Nano-spheres stabilized poly(vinyl phosphonic acid) as proton conducting membranes for PEMFCs

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 1 1 5 7 e1 1 1 6 4

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Nano-spheres stabilized poly(vinyl phosphonic acid) as proton conducting membranes for PEMFCs Fengjing Jiang a,b,c, Anke Kaltbeitzel c, Junliang Zhang a,b,*, Wolfgang H. Meyer c a

Institute of Fuel Cell, Shanghai Jiao Tong University, 800 Dongchuan Rd., Shanghai 200240, China Key Laboratory of Power Machinery and Engineering (Shanghai Jiao Tong University), Ministry of Education, China c Max Planck Institute for Polymer Research, Ackermannweg 10, Mainz 55128, Germany b

article info

abstract

Article history:

Poly(4-vinyl

Received 19 March 2014

acid) (PVPA) membranes were prepared as proton conducting membranes for fuel cells.

Received in revised form

The acidebase interactions between P4VP-NS and PVPA were utilized to improve the

6 May 2014

dimensional and thermal stability of PVPA. In this work, the effects of acidebase in-

Accepted 9 May 2014

teractions on proton conductivity and thermal stability were quantitatively determined

Available online 11 June 2014

and discussed based on the results of 1H magic-angle-spin NMR (1H MAS NMR), Fourier

pyridine)

nano-spheres

(P4VP-NS)

stabilized

poly(vinyl

phosphonic

transfer infrared spectroscopy (FTIR) and impedance spectroscopy measurements. Keywords:

Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

Fuel cells

reserved.

Proton conducting Polyelectrolyte Acidebase interaction Composite membrane

Introduction Proton exchange membranes (PEMs) for fuel cells have undergone significant developments during the past decades [1e3]. PVPA which contains high fraction of acid groups in the polymer chain, exhibits very high proton conductivity (~10
2 S cm
1) under humid conditions and has been considered as a promising proton conducting polyelectrolyte for proton exchange membrane fuel cells (PEMFCs) [4e7]. However, PVPA is very hygroscopic and water soluble, so the tensile strength and shear strength of PVPA decline dramatically in humid atmosphere. Thus, PVPA cannot be directly used as proton exchange membrane for fuel cells as

water is continuously produce at the cathode of a working fuel cell. Moreover, it was reported that PVPA forms anhydrides at low relative humidity (RH) and high temperatures, which impedes proton conductivity [8]. In order to overcome these drawbacks and make PVPA applicable as PEM for fuel cells, works of preparing copolymers of vinyl phosphonic acid with other proton acceptors (e.g. imidazole, triazole, pyridine et al.) have been reported [9e13]. Recently, statistical copolymers of poly(vinyl benzyl phosphonic acid) and poly(4-vinyl pyridine) (P4VP) were synthesized in our group and the influence of the copolymer composition on conductivity was studied [14,15]. However, the investigation of effects of acidebase interactions on properties was still insufficient.

* Corresponding author. Institute of Fuel Cell, Shanghai Jiao Tong University, 800 Dongchuan Rd., Shanghai 200240, China. Fax: þ86 21 34206249. E-mail address: [email protected] (J. Zhang). http://dx.doi.org/10.1016/j.ijhydene.2014.05.050 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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Organiceinorganic composite PEMs based on perfluorinated ionomers and different types of inorganic fillers have been intensively studied [16e19]. These studies demonstrate that inorganiceorganic composite membranes are one of the most interesting routes to prepare promising PEMs for application in PEMFCs [20e24]. However, to the best of our knowledge, studies on P4VP based organiceinorganic composite PEMs are seldom reported. In this work, silicaeP4VP coreeshell nano-spheres were prepared via surface-initiated atom transfer radical polymerization of 4-vinyl pyridine monomers. The obtained P4VP-NS were mixed with PVPA to prepare PVPA/P4VP-NS hybrid proton conducting membranes for fuel cells. The prepared P4VP-NS have very dense P4VP brushes grafted on the surface of silica nanoparticles. Therefore, most of the acidebase interactions between phosphonic acid and pyridine units tend to occur in the P4VP-NS surface. The acidebase interactions (cross-linking) between P4VP-NS and PVPA matrix will help to improve the dimensional and thermal stability of PVPA. Moreover, the effect of acidebase interaction on proton conduction was also investigated.

