The finding that direct C2O4 2-oxidation is unchanged upon introducing 4-BrBN to the solution supports the prediction that CO2 •-generated from direct C2O4 2-oxidation is immediately oxidized at the electrode, effectively preventing the reduction of 4-BrBN by CO2 •-. Conversely, the decrease in ipa for DiMeFc + -mediated C2O4 2-oxidation can be attributed to the homogeneous reduction of 4-BrBN by CO2 •-, preventing both direct CO2 •-oxidation (eq 7) as well as the regeneration and subsequent oxidation of DiMeFc (eq 15 followed by eq 12). CV studies presented in the Supporting The origin of the small voltammetric peak at ~0.2 V vs Fc/Fc + in the presence of 4-BrBN is either due to direct C2O4 2-•CO2 oxidation (Figure ) and/or Br -oxidation (Figure ), where Br -is generated from 4-BrBN reduction, eqs 1 and 2. Specifically, in a DMF solution containing C2O4 2-, direct C2O4 2-•CO2 oxidation occurs at ~0.2 V, while Br -oxidation in a DMF solution containing only tetraethylammonium bromide also occurs at ~0.2 V. As such, the oxidation of both species may be responsible for the small voltammetric peak at ~0.2 V seen in the red trace in with DiMeFc + -mediated C2O4 2-oxidation and the follow-up reduction of 4-BrBN by CO2 •-and are therefore ignored in the mechanistic analysis presented later. In addition, the potential applied during bulk electrolysis is not sufficiently positive to oxidize Br -.