In DMFCs

The effects of dispersion of the electrocatalyst and of particle size on the kinetics of electrooxidation of methanol have been the subject of numerous studies because of the utilization of carbon support in DMFC anodes. The main objective is to determine the optimum size of the platinum anode particles in order to increase the effectiveness factor of platinum. Such a size effect, which is widely recognized in the case of the reduction of oxygen, is still a subject of discussion for the oxidation of methanol. According to some investigators, an optimum of 2 nm for the platinum particle size exists, but studying particle sizes up to 1.4 nm, other authors observed no size effect. According to a recent study, the rate of oxidation of methanol remains constant for particles greater than 4.5 nm, but decreases with size for smaller particles (up to 2.2 nm). 

In this section, we summarize the kinetic behavior of the oxygen reduction reaction (ORR), mainly on platinum electrodes since this metal is the most active electrocatalyst for this reaction in an acidic medium. The discussion will, however, be restricted to the characteristics of this reaction in DMFCs because of the possible presence in the cathode compartment of methanol, which can cross over the proton exchange membrane. 

Table 4 and Fig. 18 illustrate the performance levels achieved by the active players in DMFC R D. The main goal in DMFC research in the U.S. and European programs is to achieve a stable performance level of 200 mW/cm at a cell potential of 0.5 to 0.6 V. It is because of the relatively low activity of the electrocatalyst for methanol electrooxidation that this power level is less than half that of a PEMFC with Hj as a fuel. A higher power level of the DMFC is essential for a transportation application, but the present power level goal is quite adequate for small portable power sources. 

Waszczuk et al., 2001b Tong et al., 2002]. Because Ru is deposited as nanosized Ru islands of monoatomic height, the Ru coverage of Pt could be determined accurately. In that case, the best activity with regard to methanol oxidation was found for a Ru coverage close to 40-50% at 0.3 and 0.5 V vs. RHE. However, the structure of such catalysts and the conditions of smdy are far from those used in DMFCs. Moreover, the surface composition of a bimetallic catalyst likely depends on the method of preparation of the catalyst [Caillard et al., 2006] and on the potential [Blasini et al., 2006]. 

Such bimetallic alloys display higher tolerance to the presence of methanol, as shown in Fig. 11.12, where Pt-Cr/C is compared with Pt/C. However, an increase in alcohol concentration leads to a decrease in the tolerance of the catalyst [Koffi et al., 2005 Coutanceau et ah, 2006]. Low power densities are currently obtained in DMFCs working at low temperature [Hogarth and Ralph, 2002] because it is difficult to activate the oxidation reaction of the alcohol and the reduction reaction of molecular oxygen at room temperature. To counterbalance the loss of performance of the cell due to low reaction rates, the membrane thickness can be reduced in order to increase its conductance [Shen et al., 2004]. As a result, methanol crossover is strongly increased. This could be detrimental to the fuel cell s electrical performance, as methanol acts as a poison for conventional Pt-based catalysts present in fuel cell cathodes, especially in the case of mini or micro fuel cell applications, where high methanol concentrations are required (5-10 M). 

The hnding of very substantial amounts of incomplete oxidation products for methanol and formaldehyde oxidation can have considerable consequences for technical applications, such as in DMFCs. In that case, the release of formaldehyde at the fuel cell exhaust has to be avoided not only from efficiency and energetic reasons, but in particular because of the toxicity of formaldehyde. While in standard DMFC applications the catalyst loading is sufficiently high that this is not a problem, i.e., only CO2 is detected [Arico et al., 1998], the trend to reducing the catalyst loading or applications in micro fuel cells may lead to situations where the formation of incomplete oxidation products could indeed become problematic (see also Wasmus et al. [1995]). For such purposes, one could dehne a maximum space velocity above which formation of incomplete oxidation products may become critical. 

Direct-methanol fuel cells (DMFCs) have attracted considerable attention for certain mobile and portable applications, because of their high specific energy density, low poison emissions, easy fuel handling, and miniaturization [129,130], However, the methanol permeation through electrolyte membranes (usually called methanol cross-over) in DMFCs still is one of the critical problems hindering the commercialization [131,132], Nafion , a... 