Experimental section Materials PVPA was synthesized via anionic polymerization. The details of the anionic synthesis of PVPA were reported in a separate publication [25]. The P4VP-NS were self-prepared. The preparation method and characteristics of P4VP-NS were described in another publication (reference name: SiO2-NP-Cl-4VP) [26].The diameter of the P4VP-NS ranges from 40 to 100 nm and the content of P4VP is around 94 wt%. The structure of P4VP-NS is schematically illustrated in Fig. 1.

Preparation of PVPA/P4VP-NS blends 5 g PVPA was firstly dissolved in 100 ml deionized water to obtain a PVPA aqueous solution. A certain amount of P4VP-NS suspension (5 wt% P4VP-NS in methanol) was added dropwise to the PVPA aqueous solution. The mixtures were kept stirring under N2 for another 2 h at room temperature. Then the methanol in the mixture was evaporated under reduced

pressure. The residue mixture was freeze-dried to obtain the PVPA/P4VP-NS blends. Various PVPA/P4VP-NS blends were prepared by changing the mass ratio of PVPA to P4VP-NS. The molar ratio of pyridine groups to phosphonic acid groups in the blends is listed in Table 1. The mass ratio of P4VP-NS to PVPA happens to be the same as the molar ratio of 4-vinyl pyridine (4VP) and vinyl phosphonic acid (VPA) groups.

Methods of characterization 1 H MAS NMR experiments were carried out at 500.13 MHz (Bruker ASX500 spectrometer), with the spinning frequency of 25 kHz. FTIR spectra were obtained on a Nicolet 730 FTIR spectrometer. All materials were measured in the pellet form with KBr. In order to determine the amount of water sorption, the samples were stored under an atmosphere of fixed RH and temperature for several days. The RH was set by saturated salt solutions according to literature data [27]. The uptake of water was measured on a Mettler MX5 micro balance until constant weight W was obtained; the water sorption was calculated from Eq. (1).

water uptakeð%Þ ¼

Wwet
Wdry  100 Wdry

(1)

Proton conductivity was measured using impedance spectroscopy in a two-electrode geometry using an SI 1260 impedance/gain-phase analyzer in the frequency range of 10
1e106 Hz. Direct current (DC) proton conductivities were evaluated from Bode Plots. The humidity during data acquisition was set by mixing dry nitrogen with humidity saturated nitrogen. The RH was measured using a Sensiron SHT15 sensor. For Arrhenius plots the environment was set in a Binder KBF 240 climate chamber. For dry-condition proton conductivity measurements, all the samples were dried at 50  C under vacuum for 4 days prior to the measurements. Completely dry N2 was used to flush the samples during the measurements. The samples for conductivity measurements were pressed to tablets and contacted by E-tek® to stainless steel electrodes.

Results and discussion Acidebase complex formation in PVPA/P4VP-NS blends PVPA has a very high concentration of phosphonic acid groups in the polymer chains (the chemical structure of PVPA is

Table 1 e Sample list of PVPA/P4VP-NS blends. Samples

Fig. 1 e Illustration of structure of P4VP-NS (the content of P4VP is around 94 wt%) [26].

VP/PA-1 VP/PA-2 VP/PA-3 VP/PA-4 VP/PA-5 VP/PA-6

Composition (mass ratio) (P4VP-NS:PVPA)

