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Progress in Polymer Science 36 (2011) 813–843

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Polymer membranes for high temperature proton exchangemembrane fuel cell: Recent advances and challenges

Saswata Bosea, Tapas Kuilaa, Thi Xuan Hien Nguyenb, Nam Hoon Kimc,Kin-tak Laua,d,e, Joong Hee Leea,b,∗

a Department of BIN Fusion Technology, Chonbuk National University, Jeonju, Jeonbuk, 561-756, Republic of Koreab BIN Fusion Research Team, Department of Polymer & Nano Engineering, Chonbuk National University, Jeonju, Jeonbuk, 561-756, Republic of Koreac Department of Hydrogen and Fuel Cell Engineering, Chonbuk National University, Jeonju, Jeonbuk, 561-756, Republic of Koread Centre of Excellence in Engineered Fibre Composites, Faculty of Engineering and Surveying, University of Southern Queensland, Toowoomba, Australiae Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China

a r t i c l e i n f o

Article history:Received 23 July 2010Received in revised form 8 January 2011Accepted 10 January 2011Available online 26 January 2011

a b s t r a c t

Proton-exchange membrane fuel cells (PEMFCs) are considered to be a promising tech-nology for efficient power generation in the 21st century. Currently, high temperatureproton exchange membrane fuel cells (HT-PEMFC) offer several advantages, such as highproton conductivity, low permeability to fuel, low electro-osmotic drag coefficient, goodchemical/thermal stability, good mechanical properties and low cost. Owing to the afore-mentioned features, high temperature proton exchange membrane fuel cells have beenutilized more widely compared to low temperature proton exchange membrane fuel cells,which contain certain limitations, such as carbon monoxide poisoning, heat management,water leaching, etc. This review examines the inspiration for HT-PEMFC development,the technological constraints, and recent advances. Various classes of polymers, such assulfonated hydrocarbon polymers, acid–base polymers and blend polymers, have been

Abbreviations: AB-PBI, poly(2,5-polybenzimidazole); AIBN, azobisisobutyronitrile; AIPA, 5-aminoisophthalic acid; APP 414, APP 414 membrane(APTES/PDMS/POCl3 molar ratio: 4/1/4); APTES, 3-aminopropyl triethoxysilane; Ar, Argon; BIS, polysiloxane matrix; BPO4, boron phosphate; BPSH,sulfonated poly(arylene ether sulfone); BT, Benzotriazole-5-carboxylic acid; CLs, Catalyst layers; CO, carbon monoxide; CPSf, carboxylated polysulfone;CsPOM, Cs2.5H0.5PMo12O40; DAB, 3,3′-diaminobenzidine; DBpX, dibromo-p-xylene; DCMP, dichloromethyl phosphinic acid; DDMEFC, direct dimethyl etherfuel cell; DF, decafluorobiphenyl; DI, deionized; DMAc, N,N-dimethylacetamide; DMF, dimethylformamide; DMSO, dimethyl sulfoxide; DPE, dicarboxylicacid 4,4′-diphenylether; F, 4,4′-(hexafluoroisopropylidene) diphenol; GDE, gas diffusion electrode; GLYMO, 3-glycidyloxypropyl-trimethoxysilane; HOR,Hydrogen Oxidation Reaction; HPA, heteropolyacid; HPMC, hydroxypropyl methyl cellulose; HS, hydrazine sulfate; HTFSI, trifluoromethanesulfonimide;HT-PEMFC, high temperature proton exchange membrane fuel cells; IEC, ion exchange capacity; Im, imidazole; IPA, isophthalic acid; IR, infrared; LPEI,linear polyethyleneimine; MEA, membrane electrode assembly; NF, Nafion; NF–ZrP, Nafion zirconium phosphate; NMP, N-methylpyrrilidone; ORR, OxygenReduction Reaction; PA, phosphoric acid; PAEEN, poly(aryl ether ether nitrile)s containing sulfonic acid groups; PAIPA, poly(5-aminoisophthalic acid); PBI,polybenzimidazole; PBIANI, poly(benzimidazole-co-aniline); PBIB, poly(benzimidazole-co-benzene); PDMS, poly(dimethyl siloxane); PEEK-WC, poly(oxa-pphenylene-3,3-phtalido-p-phenylenxoxa-p-phenylenexoxyp phenylene); PEI, polyethyleneimine; PEK, poly(ether ketone); PEM, proton exchangemembrane (polymer electrolyte membrane); PEMFC, proton exchange membrane fuel cells; PEO, poly(ethylene oxide); PFCB-PBI, perfluorocyclobutylcontaining polybenzimidazoles; PFSA, perfluorosulfonated acid; POM, polyoxometalate; PPA, polyphosphoric acid; PPO, poly(2,6-dimethyl-1,4 -phenyleneoxide); PS-b-PVBPA, poly(styrene-b-vinylbenzylphosphonic acid); PSf-Bim, polysulfone bearing benzimidazole side group; Pt, platinum; PTFE, polytetraflu-oroethylene; PVA, polyvinyl alcohol; PVTri, poly(1-vinyl-1,2,4-triazole); PWA, phosphotungstic acid (H3PO12O40·29H2O); PVDF, poly(vinylidene fluoride);Py-PBI, pyridine-based polybenzimidazole; RH, relative humidity; SBA-15, spherical particles of mesoporous silicates; Si-MCM-41, silica-mobile crys-talline material; SiW, silicotungstic acid; SPBIBI, sulfonated poly[bis(benzimidazobenzisoquinolinones)]; SPEEK, sulphonated polyetheretherketone; SPES,sulfonated poly(ether sulfone); SPFEK, sulfonated poly(fluorenyl ether ketone)s; SPIs, sulfonated polyimides; SPPEK, sulfonated poly(phthalazinone etherketone); SPPO, sulphonated poly(2,6-dimethyl-1,4-phenylene oxide); sPPSQ, sulfonated poly(phenylsilsesquioxane); SPSF, sulphonated polysulphone;TAB, tetraaminobiphenyl; TADE, 3,3′ ,4,4′- tetraaminodiphenyl-ether; TBT, tetrabutyl titanate (Ti(OC4H9)4); TCND, 1,4,5,8-naphthalenetetracarboxylicdianhydride; TES, tetraerthoxy silane; TEOS, tetraethyl orthosilicate; TFA, trifluoroacetic acid; Tg, glass transition temperature; TMA, trimesic acid;TMS, Tetramethylene sulfone; TMSCS, trimethylsilylchlorsulfonate; TPP, triphenylphosphite; ZrP, zirconium hydrogen phosphate; ZrOCl2, zirconium oxychloride; ZrSPP, zirconium sulphophenyl phosphate.

∗ Corresponding author. Tel.: +82 63 270 2342; fax: +82 63 270 2341.E-mail address: jhl@chonbuk.ac.kr (J.H. Lee).

0079-6700/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.progpolymsci.2011.01.003

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814 S. Bose et al. / Progress in Polymer Science 36 (2011) 813–843

Keywords:High temperature proton exchangemembraneFuel cellPolymeric membraneProton conductivityCell performanceWater retention

analyzed to fulfill the key requirements of high temperature operation of proton exchangemembrane fuel cells (PEMFC). The effect of inorganic additives on the performance of HT-PEMFC has been scrutinized. A detailed discussion of the synthesis of polymer, membranefabrication and physicochemical characterizations is provided. The proton conductivityand cell performance of the polymeric membranes can be improved by high tempera-ture treatment. The mechanical and water retention properties have shown significantimprovement., However, there is scope for further research from the perspective of achiev-ing improvements in certain areas, such as optimizing the thermal and chemical stabilityof the polymer, acid management, and the integral interface between the electrode andmembrane.

© 2011 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8141.1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8141.2. Low temperature proton exchange membrane fuel cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8141.3. Necessity of high temperature proton exchange membrane fuel cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8151.4. Challenges of high temperature proton exchange membrane fuel cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8151.5. Adopted approaches for high temperature proton exchange membrane fuel cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8161.6. Scope of the review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 816

2. Proton transport mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8162.1. Protonic defects in water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8172.2. Transport phenomenon in polymer composite membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8182.3. Proton transport through solid acid membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 819

3. Synthesis and properties of polymeric membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8193.1. Synthesis and fabrication procedures of the membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 820

3.1.1. Sulfonated aromatic hydrocarbon polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8203.1.2. Organic–inorganic composite membrane. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8223.1.3. Polymer blend membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8273.1.4. Polybenzimidazole (PBI) based acid–base membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 829

3.2. Property assessment of the membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8313.2.1. Systematic analysis of proton conductivity of polymeric membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8313.2.2. Mechanical property assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8343.2.3. Water uptake and swelling study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 835

3.3. Performance evaluation of the membrane. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8363.3.1. Single cell performance of the membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8363.3.2. Durability measurements of the membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 837

4. Application of HT-PEMFC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8375. Future design concept of HT-PEFMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 838

5.1. Pore-filling electrolyte membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8385.2. Increasing catalytic activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8385.3. Selection of electrocatalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8385.4. Hydrogen storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 839

6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 839Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 839References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 839

1. Introduction

1.1. Background

In the recent scenario, proton exchange membrane fuelcells (PEMFCs) are one of the most promising clean energytechnologies. PEMFCs have certain potential advantages,such as portable applications, power generation, high effi-ciency, etc. [1,2]. The key constituent of a PEMFC is a denseproton-exchange membrane, which is responsible for pro-ton migration from the anode to the cathode. Hydrogenis catalytically oxidized in the anode to produce protons.The membrane is generally placed between two electrodes,i.e. between the anode and cathode. The protons thus pro-

duced can migrate from the anode to cathode where theprotons react with oxygen to produce water and heat [3].The liquid electrolyte systems can be overpowered by solidproton exchange membranes due to the unique featuresof the solid proton exchange membrane, which includeeasy handling, compact, amenable to mass production, andexcellent resistance to the permeation of gaseous reactants.

1.2. Low temperature proton exchange membrane fuelcell

Typical low temperature proton exchange membranefuel cells, e.g., perflurosulfonic acid membranes (Nafion)

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Fig. 1. Adsorption of CO on Pt.

have a multiphase structure: a hydrophobic phase as acontinuous phase and sulfonic acid groups can act as ahydrophilic phase. A continuous hydrophobic phase isessential for the structural integrity of membrane, andthe hydrophilic phase acts as water reservoir [4,5]. Wateris very essential for the proton conductivity because itpromotes the dissociation of protons from the sulfonicacid groups, and provides highly mobile hydrated protons.Therefore, hydration is the key factor for maintaining theoptimal performance of the membranes. To keep the mem-brane hydrated, one or both reactant gas streams must behumidified.

1.3. Necessity of high temperature proton exchangemembrane fuel cell

Low temperature proton exchange membrane has somedisadvantages that may reduce the effectiveness of a mem-brane in fuel cell applications. This lacuna can be conqueredby adopting high temperature operation of the membrane.The need for a high temperature proton exchange mem-brane is a consequence of the following:

(i) CO catalyst poisoning: The carbon monoxide concen-tration affects the performance of a membrane at lowtemperatures. If the concentration of CO is excessive(∼10 ppm), it will strongly adsorb to the platinum (Pt)surface and poison the platinum electro-catalyst [6–9](Fig. 1). Indeed, the adsorption of CO on Pt is associatedwith high negative entropy, implying that adsorptionis favored at low temperatures, and disfavored at hightemperatures [10]. Therefore, the CO tolerance willincrease with increasing temperature and is almostnegligible at high temperatures (140 ◦C).

