Constructing pendent imidazolium-based poly(phenylene oxide)s for anion exchange membranes using a click reaction

Abstract

A series of poly(phenylene oxide)s (PPOs) bearing a flexible pendent imidazolium cation were prepared by an azide–alkyne cycloaddition between azidomethylated PPO and a novel alkyne-containing imidazolium, and their structures were confirmed by ¹H NMR, ¹³C NMR, and FT-IR. The corresponding anion exchange membranes (AEMs) showed distinct hydrophobic/hydrophilic phase-separated morphology at higher imidazolium content, as evidenced by AFM and SAXS techniques, which favors for the construction of interconnected hydroxide transport channels. As a result, the as-prepared AEMs exhibited higher conductivity (95 mS cm⁻¹, 80 °C, 100% RH) than conventional imidazolium benzylic-type AEM (55 mS cm⁻¹, 80 °C, 100% RH) with even lower IEC. Furthermore, the introduction of a 1,2,3-triazole moiety into the polymer side chain does not compromise its thermal and alkaline stability. This investigation demonstrated that the “click chemistry” strategy will benefit further tailoring of high performance AEMs with “side-chain-type” architectures.

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Introduction

With the world’s focus on reducing our dependency on fossil fuels, polymer electrolyte fuel cells (PEFCs), considered to be one of the most promising clean energy sources, have been the subject of intense research efforts in the industrial and scientific community.¹ In general, there are basically two types of PEFCs, proton exchange membrane fuel cells (PEMFCs) and anion exchange membrane fuel cells (AEMFCs), based on an acidic and basic working medium, respectively. After long-term efforts, the technology for PEMFCs has gradually matured, however, large-scale commercialization was hampered by the intrinsic drawbacks of high cost of perfluorinated membranes, high fuel permeability, and exclusive dependence on precious metal-based electrocatalysts. On the other hand, compared to PEMFC, AEMFCs have the potential to overcome these limitations by operating under alkaline conditions.^1d,2 In a basic environment, non-precious metals instead of platinum can be utilized as catalyst owing to the reduced over-potential of cathode oxygen reduction and the less corrosive basic medium.³ Further, since the ion transport is in the opposing direction to fuel transmission, the problem encountered in PEMFC of fuel crossover can be largely overcome. Consequently, AEMFCs have been gaining more and more attentions and been extensively explored.⁴

However, anion exchange membranes (AEMs), one of the key components in AEMFCs, often suffer from comparatively low ion conductivity⁵ and alkaline instability.⁶ Over the past decade, many efforts have been made and there is some consensus among researchers that the conductivity can be improved by two ways: (1) optimizing the OH⁻ transport channels via the construction of hydrophobic/hydrophilic phase separation by using multiblock copolymers and/or pendent cation ion type polyelectrolytes;⁵ (2) improving the mobility of OH⁻ from bolted cations using high basicity functional groups, such as guanidinium and quaternary phosphonium.^4a,c Unfortunately, the preparation of high basicity cations needs multi-step reactions and particular skills, and their chemical stability still lacks comprehensive evaluation. Recently, several studies demonstrate that quaternary ammonium groups, the most commonly used cations in AEMs, isolated from the polymer backbone by a flexible long linker possess an evident phase separated structure and thereby improved hydroxide conductivity.⁷ However, attaching functional cations to polymer backbones via flexible spacers is difficult to achieve and the reactions require strictly control. For example, Tomoi and coworkers^7d reported on a series of spacer-modified anion exchange resins with enhanced stability, but the specialized ω-bromoalkylstyrene monomer has only been used in polystyrene-based AEM. Jannasch et al.^7e attached quaternary ammonium groups to the PPO backbone via flexible heptyl side chains, however, the preparation of precursor bromoalkylated PPO needs to suppress the crosslinking reactions. Therefore, it is intriguing to develop a method that can facilely introduce cations with long spacers into the polymer backbone utilizing the most popular halomethylated benzylic-type polymer backbones.

