Reducing noble metal loading and increasing specific activity of oxygen evolution catalysts are omnipresent challenges in proton exchange membrane (PEM) water electrolysis, which have recently been tackled by utilizing mixed oxides of noble and non-noble elements (e.g. perovskites, IrNiO x , etc.). However, proper verification of the stability of these materials is still pending. In this work dissolution processes of various iridium-based oxides are explored by introducing a new metric, defined as the ratio between amount of evolved oxygen and dissolved iridium. The so called Stability-number is independent of loading, surface area or involved active sites and thus, provides a reasonable comparison of diverse materials with respect to stability. Furthermore it can support the clarification of dissolution mechanisms and the estimation of a catalyst's lifetime. The case study on iridium-based perovskites shows that leaching of the non-noble elements in mixed oxides leads to formation of highly active amorphous iridium oxide, the instability of which is explained by participation of activated oxygen atoms, generating short-lived vacancies that favour dissolution. These insights are considered to guide further research which should be devoted to increasing utilization of pure crystalline iridium oxide, as it is the only known structure that guarantees a high durability in acidic conditions. In case amorphous iridium oxides are used, solutions for stabilization are needed.

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Iridium is the main element in modern catalysts for the oxygen evolution reaction (OER) in proton exchange membrane water electrolyzers (PEMWE), which is predominantly due to its relatively good activity and tolerable stability in harsh PEMWE conditions. Limited abundance of iridium, however, poses limitations on widespread applications of these devices, in particular in the large scale conversion and storage of renewable energy. In this work we investigate if the electrocatalytic performance of iridium can be fine-tuned by thermal treatment of catalysts at different temperatures. The OER activity and the dissolution of two different iridium electrodes, viz. (a) flat metallic iridium surfaces prepared by electron beam physical vapor deposition (EBPVD) and (b) electrochemically prepared porous hydrous iridium oxide films (HIROF) are studied. The range of applied annealing temperatures is 100 • C-600 • C, with a general trend of decreasing activity and increasing stability the higher the temperature. Numerous peculiarities in the trend are however observed. These are discussed considering variations of oxide structure, morphology and electronic conductivity.