Operando and Ex-Situ Investigation of PEMFC Degradation

Abstract

One of the key requirements for fuel cell electric vehicles (FCEVs) is longevity with a targeted lifetime of the fuel cell of 5000 - 6000 h. This lifetime needs to be assured in dynamic PEMFC operation including critical events. To mitigate performance degradation a fundamental understanding of the degradation phenomena that occur during different operation conditions are required. Therefore operando electrochemical investigations including monitoring of local operation conditions have been carried out in a broad scope of operation conditions to i) study impact of operation conditions on degradation and ii) identify phenomena that accelerate performance decay (so called accelerated stress tests - AST). Our results show that dynamic load cycling leads to lower performance losses compared to operation at constant current [1]. Moreover, operation in the low current range (i.e. high potential range) exhibited highest irreversible and reversible degradation which is caused by a significantly increased ohmic and mass transport resistance. The observed performance losses are closely linked with inhomogeneous current density distributions. Ex-situ measurements show that MEA regions with particularly high local current densities exhibit more pronounced platinum band formation in the membrane, as shown in the Figure, which is correlated with irreversible cathode degradation. In order to understand the impact of specific operation conditions occurring during load cycling as required for automotive operation specific in-situ ASTs were performed focusing on high potential operation, mechanical stress, temperature cycling and high load operation. Although in-situ ASTs exist for the membrane and the catalyst layer, no in-situ ASTs are available for the GDL so far. Hence our current activity is focused on the development of GDL specific ASTs and quantification of their impact on degradation of individual MEA components. To quantify impact of these ASTs on degradation of individual MEA components aged components were analyzed not only by ex-situ methods, but also by electrochemical characterization in combination of with pristine MEA components. The results clearly show that, for instance, mechanical stress due to freezing water during freeze-thaw cycling has negative effect on mass transport capabilities of the MEA. In this context also mitigation strategies to avoid icing in the cell is proposed which mitigates performance decay [2]. In addition of studying degradation effects caused by operation conditions, we also provide an analysis of effects of different externally introduced contaminants. These include silicon (e.g. from gasket) and nickel (e.g. from defective coatings) which effects on local MEA degradation was studied with DLR’ segmented cell in different operation conditions [3]. Apart from understanding degradation on material level, our activities cover the development of methods for on-line fault diagnostics and state-of-health monitoring in order to identify critical operation conditions characterized by gradients in various parameters and uneven flow distribution between the single cells [4]. The presented work is focused on fault monitoring in PEMFC stacks by evaluation of local performance phenomena and impedance analysis. Combined studies of local current density measurements and impedance data for all single cells in the stack will be presented and links between impedance response, local effects within the cells, and stack fault modes will be highlighted. Acknowledgements: This project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No 779565 (ID-Fast). This Joint Undertaking receives support from the European Union’s Horizon 2020 research and innovation programme.