Oxidation of hydrogen on oxidized platinum - Part I: The tunneling current

The hydrogen oxidation reaction (HOR) on bare and oxidized platinum in alkaline solution was studied at temperatures from 22 degrees C to 220 degrees C. It is found that the oxide film formed on platinum has a profound impact on the HOR rate, with the oxidation current decreasing exponentially with increasing overpotential. This inverse Tafel relationship can be explained by combining quantum mechanical tunneling theory with the point defect model (PDM) for film growth and dissolution. The tunneling constant, which is determined by the potential barrier height across the film/solution interface, is found to be temperature independent, if the anodizing constant is assumed to be invariant with temperature, and to be independent of pH and H-2 concentration within the ranges of these independent variables studied. A barrier height of 0.31 eV is obtained from the derived tunneling constant. Based on the combined film growth-quantum mechanical tunneling theory, the HOR is employed as an in situ probe for characterizing the thin oxide film that forms on platinum in alkaline solution as a function of temperature. The calculated film thickness increases linearly with overpotential until the hydrogen oxidation current is too small to be differentiated from the total measured current, which comprises the passive current and the capacitive charging current when measured under potentiodynamic conditions. Compared with conventional techniques, this method has the advantage of being sensitive. convenient, and of an in situ character, although, at the current stage of development, it is limited to a maximum film thickness of 1-2 nm. (c) 2006 Elsevier B.V. All rights reserved.