Defining the transfer coefficient in electrochemistry: An assessment (IUPAC Technical Report)

Abstract

AbstractThe transfer coefficientαis a quantity that is commonly employed in the kinetic investigation of electrode processes. In the 3rdedition of the IUPAC Green Book, the cathodic transfer coefficientαcis defined as –(RT/nF)(dlnkc/dE), wherekcis the electroreduction rate constant,Eis the applied potential, andR, T, andFhave their usual significance. This definition is equivalent to the other, -(RT/nF)(dln|jc|/dE), wherejcis the cathodic current density corrected for any changes in the reactant concentration at the electrode surface with respect to its bulk value. The anodic transfer coefficientαais defined similarly, by simply replacingjcwith the anodic current densityjaand the minus sign with the plus sign. It is shown that this definition applies only to an electrode reaction that consists of a single elementary step involving the simultaneous uptake ofnelectrons from the electrode in the case ofαc, or their release to the electrode in the case ofαa. However, an elementary step involving the simultaneous release or uptake of more than one electron is regarded as highly improbable in view of the absolute rate theory of electron transfer of Marcus; the hardly satisfiable requirements for the occurrence of such an event are examined. Moreover, the majority of electrode reactions do not consist of a single elementary step; rather, they are multistep, multi-electron processes. The uncritical application of the above definitions ofαcandαahas led researchers to provide unwarranted mechanistic interpretations of electrode reactions. In fact, the only directly measurable experimental quantity isdln|j|/dE, which can be made dimensionless upon multiplication byRT/F, yielding (RT/F)(dln|j|/dE). One common source of misinterpretation consists in setting this experimental quantity equal toαn, according to the above definition of the transfer coefficient, and in trying to estimatenfromαn, upon ascribing an arbitrary value toα, often close to 0.5. The resultingnvalue is then identified with the number of electrons involved in a hypothetical rate-determining step or with that involved in the overall electrode reaction. A few examples of these unwarranted mechanistic interpretations are reported. In view of the above considerations, it is proposed to define the cathodic and anodic transfer coefficients by the quantitiesαc= –(RT/F)(dln|jc|/dE) andαa= (RT/F)(dlnja/dE), which are independent of any mechanistic consideration.