Numerous previous studies reported on the ORR on polycrystalline Ru (Ru(pc)) elec-trodes (particles, sheets, etc.) in aqueous acidic and alkaline electrolytes. An early study by Nekrasov and Krushcheva showed that during the ORR in alkaline electrolyte, part of the current is also related to the formation of H 2 O 2 . Furthermore, the authors reported that the amount of H 2 O 2 depends on the amount of Ru surface oxides formed during high-potential oxidation. Their results were later confirmed by Anastasijivic et al., who also observed similar trends in activity and product formation in acid electrolytes. These reports and further studies showed that the relative amount of H 2 O 2 increases with increasing overpotential, reaching a maximum between the onset of the ORR and the hydrogen evolution reaction (HER) region. Overall, however, the H 2 O 2 yields are only within a few percent but vary with the applied potential.