(ii) Heat Management: During low temperature opera-tion, the main disadvantage of PEMFCs is the coolingof a system. A PEMFC operating at 80 ◦C with an effi-ciency of 40–50% can produce a huge amount of heatthat needs to be removed from the system to main-tain the working temperature. The excess heat can berecovered as steam, which in turn can be used eitherfor direct heating or steam reforming or for pressur-ized operations. In this way, the overall efficiency ofthe system under high temperature operations can beincreased significantly, which is essential for transport

applications. An increase in temperature also reducesthe front area of radiators, which is very important forautomobile applications. However, high grade exhaustheat can be intergraded in into the fuel processingstages [11].

(iii) Direct Hydrogen: The temperature range of100–200 ◦C is the primary requirement for hydrogendesorption from a high-capacity H2 storage tank. Sucha high-capacity H2 storage tank cannot be consideredin low temperature circumstances.

(iv) Humidification: In low temperature scenarios, a highhumidification environment is essential to reach pres-surization. However, a high humidity atmosphere doesnot provide the resistance to impurities caused byfuel. Membranes that are capable of functioning atreduced humidity do not require pressurization andhence can be effective in resisting the damage causedby fuel impurities. High temperature operation doesnot require high humidity and the problems due towater management can be reduced effectively.

(v) Increase in diffusion rate: Interlayer diffusionenhances with increasing the temperature. In addi-tion, at high temperature the evaporation of watermolecule will lead to increase the exposed surfacearea, which will allow the reactants to diffuse into thereaction layer.

(vi) Prohibitive technology costs: Potential savings byreducing the loading level of the electro-catalyst usedmay provide a strong economical driving force todevelop fuel cells that can operate at high tempera-tures.

1.4. Challenges of high temperature proton exchangemembrane fuel cell

PEMFCs operating at high temperatures are consid-ered as one of the potential solution for the technicalchallenges faced during low temperature operation. Theterm ‘high temperature’ refers to the temperature range,100–200 ◦C, which does not appear to be high in an engi-neering point of view. However, in the current state ofthe art, development of high temperature PEM for fuelcells is very important in the field of materials scienceand engineering. However, there are also some technicalobstacles during high temperature treatment of polymer

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electrolyte membranes. Polymer membranes are incapableof operating at high temperatures because water from themembrane evaporates out resulting in a loss of protonconductivity [12]. Actually, HT-PEMFC operates withouthumidification which causes large Ohmic losses and inturn lowers the operating voltage, power, and efficiencyof membrane at a given current. As a result, dehydra-tion at higher temperatures could potentially offset theperformance of the membrane. High proton conductiv-ity is essential for achieving a high power density in fuelcells. Dehydration of the membrane at high temperaturesmay lead to decrease the proton conductivity value andin the process performance of polymeric membranes getshampered. However, the susceptibility towards chemicaldegradation at elevated temperatures and the high cost ofthe membrane are also the determining factor in decid-ing the performance of the membrane. For example, theconductivity of Nafion reaches up to 10−1 S/cm in fullyhydrated condition but dramatically decreases with tem-perature above the boiling temperature of water because ofthe loss of absorbed water from the membranes. These fac-tors can adversely affect fuel cell performance and hencecalls for extensive amount of research. Actually, for highperformance at elevated temperature, HT-PEMFC shouldpossess the following requirements: (1) Low-cost materi-als; (2) High proton conductivities over 100 ◦C; (3) Goodwater uptake above 100 ◦C; and (4) Durability for 10 years.

Therefore, several approaches have been adopted tomodify the polymeric membrane in order to maintain theproton conductivity as well as the performance of themembrane at high temperatures, and can be used in fuelcell applications.

1.5. Adopted approaches for high temperature protonexchange membrane fuel cell

Hydrophilic groups, i.e. inorganic materials can beincorporated into hydrophobic polymer membranes toenhance the binding capacity of the water. Watermolecules of a membrane can attach to the inorganic mate-rials through hydrogen bonding, and in that way watercan be retained in the membrane [13]. The effectivenessof these hydrophilic additives in water retention has beenshown in the case of heteropolyacids in Nafion [12,14]. Theacid helps to keep the membrane hydrated and also playsa role in improving the proton density. Hence, the mem-brane shows reasonable performance in a fuel cell at hightemperatures. Nevertheless, with time, the leaching of acidfrom the membrane will cause dehydration and reducethe life of the membrane. The use of solid materials (sil-ica gel, zirconium phosphate etc.) that can be immobilizedin the membrane may be a solution for the aforemen-tioned problem. Also, in this respect, modification of PFSAmembranes with inorganic proton conductors is anotherapproach to obtain reasonable proton conductivity, whichwill be hardly dependent on the water contents.

A second approach is the use of a non aqueous, lowvolatile solvent by replacing water as the proton accep-tor within the polymer membrane. Till date, a range ofnon aqueous solvents as the primary proton carrier insteadof water have been reported, including phosphoric acid,

imidazole, butyl methyl imidazolium triflate, etc. [15–17].Water is an excellent solvent for fuel cell applicationbecause of its ability to act as a bronsted base and posseshigh di-electric constant. Phosphoric acid (PA), which canbe used as a replacement for water as a solvent for PEM-FCs at high temperatures, exhibits similar properties towater with improved physical characteristics (low volatil-ity). Upon assembly of a PA-doped membrane and theelectrodes, PA from the membrane diffuses into the elec-trodes and acts as an ionomer in the electrodes, which willlead to increase the proton conductivity [11].

The third approach is the use of a solid state protonicconductor. The major difference between the proposedapproach and the earlier ones is the medium of conduction.In this approach, the solid state material conducts protons,whereas in the earlier approaches, liquid solvents are themedium for carrying the protons.

1.6. Scope of the review

A range of polymeric membranes have been used asproton conducting membranes for high temperature fuelcell applications till date [18–21]. In the current reviewan effort has been made to summarize the fabrication andmodification techniques of various polymeric membranesthat can be used at high temperatures for fuel cell tech-nology. Systematic examination of the electrochemical,mechanical, morphological properties as well as the waterretention capacity at high temperatures has been carriedout. The developed membrane can be classified into thefollowing groups:

(i) sulfonated aromatic hydrocarbon polymer mem-branes,

(ii) inorganic–organic composite membranes,(iii) membranes of blend polymers,(iv) acid–base polymer membranes.

Modification techniques of the PFSA membranes usingvarious inorganic fillers and the effectiveness of themembrane with the goal of achieving improved protonconduction above 100 ◦C have been reviewed extensively.Nonperfluorosulfonated membranes [22–26] have alsobeen reviewed in a detailed manner. Some of the sulfonatedhydrocarbons show high proton conductivity for poten-tial operation at 100–120 ◦C. To achieve high operationaltemperatures, several researchers have proposed a furtherstep towards the development of inorganic–organic com-posites based on these alternative polymers [27–29]. Inthis article, a critical review of inorganic–organic polymericcomposites has been carried out. Acid–base polymers areanother class of proton conducting membranes with attrac-tive performance at high temperatures. Attention-grabbingacid–base polymers are phosphoric-acid-doped PBI andionically cross-linked acid–base blends, as reviewed byWainright et al. [30].

2. Proton transport mechanism

In metals, proton transfer generally occurs betweenthe interstitial octahedral or tetrahedral sites, resulting

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S. Bose et al. / Progress in Polymer Science 36 (2011) 813–843 817

Fig. 2. Schematic diagram of the two reaction steps in long range protontransport [4] (Copyright Elsevier Science Ltd., UK, 2000).

in long-range proton transport, i.e. diffusion. However, innon-metallic environments, the proton mobility requiresnot only proton transfer reactions within the hydrogenbonds but also structural reorganization, as illustratedschematically in Fig. 2.

2.1. Protonic defects in water

The degree of hydration is the main factor which gov-erns the proton conductivity in a polymeric membrane. Theproton transport mechanism can either be described by aHopping mechanism or by a Diffusion phenomenon [31].However, proton transport mechanism through a PEM isbasically conduction through water. The dominant inter-molecular interaction in water is hydrogen bonding. The

introduction of an extra proton leads to proton defects,resulting in a contraction of hydrogen bond in the vicin-ity of such defects. The binding power of a water moleculedepends on the number of hydrogen bonds involve in it.This also leads to relaxation effects in the neighboringhydrogen bonds as a response to the formation and cleav-age of hydrogen bonds. When a hydrogen bond is formed,the surrounding bonds are weakened but the cleavageof hydrogen bonds strengthens the neighboring bonds[4,32,33]. Therefore, defects caused by the incorporationof excess protons weaken the intermolecular interactionby means of breakage and reformation of bonds in combi-nation with large variations in bond length [4,34–36].

Excess protons can be a part of a dimer (H5O2+, ‘Zun-

del’ ion) or a part of a hydrated hydronium ion (H9O4+,

‘Eigen’ ion). The central bond of H5O2+ (≈250 pm) is notice-

ably contracted compared to the average hydrogen bondlength in bulk water (≈280 pm) but elongated comparedto an isolated dimer (≈240 pm).

However, in case of H9O4+, the central bond is less con-

tracted (≈260 pm) than H5O2+. Hence, both configurations

appear with comparable probability and there is thermody-namic stability between the two configurations [35]. Nowfor the ‘Zundel’ ions, the position of the excess protoncoincides with the centre of symmetry of the coordinationpattern but for the ‘Eigen’ ion, the centre of the symmetryof the coordination pattern coincides with the oxygen ofthe hydronium ion. Fig. 3 shows the formation of an ‘Eigen’ion from a Zundel’ ion and the change in protonic chargefrom one ion to another. In a proton exchange membrane,the hydrated environment, often acidic, acts as a solventfor the diffusion of hydronium and dimer ions formed. Thediffusion phenomenon is shown in Fig. 4.

An understanding of the chemistry of the migration ofprotonic defects through the membrane is very impor-tant. The catalytically oxidized protons, acting as defects,migrate through the membrane from the anode to the cath-ode, and carry water molecules with them. The averagenumber of water molecules carried per proton is called theelectro-osmotic drag coefficient. The carried water accu-mulates in cathode/membrane interface due to this electroosmotic drag.

Fig. 3. Transport mechanism of Protonic defects in water [34] (Copyright Elsevier Science Ltd., UK, 2005).

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Fig. 4. Proton transport in a polymer membrane [34] (Copyright Elsevier Science Ltd., UK, 2005).

Reaction at anode:

H2→ H+ + 2e

Reaction at cathode:

1/2O2 + 2e + 2H+ → H2O

Water can be removed through the cathode or travelthrough the membrane and be eliminated through theanode. The flux of water from the cathode to theanode is known as back diffusion. Fig. 5 represents aschematic diagram of electro osmotic drag and backdiffusion.

2.2. Transport phenomenon in polymer compositemembrane

The following summarized a few approaches to protontransport through a composite:

(i) Hygroscopic composites: The incorporation of hydro-scopic inorganic filler (e.g., silica) may result inan increase in membrane swelling at lower rela-tive humidity, and offer resistance to fuel crossover.Hence, the transport of protons through the membranebecomes easier and reduces the methanol permeabil-ity. Fig. 6 shows proton transport through a compositemembrane.

(ii) Conducting composites: A conducting species can beintroduced into the polymer, which reduces water and

Fig. 5. Electro osmotic drag and back diffusion in PEMFC.

