Electrochemical impedance spectroscopy (EIS) is an indispensable tool for non-destructive operando characterization of Polymer Electrolyte Fuel Cells (PEFCs). However, in order to interpret the PEFC’s impedance response and understand the phenomena revealed by EIS, numerous semi-empirical or purely empirical models are used. In this work, a relatively simple model for PEFC cathode catalyst layers in absence of oxygen has been developed, where all the equivalent circuit parameters have an entire physical meaning. It is based on: (i) experimental quantification of the catalyst layer pore radii, (ii) application of De Levie’s analytical formula to calculate the response of a single pore, (iii) approximating the ionomer distribution within every pore, (iv) accounting for the specific adsorption of sulfonate groups and (v) accounting for a small H2 crossover through ~15 μm ionomer membranes. The derived model has effectively only 6 independent fitting parameters and each of them has clear physical meaning. It was used to investigate the cathode catalyst layer and the double layer capacitance at the interface between the ionomer/membrane and Pt-electrocatalyst. The model has demonstrated excellent results in fitting and interpretation of the impedance data under different relative humidities. A simple script enabling fitting of impedance data is provided as supporting information.

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Carbon corrosion represents one of the most critical degradation mechanisms within state-of-the-art polymer electrolyte membrane fuel cells. Its most prominent consequences include the loss of electrochemical active surface area (ECSA), porosity, and electrical contact within the electrode. The present study seeks a detailed understanding of the dependence between the polarization performance losses and the individual loss terms, both as a function of the amount of corroded carbon. A simplified onedimensional polarization model is developed with the aim to simulatively reconstruct the empirical polarization curve based on in situ cell characteristics during the course of carbon corrosion. It is shown that this model enables the assignment of nearly all voltage losses at various stages of the corrosion process, up to a cell current density of around 2A cm -2 . Furthermore, the "observer effect" during the carbon corrosion studies is investigated, where it is demonstrated that the application of characterization steps exerts strong adverse effects on the ECSA. This is explained by the "voltage cycling" conditions which are inevitably introduced during the characterization protocols, part of which is conducted under cell voltage as low as 0.2V.

The achievement of durability targets is an important challenge for the commercialization of fuel cell electric vehicles (FCEV). In order to meet the requirements, knowledge about the most severe degradation mechanisms of fuel cell stacks under automotive conditions is crucial. In the present work, degradation analysis of an automotive full size stack is performed. Herein, we focus on defects at the cathode catalyst layer and their interrelation including inhomogeneous adhesion of the microporous layer on the catalyst layer, crack formation, cathode catalyst layer thinning and wrinkling of the catalyst coated membrane. In addition, we report linear and circular Pt depositions on top of the cathode catalyst layer, which have to the best of our knowledge not been described in literature yet. For the latter, a degradation mechanism based on liquid water formation, local fuel starvation and current density distribution at the interface between microporous layer and cathode catalyst layer is postulated. Finally, a fast indication for stack degradation is suggested by correlating different degradation phenomena. This improved stack analysis approach allowed us to detect local differences in degradation on both cell and stack level.