molar ratio of 4VP:VPA

1:9 2:8 3:7 5:5 7:3 9:1

1:9 2:8 3:7 5:5 7:3 9:1

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shown in Fig. 2(a)). Pyridine units in P4VP are likely to form acidebase complexes with phosphonic acid (see Fig. 2(b)). Solid state 1H MAS NMR spectra were recorded to determine the acidebase interaction in the PVPA/P4VP-NS blends (see Fig. 3). Due to the limited resolution, the resonance peaks were not well separated. However, the content of NH and “free” POH moieties in the composites were quantitatively determined from the decomposition of the corresponding peaks. For example, the decomposition of 1H MAS NMR spectra of VP/PA-1, VP/PA-3 and VP/PA-4 was shown in Fig. 4. The peak around 15.5 ppm refers to NH (protonation of the pyridine unites); the peak around 11.7 ppm is the resonance of POH from the phosphonic acid group. The integral of each peak indicates the amount of the corresponding hydrogen. The results are listed in Table 2. From the spectra in Fig. 3, as a result of complex formation, the intensity of the POH peak decreases with increasing amount of P4VP-NS. In the spectrum of VP/PA-6 the peak of POH group disappeared, which is probably because most of the POH groups have reacted with pyridine units. As shown in Table 2, the concentration of POH moieties decreases from VP/ PA-1 to VP/PA-6, with increasing P4VP-NS content. The acidebase complex can also be observed in FTIR spectra. The FTIR spectra of P4VP-NS and the blends are represented in Fig. 5. The correlative assignments of the peaks of the spectra were listed in Table 3. In the FTIR spectra of the blends, the broad bands at 3300e2850 cm
1 (the OH stretching of the POH group [12]) are overlapped with the broad peak at 3100 cm
1 (NeH stretching [12]). With the increasing content of P4VP-NS, the peak of broad band shifts from 2850 cm
1 to 3100 cm
1 because the intensity of NeH stretching peak is increased accordingly. The broad peaks around 1040e910 cm
1 and 1150 cm
1 are (PeO)H and P]O stretching, respectively [12]. From VP/PA-3 to VP/PA-6, the intensity of these two peaks decreases with increasing P4VP-NS content, indicating the decreasing phosphonic acid content in the blends. The IR spectra also indicate that the amount of acidebase complex in the blends increases with the increasing content of P4VP-NS which is different from that in a copolymer. In a copolymer with both acid and base unites, the amount of acidebase complex reaches its maximum value around a composition of 1:1 M ratio (acid:base) [15]. While in the blends with basemodified nanoparticles, acidebase complexes can only form at the surface layer of P4VP-NS, therefore, the amount of the complex increases with the surface area of P4VP-NS. The morphologies of the blends are schematically illustrated in Fig. 6.

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Fig. 3 e Solid state 1H MAS NMR spectra of PVPA/P4VP-NS blends.

Dimensional stability of PVPA/P4VP-NS blends under humid conditions PVPA is hygroscopic and water soluble due to its very high density of phosphonic acid groups in the polymer chains, which makes it alone not an ideal material for proton conducting membranes for fuel cells. In order to enhance the dimensional stability of PVPA in humid atmosphere, P4VP-NS was blended with PVPA via acidebase interaction. The pyridine groups acted as proton acceptors while phosphonic acid groups donated protons to form acidebase complexes (see Fig. 2(b)). As a result, PVPA was ionically cross-linked by P4VPNS, the degree of cross-linking increased along with the P4VPNS content in the blends. It is known that a simple linear relationship between water uptake and degree of swelling is applicable over a wide range, and hence, within this range, changes in water uptake are a reliable measure of volume changes [28]. In this work, water uptake of PVPA/P4VP-NS blends and PVPA were measured to compare their dimensional stabilities. The result is shown in Fig. 7 that the degree of swelling of PVPA decreased dramatically after blending with P4VP-NS. Interestingly, all the PVPA/ P4VP-NS blends showed very low water uptake less than 10 wt % at 100% RH, including the blend that contained 10 wt% P4VP-NS. According to the FloryeRehner theory, in a crosslinked network, the swelling capacity of the network is diminished by cross-linking [29]. Thus, the significant decrease in water uptake indicates a substantial improvement of dimensional stability of the polyelectrolyte.

Proton conduction under dry conditions

Fig. 2 e Chemical structure of (a) PVPA and (b) P4VPePVPA complex.

