Segmented cells as tool for development of fuel cells and error prevention/prediagnostic in fuel cell stacks

In the past various techniques to measure the current density distribution in PEFC were developed at the DLR Institute in Stuttgart. These techniques are used successfully to advance and to support the development of fuel cell stacks. Furthermore, the current density distribution measurements can also be used as a feedback control system to optimize the operation conditions, to avoid unfavourable conditions and to detect problems and defects at an early state. For this purpose three different techniques for determination of current densities were developed. In all techniques at least one of the flow fields is segmented. In an early design the individual seg¬ments are connected together with external resistances which are used for the measurement of the current den¬sity. This design is only suitable for the current density measurements in end plates of stacks or in single cells. Secondly, hall sensors are used to determine the current inside the segments. A recent development uses a design in which the segments as well as the resistances are realized in a printed circuit board (PCB) which can be used as a bipolar plate in stacks as well as in single cells. Results can be used to validate models and to optimise components of a stack, e.g. flow field and manifold design, as well as the operating conditions of fuel cells. By applying segmented bipolar plates as sensor plates for system control integrated in stack systems high performance, safe operation and long life cycles can be achieved. The main challenge in the development of control strategies based on current density measurements is their interpretation. Therefore a simple and fast simulation of measured current density distribution has been developed taking into account the major influences on fuel cell performance (mainly the water management). As a result the actual local partial pressures e.g. of oxygen and of water can be determined in short intervals and as a consequence a malfunction of the stack can be detected and avoided