Mixed Diffusion-Kinetic Control of H2O2 Oxidation at an Oxide-Covered Platinum Electrode in Alkaline Electrolyte: Implications for Oxygen Electroreduction Studies with a Rotating Ring Disk Electrode

The rotating ring disk electrode (RRDE) with a Pt ring is widely applied for oxygen reduction reaction (ORR) studies, and it is generally believed that oxidation of H2O2 at the Pt ring is controlled by diffusion. Indeed, H2O2 oxidation at bare platinum is a diffusion-controlled reaction. However, when Pt ring is held at 1.2 V-RHE, it is being covered with surface oxide, and H2O2 oxidation turns into a reaction under mixed diffusion-kinetic control. We show that, for H2O2 oxidation at an oxide-covered Pt ring in 0.1 M KOH, the ratio of the empirical ring current to corresponding ring current under pure diffusion control varies monotonically with rotation rate. At 1.2 V-RHE, the ratio is equal to 79.9 % and 61.4 % at 400 rpm and 2500 rpm, respectively. We demonstrate that this deviation prevents the correct diagnosis of ORR mechanism and leads to overestimated electron transfer number values. To eliminate the error in RRDE measurements, we employed a procedure based on the Koutecky-Levich method, which, to our best knowledge, has not been reported yet.