Neutron Imaging and Modelling of Autonomous Hydration-Dehydration Cycles in PEFCs Operated with Saturated Anode and Dry Cathode Feed

Kurzfassung

In this work, the current fluctuations arising in Polymer Electrolyte Fuel Cells (PEFCs) operated with wet anode and dry cathode feed (RH_a^in≈100%, RH_c^in≈5%) due to the cyclic hydration/dehydration of the membrane are examined. The analysis combines liquid water distributions obtained by neutron imaging experiments and predictions of a 3D two-phase non-isothermal model. The neutron visualizations show that the nonlinear dynamics stem from the periodic shedding of water droplets at the anode inlet chamber. As shown in Figure 1, two well-differentiated processes can be distinguished in analogy to ignition and extinction phenomena in combustion. A fast autocatalytic increase of the current (decrease of HFR) in about 20 s that goes hand-in-hand with the transport of liquid water along the anode channel (ignition), followed by a slower decrease of the current (increase of HFR) in 200 s as the accumulated water is flushed out of the cell (extinction). This challenging scenario is modeled by introducing or not introducing a steady flux of liquid water at the vapor-saturated anode inlet. As can be seen in Figure 2, the low- and high-performance states of the hydration-dehydration cycles are properly captured by the model. The low-performance state is achieved when no liquid water is assumed to enter at the anode inlet, leading to a high ionic resistance and a fairly inhomogeneous current distribution. The membrane dry-out is stronger near the cathode inlet, and decays along the cathode channel due to the membrane humidification by the anode stream. By contrast, the high-performance state is reached when a flux of liquid water is introduced at the anode inlet. The water input humidifies the cell and offsets the membrane dry-out caused by the cathode stream, leading to a low ionic resistance and a homogeneous current distribution. Two important conclusions are obtained from the work: i) liquid water injected into the anode channel is an efficient means for membrane humidification, even in the presence of a dry cathode feed, without incurring in high mass-transport losses; ii) the oscillatory hydration-dehydration regime is a useful operating scenario to study membrane resistance in durability tests.