Polybenzimidazoles, a class of aromatic heterocyclic polymers, are well known due to their remarkable thermal stability, mechanical properties and chemical resistance which are often required in extreme operation conditions. Because of these properties, polybenzimidazoles are excellent candidates in various application areas including proton exchange membrane fuel cells, gas separation membranes, reverse osmosis and nanofiltration, and high performance coatings. The following studies are focused on the synthesis, characterization and related properties of polybenzimidazoles and polybenzimidazole based materials. A novel sulfonyl-containing tetraamino-substituted monomer (3,3',4,4'-tetraaminodiphenylsulfone) was synthesized and polymerized with three different diacid monomers to make polybenzimidazoles. The new monomer synthesis route with reduced steps relative to the existing literature method increased the overall yield by a factor of three. The sulfonyl-containing polybenzimidazoles have enhanced solubilities in common organic solvents including dimthylsulfoxide, dimethylacetamide and N-methyl-2-pyrrolidone in comparison with the commercial polybenzimidazole, Celazole®, poly(2,2'-(m-phenylene)-5,5'-bibenzimidazole). The improvements in solubility are attributed to the introduction of polar sulfonyl linking moiety in the monomer. Remarkable thermal stabilities (high T_g, > 428 °C) were demonstrated through Dynamic Mechanical Analysis (DMA) and Thermogravimetric Analysis (TGA). A well designed film casting process was investigated and established. Polybenzimidazoles were fabricated into transparent thin films (20-30 μm thick) for gas transport measurements. These novel polybenzimidazole films exhibited extraordinary gas separation properties, especially for H₂/CO₂ separation. There is a trade-off relationship between gas permeability and selectivity through dense, non-porous polymer membranes that was discovered by Robeson in 1991. The ultimate goal for developing gas separation membranes is to improve both permeability and selectivity simultaneously. Gas permeability is related to the free volume between polymer chains. In order to improve gas permeability, we hypothesized a concept that increasing free volume could be achieved by thermally degrading sacrificial components and volatilizing their byproducts from a glassy matrix. Volatile components were introduced into the films to preoccupy the spaces between polymer chains. Once they were degraded and removed through the thermal treatment, it was hypothesized that the preoccupied spaces would remain empty due to the glassy nature of the matrix at the heat treatment temperature, thus resulting in more free volume. Two post- modification strategies including grafting and blending were utilized to incorporate the volatile components, poly(propylene oxide) and poly(ethylene oxide). Post-modified polybenzimidazole films impressively showed significant enhancements in both gas permeability and selectivity for H₂/CO₂ separation. The H₂ permeability of the post-modified TADPS-OBA polybenzimidazole increased from 3.1-6.2 Barrers to 5.2-7.5 Barrers (up to 66% increase). The selectivity for H₂/CO₂ increased from 7.5-10.5 to 10.1-13.0 (up to 33% increase). The study on the potential effects of water vapor on the separation performance of PBI membranes was discussed in the appendix. / Ph. D. / Polybenzimidazoles represent a class of polymeric high performance materials due to their remarkable thermal stability, mechanical properties and chemical resistance. They are competitive material candidates for applications involving extreme conditions including high pressure and high temperature. The following studies are focused on the synthesis, characterization and properties of polybenzimidazoles and polybenzimidazole based copolymers and blends. Of particular importance to this dissertation are the gas transport properties. The new materials are excellent candidates for making non-porous membranes that can separate very small molecules such as nitrogen, oxygen, carbon dioxide, and hydrogen. The non-porous membranes achieve separations of such small molecules by having the gases solubilize in the upstream side of a membrane, diffuse through it, then evaporate from the downstream side. This mechanism is known as the solution-diffusion mechanism. The monomer, 3,3’,4,4’-tetraaminodiphenylsulfone, was synthesized via our designed synthesis method that was simpler than previous methods described in the literature and with a 3 times higher yield. A series of polybenzimidazoles with systematically varied chemical structures were