Proton conductivities and methanol permeabilities of membranes made from partially sulfonated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene copolymers

Current status and future perspectives of proton exchange membranes for hydrogen fuel cells

The world is on the lookout for sustainable and environmentally benign energy generating systems. Fuel cells (FCs) are regarded as environmentally friendly technology since they address a variety of environmental issues, such as hazardous levels of local pollutants, while also delivering economic advantages owing to their high efficiency. A fuel cell is a device that changes chemical energy contained in fuels (such as hydrogen and methanol) into electrical energy. A wide variety of FCs are commercially available; however, proton exchange membranes for hydrogen fuel cells (PEMFCs) have received overwhelming attention owing to their potential to significantly reduce our energy consumption, pollution emissions, and reliance on fossil fuels. The proton exchange membrane (PEM) is a critical element; it is made of semipermeable polymer and serves as a barrier between the cathode and anode during fuel cell construction. Additionally, membranes function as an insulator between the cathode and anode, facilitating proton exchange and inhibiting electron exchange between the electrodes. Due to the excellent features such as durability and proton conductivity, Nafion membranes are commercially viable and have been in use for a long time. However, Nafion membranes are costly, and their proton exchange capacities degrade over time at higher temperatures and low relative humidity. Other types of membranes have been considered in addition to Nafion membranes. This article discusses the problems connected with several types of PEMs, as well as the strategies adopted to improve their characteristics and performance.

Acidity effects of medium fluids on anhydrous proton conductivity of acid-swollen block polymer electrolyte membranes

Proton-conductive polymer electrolyte membranes (PEMs) were prepared by infiltrating sulfuric acid (Sa) or phosphoric acid (Pa) into a polystyrene-b-poly(4-vinylpyridine)-b-polystyrene (S–P–S) triblock copolymer. When the molar ratio of acid to pyridyl groups in S–P–S, i.e., the acid doping level (ADL), is below unity, the P-block/acid phase in the PEMs exhibited a moderately high glass transition temperature (T_g) of ∼140 °C because of consumption of acids for forming the acid–base complexes between the pyridyl groups and the acids, also resulting in almost no free protons in the PEMs; therefore, the PEMs were totally glassy and exhibited almost no anhydrous conductivity. In contrast, when ADL is larger than unity, the T_gs of the phase composed of acid and P blocks were lower than room temperature, due to the excessive molar amount of acid serving as a plasticizer. Such swollen PEMs with excessive amounts of acid releasing free protons were soft and exhibited high conductivities even without humidification. In particular, an S–P–S/Sa membrane with ADL of 4.6 exhibited a very high anhydrous conductivity of 1.4 × 10⁻¹ S cm⁻¹ at 95 °C, which is comparable to that of humidified Nafion membranes. Furthermore, S–P–S/Sa membranes with lower T_gs exhibited higher conductivities than S–P–S/Pa membranes, whereas the temperature dependence of the conductivities for S–P–S/Pa is stronger than that for S–P–S/Sa, suggesting Pa with a lower acidity would not be effectively dissociated into a dihydrogen phosphate anion and a free proton in the PEMs at lower temperatures.

Sulfuric acid-swollen block polymer membranes exhibit anhydrous conductivities of ∼0.1 S cm⁻¹ that is higher than those of phosphoric acid-swollen membranes, whereas temperature dependence of conductivities of the latter is stronger than the former.

100th Anniversary of Macromolecular Science Viewpoint: Block Copolymers with Tethered Acid Groups: Challenges and Opportunities

Scientific research on advanced polymer electrolytes has led to the emergence of all-solid-state energy storage/transfer systems. Early research began with acid-tethered polymers half a century ago, and research interest has gradually shifted to high-precision polymers with controllable acid functional groups and nanoscale morphologies. Consequently, various self-assembled acid-tethered block polymer morphologies have been produced. Their ion properties are profoundly affected by the multiscale intermolecular interactions in confinements. The creation of hierarchically organized ion/dipole arrangements inside the block copolymer nanostructures has been highlighted as a future method for developing advanced single-ion polymers with decoupled ion dynamics and polymer chain relaxation. Several emerging practical applications of the acid-tethered block copolymers have been explored to draw attention to the challenges and opportunities in developing state-of-the-art electrochemical systems.