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Fig. 6. Proton transport in a nanocomposite membrane [34] (Copyright Elsevier Science Ltd., UK, 2005).

methanol permeability significantly. The conductingspecies tighten the pores of the polymer, which inhibitthe molecular migration of unwanted species throughthe membrane. Furthermore, the incorporation of aconducting material also results in the recovery of con-duction loss due to a reduced fraction of water from themembrane.

(iii) Water substituted composites: These composites arebased on the polymer matrix and an alternative protontransporter, i.e., use of hetropolyacids. The aim is toimmobilize a highly conducting acid in the polymerso that proton conduction is independent of hydrationand the electro osmotic drag is reduced.

2.3. Proton transport through solid acid membrane

Proton transport through a solid acid membrane is basi-cally a surface phenomenon. Solid acids have a monoclinicstructure. At high temperatures, the structure changes

from monoclinic to triclinic and the conductivity increasesby 2–3 orders of magnitude. This transition is known asa ‘superprotonic’ transition [37]. The surface transport ofvarious metal phosphates [38,39] occurs along the exposedacid sites on the surface. The exposed acid sites can trapwater and carry the proton from one electrode to another.Fig. 7 illustrates the transport of protons through a solidacid.

3. Synthesis and properties of polymericmembranes

A summary of the procedures of polymer synthesisand membrane fabrication technique has been discussedelaborately. Various properties associated with high tem-perature proton exchange membranes, such as protonconductivity, mechanical property, water uptake, swellingstudy, and the performance of the membrane have beenanalyzed in details.

Fig. 7. Proton transport in a solid acid membrane [34] (Copyright Elsevier Science Ltd., UK, 2005).

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Fig. 8. Polymerization technique of poly(2,5-benzophenone) via nickel-catalyzed coupling polymerization [40] (Copyright Elsevier Science Ltd.,UK, 2004).

3.1. Synthesis and fabrication procedures of themembranes

The synthesis and fabrication technique of several typesof polymeric membranes for high temperature membranefuel cell are discussed.

3.1.1. Sulfonated aromatic hydrocarbon polymers3.1.1.1. Sulfonated poly (p-phenylene). Ghassemi et al.[40] reported the polymerization technique of poly(2,5-benzophenone) from 2,5-dichlorobenzophenone (Fig. 8).The obtained polymer was treated with fuming sulfuricacid at room temperature. The sulfonated polymer waswashed thoroughly with DI water and dried for 24 h at 80 ◦Cprior to use.

Membrane fabrication is quite difficult because thepolymer is quite rigid in nature. A piece of glass clothwas placed over a glass plate and the polymer solutionin NMP was casted onto the plate. The plate was thencovered with a dish and heated with an IR lamp at 60 ◦Cin a nitrogen atmosphere. The resulting membrane pro-duced in the form of glass fibers had a thickness of 150 �m.Kobayashi et al. [41] reported the synthesis and proper-ties of sulfonated poly (4-phenoxybenzoyl-1,4-phenylene)(SPPBP) membrane. In order to synthesize SPPBP, PPBPwas dissolved in concentrated sulfuric acid under nitrogenatmosphere. The solution was held at room temperature forthe desired time and poured into a large excess of water.The precipitate was then collected by filtration. The pre-cipitate was pulverized and washed thoroughly until pH ofthe solution became 7. The SPPBP was then dialyzed againstdistilled water using a cellulose acetate membrane.

3.1.1.2. Sulfonated poly(ether ketone). Several researchershave proposed that poly(ether ketone) can be synthesizedby nucleophilic aromatic substitution polycondensation[42–44]. The polymerization was carried out in presenceof K2CO3 in TMS as the solvent at 210 ◦C. High molecularweight polymers were readily obtained in 4 h. Fig. 9 showsthe synthetic route for polymer synthesis. The resultingpolymer was poured into 150 ml. of ethanol. The polymerwas heated under reflux in deionized water and ethanolseveral times to remove the salts and solvents and dried at120 ◦C for 24 h. Subsequently, 100 ml of concentrated sulfu-ric acid was added to 5 g of polymer to form a silk-like solid,which was washed thoroughly and dried under vacuum at100 ◦C for 24 h. For membrane fabrication, the sulfonatedpolymer (Fig. 10) solution in DMAc was poured onto a glassplate at 50 ◦C under a nitrogen atmosphere [45]. The result-ing membrane (100–150 �m) was dried under vacuum at120 ◦C for 24 h.

3.1.1.3. Sulfonated polysulfone. Genova-Dimitrova et al.[46] reported a technique for the sulfonation of polysul-fone using (CH3)3SiSO3Cl (TMSCS) as the sulfonating agent.Sulfonation was carried out under anhydrous conditionsin Ar. The membrane fabrication technique is mentionedbelow. A polymer solution was prepared using a mixture ofisopropanol and dichloroethane as solvent. The phospha-toantimonic acid was used as the aqueous gel. Water fromthe solution was removed at 80 ◦C under vacuum. The vis-cous suspension was casted onto a flat glass surface. Themembrane was then heated at moderate temperatures ina ventilated oven for several hours to remove the solvent.The flat glass was left at room temperature and the poly-mer film was peeled by water to remove the solvent. Themembranes were 150–200 �m thick and 10 cm in diame-ter.

3.1.1.4. Sulfonated poly(arylene ether sulfone). Nucleophilicaromatic polycondensation is the most common proce-dure for synthesizing poly(arylene ether sulfone) [47–49].Sulfonation of the resultant polymer was carried outusing chlorosulfonic acid in dichloromethane. The result-ing product was washed with hexane and water severaltimes and dried under vacuum at 60 ◦C for 15 h to obtain awhite powder of sulfonated polymer. Fig. 11 illustrates the

Fig. 9. Synthetic route of aromatic poly(ether ketone) [42] (Copyright Elsevier Science Ltd., UK, 2004).

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Fig. 10. (a) main chain acid PEK; (b) side group acid Ph-PEK [43] (Copyright the American Chemical Society, USA, 2007).

synthesis procedure. Sulfonated polymers were added toDMAc and then casted onto a clean, flat glass plate and heldfor 15 h. The colorless membranes were then immersedin HNO3, washed several times with DI water and finallyplaced under vacuum at 60 ◦C for 15 h.

3.1.1.5. Sulfonated poly(aryl ether ether nitrile). Aromaticpoly(aryl ether nitrile) has been prepared by sev-eral researchers [50–52] via a nucleophilic substitution

polycondensation reaction of bisphenols and dihaloben-zonitriles in dipolar solvents. Some researchers alsoreported the incorporation of nitrile groups into poly (arylethersulfone) to improve the quality of the membraneelectrolyte assemblies [53,54]. The synthetic route of thepolymer was shown in Fig. 12. For membrane fabrication,the potassium salt of the polymer was dissolved in DMAcand filtered. The filtered solution was then casted on a glassplate at 40 ◦C for one day under a nitrogen atmosphere. The

Fig. 11. Synthesis of poly(arylene ether sulfone) containg fluorenyl group and sulfonation of the polymer using chlorosulfonic acid [47] (Copyright theAmerican Chemical Society, USA, 2007).

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Fig. 12. Synthesis of SPAEEN [50] (Copyright the American Chemical Society, USA, 2005).

acid form of the membrane was obtained by treatment ofthe potassium form of polymer with 2N sulfuric acid. Thethickness of the membrane was 40–70 �m.

3.1.1.6. Sulfonated poly(sufide ketone). Hay et al. [55]reported the synthesis of end-functionalized poly(sulfideketone)s with six sulfonic acid moieties at the chainends for PEMs, which is illustrated in Fig. 13. Poly(sulfideketone)s were synthesized by the condensation of aro-matic dihalide and aromatic dithiol in the presenceof 1-(4-hydroxyphenyl)-2,3,4,5,6-phentaphenylbenzene.Subsequent sulfonation was performed with sulfonyl chlo-ride followed by hydrolysis with a KOH aqueous solution.Membrane fabrication was then performed via solutioncasting method.

3.1.1.7. Sulfonated polyimide. The polymerization reac-tion of bis[3-(4-sulfophenoxy)propoxy] benzidine with

1,4,5,8-naphthalenetetracarboxylic dianhydride (TCND)was carried out in m-cresol solution to give the resultingpolymer [56,57]. The reaction procedure is illustrated inFig. 14.

The polymer solution in m-cresol was casted onto a glassplate and dried at 60 ◦C for 1 day. The triethylammoniumsalt form membrane was soaked in ethanol containing 1NHNO3 for 12 h. The membrane in the acid form was thenwashed thoroughly with ethanol and dried at 60 ◦C underreduced pressure.

3.1.2. Organic–inorganic composite membrane3.1.2.1. Fluorinated polymer/SiO2 composite. Kimet al. [58] reported the synthesis of organicpolymers involving decafluorobiphenyl (DF) and 4,4′-(hexafluoroisopropylidene) diphenol (F) in the presenceof dimethylacetamide (DMAc) followed by the addition of

potassium carbonate and heating to 120 ◦C under a nitro-gen atmosphere for 2–3 h. The mixture was allowed to coolto room temperature, and then washed with distilled watercontaining acetic acid to precipitate the polymer (Fig. 15).Subsequently, sulfonation of the polymer was carried outusing fuming sulfuric acid. The sulfonated polymer (SDF-F)was precipitated in water and washed until the pH was7. Simultaneously, a silicon oxide solution was preparedusing tetraethoxysilane (TES), distilled H2O and HCl. After3 h stirring, the silicon oxide solution was added to SDF-F,dried at 100 ◦C for 24 h and finally casted overnight usingteflon plates to obtain the organic–inorganic composite(SDF-F/SiO2) membranes.

3.1.2.2. Polyalkoxysilane/phosphotungstic acid composite.The bridged polysilsesquioxanes were obtained by hydrosi-lylation addition to dienes, as shown in the followingreaction [59]:

The bridged silsesquioxane polymers were thenhydrolyzed and condensed to form hybrid compositesthrough a catalytic reaction with PWA (H3PO12O40·29H2O).The composite membrane was fabricated using a sol–geltechnique (Fig. 16).

3.1.2.3. Nafion/PTFE/Zirconium phosphate (Zr(HPO4)2) com-posite. Preparation of Nafion (NF)–ZrP composite mem-branes using “direct impregnation” process has beenreported by several researchers [60,61]. Briefly, the sub-�m porous PTFE film was impregnated directly into theNafion/ZrOCl2 blend solution at room temperature for 5 h,followed by annealing at 130 ◦C for 1 h. The membranewas then immersed in distilled water at room temper-ature to clean out the residual solvent and the residualZrOCl2 was precipitated on the membrane surface. Themembrane was then treated with phosphoric acid solu-tion at roomtemperature for 4 h to convert the ZrOCl2 to

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Fig.

13.

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Fig. 14. Synthesis of sulfonated polyimide having pendant acidic group [56] (Copyright Wiley-VCH, Germany, 2007).

Zr(HPO4)2.

ZrOCl2 + 2H3PO4 → Zr(HPO4)2 + H2O + 2HCl

The membrane was rinsed several times with distilledwater at room temperature to clean out the residual phos-phoric acid on the membrane surface. Subsequently, themembrane was swollen in distilled water at room tem-perature for 24 h, and then in sulphuric acid at room

temperature for a further 6 h. The membrane was subse-quently dried at room temperature. The thicknesses of theNF–ZrP composite membrane ranged from 20 to 22 �m.

3.1.2.4. Nafion/TiO2 composite. The Nafion/TiO2 compositeis generally prepared by a sol-gel process, as described byseveral researchers [62]. Normally, the sol–gel incorpora-tion of a catalyst is essential as it can provide an acidic

Fig. 15. Synthesis of fluorinated polymer and sulfonation of the same using fuming H2SO4 [58] (Copyright Elsevier Science Ltd., UK, 2004).

