DMFC Cathode

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. 

As discussed above, experimental spectra do not allow to unambiguously understand the effect of crossover. In this situation, modeling may give valuable hints. Du et al. (2007b) developed a kinetic model of simultaneous MOR and ORR in the 

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. 

In another work, Du et al. (2007a) measured the impedance spectra of a half-cell cathode, with and without 0.5M methanol solution on the anode side of the membrane. The resulting spectra show an increase in the charge transfer resistance due to crossover, and a somewhat larger inductive low-frequency loop. Using the equivalent circuit approach, the increase in the loop radius was attributed to the poisoning of the Pt surface by MOR intermediates. 

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Figure 13. Comparison of oxygen electrode performance in H2-02 PEMFC and DMFC ( ) potential of the H2-O2 PEMFC cathode, (o) potential of the DMFC cathode, (A) DMFC cell potential.

PEMFC)/direct methanol fuel cell (DMFC) cathode limit the available sites for reduction of molecular oxygen. Alternatively, at the anode of a PEMFC or DMFC, the oxidation of water is necessary to produce hydroxyl or oxygen species that participate in oxidation of strongly bound carbon monoxide species. Taylor and co-workers [Taylor et ah, 2007b] have recently reported on a systematic study that examined the potential dependence of water redox reactions over a series of different metal electrode surfaces. For comparison purposes, we will start with a brief discussion of electronic structure studies of water activity with consideration of UHV model systems. 

Finally, the liquid water flux found in the DMFC cathode exhaust is... 

Table 2.1 lists the measured value of the DMFC current density, the equivalent current density of methanol crossover, the total water flux at a DMFC cathode and the calculated water electro-osmotic drag coefficient from Equation 2.2 at various DMFC operating temperatures. 

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

Figure 5.39 shows the typical Nyquist plots of the EIS of a DMFC cathode operating on air and pure oxygen, respectively. The low-frequency arc is absent from the cathode spectrum operating on pure oxygen. Two arcs can be observed in the case of operation on air. 

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. 

To elucidate methanol crossover at the DMFC cathode, the active electrode surface of the cathode was divided into two separate parts one for oxygen reduction and the other for oxidation of crossover methanol. In this model, the methanol oxidation and oxygen reduction occur in parallel at different sites or pores because of the porous structure of the catalyst layer. The equivalent circuit for this model is presented in Figure 6.69. 

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.)...

Established in-house catalyst fabrication capability, by which otherwise unavailable catalysts can be S5mthesized, e g., 80 weight percent Pt/C catalyst for the DMFC cathode. 

Investigate composition of the DMFC cathode exhaust at various operating conditions. 

Meeting of the of the Electrochemical Society, Philadelphia, Pennsylvania, May 12-17, 2002. Title DMFC Cathode Catalyst with Improved Methanol Tolerance Y. Zhu, ... 

Liu Y, Ishihara A, Mitsushima S, Kamiya N, Ota K (2007) Transition metal oxides as DMFC cathodes without platinum. J Electrochem Soc 154(7) B664—B669... 

Li HQ, Xin Q, Li WZ, Zhou ZH, Jiang LH, Yang SH, Sun GQ (2004) An improved palladium-based DMFCs cathode catalyst Chem Commun 23 2776-2777... 

Yet, the most interesting results for ORR on Fe-Pd/WC catalyst are those obtained in the presence of alcohol. The electrode response for Fe-Pd/WC system in oxygen saturated acid has not been affected even at high concentrations of methanol, whereas the ORR on conventional catalyst Pt/C was completely restrained since the dominant reaction was basically methanol oxidation rally. Such results suggested that Pd-Fe/WC/C could be an excellent candidate for direct methanol fuel cell (DMFC) cathode because of its inert activity toward methanol oxidation as seen in Fig. 23.3. The utilization of a completely inert catalyst to methanol oxidation is one of the important criteria for DMFCs to operate at higher power densities since the issue of methanol crossover is unavoidable and can cause dramatic loss in cell performance, especially with common catalysts that are active... 

Noninteracting processes of oxygen reduction and methanol oxidation at the DMFC cathode result in a mixed potential corresponding to increase of cathode overpotential at some given cell current. 

In cells of various t3rpes, the flow in the channels can be purely gaseous (SOFC and anode of PEFC), two-phase (PEFC and DMFC cathodes, and DMFC anode) or purely liquid (DMFC anode at low current). A typical inlet flow velocity of liquid methanol-water solution in the anode channel of the DMFC varies between 0.1 and 1 cm s. The velocity of gaseous flow in fuel cells is between 10 and 10 cm s. With the typical channel diameter in the order of 0.1 cm, the Reynolds number varies in the range of 100-1000 and hence the flows are laminar. 

Wippermann K, Richter B, Klafki K, Mergel J, Zehl G, Dorbandt I, Bogdanoff P, Fiechter S, Kaytakoglu S (2007) Carbon supported Ru - Se as methanol tolerant catalysts for DMFC cathodes part II preparation and characterization of MEAs. J Appl Electrochem 37 1399-1411... 

Mazza F, Trassatti S (1963) Transition metal oxides as DMFC cathodes without platinum. 

Combinatorial methods have also been applied to the area of exploring methanol-tolerant Pt and non-Pt alloy eatalysts for DMFC cathodes. Liu et al. investigated a series of Pt-based and non-Pt binary alloys as ORR electrocatalysts by employing the high-throughput optical screening method. The catalyst arrays were prepared... 

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