Given the azide–alkyne click chemistry has characteristics of high efficiency, good adaptability, and solvent insensitivity,⁸ the introduction of flexible spacers bearing cation ions into polymer main chain using azide–alkyne cycloaddition is of great merit. Moreover, the resulting 1,2,3-triazole moiety has the potential to form hydrogen bonds with water and hydroxide,⁹ which facilitates improving hydroxide conductivity. Therefore, we report herein a novel PPO-based AEM with a “side-chain-type” structure of pendent imidazolium, which is formed by azide–alkyne cycloaddition between azidomethylated PPO and an alkyne-containing imidazolium, and focus on the effects of the “side-chain-type” structure on the properties of the resultant AEMs. In addition, the corresponding traditional AEM with imidazolium groups located in benzylic sites was synthesized and investigated for comparison.

Experimental

Materials

Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) was kindly supplied by Saudi Basic Industries Corporation. Tosyl chloride, 1-octyne, sodium ascorbate, triethylene glycol monomethyl ether, N-bromosuccinimide (NBS) were purchased from Aladdin Industrial Corporation and used without further purification. 1,2-Dimethylimidazole (Heowns Biochemical Technology Co., Ltd, AR) and N-methyl-2-pyrrolidone (NMP, Sinopharm Chemical Reagent Co., Ltd, AR) were purified by distillation under vacuum before use. N,N,N′,N′,N′′-Pentamethyldiethylenetriamine (PMDETA, TCI, AR) and 5-chloro-1-pentyne (TCI, AR) were used without further purification. 2,2′-Azobisisobutyronitrile (AIBN, Sinopharm Chemical Reagent Co., Ltd, AR) was recrystallized from ethanol. Copper(I) bromide (CuBr) was purified by stirring with glacial acetic acid for 2 hours, filtrating, and then washing with glacial acetic acid, ethanol and diethyl ether in succession, and finally stored in a glovebox before use. CuSO₄·5H₂O was obtained from Guangzhou Xinbo Chemical Co. Ltd. and recrystallized from water twice. Unless otherwise noted, the other chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd and used as received.

Synthesis of 1,2-dimethyl-3-(4-pentynyl)imidazolium chloride

1,2-Dimethylimidazole (12 mmol, 1.16 g), 5-chloro-1-pentyne (10 mmol, 1.02 g), and chloroform (2 mL) were added to a 25 mL round-bottomed flask equipped with a magnetic stirrer. This solution was heated to reflux and stirred for about 2 days. After cooling, the solvent was removed under vacuum. The residue was purified by chromatography on silica gel using DCM–MeOH (7/1, v/v) as eluent to provide 1,2-dimethyl-3-(4-pentynyl)imidazolium chloride (1.05 g, yield: 44%) as a white powder.

¹H NMR (600 MHz, CDCl₃) δ 2.08–2.12 (m, 2H), 2.11 (t, J = 2.6 Hz, 1H), 2.33 (td, J₁ = 6.7 Hz, J₂ = 2.6 Hz, 2H), 2.84 (s, 3H), 4.03 (s, 3H), 4.43 (t, J = 7.0 Hz, 2H), 7.71 (d, J = 2.0 Hz, 1H), 7.81 ppm (d, J = 2.0 Hz, 1H); ¹³C NMR (150 MHz, CDCl₃) δ 144.12, 123.43, 121.56, 81.94, 70.83, 47.25, 36.07, 28.20, 15.43, 10.70 ppm; HRMS: (ESI†) calcd for (C₁₀H₁₅N₂)⁺ (M⁺) 163.1235; found 163.1228.

Synthesis of 1-methoxytri(ethylene glycol)-4-hexyl-1,2,3-triazole

1-Octyne (2.81 mmol, 0.31 g), 1-azido-2-[2-(2-methoxyethoxy)ethoxy]ethane (2.38 mmol, 0.45 g), PMDETA (0.57 mmol, 0.10 g) and DMF (8 mL) were added into a Schlenk flask. After the mixture was degassed by three freeze–vacuum–thaw cycles, CuBr (0.28 mmol, 0.04 g) was quickly introduced into the flask under the protection of nitrogen flow. After degassing by three additional freeze–pump–thaw cycles, the tube was sealed under nitrogen atmosphere and then immersed in a water bath thermostated at 60 °C. After the click reaction had proceeded for 24 h, the reaction was quenched by purging with air into the system. Then the crude product was extracted by dichloromethane and the organic layer was dried over Na₂SO₄, and concentrated under reduced pressure. Finally, 0.68 g (yield: 90%) of the pure product was obtained as a pale yellow oily liquid after purification by silica gel column chromatography (PE/EtOAc = 6/1, v/v).

