(Invited) Dissolution of Iridium: Aqueous vs. Solid Polymer Electrolytes
Authors: Cherevko Serhiy , Zlatar Matej , Milosevic Maja , Geuß Moritz 
Published: December 2024

Iridium oxide possesses unique physicochemical properties, making it the only oxygen evolution reaction (OER) catalyst used in modern proton exchange membrane water electrolysis (PEMWE). It is electronically conductive and relatively active towards the OER. Moreover, it can maintain these properties for a long time, guaranteeing the required long-term operation of PEMWE. Nevertheless, iridium oxide is not entirely immune during the OER. In the currently used PEMWE, the dissolution of Ir is masked by its high loading. This way-around solution will likely not work in low Ir loading anodes. To overcome this challenge, other material or operational solutions will be required. Both should be based on a more advanced understanding of the Ir dissolution mechanisms.
In this presentation, we overview the current status of the iridium dissolution research. Aqueous and solid polymer electrolyte systems are considered as the former suits best model fundamental studies, and the latter represents real devices. As the first step in understanding Ir dissolution stability, the thermodynamics of Ir in broad potential and pH ranges, as visualized by Pourbaix diagrams, is discussed. Experimental data on Ir dissolution kinetics obtained in on-line inductively coupled plasma mass spectrometry (ICP-MS) studies is presented next. It is shown that changes in the Ir oxidation state during oxidation/reduction and OER are responsible for Ir losses.1 The OER-triggered Ir dissolution is explained by suggesting the existence of common intermediates in water oxidation and Ir leaching.2, 3

Besides mechanistic insights, Ir dissolution rates and amounts for different Ir oxides are quantified. Such quantification is more challenging in polymer electrolyte systems. Hence, a detailed analysis of various cell components to monitor Ir is performed. It is shown that in a full-cell setup, the main sink of Ir is the membrane and cathode catalyst layer.4 While providing necessary information, experiments on the full-cell level are tedious and slow. We show that the same information can be obtained using half-cell gas diffusion electrode (GDE) setups.5 The talk will be finalized by comparing and contrasting both electrolyte systems and discussing the next steps required to advance our understanding of Ir dissolution to provide mitigating strategies in stabilizing low Ir loading PEMWE anodes.

References:

M. Zlatar, D. Escalera-López, M. G. Rodríguez, T. Hrbek, C. Götz, R. Mary Joy, A. Savan, H. P. Tran, H. N. Nong, P. Pobedinskas, V. Briega-Martos, A. Hutzler, T. Böhm, K. Haenen, A. Ludwig, I. Khalakhan, P. Strasser and S. Cherevko, ACS Catalysis, 13, 15375 (2023).

O. Kasian, J. P. Grote, S. Geiger, S. Cherevko and K. J. J. Mayrhofer, Angew Chem Int Ed Engl, 57, 2488 (2018).

S. Geiger, O. Kasian, M. Ledendecker, E. Pizzutilo, A. M. Mingers, W. T. Fu, O. Diaz-Morales, Z. Li, T. Oellers, L. Fruchter, A. Ludwig, K. J. J. Mayrhofer, M. T. M. Koper and S. Cherevko, Nature Catalysis, 1, 508 (2018).

M. Milosevic, T. Böhm, A. Körner, M. Bierling, L. Winkelmann, K. Ehelebe, A. Hutzler, M. Suermann, S. Thiele and S. Cherevko, ACS Energy Letters, 8, 2682 (2023).

M. Geuß, M. Milosevic, M. Bierling, L. Löttert, D. Abbas, D. Escalera-López, V. Lloret, K. Ehelebe, K. J. J. Mayrhofer, S. Thiele and S. Cherevko, Journal of The Electrochemical Society, 170, 114510 (2023).