prepared and it was demonstrated that they all had enhanced solubilities in common organic solvents over the only known commercial polybenzimidazole, Celazole®. This is particularly important for membrane materials because they must be fabricated into thin films from solution. Remarkable thermal stabilities for polymeric materials with glass transition temperatures above 400 °C were found for these polybenzimidazoles. A well designed film casting process was investigated and established. Polybenzimidazoles were fabricated into transparent thin films (20-30 µm thick) and their gas transport properties were measured. These novel polybenzimidazole films exhibited extraordinary gas separation properties, especially for H₂/CO₂ separation. The gas transport properties involve two important parameters, permeability and selectivity. A trade-off relationship between the two parameters was discovered by Robeson in 1991. The ultimate goal for developing gas separation membranes is to improve permeability and selectivity at the same time. In order to improve gas permeability, we hypothesized a concept that increasing permeability could be achieved by creating more spaces between the polymer chains in non-porous films. Sacrificial components were introduced into the films, then thermally degraded and the byproducts were volatilized to remove them from the film. It was further hypothesized that conducting the heat treatment process at a temperature where the matrix polymer was in the glassy state would allow the matrix polymer to preserve the free volume introduced by the volatization. Two post-modification strategies including grafting and blending were utilized to incorporate the volatile components, poly(propylene oxide) and poly(ethylene oxide). Post-modified polybenzimidazole films impressively showed significant enhancements in both gas permeability and selectivity for H₂/CO₂ separation. This is an important separation that could economically be carried out at elevated temperatures (~250°C) if the polymer membrane would withstand such a temperature. It could be utilized to separate H₂ from CO₂ in pre-combustion syngas. This is the major method for H₂ production worldwide. The study on the potential effects of water vapor on the separation performance of PBI membranes was discussed in the appendix.

The thermally regenerative fuel cell (TRFC) concept that is analyzed is a polymer electrolyte membrane fuel cell (PEMFC), powered by the electro-oxidation of H2 and the electro-reduction of propiophenone. The main products of this fuel cell should be 1-phenyl-1-propanol and electricity. The 1-phenyl-1-propanol should then be converted back to propiophenone, while hydrogen is regenerated by using waste heat and a metal catalyst (Pd/SiO2). The first objective was to find a compatible polymer that would work as either an ionomer/binding agent and as a membrane in the membrane electrode assembly (MEA) of the TRFC. This was achieved by checking the compatibility of each polymer with 1-phenyl-1-propanol and propiophenone (the alcohol-ketone pair). Catalyst coated gas diffusion layers or catalyst coated membranes were made to test the stability of the polymers in the catalyst bed when exposed to the alcohol-ketone pair. If the polymer was compatible with the alcohol-ketone pair, MEAs were constructed using this polymer. The second objective was to test these MEAs inside a H2/propiophenone fuel cell that would prove the concept of our envisioned TRFC. It was found that the only polymer that was stable in the alcohol-ketone pair was mPBI (m-phenylene polybenzimidazole). The mPBI had to be doped with H3PO4 to enable H+ conductivity. Unfortunately, some H3PO4 leached out of the H3PO4-doped mPBI when in the presence of the alcohol-ketone pair. MEAs that were created using H3PO4-doped mPBI were found to work for H2/air and H2/propiophenone fed PEMFCs. The best performance achieved with the H2/propiophenone powered fuel cell was 6.23 μW/cm2. Unfortunately, the presence of the 1-phenyl-1-propanol product could not be proved by EIS or CV on the fuel cell, or by GC-FID of the cathode effluent. Other unknown products were seen in the GC-FID spectrum of the cathode effluent. Therefore, it is possible that the propiophenone did reduce at the cathode but it produced an unknown product. In conclusion, the viability of the proposed TRFC system was not verified. H3PO4 leaching from the MEA makes it impossible to use H3PO4-doped mPBI as the electrolyte in the final version of the MEA in the TRFC system. / Thesis (Master, Chemistry) -- Queen's University, 2013-04-12 17:16:37.724

No description available.