¹H NMR (600 MHz, CDCl₃) δ 0.88 (t, J = 6.7 Hz, 3H), 1.27–1.38 (m, 6H), 1.67 (m, 2H), 2.71 (t, J = 7.6 Hz, 2H), 3.38 (s, 3H), 3.54 (t, J = 4.1 Hz, 2H), 3.62 (m, 6H), 3.86 (t, J = 5.1 Hz, 2H), 4.51 (t, J = 5.1 Hz, 2H), 7.47 ppm (s, 1H); ¹³C NMR (150 MHz, CDCl₃) δ 148.41, 121.95, 72.05, 70.67, 70.65, 70.62, 69.76, 59.19, 50.23, 31.73, 29.62, 29.10, 25.82, 22.72, 14.22 ppm; HRMS (ESI†) m/z calcd for C₁₅H₃₀N₃O₃ (M + H)⁺ 300.2287; found 300.2271.

Synthesis of bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO)

A typical procedure for the preparation of BPPO was as follows: to a stirred solution of PPO (24.96 mmol, 3.00 g) in carbon tetrachloride (120 mL), N-bromosuccinimide (11.24 mmol, 2.01 g) and AIBN (0.34 mmol, 0.05 g) were added. The mixture was heated to 60 °C, stirred for 24 h and then filtered. The filtrate was concentrated and dropwise added into a 10-fold excess of ethanol to precipitate the product. The product was then redissolved in chloroform (120 mL) and reprecipitated in ethanol twice. After drying in vacuum, the product (BPPO-31) was obtained as a pale yellow powder (3.30 g, yield: 92%). Gel permeation chromatography analysis results (polystyrene standards): M_n,GPC = 20 100 g mol⁻¹ and the polydispersity index (PDI) = 2.25. ¹H NMR (CDCl₃, 600 MHz): δ 2.09 (s, 16.12H), 4.34 (s, 2H), 6.47–6.53 (m, 4.45H), 6.64–6.71 ppm (m, 1.83H). The degree of bromination under these reaction conditions was 31%, calculated from its ¹H NMR spectrum.

By the same procedure, 8.74 mmol (1.55 g) of N-bromosuccinimide and 0.26 mmol (0.04 g) of AIBN gave BPPO-19 (2.82 g, yield: 83%, bromination degree: 19%); additionally, a 40% bromination degree product (BPPO-40) was synthesized with a similar procedure, except that N-bromosuccinimide (13.73 mmol, 2.45 g) and AIBN (0.41 mol, 0.07 g) were added (3.30 g, yield: 87%).

Synthesis of azidomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (APPO)

APPO was synthesized by displacement of the corresponding benzyl bromide of BPPO with sodium azide. The typical procedure, described hereafter for the case of APPO-31, was carried out as follows: to a stirred solution of BPPO-31 (19.39 mmol, 2.80 g) in NMP (80 mL), sodium azide (49.01 mmol, 3.19 g) was added. The mixture was heated to 60 °C and stirred for 24 h, and then cooled and slowly poured into deionized water. The off-white product (APPO-31) was filtered and thoroughly washed with water and dried under vacuum at 50 °C for 24 h with a yield of 99% (2.55 g).

¹H NMR (CDCl₃, 600 MHz): δ 2.09 (s, 16.12H), 4.34 (s, 2H), 6.47–6.53 (m, 4.45H), 6.64–6.71 ppm (m, 1.83H).

Synthesis of pendent imidazolium-functionalized PPO (PPO-PIm) polymer

APPO-31 (9.04 mmol, 1.22 g), 1,2-dimethyl-3-(4-pentynyl)imidazolium chloride (3.36 mmol, 0.68 g), sodium ascorbate (0.67 mmol, 0.13 g), deionized water (4 mL), and THF (40 mL) were added into a Schlenk flask. After the mixture was degassed by three freeze–vacuum–thaw cycles, CuSO₄·5H₂O (0.34 mmol, 0.09 g) was quickly introduced into the flask under the protection of nitrogen flow. After degassing by three additional freeze–pump–thaw cycles, the tube was sealed under nitrogen atmosphere and then immersed in a water bath thermostated at 60 °C. After the click reaction had proceeded for 24 h, the reaction was quenched by purging with air into the system. The reaction mixture was poured into acetone and an off-white precipitate was collected to give PPO-PIm-31 powder with a yield of 98% (0.72 g).

