Voltammetry at a three-phase junction

Abstract

When insoluble insulating crystals adhere to an electrode, the three-phase junction – where electrolyte solution, electrode and crystal meet – is the only feasible site for an electrochemical reaction. Moreover, sustained reaction is possible only if ions from the electrolyte solution are able to enter the crystal through the three-phase junction and disperse within the crystal. Here, order-of-magnitude calculations demonstrate that diffusion to the three-phase junction is well able to support voltammetry under standard experimental conditions. A model is built for cases of adherent cubes of uniform size and thereby the shapes of chronoamperograms, chronograviograms and cyclic voltammograms are predicted. The model assumes that the ion concentration at the three-phase junction plays a crucial role in the voltammetry, being determined by quasi-steady-state ion diffusion from the bulk, the thermodynamics of the electrode reaction, and the extent to which the crystal has already undergone reaction. Depending on the crystal size and scan rate, cyclic voltammograms may mimic solution-phase voltammograms from classical thin-layer experiments or from typical stripping experiments. The effect of size heterogeneity on cyclic voltammetry is simulated for lognormal distributions.