Role of Mechanical Factors in Controlling the Structure–Function Relationship of PFSA Ionomers

Ion-conducting polymers are ideal solid electrolytes for most energy storage and conversion devices where ion transport is a critical functionality. The system performance and stability are related to the transport and mechanical properties of the ionomers, which are correlated through physiochemical interactions and morphology. Thus, there exists a balance between the chemical and mechanical energies which controls the structure–function relationship of the ionomer. In this paper, it is reported how and why thermal treatments result in different water uptakes and nanostructures for a perfluorinated sulfonic acid (PFSA) membrane. The nanostructure of the PFSA membrane is characterized using small- and wide-angle X-ray scattering experiments. These changes are correlated with water content and mechanical properties and result in fundamental relationships to characterize the membrane with different thermal histories. Moreover, quasi-equilibrium water uptake and domain spacing both decrease with predrying or preconstraining the membrane, thereby suggesting that similar mechanical energies govern the structural changes via internal and external constraints, respectively. The findings suggest that heat treatments alter the balance between the chemical–mechanical energies where the interplay of the morphology and mechanical properties controls the structure–function relationship of the membrane. Finally, a model is developed using an energy-balance approach with inputs of the mechanical and structural properties; the dependence of water uptake on pretreatment is successfully predicted

Subsecond Morphological Changes in Nafion during Water Uptake Detected by Small-Angle X-ray Scattering

The ability of the Nafion membrane to absorb water rapidly and create a network of hydrated interconnected water domains provides this material with an unmatched ability to conduct ions through a chemically and mechanically robust membrane. The morphology and composition of these hydrated membranes significantly affects their transport properties and performance. This work demonstrates that differences in interfacial interactions between the membranes exposed to vapor or liquid water can cause significant changes in kinetics of water uptake. In situ small-angle X-ray scattering (SAXS) experiments captured the rapid swelling of the membrane in liquid water with a nanostructure rearrangement on the order of seconds. For membranes in contact with water vapor, morphological changes are four orders-of-magnitude slower than in liquid water, suggesting that interfacial resistance limits the penetration of water into the membrane. Also, upon water absorption from liquid water, a structural rearrangement from a distribution of spherical and cylindrical domains to exclusively cylindrical-like domains is suggested. These differences in water-uptake kinetics and morphology provide a new perspective into Schroeder's paradox, which dictates a different water content for vapor- and liquid-equilibrated ionomers at unit activity. The findings of this work provide critical insights into the fast kinetics of water absorption of the Nafion membrane, which can aid in the design of energy conversion devices that operate under frequent changes in environmental conditions

Controlling Nafion Structure and Properties via Wetting Interactions

Proton conducting ionomers are widely used for electrochemical applications including fuel-cell devices, flow batteries, and solar-fuels generators. For most applications the presence of interfacial interactions can affect the structure and properties of ionomers. Nafion is the most widely used ionomer for electrochemical applications due to their remarkable proton conductivity and stability. While Nafion membranes have been widely studied, the behavior and morphology of this ionomer under operating conditions when confined to a thin-film morphology are still not well understood. Using <i>in situ</i> grazing-incidence small-angle X-ray scattering (GISAXS) techniques, this work demonstrates that the wetting interaction in thin-film interfaces can drastically affect the internal morphology of ionomers and in turn modify its transport properties. Thin films cast on hydrophobic substrates result in parallel orientation of ionomer channels that retard the absorption of water from humidified environments; while films prepared on SiO₂ result in isotropic orientation of these domains, thus favoring water sorption and swelling of the polymer. Furthermore, the results presented in this paper demonstrate that upon thermal annealing of Nafion thin films static crystalline domains form within the polymer matrix that restrict further water uptake. The results presented in this study can aid in the rational design of functional composite materials used in fuel-cell catalyst layers and solar-fuels devices

Understanding Water Uptake and Transport in Nafion Using X‑ray Microtomography

To develop new ionomers and optimize existing ones, there is a need to understand their structure/function relationships experimentally. In this letter, synchrotron X-ray microtomography is used to examine water distributions within Nafion, the most commonly used ionomer. Simultaneous high spatial (∼1 μm) and temporal (∼10 min) resolutions, previously unattained by other techniques, clearly show the nonlinear water profile across the membrane thickness, with a continuous transition from dynamic to steady-state transport coefficients with the requisite water-content dependence. The data also demonstrate the importance of the interfacial condition in controlling the water profile and help to answer some long-standing debates in the literature

Confinement-Driven Increase in Ionomer Thin-Film Modulus

Ion-conductive polymers, or ionomers, are critical materials for a wide range of electrochemical technologies. For optimizing the complex heterogeneous structures in which they occur, there is a need to elucidate the governing structure–property relationships, especially at nanoscale dimensions where interfacial interactions dominate the overall materials response due to confinement effects. It is widely acknowledged that polymer physical behavior can be drastically altered from the bulk when under confinement and the literature is replete with examples thereof. However, there is a deficit in the understanding of ionomers when confined to the nanoscale, although it is apparent from literature that confinement can influence ionomer properties. Herein we show that as one particular ionomer, Nafion, is confined to thin films, there is a drastic increase in the modulus over the bulk value, and we demonstrate that this stiffening can explain previously observed deviations in materials properties such as water transport and uptake upon confinement. Moreover, we provide insight into the underlying confinement-induced stiffening through the application of a simple theoretical framework based on self-consistent micromechanics. This framework can be applied to other polymer systems and assumes that as the polymer is confined the mechanical response becomes dominated by the modulus of individual polymer chains

In Situ Method for Measuring the Mechanical Properties of Nafion Thin Films during Hydration Cycles

Perfluorinated ionomers, in particular Nafion, are an essential component in hydrogen fuel cells, as both the proton exchange membrane and the binder within the catalyst layer. During normal operation of a hydrogen fuel cell, the ionomer will progressively swell and deswell in response to the changes in hydration, resulting in mechanical fatigue and ultimately failure over time. In this study, we have developed and implemented a cantilever bending technique in order to investigate the swelling-induced stresses in biaxially constrained Nafion thin films. When the deflection of a cantilever beam coated with a polymer film is monitored as it is exposed to varying humidity environments, the swelling induced stress-thickness product of the polymer film is measured. By combining the stress-thickness results with a measurement of the swelling strain as a function of humidity, as measured by quartz crystal microbalance (QCM) and X-ray reflectivity (XR), the swelling stress can be determined. An estimate of the Young’s modulus of thin Nafion films as a function of relative humidity is obtained. The Young’s modulus values indicate orientation of the ionic domains within the polymer films, which were confirmed by grazing incidence small-angle X-ray scattering (GISAXS). This study represents a measurement platform that can be expanded to incorporate novel ionomer systems and fuel cell components to mimic the stress state of a working hydrogen fuel cell