The Air-Water Interface of Water Microdroplets Formed by Ultrasonication or Condensation Does Not Produce H2O2

Recent reports on the production of hydrogen peroxide (H2O2) on the surface of condensed water microdroplets without the addition of catalysts or additives have sparked significant interest. The underlying mechanism is thought to be ultrahigh electric fields at the air–water interface; smaller droplets present larger interfacial areas and produce higher (detectable) H2O2 yields. To gain insights into this phenomenon, we performed condensation experiments and quantified H2O2 formation as a function of the vapor source. Specifically, we compared the H2O2 concentration in water microdroplets condensed from the vapor realized via (i) heating water in the range of 50–70 °C and (ii) ultrasonic humidification (as exploited in the original report). Experimental results revealed that the H2O2 level inside water microdroplets condensed via heating water was below our detection limit (≥0.25 μM), regardless of the droplet size or the substrate wettability. In contrast, water droplets condensed via ultrasonic humidification contained significantly higher (∼1 μM) H2O2 concentrations. We conclude that the ultrasonic humidifiers contribute to H2O2 production, not droplet interfacial effects.