More fundamental insights were gained from studies on single-crystal electrodes, which can be directly linked to the results of theoretical studies. Most of the existing studies, however, report on the ORR on adatom-modified Ru(0001) electrodes, both experimental and theoretical. However, to the best of our knowledge, there is only a single experimental study by Inoue et al., elucidating in more detail the ORR in O 2 saturated HClO 4 on bare Ru(0001) and Ru (10 11) electrodes (including data on Pt-modified surfaces). The authors illustrate that Ru (10 11) shows a lower overpotential for the ORR on Ru(0001) and is also more selective for the complete reduction.