We report on multiblock copolymers consisting of highly sulfonated hydrophilic poly(arylene sulfone) (SPAS) blocks combined with hydrophobic poly(arylene ether sulfone) (PAES) blocks. Thiol-terminated precursor blocks of sulfonated poly(arylene thioether sulfone) (SPATS) were first prepared via polycondensations involving a novel tetrasulfonated dichlorotetraphenyldisulfone monomer, followed by coupling with pentafluorophenyl end-capped PAES precursor blocks under mild conditions to form SPATS−PAES block copolymers. The thioether bridges of the SPATS blocks were then selectively oxidized to obtain the SPAS−PAES copolymers with hydrophilic blocks containing exclusively sulfone bridges. Thus, the SPAS blocks were designed for high chain stiffness and stability toward desulfonation and had an ion exchange capacity (IEC) of 4.2 mequiv g −1 . Membranes of the SPAS−PAES copolymers were phase separated on the nanoscale and showed an increased thermal stability and decreased water uptake in relation to the corresponding SPATS−PAES membranes. Meta-connectivity in the sulfonated blocks gave slightly higher water uptake than pure para-connectivity. At 80 °C and 30% relative humidity, the proton conductivity of a SPAS−PAES membrane with an IEC of 1.8 mequiv g −1 reached 5.1 mS cm −1 , which was comparable to that of Nafion and far exceeded that of a sulfonated statistical copolymer membrane with a similar IEC. This class of block copolymers possesses very attractive properties and has great prospective to meet the demands of various electrochemical applications.
(arylene ether sulfone)s with different block lengths and ionic contents are tailored for durable and proton-conducting electrolyte membranes. Two series of fully aromatic copolymers are prepared by coupling reactions between non-sulfonated hydrophobic precursor blocks and highly sulfonated hydrophilic precursor blocks containing either fully disulfonated diarylsulfone or fully tetrasulfonated tetraaryldisulfone segments. The sulfonic acid groups are exclusively introduced in ortho positions to the sulfone bridges to impede desulfonation reactions and give the blocks ion exchange capacities (IECs) of 4.1 and 4.6 meq. g −1 , respectively. Solvent cast block copolymer membranes show well-connected hydrophilic nanophase domains for proton transport and high decomposition temperatures above 310 °C under air. Despite higher IEC values, membranes containing tetrasulfonated tetraaryldisulfone segments display a markedly lower water uptake than the corresponding ones with disulfonated diarylsulfone segments when immersed in water at 100 °C, presumably because of the much higher chain stiffness and glass transition temperature of the former segments. The former membranes have proton conductivities in level of a perfluorosulfonic acid membrane (NRE212) under fully humidified conditions. A membrane with an IEC of 1.83 meq. g −1 reaches above 6 mS cm −1 under 30% relative humidity at 80 °C, to be compared with 10 mS cm −1 for NRE212 under the same conditions.solubilization of conductive polymers such as poly(thiophene)s. [4] Still, perhaps the most important application area of these robust polymers is currently as proton exchange membranes for polymer electrolyte fuel cells, which are highly energy efficient and potentially environmentally benign power devices. [5,6] Today, perfluorosulfonic acid polymers such as Nafion are considered as state-of-the-art membranes because of their high chemical stability and high proton conductivity under a wide humidity range at moderate operating temperatures. [7,8] Nevertheless, the Nafion membrane suffers from general disadvantages such as high cost, high fuel permeability and a low mechanical modulus. More seriously, the Nafion membrane has a tendency to dehydrate above 80 °C, leading to lower conductivities. It thus fails to meet the current industrial demands which call for operating temperatures above 100 °C. [9,10] Over the last decade, an extensive research effort has been directed toward developing alternative membranes based on sulfonated hydrocarbon polymers to overcome the drawbacks of Nafion. [11,12] The use of hydrocarbon-based monomers and building blocks has widely expanded the possibilities to vary and control the structure and function of the membranes in relation to fluorocarbon-based approaches. For example, a wide range of sulfonated high-performance aromatic polymers has been prepared and their films have subsequently been evaluated and demonstrated as prospective fuel cell membranes. Among these polymers, sulfonated poly(arylene ether sulfone)s (SPAESs) have been ex...
On the performance and stability of proton exchange membrane fuel cells (PEMFCs), the water distribution inside the membrane has a direct influence. In this study, coherent anti-Stokes Raman scattering (CARS) spectroscopy was applied to investigate the different chemical states of water (protonated, hydrogen-bonded (H-bonded) and non-H-bonded water) inside the membrane with high spatial (10 μm φ (area) × 1 μm (depth)) and time (1.0 s) resolutions. The number of water molecules in different states per sulfonic acid group in a Nafion membrane was calculated using the intensity ratio of deconvoluted O–H and C–F stretching bands in CARS spectra as a function of current density and at different locations. The number of protonated water species was unchanged regardless of the relative humidity (RH) and current density, whereas H-bonded water molecules increased with RH and current density. This monitoring system is expected to be used for analyzing the transient states during the PEMFC operation.
The counterion condensation behavior of proton conducting sulfonated polysulfones has been investigated by combining electrophoretic NMR, pulsed magnetic field gradient NMR, and conductivity measurements on monomeric and polymeric samples with concentrations of ionic groups in the range where dissociation is not complete (IEC = 4.55–7.04 mequiv g–1). In this regime, counterion condensation is shown to critically depend on details of the molecular structure, and all atom molecular dynamics (MD) simulations reveal the formation of well-defined ionic aggregates (e.g., triple ions). The corresponding global minima of the free energy are suggested to be the result of a delicate balance of the energetics involved in conformational changes, formation of ionic aggregates, and solvation. This goes beyond Manning’s counterion condensation theory and has important implications for the development of membranes with high ionic conductivity as needed for many electrochemical applications such as fuel cells and batteries.
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