Aromatic Ionomers – A Promising Alternative to Nafion

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PEM Functions and Its Requirements

The PEM is referred to as the heart of a PEMFC. Its main role is to assure the migration of hydrated protons generated at anode to the cathode (where they recombine with reduced oxygen to form water) and prevent electron as well as fuel crossover. Due to the highly aggressive conditions inside fuel cells (i.e., high temperature, high pressure, the presence of oxygen and free radicals, etc.) successful PEMs must possess good thermomechanical properties in both dry and hydrated states, excellent chemical and electrochemical stabilities, high proton conductivity, but also low cost, etc.12–14

Commercializing Barriers of PEMFCs

Although PEMFCs have been commercially available for many years, technological advances have left three major challenges: cost, performance and lifetime, which are interrelated. Over the past decades, substantial effort has led to significant reduction of PEMFC cost. But, the cost reduction is mainly contributed by auxiliary facilities and catalyst, while the PEM cost is nearly unchanged due to widely employing PFSA membranes including Nafion .15 The lifetime of PEMFC could be improved if the membrane thickness is increased, but this also increases the cell cost by adding materials and lowering specific performance.16

Nafion® based PEMs

Introduction

Introduced by E. I. du Pont de Nemours and Company in the late 1960s, Nafion® is the representative of PFSA membranes comprising a PTFE backbone bearing different side chain architectures such as NeoseptaF™ (Tokuyama), Gore-Select™ (W.L. Gore and Associates, Inc.), Flemion™ (Asahi Glass Company), Asiplex™ (Asahi Chemical Industry), and Aquivion®, Hyflon® (Solvay Plastics), etc.1,2,16–19 The application of Nafion® in PEMFCs in the early 1970s used to be considered as a major breakthrough of PEM technology.1,20
The high FC performances and the superior chemical/electrochemical stabilities of Nafion originate from (i) its chemical structure composed of an extremely hydrophobic flexible PTFE backbone coupled with a pendant perfluorosulfonic acid side chain (Figure 2.1) resulting in a neat nanophase separation between hydrophobic PTFE backbone and hydrophilic ionic domains, and (ii) the super-acidity of perfluorosulfonic acid groups as compared to other ionomers bearing aryl sulfonic acid functions.16,17

