Yeonsu Jung

Flexible and Robust Thermoelectric Generators Based on All-Carbon Nanotube Yarn without Metal Electrodes

As practical interest in flexible/or wearable power-conversion devices increases, the demand for high-performance alternatives to thermoelectric (TE) generators based on brittle inorganic materials is growing. Herein, we propose a flexible and ultralight TE generator (TEG) based on carbon nanotube yarn (CNTY) with excellent TE performance. The as-prepared CNTY shows a superior electrical conductivity of 3147 S/cm due to increased longitudinal carrier mobility derived from a highly aligned structure. Our TEG is innovative in that the CNTY acts as multifunctions in the same device. The CNTY is alternatively doped into n- and p-types using polyethylenimine and FeCl3, respectively. The highly conductive CNTY between the doped regions is used as electrodes to minimize the circuit resistance, thereby forming an all-carbon TEG without additional metal deposition. A flexible TEG based on 60 pairs of n- and p-doped CNTY shows the maximum power density of 10.85 and 697 μW/g at temperature differences of 5 and 40 K, respectively, which are the highest values among reported TEGs based on flexible materials. We believe that the strategy proposed here to improve the power density of flexible TEG by introducing highly aligned CNTY and designing a device without metal electrodes shows great potential for the flexible/or wearable power-conversion devices.

Capillarity ion concentration polarization as spontaneous desalting mechanism

To overcome a world-wide water shortage problem, numerous desalination methods have been developed with state-of-the-art power efficiency. Here we propose a spontaneous desalting mechanism referred to as the capillarity ion concentration polarization. An ion-depletion zone is spontaneously formed near a nanoporous material by the permselective ion transportation driven by the capillarity of the material, in contrast to electrokinetic ion concentration polarization which achieves the same ion-depletion zone by an external d.c. bias. This capillarity ion concentration polarization device is shown to be capable of desalting an ambient electrolyte more than 90% without any external electrical power sources. Theoretical analysis for both static and transient conditions are conducted to characterize this phenomenon. These results indicate that the capillarity ion concentration polarization system can offer unique and economical approaches for a power-free water purification system.

Metal–Phenolic Carbon Nanocomposites for Robust and Flexible Energy-Storage Devices

Future electronics applications such as wearable electronics depend on the successful construction of energy-storage devices with superior flexibility and high electrochemical performance. However, these prerequisites are challenging to combine: External forces often cause performance degradation, whereas the trade-off between the required nanostructures for strength and electrochemical performance only results in diminished energy storage. Herein, a flexible supercapacitor based on tannic acid (TA) and carbon nanotubes (CNTs) with a unique nanostructure is presented. TA was self-assembled on the surface of the CNTs by metal-phenolic coordination bonds, which provides the hybrid film with both high strength and high pseudocapacitance. Besides 17-fold increased mechanical strength of the final composite, the hybrid film simultaneously exhibits excellent flexibility and volumetric capacitance.

High-strength carbon nanotube/carbon composite fibers via chemical vapor infiltration

In this study, we have developed an efficient and scalable method for improving the mechanical properties of carbon nanotube (CNT) fibers. The mechanical properties of as-synthesized CNT fibers are primarily limited by their porous structures and the weak bonding between adjacent CNTs. These result in inefficient load transfer, leading to low tensile strength and modulus. In order to overcome these limitations, we have adopted chemical vapor infiltration (CVI) to efficiently fill the internal voids of the CNT fibers with carbon species which are thermally decomposed from gas phase hydrocarbon. Through the optimization of the processing time, temperature, and gas flow velocity, we have confirmed that carbon species formed by the thermal decomposition of acetylene (C2H2) gas successfully infiltrated into porous CNT fibers and densified them at relatively low temperatures (650-750 °C). As a result, after CVI processing of the as-synthesized CNT fibers under optimum conditions, the tensile strength and modulus increased from 0.6 GPa to 1.7 GPa and from 25 GPa to 127 GPa, respectively. The CVI technique, combined with the direct spinning of CNT fibers, can open up a route to the fast and scalable fabrication of high performance CNT/C composite fibers. In addition, the CVI technique is a platform technology that can be easily adapted into other nano-carbon based yarn-like fibers such as graphene fibers.

