Towards a Deeper Understanding of Catalyst Degradation during Intermittent Operation at Different Operating Conditions of Proton Exchange Membrane Water Electrolyzers
Authors: Kornherr Matthias , Schramm Carina , Ernst Matthias Felix , Müller Manuel , Gasteiger Hubert Andreas 
Published: December 2024

Low-temperature water electrolysis with a proton exchange membrane (PEM) enables the production of green hydrogen without direct emissions of CO2. Besides the ability to generate hydrogen at elevated pressure and very high current densities, PEM water electrolyzers (PEMWEs) offer a high load flexibility [1]. This facilitates a coupling to renewable energy sources and opens pathways for the large-scale implementation of the PEM water electrolysis technology. Considering the limited supply of iridium, current efforts target reduced iridium loadings and even higher current densities while maintaining high performance, along with the implementation of iridium recycling loops [2]. To nevertheless ensure the durability of iridium-based catalysts over the desired >50,000 hours of PEMWE lifetime, it is crucial to fundamentally understand the various degradation mechanisms as well as lifetime-affecting operating parameters and conditions. Therefore, growing efforts are put into the development of accelerated stress tests (ASTs) to predict and understand the degradation of PEMWE components and materials within reasonably short experiments. While various anode catalyst materials have for example been screened by Alia et al. in galvanostatic and potentiostatic ASTs with intermittent operation or simulated start-stop cycles [3, 4], the underlying degradation mechanisms of iridium-based catalysts for the oxygen evolution reaction (OER) in PEMWE anodes are still not fully understood.
The here presented work aims for a better understanding of anode catalyst degradation during intermittent operation of PEMWEs. Performance changes and catalyst durability of a TiO2 supported IrO2 anode catalyst are investigated in 5 cm2 single-cells at loadings of 1.7-1.8 mgIr cm-2 over the course of an AST, consisting of operation periods at 3 A cm-2 and 0.1 A cm-2 and of periods at open circuit voltage (OCV), which was previously developed by our group [5]. To mitigate the growing contact resistances between the Ti-based porous transport layer (PTL) and the anode electrode that was observed by Weiß et al. [5] and to focus on the performance changes resulting from catalyst layer degradation, a platinum-coated PTL is used here. While the catalyst activity and therefore the performance is increasing in the first cycles of the OCV-AST due to a transition of the IrO2-catalyst surface to a hydrous iridium oxide, a significant subsequent performance loss is observed upon further cycling (see blue symbols in Fig. 1). On the other hand, for an AST where the cell voltage during idle periods is held at 1.3 V to prevent the intermittent reduction of the iridium catalyst (caused by hydrogen crossover during OCV periods), the high-frequency resistance-corrected cell voltage at 0.1 A cm-2 initially remains at high values but increases by only ~5 mV over 660 cycles (see red symbols).
Besides presenting factors that affect the initial performance improvement during the OCV-AST to better understand the performance gain, we will investigate the impact of different operating conditions such as maximum current density or operating temperature on the performance decay after prolonged cycling. With different in-situ characterization methods like electrochemical impedance spectroscopy or cyclic voltammetry, we will disentangle and elucidate possible reasons for the observed voltage losses to better understand catalyst degradation and critical lifetime-affecting operating parameters and conditions.

References:

[1] A. Buttler, H. Spliethoff; "Current status of water electrolysis for energy storage, grid balancing and sector coupling via power-to-gas and power-to-liquids: A review"; Renewable and Sustainable Energy Reviews 82
(2018) 2440-2454.
[2] C. Mittelsteadt, E. Sorensen, Q. Jia; "Ir Strangelove, or How to Learn to Stop Worrying and Love the PEM Water Electrolysis"; Energy & Fuels 37
(2023) 12558-12569.
[3] S. M. Alia, K. S. Reeves, H. Yu, J. H. Park, N. N. Kariuki, A. J. Kropf, D. J. Myers, and D. A. Cullen; "Catalyst-Specific Accelerated Stress Tests in Proton Exchange Membrane Low-Temperature Electrolysis for Intermittent Operation"; J
ournal of the Electrochemical Society
171
(2024) 024505.
[4] S. M. Alia, K. S. Reeves, D. A. Cullen, H. Yu, A. J. Kropf, N. N. Kariuki, J. H. Park, and D. J. Myers; "Simulated Start-Stop and the Impact of Catalyst Layer Redox on Degradation and Performance Loss in Low Temperature Electrolysis"; J
ournal of the Electrochemical Society
171
(2024) 044503.
[5] A. Weiß, A. Siebel, M. Bernt, T.-H. Shen, V. Tileli, and H. A. Gasteiger; "Impact of Intermittent Operation on Lifetime and Performance of a PEM Water Electrolyzer"; Journal of the
Electrochemical Society
166
(8)
(2019) F487-F497.

Acknowledgements:

The authors gratefully acknowledge the financial funding from the German Federal Ministry of Education and Research (BMBF) in the framework of the Kopernikus P2X project (funding number 03SFK2V0-2) and the H2Giga IRIDIOS project (funding number 03HY129B).

Figure 1