Abstract
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 high-current operation point of the oscillating state due to the balance of drying and wetting processes in combination with water transport have been observed previously; however, several new aspects have been investigated in the present study, providing insight into the initiation processes. Oscillations are present under both galvanostatic and potentiostatic conditions, and it is demonstrated that the transitions involve local membrane drying through the comparison of impedance data with current density distributions. The oscillations are caused by periodic flow type changes from one- to two-phase flow in the anodic channels of the flow field. It has been observed that cell orientation with respect to Earth's gravity field affects the liquid water distribution in the anodic flow channels and, thus, also affects the oscillatory behavior of the cell performance. When comparing cell performance oscillations with a five-channel serpentine flow field to a cell with a one-channel serpentine flow field, the oscillating performance behavior is observed to also be affected by the gas velocity within an anodic flow channel.
Highlights
Water management has been the main focus of study for numerous groups in recent years
There are interesting features in the oscillation that have the potential for advancing the understanding of water interaction in the cell: (i) the downward transition associated with dehydration of the membrane, which has a considerable stability and can be studied using in situ techniques (e.g., electrochemical impedance spectroscopy (EIS) in segments); (ii) a transient high current operation point that corresponds to the steady state of the cell under optimum hydration conditions; and (iii) a characteristic upward transition time that does not depend on operation conditions for a membrane system
Even though the conditions of normal stack operation vary considerably compared with the conditions under which oscillatory fluctuations appear, several groups have been interested in this phenomena.[11,27,29]. The reason for this interest is that water transport plays a crucial role in fuel cell performance and reliable operation
Summary
Water management has been the main focus of study for numerous groups in recent years. The oscillatory behavior of the cell performance induced by large humidity gradients from the wet anode to dry cathode air is significant as it allows us to understand and evaluate some of the processes related to the management of water in PEMFCs. In particular, there are interesting features in the oscillation that have the potential for advancing the understanding of water interaction in the cell: (i) the downward transition associated with dehydration of the membrane, which has a considerable stability and can be studied using in situ techniques (e.g., electrochemical impedance spectroscopy (EIS) in segments); (ii) a transient high current operation point that corresponds to the steady state of the cell under optimum hydration conditions; and (iii) a characteristic upward transition time that does not depend on operation conditions for a membrane system.
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