Ether Ether Ketone - Springer Link

ISSN 0965545X, Polymer Science, Ser. A, 2011, Vol. 53, No. 12, pp. 1159–1166. © Pleiades Publishing, Ltd., 2011.

MEMBRANE

Development of Sulfonated Poly (Ether Ether Ketone) Electrolyte Membrane for Applications in Hydrogen Sensor1 Srinivasan Guhan, Narayanaswamy V. Prabhu, and Dharmalingam Sangeetha Department of Chemistry, Anna University Chennai, India email: [email protected] Received December 18, 2010; Revised Manuscript Received June 4, 2011

Abstract—The future economy is projected as hydrogen economy and fuel cells are set to become the energy source either replacing or augmenting the present oil based technology. A sulfonated poly ether ether ketone (SPEEK) membrane as the electrolyte for hydrogen sensor that operates at room temperature was developed in our lab. The electrolyte used was SPEEK, which is a proton conducting solid polymer membrane. The membranes were characterized using various available techniques like TGA, XRD, SEM, etc. The durability was studied using the Fenton’s reagent. The proton conducting ability was analyzed using impedance spec troscopy. The catalysts considered were platinum for the cathode and three different catalysts (Pt, Pt/Pd and Pd) for the anode. The MEAs were evaluated for their performance in hydrogen sensor and the one with plat inum catalyst at the anode gave the best response among the three indicating its suitability for the SPEEK membrane for hydrogen sensor. DOI: 10.1134/S0965545X11120133 1

INTRODUCTION

Hydrogen is an important industrial raw material particularly in petrochemistry and energy sources [1]. Its leakage can lead to explosion and its penetration in metal and alloy systems often results in hydrogen em brittlement [2]. Hence, handling of hydrogen requires special safety concerns. Also, hydrogen is an explosive above the lower explosive limit (LEL), 4%, in air even at room temperature [3]. Though different types of sensors are currently known [4–6], most of them show poor response in sensing low hydrogen concentrations in the parts per million (ppm) ranges. However, the in dustrial importance and operational safety of hydro gen demand reliable hydrogen sensors to ensure the effectiveness of the process and hazards control sys tems [2]. All these requirements, mainly from the in dustrial sector accelerate the development of hydrogen sensors. Among the variety of hydrogen sensors, the elec trochemical hydrogen sensors [4, 7–9] receive more concerns owing to their peculiar merits. The electro chemical hydrogen sensors that has been developed so far include potentiometric [10, 11] and amperometric [12, 13] devices. Though potentiometric sensors have a wide dynamic range, they lack for accuracy in their logarithmic response. On the other hand, the ampero metric sensors are linear in their response and are more accurate. The solid polymer electrolytes used in amperometric sensors have the advantages against the possibility of leakages, corrosion, volatilization etc., 1 The article is published in the original.

when compared with the liquid electrolytes [1]. Am perometric hydrogen sensors based on solid polymer electrolytes (SPE) are smaller in dimension and light er in weight, thus allowing miniaturization [14]. The most commonly employed SPE membranes in the electrochemical solid state gas sensors are the copoly mer (poly tetrafluoroethylene and poly sulfonylfluo ride vinyl ether) manufactured by Dupont Co., which is popularly known under the trade name as Nafion®. It has excellent ionic conductivity, good perm selectiv ity, outstanding chemical stability and good mechani cal strength. In general, the perfluorinated membranes show high conductivity under the condition of high water vapour pressure [15]. But, there are a few draw backs associated with them which include (i) high cost, (ii) loss in the proton conduction at higher tem peratures, (iii) dependence of relative humidity to maintain high proton conductivity and (iv) non eco friendly nature due to the presence of fluorine. In the past, several attempts have been made to de velop solid state hydrogen sensors operating at ambi ent temperature. Liu and coworkers [16] have report ed a solid state amperometric detector for sensing hy drogen on a Pt/C/Nafion composite electrode prepared by mechanically hot pressing a Pt/C film on Nafion. The metallization is a very critical factor for obtaining high performance. C. Ramesh and cowork ers developed an amperometric hydrogen sensor based on a blend of PVA and H3PO4 with palladium at the anode and platinum at the cathode [13] and in another study with the same blend they utilized a mixture of palladium and platinum at the anode and platinum at

