PEMFC electrolyte for

Sol-gel techniques have been successfidly applied to form fuel cell components with enhanced microstructures for high-temperature fuel cells. The apphcations were recently extended to synthesis of hybrid electrolyte for PEMFC. Although die results look promising, the sol-gel processing needs further development to deposit micro-structured materials in a selective area such as the triple-phase boundary of a fuel cell. That is, in the case of PEMFC, the sol-gel techniques need to be expanded to form membrane-electrode-assembly with improved microstructures in addition to the synthesis of hybrid membranes to get higher fuel cell performance. 

Over the last decade, several new proton exchange membranes have been developed. The new polymers in fuel cell applications are based mostly on hydrocarbon structures for the polymer backbone. Poly(styrene sulfonic acid) is a basic material in this field. In practice, poly(styrene sulfonic acid) and the analogous polymers such as phenol sulfonic acid resin and poly(trifluorostyrene sulfonic acid), were frequently used as polymer electrolytes for PEMFCs in the 1960s. Chemically and thermally stable aromatic polymers such as poly(styrene) [ 3 ], poly(oxy-1,4-phenyleneoxy-1,4-phenylenecarbony 1-1,4-phenylene) (PEEK) [4], poly(phenylenesulfide) [5], poly(l,4-phenylene) [6, 7], poly (oxy-1,4-phe-nylene) [8], and other aromatic polymers [9-11], can be employed as the polymer backbone for proton conducting polymers. These chemical structures are illustrated in Fig. 6.2. 

The electrolyte for PEMFC is an ion conduction polymer H" or proton is the mobile ion in the conduction process of PEMFC. Nation membrane forms the basis of electrolyte for PEMFC. The starting material for Nation is polyethylene. Polyethylene is modified using fluorine for hydrogen. The process of replacing hydrogen with fluorine is known as perfluorination. The process of perfluorination is shown in Fig. 1.10. AU four hydrogen molecules in the ethylene group are replaced by fluorine and the modified polymer is known as polytetrafluoroethylene (PTFE). 

PFSA membrane is by far the most studied proton electrolyte for PEMFC. There are three advantages to the use of PFSA membranes in PEMFCs. First, due to PTEE-based backbone, PFSA membranes are relatively strong and stable in both oxidative and reductive environments. In fact, durability of 60,000 h has been reported." Second, the proton conductivity achieved in PFSA manbrane can be as high as 0.13 S/cm at 75°C and 100% relative humidityA ceU resistance is as low as 0.05 cm for a 100 pm thick membrane with voltage loss of only 50 mV at 1 A/cm. " Third, PFSA has relatively good mechanical properties. For Nafion... 

Oxidation of Adsorbed CO The electro-oxidation of CO has been extensively studied given its importance as a model electrochemical reaction and its relevance to the development of CO-tolerant anodes for PEMFCs and efficient anodes for DMFCs. In this section, we focus on the oxidation of a COads monolayer and do not cover continuous oxidation of CO dissolved in electrolyte. An invaluable advantage of COads electro-oxidation as a model reaction is that it does not involve diffusion in the electrolyte bulk, and thus is not subject to the problems associated with mass transport corrections and desorption/readsorption processes. 

For PEMFCs, the solid electrolytes are polymer membranes polymers modified to include ions, usually sulfonic groups. One of the most widely used membranes today is the polymer Nafion , created by the DuPont company. These membranes have aliphatic perfluorinated backbones with ether-linked side chains ending in sulfonate cation exchange groups [6, 7], Nafion is a copolymer of tetrafluoroethylene and sulfonyl fluoride vinyl ether [8] and has a semi-crystalline structure [9], This structure (which resembles Teflon ) gives Nafion long-term stability in oxidative or reductive conditions. The sulfonic groups of the polymers facilitate the transport of protons. The polymers consist of hydrophilic and hydrophobic domains that allow the transport of protons from the anode to the cathode [10, 11],... 

Fuel cells are classified primarily according to the nature of the electrolyte. Moreover, the nature of the electrolyte governs the choices of the electrodes and the operation temperatures. Shown in table 10.1 are the fuel cell technologies currently under development. "" Technologies attracting attention toward development and commercialization include direct methanol (DMFC), polymer electrolyte membrane (PEMFC), solid-acid (SAFC), phosphoric acid (PAFC), alkaline (AFC), molten carbonate (MCFC), and solid-oxide (SOFC) fuel cells. This chapter is aimed at the solid-oxide fuel cells (SOFCs) and related electrolytes used for the fabrication of cells. 

Electrocatalysts One of the positive features of the supported electrocatalyst is that stable particle sizes in PAFCs and PEMFCs of the order of 2-3 nm can be achieved. These particles are in contact with the electrolyte, and since mass transport of the reactants occurs by spherical diffusion of low concentrations of the fuel-cell reactants (hydrogen and oxygen) through the electrolyte to the ultrafine electrocatalyst particles, the problems connected with diffusional limiting currents are minimized. There has to be good contact between the electrocatalyst particles and the carbon support to minimize ohmic losses and between the supported electrocatalysts and the electrolyte for the proton transport to the electrocatalyst particles and for the subsequent oxygen reduction reaction. This electrolyte network, in contact with the supported electrocatalyst in the active layer of the electrodes, has to be continuous up to the interface of the active layer with the electrolyte layer to minimize ohmic losses. 

In both PEMFCs and SOFCs, there is a strong need to improve the electrolyte materials and to find substitutes for the traditional electrolytes, which are Nafion for the PEMFC and yttria-stabilized zirconia (YSZ) for the SOFC. In this paper, we will focus on the discussion of new developments in the field of SOFC electrolytes. For recent developments concerning other fuel cell types, the reader is referred to review articles and books. " ... 

The first and third parentheses on the right hand side of Eq. (86) represent the electrodes and electrolyte contact resistances and the second term represent the bulk resistance of the electrolyte. Substitution of Eqs. (78-86) into (85) gives the following current-voltage relation, or polarization equation, for PEMFC... 

The materials for PEMFC electrodes should have good electrical conductivities and be stable in contact with electrolyte. Platinum-based electrodes have shown excellent electrochemical activities for PEMFC. 

Based on the literature survey, no membranes or MEAs reported so far can achieve all the above required goals. This research is directed at developing novel high-temperature, composite proton exchange membrane-electrolyte assemblies for PEMFC for building applications. 

G. M. Anilkiimar, S. Nakazawa, T. Okubo, and T. Yamaguchi. Proton conducting phosphated zirconia-sulfonated polyether sulfone nanohybrid electrolyte for low humidity, wide-temperature PEMFC operation. Elec-trochein. Commun., 8(1) 133-136, January 2006. 

Figure 22.13. Schematic of the dry production technique for PEMFC MEAs [99]. (Reprinted from Journal of Power Sources, 86(1-2), Guizow E, Schulze M, Wagner N, Kaz T, Reissner R, Steinhilher G, et al. Dry layer preparation and characterisation of potymer electrolyte fuel cell components, 352-62, 2000, with permission from Elsevier.)...

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