Water management is one of the most relevant issues for the successful operation of Polymer Electrolyte Membrane Fuel Cells (PEMFCs). An excessive humidification of the inlet gas streams produces an accumulation of liquid water in the porous electrodes, which hinders the transport of reactants towards the active catalyst sites, thereby decreasing cell performance (an effect known as flooding). On the other hand, an insufficient level of humidification lowers the ionic conductivity of the membrane, also leading to a performance reduction. The complex water management further challenges the transient operation of PEMFCs, so that fast changes of the humidification conditions should be avoided as far as possible.

 Previous work has shown the existence of periodic oscillations between high- and low-current density levels in cells operated with dry cathode and fully humidified anode streams^{1, 2}. This oscillatory behavior is associated to the periodic wetting (ignition) and drying of the membrane. The ignition process is caused by the transient accumulation of liquid water at the anode inlet, which is then transported downstream the channel. This leads to a rehydration of dried, deactivated areas of the cell, thereby decreasing the membrane (high frequency) resistance. The ignition process is then followed by a progressive drying of the membrane, so that the ignition/drying (high-/low-current density) cycle repeats itself again and again.

 In this work, the spatio-temporal distributions of liquid water during both ignition and drying processes have been visualized using in-plane high-resolution neutron imaging³. Two MEAs with different membranes (Nafion^® 111 and Nafion^® 117) were investigated to assess their impact on the transient distribution of water throughout the cell. In addition, the experimental data have been compared with the predictions of a transient two-phase macroscopic model. The combined experimental and numerical investigation conducted in this work-in-progress will provide insight on the complex water transport phenomena that take place in operating PEMFCs.

 References

1. D. G. Sanchez, D. G. Diaz, R. Hiesgen, I. Wehl, and K. A. Friedrich, J. Electroanal. 635 Chem., 649, 219 (2010).

2. D. G. Sanchez, A. Ortiz, and K. a. Friedrich, J. Electrochem. Soc., 160, F636 (2013).

3. P. Boillat, G. Frei, E. H. Lehmann, G. G. Scherer, and A.Wokaun, Electrochem. Solid St., 13, B25 (2010).