized water. The membranes were placed in a dilute (~0.05 M) HC1 solution of known volume and concentration for 12-16 h. The volume of the solution was so adjusted to contain approximately 50% more HCl than the required for the theoretical ion exchange capacity calculated from the sample mass gain. The membranes were removed from the solution and held above it while the sample surface was rinsed with deionized water. The samples were dried in an oven at 80 ºC for 48 h, weighed and marked as md. The residual solution was titrated with NaOH and the ion exchange capacity (IEC) was calculated from the following relationship 27: 9



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IEC =

,-./ 0-./ ,123- 01234

× 1000

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(4)

where VHCl and V NaOH were the volumes of HC1 and NaOH solutions, respectively; and NHCl and NNaOH were the concentration of HC1 and NaOH solutions, respectively. Methanol permeability: Methanol permeability values were determined by using a dual glass diffusion apparatus (As shown in Fig. S1), where the membrane sample with an effective area of 19.63 cm2 was clamped between the two compartments (separately contained 0.1 M methanol solution and water), and the chambers were stirred. The increase in methanol concentration of the water as a function of time was determined by a gas chromatograph (GC-6890A, Aglient,USA) equipped with a thermal-conductivity detector. The methanol permeability (P) was calculated using the following equation: 56 7 58

77;

= AP ,

(5)

6&

where CB (t) is the methanol concentration measured in the receiving compartment as a function of time, VB the volume of receiving compartment, and L and A are the thickness and effective area of membrane, respectively. P is the methanol permeability and can be determined from the slope of the plot of methanol concentration in the receiving compartment versus time. Ion permeability: The ion permeability of Na+, Ca2+ and Cu2+ was determined and calculated the same as methanol permeability. The initial concentrations of Na+, Ca2+ and Cu2+ are 0.1 M. Their concentrations in the receiving compartment were monitored by atomic absorption spectroscopy (SpectrAA 220FS, VARIAN, USA).

10


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Stability Test: The oxidative stability of the high IEC polymeric membranes under strong oxidizing condition was evaluated in Fenton’s reagent, and the alkaline stability of the IEMs was carried out using 1M NaOH. Small pieces of membrane samples of known weight were soaked in Fenton’s reagent (3% H2O2 containing 3 ppm FeSO4) or 1M NaOH at 80 ºC for 10h. Oxidative and alkaline stability was evaluated by weight loss of the samples.

3. Results and Discussion 3.1 Cation-exchange membranes (CEMs)

In the present paper, CEMs were fabricated by in-situ cross-linking polymerization of AA in the presence of hydrophobic PES. The cross-linking degree and the content of cationic AA groups were tuned via changing the content of AA. The membranes prepared from different contents of AA were referred to as CEM-X, where X is the weight (g) of AA in CEMs, as shown in Table 1. The prepared membranes are schematically represented in Scheme 1. The Fourier transform IR (FTIR) spectra of the CEMs prepared from different AA concentrations are displayed in Fig. S2 (Supporting information). The absorption band at 1726 cm-1 in the spectrum of CEMs is attributed to the characteristic absorption of the C=O in PAA, confirming the successful introduction of carboxyl groups in CEMs. The contents of the element S, confirmed by energy-dispersive X-ray spectroscopy (EDX), are 2.39, 1.48 and 1.45% in CEM-2, CEM-4 and CEM-8, respectively. The values decreased with the increase of PAA content as expected. The element S is well-distributed in

11



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CEMs, reflecting a good distribution of PES, as shown in Fig. 1 (a). The 2D distributions of C, O and S in CEMs are homogenous (See Fig. S3, S4 and S5, supporting information). As a result, PAA has been well incorporated to CEMs by using mold casting approach. Interestingly, approximately 200 nm of holes appears in the surface of CEM-2, while they do not exist in those of CEM-4 and CEM-8, as shown in Fig. 1 (b). It is caused by the poor miscibility

of

hydrophilic polymers (PAA in this case) and hydrophobic PES.28 The increasing content of PAA leads to the improving miscibility of PAA and PES, thus the macro phase separation is suppressed. Table 1 Characteristics of prepared CEMs Membrane system

Nafion 117

CEM-2

CEM-4

CEM-8

AA (g)

--

2

4

8

Solid content (%)

--

30.8

35.7

43.8

Thickness (µm)

183

173

171

197

Water uptake (%)

16

197

181

118

Length change (%)

1.2

4.2

7.2

2.7

Width change (%)

3.6

18

14

9.5

Height change (%)

2.0

8.7

4.7

12.7

Volume change (%)

6.9

33.6

28.0

26.7

12


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Fig. 1 3-demensional distribution of the element S crossover the CEMs (a), the surface morphology of CEMs (b, ×10000), and the enlargements of the surface of CEMs (b, right corner, ×30000).

