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.