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Fig. 16. Sol–gel synthetic route for composite membrane fabrication [59] (Copyright Elsevier Science Ltd., UK, 2003).

environment for the reaction to initiate. In this reaction,the addition of acid is not needed because the sulfonic acidclusters of Nafion structure act as a catalyst [63]. First, thecommercial Nafion membrane was swollen in a methanolsolution at 60 ◦C for 20 min. The swollen membrane wasthen dipped in a precursor solution that had been preparedby mixing TBT and methanol to accomplish hydrolysis inthe membrane. Finally, the membrane was dried at 60 ◦Cunder vacuum for 12 h to form a condensed titanium oxidenetwork within the membrane. Fig. 17 gives a schematicdiagram of the formation of a Nafion/TiO2 composite.

Ti(OC4H9)4 + 4H2O → Ti(OH)4 + 4C4H9OH

Ti(OH)4 → TiO2 + H2O

3.1.2.5. PVA/SiO2/Silicotungstic acid (SiW) composite. Theorganic (PVA)–inorganic (SiO2) composite was preparedusing a sol–gel method [64]. Tetraethyl orthosilicate (TEOS)and silicotungstic acid (SiW) were added to a solution ofpolyvinyl alcohol (PVA) in water and refluxed at 353K for6 h. SiW acted as an acid catalyst for hydrolysis and pro-moted the condensation of tetraethyl orthosilicate to SiO2.Cross-linking between the silica matrix and polyvinyl alco-hol occurred in the presence of polyoxometalate (POM).The transparent gel formed was drop-casted to obtain amembrane 60–100 �m in thickness.

3.1.2.6. Cs2.5H0.5PMO12O40/polybenzimidazole(PBI)/H3PO4 composite. PBI (poly[2,2′-m-(phenylene)-5,5′-bibenzimidazole]) was prepared from3,3′-diaminobenzidine and isophtalic acid in polyphos-

phoric acid (PPA) by solution poly-condensation attemperatures ranging from 170 to 200 ◦C, as shown inFig. 18 [65].

Cs2.5H0.5PMo12O40 (CsPOM) was prepared fromH3PMo12O40 and Cs2CO3 [66–68]. In this procedure, a

Fig. 17. Schematic representation of Nafion/TiO2 nanocomposite forma-tion [62] (Copyright Elsevier Science Ltd., UK, 2010).

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Fig. 18. Preparation of the PBI resin via polycondensation reaction [65] (Copyright Elsevier Science Ltd., UK, 2008).

solution of Cs2CO3 in water was added drop-wise to awater based solution of H3PMo12O40. The resulting pre-cipitate was recovered from the solution by evaporation at55 ◦C and washed with de-ionized water to obtain the pureCsPOM. The composite membrane was prepared accordingto the following method. Firstly, the CsPOM inorganicpowder was dispersed in a solution of PBI in NMP. TheCsPOM/PBI composite membrane was synthesized byevaporating the NMP solvent at 120 ◦C for 2 h. Finally, thecomposite membrane was doped using aqueous solutionof H3PO4 for 10 h at 120 ◦C. The residual H3PO4 from themembrane surface was removed prior to use.

3.1.2.7. Nafion/SiO2 composite. Nafion/silica compositemembranes are generally prepared in two ways: (1)re-casting of a mixture of silica and Nafion ionomer[69–72]; (2) impregnation of membranes with a solutionof inorganic precursors, such as tetraethoxysilane (TES),followed by in situ sol–gel reaction [73–76]. The com-posites prepared using the above mentioned methodsreduce the methanol crossover and improve the waterretention properties but suffer from reduced durability.Tang et al. [77] reported a novel route of synthesizinga self-assembled Nafion/silica composite. In the typicalroute mentioned by Tang et al., Nafion ionomers weredissolved in a N-methyl-2-pyrrolidone (NMP) solution. Aweighed amount of tetraethoxysilane (TES) was dissolvedin the Nafion/NMP mixture using a homogenizer, followedby addition of dilute HCl with vigorous stirring. Self-assembled Nafion–SiO2 nanoparticles were obtained afterstirring for 8 h (Fig. 19). The Nafion/SiO2 nanocompositemembranes were prepared by recasting.

3.1.2.8. Sulfonated poly(ether sulfone) (SPES)/boron phos-phate (BPO4) composite. Dai et al. [78] reported thesulfonation of poly(ethersulfone) using chlorosulfonic acidas a sulfonating agent and the organic–inorganic compos-ite was prepared using sol–gel technology. Equal molarproportions of Tripropylborate (C3H7O)3B and phospho-ric acid were used as precursors to synthesize BPO4.A desired amount of each precursor was added toa SPES/N,N-dimethylacetamide (DMAc) solution, stirredwith a magnetic stirrer for 30 min and degassed by ultra-sonication. To remove the impurities, the casting solutionswere filtered through a 0.2 mm pore size Teflon filter before

membrane preparation. The prepared mixture was pouredslowly into a glass dish. The thickness of the membranewas approximately 60 �m. Air-drying of the membranewas carried out at 80 ◦C for 4 h followed by a desiccatingstep at 120 ◦C for 12 h and further drying under vacuum at120 ◦C for 24 h [79].

3.1.2.9. Polyoxadiazole/silica composite. The typical syn-thesis procedure of sulfonated poly(diphenyether-1,3,4-oxadiazole) was demonstrated by several researchers[80–83]. Initially polyphophoric acid (PPA) was heated to100 ◦C under a dry argon atmosphere. HS was then intro-duced to PPA and the reaction temperature was allowedto increase to 160 ◦C. After reaching the desired reactiontemperature (160 ◦C), dicarboxylic acid 4,4′-diphenylether(DPE) was added to a solution of PPA and HS. After 5 h, thereaction medium was poured into tepid water (contain-ing 5%,w/v sodium hydroxide) to precipitate the polymer.Nanocomposite membranes were prepared by adding inor-ganic filler into the polymer solution in DMSO. The solutionwas stirred for 6 h and casted onto a glass plate followedby the evaporation of DMSO in a vacuum oven at 60 ◦C for24 h. For further residual solvent removal, the membraneswere immersed in a water bath at 60 ◦C for 48 h and thendried in a vacuum oven at 60 ◦C for 24 h. The final thick-ness of the membranes was approximately 50–70 �m. Theresulting membranes were converted into their acid formby immersing the cast membranes in H3PO4 at room tem-perature for 24 h, followed by immersion in water for 48 hto ensure the total leaching of residual phosphoric acid.

3.1.2.10. Linear PEI/SiO2 composites. Yang et al. [84]reported the preparation of linear polyethyleneimine.According to this procedure, a weighed amount of lin-ear poly(2-ethyl-2-oxazoline) and a 7 M HCl solution wereheated for 5 days. The product was cooled to room tem-perature and precipitated in a NaOH solution. The solidwas collected by filtration and washed thoroughly withDI water until it was neutral. A LPEI solution in ethanolwas treated with GLYMO and stirred for 30 min at roomtemperature. A weighed amount of HTFSI (as dopant) wasadded dropwise and stirred for a further 30 min at roomtemperature. The resulting solution was casted onto a PTFEsurface, air dried at room temperature for 1 day, and heatedto 80 ◦C overnight to produce flexible membranes (thick-

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Fig. 19. Formation of Nafion-SiO2 composite by the self-assembly method [77] (Copyright Elsevier Science Ltd., UK, 2007).

ness of 150 �m). Fig. 20 shows the synthesis of the polymerand the membrane fabrication technique.

3.1.3. Polymer blend membrane3.1.3.1. Nafion/poly(1-vinyl-1,2,4-triazole) blend. PVTri isproduced by the free radical polymerization of 1-vinyl-1,2,4-triazole in toluene using AIBN as the initiator [85,86].The reaction mixture was purged with nitrogen and thepolymerization reaction was performed at 85 ◦C for 2 h. Theresulting white powder polymer was filtered and dried in avacuum. The resulting polymer was dissolved in DMF andmixed with a commercial Nafion solution. The solutionswere then cast onto polished Teflon plates. The slow evap-oration of the solvent was carried out at 40 ◦C, followed bydrying under vacuum at 80 ◦C for at least 24 h. Transpar-ent, hygroscopic and free standing films with a thicknessof 150–300 �m were obtained.

3.1.3.2. Sulfonated PBIBI/poly(vinylidene fluoride) blend.Wang et al. [87] reported the synthesis of SPBIBI. Blendswere prepared by dissolving SPBIBI and poly(vinylidenefluoride) in DMSO for 12 h. The homogeneous blend solu-tions were filtered through a 0.5 �m PTFE membrane andpoured into a glass petri dish at 80 ◦C to cast the blendedfilms. The transparent homogeneous thin films (30–50 �m)were boiled in water thoroughly and dried in a vacuum

oven at 100 ◦C for 48 h to remove the solvent. The pro-ton exchanged membranes were washed thoroughly withdeionized water and dried in a vacuum at 100 ◦C for 10 h.

3.1.3.3. Poly(benzimidazole)/polyether blend. Li et al. [88]reported the synthesis of poly(benzimidazole). The fabrica-tion technique of sulfonated partially fluorinated polyetherhas been demonstrated by several researchers [89,90] andis shown in Fig. 21. First, both polymers were placed ina DMAc solution. The mixture solution was stirred for 2 hat room temperature. The resulting membranes were thencasted from the mixture solution using petri dishes. Themajority of the solvent was evaporated in a ventilated ovenat temperatures ranging from 60 to 120 ◦C. The membraneswere then peeled off, boiled in water and finally driedat 200 ◦C for 2 h. The blend membranes were doped withphosphoric acid in a closed container to avoid changes inthe concentration from acid evaporation. After acid dop-ing, the blended membranes were removed from the acidsolution, blotted with filter paper and dried under vacuumat 110 ◦C for 120 h.

3.1.3.4. PPO/poly(styrene-b-vinylbenzylphosphonic acid)blend. Poly(styrene-b-vinylbenzylphosphonic acid) (PS-b-PVBPA) has been prepared by stable free radicalpolymerization [91,92]. PPO and PS-b-PVBPA copolymer

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Fig. 20. Preparation of HTFSI-doped LPEI/SiO2 composite membrane [84] (Copyright Elsevier Science Ltd., UK, 2008).

Fig. 21. Synthesis of partially fluorinated polyether [88] (Copyright Elsevier Science Ltd., UK, 2010).

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Fig. 22. Synthetic scheme of polysulfone bearing a benzimidazole side group [93] (Copyright Elsevier Science Ltd., UK, 2006).

were dissolved in NMP followed by vigorous stirring at80 ◦C. The obtained solution was filtered rapidly througha 0.45 �m membrane, and casted onto a preheated (95 ◦C)glass plate. NMP was then evaporated at 95 ◦C for 6 h. Theresulting film was dried again under vacuum at 95 ◦C for24 h. To transform the film into a membrane, the film wastreated with a HCl solution for at 50 ◦C for 12 h and then inboiling deionized water for 12 h, and finally washed withdeionized water.

