The resistance value was chosen to

The resistance value was chosen to be outside the hysteresis loop shown in Fig. 3A. As depicted in Fig. 3C, when the load resistance was reduced from 25 to 5 Ω the current jumped immediately from 25 to 75 mA, and then settled at 65 mA within seconds. The current remained steady at 65 mA for almost 1500 s; after 1500 s the current jumped to 87 mA over the course of ∼100 s, where it remained steady for a period of more than 24 h. The approach relative humidity, also tracked the current changes. These results show there exist dynamic processes in the PEM fuel cell operation associated with at least three distinct time scales: processes that respond in < 1 s, processes that respond in ∼100 s, and processes that respond in > 1000 s. We suggest that the fast response corresponds to the current following the load at nearly constant membrane water content. The 100 s response time corresponds to the adjustment of membrane water content by diffusion. The long time response of 103–104 s corresponds to water absorption or desorption from the membrane and mechanical relaxation processes of the membrane to steady state when the load resistance was changed from a low load resistance (2 Ω) to a high load resistance (20 Ω) took a longer time, ∼10,000 s, as shown in Fig. 3B. The water content in the effluent streams, as measured by the relative humidity, also tracked the current changes.