Fig. 2: Free energy dependence of superoxide oxidation kinetics to 3O2 and 1O2 and their yields.
From: Marcus kinetics control singlet and triplet oxygen evolving from superoxide
a, We measured the kinetic constants k for mediated KO2 oxidation in MeCN electrolyte with mediators covering a large range of redox potentials. Plot of ln(k) compared with the mediator potential (\({E}_{{\text{RM}}^{\text{ox}/\text{red}}}^{\circ }\), top axis) and driving force (–ΔG°, bottom axis). \(-\Delta {G}^{\circ }=\left({E}_{{\text{RM}}^{\text{ox}/\text{red}}}^{\circ }-{E}_{{\genfrac{}{}{0ex}{}{3}{}{\rm{O}}}_{2}/{\text{KO}}_{2}}^{\circ }\right)\,F\), where \({E}_{{\genfrac{}{}{0ex}{}{3}{}{\rm{O}}}_{2}/{\text{KO}}_{2}}^{\circ }=2.48\,{\rm{V}}\) on the K/K+ scale. The mediators are shown in Extended Data Fig. 2. The full line best fits equation (2); the broken line parabolas represent the first and second terms in equation (2). The fitted values are Zel,3 = 1.10 × 10–2 cm s–1, Zel,1 = 7.00 × 10–2 cm s–1, λ3 = λ1 = 0.95 eV, \({\Delta G}_{1\leftarrow 3}^{\circ }=0.84\,{\rm{eV}}\) and R2 = 0.998. Based on these fits, the standard potential \({E}_{{\genfrac{}{}{0ex}{}{1}{}{\rm{O}}}_{2}/{\text{KO}}_{2}}^{\circ }\) and associated driving force \({\Delta G}_{1\leftarrow 3}^{\circ }\) are marked. They are linked by \({E}_{{\genfrac{}{}{0ex}{}{1}{}{\rm{O}}}_{2}/{\text{KO}}_{2}}^{\circ }\,=\,{E}_{{\genfrac{}{}{0ex}{}{3}{}{\rm{O}}}_{2}/{\text{KO}}_{2}}^{\circ }+{\Delta G}_{1\leftarrow 3}^{\circ }/F\). The blue- and red-shaded area indicates the transition from k3/(k1+3) = 0.99 to k1/(k1+3) = 0.99. b, 3O2 yield per mole of RMox (bars) during KO2 oxidation as measured by mass spectrometry. The dashed line and the circular markers show simulated 3O2 yields considering 1O2 quenching by solvent and redox mediator (Methods and Extended Data Fig. 3). The dashed line used the trend line for the mediator quenching rate constant kQ, whereas the markers use the individually measured values (Extended Data Fig. 3c). c, Normalized 1,270 nm NIR emission (bars) during KO2 oxidation. The dashed line and the circular markers show the simulated NIR emission considering 1O2 formation with the kinetics k1 (the right parabola in a) and 1O2 quenching by solvent and redox mediator (Methods and Extended Data Fig. 3). Data are presented as mean ± s.d. (n ≥ 3).