3.1.3.5. Sulfonated poly(ether ether ketone)/polysulfoneblend. Fu et al. [93] reported the preparation of poly-sulfone bearing a benzimidazole side group (PSf-BIm)starting from carboxylated polysulfone (CPSf). CPSf and1,2-diaminobenzene were dissolved in DMF in a three-necked flask, followed by the addition of lithium chlorideand TPP (Fig. 22). The solution was stirred at 100 ◦C for 3 hand then at 150 ◦C for 10 h under a nitrogen atmosphereand then poured into methanol to precipitate the polymer.The precipitate was collected by filtration and driedin a vacuum oven at 110 ◦C overnight. Yang et al. [94]reported the synthesis of poly(ether ether ketone) (SPEEK).SPEEK/PSf–BIm blend membrane was prepared by solutioncasting method using dimethylacetamide (DMAc) as thesolvent.

3.1.4. Polybenzimidazole (PBI) based acid–basemembrane3.1.4.1. Synthesis of PBI.

3.1.4.1.1. Heterogeneous synthesis of PBI. Choe et al. [95]proposed a single stage method to synthesize high molec-ular PBI (Fig. 23) using tetraaminobiphenyl (TAB) andisophthalic acid (IPA) as monomers.

3.1.4.1.2. Homogeneous synthesis of PBI. PBI can alsobe synthesized in homogeneous solutions using solvent,like polyphosphoric acid (PPA), as reported by Iwakuraet al. [97]. Other solvents, such as a mixture of phospho-rus pentoxide (P2O5) and methanesulphonic acid (MSA),a low-viscous liquid, were also used as a solvent for thehomogeneous synthesis of PBI [98,99].

3.1.4.2. Modification of PBI. Pyridine-based PBI (Py-PBI)was synthesized from the pyridine dicarboxylic acids,as reported by several groups [100–102]. Schuster et al.[102] synthesized OO-PBI and OSO2-PBI. The OO-PBI wasprepared by a polycondensation reaction between 3,3′,4,4′-tetraaminodiphenyl-ether (TADE) and 4,4′-oxy-dibenzoicacid, and the OSO2-PBI was synthesized using 3,3′,4,4′-tetra aminodiphenyl-sulphone and 4,4′-oxy-dibenzoicacid. Bhadra et al. [103] synthesized hyperbranchedPBIB with a honeycomb structure through the con-densation polymerization of trimesic acid (TMA) and

Fig. 23. Single stage process for PBI synthesis [96] (Copyright Elsevier Science Ltd., UK, 2009).

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Fig. 24. Synthesis of poly (benzimidazole-co-aniline) [106] (Copyright Elsevier Science Ltd., UK, 2010).

3,3′-diaminobenzidine (DAB). Xu et al. [104,105] syn-thesized amine-terminated hyperbranched PBI. Bhadraet al. [106] also synthesized Poly(5-aminoisophthalicacid) (PAIPA) by the oxidative polymerization of 5-aminoisophthalic acid (AIPA), which was then subjected tocondensation polymerization with 3,3′-diaminobenzidine(DAB) to obtain poly(benzimidazole-co-aniline) (PBIANI),a self-cross-linked, net-structured, proton conducting PBItype of polymer membrane (Fig. 24) for high temperatureproton exchange membrane fuel cells (HT-PEMFC).

Poly(2,5-polybenzimidazole) (AB-PBI) has a uncom-plicated structure compared to PBI. The polymerizationprocedure of ABPBI was carried out using a single monomer(3,4-diaminobenzoic acid). Recent efforts were made tosynthesize AB-PBI in polyphosphoric acid [96,102,107,108]or in a P2O5–MSA mixture [98]. Fig. 25 shows the syntheticroute for the fabrication of AB-PBI.

3.1.4.3. Casting of the membrane. There are two types ofdirectly cast membranes. The membranes which can becasted directly from polyphosphoric acid are called PPA-cast membranes and those can be casted from a mixtureof phosphoric acid and trifluoroacetic acids are called TFA-cast membranes. The membranes casted from an organicsolution require further doping with phosphoric acid. Thetypically used organic solvent is N,N-dimethylacetamide

(DMAc), and membranes obtained are referred to as DMAc-cast membranes.

PPA-cast membranes: As discussed above, polyphospho-ric acid (PPA) is used as an efficient condensation reagentand solvent for the synthesis of PBI. Xiao et al. [100,109]developed a sol–gel process to fabricate PBI–H3PO4 mem-branes directly from a PBI solution in PPA at approximately200 ◦C, without isolation or re-dissolution of the polymerafter synthesis. After casting, the hydrolysis of PPA was car-ried out to transform it into phosphoric acid, resulting inphosphoric acid-doped PBI membranes.

TFA-cast membranes: PBI powder was first mixed withtrifluoroacetic acid. After heating under reflux for a fewhours, a certain amount of H3PO4 was added to the PBI solu-tion. The resulting solution was filtered and casted ontoa glass plate under a nitrogen atmosphere to obtain themembrane. The membrane was then dried at room tem-perature under vacuum.

DMAc-cast membranes: DMAc casted membranesshould be doped with acid to make them suitable candi-dates for high temperature PEM. Several kinds of acidswere studied till date [110–115], however, doping withphosphoric acid has some benefits (excellent thermal sta-bility, very low vapor pressure at elevated temperatures,unique proton conductivity, etc.) over other conventionalacids, such as HCl, HNO3, HClO4 and H2SO4.

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Fig. 25. Synthesis of AB-PBI [96] (Copyright Elsevier Science Ltd., UK, 2009).

3.1.4.4. Modification of membrane. The conductivity of PBIgenerally increases with increasing acid-doping level butwith a concomitant decrease in mechanical strength. Theoptimum doping level is thus a compromise between thesetwo effects. Different methods have been employed toimprove the proton conductivity without sacrificing themechanical strength or vice versa. These methods includeionic and covalent cross-linking of the polymeric mem-branes.

Ionic cross-linking: Flexible ionomer networks can beprepared from acid–base polymers by the ionic cross-linking of polymeric acids and polymeric bases [116].Sulphonated polysulphone (SPSF) [117,118], sulphonatedpolyetheretherketone (SPEEK) [119], sulphonatedpoly(2,6-dimethyl-1,4-phenylene oxide) (SPPO) [120],and sulphonated poly(arylene thioether)s [121] are theacidic polymers typically used to modify basic PBI throughionic cross-linking. Precipitation of a polysalt occurswhen both acidic and basic polymers are dissolved ina common solvent for membrane casting. To overcomethe aforementioned problems, the acidic polymer isusually prepared in a neutralized form. Basically the ioniccross-linked membrane shows poor thermal stability andreadily ruptures at elevated temperatures, resulting inunacceptable swelling and mechanical instability.

Covalent cross-linking: The covalently cross-linked poly-benzimidazole is tougher than non-cross-linked analoguesand exhibits improved compaction resistance during pro-longed use at higher pressures. For fuel cell applications,

dibromo-p-xylene (DBpX) [122,123] is used as a cross-linker for PBI membranes. Above 250 ◦C, cross-linkedmembranes not only show higher proton conductivity butalso improved mechanical strength.

3.2. Property assessment of the membrane

A discussion of properties associated with high tem-perature proton exchange membranes, such as protonconductivity, mechanical property, water uptake, swellingstudy, and the cell performance as well as the durabilityhas been carried out in detail.

3.2.1. Systematic analysis of proton conductivity ofpolymeric membranes

Proton conductivity is a very important tool for deter-mining the performance or utility of a membrane asa candidate for fuel cells. The membranes are sand-wiched between two Teflon plates with platinum wiresas electrodes. The measurements were carried out attemperatures ranging from 20 to 150 ◦C under properhumidification. The proton conductivity was calculatedusing the following equation [124,125]:

� = L/Rdw

where, L is the distance between the electrodes; d and ware the thickness and width of the membrane, respectively;R is the measured resistance. Fig. 26 shows a schematic

Fig. 26. Cell structure for the proton conductivity measurements [138] (Copyright Elsevier Science Ltd., UK, 2008).

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832 S. Bose et al. / Progress in Polymer Science 36 (2011) 813–843

illustration of the cell used for the proton conductivity mea-surements.

Kreuer et al. [15] reported that imidazole intercalatedsulfonated polyetherketone membranes had a proton con-ductivity 0.01 and 0.02 S/cm at 120 and 200 ◦C, respectively.The polymers and liquids containing imidazole and pyra-zole involved in very fast proton conduction processes.

Sulfonation of polysulfone strongly affects both pro-tonic conductivity and lifetime of composite polyelec-trolytes. Sulfonated polysulfone with 8% of phosphatoanti-monic acid resulted in a conductivity trebling 0.06 S cm−1

versus 0.02 S cm−1 at 80 ◦C and 98% of relative humidity(RH) [46]. The study was focused on mastering PSF sulfona-tion and its composite performance optimization.

Miyatake et al. [47] found that the Poly(arylene ethersulfone)-based membranes showed comparable protonconductivity to that of the perfluorinated ionomer mem-brane (Nafion 112) under a wide range of conditions(80–120 ◦C and 20–93% relative humidity (RH)). The high-est proton conductivity of 0.3 S/cm was obtained at 80 ◦Cand 93% RH. Although the proton conductivity value wasdecreased after long treatment, but the values were still atacceptable levels for fuel cell operation. The membranesretained their strength, flexibility, and high molecularweight. the efficiency of Poly(arylene ether sulfone)-basedmembrane as compared to Nafion in a fuel cell at elevatedtemperature were demonstrated.

Miyatake et al. [56] prepared membranes containingpolyimide with pendant sulfophenoxypropoxy groups. Themembranes exhibited twice the proton conductivity (ca.0.2 S/cm) of Nafion at lower temperatures. Moreover, withincreasing temperature, the proton conductivity of themembranes increased and reached 1.00 S/cm at 120 ◦C.Synthesis and properties of a novel polyimide electrolytehaving longer pendant sulfophenoxypropoxy groups werereported. The ionomer membrane showed very high protonconductivity of 1.0 S/cm at 120 ◦C and 100% relative humid-ity, which was higher than that of a Nafion membrane.

Honma et al. [59] found that the proton conductivity ofPolyalkoxysilane/phosphotungstic acid composite exceeds0.002 S/cm level at 120 ◦C and 20% R.H. The results observedby Honma et al. revealed that the proton conductivity wasstable above 100 ◦C with a small dependence on the RH aswell as the presence of a robust conductive channel struc-ture within the flexible macromolecules.

PVA/SiO2/Silicotungstic acid (SiW) composite mem-brane exhibited reasonably high value of conductivity inthe temperature range of 80–100 ◦C [64]. The compos-ite was found to be thermally stable at high temperaturebecause of the inorganic silica framework in the matrix.The aim of the work was to synthesize and characterizethe organic–inorganic nanocomposite membrane based onsilicotungstic acid.

Ye et al. [126] inspected the change in proton conductiv-ity of sulfonated polyimides (SPIs) at temperatures rangingfrom 120 to 160 ◦C. At 120 ◦C, the conductivity of SPIs waslower than those of Nafion 115. At 140 ◦C, the SPI mem-branes demonstrated higher conductivity than the Nafion®

115 membranes, and a similar trend was observed whenthe temperature was increased to 160 ◦C. In this paper,a series of random sulfonated polyimides with controlled

sulfonation degrees were synthesized. The physical prop-erties and proton conductivities of the membranes at 70 ◦Cand high temperatures above 100 ◦C were investigated, andtheir potential application for high temperature PEM fuelcells was explored.

