Improved Understanding of PEMFC Stack Durability Testing through Comprehensive In-Situ and Ex-Situ Characterization Techniques
Authors: Schüttoff Miriam , Wachtel Christian , Wilhelm Florian , Scholta Joachim , Hölzle Markus 
Published: December 2024

The pursuit of durable Proton Exchange Membrane Fuel Cells (PEMFC) for mobile applications has seen remarkable progress in the past decade. Despite this progress, ensuring long-term durability of PEMFCs remains a complex challenge. In addition to the development of accelerated test methods that allow a faster assessment of the expected lifetime, the use of in-situ and ex-situ characterization methods is necessary to gain a better understanding of the mechanisms leading to aging processes within fuel cell stacks. By adopting the learnings from such tests, targeted material development can be fostered and the operating strategies for fuel cells can be optimized to ultimately increase the durability.

In this work, we present our recent results on the development of accelerated durability test (ADT) protocols. Starting from a detailed review of aging mechanisms under realistic operating conditions, these ADT protocols were developed obeying the limitations usually effective in a state-of-the-art vehicle system, i.e. voltage clipping at 0.85 V. Based on a 5,500 h long-term reference test under automotive conditions, four different ADT profiles were derived. Individual stressors were added step by step in order to analyzed the root causes of individual degradation effects. Stressors applied are load, temperature, humidity and potential cycling. All tests were performed using seven-cell short stacks with automotive-size active area, developed within the publicly funded Auto Stack Industry (ASI) project [1]. By using several in-situ characterization techniques, as spatially resolved current density measurement, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), different aging effects could be observed depending on the ADT profile.

Our results show that the presently used long-term tests can be effectively shortened by a factor of 3 to 6 by applying appropriately tailored ADTs. In order to relate in-situ observations to physical causes, multiple ex-situ analysis methods were performed after each test. These include infrared (IR) thermography to investigate membrane degradation, contact angle measurements, X-ray photoelectron spectroscopy (XPS) and inverse gas chromatography to determine changes in hydrophobic behavior of the gas diffusion layers (GDL). STEM measurements contribute further information on catalyst aging and FIB-SEM analyses were carried out to investigate layer thicknesses and detect further degradation phenomena within the membrane electrode assemblies (MEA). By linking all in-situ and ex-situ results, a comprehensive picture of (accelerated) aging of PEMFC components as a function of the selected stressors could be created.

The results show that state-of-the-art MEAs are very capable of achieving operating times beyond 5,000 operating hours. The membrane and the anode catalyst layer in particular showed no significant signs of ageing in any test applied here. However, the cathode catalyst layer (CCL) is affected by ECSA (electrochemically active surface area) degradation and carbon corrosion. In contrast to all other degradation phenomena investigated, the former could not be significantly accelerated by any ADT protocol, demonstrating the effectiveness of voltage clipping in preventing the cathode catalyst particles from ripening. The GDL, however, shows noticeable degradation effects and should be put further into the focus of future investigations related to PEMFC durability.

The authors wish to thank the AUDI AG for funding this work as well as the ASI consortium for their cooperative approach and (non-financial) support.

[1] A. Martin, Abschlussbericht AutoStack Industrie: Öffentliche Version https://www.greenerity.com/wp-content/uploads/PROJEKTABSCHLUSSBERICHT-ASI.pdf (in German), 2023.