Ces travaux de thèse portent sur l'optimisation des méthodes de fabrication des assemblages membrane électrodes des Piles à Membrane Echangeuse de Protons (PEMFC, Proton Exchange Membrane Fuel Cell). Ils ont pour objectif d'optimiser le dépôt des couches catalytiques sur la membrane par une méthode de transfert. Le procédé a été utilisé pour fabriquer d'une part des assemblages à membrane Nafion® pour les piles à combustible à membrane fonctionnant à 80 °C (PEMFC) et d'autre part des assemblages à membrane polybenzimidazole dopée en acide phosphorique pour les PEMFC à haute température (160 °C). Au cours de cette étude, la détermination précise de la quantité de platine a été rendue possible par des mesures non destructives en fluorescence X. Nous avons développé également une méthode originale de fabrication de suspensions de blendes Nafion-PBI qui ont été incorporées dans les électrodes des assemblages à membrane PBI. L'effet de la composition, des épaisseurs et du mode de préparation des électrodes sur les performances des assemblages a été discuté. Les assemblages membrane électrodes à membrane PBI ont été caractérisés par des mesures en polarisation et en spectroscopie d'impédance (EIS). La détermination de surface active d'électrode a été réalisée par des mesures en voltammétrie cyclique in-situ (CV). La mise au point du procédé de fabrication des électrodes par transfert de couches actives sur membrane a permis d'obtenir des informations importantes sur les conditions de préparation des électrodes. Les performances des assemblages à membrane Nafion® sont supérieures à celles obtenues sur des assemblages de référence avec des électrodes supportées sur couche de diffusion (GDE). Il a été possible de réaliser pour la première fois des assemblages avec un dépôt sur des membranes polybenzimidazole déjà dopées en acide, les premiers résultats obtenus sont extrêmement encourageants. Le procédé de transfert des couches catalytiques pourrait être adapté pour réaliser des dépôts sur d'autres variétés de membranes dopées ou non dopées en acide. / This work concerns the optimization of the fabrication processes of membrane electrode assemblies for the Proton Exchange Membrane Fuel Cell (PEMFC). The objective is to carry out the deposition of catalyst layers onto the membranes by a transfer process. The optimization of the catalyst layer compositions and its morphology is crucial for this process. Assemblies with Nafion® membranes for PEMFC working at 80 °C and phosphoric acid doped polybenzimidazole membranes for HTPEMFC (160 °C) have been prepared by this method. X-ray fluorescence spectrometry, due to its non destructive nature, was applied for precise analysis of platinum loading on the electrodes. In this work, a new method was also developed for the preparation of Nafion-PBI blend suspensions that have been incorporated in the electrodes of the PBI membrane electrodes assemblies. The PBI membrane electrode assemblies have been characterized by polarization measurements and electrochemical impedance spectroscopy (EIS). The in situ PEM Fuel Cell electrochemical surface area (ECSA) has been determined by cyclic voltammétrie measurements. The performances of Nafion membrane assemblies are higher than those obtained on reference assemblies, with gas diffusion layer supported electrodes. Promising results have been obtained on the assemblies performed for the first time with acid doped PBI membranes. The transfer process of the catalyst layer can also be used on other types of membrane.