¹H NMR (CDCl₃, 600 MHz): δ 2.01 (m, 16.57H), 2.57 (m, 4.64H), 3.74 (s, 3.00H), 4.17 (s, 2.10H), 5.38 (s, 1.87H), 6.50–6.67 (m, 6.51H), 7.68–7.82 ppm (m, 3.18H).

In addition, the traditional membrane PPO-Im-34, in which an imidazolium group is located in the benzylic sites of PPO and with a bromination degree of 34%, was prepared by the method reported in previous literature.¹⁰

Membrane preparation

The PPO-PIm-31 membrane with a counter ion of Cl⁻ was prepared by solution casting from NMP. A solution of the dried PPO-PIm-31 powder in NMP with a concentration of ∼10 wt% was filtered and then cast onto a glass plate, and dried at 80 °C overnight. After further drying in a vacuum oven for another 12 h at 80 °C, the membrane was soaked in deionized water for 2 days to remove the residual NMP. The resultant polymeric membrane was immersed in 1 M NaOH at room temperature for 24 h three times to ensure a complete conversion from the Cl⁻ to OH⁻ form. Finally, the obtained anion exchange membrane (AEM) with the counter ion of OH⁻ was immersed in deionized water for 24 h and washed thoroughly with deionized water until the pH of the residual water was neutral.

All other membranes PPO-PIm-x (where x originates from the degree of bromination) were prepared using similar procedures as described above.

Characterization and measurements

NMR spectra were recorded on an Agilent 600 MHz spectrometer using CDCl₃ or DMSO-d₆ as solvent. Fourier transform infrared (FT-IR) spectra of the polymers were recorded on a Perkin-Elmer Spectrum 100 IR spectrometer in the range of 4000–400 cm⁻¹ using KBr pellets at room temperature. Gel Permeation Chromatography (GPC) was performed on a Waters 1515 pump and Waters 2414 differential refractive index (RI) detector (set at 40 °C) using a series of linear Styragel HR1, HR2 and HR4 columns. Molecular weight and polydispersity data are reported relative to polystyrene standards. The eluent was tetrahydrofuran (THF) at a flow rate of 0.3 mL min⁻¹. Thermal analysis was carried out by Netzsch TG 209 F3 thermogravimetric analyzer (TGA) and all the samples were heated from 30 to 700 °C at a heating rate of 10 °C min⁻¹ under a nitrogen flow. The atomic force microscopy (AFM) studies were performed with a Bruker Dimension FastScan Atomic Force Microscopy in the tapping mode. Small-angle X-ray scattering (SAXS) measurements were performed using a SAXSess (Anton Paar) equipped with a Kratky block-collimation system. The scattering pattern was recorded on an imaging plate (IP) with a pixel size of 42.3 × 42.3 μm² which extended to the high-angle range (the q range covered by the IP was from 0.06 to 29 nm⁻¹). High-resolution mass spectra (HRMS) were obtained with Waters LCT Premier XE mass spectrometer by ESI on a TOF mass analyzer.

Water uptake and swelling ratio

The water uptake (WU) and the swelling ratio of the membranes can be evaluated by weight analysis and linear expansion ratio (LER), respectively. The membrane samples (4 cm in length and 1 cm in width) were immersed in water at a given temperature for 2 days, then the membranes were taken out, the excess water on the surface was wiped with tissue papers, and the weight and length of the wet membranes were quickly measured. Afterwards, the membranes were completely dried in vacuum oven at 60 °C for 24 h and the weight and length were quickly measured. Finally, the WU and LER were respectively calculated by the following equations:

where W_wet and W_dry are the mass of the wet and dry membranes; L_wet and L_dry are the length of the wet and dry membranes, respectively.

