Freezing-Thawing-Induced Hydrogen Peroxide Formation in PFA Containers

Hydrogen peroxide (H2O2) and related reactive oxygen species are increasingly reported to form under dark conditions at aqueous interfaces, yet the underlying mechanisms remain debated. Here, we demonstrate that the freezing and thawing of water in fluorinated (PFA) containers generate H2O2 without external electrical bias or mechanical agitation. H2O2 formation increases approximately linearly with the number of freezing/thawing cycles and depends strongly on polymer identity, salt concentration, dissolved oxygen, and freezing/thawing temperatures. Perfluoroalkoxy polymer (PFA) tubes produce substantially more H2O2 than polypropylene (PP) tubes, and the NaCl dependence is qualitatively consistent with previously reported trends associated with contact electrification of aqueous NaCl on fluorinated polymers. Experiments in an ozone-free glovebox demonstrate that dissolved oxygen is required, while rapid freezing and higher thawing temperatures enhance the level of H2O2 formation. Overall, these findings suggest that freezing/thawing in PFA-based systems may represent a previously underappreciated pathway for H2O2 formation with potential implications for interpreting low-concentration ROS measurements and understanding interfacial redox processes in freezing environments.