Asymmetry in Droplet Charge and H2O2 Production: Exclusive Production of Hydrogen Peroxide in Negatively Charged Microdroplets via Electron Emission

Abstract

The formation mechanism of hydrogen peroxide (H2O2) in water microdroplets has been hotly debated, with proposals ranging from strong interfacial electric fields, discharge between droplets (i.e., microlightning), and reactions with ozone or dissolved oxygen. Here, we quantify H2O2 formation from positive, negative, and neutral droplets under oxygen- and ozone-free conditions. Neutral water droplets formed by condensation and positive electrospray droplets do not produce detectable amounts of H2O2 (detection limit ∼ 60 nM), whereas negative electrospray droplets generate up to tens of micromolar H2O2. The H2O2 concentration in negative droplets scales linearly with spray current, indicating the role of excess OH– that is electrochemically produced by the electrospray process itself. More than 140 MJ/mol of energy is needed to produce micromolar concentrations of H2O2 in negative electrospray. Intrinsic interfacial electric field and interdroplet discharge mechanisms are inconsistent with the pronounced asymmetry in H2O2 production with droplet polarity. Instead, the results support a pathway for the formation of hydroxyl radicals (OH•) that is unique to activated negatively charged droplets, wherein electron ejection from OH– occurs during droplet fission and upon contact with a stainless-steel surface. This mechanism leads to abundant OH• radicals that combine to form H2O2. These findings suggest a new mechanism for the formation of H2O2 from aqueous microdroplets and further confirm that H2O2 generation is not spontaneous or a result of an inherent property of the uncharged air–water interface in the absence of reactive oxygen or activation.