Nominal dry proton conductivities of the blends were measured and shown in Fig. 8. The proton conductivities of the blends decreased with increased content of P4VP-NS. VP/ PA-1 exhibited a similar conductivity to that of PVPA. According to Eq. (2), conductivity (s) is proportional to the

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Table 2 e Percentage of NH and POH existing in the blends over the total number of phosphonic acid units by 1H MAS NMR. Sample

Molar ratio of VPA (%)a

NH (%)

POH (%)

VP/PA-1 VP/PA-3 VP/PA-4 VP/PA-6

90 70 50 10

6 30 42 N/A

94 70 58 N/Ab

a b

Molar ratio of VPA out of total repeat units (VPA þ 4VP). POH group could not be detected by 1H MAS NMR.

number of charge carriers (n) per unit volume (the chargecarrier-number density), the mobility of ions in an electric field (m) and the number of charges per carrier. sðTÞ ¼ nðTÞmðTÞq

(2)

In proton conductors, the number of charge carrier (proton) depends on the acid dissociation constant (ka) of protons at a given temperature. Ka is the equilibrium constant for a dissociation reaction, for example, as shown in Eq. (3): ka

HAcƒƒƒƒ!Hþ þ Ac


(3)

HAc is an acid (proton donator) which generates protons for proton conduction. The equilibrium can be written symbolically as: ka ¼

½Hþ ½Ac
 ½HAc

(4)

The relation between ka and temperature T is: vln ka DH ¼
2 RT vT

(5)

where R is the gas constant, DH is the enthalpy change. We can see that, at a given temperature, the number of dissociated protons is determined by ka. The pKa value is commonly used to identify the acidity of an acid which is given by Eq. (6): k

pKa ¼
log10a

(6)

For PVPA, the pKa value for the dissociation of the first proton in the appropriate monomer was determined to be 2.71 [7]. However, since the pKa value for the 4VP-homopolymer (P4VP) was reported to be 5.6 [30], protons dissociated from PVPA were trapped in complexes with P4VP as long as a new equilibrium was reached, which depended on the molar ratio of the PVPAeP4VP-blend. Therefore, the number of dissociated protons decreased with the increasing amount of acidebase complex, leading to a decrease in conductivity. To further understand the effect of acidebase complex formation on proton conduction, the activation energies of conductivity of the blends were compared. As shown in Fig. 8, the conductivities of the blends follow the Arrhenius equation (Eq. (7)),  
Ea s ¼ s0 exp RT

(7)

Fig. 4 e Decomposition of 1H MAS NMR spectra of (a) VP/ PA-1, (b) VP/PA-3 and (c) VP/PA-4.

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Fig. 6 e Schematic illustration of the morphology of PVPA/ P4VP-NS blends with (a) low content of P4VP-NS and (b) with high content of P4VP-NS.

Fig. 5 e FTIR spectra of P4VP-NS and PVPA/P4VP-NS blends. where Ea, R, T, s0 represent the activation energy scaled as a molar energy, gas constant, temperature and pre-exponential factor respectively. The apparent activation energy (Ea) of proton conduction can be calculated accordingly. It is noteworthy that the Arrhenius plots of the blends at high temperatures (>120  C) are not linear. This is more likely because of the anhydride formation of the POH groups. The Ea was calculated based on the linear part of the plots. Table 4 shows the obtained activation energies calculated from Fig. 8. The results indicate that the activation energies of the blends were almost the same (81 kJ mol
1) which was larger than that of PVPA (65 kJ mol
1). According to the Eq. (2), at a given temperature, proton conductivities of the blends were determined by both the number and the mobility of protons. Because the Ea of conductivity for all the blends turned out to be the same, the decrease of conductivity with increasing P4VP-NS content might be mainly caused by the reduction of charge carriers (mobile protons) in the blends. Actually, in the blends, the complex formation diminishes “mobile” protons in two ways. Firstly, protons are trapped by pyridine units as NH; Secondly, the formation of acidebase complexes leads to poor side chain motion raising the position barrier for proton transport. The increase in position barrier of the blends significantly decreases the number of “mobile” protons rather than increasing the Ea of conductivity. In homogeneous systems, the increase of position barrier increases the Ea of conductivity because position barrier for each proton is identical. However, in this work, the blends are inhomogeneous systems (see Fig. 6). Their conductivities are mainly

contributed by the fast-moving protons which are not blocked by the additional barriers. The protons that overcome additional barriers do not contribute much to the conductivity. Moreover, with increasing amount of complex in the blends, the number of the fast-moving protons is decreased significantly. Since the fast-moving protons in the blends still contribute the most of the conductivity, the activation energy should remain the same.

