Imidazolium functionalized polysulfone electrolyte membranes with varied chain structures: a comparative study

Abstract

The alkaline electrolyte membrane (AEM) is one of the key materials for alkaline electrolyte membrane fuel cells. While most studies focus on AEM performance in relation to cation structure, ion exchange capacity (IEC) and membrane architecture (cross-linking, composite, microphase separation), herein we report that AEMs of varied backbone structures and comparable IEC show remarkably different conductivities and alkaline stability. In particular, the AEM of imidazolium functionalized, isopropylene-containing polysulfone exhibits higher conductivity, but lower swelling- and alkali tolerance than those without such a unit. To confirm and mitigate the backbone influence, we grafted the isopropylene-containing polysulfone with imidazolium functionalized side chains. The resulting grafted membrane shows conductivity of 64 mS cm⁻¹ at 80 °C, which is higher than that of the un-grafted AEMs; its stability against water swelling and hydroxide attack is also improved. Our study provides a new understanding on the structure–property correlation of AEMs, and thus is beneficial for designing high-performance electrolyte membranes for an alkaline fuel cell.

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1. Introduction

Alkaline electrolyte membrane fuel cells (AEMFCs) are one of the next-generation cost effective and high-efficiency power sources; this is because AEMFC works under alkaline conditions, making the electrode reaction more facile than in proton exchange membrane fuel cells, and non-precious metal catalysts (such as Co, Ni and Fe) can be used to replace the noble metal catalysts such as Pt.^1–3 Meanwhile, the use of an alkaline electrolyte membrane (AEM) in place of a liquid electrolyte makes the fuel cell more compact, avoids the electrolyte leakage problem and minimizes the negative effect of carbon dioxide pollution issues.⁴

AEM is one of the core materials for AEMFC. Compared with proton exchange membranes (PEM), AEM suffers a more serious conductivity-stability conflict. The larger size of hydroxide ion relative to protons, coupled with less continuous ion transport pathway, exerts much higher resistance of hydroxide ion transport in an AEM than that of proton transport in a PEM. As such, AEM conductivity improvement relies heavily on incorporation of high contents of hydroxide exchange groups; this will lead to serious swelling and accelerated membrane degradation under the fuel cell working condition (high temperature and high pH). The degradation may occur on the polymer backbone⁵ and/or on the attached cations,^6–8 which include a head group and a tether between the backbone and the head.

Nowadays, AEM studies mainly focus on understanding and addressing the above mentioned stability and conductivity issues. One aspect is to find out or synthesize a robust polymer structure that is stable enough to withstand the hydroxide attack at elevated temperatures; the polymers that have been studied include poly(arylene ether sulfone),^9,10 poly(ether ether ketone),^11,12 poly(phthalazinone ether ketone),^13–15 poly(vinyl alcohol),¹⁶ poly(phenylene oxide),^17–19 polyphenylene²⁰ and so on. Most of these polymers, however, may lack long-term stability under hot, alkali conditions when quaternized to a level sufficient to insure appreciable membrane conductivity; this is due to cation triggered backbone scission as reported by Ramani and Arges.⁵

The second aspect is to design an alkali-stable cation for hydroxide exchange. It is well known that quaternary ammonium (QA) cations are prone to decomposition following an SN2 or Hoffmann elimination mechanism when exposed to a strong alkali medium at elevated temperatures. With such a background, none QA cations have been explored in recent years, including imidazolium,^21–24 guanidinium,^25–27 pyrrodinium²⁸ and quaternary phosphonium,^29–31 among which imidazolium and guanidinium cations are potentially advantageous due to their resonance effect.³² Resonance means charge delocalization, which may weaken the hydroxide attack on the cation structure so that the cation stability can be improved.

