Insights into the Path-Dependent Charge of Iridium Dissolution Products and Stability of Electrocatalytic Water Splitting

The electrocatalytic stability of the oxygen evolution reaction (OER) is challenging for the storage of fluctuating renewable energies using polymer electrolyte membrane water electrolyzers (PEMWEs). Investigations are commonly conducted in so-called half-cell setups and different OER-related dissolution pathways have been proposed. However, the orders of magnitude difference in dissolution rate between half-cells and PEMWE using membrane electrode assemblies (MEA) is not well understood. In this work, the charge-related absorption affinity of Iridium (Ir) dissolution products, from both half-cell and MEA setups, is investigated, using cation and anion exchange materials. In the half-cell, a roughly constant ratio of cationic to anionic dissolution species is indicative of a single, dominant OER-related Ir dissolution pathway. While Ir dissolved in half-cells is mainly cationic, the Ir species from the MEA appear mainly in anionic form. This can be explained by the transport conditions of different Ir ions inside the catalyst layer, influenced by their ionomer absorption affinity and the migration driving force. Based on this understanding, key influences of electrocatalytic stability of MEAs, the effect of confinement of dissolved Ir species and the stability discrepancy to half-cells are discussed.