Characterization of Chemical Reactions Coupled to Electron Transfer Reactions Using Cyclic Voltammetry

widely used for the initial characterization of electrochemically active systems. In addition to indicating the number of different oxidation states, and their relative energies, CV can also be used for mechanistic studies of systems in which the electron transfer reactions are coupled to chemical reactions, due to the characteristic appearance of cyclic voltammograms associated with different mechanisms (1). These characteristically shaped voltammograms are the subject of this article. The cyclic voltammogram for a reversible process (i.e., the surface concentrations of the oxidized and reduced species required by the Nernst equation are maintained at all potentials) is shown in F1 for the reaction O + e = R. The separation of the peak potentials on the forward (reduction) and reverse (oxidation) scans (∆Ep) is about 58 mV (at 25 °C) (2), and the magnitudes of the two peak currents are equal (the peak current ratio, ipa/ipc, = 1). The coupling of chemical reactions to the electron transfer reactions can lead to changes in the peak potentials and/or the peak currents, and the effect of chemical reactions is often expressed in terms of changes in the peak current ratio and/or peak potentials. In addition, in this article, the effects of chemical reactions are also shown using cyclic voltammograms generated by DigiSim, a simulation program for CV. When considering the effect of chemical reactions, it is very important to note that any variations from reversible behavior are related not to the absolute magnitude of the rate constant for the chemical reaction (k), but to the value of this rate constant relative to the time scale of the experiment. For example, decreasing the time scale of the experiment also decreases the time available for the chemical reaction, and hence the effect of the chemical reaction can be decreased. The time scale of a cyclic voltammetry experiment is determined by the scan rate ν (increasing the scan rate decreases the experimental time scale); therefore, the important parameter in determining the effect of chemical reactions is k/ν. In the following examples, simulated voltammograms are shown for different mechanisms at different values of k/ν (where k is the rate of the forward chemical reaction), unless specified otherwise (3). The nomenclature used to describe the mechanisms is based on E for an electron transfer reaction and C for a chemical reaction; for example, an EC mechanism consists of an electron transfer reaction (E) followed by a chemical reaction (C). Unless stated otherwise, the following parameter values were used for all simulations: Eo = 0 V, scan rate = 1 V s-1, ks = 1 cm s-1, α = 0.5. These values were selected so that there would be no interfering effects from slow heterogeneous electron transfer kinetics.