Electrode-Gas Diffusion Layer

For the porous electrode-gas diffusion layer, the heat equation involves primarily heat generation caused by ohmic heating and heat transfer by conduction and convection. The heat equation is expressed as 

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Mechanistic models can generally be characterized by the scope of the model. In many cases, modeling efforts focus on a specific part or parts of the fuel cell, like the cathode catalyst layer [39], the cathode electrode (gas diffusion layer plus catalyst layer) [40-42], or the membrane electrode assembly (MEA) [43, 44]. These models are very useful in that they... 

Fluid flow and pressure variation in a fuel cell play a critical role in the distribution of reactant gas concentration at electrochemical reaction sites and, hence, in the distribution of local current densities and cause mass transfer loss. The governing equations for reactant gas flows in gas flow channels and in porous electrode-gas diffusion layers are given by conservation of mass and momentum equations. Solutions to these equations result in the distribution of pressure, P, and velocity field, which is also referred to as the bulk motion in the gas flow channels and porous electrode-gas diffusion layers. 

Figure 6.10 shows a typical temperature distribution in the electrode-gas diffusion layers, membrane, and gas channels at a given cross section. 

In order to determine the reactant gas transport rates in the gas channels and in electrode/gas diffusion layers and the consumption rates at electrodemembrane interfaces, it is necessary to determine the gas concentration distributions from mass species conservation equation. Conservation of mass species over a differential element, considering mass diffusion and convection, leads to... 

For the porous electrode-gas diffusion layer with bulk fluid motion given by Darcy or Brinkman s equation, the governing species transport is given as... 

Figure 6.15 shows a typical reactant gas concentration distribution in the electrode-gas diffusion layers and gas flow channels of a tri-layer fuel cell for a typical operating current density. 

Convection mass transfer coefficients are often used as convective boundary conditions for gas diffusion in a stationary media. However, while applying mass transfer correlations to describe mass species transport from the electrode-gas diffusion layer to gas flow stream in the channel, it is assumed that species mass transport rate at the wall is small and does not alter the hydrod5mamic, thermal, and concentration boundary layers like in boundary layers with wall suction or blowing. 

Diffusive mass transfer resistance in the electrode-gas diffusion layer... 

Notice that in situations where net mass transfer resistance is controlled by the diffusion resistance in the electrode-gas diffusion layer only, the reactant gas concentration at the reaction surface is given as... 

Equation 6.80 shows that the current density depends directly on the reactant gas concentration in the channel as well as the reactant gas concentration at the electrode-electrolyte interface. The current density increases with higher gas concentration, Q in the channel and lower concentration, Q i, at the reaction surface. For a given electrode-gas diffusion layer and gas channel design with a fixed Q value, the maximum current density or the... 

As we can see, the electrical potential decreases over the thicknesses of the anode and cathode electrodes owing to the ohmic resistance of the electrode gas diffusion layers to electron transport. These losses are referred to as the anode and cathode ohmic aoerpotentials Tiohm.a lohm.c/ respectively. 

Water produced by electrochemical reactions in a fuel cell needs to be removed for efficient operation of the cell. Water transport in electrode-gas diffusion layers, electrolyte, and gas flow channels plays a critical role in the design of a fuel cell. Figure 7.8 shows water generation at the electrodeelectrolyte interface and mechanisms of water transport in a fuel cell. 

Water produced at the electrode-electrolyte interfaces transports through the electrode-gas diffusion layers by diffusion and convection toward the gas flow channels where it may be transferred to the reactant gas flow streams by convection and diffusion if the gas streams are sufficiently dry. It is essential that water is removed from the electrolyte-electrode interface either by the flowing gas streams or by some external water collection system in order to prevent any accumulation or flooding of the electrode-electrolyte interface regions ftat blocks the pores of the electrode-gas diffusion layer and prevents reactant gas to reach reaction sites causing cell concentration polarization or mass tranter loss. Water flooding issue and mass transfer loss are... 

The water transport equation in gas flow chaimels and in electrode gas diffusion layer is similar to the gas species transport equations presented in Chapter 6, and it is given as follows ... 

The interdigitated flow-field design consists of two sets of dead-ended gas flow channels as shown in Figure 10.16a. The first set of dead-ended inflow channels carries the gas stream from the inlet ports and transfers to the electrode gas diffusion layer. The gas stream is forced by advection through the porous gas diffusion layer toward the electrode-electrolyte interface and toward the second set of dead-ended outflow channels and moves toward the gas stream outlet port. 

In order to determine the reactant gas transport rates in the electrode/gas diffusion layers and the consumption rates at electrode-membrane interfaces. 

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