Effect of the bis-imidazolium-based poly(ionic liquid) on the microstructure and the properties of AAEMs based on polyvinyl alcohol

Abstract

A novel poly(ionic liquid) (PIL) based on bis-imidazolium is designed to improve the performances of alkaline anion exchange membrane (AAEM). As a result, the bis-imidazolium-based PILs AAEMs (PVA/DC_nPIL) exhibit lower swelling ratio (higher dimensional stability) and higher chemical stability than that of mono-imidazolium-based PIL AAEM (PVA/C₄PIL) under the same IEC value. Meanwhile, the length of the side chains of PILs was changed from the double imidazolium, which induces a different microstructure that can affect the ionic conductivity and stability obviously. The SAXS results show that the PVA/DC₈PIL can generate larger ionic clusters for facilitating the formation of interconnected broad ionic channels, which leads to higher conductivity in comparison with the PVA/DC_nPIL (n = 4, 12) membranes. These observations unambiguously indicate that the designed bis-imidazolium-based PIL with an appropriate length of hydrophobic chains is an effective approach to increase the ion conductivity, dimensional and chemical stability of AAEMs.

---

Introduction

Recently, alkaline anion exchange membrane fuel cells (AAEMFCs) have evoked great interest due to their much faster kinetics of fuel electro-oxidation in alkaline environment than those present in acidic media.^1,2 AAEMFCs show a more efficient oxygen reduction reaction at the cathode compared with proton exchange membrane fuel cells,³ and reduced amount of the platinum catalyst or noble catalysts used, allowing further cost reduction.^4,5 Furthermore, the use of an anion exchange electrolyte membrane also introduces an all-solid-state fuel cell, wherein the leaching out of KOH (aq.) is avoided.⁶

As a crucial component in AAEMFCs, the alkaline anion exchange membranes (AAEMs) conduct hydroxide ions from cathode to the anode where water is produced; this can help simplify the water management.^7,8 The hydroxide ions move in the opposite direction with the fuel, which reduces fuel permeability and the cost on other aspects.⁹ For fuel cell applications, AAEMs should possess several properties, such as high ionic conductivity, low swelling degree, high chemical stability, etc. The mobility of OH⁻ in AAEM seems to be much lower than that in KOH solution, and the OH⁻ conductivity of AAEMs is usually less than 1/4 of the H⁺ conductivity of Nafion under the same ion-exchange capacity (IEC) reported before.^10,11 Usually in order to enhance the OH⁻ conductivity, the IEC value is usually high. But high IEC value means high water uptake, which can induce poor chemical stability and high swelling degree (poor dimensional stability).¹² So, disentangle the dilemma of conductivity and stability in anion exchange membrane becomes critical.

The chemical stability of an AAEM is strongly dependent on the nature of cation. AAEMs typically contain quaternary ammonium functional groups (cationic groups as poly-NMe₃⁺) pendant to the polymer backbone and mobile negatively charged counter ions (OH⁻).^6,13 However, it has been demonstrated that quaternary ammonium-based AAEMs are unstable in alkaline medium, especially at elevated temperatures.^14,15 In recent years, the guanidinium cations,^16–18 quaternary phosphonium cations,^19–21 imidazolium cations^22,23 and benzimidazolium cations^24–27 based polymers have been extensively studied. Among the different kinds of AAEMs, imidazolium-based AAEMs have attracted much attention. Bencai Lin et al. reported a C2-substituted imidazolium-based cross-linked anion exchange membrane with high ionic conductivity up to 2.0 × 10⁻² S cm⁻¹ and good long-term chemical stability in 1 M KOH solution.²⁸ Congrong Yang et al. reported anion exchange membranes based on 1,2-dimethylimidazolium, which displayed excellent thermal stability and relatively good alkali stability.²⁹ These results suggest that imidazolium cations have considerable chemical stability in alkaline condition. However, the anion conductivity of the membrane fabricated with mono-imidazolium type IL is still low.

Here, we have tried to design a new type of AAEMs based on polyvinyl alcohol (PVA) and poly ionic liquids (PILs). The PVA was used as polymer backbone because of its good film-forming, low cost, low methanol permeability and good chemical stability. Meanwhile, PILs have better hydrophobicity than ILs monomers. We replaced the conventional mono-imidazolium-based PIL with bis-imidazolium-based PIL, while keeping the IEC values of all AAEMs at the same level. Compared with mono-cationic PIL, bis-cationic PIL shows better chemical stability, lower volatility and more flexibility in tuning the physicochemical properties.^30,31 Meanwhile, we changed the length of the side chain between the double imidazolium head groups. We hope that this special design about the bis-imidazolium-based PIL can elevate the chemical stability of AAEMs and facilitate the formation of microstructure-interconnected broad ionic channels, which is a benefit for the hydroxide ions conduction. This finding focused on the effect of bis-imidazolium-based PIL on the microstructure of AAEMs instead of mono-imidazolium-based PIL, which can influence the ionic conductivity, swelling ratio, and chemical stability obviously.

