Enhanced proton conductivity of proton exchange membrane at low humidity based on poly(methacrylic acid)-loaded imidazole microcapsules

Abstract

Inspired by the water reserving function of vacuoles in plant cells, poly(methacrylic acid)-loaded imidazole microcapsules (IMCs-PMAA) with high water retention were prepared by distillation–precipitation polymerization and incorporated into a sulfonated poly(ether ether ketone) (SPEEK) matrix to fabricate SPEEK/IMCs-PMAA composite membranes. Compared with the SPEEK control membrane, the composite membranes exhibit higher water uptake, lower swelling degree and better methanol barrier properties. The water retention capacity of the microcapsules is optimized by the carboxylic acid groups of PMAA and the strong electrostatic attraction between the imidazole groups and sulfonic acid groups. Proton transfer pathways are constructed both inside the microcapsules and at the IMCs–SPEEK interface to enhance proton conductivity. The highest proton conductivity of composite membranes is about 5.52 × 10⁻² S cm⁻¹ at room temperature and 100% relative humidity (RH), which is more than two times of the SPEEK control membrane conductivity (2.51 × 10⁻² S cm⁻¹). In particular, the SPEEK/IMCs-PMAA-20 membrane exhibits the highest proton conductivity of 1.93 × 10⁻² S cm⁻¹ at 20% RH, which is two orders of magnitude higher than that of the SPEEK control membrane. The high water retention and proton conduction properties demonstrate that the composite membranes have a great potential application for the direct methanol fuel cell (DMFC).

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1. Introduction

Water retention materials, which can absorb water at high humidity and retain water at low humidity, are suitable for a variety of applications such as food, agriculture and fuel cells.^1–3 The water retention property can confer stable performance and long lifetimes on materials at low humidity.^4–6 Proton exchange membrane (PEM) is the core component of the direct methanol fuel cell (DMFC), which determines the performance of DMFC and fulfills the conversion from chemical energy to electrical energy.^7–9 A representative PEM material, sulfonated poly(ether ether ketone) (SPEEK), is attractive for its acceptable proton conductivity, methanol barrier property and considerably lower price compared to Nafion®, a perfluorosulfonic acid membrane from Dupont™.^10–14 It is well known that medium temperatures (80–120 °C) and low relative humidity (RH, <50%) are desirable for DMFCs to achieve high activity of electrocatalysis, low levels of carbon monoxide poisoning, efficient water and thermal management, and enhanced energy conversion efficiencies.^15,16 However, the proton conductivity of the SPEEK membrane drops sharply at low humidity due to the severe loss of water inside ion-channels and the resultant destruction of the nanochannels for proton transfer.^17,18 Therefore, development of PEM with high water retention and proton conductivity at low humidity is needed for practical application in DMFC.

It is widely accepted that proton transfers through PEM via the vehicle mechanism and Grotthuss mechanism, in which the proton conductivity is mainly dependent on the characters of proton carriers and the amount of bound water.¹⁹ Accordingly, the high water retention property will confer PEM with high proton conductivity at low humidity. Recently, various efforts have been devoted for developing PEMs with high water retention, including for instance, (i) incorporating inorganic porous fillers (e.g. SiO₂, TiO₂ and ZrO₂) into the polymer matrix as water-retention additives;^20–23 (ii) blending with hydrophilic materials or grafting water retention groups on the membrane surface;^24–29 and (iii) constructing ordered and narrowed nanochannels to acquire the optimum free volume.^30,31 Among these methods, incorporating inorganic particles with porous or hollow structure is considered to be a facile and efficient approach. The hollow structure with high specific surface and low density enables inorganic particles to bear more functional groups as active sites and hold more water molecules. For example, Zhao et al. fabricated membranes based on chitosan and phosphorylated hollow silica submicrospheres, and the hybrid membranes exhibited enhanced proton conductivity at low humidity compared to the pristine membrane.³²

