Perfluorosulfonic-acid (PFSA) ionomers are widely used as solid electrolytes and ion-exchange membranes in electrochemical devices, wherein their properties are impacted by the interactions among the anionic sulfonate groups, mobile counter-ions (cations), and hydration levels. Cation-form and humidity collectively affect the structure/transport-property relationship, yet their interplay is still not well known. In this paper, we report changes in water uptake and conductivity of cation-exchanged PFSA in both vapor and liquid water, which are then correlated with changes in mechanical properties and nanostructure (hydrophilic-domain spacing and phase-separation). It is found that the magnitude of changes brought upon depends significantly on the membrane water content, with a master curve in terms of water volume fraction realized. Moreover, nanostructure and dynamical-mechanical behavior of the membrane is examined to establish structure/transport and transport/stability relationships. It is found that with increasing cation size (radius) and valence, the storage modulus increases, while the water uptake and conductivity decreases. In addition, regardless of the cation type, a universal relationship is found between the conductivity and modulus, indicative of a transport/stability tradeoff.

Perfluorosulfonic-acid (PFSA) ionomer membranes (most commonly Nafion ® ) are currently the prototypical proton-exchange-membrane in polymer-electrolyte fuel cells (PEFCs), for which durability still represents a technical barrier to their commercialization. In an effort to address the durability demands, PFSA membranes with reinforcement and/or stabilizers have become of great interest as they have demonstrated superior durability in PEFCs compared to their unreinforced analogues. One such particular membrane that is tailored for enhanced durability and commonly employed in PEFCs is Nafion XL, a Nafion-based ionomer membrane with mechanical reinforcement and chemical stabilizers. Despite an increasing number of recent studies demonstrating its improved lifetime in accelerated stress testing (AST), its structure and transport properties have not been investigated in a systematic fashion. In this paper, we report water uptake, dimensional change, conductivity, and mechanical properties of Nafion XL membrane, as well as its strong anisotropy, in comparison to (unreinforced) Nafion 212 membrane.Moreover, water-domain spacing and crystallinity of Nafion XL membrane, determined from small-and wide-angle X-ray scattering (SAXS/WAXS) experiments, are correlated with the measured properties to establish a structure/property relationship, and discussed within the context of composite materials. It is also found that (pre)conditioning of the membrane by heating in water at different temperatures could have significant impacts on its structure/property relationship, in particular, the mechanical stability and conductivity, which were related to morphological changes observed from microscopy 2 studies. The findings reported here not only provide a new dataset that can be used for PEFC performance and durability modeling but also benefit the efforts on developing composite ionconductive membranes. 1.Highlights  The composite Nafion XL membrane's structure/property relationship is investigated  Impact of pretreatment on membrane's transport and mechanical properties are shown  Membrane's composite morphology are revealed through TEM, SEM and SAXS/WAXS

Perfluorosulfonic‐acid (PFSA) membranes are widely used as the solid electrolyte in electrochemical devices where their main functionalities are ion (proton) conduction and gas separation in a thermomechanically stable matrix. Due to prolonged operational requirements in these devices, PFSA membranes’ properties change with time due to hygrothermal aging. This paper studies the evolution of PFSA structure/property relationship changes during hygrothermal aging, including chemical changes leading to changes in ion‐exchange capacity (IEC), nanostructure, water‐uptake behavior, conductivity, and mechanical properties. Our findings demonstrate that with hygrothermal aging, the storage modulus increases, while IEC and water content decrease, consistent with the changes in nanostructure, that is, water‐ and crystalline‐domain spacings inferred from small‐ and wide‐angle X‐ray scattering (SAXS/WAXS) experiments. In addition, the impact of aging is found to depend on the membrane's thermal prehistory and post‐treatments, although universal correlations exist between nanostructural changes and water uptake. The findings have impact on understanding lifetime, durability, and use of these and related polymers in various technologies. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 570–581