Novel composite membranes based on sulfonated poly(ether ether ketone) and adenosine triphosphate for enhanced proton conduction

Yongheng Yinab, Jiahui Wangab, Shengtao Jiangc, Xin Yangab, Xuya Zhangab, Ying Caoab, Li Caoab and Hong Wu*abd
aSchool of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China. E-mail: wuhong@tju.edu.cn; Fax: +86-22-23500086; Tel: +86-22-23500086
bCollaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China
cCollege of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China
dTianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University, Tianjin 300072, China

Received 17th July 2015 , Accepted 1st September 2015

First published on 1st September 2015


Abstract

Adenosine triphosphate (ATP) molecules, which contain proton-conductive phosphoric acid groups, imidazole groups and amino groups are incorporated into sulfonated poly(ether ether ketone) (SPEEK) to fabricate novel composite membranes. The morphology, structure and proton conduction abilities of the prepared membranes are investigated. ATP coalesces with SPEEK through ionic interactions between amino groups and sulfonic acid groups, which are beneficial to the stable existence of ATP molecules within polymer networks and enhance the thermal stability of the composite membranes. Due to the formation of abundant acid–base pairs among ATP molecules as well as between ATP and SPEEK chains, the proton conductivities of composite membranes under different conditions are increased in comparison with the plain SPEEK membrane, and this improvement becomes more evident when humidity is decreased. Particularly, the SPEEK/ATP composite membrane displays a highest proton conductivity of 0.198 S cm1 at 80 °C and 100% RH. The proton conductivity of the SPEEK/ATP-20 membrane at 41% RH is nearly 30 times higher than that of the plain SPEEK membrane. The results indicate a promising potential of such acid–base pairs which contain –PO3H2 groups in enhancing the proton conduction of membrane materials.


Introduction

Proton exchange membrane fuel cell (PEMFC) is a kind of new technology which offers great promise for energy-efficiency and environmental security. Proton exchange membrane (PEM) plays a performance-limiting role in PEMFC.1,2 Theoretically, the proton transfer through the membrane depends vitally on the Grotthuss mechanism, i.e. protons hop from one proton carrier to another.3 So building efficient proton carrier sites within the membrane is crucial to acquire a high proton conductivity.

In recent years, acid–base pairs have been demonstrated to be efficient proton conduction/carrier sites due to their amphoteric character.4–6 Generally, the acidic groups act as proton donors and the basic groups act as proton acceptors, they are linked by intermediate water bridges. The proton interacts with acidic group through hydrogen bonding, migrates along the water bridge and finally arrives at the basic group to complete the transfer.7 Intrigued by these finding, researchers attempt to fabricate PEMs with acid–base pairs. Tributsch et al. incorporated lysine-functionalized silica nanoparticles into the etched pores of polyethylene terephthalate. The power output of the as-prepared membranes approached the commercial Nafion® membrane.8 The uracil, an important genetic material in ribonucleic acid (RNA), was used by Yamada et al. to fabricate biomimetic membranes based on chitin phosphate. The proton donors (phosphate groups) in chitin phosphate interact with the proton acceptors (imino groups) in uracil to transport protons, rendering the prepared composite membranes with high proton conductivity (>10−3 S cm−1) at 160 °C under anhydrous conditions.9 Aslan et al. used adenine- and guanine-functionalized poly(glycidyl methacrylate) to fabricate proton-conducting membranes. It was found that the conductivity is improved significantly after doping with phosphoric acid, suggesting that the acidic phosphate ions and the basic imidazole form efficient proton-transfer sites in membranes.10 We previously incorporated carboxylic acid–amino groups functionalized titania–silica into chitosan and found that the acid–base pairs could serve as active sites for proton migration and the resultant hybrid membrane possessed a superior conductivity.11 However, the number of acid–base pairs formed in most hybrid membranes are limited, so facile strategies to fabricate membrane with abundant proton-conductive sites remain to be further explored.

One ion channel in living cells can conduct as high as 105 protons per second,8 the ion conduction in organisms is closely related to the proton-conductive groups.12–16 For example, in bovine heart cytochrome c oxidase, the proton is preloaded into the pump site involving histidine residues as pump elements, then the proton transfers along the amino acid-lined channels with high efficiency.17,18 The delicate structure of ion channels also demonstrates that building lined acid–base pairs inside the polymer could offer an effective way to enhance the proton-conducting ability. The ATP molecules (Fig. 1) contain both amphoteric –PO3H2 groups and basic imidazole and –NH2 groups, so they could form acid–base pairs with acidic polymer chains or among themselves through hydrogen bonding. Besides, the ionic interaction between basic groups in ATP and acidic groups in polymer is helpful to suppress the leaching of ATP from membrane.


image file: c5ra14143e-f1.tif
Fig. 1 Molecular structure of adenosine triphosphate (ATP).

