Phosphotungstic acid embedded sulfonated poly(arylene ether ketone sulfone) copolymers with amino groups for proton exchange membranes

Abstract

Sulfonated poly(arylene ether ketone sulfone) containing pendant amino groups (Am-SPAEKS)/phosphotungstic acid (HPW) composite membranes with different HPW content were prepared by the solution blend method as proton exchange membranes (PEM) for proton exchange membrane fuel cells (PEMFCs) that operate at medium–high temperatures. Am-SPAEKS was synthesized successfully by adjusting the molar feed ratio, which was confirmed by Fourier transform infrared (FT-IR) and ¹H NMR spectroscopy. FT-IR spectroscopy showed that there were acid–base interactions between the amino groups and HPW, as well as between HPW and the sulfonic groups. These interactions could stabilize HPW in the composite membranes, which was confirmed by SEM images. The water uptake and thermal stability of the composite membranes were improved compared with the Am-SPAEKS membranes. The water retention capacity and the mechanical property of the composite membranes also met the requirement for proton exchange membranes. The proton conductivity of the Am-SPAEKS/HPW30% composite membrane was as high as 0.091 S cm⁻¹ at 120 °C. These results indicate that the Am-SPAEKS/HPW composite membranes have potential to be used as alternative proton exchange membrane materials for proton exchange membrane fuel cells at medium–high temperature conditions.

---

Introduction

Proton exchange membrane fuel cells are popularly used as energy conversion devices with high energy conversion efficiency, high power density and low pollution, which accomplish the conversion of the chemical energy of fuels to electricity with low greenhouse gas emission. These properties make it possible for them to replace the other sources of energy that are applied in portable electronics and vehicles.^1–6 In particular, the operation of PEMFCs has many advantages at moderate-high temperatures, such as simple system design, fast electrode kinetics, reduced water management, high tolerance towards impurities (such as CO) and low cost.^7,8

Proton exchange membranes not only transfer protons but also isolate the oxidant and fuel, which are the key components of PEMFCs.⁹ The commercially available PEMs for PEMFCs (e.g. Nafion®) have high proton conductivity because of the proton transport channel formed by the phase separation between their hydrophilic groups and the hydrophobic domains below 80 °C.¹⁰ However, proton conductivity sharply decreases above 80 °C due to severe water loss.

Therefore, the development of PEMs for medium–high temperatures has become a research focus. Several strategies have been adopted to achieve high proton conductivity at medium–high temperatures and low relative humidity such as incorporating inorganic fillers into a polymer matrix as hydroscopic agents (TiO₂, SiO₂, and ZrO₂).^11–17 The water uptake of these composite membranes increased due to the numerous hydrogen bond sites of these inorganic fillers. However, the composite membranes exhibited relatively low proton conductivity because of the low intrinsic proton conductivity of these inorganic fillers.¹⁵ Moreover, these hybrid membranes always display poor interfacial connection between organics and inorganics, which results from the weak interactions between inorganic fillers and polymer membranes matrix; this leads to an increase in the swelling ratio of the membranes, which influences the dimensional stability of the membranes.¹⁸

A type of inorganic filler that possesses the capacity of proton conductivity itself is needed. Phosphotungstic acid (H₃PW₁₂O₄₀ abbreviated as HPW) is a solid heteropolyacid that has attracted considerable interest.¹⁹ It is well known as a proton conductor because of its unique Keggin structure.²⁰ Many attempts have been made to develop new composite membranes using HPW to improve proton conductivity. Ramani et al. embedded HPW into Nafion® to prepare composite membranes, in which the improvement of conductivity was needed to reduce the additive particles sizes, and the proton conductivity of the composite membranes at 120 °C and 35% relative humidity was 0.015 S cm⁻¹.²¹ Zhou et al. used the inorganic phosphotungstic acid as a proton carrier and mesoporous silica as the matrix (HPW-meso-silica) to develop a new PEM and its proton conductivity was 0.11 S cm⁻¹ at 90 °C and 100% relative humidity.²² Zeng et al. fabricated a series of short stacks with 2 cells, 6 cells, and 10 cells, employing HPW functionalized mesoporous silica PEMs at a high temperature with a maximum power density of 74.4 W, which occurs at 150 °C in H₂/O₂.²³ Tang et al. also developed HPW functionalized mesoporous silica composite membranes using a one-step self-assembly route, which had a high proton conductivity of 0.06–0.08 S cm⁻¹ at 70–100 °C.²⁴ Moreover, Liu et al. prepared HPW functionalized mesoporous silica using cost-effective tape-casting, incorporating phase inversion and vacuum-assisted wet impregnation techniques.²⁵ However, HPW is soluble in water, and it is a challenge to immobilize HPW for creating a new PEM material with high proton conductivity both at low relative humidity and high relative humidity, in spite of the fact that the proton conductivity of HPW is as high as 0.02–0.1 S cm⁻¹ at room temperature.^26–28 A new approach was found to immobilize HPW using amino groups, which took advantage of the overall negative charge of heteropolyanions.²⁹

