A fuel cell is an electrochemical device that converts the chemical energy of fuels directly to electricity. A popular way of evaluating a fuel cell is to measure its polarization curve, which is a plot showing the cell voltage change with current or current density. A good fuel cell should display a polarization curve with high-current density at high cell voltage, indicating high-power output. Figure 2.1 shows typical polarization curves for a proton exchange membrane fuel cell (PEMFC) (Lee et al., 1998).