Ces travaux de thèse s'inscrivent dans le contexte des efforts de recherches menés pour proposer des matériaux susceptibles de lever les verrous technologiques au développement des piles à combustible à membrane. L'un de ces enjeux est l'augmentation de la température de fonctionnement (150 - 250 °C) afin d'améliorer les cinétiques réactionnelles permettant une diminution de la quantité de catalyseur ainsi qu'une simplification de la gestion de l'eau, une réduction du système de refroidissement et une meilleure résistance à l'empoisonnement au monoxyde de carbone du platine. La motivation de cette étude a été de substituer au carbone un matériau support de catalyseur avec une plus grande résistance électrochimique.Notre choix s'est porté sur le carbure de tungstène qui, en plus d'une conductivité électronique élevée, présente une activité catalytique pour l'oxydation de l'hydrogène et la réduction de l'oxygène en milieu acide. La mise au point d'une méthode de synthèse innovante par voie hydrothermale a permis l'élaboration de microsphères de carbure de tungstène (MCT) de surface spécifique élevée (68 m2.g-1 avec 4 % de carbone résiduel) et d'architecture inusuelle. Des nanoparticules de platine de taille contrôlée ont été préparées par méthode polyol afin d'être déposées en surface des MCT. Après caractérisations électrochimiques ex-situ couplées à des analyses de surface (XPS) de ces catalyseurs Pt/WC, la mise en forme d'électrodes par enduction et transfert sur la membrane a permis la réalisation d'assemblages membrane - électrode et leurs caractérisations en pile à combustible. Des membranes polybenzimidazole dopé acide phosphorique (PBI-H3PO4) ont été utilisées pour remplacer les membranes Nafion afin d'augmenter la température de fonctionnement. / The objective of this work was to develop alternative suitable materials to increase operating temperature of a Proton Exchange Membrane Fuel Cell. The increase of the operating temperature (150 - 250 °C) is attractive for cost reduction and reliability in terms of reaction kinetics, catalyst tolerance, heat rejection and water management. Our work was focused on tungsten carbide which has an high electrical conductivity and exhibits a significant catalytic activity for hydrogen oxidation and oxygen reduction in acidic environment. We have reported a novel approach to produce tungsten carbide microspheres (TCM) with an high surface area (68 m2.g-1 including only 4 % of residual carbon) and an unusual architecture. Platinum nanoparticles were prepared by polyol method and were then deposited on TCM. Physical, chemical as well as electrochemical characterisations of WC supported platinum nanoparticles Pt/WC are described and discussed in comparison with a platinum electrocatalyst on a commercial carbon support (Vulcan XC-72R). Membrane Electrode Assembly was then prepared by coating - decal process, and characterised by single cell test and compared to conventional Pt/C assembly. Phosphoric acid doped polybenzimidazole PBI(H3PO4) was used as electrolyte to replace Nafion membrane in order to carry out fuel cell testing at higher temperature.

1. Membranes échangeuses de protons à haute température (HT-PEM) pour application dans les piles à combustible:Le succès des piles à combustible à base de HT-PEM dépend fortement du matériau membranaire. D’importants progrès ont été accomplis dans la conception de PEMs à transport facilité de protons. L'objectif de la première partie de ce travail de thèse était de fabriquer des membranes électrolytes à haute conductivité, capables de fonctionner au-dessus de 120°C dans des conditions anhydres, sans acides minéraux, et sans sacrifier la résistance mécanique. La stratégie suivie combine l’utilisation de micro-filtres (support) à base de polybenzimidazole (PBI) présentant un réseau de pores ordonnés, et de liquides ioniques (ILs)à base de polyimidazolium comme phase conductrice. Deux types de micro-filtres de PBI ont été préparés: avec un réseau de pores droits (SPBI), ou avec une structure poreuse hiérarchique (HPBI). Les ILs polymérisés (PIL) suscitent un grand intérêt comme tous les électrolytes flexibles à l'état solide en raison de leur sécurité d’utilisation et de leur bonne stabilité thermique, chimique et électrochimique. Dans ce travail, un IL monomèrique protique 1-H-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide a été choisi pour sa conductivité protonique élevée, sa faible rétention d'eau et sa bonne stabilité thermique. Puisque les performances d’une PEM formée par immersion d’un support poreux dans un IL dépendent surtout de la structure poreuse du support, il