On the basis of using sulfonated poly(arylene ether ketone sulfone) containing pendant amino groups (Am-SPAEKS), we developed a new medium–high temperature composite PEM with HPW. The N atoms on the Am-SPAEKS act as both a proton donor and receptor.^30,31 Thus, they provide a new proton transport channel at medium–high temperatures. In particular, the acid–base interaction between HPW and amino groups was utilized to immobilize HPW for maintaining high proton conductivity at medium–high temperatures. The interaction between sulfonic acid groups and amino groups not only plays a role in immobilizing HPW, but also enhances water retention capacity and mechanical properties, thus improving the dimensional stability and reducing the methanol permeability coefficient of PEMs.

Experimental

Material

3,3′-Disulfonated 4,4′-dichlorodiphenyl sulfone (SDCDPS, 99%) and 4-aminophenyl hydroquinone (4Am-PH, 99%) were synthesized in-house.^32,33 Tetramethylenesulfone (TMS, CP grade), 2,2-bis(4-hydroxyphenyl)propane (bisphenol A, AR grade), and potassium carbonate (K₂CO₃) (AR grade) were purchased from Tianjin XingFu Fine Chemical Industry Research Institute, China. Toluene (AR grade) and HCl (AR grade) were obtained from Beijing Chemical Plant, and N-methyl-2-pyrrolidinone (NMP, AR grade) was purchased from Tianjin Fu Chen Chemical Reagent Factory, China. HPW was provided by Aladdin Industrial Co (AR grade). 4,4′-Difluorobenzophenone (DFB, AR grade) was obtained from YanBian Long Jing Chemical Company, China, and solid reagents were used after drying for 12 h at 60 °C under vacuum.

Synthesis of Am-SPAEKS

Am-SPAEKS (DS = 80%) were synthesized by adjusting the mole ratio of SDCDPS and DFB, as shown in Scheme 1. The content of the amino groups of Am-SPAEKS was controlled by the mole ratio of 4Am-PH to bisphenol A (2 : 8). The direct polycondensation reaction of Am-SPAEKS was carried out as follows: DFB (9.0 mmol), bisphenol A (12 mmol), SDCDPS (6.0 mmol), 4Am-PH (3.0 mmol), K₂CO₃ (15 mmol), TMS (16 mL), and toluene (15 mL) were placed in a 100 mL three-neck round-bottom flask equipped with a heating jacket, a reflux condenser and a nitrogen inlet. The mixture was refluxed at 130 °C for 4 h with stirring to remove the water produced in the reaction by azeotropic distillation. Subsequently, toluene was removed, and the temperature of the reaction mixture was increased to 175 °C and maintained at 175 °C for several hours. When the system viscosity was high enough, heating was stopped, and the solution was poured into a beaker containing deionized water. The obtained strip copolymer was cut into pieces and collected after washing with boiling deionized water to remove solvents and inorganic water-soluble salts, and then dried under vacuum to obtain the product (in the sodium form). The salt copolymer was then immersed in 2 M HCl for 48 h to transform it into an acid copolymer, which was washed with deionized water to remove excess HCl, and dried at 80 °C for 24 h before use.

Scheme 1 Synthesis route of Am-SPAKES copolymer.

Membrane preparation

A series of composite membranes with different content of HPW was prepared, which were denoted as Am-SPAEKS, Am-SPAEKS/HPW10%, Am-SPAEKS/HPW20%, Am-SPAEKS/HPW30%. Membrane thickness was in the range of 60–80 μm. Taking the preparation of Am-SPAEKS/HPW10% as an example, the procedure used is as follows: the acid copolymer (0.9 g) and HPW (0.1 g) were dissolved in 10 mL NMP with stirring and heating to form a homogeneous solution. Then, the solution was poured on a clean and dry glass pane and dried in a vacuum oven at 60 °C for 48 h to remove the solvent, and the composite membranes were thus obtained.

