Fundamental Studies of the Chemistry and Charge Exchange at a Plasma-Liquid Interface

Plasma electrolysis consists of an electrolytic cell where a direct current (DC) nonthermal plasma replaces the cathode, anode, or both electrodes. As opposed to conventional electrolysis, where surface reactions are controlled by the catalytic properties of the electrode material, the chemistry is brought about in the volume of the liquid via the introduction of highly reactive species such as the solvated electron (e^-_aq), and the hydroxyl (•OH) radical.

This chemistry is not unique to plasma-liquid systems, and has been studied in the field of radiation chemistry for decades. However, while the chemistry is the same, the experimental systems used to generate it are very different which, in addition to other factors like cost and safety, makes it worthy of study. For example, while pulses of radiation may penetrate deeply into a solvent, the much lower energies of either electrons or positive ions going into the solution ensures that these reactive species will be formed uniquely in a very thin layer (~10-100 nm) of liquid close to the surface. On an opposite note, while the chemistry is different between plasma and solid electrodes, the experimental system can be very similar, and similar issues are experienced in both systems. These include the decrease in Faradaic efficiency due to competing reactions at an electrode, the requirement to exchange charge at the interface, and the added effect of reactant transport toward the interface, in addition to the kinetics.

In this work, an electrolytic cell consisting of a DC nonthermal, atmospheric-pressure argon plasma was used as with a submerged platinum counter electrode as an experimental system to study some of these issues. Experiments were conducted using both, a plasma cathode, and a plasma anode configuration. In addition to experiments, MATLAB simulations were used to study the combined effect of chemical reactions and diffusion as a function of time in a plasma cathode.

First, measurements of the Faradaic efficiency in the reduction of chloroacetate and ferricyanide at a plasma cathode are used to qualitatively show that the system is transport-limited. Additionally, it is shown that •OH introduced from the plasma significantly decrease the Faradaic efficiency for reversible reactions, but not for non-reversible reactions. Second, the combined use of mathematical scaling and simulations of reaction-diffusion equations show that depletion of a scavenger close to the interface can be predicted with a characteristic time, t_c, which scales proportionally with the scavenger diffusivity (D_s) and the square of the scavenger bulk concentration (S_B) and inversely proportional to the electron flux (J) squared; that is t_c ~ D_sS_B²F²/J², where F is Faraday’s constant. And third, measurements of the plasma voltage and breakdown voltage were used to study the charge exchange via electron emission from an aqueous solution into the plasma under a plasma anode configuration. Results show that emitted electrons are never solvated, or even pre-solvated, and are independent of the liquid chemistry. The secondary emission coefficient is also quantified, and it is estimated that roughly only one out of a million positive ions colliding with the liquid generate an electron which escapes into the plasma.