Tandem Effect of Two Sulfophenylene Groups in Aromatic Polymers for Fuel Cell Membranes

While the structural design of sulfonated polyphenylenes for high-temperature operable fuel cell membranes has been mainly focused on the hydrophobic components, there have been few studies on the hydrophilic components. Herein, we designed a hydrophilic monomer (BSP) containing two tandemly linked sulfonated phenylenes (disulfo-biphenylene groups) and copolymerized with fluorinated terphenylene monomer for chemically/mechanically stable, highly proton conductive membranes for fuel cells. Compared with the typical hydrophilic monomer (SP) composed of a single sulfophenylene, the BSP monomer provided the resulting polymer and membrane with higher molecular weight, better mechanical stability, and combined fuel cell durability

Sulfonated Terpolymers Containing Alkylene and Perfluoroalkylene Groups: Effect of Aliphatic Groups on Membrane Properties and Interface with the Catalyst Layers

Two series of terpolymers (SPA-A, -B, and -C) composed of perfluoroalkylene, alkylene, and sulfonated phenylene (SP) groups were prepared to investigate the effect of aliphatic groups and their compositions onto the membrane properties for proton exchange membrane fuel cells. The composition of perfluoroalkylene (<i>m</i>) and alkylene groups (<i>n</i>) in SPA terpolymers was controlled to be <i>m</i>/<i>n</i> = 0.5/0.5 for SPA-A, 0.65/0.35 for SPA-B, and 1/0 for SPA-C. SPA terpolymers provided thin and bendable membranes with ion exchange capacity (IEC) ranging from 1.61 to 3.18 mequiv g^–1. Introducing alkylene groups into the polymer main chain was effective in achieving high IEC values. SPA-B membranes with lower alkylene group content showed slightly more developed phase-separated morphology than that of SPA-A membranes with higher alkylene group content. The developed phase separation with interconnected ionic channels resulted in high proton conductivity for SPA-B membranes. The alkylene groups in the main chain also contributed to improving mechanical properties as suggested by stress versus strain curves, in which SPA membranes exhibited higher Young’s moduli and higher yield strength than those of copolymer (SPA-C) membranes with no alkylene groups. An H₂/O₂ (or air) fuel cell with SPA-B membrane exhibited high open circuit voltage (OCV, 0.99 V at 100% RH with O₂), low ohmic resistance (0.05 Ω cm² at 100% RH with O₂), and good current/voltage performance, reflecting the properties of SPA-B membrane. However, interfacial compatibility with the catalyst layers was somewhat deteriorated with SPA-B membrane to cause lower mass activity (70 A g^–1 at 100% RH) of the cathode compared to that with SPAF membrane (102 A g^–1 at 100% RH). SPA-B membrane was durable in OCV hold test for 1000 h with slight degradation in alkylene groups

Tuning the Hydrophobic Component in Reinforced Poly(arylimidazolium)-Based Anion Exchange Membranes for Alkaline Fuel Cells

A series of imidazolium-based aromatic copolymers were synthesized using fluorinated and non-fluorinated hydrophobic monomers. The quaternized copolymers were reinforced with the plasma-treated porous polyethylene substrate to provide flexible, homogeneous membranes. Cross-sectional SEM images revealed a triple-layer structure. The reinforced membranes exhibited phase-separated morphology as confirmed through TEM images. Among the membranes, QQP-MEIm-PE-containing quinquephenylene hydrophobic groups exhibited the most balanced properties (ion conductivity, mechanical strength, and alkaline stability). In particular, QQP-MEIm-PE exhibited excellent elongation properties with 24 MPa maximum stress and 205% elongation at break. A single H2/O2 fuel cell using the QQP-MEIm-PE membrane (1.26 meq g–1) and non-PGM-(Fe–N–C) cathode achieved 222 mW cm–2, which accounted for 888 mW mg–1Pt at 560 mA cm–2. Reasonable durability was confirmed with the membrane in the operating fuel cell

Synthesis and Properties of Sulfonated Block Copolymers Having Fluorenyl Groups for Fuel-Cell Applications

A series of sulfonated poly(arylene ether sulfone)s (SPEs) block copolymers containing fluorenyl groups were synthesized. Bis(4-fluorophenyl)sulfone (FPS) and 2,2-bis(4-hydroxy-3,5-dimethylpheny)propane were used as comonomers for hydrophobic blocks, whereas FPS and 9,9-bis(4-hydroxyphenyl)fluorene were used as hydrophilic blocks. Sulfonation with chlorosulfonic acid gave sulfonated block copolymers with molecular weight (Mw) higher than 150 kDa. Proton conductivity of the SPE block copolymer with the ion exchange capacity (IEC) = 2.20 mequiv/g was 0.14 S/cm [80% relative humidity (RH)] and 0.02 S/cm (40% RH) at 80 °C, which is higher or comparable to that of a perfluorinated ionomer (Nafion) membrane. The longer hydrophilic and hydrophobic blocks resulted in higher water uptake and higher proton conductivity. Scanning transmission electron microscopy observation revealed that phase separation of the SPE block copolymers was more pronounced than that of the SPE random copolymers. The SPE block copolymer membranes showed higher mechanical properties than those of the random ones. With these properties, the SPE block copolymer membranes seem promising for fuel-cell applications

Highly Conductive and Ultra Alkaline Stable Anion Exchange Membranes by Superacid-Promoted Polycondensation for Fuel Cells

