Coupled Electron- and Phase-Transfer Reactions at a Three-Phase Interface.

Coupled electron- and phase-transfer reactions are fundamentally important in electrochemical energy conversion and storage, e.g., intercalation of Li+ in batteries and electrochemistry at the three-phase boundary in fuel cells. The mechanism, energetics, and kinetics of these complex reactions play an important role in device performance. Herein, we describe experimental methodology to quantitatively investigate coupled electron- and phase-transfer reactions at an individual, geometrically well-defined, three-phase interface. Specifically, a Pt-Ir wire electrode is placed across a H2O/1,2-dichloroethane (DCE) interface, creating a Pt-Ir/H2O/DCE boundary that is defined mathematically by a line around the surface of the wire. We investigated the oxidation of ferrocene (Fc), initially present in DCE (but essentially insoluble in water), at the three-phase boundary, and the transfer of its charged reaction product ferrocenium (Fc+) across the interface into the aqueous phase. In cyclic voltammetry, a reversible wave at E1/2 ∼ 0.58 V is observed for Fc oxidation in DCE on the first scan. The Fc+ produced near the H2O/DCE interface transfers into the aqueous phase. On the second and subsequent cycles, a second reversible wave at more negative potentials, E1/2 ∼ 0.33 V, is observed, corresponding to the reduction of Fc+ (and reoxidation back to Fc) in the aqueous phase. Finite-element simulations quantitatively capture the voltammetric response of coupled electron and phase transfer at the three-phase interface and indicate that the electrochemical response observed in the aqueous phase occurs within ∼200 μm of the Pt-Ir/H2O/DCE boundary. Finally, we demonstrate that the rate of transfer of Fc+ is strongly dependent on the concentration of supporting electrolyte, reaching a maximum at an intermediate electrolyte concentration, suggesting a critical role of the electric field distribution in determining the reaction rates at the three-phase interface.