Experimental

Materials

PVA (98–99%, hydrolyzed, average molecular weight M_w = 88 000–97 000) was purchased from the Alfa Aesar. The compound 1-methylimidazole, 1-vinylimidazole, 1-bromobutane, 1,4-dibromobutane, 1,8-dibromooctane and 1,12-dibromododecane were purchased from Accelerating Scientific and Industrial Development thereby Serving Humanity. All the other chemicals, including acetonitrile (AR), diethyl ether (AR), acetone (AR), ethyl acetate (AR), methanol (AR) and ethanol (AR), were obtained from Tianjin Fuyu Fine Chemical Co., Ltd.

Synthesis of PILs

Synthesis of C₄PIL. The compound 1-butyl-3-vinylimidazolium bromide (C₄IL) was firstly synthesized via the reaction of 1-bromobutane with 1-vinylimidazole at 70 °C for 48 h. Then, the mixed solution was washed with ethyl acetate at least three times. After evaporating in vacuo, the resulting solid was collected. The synthetic route of C₄IL was shown in Fig. 1. ¹HNMR (300 MHz, D₂O): 7.83 (d, 1H), 7.59 (d, 1H), 7.11 (dd, J = 15.6 and 8.7 Hz, 1H), 5.78 (dd, J = 15.6 and 2.7 Hz, 1H), 5.41 (dd, J = 8.7 and 2.7 Hz, 1H), 4.23 (t, 2H), 1.83 (m, 2H), 1.28 (m, 2H), 0.90 (t, 3H).

Fig. 1 Synthetic route of the mono- and bis-imidazolium based PILs.

Poly(1-butyl-3-vinylimidazolium bromide) (C₄PIL) was prepared via free radical polymerization of 1-butyl-3-vinylimidazolium bromide in ethanol at 70 °C under a nitrogen atmosphere for 48 h. Then, the product was precipitated from ethanol by acetone.

Synthesis of DC_nPILs. The products 3-(4-bromobutyl)-1-methyl-imidazolium bromide (C₄MIL), 3-(8-bromooctyl)-1-methyl-imidazolium bromide (C₈MIL) and 3-(12-bromododecyl)-1-methyl-imidazolium bromide (C₁₂MIL) were synthesized via the reaction of 1-methylimidazole and dibromoalkane in a molar ratio of 1-methylimidazole/dibromoalkane being 1 : 7 in acetonitrile (Fig. 1), and the solution was reacted at 70 °C under a nitrogen atmosphere for 48 h. Then the solution was washed with ethyl acetate and diethyl ether at least three times, respectively.³² The products were dried in vacuum at room temperature.

C₄MIL, ¹HNMR (300 MHz, D₂O): 9.21 (s, 1H), 7.84 (s, 1H), 7.77 (s, 1H), 4.23 (t, 2H), 3.88 (s, 3H), 3.48 (t, 2H), 1.70–1.97 (m, 4H).

C₈MIL, ¹HNMR (300 MHz, D₂O): 8.59 (s, 1H), 7.30 (m, 1H), 7.30 (s, 1H), 4.03 (t, 2H), 3.48 (s, 3H), 3.36 (t, 2H), 1.66–1.79 (m, 4H), 1.12–1.23 (m, 8H).

C₁₂MIL, ¹HNMR (300 MHz, D₂O): 10.26 (s, 1H), 7.40 (s, 1H), 7.30 (s, 1H), 4.03 (t, 2H), 4.13 (s, 3H), 3.39 (t, 2H), 1.80–1.94 (m, 4H), 1.26–1.44 (m, 16H).

The bis-imidazolium ionic liquids (DC_nILs) were synthesized via the reaction of 3-(n-bromide alkyl)-1-methyl-imidazolium bromide (C_nMIL) with 1-vinylimidazole (molar ratio = 2 : 3) in acetonitrile at 70 °C for 72 h.

DC₄IL, ¹HNMR (300 MHz, D₂O): 8.98 (s, 1H), 8.64 (s, 1H), 7.74 (s, 1H), 7.48 (s, 1H), 7.35 (m, 2H), 7.03 (dd, J = 15.6 and 8.7 Hz, 1H), 5.67 (dd, J = 15.6 and 2.7 Hz, 1H), 5.31 (dd, J = 8.7 and 2.7 Hz, 1H), 4.12 (m, 4H), 3.77 (s, 3H), 1.80–1.86 (m, 4H).

DC₈IL, ¹HNMR (300 MHz, D₂O): 8.92 (s, 1H), 8.58 (s, 1H), 7.65 (s, 1H), 7.45 (s, 1H), 7.34 (m, 2H), 7.01 (dd, J = 15.6 and 8.7 Hz, 1H), 5.64 (dd, J = 15.6 and 2.7 Hz, 1H), 5.28 (dd, J = 8.7 and 2.7 Hz, 1H), 4.13 (m, 4H), 3.76 (s, 3H), 1.68–1.82 (m, 4H), 1.10–1.19 (m, 8H).

DC₁₂IL, ¹HNMR (300 MHz, D₂O): 8.61 (s, 1H), 7.83 (s, 1H), 7.70 (s, 1H), 7.50 (s, 1H), 7.34 (m, 2H), 7.00 (dd, J = 15.6 and 8.7 Hz, 1H), 5.69 (dd, J = 15.6 and 2.7 Hz, 1H), 5.33 (dd, J = 8.7 and 2.7 Hz, 1H), 4.08 (m, 4H), 3.81 (s, 3H), 1.76–1.84 (m, 4H), 1.18–1.23 (m, 16H).

