Relationships between Structure and Alkaline Stability of Imidazolium Cations for Fuel Cell Membrane Applications

Anion exchange membranes have substantial potential to be useful in methanol fuel cells due to the viability of non-noble metal electrocatalysts at high pH and increases in the oxidation rate of methanol in alkaline conditions. However, long-term stability of the cationic moiety has been an issue, and imidazoliums have recently attracted attention as candidates for stable cations. The prevailing strategy for increasing the stability of the imidazolium has involved adding sterically hindering groups at the 2 position. Surprisingly, the findings of this study show that steric hindrance is the least effective strategy for stabilizing imidazoliums. We propose that the most important stabilizing factor for an imidazolium is the ability to provide alternative, reversible deprotonation reactions with hydroxide and outline other structure–property relationships for imidazolium cations

Highly Frustrated Poly(ionic liquid) ABC Triblock Terpolymers with Exceptionally High Morphology Factors

In this work, we report the successful synthesis of 17 unique compositions of a poly(ionic liquid) (PIL) ABC triblock terpolymer, poly(S-b-VBMIm-TFSI-b-HA), where S is styrene, VBMIm-TFSI is vinylbenzyl methylimidazolium bis(trifluoromethanesulfonyl)imide, and HA is hexyl acrylate. Nine distinct morphologies were observed, including two-phase and three-phase disordered microphase separated (D2 and D3), two-phase hexagonally packed cylinders (C2), core–shell hexagonally packed cylinders (CCS), three-phase lamellae (L3), two-phase lamellae (L2), core–shell double gyroid (Q230), spheres-in-lamellae (LSI), and a three-phase hexagonal superlattice of cylinders (CSL). The LSI morphology was unambiguously confirmed using small-angle X-ray scattering and transmission electron microscopy. Morphology type significantly impacted the ion conductivity of the PIL ABC triblock terpolymers, where remarkable changes in morphology factor (normalized ion conductivity) were observed with only small changes in the conducting volume fraction, i.e., PIL block composition. An exceptionally high morphology factor of 2.0 was observed from the PIL ABC triblock terpolymer with a hexagonal superlattice morphology due to the three-dimensional narrow, continuous PIL nanodomains that accelerate ion conduction. Overall, this work demonstrates the first systematic study of highly frustrated single-ion conducting ABC triblock terpolymers with a diverse set of morphologies and exceptionally high morphology factors, enabling the exploration of transport–morphology relationships to guide the future design of highly conductive polymer electrolytes

Highly Conductive Anion Exchange Membrane for High Power Density Fuel-Cell Performance

Anion exchange membrane fuel cells (AEMFCs) are regarded as a new generation of fuel cell technology that has the potential to overcome many obstacles of the mainstream proton exchange membrane fuel cells (PEMFCs) in cost, catalyst stability, efficiency, and system size. However, the low ionic conductivity and poor thermal stability of current anion exchange membranes (AEMs) have been the key factors limiting the performance of AEMFCs. In this study, an AEM made of styrenic diblock copolymer with a quaternary ammonium-functionalized hydrophilic block and a cross-linkable hydrophobic block and possessing bicontinuous phases of a hydrophobic network and hydrophilic conduction paths was found to have high ionic conductivity at 98 mS cm^–1 and controlled membrane swelling with water uptake at 117 wt % at 22 °C. Membrane characterizations and fuel cell tests of the new AEM were carried out together with a commercial AEM, Tokuyama A201, for comparison. The high ionic conductivity and water permeability of the new membrane reported in this study is attributed to the reduced torturosity of the ionic conduction paths, while the hydrophobic network maintains the membrane mechanical integrity, preventing excessive water uptake

Influence of Zwitterions on Thermomechanical Properties and Morphology of Acrylic Copolymers: Implications for Electroactive Applications

<i>n</i>-Butyl acrylate-based zwitterionomers and ionomers containing 3-[[2-(methacryloyloxy)ethyl](dimethyl)ammonio]-1-propanesulfonate (SBMA) and 2-[butyl(dimethyl)amino]ethyl methacrylate methanesulfonate (BDMAEMA MS), respectively, were synthesized using conventional free radical polymerization. Size-exclusion chromatography confirmed the molecular weights of the copolymers exceeded the critical molecular weight between entanglements (<i>M</i>_e) for poly(<i>n</i>-butyl acrylate). Differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), and atomic force microscopy (AFM) revealed that zwitterionomers promoted more well-defined microphase separation than cationic analogues. Dynamic mechanical analyses (DMA) of the copolymers showed a rubbery plateau region due to physical cross-links between charges for zwitterionomers only. Since SBMA and BDMAEMA MS have very similar chemical structures, we attributed improved microphase separation and superior elastomeric performance of the zwitterionomers to stronger association between covalently tethered charged pairs

Role of Metal–Ligand Bond Strength and Phase Separation on the Mechanical Properties of Metallopolymer Films

This work studies the properties of poly­(<i>n</i>-butyl acrylate) functionalized with 2,6-bis­(1′-methylbenzimidazolyl)­pyridine ligand and cross-linked with either copper­(II), zinc­(II), or cobalt­(II) metal ions. Because of phase separation between the metal–ligand complex and the polymer matrix, these polymers have a rubbery plateau modulus that is 10 times higher than expected based on the theory of rubber elasticity. Differences in the metal–ligand bond strength influence the mechanical behavior at high temperature and strains. Because of the particularly weak bond strength associated with the copper–ligand bond, the metallopolymer containing copper degrades at a lower temperature and has lower yield strength, ultimate tensile strength, and creep resistance than polymers containing cobalt and zinc. To tune the properties of the polymer further, a polymer is made with both copper and cobalt ions. The hybrid polymer combines the properties of the stiffer cobalt-containing polymer with the more compliant copper-containing polymer