Sulfonated poly(ether sulfone) (SPES)/boron phosphate(BPO4) composite membranes are promising candidates asPEMFCs at high temperatures, as reported by Wen et al.[79]. SPES/BPO4 composites exhibited lower proton con-ductivity than Nafion below 100 ◦C, but the conductivityof Nafion decreased sharply above 100 ◦C due to dehydra-tion. On the other hand, a steady increase in conductivitywas observed for SPES/BPO4 composite membranes, whichmight be due to the increased mobility of water, structuralreorientation and increased molecular mobility [32,127].For example, the conductivity of a SPES/BPO4 membraneincreased from 0.032 to 0.038 S/cm when the temperaturewas increased from 100 to 120 ◦C. SPES/BPO4 compositemembranes were synthesized by sol–gel method for usein high-temperature PEMFCs, followed by investigation ofthe thermal stability, mechanical behavior, water uptake,oxidative stability, morphology and proton conductivity ofthe composite membranes.

Zhang et al. [128] reported that the proton conduc-tivity of the SPFEK–SiO2–HPMC hybrid membranes weredecreased from 0.0254 to 0.0198 S/cm with increasingtemperature from 80 to 120 ◦C at 50% RH, which werestill 10.4% and 11.2% higher than that of Nafion 117. Theabovementioned phenomenon could be explained by thestate of water molecules and the properties of SiO2 andHPMC. Water molecules exist as a liquid at temperaturesbelow 100 ◦C but evaporate at temperatures above 100 ◦C.Because of the presence SiO2 with Lewis acid sites andHPMC with water retention properties, the hybrid mem-brane showed better electrochemical performance at hightemperatures.

Variation in protonic conductivity of zirconium hydro-gen phosphate/disulfonated poly(arylene ether sulfone)with temperature was demonstrated by Hill et al. [129].The study dealt with the in situ blending of zirco-nium hydrogen phosphate with directly polymerizeddisulfonated poly(arylene ether sulfone) copolymers toform transparent organic/inorganic nanocomposite hybridPEMs. Introduction of an inorganic component into pro-ton exchange membranes led to improve the propertiesby potentially decreasing the water uptake, while increas-ing the protonic conductivity, modulus and mechanicalstrength of the concerned membrane.

Ghil et al. [130] synthesized phosphonic acid func-tionalized poly(dimethyl siloxane) membrane for hightemperature proton exchange membrane fuel cells andreported that the proton conductivity of the membrane(APP 414) was 0.072 S/cm at 130 ◦C, which is fairly goodfor high temperature operation of PEMFC. However, theproton conductivity of Nafion 112 decreased significantlyabove 100 ◦C (Fig. 27).

Protonic conductivity of the BPSH/ZrP compositemembranes was lower than unmodified BPSH at roomtemperature, but, the conductivity was improved at hightemperatures, thereby corroborating the stabilizing behav-ior of ZrP and hence the degree of dehydration reduced at

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Fig. 27. Proton conductivity of the Nafion112 and APP membrane [130] (Copyright Elsevier Science Ltd., UK, 2006).

high temperatures. The decrease in protonic conductivityabove 110 ◦C for the pure BPSH-40 copolymer was proba-bly due to the hydrated glass transition temperature (Tg)of the copolymer [131].

Kim et al. [132] used zirconium sulphophenyl phos-phate (ZrSPP) particles as a solid proton conductor anda hygroscopic material to enhance the proton conductiv-ity of Nafion at high temperatures (>100 ◦C). The protonconductivity as a function of temperature and RH% wasinvestigated. It was concluded that ZrSPP particles in theNafion matrix did not impede proton conductivity of matrixbelow 90 ◦C at high RH% but acted as a hygroscopic materialin the temperature range of 90–110 ◦C.

He et al. [133] established the mechanism of protonconductivity upon phosphoric acid doping and the protonconductivity of phosphoric acid-doped polybenzimidazoleand its composites with inorganic proton conductors. Theconductivity of phosphoric acid-doped PBI and PBI com-posite membranes was found to be dependent on theacid doping level, relative humidity (RH) and temperature[110,134]. A conductivity of 6.8 × 10−2 S/cm was observedfor PBI membranes with a H3PO4 doping level of 5.6 (molenumber of H3PO4 per repeat unit of PBI) at 200 ◦C and 5%RH. Moreover, a higher conductivity of 9.6 × 10−2 S/cm wasobtained for a ZrP/PBI membrane under the same condi-tions. H3PO4 exhibits effective proton conductivity even inan anhydrous form due to its unique proton conductionmechanism by self-ionization and self-dehydration [135]

5H3PO4 = 2H4PO4+ + H3O+ + H4PO4

− + H2P2O72−

In the case of the acid-doped PBI, hydrogen bonds wereestablished between the polymer and acid. In other words,PBI can act as a solvent that facilitated the dissociation ofphosphoric acid.

Lin et al. [60] suggested that the hybridization of Nafionmembranes using ZrP nano-particles helped to retain mois-ture at temperatures of 110–130 ◦C with low humidity. Thelower conductivity of NF–ZrP 117 and NF–ZrP 234 thanNF–ZrP 39 could be attributed to less impregnation of theNafion/ZrOCl2 solution into porous PTFE film.

Gosalawit et al. [136] proposed a Krytox-Si–Nafionhybrid membrane that can maintain the proton conduc-tivity over a wide range of temperatures (80–130 ◦C). Atroom temperature, the composite membrane under con-sideration showed lower proton conductivity (∼10−4 S/cm)compared to the Nafion membrane. However, the mem-brane could maintain its proton conductivity at 130 ◦C,whereas the Nafion membrane at elevated temperatureshowed a significant decrease in conductivity due to dehy-dration. Indeed, at high temperatures, the absorbance ofwater by the hybrid membrane might play a key role inmaintaining the proton conductivity [137]. However, theproton conductivity increased with increasing Krytox-Sicontent compared to hybrid membrane with low silica con-tent, which was probably due to the retention of water bysilica at higher temperatures [73].

The proton conductivity of Nafion/sPPSQ nanocompos-ite was examined by Nam et al. [138]. Nafion/5 wt% sPPSQcomposite membrane showed a proton conductivity of1.57 × 10−1 S/cm at 120 ◦C, which was higher than thatof Nafion. The decrease in the conductivity of Nafion 115above 100 ◦C might be attributed to the decrease in unas-sociated sulfonic groups. The reason behind the decreasein sulfonic groups at high temperature was the deforma-tion of fluoro-backbone [139,140]. An attempt was madeto utilize a Nafion-based composite membrane for directdimethyl ether fuel cell (DDMEFC) applications operatedabove 100 ◦C.

The proton exchange membrane based on Nafionand silica (SBA-15) had a proton conductivity of8.52 × 10−4 S/cm at low humidity of ∼10% and 140 ◦C[142]. At higher temperatures, the triazole groups,attached to the nanopores of SBA-15, played a key role inaligning the SBA-15 channels through which proton trans-fer might be facilitated. SBA-15 was utilized to immobilizeheterocyclic compounds having carboxylic acid, such asbenzotriazole-5-carboxylic acid (BT) by covalent bondingand in the process proton conductivity was improvedat elevated temperature. SBA-15 is well known to havehigh water retention capability in its nano-sized pores at

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high temperatures as compared with membranes such asNafion, and this would be an additional help to increasethe proton conductivity at high temperatures.

Pu et al. [141] revealed that the water evaporation fromthe composite membrane with a hollow nanosphere wasquite difficult compared to that from the correspondingpure polymer membranes at higher temperatures. The rea-son behind the water retention by hollow nanospheres in acomposite membrane was that hollow nanospheres with ahole on the shell acted like a water reservoir and providedthe necessary water after 100 ◦C because the boiling pointof water in a nano-space or through a nano-hole might beabove 100 ◦C.

Kim et al. [143] evaluated the feasibility of HPA/BPSHcomposite membranes for use in proton exchange mem-brane (PEM) fuel cells. Hydrated membranes consistingof 30 wt% HPA and 70 wt% BPSH had a conductivity of0.15 S/cm at 130 ◦C. In contrast, the pure copolymer had aproton conductivity of 0.07 S/cm at room temperature andcan reach a maximum conductivity of only 0.09 S/cm, mostlikely due to dehydration at elevated temperatures.

Wilhelm et al. [144] synthesized hybrid membranescontaining SO3H-functionalized mesoporous Si-MCM-41as a hydrophilic inorganic modifier in a polysiloxane matrixand measured the proton conductivity at temperatures ashigh as 180 ◦C. At 180 ◦C, pure polysiloxane showed protonconductivity one order of magnitude lower than Nafion.However, in the presence of SO3H-modified Si-MCM-41,the proton conductivity was higher than that of Nafionmembranes by a factor of 10. The proton conductivity ofsulfonated particles was modelled on the atomic scale inorder to understand the influence of the density of sulfonicacid groups and of the presence of water molecules.

Gomes et al. [80] synthesized novel nanocompositemembranes using sulfonated polyoxadiazole and sul-fonated dense and mesoporous (MCM-41) silica particles. Aproton conductivity of 0.034 S/cm at 120 ◦C was obtained,which was approximately twice the value of the pristinepolymer membrane.

The proton conductivity of SulfonatedPBIBI/poly(vinylidene fluoride) blend membrane con-taining 10 wt% PVDF was comparable to that of Nafion 117in the temperature range of 20–140 ◦C [87]. Introductionof PVDF in the SPBIBI matrix altered the morphologicalstructure of the blend membranes, which led to theformation of improved connectivity channels and in theprocess the proton conductivity of the blend was enoughto be effective in a fuel cell.

As reported by Fu et al. [93], the conductivity of theSPEEK/PSf–BIm blend membranes increased with increas-ing temperature due to the presence of benzimidazoletethered onto polysulfone. The pendant benzimidazolecould act as a ‘bridge’ to promote proton conductionbetween sulfonic acid groups under low relative humid-ity conditions. Also, the proton conductivity increases asthe DC of polysulfone to which benzimidazole is tetheredincreases, confirming the role played by benzimidazole onproton conduction. The author proposed a novel strategy inwhich the benzimidazole group is attached to an aromaticpolymer like poly(sulfone), which exhibits good stabilityand local mobility.

A detail demonstration of literature survey has beenpresented in a systematic way analyzing the proton con-ductivity of different types of membrane. The protonconductivity membranes are sufficient enough for hightemperature fuel cell applications. However, further scopeof research still exists from the view point of commer-cialization of the membranes. Table 1 summarizes thedeviation in the proton conductivity of different mem-branes used in fuel cell under high temperature operation.

3.2.2. Mechanical property assessmentThe mechanical properties of the membrane affect

the manufacturing conditions of the membrane electrodeassembly (MEA) and the durability of the high-temperaturePEMFC, since the temperature, pressure and humidificationvary frequently during PEMFC operation. Therefore, deter-mination of mechanical stability of the membrane is one ofthe crucial parameter in determining the performance ofthe membrane under elevated temperature.

Wen et al. [79] reported an improvement in themechanical properties of SPES/BPO4 composite membranecompared to Nafion at 100 ◦C. The Young’s modulus ofSPES/BPO4 (40wt%) membrane was 1.10 GPa, which washigher than that of the Nafion 112 membrane. The tensilestrength of the abovementioned composite was approxi-mately 17.1 MPa at 100 ◦C, which was higher than Nafion.At high temperatures, the Young’s modulus and tensilestrength of the composite membranes decreased slightlycompared to that at room temperature but the elongationat break increased considerably at elevated temperatures.This was attributed to the increase in chain flexibilityat high temperature. The observation corroborated thatthe composite membranes were strong and tough at hightemperatures, making them promising for use in high tem-perature PEMFCs.

Shao et al. [147] reported that the Nafion aided byinorganic oxides resulted in the formation of brittle mem-branes that exhibited improvement in tensile propertiesat elevated temperatures, indicating that inorganic parti-cles have a significant degree of interconnection with themembrane at elevated temperatures.