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O

O C

O

n H2SO4

O

O

O C n

SO3H Fig. 1. Conversion of PEEK to SPEEK.

the cathode [17]. Velayutham et al. worked on amper ometric sensors based on Nafion with Palladium cata lyst at the anode and platinum at the cathode [18]. There are many methods available in the literature de scribing the metallization, e.g., mechanical, electro chemical and chemical reduction processes [19]. A more efficient way to deposit the noble metals onto the membrane appears to be achievable by means of chemical reduction. Also, the chemical reduction process is of low investment and production cost. The chemical reduction process can be divided into two types, the Takenata–Torikai method [20] and the im pregnationreduction method [21]. Energy consumption is more for catalytic sensors which work at higher temperatures. The most charac teristic feature of the present work is that the electro chemical sensor reported here does not require power to operate since it works at room temperature on the fuelcell principle which makes the sensor cost effec tive and the developed sensor is also relatively low in maintenance. In the current investigation, sulfonated polyether ether ketone (SPEEK) has been used as the ionomer membrane because of its excellent thermal and me chanical stabilities, appreciable proton conductivity etc. In the cathode, platinum catalyst was taken and in the anode three different catalysts (Pt, Pd/Pt, Pd) were taken and their performance was evaluated. The catalysts were deposited by the simple impregnation reduction method. Various other parameters like ion exchange capacity, water absorption, durability and proton conductivity were also examined. The ionomer membrane was characterized using TGA, XRD, SEM and discussed. EXPERIMENTAL PEEK was obtained from Victrex, England and was dried overnight at 100°C before use. A 30 weight per centage Nafion solution was procured from Sigma Al drich, USA. Sulfuric acid (36 N), NMethyl Pyrolli done (NMP), chloroplatinic acid and palladium chlo ride (AR grades) were procured from SRL industries, India and were used as received. Isopropanol (AR grade) was obtained from Merck.

Sulfonation of PEEK and Membrane Synthesis PEEK polymer was sulfonated using sulfuric acid as described elsewhere [22]. The SPEEK obtained from the above process was dried in a vacuum oven at 80°C overnight. It was then dissolved in a suitable quantity of NMP and cast onto a clean, dry petri dish. The membrane was obtained by evaporating the sol vent in vacuum oven at 80°C for 24 h. The obtained membranes with thickness 70 µm, were pale brown in colour and were peeled off from the dish and stored for further analysis. The conversion of PEEK to SPEEK is given in Fig. 1. Characterization Techniques The ion exchange capacity (IEC) indicates the number of milli equivalents of ions in 1 g of the dry polymer. It was determined by titration method. The membrane in its acid form was weighed and then soaked in an aqueous solution containing a large ex cess of KCl in order to extract all the protons from the membrane. The electrolyte solution was then neutral ized using a very dilute Na2CO3 solution of known concentration (0.01 N). The EW (equivalent weight) values were calculated from the dry weight of the membrane divided by the volume and the normality of the Na2CO3 solution. The IEC values were expressed as number of meq. of sulfonic groups per gram of dry polymer. The amount of solvent intake by the membranes was studied. The dried membranes were weighed and soaked in water and allowed to get equilibrated at room temperature for 24 h, above which the weight was constant. The swollen membranes were then quickly weighed after blotting the surface water and the values noted. The swelling degree was determined using the formula, M − M dry SW = wet × 100%, M dry where Mwet—weight of wet membrane, Mdry—weight of dry membrane. The measurements of proton conductivity, σ (S/cm) of the membranes were carried out using Au tolab Potentiostat Galvanostat impedance analyser. Membranes with required dimensions were cut and pretreated with 0.01 N sulfuric acid and kept in water for 100% hydration. Then it was placed between two silver electrodes with an area of 1.33 cm2 with a uni form pressure applied to hold the system. The resis tance offered by the membrane was calculated and then converted to conductivity values using the for mula: σ = L/(R × A), where σ is the conductivity in S/cm, R is the resistance offered by the membrane in ohms, L is the thickness of the membrane in cm and A is the area of the mem brane in cm2. POLYMER SCIENCE

