Catalyst layer activation resistivity

COMMENTS We could also have added the catalyst layer diffusion resistance. This model, while serving as a useful qualitative tool, is not precise, simply because we have no idea of the thickness of any layer of liquid water of ionomer locally in the electrode structures. Also, some local flooding simply turns off the current in this location by these effects, but this only means areas which are flooding have reduced performance. Other areas in the fuel ceU may not be flooded, so that the net effect of the flooding is actually to reduce the electrochemicaUy active surface area, which is an approach taken by modely including flooding, discussed later in this chapter. 

It is well known that though NO conversion is unaffected by the thickness of the monolith wall beyond a small critical value, SO2 conversion increases linearly with increasing wall thickness. This is indicated in Fig. 9 such trends reflect the different influence of internal diffusional resistances on DeNOx reaction and SO2 oxidation, which, as discussed previously, are respectively confined to a superficial layer of the catalyst and active inside the whole wall. Consequently, the design of SCR monoliths should pursue the realization of very thin catalytic walls Fig. 9, for example, shows that reducing the catalyst half-thickness from 0.7 mm to 0.2 mm does not alter the DeNOxing performance but causes a decrease of SO2 oxidation as significant as 78%. 

In addition to loss of the platinum, the carlxm support that anchors the platinum crystallites and provides electrical coimectivity to the gas-diffusion media and bipolar plates is also subject to degradation. In phosphoric acid fuel cell, graphitized carbons are the standard because of the need for corrosion resistance in high-temperature acid environments [129], but PEM fuel cells have not employed fully graphitized carbons in the catalyst layers, due in large part to the belief that the extra cost could be avoided. Electrochemical corrosion of carbon materials as catalyst supports will cause electrical isolation of the catalyst particles as they are separated from the support or lead to aggregation of catalyst particles, both of which result in a decrease in the electrochemical active surface area of the catalyst and an increase in the hydrophUicity of the surface, which can, in turn, result in a decrease in gas permeability as the pores become more likely to be filled with liquid water films that can hinder gas transport. 

Results during start-up from -5°C, residual water did not alter the electrochemical active surface area or charge resistance at low current density less water was stored in the catalyst layer than in the cell... 

The potential loss in the membrane is due to resistance to proton transport across the membrane from anode catalyst layer to cathode catalyst layer. The distribution pattern of the protonic overpotential is dependent on the path traveled by the protons and the activities in the catalyst layers. Figure 3.18 shows the potential loss distribution in the membrane for three different nominal current densities. It can be seen that the potential drop is more uniformly distributed across the membrane. This is because of the smaller gradient of the hydrogen concentration distribution imder the channel and land areas at the anode catalyst layer due to the higher diffusivity of the hydrogen. 

The most significant contributions to the ohmic losses are due to the membrane, the bipolar/cooling plates and the electrodes (GDM + catalyst layer), including contact resistance between components. In a H2/air-fed fuel cell, the activation losses are mainly at the cathode. The catalyst layer, the microporous layer (MPL), and the macroporous gas diffusion layer (GDL) of the gas diffusion medium (GDM) as well as the gas channel design (i.e., the bipolar plate) all contribute to transport losses of reactants. 

More complex empirical equations based on Equation 1.155 have also been developed. For example, Amphlett et al. [167] presented empirical equations and terms that relate activation losses, internal resistance, and all temperature dependencies through fitting parameters. Sena et at. [168] analyzed the catalyst layer and considered it as a thin-film flooded agglomerate thus, the GDE is assumed to be formed by an assembly of flooded zones (catalytic zones) and empty zones (no catalyst present). The final equation relates the oxygen diffiision effects in the GDE ... 

The effect of impurities on fuel cells, often referred to as fuel cell contamination, has been identified as one of the most important issues in fuel cell operation and applications. Studies have shown that the component most affected by contamination is the MEA [3]. Three major effects of contamination on the MEA have been identified [3,4] (1) the kinetic effect, which involves poisoning of the catalysts or a decrease in catalytic activity (2) the conductivity effect, reflected in an increase in the solid electrolyte resistance and (3) the mass transfer effect, caused by changes in catalyst layer structure, interface properties, and hydrophobicity, hindering the mass transfer of hydrogen and/or oxygen. 

Although the catalyst activity is the most important factor in improving cell performance of HT-PEMFC, the catalyst layer structure can also be optimized to increase the concentration of O2 in phosphoric acid near the catalyst sites. To establish a diffusion path for O2 in gas phase, two possible approaches can be taken. One is the dispersion of the hydrophobic binder such as PTFE within the catalyst layer. For the MEAs that contain the high content of phosphoric acid, it would be best to use the PTFE binder in the catalyst layer. The other way is to control the pore size within the catalyst layer so that the pores that are filled with phosphoric acid and pores that provide path for O2 diffusion in the gas phase can be separated according to the pore size. The piimaiy pores between catalyst particles are known to be filled with phosphoric acid, while the larger secondary pores between the agglomerates of catalyst particles provide O2 diffusion path [47]. If the O2 diffusion path within the catalyst layer can be established and maintained without the use of the hydrophobic binder which increases the ohmic resistance in MEAs [45], the cell performance of HT-PEMFC can be improved. 

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