Thermal degradation kinetics of ionic liquid [BMIM]BF4/TEA/PFSA composite membranes for fuel cell

Abstract

20% ILs-TEA-PFSA composite membranes were prepared using the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM]BF4 (ILs) and were doped with triethylamine (TEA) to modify the perfluorosulfonic acid (PFSA). The thermal decomposition kinetics of the composite membrane was investigated using nonisothermal thermogravimetry in order to study the high-temperature durability of the ionic-liquid-doped industrial-grade perfluorosulfonic acid ion membrane for fuel cells. The results showed that the thermal degradation of the composite membranes occurs over three stages, for which the conversion rates are in the ranges of 0.01–0.1, 0.15–0.4 and 0.45–0.7. The average apparent activation energies of membrane degradation in the first, second and third stages are 151.3, 166.5 and 170.1 kJ mol−1, respectively. At a heating rate (β) of 20 °C min−1, the thermal degradation process of the composite mechanism follows an n = 4 reaction order mechanism, and the mechanism function f(α) is 1 − (1 − α)4 and g(α) is 1/4 (1 − α)−3. The heat resistance showed that if β is equal to 15 °C min−1, the lowest temperature at which the composite membrane decomposes by 1% is 363.5 °C. Similarly, when α is 0.32%, the thermal decomposition of the composite membrane occurs above 350 °C. Isothermal thermogravimetric analysis showed that the thermal lifetimes (t5% and t10%) of the composite membrane were 4.83 × 105 and 9.80 × 105 h, respectively. If α reached 5 and 10% under a nitrogen atmosphere at 180 °C, the isothermal decomposition process underwent a first-order reaction mechanism. The apparent activation energy (Ea) of the thermal degradation of the composite membrane was 166.8 kJ mol−1. In addition, using isothermal data from a 20% [BMIM] BF4/TEA/PFSA composite membrane at 688 K, based on a first-order reaction, the respective theoretical master curves were compared with the experimental master plots of dα/dθ/(dα/dθ)α=0.5, θ/θ0.5 and (dα/dθ)θ versus α.