The DC_nPILs were prepared via free radical polymerization of DC_nILs in ethanol at 70 °C under a nitrogen atmosphere for 48 h. Then, the product was precipitated from ethanol by acetone. The obtained products were dried in vacuum at room temperature.

Membrane preparation

The polymer membranes were formed by solution casting method. A PVA aqueous solution (10 wt%) was prepared by dissolving PVA in water at 90 °C with vigorous stirring until a transparent solution was obtained. The PIL was added into water at room temperature to prepare a 10 wt% solution, and appropriate amount of PVA solution was added according to the molar ratio as Table 1. The mixed solution was heated to 30 °C for 6 h. After that, the solution was poured and cast onto a flat glass plate, dried under ambient condition in air for 1 day. Then, the membrane was annealed at 85 °C for 6 h in a vacuum drying. The membrane that peeled off from the flat glass plate was kept in 1 mol L⁻¹ NaOH solution for 48 h to convert its X⁻ form to OH⁻ form. The converted membrane was washed with deionized water three times until pH neutral, then stored in deionized water for 2 days before use. The thickness of AEMs is about 100–130 μm.

Table 1 The water uptake, swelling ratio and IEC value of AAEMs

Membrane sample	Molar ratio of constitutional unit about PVA/PIL	Water uptake (%)	Swelling ratio (%)	Measured IEC (mmol g^−1)	Theoretical IEC (mmol g^−1)
PVA + C4PIL	1 : 0.2	728 ± 27	349	2.10 ± 0.02	2.21
PVA + DC4PIL	1 : 0.1	675 ± 25	296	2.27 ± 0.08	2.37
PVA + DC8PIL	1 : 0.1	621 ± 24	241	2.15 ± 0.07	2.25
PVA + DC12PIL	1 : 0.1	518 ± 16	168	2.06 ± 0.10	2.12

---

Characterization and measurement

Structural and morphology characterization

The structures of AAEMs were performed under vacuum using Anton-paar SAX Sess mc2 system (SAXS) with Ni-filtered Cu Kα radiation (0.154 nm) operating at 50 kV and 40 mA at 25 °C. The morphologies of AAEMs were characterized by scanning electron microscopy (SEM, JEOL JSM-7600F). Thermogravimetric analyses (TGA) were used to investigate the thermal properties of membranes with a Rheometric Scientific TGA 1500 (Piscataway, NJ). The membranes were placed under an inert atmosphere of nitrogen with a heating rate of 10 °C min⁻¹ over the temperature range from room temperature to 600 °C.

Ion exchange capacity

The ion exchange capacity (also known as the IEC value) of the membrane was measured by the classical titration technique. Firstly, the membrane was soaked in a large volume of 0.01 M HCl solution under room temperature for 48 h. Thereafter, the membrane was removed and washed with deionized water. At last, the ion-exchanged solution was titrated with 0.01 M NaOH solution. The IEC value was calculated using the following equation:

IEC = (V_ini,NaOH − V_fin,NaOH)C_NaOH/W_dry,membrane (1)

where V_ini,NaOH is the initial volume of NaOH, V_fin,NaOH is the residual volume of NaOH, C_NaOH is the concentration of NaOH, W_dry,membrane is the quality of dry membrane.

Swelling ratio and water uptake

The swelling ratio (SR) was determined from the volume difference between dry and wet membranes. The dimension of the dry membrane (in X⁻ form) was measured first. The membrane was then immersed in 1 mol L⁻¹ NaOH solution for 48 h to convert X⁻ into OH⁻, and washed with deionized water for several times to remove the remaining NaOH. Then, the length, width and thickness of the membrane were quickly measured. SR was calculated using the expression:

SR = (V_wet,OH⁻ − V_dry,X⁻)/V_dry,X⁻ (2)

where V_wet,OH⁻ and V_dry,X⁻ are the volumes of wet and dry membranes contain OH⁻ and X⁻, respectively.

Water uptake (WU) was determined by measuring the variation in the weight of the wet and dry membranes in OH⁻ form. The OH⁻ form membrane was immersed in deionized water for 48 h; the surface-attached liquid on both sides was carefully wiped, quickly weighted and measured. Then, the wet membrane was dried at 60 °C until the constant weight was obtained. WU was calculated by the following expression:

WU = (W_wet,OH⁻ − W_dry,OH⁻)/W_dry,OH⁻ (3)

where W_wet and W_dry are the qualities of the wet and dry membranes in OH⁻ form, respectively.

Ionic conductivity

The ionic conductivity was measured using the electrochemical workstation (CHI760D) with electrochemical impedance taken between 1 MHz and 0.1 Hz at the voltage amplitude of 0.01 mV.³³ All the membranes were soaked in deionized water for at least 48 h before the test. The ionic conductivity was calculated according to the following equation:

σ = l/RWT (4)

where σ is the ionic conductivity, l is the distance between the electrodes, R is the membrane resistance, W and T is the width and thickness of the membrane, respectively.