In our previous work, we designed and synthesized two types of non-QA AEMs, one based on bis-guanidinium-bridged polysilsesquioxane,²⁶ and the other on imidazolium-functionalized bisphenol-A-derived polysulfone.²⁴ The former displayed improved alkali tolerance due to enhanced resonance of the bis-guanidinium structure. The latter, however, underwent significant conductivity decay after a 24 h treatment with 3 M NaOH at 60 °C due to imidazolium decomposition; this is unexpected, and does not agree with some of the literature reported findings. For example, Fang et al. reported 1-ethylimidazole-quaternized aliphatic AEMs stable at 60 °C in 6 M NaOH for 120 h;³³ Yan et al. reported highly stable 1-methylimidazolium aliphatic AEMs that maintained conductivity over 1000 h in 1 M KOH at 60 °C.²³ Correlating these results with ours on bisphenol-A polysulfone AEM,²⁴ it is inferred that backbone structure may have a significant influence on the imidazolium cation stability such that imidazolium head group attached to aliphatic backbone shows better alkali tolerance than that attached to aromatic backbone. This inference, however, is still preliminary and deserves further investigation. Before probing into the in-depth influencing mechanism, we first need to confirm whether imidazolium AEMs with different polysulfone structures show similarly low stability as bisphenol-A polysulfone AEM, and whether backbone structure influences other important properties (conductivity and swelling behavior) of imidazolium AEMs apart from alkali stability.

In this work, a series of polysulfones with or without isopropylene and hexafluoroisopropylene group in the backbone were synthesized and used for AEM fabrication. The resulting membranes were studied for their hydroxide ion conductivity, water uptake, swelling behavior and alkaline stability, all of which are correlated with the backbone structure. Our investigations suggest that the bulky isopropylene group in the backbone near the cation head group facilitates hydroxide conduction but undermines cation stability of the membrane. To confirm and mitigate the adverse effect of isopropylene group, we grafted the polysulfone with aliphatic side chains carrying imidazolium cations, whose head group is relatively far away from the bulky group in the main chain. The grafted membrane showed better conductivity and alkali stability than the un-grafted one.

2. Experimental

2.1 Materials

Difluorodiphenylsulfone (98%, Energy Chemicals, Shanghai) was re-crystallized from ethanol, bisphenol-A (AR, Tianjin Damao Chemicals) from toluene, 4,4′-biphenol (97%, Aladdin Reagent) from acetone, and hexafluorobisphenol-A (97%, Aladdin Reagent) from ethanol, respectively, before use. N,N-Dimethylacetamide (AR) and toluene (99%) were distilled before use. 1,1,2,2-Tetrachloroethane (AR, Tianjin Damao), anhydrous K₂CO₃ (AR, Tianjin Bodi Chemicals), chloromethyl methyl ether (98%, Xiya Reagent), methanol (AR), 2-methallylchloride (97%, Aladdin Reagent), CuCl (97%, Aladdin Reagent) and 2,2′-bipyridyl (98%, Aladdin Reagent) and 1-methylimidazole (99%, Aladdin Reagent) were all used as received.

2.2 Synthesis of polysulfone (PSf) with varied backbone structures

PSfs of different chain structures were synthesized following the typical condensation polymerization route. Take PSf-A (containing isopropylene group in the backbone) as an example. Under a N₂ atmosphere, 1.017 g (4 mmol) difluorodiphenylsulfone, 0.914 g (4 mmol) bisphenol-A and 1.382 g (10 mmol) anhydrous K₂CO₃ were added into a 3-necked round-bottom flask equipped with a Dean–Stark trap, a N₂ inlet, a condenser and a mechanical stirrer. Then 30 ml N,N-dimethylacetamide (DMAc) and 10 ml toluene (distilled) were added. After three cycles of evacuation and nitrogen purge, the mixture was stirred and refluxed at 70–80 °C for 3–5 h, and then the temperature ramped to 140 °C. The water produced in the above process could form an azetrope with toluene and got removed together with it. Upon complete removal of the azetrope, temperature was raised to 170 °C and maintained for 16 h to complete the polymerization. The resultant viscous mixture was diluted with 30 ml DMAc and then poured into a copious amount of ice water for precipitation of the polymer. After vigorous stirring for 2 h, the precipitate was collected by filtering, washed repeatedly with deionized water and then soaked with methanol for 12 h. Finally the polymer was vacuum dried at 60 °C for 24 h.

Following a similar route as above, PSf-B (no isopropylene group in the backbone) and PSf-C (containing hexafluoroisopropylene group in the backbone) were synthesized by polycondensation of difluorodiphenylsulfone with 4,4′-biphenol and hexafluorobisphenol-A, respectively.

