Cathode catalyst layer impedance, 405

The electric circuit of membrane electrode assemblies is a combination of anode and cathode catalyst layers plus the membrane. In general, the anode catalyst layer is considered an electric circuit, the cathode catalyst layer is considered another electric circuit similar to that of the anode but with different RC values, and the membrane is treated as a resistance. These three electric circuits are connected in series to construct a whole-cell equivalent circuit. A typical impedance spectrum is shown in Chapter 1 as Figure 1.16. Since the anode reaction is significantly faster than the cathode, the RC electric circuit of the anode can be disregarded. 

Figure 5.30. Schematic of the catalyst layer geometry and its composition, exhibiting the different functional parts, a A sketch of the layer, used to construct a continuous model, b A one-dimensional transmission-line equivalent circuit where the elementary unit with protonic resistivity Rp, the charge transfer resistivity Rch and the double-layer capacitance Cj are highlighted [34], (Reprinted from Journal of Electroanalytical Chemistry, 475, Eikerling M, Komyshev AA. Electrochemical impedance of the cathode catalyst layer in polymer electrolyte fuel cells, 107-23, 1999, with permission from Elsevier.)...

Eikerling M, Komyshev AA (1999) Electrochemical impedance of the cathode catalyst layer in polymer electrolyte fuel cells. J Electroanal Chem 475 107-23... 

Figure 8.15. Plot of cell potential vs. fuel cell current density, (/o), indicating the effect of liquid water accumulation in the CCL on performance (soUd hne). The interplay of liquid water accumulation in pores and impeded oxygen transport causes the transition from the ideally wetted state to the fully saturated state (dotted tines), as indicated [51]. (Reprinted from Electrochimica Acta, 53.13, Liu J, Eikerting M. Model of cathode catalyst layers for polymer electrolyte fuel cells The role of porous structure and water accumulation, 4435— 46, 2008, with permission from Elsevier.)...

Below, the model for DMFC cathode impedance is presented, assuming the electrochemical mechanism of MOR on the cathode side (Kulikovsky, 2012b). In this section, the nonstationary version of the DMFC cathode performance model (the section Cathode Catalyst Layer in a DMFC ) is used to calculate the cathode impedance. As discussed in the section Cathode Catalyst Layer in a DMFC, the model takes into account spatial distribution of the MOR and ORR, through the cathode thickness. It is shown below that the spatial separation of MOR and ORR, discussed in the section Cathode Catalyst Layer in a DMFC, leads to the formation of a separate semicircle in the impedance spectrum. 

Kulikovsky, A. A. and Eikerling, M. 2013. Analytical solutions for impedance of the cathode catalyst layer in PEM fuel cell Layer parameters from impedance spectrum without fitting. J. Electroanal. Chem.. 691, 13-17. 

Nara, H., Tominaka, S., Momma, T., and Osaka, T. 2011. Impedance analysis counting reaction distribution on degradation of cathode catalyst layer in PEFCs. 158,... 

M. Eikerling and A. A. Kornyshev, Electrochemical Impedance of the Cathode Catalyst Layer in Pol5mer Electrolyte Fuel Cells, J. Electroanal. Chem., Vol. 475, pp. 107-123,1999. 

Two impedance arcs, which correspond to two relaxation times (i.e., charge transfer plus mass transfer) often occur when the cell is operated at high current densities or overpotentials. The medium-frequency feature (kinetic arc) reflects the combination of an effective charge-transfer resistance associated with the ORR and a double-layer capacitance within the catalyst layer, and the low-fiequency arc (mass transfer arc), which mainly reflects the mass-transport limitations in the gas phase within the backing and the catalyst layer. Due to its appearance at low frequencies, it is often attributed to a hindrance by finite diffusion. However, other effects, such as constant dispersion due to inhomogeneities in the electrode surface and the adsorption, can also contribute to this second arc, complicating the analysis. Normally, the lower-frequency loop can be eliminated if the fuel cell cathode is operated on pure oxygen, as stated above [18],... 

