Probing time-resolved plasma-driven solution electrochemistry in a falling liquid film plasma reactor: Identification of HO2− as a plasma-derived reducing agent

Many applications involving plasma–liquid interactions depend on the reactive processes occurring at the plasma–liquid interface. We report on a falling liquid film plasma reactor allowing for in situ optical absorption measurements of the time-dependence of the ferricyanide/ferrocyanide redox reactivity, complemented with ex situ measurement of the decomposition of formate. We found excellent agreement between the measured decomposition percentages and the diffusion-limited decomposition of formate by interfacial plasma-enabled reactions, except at high pH in thin liquid films, indicating the involvement of previously unexplored plasma-induced liquid phase chemistry enabled by long-lived reactive species. We also determined that high pH facilitates a reduction-favoring environment in ferricyanide/ferrocyanide redox solutions. In situ conversion measurements of a 1:1 ferricyanide/ferrocyanide redox mixture exceed the measured ex situ conversion and show that conversion of a 1:1 ferricyanide/ferrocyanide mixture is strongly dependent on film thickness. We identified three dominant processes: reduction faster than ms time scales for film thicknesses 100 µm, •OH-driven oxidation on time scales of 10 ms, and reduction on 15 ms time scales for film thickness 100 µm. We attribute the slow reduction and larger formate decomposition at high pH to HO2− formed from plasma-produced H2O2 enabled by the high pH at the plasma–liquid interface as confirmed experimentally and by computed reaction rates of HO2− with ferricyanide. Overall, this work demonstrates the utility of liquid film reactors in enabling the discovery of new plasma-interfacial chemistry and the utility of atmospheric plasmas for electrodeless electrochemistry.