Imidazolium-Functionalized Poly(arylene ether sulfone) Anion

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Imidazolium functionalized poly (arylene ether sulfone)s anion exchange membranes densely grafted with flexible side chains for fuel cells Dong Guo, Ao Nan Lai, Chen Xiao Lin, Qiugen Zhang, Aimei Zhu, and Qinglin Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07711 • Publication Date (Web): 31 Aug 2016 Downloaded from http://pubs.acs.org on September 6, 2016

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ACS Applied Materials & Interfaces

Imidazolium functionalized poly (arylene ether sulfone)s anion exchange membranes densely grafted with flexible side chains for fuel cells Dong Guo, Ao Nan Lai, Chen Xiao Lin, Qiu Gen Zhang, Ai Mei Zhu, Qing Lin Liu* Department of Chemical & Biochemical Engineering, Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, The College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, P. R. China *Corresponding author: Q.L. Liu, E-mail: [email protected], Tel: 86-592-2188072, Fax: 86-592-2184822

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ABSTRACT With the intention of optimizing the performance of anion exchange membranes (AEMs), a set of imidazolium functionalized poly (arylene ether sulfone)s with densely distributed long flexible aliphatic side chains was synthesized. The membranes made from the as-synthesized polymers are robust, transparent and endowed with microphase segregation capability. Ionic exchange capacity (IEC), hydroxide conductivity, water uptake, thermal stability and alkaline resistance of the AEMs were evaluated in detail for fuel cell applications. Morphological observation with the use of atomic force microscopy (AFM) and small angle X-ray scattering (SAXS) reveals that the combination of high local density type and side chain type architecture induces distinguished nanophase separation in the AEMs. The as-prepared membranes have advantages in effective water management and ion conductivity over traditional main chain polymers. Typically, the conductivity and IEC were in the ranges of 57.3–112.5 mS cm-1 and 1.35–1.84 meq g-1 at 80 ºC, respectively. Furthermore, the membranes exhibit good thermal and alkaline stability, and achieve a peak power density of 114.5 mW cm-2 at a current density of 250.1 mA cm-2. Therefore, the present polymers containing clustered flexible pendent aliphatic imidazolium promise to be attractive AEM materials for fuel cells. KEYWORDS: Anion exchange membranes; Flexible side chains; Phase separation; Ion conduction; Fuel cell application


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1. Introduction As an attractive energy conversion technology, fuel cells generate electric power from fuels with high energy density and slight chemical emissions. Proton exchange membrane fuel cells (PEMFCs) have been extensively explored because of their superiority including desirable proton transfer speed and satisfactory cell performance1-2. However, high material cost arising from both the required noble metal catalysts and perfluorosulfonated polymer electrolytes has impeded the practical application of PEMFCs3-4. Anion exchange membrane fuel cells (AEMFCs) are developed as a desirable alternative to PEMFCs because they have faster electrochemical reaction kinetics in alkaline media and take advantageous of utilizing non-precious metal materials as the electro catalysts4-7. As a pivotal module of AEMFCs, AEMs serve as both the fuel/oxidant barrier and hydroxide carrier. An inherent challenge for AEMFCs applications is the production of high-performance AEMs that can simultaneously maintain superb hydroxide conductivity, good dimensional and mechanical stability as well as prominent alkaline resistance property. Various AEMs have been designed and prepared for AEMFCs in the past few years, the most are constructed with cationic groups directly attached and randomly distributed along the polymer backbones. The resulting polymer backbone is rigid, which normally prevents the occurrence of continuous ion clustering, leading to the formation of dead end ionic channels and low ion conductivity8. A desirable ionic conductivity of AEMs for fuel cell applications would normally accompany with high IECs. As a consequence, the AEMs would suffer from excessive swelling and loss of the mechanical properties, thus rendering them unsuitable for practical AEMFCs applications. 3 ACS Paragon Plus Environment


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To improve ionic conducting efficiency and to achieve high conductivity at low IEC and water uptake, a versatile and feasible approach is to create networks of ionic channels within the material. Such method would necessitate the design of typical polymer structures capable of creating hydrophilic/hydrophobic phase segregation. Recently, a feasible approach to design the target polymer structure involves introducing ionic groups locally and densely into the main chain of a random polymer or specified segments in block copolymers (local density type).9-11 For example, Chen and co-workers12 prepared a class of poly (arylene ether ketone)-based AEMs containing methylated bisphenol monomers. This approach allows quaternary ammonium to be densely anchored on the benzyl of the monomer units and the resultant AEMs displayed enhanced ionic conductivity with the existence of distinct ionic clusters as confirmed by SAXS. Another strategy to enhance ionic conductivity is to position ionic groups into the pendant side chains grafted on the copolymer main chain (side chain type).13-16 Compared with the ionic groups directly fixed on copolymer main chain, side chain type ionic groups have stronger self-assembly tendency to form more and larger ionic clusters, which are conductive to the formation of efficient ionic transfer networks.17-18 Wang et al.13 prepared a set of side chain type poly (arylene ether sulfone)-based AEMs containing quaternary ammonium groups in pendant stiff phenyl rings. These AEMs exhibited high hydroxide conductivity with the highest value of 82 mS cm-1 (IEC: 2.36 meq g−1) at 80 °C. Recently, a set of AEMs possessing flexible side chains was prepared via introduction of imidazolium-based ionic liquids into the polymer backbone.19 The flexible side chains structure facilitates the ionic cluster assembly resulting in arising of microphase separation 4 ACS Paragon Plus Environment

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and wide ionic channels. Subsequently, the as-prepared AEMs exhibit favorable hydroxide conductivity in the range of 51.66–108.53 mS cm-1 accompanied by IEC of 1.01–1.90 meq g-1 at 80 °C. Herein, inspired by the attractive features of above mentioned design architecture, we seek to combine the superiority of local density type and flexible side chain type ionic groups to further optimize the performance of AEMs. A series of AEMs with four flexible side chains containing imidazolium grafted onto fluorene-based copolymer main chain were constructed. It is anticipated that the novel macromolecular design would be in favor of low dimensional change and high hydroxide conductivity simultaneously as a consequence of remarkable hydrophilic/hydrophobic difference between the ionic domain and polymer backbone. The morphology and properties of the membranes were investigated in detail.

