Accelerated Iridium Dissolution in Proton Exchange Membrane (PEM) Water Electrolyzers by Inert Mobile Anions Adsorbed in the Double Layer

Iridium oxides display state-of-the-art electrocatalysts for anodes in proton exchange membrane water electrolyzers (PEM-WE), combining electrocatalytic activity for the oxygen evolution reaction (OER) and reasonable stability. During OER with liquid electrolytes, iridium dissolution rates were reported as orders of magnitude higher than those of operating PEM-WE cells, while the reasons for these differences are not well understood. Here, iridium oxide dissolution in an operating PEM-WE cell is examined with different feeds, including pure water, 0.1 M sulfuric acid, and 0.1 M perchloric acid. With sulfuric acid feed, the electrically contacted iridium oxide at the anode is found to dissolve within 22 h. In comparison, the dissolution rates with perchloric acid addition and pure water are approximately 120 and 1500 times smaller, respectively. These differences are explained with a novel theory that correlates the influence of inert mobile anions on dissolution rates by their adsorption in the electrochemical double layer. This physicochemical effect also explains previously reported discrepancies of reported iridium dissolution rates with different electrolytes. Based on the results, the quality of the feed water in terms of inert anion pollution is highlighted as a critical factor for achieving long life of PEM-WE cells with low iridium loadings.