Characterization

¹H NMR spectra were recorded using a Bruker Avance spectrometer with deuterated dimethyl sulfoxide (DMSO-d₆) as the solvent and tetramethylsilane as the internal standard. FT-IR spectroscopy was used to characterize the structures of the samples using a Vector-22 spectrometer (Bruker, Germany) in the spectral range of 4000–500 cm⁻¹. The thermal stability of the membranes was determined using thermogravimetric analysis (TGA) on a Perkin Elmer Pyris 1 thermal analyzer under nitrogen atmosphere in the temperature range of 50–600 °C at a heating rate of 10 °C min⁻¹. The microstructure of the membranes was observed using a JSM6510 scanning electron microscope (SEM).

Measurements

Mechanical property

The mechanical property of the membranes was determined on an Instron 5965 machine, and the samples were cut into a dumbbell (25 × 4 mm²) and fixed in the testing instrument at a tensile rate of 1 mm min⁻¹.

Stability of HPW in the membranes, water uptake and swelling ratio

The prepared membranes were immersed in deionized water at 20 °C for 40 days and dried. The weight loss of HPW was calculated according to the following formula:³⁴where W_HPW is the weight of HPW initially added, and W_dry1 and W_dry2 are the dry weights of the membranes before and after immersion in water, respectively.

The water uptake (WU) of the membranes was measured after testing the stability of HPW in the membranes, which was determined by the change in the weights between the dry and the wet membranes. The detailed procedure was previously reported.³³ The WU was calculated using the following formula:

where W_wet and W_dry are the weight of the wet and the dry membranes, respectively.

The test for the swelling ratio (SR) for the membranes was similar to the WU, and was determined by the change in the lengths and the thickness between the wet and the dry membranes. SR was calculated according to the following formula:

where h_wet and h_dry are the lengths of the dry and wet samples, and l_wet and l_dry are the thickness of the wet and dry samples, respectively.

Water desorption coefficient of the membrane

The water desorption curves represent the water retention capacity of the membranes and are reflected by the change in weight of water in the membranes with time at a certain temperature. TGA was used to record the water desorption curves at 80 °C for 1 h. The initial stage of the curves obeys the Fick's laws of diffusion.³¹ The water desorption coefficient was calculated using the following formula:where, D_t is the water desorption coefficient of the membrane, M_t/M_∞ is the weight change of the water with time, and L is the width of the membrane.

Oxidative stability

The oxidative stability of the membranes was characterized by the residual weight after immersion in Fenton's reagent (3% H₂O₂, 2 ppm FeSO₄) at 80 °C for 1 h. The oxidative stability was evaluated from the percentage of remaining weight of the total weight.

Proton conductivity

The membranes (40 mm × 10 mm) were kept at a constant temperature and the proton conductivity of the membranes was determined using the four-electrode AC impedance method in the frequency range of 100 kHz to 0.1 Hz, ac perturbation of 10 mV, and dc rest voltage of 0.0 V using a Princeton Applied Research Model 2273A Potentiostat (Model 5210 frequency response detector, EG&G PARC, Princeton, NJ).³² Before the test, the membranes were immersed in water for 24 h to obtain 100% relative humidity (RH). Subsequently, the samples were sandwiched between molds, and the temperature was controlled using a constant temperature chamber. The proton conductivity (σ) was calculated using the following equation:

σ = L/(R × S) (5)

where L (cm) is the distance between the two electrodes, R (Ω) is the membrane resistance, and S (cm²) is the cross-sectional area of the membrane.

Methanol permeability

The methanol permeability coefficient of the membranes was tested using a custom iron diffusion device, which contained two cells; one was filled with 190 mL methanol solution (10 M), the other one was filled with 190 mL pure water, and they were separated by the membranes. The solutions of the two diffusion cells were magnetically stirred at different temperatures. Methanol concentration in the water reservoir was measured using a Shimadzu GC-8A chromatograph. The methanol permeability coefficient of the membranes at 25 °C and 60 °C was calculated using the following formula:³²where C_A and C_B(t) (mol m⁻³) are the methanol concentrations in methanol and water reservoirs, respectively; A (cm²) is the effective area; DK (cm⁻² s⁻¹) and V_B (mL) are the methanol diffusion coefficient and volume of permeated reservoirs, respectively; and L (cm) is the thickness of the membranes.

Results and discussion

Characterization of Am-SPAEKS copolymer

The structures of the copolymers were characterized using ¹H NMR and FT-IR spectroscopy. Fig. 1(a) shows the ¹H NMR spectrum of the copolymer. The peaks from δ 6.5 to 8.5 ppm are assigned to the protons on the benzene rings of Am-SPAEKS, as shown in Fig. 1(a), which is consistent with those reported in the literature.^31,33 Thus, Am-SPAEKS were synthesized successfully, as shown by ¹H NMR spectroscopy.

