Electrochemical S2O8 2-reduction is well-established to proceed through an ECE mechanism (eqs 7-9). Briefly, S2O8 2-is first reduced to S2O8 3•-(eq 7) which dissociates within ~1 ps to yield SO4 2-and SO4 •-(eq 8). The strongly oxidizing SO4 •-can then be reduced to SO4 2-(eq 9), resulting in the overall two-electron reduction of S2O8 2-to two SO4 2-(eq 10). Figure shows the CV response of S2O8 2-reduction at a GC electrode. All voltammetric data were collected using a single-compartment three-electrode cell in an O2-free aqueous 0.1 M Na2SO4 solution. A notable feature of S2O8 2-reduction is that its direct electrochemical reduction is observed at ~ -1.2 V vs Ag/AgCl (Figure , blue trace), meaning that an ~1.5 V kinetic overpotential, due to slow heterogenous electron transfer, is required to observe its direct reduction at the GC electrode. However, the homogenous 1e -reduction of S2O8 2-is theoretically possible using an outer-sphere redox mediator with a standard potential (E 0 ) more negative than E 0 for eq 7, a strategy introduced later for initiating the autocatalytic reaction. 20 S2O8 2-+ e -→ S2O8