The resultant chemical/physical structures are strongly associated with the performance of CEMs, such as water uptake and swelling. The thickness, water uptake and swelling data of the prepared CEMs are shown in Table 1. The consistent thickness of the CEMs indicates the well-controlled performance of mold casting method. The water uptake of the CEMs decreases from 197 to 118 % with increasing AA content while the swelling of the prepared membranes does not change significantly with increasing AA content due to the compact structure of CEMs (Fig. 1a). Cross-linked PAA is commonly used as the superabsorbent attributed to its high swelling. In order to prevent the high swelling degree, the strategy in current study is using high concentration of cross-linker (ethylene glycol di-methacrylate). The swelling ratios of corresponding hydrogels without PES are only 2.3, 1.1 and 0.7 for CEM-2, CEM-4 and CEM-8, respectively. Therefore, the low volume change of the CEMs are due to the combined effects from high cross-linking and hydrophobicity. In other words, with higher content of hydrophilic AA groups, more water is expected to be absorbed, while the high cross-linking degree derived from the high content of AA groups and the hydrophobic PES will suppress the swelling.

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The high content of AA groups benefits the high IECs. Fig. 2 shows the IECs of the CEMs in both theoretical and titration values. We fabricated cationic exchange membrane with ultra-high IEC as high as 7.88 mequiv g-1. The IECs is about 8-9 times greater than that of commercial Nafion 117 (~0.89 mequiv g-1).20 Both the titration values and theoretical calculations of IECs increase with increasing the content of AA. The titration values of IECs are a little bit less than theoretical calculations, since the little amount of unreacted functional monomers is eluted during phase separation process.

Fig. 2 Ion-exchange capacities (IECs) of CEMs: Theoretical IECs (in black pattern) and titration IECs (in light blue pattern).

The incorporation of high content of AA into CEMs also leads to an increasing conductivity and low permeability. The conductivity of the CEMs is summarized in Table 2. As the AA content increasing, the conductivity increases from 2.23 to 3.25 mS cm-1 due to the increased content of acrylic acid groups and hence of the IEC. Table 2 also shows the sodium (Na+), calcium (Ca2+) and copper (Cu2+) ion permeability of the CEMs. All the CEMs

14


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exhibited the low order of magnitude (10-7) of permeability among all ions. These low values of the integral permeability confirm that a high capacity ion-exchange membrane between diluted solutions acts as a barrier to suppress the electrolyte diffusion. Taken the permeability of Cu2+ as an example, at the beginning, the solution containing Cu2+ is blue at one side of the CEMs, and the pure water container is transparent at another side. The water container keeps its transparency after several days, while the CEMs turn blue due to the adsorbed Cu2+ ions (See Fig. S6 and S7, supporting information). This result is mainly due to two factors: (1) owing to the Donnan exclusion effect, the existence of negatively charged carboxyl groups prevents the cationic ions permeation through the membrane and (2) the well-connected cross-linking and compact networks (as shown in Fig. 1) greatly restrict the cationic ions permeation as well.29 With increasing the acrylic acid content, the permeation of cationic ions and methanol hardly changes. Therefore, the combined effect of cross-linking and the hydrophilic nature of the acrylic acid groups leads to the membranes with acceptable conductivity and low permeability against Na+, Ca2+ and Cu2+ ions, as well as methanol. Compared to CEMs, Nafion 117 has higher conductivity, and better selectivity between Na+ and Ca2+/Cu2+. It is because that the multivalent cationic ions have the larger size in comparison with that of monovalent ones.30 It has been found that counter-ions with larger electrostatic attraction toward ion-exchange membranes showed reduced mobility through the membrane.31, 32 15



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Table 2 Conductivity, and ion and methanol permeabilities for the CEMs. Membrane system

Nafion117 CEM-2 CEM-4 CEM-8

Conductivity (mS cm-1)

8.41

2.23

2.69

3.25

Na+ permeability (10-7 cm2 sec-1)

1.82

2.46

2.31

2.06

Ca2+ permeability (10-7 cm2 sec-1)

11.20

3.39

2.66

2.57

Cu2+ permeability (10-7 cm2 sec-1)

9.30

4.67

1.53

1.45

Methanol permeability (10-7 cm2 sec-1)

