PEFC Schematic

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... 

A typical PEFC, shown schematically in Fig. 1, consists of the anode and cathode compartments, separated by a proton conducting polymeric membrane. The anode and cathode sides each comprises of gas channel, gas diffusion layer (GDL) and catalyst layer (CL). Despite tremendous recent progress in enhancing the overall cell performance, a pivotal performance/durability limitation in PEFCs centers on liquid water transport and resulting flooding in the constituent components.1,2 Liquid water blocks the porous pathways in the CL and GDL thus causing hindered oxygen transport to the... 

Figure 1. Schematic illustration of electrochemical and transport processes in a PEFC.

Figure 8.3 shows a schematic representation of a polymer electrolyte fuel cell (PEFC). In this instance, the oxidation reaction in the cathode is given by... 

Figure 1 is a schematic presentation of the cross-section of a single polymer electrolyte fuel cell (PEFC). This scheme will be used to discuss the key materials and processes in the PEFCs. The heart of the cell, which is magnified in the... 

Fig. 1. Schematic presentation of a PEFC cross-section. The cell (left) consists of a membrane catalyzed on both sides (referred to as a membrane/electrode (M E) assembly ), gas-diffusion backing layers and current collectors with flow fields for gas distribution. The latter become bipolar plates in a fuel cell stack. The M E assembly described schematically here (right) shows catalyst layers made of Pt/C catalyst intermixed with ionomer and bonded to the membrane (large circles in the scheme correspond to 10 nm dia. carbon particles and small circles to 2 nm dia. platinum particles).

Figure 24 describes schematically the three recent modes of preparation of membrane/electrode assemblies based on commercially available dispersed platinum catalysts. Comparison of catalyst utilization obtained with the different PEFC catalyzation techniques is given in Fig. 25. The advantage in catalyst utilization of the thin-layer approach is clearly seen, increasing at the higher cell currents (lower cell voltage) thanks to minimized mass-transport limitations in the thin catalyst layer.

The initial emphasis on evaluation and modeling of losses in the membrane electrolyte was required because this unique component of the PEFC is quite different from the electrolytes employed in other, low-temperature, fuel cell systems. One very important element which determines the performance of the PEFC is the water-content dependence of the protonic conductivity in the ionomeric membrane. The water profile established across and along [106]) the membrane at steady state is thus an important performance-determining element. The water profile in the membrane is determined, in turn, by the eombined effects of several flux elements presented schematically in Fig. 27. Under some conditions (typically, Pcath > Pan), an additional flux component due to hydraulic permeability has to be considered (see Eq. (16)). A mathematical description of water transport in the membrane requires knowledge of the detailed dependencies on water content of (1) the electroosmotic drag coefficient (water transport coupled to proton transport) and (2) the water diffusion coefficient. Experimental evaluation of these parameters is described in detail in Section 5.3.2. 

Fig. 42. Schematic of regions considered in PEFC air electrode modeling, including (from left to right) gas flow channel, gas-diffusion backing, and cathode catalyst layer. Oxygen is transported in the backing through the gas-phase component of a porous/tortuous medium and through the catalyst layer by diffusion through a condensed medium. The catalyst layer also transports protons and is assumed to have evenly distributed catalyst particles within its volume [100]. (Reprinted by permission of the Electrochemic Society).

In this section, recent advances in the field of polymer electrolyte direct methanol fuel cells, i.e., PEFCs based on direct anodic oxidation of methanol are discussed. A schematic of such a ceU is shown in Fig. 48, together with the processes that take place in the cell. The DMFC has many facets, electrocatalysis materials and components which deserve a detailed treatment. The discussion here will be confined, however, to the very significant performance enhancement demostrated recently with polymer electrolyte DMFCs, and, as a result, to possible consideration of DMFCs as a nearer term technology. 

Fig. 48. Schematic of a PEFC based on the direct oxidation of methanol.

Polymer-electrolyte fuel cells (PEFC and DMFC) possess a exceptionally diverse range of applications, since they exhibit high thermodynamic efficiency, low emission levels, relative ease of implementation into existing infrastructures and variability in system size and layout. Their key components are a proton-conducting polymer-electrolyte membrane (PEM) and two composite electrodes backed up by electronically conducting porous transport layers and flow fields, as shown schematically in Fig. 1(a). 

Sources of water and the various flux components that redistribute water in an operating PEFC are schematically shown in Fig. 14. 

Fig. 14 Schematic representation of water sources and waterfluxes that determine the water distribution profile across an operating PEFC.

Such combined modeling/experimental diagnostics work for PEFC cathodes was described in [13]. The latter model addresses the cathode catalyst layer and gas-diffusion backing, as schematically presented in Fig. 44. The catalyst layer is considered in the model a composite film, typically 4-7 pm thick, of Pt/C catalyst intermixed with recast ionomer. The... 

Fig. 44 Schematic of regions considered in comprehensive modeling of the PEFC air electrode. From left to right gas flow channel, GDL, cathode catalyst layer. Oxygen in transported through the porous GDL by gas-phase diffusion through an inert mixture of nitrogen and water vapor. The catalyst layer is described in terms of effective transport characteristics of gas, protons, and electrons [13].

A serious candidate for transportation application is also the direct methanol fuel cell (DMFC) which has been realized already on a laboratory scale. A catalytic burner is requited to evaporate the methanol/water mixture and to bum the exhaust gas at the anode [43]. Considering the complete energy chain, a PEFC is by 50 % more efficient than a diesel engine which consumes 4 1 per 100 km this is also valid for a natural gas driven engine [37]. Fig. 7-6 presents the processing schematics of both IMFC and DMFC. The DMFC offers a much simpler system than the PEFC. The DMFC is currently at an early development stage. It is perceived to offer improved solutions to the need for a small-scale power supply. A program for the construction of a 30 kW stack has recently started [29]. 

Fig. 8.7 Schematic of the active layer of a gas diffusion electrode PEFC.

Figure 8.11 shows a (simplified) schematic of a mobile PEFC system with the three subsystems for fuel, oxidant and cooling. 

Fig. 3 Components of the polymer electrolyte fuel cell (PEFC) membrane electrode assembly (MEA) on the left, including separator plates and gasket. A schematic of a PEFC stack is shown on the right, comprising a number of single cells in series...

The molecular structure of a conventional polymer used for a PFSA membrane is shown in Fig. 1. Membranes registered as Nafion (DuPont), Flemion , (Asahi Glass), and Aciplex (Asahi Chemical) have been commercialized for brine electrolysis and they are used in the form of alkali metal salt. Figure 4 shows a schematic illustration of a membrane for chlor-alkali electrolysis. The PFSA layer is laminated with a thin perfluorocarboxylic acid layer, and both sides of the composite membrane are hydrophilized to avoid the sticking of evolved hydrogen and chlorine. The membrane is reinforced with PTFE cloth. The technology was applied to PEFC membranes with thickness of over 50 xm [14]. 

Fig. 9.2 Current-voltage curves of a hydrogen operated PEFC and a DMFC, schematic comparison...

Fig. 14.12 Schematic of a PEFC expeiitaicing H2/air-from t stan/stop and major electrochemical reactions taking place [60]...

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