Charge Carrier Tunneling Across the Passive Film on Platinum

The reduction of oxygen on platinum in 0.1M KOH has been studied at ambient temperature using a rotating ring disk electrode and two Tafel slopes are observed. The structural and electronic properties of this film were investigated over an extended temperature range (25oC to 220oC) using the rate of hydrogen oxidation as an experimental probe in a pressurized, controlled hydrodynamic flow channel. By combining Quantum Mechanical Tunneling theory and the Point Defect Model, a new method was devised for determining the thickness of the extremely thin oxide film on platinum. This QMT-PDM method yields a temperature-invariant tunneling constant (which defines the blocking character of the oxide film to charge transfer) as 0.57x108cm-1 and yields a barrier height of 0.31eV. Finally, the barrier oxide layer on platinum is postulated to comprise a columnar, nano crystalline structure with tetragonal unit cells oriented with the c-axis perpendicular to the metal surface.