5.05

2.80

2.67

2.51

It is well-known that the cross-sectional and surface morphology and the chemical compositions of CEMs are contributed to their ion permeability as well as their chemical stability. The chemical stability of membranes is of much concern to the lifetime of IEMs. For CEMs, the membranes could consume certain amounts of HCl, therefore, a strong NaOH solution as alkaline media was used to test the chemical stability of CEMs. Fenton`s reagents are also commonly used for testing the chemical/oxidative stability of IEMs. In situ formed H2O2 and •OH or •OOH radicals from its decomposition are believed to attack the hydrogen containing bonds in polymer membranes. This is assumed to be the principal degradation mechanism of common PEMFC membranes. Experimentally, the generation of these radicals can be achieved by Fe2+/Fe3+ catalysed H2O2 decomposition. Based on this method, the so-called Fenton test is used for the stability evaluation of PEMFC membranes.33 In general, cation exchange membranes (CEMs) are more durable than anion exchange membranes (AEMs) in terms of both thermal stability and chemical stability in strongly alkaline solutions since the quaternary ammonium groups in AEMs 16


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tend to decompose at elevated temperatures and in highly concentrated alkali solutions.34 Herein, we investigate the stability of the membranes under alkaline condition (1M NaOH) and oxidative condition (Fenton`s reagent), commonly accepted harsh circumstance for the membrane application. Under alkaline solution, there are almost no weight changes before and after the treatment for all CEMs (data are not shown). In addition, the weight loss of CEMs in Fenton`s reagent is also low with the maximum of 3.4% for CEM-8, as shown in Fig. 3. The results proved that the CEMs have high chemical stability over alkaline and oxidative solutions, which are due to the highly cross-linked network and compact structure (see Fig. 2) of the CEMs. The cleaning-in-place (CIP) procedure is also commonly used in industrial electrodialysis stacks.35-37 Membranes underwent the following cycle: 1) 30 min in 0.1 M HCl at 40 °C under stirring; 2) Rinsing with deionized water at room temperature; 3) 30 min in 0.1 M NaOH at 40 °C under stirring; 4) Rinsing with deionized water at room temperature before restarting the cycle. In Garcia-Vasquez`s work, the indicators for measuring the chemical stability could be conductivity, FTIR, SEM, extracted content and so on. A combination of these measurements are promising contribution in this field.

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Fig. 3 Oxidative stability (weight loss, %) for developed CEMs.

Besides the chemical stability, the thermal stability plays an important role for the application of CEMs. The thermogravimetric analysis (TGA) curves of the CEMs are shown in Fig. S8, S9 and S10 (Supporting information). The temperature of the PES/PAA pyrolysis evidences that the present mold casting approach does not decrease the thermal stability of the ion exchange membranes, although the thermal stability of cross-linked PAA is lower. The thermal-oxidative stability is good at least up to 200 °C for polymer electrolyte membranes.38 In the current study, the PES/PAA membranes can be thermally stable up to 304 °C and will be stable enough within conceivable temperature range of proton exchange membrane fuel cell (PEMFC) application. The introduction of high content of AA affects the mechanical property of CEMs as well. Stress-strain curves of free-standing CEMs are shown in Fig. 4. The wet, free-standing CEMs exhibit elastic-plastic behavior and yield stress values ranging from 0.02-0.35 MPa depending on the processing conditions. Mold casting approach at higher PAA concentration forms a softer network due to the flexible segments of 18


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PAA, resulting in membranes with higher elastic-plastic behavior and lower yield stress values than the membranes cross-linked with lower PAA concentrations, which form highly cross-linked materials.

Fig. 4 The mechanical property (strain-stress) of CEMs. 3.2 Anion-exchange membranes (CEMs) Same as the fabrication of CEMs, AEMs were also prepared by in-situ cross-linking polymerization of the monomer (2VP in this case) in the presence of hydrophobic PES. 2VP is the functional material serving anion-exchange capacity. The FTIR spectra of the AEMs prepared from different 2VP concentrations are displayed in Fig. S11 (Supporting information). The peaks at 1589 and 1473 cm-1 are attributed to the C=C– and C=N– bonds from pyridine in P(2VP),39 confirming the successful introduction of pyridine groups in AEMs. The contents of the element S and N, confirmed by energy-dispersive X-ray spectroscopy (EDX), are 1.41 and 0.93 in AEM-4, 3.37 and 2.09 in AEM-8, and 3.27 and 6.71 in AEM-12, respectively. The increasing content of S is due to the aggregation of PES with the increase of P2VP content,

19



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especially in the area close to the surface of the AEMs (Fig. 5 a). With the increase of P2VP concentration, the hydrophilicity of the composite membranes increased, leading to a higher rate of solvent exchange, and further resulting in the aggregation of PES. The content of N increases with the increase of P2VP content as expected, and it is well-distributed in the AEMs as shown in Fig. 5 (b). The 2D distributions of C, O, N and S in AEMs are shown in Fig. S12, S13 and S14 (Supporting information). As a result, P2VP has been well incorporated to AEMs by using mold casting approach. Contrary to the CEMs, No large holes appear in the surface of the AEM, while the roughness of the surface increases with the increase of P2VP, as shown in Fig. 5 (c). The compact cross-section and rough surface in polymeric membranes affects their swelling, permeability and chemical stability.

