Double-Layer Inhibition of Peroxydisulfate Reduction at Mercury Ultramicroelectrodes. A Quantitative Analysis of the Frumkin Effect Including Molecular Transport and Long-Range Electron Transfer

Abstract

The electrical double layer (EDL) can lead to unexpected voltammetric phenomena. A notable example is the Frumkin effect, observed for the reduction of peroxydisulfate (S 2 O 8 2−). In the absence of excess supporting electrolyte, the rate of S 2 O 8 2− reduction decreases as the cathodic overpotential is increased, contrary to expectations for faradaic electron-transfer processes. Here, we report a demonstration of the Frumkin effect for S 2 O 8 2− reduction at Hg ultramicroelectrodes, revealing steady-state voltammetry behavior consistent with prior observations at rotating disk electrodes. A finite element model is used to simulate the effect of the EDL on S 2 O 8 2− reduction in the presence and absence of excess supporting electrolyte (K 2 SO 4 ). Semiquantitative agreement between experimental and simulated voltammograms is obtained, including capturing the decrease in current for S 2 O 8 2− reduction with increasing overpotential. We show that S 2 O 8 2− migration within the EDL, at electrode potentials negative of the potential of zero charge, is the primary process responsible for the observed decrease in current with increasing driving force. These combined experiments and finite element simulations represent the first quantitative description of the Frumkin effect with rigorous inclusion of the EDL, migrational and diffusional mass-transport, and long-range electron transfer kinetics.