Fig. 1 The characterization of Am-SPAEKS copolymer.

Fig. 1(b) shows the FT-IR spectrum of the copolymer, in which the absorption bands centered at 1231 cm⁻¹ and 1095 cm⁻¹ can be assigned to the O=S=O and S=O stretching vibrations of the sulfonic groups of Am-SPAEKS membrane, while the peaks at 1162 cm⁻¹ can be attributed to C–S–O bonds. The N–H stretching vibration peaks appeared at 3458 cm⁻¹ and 3352 cm⁻¹. This result further confirms that the target copolymer was synthesized.

FT-IR characterization of HPW, Am-SPAEKS and Am-SPAEKS/HPW membranes

The FT-IR spectra of HPW, Am-SPAEKS and Am-SPAEKS/HPW membranes are shown in Fig. 2. The characteristic peaks of the P–O, W=O_t, W–O_c–W, and W–O_e–W stretching vibrations of HPW can be attributed to the peaks at 1071, 973, 895 and 768 cm⁻¹, respectively, which are consistent with those reported in the literature for the [PW₁₂O₄₀]³⁻ Keggin unit.^35,36 The IR absorption peaks at 3458 cm⁻¹ and 3352 cm⁻¹ are attributed to N–H stretching vibration, and they shift to 3469 cm⁻¹ and 3381 cm⁻¹ in the IR spectra of Am-SPAEKS/HPW composite membranes. Moreover, the characteristic peaks of the sulfonic groups shift to 1235 cm⁻¹ and 1097 cm⁻¹. In addition, the absorption peaks of HPW shift from 1071, 973 and 895 cm⁻¹ to 1079, 980 and 893 cm⁻¹, respectively. This demonstrates that there are strong hydrogen-bond interactions between the sulfonic groups and HPW, and acid–base interaction between amino groups and HPW. These results indicate that Am-SPAEKS/HPW composite membranes were successfully prepared.

Fig. 2 FT-IR spectra of HPW, Am-SPAEKS and Am-SPAEKS/HPW.

Thermal and mechanical performance of the membranes

The thermal properties of the membranes were determined using TGA by recording the weight loss of the membranes with temperature. The TGA curves of the membranes are shown in Fig. 3. Am-SPAEKS membranes showed three weight loss steps. The first step at around 100 °C is due to the evaporation of residual solvent and water molecules. The second step at around 200 °C corresponds to the degradation of the sulfonic groups from the copolymers and the last step, which started around 400 °C, is attributed to the decomposition of the polymer main chain. The composite membranes present similar decomposition patterns, but the temperature at which the sulfonic acid groups degraded is higher than that of the Am-SPAEKS membranes. This can be explained by the fact that HPW has excellent thermal stability, as shown in Fig. 3, and the interaction between the sulfonic groups and HPW improved the stability of the sulfonic groups. This result demonstrated that the composite membranes exhibited favorable thermal properties.

Fig. 3 TGA curves of the membranes and HPW.

PEMs are required to possess good tensile strength in the application of PEMFCs. The mechanical properties of the membranes are listed in Table 1. With increasing HPW content, the Young's modulus and tensile strength of the composite membranes decreased because of the reduced content of Am-SPAEKS, which acted as a network in the membranes. Thus, the stretch of the backbone was restricted and this hindered the strain.²⁵ However, the Young's modulus and tensile strength of the composite membranes can also reach 1392.76 MPa and 38.29 MPa, respectively. This shows that the composite membranes possess the appropriate mechanical performance to meet the requirements of PEMs.

Table 1 T_d5%^a, oxidative stability and mechanical properties of membranes

Samples	Td5% (°C)	Oxidative stability (%) 80 °C	Young's modulus (MPa) 25 °C	Tensile strength (MPa) 25 °C
a Td5% is the temperature when the membrane weight loss is 5%.
Nafion®	—	98	186	36.64
Am-SPAEKS	162	94.74	1765.76	62.17
Am-SPAEKS/HPW10%	182	93.18	1702.15	59.19
Am-SPAEKS/HPW20%	185	90.48	1594.61	39.30
Am-SPAEKS/HPW30%	242	88.75	1392.76	38.29

---

Morphology

Morphology has a direct impact on the performances of membranes. The microstructure of membranes were observed using SEM. As shown in Fig. 4, Am-SPAEKS membranes were smooth and compact, on which their favorable mechanical property and thermal stability are based. The white parts in the images of the composite membranes are HPW particles. It can be observed that the HPW particles disperse evenly on the cross-section and surface of the composite membranes, which results from not only the interaction between the amino groups and HPW, but also the interaction between the sulfonic groups and HPW. In addition, with increasing HPW content, the white parts become more and intensive. This indicates that the HPW particles were dispersed evenly in the composite membranes.

Fig. 4 SEM images of Am-SPAEKS and Am-SPAEKS/HPW membranes.

Stability of HPW in the membranes, water uptake and swelling ratio

The stability of HPW in the membranes was evaluated by the weight loss of HPW in the composite membranes after immersion in water at 25 °C for 40 days. The HPW release was calculated and is displayed in Table 2. The release ratio of HPW increases from 8.3% to 13.9% with the increasing HPW content, but remains at a low level as expected. This could be explained by the interaction between HPW and amino groups as well as the acid–base interaction between sulfonic groups and amino groups. The acid–base interaction between HPW and amino groups could stabilize HPW particles, and the acid–base interaction between the sulfonic groups and amino groups formed a network to surround the HPW particles. Besides, the hydrogen bond interaction (between sulfonic groups and HPW) further immobilized HPW in the membranes.

Table 2 Analytical data of Am-SPAEKS and Am-SPAEKS/HPW membranes

Samples	HPW release (%)	Methanol diffusion coefficient (×10^−7 cm^2 s^−1)		Water diffusion coefficient (×10^−8 cm^2 s^−1) 80 °C	Ea (kJ mol^−1) ≥80 °C	Proton conductivity (S cm^−1)
		25 °C	60 °C			80 °C	120 °C
Nafion®	—	24.8	—	—	—	0.063	0.018
Am-SPAEKS	—	8.36	17.13	9.1	5.7	0.086	0.072
Am-SPAEKS/HPW10%	8.3	10.25	20.02	4.2	2.8	0.088	0.080
Am-SPAEKS/HPW20%	12.5	12.51	22.76	3.9	1.9	0.092	0.086
Am-SPAEKS/HPW30%	13.9	15.83	25.34	2.6	1.2	0.095	0.091

---

Efficient water uptake is necessary for electrolyte membranes to transfer protons because water acts as the proton carrier, and protons are transferred in PEMs by forming hydrated protons with water molecules (H₃O⁺ and H₅O₂⁺).³⁷ The water uptake of the membranes at different temperatures is shown in Fig. 5. The uptake of the Am-SPAEKS/HPW composite membranes increases with an increase in HPW content. The maximum WU of the Am-SPAEKS/HPW30% membranes is as high as 22.58% at 80 °C. This is due to the fact that HPW acts as a conductor and facilitates the permeation of H₂O molecules into the membranes, and HPW is highly hydrophilic, thus it is beneficial to absorb water molecules and increase the WU.³⁸

Fig. 5 Water uptake of Am-SPAEKS and Am-SPAEKS/HPW membranes.

Abundant hydration is essential for PEMs to maintain high proton conductivity. However, excess water uptake causes extreme swelling ratios, leading to a decline in mechanical performance and dimensional stability.³³ Thus, a moderate swelling ratio is also critical and is beneficial to prevent fuel from permeating through PEMs. Fig. 6 shows the swelling ratio of the membranes. It can be found that the swelling ratio of the composite membranes is higher than that of Am-SPAEKS membranes. This is explained by the fact that the interaction between the amino groups and the sulfonic acid groups is weakened by the addition of HPW, which results in an increase in the free volume of the composite membranes and leads to an increase in the swelling ratio, compared with Am-SPAEKS membranes. However, the swelling ratios of composite membranes are enough to meet the requirement for PEMs (the swelling ratio of Nafion® is 27% at 80 °C).

Fig. 6 Swelling ratio of Am-SPAEKS and Am-SPAEKS/HPW membranes.

Water retention capacity

The water retention capacity of PEMs plays an important role in proton conductivity because PEMs transfer protons among sulfonic acid groups by the formation and cleavage of hydrogen bond networks by water molecules. Fig. 7 shows water absorption curves, which are obtained by recording the weight loss of the membranes using TGA at 80 °C for 1 h. The water diffusion coefficients are listed in Table 2. A lower water diffusion coefficient indicates a better water retention capacity, while a higher water diffusion coefficient indicates a poor water retention capacity. The water diffusion coefficients of the composite membranes are lower (the maximum water diffusion coefficient is 4.2 × 10⁻⁸ cm² s⁻¹) than that of Am-SPAEKS (9.1 × 10⁻⁸ cm² s⁻¹), and they decrease with increasing HPW content. This is because the acid–base interaction between the amino groups and the sulfonic groups form a network to prevent water molecules from moving freely, which limits the evaporation of water in the composite membranes. Moreover, HPW was immobilized by the amino groups and the unique structure of the anion can bound water molecules in the composite membranes. The water retention principle of the composite membranes is shown in Scheme 2.

