Engineers at Case Western Reserve University have demonstrated that an atmospheric-pressure microplasma can act as a gaseous, metal-free electrode to mediate electron-transfer reactions in aqueous solutions—i.e., an electrochemical cell that uses a plasma for an electrode. A paper on their work appears in the Journal of the American Chemical Society.
The ability to initiate and control electrochemical reactions at a plasma-liquid interface could open a new direction for electrochemistry based on interactions between gas-phase electrons and ionic solutions, although senior author Professor Mohan Sankaran notes that “this is a basic idea. We don’t know where it will go.”
Electrochemical systems are characterized by charge-transfer reactions across dissimilar phases, most commonly solid liquid interfaces. In a typical electrochemical cell, two metal electrodes are separated by an aqueous ion-conducting electrolyte, and electric potential differences lead to charge-transfer reactions at the metal electrode/ionic electrolyte interfaces. However, electrochemical reactions are not limited to those that occur at the interface of solid metals and liquids.
...We are interested in understanding charge-transfer reactions at the plasma-liquid interface. The initiation of electrochemical reactions by a gaseous electrode is of technological interest because metals such as Pt, which are commonly used as electrodes, are expensive and limited in supply, and there has been growing interest to eliminate them. There are potential materials applications, as well, of plasma-assisted electrochemistry such as the synthesis of nanostructured materials. However, it is not clear how gas-phase electrons interact with ions in solutions in comparison to electrochemical systems involving metal electrodes.
Here, we report for the first time evidence of electron-transfer reactions at the plasmaliquid interface. Our study is enabled by the recent development of a non-thermal, atmospheric-pressure microplasma source which can be stably formed on a solution surface at ambient conditions. Using the ferricyanide/ferrocyanide redox couple as a model system, we show that charge transfer depends on the properties of the discharge; for example, the reduction rate of ferricyanide is found to increase with discharge current, which in turn is related to the flux of plasma electrons to the solution surface. These findings open a new direction for electrochemistry where gas-phase electrons with tunable fluxes or energies are used to initiate and control electrochemical reactions in solution.—Reynolds et al.
The team used a glass electrochemical cell—two glass jars joined with a glass tube—with an electrolyte solution of potassium ferricyanide and potassium chloride with a fritted plug in the tub to keep the anode and cathode electrolytes from mixing. Separate compartments for the cathode and anode permitted isolation of the half-cell reaction occurring at the plasma-liquid interface.
In the anode side, an Ag/AgCl mesh was partly immersed in the electrolyte and served as the counter electrode. A stainless steel capillary tube was positioned 2 mm away from and normal to the surface of the electrolyte in the cathode side. A microplasma was ignited in the exit argon ﬂow. When a current was passed through the plasma, electrons reduced ferricyanide to ferrocyanide.
Monitoring with ultraviolet-visible spectrophotometry showed the solution was reduced at a relatively constant rate and that each ferrycyanide molecule was reduced to one ferrocyanide molecule. As the current was raised, the rate of reduction increased. And testing at both electrodes showed no current was lost.
The researchers, however, found two drawbacks:
Only about one in 20 electrons transferred from the plasma was involved in the reduction reaction. They speculated the lost electrons were converting hydrogen in the water to hydrogen molecules, or that other reactions they were unable to monitor were taking place.
The power needed to form the plasma and induce the electrochemical reactions was substantially higher than that required to induce the reaction with metal cathodes.
The researchers are fine-tuning the process and testing for optimal combinations of electrode design and chemical reactions for different uses.
Carolyn Richmonds, Megan Witzke, Brandon Bartling, Seung Whan Lee, Jesse Wainright, Chung-Chiun Liu, and R. Mohan Sankaran (2011) Electron-Transfer Reactions at the Plasma–Liquid Interface. Journal of the American Chemical Society DOI: 10.1021/ja207547