Cathode impedance

DMFC cathode impedance spectra can be obtained by the following steps ... 

Subtract the anode impedances from the complete cell impedances, thereby obtaining a cathode impedance spectrum [43],... 

Figure 5.39. Nyquist plots of a typical DMFC cathode impedance spectrum ( ), operation on air ( ), operation on oxygen [43], (Reprinted from Journal of Power Sources, 75, Muller JT, Urban PM. Characterization of direct methanol fuel cells by AC impedance spectroscopy, 139M3, 1998, with permission from Elsevier and the authors.)...

Figure 5.42 shows an example of the effect of humidity on the EIS spectra. It can be seen that the cut-off for anode humidification does not affect the spectra too much compared with the cut-off for cathode humidification. As we know, the fuel cell EIS primarily represents the cathode behaviour. Therefore, cathode humidification can greatly affect the whole impedance spectra. The humidification cut-off at the cathode causes a large difference in both the membrane resistance and the kinetic resistance. Dehydration of the anode also brings about a substantial increase in cathode impedance because a dry anode pulls water away from the cathode and across the membrane, which makes it hard to keep the cathode well hydrated [18],... 

To separate the contribution of the anode from that of the cathode, Wagner et al. provided a more general equivalent circuit, as shown in Figure 6.33. The anode, the cathode, and the membrane resistance were in series. A parasitic inductance due to the mutual induction effect was also included. The cathode impedance was given by charge-transfer resistance (Rct0i) and constant phase element (CPE) in... 

Methanol oxidation in DMFCs has also been investigated with impedance spectroscopy. The anode impedance spectrum was usually obtained using the cathode as a reference electrode, by supplying hydrogen to the cathode, or by using a reference electrode to separate the anode and cathode impedance spectra. The... 

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... 

Typical impedance spectra of a DMFC cathode operating on air and pure 02 were shown in Chapter 5, Figure 5.39. Two arcs were observed when air was used as the oxidant, while only one arc was observed for 02. According to the previous equation, the membrane resistance (or the arc at high frequencies caused by the membrane) was not present in the cathode impedance spectra. Since the membrane impedance was included in both the total cell impedance ZDMFC and the anode impedance subtracting the two cancelled out the membrane impedance. 

Unlike the cathode reaction in hydrogen/air PEM fuel cells, the cathode reaction in a DMFC also involves poisoning of the catalyst due to methanol crossover. Methanol oxidation on the electrode can lead to CO adsorption, which usually results in inductance at low frequencies. In some cases, an inductance loop is observed in cathode impedance spectra, as shown in Figure 6.68. 

In spite of the bulk electrolyte resistance cancelling out in Equation 6.68, small differences between the series resistance in the measurements of ZDMPC and Zanode also contribute to cathode impedance. Considering the mass transport resistance of 02 in the pores and the bulk electrolyte resistance, the cathode impedance is given by... 

From this physical model, an electrical model of the interface can be given. Free corrosion is the association of an anodic process (iron dissolution) and a cathodic process (electrolyte reduction). Ther ore, as discussed in Section 9.2.1, the total impedance of the system near the corrosion potential is equivalent to an anodic impedance Za in parallel with a cathodic impedance Zc with a solution resistance Re added in series as shoxvn in Figure 13.13(a). The anodic impedance Za is simply depicted by a double-layer capacitance in parallel with a charge-transfer resistance (Figure 13.13(b)). The cathodic branch is described, following the method of de Levie, by a distributed impedance in space as a transmission line in the conducting macropore (Figure 13.12). The interfacial impedance of the microporous... 

The calculation gives, for the cathodic impedance, the general form... 

Furthermore, by varying other experimental conditions such as current load, temperature, gas composition, and as recently shown by Andreaus et al. [2002] hydrogen humidification and membrane thickness, measured cell impedance can be split into anode impedance, cathode impedance and electrolyte resistance, without using reference electrodes. These results were used to derive appropriate equivalent circuits for the analysis of impedance spectra measured on fuel cells operating with H2/O2, H2/air and H2 + lOOppm CO/O2. The variation of the experimental conditions is also a useful method to confirm the accuracy of the equivalent circuit. 

Fig. U Schematic of a bipolar lead illustrating the factors involved in determining system impedance. The arrows denote current flow. Resistance to current flow occurs at the lead conductor (conductor resistance), at the cathode-tissue interface (cathode impedance and polarization), in the myocardium (tissue impedance) and at the anode (anode impedance). The largest contributors to system impedance are the cathode impedance and polarization effects.

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]. 

The electrode impedance equation has positive signs for the anode impedance and negative for the cathode impedance. Equation (8.56) was the time... 

NO2 in the air stream degrades the performance of the fuel cell. The poisoning effects of NO2 are mainly due to the overlapping of the ORR, NO, and HNO2 oxidation reactions, and increased cathodic impedance [141,147]. 

In both cases, the LiF forms directly on the active LiNi gCOgP surface, which will have a more serious effect on cell performance than if LiF is formed from decomposition reactions throughout the electrolyte. The observed increase in cathode impedance with temperature thus results from the combined effects of a thickening of the polymeric surface layer and LiF formation adjacent to the LiNi gCOo jOj surface. 

The feature of DMFC cathode is a large flux of methanol permeated through the polymer membrane from the anode. The methanol crossover may be expected to affect DMFC cathode impedance spectra. These spectra have been measured by Muller and Urban (1998), Diard et al. (2003), Furukawa et al. (2005), and Piela et al. (2006). However, up to now, the effect of crossover on DMFC cathode spectra has not been fully understood in none of these works, the features of the spectra, due to methanol crossover, have been clearly identified. This suggests that the effect of crossover is either small or it is masked by other effects. 

Measuring DMFC cathode impedance is difficult for two reasons. First, it is hard to achieve tme steady-state DMFC operation. Typically, the cell potential slightly varies with time (drift). Another problem is that in a cell without a reference electrode, the cathode impedance cannot be measured directly. Usually, the impedance of the whole cell is measured first. The oxygen on the cathode is then replaced by hydrogen, and the impedance of this quasi-half cell (anode impedance) is subtracted from the whole-cell curve to obtain the cathode spectrum. 

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. 

In this section, a model for the PEFC cathode impedance is discussed, including oxygen transport in the channel (Kulikovsky, 2012d). The model is based on the transient CCL performance model from the section Basic Equations linked to the nonstationary extensions of the models for oxygen transport in the GDL and in the channel, discussed in the section Performance Modeling of a Fuel Cell. ... 

Diard, J.-R, Glandut, N., Landaud, R, Gorrec, B. Le, and MonteUa, C. 2003. A method for determining anode and cathode impedances of a direct methanol fuel cell running on a load. Electrochim. Acta. 48, 555-562. 

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