2. Experimental 2.1 Materials 2,6-dimethoxyphenol (99%, Sigma-Aldrich, USA), 3-mercaptopropionic acid (MPA) (99%, Sigma-Aldrich, USA), 4,4'-(hexafluoroisopropylidene) diphenol (BPHF) (98.0%, Aladdin, China), 9-fluorenone (99%, Aladdin, China), 4,4' -difluorodiphenyl sulfone (BPMF) (99%, TCI, Japan) and boron tribromide (BBr3) (99%, Aladdin, China) were used as received. N,N-dimethylacetamide (DMAc) (99.8%, Aladdin, China) and toluene were purified by stirring

over

CaH2

for

24

h,

filtering

and

distilling

prior

to

use.

1-(6-bromohexyl)-3-methylimidazolium bromide (6BrIm) was synthesized as previous report.19 All other reagents were provided by Shanghai Sinopharm Chemical Reagent Co., Ltd (China) and used without further treatment. 5 ACS Paragon Plus Environment


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2.2 Preparation of the AEMs 2.2.1 Synthesis of 9, 9-bis(3,5-dimethoxy-4-hydroxyphenyl)fluorene (DMHF) DMHF was prepared from 9-fluorenone and 2, 6-dimethoxyphenol by an approach noted by Wang et al.17 Briefly, A 150 mL four-necked flask equipped with a thermometer, a condenser was charged with 2,6-dimethoxyphenol (23.130 g, 0.15 mol), 9-fluorenone (10.800 g, 0.06 mol), MPA (0.15 mL) and toluene (20 mL). The solution was then mechanically stirred at 30 °C for 30 min with N2 flow followed by dropwise addition of 98 wt% H2SO4 (2.3 mL) in 10 min. The system was further heated to 55 °C and stirred for another 3~4 h. After solidification and being cooled down, the mixture was transferred into ice water to get crude product. The crude product was purified by recrystallizing from toluene, followed by vacuum drying at 80 °C to obtain final product. 2.2.2 Synthesis of MPES-x The MPES-x was prepared via condensation polymerization shown in Scheme 1, where x denotes the molar ratio of DMHF to BPMF. The representative preparation of MPES-0.4 is detailed below. Firstly, 0.9410 g (2 mmol) of DMHF, 1.2713 g (5 mmol) of BPMF, 1.0087 g (3 mmol) of BPHF, 1.3820 g (10 mmol) of K2CO3, 18 mL of DMAc and 10 mL of toluene were charged into a 100 mL round-bottomed flask with a Dean–Stark trap. The mixture was stirred at 140 °C for 4 h then slowly raised to 155 °C for additional 15 h with N2 flow. Afterwards, to the system was introduced a trace amount of DMHF (0.14 mmol, 0.0600 g) for end capping for 1 h. The resulting solution was then cooled down and transferred into aqueous methanol (methanol/deionized water=1/1, v/v) to get crude fibrous product. The product was then purified by Soxhlet extraction with methanol followed by vacuum drying at 6 ACS Paragon Plus Environment

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80 °C overnight. 2.2.3 Demethylation of MPES-x The demethylation reaction of MPES-x was conducted in chloroform, using BBr3 as the demethylation agent (Scheme 1). Briefly, 1.5000 g of MPES-0.4 was dissolved in 30 mL of chloroform to form a homogeneous solution. Then a mixture of BBr3 (2 mL) and chloroform (30 mL) was dropped slowly into the solution at 0 °C. The solution was then maintained at 0 °C with stirring for 5 h to precipitate solid product. Thereafter, the precipitate was purified by washing thoroughly with hot water and chloroform followed by vacuum drying at 60 °C overnight. The obtained product is designated HPES-x, where H refers to hydroxyl groups. 2.2.4 Synthesis of ImPES-x copolymers and AEMs fabrication The synthetic route of ImPES-0.4 is typically presented below. 1.0000 g of HPES-0.40, 0.0500 g of KI (0.29 mmol), 0.9400 g of K2CO3 (6.8 mmol), and 20 mL of DMSO were placed into a 50 mL flask under N2 atmosphere. When a homogeneous solution was formed at 60 °C, 1.3690 g of 6BrIm (4.2 mmol) was placed into the solution with stirring. After the reaction was held at 100 °C for 12 h, the solution was cooled down and precipitated in acetone to give a solid product. The crude product then was purified by washing thoroughly with acetone and deionized water, followed by drying under vacuum at 60 °C overnight to acquire the final ImPES-x polymers. For the membrane formation, 0.8000 g of ImPES-x was dissolved in DMSO (20 mL). Then a 0.45 µm PTFE filter was utilized to give clear and brown solution. After casting onto a flat glass plate and vacuum drying at 60 °C for 48 h, the casting solution was transformed into a transparent film by solvent evaporation. Then the Br− type membrane was converted to OH‒ 7 ACS Paragon Plus Environment


