Detection of leakages by current density measurements

Kurzfassung

The formation of leakages in the membrane in PEFC is a relatively frequent failure mode which leads to a significant loss of performance or loss of the fuel cell functionality. The leakages occur due to a malfunction during the manufacturing process of the membrane or the membrane-electrode-assembly (e.g. penetration of a carbon fiber from the GDL), the assembling the fuel cell (mechanical stress) or be dry-wet cycling of the membrane. Leakages in the membrane allow the reactant gases to cross the membrane and to react directly without generation of electricity. This reaction takes place on the catalyst surfaces like the electrochemical reactions. Consequently, at the leakage a local mixed potential will be formed. Caused by the electrical in-plane conductivity of the electrodes, gas diffusion layer, bipolar plates and membrane the local potential influences the residual electrode, and consequently, an electrochemical mass conversion depending on the local potentials starts. In current density measurements the local electrochemical mass conversion is measured. Alterations in the current density distributions are related to varying local operating conditions. For a cell at open circuit condition the total (integral) current is zero. However, in the cell at open circuit conditions different current directions will be occur in the cell in some areas. This can be clearly observed in the current density distribution measurements. Typically in the cell at open circuit conditions, the currents in the individual segments are all nearly zero, because the local conditions in the cell do not varies significantly. In contrast, if the cell has a leakage, at the leakage a mixed potential will be formed. Consequently, the local conditions in the cell at the leakage are significant changed compared with the undamaged areas. The lateral differences in the local potentials induce equalizing currents which can clearly detect in the current density measurement. In the current density measurement the equalizing current in the bipolar plate, which is the main equalizing current caused by the better in-plane conductivity compared with the in-plane conductivity of the membrane and the gas diffusion layer, is measured. Under load the current density distribution are changed with the cell voltages. If the cell has a leakage the changes are more significant as in an undamaged cell. The current density measurements on the cell with a leakage show clearly, that current density measurements are a powerful tool for error diagnostic in fuel cells.