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DEVELOPMENT OF SULFONATED POLY (ETHER ETHER KETONE) ELECTROLYTE

For checking the durability of the membranes, the following procedure was adopted. Initially a 4 ppm ferrous ammonium sulfate in 3% H2O2 was freshly prepared and the temperature of the solution was maintained at 80°C. The membrane with the dimen sion of 0.5 cm2 was cut and soaked in the solution. The time required for the physical disintegration of the membrane is carefully noted down and reported. To know the amount of crystallinity, XRD mea surements were performed using a X’ Pert Pro diffrac tometer. The dried samples were mounted on an alu minium sample holder. The scanning angle ranged from 1° to 80° with a scanning rate of 2° per min. The spectra were taken at ambient temperatures (25 ± 2°C). TGA analysis is mainly carried out to determine the thermal stability of the membranes. The change in weight of the membrane with increase in temperature at a heating rate of 20°C/min in the range of the tem perature between 30°C and 900°C is followed using a SDT Q600 US analyser. The solution viscosities were measured by using sulfuric acid (95–98 wt %) at 25°C using an Ubbel hode viscometer. The intrinsic viscosity was taken as the arithmetical mean of reduced and inherent viscos ities extrapolated to zero concentration. Material Preparation and Sensor Assembly Metals are chosen to function as catalysts in electro membrane process due to their multifaceted molecu lar orientations. These facets serve as active sites for rapid and efficient oxidation and reduction reactions. Each metal has its own unique structure and chemical properties. This gives metals some aspects of selectivi ty for certain fuels. Most of the optical fiber sensors use palladium (Pd) film as transducer to detect the con centration of hydrogen. The sensor uses SPEEK as the proton conducting solid polymer electrolyte membrane. The membrane was pretreated with 3% hydrogen peroxide at 100°C for 30 min to remove organic impurities. After washing in water, the membrane was treated with 1 M sulfuric acid for 30 min to remove metallic impurities. Finally the membrane was soaked in boiling water for one hour and then washed with deionised water. Now, the membrane is ready for the catalyst coating. In this study both palladium and platinum were attempted on the sensing side as electrode material. Pt paste was pre pared by dissolving the chloroplatinic acid in mini mum quantity of alcohol. Chloroplatinic acid was dis solved in few drops of propanol and then the mixture was coated onto the membrane sheets by means of brushing technique. Next to that, the catalyst was re duced by passing hydrogen gas for 5–10 min. Similarly the Pd catalyst was prepared from palladium chloride and reduced with hydrogen. The hydrogen sensing ef fect of catalyst was studied with different combination POLYMER SCIENCE

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Hydrogen SPEEK

A

Catalyst

Oxygen Fig. 2. Schematic representation of the sensor assembly.

of Pt and Pd catalysts in anode and cathode sides. (i) The anode and cathode was modified with multiple layers of Pt particles. (ii) The anode was modified with 80% Pt and 20% Pd catalyst and the cathode with Pt. (iii) Anode and cathode were coated with Pd and Pt catalysts, respectively. The catalyst coated membranes were hot pressed and the temperature used for the hot pressing is 50°C with pressure of 5 tonnes. The goal of hot pressing is to provide better contact between the catalyst and the membrane. After the fabrication of the MEA, the sensor performance for hydrogen gas was investigated. The catalyst loading was 0.25 mg/cm2 for the anode and 0.50 mg/cm2 for the cathode. The area of the electrode was 9 cm2. It is known that in a 1 cm2 sensor, for every 2393 A/cm2 of power generated 1 m3/h of hydrogen is consumed. The concentration of hydrogen was calcu lated accordingly. The structure of the sensor was developed as fol lows. For the sensor assembly, on either side of the MEAs, the following materials were fixed in the same order. First the meshes, silicone gaskets, Teflon gaskets and finally the endplates were fixed. The entire assem bly was tightened by means of bolts and nuts. The sen sor assembly is schematically shown in Fig. 2. The operating temperature of the sensor was RT, no carrier gas was used and the relative humidity is 100% as the gases are pumped through boiling water. The surface morphology of the membranes after catalyst coating was investigated using Jeol 6360 Scan ning Electron Microscope instrument. Prior to the in vestigation, the samples were coated with a thin layer of gold. RESULTS AND DISCUSSION IEC, Durability, Proton Conductivity and Water Uptake The IEC value of the synthesized SPEEK mem brane was found to be 1.87 mequiv/g which is higher than that of Nafion 117 (1.23 mequiv/g) at the same experimental condition. The IEC value is indirectly a

