Hydrogen peroxide formation mechanisms in liquid anode and cathode discharges

Abstract

Hydrogen peroxide (H2O2) plays a key role in plasma-induced chemistry for various applications. The mechanisms governing the production of aqueous hydrogen peroxide, H2O2aq, 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 H2O2aq concentration show that the H2O2aq 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 H2O2aq yield in the liquid cathode configuration being predominantly driven by the production of aqueous hydroxyl radicals, OHaq, that are formed by incident positive ions. This hypothesis is further supported by experiments using various OHaq 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.