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.