Hydrogen peroxide formation mechanisms in liquid anode and cathode discharges

Abstract Hydrogen peroxide (H 2 O 2 ) plays a key role in plasma-induced chemistry for various applications. The mechanisms governing the production of aqueous hydrogen peroxide, H₂O₂ aq , in non-thermal plasma–liquid interactions, particularly the significant production rate dependence on liquid electrode polarity, have resulted in persistent controversy in the literature. We conduct spatiotemporal measurements of the gas phase H₂O₂ density at the interface of a pulsed helium plasma jet impinging on a liquid electrode, using photo-fragmentation laser-induced fluorescence. We show that gas phase H₂O₂ densities are unexpectedly similar for both discharge polarities. In contrast, complementary measurements of the liquid phase H₂O₂ aq concentration show that the H₂O₂ aq production is more than tenfold higher for a liquid cathode discharge compared to the liquid anode discharge. While this disparity cannot be reconciled by the solvation of gas phase H₂O₂, it can be explained by the enhanced H₂O₂ aq yield in the liquid cathode configuration being predominantly driven by the production of aqueous hydroxyl radicals, OH aq , that are formed by incident positive ions. This hypothesis is further supported by experiments using various OH aq scavengers as well as modeling results. The reported findings highlight that ion-driven liquid-phase chemistry is the dominant mechanism responsible for the polarity dependence in H₂O₂ synthesis by plasmas in contact with an aqueous electrode.