Dong Won Shin

Nanocrack-regulated self-humidifying membranes

The regulation of water content in polymeric membranes is important in a number of applications, such as reverse electrodialysis and proton-exchange fuel-cell membranes. External thermal and water management systems add both mass and size to systems, and so intrinsic mechanisms of retaining water and maintaining ionic transport in such membranes are particularly important for applications where small system size is important. For example, in proton-exchange membrane fuel cells, where water retention in the membrane is crucial for efficient transport of hydrated ions, by operating the cells at higher temperatures without external humidification, the membrane is self-humidified with water generated by electrochemical reactions. Here we report an alternative solution that does not rely on external regulation of water supply or high temperatures. Water content in hydrocarbon polymer membranes is regulated through nanometre-scale cracks (‘nanocracks’) in a hydrophobic surface coating. These cracks work as nanoscale valves to retard water desorption and to maintain ion conductivity in the membrane on dehumidification. Hydrocarbon fuel-cell membranes with surface nanocrack coatings operated at intermediate temperatures show improved electrochemical performance, and coated reverse-electrodialysis membranes show enhanced ionic selectivity with low bulk resistance.

Hydrocarbon-Based Polymer Electrolyte Membranes: Importance of Morphology on Ion Transport and Membrane Stability

A fundamental understanding of polymer microstructure is important in order to design novel polymer electrolyte membranes (PEMs) with excellent electrochemical performance and stabilities. Hydrocarbon-based polymers have distinct microstructure according to their chemical structure. The ionic clusters and/or channels play a critical role in PEMs, affecting ion conductivity and water transport, especially at medium temperature and low relative humidity (RH). In addition, physical properties such as water uptake and dimensional swelling behavior depend strongly on polymer morphology. Over the past few decades, much research has focused on the synthetic development and microstructural characterization of hydrocarbon-based PEM materials. Furthermore, blends, composites, pressing, shear field, electrical field, surface modification, and cross-linking have also been shown to be effective approaches to obtain/maintain well-defined PEM microstructure. This review summarizes recent work on developments in advanced PEMs with various chemical structures and architecture and the resulting polymer microstructures and morphologies that arise for potential application in fuel cell, lithium ion battery, redox flow battery, actuators, and electrodialysis.

Morphological transformation during cross-linking of a highly sulfonated poly(phenylene sulfide nitrile) random copolymer

We present a new approach of morphological transformation for effective proton transport within ionomers, even at partially hydrated states. Highly sulfonated poly(phenylene sulfide nitrile) (XESPSN) random network copolymers were synthesized as alternatives to state-of-the-art perfluorinated polymers such as Nafion®. A combination of thermal annealing and cross-linking, which was conducted at 250 °C by simple trimerisation of ethynyl groups at the chain termini, results in a morphological transformation. The resulting nanophase separation between the hydrophilic and hydrophobic domains forms well-connected hydrophilic nanochannels for dramatically enhanced proton conduction, even at partially hydrated conditions. For instance, the proton conductivity of XESPSN60 was 160% higher than that of Nafion® 212 at 80 °C and 50% relative humidity. The water uptake and dimensional swelling were also reduced and mechanical properties and oxidative stability were improved after three-dimensional network formation. The fuel cell performance of XESPSN membranes exhibited a significantly higher maximum power density than that of Nafion® 212 under partially hydrated environments.

Highly conductive and durable poly(arylene ether sulfone) anion exchange membrane with end-group cross-linking

Here, we demonstrate the improved electrochemical performance and stability of end-group cross-linked anion exchange membranes (AEM) for the first time via the introduction of imidazolium groups in poly(arylene ether sulfone) (Imd-PAES). A novel feature of the cross-linking reaction is that basic additives are not required to prevent gelation with the cationic functional groups. In this work, the sodium salt of 3-hydroxyphenylacetylene acted directly as the end-group cross-linker, and it was cross-linked by thermal treatment at 180 °C. The gel fraction and hydroxide conductivity of the cross-linked membranes (XE-Imds) depended on the cross-linking temperature and time. The prepared XE-Imd70 (70 refers to the degree of functionalization) membranes with an ion exchange capacity (IEC) of 2.2 meq g−1 achieved a high hydroxide conductivity (107 mS cm−1). This material also showed good single cell performance (XE-Imd70: 202 mA cm−2 at 0.6 V and a maximum power density of 196.1 mW cm−2) at 80 °C, 100% relative humidity (RH), and improved durability and alkaline stability. The excellent hydroxide conductivity and electrochemical performance of XE-Imd70 was due to the fact that the ion cluster size of XE-Imd membranes was larger (12.1–14 nm) than that of E-Imd (5.5–8.14 nm), indicating that XE-Imd membranes have a closely associated ion-clustered morphology, which was confirmed by transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) measurements.