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Scheme 1 Synthesis of the MPES-x, HPES-x and ImPES-x polymers. type by immersion in 1 M KOH at room temperature for 36 h. Finally, the AEM was rinsed with deionized water and stored in deionized water prior to characterization. 2.3 Instrument and characterization 8 ACS Paragon Plus Environment

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2.3.1 Chemical structure analysis The Avancell 500 MHz spectrometer (Bruker, Switzerland) was applied for collection of nuclear magnetic resonance spectra (1HNMR) with deuterated DMSO as the NMR solvent. FT-IR measurement (32 scans) was conducted with Nicolet Avatar 330 spectrophotometer (Thermo Electron Corporation, USA) in the range of 4000–500 cm-1. 2.3.2 Membrane morphology characterization The SEM images of the samples were acquired on a field emission scanning electron microscope (SEM, Zeiss Sigma, Germany). AFM (DI Multimode V, Bruker Co.) and SAXS (SAXSee-MC2 X-ray diffractometer) were employed for detection of the morphology structure of the AEMs in OH− form. The related characterization method was utilized as previously reported 19. 2.3.3 Ionic exchange capacity (IEC) The IECs were determined via the back titration method. Briefly, the dried alkaline sample was equilibrated in a 0.1 M hydrochloric acid solution for at least 48 h to fully exchange the OH− ions with Cl− ions. Afterwards, the surplus H+ within the system was titrated by a standard 0.05 M potassium hydroxide solution. The IEC (meq g-1) is described as

IEC =

M1 − M 2 md

(1)

where M1 and M2 (meq) are the amount of hydrochloric acid before and after equilibrium, respectively, and md (g) refers to the mass of dry samples. 2.3.4 Water uptake (WU), static water contact angle and swelling ratio (SR) In order to evaluate the WU of the membranes, the quartz spring balance was employed as illustrated in literature.19 The WU is described via the equation 9 ACS Paragon Plus Environment


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WU =

kL3 − kL2 L − L2 ×100% = 3 ×100% kL2 − kL1 L2 − L1

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(2)

In equation (2), k indicate the elasticity coefficient of quartz spring, and L1, L2, L3 refer to the length of the quartz spring with non-loading, and loading with dry and swollen AEMs, respectively. The hydration number, λ, which refers to the average mole of water molecule per ionic group, can be described by

λ=

WU 1000 × M H 2O IEC

(3)

where MH2O is the molecular weight of water (18.0 g mol-1). The static contact angle meter (SL200B, SOLONTECH, China) was performed with pendent-drop method at RT to acquire water contact angles of the AEMs. SR of the membranes (OH− form) was characterized by checking dimensional difference between the wet and dry samples at certain temperature. The SR is described via

SR =

Lw − Ld × 100% Ld

(4)

Where, Lw and Ld is the length of the samples, before and after swelling in water at certain temperature, respectively. 2.3.5 Mechanical property and thermal stability The mechanical property was assessed at RT with the use of a test machine (WDW-1E Testing System). The AEM samples with 10 mm width were stretched at a rate of 5 mm min-1. A thermo gravimetric analyzer (TGA, SDQT600, USA) was employed for the assessment of thermal stability of the polymers (OH– form) under a N2 environment. The testing temperature varied from RT-800 ºC with the heating rate of 10 ºC min-1. The samples were treated by 10 ACS Paragon Plus Environment

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vacuum drying at 80 °C for 24 h prior to the test. 2.3.6 Ion conductivity. The hydroxide conductivity was estimated by the two-electrode AC impedance spectroscopy method according to the report19. The impedance of the AEMs was detected in DI water at given temperature, and the conductivity (σ, mS cm-1) of the samples is obtained by

σ=

l AR

(5)

where l (cm) is the space between the electrodes, A (cm2) refers to the cross-sectional area of the tested samples and R (Ω) indicates the AEM resistance. 2.3.7 Alkaline stability To investigate the alkali resistance, the change of membrane conductivity was tracked after the sample was soaked in a 1 M potassium hydroxide solution at 60 °C. The chemical structure of the tested sample was also recorded by 1H NMR. Prior to testing, the AEMs were rinsed thoroughly with DI water followed by immersing in water for at least 24 h. 2.3.8 Fuel cell assembly and measurement The alkaline ImPES-0.45 membrane with highest conductivity was employed for single fuel cell test. Firstly, the carbon paper with an active area of 4 cm2 was sprayed with catalyst ink, which was obtained from a mixture of Pt/C (40 wt% Pt, Johnson Matthey) and 5 wt% homemade ionomer in DMF and glycerol. The AEM was sandwiched between the pre-treated carbon papers (1 mg cm‒2 Pt), followed by hot-pressing at 50 ºC and 0.8 MPa for 5 min. The MEA was subsequently set into a single cell test fixture (ZY8714, ZHONGYING Electronic Co., Ltd.) with H2 (anode) and O2 (cathode) at a feed rate of 50 and 100 mL min-1, 11 ACS Paragon Plus Environment


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respectively. The test condition was controlled at 60 ºC and 100% RH.