Proton conductivity under humid conditions PVPA is a very hygroscopic polymer and can absorb a lot of water before it is totally dissolved in water. As already demonstrated, after blending with P4VP-NS, the water uptake was dramatically changed. The humidity dependence of proton conductivities of the blends at 25  C is shown in Fig. 9. After being exposed to water vapor, all the conductivities increase rapidly with RH. VP/PA-1 had a similar conductivity like that of PVPA which reached as high as 0.05 S cm
1, at 90% RH. The conductivity decreased when the PAVP-NS content increased. Proton conductivities of VP/PA-1, VP/PA-2 and VP/PA-6 were also measured at 80% RH, at various temperatures (as shown in Fig. 10). VP/PA-1 showed very high proton conductivity (~0.3 S cm
1) at 85  C and 80% RH. In the temperature

Table 3 e Assignment of the FTIR peaks shown in Fig. 5 [12]. Wave number (cm
1) 3300e2850 3100 1150 1040e910

Assignment eOH stretching of POH NeH stretching P]O stretching (PeO)H stretching

Fig. 7 e Water uptake of PVPA and PVPA/P4VP-NS blends.

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Fig. 8 e Arrhenius plot of the conductivities of PVPA and the PVPA/P4VP-NS blends measured in dry conditions.

range from 25  C to 85  C, both VP/PA-1 and VP/PA-2 exhibited high proton conductivities. The Arrhenius plots of the conductivities in Fig. 10 showed that the temperature dependence of the conductivities followed Arrhenius type behavior. The calculated activation energies of the blends were identical (~40 kJ mol
1), much smaller than that in dry condition (~81 kJ mol
1). This is because all the samples are hygroscopic. The adsorbed water in the samples plays a role of plasticizer, facilitating the structure diffusion of protons. Proton transport can also be enhanced by diffusion of small and mobile proton vehicles (e.g. water molecules or hydronium ions). Moreover, water molecules can bridge the proton donor and acceptor sites, giving rise to additional pathways for hydrogen bond breaking and forming processes. All these effects significantly decrease the Ea of proton conduction in humid conditions and therefore lead to high proton conductivity.

Fig. 9 e Proton conductivity of PVPA and PVPA/P4VP-NS blends at 25  C versus RH.

By blending with P4VP-NS, part of the POH groups reacts with the pyridine units to form complexes, which may prevent these POH groups from condensing at elevated temperatures. This could be another advantage that can be expected by blending PVPA with P4VP-NS. In order to compare the thermal stability of the materials, proton conductivities of the blends were measured during heatingecooling cycles. The proton conductivities measured in the heating procedure were compared with that measured in the cooling procedure. We can see from Fig. 12 that, in VP/ PA-1, there was a big discrepancy between the conductivities measured during heating and cooling procedures. This may be caused by the anhydride formation between the POH groups at high temperatures. With increasing P4VP-NS content, the

Thermal stability of PVPA and PVPA/P4VP-NS blends It is well known that phosphonic acids undergo selfcondensation at high temperature and form phosphonic acid anhydrides [7,8,15]. This reaction eliminates protons as charge carriers and, meanwhile, causes barriers for proton transport which consequently decreases proton conductivity. The reaction of self-condensation of PVPA is shown in Fig. 11.

Table 4 e Activation energies of conductivity calculated from Fig. 8. Samples PVPA VP/PA-1 VP/PA-2 VP/PA-3 VP/PA-4

Ea (kJ mol
1) 65 81 82 81 81

Fig. 10 e Arrhenius plots of conductivities of the blends measured at 80% RH.

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Fig. 11 e Reaction of self-condensation of PVPA. Sieber is acknowledged for the conductivity measurements. Dr. Guangjin Hou and Dr. Haijin Zhu are thanked for the NMR measurements and discussion.

references

Fig. 12 e Proton conductivities of PVPA/P4VP-NS blends measured in heatingecooling cycles.

discrepancy diminished, probably due to the acidebase interactions. In VP/PA-5, no obvious decrease of conductivity was detected, indicating that most of the POH groups might be trapped in the complex with 4VP.

