 Solution

Peak I is the cathodic reduction shifted positive of the unbound AQ${}^{\bullet -}$/AQ${}^{2-}$ event, consistent with reduction of the CO${}_2$-bound semiquinone to mono-carbonate dianion. Peak II is the corresponding relatively negative-potential return oxidation of mono-carbonate dianion. Peak III, appearing farther positive on the faster return scan, is consistent with direct oxidation of the more strongly protected bis-carbonate dianion, followed by CO${}_2$ release.

This assignment is consistent with binding making the reduced species harder to oxidize. At slow scan, chemical interconversion and CO${}_2$ dissociation can feed the easier mono-carbonate oxidation pathway; at fast scan a larger fraction of kinetically retained bis-adduct survives to its direct oxidation potential. The two separated reductions also show that initial capture and its associated potential shift are not sufficient to make every ECEC step a single equilibrated two-electron wave.

These traces support coupled electron-transfer/binding chemistry but do not determine every microscopic rate constant or exclude EECC pathways. The peak assignments are species-family assignments within the stated mechanism, rather than proof of a unique elementary sequence from voltammetry alone.