Plasma Electrochemistry: Redox Reactions in Gases
Authors: Caruana Daren 
Published: November 2022

Electrochemistry underpins numerous commercially important processes ranging from energy generation to sensors and coating deposition. A common and –so far– essential feature of all these processes involve a redox active species dissolved in a solvent which serves to allow charge exchange between electrodes. Rather than being inert, the solvent is without exception redox-active at extreme potentials and thus restrict the breadth of observable reactions to a narrow potential window of approx. -2.5 to +2.5 V vs. NHE. 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 scope of this work is to apply the well-healed cornerstones of electrochemistry developed almost exclusively in liquids, to the new context of gaseous plasma.
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&4 We show that free electrons do indeed reduce a copper oxide film, which may be carefully controlled by surface bias.5

A gaseous flame doped with electroactive species may be considered as electrolytes. Using a three-electrode system6, we may measure unique voltammograms for a series of small organic molecules and amino acids.7 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.

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