In this study, ATP is incorporated into SPEEK by solution blending to prepare novel composite membrane materials. SPEEK is a kind of promising material for PEM due to its high chemical stability, low cost, tunable sulfonation degree and low fuel permeability.19–23 The direct incorporation of ATP biomolecules into SPEEK endows the membrane with abundant proton-conducting sites. The proton conductivity of the resultant composite membranes, especially under low humidity and elevated temperature, is evaluated. The influence of filler content of ATP on membrane performance and the proton conduction mechanism in membranes are investigated and discussed.

Experimental

Materials and chemicals

Poly(ether ether ketone) (PEEK) was purchased from Victrex High-performance Materials Co., Ltd (Shanghai, China). Adenosine 5′-triphosphate disodium salt hydrate (ATP) was obtained from Aladdin. N,N-Dimethylformamide (DMF), methanol and other reagents with analytical grade were obtained from Guangfu Fine Chemical Research Institute (Tianjin, China) and used without any further purification. Deionized water was used in all the experiments.

Preparation of sulfonated poly(ether ether ketone) (SPEEK)

SPEEK was prepared by a direct sulfonation method.24 PEEK was dried overnight at 80 °C in an oven. 14 g dried PEEK was gradually dissolved in 100 mL concentrated sulfuric acid (95–98%) for ∼3 h at room temperature, followed by vigorously stirring at 45 °C for ∼6 h. After sulfonation, the solution was gradually precipitated in ice-cold water under mechanical agitation. Then the obtained SPEEK precipitate was filtrated and washed several times with water until the pH was neutral. Finally the SPEEK was dried at room temperature for 24 h and at 60 °C for another 24 h.

Membrane preparation

SPEEK was dissolved in DMF at room temperature to a final concentration of 0.1 g mL−1. A premeasured amount of ATP was dissolved in 2 g water, subsequently the ATP solution was added into SPEEK slowly to form a clear solution in which the ATP was supersaturated. The mixed suspension was stirred for ∼0.5 h to ensure complete mixing, then the mixture was degasified and cast onto a glass plate. The resultant membranes were successively dried overnight at 60 °C for 12 h, followed by annealing at 80 °C for another 12 h. The prepared plain SPEEK membrane and composite membranes were designated as SPEEK and SPEEK/ATP-X respectively, where X (=10, 15 or 20) was the weight ratio percentage of ATP to SPEEK. The thickness of membranes was in the range of 50–60 μm.

Characterizations

The cross-sectional morphology of the membranes was observed using field emission scanning electron microscope (FESEM, Hitachi S-4800) operated at 3 kV. The cross-sections were prepared by freeze-fracturing samples in liquid nitrogen and then coated with a thin layer of sputtered gold. The structure of the membranes was characterized by Fourier transform infrared spectrometer (FTIR, Nicolet MAGNA-IR 560) in the wavenumber range of 1000–2000 cm−1. The crystalline structure of the membranes was monitored using an X-ray diffractometer (XRD, RigakuD/max2500v/Pc) equipped with Cu Kα radiation, the scanning angle ranged from 2° to 50° with a scanning rate of 2° min−1. The glass transition temperature (Tg) of the membranes was measured by differential scanning calorimetry (DSC, Netzsch 204 F1), the membrane sample was heated from 25 to 130 °C at a heating rate of 10 °C min−1 under N2 atmosphere, then cooled to 90 °C and reheated to 260 °C. The thermal stability of membranes was investigated using Thermo gravimetric analyzer (TGA, Perkin-ElmerPyris) in the temperature range of 40–800 °C at a heating rate of 10 °C min−1 under N2 atmosphere. Before measurement, all the membranes were vacuum-dried for 24 h. The leakage of ATP molecules from composite membranes at room temperature and 45 °C was measured as follows. 0.2 g SPEEK/ATP-20 composite membrane was immersed in 10 mL 2 M methanol solution at constant temperature for 3–15 days. The amount of ATP was determined from phosphorus in solution by using Inductively Coupled Plasmaoptical Emission Spectrophotometer (ICP, Vista-MPX).

