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

The electrical double layer (EDL) can lead to unexpectedvoltammetricphenomena. A notable example is the Frumkin effect, observed for thereduction of peroxydisulfate (S2O8 (2-)). In the absence of excess supporting electrolyte, the rate of S2O8 (2-) reduction decreases as thecathodic overpotential is increased, contrary to expectations forfaradaic electron-transfer processes. Here, we report a demonstrationof the Frumkin effect for S2O8 (2-) reduction at Hg ultramicroelectrodes, revealing steady-state voltammetrybehavior consistent with prior observations at rotating disk electrodes.A finite element model is used to simulate the effect of the EDL onS(2)O(8) (2-) reduction in the presenceand absence of excess supporting electrolyte (K2SO4). Semiquantitative agreement between experimental and simulatedvoltammograms is obtained, including capturing the decrease in currentfor S2O8 (2-) reduction withincreasing overpotential. We show that S2O8 (2-) migration within the EDL, at electrode potentialsnegative of the potential of zero charge, is the primary process responsiblefor the observed decrease in current with increasing driving force.These combined experiments and finite element simulations representthe first quantitative description of the Frumkin effect with rigorousinclusion of the EDL, migrational and diffusional mass-transport,and long-range electron transfer kinetics.