Hydroxide conductivity

The hydroxide conductivity was obtained in a frequency range from 1 Hz to 1 MHz using an AUTOLAB PGSTAT 302 by a four-point probe ac impedance method. The samples (4 cm in length and 1 cm in width) were measured in 100% relative humidity (RH) at different temperatures, and the hydroxide conductivity (σ) was calculated by the following equation:
where R is the membrane resistance, L is the distance between potential-sensing electrodes, W and d refer to the width and thickness of the membrane, respectively.

Alkaline stability

The alkaline stability of the obtained AEMs was evaluated by the changes of ionic conductivity after soaking the membrane in 1 M NaOH aqueous solution at 80 °C for several days. Moreover, in order to evaluate the alkaline stability of 1,2,3-triazole group, 1-methoxytri(ethylene glycol)-4-hexyl-1,2,3-triazole as a model compound was dissolved in 1 M NaOH–D₂O at 80 °C and the ¹H NMR spectral variations detected in situ after several days alkaline hydrolysis.

Results and discussion

Synthesis and characterization of terminal alkyne-containing imidazolium and the model compound

The terminal alkyne-containing monomer, 1,2-dimethyl-3-(4-pentynyl)imidazolium chloride, was synthesized by quaternization of 5-chloro-1-pentyne and 1,2-dimethylimidazole in chloroform. 1,2-Dimethylimidazole was selected as a precursor because of its higher alkaline stability than the commonly used 1-methylimidazole,^6a,11 where alkaline instability is a catastrophic obstacle facing AEMs. Owing to the poor nucleofuge behavior of alkyl chlorides, the increase of the reaction time and the reaction temperature would contribute to improving the productivity. The chemical structure of 1,2-dimethyl-3-(4-pentynyl)imidazolium chloride was confirmed by ¹H NMR, ¹³C NMR, and HRMS, as shown in the ESI.†

In order to evaluate the alkaline stability of the 1,2,3-triazole ring, which acts as a “linker” between the polymer backbone and the functional imidazolium, 1-methoxytri(ethylene glycol)-4-hexyl-1,2,3-triazole as a model compound was synthesized, as illustrated in Scheme S1.† In addition, to improve the solubility of the model compound in water, it had a triethylene glycol group designed and introduced into it, and the chemical structure was evidenced by ¹H NMR, ¹³C NMR, and HRMS (see ESI†).

Synthesis and characterization of the polymers

A series of imidazolium-functionalized PPO polymers, PPO-PIm-x were synthesized in three steps, that is, bromination, azidation, and azide–alkyne cycloaddition (Scheme 1). Since the bromination reaction is often accompanied by slight but undesirable formation of methyl-dibrominated derivatives, the reaction conditions had to be carefully controlled. The optimized conditions were at 60 °C and AIBN as radical initiator to achieve a desirable degree of bromination without dibromomethyl byproducts. The bromination degree of PPO was controllable by changing the amounts of N-bromosuccinimide, and was kept in the range of 20–40% to ensure the IEC value of the later imidazolium-functionalized PPO was within the proper range, which is the key factor to determine the membrane’s hydroxide conductivity, water uptake, and dimensional stability. The chemical structure and the degree of bromination of the brominated PPO (BPPO) were analyzed by ¹H NMR spectroscopy. As shown in Fig. 1, a new peak assignable to the methylene protons of the bromomethyl groups was observed at 4.34 ppm, which demonstrates the success of bromination.¹² The degree of bromination was estimated from the integral ratio of the methylene protons of the bromomethyl groups (4.34 ppm) to the protons of the residue methyl protons at 2.09 ppm.

Scheme 1 Synthetic routes employed for the preparation of PPO-PIms.

Fig. 1 ¹H NMR spectra of BPPO-31 (CDCl₃), APPO-31 (CDCl₃) and PPO-PIm-31 (DMSO-d₆) in Cl⁻ form.

