PEFCs

AFC = all line fuel ceU MCFC = molten carbonate fuel ceU PAFC = phosphoric acid fuel ceU PEFC = polymer electrolyte fuel ceU and SOFC = solid oxide fuel ceU. 

Polymer Electrolyte Fuel Cell. The electrolyte in a PEFC is an ion-exchange (qv) membrane, a fluorinated sulfonic acid polymer, which is a proton conductor (see Membrane technology). The only Hquid present in this fuel cell is the product water thus corrosion problems are minimal. Water management in the membrane is critical for efficient performance. The fuel cell must operate under conditions where the by-product water does not evaporate faster than it is produced because the membrane must be hydrated to maintain acceptable proton conductivity. Because of the limitation on the operating temperature, usually less than 120°C, H2-rich gas having Htde or no (

The porous electrodes in PEFCs are bonded to the surface of the ion-exchange membranes which are 0.12- to 0.25-mm thick by pressure and at a temperature usually between the glass-transition temperature and the thermal degradation temperature of the membrane. These conditions provide the necessary environment to produce an intimate contact between the electrocatalyst and the membrane surface. The early PEFCs contained Nafton membranes and about 4 mg/cm of Pt black in both the cathode and anode. Such electrode/membrane combinations, using the appropriate current coUectors and supporting stmcture in PEFCs and water electrolysis ceUs, are capable of operating at pressures up to 20.7 MPa (3000 psi), differential pressures up to 3.5 MPa (500 psi), and current densities of 2000 m A/cm. ... 

Very substantial advances have been made in terms of improvements in electrode stmctures and increases in the Pt utili2ation as illustrated in Figure 1. It appears that Pt loadings of less than 0.2 mg Pt/cm are adequate to obtain acceptable performance in PEFCs using pure H2 as the fuel (see Thin films). Whereas early electrodes contained 4 mg Pt/cm, the most recent developments in electrode fabrication have permitted Pt loadings to be reduced to 0.13 mg Pt/cm in a thin-film stmcture, while maintaining high performance. 

Fig. 1. Increase in Pt utiH2ation in PEFCs, where A represents the GE space technology fuel ceU, 4 mg Pt/cm B represents Prototech, 0.45 mg Pt/cm ...

As of this writing, the primary focus of research and development in PEFC technology is a fuel-ceU system for terrestrial transportation appHcations... 

Siemens AG has been involved in R D on PFFCs, and Vickers Shipbuilding Engineering Ltd. (United Kingdom) is evaluating PFFCs from Ballard Power Systems for power generation. A 35-ceU stack was successfully tested for more than 300 h. Plans are under way to test a 20-kW PEFC. 

Being acidic, fluorocarbon ionomers can tolerate carbon dioxide in the mel and air streams PEFCs, therefore, are compatible with hydrocarbon fuels. However, the platinum catalysts on the fuel and air elec trodes are extremely sensitive to carbon monoxide only a few parts per million are acceptable. Catalysts that are tolerant to carbon monoxide are being explored. Typical polarization curves for PEFCs are shown in Fig. 27-64. 

A schematic diagram of a methanol-fueled PEFC system is shown in Fig. 27-65. A methanol reformer (to convert CH3OH to H9 and CO9... 

Polymer electrolyte fuel cells can be obtained from several developers. These fuel cells deliver about 5 kW of power and measure 30 by 30 by 70 cm (12 X 12 X 28 in.). For the large produc tion volume anticipated if the automotive industry were to adopt the PEFC, a system cost of less than 100/kW may be reached eventually. 

Specific aspects examined here include insights and conclusions derived from the most recently performed density functional theory (DFT) calculations, which have been based on a comprehensive model of the electrochemical interface, and the strong disagreements (which seem to defy all recent theoretical efforts) that remain regarding proper interpretation of experimental ORR results and proper identihcation of the ORR mechanism in a PEFC cathode employing Pt catalysts. 

Implementation of Pt/C catalysts in PEFC technology using recast Nafion as a proton conducting and bonding agent [Raistrick, 1986 Wilson and Gottesfeld, 1992]. 

Optimization of the catalyst layer composition and thickness in PEFCs for maximum catalyst utilization in operation on air and on impure hydrogen feed streams [Wilson, 1993 Springer et al., 1993]. 

What is behind the apparent disagreements between Tafel slopes and reaction orders reported from recent investigations of the ORR at PEFC cathode catalysts and the slopes and reaction orders measured earlier for model systems of low Pt surface area Is the ORR process at a dispersed Pt catalyst possibly different in nature from the ORR process at low-surface-area Pt ... 

Can an ORR mechanism at Pt metal in an acid electrolyte with the Reaction (1.2) as the first and rate-limiting step be defended in light of the recently reported apparent Tafel slope and reaction order for ORR in the PEFC cathode ... 

The lesson to be taken from this report by Paik et al. [2004] is that a Pt catalyst in contact with a hydrous electrolyte is so active in forming chemisorbed oxygen at temp-eramres and potentials relevant to an operating PEFC, that the description of the cathode catalyst surface as Pt, implying Pt metal, is seriously flawed. Indeed, that a Reaction (1.4) acmally takes place at a Pt catalyst surface, exposes, Pt to be less noble than usually considered (although it remains a precious metal nevertheless. ..). Such a surface oxidation process, taking place on exposure to O2 and water and driven by electronically shorted ORR cathode site and metal anode site, is ordinarily associated with surface oxidation (and corrosion) of the less noble metals. 

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