Despite decades of research, no consensus has been reached regarding basic aspects of the mechanism of the oxygen reduction reaction (ORR) on Au electrodes in aqueous neutral and alkaline solutions^{1, 2}., Most of the problems from an experimental viewpoint stem from the lack of techniques capable of identifying species believed to be involved as intermediates in the reaction, including both adsorbed and solution-phase superoxide and peroxide. Recently, we developed a rotating ring-disk technique incorporating a judiciously functionalized Au ring displaying remarkably high specificity toward solution-phase superoxide in neutral solutions which made it possible to detect such species generated by the reduction of dioxygen at a bare Au disk electrode as a function of the applied potential³. Our contribution illustrates the use of in situ differential reflectance spectroscopy to monitor the adsorption of hydroxyl ion on a Au rotating disk electrode during dioxygen reduction, as well as that of on line mass spectrometry under forced convection to measure the rates of heterogeneous peroxide disproportionation on the same electrode surface using a jet impinging electrolyte arrangement⁴.

In basic solution, reduction of oxygen on Au electrodes is thought to proceed at least in part through a hydrogen peroxide intermediate. Many authors have speculated that subsequent disproportionation of the peroxide intermediate to oxygen and water plays an important mechanistic role, but measurements of disproportionation rates have not been attempted^{1, 2}. This is most likely due to the difficulty of deconvoluting this reaction rate from the rates of oxygen / peroxide reduction. To this end, we measured both the consumption rate of peroxide using a rotating ring disk electrode (RRDE) and the production rate of oxygen by on-line mass spectrometry on a Au electrode at open circuit in 0.1 M NaOH. Shown in Figure 1 are plots of the partial pressure of dioxygen monitored with the mass spectrometer at two fixed disk potentials, i.e. 0.2 and -1.2 V, at which peroxide is oxidized and reduced respectively under diffusion limited conditions, yielding maximum and minimum O₂ signal. The cell was then disconnected and the open circuit potential, after which the O₂ signal settled at an intermediate value 1/3 of the way between that seen at 0.2 and -1.2 V. This spontaneous production of O₂ from H₂O₂ without net current through the disk can only be explained by invoking H₂O₂ disproportionation. Furthermore, since only 1 O₂ is produced per 2 H₂O₂ via disproportionation, the O₂ signal at OCP represents consumption of 2/3 of the total H₂O₂ reaching the Au electrode.

Fig. 1: A Au disk electrode surrounded by a porous, gas-permeable Teflon ring was mounted in the wall-jet configuration (flow rate 0.45 mL/s), and the O₂ pressure passing through the Teflon ring was monitored by mass spec while the following electrochemical experiment was conducted: the Au electrode potential was held at either 0.2 (black) or -1.2 V vs Ag/AgCl (red) for 40 s, and then the circuit was opened. The solution was 0.1 M NaOH with 1 mM H₂O₂ (solid lines) or without H₂O₂ (dotted lines). The selected potentials of 0.2 and -1.2 V induce mass transport limited oxidation and reduction of H₂O₂, respectively, and so will yield the maximum and minimum O₂ signal.

Other authors have invoked superoxide as yet another intermediate in the reduction of oxygen on Au in alkaline solutions. Also to be discussed in this presentation is a theoretical analysis of the mechanism shown in Scheme I, which accounts for the contributions associated with all three species to the measured currents.

References:

1. Zurilla, R. W.; Sen, R. K.; Yeager, E. Electrochem Soc. 1978, 125, 1103-1109.
2. Kim, J.; Gewirth, A. A. Phys. Chem. B. 2006, 110, 2565-2571.
3. Feng, Z.; Georgescu, N. S.; Scherson, D. A. Chem. 2016, 88, 1088-1091.
4. Treufeld, I.; Jebaraj, A. J. J.; Xu, J.; Martins de Godoi, D.; Scherson, D. A. Chem. 2012, 84, 5175-5179.