Kim et al. [58] examined the mechanical stability ofSiO2 aided- sulfonated poly(fluorinated aromatic ethers)membranes at elevated temperatures. They found thatthe membrane had excellent flexibility without tearingwhen rolled. However, at a 4 wt% SiO2 loading, the ten-sile strength of the composite membrane (20.1 MPa) wascomparable to that of Nafion (21.04 MPa) at elevated tem-peratures. Therefore, a membrane with higher tensilestrength may endure high temperature operation for alonger time.

Bhadra et al. [103] reported the effect of phos-phoric acid doping on the mechanical properties ofa hyperbranched poly(benzimidazole-co-benzene) (PBIB)membrane. According to their observation, the stress atbreak for the phosphoric acid doped PBIB (DPBIB) mem-brane showed a value of 29 ±3 MPa, which was comparableto that of Nafion (28 ±2 MPa).

Wang et al. [87] examined a new blend system consist-ing of an amorphous sulfonated SPBIBI and semi-crystallinepoly(vinylidene fluoride) (PVDF) for proton exchange

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Table 1Proton conductivity of different membranes.

Types of membrane Operationaltemperature (◦C)

Relative humidity (%) Protonconductivity (S/cm)

Reference

Functionalized PDMS (APP 414) 130 100 0.072 [130]SPES/BPO4 composite 120 – 0.038 [79]SPFEK–SiO2– HPMC hybrid membrane 120 50 0.0198 [128]Disulfonated poly(arylene ether sulfone)/ZrP

composite130 100 0.130 [129]

Sulfonated polyimides 140 10–20 .0005 [126]160 5–12 .002

Nafion/ZrSPP composite 110 50 ≥0.005 [132]98 ≤0.05

PBI/ZrP composite 200 5 0.096 [133]S-polyoxadiazole/mesoporous silica (MCM-41) 120 25 0.034 [80]Krytox-Si–Nafion hybrid membrane 130 Ambient condition 1.72 × 10−4 [136]Nafion/sulfonated poly(phenylsilsesquioxane) (sPPSQ)

nanocomposite120 100 0.157 [138]

Nafion/silica (SBA-15) 140 10 8.52 × 10−4 [142]Heteropolyacid (HPA)/sulfonated BPSH composite 130 – 0.15 [143]Polyimide Containing Pendant Sulfophenoxypropoxy

Groups120 100 1.00 [56]

poly(benzimidazole-co-aniline) 120 100 0.167 [106]PPO/poly(styrene-b-vinylbenzylphosphonic acid) 140 100 0.28 [91]Perfluorocyclobutyl containing polybenzimidazoles 140 Without humidification 0.12 [145]polybenzimidazole (PBI) containing bulky basic

benzimidazole side groups180 Without humidification 0.16 [146]

Imidazole intercalated into sulfonatedpolyetherketone membrane

120 Without humidification 0.01 [15]200 Without humidification 0.02

membranes, and the mechanical properties of the blendwere analyzed. The tensile strength, Young’s modulus andelongation at break of the materials ranged from 40.2 and98.4 MPa, 0.32–1.07 GPa and 35.8–100.6%, respectively,indicating sufficient strength and toughness for fuel cellapplications. The fascinating flexibility of the concernedmembrane indicated the role of sulfonic groups and rota-tion about the central biphenyl bond [148,149].

Noye et al. [150] synthesized phosphoric acid dopedpolybenzimidazole (PBI) membranes cross-linked withdichloromethyl phosphinic acid (DCMP), and found thatthe cross-linked membranes showed increased mechanicalstrength. The tensile strengths of the cross-linked PBI-DCMP membranes were significantly higher than those ofthe linear PBI.

Shang et al. [151] demonstrated the change in tensilestrength of a sulfonated fluorene-containing poly(aryleneether ketone) membrane. Both the maximum tensilestrength and tensile strength at break were decreasedslightly with increasing degree of sulfonation due to theincrease in level of water absorbance in the membrane.The absorbed water acts as a plasticizer or solvent forthe membrane resulting in a slight reduction of mechan-ical strength. Although there was a slight decrease in themechanical properties with increasing sulfonation level,the values were still higher than those of Nafion at highertemperatures.

Chuang at al. [152] reported the effectiveness ofthe Polybenzimidazole (PBI)/imidazole (Im) hybrid mem-branes as PEMs at elevated temperature. Pure PBI film had atensile modulus of 1.74 GPa, a tensile strength of 90.7 MPa,and an elongation at break of 10.0%. However, the mechan-ical properties deteriorated in the presence of excess Im(30 wt%). Indeed, in presence of excess filler, inhomo-

geneity in the structure caused phase separation and adecrease in mechanical properties. However, the mechan-ical properties and proton conductivity were reasonablein the presence of 10–20 wt% Im. If both the conductivityand mechanical properties were considered, the appropri-ate Im content in the fluorine-containing PBI should be 20per repeat unit of PBI.

Phosphoric acid-doped PFCB–PBI membranes preparedusing a direct casting method exhibited a tensile strengthand elongation at break of 0.35 MPa and 130%, respectively[145]. These membranes are strong enough to be fabricatedinto membrane electrode assemblies.

3.2.3. Water uptake and swelling studyThe ability of water uptake by various types of polymeric

membranes is one of the important factors in determin-ing the effectiveness of membrane under high temperatureoperation. The water uptake of the polymer membranes ata given RH is affected by various parameters as follows:

I. Temperature [153],II. Dissociation constant and number of conducting groups

[154],III. Type of counter ions [155],IV. Elasticity of the polymer matrix,V. Hydrophobicity of the polymer surface, and pre-

treatment of the membrane [156].

In general, the proton conductivity of the polymer mem-brane increases with increasing water uptake. Basically, thecreation of a hydrophilic domain facilitates water uptakeand in the process the proton conductivity is increased[157]. However, excess water uptake most often results in ahigher degree of membrane swelling, leading to a slump in

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mechanical stability. Moreover, a higher degree of swellingalso leads to high methanol permeability. Therefore, opti-mization of the water uptake and polymer swelling isessential for successful operation in a fuel cell at high tem-peratures. To measure the percentage water uptake, themembranes are dried in a vacuum oven, weighed (Wdry),and then immersed in deionized water at different tem-peratures. The wet membranes are then blotted to removethe surface water droplets and weighed quickly (Wwet). Thewater uptake of the membranes is calculated as follows:

Water uptake (%) =[

Wwet − Wdry

Wdry

]× 100%

Wen et al. [79] reported the percentage water uptake inthe SPES/BPO4 composite membranes with different BPO4loadings. The water uptake also increased with increas-ing BPO4 content, which was similar to Krishnan et al.[127]. An increase in water uptake can be attributed tothe excellent hydrophilicity of boron phosphate because itcontains various types of hydroxyl groups [158]. The com-posite membranes showed significantly more water uptakeat higher temperatures than the Nafion 112 membraneindicating that the SPES/BPO4 composite membranes canbe used as promising electrolytes for PEMFCs, particularlyunder high temperatures and low humidity conditions.

Hybrid membranes of polysiloxane matrix (BIS) aidedby SO3H-functionalized mesoporous Si (MCM-41) particleshad higher surface areas compared to that of particle-free polysiloxane and Nafion 117 membranes. The initialwater uptake of pure polysiloxane was much lower andwas affected by the incorporation of inorganic particles,as observed by Wilhelm et al. [144]. Furthermore, theisotherms of pure polysiloxane and hybrid membranewith 9.6% particles exhibited water uptakes of 8.2 wt.%and 8.7 wt.%, respectively. However, at higher filler load-ings (17.1% of SO3H-MCM), the water uptake increased to12.5 wt% due to capillary condensation of water vapor inthe mesopores of embedded particles.

Kannan et al. [159] demonstrated the water retentionproperties of Nafion/phospho-silicate hybrid membranes.The water uptake of hybridized Nafion membranes wassubstantially higher than that of raw Nafion. For the hybridmembrane and raw Nafion, the water retention was 15.8and 4.5 wt%, respectively. This might be due to the stronghydrophilic nature of phospho-silicate gels, holding waterby forming hydrogen bonds. Similar observations weremade with phospho-silicate-incorporated sulphonatedpoly(2,6-dimethyl-1,4-phenylene oxide) [160,161].

Jalani et al. [162] synthesized nafion-MO2 (M = Zr,Si, Ti) nanocomposite membranes and examined thewater retention properties at high temperatures. At120 ◦C, all Nafion–MO2 nanocomposites exhibited betterwater uptake than the unmodified Nafion membrane. Anafion–ZrO2 membrane demonstrated 45% higher wateruptake, whereas the Nafion–SiO2 membrane exhibited 15%improvement in water uptake compared to pure Nafion at120 ◦C. The enhanced water uptake can be attributed to thehydrophilic nature of the acidic inorganic additives withinthe pores of the Nafion membrane.

Tian et al. [163] reported water retention of 50.3% forsulfonated poly(phthalazinone ether ketone) (SPPEK) at100 ◦C compared to 22.8% at 30 ◦C. The swelling ratio ofthe membrane under consideration was 49.6% and 31.6% at100 and 30 ◦C, respectively. The water uptake and swellingratio increased with increasing sulfonation indicating thestrong hydrophilicity of sulfonic acid groups.

Bai et al. [164] evaluated poly(ethylene oxide) softsegment-containing sulfonated polyimide (SPI) copoly-mers for high temperature PEM. The water retentionproperties of PEO-containing SPI membranes were muchhigher (42.9%) than those of Nafion 115 (22.9%) due totheir higher IEC values. However, increment in sulfonic acidconcentration (2.02–2.87 mmol/g) with decreasing PEOcontent (20–2 mol%) lead to increase in membrane wateruptake from 42.9 to 65.5%. Moreover, as observed from theliterature [165], the water uptake of these new SPI mem-branes was much higher than other non-PEO-containingSPI membranes.

3.3. Performance evaluation of the membrane

The high temperature polymer electrolyte membraneshould not only have excellent proton conductivity ormechanical stability but also should exhibit better perfor-mance. Optimum performance of the PEM is one of thecrucial requirements for high temperature utilization.

3.3.1. Single cell performance of the membraneSingle cell test is one of the important parameter from

the perspective of performance of the membrane at ele-vated temperature.

Shao et al. [72] investigated the single cell performanceof a Nafion–silica membrane doped with phosphotungsticacid at 110 ◦C, and observed that the composite membranesshow better cell performance than Nafion 115 membranes.At a potential of 0.4 V, the cell constitutes of PWA-dopedNafion/SiO2 and the un-doped Nafion/SiO2 membranes hadcurrent densities of 540 and 320 mA/cm2, respectively. Onthe other hand, the cell with the Nafion 115 membranedelivered only a nominal current density of 95 mA/cm2. Thereduced cell performance of Nafion 115 was attributed tothe higher membrane resistance in the cell.

A silica-doped sulfonated poly(fluorenyl ether ketone)s(SPFEK) membrane using hydroxypropyl methyl cellulose(HPMC) as a dispersant showed the highest power den-sity of 260 mW/cm2 at 120 ◦C and 50% RH, whereas the cellwith the Nafion 117 membrane had a highest power den-sity of 16 mW/cm2 under the same conditions mentionedabove [128]. However, at 80 ◦C and 50% RH, the cell perfor-mance of the membrane was only slightly better than theperformance at 120 ◦C, which may be due to the followingfactors:

I. An increment of �G (change in Gibbs free energy) valueat high temperatures, and

II. An increase in the stack resistance due to rapid waterloss in the membrane at high temperatures [72].

Amjadi et al. [62] measured the cell performance ofNafion/TiO2 membrane at 110 ◦C. At this temperature,

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Table 2Brief details of the water retention property and single cell performance.

