## Characteristics of Fuel Cells

Reversible fuel cells will deliver to a load a voltage, VL = Vrev, which is independent of the current generated. Their V-I characteristic is a horizontal line. Such ideal cells require reaction kinetics fast enough to supply electrons at the rate demanded by the current drawn. Clearly, reversible cells cannot be realized. In practical fuel cells, two major deviations from the ideal are observed:

1. The open-circuit voltage, Voc, is smaller than Vrev.

2. The load voltage, VL, decreases as the load current, IL, increases.

Frequently, the V-I characteristics approaches a straight line with some curvature at low currents as, for example, those of Figure 9.26 (left), while other cells exhibit characteristics in which the linearity can be seen only in a limited region (right).

To facilitate fuel cell performance calculations, it is useful to express the cell's characteristics in a mathematical form, that is, to describe the load voltage, VL, as an analytical function of the load current, IL,

Figure 9.26 The typical modern fuel cell tends to have V-I characteristics consisting of a long stretch of apparently linear relationship between current and voltage, with a small curvature at the low current end (left, above). The small Engelhard liquid-electrolyte demonstration cell had a limited region in which voltage decreases linearly with current. At both lower and higher currents, the V-I characteristic exhibited marked curvature (right, above).

Figure 9.26 The typical modern fuel cell tends to have V-I characteristics consisting of a long stretch of apparently linear relationship between current and voltage, with a small curvature at the low current end (left, above). The small Engelhard liquid-electrolyte demonstration cell had a limited region in which voltage decreases linearly with current. At both lower and higher currents, the V-I characteristic exhibited marked curvature (right, above).

One way to accomplish this is to fit, empirically, a curve to the observed data, that is, to treat the cell as a black box without inquiring what is happening internally. It turns out that good fits can, in general, be achieved. They describe accurately what the load voltage is for a given load current, as long as such parameters as temperatures and pressures are the same as those used in obtaining the data. However, such empirically obtained mathematical expressions fail to provide adequate insight on the workings of the fuel cell, and, thus, they provided limited guidance as how to improve its design. For that, one must be able to interpret the relationship between the empirical parameters and the physical processes inside the cell.

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