Nyquist plot Catalyst layer impedance

Guo et al. [7], as shown by the Nyquist plots in Figure 6.10. In their impedance measurements, different amounts of Nafion ionomer in the catalyst layer, ranging from 0.33 to 1.13 mg/cm2 (dry weight) were examined. The active area of their fuel cells was 1.0 cm2. The fuel cells were operated in H2/air gas feeding mode with a flow rate of 220 cm3/min (at standard temperature and pressure) for both sides. The cell temperature as well as the humidification temperature for both electrodes were controlled at 70°C. The cell s AC impedance was measured using a Gamry PC4/750-DHC2 potentiostat. The perturbation amplitude was set at 5 mV in potentiostatic mode, and the frequency was scanned from 0.01 Hz to 100 kHz with 10 points per decade. 

Figure 6.10. Comparison of Nyquist plots of fuel cells with different Nafion loadings in the catalyst layers of both the cathode and the anode [7], (Reproduced by permission of ECS—The Electrochemical Society, from Guo Q, Cayetano M, Tsou Y, De-Castro ES, White RE. Study of ionic conductivity profiles of the air cathode of a PEMFC by AC impedance spectroscopy.)...

The reaction penetration depths. Id or la, are highly insightful parameters to evaluate catalyst layer designs in view of transport limitations, uniformity of reaction rate distributions, and the corresponding effectiveness factor of Pt utilization, as discussed in the sections Catalyst Layer Designs in Chapter 1 and Nonuniform Reaction Rate Distributions Effectiveness Factor in Chapter 3. Albeit, these parameters are not measurable. The differential resistances, Rd or Ra, can be determined experimentally either as the slope of the polarization curve or from electrochemical impedance spectra (Nyquist plots) as the low-frequency intercept of the CCL semicircle with the real axis. The expressions in Equation 4.33 thus relate the reaction penetration depths to parameters that can be measured. 

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