Impedance cathode side

While a good equivalent-circuit representation of the transport processes in a fuel cell can lead to an increased understanding, it is not as good as taking a 1-D sandwich model and taking it into the frequency domain. These models typically analyze the cathode side of the fuel cell. °2.3i3 3i4 pj g j ost comprehensive is probably that of Springer et al. °2 The use of impedance models allows for the calculation of parameters, like gas-phase tortuosity, which cannot be determined easily by other means, and can also allow for the separation of diffusion and migra-... 

Figure 6.6. Polarization curves of fuel cells with electrodes containing 40 wt% PTFE in the gas diffusion layer. The temperature of the humidifier on the cathode side was maintained at ( ) 65°C and ( ) 80°C. For comparison, the polarization curve of the fuel cell with the electrode containing 30 wt% PTFE in the gas diffusion layer is shown at the cathode humidification temperature of 65°C (A) [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 impedance spectra of the DMFC cathode electrodes are obtained by subtracting the anode impedance from the total cell impedance. The cell impedance, ZDMFC, was obtained from normal operation of the DMFC (i.e., the cathode side was fed with air or 02 and the anode side was fed with methanol solution). The anode impedance was measured by supplying H2 to the cathode compartment, which was used as a dynamic hydrogen reference electrode. Since the impedance of the H2 electrode is negligible, the measured impedance is considered to be the anode impedance, Zanode. The cathode impedance is therefore... 

Wang, X., Lau, K.C., Turner, C.H., Dunlap, B.I. Kinetic Monte Carlo simulation of AC impedance on the cathode side of a solid oxide fuel cell. J. Electrochem. Soc. 2010,157, B90-8. 

Usually, the starting point of model derivation is either a physical description along the channel or across the membrane electrode assembly (MEA). For HT-PEFCs, the interaction of product water and electrolyte deserves special attention. Water is produced on the cathode side of the fuel cell and will either be released to the gas phase or become adsorbed in the electrolyte. As can be derived from electrochemical impedance spectroscopy (EIS) measurements [14], water production and removal are not equally fast Water uptake of the membrane is very fast because the water production takes place inside the electrolyte, whereas the transport of water vapor to the gas channels is difiusion limited. It takes several minutes before a stationary state is reached for a single cell. The electrolyte, which consists of phosphoric add, water, and the membrane polymer, changes composition as a function of temperature and water content [15-18]. As a consequence, the proton conductivity changes as a function of current density [14, 19, 20). 

Solartron 12SS and Solartron 1287 are applied for the AC impedance measurement, and Prodigit 3356 DC electronic load is applied for the power measurement. Platinum grids and leads at die anode and cathode side of the cell are used to measure cell current and vohage. 

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 a DMFC, methanol can transport through the membrane to the cathode side, where it is oxidized. This results in a much lower mixed cathode potential. Piela et al. found that the cathode impedance spectrum in a DMFC also showed pseudo-inductive behavior. They modeled such a behavior by treating the cathode as a highly non-equipotential electrode consisting of the ORR and the methanol oxidation [35]. 

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. 

Springer et al. (1996) developed a physical model for the impedance of the cathode side of a PEFC, taking into account oxygen transport in the GDL. They fitted the... 

The results of the section Impedance of the Cathode Side of a PEM Fuel Cell, have been obtained assuming constant oxygen stoichiometry X of the flow. However, impedance experiments are usually performed at a constant oxygen flow rate, rather than at a constant X. Indeed, keeping constant X means that the inlet flow rate must be changed in phase with the mean current density perturbation, which is hardly possible. 

Kulikovsky, A. A. 2012d. A model for local impedance of the cathode side of PEM fuel ceU with segmented electrodes. dElectwchejjii., 159, F294—F300. 

The preceding analysis helps us roughly quantify the contribution of the individual reactions with different time constants however, a two-electrode electrochemical cell places a serious limitation on the reliable differentiation of the time constants of the real reaction steps. That is, the impedance spectrum obtained from a two-electrode electrochemical cell is significantly distorted from the spectrum of the electrode of concern (i.e., the cathode) due to the overlap of the relaxation times for all the reactions on the anode and cathode sides. The separation in the contributions of the cathode and anode, together with setting the design strategy of the materials, will be discussed in a subsequent section. 

The V-I characteristic of a single cell A is shown in Fig. 14.8(a). Although the power density has not reached the target, it is proved that the HMFC concept works without any serious problems. The cell resistance was divided into an IR resistance and a polarization resistance by the AC impedance method. The composition of cell resistance is shown in Fig. 14.8(b). The polarization resistance is further analyzed by an AC impedance method because it is difficult to introduce a reference electrode to a fuel cell with a thin-film electrolyte [12]. AC impedance spectra were measured for various Ph2 of anode side and Pq2 of the cathode side to determine the polarization (Fig. 14.9). In all conditions, two polarization semicircles were seen. One semicircle has a peak around 3 kHz (Ra), and the other has a peak around 30 kHz (Rb). Both of these are affected by Po2 of cathode gas, and none of these is affected by Ph2 of anode gas. So, both Ra and Rb correspond to the cathode polarization, and the anode polarization is very small compared to the cathode polarization. As mentioned, Pd has very high hydrogen permeation capability, and it is reasonable that Pd has high activity as a fuel cell anode. The Po2 dependence of Ra and Rb is analyzed... 

The polarisation part contributes mainly to the impedance part The ohmic resistances increased a few milliohms within the first 500 h and then remained almost constant. The conductivity of Cr203 is almost 10 -10 " S/cm at 800°C and by the end of the stack degradation test, the Cr scale is almost 5 pm. For an interconnect plate of area 25 cm and oxide scale 5 pm thick, an increase in ohmic resistance up to 10-100 mfl could be expected. The cathode side of (Mn, Co) spinel-coated interconnect plate might contribute to the ohmic resistance. 

In the cathode side, continuous acid-base reactions, ionic exchange, and possible dissolution of the transition metal ions also lead to an increase in the impedance (in addition to capacity-fading due to the structural changes [201]). Highly critical in this respect is the contamination level in solutions, especially the HF concentration. 

Furthermore, Boukamp and Adler showed that when the electrodes on opposite sides of a cell are different from each other (as they are in a fuel cell), errors may not only involve a numerical scaling factor but also cross-contamination of anode and cathode frequency response in the measured half-cell impedances. For example. Figure 55a shows the calculated half-cell impedance of the cell idealized in Figure 53a, assuming alignment errors of 1 electrolyte thickness. Significant distortion of the halfcell impedances (Za and Zb) from the actual impedance of the electrodes are apparent, including cross-talk of anode and cathode frequency response (1 and 10 Hz, respectively), as well as a... 

Example 11.1 Diffusion with First-Order Reaction Develop an expression for the impedance response for the reduction of ferricyanide at potentials sufficiently cathodic to allow the anodic reaction to be ignored, yet sufficiently anodic to avoid reduction of oxygen (as a side reaction). Under these conditions, the reaction... 

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