Fig. 5 3-demensional distribution of the elements S (a, yellow dots) and N (b, white dots) crossover the AEMs, and the surface morphology of AEMs (c).

20


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Table 3 lists the compositions, thickness, water uptake and the changes in three dimensions of the AEMs with different P2VP loadings. As the P2VP loading increases, the membrane thickness keeps consistence, indicating the well-controlled fabrication of the AEMs via the universal mold casting approach. The water uptake of pure Nafion 117 membrane is 16 wt. %, which is comparable to previous literatures.40, 41 The water uptakes of the AEMs are larger than that of Nafion 117 due to the higher amount of the hydrophilic polymer of P2VP. In addition, the water uptake of the membrane is substantially reduced with increasing the P2VP loading, and this effect is related to the increased solid content of the P2VP in AEMs. Due to the highly cross-linked network, the swelling ratios of corresponding hydrogels without PES for AEM-4, AEM-8 and AEM-12 are only 1.7, 0.8 and 0.6, respectively. Therefore, the low volume change of the CEMs is due to the combined effects from the highly cross-linked (compact) network and hydrophobicity. Compared to Nafion 117, the CEMs has higher dimensional changes due to their higher swelling ability. Table 3 Characteristics of the prepared AEMs. Membrane system

Nafion 117

AEM-4

AEM-8

AEM-12

2VP (g)

--

4

8

12

Solid content (%)

--

35.7

43.8

50

Thickness (µm)

183

180

172

173

Water uptake (%)

16

125

112

108

Length change (%)

1.2

7.6

7.4

4.3

Width change (%)

3.6

5

14.5

2.5 21



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Height change (%)

2.0

3.5

5.6

6.3

Volume change (%)

6.9

17.0

30.3

10.9

As shown in Fig. 6, the AEMs composed of P2VP, have much larger IEC values compared to those of anion membranes in the literatures (0.19 mequiv g-1 for Aciplex A-172

18

and 1.23 mequiv g-1 for PFAM 3020

19

). Both the

titration values and theoretical calculations of IECs increase with increasing the content of 2VP. The IECs determined by titration were slightly lower than the theoretical IECs, probably due to the elution of little unreacted 2VP. Jho et al. fabricate 4-vinylpyridine-grafted poly-(ethylene-co-tetrafluoroethylene) via radiation-grafting, receiving an IEC range of 3.45-5.93 mequiv g-1.20 However, the γ-rays have the risk of damaging the membranes, and the reported method is limited by the shape of membranes and the tedious purification process.

Fig. 6 Ion-exchange capacities (IECs) of AEMs: Theoretical IECs (in black) and titration IECs (in light blue).

The increasing incorporation of P2VP not only results in the increasing IECs but also benefits the increasing conductivity. The conductivity and methanol 22


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permeability of the AEMs are shown in Fig. 7. The conductivity of the membrane initially increases and reaches its maximum (3.66 mS cm-1) at 12 g of P2VP loading. Moreover, the conductivities of the composite membranes are all comparable with that of Nafion 117 membrane (4.95 mS cm-1), which indicates that the increase of the hydrophilic P2VP amount results in high IECs and assists in conductivity. On the other hand, methanol permeability of the membrane increases continuously with increasing the P2VP loading but keep at a certain level. As shown in Fig. 7, the AEM-4 membrane possesses the lowest methanol permeability of 7.72 ×10−7 cm2 s-1 which is about three times lower than that of Nafion membrane (2.3×10−6 cm2 s-1) as previously reported

40

. The

increased methanol permeability of the composite membranes might partially arise from the higher volume changes (Table 1), increased hydrophilicity, and the morphology changes as indicated in Fig. 5 (SEM images).

Fig. 7 Conductivity (black spherical marker) and methanol permeability (blue square marker) of AEM-4, AEM-8 and AEM-12.

