Abstract
<jats:p> The formation mechanism of hydrogen peroxide (H <jats:sub>2</jats:sub> O <jats:sub>2</jats:sub> ) 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 H <jats:sub>2</jats:sub> O <jats:sub>2</jats:sub> 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 H <jats:sub>2</jats:sub> O <jats:sub>2</jats:sub> (detection limit ~60 nM), whereas negative electrospray droplets generate up to tens of micromolar H <jats:sub>2</jats:sub> O <jats:sub>2</jats:sub> . The H <jats:sub>2</jats:sub> O <jats:sub>2</jats:sub> 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 H <jats:sub>2</jats:sub> O <jats:sub>2</jats:sub> in negative electrospray. Intrinsic interfacial electric field and inter-droplet discharge mechanisms are inconsistent with the pronounced asymmetry in H <jats:sub>2</jats:sub> O <jats:sub>2</jats:sub> production with droplet polarity. Instead, the results support a pathway for formation of hydroxyl radicals (OH <jats:sup>•</jats:sup> ) 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 <jats:sup>•</jats:sup> that combine to form H2O2. These findings suggest a new mechanism for formation of H <jats:sub>2</jats:sub> O <jats:sub>2</jats:sub> from aqueous microdroplets and further demonstrate that H <jats:sub>2</jats:sub> O <jats:sub>2</jats:sub> 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. </jats:p>