3. Results and discussion 3.1. Structure analysis of the monomer and polymers 2, 6-dimethoxyphenol was reacted with 9-fluorenone to synthesize the monomer DMHF utilizing MPA and sulfuric acid as the catalyst.17 As confirmed by 1H NMR analysis in Figure S1 (Supporting information), The signal at 3.53 ppm (H6) is ascribed to the protons in ‒OCH3, and the clear signals H1 to H5 correspond to protons on phenyl rings, meanwhile the phenolic hydroxyl signal appears at around 8.34 ppm. 1-(6-bromohexyl)-3-methylimidazolium bromide (6BrIm), an imidazolium based monomer, was prepared according to a previous method.19 As plotted in Figure S2, the signals at 9.27 (Hi), 7.83 (Hg) and 7.75 (Hh) ppm correspond to the imidazolium group and the resonance at 3.85 ppm (Hj) is associated with the methyl on imidazolium ring. In addition, the resonance at 4.18 (Hf), 3.53 (Ha), 1.73–1.83 (Hb, He), and 1.27–1.41 ppm (Hc, Hd) arise from methylene of the aliphatic chain. This suggests successful synthesis of the 6BrIm. A set of MPES-x copolymers was prepared via polycondensation with varying feed molar ratios of DMHF to BPMF (0.30, 0.35, 0.40 and 0.45). The 1H NMR spectra of MPES-0.4 is shown in Figure S3. The signals at around 6.50 (H5) and 6.81–6.92 ppm (H2, H3) are in response to the protons on phenyl rings. Meanwhile the other aromatic protons can be clearly correlated with the polymer backbone. Moreover, the characteristic signal appearing at 3.48 ppm (H6) verifies the presence of methoxy group. Hence, the well-assigned peaks suggest the successful synthesis of the copolymers. Furthermore, Table S1 shows the GPC data of the polymers. Their high molecular weight normally guarantees desirable mechanical 12 ACS Paragon Plus Environment

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and thermal stability of resultant AEMs. The methoxy-bearing poly (ether sulfone) was converted to hydroxyl-bearing copolymer after treating with BBr3 in an ice-bath. The representative 1H NMR spectra of HPES-0.4 are shown in Figure S4. A comparison with MPES-0.4 reveals that a new peak assigned to the phenolic hydroxyl emerged at 9.45 ppm. More importantly, the disappearance of the signal at 3.48 ppm (H6 in Figure S3) from the protons of methoxy confirms the complete demethylation reaction. Imidazolium-functionalized copolymers with dense flexible side chain were synthesized by grafting the imidazolium-bearing ILs onto the HEPS-x, as depicted in Scheme 1. The reaction was accomplished with the catalysis of K2CO3 and KI. The structure of ImPES-0.4 was identified in Figure S5. The characteristic peak ascribed to –OH at 9.45 ppm disappeared as expected, and new signals emerged at 9.12–9.37 ppm (Hi) arising from the methylene protons on the imidazolium ring. Accordingly, the signal from the methylene groups of aliphatic chain appeared at around 0.9−1.8 ppm (Hb–He) and 3.65−4.36 ppm (Ha, Hf, Hj) with lower frequency. The results from 1H NMR indicate that the phenolic hydroxyl reacted completely with ILs with introduce of excessive ILs to the reaction system. The functional group of MPES, HPES, and ImPES was further verified by FT-IR analysis. As shown in Figure S6a, the strong absorption band at around 1240 cm-1 is ascribed to the asymmetric vibration of phenoxy groups, suggesting the polymerization of MPES. In addition, the broad absorption band at 3420 cm-1 in Figure S6b is in response to phenolic hydroxyl, meanwhile the new signal appearing at 1664 cm-1 in Figure S6c indicates the existence of imidazolium ring. Combining with the 1H NMR spectra, these results suggest the 13 ACS Paragon Plus Environment


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successful preparation of ImPES-x membranes. 3.2. Membrane morphology

Figure 1. (a) Photograph, SEM images (b) cross-section and (c) surface of ImPES-0.4. Figure 1 shows the SEM images of ImPES-0.4 membrane. The digital photograph in Figure 1a clearly shows that the membrane is transparent with good flexibility. The thickness of the membrane from the cross-section image is approximately 30µm (Figure 1b). Meanwhile the surface image reveals the formation of homogeneous and compact structure without any visible defects (Figure 1c). To clarify the architecture-morphology correlation of the as-prepared membranes, AFM was utilized for investigation of microphase separation structure in a tapping mode. Figure 2 shows that the ImPES-x membranes exhibit distinguished microphase segregation. The darker regions correspond to soft hydrophilic domains that are composed of imidazolium ionic clusters. The brighter ones are generally associated with the hard segments of hydrophobic polymer backbones. As shown from Figure 2d, ImPES-0.45 reveals developed nanoscale ionic transport networks, which are well inter-connected to each other throughout the field of view. The distinct self-assembly morphology is regarded to be induced by the remarkable hydrophilic/hydrophobic discrepancy between the densely functionalized side chains and the polymer backbone. It is well-known that long flexible ionic side chain with high mobility is conducive to the aggregation of hydrophilic ionic cluster. Furthermore, with side chains 14 ACS Paragon Plus Environment

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densely anchored on the polymer backbones, the ratio of hydrophobic to hydrophilic segment can be increased at a similar cationic content. Hence, an increase in both the side chain hydrophilicity and the main chain hydrophobicity tends to result in the aggregation of larger ionic domains, which is crucial for hydroxide transport.20-22

