Temperature dependence of the Tafel slope for oxygen reduction on platinum in concentrated phosphoric acid

Oxygen reduction on bright platinum in concentrated H sub 3 PO sub 4 has been investigated with the rotating disc electrochemical technique at temperatures from 25-250 deg C and O pressures up to 1.77 MPa. Cyclic voltammetry has been employed to study the anodic film formed on Pt in concentrated H sub 3 PO sub 4 and the possible electroreduction of H sub 3 PO sub 4 on Pt. The apparent transfer coefficient for the O reduction has been found to be approximately proportional to temperature rather than independent of temperature. Such behaviour is difficult to reconcile with accepted theories for the effect of electrode potential on the energy barriers for electrode processes. It is of importance to establish an understanding of this phenomenon. Possible factors which can contribute to the temperature dependence of the transfer coefficient but which would not necessarily result in a direct proportionality to temperature include potential dependent adsorption of solution phase species, restructuring of the solution in the compact layer, proton and electron tunnelling, a shift in rate-determining step, changes in the symmetry of the potential energy barrier, penetration of the electric field into the electrode phase, insufficient correction for ohmic losses, and impurity effects.