Diagnostics of PEM fuel cells

Kurzfassung

The analysis of processes in low temperature fuel cells (PEFC, DMFC) is a main topic at the Ger-man Aerospace Center (DLR). The degradation of cell components, especially electrodes, and changes during start-up processes are of foremost interest. For this purpose physical and elec-trochemical methods are used individually as well as in combination in order to study the cell behaviour in terms of performance and durability. In addition to routinely applied electrochemi-cal methods such as I-V characteristics, impedance spectroscopy and chronopotentiometry and chronoamperometry, different methods for locally resolved current density measurements by means of segmented cell technology and integrated temperature sensors have been developed. The latest development with segmented bipolar plates based on printed circuit boards (PCB) is used both in single cells and different fuel cell stacks. In addition, the current density distribution measurements have a high potential for the application of controlling fuel cell systems and early detection of malfunctions and failures in fuel cell stacks. The use of ex-situ methods before and after operation is needed to complete the interpretation of in-situ electrochemical data and to identify degradation mechanisms. Physical characterization methods such as SEM/EDX, XPS, XRD, TPDRO, porosimetry etc. are applied to investigate electrodes, gas diffusion layers and bi-polar plates. Recently, a test facility for the investigation of time-resolved water sorption of membrane materials in combination with electrochemical impedance spectroscopy and also a test rig which enables electrochemical measurements at low pressure has been set up. The elec-trochemical characterization at low pressure can simulate fuel cell operation at high altitudes as it is relevant for aircraft application (0.2 bar (abs.) corresponds to approximately 12.000 m). The main objectives with regard to the use of the analytical tools mentioned are the understand-ing of the fuel cell’s behaviour and its processes and the support of the development of fuel cell components. The investigation of degradation processes allows identifying critical operating conditions and, in combination with the in-situ diagnostics, to develop intelligent control strate-gies. The presentation summarizes the capabilities at DLR with respect to the analysis of fuel cell’s behaviour and gives examples of analytical studies to discuss the potentials and limitations of the diagnostic methodology that is applied.