Electrode proton resistance

The standard electrode potential and its temperature coefficient are found in the literature.36 Kinetic parameter values were measured in-house for HOR,33 ORR,34 OER,35 and COR.12 22 Table 2 gives cell component materials and transport properties. The membrane and electrode proton conductivity in Table 2 are based on the measured membrane and electrode resistance,42,43 which is a strong function of relative humidity (RH). In what follows next, we will describe the... 

In situ resistance measurements by either high-frequency AC perturbation (referred to as HFR) or current interrupt are popular for quantifying the sum of the protonic membrane resistance and the electronic bulk and contact resistances across the electrodes, DMs, and BPs (RJ. Note that these do not include the proton resistance in the anode and cathode electrodes, though the separation is often... 

In general, thin catalyst layers will have the lower proton resistance. However, they may also fill up more easily with water as is, for example, the case with the very thin NSTF electrodes by 3M, which only seem to function well at non-saturated conditions. Also for start-up under freezing conditions, a thicker electrode, or at least a higher pore volume, seems to be an advantage as complete filling with ice is even more detrimental. 

The term Ici/crp in Equation 5.110 is the proton resistivity of the CCL. This will be discussed in the section Finite but Small Current An Analytical Solution. Interestingly, if e is small. Red does not contain this term. Physically, if the reaction penetration depth is small (hydrogen electrode), the reaction runs close to the membrane and, therefore, protons do not need to be transported deep into the catalyst layer. Thus, the contribution of the proton transport to the cell resistivity does not appear as a separate term. The proton transport represented by ap is included into the total CL resistivity bf 2hap). 

While the conductivity decrease is a shortcoming of polar groups, their incorporation may lead to advantages in terms of (i) mechanical performance, (ii) decrease in membrane-electrode interfacial resistance, and (iii) decrease in methanol crossover, which is an asset for an implementation in direct methanol fuel cells (DMFC), provided high selectivity (ratio of proton conductivity to methanol permeability) is retained [52]. While comparing the effects of polar groups, it appears that phenylphosphine incorporation. 

The desired membrane must also have the following characteristics (1) the proton conductivity of the membrane material should be high and the membrane should be thin to minimize membrane resistance (2) the membrane must be mechanically strong and the change in membrane area between the dry and the swollen states should be negligible, to minimize any membrane/electrode interface resistance (3) the membrane material must be chemically stable during DMFC operation and (4) the membrane mnst be economical to manufacture. 

Like many other fluoropolymers, Nafion is quite resistant to chemical attack, but the presence of its strong perfluorosulfonic acid groups imparts many of its desirable properties as a proton exchange membrane. Fine dispersions (sometimes incorrectly called solutions) can be generated with alcohol/water treatments. Such dispersions are often critical for the generation of the catalyst electrode structure and the MEAs. Films prepared by simply drying these dispersions are often called recast Nafion, and it is often not realized that its morphology and physical behavior are much different from those of the extruded, more crystalline form. 

As mentioned, the reaction distribution is the main effect on the catalyst-layer scale. Because of the facile kinetics (i.e., low charge-transfer resistance) compared to the ionic resistance of proton movement for the HOR, the reaction distribution in the anode is a relatively sharp front next to the membrane. This can be seen in analyzing Figure 10, and it means that the catalyst layer should be relatively thin in order to utilize the most catalyst and increase the efficiency of the electrode. It also means that treating the anode catalyst layer as an interface is valid. On the other hand, the charge-transfer resistance for the ORR is relatively high, and thus, the reaction distribution is basically uniform across the cathode. This means... 

Figure 4.36. Normalized resistivity (open points) and conductivity (solid points) profiles for Pt/C cathodes with (triangles) and without (circles) Nafion impregnation [8], (Reproduced by permission of the authors and of ECS—The Electrochemical Society, from Lefebvre MC, Martin RES, Pickup PG. Characterization of ionic conductivity within proton exchange membrane fuel cell gas diffusion electrodes by impedance spectroscopy.)...

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