Polymeric microcapsules is a novel water-retention material, which possess not only hollow structure but more adjustable characteristics compared to hollow inorganic spheres. The thickness of polymeric shells can be controlled by altering the polymerization process, and the interfacial compatibility between microcapsules and organic polymer membrane matrix can be improved due to the flexible shells of the microcapsules. Furthermore, polymeric microcapsules in membrane play the role of water reservoirs, which can regulate the balance of water uptake and water release.^33,34 Several different species of microcapsules were synthesized in previous work, such as zwitterionic microcapsules based on sulfobetaine and polymeric microcapsules with tunable hydrophilicity–hydrophobicity,^35,36 and the resultant microcapsules conferred membranes with high water retention and enhanced proton conductivity. However, improving water retention only by the hollow structure of microcapsules may easily result in loss of free water. A good method to improve the water retention property is incorporating a large quantity of acid groups with high hydration energy into the microcapsules. The acid groups as proton conducting sites inside microcapsules can facilitate proton transfer. Moreover, they can also improve the bound water content in the microcapsules and form hydrogen bond networks with water molecules. Thus, proton transfer pathways can be constructed both inside the microcapsules and on the microcapsules surface, which contribute to stable proton conductivity at low humidity.

In this study, the hollow structure of polymeric microcapsules was utilized to construct proton transfer pathways to further optimize the proton conductive performance. Inspired by the high hydration energy of carboxylic acid groups and high water retention of microcapsules, poly(methacrylic acid)-loaded imidazole microcapsules (IMCs-PMAA) were synthesized by distillation–precipitation polymerization, and then incorporated into the SPEEK matrix to fabricate SPEEK/IMCs-PMAA composite membranes. The microstructure, compositions and physicochemical properties of IMCs-PMAA microcapsules and the composite membranes were investigated. The water uptake, swelling degree, proton conductivity and methanol barrier properties of the membranes were extensively evaluated.

2. Experimental

2.1. Materials and chemicals

Methacrylic acid (MAA), acetonitrile and 2,2′-azobisisobutyronitrile (AIBN) were of analytical grade and obtained from Tianjin Jiangtian Chemical Technology Co., Ltd. Vinyl imidazole (VI, >99 wt%) was supplied from Sigma Co., Ltd. Ethyleneglycol dimethacrylate (EGDMA, >99.5%) was purchased from Alfa Aesar Co. and used as received. Dimethylformamide (DMF, >99.5 wt%) and methanol (analytical grade) were purchased from Tianjin Guangfu chemical industry. Concentrated sulfuric acid (>98 wt%) was supplied from Beijing chemical factory. Poly(ether ether ketone) (PEEK, Victrex 381 G) pellets were purchased from Victrex, England. Deionized water was produced by laboratory water purification system (Millipore Elix® Advantage 5) and used throughout the experiment.

2.2. Preparation of IMCs-PMAA and SPEEK

IMCs-PMAA were synthesized via distillation–precipitation polymerization,³⁷ and the procedure is shown in Scheme 1. MAA (2 mL) was dissolved in acetonitrile (80 mL) under ultrasonic treatment for 5 min. Then, AIBN (0.04 g) was employed as initiating agent and added to the mixture. The mixture was maintained at 90 °C for 2 h under reflux. After the polymerization, the resultant polymer carboxylic acid spheres (PCASs) were washed by deionized water, and then dried under vacuum at 50 °C to achieve constant weight. The PCASs (0.2 g) were dispersed in acetonitrile (80 mL) under ultrasonic treatment. Then, VI (0.35 mL) as comonomer, EGDMA (0.35 mL) as cross linker and AIBN (0.014 g) as initiating agent were added to the mixture. The mixture was maintained at 100 °C for 1.5 h under reflux. The PCASs-loaded imidazole spheres were obtained and immersed into deionized water for 24 h to get the final IMCs-PMAA hollow microcapsules.

Scheme 1 Preparation protocol of IMCs-PMAA.