Fig. 7 Water diffusion coefficient of Am-SPAEKS and Am-SPAEKS/HPW membranes.

Scheme 2 The water retention principle of the composite membranes.

Oxidative stability

Oxidative stability is another important parameter to evaluate the performance of PEMs. The oxidative stability of the available PEMs Nafion® was determined by residual weight after immersion in Fenton's reagent at 80 °C for 1 h. The test results of the oxidative stability for the membranes are displayed in Table 1. It can be found that the composite membranes have weaker oxidative stability than that of Am-SPAEKS, and with increasing content of HPW, their oxidative stability declines. This is because the interaction between HPW and the amino groups stabilizes HPW, and HPW is hydrophilic, which increases the WU efficiently, and the increased water content benefits the transport of peroxides, thus accelerating degradation.³⁹ This is why the oxidative stability of the Am-SPAEKS/HPW composite membranes (residual weight from 93.18% to 88.75%) is lower than that of Am-SPAEKS membranes (94.74%). In general, the composite membranes exhibited satisfactory antioxidant stability compared with Nafion®, which has an oxidative stability of 98%.⁴⁰

Proton conductivity and methanol permeability

Proton conductivity has a significant impact on the performance of PEMs, and PEMs must possess high proton conductivity as the core component of PEMFCs. The proton conductivity of the membranes is shown in Fig. 8. It can be observed that the proton conductivity of the membranes increased with increasing HPW content at the same temperature and relative humidity. This is due to the fact that the special anion structure of HPW can combine with hydrated protons to transfer them across the membranes like sulfonic acid groups.³⁸ Although the proton conductivity decreases slightly with increasing test temperature above 80 °C, it is higher than that of Am-SPAEKS membranes and Nafion® (in the range of 0.063–0.018 S cm⁻¹ from 80 °C to 120 °C). At low temperatures, protons are transferred according to the “Vehicular” mechanism by HPW and the sulfonic acid groups, which is dependent on the water environment,⁴¹ as shown in Scheme 3(a). Above 80 °C, the protons are transferred based on both the “Grotthuss” and “Vehicular” mechanism because of the enhancement of the water retention capacity of the composite membranes when the proton transmission is accomplished not only by HPW and the sulfonic acid groups, as shown in Scheme 3(a), but also by the formation and fracture of the hydrogen bonding between N atoms and HPW, as shown in Scheme 3(b). Thus, the Am-SPAEKS/HPW30% composite membranes maintain high proton conductivity of 0.095–0.091 S cm⁻¹ in the temperature range of 80–120 °C.

Fig. 8 Proton conductivity of the membranes.

Scheme 3 Illustration of proton transfer mode.

The Arrhenius plot of the membranes is shown in Fig. 9. The activation energies of proton transfer for the membranes were calculated using the Arrhenius equation. It is found that the activation energies for Am-SPAEKS, Am-SPAEKS/HPW10%, Am-SPAEKS/HPW20% and Am-SPAEKS/HPW30% are similar below 80 °C, and this indicates that proton transfer might occur mainly in the aqueous environment.⁴² With an increase in HPW content, the activation energies of the composite membranes decrease, which are lower than that of the Am-SPAEKS membranes, and this phenomenon becomes obvious from 80 °C to 120 °C (the activation energies of Am-SPAEKS to the Am-SPAEKS/HPW30% was from 5.7 kJ mol⁻¹ to 1.2 kJ mol⁻¹). This means that the composite membranes transported protons more easily than the Am-SPAEKS membranes at this temperature range and this is caused by three aspects: first, the acid–base interaction improves the water retention capacity of the composite membranes, thus maintaining a low activation energy; second, the N atoms act as proton donors and acceptors,³¹ thus shorten the distance of proton transmission and provide a new channel; third, the HPW immobilized by the amino groups provides more active sites for the transmission of protons among the functional groups (amino groups and sulfonic acid groups), which is based on its hydrophilic nature. These factors resulted in the lower activation energy of proton conductivity for the composite membranes than that of the Am-SPAEKS membranes from 80 °C to 120 °C.

