Voltage response analysis of polymer electrolyte fuel cells during dynamic load changes

Polymer electrolyte fuel cells reach high efficiency and zero emissions even under highly dynamic loads. Power fluctuations, sudden load changes, start up and shut down procedures cause locally very high as well as very low voltages. These undesired voltage levels accelerate the degradation of the cells in terms of carbon corrosion, platinum oxidation and membrane decomposition. Experiments with single cells are capable to indicate, which conditions are most harmful to the cell.
In Fig.1 the voltage responses directly after step load changes are shown. The voltage increases above the steady state level and follows an exponential course. For an increasing load step, the curve does not reach a constant value. For a decreasing load step the time and the conditions are sufficient to reach a constant value according to steady state operation. The measurements shown in Fig.1 were conducted with partially humidified gases, constant temperature and constant gas flow.
The voltage over- and undershoots are proportional to the range of the load change. Considering the first current ramp, no voltage undershoot is observed. This leads to the assumption that the amount of the over/under shoot depends on the velocity of the current increase and respectively the current decrease. Measurements with varying operation conditions show that these effects can be mitigated by correct adjusted operation parameters like stoichiometry, humidity and pressure.
These measurements show that higher humidity and higher gas flows are beneficial for the voltage response behavior of the cell. To determine the local operating conditions, the current distribution is measured in a segmented cell (rectangular 25 cm2) with parallel flow field (Fig.1). The results show an inhomogeneous behavior of the segments caused by varying relative humidity along the gas channels. The cell was operated in counter flow. The uneven current distribution indicates different mass transport mechanisms in the segments. To investigate the influence of the geometry of the cell further, measurements with a segmented square triple serpentine flow field with the same active area and with different spatial current distributions are performed. Based on the results, mass transport phenomena that cause these differences are proposed and discussed.