Chemical stability

The OH⁻ form AEMs were immersed into 2, 4, 6, 8 mol L⁻¹ NaOH solutions at 60 °C for 24 h.⁷ Each type of membranes were immersed into 1 mol L⁻¹ NaOH solution (the concentration of membrane prepared) at 60 °C for 24 h as a comparison. The membranes were washed with deionized water until the free NaOH inside the samples was completely removed. The ionic conductivities of four samples were investigated at 30 °C.

Methanol permeability

The methanol permeability coefficient D_K (cm² s⁻¹) was measured using a two-chamber diffusion cell;^34–36 this glass cell consisted of two reservoirs separated by a vertical membrane. The membrane was fixed and clamped between both reservoirs. One reservoir (V_A) was filled with pure deionized water (50 mL), and the other reservoir (V_B) was filled with 50 mL methanol/water solution (10.0 M). To ensure uniformity, magnetic stirrers were applied in each compartment at 25 °C. The concentration of methanol (C_B) in water was measured by gas chromatography (Agilent 7890A GC). The methanol permeability was calculated as follows:

C_B(t) = (AC_AD_K)t/V_BL (5)

where C_B(t) is the methanol concentration in the receiving reservoir at the moment of t, C_A is the methanol concentration in the source reservoir at the initial time, D_K is the methanol permeability, V_B is the solution volume in the receiving reservoir, A and L are the area and thickness of membrane.

Mechanical property

Mechanical tensile tests were performed using a Universal Testing Machine (Yashima Works Ltd. Co., model RTM-IT) at room temperature. The crosshead displacement speed of testing was set at the rate of 10 mm min⁻¹. The membranes with thickness around 100 μm and size of 6 mm × 25 mm were used for testing in 50% RH.³⁷

Tensile strength (TS) is the stress at the maximum in the plastic portion of the stress–strain curve that may be sustained by the membrane. Tensile strength was calculated as follows:

TS = F/A₀ (6)

where F is the maximum load, A₀ is the cross section area.

Moreover, ductility is another important mechanical property. It is a measure of the degree of plastic deformation that has been sustained at fracture. Ductility (% EL) may be expressed as the percentage of elongation, which was the plastic strain percent at fracture and calculated as follows:

% EL = (l_f − l₀)/l₀ (7)

where l_f and l₀ are the fracture length and original gauge length, respectively.

Results and discussion

Ion exchange capacity, water uptake and swelling ratio

IEC is an important factor directly affecting water uptake and swelling in AAEMs.³⁷ In order to investigate the effect of bis-imidazolium-based PIL on the water uptake and swelling ratio, the molar ratio of constitutional unit about PVA and C₄PIL is 1 : 0.2, which has the same mole number of hydroxide ion with PVA/DC_nPIL (molar ratio = 1 : 0.1) membrane. As shown in Table 1, the measured IEC values of all membranes are around 2.1 mmol g⁻¹, which are close to the theoretical IEC values. However, the water uptake and swelling ratio of PVA/DC₄PIL are the smaller one in comparison with those properties of PVA/C₄PIL with the same IEC value. This observation is not in line with our ordinary understanding about the IEC-dependent swelling behavior of AAEMs. Such an interesting property of PVA/DC₄PIL can only be ascribed to its peculiar structure of double-cation functional group.³⁸ It has been recognized that the intermolecular interaction between polymer backbones is enhanced when functionalized with charged side chains, most probably because of the charge entanglement. The increased charge density of side chains of DC₄PIL may enhance the intermolecular interaction between the backbones.³⁹ This would leave less space for water uptake, and the swelling ratio will decrease compared with that of PVA/C₄PIL membrane. Meanwhile, the hydrophobic side chains in the PVA/DC_nPIL membranes have clearly enhanced the hydrophobic matrix. With the length of side chain increase, the swelling ratio and water uptake decrease obviously, which effectively restricts the polymer swelling.

Structural and morphological characterization

The SAXS results of membranes are shown in Fig. 2. The scatting pattern of PVA/C₄PIL and PVA/DC₄PIL do not show any peaks, indicating that no ionic cluster is present in these samples.⁴⁰ While for PVA/DC_nPIL (n = 8, 12) membranes, SAXS peaks emerge at 3.7 and 2.9 nm⁻¹, corresponding to a Bragg spacing of about 1.7 and 2.2 nm, respectively. This suggests the ionic clusters of PVA/DC₄PIL (x = 8, 12) assemble to some degree and the size of ionic clusters enlarged.¹² These bigger ionic clusters could facilitate the formation of broad ionic channels, but over aggregation may result in partitioned hydrophilic domains.

Fig. 2 SAXS spectra of PVA/PIL membranes. (a) PVA/C₄PIL; (b) PVA/DC₄PIL; (c) PVA/DC₈PIL; (d) PVA/DC₁₂PIL. The inset shows the SAXS spectrum of PVA/DC₈PIL.