Recent Progress in Conducting Polymers for Hydrogen Storage and Fuel Cell Applications

Hydrogen is a clean fuel and an abundant renewable energy resource. In recent years, huge scientific attention has been invested to invent suitable materials for its safe storage. Conducting polymers has been extensively investigated as a potential hydrogen storage and fuel cell membrane due to the low cost, ease of synthesis and processability to achieve the desired morphological and microstructural architecture, ease of doping and composite formation, chemical stability and functional properties. The review presents the recent progress in the direction of material selection, modification to achieve appropriate morphology and adsorbent properties, chemical and thermal stabilities. Polyaniline is the most explored material for hydrogen storage. Polypyrrole and polythiophene has also been explored to some extent. Activated carbons derived from conducting polymers have shown the highest specific surface area and significant storage. This review also covers recent advances in the field of proton conducting solid polymer electrolyte membranes in fuel cells application. This review focuses on the basic structure, synthesis and working mechanisms of the polymer materials and critically discusses their relative merits.

Hyperbranch-Crosslinked S-SEBS Block Copolymer Membranes for Desalination by Pervaporation

Sulfonated aromatic polymer (SAP) featuring hydrophilic nanochannels for water transport is a promising membrane material for desalination. SAPs with a high sulfonation degree favor water transport but suffer from reduced mechanical strength and membrane swelling. In this work, a hyperbranched polyester, H302, was introduced to crosslink a sulfonated styrene-ethylene/butylene-styrene (S-SEBS) copolymer membrane. The effects of crosslinking temperature and amount of H302 on the microstructure, and the pervaporation desalination performance of the membrane, were investigated. H302/S-SEBS copolymer membranes with different crosslinking conditions were characterized by various techniques including FTIR, DSC, EA, SEM, TEM and SAXS, and tensile strength, water sorption and contact angle measurements. The results indicate that the introduction of hyperbranched polyester enlarged the hydrophilic microdomain of the S-SEBS membrane. Crosslinking with hyperbranched polyester with heat treatment effectively enhanced the mechanical strength of the S-SEBS membrane, with the tensile strength being increased by 140–200% and the swelling ratio reduced by 45–70%, while reasonable water flux was maintained. When treating 5 wt% hypersaline water at 65 °C, the optimized crosslinked membrane containing 15 wt% H302 and heated at 100 °C reached a water flux of 9.3 kg·m⁻²·h⁻¹ and a salt rejection of 99.9%. The results indicate that the hyperbranched-S-SEBS membrane is promising for use in PV desalination.

Ionic complexation of endblock-sulfonated thermoplastic elastomers and their physical gels for improved thermomechanical performance

Thermoplastic elastomers (TPEs) composed of nonpolar triblock copolymers constitute a broadly important class of (re)processable network-forming macromolecules employed in ubiquitous commercial applications. Physical gelation of these materials in the presence of a low-volatility oil that is midblock-selective yields tunably soft TPE gels (TPEGs) that are suitable for emergent technologies ranging from electroactive, phase-change and shape-memory responsive media to patternable soft substrates for flexible electronics and microfluidics. Many of the high-volume TPEs used for these purposes possess styrenic endblocks that are inherently limited by a relatively low glass transition temperature. To mitigate this shortcoming, we sulfonate and subsequently complex (and physically crosslink) the endblocks with trivalent Al³⁺ ions. Doing so reduces the effective hydrophilicity of the sulfonated endblocks, as evidenced by water uptake measurements, while concurrently enhancing the thermomechanical stability of the corresponding TPEGs. Chemical modification results, as well as morphological and property development, are investigated as functions of the degree of sulfonation, complexation and TPEG composition.

SAXS signature of the lamellar ordering of ionic domains of perfluorinated sulfonic-acid ionomers by electric and magnetic field-assisted casting

The origins of the matrix and ionomer peaks of Nafion SAXS pattern are revisited.

Cross-Linked Quaternized Poly(styrene-b-(ethylene-co-butylene)-b-styrene) for Anion Exchange Membrane: Synthesis, Characterization and Properties

Poly(styrene-b-(ethylene-co-butylene)-b-styrene) triblock copolymer (SEBS) was selected for functionalization and cross-linking reaction to prepare the anion exchange membrane. The cross-linked quaternized SEBS (QSEBS-Cn) membranes were synthesized by simultaneous of quaternization and cross-linking of chloromethylated SEBS with α,ω-difunctional tertiary amines. The spacer groups of (-CH2-)n in diamines did affect the functionalization, micromorphology and properties of the resulting QSEBS-Cn membranes. The ionic conductivity of QSEBS-Cn membranes greatly increased and methanol resistance slightly decreased with increasing the length of spacer groups in the cross-linked structures from -(CH2)- to -(CH2)6-. Compared to the un-cross-linked QSEBS, the QSEBS-Cn membranes behaved much higher mechanical property, service temperature, chemical stability and thermal stability. Moreover, the hybrid composite membrane of QSEBS-C6 with 0.5% of graphene oxide could also be in situ prepared. This hybrid membrane had both relatively high ionic conductivity of 2.0 × 10(-2) S·cm(-1) and high selectivity of 7.6 × 10(4) S·s·cm(-3) at 60 °C due to its low methanol permeability.