A clustered sulfonated poly(ether sulfone) based on a new fluorene-based bisphenol monomer

A new fluorene-based bisphenol monomer containing two pendant phenyl groups, 9,9-bis(3-phenyl-4-hydroxy)phenyl-fluorene, was readily synthesized in high yield by a one-step reaction from inexpensive starting materials. A series of poly(ether sulfone)s with clustered sulfonic acid groups was prepared for fuel cell applications by polycondensation of the new monomer with bis(4-hydroxyphenyl)sulfone and bis(4-fluorophenyl)sulfone, followed by sulfonation exclusively on the fluorene rings and pendant phenyl rings, using concentrated sulfuric acid at room temperature. The sulfonated polymers gave tough, flexible, and transparent membranes by solvent casting. The ionic exchange capacity (IEC), water-uptake, dimensional stabilities, mechanical properties, thermal and oxidative stabilities as well as proton conductivities and single fuel cell properties of the membranes were investigated. The membranes with high IEC values show high proton transport properties, and their proton conductivities exhibit lower dependence on relative humidity compared with typical aromatic ion exchange membranes. 4-SPES-38 with an IEC value of 2.23 mequiv. g−1 displays comparable fuel cell performance with Nafion 212 under low humidity conditions.

Modified Sulfonated Poly(arylene ether sulfone) Membranes Prepared via a Radiation Grafting Method for Fuel Cell Application

Polymer modifications involving crosslinking and grafting by radiation have been widely researched for use in biopolymers, hydrogels, heat-resisting electric wires, vulcanization, polymer recycling, gas separation, and pervaporation membranes because of their advantages over traditional chemical crosslinking and grafting methods, including a catalyst-free reaction, post-modification at room temperature for solid polymers, and short modification times and steps. Various recent studies have also utilized radiation modification techniques to prepare proton exchange membranes (PEMs). PEMs are membranes which have the ability to selectively transfer protons generated by electrochemical reactions from the anode to the cathode in fuel cells. For this purpose, polymers having strong acidic functional group as proton carriers (e.g. sulfonic acid groups) have generally been used. There are two main strategies to introduce sulfonic acid groups into polymers. One is to introduce the groups directly to a polymer (post-sulfonation or polymersulfonation) and the other is to perform polymerization with a sulfonated monomer (monomer-sulfonation). To date, preirradiation and post-sulfonation methods, in which sulfonic acid groups are introduced after crosslinking or grafting non-sulfonated polymers by irradiation, have been used in radiation-induced processes of PEMs in order to prevent unfavorable membrane damage such as decomposition of sulfonic acid groups. However, this method has structural limitations for PEMs because the types of target polymer for irradiation are restricted to perfluorinated or partially fluorinated polymers and aliphatic polymers. In addition, crosslinking and grafting agents should have sites available for post-sulfonation. Whereas, few studies have been conducted on radiation-induced sulfonated polymers (post-irradiation method) despite the advantage that various types of polymers, prepared with various sulfonated and nonsulfonated monomers, can be used as a matrix for radiation modification. In this study, we investigated a post-irradiation method for PEMs and tried to improve PEM performance by preparing a radiation-grafted sulfonated polymer. Sulfonated polyarylene ether sulfone (SPAES) was used as a polymer matrix for irradiation. Here, the sulfonated polymer with a perfluorinated backbone (e.g. Nafion) was excluded due to its serious decomposition resulting from the chain scission effect caused by irradiation. Due to its additional acid group, acrylic acid was used as a grafting agent to increase the proton conduction properties of the resulting polymer.