When the concentration of added H2Q ([H2Q]) was higher than that of O2 ([O2]), pc2 (Q •-/Q 2-) considerably shifted to the positive potential side due to the HB effect from unreacted H2Q, resulting in an apparently one additive reduction curve (pc1: O2/O2 •-and pc2: Q •-/Q 2-), under an equilibrium state as observed in Figure . The fact that pc2 is significantly affected by HB in the CVs suggests that the reaction rate at step (iii), at which PC formed in step (ii) becomes Q •-(dissociation of HBs), is not slow. Next, we further measured CVs for the same solutions containing O2 and H2Q using various scan rates to improve the simulation parameters and then analyzed the kinetics and diffusion details. Figure shows the CVs at higher scan rates (a) at 1.0 V s -1 and (b) at 4.0 V s -1 with H2Q concentrations of 0, 1.0, 2.0, 3.0, 5.0, and 10.0 × 10 -3 mol dm -3 . As the scan rate increased, all peaks broadened in the CVs because the electrode process was slower and chemical species diffused more rapidly than at the scan rate. It is noteworthy that the concentration dependence of the added H2Q on the loss of the reversibility of O2/O2 •- (reoxidation current: pa1) remained almost unchanged even at higher scan rates. This suggests that the kinetics of the homogeneous 2PCET, which archived the irreversibility, was sufficiently fast that it had little effect on increasing the scan rate. Additionally, CVs for the same solution containing O2 and H2Q (c: 1.0, d: 5.0 × 10 -3 mol dm -3 ) at various scan rates (0.1-1.0 V s -1 , Supporting Information, Figure ) with the results (current, vertical axis) divided by the square roots of the scan rates are shown in Figure ). In these CVs, the peaks of O2/O2 •- (pc1/pa1) become broaden (showed by arrows) with scan rate dependency, which is a typical observation of quasireversible redox because ET reactions are sluggish, called peak-to-peak separation. The peak of Q/Q •-show similar peak-to-peak separation to that of O2/O2 •-, but the peak of Q •-/Q 2-show a somewhat larger change. Considering that the barrier to ET (electrochemical irreversibility) is the same in the first and second waves of Q, the difference between the peaks (Q/Q •-and Q •-/Q 2-) may be due to the effects of the HB formation and dissociation steps. It is well documented that Q 2-interact with hydrogen donor via a specific interaction, i.e., hydrogen bonding involving a strong n-σ charge transfer interaction. Therefore, the peaks of Q/Q •- showing typical peak-to-peak separation in a scan ratedependent manner without being affected by HBs suggest that the dissociation of PC forming FPs (Q •-, H2O2) efficiently occurs.