The post-sulfonation method was used to prepare SPEEK according to the procedure reported in ref. 38. PEEK (30 g) was slowly added into concentrated sulfuric acid (200 mL), and the entire process was under vigorous stirring. After continuously stirring at 50 °C for 10 h in water bath, the mixture was poured into excess of cold water to obtain white precipitation. The precipitation was rinsed with deionized water until a pH of 7.0 was reached, and then dried at 60 °C for 24 h in a vacuum oven. The degree of sulfonation (DS) of the resultant SPEEK is 66%, measured by the titration method.

2.3. Preparation of membranes

A certain amount of IMCs-PMAA microcapsules and DMF (8 mL) were mixed, and then the mixture was placed in an ultrasonic bath to ensure uniform dispersion of microcapsules in the solvent. Subsequently, SPEEK was added (0.6 g) to the suspension prepared above and maintained under stirring for 24 h to obtain the well-distributed casting membrane solution. The membrane was formed by casting the solution onto a square glass plate and dried at 60 °C in an oven for 24 h. Then, the formed membranes were immersed into 2 M hydrochloric acid solution at room temperature for 48 h in order to be sufficiently acidified. Afterward, the membranes were repeatedly rinsed with deionized water until a neutral pH was reached and dried for 24 h in a vacuum oven. The resultant composite membranes are designated as SPEEK/IMCs-PMAA-X, where X (X = 2.5, 5, 10, 15 or 20) is the weight percentage of the IMCs-PMAA to SPEEK. The SPEEK control membrane was prepared in the similar manner but without incorporating IMCs-PMAA. The averaged thickness of the membranes is in the range of 52–72 μm.

2.4. Characterization

The morphology of PCASs and IMCs-PMAA was characterized by transmission electron microscopy (TEM, TEM-100 CX II). Field emission scanning electron microscope (FESEM, X-650) was used to observe the cross-sectional microstructure of the composite membranes. The chemical compositions of the IMCs-PMAA and membranes were characterized by Fourier transform infrared spectra (FTIR, Nicolet-560, 400–4000 cm⁻¹) with a resolution of 4 cm⁻¹. The small-angle X-ray scattering (SAXS) analysis was utilized to determine the phase separation of composite membranes. SAXS patterns of membranes were recorded on a Rigaku D/max 2500v/pc diffractometer (CuK 40 kV, 100 mA) in the range of 0.05°–5° with a scanning speed of 0.2° min⁻¹. The glass transition temperature (T_g) of membranes was determined by a differential scanning calorimetry (DSC, 204 F1, NETZSCH instrument). The sample was preheated under a nitrogen atmosphere from room temperature to 150 °C at a heating rate of 10 °C min⁻¹, and then cooled to 90 °C and reheated to 260 °C. Thermogravimetric analysis (TGA) of membranes was carried out on a Pyris thermal analyzer (TA Instrument, Perkin-Elmer), the test temperature increased from 30 °C to 800 °C at a heating rate of 10 °C min⁻¹ and the entire process was under a N₂ atmosphere.

2.5. Water uptake, swelling degree and ion-exchange capacity

The water uptake and swelling degree refer to the weight and volume of membrane varying after complete hydration, respectively. The sample membrane was first immersed into deionized water at 25 °C for 24 h, and then its weight (W_wet) and volume (V_wet) were measured. Subsequently, the sample was dried at 50 °C under vacuum for 24 h, weighed (W_dry) and measured (V_dry) again. The procedure was repeated three times, and the error was within ±5%. Then, the water uptake and swelling degree were calculated according to the following equations (eqn (1) and (2)), respectively.

The ion-exchange capacity (IEC) of membranes was measured by the traditional titration method. The dried membrane sample was first immersed into 2 M NaCl solution (20 mL) for 24 h to ensure that H⁺ ions completely exchanged with Na⁺ ions. Then, the exchanged H⁺ ions within the solution were titrated with 0.01 M NaOH solution and phenolphthalein was used as indicator. The IEC value of the membrane was calculated according to eqn (3):

where m is the weight of dried membrane (g) and V_NaOH is the titrimetric volume of the NaOH solution (mL).