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measure of the proton conducting ability of the mem brane. Greater the IEC value greater will be the proton conductivity of the membrane. The membranes were found to be stable for more than 6 h in the peroxide so lution indicating a very good stability.

tion peak of XRD pattern. An empirical relation be tween Xc and the β was deduced,

IEC provides an indication of the content of acid groups present in a polymer matrix, which are respon sible for the conduction of protons and thus is an indi rect and reliable approximation of the proton conduc tivity. Sulfonation is commonly used to modify the polymers to increase their hydrophilicity. Basically PEEK is hydrophobic in nature and when sulfonated, it becomes hydrophilic. The sulfonated groups might aggregate into hydrophilic cluster in the polymeric membrane, which could provide cation transport pathways or ionic transport channels. The conductivi ty was determined using impedance spectroscopy. The proton conductivity of the membranes was found to be 0.148 × 10–3 S/cm. The conductivity was measured at 100% relative humidity and at room temperature. The measured proton conductivity was close to those re ported earlier [23–26].

where Xc is the crystallinity degree, β is the full width of the peak (0 0 2) at half intensity, KA is a constant (0.24).

The water absorption of the synthesized SPEEK membrane was found to be 15.8%. Earlier studies re ported the water absorption values between 13% and 22% which is close to the one determined by us [26– 28]. The water uptake is very crucial as it is closely re lated to the proton conducting ability [29]. On the other hand, too much of water uptake is also not desir able as it may cause mechanical failure. The water sorption of the membranes was usually defined in weight percentage with respect to the weight of the dry membrane. The absorption of water depends on the degree of sulfonation (DS). Higher the DS, greater is the absorption of the solvents due to the increase in the number of sulfonic acid groups. PEMFCs use a proton conducting polymer membrane as an electrolyte, which is typically a poor proton conductor unless wa ter is present. Therefore, the hydration of a PEM is very important to the performance of the sensor. XRD and TGA The XRD pattern of PEEK and SPEEK are shown in Fig 3. PEEK is semicrystalline in nature. There are peaks available at low 2θ values (20°, 22°, 28°, 33° and 38°) in the XRD spectrum of PEEK which are com pletely absent in the XRD spectrum of SPEEK. It is well known that the crystalline peaks will appear at low 2θ values and hence it may be concluded that the pro cess of sulfonation decreases the crystallinity of PEEK with SPEEK becoming amorphous. The crystallinity noted by Xc, corresponds to the fraction of crystalline phase in the investigated volume of sample. The Xc and the average crystallite size are related structural parameters of materials since both are extracted from the width of a corresponding reflec