The methyl azidation reaction of the PPO was carried out with BPPO and NaN₃ in NMP as solvent for 24 h at 60 °C, and the complete azidation was confirmed by the shift of the ¹H NMR signal at δ = 4.34 ppm to a higher magnetic field at 4.21 ppm of methylene protons (Fig. 1). Furthermore, the azidation reaction was also confirmed by the appearance of the characteristic peak specific to the azide group at 2102 cm⁻¹ in the FT-IR spectrum,¹³ as shown in Fig. S1.†

The terminal alkyne-containing imidazolium, 1,2-dimethyl-3-(4-pentynyl)imidazolium chloride, was synthesized and introduced into the backbone of the PPO for the formation of the PPO-PIm polymer by a click reaction. The ¹H NMR spectrum showed that new peaks appeared at 7.80 ppm (H of the triazole), 4.17 and 2.57 ppm (H of alkyl group); at the same time, the methylene protons (peak f) connected to the PPO backbone shifted to low field after the click reaction. Interestingly, the ¹H NMR signals of the C2-methyl protons of the imidazolium ring completely disappeared, which may be attributed to the fact that considerable hydrogen/deuterium (H/D) exchange occurred at the 2-methyl group with the presence resultant 1,2,3-triazole moiety, acting as a weak base.¹⁴ Furthermore, the grafting degree of PPO-PIm calculated from its ¹H NMR spectrum was in good agreement with the aforementioned bromination degree of PPO, indicating a quantitative alkyne–azide click reaction had occurred. The FT-IR absorption of the azide at 2102 cm⁻¹ disappeared completely after the click reaction (see Fig. S1†), also confirming the quantitative transformation.

Water uptake, swelling ratio, and thermal property

Water uptake is known to have a vital effect on the hydroxide conductivity and dimensional stability of AEMs. The water uptake, hydration number (λ, designated as the number of absorbed water molecules per imidazolium group), and in-plane swelling ratio of the hydrated PPO-PIm membranes along with the PPO-Im-34 (IEC = 1.89 meg g⁻¹) at 30 °C are summarized in Table 1. As expected, the water uptake and swelling ratio increases with increasing IEC. For example, PPO-PIm-40 showed 83% water uptake and 27% in-plane swelling ratio, which was much higher than PPO-PIm-19 (9% water uptake and 5% swelling ratio) with a low IEC value of 1.15 meq g⁻¹. It could also be found that the water uptake of the PPO-PIm-40 membrane is higher than that of the PPO-Im-34 membrane with a comparable IEC value, which is considered to be related to the triazole groups absorbing more water molecules via hydrogen bonds to its nitrogen atoms.⁹ Usually, appropriate water uptake facilitates hydroxide conductivity, while excessive water absorption would lead to an undesirable membrane swelling and lower the conductivity because of the decline of ionic concentration.¹⁵ Although PPO-PIm-40 showed high water absorption, the membrane showed good membrane toughness and could be free-standing (Fig. S2†), and the conductivity with the value of 68 ms cm⁻¹ at 60 °C was still higher than that of PPO-Im-34 (IEC = 1.89 meg g⁻¹, 45 ms cm⁻¹), suggesting good hydroxide transportation efficiency of PPO-PIm membrane.

Table 1 Ion exchange capacity (IEC), water uptake (WU), swelling ratio, hydration number, and hydroxide ion conductivity of PPO-PIm-x and PPO-Im-34 AEMs

Membrane	IEC^a (meg g^−1)	WU^b (%)	Swelling ratio^b (%)	λ^c	Conductivity^d (mS cm^−1)
					30 °C	60 °C
a Calculated from ^1H NMR spectra.b After immersing in water at 30 °C for 48 h.c Number of absorbed water molecules per imidazolium group.d Measured in water.
PPO-PIm-19	1.15	18	5	8.7	26	41
PPO-PIm-31	1.59	45	19	15.8	32	49
PPO-PIm-40	1.85	83	27	24.9	38	68
PPO-Im-34	1.89	38	8	11.1	29	45

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The thermal stabilities of the membranes were evaluated by TGA. Fig. 2 shows the TGA and DTG curves of PPO-PIm-x and PPO-Im-34 AEMs in OH⁻ form under a nitrogen flow. Different from PPO-Im-34 with three weight loss steps, the PPO-PIm-x membranes appear to have four distinct weight loss steps. The initial weight loss step observed below 120 °C is likely to be associated with the loss of water absorbed by hygroscopic polymers, and is dependent on their IEC values. By comparing with the TGA curve of PPO-Im-34,¹⁰ the weight loss at about 250 °C and above 400 °C can be inferred to be due to the degradation of the imidazolium groups and polymer backbone, respectively. Therefore, the additional weight loss at about 320 °C may be from the decomposition of the 1,2,3-triazole moiety or alkyl spacer units.^7a Therefore, the introduction of a triazole group by alkyne–azide click chemistry should not deteriorate the thermal stability of AEMs, because its thermal decomposition temperature was above the operating temperatures of typical AEMFC.