est essentiel d’optimiser l’architecture des pores réservoirs. Ainsi, nos travaux visent à améliorer à la fois la conductivité ionique et la stabilité dimensionnelle des PEMs à base de PIL par une conception appropriée de l'architecture poreuse. En effet, la faible stabilité dimensionnelle et mécanique du poly[1-(3H-imidazolium)éthylène] bis(trifluorométhanesulfonyl) imide est améliorée grâce à son infiltration dans un support PBI architecturé. La configuration d'infiltration, l'addition d’agent réticulant et les conditions de polymérisation UV "in situ" ont été considérées comme paramètres d'optimisation pour les deux types de micro-tamis en PBI.2. Membranes à base de liquide ionique supporté (SILM) pour la valorisation du méthane:La valorisation du gaz naturel, intégrant l'élimination de CO2 et N2, est l’une des applications de séparation des gaz industriels où les membranes sont une alternative prometteuse à petite échelle. L'objectif de nos travaux était de développer des membranes de type SILM, sélectives au CH4. Notre stratégie combine des micro-tamis à base polybenzimidazole (PBI) comme supports présentant une bonne endurance et de bonnes propriétés thermiques, et des liquides ioniques (ILs) protiques avec des ions imidazolium et trifluorométhane sulfonylimide pour la solubilité du CH4. Bien que la faible pression de vapeur du IL protique atténue sa volatilité dans les SILMs traditionnels, son expulsion hors des pores reste une préoccupation majeure. Un design approprié du support, avec des pores submicroniques, combiné à un IL de tension superficielle élevée, devrait générer des SILMs plus stables, adaptées aux applications à pression transmembranaire modérée ou élevée. Ainsi, des supports PBI à porosité aléatoire (RPBI), obtenus par séparation de phase, ont été largement utilisés. En outre, la polymérisation des RTILs peut fournir d’autres avantages en termes de sécurité, de stabilité et de propriétés mécaniques. Dans cette étude, trois classes de SILMs à base de PBI, avec le IL protique 1-H-3-methylimidazolium bis(trifluoromethane sulfonyl)imide (IL), le monomérique 1-H-3-vinyllimidazolium bis(trifluoromethane sulfonyl)imide (MIL) et le polymérique poly[1-(3H-imidazolium)ethylene] bis(trifluoromethanesulfonyl)imide (PIL) ont été fabriqués avec succès et caractérisées en perméation de gaz purs. Des membranes hautement permsélectives au méthane ont été obtenues, qui sont très prometteuses pour la séparation de mélanges de gaz tels que CH4/N2 / 1. High temperature Proton Exchange Membranes (HT-PEMs) for Fuel Cell applications:The success of the High temperature proton exchange membrane fuel cell (HT-PEMFC) direction is very much dependent on the development of the membrane material. With facilitated proton transport chemistries, great progresses in designing and fabricating facilitated PEMs have been accomplished. The objective of this first part of the PhD work was to fabricate highly conductive electrolyte membranes capable to operate above 120°C under anhydrous conditions and in the absence of mineral acids, without sacrificing the mechanical behavior. The followed rationale is based on the combination of polybenzimidazole (PBI) microsieves as structural supports and poly-imidazolium based ionic liquid (IL) moieties as conducting phase. Two types of PBI microsieves have been prepared following two different microfabrication processes: straight porous PBI and hierarchically structured PBI microsieves.Polymeric ionic liquids (PILs) have triggered great interest as all solid-state flexible electrolytes because of safety and superior thermal, chemical and electrochemical stability. In this part, the 1-H-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide has been mainly selected due to its high proton conductivity, low water uptake values as well as thermal stability.The consecution of a polymeric container with optimized pore architecture is extremely essential since the performance of PEM based on immersing a porous support into ILs, mainly depends on the porous structure. Thus, our research efforts have been directed to improve both, the ion conductivity and the dimensional stability of the PIL supported PEMs by a proper design of the porous architecture. Herein, the diminished dimensional and mechanical stability of poly[1-(3H-imidazolium)ethylene]bis(trifluoromethanesulfonyl)imide has been improved thanks to its infiltration on a PBI support with specific pore architecture. The infiltration configuration, cross-linker addition and “in situ” UV polymerization conditions were taken as optimization parameters for both PBI type microsieves.2. Supported