Fig. 9 Arrhenius plot of proton conductivity vs. temperature.

The proton conductivity at different RH conditions was measured to further verify the effect of inorganic materials on proton transport, as shown in Fig. 10. At 100% RH condition, the proton conductivity of all the samples were similar. However, with a decrease in RH, the composite membranes displayed higher proton conductivity compared to the Am-SPAEKS membranes. Moreover, the proton conductivity of the Am-SPAEKS membranes decreased significantly at low RH (<80%). This is due to the following aspects: (i) the composite membranes had a better water retention capacity to maintain high proton conductivity at low RH and (ii) the HPW had an intrinsic capacity of transporting protons, which played an important role in proton transport at low RH.

Fig. 10 Proton conductivity of the membranes at different relative humidities.

Both hydrogen and methanol are used in PEMFCs as fuel. Thus, the methanol permeability coefficient of the membranes needs to be studied. In this study, the methanol permeability coefficient of the membranes was investigated in a 10 M methanol solution at 25 °C and 60 °C and the results are presented in Table 2. The methanol permeability coefficients of composite membranes are in the range of 10.25 × 10⁻⁷ cm² s⁻¹ to 15.83 × 10⁻⁷ cm² s⁻¹ at 25 °C, and it is found that the methanol permeability coefficients of the composite membranes increase with increasing HPW, and are higher than those of Am-SPAEKS membranes. This might be related to the fact that the addition of HPW reduces the acid–base interaction between the amino groups and the sulfonic acid groups, which increases the free volume absorbed water molecules and this leads to the loose structure of the composite membranes, which induces a higher methanol permeability coefficient. Despite this, the composite membranes display a low methanol diffusion coefficient.

Conclusion

In a word, Am-SPAEKS with a certain DS was synthesized by direct polycondensation reactions, where the content of amino groups was constant. The composite membranes were prepared by the solution blend method. FT-IR spectra indicated that there were acid–base interactions between amino groups and sulfonic groups, as well as between amino groups and HPW, which could immobilize HPW in the composite membranes. SEM images showed that HPW was dispersed evenly, and there was no phase separation. The composite membranes exhibited a favorable performance such as thermal property, dimension stability, oxidative stability and mechanical performance. In particular, the proton conductivity of the composite membranes was improved at medium–high temperatures.

Acknowledgements

The authors thank the Natural Science Foundation of China (Grant No. 51273024 and 51303015), the Department of Education of Jilin Province (Grant No. 2014119), and the Scientific and Technological Planning Projects of the Jilin Province (Grant No. 20130101021JC) for the financial support for this study.

