Thin Layer Sonoelectrochemistry in Aqueous and Non-Aqueous Systems

Abstract

In thin layer sonoelectrochemistry, ultrasonic sound waves introduced into an electrolyte layer increases the rates of heterogeneous processes [1, 2]. From previous studies of thin layer sonoelectrochemistry, the rates of heterogeneous electron transfer, oxygen reduction reaction (ORR), and methanol electrolysis were facilitated under sonication. In the thin layer of electrolyte, ultrasound energy is reected into the electrolyte at the electrolytejair meniscus to minimize the losses from attenuation found in bulk systems. This harvests the ultrasonic energy more e¢ ciently. The observed e/ects are on interfacial rates and not mass transport. No cavitation is thus far observed. These e/ects have been observed at platinum electrodes in aqueous electrolytes when the interfacial reactions are slow. The studies were done at a single frequency and varied intensity. The fundamental mechanism of thin layer sonoelectrochemistry is not yet fully understood. Here, studies are undertaken to broaden inspection of experimental parameters. This identies critical parameters of the ultrasonic input (e.g., frequency, intensity, duty cycle) and the electrochemical system (e.g., solvent properties of viscosity and density, electrode materials). The data informs the models. In this study, thin layer sonoelectrochemistry was undertaken on academically interesting reactions in aqueous and non aqueous electrolytes. Redox probes include tris(bipyridine)ruthenium(II) chloride (Ru(bpy)2+ 3 ), iron ( III ) (Fe3+) and p-benzoquinone (p-BQ). These choices allow comparison of fast (Ru(bpy)2+ 3 ) and slower (Fe3+) heterogeneous electron transfer rates as well as coupled proton electron (p-BQ) processes in different solvents. Cyclic voltammetry measurements were performed on a Pt wire working electrode against a shrouded Pt/PtO2 quasireference electrode (QRE). The ultrasonic transducer is placed in the cell but is electrically isolated from the electrochemical measurement circuit. The electrolytes are HNO3/H2SO4 in aqueous and tetrabutylammonium tetrauoroborate (TBABF4) in non-aqueous solvent DMF. The outcomes of these studies will support fundamental understanding of the thin layer sonoelectrochemical mechanisms.