Ju-Won Jeon

In Situ One-Step Synthesis of Hierarchical Nitrogen-Doped Porous Carbon for High-Performance Supercapacitors

A hierarchically structured nitrogen-doped porous carbon is prepared from a nitrogen-containing isoreticular metal-organic framework (IRMOF-3) using a self-sacrificial templating method. IRMOF-3 itself provides the carbon and nitrogen content as well as the porous structure. For high carbonization temperatures (950 °C), the carbonized MOF required no further purification steps, thus eliminating the need for solvents or acid. Nitrogen content and surface area are easily controlled by the carbonization temperature. The nitrogen content decreases from 7 to 3.3 at % as carbonization temperature increases from 600 to 950 °C. There is a distinct trade-off between nitrogen content, porosity, and defects in the carbon structure. Carbonized IRMOFs are evaluated as supercapacitor electrodes. For a carbonization temperature of 950 °C, the nitrogen-doped porous carbon has an exceptionally high capacitance of 239 F g(-1). In comparison, an analogous nitrogen-free carbon bears a low capacitance of 24 F g(-1), demonstrating the importance of nitrogen dopants in the charge storage process. The route is scalable in that multi-gram quantities of nitrogen-doped porous carbons are easily produced.

Oxidatively stable polyaniline:polyacid electrodes for electrochemical energy storage

Conjugated polymers, such as polyaniline, have been widely explored as sensors, electrodes, and conductive fillers. As an electrode material in electrochemical energy storage systems, polyaniline can be subject to irreversible oxidation that reduces cycle life and electrode capacity, thus, limiting its widespread application. Here we present a simple route to produce and prepare polyaniline-based electrodes that are oxidatively stable up to 4.5 V vs. Li/Li(+). The route uses a polyacid to stabilize the fully oxidized pernigraniline salt form of polyaniline, which is normally highly unstable as a homopolymer. The result is an organic electrode of exceptionally high capacity, energy density, power density, and cycle life. We demonstrate that the polyaniline:polyacid electrode stores 230 mA h g(-1) of polyaniline for over 800 cycles, far surpassing homopolymer polyaniline under equivalent conditions. This approach provides a highly stable, electrochemically reversible replacement for conventional polyaniline.

Charge Storage in Polymer Acid-Doped Polyaniline-Based Layer-by-Layer Electrodes

Polymeric electrodes that can achieve high doping levels and store charge reversibly are desired for electrochemical energy storage because they can potentially achieve high specific capacities and energies. One such candidate is the polyaniline:poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PANI:PAAMPSA) complex, a water-processable complex obtained via template polymerization that is known to reversibly achieve high doping levels at potentials of up to 4.5 V versus Li/Li+. Here, for the first time, PANI:PAAMPSA is successfully incorporated into layer-by-layer (LbL) electrodes. This processing technique is chosen for its ability to blend species on a molecular level and its ability to conformally coat a substrate. Three different polyaniline-based LbL electrodes comprised of PANI/PAAMPSA, PANI/PANI:PAAMPSA, and linear poly(ethylenimine)/PANI:PAAMPSA are compared in terms of film growth, charge storage, and reversibility. We found that the reversibility of PANI:PAAMPSA is retained within the LbL electrodes and that the PANI/PANI:PAAMPSA electrode exhibits the best performance in terms of capacity and cycle life. These results provide general guidelines for the assembly of PANI:PAAMPSA in LbL films and also demonstrate their potential as electrochemically active components in electrodes.

Polyaniline nanofiber/electrochemically reduced graphene oxide layer-by-layer electrodes for electrochemical energy storage

