High polarization curve modeling

In order to prove the S-shaped character of the polarization curve, the system was studied galvanostatically. The model predicts that the sandwiched branch of the polarization curve should be stable, and therefore measurable under galvanostatic conditions. Figure 6.10 shows the results of the experiment depending on the scan rate, an S-shaped curve can be observed in the back scan, i.e., from high to low current. At low... 

Ahn et al. have developed fibre-based composite electrode structures suitable for oxygen reduction in fuel cell cathodes (containing high electrochemically active surface areas and high void volumes) [22], The impedance data obtained at -450 mV (vs. SCE), in the linear region of the polarization curves, are shown in Figure 6.22. Ohmic, kinetic, and mass transfer resistances were determined by fitting the impedance spectra with an appropriate equivalent circuit model. 

Another consequence of membrane contamination by cationic impurity can be a decrease in the limiting current on polarization curve measured at high contamination levels. Due to proton deficiency at the cathode, the ORR current may become limited by diffusion of protons, but not oxygen diffusion through the CCL. This effect observed in experimental systems (Uribe et al, 2002 Halseid et al, 2006b) was qualitatively described using model assumptions proposed by Kienitz et al. (2009). 

The coupled hierarchical model was evaluated by comparison with experimental data of Suzuki et al. (2011) and Soboleva et al. (2011). Both of these studies provided experimental data on CL structure as well as electrochemical performance, which were used to parameterize the model. The pore size distributions of the catalyst layers are depicted in Figure 3.42. Figure 4.12a shows polarization curves from both experimental studies compared to the curves obtained from the hierarchical model. Experimental trends are reproduced within the model. It is evident that flooding of the GDL is responsible for the knee in fuel cell voltage at high current density. 

The model allows deriving the approximate polarization curve of the CCL in the supercritical (high-current) regime. In this regime, Coxfi — 0 and hence... 

Chang et al. [2, 3] provided an extended model with Butler-Volmer electrochemical reaction kinetics and the capabihty of predicting complete polarization curves. The results obtained for Y-shaped [2] andF-shaped [3] formic acid/dissolved oxygen-based cells were in good agreement with previous experimental studies [4, 5] and confirmed the cathodic activity and mass transport limitation of these cells. Consequently, the predicted cell performance was essentially independent of anodic fonnic add concentration. The numerical results also recommended high aspect... 

COMMENTS Although low-temperature PEFCs tend to be limited by reactant availability (concentration polarization), many high-temperature fuel cells are ultimately limited by the ionic conductivity of the electrolyte, because mass transport and kinetics are facilitated at high temperature (e.g., see polarization curve in Example 4.6). Also, note the relative importance of the electrolyte in the overall loss. Considering that some PEFC electrolytes can be as thin as 18 pm, the electrode resistance can even be larger than that of the main electrolyte. Also it should be noted that the use of ionomer percentage to adjust ionic conductivity is an approximation, as well as the approximation that reaction occurs in the middle of the catalyst layer. More complex modeling of these reactions and losses are available in literature but are beyond the scope of this text. 

The final piece of the polarization curve to be modeled is the departure from the expected OCR given by the Nemst equation. For low-temperature PEFCs, the OCV is predicted to be around 1.2 V, but in practice, only about 1.0 V is observed. For a high-temperature SOFC, however, the actual OCV can be very close to the theoretical OCV. For the PEFC, the 0.2 V represents an incredibly significant efficiency loss before any useful current is even drawn. The departure from the theoretical OCV is typically a result of two phenomena ... 

In this expository article, the basic mathematical model of some simple electrochemical processes was discussed. The model is based on the concept of conservation of charge within the electrolyte. The boundary conditions, on the other hand, are problem-specific. The subject of electrode kinetics is central to the proper specification of the boundary conditions. In their most general form, the conditions are nonlinear, leading to a nonlinear boundary value problem. This is closely tied to the nonlinear polarization curves. The analytical solution of the mathematical model is formidable and for moderately simple two-dimensional regions is impossible to obtain. The only feasible approach is numerical simulation. The use of high-speed digital computers is an essential tool in solving such problems. 

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