References

1. M. Z. Jacobson, W. G. Colella and D. M. Golden, Science, 2005, 308, 1901–1905.
2. M. Nogami, H. Matsushita, Y. Goto and T. Kasuga, Adv. Mater., 2000, 12, 1370–1372.
3. P. Agnolucci, Int. J. Hydrogen Energy, 2007, 32, 4306–4318.
4. P. Costamagna and S. Srinivasan, J. Power Sources, 2001, 102, 253–269.
5. S. P. Jiang, Z. Liu and Z. Q. Tian, Adv. Mater., 2006, 18, 1068–1072.
6. Y. Yang, H. Gao and L. Zheng, RSC Adv., 2015, 5, 17683–17689.
7. L. Cindrella, A. M. Kannan, J. F. Lin, K. Saminathan, Y. Ho, C. W. Lin and J. Wertz, J. Power Sources, 2009, 194, 146–160.
8. A. M. Kannan, L. Cindrella and L. Munukutla, Electrochim. Acta, 2008, 53, 2416–2422.
9. X. Xu, L. Li, H. Wang, X. Li and X. Zhuang, RSC Adv., 2015, 5, 4934–4940.
10. C. Zhao, D. He, Y. Li, J. Xiang, P. Li and H.-J. Sue, RSC Adv., 2015, 5, 47284–47293.
11. A. Saccà, A. Carbone, E. Passalacqua, A. D'Epifanio, S. Licoccia, E. Traversa, E. Sala, F. Traini and R. Ornelas, J. Power Sources, 2005, 152, 16–21.
12. A. K. Sahu, G. Selvarani, S. Pitchumani, P. Sridhar and A. K. Shukla, J. Electrochem. Soc., 2007, 154, B123–B132.
13. A. Saccà, I. Gatto, A. Carbone, R. Pedicini and E. Passalacqua, J. Power Sources, 2006, 163, 47–51.
14. C. H. Rhee, H. K. Kim, H. Chang and J. S. Lee, Chem. Mater., 2005, 17, 1691–1697.
15. H. M. L. Thijs, C. R. Becer, C. Guerrero-Sanchez, D. Fournier, R. Hoogenboom and U. S. Schubert, J. Mater. Chem., 2007, 17, 4864–4871.
16. A. M. Herring, J. Macromol. Sci., Part C, 2006, 46, 245–296.
17. E. A. Mistri and S. Banerjee, RSC Adv., 2014, 4, 22398–22410.
18. S. S. Kulkarni, S. M. Tambe, A. A. Kittur and M. Y. Kariduraganavar, J. Membr. Sci., 2006, 285, 420–431.
19. S. V. Sambasivarao, Y. Liu, J. L. Horan, S. Seifert, A. M. Herring and C. M. Maupin, J. Phys. Chem. C, 2014, 118, 20193–20202.
20. V. Ramani, H. R. Kunz and J. M. Fenton, J. Membr. Sci., 2005, 266, 110–114.
21. V. Ramani, H. R. Kunz and J. M. Fenton, J. Membr. Sci., 2004, 232, 31–44.
22. Y. Zhou, J. Yang, H. Su, J. Zeng, S. P. Jiang and W. A. Goddard, J. Am. Chem. Soc., 2014, 136, 4954–4964.
23. J. Zeng, B. Jin, P. K. Shen, B. He, K. Lamb, R. de Marco and S. P. Jiang, Int. J. Hydrogen Energy, 2013, 38, 12830–12837.
24. H. Tang, M. Pan, S. Lu, J. Lu and S. P. Jiang, Chem. Commun., 2010, 46, 4351–4353.
25. L. Bai, L. Zhang, H. Q. He, R. K. S. O. A. Rasheed, C. Z. Zhang, O. L. Ding and S. H. Chan, J. Power Sources, 2014, 246, 522–530.
26. C. Pazé, S. Bordiga and A. Zecchina, Langmuir, 2000, 16, 8139–8144.
27. S. Uchida, K. Inumaru and M. Misono, J. Phys. Chem. B, 2000, 104, 8108–8115.
28. E. Fontananova, F. Trotta, J. C. Jansen and E. Drioli, J. Membr. Sci., 2010, 348, 326–336.
29. G. Luo, L. Kang, M. Zhu and B. Dai, Fuel Process. Technol., 2014, 118, 20–27.
30. W.-Q. Deng, V. Molinero and W. A. Goddard, J. Am. Chem. Soc., 2004, 126, 15644–15645.
31. J. Xu, H. Cheng, L. Ma, H. Han, Y. Huang and Z. Wang, J. Polym. Res., 2014, 21, 1–11.
32. Y. Zhang, Y. Wan, G. Zhang, K. Shao, C. Zhao, H. Li and H. Na, J. Membr. Sci., 2010, 348, 353–359.
33. H. Cheng, J. Xu, L. Ma, L. Xu, B. Liu, Z. Wang and H. Zhang, J. Power Sources, 2014, 260, 307–316.
34. D. Xu, G. Zhang, N. Zhang, H. Li, Y. Zhang, K. Shao, M. Han, C. M. Lew and H. Na, J. Mater. Chem., 2010, 20, 9239–9245.
35. B. B. Bardin, S. V. Bordawekar, M. Neurock and R. J. Davis, J. Phys. Chem. B, 1998, 102, 10817–10825.
36. J. Yang, M. J. Janik, D. Ma, A. Zheng, M. Zhang, M. Neurock, R. J. Davis, C. Ye and F. Deng, J. Am. Chem. Soc., 2005, 127, 18274–18280.
37. G. Wang, Y. Yao, G. Xiao and D. Yan, J. Membr. Sci., 2013, 425–426, 200–207.
38. H. T. Li, G. Zhang, J. Wu, C. J. Zhao, Y. Zhang, K. Shao, M. M. Han, H. D. Lin, J. Zhu and H. Na, J. Power Sources, 2010, 195, 6443–6449.
39. H. Wu, X. Shen, Y. Cao, Z. Li and Z. Jiang, J. Membr. Sci., 2014, 451, 74–84.
40. J. Kim and D. Kim, J. Membr. Sci., 2012, 405–406, 176–184.
41. R. Subbaraman, H. Ghassemi and T. A. Zawodzinski, J. Am. Chem. Soc., 2007, 129, 2238–2239.
42. Q. Zhang, S. Zhang and S. Li, Int. J. Hydrogen Energy, 2011, 36, 5512–5520.

---