β×

3

X c = K A,

Based on the above equation, the crystallinity of PEEK (0.0827) was found to be higher than that of SPEEK (0.0164). The amorphous nature of SPEEK makes it less brittle and more flexible which is a desir able character for an electrolyte membrane in the electrochemical environment. The TGA thermogram of PEEK and SPEEK are shown in Fig. 4. In the case of PEEK, a single step degradation was observed above 450°C. This may be attributed to the degradation of the polymer back bone. On the other hand, SPEEK membranes showed a three step degradation. The first weight loss between 50°C and 150°C may be due to the evolution of phys ically and chemically bound water along with trace amounts of solvent. The presence of water could be due to the hydrophilic nature of the sulfonic acid groups. The second loss that occurred after 225°C may be due to the detachment of the sulfonic acid groups from the chain. The third weight loss that occurred af ter 400°C may be attributed to the main chain degra dation. Though PEEK is more thermally stable when compared to SPEEK, the thermal stability of SPEEK is quite higher for a successful operation as an electro lyte membrane in hydrogen sensor. SEM The SEM images of the membranes coated with platinum, platinum/palladium and palladium are shown in Figs. 5a, 5b and 5c, respectively. It is evident from the images that in the case of the membrane coated with platinum catalyst (Fig. 5a), there is a fine and uniform distribution of the catalyst over the entire surface of the polymer matrix. The uniform distribu tion enables the catalyst in a best possible way for the electrode reactions. In the case of membrane coated with palladium catalyst (Fig. 5c), the catalyst though capable of sensing the hydrogen gas, is not effective in splitting off the hydrogen. This could be due to the fact that the catalyst layer looks more like a film rather than tiny particles and hence the surface area of the catalyst is very much smaller causing an inferior performance. With the catalyst mixture coated on the membrane, the formation of agglomerates was clearly seen as evi denced by the SEM image shown in Fig. 5b. Once again, the formation of agglomerates may not be effec tive in splitting the hydrogen gas into its constituents, causing an inferior performance of the resulting sensor assembly. POLYMER SCIENCE

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SPEEK

cps

002 002

PEEK

10

20

30

40 50 2θ, deg

60

70

Fig. 3. XRD Pattern of PEEK and SPEEK.

Weight residue, % 120 PEEK

80

SPEEK

40

0

200

400

600

800 1000 Temperature, °C

Fig. 4. TGA thermogram of PEEK and SPEEK.

Molecular Mass It has been reported already [30, 31] that no chem ical degradation and no sulfone crosslinking were ob served when concentrated sulfuric acid (concentration POLYMER SCIENCE

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below 100%) was used for sulfonation at ambient tem perature. Therefore the weight average molecular weight (Mw) values of both PEEK extrapolated from SPEEK with Degree of Sulfonation (DS), 1.0, sul

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10 μm

(a)

10 μm

(b)

10 μm

(c)

Fig. 5. SEM images of membranes coated with (a) Pt/Pt, (b) PdPt/Pt and (c) Pd/Pt.

fonated at room temperature should be regarded as a credible approximation of the Mw of initial PEEK samples. Accordingly, the Mw of PEEK was found to be 32200. Hydrogen Sensor Performance In general, the electrochemical processes taking place in the sensor are the oxidation of hydrogen at the anode and reduction of oxygen at the cathode. After the oxidation of hydrogen, the protons move towards the cathode through the proton conducting SPEEK SPEEK electrolyte Anode catalyst layer

Cathode catalyst layer

membrane by means of the socalled Grothus mecha nism or hopping mechanism. Meanwhile, the elec trons produced due to the oxidation of hydrogen take up the path of an external circuit and thus electric cur rent is produced. The amount of electrons produced depends on the concentration of the hydrogen gas that enters the anode. In other words, the external current flow is proportional to the hydrogen concentration in the gas phase [32]. The sample gas is passed over the sensing electrode. Sensor testing was performed at ambient pressure and Limiting current density, mA/cm2 0.25 Electrolyte: SPEEK 0.20

Temperature: RT Carrier gas: Nil Humidification: 100% RH

0.15 H2 → 2H+ + 2e−1

2H+ + 2e−1 + 1/2O2 → H2O

H+→

0.10

Pd–Pt Pt/Pd–Pt

0.05

Pt–Pt

0

Fig. 6. Reactions occurring at the electrodes.