Fig. 2 (a) TGA and (b) DTG curves of the PPO-PIm-x and PPO-Im-34 membranes.

Hydroxide conductivity

The hydroxide conductivity of AEMs is crucial to the development of AFCs, and the data of the PPO-PIm-x membranes along with PPO-Im-34 for comparison are plotted in Fig. 3 as a function of temperature. Similar to previous reports,^4f,16 the conductivity of the AEMs monotonously increased with increasing IEC, because more ion groups will directly participate in ion transport and the volume fraction of water also increased in the membrane matrix which is helpful for ion transportation. On the other hand, with an increase in temperature, the ion conductivities of all membranes increased significantly, resulting from the simultaneous enhancement of free volume and mobility ability for ion transport as the temperature increased.^6a In the entire temperature range studied, it should be pointed out that the PPO-PIm-40 showed much higher hydroxide conductivity than that of PPO-Im-34, even though the IEC value of PPO-Im-34 (1.89 meq g⁻¹) is slightly higher than that of PPO-PIm-40 (1.85 meq g⁻¹). This may be explained by the fact that the hydrophilic imidazolium cation is separated from the hydrophobic PPO backbone by a flexible linker, which facilitates imidazolium ion aggregation, inducing a considerable hydrophilic–hydrophobic microphase separation and thereby improving ion conductivity.¹⁷ Furthermore, the 1,2,3-triazole moieties can provide proper sites for hydrogen bonds, favour ionic cluster aggregation and large hydration numbers for the rapid movement of hydroxide carriers.^9,18 In addition to high hydroxide conductivity, it is worth noting that the ion conductivity of PPO-PIm-x membranes exhibited higher growth rates than that of PPO-Im-34 at higher temperatures (60–80 °C), indicating the more adaptive nature of the PPO-PIm membranes in high temperature AFCs. These results point out the potential benefits of designing an AEM bearing a flexible linker separating the hydrophobic backbone and hydrophilic cations, and the impact of the linker’s length will be systematically studied in our future work.

Fig. 3 Temperature dependence of the hydroxide conductivity of the PPO-PIm-x membranes and PPO-Im-34 at 100% RH.

Morphologies of the membranes

The morphologies of the PPO-PIm membranes were investigated by atomic force microscopy (AFM) and small-angle X-ray scattering (SAXS) analyses. The tapping mode AFM phase images of PPO-PIm membranes were recorded under ambient conditions on a 1 μm × 1 μm size scale, as shown in Fig. 4. All the AFM images exhibited a clear microphase separation morphology, in which the bright and dark regions correspond to hydrophobic polymer backbone and hydrophilic ionic clusters containing a certain amount of water, respectively.^6b Generally, the hydrophobic domains are responsible for the dimensional stability of the membrane, while the hydrophilic counterparts absorb water and are in charge of hydroxide transport. By comparing the AFM images of the membranes with different imidazolium content, it is clear that the interconnectivity of the hydrophilic domain improves with increasing imidazolium content. In the case of PPO-PIm-19, which has an IEC value of 1.15 meq g⁻¹, irregular and discrete hydrophilic clusters were observed. However, the interconnectivity of hydrophilic clusters appears to be more pronounced for PPO-PIm-40 membranes with an IEC value of 1.85 meq g⁻¹, and the interconnected ionic channels in the membrane like an “expressway network” could efficiently facilitate hydroxide transport, resulting in the improvement of the conductivity. It is worth noting that although the connectivity of the hydrophilic domains improved with increasing imidazolium content, the size remains substantially constant.

Fig. 4 AFM phase images of (a) PPO-PIm-19, (b) PPO-PIm-31, (c) PPO-PIm-40 in Cl⁻ form.

