Miniaturized DMFC

DMFCs convert methanol directly into power, heat, and carbon dioxide. There are several issues particular to this type of ceU that must be addressed. 

As the methanol molecule is chemically very similar to the water molecule, methanol also diffuses into the typically used Nation membrane, which leads to a loss of the fuel. Even more critical, however, is the mixed potential, due to the presence of methanol on both the anode and cathode. Additionally, methanol is toxic and release of methanol into the environment should be minimized. There are two conventional, well-estabHshed methods to treat this problem. First, thick membranes are used. This means that the ionic resistance is also relatively high, which leads to a lower power density compared with a PEMFC. Second, methanol is used in a very low-concentration solution at the anode (usually 1-2 moll ). To take advantage of highly concentrated methanol as a fuel in the cartridge and the high energy density, the fuel within the system must be diluted with product water. 

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Fuel cells incorporating lithographic methods and masking/deposition/etching protocols have been fabricated on Si wafers and thereby satisfy two critical needs in a standard fuel cell collection of electrons (current collectors) and controlling the flow field of fuel and oxidant. Kelley et al. produced a miniature direct methanol fuel cell (DMFC) with a current— voltage and fuel utilization performance that matched standard-sized DMFCs prepared in-lab.A working volume for the miniature DMFC of 12 mm was reported, with an operational performance of 822 W h kg at 70 °C. ... 

Recently, there has been a growing interest in miniaturizing DMFCs and exploring their potential as a long-life and charge-free power source for portable and mobile... 

Many questions and aspects requiring regulations and standards remain to be answered. These include the mating of fuel cell stacks with their reactant containers (more particularly, standard junctions for cartridges holding methanol solution in miniature DMFCs) and the reporting of parameters (voltage, freqnency, etc.)... 

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... 

The fuel cell has a proven history with solid oxide based systems. Recent needs for miniaturization have generated new ideas of fuel sources and structures. These include direct methanol fuel cells (DMFCs) and polymer electrolyte membrane fuel cells (PEMFCs). These fuel cells have advantages over others for several reasons. A key quality is the potential energy that the fuel sources provide with respect to the fuel costs. 

PEM fuel cells operate with hydrogen. At the moment, there is no hydrogen storage available which is suitable for miniature applications. For direct methanol fuel cells (DMFCs) a better storage opportunity exists in form of methanol cartridges. 

Abstract Many companies are making significant efforts in the development of prototypes of DAFC (mainly DMFC) for replace batteries (battery charge and auxiliary power units) in portable devices. Some of the most relevant prototypes are summarized however, most of these devices are not ready to be commercialized due to the high cost and low power reached. Furthermore, for the massive application of the DAFC technologies is necessary solve some of the drawbacks (as miniaturization, products balance, cost reduction, etc.). The cost of the prototypes is analyzed as well as the degradation of the components that affects the durability of the devices. 

In Chap. 1 (Sects. 1.5.5 and 1.5.6) we described the active and passive mode of DAFC operation and different configurations reported in R D works. Here we will briefly summarize the configuration and operation modes of DMFC prototypes, which are closely related to the fuel management and the miniaturization of the system. 

During the past years, portable versions of fuel cells have emerged. The most promising miniature fuel cell is the DMFC, which is inexpensive and convenient, has a reasonable electrochemical performance, and does not require pressurized hydrogen gas. 

In a review by Wee 2006, the parameters of DBFCs are compared with those of DMFCs. It was shown that a DBFC system is superior to that of a DMFC system in terms of cell size and fuel solution consnmption. From an economic point of view (total operating costs) a DBFC system is more favorable in specific portable applications such as miniaturized or micro power systems with short operational time spans. In a review paper, Liu and Li (2009) mention that some problems in the field of DBHFCs (e.g., hydrogen evolution, BH crossover, and nonplatinum catalysts) remain to be investigated further and solved. 

Of all the system characteristics of DMFC with regard to commercialization as a mobile device energy source, energy efficiency, energy density, power density, and cost have to be competitive with rechargeable batteries. These requirements are somewhat different from other types of fuel cells for residential or vehicle application, which can utilize auxiliary compartments more freely. These compartments include a balance of plant (BOP), such as a compressor or pump, for fuel and oxidant supply, humidifier, heat exchanger, and so on. These comparnnents have to be miniaturized effectively or eliminated. This means that the materials in the stack should themselves have the functions of BOP. 

Typically, micro fuel cells use methanol as fuel alfhough hydrogen-fed micro fuel cells have also been developed. The choice of the type of fuel cell to use in portable devices may be limited to low-temperature fuel cells such as PEMFC (proton exchange membrane fuel cell/polymer electrolyte membrane fuel cell) and DMFC. However, micro reformed methanol fuel cells and miniature SOFCs have also been developed. 

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