The cross-section morphological characterization of PVA/PIL membrane was performed by SEM (Fig. 3). In the cross-section images of PVA/C₄PIL and PVA/DC₄PIL membranes (Fig. 3a and b), some wrinkled and spongy structures can be observed obviously. It is important to note that no large agglomerates are visible through the section. This means that the PILs and PVA can be dispersed homogeneously in these PVA/PIL membranes. With the increase of hydrophobic chain length of PILs, the hole-work structure is formed in PVA/DC₈PIL membrane. These holes represent the hydrophilic domains containing water. After the membrane being annealed at high temperature, the free water would evaporate out of the hydrophilic domains and the holes are formed.⁴¹ As shown in the inset of Fig. 3c, the hydrophobic side chain of DC₈PIL can drive the ionic clusters to aggregate,^42,43 which are forming a interconnected channels (the red section). This is beneficial for the conductivity and chemical stability of PVA/DC₈PIL membrane. In PVA/DC₁₂PIL (Fig. 3d) membrane, some agglomerates appear. This may be due to the extensive assembly of hydrophilic species, leading to the formation of partitioned hydrophilic micro-domains. These are consistent with the SAXS results.

Fig. 3 The cross-section SEM images of PVA/PIL membranes. (a) PVA/C₄PIL; (b) PVA/DC₄PIL; (c) PVA/DC₈PIL; (d) PVA/DC₁₂PIL.

Ionic conductivity and thermal stability

The ionic conductivity is one of the most crucial parameters for characterizing anion exchange membranes. Under the same IEC value, the ionic conductivity of bis-imidazolium-based PVA/DC₄PIL membrane is higher than that of mono-imidazolium-based PVA/C₄PIL membrane at the same temperature (Fig. 4). This may be due to the higher charge density of the side chain, which affords closer proximity between neighboring ionic groups, leading to higher ion mobility.⁴⁴ Meanwhile, when the PVA/C₄PIL membrane is present at 80 °C, the membrane becomes break. This may be related to the poor electrostatic interaction in the membrane, which results in the destruction of the blending at 80 °C. In other words, the DC₄PIL can improve the operating temperature of AEM. In addition, the ionic conductivities of PVA/DC₈PIL are higher than that of those membranes at the same temperature, although the IEC is the same. These may be due to the fact that the ionic conductivity depends not only on the IEC, but also on the microstructure. Apparently, it is the ion-aggregating structure in PVA/DC₈PIL that promotes the OH⁻ mobility. The resulting interconnected and broad ionic channels have served as a “highway” for express OH⁻ transport.⁴⁵ In PVA/DC₁₂PIL, the ionic clusters seem to be over-assembled, which may result in partitioned hydrophilic domains. According to previous reports, the over-aggregation can lower the probability of forming interconnected ionic channels and break the macroscopic uniformity of the membrane, resulting in lower OH⁻ conductivity.¹² This result has been verified by SAXS and SEM. The largest ionic conductivity of PVA/DC₈PIL can reach to 43.5 mS cm⁻¹ at 80 °C, which is higher than that of chloromethylated polysulfone/quaternized graphenes membrane (18.73 mS cm⁻¹),⁴⁶ quaternized ammonio groups-substituted poly(arylene ether)s membrane (35 mS cm⁻¹)⁴⁷ and quaternary amine chitosan derivatives-(butanediyl-1,4-bis(N-dodecylimidazole bromide))₁₅ membrane (41.9 mS cm⁻¹) at 80 °C.⁴⁸

Fig. 4 Ionic conductivities of PVA/PIL membranes. (a) PVA/C₄PIL; (b) PVA/DC₄PIL; (c) PVA/DC₈PIL; (d) PVA/DC₁₂PIL.

The Grotthuss mechanism, diffusion and convection are considered to be the dominant transport mechanisms for OH⁻ transport through AAEMs.^49–51 The Grotthuss mechanism⁵² is based on the fact that OH⁻ exhibit Grotthuss behavior in aqueous solutions.^53,54 Diffusive transport occurs in the presence of a concentration or electrical potential gradient. Convective transport across the membrane appears since OH⁻ moving through the membrane, drag water molecules with them through the membrane, thus generating a convective flow of water molecules within the membrane.⁵⁵ So, OH⁻ transport mechanisms in the membrane should be the synergistic effect of these three transport mechanisms.

The apparent activation energy (E_a) for PVA/PIL membranes was estimated using the following equation:⁴⁵

ln σ = −E_a/RT (8)

where σ is the proton conductivity, R is the universal gas constant, and T is the absolute temperature.

The E_a values of PVA/C₄PIL, PVA/DC₄PIL, PVA/DC₈PIL and PVA/DC₁₂PIL membranes are 22.45, 16.09, 13.26, 19.22 kJ mol⁻¹, respectively. The lower E_a value of proton conductivity indicates the lower energy needed for ionic transfer in the membranes. The E_a value of PVA/DC₈PIL is the lowest one which can be attributed to the “highway” of this membrane, it's benefit for the conducting of OH⁻.

The TGA curves and derivative curves of membranes are shown in Fig. 5A, all membranes show three decomposition temperatures at 190–280, 310–360 and 448 °C, which could be attributed to the degradation of imidazolium side chain,²³ the backbone of PILs³¹ and PVA,⁴⁵ respectively. The derivative curves (Fig. 5B) indicate that the thermal stability of mono-imidazolium-based PVA/C₄PIL membrane is poorer than that of bis-imidazolium-based PVA/DC_nPIL membrane.

