Determination of the Electron Transfer Number for the Oxygen Reduction Reaction: From Theory to Experiment

The forced convection methods on the rotating disk and ring-disk electrodes are carefully analyzed toward their use for calculation of the electron transfer number (n) for the oxygen reduction reaction (ORR) on various catalysts. It is shown that the widely used Koutechy-Levich (KL) method is not suitable to determine n for the ORR either theoretically or experimentally. From a theoretical viewpoint, the ORR is neither a single-step nor a one-way reaction and, therefore does not fulfill the assumptions of the KL method. From an experimental viewpoint, n is significantly dependent on the angular velocity of the rotating disk electrode, contradicting the assumption of constant n in the KL theory. An improved model is used to establish the aforementioned relationship between n and angular velocity. The recommended way to determine n for the ORR in alkaline electrolytes is to use the rotating ring-disk electrode with a properly biased Au ring, supplemented by the calibration of the collection efficiency.