Effect of Hygrothermal Ageing on PFSA Ionomers' Structure/Property Relationship

Abstract

Perfluorosulfonic-acid (PFSA) membranes are frequently subjected to high humidity and temperature cycles during fuel-cell operation. It is of great interest to understand how the properties of the membrane change with ageing conditions and time. In this study, we investigate how the properties of as-received and pretreated Nafion membranes change after exposure to hygrothermal ageing, including the chemical structure, mechanical properties, water uptake, ionic conductivity, and morphology. Our findings demonstrate that anhydrides form during ageing via a condensation reaction, which results in chemical crosslinks that impair the membrane functionalities by reducing water uptake and conductivity and increasing the storage modulus and α relaxation temperature. In addition, a membrane aged in a 75% relativehumidity environment exhibits more dramatic changes compared to that aged in 100% conditions. It is also shown that the impact of ageing can be recovered through a post-treatment by boiling the membrane in strong acid.

New Insights into Perfluorinated Sulfonic-Acid Ionomers

“… However, unlike thermal annealing, aging does not change crystallinity. , The SAXS data are consistent with this in that after aging the ionomer peak shifts significantly to the right and broadens, implying a wider distribution of smaller and maybe even isolated water domains. Also, the d w –λ trend (discussed in section ) shifts downward upon aging ( d w decreases more than λ) indicating a break in the hydration vs d -spacing correlation (Figure ), where the aging-induced cross-links prevent expansion and perhaps connectivity of the water domains. ,,,, This is consistent with the more severe decrease in ionic conductivity than can be solely explained by changes in EW and water uptake as observed by Collette et al and Clapham et al, who reported a conductivity loss of 80% versus a 49% increase in EW.…”

“…Since the early SAXS works by Roche et al, , Gierke and co-workers, , and Fujimura et al , reported a humidity-dependent ionomer peak associated with a phase-separated nanostructure, a large number of studies have employed SAXS/SANS to delineate PFSA morphological features. Such investigations include the effect of hydration, ,,,,,,,,,,,,,− ,,,,, temperature and thermal history, ,,,,,,,,,, freeze, , aging, ,, mechanical loads, stretching and orientation, ,,,,,,,,,,, time, − ,, cations, ,,,, solvents, ,,, and PFSAs in dispersion state. ,− Early studies employed thermodynamics-based models to predict the hydrophilic domain formation in the hydrophobic matrix based on an energy balance between the electrostatic interactions and deformation of the surrounding matrix as mentioned in section . ,,,…”

“…Possible scenarios affecting the phase-separated morphology of PFSA resulting in inaccessible ionic sites, such as ionic cross-linking, contamination or the process of reversible anhydride formation. Impact of hygrothermal aging on FTIR spectra, which shows the formation of the 1440 cm –1 peak, and liquid-water conductivity of Nafion 212 membrane (from Shi et al , ). For comparison, decrease in conductivity of Nafion in the presence of ammonium is shown (from Coms et al).…”

“…As discussed throughout this review, PFSA ionomers are not in true equilibrium and their morphology tends to change over time and under certain environmental conditions; such changes over longer time scales is termed aging. ,,,,, During such aging, a PFSA membrane was observed to exhibit a decay in conductivity, ,,,, gas permeability, water permeance and water uptake, as well as an increase in tensile modulus, ,,,,, and T α, , with implications on membrane performance in PEFCs , and humidifiers . Many of these changes are attributed to the formation of functional species (e.g., anhydrides) that form cross-links between the ionic groups, which are catalyzed by contaminants.…”

“… ,, The recovery process is thought to be promoted by protonation of unaccesible SO 3 – groups in acid (whether hydrolysis of S–O–S or removal of contaminants), thereby breaking cross-links, which is corroborated by the disappearance of their fingerprint in FT-IR peaks and recovery of their baseline EW. , Moreover, the partial recovery in water uptake and conductivity upon heating in water was also observed for d -spacing, indicating, once again, a universal correlation between nanostructure and transport properties. In addition, it was recently reported that aging-induced changes in Nafion EW, structure, and properties are more pronounced if the as-received membrane is hot-pressed, and aging rate is slower for Nafion 212, compared to Nafion 112 that has higher concentration of metal ion contaminants …”