Conclusion Proton conducting polyelectrolyte based on blends of P4VP-NS and PVPA were prepared. The effects of acidebase interaction on dimensional stability, proton conduction and thermal stability were studied. The results show that the dimensional stability of PVPA was significantly improved by blending with P4VP-NS. Acidebase complex formation in the blends is not favorable for proton conduction due to the smaller acid dissociation constant of protonated pyridine units. However, with low content of P4VP-NS (less than 20 wt%), high proton conductivity above 10
2 S cm
1 were obtained under humid conditions. Moreover, the acidebase interaction was helpful for improving the thermal stability of PVPA by preventing the POH groups from self-condensation.

Acknowledgment This work was supported by the MPIP scholarship, the National Natural Science Foundation of China (Grant no. 21206090) and the Ph.D. Programs Foundation of Ministry of Education of China (Grant no. 20120073120065). Christoph

[1] Hickner MA, Ghassemi H, Kim YS, Einsla BR, McGrath JE. Alternative polymer systems for proton exchange membranes (PEMs). Chem Rev 2004;104:4587e612. [2] Ahmad H, Kamarudin SK, Hasran UA, Daud WRW. Overview of hybrid membranes for direct-methanol fuel-cell applications. Int J Hydrogen Energy 2010;35:2160e75. [3] Schuster MFH, Meyer WH. Anhydrous proton-conducting polymers. Ann Rev Mater Sci 2003;33:233e61. [4] Steininger H, Schuster M, Kreuer KD, Kaltbeitzel A, Bingoel B, Meyer WH, et al. Intermediate temperature proton conductors for PEM fuel cells based on phosphonic acid as protogenic group: a progress report. Phys Chem Chem Phys 2007;9:1764e73. [5] Lee YJ, Murakhtina T, Sebastiani D, Spiess HW. 2H solid-state NMR of mobile protons: it is not always the simple way. J Am Coll Surg 2007;129:12406e7. [6] Lee YJ, Bingoel B, Murakhtina T, Sebastiani D, Meyer WH, Wegner G, et al. High-resolution solid-state NMR studies of poly(vinyl phosphonic acid) proton-conducting polymer: molecular structure and proton dynamics. J Phys Chem B 2007;111:9711e21. [7] Bingoel B, Meyer WH, Wagner M, Wegner G. Synthesis, microstructure, and acidity of poly (vinylphosphonic acid). Macromol Rapid Comm 2006;27:1719e24. [8] Kaltbeitzel A, Schauff S, Steininger H, Bingoel B, Brunklaus G, Meyer WH, et al. Water sorption of poly(vinylphosphonic acid) and its influence on proton conductivity. Solid State Ionics 2007;178:469e74. [9] Gunday ST, Bozkurt A, Meyer WH, Wegner G. Effects of different acid functional groups on proton conductivity of polymer-1,2,4-triazole blends. J Polym Sci B 2006;44:3315e22. [10] Kufaci M, Bozkurt A, Tulu M. Poly(ethyleneglycol methacrylate phosphate) and heterocycle based proton conducting composite materials. Solid State Ionics 2006;17:1003e7. [11] Bozkurt A, Meyer WH, Gutmann J, Wegner G. Proton conducting copolymers on the basis of vinylphosphonic acid and 4-vinylimidazole. Solid State Ionics 2003;164:169e76. [12] Sevil F, Bozkurt A. Proton conducting polymer electrolytes on the basis of poly (vinylphosphonic acid) and imidazole. J Phys Chem Solids 2004;65:1659e62. [13] Jiang F, Pu H, Meyer WH, Guan Y, Wan D. A new anhydrous proton conductor based on polybenzimidazole and tridecyl phosphate. Electrochim Acta 2008;53:4495e9. [14] Jiang F, Kaltbeitzel A, Meyer WH, Wegner G. Protonconducting a polymers via atom transfer radical polymerization of diisopropyl-p-vinylbenzyl phosphonate and 4-vinylpyridine. Macromolecules 2008;41:3081e5. [15] Jiang F, Kaltbeitzel A, Fassbender B, Brunklaus G, Meyer WH, Spiess HW, et al. Effect of polymer composition and water