2.3 Synthesis of chloromethylated PSf with controlled degree of chloromethylation

A typical synthesis is as follows. 1.0 g PSf-A and 50 ml 1,1,2,2-tetrachloroethane was added into a 3-necked round bottom flask equipped with a condenser, a magnetic stirrer and a nitrogen inlet. The system was evacuated and nitrogen purged for three cycles; upon complete dissolution of the polymer, 30 μl SnCl₄ and 1.7 ml chloromethyl methyl ether (CMME) were added slowly (CMME is a strong toxic; great caution should be taken to avoid direct contact or inhalation). The mixture was refluxed at 55 °C for 12 h under nitrogen atmosphere and then cooled to room temperature and poured slowly into 200 ml methanol. The resulting white precipitate, named CMPSf-A, was stirred vigorously, filtered, washed repeatedly with methanol, and then vacuum dried at 60 °C for 24 h. Chloromethylation of PSf-B and PSf-C were carried out following the above procedure. The products were denoted as CMPSf-B and CMPSf-C, respectively.

Degree of chloromethylation was controlled by adopting proper temperature and time for the chloromethylation reaction as has been reported in our previous work.²⁴

2.4 PSf quaternization and AEM fabrication

Take CMPSf-A as an example. 0.6 g CMPSf-A was dissolved in DMAc, making a 5–10% (m/v) solution, to which slightly excessive 1-methylimidazole (MIm) was added. The mixture was stirred at 30 °C for 8 h, degassed by sonication and then cast on a pre-cleaned glass plate. The cast solution was heated by ramping to 80 °C; the resulting membrane was further vacuum dried 80 °C for 24 h. Finally, the membrane was alkalized by soaking in a 1 M NaOH solution (aq.) for 24 h followed by copious rinsing with deionized water.

2.5 PSf-A-g-poly(methallyl imidazolium) synthesis and AEM fabrication

In a typical synthesis, the CMPSf-A (0.628 g) and 2-methallylchloride (MAC) (0.5 ml) were dissolved in 25 ml DMAc under vigorous stirring at room temperature. Upon formation of a homogeneous solution, CuCl (0.014 g) and 2,2′-bipyridyl (0.035 g) were added under nitrogen and stirred for 1 h. The flask was then sealed and placed in a 90 °C oil bath for predetermined time duration. The mixture was precipitated into a large excess of ethanol, and the collected polymer was washed with an ethanol/water (1 : 1 in vol) mixture for several times. The resulting PSf-A-g-poly(methallylchloride), or (PSf-A-g-PMAC) was vacuum-dried at 80 °C for 24 h.

For quaternization, PSf-A-g-PMAC (0.619 g, 1.43 mmol) was dissolved in 10 ml DMAc, and then 1-methylimidazole (2 ml, 0.025 mmol) was added. The reaction mixture was stirred at room temperature for 8 h and then poured into distilled water. The precipitate, PSf-A-g-poly(methallyl imidazolium), or PSf-A-g-PMAIm was filtered off, washed with deionized water thoroughly, and then vacuum dried at 60 °C for 24 h.

For membrane fabrication, PSf-A-g-PMAIm was dissolved in DMAc to afford a 5–10% transparent solution, which was then cast onto a pre-cleaned glass substrate. The membrane was carefully dried on a hot plate (60–80 °C) and then vacuum-dried at temperatures up to 80 °C for 24 h. The dry membrane obtained was rendered into alkaline form by soaking in a 1 M aqueous solution of NaOH for 24 h.

2.6 Membrane characterizations

2.6.1 Spectroscopic studies. Fourier transform infrared spectroscopy (FTIR) study of the membranes was performed using a Lambda-950 spectrometer with an ATR accessory containing a Ge crystal. FTIR spectra were recorded in the wave number range of 600–4000 cm⁻¹ with a resolution of 4 cm⁻¹. ¹H-NMR was performed using INOVA-400 spectrometer with deturated dimethylsulfoxide (DMSO) as the solvent and tetramethylsilane as the internal standard.

2.6.2 Ion exchange capacity (IEC) determination. The membranes in the OH form were immersed in 50 ml of 0.1 M HCl standard for 48 h. The solutions were then titrated with a standardized NaOH solution using phenolphthalein as an indicator. The IEC of membranes was calculated from:
where V₁ and V₂ are the volume of the standard NaOH solution consumed in the titration without and with membranes, respectively, C_NaOH is the molar concentration of standard NaOH solution, and M_dry is the mass of the dried membranes.

2.6.3 Water uptake and swelling ratio measurements. The membranes were vacuum-dried at 60 °C for 24 h and then immersed in deionized water at given temperature for 4 h. The soaked membranes were wiped with tissue paper, and quickly weighed. The water uptake was calculated from:
where, W_dry and W_wet are the weight of the dry and the corresponding water-swollen membranes, respectively. The swelling ratio was calculated from:
where, L_dry and L_wet are the length of the dry and wet samples, respectively.