Types of membrane Operational temperature (◦C) Water retention (%) Cell voltage (V) Current density (mA/cm2) Reference

PWA-doped Nafion/SiO2 110 38 0.4 540Nafion/SiO2 110 34 0.4 320 [72]Nafion 115 110 26 0.4 95SPFEK–SiO2–HPMC 120 48.51 0.1 700SPFEK–SiO2 120 38.94 0.1 250 [128]SPFEK 120 30.15 0.1 150Nafion/TiO2 110 ∼ 35 (at 80 ◦C) 0.4 200 [62]sulfonated PEEK-WC 120 14 (at 80 ◦C) 0.2 900 [45]Doped-PBIANI 130 – 0.4 100 [106]ABPBI/LiCl (1:2) 150 – 0.3 1180 [167]H3PO4-doped PFCB–PBI 140 46–47 0.5 200 [145]

PEO-containing SPI 120 42.9 0.5 640 [164]0.4 880

pure Nafion showed very poor performance, whereasthe composite membranes exhibited a current density of∼200 mA/cm2 at 0.4 V. However, the performance deteri-oration of pure Nafion at 110 ◦C might be related to theglass transition temperature of Nafion. Increasing the tem-perature above the Tg of Nafion (91 ◦C) leads to membraneshrinkage and a significant decrease in cell performance.Another reason for the deterioration of cell performance atelevated temperatures is probably due to augmentation inmembrane resistance.

The performance of the phosphoric acid-doped(CsPOM)/polybenzimidazole (PBI) membrane as a proton-exchange membrane fuel cell (PEMFC) was better than thatwith a phosphoric acid-doped PBI membrane at 150 ◦C, asreported by Li et al. [65].

Paturzo et al. [45] demonstrated the cell performanceof a sulfonated PEEK-WC membrane at 120 ◦C. The sul-fonated PEEK-WC membrane showed a higher currentdensity (∼900 mA/cm2) than that obtained for Nafion117 (700 mA/cm2). The lower cell voltage exhibited bythe sulfonated PEEK-WC membrane in the low currentdensity zone might be due to the occurrence of para-sitic reactions, as suggested by Lufrano et al. [166]. At120 ◦C, the Nafion membrane showed a power density of275 mW/cm2, whereas, the value for the sulfonated PEEK-WC membrane was 284 mW/cm2.

Ong et al. [167] examined the single-step fabricationof ABPBI-based GDE as well as its MEA characteristics forhigh-temperature PEM fuel cells. The best fuel cell perfor-mance was recorded at a ABPBI/LiCl ratio of 1:2 with a cellpower density of 0.37 W/cm2. The performance began todecrease with further increasing LiCl content.

Wang et al. [168] investigated the fuel cell performanceof phosphonic acid-functionalized silica/Nafion compositemembrane. At 120 ◦C and 35% RH, the cell performancewas improved by 80 mV at 0.8 A/cm, but under wet condi-tions, the cell performance was not adequately improvedbecause the loss in mass transport in the stack was morepronounced under wet conditions. Table 2 lists the waterretention properties and single cell performance of the dif-ferent membranes at high temperatures.

3.3.2. Durability measurements of the membraneDurability plays an important part in deciding the

life time of PEMs at elevated temperatures. Basically,

degradation of the membranes is classified as chemi-cal/electrochemical degradation and physical degradation.During the chemical/electrochemical process, the decom-position of hydrogen peroxide results in the formationof intermediate products, such as HO• and HO2

•, whichcauses membrane degradation [169]. Indeed, there aretwo pathways for the generation of free radical speciesfrom hydrogen peroxide. Firstly, generation at the cathodedue to the electrochemical two-electron reduction of oxy-gen [170], and secondly, generation at the anode owingto chemical combination of crossover oxygen and hydro-gen at the anode [171,172]. Till date several approacheswere adopted to improve the membrane durability. Thepassive approach was to improve the polymer stabilityby synthesizing short side chain polymers [173,174], andadopting novel hydrocarbon polymer electrolytes [175],and the active approach was to suppress free radicals attackby avoiding hydrogen peroxide formation or by destroy-ing hydrogen peroxide [176–179], and by scavenging freeradicals [180].

4. Application of HT-PEMFC

PEM fuel cell systems are currently used in differentapplications however, the end-uses can be classified in tothree main groups:

I. Transportation (including niche applications, light dutymarkets and buses),

II. Stationary (large and small applications), andIII. Portable.

However, owing to the problems associated with thelow temperature operation it is the necessary to change thedirection of research from low temperature PEMFC to hightemperature PEMFC. In the near future HT-PEMFC technol-ogy could be a commercialization game changer. Protonexchange membrane fuel cells (PEMFCs) operating at rela-tively high temperatures are very promising for industrialuse. They have high power density, can vary their out-put rapidly to meet shifts in the power demand and arewell suited especially for applications in the automotiveindustry. Moreover, working at a high temperature couldincrease drastically the fuel cell CO tolerance, simplifyinga reformer based fuel cell system and drastically reducing

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838 S. Bose et al. / Progress in Polymer Science 36 (2011) 813–843

Fig. 28. An electrode-electrolyte membrane integrated system using a pore-filling membrane with inorganic substrates.

the cost. HT-PEMFC stacks can be utilized for OEM integra-tors.

5. Future design concept of HT-PEFMC

Various polymers were synthesized and tested fortheir proton conductivity, mechanical stability, electrode-membrane interface connectivity, etc., aiming at improve-ment in membrane performance for high temperaturefuel cell applications. These efforts seem to continue withinsightful vision for commercialization the membrane aswell as to reduce the cost of PEM at elevated temperature.The future design concept of HT-PEMFC will open up newpromising avenues for further research and developmentin the following areas.

5.1. Pore-filling electrolyte membranes

Pore-filling membrane concept can be applied tohigher-temperature PEMFCs using different substrates andfilling polymer materials having thermal and electrochem-ical durability. In addition, incorporation of an inorganicsubstrate makes it possible to further enhance the thermalstability and a thin fragile ceramic substrate can be used foran integrated membrane-electrode assembly system. Thisconcept can be utilized to design a suitable high tempera-ture PEM for different applications by choosing a substrateand filling polymer electrolyte (Fig. 28).

5.2. Increasing catalytic activity

Platinum is by far the most effective element used forPEM fuel cell catalysts and nearly all current PEM fuel cellsutilize platinum on porous carbon to facilitate both hydro-gen oxidation and oxygen reduction. There are several waysto improve the catalytic activity of platinum:

I. Optimizization of the size and shape of the platinumparticles.

II. High-index facets of platinum nanoparticles provide agreater density of reactive sites for oxygen reductionthan typical platinum nanoparticles [181].

III. Alloying of Pt with other metals. For example, it wasrecently shown that the Pt3Ni(111) surface has a higheroxygen reduction activity than pure platinum by a fac-tor of ten [182]. The authors attribute this dramaticperformance increase to the number of available sitesfor oxygen adsorption and reduction.

IV. Graphene-based clusters as support material for plat-inum. The nanometer-sized hydrographene specieshave a small energy gap which is the driving force forPt–C bond formation [183] and in the process catalyticactivity of platinum can be retained.

5.3. Selection of electrocatalyst

The Oxygen Reduction Reaction (ORR) at the cathode isthe kinetically slow process as compared to the HydrogenOxidation Reaction (HOR). Consequently, developing activecatalysts for the ORR is the focus of PEMFC electrocatalysis

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S. Bose et al. / Progress in Polymer Science 36 (2011) 813–843 839

research and development. Currently two approaches areunderway to address the issues of sluggish ORR activity:

I. Reduction in platinum loading in PEMFC catalyst layers(CLs) while maintaining high performance of HT-PEMFC,

II. Exploration of non-precious metal catalysts as a replace-ment of platinum. Owing to their high cost, currentPt/C catalysts are not feasible for commercialization.Therefore more researches should be performed withnon-precious, nano-sized metal electrocatalysts as asubstitute of platinum.

5.4. Hydrogen storage

Another important objective of the research revolvesaround the storage of hydrogen in a most convenient andsafe manner. The hydrogen as fuel for PEMFC should bestored carefully as it is highly flammable. Hydrogen canbe stored in a number of ways, including compressed gas,chemical compounds, liquid hydrogen or metallic hydro-gen. Metal hydrides can be stored till about two percenthydrogen by weight and, thus, can be bulky. Some carbonstructures are under investigation for hydrogen storage butthese have yet to be commercialized. Another approach,involves storing hydrogen as a liquid or a solid. In theprocess, high-purity hydrogen is being generated fromenvironment-friendly raw materials. Hydrogen storage asa solid involves the use of recycled plastics. In this process,pellets of sodium hydride that is coated with a waterproofplastic coating or skin. Once the plastic coating is fractured,the sodium hydride inside reacts vigorously with water toproduce hydrogen. In the coming years more effective wayof storing hydrogen in a safe way should be examined indetail.

However, in order to commercialize HT-PEM for fuelcell application extensive researches are also required inthe following areas: (a) Ceramic membranes for high-temperature fuel cells in order to improve sulphurtolerance and long-term stability, (b) Capture, storageand subsequent use of high-grade waste heat, and (c)Fuel chemical conversion using oxide materials for high-temperature catalysis.

6. Conclusions

The problems associated with the energy crisis andenvironmental pollution has been the issues of consid-erable concern. PEMFCs have been chosen as one of thepotential solution to overcome the afore-mentioned prob-lems. Fuel cells, Owing to their particular properties, are onthe verge of creating a vast revolutionary change in the fieldof electricity. The idea behind the development of high tem-perature PEMFCs originates from the drawbacks exhibitedby low temperature PEMFCs, such as, CO catalyst poisoning,heat and water management, problems regarding humid-ification etc. However, high temperature operation oftenleads to dehydration, which deteriorates the membraneperformance. Accordingly, several approaches have beenadopted to retain the water at high temperatures including

I. Incorporation of inorganic additives to hydrophobicpolymer membranes,

II. Use of non aqueous, low volatile solvents to replacewater as the proton acceptor within the polymer mem-brane,

III. Utilization of a solid state protonic conductor.

Different aspects of polymeric membranes whichcan be used as a high temperature proton exchangemembrane were reviewed in this article. The syntheticroutes and techniques for the membrane casting ofsulfonated hydrocarbon polymers, acid–base complexes,blend polymers and organic-inorganic composites mem-brane were explored in detail. Sulfonated hydrocarbonpolymers as well as blend polymers showed remarkableimprovement in mechanical properties and proton con-ductivity at elevated temperatures. The introduction ofmetallic oxides, such as MO2, into the polymer matrixincreased the water retention properties of membrane.The effect of phosphoric acid doping on the perfor-mance of PBI type membranes were analyzed. PBI typeacid–base complexes showed substantial improvement inthe electrochemical properties at temperatures as high as140 ◦C. Sulfonated hydrocarbons also showed good per-formance as high temperature PEMFCs owing to theirpromising proton conductivity and mechanical stabil-ity.

Acknowledgement

The study was supported by the NSL (NationalSpace Lab) program (S108A01003210) and WCU (WorldClass University) program (R31-2008-20029) through theNational Research Foundation of Korea funded by the Min-istry of Education, Science and Technology.

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