We tested six structurally relevant PFESAs (Figure ). NBP-COOH is the hydrolysis product of the Nafion copolymer precursor. NBP-H (commonly named "Nafion by-product 2") is the decarboxylation product of NBP-COOH. TP1 is a shorter analog of NBP-COOH. TP2 can be considered as the further shortened "analog" of TP1. NBP-F is a fully fluorinated structure without either -COO -group or C-H bond on the terminal. 2+2 PFESA can be considered a shorter and linear analog of NBP-F. Instead of meticulously measuring second-order rate constants, we evaluated the reactivity using F -release as an established rapid probe of oxidative degradation. 17