Figure 2. AFM phase images of the ImPES-x membrane, x= (a) 0.3, (b) 0.35, (c) 0.4 and (d) 0.45. Scale: 500×500 nm. The ionic domain within the materials was further characterized with the use of maximum scattering ionomer peak (qmax) in SAXS profiles. As plotted in Figure 3, the typical scattering

peaks from ImPES-0.35 and ImPES-0.45 are indicative of nanophase separation, as noted by AFM observation. The peaks from ImPES-0.35 and ImPES-0.45 are located at qmax=3.05 and 1.55 nm-1, respectively. Their corresponding average inter-domain spacing (d), derived from Bragg’s equation (d=2π/qmax), is 2.06 and 4.05 nm, respectively. Both of the membranes exhibit scattering peaks in Figure 3, while ImPES-0.45 has a higher d value and a clearer ionomer peak than ImPES-0.35. This indicates the existence of more uniform aggregation of ionic cluster and expanded ionomer domain. The result is in accordance with the related 15 ACS Paragon Plus Environment


studies23-24 that AEMs with more hydrophilic side chains exhibit the tendency of developing broader hydrophilic ionic domains. In a nutshell, it is shown that both the AFM images and SAXS profiles witness the phase separation in the membranes. (a)

(b)

ImPES-0.45

ImPES-0.35

Intensity (a. u.)

Intensity (a. u.)

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1

2

-1

3

4

2

3

4

5

-1

q (nm

q (nm )

)

Figure 3. SAXS profiles of the (a) ImPES-0.45 and (b) ImPES-0.35 AEMs. 3.3. Properties of the ImPES-x membranes 3.3.1. Analysis of IEC, water uptake (WU) and swelling ratio (SR) Table 1 IEC, water uptake, swelling ratio and λ of the alkaline membranes.

Membranes

a

IEC (meq g-1)

WU (%)

Cal. a

Exp. b

30 °C

60 °C

ImPES-0.30

1.50

1.35

14.6

ImPES-0.35

1.67

1.58

ImPES-0.40

1.82

ImPES-0.45

1.95

SR (%)

λc

30 °C

60 °C

18.2

15.3

19.2

6.0

22.3

29.1

18.0

27.1

7.8

1.71

31.5

40.9

24.2

35.3

10.2

1.84

44.3

57.3

33.3

46.7

13.4

determined by the feed monomer ratio, b measured by back titration, c the moles of water

molecule per ionic group (30 °C) . The IEC, water uptake and swelling ratio of ImPES-x membranes are collected in Table 1. IEC reflects exchangeable ionic density and plays a critical effect on both water absorption and ion conductivity. As presented in Table 1, the theoretical IEC calculated from feed monomer ratio is in the range of 1.50–1.95 meq g-1. The experimental values obtained by the 16 ACS Paragon Plus Environment

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titration method vary from 1.35 to 1.84 meq g-1. The experimental IEC of the ImPES-x membranes is consistent with the theoretical value, implying the successful alkalization of the Br– type membranes. 70 60

Water uptake (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


66.4 °

30 °C 60 °C

74.7 °

50 80.7 °

40 30

88.5 °

20 10 0 ImPES-0.3 ImPES-0.35 ImPES-0.4 ImPES-0.45

Figure 4. Water uptake and static water contact angles of the membranes. Water uptake is a significant factor and dominates the swelling and ion conductivity. Superfluous water uptake is normally accompanied by over-swelling and mechanical weakness. Fortunately, the water uptake of the membranes is considerably low in terms of their IECs. As listed in Table 1, the water content increases from 14.6% to 44.3% at 30 °C and 18.2% to 57.3% at 60 °C due to increment of ionic groups. In comparison with some main chain and even side chain polymers, the dense flexible side chain type ImPES-x membranes exhibit much lower water uptake with similar IECs. For example, the fluorine-based F-QPES membrane25 (IEC=1.89 meq g-1) and side chain ImPES-1.019 (IEC=1.83 meq g-1) have water content of 109.4% and 83.95% at 30 °C, which are nearly 2.5 and 2 times more than that of ImPES-0.45, respectively. The lower water uptake probably benefits from the unique polymer architecture. It is believed that the positioning of ionic conducting group on side chain instead of main chain normally induces restraint of water uptake.26-27 Furthermore, by grafting side 17 ACS Paragon Plus Environment


chain locally and densely on polymer main chains, more hydrophobic segments on backbones can be reserved at similar IECs, thus also restricting water uptake effectively. The corresponding λ (number of water molecules per mole of ionic group) is in the range of 6.0– 13.4 and increases with IEC. In addition, the static water contact angle, which is an indicative of hydrophilicity of membranes, is shown in Figure 4. The water contact angle decreased from 88.5° to 66.4° for the membranes from ImPES-0.3 to ImPES-0.45. This means that higher ionic content endows higher hydrophilicity to the AEMs. 50

Swelling ratio (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

30 °C 60 °C

40 30 20 10 0

Nafion 117 0.30

0.35

0.40

0.45

Figure 5. Swelling ratio of the ImPES-x and Nafion 117. Higher dimensional stability results from lower water uptake. All the alkaline AEMs exhibit low swelling ratio ranging from 15.3% to 33.3% at 30 °C (Table 1), which is consistent with water uptake. This could be attributed to the well-aggregated ionic clusters in the side chains holding water molecules away from the polymer backbone, unlike some random main chain polymers. Thus, the obtained robust hydrophobic frame serves to mitigate excessive swelling. In addition, high density side chains with tethered imidazollium can also significantly strengthen the intermolecular interaction of polymer chains, which further induce the depression of swelling ratio.21 Overall, the comparison of SR of all the membranes with that of Nafion 117 (Figure 5) suggests that the dimensional stability of the ImPES-x 18 ACS Paragon Plus Environment

