Abstract
The kinetics of growth of thin anodic oxide films at platinum electrodes is studied in alkaline solutions of different pH. At all pH's, kinetics above 1.0Vvs. RHE follow the formalism of the high field assisted formation and migration of ions through an oxide film, i.e., , where c is a constant independent of pH, and is the thickness of the film, modified by the thickness of the inner Helmholtz layer, expressed as charge density. is pH dependent; it decreases 60 mV as pH increases one unit. However, the exchange current density, , decreases by one order of magnitude as pH decreases one unit. The dependence of on pH is unexpected for the high field model of growth with either a process at the metal/ oxide film interface, or a process within the oxide phase as the rate‐determining step. It can be explained only if a process at the oxide film/solution interface is rate determining. It is shown that in acid solution, too, this process is rate determining and not a process at the metal/oxide film interface as previously concluded. The observed difference in the activation energies for the growth in acid (25 kcal/mol) and alkaline (18.5 kcal/mol) solutions, which previously could not be accounted for, is due to different species reacting in the rate‐determining step. In acid and neutral solutions, and in alkaline solutions, OH− are the reacting species in the rate‐determining step.
Published Version
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call