1

2 3 4 5 6 Concentration of hydrogen, ppm

Fig. 7. Response curve of MEAs with different catalysts. POLYMER SCIENCE

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room temperature at a gas flow rate of 0.0116 ml/s to 0.0694 ml/s. Hydrogen gas detection in different mea suring environments has recently become a very im portant problem and hence lot of efforts have been made to develop highperformance hydrogen sensors with safety and longer life. The sensor operates in am perometric mode and the limiting current is linearly dependent on hydrogen concentration in sample gas. The electrochemical reactions occurring in the sensor are given in Fig. 6 and the amount of current generated with respect to various concentrations of hydrogen us ing platinum catalyst in the cathode and three differ ent catalysts (Pt, Pt/Pd and Pd) in the anode are de picted in Fig. 7. Among the three different catalysts in the anode, the platinum catalyst was found to produce a better electric current than the other two catalysts with the hydrogen concentration remaining constant. It has been reported in the literature that hydrogen has a high solubility in palladium and hence the pro cess was observed to be diffusion controlled [13]. Sie bert et al. in their work stated that according to the simplified kinetic laws, the oxidation of hydrogen at the anode is a diffusion limited process [33]. Ramesh et al. explained that the supply of hydrogen at the an odic interface is the deterministic factor for the mag nitude of the limiting current. They reported that the short circuit current is a diffusion controlled process and hence an increase in the limiting current was ob served with increase in the supply of hydrogen [17]. It has been reported that when the rate of oxidation of hydrogen at the sensing electrode is higher than the rate of supply of hydrogen to the electrode, the elec trochemical process becomes diffusion controlled [34]. Hence, the oxidation of hydrogen at the sensing electrode is a diffusion controlled process and is the rate determining step. Since palladium has high stick ing coefficient for hydrogen, the solubility of hydrogen is also high. The high solubility leads to decreased con centration gradient which in turn makes the anodic process no more a diffusion controlled process. Due to the solubility factor, pure platinum acts as a better cat alyst when compared to palladium as well as mixture of palladium and platinum. With palladium catalyst, both as a mixture with Pt and when used alone, ag glomerates were found, thereby decreasing the catalyt ic performance. With pure platinum as the catalyst, oxidation of hydrogen is at a higher rate, the amount of electrons that are produced is also higher and hence the output electric current is also higher. CONCLUSION A polymer electrolyte fuel cell based amperometric hydrogen sensor was fabricated using SPEEK mem brane as the electrolyte and platinum catalyst at the cathode and three different catalysts (Pt, Pt/Pd and Pd) at the anode to study the hydrogen response. The electrodes were prepared by the simple reductionim POLYMER SCIENCE

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pregnation method. This method is an inexpensive method for the fabrication of the electrodes. The sen sor was operated at room temperature. The platinum catalyst at the anode produced a higher response to the same hydrogen concentration than the other two cat alysts. The electrolyte membrane was also character ized using the various techniques available, such as, TGA, XRD, SEM (SPEEK membrane after catalyst coating) etc. A simple construction of the electrodes with subsequent fabrication of hydrogen sensor and its performance make the device quiet promising in a va riety of technologically relevant areas. ACKNOWLEDGMENTS The authors would like to thank the generous fund ing provided by the Board of Research in Nuclear Sci ences (BRNS), India and CTDT, Anna University Chennai, India to carry out the research work. REFERENCES 1. W. Lu, S. Wu, L. Wang, and Z. Su, Sens. Actuators B 107, 812 (2005). 2. Y. Tan and T. C. Tan, J. Electrochem. Soc. 142, 1923 (1995). 3. M. Sakthivel and W. Weppner, J. Solid State Electro chem. 11, 561 (2007). 4. R. Bouchet, S. Rosini, G. Vitter, and E. Siebert, Sens. Actuators B 76, 610 (2001). 5. F. Favier, E. C. Walter, M. P. Zach, T. Beuter, and R. M. Penner, Science (Washington, D. C.) 293, 2227 (2001). 6. D. R. Baselt, B. Fruhberger, E. Klaassen, S. Cemalovic, C. L. Britton, S. V. Patel, T. E. Mlsna, D. McCorkle, and B. Warmack, Sens. Actuators B 88, 120 (2003). 7. J. J. Podesta, A. M. G. Navarro, C. N. Estrella, and M. A. Esteso, Res. Microbiol. 148, 87 (1997). 8. R. Bouchet, E. Siebert, and G. Vitter, J. Electrochem. Soc. 147, 3548 (2000). 9. E. Opekar, J. Langmaier, and Z. Samec, J. Electroanal. Chem. 379, 301 (1994). 10. G. Y. Lu, N. Miura, and N. Yamazoe, J. Electrochem. Soc. 143, L154 (1996). 11. N. Maffei and A. Kuriakose, Sens. Actuators B 56, 243 (1999). 12. N. Miura, T. Harada,Y. Shimizu, and N. Yamazoe, Nippon Kagaku Kaishi 6, 736 (1991). 13. C. Ramesh, G. Velayutham, N. Murugesan, V. Gane san, K. S. Dhathathreyan, and G. Periaswami, J. Solid State Electrochem. 7, 511 (2003). 14. M. Sakthivel and W. Weppner, Sens. Actuators B 113, 998 (2006). 15. A. V. Anantaraman and C. L. Gardner, J. Electroanal. Chem. 414, 115 (1996). 16. Y. C. Liu, B. J. Hwang, and I. J. Tzens, J. Electrochem. Soc. 149, H173 (2002). 17. C. Ramesh, G. Velayutham, N. Murugesan, V. Gane san, V. Manivannan, and G. Periaswami, Ionics 10, 50 (2004).

