Reaction pathway: Usually, the ORR is described as an inner-sphere electron transfer reaction, where O 2 adsorbs on adjacent, available surface sites and reacts in a multistep reaction with the surface via intermediates such as *OOH or *OH to form either H 2 O 2 or H 2 O. From this reaction pathway, and using the concept of linear scaling, it is expected that for metals that bind atomic oxygen strongly, such as Ru, the ORR should predominantly proceed via the four-electron pathway to form H 2 O. As stated already in Ref. 48, this relation might, however, not be applicable for surfaces covered by other adsorbates. In that case, the reaction rate should be zero, even though the bare surface would be active. Moreover, on surfaces partially covered by adsorbates other than reactants or reaction intermediates, the adsorbate-covered sites/areas can contribute via different reaction pathways to the total reaction than those dominant on bare surface sites. Therefore, knowing the surface adlayer composition and coverage is imperative for an in-depth discussion of the respective reaction mechanism.