Cross-linked, sulfonic-acid-substituted, polyphosphazene-based PEMs have primarily been examined for potential use in DMFC applications due to their low MeOH crossover with reported values 2.5 times lower than that of Nafion. These materials have also been shown to display good thermomechanical and chemical stability (in a Fenton test). Sulfonamide-substituted polyphosphazenes have exhibited very high power densities that are comparable with Nation and may be suitable for use in PEMFC applications. ... 

A more recent example from this group is 23b. Conductivity measurements under fully humidified conditions for 23b were on average about an order of magnitude lower than for Nation. However, under dry conditions, the values were only slightly lower for 23b in comparison to Nation. Interestingly, the MeOH diffusion coefficients for 23 were 20-40 times lower than for Nation. This might make these membranes potentially suitable for use in DMFC applications (see Figure 3.25). ... 

Metal foams have been used in the past in the development of FF plates. However, Gamburzev and Appleby [53] used Ni foams as both a DL and a flow field plate with an MPL layer on one of its surfaces. They observed that such a design had high contact resistance between the nickel foam and the MPL and also increased gas diffusion resistance due to the required MPL thickness. Arisetty, Prasad, and Advani [54] were able to demonstrate that these materials can also be used as potential anode diffusion layers in DMFCs (see Figure 4.10). In fact, the nickel foam used in this study performed better than a carbon cloth (Avcarb 1071HCB) and a stainless steel mesh. However, it was recognized that a major drawback for these foams is their susceptibility to corrosion. 

Unfortunately, few experimental data have been published regarding these types of diffusion layers. Yazici [65] presented a study in which the graphite foils made by Graftech Inc. were used as cathode diffusion layers in DMFCs. Two foils were used one was made out of 80% expanded graphite and 20% PTFE coated carbon particles to form a porous sheet, and the other was identical to the first except that it was perforated for more permeability with 2,500 tips per square inch (15% open area). 

In DMFCs, methanol crossover and carbon dioxide gas management are critical issues that have be dealt with. Argyropoulos, Scott, and Taama [98] used a transparent fuel cell (fhe anode end plate was made out of acrylic) to visualize the CO2 evolution and management on the anode side. Both CFPs and CCs were used as anode DLs and it was observed that CFP (Toray carbon paper) was not a suitable material due to its poor gas removal properties. 

In DMFCs, Scott, Taama, and Argyropoulos [117] changed the PTFE content (from 0 to 40 wt%) of the anode DL (E-TEK type A CC) in order to observe how this affected the methanol and carbon dioxide transport through the DL. At very high levels of PTFE, the performance of the cell decreases due to an increase in resistance losses. On the other hand, when an untreated CC was used, the observed performance was the lowest of all the materials investigated. In this study it was concluded that the ideal amount of hydro-phobic agent for the anode DL is around 13-20 wt% (see Figure 4.17). 

In DMFCs, Xu et al. [119] tested various carbon fiber papers with different thicknesses (TGP-H-030, TGP-H-060, TGP-H-090, and TGP-H-120) as anode... 

In DMFCs, the water balance analytical method has been used as a tool to study the fuel (methanol) and water crossover from the anode toward the cathode. Xu, Zhao, and He [120] and Xu and Zhao [180] performed a thorough investigation of how different cathode DLs and MPLs affected the total water crossover from the anode side. In order to be able to perform the water balance equations, they also collected the water at both outlets of the cell. This analysis technique was vital for them to be able to observe how different characteristics for fhe cafhode DL affect not only the overall performance of the fuel cell buf also fhe nef wafer drag coefficient and water crossover in DMFCs. 

G. Lu, F. Liu, and G. Y. Wang. Water transport through Nation 112 membrane in DMFCs. Electrochemical and Solid State Letters 8 (2005) A1-A4. 

H. Yang, T. S. Zhao, and Q. Ye. In situ visualization study of CO2 gas bubble behavior in DMFC anode flow fields. Journal of Power Sources 139 (2005) 79-90. 

