Diagnosis of the Mechanism of Anodic Oxide Film Growth on Platinum in H2SO4

Abstract

Diagnostic criteria for the growth of the anodic oxide film on platinum in H2SO4 are reported. The criteria apply to the generalized High Field Model (HFM), which postulates that the electric field within the film is dependent on the applied voltage and film thickness, and the Point Defect Model (PDM), which describes the electric field as being constant (independent of voltage and film thickness) during film growth. The constancy of the electric field in the passive film on platinum, as demonstrated in this work, is attributed to band-to-band Esaki tunneling, which buffers the electric field against changes in the applied voltage and film thickness. The Place Exchange Model (PEM), which is often invoked to describe the growth of the anodic film on Pt is rejected, because it does not predict a steady state in barrier layer thickness, which is observed experimentally. In this paper, we apply an analytical analysis of the oxide film growth transient on platinum in H2SO4 and demonstrate, unequivocally, that the PDM provides a superior theoretical framework than does the HFM or the PEM for interpreting oxide film growth on platinum. Importantly, we argue that the diagnostic criteria also apply to metal interstitial conductors, which is the mechanism of conduction proposed in the HFM for formation of the PtO oxide film on platinum, but with a thickness-dependent electric field, rather than being restricted to oxygen vacancy conductors alone, as originally derived for the PDM. Thus, the ability of the diagnostic criteria to differentiate between the HFM and the PDM, in this case, is a direct assessment of the dependence of the electric field strength on the applied voltage and barrier layer thickness, with the experimental results coming down on the side of the PDM.