Further Insights into PEMFC Stack Degradation Modes Depending on Operating Strategies
Authors: Micoud Fabrice , Rosini Sébastien , Gaudemer Guillaume , Finkler Anaïs , Laforêt Hortense , Decoopman Benjamin , Paganelli Gino , Jacques Pierre-André , Escribano Sylvie 
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. Whereas the overall electrical performances and power densities have now reached most of the targeted specifications for integration, the overall durability and cost are still major hurdles to the mass market. The mechanisms of performances loss under PEMFC operation on individual components (catalyst, membrane, gas diffusion layer) are well-known in the literature at the material or small electrode scales. However in real stack configuration, the degradations may be strongly accelerated and localized since all phenomena are highly inter-correlated and very dependent of both the stack/cell designs and the operating conditions.

The decrease in performances (under continuous or cycling modes) is composed of both irreversible losses (essentially due to materials degradations) and reversible losses which are attributed to the evolution of cell water management and electrode pollution (CO-like contamination, Pt-oxide formation) over time. Long stop phases make it possible to rejuvenate these reversible degradations but may not always be convenient or possible according to the applications or user needs. Developing reliable operation strategies either to recover or avoid reversible performance loss are necessary to adjust system dimensioning and prevent from using oversized stacks.

When considering particularly the reversibility losses hypothetically caused by PtOx formation at high cathode potential, a modification during a short period of local conditions enabling to remove the oxides can be proposed to stabilize the performances [1]. Air-starvation have already been studied at single cell level to regenerate the reversible losses in Direct Methanol Fuel Cell (DMFC) [2] but the impact on the long-term durability remains uncertain. Our work is thus on PEMFC stack operation under both automotive Fuel Cell Dynamic Load Cycles (FC-DLC) and stationary conditions at fixed load to evaluate and discuss the interest and reliability of transient and repeated air-starvation periods.

Start-up and shut-down protocols must also be taken into account since they can cause harsh degradation phenomena in the fuel cell caused by the so-called reverse current mechanism arising from the H2|O2 (or air) front at the anode side. Since this configuration is hardly avoidable after long stops in real systems, mitigation strategies must also be developed to enhance the lifetime of the PEMFC stack despite the embedded system integration constraints. In this perspective, we determined in-situ current density distribution mapping using an S++® equipment inside our stack in order to localize the degradation under repetitive fuel starvation conditions. Based on the degradation mapping, we propose to discuss the possibility to tune and adapt the 2D/3D structure and composition of the Membrane Electrode Assembly to modify internal current distribution and mitigate corrosion phenomena within the cells.

References

[1] Y. Huang et al. , J. Electrochem. Soc., 2013, vol. 161, no1, p. F10

[2] C. Eickes et al. , J. Electrochem. Soc., 2006, vol. 153, no 1, p. A171