Influence of the Start-Up Procedure on the Performance of Polymer Electrolyte Fuel Cells

Kurzfassung

The performance of polymer electrolyte fuel cells (PEFC) depends on many different aspects: Besides the operating parameters, which influences on performance are mainly understood, the history of the fuel cell operation has an essential impact. This fact is reflected in the strong increase of the investigations of degradation processes. During long-term operation, the performance of polymer electrolyte fuel cells shows a continuous descent while the operating conditions remain the same. In an analogous manner, the start-up procedure of fuel cells and the conditioning of their components - especially of the membrane electrode assembly during to the start-up procedure - have also a significant influence on the cell performance, but are frequently neglected in these studies. Therefore, investigations of the start-up procedure, which are comparable to the degradation investigations, are necessary. During the corresponding start-up process the membrane is humidified for enabling a sufficient proton conductivity of the polymer electrolyte. Simultaneously, the first electrochemical operation leads to a closer connection of the different components of the MEA. The DLR has investigated the influence of different start-up procedures on the performance of a PEFC. The MEAs used in these experiments consisted of commercial Nafion membranes and electrodes from ETEK which were pressed together during the assembly of the cells. In this contribution the characterisation of different start-up procedures by in-situ methods like polarization curves and electrochemical impedance spectroscopy (EIS) (at the end of each start-up procedure) will be presented. By means of these investigations two basic results can be proven: In comparison to a constant current at start-up, sequentially alternating operating points with increasing current densities yield higher performance at the end of the procedure. Thereby, an increase of the cell performance is coupled with a reduction of the time requirement for steady-state operation.