Ionic liquid membranes (SILMs) for methane upgrading:The natural gas upgrading, i.e. removal of CO2 and N2, is one of the major industrial gas separation application where membranes arise as promising alternative at small scale.The objective of this second part of the work was to develop CH4 selective Supported Ionic Liquid Membranes (SILMs). Once again, the rationale followed is based on the combination of PBI microsieves as structural supports, to take advantage of its endurance and thermal properties, and protic ILs with imidazolium and trifluoromethane sulfonyl)imide ions due to their CH4 solubility properties. Although the negligible protic IL vapor pressure alleviates one of the problems associated with traditional SILMs, namely liquid volatility; expulsion of the liquid from the membrane pores is a major concern. A proper design of the support, with sub-micron pores, combined with IL having high surface tension could lead to SILM with adequate physical stability for applications involving moderate to high trans-membrane pressures. Therefore, random porous PBI supports, obtained by phase separation method, have been extensively used. In addition, polymerization of RTILs could provide additional advantages in terms of safety, stability and mechanical properties.In this study, three classes of SILMs, based on PBI with the 1-H-3-methylimidazolium bis(trifluoromethane sulfonyl)imide, the 1-H-3-vinylimidazolium bis(trifluoromethane sulfonyl)imide and the poly[1-(3H-imidazolium)ethylene] bis(trifluoromethanesulfonyl)imide have been successfully fabricated and characterized by single gas permeation measurements. Results revealed that the prepared membranes were highly selective to CH4 and thus very promising for CH4/N2 gas mixture separation.

Doctor of Philosophy / Department of Chemical Engineering / Mary E. Rezac / Polyimide membranes have been used extensively in gas separation applications because of their attractive gas transport properties and the ease of processing these materials. Other applications of membranes, such as membrane reactors, which could compete with more traditional packed and slurry bed reactors across a wider range of environments, could benefit from improvements in the thermal and chemical stability of polymeric membranes. This work focuses on blending polyimide and polybenzimidazole polymers to improve the thermal and chemical stability of polyimide membranes while retaining the desirable characteristics of the polyimide. Blended dense films and asymmetric membranes were fabricated and characterized. Dense film properties are useful for studying intrinsic properties of the polymer blends. Transport properties of dense films were characterized from room temperature to 200°C. Properties including miscibility, density, chain packing and thermal stability were investigated. A process for fabricating flat sheet blended integrally skinned asymmetric membranes by phase inversion was developed. The transport properties of membranes were characterized from room temperature to 300°C. A critical characteristic of gas separation membranes is selectivity. Post-treatments including thermal annealing and vapor and liquid surface treatments were investigated to improve the selectivity of blended membranes. Vapor and liquid surface treatments with common, benign solvents including an alkane, an aldehyde and an alcohol resulted in improvements in selectivity.

A test system for the performance analysis of a novel thermally regenerative fuel cell (TRFC) using propiophenone and hydrogen as the oxidant and fuel respectively was designed and built. The test system is capable of either hydrogen-air or hydrogen-propiophenone operation. Membrane electrode assemblies (MEAs) were made using commercial phosphoric acid-doped polybenzimidazole (PBI) membranes and commercial electrodes. Using Pt/carbon paper electrodes with a catalyst loading of 1mg/cm2 and a membrane with an acid doping level of 10.2 mol acid/mol of polymer repeat unit, a maximum performance of 212 mW/cm2 at a current density of 575 mA/cm2 was achieved for baseline hydrogen-air testing at 110°C. Problems were encountered, however, in achieving consistent, reproducible performance for in-house fabricated MEAs. Furthermore, ex-situ electrochemical impedance spectrometry (EIS) showed that the phosphoric acid-doped PBI was unstable in the propiophenone and that acid-leaching was occurring. In order to have MEAs with consistent characteristics for verifying the test system performance, commercial phosphoric acid-doped PBI membrane electrode assemblies were used. At a temperature of 160°C and