UNCONVENTIONAL TEMPERATURE-DEPENDENCE OF THE BETA-FACTOR FOR THE OXYGEN EVOLUTION REACTION AT PT ELECTRODES IN ACID-SOLUTION - SIGNIFICANCE OF THE RECOVERED ACTIVATION-ENERGIES

Oxygen evolution reaction is examined at Pt electrodes in acid solutions at temperatures ranging from 273 to 348 K. Tafel slopes at all temperatures are close to 120 mV, i.e., the symmetry factor β = βST (≃1/2 at room temperature). When β = βST, the Arrhenius plots, log i at a given E vs 1/T, are not linear. The common assumption that, from quasi-linear plots and isothermal conditions, the enthalpy of activation at the reversible potential, ΔH‡R, and/or at zero Galvani potential difference, ΔH‡Δφ=0, are recovered does not hold. It is suggested that ΔH‡ at any potential is close to zero when β = βST and that the activation energy is predominantly given by the entropic term, TΔS‡, which is potential dependent. The rate equation for the reaction is given in terms of the electron energies of the reacting complex in its ground state, ε0, and activated state, ε. It is shown that, for the case β = βST, ε0 becomes less negative as temperature increases, and this results in the increased rates. In turn, the change in ε0 with temperature arises from the entropic term, TΔS0, in the ground state of the reacting complex. This leads to the linear dependence of ΔS‡ at a given potential on temperature. From the plot of E vs T, or log i vs T, both of which are now linear, ΔS0 is determined. © 1990 American Chemical Society.