In PEFCs

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... 

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]. 

Carbon, carbon hybrids and carbon composites in PEFCs... 

In a fuel cell, the electrocatalysts generate electrical power by reducing the oxygen at the cathode and oxidizing the fuel at the anode [1], Pt and Pt alloys are the most commonly used electrocatalysts in PEFCs due to their high catalytic activity and chemical stability [99-103]. 

CNTs (single-walled or multi-walled) are the most common material used as catalyst support in PEFCs. SWCNTs have large surface areas while MWCNTs are more conductive than SWCNTs [111, 112]. 

Graphene is also used as catalyst support in PEFCs as it offers high conductivity, facile electron transfer and large surface area [151,152]. The planar structure of graphene allows its edge and basal planes to interact with the nanoparticles of the electrocatalyst [100],... 

Stable performance was demonstrated to 4,000 hours with Nafion membrane cells having 0.13 mg Pt/cm and cell conditions of 2.4/5.1 atmospheres, H2/air, and 80°C (4000 hour performance was 0.5 V at 600 mA/cm ). These results mean that the previous problem of water management is not severe, particularly after thinner membranes of somewhat lower equivalent weight have become available. Some losses may be caused by slow anode catalyst deactivation, but it has been concluded that the platinum catalyst "ripening" phenomenon does not contribute significantly to the long-term performance losses observed in PEFCs (5). 

Figure 3-5 Evolutionary Changes in PEFCs Performance [(a) H2/O2, (b) Reformate...

When used in a PEFC system, the reformate must pass through a preferential CO catalytic oxidizer, even after being shifted in a shift reactor. Typically, the PEFC can tolerate a CO level of only 50 ppm. Work is being performed to increase the CO tolerance level in PEFC. At least two competing reactions can occur in the preferential catalytic oxidizer ... 

Pozio, A., Silva, R. R, De Francesco, M. and Giorgi, L. 2003. Nafion degradation in PEFCs from end plate iron contamination. Electrochimica Acta 48 1543-1549. 

E. C. Kumbur, K. V. Sharp, and M. M. Mench. Validated Leverett approach for multiphase flow in PEFC diffusion media. I. Hydrophobicity effect. Journal of the Electrochemical Society 154 (2007) B1295-B1304. 

J. J. Kowal, A. Turhan, K. Heller, J. Brenizer, and M. M. Mench. Liquid water storage, distribution, and removal from diffusion media in PEFCS. Journal of the Electrochemical Society 153 (2006) A1971-A1978. 

From the operational point of view, the major challenge is to understand the versatile role of water for structure and processes in fuel cells. As the main product of the reaction, the presence of water is unavoidable in any type of fuel cell running on hydrogen, methanol, or other hydrocarbon-based fuels. In PEFCs in particular, water molecules determine the interactions between molecular... 

The factors 4 and 4 accormt for the heterogeneity of the interface. The interfacial flux conditions. Equations (6.56) and (6.57), can be straightforwardly applied at plain interfaces of the PEM with adjacent homogeneous phases of water (either vapor or liquid). However, in PEFCs with ionomer-impregnated catalyst layers, the ionomer interfaces with vapor and liquid water are randomly dispersed inside the porous composite media. This leads to a highly distributed heterogeneous interface. An attempt to incorporate vaporization exchange into models of catalyst layer operation has been made and will be described in Section 6.9.4. 

This section provides a comprehensive overview of recent efforts in physical theory, molecular modeling, and performance modeling of CLs in PEFCs. Our major focus will be on state-of-the-art CLs that contain Pt nanoparticle electrocatalysts, a porous carbonaceous substrate, and an embedded network of interconnected ionomer domains as the main constituents. The section starts with a general discussion of structure and processes in catalyst layers and how they transpire in the evaluation of performance. Thereafter, aspects related to self-organization phenomena in catalyst layer inks during fabrication will be discussed. These phenomena determine the effective properties for transport and electrocatalytic activity. Finally, physical models of catalyst layer operation will be reviewed that relate structure, processes, and operating conditions to performance. 

Eikerling i has demonsfrafed capabilities of this approach. A simple representation of fhe pore space by a bimodal 5-distribution reveals the role of fhe CCL as a "wafershed" in PEFCs. For this case, a full analytical solution could be found. Af fhe same fime, it still captures essential physical processes and major structural features such as typical pore sizes (r, r ), and distinct contributions to porosity from primary and secondary pores (X,Xm). 

In the PEFC system, the mean pore radii of catalyst layers are of the order of 0.1 pm. The Knudsen diffusion coefficients at 80 °C for O2 and H2O through the catalyst layer are thus estimated to be 0.32 and 0.43 cm /s, respectively. These values are comparable to the respective ordinary diffusion coefficients, indicating that Knudsen diffusion further restricts the rates of oxygen and water transport through the cathode catalyst layer in PEFCs and should be taken into account. 

Much effort has been expended in the last 5 years upon development of numerical models with increasingly less restrictive assumptions and more physical complexities. Current development in PEFC modeling is in the direction of applying computational fluid dynamics (CFD) to solve the complete set of transport equations governing mass, momentum, species, energy, and charge conservation. 

Various forms of governing equations have been used in PEFC modeling, although all fall under the single-phase assumption. To clarify important subtleties with theoretical rigor, in this subsection we summarize a set of conservation equations and provide detailed comments of various terms that should be used. 

Water management is one of the most critical and widely studied issues in PEFC. Water management is referred to as balancing membrane hydration with flooding avoidance. These are two conflicting needs to hydrate the polymer electrolyte and to avoid flooding in porous electrodes and GDL for reactant/ product transport. 

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