Hydroxide-Ion-Promoted Complete Mineralization of Perfluorinated Ion-Exchange Membranes in Subcritical Water

Perfluorinated ion-exchange membranesPFS and PFC membranesbearing sulfonic acid and carboxylic acid end groups, respectivelyare widely used in polymer electrolyte membrane fuel cells, water-splitting electrolyzers for solar hydrogen production, and brine electrolysis. In this work, their decomposition in subcritical water with alkaline reagents was investigated for fluorine recycling. Only negligible amounts of F^- were generated in pure subcritical water, whereas the addition of KOH dramatically accelerated mineralization. The PFS membrane afforded a 96% F^- yield after reaction in a 1.0 M KOH solution at 300 °C for 6 h. The sulfur content was also nearly completely mineralized, with the combined yields of SO₃ ^2- and SO₄ ^2- reaching 97%. Although the PFC membrane was more stable, virtually complete mineralization was achieved with a 99% F^- yield after it was reacted in a 1.0 M KOH solution at 350 °C for 18 h. A precursor membrane bearing SO₂F end groups was likewise completely mineralized, giving a 102% F^- yield after reaction in a 1.0 M KOH solution at 300 °C for 6 h. Furthermore, subsequent addition of Ca-(OH)₂ to the reaction solutions followed by purification produced pure CaF₂.