Heinz Sander

Investigation of Locally Resolved Current Density Distribution of Segmented PEM Fuel Cells to Detect Malfunctions

With the aid of PCB technology, current density distribution could be achieved by measuring the local current density inside the fuel cell, which would be more precisely reflecting the local electrochemical reaction. An important application of segmented cell technology was used for in-situ error detection of the evolution of membrane leakage. From the evolution of current density distributions, starting locations of membrane leakages and their spreading could be detected. The effect of operation conditions could be found: the steeply decreased pressure and the fast cooling of the local membrane was the main reason for the degradation of the membrane. It caused large temperature gradient in different local membranes, which resulted in inhomogeneous current density distribution and mechanical strain and stress. With the aid of segmented cell, malfunction could be detected at an early stage and thereby catastrophic failure of the whole stack may be avoided.

Dynamic Behavior and In-Situ Diagnostics of PEFCs

During fuel cell operation the electrochemical activity often is not homogenous over the electrode area. This may be caused by an non-uniform water content in the membrane, an inhomogeneous temperature distribution, and reactant gradients in the cell. Consequently a variation of the current density over the cell area occurs which tends to result in inferior performance. For in situ measurements of the current density distribution in fuel cell stacks a segmented bipolar plate was developed. The segmented bipolar plate was first tested in single cells with stack endplates to verify the function of all components. The tests showed that the measurement tool works very reliable and accurate. The insight in an operating fuel cell stack via current density distribution measurement is very helpful to investigate interactions between cells. Results can be used to validate models and to optimise stack components, e.g. flow field and manifold design, as well as to detect the best stack operating conditions. By applying segmented bipolar plates as sensor plates for stack system controls an improved performance, safe operation and longer life cycles can be achieved. The developed segmented bipolar plates with integrated current sensors were used to assemble a short stack consisting of 3 cells; each of them having an active area of 25cm2 divided into 49 segments. The design of the bipolar plate proofed very suitable for easy assembling of single cells and stacks. First measurement results show that different current distributions can appear in the cells and these can vary from cell to cell, depending on the operating conditions of the stack. Electrical coupling between the cells was investigated and found to be only marginal for the assembly used.Copyright