Evidence for OH radical production during electrocatalysis of oxygen reduction on Pt surfaces: consequences and application.

Multielectronic O(2) reduction reactions (ORR) at Pt surface (and at Au surface for comparison purpose) were examined both in water and in organic solvents using a strategy based on radical footprinting and scanning electrochemical microscopy (SECM). Experiments reveal a considerable and undocumented production of OH radicals when O(2) is reduced at a Pt electrode. These observations imply that the generally admitted description of ORR as simple competitive pathways between 2-electron (O(2) to H(2)O(2)) and 4-electron (O(2) to H(2)O) reductions is often inadequate and demonstrate the occurrence of another 3-electron pathway (O(2) to OH radical). This behavior is especially observable at neutral and basic pH's in water and in organic solvents like dimethylformamide or dichloromethane. In view of the high reactivity of OH radical versus organic or living materials, this observation could have important consequences in several practical situations (fuel cells, sensors, etc.) as far as O(2) reduction is concerned. This also appears as a simple way to locally produce highly reactive species as exemplified in the present work by the micropatterning of organic surfaces.