Fig. 5 TGA curves (A) and derivative curves of the TG profiles (B) for PVA/PIL membranes. (a) PVA/C₄PIL; (b) PVA/DC₄PIL; (c) PVA/DC₈PIL; (d) PVA/DC₁₂PIL.

Chemical stability

The chemical stability of anion exchange membranes is considered to be another key property for applications in AAEMFCs. In order to simulate the working environment of AAEMFCs, the membranes were treated at a relatively high temperature. Also the AAEMs were immersed into different concentrations of NaOH solution to examine the chemical stability of the cationic groups when suffering constant chemical attack under high pH conditions. Each type of membrane was immersed into 1 mol L⁻¹ NaOH solution for 24 h, and the ionic conductivity (σ₁) at 30 °C was used as a comparison with that (σ_x, x = 2, 4, 6, 8) of the same type of membrane, which was immersed into NaOH solution with higher concentrations. The ionic conductivities of all membranes decrease obviously at NaOH solution with high concentration (Fig. 6). However, the ionic conductivity of bis-imidazolium-based PVA/DC₄PIL membrane decreases at a lower rate compared with mono-imidazolium-based PVA/C₄PIL membrane. Such a chemical stability enhancement should benefit from the electrostatic attractions of higher charge density of the side chain, which reduces the vacant space for absorbing the OH⁻ to attack the polymer backbone.⁵⁶ Also, the bis-imidazolium-based PIL can enhance the conjugation and steric hindrance to decrease the occurrence possibility of S_N2 reaction.^57,58 Besides, the bis-imidazolium-based PVA/DC₈PIL membrane also displays better chemical stability, which may be due to its special microstructure. The hydrophilic/hydrophobic separation of PVA/DC₈PIL membrane can confine the OH⁻ in hydrophilic domains (the ionic channel) and the polymer backbone can be well protected by the additional hydrophobic structure.³⁸

Fig. 6 The conductivities of PVA/PIL membranes after treatment with different concentrations of NaOH solution at 60 °C. (a) PVA/C₄PIL; (b) PVA/DC₄PIL; (c) PVA/DC₈PIL; (d) PVA/DC₁₂PIL.

Mechanical property and methanol permeability

Good mechanical property of AAEMs is one of the necessary demands for their applications.⁵⁹ The mechanical stabilities of PVA/PIL membranes were tested in terms of tensile strength and elongation at break. As seen in Table 2, the tensile strength of bis-imidazolium-based PVA/DC₄PIL membrane is higher than that of mono-imidazolium-based PVA/C₄PILs membrane. This behavior may be due to the fact that the introduction of bis-imidazolium enables the electrostatic interaction of the copolymer, leading to a more compact membrane structure.⁶⁰ The tensile strength of PVA/DC₁₂PIL membrane is higher than that of other PVA/DC_nPIL membranes. This is due to the additional hydrophobic side chains in PVA/DC₁₂PIL, which have clearly enhanced the hydrophobic matrix in the membrane. This can effectively restrict the polymer swelling and intensify the mechanical strength.⁴⁴ The elongation at break of PVA/DC₁₂PIL is highest, indicating the flexible structure of PVA/DC₁₂PIL membrane.

Table 2 The mechanical stability and methanol permeability of AAEMs

Membrane sample	Molar ratio of constitutional unit about PVA/PIL	TS (MPa)	EL (%)	DK (cm^2 s^−1)
PVA + C4PIL	1 : 0.2	2.72	350	2.09 × 10^−6
PVA + DC4PIL	1 : 0.1	3.35	531.4	1.83 × 10^−6
PVA + DC8PIL	1 : 0.1	6.99	620.8	1.51 × 10^−6
PVA + DC12PIL	1 : 0.1	8.34	705.6	1.36 × 10^−6

---

The methanol permeability is regarded as a key factor for direct methanol fuel cells. The methanol crossover not only causes loss of fuel, but also decreases the performance at the cathode due to the mixed reaction of methanol oxidation and oxygen reduction.⁶¹ The membranes of PVA/DC_nPIL (n = 8, 12) exhibit methanol permeability with 1.51 × 10⁻⁶ and 1.36 × 10⁻⁶ cm² s⁻¹, respectively, which are the lower class among the traditional PVA composite membranes (0.568 to 4.42 × 10⁻⁶ cm² s⁻¹).^62–64 The methanol permeability values of all PVA/DC_nPIL (n = 4, 8, 12) membranes are lower than that of PVA/C₄PIL membrane due to the electrostatic attractions of higher charge density of the side chains, inducing a much denser structure to act as the methanol barrier.⁶⁵ Meanwhile, the methanol permeability of PVA/DC₁₂PIL is lower than that of PVA/DC₄PIL and PVA/DC₈PIL membranes. This is related to the lower water uptake of PVA/DC₁₂PIL membrane.

