Understanding the Chemistry and Physics of Charge Transfer in Electrolysis with a Plasma Electrode

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, where reactions depend on the catalytic properties of the electrode, highly reactive species including solvated electrons (e- aq), hydroxyl radicals (•OH) and atomic hydrogen (H•) are introduced into, or are generated in, the volume of the liquid. This novel interfacial interaction has led to a variety of applications including the chemical analysis of dissolved elements in an aqueous sample, the synthesis of both nanomaterials and useful chemicals, and the destruction of organic contaminants for water purification. With reported faradaic efficiencies greater than 100%, plasma electrolysis is a compelling alternative to conventional electrolysis. However, there are many things that are not understood about the charge transfer process, particularly when the plasma serves as the anode in the electrolytic cell. This includes how these chemical species are generated and in what amounts, as well as how electrons are emitted from the liquid into the plasma (in a process called secondary emission) to complete the electrolytic circuit. In this work we use the plasma voltage and breakdown voltage to study electron emission and show that emitted electrons are never solvated, or even pre-solvated, and are independent of the liquid chemistry. We find that secondary emission is very inefficient, with roughly 1 out of a million electrons escaping the solution. Finally, reactions with chloroacetate and chloroacetic acid were used to narrow down the potential yields of each of the chemical species and refine our understanding of the faradaic efficiency.