2.6. Methanol permeability

A glass diffusion cell containing two compartments was utilized to measure the methanol permeability of membranes according to the previous literature.³⁹ The membrane was soaked in deionized water for at least 24 h, and then fastened tightly by two same compartments. Then, the sandwich structure was fixed by two magnetic stirrers. One compartment (A) was initially filled with methanol solution (2.0 M, 30 mL) and the other (B) was filled with deionized water (30 mL). The methanol concentration in the compartment (B) was determined every 3 min by a gas chromatography (Agilent 7820) with a DB-624 column. The methanol permeability (P, cm² s⁻¹) was calculated by eqn (4).where K is the methanol concentration gradient in the compartment (B) (mol L⁻¹ s⁻¹), V_B is the volume of the compartment (B) (cm³), C_A0 is the feed concentration of methanol in compartment (A) (mol L⁻¹). A and l are the effective area (cm²) and thickness (cm) of the membrane, respectively.

2.7. Proton conductivity and selectivity

A two-point-probe conductivity cell (FRA, Compactstat, IVIUM Tech.) with an oscillating voltage of 20 mV and a frequency range of 1 MHZ to 1 HZ was used to measure the proton conductivity of membranes. The sample membrane was first immersed into deionized water for 48 h, and then characterized from 30 °C to 80 °C at 100% RH or at 40 °C and 20% RH. The proton conductivity (σ, S cm⁻¹) was calculated by eqn (5).where l is the distance between the anode and cathode electrodes (cm), A is the cross-sectional area across which proton transfers (cm²) and R is the membrane resistance (Ω).

The methanol barrier performance and proton conductivity of the proton exchange membrane can be evaluated by selectivity Φ (S s cm⁻³), which was calculated by eqn (6).

where σ is proton conductivity (S cm⁻¹) calculated by eqn (5), and P is methanol permeability (cm² s⁻¹) calculated by eqn (4).

3. Results and discussion

3.1. Characterization of PCASs and IMCs-PMAA

Fig. 1 is the TEM images of the as-prepared PCASs and IMCs-PMAA. The resultant PCASs particles are spherical and smooth with a uniform size of about 150 nm in diameter (Fig. 1(a)). No evident interface can be observed in the TEM image of IMCs-PMAA (Fig. 1(b)) because both of PMAA and the imidazole shells are organic polymer. Moreover, due to the dissolvable behavior of PCASs assembled by non-crosslinked PMAA polymer chains, the diameters of these particles swell to about 260 nm in water. The size of microspheres can be controlled by adjusting the concentration of the comonomer and heating rate.

Fig. 1 TEM images of (a) PCASs and (b) IMCs-PMAA.

The chemical composition of PCASs and IMCs-PMAA were analyzed by FTIR. Fig. 2 shows the FTIR spectra of PCASs and IMCs-PMAA. The FTIR spectrum of PCASs displays absorption bands at 1390 cm⁻¹ and 1718 cm⁻¹ corresponding to the bending vibration of COO–H and the stretching vibration of C=O, respectively.⁴⁰ For the FTIR spectrum of IMCs-PMAA, the absorption bands at 3156 cm⁻¹ and 1640 cm⁻¹ are assigned to the stretching vibration of –N–H and heteroaromatic ring of imidazole, respectively. The band at 1640 cm⁻¹ is not obvious due to the overlap with the band of C=O.

Fig. 2 FTIR spectra of PCASs and IMCs-PMAA.