Graphene-containing layer-by-layer (LbL) electrodes are promising for thin film electrochemical energy storage. However, common practice centers on assembly with chemically reduced graphene oxide sheets, which have a tendency to severely aggregate during processing. More direct and facile is the LbL assembly of graphene oxide (GO) sheets and their subsequent electrochemical reduction. Here, we demonstrate porous (void fraction = 0.625) LbL electrodes comprised of electrochemically reduced GO (ERGO) sheets and polyaniline nanofibers (PANI NFs) for use in non-aqueous energy storage systems. Our approach is also promising for deposition onto complex surfaces, as demonstrated here by the successful assembly onto cotton fabric. Both PANI NFs and ERGO sheets store charge, bear conductivity, and provide a highly porous architecture, which facilitates the mass transport of ions. The nature of PANI NF/GO LbL assembly and growth is first presented, which we find to be affected by assembly pH. The confirmation of the electrochemical reduction step is then discussed, followed by the electrochemical performance of the resulting electrodes in a non-aqueous lithium metal battery. Capacity varies from 85 to 184 mA h cm−3 (188 to 461 mA h g−1) at 0.1 A g−1 (electrode mass basis), depending on the electrode thickness. The highest specific energy measured was 1395 mW h g−1 at a specific power of 1590 mW g−1, and the highest specific power was 60 252 mW g−1 at a specific energy of 927 mW h g−1. These results demonstrate that electroactive polyaniline nanofiber/graphene coatings from aqueous layer-by-layer assembly are attainable for energy storage.

Electric field induced morphological transitions in polyelectrolyte multilayers.

In this work, the morphological transitions in weak polyelectrolyte (PE) multilayers (PEMs) assembled from linear poly(ethylene imine) (LPEI) and poly(acrylic acid) (PAA) upon application of an electric field were studied. Exposure to an electric field results in the creation of a porous structure, which can be ascribed to local changes in pH from the hydrolysis of water and subsequent structural rearrangements of the weak PE constituents. Depending on the duration of application of the field, the porous transition gradually develops into a range of structures and pore sizes. It was discovered that the morphological transition of the LbL films starts at the multilayer-electrode interface and propagates through the film. First an asymmetrical structure forms, consisting of microscaled pores near the electrode and nanoscaled pores near the surface in contact with the electrolyte solution. At longer application of the field the porous structures become microscaled throughout. The results revealed in this study not only demonstrate experimental feasibility for controlling variation in pore size and porosity of multilayer films but also deepens the understanding of the mechanism of the porous transition. In addition, electrical potential is used to release small molecules from the PEMs.

Polyaniline nanofiber/vanadium pentoxide sprayed layer-by-layer electrodes for energy storage

Layer-by-layer assembly, as a low-cost process to create high-performance coatings, has been widely studied over the past 20 years. However, conventional layer-by-layer assembly is not well suited to large-area, large-scale and rapid application because of the long time scale required to complete a multilayer coating. Here, we develop a simple, water-based, rapid spray-on method to produce and prepare polyaniline/vanadium pentoxide layer-by-layer thin film cathodes for Li-ion batteries. This method uses spray-assisted LbL assembly, which is suitable to coating over large areas rapidly. The result is a water-processable hybrid cathode with high capacity (up to 232 mA h g−1 at a discharge current of 5 μA cm−2), specific energy (up to 650 mW h g−1 at a discharge current of 0.5 μA cm−2), specific power (up to 3395 mW g−1 at a discharge current of 25 μA cm−2), and good cycle life. The performance is dependent on thickness and discharge rate. Compared to the traditional polyaniline/vanadium pentoxide prepared by dipping at a rate of 0.0373 nm s−1, sprayed electrodes grow at a significantly high rate of 0.42 nm s−1 – 11 times faster. This approach demonstrates the rapid layer-by-layer assembly of Li-ion battery electrodes without sacrificing performance.

Design of Hybrid Electrochromic Materials with Large Electrical Modulation of Plasmonic Resonances

We present a rational approach to fabricating plasmonically active hybrid polymer-metal nanomaterials with electrochemical tunability of the localized surface plasmon resonances (LSPRs) of noble metal nanostructures embedded in an electroactive polymer matrix. The key requirement for being able to significantly modulate the LSPR band position is a close overlap between the refractive index change [Δn(λ)] of a stimuli-responsive polymeric matrix and the intrinsic LSPR bands. For this purpose, gold nanorods with a controlled aspect ratio, synthesized to provide high refractive index sensitivity while maintaining good oxidative stability, were combined with a solution-processable electroactive and electrochromic polymer (ECP): alkoxy-substituted poly(3,4-propylenedioxythiophene) [PProDOT(CH2OEtHx)2]. Spectral characteristics of the ECP, in particular the Δn(λ) variation, were evaluated as the material was switched between oxidized and reduced states. We fabricated ultrathin plasmonic electrochromic hybrid films consisting of gold nanorods and ECP that exhibited a large, stable, and reversible LSPR modulation of up to 25-30 nm with an applied electrical potential. Finite-difference time-domain (FDTD) simulations confirm a good match between the experimentally measured refractive index change in the ECP and the plasmonic response during electrochemical modulations.

