Insights into the cyclic voltammetry of surface-confined molecules undergoing two-electron transfers of any reversibility and any ordering of the formal potentials: unravelling the apparent governing factors

Abstract

The cyclic voltammetry (CV) of surface-immobilized electroactive species undergoing two-electron transfers (so-called surface EE mechanism) is comprehensively studied in this work, considering any electron transfer kinetics (k0′ 1 , k0′ 2 ) and any values of their formal potentials (E0′ 1 , E0′ 2 ). Hence, a large number of possible situations arise, which are systematized through the definition of apparent electrochemical rate constants for both electron transfers (which account for the combined effect of k0′ i (i = 1, 2), ΔE0′ (= E0′ 2 - E0′ 1) and the scan rate), with their values and ratio determining the features of the voltammetric signal. Guidelines are established for the elucidation of the reversibility of the process as a function of the relative values of such apparent rate constants, which reflects on the symmetry of the CV signals in the forward and reverse scans. Thus, as the second transfer is more favorable than the first one, kinetically and/or thermodynamically, the process behaves as more irreversible. Also, simple protocols of quantification of the corresponding kinetic and thermodynamic parameters are given on the basis of the analysis of the height, width, number and position of the CV peaks.