Oscillations in PEM fuel cells� �at low cathode humidification

Kurzfassung

Oscillatory fluctuations of a single proton exchange membrane fuel cell appear upon operation with a dry cathode air supply and a fully humidified anode stream. Periodic transitions between a low and a high current operation point of the oscillating state were observed. The transition time for the change from the low to the high operation is fast and does not depend on the operating parameters, while the downward transition depends strongly on the operating conditions. An insight into the transitions is obtained by current density distributions at distinct times indicating a propagating active area with defined boundaries. The high current operation possesses a high electro-osmotic drag and a high permeation rate (corresponding to liquid-vapor permeation) leading to a large water flux to the cathode. Subsequently, the liquid reservoir at the anode is consumed leading to an anode drying. The system establishes a new quasi-stable operation point associated with a low current, low electro-osmotic drag coefficient, and a low water permeation (corresponding to vapor-vapor permeation). When liquid water is formed at the anode interface after some time the fast transition to the high current operation occurs due also to a flooding effect leading to blockage of channels.