A Critical Analysis of Spontaneous Hydrogen Peroxide (H2O2) Formation at the Air–Water Interface: Reconciling Conflicting Reports

Recent reports of spontaneous hydrogen peroxide (H2O2) formation in microdroplets have sparked productive debate, with mechanisms falling into two families: those invoking the air–water interface – (i) ultrahigh electric fields driving the formation of HO• from HO−, (ii) comproportionation of water into H2O•+ and H2O•−, and (iii) partial dehydration of interfacial H+ and HO− – and those invoking other pathways, namely (iv) O2 reduction at water–solid contacts, (v) sonochemistry, and (vi) tribochemistry. Drawing on our experiments and established radiation and interfacial chemistry, we assess each proposal against four diagnostic criteria based on energy budget, mass balance, O2-dependence, and substrate specificity. Mechanisms (i–iii) demand energies unattainable at ambient interfaces and obligatory co-products that have never been observed, whereas (iv–vi) satisfy all four criteria and align with the broader literature. This framework repositions spontaneous H2O2 within established chemistry and extends naturally to other claims of anomalous microdroplet reactivity.