Water uptake and swelling degree

The water uptake and swelling degree of the membranes were measured according to the methods used in our previous work.25 The values were calculated by
 
image file: c5ra14143e-t1.tif(1)
 
image file: c5ra14143e-t2.tif(2)

Before measurement, each membrane sample (∼4 × 4 cm2) was vacuum-dried at 60 °C for 24 h, the dried membrane was weighed (Wd, g). Then the membrane was soaked in water for 24 h at room temperature, after fully hydration, the membrane was reweighed (Ww, g). The measurement errors were within ±5%.

Ion exchange capability (IEC)

The IEC value of the membranes was measured with a classical back-titration method26 and calculated by eqn (3):
 
image file: c5ra14143e-t3.tif(3)
in which VNaOH (L) is the volume of NaOH solution consumed for titration and Wd (g) is the weight of dried membrane sample. The measurement errors were within ±4.0%.

Proton conductivity

The proton impedance of membranes was measured over the frequency range of 1 Hz to 1 MHz on an electrochemical workstation (PARSTAT 2273, Princeton). During the test of proton conductivity at 100% relative humidity (RH), the temperature was controlled by the water vapor from room temperature to 80 °C. The membrane was soaked in 0.2 M H2SO4 solution for 24 h and then immersed in distilled water until pH reached neutral, then the fully hydrated membranes were put into a climate box under 40 °C and 20% RH to measure the time-dependent proton conductivity. For the measurement of proton conductivity at low humidity, the saturated steam of KBr, NaNO2 and K2CO3 was used to keep the RH at 78%, 57% and 41%, 6 hours are needed to stabilize the system before testing. The proton conductivity (σ, S cm−1) was calculated by
 
image file: c5ra14143e-t4.tif(4)
where l0 is the distance between the electrodes used to measure the potential, A is the effective surface area and R is the membrane impedance.

Results and discussion

Structure characterization of membranes

SEM images in Fig. 2 display the morphology of the membranes. It can be seen the SPEEK/ATP composite membranes possess a uniform structure when the loading content is below 15 wt%, indicating that the ATP molecules are incorporated into the membranes without aggregation. The cross-section of composite membrane becomes rougher and some wrinkles can be found as the ATP content is increased up to 20 wt%. The organic ATP biomolecules possess hydrophilic groups such as –OH, –O–, etc., showing good compatibility with the hydrophilic polymer matrix,27,28 so the composite membranes are fabricated without any observable defects.
image file: c5ra14143e-f2.tif
Fig. 2 SEM images of the cross-sections of (a) plain SPEEK membrane, (b) SPEEK/ATP-10 composite membrane, (c) SPEEK/ATP-15 composite membrane and (d) SPEEK/ATP-20 composite membrane.

To demonstrate the binding interaction between ATP and SPEEK, the membranes are characterized by FTIR. Both the transmission and reflection-absorption spectra ranging from 1000–2000 cm−1 are illustrated in Fig. 3a and Fig. 3b. The absorption bands at ∼1020 and 1080 cm−1 are assigned to the asymmetric stretching vibrations of O[double bond, length as m-dash]S[double bond, length as m-dash]O. After introducing ATP into SPEEK membrane, the P[double bond, length as m-dash]O stretching vibration can be observed at ∼1200 cm−1 in Fig. 3a.29 In addition, the presence of the C–N stretching peak can be observed at ∼1650 cm−1 in Fig. 3b due to the –NH2 groups.30 It is worth noting that the original –SO3 stretching peak at 1079 cm−1 shifts towards lower frequency as the content of ATP increases, suggesting that the electrostatic interaction between the –NH2 groups in ATP molecules and the –SO3H groups in SPEEK chains is enhanced within the composite membranes.31 This is beneficial to the stable existence of ATP molecules along the SPEEK chains.


image file: c5ra14143e-f3.tif
Fig. 3 (a) FTIR transmission spectra, (b) FTIR reflection-absorption spectra, (c) XRD patterns and (d) DSC curves of the plain membrane and the composite membranes; (e) ATP release from SPEEK/ATP-20 composite membrane as a function of time at room temperature and 45 °C.

The XRD patterns of the plain and composite membranes show the effect of ATP molecules on the crystalline structure of polymer matrix (see Fig. 3c). PEEK displays sharp peaks at 2θ from 20–30° due to the semicrystalline structure, while SPEEK is generally amorphous because of high sulfonation degree.32,33 The SPEEK used in this study shows crystalline peaks at 2θ of 19° and 21° although they are overlaid by a broad amorphous halo. These two peaks correspond to the reflections from (1 1 0) and (1 1 1) planes, respectively. As the ATP content increases, the intensity of these two crystalline peaks becomes weaker, and the peaks almost disappear when the ATP load reaches 20 wt%. The decrease in crystallinity of the membrane indicates that the filling of ATP molecules disturbs the originally ordered packing of SPEEK chains.

