Date of Award
Doctor of Philosophy (PhD)
The increasing availability and decreasing cost of electricity generated by renewable resources have motivated research into electrified chemical processing, whereby electrical energy is used to drive chemical transformations. Electricity-intensive processing techniques such as electrochemistry using solid electrodes has attracted attention in this context for the synthesis of organic compounds, such as high-value pharmaceuticals and renewable chemical production. Selective chemical transformations are achieved in conventional aqueous electrochemical systems by using external circuitry to bias solid electrodes, allowing for the preferential transfer of electrons between the electrode-liquid interface. Despite having the ability to promote controlled electrochemical reactions, configurations using solid electrodes are known to suffer from complications including surface fouling by carbonaceous coatings and are limited by material-dependent operational conditions. Another way to utilize electricity to drive chemical reactions is by employing nonthermal plasma, which is a partially ionized gas comprised of hot electrons (T ≥ 10000 K) and relatively cold ions and neutrals that remain at, or near, ambient temperature. Nonthermal atmospheric plasma interacting with liquids act as a potent source of highly reactive species including radicals, photons, atomic and molecular ions, and electrons, which participate in charge-transfer processes across the interface formed between the plasma and electrolyte solution (i.e. plasma-liquid interface). In turn, these reactive species facilitate reduction-oxidation (redox) reactions within solution without the need of a counter electrode. Thus, plasma-liquid systems are designated as electrodeless electrochemical systems, an approach that is hypothesized to ameliorate the issues of electrode fouling experienced using solid electrodes. Despite these features compared to conventional electrochemical systems, remarkably little is known about the electrochemical nature of the plasma-liquid interface, hindering the ability to rationally design redox reactions. The work of this dissertation is focused on understanding the fundamental electrochemical structure at a plasma-liquid interface. The first aim of this thesis is the development of a methodology capable of performing in situ measurements of the reduction potential in plasma-liquid systems. While the reduction potential is a key parameter in electrochemistry that determines the rate and direction of the redox reaction, a standard protocol for characterization of the reduction potential has not been established for plasma-liquid systems. The second aim of this dissertation is to elucidate the spatial locations of the reduction and oxidation half-reactions in an electrodeless electrochemical system. While the redox half-reactions simultaneously occur on the surfaces of well-defined, solid electrodes in conventional electrochemistry configurations, the absence of solid electrodes obfuscates the locations of the half-reactions (i.e. the electrodeless cathode and anode) in plasma-liquid systems. The final aim of this work is to develop a framework for understanding how to tune the reduction potential in solution, analogous to the external circuitry used in conventional electrochemical systems. More specifically, establishing a correlation between the state variables of the plasma (i.e. electron temperature and electron density) and the reduction potential in solution.