Conclusions

The relationship between the property of the membrane and the microstructure induced by the PIL structure has been investigated systematically. As designed, the PVA/DC_nPIL membranes based on bis-imidazolium PILs exhibited lower swelling ratio (higher dimensional stability) and higher chemical stability than those of PVA/C₄PIL membrane based on mono-imidazolium PIL under the same level of IEC value. The SAXS results showed that the PVA/DC₈PIL can generate larger ionic clusters for facilitating the formation of interconnected broad ionic channels, whose conductivity was higher than that of PVA/DC_nPIL (n = 4, 12) membranes. These observations unambiguously indicate that design of bis-imidazolium-based PILs with appropriate length of hydrophobic side chain is an effective approach to increase the ion conductivity, dimensional and chemical stabilities of AAEMs. We hope this finding could provide a strategy for the rational design of polymer membranes.

Acknowledgements

The authors are grateful to the National Basic Research Program (2013CB834505), Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20120131130003).

Notes and references

1. T. J. Clark, N. J. Robertson, H. A. Kostalik Iv, E. B. Lobkovsky, P. F. Mutolo, H. D. Abruña and G. W. Coates, J. Am. Chem. Soc., 2009, 131, 12888.
2. X. Liao, Y. Gong, Y. Liu, D. Zuo and H. Zhang, RSC Adv., 2015, 5, 99347.
3. E. H. Yu and K. Scott, J. Power Sources, 2004, 137, 248.
4. J. Kim, T. Momma and T. Osaka, J. Power Sources, 2009, 189, 999.
5. Y. S. Li, T. S. Zhao and Z. X. Liang, J. Power Sources, 2009, 187, 387.
6. R. C. T. Slade, J. P. Kizewski, S. D. Poynton, R. Zeng and J. R. Varcoe, Alkaline membrane fuel cells, Springer, New York, 2013.
7. J. Wang, H. Wei, S. Yang, H. Fang, P. Xu and Y. Ding, RSC Adv., 2015, 5, 93415.
8. H. Chen, J. Wang, H. Bai, J. Sun, Y. Li, Y. Liu and J. Wang, RSC Adv., 2015, 5, 88736.
9. J. S. Spendelow, J. D. Goodpaster, P. J. A. Kenis and A. Wieckowski, J. Phys. Chem. B, 2006, 110, 9545.
10. M. E. Tuckerman, D. Marx and M. Parrinello, Nature, 2002, 417, 925.
11. M. R. Hibbs, M. A. Hickner, T. M. Alam, S. K. McIntyre, C. H. Fujimoto and C. J. Cornelius, Chem. Mater., 2008, 20, 2566.
12. J. Pan, C. Chen, Y. Li, L. Wang, L. Tan, G. Li, X. Tang, L. Xiao, J. Lu and L. Zhuang, Energy Environ. Sci., 2014, 7, 354.
13. H. Liu, Z. Guo, S. He, H. Yin, C. Fei and Y. Feng, Polym. Chem., 2014, 5, 4756.
14. G. Wang, Y. Weng, D. Chu, D. Xie and R. Chen, J. Membr. Sci., 2009, 326, 4.
15. M. Zhang, H. K. Kim, E. Chalkova, F. Mark, S. N. Lvov and T. C. M. Chung, Macromolecules, 2011, 44, 5937.
16. Q. Zhang, S. Li and S. Zhang, Chem. Commun., 2010, 46, 7495.
17. D. S. Kim, A. Labouriau, M. D. Guiver and Y. S. Kim, Chem. Mater., 2011, 23, 3795.
18. C. Qu, H. Zhang, F. Zhang and B. Liu, J. Mater. Chem., 2012, 22, 8203.
19. S. Gu, R. Cai and Y. Yan, Chem. Commun., 2011, 47, 2856–2858.
20. K. J. T. Noonan, K. M. Hugar, H. A. Kostalik, E. B. Lobkovsky, H. D. Abruña and G. W. Coates, J. Am. Chem. Soc., 2012, 134, 18161.
21. C. G. Arges, J. Parrondo, G. Johnson, A. Nadhan and V. J. Ramani, J. Mater. Chem., 2012, 22, 3733.
22. Y. Wu, L. Han, X. Zhang, J. Mao, L. Gong, W. Guo, K. Gu and S. Li, Polym. Chem., 2015, 6, 2560.
23. B. Qiu, B. Lin, Z. Si, L. Qiu, F. Chu, J. Zhao and F. Yan, J. Power Sources, 2012, 217, 329.
24. O. D. Thomas, K. J. Soo, T. J. Peckham, M. P. Kulkarni and S. Holdcroft, J. Am. Chem. Soc., 2012, 134, 10753.
25. D. Henkensmeier, H. J. Kim, H. J. Lee, D. H. Lee, I. H. Oh, S. A. Hong, S. W. Nam and T. H. Lim, Macromol. Mater. Eng., 2011, 296, 899.