Morphology of Nafion

A wealth of morphological models of Nafion has been reported in an attempt to precisely describe its morphology, the key contribution to the high performances.17,21–24 Most of them were proposed based on small-angle scattering (SAS) profiles of Nafion in different physical states (dry and hydrated membranes, or solution). Both neutron (SANS) and X-rays (SAXS) scattering spectra of Nafion in hydrated state generally exhibit a number of hydration-dependent characteristic features related to a complex multi-scale organization directly impacted by water content. There are typically two correlation peaks, the so-called ionomer peak in the wave vector (Q) range of 0.1–0.2 Å–1 and the so-called “matrix knee” observed at lower Q values (≈ 0.05−0.06 Å–1). The presence of these peaks indicates a regular organization at two different length scales. Their position, shape and intensity are extremely sensitive to the hydration level, with systematic variations characteristic of the microscopic swelling behavior. While the ionomer peak is the fingerprint of all PFSA membranes and was attributed to the organization of ionic domains, the origin of the “matrix knee” is not completely elucidated. This feature was reported with a variety of shapes, positions and intensities depending on chemical architecture and membrane processing conditions.17 In addition, another structural feature is a certain degree of crystallinity arising from ordering of PTFE backbone. It was shown that the crystallinity depends on the type of polymer and/or membrane processing condition.25–27 Its influence on the quality of hydrophilic/hydrophobic separation and large-scale organization was observed but has not been elucidated. In particular, correlations between the degree of crystallinity and the matrix knee features were reported,28–30 although they could not be rationalized in terms of long-range correlations of crystallites nor peculiarities of semi-crystalline versus amorphous regions.
To date, different structural models were proposed to account for the shape of the SAS spectra, particularly the origin of two scattering maxima and their variations along hydration level. In the following, a brief overview of the main models is presented.
In 1981, Gierke et al.31–33 proposed the cluster-network model in which Nafion has inverted-micelle structure with spherical ionic clusters around 3−5 nm in diameter, and the clusters are interconnected by short channels about 1 nm in diameter (Figure 2.2a). At the same time, another model based on spherical ionic clusters, e.g., modified core-shell model, was proposed by Fujimura et al.28,34 By analyzing SAXS and WAXD (wide-angle X-ray diffraction) of Nafion with different equivalent weight and counter cations, the authors concluded that the scattering behavior of Nafion has been best described by an intra-particle core-shell model (Figure 2.2c) similar to that proposed by MacKnight35 and co-workers36, rather than an inter-particle model (Figure 2.2b) as proposed by Cooper et al.37 In the former, the ion-rich core is surrounded by the ion-poor shell composed mostly of perfluorocarbon chains, the core-shell particles being dispersed in a matrix of fluorocarbon chains and non-clustered ions.
Figure 2.2. (a) Cluster-network, (b) inter-particle, and (c) intra-particle core-shell model for morphology of hydrated Nafion. Schemes adapted with permission from ref.17 Copyright 2004 American Chemical Society.
The cluster-network model was widely accepted and considered as the most prevalent model in the literature until the late 90s as its accordance with available scattering results. However, new insights into morphological and dimensional evolution during swelling were gained later, due to progress in scattering techniques, capability to measure SAS spectra over extended Q-range. Additional investigations, and increasing information by numerical simulations23,24,38 have gradually raised doubt about the spherical shape and spatial distribution of ionic clusters inside the semicrystalline matrix of Nafion.17
In 1997, Litt39 proposed the so-called simple lamellar model, where ionic clusters of lamellar shape are introduced. In this model, the ionic domains are locally planar and parallel to each other. As water is absorbed, the ionic domains swell and spread apart the nonpolar domains, leading to a parallel shift of the scattering maxima associated to lamellar ordering on non-polar domains correlations. The expansion is supposed to be impeded by the macromolecule tie that connect the conducting channels. This model has provided a more rational explanation for the reversible swelling behavior of Nafion, and the linear dependence of the ionomer peak with hydration degree (in contrast to the 1/3-power dependence for isotropic swelling of spherical structures). However, further work on swelling behavior reported by Gebel40 and Young41 revealed a dissimilar shift of the two maxima, incompatible with the lamellae-based structure proposed by Litt.39 Therefore, a more sophisticated morphology was postulated. By studying the microstructure and the swelling behavior of Nafion membranes immersed in water–methanol under in-situ conditions, Haubold et al.42 proposed a modified lamellar model, the sandwich-like model, which composes of a core region embedded by a shell, the former being either empty or filled by water-methanol (Figure 2.3). Although this model interestingly depicts details of a possible local structure, it does not provide an overall 3D pattern of hydrophilic/hydrophobic organization.
Figure 2.3. The sandwich-like model. Scheme adapted with permission from ref.42 Copyright 2001 Elsevier.