The microphase separation structure of the membrane was also confirmed by SAXS measurements. As illustrated in Fig. 5, the membranes of PPO-PIm-31 and PPO-PIm-40 showed characteristic peaks, indicating the formation of microphase separation with ionic domains. However, the broad peak profile and the absence of higher order scattering peaks in the SAXS curves, suggests that a weak separated phase and no long-range ordered structure. The interdomain spacings between the ionic clusters calculated from d = 2π/q_max of PPO-PIm-31 and PPO-PIm-40 were both ca. 4.3 nm in size, and seem to be independent of IEC value, which may be attributed to the same “spacer” length between polymer backbone and imidazolium ions. Furthermore, even longer “spacers” will be studied in our future work. Different from the aforementioned PPO-PIm-31 and PPO-PIm-40, PPO-PIm-19, for the membrane with lowest IEC of 1.15 meq g⁻¹, almost no scattering peak was observed. These results reveal that the ion content and the “spacer” length between the ion and polymer backbone have a strong influence on the membrane’s morphology.

Fig. 5 SAXS profiles of dry PPO-PIm-x AEMs in the Cl⁻ form.

Alkaline stability

The alkaline stability is a critical issue for evaluating the practical use of AEM in AFCs.^1c Owing to the introduction of a 1,2,3-triazole moiety into the polymer side chains, its alkaline stability should be initially evaluated in the AEM operating environment. Therefore, a model molecule, 1-methoxytri(ethylene glycol)-4-hexyl-1,2,3-triazole, containing a 1,2,3-triazole ring was designed and synthesized, and its alkaline stability was investigated in situ by ¹H NMR spectroscopy. ¹H NMR spectra of model molecule in 1 M NaOD aqueous solution at 80 °C are illustrated in Fig. S3.† Apart from the triazole proton disappearing (H/D exchange), all proton signals were keep intact and no new peaks were detected after 7 days (168 h) of alkaline hydrolysis, indicating a good alkaline stability of the 1,2,3-triazole moiety.

Then, the alkaline stability of PPO-PIm-x membranes was investigated by monitoring the variation in conductivity over time for membranes soaked in 1 M NaOH solution at 80 °C. As illustrated in Fig. 6, PPO-PIm-19 and PPO-PIm-31 showed a slight loss of conductivity (∼5%) after seven days of aging test. This good alkaline stability can be interpreted by the imidazolium group being attached via alkyl spacers to the PPO backbone, in accordance with the previous reported quaternary ammonium (QA)-based AEM results.^7e,19 However, a considerable decrease of about 11% in conductivity for the PPO-PIm-40 membrane with higher IEC, suggests that the alkaline stability of the membrane is dependent on its ion content. Considering the stability of the aforementioned 1,2,3-triazole group, the attenuation in conductivity mainly resulted from the degradation of 1,2,3-trialkylimidazolium cations.^20,21 Nevertheless, take into account the tailorability of click chemistry and the stability of the concomitant 1,2,3-triazole moiety, the stability of the membrane can be improved by using high stable cations, such as the recent reported pyrrolidinium and modified imidazolium cations.^6c,21,22

Fig. 6 Hydroxide conductivity of PPO-PIm-x AEMs after immersion in 1 M NaOH solution at 80 °C for several days.

Conclusions

In summary, novel PPO ionomers (PPO-PIm-x) with imidazolium moieties linked via flexible spacer units were prepared by using typical click chemistry. The TGA results showed that the incorporation of a flexible linkage and clicked 1,2,3-triazole group did not deteriorate the thermal stability of the membrane. The as-prepared AEMs exhibited higher hydroxide conductivity than the traditional benzylic dimethylimidazolium AEM with a similar IEC, which is favorable in practical applications. This is probably due to the formation of continuous ionic nanochannels resulting from hydrophilic ion cluster aggregation and considerable microphase separated structure, as evidenced by high water uptake, AFM, and SAXS results. Furthermore, alkaline stability tests confirmed that the 1,2,3-triazole ring was quite stable in 1 M NaOH solution at 80 °C for several days and the PPO-PIm-x AEMs showed good alkaline stability.

Acknowledgements

The work was supported by National Natural Science Foundation of China (No. 21404030, 51373045) and Fundamental Research Funds for the Central Universities of China (No. 2014HGQC0014). H. W. thanks Prof. Tongwen Xu at University of Science and Technology of China for all tests of hydroxide conductivity and Mr Sheng Wang at Peking University for SAXS measurement.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra17748k

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