26. D. Henkensmeier, H. R. Cho, H. J. Kim, C. N. Kirchner, J. Leppin, A. Dyck, J. H. Jang, E. Cho, S. W. Nam and T. H. Lim, Polym. Degrad. Stab., 2012, 97, 264.
27. O. D. Thomas, K. J. W. Y. Soo, T. J. Peckham, M. P. Kulkarni and S. Holdcroft, Polym. Chem., 2011, 2, 1641.
28. B. Lin, F. Chu, Y. Ren, B. Jia, N. Yuan, H. Shang, T. Feng, Y. Zhu and J. Ding, J. Power Sources, 2014, 266, 186.
29. C. Yang, S. Wang, L. Jiang, J. Hu, W. Ma and G. Sun, Int. J. Hydrogen Energy, 2015, 40, 2363.
30. H. Shirota, T. Mandai, H. Fukazawa and T. Kato, J. Chem. Eng. Data, 2011, 56, 2453.
31. X. Chen, J. Zhao, J. Zhang, L. Qiu, D. Xu, H. Zhang, X. Han, B. Sun, G. Fu, Y. Zhang and F. Yan, J. Mater. Chem., 2012, 22, 18018.
32. J. A. Vicente, A. Mlonka, H. Q. N. Gunaratne, M. Swadźba-Kwaśny and P. Nockemann, Chem. Commun., 2012, 48, 6115.
33. P. Wang, Y. N. Zhou, J. S. Luo and Z. H. Luo, Polym. Chem., 2014, 5, 882.
34. C. P. Liu, C. A. Dai, C. Y. Chao and S. J. Chang, J. Power Sources, 2014, 249, 285.
35. G. S. Sailaja, S. Miyanishi and T. Yamaguchi, Polym. Chem., 2015, 6, 7964.
36. J. Kim and D. Kim, J. Membr. Sci., 2012, 405–406, 176.
37. A. H. N. Rao, H. J. Kim, S. Nam and T. H. Kim, Polymer, 2013, 54, 6918.
38. J. Pan, Y. Li, J. Han, G. Li, L. Tan, C. Chen, J. Lu and L. Zhuang, Energy Environ. Sci., 2013, 6, 2912.
39. K. K. Chee, J. Appl. Polym. Sci., 1991, 43, 1205.
40. D. Chen and M. A. Hickner, Macromolecules, 2013, 46, 9270.
41. L. Lebrun, E. Da Silva and M. Metayer, J. Appl. Polym. Sci., 2002, 84, 1572.
42. J. Xu, C. Ren, H. Cheng, L. Ma and Z. Wang, J. Polym. Res., 2013, 20, 1.
43. H. K. Kim, M. Zhang, X. Yuan, S. N. Lvov and T. C. Mike Chung, Macromolecules, 2012, 45, 2460.
44. G. He, Z. Li, J. Zhao, S. Wang, H. Wu, M. D. Guiver and Z. Jiang, Adv. Mater., 2015, 27, 5280.
45. Y. Yang, H. Gao and L. Zheng, RSC Adv., 2015, 5, 17683.
46. L. Liu, C. Tong, Y. He, Y. Zhao and C. Lü, J. Membr. Sci., 2015, 487, 99.
47. M. Tanaka, M. Koike, K. Miyatake and M. Watanabe, Polym. Chem., 2011, 2, 99.
48. J. L. Wang, L. L. Wang, R. Feng and Y. Zhang, Solid State Ionics, 2015, 278, 144.
49. M. Dominik, ChemPhysChem, 2006, 7, 1848.
50. S. J. Paddison and R. Paul, Phys. Chem. Chem. Phys., 2002, 4, 1158.
51. P. Choi, N. H. Jalani and R. Datta, J. Electrochem. Soc., 2005, 152, E84.
52. K. N. Grew and W. K. S. Chiu, J. Electrochem. Soc., 2010, 157, B327.
53. M. E. Tuckerman, D. Marx and M. Parrinello, Nature, 2002, 417, 925.
54. N. Agmon, Chem. Phys. Lett., 2000, 319, 247.
55. G. Merle, M. Wessling and K. Nijmeijer, J. Membr. Sci., 2011, 377, 1.
56. M. Y. Li, G. Zhang, S. Xu, C. J. Zhao, M. M. Han and L. Y. Zhang, J. Power Sources, 2014, 255, 101.
57. O. D. Thomas, K. Soo, T. J. Peckham, M. P. Kulkarni and S. Holdcroft, Polym. Chem., 2011, 2, 1641.
58. W. H. Awad, J. W. Gilman, M. Nyden, R. H. Harris, T. E. Sutto, J. Callahan, P. C. Trulove, H. C. DeLong and D. M. Fox, Thermochim. Acta, 2004, 409, 3.
59. Y. Yang, F. Lu, X. Gao, S. Xie, N. Sun and L. Zheng, J. Membr. Sci., 2015, 490, 38.
60. Y. W. Kim, J. T. Park, J. H. Koh, D. K. Roh and J. H. Kim, J. Membr. Sci., 2008, 325, 319.
61. Y. Yang, H. Gao, F. Lu and L. Zheng, Int. J. Hydrogen Energy, 2014, 39, 17191.
62. T. Zhou, J. Zhang, J. Qiao, L. Liu, G. Jiang, J. Zhang and Y. Liu, J. Power Sources, 2013, 227, 291.
63. Y. Xiong, Q. L. Liu, Q. G. Zhang and A. M. Zhu, J. Power Sources, 2008, 183, 447.
64. J. M. Yang and S. A. Wang, J. Membr. Sci., 2015, 477, 49.
65. H. Cheng, J. Xu, L. Ma, L. Xu, B. Liu, Z. Wang and H. Zhang, J. Power Sources, 2014, 260, 307.

---