More comprehensive models were developed after the years 2000, with the objective to unify the body of information accumulated on local, nanoscopic and mesoscopic structure. The focus was put on rationalizing under-looked details of SAS spectra, as asymptotic behaviors, form factors and hydration-dependent variations of peak positions, as well as elucidating the structure of ionomer solutions. The structure evolution of PFSA ionomers during swelling40 as well as solution-cast process43–45 was systematically studied by the group of Gebel using SAS technique.
In swollen membranes, a significant change in swelling behavior was found at polymer volume fraction (Φp) around 0.5.46 This was attributed to a continuous transformation from water-in-polymer state to polymer-in-water state. Moreover, the high-Q asymptotic behavior in SAS spectra was scrutinized. A Porod’s law (a Porod’s slope of Q-4 revealing a sharp interface between two scattering objects),47 was evidenced for all samples. This behavior typically indicates the presence of a sharp interface between two phases, e.g., polymer and ionic phases. The corresponding specific surface (σ) could be extracted using Guinier approximation48,49 and was found to be 55 Å2.
Evidence of rod-like elongated polymeric aggregates (Porod’s slope of Q-1)47,49 was reported in diluted ionomer solutions.50 The local structure on the scale of a few nanometers is identical to that of swollen membrane (at equivalent Φp) implying the existence of a network of rod-like polymer particles in the latter.
Based on these key observations, Gebel proposed the so-called elongated-aggregate model.46 In this model (see Figure 2.4), dry membranes contain separately spherical ionic clusters of ≈ 1.5 nm in diameter with center-to-center distance of ≈ 2.7 nm. During hydration, the clusters swell to form bulks of water surrounded by ionic groups at the interface to minimize interfacial energy. As water content (Φw) increases (between 0.3 and 0.5), structural reorganization occurs to retain specific surface area, forming water cylinders connecting the spherical clusters. At Φp < 0.5, a structure inversion occurs to form a connected network of rods. In over-swollen state, these rod-like structures are separated to yield a colloidal dispersion of isolated rods.
Despite proposing a more rational mechanism for the structure evolution from the isolated clusters to rod-like structures in solution, this model lacks a thermodynamic justification for the phase inversion process.17 Therefore, it was further questioned, but it has already contained the primary ingredient of the nowadays widely accepted structural models, i.e., Nafion constituted of elongated polymer particles, and these reorganizing along hydration and dehydration sequences. The group of Gebel51 later proposed the ribbon-like model, which originates from these ideas and was sustained by new sets of SAS data taken on i) extended range of Q-vectors, including the ultra-small angle region, and ii) using advanced contrast variation techniques available by SANS to evaluate the condensation of ions at the water-polymer interface. On analyzing these data, they proposed that Nafion is composed of flat elongated particles embedded in a continuous ionic medium, and organized in large scale bundles.
Figure 2.4. Schematic representation of the structural evolution as a function of water content. Schemes reprinted and adapted with permission from ref.46 Copyright 2000 Elsevier.
In 2008, Schmidt-Rohr and Chen52 proposed a parallel cylinder model (Figure 2.5) by simulating the SAS data of hydrated Nafion published by the Gebel’s group (Rubatat et al.50). This model was supported by a NMR study.53 The key feature of this microstructure is inverted micelle cylinders with large diameters even at low water content (e.g., 2.4 nm for a Φw ≈ 0.2 corresponding to about 80% RH).
Figure 2.5. Schematic diagram of parallel cylinder model. (a) An inverted-micelle cylinder with the polymer backbones outside and the ionic side groups lining the water channel. (b) An approximately hexagonal packing of inverted-micelle cylinders. (c) Cross-sections through the cylindrical water channels (white) and the Nafion crystallites (black) in the non-crystalline Nafion matrix (dark grey). Schemes reprinted and adapted with permission from ref.52 Copyright 2008 Nature Publishing Group.
However, Kreuer et Portale54 raised doubts about this model since a constant number of cylinders requires significant structural reorganization to adjust changes of the water content, in contrast to extremely fast equilibration when water absorbs into the membrane.55 Furthermore, the accumulation of equal charges is energetically unfavorable when they are not completely covered by water molecules. Therefore, they have sustained the locally flat and narrow structures similar to lamellar model previously proposed by Litt.39 or polymer ribbon morphology, as suggested by Rubatat et al.51 as it allows protonic charge carriers to electrostatically interact with several sulfonic groups.
In conclusion, although details of the Nafion morphology are still debated and not fully elucidated, there is a consensus on the main features of Nafion’s morphology.
Due to their amphiphilic character, hydrated PFSA membranes exhibit highly separated hydrophobic and ionic domains. The neat separation between polar and non-polar regions yields the formation of a well-defined scattering maximum, the ionomer peak. The interfacial region is sharp, leading to a typical Porod’s behavior in the high-Q region of the scattering spectra.
Increasing the water content results in more extended and better connected ionic domains. The size and shape of ionic domains is rather irregular, with indications of a preferred locally flat topology (in particular at low hydration).
The domain expansion is accompanied by modifications of the topology of the interface between the ionic domains and the polymer matrix. Recently, the morphology and swelling of PFSAs have been shown to resemble that of ionic surfactants,56–58 highlighting the predominant effect of side-chains and acidic functions in controlling the size, shape and organization of ionic domains.

