Oxygen Absorption in Electrocatalyst Layers Detected by Scanning Electrochemical Microscopy

Scanning electrochemical microscopy (SECM) is able to track the local electrochemical activity of an electrolyte‐immersed substrate employing an ultra‐micro‐electrode (UME) in micrometer‐scale spatial resolution. In this study, SECM is employed to investigate the presence of oxygen in the electrocatalyst layers of polymer electrolyte membrane fuel cells and electrolyzers. Approach curves on electrocatalyst layers with the tip potential set for oxygen reduction reveal that a significant amount of oxygen is absorbed in the catalyst layer. We confirm that the coexistence of Nafion ionomer and carbon black leads to oxygen confinement. It is suggested that this oxygen is confined within the hydrophobic parts of the self‐assembled Nafion on the graphitic surfaces of the carbon black. Absorption of oxygen into the electrocatalyst layer only in the presence of Nafion (Nafion+Catalyst) is confirmed using scanning electrochemical microscopy. This is justified from a positive feedback when the tip approaches the layer containing both catalyst and Nafion. In contrast, a negative feedback is observed for both the catalyst and Nafion, when tested separately, indicating no significant amount of oxygen is absorbed into either of them.