Impedance DMFC cathode

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

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.)...

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. 

Figure 6.68. Impedance spectra of a DMFC cathode. Experimental conditions 75°C, air stoichiometric ratio 10, 69 mA cnT2, 0.879 V, and frequency range 10 kHz-0.1 Hz [57], (Reproduced by permission of ECS—The Electrochemical Society, from Piela P, Fields R, Zelenay P. Electrochemical impedance spectroscopy for direct methanol fuel cell diagnostics.)...

Figure 5.19a shows the impedance spectra corresponding to the DMFC cathode (the last column in Table 5.6), in which both oxygen and proton transport contribute to potential loss. For simplicity, the effect of fuel (methanol) crossover peculiar to... 

DMFC cathodes is ignored (this effect will be considered in the section Impedance of DMFC Cathode ). Figure 5.19b depicts the respective shapes of the oxygen concentration and local overpotential through the CCL. 

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. 

DMFC cathode. The model was used to fit experimental impedance spectra the contributions due to catalyst poisoning by MOR intermediates and due to parasitic current produced in the MOR have been calculated. However, Du et al. (2007b) assumed that the ORR and MOR are running uniformly over the cathode thickness. In that case, the above effects only weakly distort the shape of the cathode semicircle. 

An impedance model of the DMFC cathode, with a detailed description of the MOR and ORR reaction mechanisms, has been reported by Chen et al. (2009). However, in this work, the cathode was treated as an infinitely thin interface and no space-resolved transport processes within the CCL were taken into account. 

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. 2012b. A model for DMFC cathode impedance The effect of methanol crossover. 24, 65-68. 

For EIS measurements of a direct methanol fuel cell (DMFC), the anode is supplied with an aqueous solution of methanol at a concentration such as 1 M and using controlled flow rates. The cathode is operated on either air, oxygen, or hydrogen, with controlled flow rate and pressure [43], In order to measure the anode EIS, the DMFC is fed with hydrogen gas instead of air or oxygen, to eliminate the contributions of the cathode. This cathode is normally denoted as a dynamic hydrogen electrode (DHE). Thus, the anode impedance spectra between the anode and the DHE can be obtained in a complete fuel cell. 

Record the impedance spectrum of a complete DMFC with the cathode operated as usual on air or oxygen. 

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... 

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. 

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. 

Figure 9-2 shows the discharge curve of borosilicate electrode versus the standard carbon electrode of the H-Tec fuel cell. The load across the fuel cell for this test is 10 Q, resulting in a discharge of 100 mA cm. The cermet electrode demonstrates minimal increases of impedance over the discharge period and the higher overall voltage. The maximum developed by borosihcate substrate is 0.3489 V. This demonstrates that Ag metallization with a platinum/ruthenium catalyst can be developed as a cathode structure in DMFCs. 

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. 

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]. 

Yang et al. (2010) developed an EIS technique to characterize a DMFC under various operating conditions. A silver/silver chloride electrode was used as an external reference electrode to probe the anode and cathode during fuel cell operation. The external reference was sensitive to the anode and cathode as current was passed in the working DMFC. The impedance spectra and DMFC polarization curves were investigated systematically as a function of air and methanol flow rates, methanol concentration, temperature, and current density. Water flooding in the cathode was also examined. 

Du, C. Y, Zhao, T. S., and Xu, C. 2007h. Simultaneous oxygen-reduction and methanol-oxidation reactions at the cathode of a DMFC A model-hased electrochemical impedance spectroscopy study. J Power ource, 167, 265-271. 

AC impedance studies have indicated that DMFC performance loss due to interfacial failure can be linked to (1) increased ohmic resistance of the cell, (2) enhanced electrode overpotential, and (3) electrode flooding. Ohmic loss due to the interfacial resistance buildup can contribute several tens of milliohm square centimeters to the total ceU resistance. An increase in the overpotential is caused by the loss of contact between the membrane and the catalyst, which renders part of the catalyst layer unusable (a possible major performance loss). Electrode flooding can be attributed to the nonuniform current distribution, which is more significant when membrane-cathode delamination occurs. 

It should be noted that the inductive part of the complex-plane impedance plots is not unique to the response of the anode in a DMFC. In Figure 16.11 we show similar behavior for the cathode of a DMFC. Moreover, pseudoinductance at very... 

Figure 16.11 Complex-plane impedance of the cathode in a DMFC, showing pseudoinductance. Reprinted with permission from P. Plela, R. Fields and P. ZelenayJ. Electrochem. Soc. A 153, (2006) 1902.

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