Quantitative cyclic voltammetry: Coupling in vitro studies and time-dependent numerical approximations to determine heterogeneous electron transfer kinetics and diffusion coefficients.
by Abrego Tello, Miguel A.; Lotfi Marchoubeh, Mahsa; Fritsch, Ingrid
Cyclic Voltammetry (CV) is an electrochem. technique known for its diagnostic capabilities to investigate redn. and oxidn. processes of chem. species. However, it is often considered a semi-quant. technique because of the varying applied potential and the superposition of both charging and faradaic current. Charging current adds information about soln. resistance and double-layer capacitance. Faradaic current is rich in information about electron transfer thermodn. (through the applied potential), kinetics (time scale through scan rate), and mass transfer. In this work, the focus is on the interpretation of the faradaic response. Exptl. and simulated results are coupled to evaluate and ext. the quant. capabilities of cyclic voltammograms. Expts. with microdisk electrodes (gold, 25-µm diam.) were performed in 5.00 mM potassium ferricyanide (K3Fe(CN)6) solns. with three KCl concns. (0.10, 0.50, 1.00 mM) at three temps. (22±1, 25±1, 30±1 °C) and at 0.02 V/s (scan rate). Electrochem. behavior was also investigated in 5.00 mM solns. of the reduced form, potassium ferrocyanide (K4Fe(CN)6), and the same three KCl concns. at one temp. (22±1 °C). Numerical approxns. of both systems (K3Fe(CN)6 and K4Fe(CN)6) were compared with exptl. results to det. heterogeneous electron transfer kinetics and diffusion parameters. The models applied the finite element method. Chronoamperometry expts. under conditions involving the oxidized species (K3Fe(CN)6) were performed to obtain the diffusion coeffs., which were compared to those obtained from the CV expts. Parameters describing heterogeneous electron transfer kinetics (transfer coeff., rate const., and formal potential) are validated under conditions involving the reduced species (K4Fe(CN)6). The transfer coeff., heterogeneous rate const., and diffusion coeff. showed strong dependence on electrolyte concn. The outcomes of this work could expand the use of CV for other applications. It could also provide a basis for a quant. anal. that allows for the measure of addnl. phys. consts.