Catastrophe situation for potential‐assisted electron transfer in relation to temperature‐dependence of tafel slopes

Abstract

AbstractThe conventional representation of Tafel slopes, b, for activation‐controlled electrode processes as b = RT/βF with β a constant having the significance of a barrier symmetry factor or Brønsted type coefficient, is rarely followed with respect to the observed dependence of b on T. Rather, b is represented more generally in the form b = RT/(βH + TβS) F where βH and Tβs are formally enthalpic and entropic components of the overall β that determines the effect of changing electrode potentials (V) on the electrochemical Gibbs energy of activation and hence on the rate. — Special attention is directed to a “catastrophe” situation that arises in some experimentally observed behaviour where b is virtually independent of T, i.e. when TβS ≫ βH, so that apparently the normal effect of V on the electron exit energy that determines the potential‐dependence of the activation energy, is obscured. — Discussion of this situation, and presentation of recent new results for proton transfer at Hg, indicates that there are important effects of V “external” to the metal on both the entropy and enthalpy of activation, in addition to the effect of V on the electron Fermi level, relative to vacuum. Compensation effects between variations of the enthalpy and entropy of activation are related to this “external” effect of V.