Direct methanol fuel cells issues

DMFC research at LANE in FY 2002 has focused primarily on fundamental issues relevant to potential portable and transportation applications of direct methanol fuel cells, such as cathode and anode electrocatalysis, electrode composition and structure, membrane properties and MEA design. Substantial progress has been achieved in cathode research. 

MRS Spring Meeting, San Francisco, CA, April 2, 2002. Title System and Material Issues in Direct Methanol Fuel Cells B. S. Pivovar... 

The aim of this book is to discuss the development, characterization, fuel cell performance, and applications of the PEM direct methanol fuel cells. Regarding the last issue, the lower power density and higher costs of DMFCs compared to conventional hydrogen-feed PEM fuel cells, have reoriented their short-term applications towards portable electronics. 

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... 

We conclude that carbon nanocoils (CNCs) are an excellent support for flie electrodes of direct methanol fuel cells. Further, the CNC maintains stable catalytic activity by measuring power density versus time for up to 100 hr in a DMFC unit cell in the performance of fuel cells since the durability of the electrode is significant for long-term applications, it is believed that the CNC is a highly promising electrode for long-term fuel cells. However, since the issue of durability and stability of the fuel cell electrode is very important, more in-depth study of these characteristics is still underway. 

In comparison with the extensive research and development of catalysts for PEMFC and DMFC, the development of catalysts for DEFCs has drawn a surge of interest recently in recent years due to its bio-fuel characteristic, its ability to eliminate the toxicity issue of methanol as in direct methanol fuel cells, and its high energy density. Carbon-supported PtRu, a well-known catalyst for DMFCs, was naturally studied for DEFCs [59], but lacks high activity for ethanol oxidation due to a high propensity of Ru to form RuOH at the oxidation potential region. 

Despite advancement in the development of direct methanol fuel cells (DMFCs), some restrictions still inhibit their large-scale commercialization. This chapter has discussed one of the primary constraints, that is, identification of appropriate membrane materials. Nafion membranes that dominate the market of polymer electrolyte membranes allow methanol permeation from the anode to the cathode side of a DMFC. This results in serious negative consequences. Three approaches have been pursued in order to resolve the methanol permeation problem. These include Nafion membranes modification, development of alternative membranes and provision of high activity anode catalysts or methanol tolerant cathode catalysts. All the three options have achieved certain degree of success in solving the problan. Of particular interest are the Nafion membranes modification and development of alternative membranes in which membranes with permeability values of 10 to 70 times lower than the pure Nafion membranes have been developed. In general, based on the tremendous research efforts being made to develop DMFCs membranes with the best qualities, we are optimistic that very soon the issue of methanol permeation shall become a history. 

Portable power—Many companies (MTI, Motorola, NEC, Fuji, Matsushita, Medis, Manhattan Scientific, Polyfuel) are developing miniature fuel cells as battery replacements for various consumer and military electronic devices. Because of fuel storage issues, most of them use methanol in either direct methanol fuel cells or through microreformer in regular PEM fuel cells. 

In recent years, PVA has been used as PEM for fuel cell applications [11-16]. Furthermore, it shows unique characterizations of good resistance to organic solvent and superior methanol barriers to Naflon, where the alcohol cross leak is a key issue in practical uses for direct methanol fuel cells (DMFCs). Since no ionic group is contained in the side chains, acid or basic groups should be introduced through ionic polymers or ionic solution. 

The worst aspect of the Nation membrane is its high cost Massive R D efforts have been going on for years to develop low cost cation-selective membranes having a performance comparable to that of Nation, at a much lower price. This is a critical issue, because the commercial success or failure of the direct methanol fuel cell hinges on our ability to develop such membranes at (1-2)% of their present cost... 

In direct liquid fuel cells, the use of MPLs is also very popular and most of the details explained earlier also apply to the liquid fuel cells. However, some of the parameters differ from those in PEM fuel cells because there are other mass transfer-based issues in DLFCs, especially on the anode side related to methanol crossover and CO2 production. 

Microfabrication processes have been used successfully to form micro-fuel cells on silicon wafers. Aspects of the design, materials, and forming of a micro-fabricated methanol fuel cell have been presented. The processes yielded reproducible, controlled structures that performed well for liquid feed, direct methanol/Oj saturated solution (1.4 mW cm ) and direct methanol/H O systems (8 mA cm" ). In addition to optimizing micro-fuel cell operating performance, there are many system-level issues to be considered when developing a complete micro power system. These issues include electro-deposition procedure, catalyst loading, channel depth, oxidants supply, and system integration. The micro-fabrication processes that have... 

One of the main issues in the direct alcohol fuel cells (DAFCs) is that the fuel can easily permeate into the cathode through the proton exchange membrane, which causes dramatic performance loss since the currently used Pt-containing cathode catalysts have no or little methanol tolerance. One of the advantages of Pd-M alloys over Pt in DAFCs is their high methanol and ethanol tolerance in acid. In particular, methanol tolerance was demonstrated for Pd-Fe, Pd-Co, Pd-Cr, Pd-Ni, and Pd-Pt alloys [19, 41, 53, 77-80]. 

Direct ethanol fuel cells (DEFCs) produce power directly from ethanol without prior reforming. Compared with methanol used as the fuel for DMFCs, ethanol is nontoxic, environmentally friendly, and universally available, and making the handling easy. Since ethanol is also a liquid alcohol like methanol, the technological issues of crossover, discharge of carbon dioxide, and passive operation are comparable. 

Some developers have investigated a direct methanol alkaline cell to circumnavigate hydrocarbon fuel separator issues. These cells exhibit a reduced performance, and have not been as thoroughly investigated as the hydrogen-fueled cells. 

Better established is their use as photocatalysts, in the photoelectro-catalytic production of H2 or the elimination of pollutants, and in developing advanced electrodes for fuel cells, particularly for direct methanol or ethanol oxidation. Nevertheless, also in this case the field can be still considered to be at an earlier stage. It has been shown how several of the results have to be further demonstrated, and issues and limits better defined. However, there are clear indications that this will be a major area of research not only for this specific field, but in general for all catalysis. The recent US DoE report Catalysis for Energy also indicates that the development of better tailored nanostructures for photo- and electro-catalytic applications, particularly for better use of renewable resources, is one of the priority areas of research in catalysis and in general of science. 

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