Spray-On Polyaniline/Poly(acrylic acid) Electrodes with Enhanced Electrochemical Stability.

Polyaniline (PANI)-based electrodes are promising candidates for energy storage, but their cycle life remains poor. Recent work suggests that secondary interactions may enhance polyanilines electrochemical stability and cycle life, but evidence to date is not conclusive. Here, we investigate spray-assisted layer-by-layer assemblies containing polyaniline nanofibers (PANI NFs) or conventional PANI and poly(acrylic acid) (PAA), which provides hydrogen bonding and electrostatic interactions. This spray-on approach may be suitable for the deposition of PANI onto a variety of surfaces. The effects of PANI type, PAA pH, and PAA molecular weight on the growth behavior, conductivity, and electrochemical performance are examined. It is shown that LbL films with PANI NFs, higher molecular weight PAA, and lower PAA pH yield the thickest films, whereas the thinnest films come from conventional PANI assembled under similar conditions. Electron microscopy imaging and density measurements show that LbL films containing PANI NFs are very porous, whereas those containing conventional PANI are very dense (0.28 vs 1.33 g/cm(3), respectively). The difference in density dramatically affects the electrochemical properties in terms of capacity and long-term cycling behavior. Upon extended cycling, PANI NFs alone rapidly lose their electrochemical activity. On the other hand, PANI NF-based LbL films exhibited somewhat enhanced stability, and PANI-based LbL films were exceptionally stable, maintaining 94.7% of their capacity after 1000 cycles when cycled up to 4.2 V vs Li/Li(+). These results show that secondary interactions from PAA enhance stability, as does the selection of PANI type and the electrodes density.

The effect of plasmon resonance coupling in P3HT-coated silver nanodisk monolayers on their optical sensitivity

We report on the optical properties of silver nanodisk (Ag ND) Langmuir Blodgett monolayers that were transferred to substrates in different coupling regimes. Ag ND monolayers deposited in the liquid expanded–gaseous (Le–G) phase demonstrated individual plasmon resonance behavior while monolayers deposited in the liquid condensed–liquid expanded (Lc–Le) and solid–liquid condensed (S–Lc) phases exhibited plasmon coupling between closely packed adjacent nanoparticles, which caused a red shift in their localized surface plasmon resonance (LSPR) spectra. The initial presence of excess polyvinylpyrrolidone (PVP) surfactant micelles on the Ag ND monolayers could be eliminated by first compressing the monolayers to high surface pressures, resulting in blue shifted extinction spectra and increased sensitivity as micelles depleted into the subphase. Ag ND monolayers were then used in conjunction with a conjugated poly(3-hexylthiophene-2,5-diyl) (P3HT) medium to reversibly modulate the LSPR by changing the local refractive index around the nanoparticles. Ultimately, a high reversible LSPR shift of 27 nm was observed with an applied electropotential of ±500 mV to the P3HT-coated Ag ND monolayer. A high refractive index sensitivity (RIS) of 141 nm per RIU was found for monolayers deposited in the Lc–Le phase due to an increase in hot spot formation.

Recent developments in bio-monitoring via advanced polymer nanocomposite-based wearable strain sensors

Recent years, an explosive growth of wearable technology has been witnessed. A highly stretchable and sensitive wearable strain sensor which can monitor motions is in great demand in various fields such as healthcare, robotic systems, prosthetics, visual realities, professional sports, entertainments, etc. An ideal strain sensor should be highly stretchable, sensitive, and robust enough for long-term use without degradation in performance. This review focuses on recent advances in polymer nanocomposite based wearable strain sensors. With the merits of highly stretchable polymeric matrix and excellent electrical conductivity of nanomaterials, polymer nanocomposite based strain sensors are successfully developed with superior performance. Unlike conventional strain gauge, new sensing mechanisms include disconnection, crack propagation, and tunneling effects leading to drastically resistance change play an important role. A rational choice of materials selection and structure design are required to achieve high sensitivity and stretchability. Lastly, prospects and challenges are discussed for future polymer nanocomposite based wearable strain sensor and their potential applications.