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membranes is promising for fuel cell applications. 3.3.2. Ionic transport High ionic conductivity is significant to AEMs and dominates the performance of fuel cells. As shown in Figure 6, all the ImPES-x samples revealed a steady enhancement in conductivity with temperature owing to improved water diffusion and ionic migration. The conductivity also expectably increased with IEC owning to higher water uptake and long-range ion domain percolation. In particular, ImPES-0.45 (IEC=1.84 meq g-1) exhibits hydroxide conductivity of 57.3 and 112.5 mS cm-1 at 30 and 80 °C, respectively. One of the primary reasons for the excellent conductivity can be that the ionic aggregation of densely hydrophilic side chains promotes OH– mobility through the materials. The resultant enlarged and continuous ionic networks, as detected by AFM, serve as highway for OH– transportation. 120

Ionic conductivity (mS cm -1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ImPES-0.30 ImPES-0.35 ImPES-0.40 ImPES-0.45

100 80 60 40 20 30

40

50

60 70 Temperature (° C)

80

Figure 6. Hydroxide conductivity versus temperature for ImPES-x membranes. Meanwhile, the apparent activation energy (Ea) for ionic transport is estimated via Ea= -b×R, where b is the slope of the line regression of ln σ vs. 1000/T plots, and R is the gas constant (8.314 J mol-1 K-1). The apparent Ea (Figure 7) in the range 17.4–21.9 J mol-1 K-1 presents a reverse trend with hydroxide conductivity. A lower Ea for the AEMs with higher 19 ACS Paragon Plus Environment


conductivity probably originates from the fact that more ionic content and developed nano-channels exist in these membranes (such as ImPES-0.45), thereby requiring less energy for hydroxide transfer. -2.0

-1

lnσ (S cm )

-2.5 -3.0 -3.5

ImPES-0.30 (21.9 KJ mol-1) -1 ImPES-0.35 (19.5 KJ mol ) -1 ImPES-0.40 (18.9 KJ mol ) -1 ImPES-0.45 (17.4 KJ mol )

-4.0 -4.5 2.8

2.9

3.0

3.1

3.2

3.3

-1

1000/T (K )

Figure 7. Arrhenius plots of the ImPES-x AEMs.

-2

-1

Ionic conductivity (10 S cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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12 11 10 9 8 7 6

ImPES-0.30 ImPES-0.35 ImPES-0.40 ImPES-0.45 QPAE-a PAES-Q-12 QSPES-40 membrane-10 NAPAEK-Q-90 membrane D QPEN-0.6 C-FPAEO-75-MIM bQAPDHTPE-OH 25

5 4 3

2 1.4

1.6

1.8

2.0

2.2

2.4

-1

IEC (meq g )

Figure 8. Hydroxide conductivity of AEMs versus IECs at 80 ºC. A comparison of the OH–conductivity of some main chain type,28-29 side chain type,13,30 cross-linked31 and hybrid AEMs32 reveals that the as-synthesized ImPES-x polymers demonstrate considerably higher conductivity at similar conditions. As illustrated in Figure 8, the ideal area on upper left-hand corner of the graph is mainly occupied by the ImPES-x membranes, which means higher conductivity at lower IEC. It thus can be concluded that the 20 ACS Paragon Plus Environment

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ImPES-x AEMs possess even higher conductivity than that of some rigid side chain membranes13 and local density type membranes.9 Therefore, the structure design in this work tends to be more viable for conductivity enhancement. Furthermore, for an in-depth comparison of AEMs with different ionic contents, λ-normalized conductivity is introduced to evaluate the ionic transport efficiency. The ImPES-x membranes display desirable normalized conductivity. For example, ImPES-0.45 exhibits remarkable λ-normalized hydroxide conductivity of 3.24 mS cm–1 at 30 °C (Table 2), which is several times higher than that of other AEMs including block type (QSPES-4033, bQAPDHTPE-OH 2534), random type (membrane D29), side chain type membranes (ImPES-1.019 and NAPAEK-Q-9030). The observations suggest that the ImPES-x with phase separation can utilize water molecules more effectively for ionic transport without frustrating swelling ratio. Nevertheless, it should be noted that the newly self-crosslinked X30Y10 membrane developed by Li et al.35 displayed a normalized conductivity up to 3.73 mS cm–1. Hence, a combination of crosslinking technique and dense flexible side chain architecture seems to be worthy of future investigation. Table 2 IEC, hydroxide conductivity, and λ-normalized conductivity (σ') of reported AEMs. IEC (meq g-1)

λ

σ (mS cm-1)

σ' (mS cm-1)

ImPES-0.35 (this work)

1.58

7.8

24.4 (30 °C)

3.11

ImPES-0.45 (this work)

1.84

13.4

43.4 (30 °C)

3.24

membrane D29

1.52

30.2

13.2 (30 °C)

0.44

1.73

8.1

9.5 (20 °C)

1.17

1.90

25.5

42.7 (30 °C)

1.74

1.29

5.3

19.8 (RT)

3.73

1.80

15.7

16.9 (20 °C)

1.08

1.89

23

29.3 (80 °C)

1.27

Membranes

9

QPAE-a

19

ImPES-1.0 X30Y1035

31

C-FPAEO-75-MIM

bQAPDHTPE-OH 25

35



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

c4PAES-2.5Im11

2.5

18.1

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14.6 (20 °C)

