Portable fuel cells characteristics

TABLE 42.3 Characteristics of Fuels for Use In Portable Fuel Cells... 

These characteristics explain why the PEMFC has become such a promising fuel cell candidate for powering vehicles, and for both stationary and portable power generation [1],... 

With the experience gathered in the development of direct methanol fuel cells, platinum-ruthenium catalysts were used for the anodic process in the first studies on direct formic acid fiiel cells. Then, it was shown that much better electrical characteristics can be obtained with palladium black as the catalyst. Importantly, with this catalyst, one can work at much lower temperatures. In particular, at a temperature of 30°C power densities of 300 mW/cm were obtained with a voltage of 0.46 V, and about 120 mW/cm with a voltage of 0.7 V. Considering all these special features, it will be very convenient to use formic acid as a reactant in fuel cells of small size, for power supply in portable equipment, ordinarily operated at ambient temperature. 

Fuel Cell Unit Size The size of the fuel cell is a characteristic that impacts fuel processor selection. There is a lower level of power output at which it is no longer advantageous to incorporate a fuel processor. The decision is also application-specific. It is likely that releasing H2 by chemical reaction from a solid compound when mixed with water is economical for small portable units (below 100 W). An H2 storage cartridge can be replaced in seconds (15). 

In practice, the suitability of a reaction system is determined by the kinetics of the reaction, which depends on temperature, pressure of gases, electrode polarization, surface area of electrodes, and presence of a catalyst. A fuel cell that is thermodynamically and kinetically feasible must be considered from an econonuc viewpoint before it is accepted. Thus, since hydrogen, hydrazine, and methanol are too expensive for general application, their use in fuel cells has been limited to special cases. Hydrogen has been used for fuel cells in satellites and space vehicles, in which reliability and lightness are more important than cost. Hydrazine fuel cells have been used in portable-radio power supplies for the United States Army because of their truly silent operation. Methanol fuel cells have been used to power navigation buoys and remote alpine television repeater stations because such power systems are comparatively free from maintenance problems over periods of a year or more. The polarization at the electrodes of a fuel cell is the most important single factor that limits the usefulness of the cell. The various polarization characteristics for a typical fuel cell are plotted separately as a function of current density in Fig. 9.11. 

Of all the fuel cells, polymer electrolyte fuel cells have very interesting characteristics, and they owe a great deal to solid electrolyte gels. In the future, establishment of a fuel supply infrastructure and progress in solid electrolyte technology will make possible relatively small-scale power generation for automobiles, computers, portable telephones, homes, buildings, schools, and hospitals, all to be derived from fuel cells. 

The preparation complexity of perfluorosulfonated membrane and the high cost have restricted PEMFC from commercialization. Many researchers are dedicated to the development of nonflnorinated PEM. The American company Dais has developed styrene/ethylene-bntylene/styrene triblock polymer [51]. This membrane is especially snitable for small power PEMFC working at room temperature. The lifetime of the membrane is up to 4000 h. Baglio did some experiments to test the performance comparison of portable direct methanol fuel cell mini-stacks between a low-cost nonfluorinated polymer electrolyte and Nafion membrane. He found that at room temperature, a single-cell nonfluorinated membrane can achieve maximum power density of about 18 mW/cm. As a comparison, the value was 31 mW/cm for Nafion 117 membrane. Despite the lower performance, the nonfluorinated membrane showed good characteristics for application in portable DMFCs especially regarded to the perspectives of significant cost reduction [52]. 

Many new types of batteries have been introduced in recent years, as demand for portable electronic devices such as cell phones, music players, and laptop computers has soared. What factors influence the choice of battery types for a particular application One of the most important considerations is energy density, just as we discussed earlier for fuels. Table 9.5 summarizes typical characteristics and uses of several types of batteries. 

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