The relative humidity measurements

The relative humidity measurements also tracked the cell current when the load resistance was switched from a low load to a high load. The change in anode relative humidity was coincident with the decrease in current, while the relative humidity at the cathode lagged the decrease in current. The existence of multiple ignited states with the PEM fuel cell was surprising. To understand how this phenomenon was affected by temperature, steady state polarization data were obtained at fuel cell temperatures of 50, 65, 80 and 95 ◦C. The procedure was time consuming: the load resistance was changed stepwise by ∼ 1 every 4 h and steady state current and voltage were recorded. “Instantaneous” polarization curves were also recorded at constant water content before each step change in the load resistance. The “instantaneous“ polarization curves were recorded by sweeping the load resistance from 0.2 to 20 Ω over a period of < 100 s. Two stable “ignited” steady states were only observed at 80 and 95 ◦C; at65 and 50 ◦C only a single ignited steady state was observed in our studies. Hysteresis loops were observed at80 and 95 ◦C. Fig. 4 shows the steady state polarization curves at 80 ◦C; also plotted are data for a number of the “instantaneous” polarization curves obtained at “constant water content”. The “instantaneous" data are nearly coincident with either the low water content or high watercontent branches of the steady state polarization curves, and show a smooth extension of the two branches beyond the region where they are long-term stable. If the load resistance is maintained in the extension region, the fuel cell current and voltage will eventually transition to the steady state values.