Recovery of proton-conducting functionality of thermally degraded Nafion membranes via sulfuric acid treatment

Nafion membrane activation, or pretreatment, involves immersion in a hot sulfuric acid solution to improve proton conductivity by replacing non-proton X + ions in –SO 3 X groups with H + ions. This study demonstrates that nonacidic –CF 2 H terminal groups, formed during the thermal degradation of Nafion membranes, can also be converted to acidic –CF 2 SO 3 H groups through sulfuric acid treatment. The –CF 2 H signals were quantitatively measured using 1 H magic angle spinning nuclear magnetic resonance spectroscopy to estimate the degree of –SO 3 H group detachment and the extent of its recovery. The disappearance of these signals directly correlated with the recovery of the proton conductivity, the spin–lattice relaxation time of 1 H signals, X-ray photoelectron spectroscopy, ion exchange capacity, and water uptake data. These findings indicate that –SO 3 H groups can be regenerated by substituting the nonacidic hydrogen in terminal groups such as –CF 2 H with –SO 3 H groups, and that simple immersion in an H 2 SO 4 solution at 80 °C is an effective method for restoring thermally degraded perfluorosulfonic acid membranes. Single–cell polymer electrolyte membrane fuel cell measurements showed that the electrochemical performance of thermally degraded membranes was largely restored after sulfuric acid treatment, and further indicated that this treatment is effective even for chemically degraded membranes. Thermally degraded Nafion membranes were restored through immersing in a hot H 2 SO 4 solution, which replaced the nonacidic H of –CF 2 H with acidic –SO 3 H groups. The recovery of acid-treated PEM was confirmed with PEMFC single-cell performance. • H 2 SO 4 treatment reproduced –SO 3 H by replacing the nonacidic H of –CF 2 H with –SO 3 H. • 1 H NMR signals for –CF 2 H were used to estimate degradation and restoration. • Nafion membranes were recovered by immersing in 1 M H 2 SO 4 solutions at 80 °C. • The recovery of acid-treated PEMs was confirmed with PEM PEMFC performance. • Our results offer a new direction for PFSA PEM regeneration.