The electron transfer kinetics of adsorbed species derived by sampled current voltammetry

Abstract

In sampled current voltammetry, a sampled current voltammogram (SCV) is constructed by performing a number of potential steps and plotting the currents taken at a specific sampling time against the corresponding step potentials. For diffusion-controlled processes, the technique produces sigmoidal current-potential curves that are easily analysed to obtain kinetic information. Here, we extend this approach to study the electron transfer kinetics of adsorbed species (O _ads+ne ⁻⇌k _bk _fR _ads) and show that the SCVs have a characteristic peak whose shape, potential and current depend on the sampling time. We also discuss the results in terms of chronocoulograms and show that the sampled charge voltacoulograms (SQVs) are also affected by kinetics. Analysis of the SCVs and SQVs is particularly useful to unravel the dependence of the electrode response on potential and time. We investigate different data treatments and show that a logarithmic SCV, a plot of ln(j) against E, is highly sensitive to the electron transfer rate constant and to the sampling time. The time dependence of the current yields Γ _O ⁰ (initial surface coverage) and E ⁰, while the potential dependence of the current extrapolated to very short sampling times yields α and k _s. This extrapolation bypasses the double layer charging distortion, provided the sampling times are selected where the current transients are free from capacitive distortion. From the analysis of the SCVs, we propose a simple protocol to derive Γ _O ⁰, E ⁰, α and k _s, from only two current transients, the only constraint being that one of the chronoamperograms must be recorded with a large overpotential, circa ±120 mV. We also propose a simpler alternative involving non-linear regression of the current transients to a single expression. The chronoamperometric approach offers advantages compared to the voltammetric methodology proposed by Laviron. 1) Capacitive distortions only affect the current transients at short times but affect the whole potential window in voltammetry. 2) The current transients can be extrapolated to very short sampling times where the backward reaction has no influence. 3) The extrapolation to very short sampling times bypasses the double layer distortion. 4) Whereas a chronoamperogram holds a wide range of timescales, the voltammetric approach requires a wide range of scan rates to distinguish the kinetic regimes. 5) The voltammetric method only yields the values of k _s and α, while the chronoamperometric method also yields Γ _O ⁰ and E ⁰. 6) Since electrochemical workstations allow shorter time scales by chronoamperometry than voltammetry, the method can assess faster electron transfer kinetics than conventional cyclic voltammetry.

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