Analysis of PEMFC Stack Components Degradation at Multiple Scales: Impact of the Ageing Modes on the Local Losses
Authors: Escribano Sylvie , Micoud Fabrice , Guetaz Laure 
Published: February 2020

Proton Exchange Membrane Fuel Cell (PEMFC) technologies are considered as a promising and clean energy supply for both transportation and stationary applications. State of art components present performance enabling integration of stacks in actual systems and vehicles. However, durability still needs to be improved as well as overall system cost still needs to be reduced to enable large scale competitive deployment. To this end, extensive investigations are required and conducted to improve understanding of the degradation mechanisms. The work presented is dedicated to the degradation phenomena occurring on MEA components at stack level, taking into account the interactions between the overall and local ageing conditions, respectively induced by the ageing mode and the design, and the local losses. The intention is to support the development of accelerated stress tests for automotive application to be validated on single cells and later-on on specific stack designs in the frame of a dedicated European project [1].

Performance and MEAs degradation are considered to identify causes and mechanisms for both reversible and non-reversible losses. Our main focus is to clarify interactions between global and local conditions as well as between performance losses and components degradation.
The approach is applied on aged samples after their ageing in stacks following different load profiles. Pre-ageing, during ageing and post-ageing characterizations of MEA are performed at multiple scales considering in-situ averaged performance and electrochemical properties, in-situ distribution of local conditions such as current density, temperature and water when available, ex-situ local electrochemical properties and components microstructure.
Among ageing modes considered in our studies, post-mortem data are available for MEA components after ageing in stacks following long term ageing on test bench, with fixed load steps or adapted drive cycles, or after use in a vehicle as range extender; for comparison, tests are performed with same stack design (active area 220 cm²), similar components and operating conditions. At stack scale, full understanding requires to add local measurements for correlating performance degradation and cell design. Two methods are applied and as far as possible combined to get these local data: current density distribution maps are performed using a segmented cell implemented in the middle of the stacks and post-mortem analyses are conducted on aged samples selected from specific zones (near gases inlet or outlets, middle) of stack-size aged MEAs, by electrochemical measurements in small differential single cells and by advanced electron microscopy techniques, then compared to non-aged samples. Local analyses are conducted so as to identify known mechanisms depending on the zones such as mainly carbon corrosion, platinum or cobalt dissolution and induced ionomer contamination. Results show differences in local effects when applying different ageing modes and the correlation between local conditions related to the design, alterations in the electrodes and performance losses.

The image illustrates the multiple-scales characterization approach with a full stack, one 220 cm² MEAs used for post-mortem analyses, one 2 cm² MEA as tested in the small differential cell for electrochemical post-ageing measurements and examples of cyclic voltammograms and SEM/TEM images obtained on local samples from selected zones.

[1]

Acknowledgements - This work is mainly conducted in the frame of the project ID-FAST. This project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under the European Union’s Horizon 2020 research and innovation program under grant agreement No. 779565.

Figure 1