TABLE OF CONTENTS In addition to modeling, which is described in Chapter 7, diagnostics is another tool that can help elucidate the complex processes taking place in an operational fuel cell. As shown in Figure 8-1, diagnostics can be used in the fuel cell design process to determine if there is anything wrong with the fuel cell and point to possible causes, as well as to calibrate or validate the model. A PEM fuel cell in operation is always between having too much water and not having enough water. Too much water may cause flooding in either the catalyst layer or the gas diffusion layer or even in the gas channels. The result of cell flooding is loss of cell potential due to difficulty of one or both reactant gases reaching the catalyst sites (concentration polarization). Too little water is likely to cause membrane drying, which in turn increases cell resistance and reduces cell potential. A number of diagnostic techniques have been developed to identify and quantify cell flooding and drying.
As discussed in Chapter 3, fuel cell performance is characterized by its polarization curve, that is, a plot of cell potential vs current density. Three distinct regions of a fuel cell polarization curve are noticeable (Figure 8-2):
At low current densities, the cell potential drops sharply as a result of the activation polarization.
At intermediate current densities, the cell potential drops linearly with current; clearly as a result of...
