Oxygen reduction in alkaline solution at glassy carbon surfaces and the role of adsorbed intermediates

- Oxygen reduction at glassy carbon is faster at high pH.
- This shift is the opposite of that predicted by the Nernst equation.
- Combined voltammetric experimental and numerical simulation
- The mechanistic importance of surface adsorbed superoxide is evidenced.

Oxygen reduction at glassy carbon (GC) exhibits distinctively different voltammetric behavior at high (> 10) and low (< 10) pH. The peak potential is found to be around − 0.4 V at pH 13, compared to − 0.6 V at pH 7.4 as measured against a saturated calomel reference electrode. Using experimental voltammetry and numerical simulation, the difference in peak potential is interpreted in terms of a difference in reaction mechanism. At low pH, O2 reduction is evidenced to proceed via a solution phase pathway initially resulting in the reduction of oxygen to superoxide. Conversely, at higher pH, a different mechanism is favored involving the formation of a surface bound superoxide species. The switch between the two mechanisms is related to the protonation of the surface bound intermediate under less basic conditions.