3.2. Characterization of composite membranes

The FESEM images of the cross-section of the SPEEK control and SPEEK/IMCs-PMAA composite membranes are demonstrated in Fig. 3. It can be seen that the SPEEK control membrane is dense and homogeneous (Fig. 3(a)), and when the filler content of IMCs-PMAA up to 20%, the as-prepared composite membranes are still void-free (Fig. 3(f)). Moreover, the IMCs-PMAA with spherical structures disperse homogeneously in all composite membrane, which is because the strong hydrogen bonding and electrostatic interaction between the imidazole groups and SPEEK chains contribute to good interfacial compatibility between SPEEK and IMCs-PMAA.

Fig. 3 FESEM images of the cross-section of SPEEK control and SPEEK/IMCs-PMAA composite membranes: (a) SPEEK, (b) SPEEK/IMCs-PMAA-2.5, (c) SPEEK/IMCs-PMAA-5, (d) SPEEK/IMCs-PMAA-10, (e) SPEEK/IMCs-PMAA-15, (f) SPEEK/IMCs-PMAA-20.

The FTIR spectra of the SPEEK control and SPEEK/IMCs-PMAA composite membranes are shown in Fig. 4. The presence of bands at 1253 cm⁻¹, 1080 cm⁻¹ and 1020 cm⁻¹ can be attributed to the stretching vibration of S=O, symmetric and asymmetric stretching vibration of O=S=O, respectively. A new band appears at 1720 cm⁻¹ for composite membranes corresponding to the stretching vibration of C=O.⁴⁰ In addition, the characteristic peak intensity of –SO₃ in composite membranes diminishes with the increase in the IMCs-PMAA content, which is attributed to the hydrogen bonding and electrostatic interaction between imidazole groups of IMCs-PMAA and sulfonic acid groups of SPEEK chains.

Fig. 4 FTIR spectra of SPEEK control and SPEEK/IMCs-PMAA composite membranes.

The glass transition temperatures (T_gs) of SPEEK control and SPEEK/IMCs-PMAA composite membranes are determined by DSC curves as shown in Fig. 5. The SPEEK control membrane displays a T_g of 193.6 °C, and the T_g of SPEEK/IMCs-PMAA composite membrane increases from 196.9 to 207.3 °C when the IMCs-PMAA content increases from 2.5 to 20 wt%. The increase in T_g indicates that acid–base pairs are formed between imidazole groups and sulfonic acid groups in the composite membranes. The strong electrostatic interaction between IMCs-PMAA and SPEEK chains inhibits the motility of SPEEK chains, which results in the requirement of more energy for SPEEK chains while occurring glass transition.

Fig. 5 DSC curves of SPEEK control and SPEEK/IMCs-PMAA composite membranes.

During the solution evaporation procedure, nanophase separation occurs between the hydrophobic backbone and hydrophilic side chains of SPEEK, and hydrophilic ion-channels composed of sulfonic acid groups are formed inside the membrane. The phase separation and ion-channel size were detected by SAXS. Based on the fundamental theory of SAXS, the equation q = 4π sin θ/λ is used to calculate the scattered intensity q, where λ and θ are the wavelength and scattering angle, respectively. The Bragg spacing d was calculated by the equation d = 2π/q. Fig. 6 shows the results of SAXS patterns of SPEEK control and SPEEK/IMCs-PMAA composite membranes. A peak at 0.0823 nm⁻¹ is observed for the SPEEK control membrane, which indicates the existence of the nanophase separation and the ion-channels.⁴¹ For SPEEK/IMCs-PMAA composite membranes, the increase in q values indicates that the size of ion-channel decreases with the incorporation of IMCs-PMAA. This can be explained by the strong electrostatic interaction between IMCs-PMAA and SPEEK chains, which inhibits the motility and configuration of SPEEK chains, and therefore decreasing the extent of nanophase separation.^25,41

Fig. 6 SAXS curves of SPEEK, SPEEK/IMCs-PMAA-10 and SPEEK/IMCs-PMAA-20 membranes.

