Local Activities of Hydroxide and Water Determine the Operation of Silver‐Based Oxygen Depolarized Cathodes

Local ion activity changes in close proximity to the surface of an oxygen depolarized cathode (ODC) were measured by scanning electrochemical microscopy (SECM). While the operating ODC produces OH − ions and consumes O 2 and H 2 O through the electrocatalytic oxygen reduction reaction (ORR), local changes in the activity of OH − ions and H 2 O are detected by means of a positioned Pt microelectrode serving as an SECM tip. Sensing at the Pt tip is based on the pH‐dependent reduction of PtO and obviates the need for prior electrode modification steps. It can be used to evaluate the coordination numbers of OH − ions and H 2 O, and the method was exploited as a novel approach of catalyst activity assessment. We show that the electrochemical reaction on highly active catalysts can have a drastic influence on the reaction environment.