Pseudo-Capacitance Model

Therefore, for equal H2/air-front residence times, the pseudo-capacitive model would suggest lower rates of carbon-support oxidation, i.e., lower rates of C02 formation for the stop process if compared to the start process, which is consistent with on-line C02 measurements of the air exiting the cathode flow-field during H2/air-front start-stop events, as shown in Fig. 16. 

Figure 19. Predicted carbon loss distribution along anode flow-field channel over a complete H2/air-front start—stop cycle using the pseudo-capacitance model in comparison with one-dimensional, normalized mass activity from Fig. 17. The pseudo-capacitance value used in the model is obtained from AC-impedance measurements as described in references (42, 43).

Figure 16. General transmission-line model for a conducting polymer-coated electrode. CF is the faradaic pseudo-capacitance of the polymer film, while Rt and Rt are its electronic and ionic resistance, respectively. R, is the uncompensated solution resistance.

In this chapter, we will review the fundamental models that we developed to predict cathode carbon-support corrosion induced by local H2 starvation and start-stop in a PEM fuel cell, and show how we used them to understand experiments and provide guidelines for developing strategies to mitigate carbon corrosion. We will discuss the kinetic model,12 coupled kinetic and transport model,14 and pseudo-capacitance model15 sequentially in the three sections that follow. Given the measured electrode kinetics for the electrochemical reactions appearing in Fig. 1, we will describe a model, compare the model results with available experimental data, and then present... 

As shown in Fig. 14, the cathode potential changes abruptly across the H2/air-front. This fact warrants the inclusion of the pseudocapacitance into the previous steady-state kinetic model.12 It is clear that the electrode s pseudo-capacitance can supply protons in transient events and thereby reduce the cathode carbon-support corrosion rate in the case of fast moving H2/air- ronts. Figure 18... 

The pseudo-capacitive effect can be incorporated in the coupled kinetic and transport model through Eqs. (19) and (20). Here we choose to illustrate the effect through the kinetic model for simplicity. With considering the pseudo-capacitive current density, the kinetic model becomes... 

Figure 20. The impact of a faster H2/air-front moving through die anode flow-field. 100% relative carbon loss is defined as the localized damage prediction when no pseudo-capacitance is considered in die model. Significantly less carbon corrosion is expected at the anode inlet region as the speed of H2/air-front increases but much less benefit at the anode outlet region.

As a model for patchwise heterogeneity Gibb and Koopal [54] have studied proton adsorption on mixtures of hematite and rutile. Both hematite and rutile could be described with the pseudo-homogeneous one-pKn SGC model. The salt concentration. For the total differential capacitance (dcTs/dpH) a similar additivity rule could be applied. 

The variation of the impedance with frequency has been examined for various geometries of a single pore, and the results are summarized in Figure 7-21 for cases with no Faradaic charge transfer, where all interfacial impedance is determined by the double-layer capacitance (Eq. 7-68) [69]. It can be shown that the more occluded the shape of a pore, the more the impedance exhibits a pseudo-transfer resistance. The RC transmission line model was also successfully applied to the impedance analysis of more complicated electrodes representing fractal structures composed of variable sizes of double and triple pores [70]. 

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