atmospheric pressure with hydrogen and air flowrates of 150 mL/min and 900 mL/min respectively a maximum power density of 387 mW/cm2 at a current density of 1.1 A/cm2 was achieved. This performance was consistent with the manufacturer’s specifications and these MEAs were subsequently used to verify the performance of TRFC test system despite the EIS results that indicated that acid-leaching would probably occur. The Pt catalyzed commercial MEAs achieved very limited performance for the hydrogenation of the ketone. However, the performance was less than but comparable to similar results previously reported in the literature by Chaurasia et al. [1]. For pure Pt catalyst loading of 1 mg/cm2, using a commercial PBI MEA operating at 160°C and atmospheric pressure, the maximum power density was 40 µW/cm2 at a current density of 1.3 mA/cm2. A 16 hour test was conducted for these conditions with a constant 1 ohm load, successfully demonstrating the operation of the test system. The test system will be used in the development of better catalysts for ketone hydrogenation. / Thesis (Master, Chemical Engineering) -- Queen's University, 2012-10-12 10:00:58.854

Acid Doped Polybenzimidazole Membranes for High Temperature Proton Exchange Membrane Fuel Cells Author: Ahmet &Ouml / zg&uuml / r Yurdakul One of the most popular candidates for high temperature PEMFC&rsquo / s is phosphoric acid doped polybenzimidazole (PBI) membrane due to its thermal and mechanical stability. In this study, high molecular weight PBI was synthesized by using PPA polymerization. The stirring rate of reaction solution was optimized to obtain high molecular weight. The inherent viscosity of polymer was measured at four points in 96 percent sulphuric acid solution at 30 degree centigrade by using an Ubbelohde viscometer. The highest average molecular weight was found as approximately 120,000 using the Mark-Houwink equation. The polymer was dissolved in N,N-dimethylacetamide at 70 degree centigrade with an ultrasonic stirrer. The membranes cast from this solution were doped with phosphoric acid solutions at different concentrations. The doping levels of the membranes were 6, 8, 10 and 11 moles phosphoric acid/PBI repeat unit. The mechanical strength of the acid doped membranes measured by tensile tests were found as 23, 16, 12 and 11 MPa, respectively. Conductivity measurements were made using the four probe technique. The membranes were placed in a conductivity cell and measurements were taken in humidity chamber with temperature and pressure control. The conductivity of membranes was measured at 110, 130 and 150 degree centigrade in both dry air and water vapor. The highest conductivity was 0.12 S/cm at 150 degree centigrade and 33 percent relative humidity for the membrane doped with 11 moles of H3PO4. The measurements showed that conductivity increased with increasing doping and humidity. Moreover, membranes had acceptable conductivity levels in dry air.

It is desirable to increase the operation temperature of proton exchange membrane fuel cells above 100oC due to fast electrode kinetics, high tolerance to fuel impurities and simple thermal and water management. In this study / the objective is to develop a high temperature proton exchange membrane fuel cell. Phosphoric acid doped polybenzimidazole membrane was chosen as the electrolyte material. Polybenzimidazole was synthesized with different molecular weights (18700-118500) by changing the synthesis conditions such as reaction time (18-24h) and temperature (185-200oC). The formation of polybenzimidazole was confirmed by FTIR, H-NMR and elemental analysis. The synthesized polymers were used to prepare homogeneous membranes which have good mechanical strength and high thermal stability. Phosphoric acid doped membranes were used to prepare membrane electrode assemblies. Dry hydrogen and oxygen gases were fed to the anode and cathode sides of the cell respectively, at a flow rate of 0.1 slpm for fuel cell tests. It was achieved to operate the single cell up to 160oC. The observed maximum power output was increased considerably from 0.015 W/cm2 to 0.061 W/cm2 at 150oC when the binder of the catalyst was changed from polybenzimidazole to polybenzimidazole and polyvinylidene fluoride mixture. The power outputs of 0.032 W/cm2 and 0.063 W/cm2 were obtained when the fuel cell operating temperatures changed as 125oC and 160oC respectively. The single cell test presents 0.035 W/cm2 and 0.070 W/cm2 with membrane thicknesses of 100 &micro / m and 70 &micro / m respectively. So it can be concluded that thinner membranes give better performances at higher temperatures.