Comparing Fuel Cell Parameters

Electrolyte Polymer H3PO4 KOH/HjO Molten salt Ceramic 

Application EV/HEV, small utility Small utility Aerospace Utility Utility 

Source Cilcrist, T., Fuel cell to the fore, IEEE Spectrum, 1998 IEEE. With permission. 

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Often a need arises to compare electrical and other characteristics of fuel cells that differ in their nature or size or to compare fuel-cell-based power generators with others. This is most readily achieved when using reduced or normalized parameters. 

Table 17.8 compares fuel cells in terms of fuels, oxidants, electrolytes, operating temperatures, efficiencies, current power output levels, and specific applications. The performance parameters, such as efficiency, temperature, and current estimates of power output levels, shown in Table 7.18 are accurate within 5% [13]. More research and development efforts are required in the areas of reliability and safety of the device. 

In industrial electrochemical cells (electrolyzers, batteries, fuel cells, and many others), porous metallic or nonmetallic electrodes are often used instead of compact nonporous electrodes. Porous electrodes have large trae areas, S, of the inner surface compared to their external geometric surface area S [i.e., large values of the formal roughness factors y = S /S (parameters yand are related as y = yt()]. Using porous electrodes, one can realize large currents at relatively low values of polarization. 

In Song et al. s same work [5], the effect that Nafion content in the catalyst layer had upon electrode performance was also investigated, following their work on the optimization of PTFE content in the gas diffusion layer. The optimization of Nafion content was done by comparing the performance of electrodes with different Nafion content in the catalyst layer while keeping other parameters of the electrode at their optimal values. Figures 6.8 and 6.9 show the polarization curves and impedance spectra of fuel cells with electrodes made of catalyst layers containing various amounts ofNafion . 

Like the reformer systems, four PEM fuel cell stacks of 110 cells each are used. These stacks are smaller and have lower catalyst loading than those for the reformer systems because the hydrogen is 99.99% pure, containing less than 10 ppm carbon monoxide. Stack parameters for the direct hydrogen systems are compared to those for the reformer systems in Table 2. 

Figure 3.6B compares three experiments where the initial water loading in the membrane was the same (l.Omg/cm ), the difference being a change in either the feed flow rate, the temperature, or the external load resistance. Increasing any one of those parameters extinguished the fuel cell current. 

In conclusion, all of these observations indicate that there is still much room to improve ADAFC performance by developing novel materials and, on the other hand, by optimizing the operational conditions of the fuel cell. Future work should look into a wider range of potential low-cost materials and composites with novel structures and properties, presenting catalytic activity comparable to that of noble metals. The development of new catalyst systems is more likely in alkaline media because of the wide range of options for the materials support and catalyst, as compared to acidic media which offer more limited materials choice. Moreover, efforts have to be addressed to meet the durability targets required for commercial application. More work is needed to optimize the operational fuel cell conditions, by achieving suitable chemical (OH concentration, hydroxyl/alcohol ratio in the fuel stream) and physical (temperature, pressure, flow rate) parameters. 

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