iii Considering the above, the formation of PRC (step (i)), dissociation of PC (step (iii)), and intra-complex 2PCET (step (ii)) are assumed to occur in consecutive stages according to the reactions shown in Figure . Then, we conducted the digital simulation for various scan rates (Supporting Information, Figure ), focusing on the redox equilibrium of O2/O2 •-(before the 2PCET, above the red dotted line in Figure ) and that of Q •-(after the 2PCET, below the red dotted line). Then, the best fitting parameters for the kinetics and diffusion (Table ) were obtained based on the parameters listed in Table . Here, it should be noted that the disproportionation reaction of Q •-to give Q and Q 2- (Equation ( )) affects the peaks. Our attempts to simulate these experimental CVs (Figure ) did not give a clear display of the involvement of the disproportionation reaction because of the lack of concentration of the HB complex (O2-H2Q) in the cathodic potential sweep, as shown in Figure , in addition to the large number of unknown parameters. Thus, a digital simulation was performed without the disproportionation reaction. However, the gross features of the CVs can be reproduced by our simulation (Figure ), and it can be concluded that the 2PCET mechanism is characterized by superior kinetics in steps (i)-(iii). Notably, the reaction kinetics of step (iii) (kf, 4.2 × 10 2 s -1 ) is important for the best fit to the digital simulation because it facilitates the steps (i)-(ii). which embodies the superior kinetics of 2PCET. This implies that step (iii) embodies the superior kinetics of 2PCET, rather than step (ii) (rate-determining step). chemical reaction Keq kf (s -1 ) kb (mol