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 SO32– and SO42– 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.