DIFFUSION AND KINETIC CONTROL IN THE DISCHARGE OF HYDROGEN ON PLATINUM AND PLATINUM ALLOYS.

Abstract : Hydrogen discharge from acid solutions on bright platinum is characterized by a Tafel slope of -0.03 V/decade with an apparent limiting current or transition to a much higher slope at high current densities (>100 mA/sq cm). Similar measurements on gold-platinum alloys indicate that the Tafel slope and apparent exchange current are surprisingly independent of composition for alloys containing less than 0.8 atom fraction gold. For such alloys the apparent limiting current density was found to decrease with increasing gold content. Polarization studies with the rotating disc technique show that the current at constant potential is strongly dependent on rotation rate but reveal no significant dependence of Tafel slope on rotation rate. These results provide evidence that diffusion control involving dissolved H2 is predominant in the region of the -0.03 V/decade slope with kinetic control predominant only above this region. Further evidence of substantial super-saturation of the solution adjacent to the electrode with H2 was obtained with the rotating disc-ring technique. (Author)