2.6.4 Conductivity measurement. Hydroxide conductivities (σ, S cm⁻¹) of the membranes (sample size: 1.5 cm × 3 cm) were obtained from σ = d/(SR), where d is the distance between reference electrodes, and S and R are the cross-section area and impedance of the membrane sample, respectively. R was measured by four-electrode impedance spectroscopy using an impedance/gain-phase analyzer (Solatron 1260) coupled with an electrochemical interface (Solatron1287). The sample and the electrodes were mounted in a PMMA cell, where the distance between the electrodes was 1 cm. The cell was immersed in deionized water at different temperatures for impedance measurement.

3. Results and discussion

3.1 Syntheses of imidazolium polysulfones of varied backbone structures

To look into the effect of backbone structure on the cation stability, conductivity and swelling behavior of AEM, three types of polysulfone were synthesized and functionalized with imidazolium. Polysulfone syntheses were achieved via polycondensation of difluorodiphenylsulfone with different bis-phenol monomers as shown in Fig. 1. Specifically, polysulfone-A, -B and -C (PSf-A, -B, -C) were synthesized from bisphenol-A, 4,4′-biphenol, and hexafluorobisphenol-A, respectively. Number averaged molecular weights of the synthesized polymers are given in Table 1. They are of the same order of magnitude, making subsequent membrane property comparison on a reasonable basis.

Fig. 1 Synthetic route and structure of PSf-A, PSf-B and PSf-C.

Table 1 Molecular weight of the polysulfones with different structures

Polymer	Mn (g mol^−1)
PSf-A	9.17 × 10^4
PSf-B	3.18 × 10^4
PSf-C	4.03 × 10^4

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The synthesized polysulfones were chloromethylated using chloromethyl methyl ether (catalyzed by tin chloride). The degree of chloromethylation was carefully controlled following our previously reported method,²⁴ i.e. by tuning the temperature and time for the chloromethylation reaction. The chloromethylated polysulfones were analyzed with ¹H-NMR (Fig. 2), where the chemical shifts at ca. 4.54, 4.64 and 4.59 are assignable to the chloromethyl protons,³⁴ while the slight difference among these values results from the different chain units where the chloromethyl group is attached.

Fig. 2 ¹H-NMR spectra for CMPSf-A, -B and -C in comparison with PSf-A, -B and -C.

By carefully controlling temperature and reaction time for the chloromethylation reaction, we got chloromethylation degrees (DC) of 0.91, 0.89, 0.87 for PSf-A, -B and -C, respectively. The DC values are calculated on basis of the H-NMR results by comparing the peak areas corresponding to the chloromethyl and the aromatic protons.

The chloromethylated PSfs were quaternized by reacting with 1-methylimidazole for AEM fabrication. FTIR spectra for the imidazolium functionalized polysulfone AEMs (ImPSf-A, -B and -C, all in hydroxide form) are given in Fig. 3. All the spectra display a broad band at ca. 3360 cm⁻¹ due to water absorption of the membranes, indicating successful quaternization of polysulfones with imidazolium cation. The presence of imidazolium group can also be confirmed by the bands at ca. 1540 and 1560 cm⁻¹ in the spectra for ImPSf-A and -B, while that of ImPSf-C is detected at 1507 cm⁻¹; such a discrepancy probably results from the presence of hexafluoroisopropylene group (–C(CF₃)₂–) in ImPSf-C, which is strongly electro-withdrawing and may cast a significant effect on the infrared absorption.

Fig. 3 FTIR spectra for the ImPSf-OH AEMs in comparison with CMPSf.

3.2 Backbone dependent properties of PSf AEMs bearing imidazolium cation

Properties of an AEM are closely related to the contents of cations in the membrane, i.e. the ion exchange capacity (IEC). High IEC normally will give high conductivity, but often results in compromised membrane stability and mechanical strength. A sound comparison of membrane performance among different AEMs should be based on comparable IEC. Therefore, we fabricated different PSf AEMs with comparable IEC values by employing chloromethylated PSf with similar degrees of chloromethylation as above mentioned. IEC values for the resulting ImPSf-A, -B and -C membranes are 1.78, 1.81 and 1.75 mmol g⁻¹, respectively.

