Solution phase superoxide as an intermediate in the oxygen reduction reaction on glassy carbon in alkaline media

On the Mechanism of the Oxygen Reduction Reaction on Au(poly) in Aqueous Alkaline Electrolytes: A Critical Reassessment

“…The currents recorded with the concentric Au ring electrode polarized at E ring = 1.15 V, i.e., i ring,O 2 , while the Au­(poly) disk was being scanned, are shown in Figure B. The blue curve in this panel was obtained under identical conditions, except that the Au­(poly) ring was functionalized with a monolayer of 3M1P, a surface modification that imparts Au­(poly) extraordinary specificity toward O 2 – (aq) oxidation, , and polarized at E ring = 0.90 V. It should be mentioned that for E ring > 0.90 V, the thiol monolayer was found to oxidatively desorb from the electrode surface at increasing rates and, therefore, higher potentials were avoided. The fact that i ring,O 2 3M1P was undetectable does not necessarily mean that O 2 – (aq) is not involved in the ORR on Au­(poly) under these experimental conditions.…”

“…As indicated, the first, and rate-determining step, eq , involves an outer-sphere one-electron transfer to yield solution-phase superoxide ion, O 2 – (aq), which is then followed by a fast and reversible one-electron transfer to generate HO 2 – (aq), eq . Also included in this scheme are two additional highly irreversible reactions, i.e., the four-electron reduction of O 2 , eq , and the two-electron reduction of HO 2 – (aq), eq , which yield, in both cases, OH – (aq) as the product. This scheme neglects the possible homogeneous dismutation of O 2 – (aq), as its rate, under the conditions selected for the experiments herein described, is too small for this reaction to proceed to any significant extent during the transit time from the disk to the ring . It should be emphasized that both solution-phase intermediates, O 2 – (aq), and HO 2 – (aq), would be oxidized on a conventional Au ring; however, functionalization of Au with a monolayer of 3M1P imparts the electrode extraordinary specificity for O 2 – (aq), oxidation, while remaining impervious to the presence of HO 2 – (aq), , as illustrated in our recent studies of the ORR on glassy carbon electrodes in our laboratories.…”

Origins of hydrogen peroxide selectivity during oxygen reduction on organic mixed ionic–electronic conducting polymers

“…On glassy carbon, which is apparently less catalytic for superoxide disproportionation, the limiting current density corresponds to a single electron transfer to form superoxide which builds up at the electrode surface, excluding O 2 and resulting in a decrease in current density with increasing potential. 50,53…”

“…On glassy carbon, which is apparently less catalytic for superoxide disproportionation, the limiting current density corresponds to a single electron transfer to form superoxide which builds up at the electrode surface, excluding O 2 and resulting in a decrease in current density with increasing potential. 50,53 While the side chains for the tested OMIECs are not identical, they all contain a polar (hydrophilic) portion, which is essential for bulk ion-insertion redox in aqueous electrolytes. In contrast, N2200, the NDI-T2 polymer with entirely nonpolar side chains, was found to be incapable of electrochemical activation and oxygen reduction in aqueous electrolytes (Fig.…”

Cyclopentadienone–NHC iron(0) complexes as low valent electrocatalysts for water oxidation

“…It is well-known that O 2 can be reduced at a GC electrode, and thus its signal should be visible in a cyclic voltammogram, after the occurrence of the electrocatalytic process. 76 Figure 5 shows the CV obtained in the aerated and de-aerated electrolytes and during the electrocatalytic process. The irreversible wave ascribable to O 2 reduction takes place at -1.13 V and it is clearly visible in the CV recorded in the aerated electrolyte solution.…”