Cathode side, microporous layer

Figure 4.1 shows a schematic of a typical polymer electrolyte membrane fuel cell (PEMFC). A typical membrane electrode assembly (MEA) consists of a proton exchange membrane that is in contact with a cathode catalyst layer (CL) on one side and an anode CL on the other side they are sandwiched together between two diffusion layers (DLs). These layers are usually treated (coated) with a hydrophobic agent such as polytetrafluoroethylene (PTFE) in order to improve the water removal within the DL and the fuel cell. It is also common to have a catalyst-backing layer or microporous layer (MPL) between the CL and DL. Usually, bipolar plates with flow field (FF) channels are located on each side of the MFA in order to transport reactants to the... 

Figure 6.7. Impedance spectra for the oxygen reduction reaction at a cathode potential of 0.6 V. The microporous layer (loading 3.5 mg/cm2) of the electrode contained 40 wt% PTFE. The temperature of the humidifier on the cathode side was maintained at ( ) 65°C and ( ) 80°C. For comparison, the spectra of the electrode with the microporous layer containing 30 wt% PTFE is shown at the cathode humidification temperature of 65°C ( ) [5], (Reprinted from Journal of Power Sources, 94(1), Song JM, Cha SY, Lee WM. Optimal composition of polymer electrolyte fuel cell electrodes determined by the AC impedance method, 78-84, 2001, with permission from Elsevier and the authors.)...

On the cathode side of low-T cells, there is usually a microporous layer (MPL) between the CL and GDL. The role of this layer is to prevent the leakage of water produced in the ORR in order to keep the membrane well humidified. In this book we will not consider MEAs with MPLs however, transport loss in the MPL can be calculated using the relations from this chapter. 

Hickner et al. (2008) used high-resolution neutron radiography to image an operating PEMFC. The cross-sectional liquid water profile of the cell was quantified as a function of cell temperature, current density, and anode and cathode gas feed flow rates. At low current densities, liquid water tended to remain on the cathode side of the cell. Significant liquid water in the anode gas flow channel was observed when the heat and water production of the cell were moderate. At 60°C, the liquid water content in the center of the GDLs decreased compared to the electrolyte and catalyst layers. This profile is indicative of water condensation within the GDL and was explained by considering the microporous layers as a barrier to liquid water transport. 

The general layout of a cell includes a proton-conducting polymer electrolyte membrane (PEM), sandwiched between the anode and the cathode. Each electrode compartment is composed of (i) an active catalyst layer (CL), which accommodates finely dispersed nanoparticles of Pt that are attached to the surface of a highly porous and electronically conductive support, (ii) a gas diffusion layer (GDL), and (iii) a flow field (FF) plate that serves at the same time as a current collector (CC) and a bipolar plate (BP). This plate conducts current between neighboring cells in a fuel cell stack. At the cathode side, usually a strongly hydrophobic microporous layer (MPL) is inserted between CL and GDL, which facilitates the removal of product water from the cathode CL. The central unit including PEM and porous electrode layers, excluding the bipolar plates, is called the membrane electrode assembly (MEA). 

Microporous Layer Cathode Side A MPL on the cathode side has been experimentally determined by many to enhance performance under high-current-density and high humidity... 

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