0.81

3.3.3. Mechanical property and thermal stability Table 3 Mechanical properties of the ImPES-x membranes. Membranes

Tensile strength (MPa)

Elongation at break (%)

Young’s modulus (MPa)

ImPES-0.30

36.4±2.1

10.1±0.5

705±37

ImPES-0.35

30.9±2.4

10.7±0.8

642±25

ImPES-0.40

28.4±1.6

12.1±1.2

480±32

ImPES-0.45

25.5±1.9

14.2±0.9

315±28

Table 3 lists the mechanical properties of the membranes. The ImPES-x at 60% RH and RT owned tensile strengths and elongation at break in the ranges of 25.5–36.4 MPa and 10.1– 14.2%, respectively. These data are comparable with or even higher than the values of reported AEMs.17,36 The tensile strength generally declines from ImPES-0.3 to ImPES-0.45. This is reasonable considering the plasticization effects from increased water uptake. Although incorporation of ionic groups results in a depression in tensile strength and Young's modulus, the as-synthesized ImPES-x membranes still exhibits sufficient mechanical capacities. The thermal degradation was analyzed via TGA under N2 flow. The first stage (< 5%) below 180 °C (Figure 9) can originate from the evaporation of water in samples. The second degradation stage starting from 210 to 340 ºC is likely in response to degradation of imidazolium groups and aliphatic side chain. The third degradation step occurring above 360 ºC arises from decomposition of polymer main chains. Overall, the TGA analysis implies that the as-synthesized polymers maintain adequate thermal endurance below 200 ºC, which is desirable for fuel cell applications. 22 ACS Paragon Plus Environment

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100

ImPES-0.30 ImPES-0.35 ImPES-0.40 ImPES-0.45

Weight (%)

90 80 70 60 50 40 30 20 10 100

200

300

400

500

600

700

Temperature (°C)

Figure 9.TGA graph of the membranes under a N2 atmosphere. 3.3.4. Alkaline stability Remaining conductivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

90

80

70

60

0

100

200

300

400

500

Time (h)

Figure 10. The alkali resistance of ImPES-0.45 in a 1M KOH solution at 60 ºC. Achieving a long-term chemical stability of AEMs remains a great challenge owing to the degradation of imidazolium cationic high-pH environment.37-38 In the present study, the alkaline resistance of ImPES-0.45 was evaluated in a 1 M KOH solution at 60 ºC for over 400 h. The variation of conductivity was monitored at certain intervals, and the change in chemical structure was recorded using 1H NMR spectroscopy. As shown in Figure 10, ImPES-0.45 shows a rapid decrease in conductivity within initial 100 h, after which the decrease in the conductivity has decelerated. After immersion in 1 M KOH solution at 60 ºC for 408 h, the membranes still maintained 68.2% of the original conductivity at 60 ºC. The 23 ACS Paragon Plus Environment


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conductivity decline was accompanied by similar reduction in IEC. As shown in Figure 11, the remained IEC is precisely calculated from the ratio of the integral of peak i in membrane after the testing to that of the initial membrane. In comparison with Figure 11a, all peaks in Figure 11b are well-assigned except for the integral area of peak i (protons on imidazolium), which decreases from 0.97 in the original sample to 0.74. This suggests that about 74% cationic group was reserved in a 1 M KOH solution at 60 ºC for 408 h. The reduction in imidazolium groups is mainly because of the nucleophilic attack of OH–. The degradation mechanism is shown in Figure S7. Briefly, the C2 position of imidazolium ring is attacked by OH– for its highest positive value of the Mulliken charge39. Consequently, non-aromatic structure intermediate is formed by introduction of OH– into the cation ring. Since the transition state structure by ring-opening reaction is unstable, the final formyl bearing product irreversibly generates and thus arouses decomposition of the imidazole-based AEMs40. The main chain type AEMs in Jannasch’s work11 showed that all imidazolium groups were lost after being soaked in 1 M NaOH at 40 ºC for 7 days. The MPAES-Q-1 AEM even broke into pieces after immersion in 1 M NaOH at 60 ºC for merely 336 h.41 In this study, the long side chain type polyelectrolyte with an alkyl spacer of >3 carbon atoms between benzene ring and cationic groups could be responsible for the slight cationic degradation. Moreover, the distinct microphase separation morphology, as confirmed by AFM images, may have partly contributed to the structural stability. This is because an ideal solvation micro-environment for OH– in the developed hydrophilic ionic domains could retard the degradation of cationic group.19,28 Nevertheless, the chemical stability could be improved with further exploration. It is reported that imidazolium with C2 substituted induces higher stability due to the steric 24 ACS Paragon Plus Environment

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hindrance.43 A recent study by Yan and coworkers44 suggests that N3-substituent, especially the isopropyl substituted imidazolium cation exhibits exceptional stability in aqueous NaOH. In light of these studies, an investigation is underway to seek for functional cationic groups with improved chemical stability.