3.2.1 Hydroxide conductivity. Hydroxide conductivity is determined by its cation structure (basicity), ion exchange capacity (IEC) and also the continuity of the ion transport channels (microphase separated morphology) in the membrane. These aspects have been well studied in the literature; apart from these, we find that polymer backbone structure is also an important factor.

Fig. 4A shows the conductivity profiles of the three polysulfone AEMs; their conductivities all increase with temperature as expected. It is worth noting that the ImPSf-A-OH membrane exhibits significantly higher conductivity than the other two. At 20 °C, the conductivity of ImPSf-A-OH is 19.68 mS cm⁻¹, while that of -B and -C membranes are 10.33 and 11.48 mS cm⁻¹, respectively. The three membranes have the same cation head groups and comparable IEC (1.78, 1.81 and 1.75 mmol g⁻¹ for ImPSf-A, -B and -C, respectively), so the remarkably different conductivities must result from their chain structure difference. The only difference among the three PSf chains is the presence or absence of the isopropylene group (–C(CH₃)₂–); this group is bulky and electro-repulsive, which will hinder chain packing and lead to low packing density. Low packing density means more free volume between the main chains and lower resistance of hydroxide transport in the membrane so that the ImPSf-A-OH conductivity is higher than the -B membrane. Very recently, Xu et al. reported an AEM containing N-methyl pyrrolidinium-C60 cations; this membrane shows greatly improved conductivity due to the “nano-cavity” formation (or increased free volume) in the membrane, which results from the presence of the bulky C60 moiety.³⁵ Similar effect also occurs in separation membranes where bulky pendant groups result in higher permeation.³⁶ As for ImPSf-C-OH, although it contains a large fluorinated isopropylene (–C(CF₃)₂–) group, the highly polar fluorine atoms may enhance inter-chain forces and offset the steric effect such that the hydroxide transport resistance is higher than that in ImPSf-A-OH membrane.

Fig. 4 (A) Temperature dependence and (B) Arrhenius plots of conductivities (σ) of different ImPSf-OH membranes.

The conductivity of ImPSf-A-OH membrane (19.68 mS cm⁻¹ @ 20 °C) is close to that of other membranes with similar IEC reported in the literature. Yan et al.'s PEEK-ImOH membrane (IEC = 1.73 mmol g⁻¹) exhibited a conductivity of ca. 20 mS cm⁻¹.¹¹ Guo et al. reported imidazolium functionalized AEM with a conductivity of 18.4 mS cm⁻¹ at IEC of 1.72 mmol g⁻¹.³³ A stable hydroxide-conducting membrane based on benzimidazolium hydroxide was reported by Holdcroft and coworkers; its conductivity was ca. 10 mS cm⁻¹ (IEC = 1.5 mmol g⁻¹).³⁷ Fig. 4B shows the Arrhenius plots of the membrane conductivities, where ln σ is plotted against 1000/T in the temperature range of 20–80 °C. From this figure, the ion transport activation energy (E_a) of different membranes can be obtained by the Arrhenius equation: E_a = −b × R, where b is the slope of the linear regression of ln σ-versus-1000/T curves, and R is the gas constant. The calculated E_a of ion transport in ImPSf-A-OH is 15.88 kJ mol⁻¹, evidently lower than the values of ImPSf-B-OH and -C (23.20 and 19.12 kJ mol⁻¹, respectively); this means more facile ion transport in the ImPSf-A-OH membrane. As for the comparison between -B and -C membranes, there is possibly a tradeoff between the bulky group related steric effect and the fluorine-related hydrophobicity of the main chain, so the ion transport behavior seems more complicated and needs further study in our future work.

3.2.2 Alkaline stability. Due to loose chain packing associated with the isopropylene moieties, the ImPSf-A-OH membrane exerts a lower resistance for hydroxide ion transport, and thus leads to higher conductivity but at the meantime, harsher hydroxide attack on the imidazolium cations will occur. Therefore, the ImPSf-A-OH membrane exhibits the poorest alkali stability among the three membranes as can be seen from Fig. 5: its conductivity decayed ca. 18% upon 168 h NaOH treatment, while that of the -B and -C membranes dropped by 11% and 7%, respectively. The conductivity decay is due to degradation of the imidazolium cations instead of backbone scission, because the condition for alkaline stability assessment is relatively mild (3 M NaOH, room temperature, 24 h) and not harsh enough to induce chain scission. According to the literature, cation induced backbone scission occurs when the membrane is exposed to high temperature and alkaline treatment for prolonged period. In our case, the membrane after the mild alkali treatment is still mechanically strong, showing intactness of the backbone, so the conductivity drop is caused mainly by decomposition of the imidazolium cation group.

