(Invited) Electrolysis with Plasma Cathodes: Modeling and Experiments to Understand the Electrochemical Interface

Tuesday, 30 May 2017: 08:40
Trafalgar (Hilton New Orleans Riverside)
D. B. Go (University of Notre Dame)
In the conventional picture of electrolysis, two metal electrodes are submerged into an electrolyte solution and an applied voltage drives redox reactions at the cathode and anode, respectively. Plasma electrolysis, also called glow discharge electrolysis, changes this convention by replacing one or both of the solid electrodes with a plasma or gas discharge. Reduction and/or oxidation reactions, therefore, are no longer mediated by a solid/liquid interface but by a gaseous/liquid interface, requiring that free charges traverse from the gas to liquid and vice versa. The plasma cathode configuration replaces the typical metal cathode with a plasma, such that free gaseous electrons in the plasma must be injected into the solution phase to initiate reduction reactions at the “cathode”. Recently, this approach to electrolysis has received significant attention for nanomaterials synthesis and emerging applications include the ‘catalyst-free’ electrochemical processing of carbon dioxide (CO2) into useful chemicals.

However, there are significant questions that persist about the nature of the electron reactions at the plasma-liquid interface. Recent studies have confirmed that the plasma electrons solvate in the solution before reacting with other dissolved species, and interfacial measurements of the absorption spectrum show that it is similar to, but different from, the standard solvated electron spectrum measured using radiolysis techniques. This anomaly suggests that the plasma-liquid interface is a wholly different environment for solvated electrons from that they experience in bulk radiolysis. To understand this interface, a model of the interfacial electrostatic Debye layer was constructed by considering both the electrostatic potential variation on the plasma side and the liquid side along with electron injection across the liquid interface. This analytical model predicts a non-linear scaling relationship between the plasma current density and electrolyte conductivity, and experimental measurements confirm the model. Extracted values from the model, including the interfacial electric field (~105 V/m) and the electrolyte cation concentration, offer insight into the electrochemical interface, but do not fully resolve questions regarding the anomalous absorption spectrum of plasma-injected solvated electrons.