Poro-elasto-capillary wicking of cellulose sponges

Capillary rise of water in porous cellulose sponges is investigated considering hygroscopic shape evolutions of micropores. We mundanely observe cellulose (kitchen) sponges swell while absorbing water. Fluid flows in deformable porous media, such as soils and hydrogels, are classically described on the basis of the theories of Darcy and poroelasticity, where the expansion of media arises due to increased pore pressure. However, the situation is qualitatively different in cellulosic porous materials like sponges because the pore expansion is driven by wetting of the surrounding cellulose walls rather than by increase of the internal pore pressure. We address a seemingly so simple but hitherto unanswered question of how fast water wicks into the swelling sponge. Our experiments uncover a power law of the wicking height versus time distinct from that for nonswelling materials. The observation using environmental scanning electron microscopy reveals the coalescence of microscale wall pores with wetting, which allows us to build a mathematical model for pore size evolution and the consequent wicking dynamics. Our study sheds light on the physics of water absorption in hygroscopically responsive multiscale porous materials, which have far more implications than everyday activities (for example, cleaning, writing, and painting) carried out with cellulosic materials (paper and sponge), including absorbent hygiene products, biomedical cell cultures, building safety, and cooking.

Morphochemical imprinting of melamine cyanurate mesocrystals in glucose-derived carbon for high performance lithium ion batteries

A novel dual imprinting method is suggested to synthesize pyridinic N-enriched, hierarchically porous carbon. Rose-like melamine cyanurate (MCA) mesocrystals are prepared by simple self-assembly and utilized as a sacrificial template. The unique morphological and chemical features of the MCA are imprinted in the carbon source during carbonization. Curled graphene-like layers with a high nitrogen content (19.9 at%, especially 8.9 at% of pyridinic N) are interconnected to form a unique red blood cell-shaped morphology with a hierarchical pore structure. The resulting material exhibits an outstanding electrochemical performance (2019 mA h g−1 at 100 mA g−1 and 643 mA h g−1 at 2000 mA g−1 after 250 cycles) when evaluated as an anode material for lithium ion batteries. Furthermore, this novel imprinting strategy can provide a simple and efficient methodology to produce pyridinic N-enriched, hierarchically porous carbonaceous materials for extensive applications.

The influence of microstructure of carbon nanotubes on the degree of length reduction during melt processing with polycarbonate

The length of fiber has strong effect on mechanical strength of fiber reinforced polymer nanocomposites. Previous work has shown that the length of multi-walled carbon nanotubes are reduced during melt mixing process with polyamide 6, 6. In this work, MWCNT/polycarbonate (PC) composites were prepared with four types of MWCNTs by injection molding. The severe melt mixing condition for dispersion of MWCNTs in the polymer matrix resulted in length reduction of MWCNTs. The microstructure and morphological properties of MWCNTs that influenced the degree of breakage and length reduction were experimentally investigated.

Relationship between polymer-CNT affinity and properties of polymer infiltrated carbon nanotube yarns

Polymers which have different affinity with carbon nanotube (CNT) were infiltrated into carbon nanotube yarns (CNTYs) to improve mechanical and electrical properties. Those properties were highly related with the internal structure and the affinity of CNT-polymer. Polymer with the poorest affinity toward CNTs showed the greatest advancement in the mechanical and electrical properties of the CNTYs among three different polymers. Deformation behavior of CNTY at each stage of stress-strain curves could be defined based on the structure and properties of polymer infiltrated CNTYs.

Preparation of aligned carbon nanotubes: Shape-dependence on isotropic to nematic phase transition

Liquid crystalline behavior of one-dimensional nanomaterial opens new paths to align carbon nanotubes (CNTs). Recently, our previous work has shown that the isotropic to nematic transition can be triggered by a facile filtration of CNT suspension with high concentration. In this work, we report the effect of shape of CNTs on the phase transition through four types of CNTs with different morphologies. The prepared CNT buckypapers exhibited different packing and mechanical performances.