Morphological Factors Affecting the Behavior of Water in Proton Exchange Membrane Materials

Abstract

Perfluorosulfonic acid ionomers (PFSAs), such as Nafion®, have become the benchmark membrane material for proton exchange membrane fuel cells (PEMFCs). Despite their commercial success, little is known about the complex morphology-property relationships governing the chemical and physical properties. For example, the detailed structure of PFSA crystallites within the amorphous phase and the spatial arrangement of the crystallites around the ionic aggregates of PFSA materials are virtually unknown. In addition, the effect of processing on the membrane performance and durability in PEMFC applications is not yet fully understood. The first part of this work focuses on the significance of the crystalline component with respect to polymer-water interactions. The second part of this work addresses the impact on changes in morphology that could occur during long-term operation in a fuel cell including the effects of heat and humidity.

New Insights into Perfluorinated Sulfonic-Acid Ionomers

“…In this regime (λ > 6), hydration enthalpy plateaus to −40 to −45 kJ/mol, ,,,− and approaches the heat of condensation of water (40 kJ/mol), which exhibits liquid-like properties, although confinement effects within nanodomains are still present (see section ). Thus, the total water content in the ionomer at a given water activity, a w , and T , can be expressed as where the residual water is a strongly bound water molecule participating in the primary hydration shells that can be removed only at elevated temperatures, i.e., λ 0 = 1 to 2 at 120 °C, ,,,, with a small trace of water possibly surviving up to 200 °C, − at the onset of SO 3 decomposition (section ). Existence of a strongly bound water molecule is also consistent with a single immobile water determined in dynamic QENS studies. , (also see section ).…”

“…It has been well documented that the morphology and properties of PFSA membranes (whether extruded or cast) are strongly influenced by their thermal history, which can be altered through heat-treatment processes such as pretreating (PT) or preboiling (PB), a.k.a. expanded form, in liquid water or heating at higher temperatures to induce annealing (i.e., higher crystallinity). ,− PFSA membranes stored and used as-received (AsR, from the manufacturer) are prone to containing contaminates and may exhibit continuous changes in structure and properties over time (i.e., physical aging phenomenon). Thus, it has been a common practice to pretreat the membrane to set a baseline by erasing its thermal history via boiling in peroxide agents (cleaning), sulfuric acid (reprotonating), and water (boiling-induced rehydration), with the exact duration and concentrations changing from one study to another, although with similar end states.…”

“…Thus, it has been a common practice to pretreat the membrane to set a baseline by erasing its thermal history via boiling in peroxide agents (cleaning), sulfuric acid (reprotonating), and water (boiling-induced rehydration), with the exact duration and concentrations changing from one study to another, although with similar end states. Such heat-treatment processes have been shown to alter the water uptake, ,− ,,− and transport coefficients (e.g., diffusivity), ,, density, permeability, ,, ionic conductivity, ,,,,,,− ,− selectivity, ,, mechanical properties, − ,− ,, thermal stability, phase-separated nanostructure and/or crystallinity, ,− ,− ,,,,− surface topology and energy. − , Such changes make it hard to compare literature studies if the pretreatment conditions have not been noted. For example, surface ionic activity of extruded Nafion membranes is lower in AsR or annealed form, however, it increases upon treatment in acid solutions. , In addition, in situ SAXS experiments showed that preboiling a PFSA membrane increases and (pre)­sets its water-domain spacing at a maximum value compared to an AsR membrane, consistent with its water uptake behavior. ,,, Overall, there is a consensus that the appropriate pretreatm...…”

Morphology Changes in Perfluorosulfonated Ionomer from Thickness and Thermal Treatment Conditions

“…Crystalline structures can be caused by thermal annealing, which involves heating a membrane above its T α , thereby giving the polymer chains enough mobility to reorient and pack themselves into a semicrystalline structure . Increased crystallinity of Nafion membranes has been studied in relation to its impact on smaller d -spacing, − water uptake reduction, ,− lower gas permeability, and proton conductivity. ,, The morphology of annealed Nafion thin films with a thickness of 55 nm on Au substrate was investigated via GISAXS by Kusoglu et al The hydrophilic ionic domain was constrained compared to the pristine Nafion after a 146 °C annealing treatment above T α . Several studies have been conducted to understand the relationship between the morphological information and physical properties of a free-standing membrane.…”

Comprehensive Picture of Water Dynamics in Nafion Membranes at Different Levels of Hydration

“…Therefore, λ water = 0 applies to the fully dried sample according to our definition and the term apparent hydration level is used in the following to stress this fact. However, we note that several groups have reported that the complete drying of Nafion membranes is not achievable, resulting in a minimum hydration level of λ water ≈ 1. ,− …”