11164

[16]

[17]

[18]

[19]

[20]

[21]

[22]

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 1 1 5 7 e1 1 1 6 4

content on proton conductivity in vinyl benzyl phosphonic acid-4-vinyl pyridine copolymers. Macromol Chem Phys 2008;209:2494e503. Di Noto V, Boaretto N, Negro E, Pace G. New inorganiceorganic proton conducting membranes based on nafion and hydrophobic fluoroalkylated silica nanoparticles. J Power Sources 2010;195:7734e42. Di Noto V, Boaretto N, Negro E, Giffin GA, Lavina S, Polizzi S. Inorganiceorganic membranes based on Nafion, [(ZrO2)$(HfO2)0.25] and [(SiO2)$(HfO2)0.28]. Part I: synthesis, thermal stability and performance in single PEMFC. Int J Hydrogen Energy 2012;37:6199e214. Di Noto V, Boaretto N, Negro E, Stallworth PE, Lavina S, Giffin GA, et al. Inorganic-organic membranes based on nafion, [(ZrO2)$(HfO2)0.25] and [(SiO2)$(HfO2)0.28]. Part II: relaxations and conductivity mechanism. Int J Hydrogen Energy 2012;37:6215e27. Di Noto V, Gliubizzi R, Negro E, Vittadello M, Pace G. Hybrid inorganic-organic proton conducting membranes based on nafion and 5 wt.% of MxOy (M ¼ Ti, Zr, Hf, Ta and W). Electrochim Acta 2007;53:1618e27. Pan J, Zhang H, Chen W, Pan M. Nafionezirconia nanocomposite membranes formed via in situ sol-gel process. Int J Hydrogen Energy 2010;35:2796e801. Di Noto V, Zawodzinski TA, Herring AM, Giffin GA, Negro E, Lavina S. Polymer electrolytes for a hydrogen economy. Int J Hydrogen Energy 2012;37:6120e31. Thiam HS, Daud WRW, Kamarudin SK, Mohammad AB, Kadhum AAH, Loh KS, et al. Overview on nanostructured

[23]

[24]

[25]

[26]

[27]

[28]

[29] [30]

membrane in fuel cell applications. Int J Hydrogen Energy 2011;36:3187e205. Helen M, Viswanathan B, Murthy SS. Synthesis and characterization of composite membranes based on azirconium phosphate and silicotungstic acid. J Membr Sci 2007;292:98e105. Di Noto V, Negro E, Sanchez J-Y, Iojoiu C. Structure relaxation interplay of a new nanostructured membrane based on tetraethylammonium trifluoromethanesulfonate ionic liquid and neutralized nafion 117 for high-temperature fuel cells. J Am Chem Soc 2010;132:2183e95. Wagner T, Manhart A, Deniz N, Kaltbeitzel A, Wagner M, Brunklaus G, et al. Vinylphosphonic acid homo- and block copolymers. Macromol Chem Phys 2009;210:1903e14. Jiang F, Meyer WH, Zhang J. Dense poly(4-vinyl pyridine) brushes grafting from silica nanoparticles via atom transfer radical polymerization. Colloids Surf A 2013;436:302e8. Greenspan L. Humidity fixed points of binary saturated aqueous solutions. J Res Nat Bur Stand Sect A 1977;81:89e96. Pepper KW, Reichenberg D, Hale DK. Properties of ionexchange resins in relation to their structure. Part IV. Swelling and shrinkage of sulphonated polystyrenes of different cross-linking. J Chem Soc 1952;0:3129e36. Flory PJ, Rehner J. Statistical mechanics of cross-linked polymer networks II. Swelling. J Chem Phys 1943;11:521e6. Narayanan SR, Yen S, Liu L, Greenbaum SG. Anhydrous proton-conducting polymeric electrolytes for fuel cells. J Phys Chem B 2006;110:3942e8.