Electrochemistry of Oxygen in Concentrated NaOH Solutions: Solubility, Diffusion Coefficients, and Superoxide Formation

The diffusion coefficient (D-O2), solubility (C-O2), and electrochemical behavior of oxygen reduction were investigated by cyclic voltammetry and transient amperometry on a Pt ultramicroelectrode in aqueous solutions containing various concentrations of NaOH (1-12 M). The results show that both D-O2 and C-O2 decrease as the solution viscosity (eta) increases significantly with increasing concentration of NaOH. The Stokes-Einstein relationship (D-O2 vs 1/eta) is followed, yielding a radius for the O-2 molecule, 2.8 angstrom, that does not change over the concentration range of NaOH studied. From results reported previously for C-O2 in more dilute NaOH solutions and the new results here, the number of electrons, n, involved in the first step of the oxygen reduction reaction (ORR) was found to change with the concentration of NaOH, from n = 2 at low NaOH concentrations (1-2 M) to n = 1 at high concentrations (>6 M), in line with the changing water activity. Scanning electrochemical microscopy (SECM) was employed as a sensitive tool to investigate the electrochemical behavior of the product of the ORR in 10 M NaOH solution, O-2(center dot-), as a function of potential. The SECM approach curves depend on the substrate bias and state of the Pt substrate surface, and the apparent rate constant for the redox couple of O-2/O-2(center dot-) is determined to be about 2.6 x 10(-4) cm/s in this solution.