5

i

(a)

1.00

0.97

5

i

(b)

1.00

0.74

10

9

8

7

6

ppm Figure 11. 1H NMR spectra of the ImPES-0.45: (a) initial Br– type membrane (b) immersed in 1M KOH solution at 60 ºC for 408 h. 3.3.5. Fuel cell performance For AEMs to be employed in electrochemical devices, the fuel cell performance should be evaluated as a prerequisite. Figure 12 illustrates the polarization curves of single H2/O2 cell employing ImPES-0.45 as electrolyte membrane. The open circuit voltage (OCV) is observed to be 1.01 V. The power density increased gradually with increasing current density, achieving a peak power density (Pmax) of 114.5 mW cm-2 at a current density of 250.1 mA cm-2. This is notably higher than that of random type AEM at 60 ºC (48 mW cm-2)45 and is also comparable with that of crosslinked AEMs (124.7 mW cm-2)46. A comparison of single cell performances including test condition in the presented work and the literatures is listed in Table S2. The 25 ACS Paragon Plus Environment


results therefore suggest a good cell performance, which is likely to benefit from high conductivity as well as good stability of the membranes for MEA fabrication.

100

0.8

80 0.6 60 0.4

40

0.2 0.0

20 0

50

100

150

200

250

300

Power density (mV cm −2)

120

1.0

Cell voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 350

-2

Current density (mA cm )

Figure 12. Polarization curves of H2/O2 fuel cell with ImPES-0.45 at 100% RH and 60 ºC.

4. Conclusion In summary, a new class of dense and flexible side chain type copolymers was successfully synthesized by polycondensation followed by grafting reaction at moderate temperature. The unique structural features are likely responsible for the distinct microphase separation between extreme opposing hydrophobic and hydrophilic domains. This structure serves to promote the efficiency of hydroxide conduction and controllable water management. Subsequently, advantageous ion conductivity in the range of 57.3–112.5 mS cm-1 at 80 ºC was observed against most main chain polymers. The ImPES-x membranes also have superior ratio of conductivity to IEC and λ values. Moreover, the membranes exhibit fairly low water uptake in the range of 14.6%–44.3% with corresponding swelling ratio of 15.3%–33.3% at 30 ºC. Likewise, The polymers demonstrate good thermal, mechanical and chemical stabilities. Furthermore single H2/O2 fuel cell test for ImPES-0.45 warrants its applicability with peak power density of 114.5 mW cm-2 at 60 ºC. Therefore, the synthesis strategy provided in this work can be a promising route to develop desirable AEMs for fuel cells materials. 26 ACS Paragon Plus Environment

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Author Information Qing Lin Liu (Liu QL), corresponding author, E-mail: [email protected], Tel: 86-592-2188072, Fax: 86-592-2184822

Notes The authors declare no competing financial interest.

Acknowledgements Financial support from the National Nature Science Foundation of China (grant no. 21376194 & 21576226), the Nature Science Foundation of Fujian Province of China (grant no. 2014H0043), and the research fund for the Priority Areas of Development in Doctoral Program of Higher Education (no. 20130121130006) is gratefully acknowledged.

Supporting Information 1

H NMR spectra of the DMHF, 6BrIm ionic liquid and copolymers, FT–IR spectra of the

copolymers. This material is available free of charge via the Internet at http://pubs.acs.org.

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[36] Han, J.; Liu, Q.; Li, X.; Pan, J.; Wei, L.; Wu, Y.; Peng, H.; Wang, Y.; Li, G.; Chen, C.; Xiao, L.; Lu, J.; Zhuang, L. An Effective Approach for Alleviating Cation-Induced Backbone Degradation in Aromatic Ether-Based Alkaline Polymer Electrolytes. ACS Appl. Mater. Interfaces 2015, 7, 2809-2816. [37] Couture, G.; Alaaeddine, A.; Boschet, F. Ameduri, B. Polymeric Materials as Anion-Exchange Membranes for Alkaline Fuel Cells. Prog. Polym. Sci. 2011, 36, 1521-1557. [38] Price, S. C.; Williams, K. S.; Beyer, F. L. Relationships between Structure and Alkaline Stability of Imidazolium Cations for Fuel Cell Membrane Applications. ACS Macro Lett. 2014, 3, 160-165. [39] Dong, H.; Gu, F.; Li, M.; Lin, B.; Si, Z.; Hou, T.; Yan, F.; Lee, S.-T.; Li, Y. Improving the Alkaline Stability of Imidazolium Cations by Substitution. ChemPhysChem 2014, 15, 3006-3014. [40] Meek, K. M.; Elabd, Y. A. Alkaline Chemical Stability of Polymerized Ionic Liquids with Various Cations. Macromolecules 2015, 48, 7071-7084. [41] Li, X.; Nie, G.; Tao, J.; Wu, W.; Wang, L.; Liao, S. Assessing the Influence of Side-Chain and Main-Chain Aromatic Benzyltrimethyl Ammonium on Anion Exchange Membranes. ACS Appl. Mater. Interfaces 2014, 6, 7585-7595. [42] Tomoi, M.; Yamaguchi, K.; Ando, R.; Kantake, Y.; Aosaki, Y.; Kubota, H. Synthesis and Thermal Stability of Novel Anion Exchange Resins with Spacer Chains. J. Appl. Polym. Sci. 1997, 64, 1161-1167. [43] Hugar, K. M.; Kostalik, H. A. t.; Coates, G. W. Imidazolium Cations with Exceptional Alkaline Stability: A Systematic Study of Structure-Stability Relationships. J. Am. Chem. Soc. 32 ACS Paragon Plus Environment

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Table of Contents Graphic

80

0.6

60 0.4

40

0.2 0.0

20 0

50

-2

100

0.8

Power density (mV cm )

120

1.0 Cell voltage (V)

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0 100 150 200 250 300 350 -2 Current density (mA cm )

ACS Paragon Plus Environment

100 nm

Imidazolium-Functionalized Poly(arylene ether sulfone) Anion

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