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Alternative Ionomers to PFSA

Despite excellent proton conductivity, and unsurpassed longevity in a fuel cell environment, Nafion® and other PFSA ionomer membranes suffer from certain drawbacks such as high cost, production process including strongly toxic and environment-unfriendly intermediates,13 low conductivity at low relative humidity, drop of conductivity at temperatures above 80−90 0C, and low mechanical properties at high temperatures.12,59 Other shortcomings of the perfluorinated ionomers are related to their high methanol permeability allowing methanol crossover from the anode to the cathode in DMFCs,13 high osmotic drag, which makes water management at high current densities difficult. As a consequence, considerable effort has been dedicated to developing alternative PEM systems for PEMFCs.11,12,15,20,60,61

Aromatic Ionomers – A Promising Alternative to Nafion

Among numerous alternative to PFSA, investigated over the past decades, aromatic ionomers are more promising for the next-generation PEM materials due to their availability, processability, wide variety of chemical compositions, and stability in the fuel cell environment. On the view of chemical structure, aromatic ionomers compose of an aromatic polymer backbone bearing ion-conducting groups, mostly aryl sulfonic acid. Based on the distribution of ionic groups along the polymeric backbone, aromatic ionomers can be classified into random ionomers with statistical distribution of the ionic functions, or block ionomers with segmented distribution of ionic functions. For each type, the ionic functions can be directly attached onto the polymer backbone or separated by a spacer. These molecular features play an important role in final morphologies and PEM performances. As concerning the structure backbone a large variety of ionomers have been studied, i.e., polysulfone (PES), poly(arylene ether) (PAE), and poly(ether ketone) (PEK), polyimide, polybenzimidazole, etc. The aim of this part is to review the state of the arts on prevalent aromatic ionomers to outline the advantages and drawbacks of each material as well as to establish their structure-morphology-property relation. The study has been focused mainly on aromatic ionomers based on PES, PAE, and PEK.

Table of contents :

