Model-assisted analysis and prediction of activity degradation in PEM-fuel cell cathodes

This work presents a model for the prediction and analysis of voltage losses in proton exchange membrane fuel cells arising from accelerated stress testing. It consists of two submodels. The first submodel uses a statistical physics-based population balance approach to describe the degradation of the catalyst active surface. It is combined with a performance submodel that allows incorporating the degradation of the catalyst activity. During testing, a dedicated diagnostic procedure is used to determine the cell performance and the cathode properties, like the electrochemical active surface area, during the stress tests. It was found that the change of the catalyst activity, described by Tafel slope and exchange current density, correlates with the change in active surface area. The model allows the description of catalyst surface reduction, changes of Tafel slope and exchange current density as well as voltage losses. We find that the voltage losses attributed to the loss of electrochemical active surface area are minor, while the dominant factor is the change of the Tafel-slope. Accordingly, this study shows that during PEM-FC cathode degradation studies the Tafel slope should be the most relevant metric. The model describes the experimental data with a standard deviation of 7.1 mV in a range of 0–2.0 A/cm2. The model is intended to be used as a building block for the prediction of performance losses of PEM fuel cells under drive cycle conditions.