TGA curves of SPEEK control and SPEEK/IMCs-PMAA composite membranes are shown in Fig. 7, which reveal a three-steps weight loss. The first weight loss below 230 °C is due to the evaporation of absorbed water in the membranes. The second weight loss around 280–380 °C is mainly attributed to the thermal decomposition of sulfonic acid groups in the SPEEK polymer. In this range, the thermal stability of composite membranes is improved by the strong interaction between imidazole groups of IMCs-PMAA and sulfonic acid groups of SPEEK, which inhibits the thermal decomposition of sulfonic acid groups. The third weight loss over 400 °C is attributed to the decomposition of the polymer backbone. It can also be observed that the weight loss of composite membranes is higher than that of the SPEEK control membrane because the carbon residual amount of IMCs-PMAA is lower than that of SPEEK after thermal decomposition. The TGA curves demonstrate that the SPEEK/IMCs-PMAA composite membranes can meet the formal operating temperature (<120 °C) for DMFC application.

Fig. 7 TGA curves of SPEEK control and SPEEK/IMCs-PMAA composite membranes.

3.3. Water uptake and swelling degree

Water uptake and swelling degree have an important influence on proton conductivity and methanol permeability of membrane. The water uptake and volume swelling degree of the as-prepared membranes as a function of temperature are shown in Fig. 8. It can be seen that both water uptake and swelling degree of the membranes increase with the increasing temperature. The water uptake of the SPEEK control membrane increases from 33.6% to 88.9% when temperature increases from 30 to 55 °C. Due to high water uptake and high water retention of the IMCs-PMAA, the composite membranes demonstrate a significant enhancement in water uptake compared with the SPEEK control membrane. The swelling degree of composite membranes decreases with the increase in IMCs-PMAA content. Especially, the swelling degree of composite membranes decreases from 155.1% to 56.7% with IMCs-PMAA content increasing from 0 to 20 wt% at 55 °C. The strong interaction between imidazole groups of IMCs-PMAA and sulfonic acid groups of SPEEK restricts the motility of SPEEK chains and decreases the free volume of membranes. Therefore, the swelling of composite membranes can be inhibited. Furthermore, compared to the SPEEK control membrane, the increased water uptake and decreased swelling degree of composite membranes demonstrate that the structure of membranes could remain stable in water for a long time. Therefore, the composite membranes possess good water stability properties.

Fig. 8 Water uptake and swelling degree of the membranes as a function of temperature.

3.4. IEC and proton conductivity of the membranes

IEC values and proton conductivity of the membranes at room temperature and 100% RH are illustrated in Fig. 9. The IEC value of the SPEEK control membrane is 1.88 mmol g⁻¹, and all IEC values of the composite membranes are lower than that of the SPEEK control membrane. On the one hand, the incorporation of IMCs-PMAA with lower IEC values compared with SPEEK has diluted the concentration of sulfonic acid groups inside the membrane. On the other hand, the strong electrostatic interaction between imidazole groups of IMCs-PMAA and sulfonic acid groups of SPEEK reduces the concentration of H⁺ dissociated from sulfonic acid groups. Therefore, the IEC values of the composite membranes decrease with the increase in IMCs-PMAA content.⁴² In addition, the proton conductivity of the SPEEK control membrane at room temperature and 100% RH is 2.51 × 10⁻² S cm⁻¹. Compared with the SPEEK control membrane, the proton conductivity of SPEEK/IMCs-PMAA composite membranes increases from 3.01 × 10⁻² to 5.52 × 10⁻² S cm⁻¹ with the IMCs-PMAA content increasing from 2.5 to 20 wt%. It can be concluded that the proton conductivity is effectively improved by the incorporation of IMCs-PMAA microcapsules.

Fig. 9 IEC and proton conductivity of the membranes at room temperature and 100% RH.