Introduction
Chapter 1. Literature review
1. Fuel Cell and Its Current Status
1.1. Fuel Cells (FCs)
1.2. Proton Exchange Membrane Fuel Cells (PEMFCs)
1.3. PEM Functions and Its Requirements
1.4. Commercializing Barriers of PEMFCs
2. Nafion based PEMs
2.1. Introduction
2.2. Morphology of Nafion
2.3. Alternative Ionomers to PFSA
3. Aromatic Ionomers – A Promising Alternative to Nafion
3.1. Ionomers with Sulfonic Acid Directly Attached to Polymer Backbone
3.1.1. SO3H Attached to ‘Ortho-to-Ether’ Positions
3.1.1.1. Post-Sulfonation
3.1.1.2. Bottom-up Copolymerization
3.1.2. SO3H Attached to Positions other than ‘Ortho-to-Ether’
3.1.3. Conclusions
3.2. Ionomers with Sulfonic Acid Attached to Fluorenyl Groups
3.2.1. Homopolymers and Random Copolymers
3.2.2. Multi-Block Copolymers
3.2.3. Conclusions
3.3. Ionomers with Sulfonic Functions Spaced from Backbone
3.3.1. Ionic Functions Spaced by a Phenylene Spacer
3.3.1.1. Spacer Directly Connected to Main Chain
3.3.1.2. Spacer Connected via Ketone Bridge
3.3.1.3. Conclusions
3.3.2. Ionic Functions Spaced by an Alkyl Spacer
3.3.3. Ionic Functions Spaced by a Perfluoroalkyl Spacer
3.3.3.1. Influence of Main Chain Structure
3.3.3.2. Influence of Side Chain Structure
3.3.3.3. Influence of Counter Cation
3.3.3.4. Conclusions
3.4. Ionomers with Sulfonimide-based Acidic Moieties
3.5. Overall Conclusions and Thesis Objectives
Chapter 2
Chapter 2A
1. Introduction
2. Experimental Section
2.1. Materials
2.2. Column Preparation and IGC Setup
2.3. Preparation of ps-PES and InX/Y Membranes
2.4. Water Uptake
2.5. Proton Conductivity
2.6. Differential Scanning Calorimetry (DSC)
2.7. Small Angle Neutron Scattering (SANS)
2.8. NMR Spectroscopy
2.9. Gas Permeability
2.10. Dynamic Mechanical Analysis (DMA)
3. Results and Discussion
3.1. Solvent Selectivity
3.2. Morphology of PEMs
3.3. Thermo-Mechanical Properties
3.4. Water Uptake and Conductivity
3.4.1. “As-Casting Membranes”
3.4.1.1. Water Uptake
3.4.1.2. Conductivity
3.4.2. Annealed Membranes
3.4.2.1. Conductivities and Water Uptakes
4. Conclusions
Chapter 2B
1. Introduction
2. Materials and Methods
2.1. Materials
2.2. Membrane Preparation
2.3. Small Angle Neutron Scattering (SANS)
2.4. Conductivity Measurements
2.5. Proton Diffusion Coefficients
3. Results and Discussion
3.1. Main Morphological Features of Block Copolymers
3.2. Transport Properties
3.3. Structure-to-Transport Correlations
4. Conclusions
Chapter 3
1. Introduction
2. Experimental Section
2.1. Materials
2.2. Membrane Preparation
2.3. Water Uptake
2.4. Differential Scanning Calorimetry (DSC)
2.5. Dynamic Mechanical Analysis (DMA)
2.6. Small-Angle Neutron Scattering (SANS)
2.7. Proton Conductivity of Membranes
3. Results and Discussion
3.1. Thermomechanical Properties
3.2. Water Uptake
3.3. Proton Conductivity
3.4. Morphology of Hydrated Blend PEMs
3.5. Dilution laws
3.6. Morphological Model of Blend Membranes
4. Conclusions
Chapter 4
1. Introduction
2. Experimental Section
2.1. Materials
2.2. Synthesis of Ionomers
2.2.1. Synthesis of Perfluorosulfonimide Ionic Compound (I-psiLi)
2.2.2. Synthesis of PES-FPES, BrPES-FPES, and Si Ionomers
2.3. Membrane Preparation
2.3.1. SiX/Y Membranes
2.3.2. Nafion Membranes
2.4. Characterization
2.4.1. NMR Spectroscopy
2.4.2. Ion-Exchange Capacity (IEC)
2.4.3. SEC-MALLS
2.4.4. Water Uptake
2.4.5. Density Measurement
2.4.6. Differential Scanning Calorimetry (DSC)
2.4.7. Thermal Gravimetrical Analysis (TGA)
2.4.8. Dynamic Mechanical Analysis (DMA)
2.4.9. Water Sorption
2.4.10. Proton Conductivity
2.4.11. Small Angle Neutron Scattering (SANS)
2.4.12. Proton Diffusion Coefficients
3. Results and Discussion
3.1. Synthesis of SiX/Y Ionomers
3.2. Thermal and Thermomechanical Properties
3.3. Morphology
3.4. Water Uptake and Proton Conductivity
4. Conclusions
Overall Conclusions and Perspectives
1. Conclusions
2. Perspectives
Annexes

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