During the ORR, four electrons per oxygen molecule must be transferred. At the same time, several chemical bonds rearrange, making it comparatively slow even on the most catalytically active surfaces, such as platinum and its alloys. Thus, understanding the mechanism of the ORR to improve and develop catalysts has been the focus of many studies for many decades. During this time, the scientific community has converged towards two mechanisms that differ in the initial activation of the oxygen molecule: