Kang Hyuck Lee

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.

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.

Thermally rearranged polymer membranes for desalination

Herein, we demonstrate thermally rearranged polybenzoxazole-co-imide (TR-PBOI) electrospun nanocomposite membranes for membrane distillation and membrane crystallization applications. We seek to demonstrate that a synergistic combination of TR polymers, porous nanofibrous membranes, and particle coating improves the long-term stability while maintaining high porosity and water flux. The fabricated membranes exhibit an excellent water flux (80 kg m−2 h−1) and NaCl rejection (>99.99%) with steady performance over more than 186 hours. In addition, for the first time, controlling the heterogeneous nucleation phenomena in membrane crystallization was clearly demonstrated using TR membrane morphology.