Understanding the Role of Energy Transfer Processes in Chemical Reactions at Plasma-Liquid Interfaces

Electrochemistry can enable sustainable chemical manufacturing but is limited by the reactions possible with conventional metal electrodes. Plasma electrochemistry, which replaces a conventional solid electrode with plasma in electrochemical cells, offers new avenues for chemical synthesis by combining charge transfer with alternative energy transfer processes at the plasma-liquid interface. To understand how plasma electrochemistry differs from conventional electrochemistry, we investigated plasma reactions with acrylonitrile, an industrially relevant molecule used as the precursor in the well-characterized electrosynthesis of adiponitrile. We demonstrate that non-charge transfer processes dominate plasma-driven chemistry through systematic variation of plasma polarity, current, and reactant concentration, combined with comprehensive quantitative analysis of solid, liquid, and gas products. Most notably, we observed no adiponitrile formation (the desired electrochemical product) while total product yields exceeded the theoretical charge-transfer maximum by up to 32-fold. Substantial polyacrylonitrile formation occurred under all conditions, a product not typically seen in conventional electrochemistry. The plasma anode yielded consistently higher products than the plasma cathode, producing hydrogen and propionitrile at 21 and 2 times the charge-transfer maximum, respectively. Electron scavenger experiments confirmed these transformations occurred primarily through non-Faradaic processes rather than charge transfer. These results demonstrate that plasma electrochemistry is primarily driven by energy transfer at plasma-electrolyte interfaces, providing fundamental insights for harnessing these interactions in chemical synthesis.