Chemical stability depends on the durability of the membrane in various oxidative or alkaline solutions. The alkaline stability of membranes for AEMs 23



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is commonly investigated.2 The same as that of CEMs, their weights exhibited no significant differences before and after the treatment of 1M NaOH aqueous solution. The weight losses of the AEMs have a peak level of 2.7% for the AEM-12 membrane (Fig. 8). In conclusion, the AEMs have both high alkaline and oxidative stability, which probably due to the highly cross-linked networks, and had a wide spectrum of applications.

Fig. 8 Oxidative stability (weight loss, %) for the AEMs.

Besides the chemical stability, the thermal stability (thermogravimetric analysis, TGA) of AEMs is also evaluated as shown in Fig. S15, S16 and S17 (Supporting information). The temperature of the PES/P2VP pyrolysis evidences that the present mold casting approach does not decrease the thermal stability of the polymeric membranes. The PES/P2VP membranes can be thermally stable up to 340 °C and will be stable enough within conceivable temperature range for an alkaline-type direct ethanol fuel cell (DEFC). Stress-strain curves of free-standing AEMs are shown in Fig. 9. The wet, free-standing AEMs exhibit elastic-plastic behavior and yield stress values ranging 24


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from 0.01-4.45 MPa depending on the processing conditions. Mold casting approach at higher P2VP concentrations forms a more compact network due to the effective crosslink density, resulting in the membranes with lower elastic-plastic behavior and higher yield stress values than the membranes

cross-linked with lower P2VP

concentrations, which form highly cross-linked, rigid materials. Overall, the yield stress values of the AEMs are lower than those of Nafion (12 MPa)

42

and other

polymeric membranes (40 MPa).43

Fig. 9 The mechanical property (strain-stress) of AEMs. There are some results of polyethersulfone (PES) membranes for ion-exchange membranes by other techniques. Matsumoto et al. prepared high branched PES by Freidel-Craft acylation using FeCl3 as the catalyst, and then films were prepared by casting from a solution of the hyperbranched sulfonyl chloride and 20 wt. % linear polymers in DMAc. The solution also contained a small amount of FeCl3 to catalyze the reaction, and the linear polymer was very sensitive to protic moieties in solution.44 Compared to our method, the catalyst was used in the synthesis procedure and film fabrication, thus the purification of catalyst might take tedious work to guarantee a 25



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Page 26 of 34

high purity of IEC membranes. Quellmalz et al. fabricated bipolaren membrane by a layer-coating method.1, 45 When solution of sulfonated PES was used for the cation selective layer and chloromethylated PES as precursor for the anion-exchange layer, the solvent of the second layer should be a non-solvent for the material of the first layer. The proposed mold casting approach is one-pot and one-step method, which might save the energy and labor work for fabricating the ion-exchange membranes.

4. Conclusions CEMs and AEMs were designed and successfully fabricated via mold casting for ion-exchange and methanol fuel cell applications. SEM observations of the CEMs and AEMs convinced that these membranes had a well-connected internal cross-linking structure. The CEMs presented a low ion and methanol permeabilities in an order of magnitude of 10−7 cm2 s-1, and the AEMs also exhibited low methanol permeability. All the IEMs provided an excellent chemical stability against strong alkaline solution and Fenton`s reagent. The introduction of positively charged pyridine and negatively charged carboxyl groups can effectively provide high ion-exchange capacities, which reach the maximum of 7.88 mequiv g-1 for CEM and 6.27 mequiv g-1 for AEM, respectively. As a result, the ion-exchange membranes show ultrahigh ion-exchange capacity, and keep the low swelling, low water uptake and acceptable conductivity. This work could thus provide a strategy toward the design and fabrication of high performance CEMs and AEMs.

26


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ASSOCIATED CONTENT Supporting Information. Methanol permeability test cell. ATR-FTIR of CEMs and AEMs. 2-demensional distribution of C, O and S in CEM-2, CEM-4 and CEM-8. 2-demensional distribution of C, O, N and S in AEM-4, AEM-8 and AEM-12.The permeability test cells for Cu2+ after 24 h. The CEM-8 after 24 h of Cu2+ permeability test. TGA and DTG curves for CEMs and AEMs. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Tel.:+86-28-85400453. Email: [email protected], [email protected].

Author Contributions Dr. Weifeng Zhao and Dr. Chao He contributed equally to this work. All authors have given approval to the final version of the manuscript. Funding Sources National Natural Science Foundation of China (No. 51225303, 51433007 and 51503125)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially sponsored by the National Natural Science Foundation of China (No. 51225303, 51433007 and 51503125). We thank Prof. Xiaobo Liu and Dr. Mengna Feng from University of Electronic Science and Technology of China for help measuring ion conductivity of polymeric membranes. 27



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table of contents (TOC)

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