In the positive-going scan, the ORR remains active up to about 0.0 V, followed by a rather steep decay. Hence, there is a pronounced hysteresis in the potential region up to about 0.1 V. The almost inhibition of the ORR in the peak centered at about 0.1 V is caused by the adsorption of *HSO 4 , which displaces the *H to form anodic H 2 . This process was was verified by preliminary DEMS measurements shown in the SI in Section S3.The ORR is also almost inhibited at higher potentials (complete inhibition at 0.4 V, see Fig. )in regions IIIa and IIa, where *HSO 4 is gradually replaced by *OH. Complete removal of *HSO 4 and the transformation of *OH into *O, only occurs at potentials larger 0.6 V, hence at about the onset potential for the ORR in the negative-going scan (Fig. ).The absence of ORR activity in the positive-going scan seems to be specific for Ru(0001) electrodes in H 2 SO 4 . In comparison, Ru(pc) electrodes show a similar ORR activity in both scan directions in this potential range (see Section S5 in the SI and elsewhere , with a steady decrease in the negative-going scan, from approximately 60% close to the onset potential of the ORR to a few percent at around 0.2 V, and remaining this low also in the HER region. The continuous decay is superimposed by a broad peak at about 0.4 V, which coincides with the peaks in the ORR and H 2 O 2 currents in Figs. and. As indicated already by the smaller ORR peak (Fig. ) and the larger H 2 O 2 current, this peak is more pronounced than the similar peak in the H 2 O 2 yield in HClO 4 electrolyte. In the positive-going scan, the H 2 O 2 yield is very low in the entire potential range up to 0.0 V, i.e., in the range where the electrode is active for the ORR (Fig. ).