Thin Layer Sonoelectrochemistry

Abstract

Ultrasound irradiation of electrodes in a thin electrolyte layer improves heterogeneous electron transfer kinetics.1,2 A bulk fluid irradiated with ultrasonic energy of sufficient intensity generates bubbles by compression and rarefaction. On bubble collapse, extremes of temperature (T ≳ 5000 K) and pressure (P ≳ 10³ atm) are generated at the narrow interface between the fluid and bubble void3. In bulk fluids, ultrasound enhances transport and impacts homogeneous reactions, but the energy attenuates. In prior sonoelectrochemical studies (first by Coury), bulk electrolyte was irradiated with sufficient ultrasonic intensity to generate cavitation. Cavitation increased homogeneous reaction rates, transport, and turbulence. No effects on heterogeneous electron transfer were found. In a thin layer sonoelectrochemical system (Figure 1), heterogeneous electron transfer rates increase. Thin layers have two advantages for harvesting sound energy2. Sound waves reflect back into the thin layer | air interface to retain energy. Losses from attenuation are less within thin layers. No frank cavitation is observed in thin layers. Electrolyte fills the well to form a meniscus. Cyclic voltammograms are recorded for each Ru(bpy)3 2+ and Fe3+ with and without sonication (Figure 2). Sonication intensity can be varied. Sonication has no impact on the voltammetry for Ru(bpy)3 2+ for which electron transfer rates are rapid compared to the scan rate. Sonication does not impact transport. Rates of Fe3+ reduction increases with sonication intensity, as shown by the more rapid rise in current on sonication. Peak potentials shift because the reference is shifting. Where rates are slow, sonication increases heterogeneous electron transfer rates without significantly impact on diffusional transport1. References and Acknowledgments: Support of the National Science Foundation and the University of Iowa is gratefully acknowledged. With thanks to Andrej Wieckowski for his explorations and contributions to understanding electrocatalysis. [1] Chester G. Duda, Ph.D. dissertation, University of Iowa, 2012. [2] J. Leddy, C.G. Duda, and W.J. Leddy III, Provisional U.S. Patent application, 2013. [3] K.S. Suslick, Y. Didenko, M.M. Fang, T. Hyeon, K.J. Kolbeck, M.M. Mdleleni, and M. Wong, Acoustic Cavitation, Phil. Trans. Royal Soc. A 1999.