Fig. 5 Retention of hydroxide conductivity of different ImPSf AEMs after being treated in 3 M NaOH at room temperature for 24 h.

3.2.3 Water uptake and swelling behavior. To substantiate the backbone influence on membrane conductivity and alkali stability, we further studied water uptake (WU) and swelling ratio (SR) of the three membranes. Again, they have comparable IEC values (1.75–1.81 mmol g⁻¹). As can be seen in Fig. 6, the ImPSf-A-OH membrane displays higher WU and SR than the other two at all temperatures studied (20, 40 and 60 °C). This agrees with the conductivity and alkaline stability profiles shown in Fig. 4 and 5, and confirms the effect of bulky isopropylene group. With the isopropylene group, the PSf-A chains are looser than PSf-B so that water is absorbed more easily into the ImPSf-A-OH membrane, leading to higher water uptake and swelling ratio. High water uptake in ImPSf-A-OH is beneficial for hydroxide ion transport in the membrane since water can function as a vehicle to facilitate the transport process. In ImPSf-C-OH, however, the strong electron negativity of fluorinated isopropylene group may strengthen the inter-chain forces and chain packing, and meanwhile, the fluorine atoms make the main chain more hydrophobic so that the water uptake and swelling ratio are low compared with ImPSf-A-OH.

Fig. 6 Water uptake (a) and swelling ratio (b) of different ImPSf-OH AEMs (IEC being ca. 1.75 mmol g⁻¹) at varied temperatures.

The fluorinated isopropylene contribution to the inter-chain forces and chain packing can be confirmed by comparing thermal stability of the three membranes. As shown in Fig. 7, they all underwent two main stages of weight loss at 100–800 °C: the first started at ca. 200 °C and was caused by imidazolium decomposition; the second started at ca. 400 °C due to main chain scission. Apparently the ImPSf-C-OH membrane shows the best thermal stability, and it provides better shield to the vulnerable imidazolium cations than the other two. Considering that these membranes have virtually the same cation ions (IEC), the high stability of ImPSf-C-OH may be related to the fluorinated isopropylene group, and the mechanism needs further investigation.

Fig. 7 The TGA curves of ImPSf-OH AEMs with similar IEC (ca. 1.75 mmol g⁻¹).

3.3 Mitigation of the main chain effect by side chain grafting

The above investigations suggest that the PSf-A backbone gives low alkali-stability of AEM relative to the other two backbone structures, and this is probably related to the loose packing effect of the bulky isopropylene group so that hydroxide ions attack the cations more easily than in the other two membranes. To further ascertain and mitigate the adverse effect of isopropylene group, we grafted PSf-A with poly(methallyl imidazolium) side chains; by doing so, the cations are placed relatively far away from the main chain and thus the isopropylene-related loose packing effect can be alleviated.

3.3.1 PSf-A-g-poly(methallyl imidazolium) or PgPIm synthesis. As shown in Fig. 8, CMPSf-A was employed as a macroinitiator to initiate atom transfer radical polymerization (ATRP) of methallylchloride (MAC) for synthesis of the grafted polymer PSf-A-g-PMAC; this polymer is then quaternized with imidazole to yield PSf-A-g-poly(methallhyl imidazolium), or PgPIm; subsequent alkalization with NaOH will give the final membrane structure, PgPIm-OH.

Fig. 8 Synthetic route of PgPIm-OH membrane.

Fig. 9A shows the ¹H-NMR spectrum of PSf-A-g-PMAC, where the peak at 4.48 ppm is assignable to the chloromethyl proton; this is consistent with the peak at 4.52 ppm corresponding to the proton of CH₂Cl group in CMPSf-A, demonstrating successful grafting of the side chains onto the PSf-A backbone. The chemical structure of PgPIm-OH (the imidazolium form of PSf-A-g-PMAC) is characterized by FTIR (Fig. 9B), where the absorption bands at 1540 and 1647 cm⁻¹ correspond to C–N and C=N stretch vibration in the imidazolium group respectively, and the band 3365 cm⁻¹ is assignable to the O–H stretch vibration. Similar bands are also detected in the spectra for ImPSf-A-OH because they have the same functional group.

