Microenvironment Matters: Destabilization of Iridium Anode Catalyst by CO Reduction Products.

Iridium is one of the most frequently employed anode electrocatalysts in CO2 and CO electrolysis, thanks to its reasonable overpotential for the oxygen evolution reaction (OER) and high stability under operating conditions. The latter has been challenged recently by a handful of studies where destabilization of iridium was observed, which was explained solely by thermodynamics (iridium is unstable at strong alkaline pH and OER potentials). In this study, we demonstrate that liquid CO and CO2 electrolysis products (such as ethanol and acetate) crossing over to the anode side under long-term operation have a severe effect on the stability of iridium. Its dissolution was studied by both ex-situ inductively coupled plasma mass spectrometry (ICP-MS) and in situ (online ICP-MS) techniques. Based on our electrolysis experiments carried out in a broad pH range (pH = 4-14), ethanol, and its partially oxidized counterpart, acetaldehyde, decreases the stability of the anode catalyst. Ethanol/acetaldehyde oxidation competes with the OER and starts in conjunction with the surface oxidation of the Ir catalyst particles. The oxygenated species are consumed by the alcohol/aldehyde oxidation process, preventing the formation of a passivating surface oxide layer, resulting in an increased iridium dissolution rate.