Phosphonium-containing diblock copolymers from living anionic polymerization of 4-diphenylphosphino styrene

Living anionic polymerization of 4-diphenylphosphino styrene (DPPS) achieved well-defined homopolymers, poly(DPPS-b-S) styrenic block copolymers, and poly(I-b-DPPS) diene-based diblock copolymers with predictable molecular weights and narrow polydispersities. In situ FTIR spectroscopy monitored the anionic polymerization of DPPS and tracked monomer consumption for kinetic analysis. Post-alkylation enabled controlled placement of phosphonium functionality in poly(I-b-DPPS) diblock copolymers, producing well-defined phosphonium-containing block copolymers with low degrees of compositional heterogeneity. Incorporating phosphonium charge disrupted the lamellar bulk morphology of the neutral diblock precursor and provided morphologies with interdigitated packing of alkyl chains on the phosphonium cation.

Characterization of Polyethylene-Graft-Sulfonated Polyarylsulfone Proton Exchange Membranes for Direct Methanol Fuel Cell Applications

This paper examines polymer film morphology and several important properties of polyethylene-graft-sulfonated polyarylene ether sulfone (PE-g-s-PAES) proton exchange membranes (PEMs) for direct methanol fuel cell applications. Due to the extreme surface energy differences between a semi-crystalline and hydrophobic PE backbone and several amorphous and hydrophilic s-PAES side chains, the PE-g-s-PAES membrane self-assembles into a unique morphology, with many proton conductive s-PAES channels embedded in the stable and tough PE matrix and a thin hydrophobic PE layer spontaneously formed on the membrane surfaces. In the bulk, these membranes show good mechanical properties (tensile strength >30 MPa, Young’s modulus >1400 MPa) and low water swelling (λ < 15) even with high IEC >3 mmol/g in the s-PAES domains. On the surface, the thin hydrophobic and semi-crystalline PE layer shows some unusual barrier (protective) properties. In addition to exhibiting higher through-plane conductivity (up to 160 mS/cm) than in-plane conductivity, the PE surface layer minimizes methanol cross-over from anode to cathode with reduced fuel loss, and stops the HO• and HO₂• radicals, originally formed at the anode, entering into PEM matrix. Evidently, the thin PE surface layer provides a highly desirable protecting layer for PEMs to reduce fuel loss and increase chemical stability. Overall, the newly developed PE-g-s-PAES membranes offer a desirable set of PEM properties, including conductivity, selectivity, mechanical strength, stability, and cost-effectiveness for direct methanol fuel cell applications.

Novel multiarm star block copolymer ionomers as proton conductive membranes

Novel multiarm star block copolymer ionomers containing hydrophobic fluorinated block at the periphery and partially sulfonated polystyrene block at the core with varying ion exchange capacities (IECs) were synthesized.

Measuring the proton conductivity of ion-exchange membranes using electrochemical impedance spectroscopy and through-plane cell

The role of the incorporation of conducting polymer (CP), doped with different sulfonic acid organic molecules, in polystyrene (PS) and high-impact polystyrene (HIPS) with poly(styrene-ethylene-butylene) (SEBS) triblock copolymer has been investigated. Two factors associated with this model membrane system are addressed: (i) the influence of the presence of a low concentration of doped conducting polymer and (ii) the influence of the membrane preparation method. Membrane characterization and bulk conductivity measurements allowed the conclusion that proton conductivity has been promoted by the addition of CP; the best results were achieved for PAni-CSA, in either PS/SEBS or HIPS/SEBS blends. Additionally, the water uptake only decreased with the addition of PAni-doped molecules compared to the pure copolymer, without loss of ion-exchange capacity (IEC). Electrodialysis efficiency for HIPS/SEBS (before annealing) is higher than that for HIPS/SEBS (after annealing), indicating that membrane preparation method is crucial. Finally, through-plane cell arrangement proved to be an effective, quick, and time-saving tool for studying the main resistance parameters of isolating polymers, which is useful for application in industry and research laboratories working with membranes for electrodialysis or fuel cells.