A series of anion exchange membranes (4-QPPAF-TMA) were prepared by a metal-free, superacid-promoted polymerization reaction. The polymers were obtained with high molecular weight (Mn = 9.5–19.6 kDa, Mw = 44.9–622.5 kDa). 4-QPPAF-TMA membranes exhibited high hydroxide ion conductivity (up to 115 mS cm–1) at 80 °C, reasonable water absorbability (45% water uptake at 30 °C for 1.7 meq g–1), low to moderate dimensional swelling (5–15% at 30–80 °C for 1.7 meq g–1), and mechanical robustness (12.8 MPa maximum stress and 32% elongation at break for 1.7 meq g–1). Furthermore, 4-QPPAF-TMA membranes exhibited excellent alkaline stability in 8 M KOH at 80 °C for 1000 h, maintaining high conductivity (105 mS cm–1, 97% remaining). density functional theory (DFT) calculations suggested that the unique molecular configuration of the pendant ammonium head groups was responsible for high resistivity to the hydroxide ion attack. A fuel cell was operated with the 4-QPPAF-TMA membrane and an ionomer using a non-PGM cathode catalyst (Fe–N–C) to achieve a peak power density of 215 mW cm–2 accountable for 860 mW mg–1 Pt at a 590 mA cm–2 current density and 0.40 V (Pt–C cathode achieved 370 mW cm–1 at 810 mA cm–2 and 0.50 V). The fuel cell was operated at constant current density (15 mA cm–2) for 240 h with −0.79 mV h–1 average cell voltage decay. The postdurability analyses revealed that the membrane did not deteriorate while the degradation of the cathode catalysts/ionomer caused the performance loss

Effect of the Hydrophilic Component in Aromatic Ionomers: Simple Structure Provides Improved Properties as Fuel Cell Membranes

To elucidate the effect of the hydrophilic component on the properties of aromatic ionomers, we have designed for the first time one of the simplest possible structures, the sulfo-1,4-phenylene unit, as the hydrophilic component. A modified Ni-mediated coupling polymerization produced the title aromatic ionomers composed of sulfonated <i>p</i>-phenylene groups and oligo­(arylene ether sulfone ketone)­s, as high-molecular-weight polymers (<i>M</i>_w = 202–240 kDa), resulting in the formation of tough, flexible membranes. The aromatic ionomer membranes showed well-developed hydrophilic/hydrophobic phase separation. Comparison with our previous aromatic ionomer membrane containing sulfonated benzophenone groups as a hydrophilic component revealed that the simple sulfophenylene structure (i.e., no polar groups such as ether, ketone, or sulfone groups in the hydrophilic component) was effective for the improvement of the membrane properties, i.e., reduced water uptake and excellent mechanical stability under humidified conditions. Furthermore, because of the high local ion exchange capacity (IEC), the simple structure led to high proton conductivity, especially at low humidity (reaching up to ca. 7.3 mS/cm at 80 °C and 20% RH), which is one of the highest values reported thus far. The improved properties of the membranes were also confirmed in an operating fuel cell

Sulfonated Poly(arylene ether sulfone ketone) Multiblock Copolymers with Highly Sulfonated Block. Synthesis and Properties

Poly(arylene ether sulfone ketone) (SPESK) multiblock copolymer membranes having highly sulfonated hydrophilic blocks were synthesized. The degree of polymerization of hydrophobic blocks (X) was controlled to be 15, 30, and 60 and that of hydrophilic blocks (Y) to be 4, 8, 12, and 16. Morphological observation by scanning transmission microscopy (STEM) and small-angle X-ray scattering (SAXS) showed that high local concentration of sulfonic acid groups within the hydrophilic blocks enhanced phase separation between the hydrophobic and hydrophilic blocks. Rodlike hydrophilic aggregates were found to be interconnected very well, which resulted in high proton conductivity even at low relative humidity (RH). The ionomer membrane with X30Y8 and 1.86 mequiv/g of ion exchange capacity (IEC) showed 0.03 S/cm at 80 °C and 40% RH, which was a comparable or higher proton conductivity than that of the state-of-the-art perfluorinated ionomer (Nafion) membrane. The longer blocks induced higher proton conductivity; however, excessively long block length offset mechanical properties. Low hydrogen and oxygen permeability was also observed

Hydrolytically Stable Polyimide Ionomer for Fuel Cell Applications

Hydrolytically Stable Polyimide Ionomer for Fuel Cell Application

Chemically Stable, Highly Anion Conductive Polymers Composed of Quinquephenylene and Pendant Ammonium Groups

To evaluate the effect of five consecutive phenylene units on the properties of anion conductive polymer membranes, a series of quaternized copolymers (QP-QAF) composed of quinquephenylene and fluorene groups functionalized with pendant hexyltrimethyl­ammonium groups were designed and synthesized. Precursor copolymers, QP-AF, with controllable copolymer compositions were synthesized by Yamamoto coupling reaction as high molecular weights. Quaternization of QP-AF using dimethyl sulfate followed by solution casting provided bendable and tough QP-QAF membranes with the ion exchange capacity (IEC) ranging from 0.80 to 2.78 mequiv g–1. The QP-QAF membranes contained phase-separated morphology due to the hydrophilic/hydrophobic differences in the components as confirmed by TEM images. High hydroxide ion conductivity (up to 134 mS cm–1 in water at 80 °C) was obtained with the membrane (IEC = 2.25 mequiv g–1). The membranes were stable in strongly alkaline conditions (∼4 M KOH at 80 °C) for 1000 h. An H2/O2 alkaline fuel cell using the QP-QAF membrane exhibited 248 mW cm–2 of the maximum power density at 60 °C under fully humidified conditions

Novel Sulfonated Poly(arylene ether): A Proton Conductive Polymer Electrolyte Designed for Fuel Cells

Novel Sulfonated Poly(arylene ether):  A Proton Conductive Polymer Electrolyte Designed for Fuel Cell