Figure 6.5 shows the AC impedance spectra of the same fuel cells measured at different cathodic potentials. It is evident that as the overpotential increases, the diameter of the kinetic arc decreases due to the increasing kinetic rate. At low overpotential, the kinetics dominates and only the kinetic arc appears. At high overpotentials, the low-frequency region shows additional arcs, which are associated with mass-transport limitations across the gas diffusion layer and within the catalyst layer. 

The function of a proton-conducting ionomer such as Nafion in the catalyst layer is to provide an ionic path for proton migration from the membrane to the reaction site at the catalyst surface. Therefore, the content of the proton-conducting ionomer in the catalyst layer will greatly influence the transport of protons to the catalyst sites. The impedance spectra of fuel cells with different Nafion loadings in the catalyst layers of both the cathode and the anode at OCV were compared by... 

This chapter has examined a variety of EIS applications in PEMFCs, including optimization of MEA structure, ionic conductivity studies of the catalyst layer, fuel cell contamination, fuel cell stacks, localized impedance, and EIS at high temperatures, and in DMFCs, including ex situ methanol oxidation, and in situ anode and cathode reactions. These materials therefore cover most aspects of PEMFCs and DMFCs. It is hoped that this chapter will provide a fundamental understanding of EIS applications in PEMFC and DMFC research, and will help fuel cell researchers to further understand PEMFC and DMFC processes. 

Fig. 44. (a) Measured impedance spectra for a PEFC air cathode potentials and (b) a simultaneous fit of these six spectra [108]. The two distinguishable features of the spectrum correspond to catalyst layer processes (high-frequency) and backing layer transport (low-frequency). (Reprinted by permission of the Electrochemical Society). 

In addition to liquid water, water vapor may also decrease the limiting current by lowering the reactant pressure in the catalyst layer and by potentially impeding the diffusion of oxygen toward the catalyst sites. When a stack is in operation, the water vapor in both the anode and the cathode is around the saturation pressure of the stack temperature. This is important for the stack to operate normally in order to avoid drying up of the membrane electrode assemblies (MEAs). Table 2.2 lists the saturated vapor pressures at different temperatures. A PEMFC normally operates at temperatures between 50°C and 70°C, which means that the saturated vapor pressure is between 0.1 and 0.3v bars. For a stack operating at a temperature near or higher than 100°C, the reactants must be pressurized. 

Nitrogen may also lower the reactant pressure in the catalyst layer and impede the diffusion of oxygen toward the catalyst sites in the catalyst layer. At the cathode, O2 in air is consumed within the catalyst layer, but N2 is not. Therefore, the N2 concentration or partial pressure within the catalyst layer may be much higher than that in fresh air, which in turn could reduce the partial pressure of O2 in the catalyst layer or impede the diffusion of O2 in the catalyst layer. 

In a DMFC, methanol is directly oxidized at the anode, as shown by Reaction 7.3. Each methanol molecule requires one water molecule for the reaction to proceed. Without water, the reaction cannot proceed, and this must be remembered when designing a DMFC system. The product is gaseous CO2, and it must be vented out so it does not impede the diffusion of methanol to the anode catalyst layer. At the cathode, oxygen combines with protons and electrons to form water, as shown by Reaction 7.4. The overall reaction is shown by Reaction 7.5, where each methanol molecule reacts with 1.5 O2 molecules to produce 1 CO2 molecule and 2 H2O molecules. 

However, there are still unresolved issues regarding the explanation of the impedance spectra. For example, it is difficult to distinguish the individual contributions from the anode and cathode sides, although it is generally considered that the rapid kinetics and mass transport of the HOR result in a negligible impedance contribution from the anode catalyst layer. In addition, the interpretation of the low-frequency feature can be very sophisticated. 

In addition to the equivalent circuit method, the impedance results can also be analyzed using mathematical models based on physicochemical theories. Guo and White developed a steady-state impedance model for the ORR at the PEM fuel cell cathode [15]. They assumed that the electrode consists of flooded ionomer-coated spherical agglomerates surrounded by gas pores. Stefan-Maxwell equations were used to describe the multiphase transport occurring in both the GDL and the catalyst layer. The model predicted a high-frequency loop due to the charge transfer process and a low-frequency loop due to the combined effect of the gas-phase transport resistance and the charge transfer resistance when the cathode is at high current densities. 

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