There are two acceptable proton conduction mechanisms proposed as the vehicle mechanism and the Grotthuss mechanism, according to which the proton transfer mechanism in the SPEEK/IMCs-PMAA composite membranes is tentatively illustrated in Scheme 2. The enhanced proton conductivity of composite membranes could be interpreted as follows. (i) The addition of IMCs-PMAA increases water uptake of the membranes, which consequently provides more proton carriers and facilitates the H⁺ dissociation of the sulfonic acid groups in SPEEK bulk,³⁴ thereby enhancing both the Grotthuss-type and vehicle-type proton transfer. (ii) The acid–base pairs are formed between sulfonic acid groups of SPEEK and imidazole groups of IMCs-PMAA, acting as proton donors and proton acceptors, respectively. Affected by the electrostatic attraction between sulfonic acid groups and imidazole groups, the protonation/deprotonation rate of the functional groups in acid–base pairs is accelerated. Therefore, new effective proton channels at IMCs-SPEEK interface are constructed, enabling fast proton transfer by the Grotthuss mechanism.^34,43 (iii)–CO₂H group is a moderate acid and its proton-donating and proton-accepting capabilities are well matched. The PMAA polymer chains in IMCs-PMAA contain a great quantity of –CO₂H groups, which can form new pathways for proton transfer with water molecules in the microcapsules. Hence, the energy barrier of proton transport from one carrier to another is low, which facilitates both vehicle and Grotthuss mechanisms and results in significantly improved proton conductivity.⁴⁰

Scheme 2 Proton transfer mechanism of the IMCs-PMAA in the composite membranes.

Proton conductivity of the membranes as a function of temperature at 100% RH is illustrated in Fig. 10. The proton conductivity of all membranes increases with the increasing temperature in accordance with the Arrhenius relationship. The values of activation energy (E_a) for proton conducting in membranes were calculated by the Arrhenius equation, which is generally utilized to analyze the proton transfer model and energy barrier. All the E_a values for the membranes are in the range of activation energy for Grotthuss mechanism (0.150–0.418 eV). The E_a value of the SPEEK control membrane is 0.259 eV. The E_a values for composite membranes are reduced to 0.168–0.244 eV depending on the different filler content of IMCs-PMAA, which indicates that the proton transfer barrier in composite membranes decreases by the incorporation of IMCs-PMAA. The IMCs-PMAA microcapsules in the membranes behave as “acid reservoirs” and form acid–base pairs with SPEEK. Effective pathways for proton transfer are constructed to enable fast proton transport with low energy barrier, and thus results in improved proton conduction properties.

Fig. 10 Proton conductivity of the membranes as a function of temperature at 100% RH.

The influence of the water environment inside membranes on the proton conducting performance was analyzed by characterizing proton conductivity of membranes at low humidity. Proton conductivity of the membranes as a function of time, at 40 °C and 20% RH, is illustrated in Fig. 11. The proton conductivity of the SPEEK control membrane is reduced sharply from 1.17 × 10⁻² S cm⁻¹ to 2.85 × 10⁻⁵ S cm⁻¹, within 90 min, at low humidity. The decrease in proton conductivity is due to the quick and serious water loss in the membrane, which not only changes the structure of the hydrophilic channels (such as decreasing size and connectivity of channels), but disrupts the water molecule network and weakens the dissociation ability of acid groups to a certain extent.⁴⁴ The IMCs-PMAA microcapsules significantly slow down the decrease rate of proton conductivity with time. When the filler content of IMCs-PMAA in the membranes is higher than 15 wt%, the proton conductivity of SPEEK/IMCs-PMAA composite membranes remains almost constant at 40 °C and 20% RH within 90 min. The SPEEK/IMCs-PMAA-20 membrane exhibits a considerably higher proton conductivity of 1.93 × 10⁻² S cm⁻¹ compared to that of the SPEEK control membrane (2.85 × 10⁻⁵ S cm⁻¹) after 90 min. Such observations indicate that the IMCs-PMAA, acting as water reservoirs, can reserve water in membranes and release water in a controllable manner when the membranes are suffering dehydration. The function of the IMCs-PMAA microcapsules in membranes is similar to the vacuoles in plant cells, and the –CO₂H groups on PMAA polymer chains with high hydration energy further improve water uptake and water retention capability.⁴⁰ The water molecules existing in composite membranes cooperate with the acid–base pairs at the IMCs-SPEEK interface, the acid groups in microcapsules and the main channels of SPEEK, which contribute to the efficient proton transfer and relatively stable proton conductivity at low humidity.