Fig. 3d illustrates the glass transition process detected for the prepared membranes. All the composite membranes exhibit one single glass transition temperature (Tg), demonstrating the well mixing of the two components. The Tg value of plain SPEEK membrane is 144.0 °C. In comparison, the Tg increases to about 180.4 °C upon 10 wt% incorporation of ATP molecules, and increase further with the elevated ATP content. The movement of Tg of composite membranes verifies that after the incorporation of a large number of ATP molecules, strong hydrogen bonding formed between the phosphoric acid groups, hydroxyl groups and sulfonic acid groups restricts the overall chain mobility of SPEEK.

The retention of ATP molecules within the composite membranes is an important factor ensuring the long-term performance of membranes. The ATP release behavior of SPEEK/ATP-20 composite membrane is shown in Fig. 3e. The release of ATP molecules at room temperature is kept at a low level and only 5.4% loss is observed after 15 days. Even though the leakage is enhanced to 9.5% when the temperature rises to 45 °C, it can be found that release is approximately constant after 3 days immersion, this indicates that most ATP molecules are retained in the polymer network through mutual interaction with SPEEK chains as discussed above.

Thermal stability of the composite membranes

The thermal stability of the membranes, which is important for the lifetime of PEMs, is investigated by TGA and shown in Fig. 4. For ATP molecules, the weight loss at 130 °C is caused by the evaporation of bound water. The decomposition between 185–220 °C is due to the debonding of first phosphoric acid group with a weight loss of ∼16%. When the ATP molecules are mixed with the SPEEK matrix, all the composite membranes display a similar decomposition process to the plain SPEEK membrane with three weight loss stages. The weight loss before 260 °C is caused by the evaporation of water (free and bound water) and residual solvent. The second sharp weight loss in the range of 280–400 °C is ascribed to the pyrolysis of functional groups within the membranes. For plain SPEEK membrane, this stage is attributed to the decomposition of sulfonic acid groups. For the composite membranes, the decomposition temperature of phosphoric acid groups is delayed because of the hydrogen-bond interactions among ATP molecules. The following third weight loss beginning at approximately 420 °C is due to the decomposition of the polymer main chains. It can be found that the last weight loss stage is inhibited by the addition of organic filler and the composite membrane exhibits higher ultimate weight at 800 °C as the ATP content increases. This is owing to the fact that the molecular-level interaction between polymer and ATP molecules retards the degradation of polymer main chains and leads to improved thermal stability. Notably, all the composite membranes are sufficiently stable below 280 °C, which is adequate for the application of fuel cells.
image file: c5ra14143e-f4.tif
Fig. 4 TG and DTG curves of the ATP, plain SPEEK membrane and SPEEK/ATP composite membranes.

Water uptake and swelling degree

The water uptake reflects the hydrophilicity of membranes, while excess swelling caused by too much water adsorption will affect the stability of membranes. The water uptake and swelling degree at room temperature are tested and the results are shown in Table 1. It is found that the water uptake values of the composite membranes are higher than that of plain SPEEK membrane, the swelling degree is kept at a similar level after the introduction of ATP molecules. Since the phosphoric acid groups have a similar water binding energy (44.4 kJ mol−1) to sulfonic acid groups (47.3 kJ mol−1),4 abundant hydrophilic phosphoric acid groups provided by ATP molecules will increase the water uptake of membranes. The size enlargement of the proton transport channels after the introduction of ATP will lead to more absorption of water molecules as well. However, the interactions between ATP and SPEEK suppress the swelling degree, endowing the composite membranes with good stability.
Table 1 Water uptake, swelling degree and ion exchange capability of the membranes at 25 °C
Membrane Water uptake (%) Swelling degree (%) IEC (mmol g−1)
SPEEK 20.3 13.4 1.655
SPEEK/ATP-10 22.4 13.7 1.702
SPEEK/ATP-15 23.9 14.3 1.918
SPEEK/ATP-20 26.0 14.4 1.964


Ion exchange capability

The ion exchange capability (IEC) implies the density of ion exchangeable functional groups in the membrane, which can be used as an indicator of proton transfer ability. The measured IEC values of the plain SPEEK and composite membranes at room temperature are listed in Table 1. The IEC values SPEEK/ATP-10, SPEEK/ATP-15, SPEEK/ATP-20 composite membranes are 2.8%, 15.9% and 18.7% higher than that of the plain SPEEK membrane, respectively. This finding demonstrates that although the incorporation of ATP molecules might dilute the concentration of H+ ions dissociated by –SO3H groups in SPEEK, the –PO3H2 groups in ATP contribute to increasing the acid dissociation ability of the composite membranes. The IEC results show that the ATP molecules embedded in the polymer matrix help to increase the IEC, this will enhance the proton conductivity of the membranes to some extent.