Most of the catalysts employed in PEM and direct methanol fuel cells, DMFCs, are based on Pt, as discussed above. However, when used as cathode catalysts in DMFCs, Pt containing catalysts can become poisoned by methanol that crosses over from the anode. Thus, considerable effort has been invested in the search for both methanol resistant membranes and cathode catalysts that are tolerant to methanol. Two classes of catalysts have been shown to exhibit oxygen reduction catalysis and methanol resistance, ruthenium chalcogen based catalysts " " and metal macrocycle complexes, such as porphyrins or phthalocyanines. ... 

DMFC modeling thus aims to provide a useful tool for the basic understanding of transport and electrochemical phenomena in DMFC and for the optimization of cell design and operating conditions. This modeling is challenging in that it entails the two-phase treatment for both anode and cathode and that both the exact role of the surface treatment in backing layers and the physical processes which control liquid-phase transport are unknown. 

Pt/Ru electrocatalysts are currently used in DMFC stacks of a few watts to a few kilowatts. The atomic ratio between Pt and Ru, the particle si2 e and the metal loading of carbon-supported anodes play a key role in their electrocatalytic behavior. Commercial electrocatalysts (e.g. from E-Tek) consist of 1 1 Pt/Ru catalysts dispersed on an electron-conducting substrate, for example carbon powder such as Vulcan XC72 (specific surface area of 200-250 m g ). However, fundamental studies carried out in our laboratory [13] showed that a 4 1 Pt/Ru ratio gives higher current and power densities (Figure 1.6). 

In this chapter, we reviewed the structure-controlled syntheses of CNFs in an attempt to offer better catalyst supports for fuel cell applications. Also, selected carbon nanofibers are used as supports for anode metal catalysts in DMFCs. The catalytic activity and the efficiency of transferring protons to ion-exchange membranes have been examined in half cells and single cells. The effects of the fiber diameter, graphene alignment and porosity on the activity of the CNF-supported catalysts have been examined in detail. 

Combinations of platium catalyst and Fe-teteraphenylparpherine complex increased the efficiency of oxygen reduction [21]. The decay product of methanol in DMFC... 

The major problem associated with the operation of DMFCs is the gradual diffusion of methanol through the membrane - known as methanol crossover - that leads to the establishment of a mixed potential at the cathode and, consequently, to a decrease of the working voltage of the cell. Because of the methanol crossover phenomenon, the maximum methanol concentration used in DMFCs is about 2M and membranes as thick as 175 pm are used. 

The theoretical cell voltage of a DMFC at standard conditions is 1.20 V. The materials used in DMFCs are similar to those in PEMFCs. Pt, PtRu, and Nafion membrane are used as cathode catalyst, anode catalyst, and proton transfer membranes, respectively. However, the catalyst loading in a DMFC is much higher than the loading used in H2/air fuel cells, because both side reactions are slow (Pt loadings 4 mg/cm2 for a DMFC, 0.8 mg/cm2 for a H2/air fuel cell). 

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 typical impedance spectra do not illustrate the inductance effect that is normally observed in DMFCs, especially at high overpotentials, where the methanol oxidation rate is higher and the CO coverage decreases with increasing potential. Figure 6.66 shows the DMFC anode impedance spectra at different current densities the corresponding equivalent circuit is shown in Figure 6.67. 

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. 

United Technologies Fuel Cells is engaged in DMFC development, in competition with Ballard/Johnson Matthey. It is a part in the project by Renault to develop the Scenic vehicle fuel cell. Neither for its PEFC, nor for its DMFC (and MCFC), does UTC Fuel Cells offer product-coloured illustrations. Moreover, its literature or listed web site does not deal with the cell voltage reversal problem, mentioned in Ballard patents above in connection with fuel cell bus operation. Accordingly it is not possible for the author to portray the UTC Fuel Cells scheme of things. 

Reddington et al. (66) reported the synthesis and screening of a 645-member discrete materials library L9 as a source of catalysts for the anode catalysis of direct methanol fuel cells (DMFCs), with the relevant goal of improving their properties as fuel cells for vehicles and other applications. The anode oxidation in DMFCs is reported in equation 1 (Fig. 11.12). At the time of the publication, state-of-the-art anode catalysts were either binary Pt-Ru alloys (67) or ternary Pt-Ru-Os alloys (68). A systematic exploration of ternary or higher order alloys as anode catalysts for DMFCs was not available, and predictive models to orient the efforts were also lacking. 

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