Fuel cell performance charge transfer resistance

Figure 5.34. Electric equivalent circuit for the impedance spectra in Figure 5.37. Ref. ohmic resistance Rct charge-transfer resistance CPE constant phase element IV Warburg element. The subscripts a and c denote anode and cathode, respectively [36]. (Modified from Boillot M, Bonnet C, Jatroudakis N, Carre P, Didierjean S, Lapicque F. Effect of gas dilution on PEM fuel cell performance and impedance response. Fuel Cells 2006 6 31-7. 2006 John Wiley Sons Limited. Reproduced with permission, and with the permission of the authors.)...

Figure 6.6 proves that increasing the humidification temperature does improve fuel cell performance. Figure 6.7 also confirms that the size of the kinetic arc does decrease with increasing humidification temperature. From these results the authors concluded that it was the reduced water content at the interface that caused the increased charge-transfer resistance of the electrode with excessive PTFE content (40 wt%). 

Figure 6.51. Charge-transfer resistance as a function of current density at 80°C, 100°C, and 120°C, 30 psig back-pressure, and 100% RH [44], (Reproduced by permission of ECS—The Electrochemical Society, and of the authors, from Tang Y, Zhang J, Song C, Liu H, Zhang J, Wang H, MacKinnon S, Peckham T, Li J, McDermid S, Kozak P, Temperature dependent performance and in situ AC impedance of high temperature PEM fuel cells using the Nafion 112 membrane.)...

Cho et al. (2003) were the first to report on freeze-thaw testing. They investigated fuel cell performance under thermal cycling between 80 °C and -10 °C using H2/O2 as fuel and oxidant respectively. After four thermal cycles, the cell performance decreased by about 100 mV and impedance measmements revealed an increase in ohmic and charge transfer resistance. Ohmic resistance was increased from -0.3 to 0.7 Q cm at 0.7 V and 0.9 Q cm at 0.8 V applied potentials, while charge transfer resistance was increased from -0.2 to 0.3 2 cm at 0.7 V and -0.6 to 0.9 Q cm at 0.8 V. Since membrane proton conductivity remained constant throughout the experiment, it was concluded that the increase in ohmic resistance was due to the enhancement of contact resistance between the electrodes, the membrane and the flow fields. 

As shown in Fig. 1.4 of Chapter 1, under a load, PEM fuel cell performance is determined by four voltage losses the voltage loss caused by mixed potential and hydrogen crossover, which is related to the Pt catalyst status and the membrane properties the activation loss, which is related to the electrode kinetics the ohmic loss, which is determined by ohmic resistance and the voltage loss caused by mass transfer, which is affected by the characteristics of the gas diffusion layer and catalyst layer. The voltage loss caused by mixed potential and hydrogen crossover will be discussed in detail in Chapter 7. The activation loss, ohmic loss, and mass transfer loss can be calculated from the charge transfer resistance, ohmic resistance, and mass transfer resistance, which can be determined by EIS measurement and simulation. 

In Chapter 1, Figure 1.4 shows a typical polarization curve of a PEM fuel cell. The voltage loss of a cell is determined by its OCV, electrode kinetics, ohmic resistance (dominated by the membrane resistance), and mass transfer property. In experiments, the OCV can be measured directly. If the ohmic resistance (Rm). kinetic resistance (Rt, also known as charge transfer resistance), and mass transfer resistance (Rmt) are known, the fuel cell performance is easily simulated. As described in Chapter 3, electrochemical impedance spectroscopy (EIS) has been introduced as a powerfiil diagnostic technique to obtain these resistances. By using the equivalent circuit shown in Figure 3.3, Rm, Rt, and R t can be simulated based on EIS data. 

FIGURE 5.7 The equivalent circuit for a fuel cell that is used to perform circuit interruption measurements Rceii- hisl csU resistance including electronic resistance and ionic resistance and the charge transfer resistance for fuel cell reactions [43]. 

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