Fig. 9 (A) ¹H-NMR of PgPIm-OH precursor in comparison with PSf-A; (B) FTIR of PgPIm-OH in comparison with CMPSf-A and ImPSf-A-OH.

3.3.2 Conductivity and alkaline stability of the PgPIm-OH membrane. With experimental conditions carefully adjusted, we got a PgPIm-OH membrane whose IEC is close to that of ImPSf-A-OH. Fig. 10A shows the hydroxide conductivities of the PgPIm-OH and ImPSf-A-OH membranes at varied temperatures. Despite comparable IEC (ca. 1.75 mmol g⁻¹), the former exhibits much higher conductivity (21–65 mS cm⁻¹) than the latter (20–54 mS cm⁻¹) at 20–80 °C. Fig. 10B shows the Arrhenius plots of the membrane conductivities; they show virtually the same slope of linear regression, meaning their ion transport activation energy values are close to each other according to the Arrhenius equation. Alkali stability of the two membranes was assessed by treating with 3 M NaOH solution at room temperature for different time durations and monitoring their change of conductivity. As shown in Fig. 10C, the PgPIm-OH membrane had its conductivity virtually unchanged after 48 h treatment while the un-grafted membrane experienced a 5% conductivity drop; when the alkali treatment was prolonged to 168 h, the PgPIm-OH membrane's conductivity dropped by 15.4% while that of the un-grafted membrane dropped by 17.6%. These stability results, albeit not significant enough compared with some of those reported in the literature for imidazolium AEMs,^{21,23,33,38–40} confirm that side-chain grafting has mitigated the adverse effect of the isopropylene group and improved alkaline stability of the AEM. With the imidazolium functionalized side chain, the membrane will undergo higher degree of microphase separation and thus have higher conductivity;^41–43 meanwhile, the imidazolium cation is located relatively far away from the loosely packed main chain so that the free volume in the vicinity of cation is reduced and therefore, the hydroxide access to and attack on the cation become more difficult, giving rise to improved alkali stability of the membrane.

Fig. 10 (A) Temperature dependence and (B) Arrhenius plots of conductivities (σ) of PgPIm-OH and ImPSf-A-OH membranes. (C) Retention of conductivities of these membranes after being treated with 3 M NaOH at room temperature for different time durations.

The mitigation effect of side chain grafting is further reflected from the swelling behavior of AEMs (Fig. 11A). The grafted membrane shows lower swelling ratio than the un-grafted one with similar IEC at all temperatures studied. This is because the cations are located away from the backbone and have less swelling power on the main chain, which is the main factor of swelling behavior. The off-backbone cations can also be better protected by the framework of backbone, leading to superior thermal stability relative to the un-grafted one as shown by the thermogravimetric analysis in Fig. 11B.

Fig. 11 (A) swelling ratio and (B) thermogravimetric curves of PgPIm-OH and ImPSf-A-OH membranes.

4. Conclusions

We have fabricated and studied properties of polysulfone AEMs with varied backbone structures, the same cation and comparable IECs; these membranes display markedly different conductivity and alkaline stability. In particular, the presence of isopropylene group in the main chain led to improved hydroxide conductivity but lowered alkali stability of the AEM; this is because the isopropylene steric effect may cause loose chain packing so that ion transport resistance is reduced and hydroxide attack on the cation becomes more severe. For confirmation and mitigation of the above effect, we grafted imidazolium functionalized side chains on the isopropylene-containing polysulfone, where the cations are located relatively far from the loosely packed backbone. In this way, the free volume in the vicinity of cations is reduced, and therefore, hydroxide access to the cation is hindered and its attack on the cations can be alleviated. The side-chain grafted AEM shows improved alkali tolerance; its conductivity maintained stable after treatment with 3 M NaOH for 48 h while that of the un-grafted membrane dropped significantly. The grafted membrane also exhibited good conductivity (21–64 mS cm⁻¹ at 20–80 °C), higher than that of the un-grafted membrane. This work provides new understanding on influencing factors of AEM conductivity and stability, and is beneficial for designing high-performance electrolyte membranes for alkaline fuel cell application.

Acknowledgements

We acknowledge the financial supports from the Natural Science Foundation of China (grant No. 21276252), the Basic Research Fund of Central Universities China (grants No. DUT14RC(3)020), the State Key Laboratory of Fine Chemicals (Panjin) (Grant No. JH2014009) and the Natural Science Foundation of Liaoning, China (2015020630).

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