Temperature-dependent proton conductivity

As discussed above, the ATP molecules are anchored to the SPEEK chains by intermolecular forces. The abundant functional groups provided by homodispersed ATP molecules are assumed to construct continuous proton transfer sites within the membrane, thus enhancing the proton conducting ability of membrane materials. The proton conductivity of membranes at 100% RH and the corresponding Arrhenius plots are given in Fig. 5 and 6 respectively. All the composite membranes show higher proton conductivity than the plain SPEEK membrane. The proton conductivity increases with the increase of ATP content, the SPEEK/ATP-10, SPEEK/ATP-15, SPEEK/ATP-20 composite membranes exhibit proton conductivities of 1.93 × 10−2, 2.46 × 10−2, 2.81 × 10−2 S cm−1 at 25 °C, which are 14.2%, 45.6% and 66.3% higher than the plain SPEEK membrane, respectively. Such increase in proton conductivity is much higher than the increase in IEC, which implies that the proton-conducting ability is not only affected by the number increase of acidic groups. In order to further investigate the proton conduction mechanism inside the composite membranes, the activation energy values for proton conduction are derived by linear fitting of the data exhibited in Fig. 6 using Arrhenius equation,
 
image file: c5ra14143e-t5.tif(5)
where σ0 is the pre-exponential factor, Ea is the activation energy, R is the gas constant and T is the Kelvin temperature. The activation energy values of all the composite membranes are in the range of 0.32–0.33 eV, which are much higher than the activation energy for merely Vehicle mechanism (0–0.150 eV).11 This result reveals that the proton migration in composite membranes is through both Grotthuss mechanism and Vehicle mechanism, whereas the former is predominant.34,35 The introduction of ATP molecules provides new proton hopping sites, thus lowering the energy-barrier for proton transport.

image file: c5ra14143e-f5.tif
Fig. 5 Proton conductivity of plain and composite membranes as a function of temperature at 100% RH.

image file: c5ra14143e-f6.tif
Fig. 6 Arrhenius plot of the proton conductivity for plain and composite membranes at 100% RH. The dots represent the experimental data and the straight lines represent the linear fitting of the data.

Based on the above analysis, it can be concluded that incorporating ATP into SPEEK creates more proton transfer sites and increases the proton conductivity. The proton migration mechanism in SPEEK/ATP composite membranes is schematically illustrated in Fig. 7. Direct blending endows the composite membranes with numerous functional groups, including –SO3H groups (14–15 wt%), –PO3H2 groups (4.5–9 wt%), imidazole groups (1.2–2.3 wt%) and –NH2 groups (0.3–0.6 wt%). Particularly, when the ATP content is up to 20%, the average distance between each ATP molecule inside the composite membrane is about 1.66 nm, so abundant acid-based pairs can be easily formed along the SPEEK chains through hydrogen bonding. The protons are transferred in two ways: (i) tight acid–base pairs, in this way, the proton is donated by an acidic group and accepted by a basic group directly to complete the transfer; (ii) loose acid–base complexes, in this manner, the proton donors and proton acceptors are linked through water bridges, usually the number of water molecules forming water bridges is below 3. The proton first interacts with acidic groups by hydrogen bonding, then is diffused by the water bridge to arrive at the basic groups, finally moves out of the acid–base pairs through hydrogen bonding with another water molecule. In these two ways, the proton donors and acceptors are linked effectively to create more continuous pathways for proton conduction. Consequently, the proton conductivity of composite membranes is improved compared with that of the plain SPEEK membrane.


image file: c5ra14143e-f7.tif
Fig. 7 The schematic depiction of the proton migration in SPEEK/ATP composite membrane.