Bicontinuous Alkaline Fuel Cell Membranes from Strongly Self-Segregating Block Copolymers

Alkaline fuel cell membranes have the potential to reduce the cost and weight of current fuel cell technology, but they still have not been broadly commercialized due to poor hydroxide conductivities and mechanical properties, in addition to low chemical stability. One approach to address these mechanical and transport shortcomings is forming a morphologically bicontinuous network of an ion transporting phase and a hydrophobic phase to provide mechanical strength. In this report, membranes having bicontinuous morphologies are fabricated by cross-linking cation-containing block copolymers with hydrophobic constituents. This is accomplished in a single step and does not require postpolymerization modification. The resulting materials conduct hydroxide ions very rapidly, as high as 120 mS cm^–1 in liquid water at 60 °C. The methodological changes required to obtain a bicontinuous morphology from such strongly self-segregating block copolymers, relevant to other applications in which bicontinuous structures are desired, are also described

Influence of Metal Ion and Polymer Core on the Melt Rheology of Metallosupramolecular Films

Detailed rheological studies of metallosupramolecular polymer films in the melt were performed to elucidate the influence of the metal ion and polymer components on their mechanical and structural properties. 4-Oxy-2,6-bis(<i>N</i>-methylbenzimidazolyl)pyridine telechelic end-capped polymers with a low-<i>T</i>_g core, either poly(tetrahydrofuran) or poly(ethylene-<i>co</i>-butylene), were prepared with differing ratios of Zn²⁺ and Eu³⁺ to determine the influence of polymer chain chemistry and metal ion on the properties. Increasing the amount of the weaker binding europium yielded more thermoresponsive films in both systems, and results show that the nature of the polymer core dramatically affected the films mechanical properties. All of the films studied exhibited large relaxation times, and we use this to explain the pure sinusoidal behavior found in the “nonlinear” viscoelastic region. Basically, the system cannot relax during a strain cycle, allowing us to assume the network destruction and creation rates to be only a function of the strain amplitude in a simplified network model used to rationalize the observed behavior

Polyethylene-Based Block Copolymers for Anion Exchange Membranes

Block copolymer membranes with a semicrystalline polyethylene component were prepared by anionic polymerization and postpolymerization functionalization reactions. Polybutadiene-<i>b</i>-poly­(4-methylstyrene) (PB-<i>b</i>-P4MS) precursors with four different block compositions and 92–95% 1,4-content in the polybutadiene block were produced by living anionic polymerization in a nonpolar solvent. The polybutadiene block was subsequently hydrogenated to prepare a polyethylene block, and the hydrogenated block copolymers were then brominated at the arylmethyl group of the P4MS block. Subsequent quaternization reaction with trimethylamine led to the anion exchange membranes. The degree of crystallinity in the polyethylene block was determined by differential scanning calorimetry to be approximately 24–27%. The postpolymerization modification reactions were examined by ¹H NMR and IR spectroscopy. The amount of quaternary ammonium groups was quantified by ion exchange capacity (IEC) measurements. Membranes with IEC’s ranging from 1.17 to 1.92 mmol/g were prepared. The IEC was varied by changing the relative amount of P4MS in the precursor block copolymer, and water uptake and ionic conductivity were found to increase with increasing IEC. Small-angle X-ray scattering (SAXS) experiments and transmission electron microscopy (TEM) showed phase-separated, bicontinuous structures at all compositions. Materials with higher IEC show improved ionic conductivity as well as lower activation energy of ion conduction compared to less functionalized membranes. The hydroxide conductivity of the block copolymer membrane with an IEC of 1.92 mmol/g reached 73 mS/cm at 60 °C in water. Tensile measurements indicated excellent mechanical properties of the semicrystalline membranes for potential use as alkaline fuel cell membrane materials

Metallopolymers Containing Excess Metal–Ligand Complex for Improved Mechanical Properties

This work incorporates ML complexes as unbound entities that interact with ML complexes bound to the backbone of the polymer. The π–π interactions and Coulombic forces between bound and unbound ML complexes hold the ML-rich phase together and result in improved mechanical properties over polymers containing only the bound ML complexes. The ML-rich phase formed ordered, cylindrical domains. The storage modulus, surface elastic modulus, and high temperature stability of these metallopolymers increased with increasing concentration of ML complex in the polymer while an optimal concentration and morphology are necessary to improve the strength and creep resistance of the polymer. Ultimately, the successful addition and patterning of unbound ML complexes as a hard phase in a polymer matrix provides an important template for the design of a new type of supramolecular nanocomposite

Sulfonimide-Containing Triblock Copolymers for Improved Conductivity and Mechanical Performance

Ion-containing block copolymers continue to attract significant interest as conducting membranes in energy storage devices. Reversible addition–fragmentation chain transfer (RAFT) polymerization enables the synthesis of well-defined ionomeric A–BC–A triblock copolymers, featuring a microphase-separated morphology and a combination of excellent mechanical properties and high ion transport. The soft central “BC” block is composed of poly­(4-styrene­sulfonyl­(trifluoro­methyl­sulfonyl)­imide) (poly­(Sty-Tf₂N)) with −SO₂–N^––SO₂–CF₃ anionic groups associated with a mobile lithium cation and low-<i>T</i>_g di­(ethylene glycol)­methyl ether methacrylate (DEGMEMA) units. External polystyrene A blocks provide mechanical strength with nanoscale morphology even at high ion content. Electrochemical impedance spectroscopy (EIS) and pulse-field-gradient (PFG) NMR spectroscopy have clarified the ion transport properties of these ionomeric A–BC–A triblock copolymers. Results confirmed that well-defined ionomeric A–BC–A triblock copolymers combine improved ion-transport properties with mechanical stability with significant potential for application in energy storage devices