Fig. 11 Proton conductivity of the membranes as a function of time at 40 °C and 20% RH.

3.5. Methanol permeability and selectivity of the membranes

The methanol permeability of the membrane should be as low as possible because the methanol crossover through the membrane will result in low open-circuit potential and catalyst poisoning.⁴⁵ As shown in Fig. 12, the methanol permeability of the SPEEK control membrane is 4.39 × 10⁻⁷ cm² s⁻¹. Compared with the SPEEK control membrane, the methanol permeability of SPEEK/IMCs-PMAA composite membranes decreases by the incorporation of IMCs-PMAA. The methanol permeability is reduced from 4.36 × 10⁻⁷ cm² s⁻¹ to 3.51 × 10⁻⁷ cm² s⁻¹ when the filler content of IMCs-PMAA increases from 2.5 to 20 wt%. On the one hand, the electrostatic interaction between imidazole groups and sulfonic acid groups narrows the methanol diffusion pathway. On the other hand, the existence of IMCs-PMAA microcapsules further prolongs the methanol diffusion pathway. Due to the above-mentioned factors, the diffusion resistance for methanol increases, thus leading to decreased methanol permeability.³⁴ The considerably lower methanol permeability of IMCs-PMAA membranes compared to that of Nafion 117 (higher than 10⁻⁶ cm² s⁻¹) demonstrates their potential in practical applications.⁴⁶

Fig. 12 Methanol permeability and selectivity of the membranes.

The comprehensive performance of the membrane was evaluated by selectivity (Φ = σ/P). As shown in Fig. 12, the selectivity of the SPEEK control membrane is 5.72 × 10⁴ S s cm⁻³. The SPEEK/IMCs-PMAA composite membranes demonstrate higher selectivity than the SPEEK control membrane mainly due to both the enhanced proton conductivity and methanol resistance by the incorporation of IMCs-PMAA. The selectivity of the SPEEK/IMCs-PMAA-20 membrane is 1.57 × 10⁵ S s cm⁻³, which is nearly two times higher than that of the SPEEK control membrane.

4. Conclusions

Poly (methacrylic acid)-loaded imidazole microcapsules (IMCs-PMAA) were successfully synthesized via distillation–precipitation polymerization, and were incorporated into a SPEEK matrix to fabricate SPEEK/IMCs-PMAA composite membranes. The introduction of microcapsules increases the water uptake of the membranes. The strong electrostatic attraction between imidazole groups of microcapsules and sulfonic acid groups of SPEEK restricts the mobility of polymer chains and decreases the hydrophilic nanochannel size, consequently improving the anti-swelling ability and methanol resistance property of the composite membranes. The sulfonic acid–imidazole pairs form efficient pathways for proton transfer at the IMCs–SPEEK interface. The carboxylic acid groups with high hydration energy further optimize the water environment inside the membranes and construct additional proton transfer pathways inside the microcapsules. Similar to the water reserving function of the vacuoles in plant cells, the IMCs-PMAA microcapsules can reserve water in membranes and release water to the SPEEK matrix in a controllable manner. Moreover, the acidic channels inside microcapsules provide continuous proton hopping sites. Accordingly, the incorporation of the IMCs-PMAA enhances the proton conductivity of composite membranes. Especially, the SPEEK/IMCs-PMAA-20 membrane with significantly higher proton conductivity demonstrates its promising prospects in PEMFC under low humidity condition.

Acknowledgements

The work is supported by Program for New Century Excellent Talents in University (NCET-10-0623), the National Science Fund for Distinguished Young Scholars (21125627) and the Programme of Introducing Talents of Discipline to Universities (no. B06006).

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