The nature of active centers catalyzing oxygen electro-reduction at platinum surfaces in alkaline media

Energy conversion devices that use OH-conducting electrolytes are becoming more and more attractive with recent substantial progress in the development and commercialization of relevant membrane materials and systems. However, the activities of numerous catalysts towards oxygen electro-reduction taking place at the cathodes of such devices in alkaline media are surprisingly different from those in acidic electrolytes. This is for instance the case for Pt and Pt-alloy electrodes, which demonstrate unexpected and drastic variations in activity depending on the electrolyte pH and surface structure. Here we apply recently introduced electrochemical scanning tunnelling microscopy noise measurements to directly identify active centers at Pt(111)-based surfaces in three alkaline electrolytes (LiOH, KOH and CsOH) under reaction conditions. For all three solutions it was found that the most active sites are located on the Pt(111) terraces, in contrast to the acidic media, where concave defects significantly increase the oxygen reduction activity. These defect-centers are to all practical purposes deactivated in alkaline media. This not only explains the above-mentioned activity differences between acidic and alkaline electrolytes but also suggests strategies to design nanostructured Pt-electrocatalysts for applications at high pH values.