Small fuel cells characteristics

During the 1960s, an American company du Pont de Nemours started to develop a new polymeric ion-exchange membrane marketed as Nation . This had a drastic effect on the further development of fuel cells, particularly for applications of the second type. It was soon obvious that this membrane could help to strongly enhance the characteristics and lifetime of relatively small fuel cells. At the same time, success was achieved toward substantially lower platinum catalyst outlays in such fuel cells. These developments aroused the interest of potential fuel-cell users. 

Small fuel cell systems, especially those operating on hydrogen fuel, are characteristically simpler than larger systems, as discussed earlier. Nevertheless, certain control elements must be imposed to foster stable operation. Representative methodology for these is as follows ... 

Natural Gas. Small fuel cell systems that are stationary and have ready access to a natural gas pipeline will predominantly take advantage of the natural gas availability (just as in the case of a larger stationary fuel cell). The principal constituent of natural gas is generally methane, CH4. The cost per unit energy for natural gas is the most attractive, and compactness is presumably not a major issue. Since its storage characteristics are not attractive and its processing system is no more favorable than that of LPG, these are likely to be the only circumstances under which natural gas would be utilized in small fuel cells. 

Potentiometric mode There is no essentially different principle involved from that on which the fuel cell is based. The distinction is that in the case of the fuel cell the required output is power whereas with the sensor it is either a small voltage or small current that monitors some chemical characteristic of the ambient. 

Subsequent deployment of the new catalyst in the cathode layer of small-area MEAs first, then large-area MEAs, and finally fuel cell stacks represents the typical series of performance tests to check the practical viability of novel ORR electrocatalyst materials. Figure 3.3.15A shows the experimental cell voltage current density characteristics (compare to Figure 3.3.7) of three dealloyed Pt-M (M = Cu, Co, Ni) nanoparticle ORR cathode electrocatalysts compared to a state-of-the-art pure-Pt catalyst. At current densities above 0.25 A/cm2, the Co- and Ni-containing cathode catalysts perform comparably to the pure-Pt standard catalyst, even though the amount of noble metal inside the catalysts is lower than that of the pure-Pt catalyst by a factor of two to three. The dealloyed Pt-Cu catalyst is even superior to Pt at reduced metal loading. 

Equation (8) gives the limiting, potential-independent current density predicted for complete control of sequence (5b) -h (5c) by the dissociative chemisorption of H2 (process (5b)) at a catalyst surface with a small number of CO-free sites (see 18a). Such a limiting rate of hydrogen electro-oxidation at low anodic overpotentials has been observed recently in RDE experiments with H2/CO mixtures, performed with platinum and PtRu RDEs [18d,e]. This limiting current density (Eq. (8)) explains the PEFC characteristic observed with low CO levels in the fuel feed stream, depicted in Fig. 13. Under such conditions, the fuel cell will exhibit ordinary anode losses up to the current density defined by Eq. (8), but higher current demands would require a... 

The fuel cell is most efficient at small loads. The efficiency decreases as the load increases and it is least efficient at full load. Because of this characteristic, the fuel cell operates efficiently under the most common driving conditions of partial load. The fuel cell drive system can deliver the same power to the wheels at zero revolutions per minute as at high speeds. Because of these relationships, the fuel cell is better suited as an automobile power source than is the internal combustion engine. 

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