Durability Testing of Polymer Electrolyte Fuel Cells Under Stationary and Automotive Conditions

Kurzfassung

Our presentation focuses on durability testing and degradation of fuel cells. A major motivation of our work is the lack of common description procedures and determination approaches of voltage losses in durability tests of fuel cell for both stationary and automotive applications; this issue leads to severe difficulties in the comparison of results obtained by different institutions or within different projects, especially if only a single value for the degradation rate is reported. In this context, special attention is devoted to the discrimination between so called reversible and irreversible voltage losses. The first are permanent and determine the maximum lifetime of a fuel cell. The latter strongly depend on the chosen operation conditions and can be recovered by specific procedures. In order so systematically address voltage losses we have performed single cell durability measurements of several hundreds of hours in 25 cm2 lab-scale cells using different test protocols containing regular refresh procedures (soak time) allowing to distinguish between reversible and irreversible losses. Furthermore, operation strategies to minimize reversible degradation without using the time consuming refresh procedures are provided. To test the refresh procedures and analyze their effect on cell performance, parameters such as duration of the soak time steps have been varied. Between these refresh steps the cells were typically operated for 50 to 150 h. As samples conventional 5-layer membrane electrode assemblies were used with PFSA membranes, Pt-based catalysts and hydrophobized carbon fiber substrates with micro porous layers as GDLs. For in-situ diagnosis of the operated cells polarization curves, electrochemical impedance spectra, and cyclic voltammograms were recorded in order to determine the impact of the operation conditions and the refresh procedures on degradation. The interpretation of the degradation of the measured membrane electrode assemblies is supported by post-mortem analysis using physical characterization techniques. Additionally, we provide possible approaches to quantitatively determine irreversible voltage decay rates. For instance, voltage values before or after voltage recovery steps can be used to calculate the irreversible loss rate. The advantages and drawbacks of different approaches are discussed. One clear conclusion is that short time tests in the range of 100 hour are not conclusive since this time is too short to make a reliable discrimination between reversible and irreversible losses; also, the decay rate of reversible loss observed after each refresh step increases substantially upon long time operation independent on the type of the refresh procedure. In summary, in our presentation strategies for determination of fuel cell voltages loss rates are compared, evaluated and assessed according to their suitability to distinguish between reversible and irreversible degradation rates; a description of voltage loss rates is proposed. Moreover, operation strategies to minimize reversible degradation are provided. The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) for Fuel Cell and Hydrogen Joint Technology Initiative under Grant No. 621216 (SecondAct) and No. 303452 (Impact).