Time-dependent proton conductivity

The time-dependent proton conductivity of membranes at 40 °C and 20% RH is shown in Fig. 8. The proton conductivity of both plain SPEEK and SPEEK/ATP composite membranes show gradual decline over time, indicating that the proton transport inside membranes depends heavily on water content. For plain SPEEK membrane, the proton conductivity decreases from 2.95 × 10−2 to 1.97 × 10−4 S cm−1 after 90 min testing. The sharp reduction of 99.33% is caused by the dehydration of membrane, which further leads to the shrinkage and disconnection of hydrophilic channels. By contrast, the proton conductivities decrease by 98.55% (from 3.56 × 10−2 to 5.15 × 10−4 S cm−1), 93.30% (from 4.39 × 10−2 to 2.94 × 10−3 S cm−1) and 83.01% (from 5.09 × 10−2 to 8.65 × 10−3 S cm−1) for SPEEK/ATP-10, SPEEK/ATP-15 and SPEEK/ATP-20, respectively. The ATP molecules retard the proton conductivity decline of membrane because the proton-conductive, hydrophilic groups in ATP contribute to the proton transfer and retain the proton-conducting ability of composite membranes.
image file: c5ra14143e-f8.tif
Fig. 8 Proton conductivity of plain and composite membranes as a function of time at 40 °C and 20% RH.

Humidity-dependent proton conductivity

The low humidity proton conductivity of the plain and composite membranes at 65 °C is shown in Fig. 9. For the plain SPEEK membrane, the proton conductivity declines sharply from 0.0696 (RH = 100%) to 1.41 × 10−4 (RH = 41%) S cm−1 with a reduction of 99.80%. This remarkable decrease is caused by the serious water loss in plain membrane under low humidity conditions. The shrinkage and disconnection of proton transport channels leads to great energy penalty in transferring protons for –SO3H groups. Compared with the plain SPEEK membrane, the composite membranes form Brønsted acid–base pairs using –PO3H2 groups, which promote the proton transfer between protonated and non-protonated sites under low humidity.36 As a result, the SPEEK/ATP composite membranes show a much slower decreasing rate of proton conductivity as the humidity decreases. In comparison, the proton conductivity of SPEEK/ATP-20 membrane at RH = 41% (4.21 × 10−3 S cm−1) is nearly 30 times higher than that of the plain SPEEK membrane (1.41 × 10−4 S cm−1). The significant improvement in proton conductivity under low humidity can be attributed to the following two aspects. First of all, the –PO3H2 groups in ATP molecules possess amphoteric character.37,38 Each –PO3H2 group has three proton-donor sites and one proton-acceptor site,39 in this sense, the –PO3H2 group can form acid–base pairs with either acidic –SO3H groups, basic groups or other –PO3H2 groups. The activation energy of proton transfer between acid–base pairs is only about one third of that of proton diffusion process,40 thus the adjacent acidic and basic groups interact with each other to compensate for proton conduction through Grotthuss mechanism under low humidity. In addition, the –PO3H2 groups have a lower average zero point energy (37.2 kJ mol−1)4 compared with –SO3H groups (69.6 kJ mol−1) in SPEEK. It means that the proton transfer barrier is relatively low for –PO3H2 groups, this is also favorable for proton migration under low humidity.
image file: c5ra14143e-f9.tif
Fig. 9 Proton conductivity of plain and composite membranes as a function of humidity at 65 °C.

Conclusion

Based on the researches that lined acid–base pairs can construct efficient proton-conducting channels, a new type of PEM materials has been successfully prepared by incorporating ATP into SPEEK matrix. The resultant composite membranes show homogeneous structure and enhanced thermal stability due to the ionic interaction between the basic groups in ATP and the acidic groups in SPEEK. Abundant ATP molecules facilitate the hydrogen bonding linkage between acidic and basic groups within the composite membranes. The acid–base pairs formed by ATP play two functions: (i) the protons are transferred through acidic groups, water bridge, basic groups with a low energy barrier; (ii) the amphoteric –PO3H2 groups possess the ability of self-dissociation and self-association of H+, thus promoting the proton-conducting efficiency of acid–base pairs under low humidity. To sum up, more proton transport sites are created by proton donors and receptors along polymer chains in the composite membranes, which promote the temperature-dependent, time-dependent and humidity-dependent proton conduction via Grotthuss mechanism. Particularly, the proton conductivity of the SPEEK/ATP-20 membrane under low humidity is about 30 times higher than that of the plain SPEEK membrane. Overall, the SPEEK/ATP composite membranes possess a strong thermal stability and well performance. This study shows that acid–base pairs containing amphoteric –PO3H2 groups may be applicable to prepare PEM materials with enhanced proton conductivity.

Acknowledgements

The authors gratefully acknowledge financial support from 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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