(Digital Presentation) Investigation of Reversal Tolerant Anode Catalysts Ageing during Start-up/Shut-Down Events on a PEM Fuel Cell
Authors: Mahmood Rafat , Hofmann Jan Philipp , Jaegermann Wolfram , Reindl Michael , Weidler Natascha 
Published: July 2022

Durability of automotive polymer electrolyte membrane fuel cells (PEMFC) strongly depends on transient operating conditions such as start-up/shut-down (SUSD). During SUSD events, the hydrogen atmosphere at the anode compartment can be replaced by air (shut-down) and vice-versa (start-up), which can lead to the formation of a hydrogen and air interface in the anode side resulting in a lack of hydrogen supply situation. In the event of a hydrogen-starved situation, the carbon-based support material can be oxidized to supply the necessary electron to complete the load circuit resulting in cell reversal mode, when the anode potential rises above 1.0 V (vs. reversible hydrogen electrode). Such events can damage the anode catalyst layer and reduce the fuel cell lifetime. One of the many ways to mitigate the cell reversal condition is to incorporate reversal tolerant anode catalysts with hydrogen oxidation reaction (HOR) catalysts to facilitate the oxygen evolution reaction (OER) over the carbon oxidation reaction (COR) 1.
IrO2 is commonly used as a reversal tolerant anode catalyst because of its high activity and stability under acidic conditions. Several studies have shown the effectiveness of IrO2 as an OER catalyst to alleviate cell reversal events 2. Recent studies have shown that IrO2 can be reduced to metallic Ir under highly reducing hydrogen atmosphere in PEM fuel cell anodes 3. While IrO2 serves its purpose as an OER catalyst, the stability of IrO2 under the transient operation of a PEM fuel cell, such as SUSD, and the impact on overall catalyst durability as well as cell performance are still unresolved.
In this work, we have developed an accelerated stress test (AST) which allows to simulate the transient operation during SUSD analogous to real operation of a PEM fuel cell by an active gas switch from hydrogen to air at the anode side. We discuss the consequences of this transient SUSD condition on the anode catalyst as well as cathode catalyst layer characterized by cyclic voltammetry, where hydrogen under potential deposition (Hupd) features show the change in effective catalyst surface area. The impact on reversal tolerance performance of the IrO2 OER catalyst has been determined by the extended exposure to reversal conditions. Post-characterization surface analysis by X-ray photoelectron spectroscopy (XPS) reveals the change of chemical state of the materials on both anode and cathode catalyst layers resulting from active gas switching at the anode. Finally, the overall PEM fuel cell performance and durability are discussed.

References

Hong, B. K., Mandal, P., Oh, J.-G. & Litster, S. On the impact of water activity on reversal tolerant fuel cell anode performance and durability. Journal of Power Sources
328, 280–288 (2016).

Moore, C. E., Eastcott, J., Cimenti, M., Kremliakova, N. & Gyenge, E. L. Novel methodology for ex situ characterization of iridium oxide catalysts in voltage reversal tolerant proton exchange membrane fuel cell anodes. Journal of Power Sources
417, 53–60 (2019).

Fathi Tovini, M. et al. Degradation Mechanism of an IrO2 Anode Co-Catalyst for Cell Voltage Reversal Mitigation under Transient Operation Conditions of a PEM Fuel Cell. J. Electrochem.
Soc.
168, 064521 (2021).