(Invited) Electrochemistry in Plasmas: From Flames to RF Plasma Jets
Authors: Caruana Daren , Sener M Emre , Calleja Maria , Elahi Atif 
Published: May 2020

As electrochemists, we are interested in electron attachment and detachment processes. Traditionally, we control the availability of electrons via an electrically conducting solid and measure electron transfer across the solid/liquid interface. Of course, there are exceptions to this picture, e.g. liquid/liquid interfaces, but often liquids are involved to provide an electrolyte medium to support the chemical species. Gaseous electrolytes have typically been ignored due to their feeble electrical conductivity. However, recently with the advent of new accessible approaches to form stable plasmas, these electrically conducting gases are attracting some significant interest and are now being investigated as exotic electrochemical environments.

The defining property of plasmas is presence of free electrons; because of this they may be considered as both electrodes or electrolytes1, 2. We describe results supporting both modes.

As electrodes, we show that metal oxides on surfaces may be reduced to zero valent metals using a helium atmospheric plasma jet.3 We show that free electrons do indeed reduce a copper oxide film, which may be carefully controlled by surface bias.4

A gaseous flame doped with electroactive species may be considered as electrolytes. Using a three-electrode system5, we may measure unique voltammograms for a series of small organic molecules and amino acids.6 Except for leucine and isoleucine, all were distinguishable. The reduction signatures originate from specific electron attachment reactions of radicals formed via incomplete combustion and fragmentation of the parent molecules. In this case without a solvent we have an extended potential window and our voltammograms extend between 0 and -10 V, which gives unprecedented access to chemistry not previously accessible in liquids. Moreover, mass transport properties are far better than in liquids, as such the fluxes of electroactive species to the electrode a much greater.

References

1. Rumbach, P., Bartels, D.M., Sankaran, R.M. & Go, D.B. The solvation of electrons by an atmospheric-pressure plasma (vol 6, pg 7248, 2015). Nature Communications
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2. Elahi, A., Fowowe, T. & Caruana, D.J. Dynamic Electrochemistry in Flame Plasma Electrolyte. Angewandte Chemie-International Edition
51, 6350-6355 (2012).

3. M. Emre Sener, S.S., Robert Palgrave, Raul Quesada Cabrera, Daren J. Caruana Patterning of Metal Oxide thin Films using H2/He Atmospheric Pressure Plasma Jet. Green Chemistry
Submitted (2019).

4. Sener, M.E. & Caruana, D.J. Modulation of copper(I) oxide reduction/oxidation in atmospheric pressure plasma jet. Electrochemistry Communications
95, 38-42 (2018).

5. Fowowe, T., Hadzifejzovic, E., Hu, J.P., Foord, J.S. & Caruana, D.J. Plasma Electrochemistry: Development of a Reference Electrode Material for High Temperature Plasma. Advanced Materials
24, 6305-6309 (2012).

6. Calleja, M., Elahi, A. & Caruana, D.J. Gas phase electrochemical analysis of amino acids and their fragments. Communications Chemistry
1 (2018).