a, CV of 5 mM DHAQ and 5 mM DBEAQ in D 2 O with 1 M KOH. The potential was scanned at 20 mV s−1, the scan starting by sweeping from positive to negative potentials. b, Fraction of DHAQ3•− radicals calculated on the basis of the Evans method as a function of SOC for DHAQ (100 mM) under a charging current of 100 mA and a flow rate of 13.6 cm3 min−1 (measured twice; red and blue), and a charging current of 150 mA and a flow rate of 33.3 cm3 min−1 (green). The figures on the right and left are the same data plotted without and with error bars. The errors are ± half the FWHM of the water signal. c, Concentration of DHAQ3•− radicals as a function of SOC for 100 mM and 200 mM DHAQ. d, Fraction of radicals as a function of SOC for 100 mM DHAQ and 100 mM DBEAQ. e, Relative concentrations of AQ2−, AQ3•− and AQ4− as a function of SOC of the system and the equilibrium constant, K c . AQ2−, AQ3•− and AQ4− are represented by dashed, dotted and solid lines, respectively. K c = 1.26 and K c = 10.35 correspond to values derived for DBEAQ and DHAQ, respectively (corresponding to E 1 – E 2 values of 6 mV and 60 mV)5,13. Curves corresponding to K c = 0.1 and K c = 20 are also shown, to illustrate the effect using the smaller and larger values E 1 – E 2 = −58.3 mV and 76.6 mV, respectively. The former negative value illustrates the case in which the radical is strongly disfavoured. f, The AQ3•− and AQ4− concentration at 50% SOC, as a function of K c . AQ3•− and AQ4− are the black dotted and blue solid lines, respectively. g, Experimentally derived radical concentrations as a function of SOC. Supplementary Information equations S6 and S7 were used to fit the experimental data, along with an SOC lag parameter ranging from 8% to 12%, where x is the fraction of AQ3•− for a given number of electrons n (discussed, as is the derivation of the equations, in the Supplementary Information). The fit to the data was done in SOC steps of 1%. h, The CV of a 100 mM DHAQ. ‘2e-Rev’ and ‘2e-QRev’ refer to a two electron reversible and two-electron quasi-reversible model, respectively, and the ‘1e+1e’ curves (with and without fitting constraints) refer to a two-step, single-electron, quasi-reversible process.