Diagnosis methods dedicated to the localisation of failed cells within PEMFC stacks

Too low humidification levels and too high stack temperatures, insufficient reactant gas flows causing starvation phenomena, bad controls of pressure gaps between anode and cathode, further mechanical stress in the electrolyte can lead to severe and irreversible damages in PEM fuel cells, i.e., scaling and gasket deterioration, and/or break in the membranes. Overboard leak checks, sometimes called pressure tests, performed with nitrogen and air are usually employed to highlight gas leakages inside the stack between anode and cathode stack compartments. This procedure allows perfectly well the detection of crossover leaks occurring within the stack assembly. However, it is not appropriate to locate which cell or cells have failed, to inform actually where the fault has occurred. The actual need of diagnosis procedures meeting these requirements is important. Indeed, once the failed cells are identified, they may be removed and replaced by safe ones. The article presents some data and descriptions of two failed fuel cell stacks with different power levels, namely 100 W and 5 kW. Some leak checks are first performed and then some experimental investigations are carried out with the goal to locate the defective cells. Some electrical signals which reflect the faults are generated. The open current voltages of the cells are monitored in different operating conditions linked with the gas supply (flow rates, pressures). The procedures allow the detection of the failed cells, which show abnormal performance and voltage pattern when compared to the other cells in the stack. The tests are also conducted in conditions that preserve as much as possible the integrity of the other safe cells. The experimental analysis is supported by the proposition of modelling actions. (c) 2007 Elsevier B.V. All rights reserved.