Competition Between Electrons and OH Radicals in Plasma Electrolysis

Abstract

Plasma electrochemistry consists of an electrolytic cell where the cathode or the anode is replaced by a direct current (DC) plasma. Unlike conventional electrolysis, highly reactive species such as the solvated electron (e- aq), and the hydroxyl (OH) radical dominate the surface reactions, instead of the properties of the solid electrode. Using this configuration, plasmas have been used to process chemicals, create long-lived chemical species, and synthesize nanoparticles in electrolytic cells. However, which chemical species are introduced to the electrolyte, in what amounts, and how their respective reactions compete, are lingering questions in the field. Here, a non-thermal, atmospheric argon DC plasma was used as a cathode coupled with a platinum anode and aqueous sodium perchlorate (NaClO4) or sodium chloride (NaCl) solutions as background electrolytes. Two chemical compounds, ferricyanide (Fe(CN)6 3-) or methylene blue (MB), were used as chemical indicators to study the competitive reactions driven by e- aq and OH, and various scavengers were used to help identify plausible reaction pathways. A simple kinetic model was used to analyze and interpret the data, revealing that the Faradaic efficiency in these systems is severely limited by the ubiquitous presence of OH.