Relating the N-shaped polarization curve of a PEM fuel cell to local oxygen starvation and hydrogen evolution
In this study, we experimentally investigate the appearance of a local negative differential resistance (N-NOR) branch in polarization curves of a segmented 7 by 7 cell measured under the steady and highly-dynamic conditions. Under both conditions, a comma shaped polarization curve, corresponding to depletion of oxygen, was followed by an increase in current as the cell voltage was lowered. This characteristic was measured under potentiostatic mode, where no current is forced through the cell, and at a positive cell voltage (<100 mV in steady-state and similar to 300 mV in dynamic condition). With a theoretical model, we show that at these positive cell voltages and upon the depletion of oxygen, a shift in the Nernst potential occurs allowing for the hydrogen evolution reaction to take place in the cathode catalyst layer. The results of the model are complemented with experimental measurements of produced hydrogen at the cathode outlet.