Comparison between Iridium Oxide- and Iridium Metal-Based Anode Catalysts during Intermittent Operation in PEM Water Electrolyzers

While lifetimes of up to 100,000 h have been reported for proton exchange membrane water electrolyzers (PEMWEs), the impact of dynamic operation on PEMWE lifetime requires further investigation [1]. Therefore, accelerated stress tests (ASTs) have been developed to assess degradation effects triggered by dynamic operation on a shorter time scale. Although voltage cycling can be easily employed to focus on catalyst degradation, many operation effects of a real system, such as gas crossover during idle periods, can only be simulated by such tests [2]. Weiß et al. [3] developed an AST comprised of steps at high (3 A cm-2) and low (0.1 A cm-2) current densities, followed by an idle period at open circuit voltage (OCV) in order to mimic dynamic operation and shutdown of a PEMWE. When operating the cell at differential pressure (absolute pressures of 1 bar | 10 bar in anode/cathode compartment), H2 permeating through the membrane from the cathode into the anode compartment during the idle periods reduces the surface of the crystalline IrO2 catalyst (the most commonly used catalyst material for PEMWEs) to metallic Ir. Upon subsequent operation at high current densities and thus potentials, Ir is re-oxidized to an amorphous, hydrous iridium oxide. This amorphous oxide is known to exhibit a higher activity for the oxygen evolution reaction (OER), but a lower stability than crystalline IrO2 [4]. This study aims to investigate the degradation of iridium oxide- and iridium metal-based anode catalyst layers during the above described OCV-AST developed by Weiß et al. [3]. A Pt-coated Ti-PTL is used to mitigate the buildup of contact resistances at the electrode/PTL interface [5]. IrO2/TiO2- and Ir/TiO2-based anode electrodes (with loadings of 1.7 – 1.9 mgIr cm-2) are coated onto a Nafion™ 212 membrane by using the decal transfer method, together with a Pt/C-based cathode (0.3 mgPt cm-2). These membrane electrode assemblies (MEAs) with the different anode catalysts are tested in 5 cm2 single-cells for approximately two weeks of intermittent operation. The Ir/TiO2 catalyst loses its metallic character in the two-week OCV-AST, as evidenced by the loss of hydrogen underpotential deposition features in the cyclic voltammogram (see region I in Fig. 1), which are well-defined at the beginning-of-test (BoT; blue line in Fig. 1) and smeared out at end-of-test (EoT, red line) [6]. Instead, features attributed to hydrous iridium oxide appear (see region II in Fig. 1), as this oxide is grown electrochemically at high potentials [4,6]. The evolution of this redox feature has been reported to also occur with the IrO2/TiO2 catalyst, indicating the transition of the less active crystalline to the amorphous iridium oxide with a higher OER activity [3]. This study aims to deepen the understanding of change in the iridium oxidation state and in its OER activity during intermittent operation as well as its implications on voltage losses. We will further investigate the impact of different operating conditions on the degradation of the two different catalysts. A detailed electrochemical characterization will be supported by ex situ X-ray diffraction and -absorption spectroscopy measurements, further exploring the oxidation state of the catalyst materials. References: [1] A. Buttler, H. Spliethoff, Renew. Sust. Energ. Rev., 82 (3), 2440-2454 (2018). [2] S.M. Alia, K.S. Reeves, D.A. Cullen, H. Yu, A.J. Kropf, N.N. Kariuki, J.H. Park, D.J. Myers, J. Electrochem. Soc., 171 (4), 044503 (2024). [3] A. Weiß, A. Siebel, M. Bernt, T.-H. Shen, V. Tileli. H.A. Gasteiger, J. Electrochem. Soc., 166 (8), F487-F497 (2019). [4] S. Geiger, O. Kasian, B.R. Shrestha, A.M. Mingers, K.J.J. Mayrhofer, S. Cherevko, J. Electrochem. Soc., 163 (11), F3132-F3138 (2016). [5] C. Liu, M. Shviro, A.S. Gago, S.F. Zaccarine, G. Bender, P. Gazdzicki, T. Morawietz, I. Biswas, M. Rasinski, A. Everwand, R. Schierholz, J. Pfeilsticker, M. Müller, P.P. Lopes, R.-A. Eichel, B. Pivovar, S. Pylypenko, K.A. Friedrich, W. Lehnert, M. Carmo, Adv. Energy Mat., 11 (8), 2002926 (2021). [6] J. Mozota, B.E. Conway, Electrochimica Acta, 28 (1), 1-8 (1983). 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), as well as the Fonds der Chemischen Industrie(Kekulé fellowship to CS). Figure 1