Impedance catalyst layer

Kulikovsky, A. A. 2012g. A physical model for the catalyst layer impedance. 

Makharia R, Mathias ME, Baker DR. 2005. Measurement of catalyst layer electrolyte resistance in PEECs using electrochemical impedance spectroscopy. J Electrochem Soc 152 A970-A977. 

Figure 4.33. Equivalent circuit of a catalyst layer [8]. (Reproduced by permission of the authors and of ECS—The Electrochemical Society, from Lefebvre MC, Martin RB, Pickup PG. Characterization of ionic conductivity within proton exchange membrane fuel cell gas diffusion electrodes by impedance spectroscopy.)...

Conductive polymers are not used in fuel cells. However, the equivalent circuit of conductive polymers is similar to that of catalyst layers, which may help to understand impedance spectra in fuel cells. In general, the electric circuits of... 

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. 

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 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... 

Fuel cell performance is affected by MEA composition, including catalyst loading, PTFE content in the gas diffusion layer, and Nafion content in the catalyst layer and membrane, each of which affects the performance in different ways, yielding distinct characteristics in the electrochemical impedance spectra. Even different fabrication methods may influence a cell s performance and electrochemical impedance spectra. With the help of the model described above, impedance spectra can provide us with a useful tool to probe structure-performance relationships and thereby optimize MEA structure and fabrication methods. 

Figure 6.3 shows the impedance spectra with catalyst loading of 0.4 and 0.1 mg Pt/cm2 at different potentials. As can be clearly seen there, the potential-independent branch, which indicates the presence of the distributed resistance inside the catalyst layer of the electrode, is much less prominent for the electrode... 

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. 

In Song et al. s same work [5], the effect that Nafion content in the catalyst layer had upon electrode performance was also investigated, following their work on the optimization of PTFE content in the gas diffusion layer. The optimization of Nafion content was done by comparing the performance of electrodes with different Nafion content in the catalyst layer while keeping other parameters of the electrode at their optimal values. Figures 6.8 and 6.9 show the polarization curves and impedance spectra of fuel cells with electrodes made of catalyst layers containing various amounts ofNafion . 

Figure 6.8. Polarization curves of fuel cells with electrodes made of catalyst layers containing various amounts of Nation ( ) 0.2 ( ) 0.8 (A) 2.0 mg/cm2 [5], (Reprinted from Journal of Power Sources, 94(1), Song JM, Cha SY, Lee WM. Optimal composition of polymer electrolyte fuel cell electrodes determined by the AC impedance method, 78-84, 2001, with permission from Elsevier and the authors.)...

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... 

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. 

To increase fundamental knowledge about ionic resistance, it is important to develop a methodology to experimentally isolate the contributions of the various cell components. Electrochemical impedance spectroscopy has been widely used by Pickup s research group to study the capacitance and ion conductivity of fuel cell catalyst layers [24-27] they performed impedance experiments under a nitrogen atmosphere, which simplified the impedance response of the electrode. Saab et al. [28] also presented a method to extract ohmic resistance, CL electrolyte resistance, and double-layer capacitance from impedance spectra using both the H2/02 and H2/N2 feed gases. In this section, we will focus on the work by Pickup et al. on using EIS to obtain ionic conductivity information from operational catalyst layers. 

The results also suggest that through AC impedance measurements, the performance drops caused by individual processes such as electrode kinetic resistance, membrane resistance, and mass transfer resistance can be correlated to either reduction or improvement in cell performance. If individual impedances are known, the contribution to the change in performance can be identified, which is very important in the design and optimization of high-temperature MEA catalyst layer components, structure down-selection, and MEA architecture. 

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