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18. G. Velayutham, C. Ramesh, N. Murugesan, V. Mani vannan, K. S. Dhathathreyan, and G. Periaswami, Ion ics 10, 63 (2004). 19. F. T. A. Vork, L. J. J. Janssen, and E. Barendrecht, Elec trochim. Acta 31, 1569 (1986). 20. Z. Ogumi, K. Inatomi, J. T. Hinatsu, and Z. I. Take hara, Electrochim. Acta 37, 1295 (1992). 21. P. Millet, R. Durand, E. Dartyge, G. Tourillon, and A. Fontaine, J. Electrochem. Soc. 140, 1373 (1993). 22. S. Guhan and D. Sangeetha, Int. J. Polym. Mater. 58, 87 (2009). 23. H. Cai, K. Shao, S. Zhong, C. Zhao, G. Zhang, X. Li, and H. Na, J. Membr. Sci. 297, 162 (2007). 24. R. K. Nagarale, G. S. Gohil, and V. K. Shahi, J. Membr. Sci. 280, 389 (2006). 25. H. L. Wu, C. C. M. Maa, C. H. Li, T. M. Lee, C. Y. Chenb, C. L. Chiang, and C. Wu, J. Membr. Sci. 280, 501 (2006).

26. C. Zhao, X. Li, Z. Wang, Z. Dou, S. Zhong, and H. Na, J. Membr. Sci. 280, 643 (2006). 27. P. Krishnan, J. S. Park, and C. S. Kim, J. Membr. Sci. 279, 220 (2006). 28. Z. Gaowen and Z. Zhentao, J. Membr. Sci. 261, 107 (2005). 29. G. Pourcelly, C. Gavach, and P. Colomban, Proton Conductors (Cambridge Univ. Press, New York, 1992). 30. X. Jin, M. T. Bishop, T. S. Ellis, and F. E. Karasz, Br. Polym. J. 17, 4 (1985). 31. M. T. Bishop, F. E. Karasz, P. S. Russo, and K. H. Lan gle, Macromolecules 18, 86 (1985). 32. N. Miura, H. Kato, N. Yamazoe, and T. Seiyama, Proc. ACS, Div. Polym. Mater. Sci. Eng., 203 (1986). 33. E. Siebert, S. Rosini, R. Bouchet, and G. Vitter, Ionics 9, 168 (2003). 34. C. Ramesh, N. Murugesan, M. V. Krishnaiah, V. Gane san, G. Periaswami, J. Solid State Electrochem. 12, 1109 (2008).

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