Oxygen transport losses

In Chapters 2-4 we will show that 77° at the membrane/CCL interface comprises all the t3rpes of voltage loss on the cathode side (the ORR activation, the oxygen transport loss in the GDL, CCL and in the flow field, and the voltage loss due to poor proton transport in the CCL). The same is true for 77 at the ACL/membrane interface. In other words, any transport loss in a fuel cell translates into a higher r] or 77 . 

Above we have neglected oxygen transport loss in the GDL. It is easy to show that accounting for this loss does not change the result (4.148). Consider the low-current polarization curve of the catalyst layer (4.140). In the presence of transport loss, the oxygen concentration in the catalyst layer cj is related to this concentration in the channel Ch by Elq. (3.2). In dimensionless variables this equation reads... 

This result differs from (4.141) by a constant factor 1 — J/jo in the denominator. However, after normalization (4.134) this factor vanishes and we again arrive at (4.143). A similar procedure for the high-current regime leads to (4.148). Thus, accounting for oxygen transport loss in the GDL does not affect the optimal shape of catalyst loading along the channel. 

Oxygen transport loss in a cell lowers the oxygen concentration Ct in the catalyst layer. From the equation above it is obvious that if Ct decreases, /jo must increase to maintain jo. 

POLARIZATION CURVES FOR SMALL TO MEDIUM OXYGEN TRANSPORT LOSS... 

This equation takes into account all the potential losses in the CCL the first term accounts for the ORR activation overpotential and the proton transport loss, while the second term represents the oxygen transport loss. 

What are the limits-of-validity of Equation 4.189 Clearly, a necessary (though not sufficient) condition is the smallness of the second term as compared to the first term in this equation. Taking the leading-order approximation for the oxygen transport losses jo/ cib) and the high-current approximation for the first term, results in the... 

Equation 4.189 can be used for the fitting at experimental polarization curves, provided that (i) the oxygen stoichiometry is large and (ii) the potential loss caused by oxygen transport in the GDL is minimal. The validity of the second condition is usually not known a priori. However, it can be easily relaxed by incorporating the respective transport loss into the polarization equation, as discussed in the section Oxygen Transport Loss in the Gas-Diffusion Layer in Chapter 5. 

The polarization curve, which takes into account the oxygen transport loss in the GDL, results from the substitution of Equation 5.41 into the CL polarization curves obtained in the previous sections. Consider first the simplest Tafel equation. Equation... 

Makharia et al. (2005) developed a physical model for the CCL impedance, neglecting the oxygen transport losses in the CCL. The model was used for fitting the experimental impedance spectra acquired at several cell currents. 

Recently, the problem discussed has been considered by Maranzana et al. (2012). They neglected oxygen transport loss in the CCL and solved the reduced problem analytically. The impedance spectra of individual segments derived in their work also exhibit closed loops. Unfortunately, the spectra measured by Schneider et al. do not show the loops. This, perhaps, is a consequence of poor spectra resolution in their experiments. The measurements of Schneider et al. (2007a,b) have been performed with 10 points per decade, while the resolution of the loops in Figure 5.24 requires about a hundred points per decade. 

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