Project Details Definitions of transfer coefficient and of partial charge transfer coefficient in electrode kinetic

Project No.: 
2011-038-1-100
Start Date: 
01 January 2012
End Date: 
01 March 2014

Objective

The transfer coefficient is a quantity that is commonly employed in the kinetic investigation of electrode processes. An unambiguous definition of the transfer coefficient, independent of any mechanistic consideration and exclusively based on experimental data, will be proposed and the appropriate use of this quantity for the elucidation of electrode kinetics will be critically examined.
The concept of partial charge transfer is frequently employed in the study of chemical adsorption (chemisorption) of molecules on electrodes. Some confusion arises from the use of this extrathermodynamic quantity, and a correct definition will be critically evaluated. The inappropriate usage of certain electrochemical terms will be pointed out. The results obtained will be described in a technical report and in a recommendation.

Description

In the first two editions of the Green Book, the transfer coefficient for an electroreduction reaction, denoted by αc, was defined as –(ν/n) (RT/F)∂lnIc/∂E, where Ic is the cathodic current at constant reactant concentration on the electrode surface, E is the applied potential, n is the number of electrons involved in the overall reaction and ν is “the number of identical activated complexes formed and destroyed in the completion of the reaction”. While the dimensionless quantity (RT/F)∂lnIc/∂E is a directly measurable experimental quantity, the estimate of the two quantities n and ν requires some mechanistic considerations; this explains why this definition has been almost never used in the literature. In the third and most recent edition of the Green Book, the quantity ν was removed from the definition of the transfer coefficient, to comply with a definition adopted in many textbooks and dating back to the pioneering work of Butler and of Erdey-Gruz and Volmer. However, the deletion of the correlation between n and ν makes the significance of n ambiguous; thus, n has been identified either with the number of electrons involved in the overall electrode reaction (which depends on the way in which the reaction is written), or with the number of electrons taken up in a single rate determining elementary step (whose probability of being greater than unity is extremely low, on the basis of quantum mechanical considerations). This ambiguity has led to misinterpretations and erroneous mechanistic conclusions by many inexperienced researchers. A different school of thought, pioneered by Bockris and Gileadi, has identified the transfer coefficient with the directly measurable quantity -(RT/F)∂lnIc/∂E, independent of any mechanistic consideration. The present collaborative effort is aimed at critically evaluating the concept of transfer coefficient, proposing that its definition be based entirely on measurable quantities, and indicating correct procedures to exploit the experimental value of αc for the elucidation of electrode reaction mechanisms.

The partial charge transfer coefficient, denoted by λ, is commonly regarded as the positive or negative fraction of the electronic charge |e| that a molecule transfers to the electrode upon its adsorption on it. This is clearly an extrathermodynamic quantity, in that it cannot be estimated without having recourse to some modelistic assumption. In fact, the division of the bonding electrons into parts pertaining to the adsorbate and to the electrode is somewhat arbitrary. λ is often estimated by measuring the thermodynamic quantity l ≡ –(∂σM/∂Γ)E/F called “electrosorption valency”, where σM is the charge density on the metal and Γ is the adsorbate surface excess; l includes contributions other than λ that must be estimated and subtracted from l in order to estimate λ. The partial charge transfer coefficient can also be estimated by measuring the dipole moment of the metal-adsorbate bond (e.g., the Me-S bond for thiol adsorption on a metal Me), since λ is expected to increase with the covalent nature of this bond, and hence with a decrease in its polarity. A further goal of this project is the definition of the partial charge transfer coefficient and a critical evaluation of the extrathermodynamic procedures adopted for its estimate. Finally, the incorrect usage of certain electrochemical notations and terminologies will be pointed out and their emendation will be recommended.

Progress

April 2012 – project announcement published in Chem. Int. Mar-Apr 2012, p. 21.

Nov 2012 – the provisional recommendations titled “Definition of the Transfer coefficient” was made available for public review until by 31 March 2013.

Feb 2014 – A technical report and recommendations have been published in Pure and Applied Chemistry, Vol 81, No 2: The Technical Report is pp 245-258, DOI:10.1515/pac-2014-5026, and the IUPAC Recommendations 2014 is pp 259-262, DOI:10.1515/pac-2014-5025

Project completed

last update 3 June 2014