Min Eui Lee

Hierarchically porous carbon nanosheets from waste coffee grounds for supercapacitors.

The nanostructure design of porous carbon-based electrode materials is key to improving the electrochemical performance of supercapacitors. In this study, hierarchically porous carbon nanosheets (HP-CNSs) were fabricated using waste coffee grounds by in situ carbonization and activation processes using KOH. Despite the simple synthesis process, the HP-CNSs had a high aspect ratio nanostructure (∼20 nm thickness to several micrometers in lateral size), a high specific surface area of 1945.7 m(2) g(-1), numerous heteroatoms, and good electrical transport properties, as well as hierarchically porous characteristics (0.5-10 nm in size). HP-CNS-based supercapacitors showed a specific energy of 35.4 Wh kg(-1) at 11250 W kg(-1) and of 23 Wh kg(-1) for a 3 s charge/discharge current rate corresponding to a specific power of 30000 W kg(-1). Additionally, the HP-CNS supercapacitors demonstrated good cyclic performance over 5000 cycles.

Cellulose nanofiber-reinforced silk fibroin composite film with high transparency

We successfully prepared optically transparent silk fibroin-cellulose nanofiber (CN) composite films from solvent casting using a stable CN suspension in an aqueous silk fibroin solution. The transmittance of the silk fibroin composite films was observed by a UV-visible spectrophotometer. The secondary structural change of the silk fibroin caused by the incorporation of CNs was characterized using Fourier transform infrared spectroscopy. A tensile test was carried out to investigate the mechanical properties. The results showed that the composite film exhibited visible-light transmittance of 75 %, and its mechanical strength and Young’s modulus were increased by 44 % and 35 %, respectively, as compared to a neat silk fibroin film.

Pentacene crystal formation on the surface of silk fibroin films

Silk fibroin (SF) has attracted interest as a gate dielectric due to its electrical insulation and high mobility in pentacene based organic thin film transistors (OTFTs). In this research, the surface energy of SF is controlled by water annealing, ethanol, and methanol solution treatments in order to study the effect of pentacene morphology on SF thin films with various treatments. For different crystallization methods, the crystal structures and surface energies of SF were investigated in detail by FT-IR and contact angle. Methanol treated SF thin film has a lower surface energy than the other two thin films. Topologies of pentacene on the SF thin films with various surface energies were obtained by atomic force microscopy (AFM). The AFM results showed that the smallest grain size of pentacene was that on methanol treated SF thin film which demonstrated that methanol treated SF thin film can be a proper candidate for a gate dielectric in OTFTs.

Synergistic catalytic effects of oxygen and nitrogen functional groups on active carbon electrodes for all-vanadium redox flow batteries

All-vanadium redox flow batteries (VRFBs) have attracted a great deal of attention as large-scale energy storage devices. However, VRFBs suffer from unfavorable energy efficiency, originating from large polarization effects that influence the redox reaction of the vanadium ions. Although the catalytic effects of the oxygen and nitrogen functional groups in carbon-based electrodes on the redox reactions of the vanadium ion have been documented in several reports, the specific catalytic effects are not understood due to the lack of systematic studies. In this study, pyroproteins having dual functionality (i.e., nitrogen and oxygen functional groups) were coated onto the surface of conventional carbon felts (the composite is hereafter termed P-CFs) and their catalytic effects on the V2+/V3+ redox reaction were characterized and compared with those of carbon felts possessing a coating layer with only oxygen or nitrogen functional groups. The synergistic catalytic effects of nitrogen and oxygen in the pyridonic structure of the P-CFs led to the lowest anodic and cathodic peak potential separation of ∼0.17 V among all the evaluated samples, having various chemical configurations with oxygen or nitrogen only. This study provides insight for the design of carbon-based active electrode materials in VRFBs and their feasibility was demonstrated in VRFB full-cells.

Morphologies and surface properties of cellulose-based activated carbon nanoplates

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Pyrolytic Carbon Nanosheets for Ultrafast and Ultrastable Sodium‐Ion Storage

Na-ion cointercalation in the graphite host structure in a glyme-based electrolyte represents a new possibility for using carbon-based materials (CMs) as anodes for Na-ion storage. However, local microstructures and nanoscale morphological features in CMs affect their electrochemical performances; they require intensive studies to achieve high levels of Na-ion storage performances. Here, pyrolytic carbon nanosheets (PCNs) composed of multitudinous graphitic nanocrystals are prepared from renewable bioresources by heating. In particular, PCN-2800 prepared by heating at 2800 °C has a distinctive sp2 carbon bonding nature, crystalline domain size of ≈44.2 Å, and high electrical conductivity of ≈320 S cm-1 , presenting significantly high rate capability at 600 C (60 A g-1 ) and stable cycling behaviors over 40 000 cycles as an anode for Na-ion storage. The results of this study show the unusual graphitization behaviors of a char-type carbon precursor and exceptionally high rate and cycling performances of the resulting graphitic material, PCN-2800, even surpassing those of supercapacitors.

Sericin-derived activated carbon-loaded alginate bead: An effective and recyclable natural polymer-based adsorbent for methylene blue removal

Activated carbon has been widely used as an effective adsorbent for removing contaminants from the water stream. Preparation of activated carbon using agricultural by-products is environmentally friendly and can greatly contribute to the virtuous cycle of natural polymers. In this study, highly porous activated carbon was prepared using silk sericin, a secondary protein of the sericulture industry. For easy processability and regeneration stability, the bead-type adsorbent was prepared using alginate (Alg) as a matrix. After that, methylene blue (MB) removal behavior of sericin-derived activated carbon (S-AC)/Alg beads was investigated. S-AC obtained by NaOH chemical activation had a larger BET surface area (2150.1 m2/g), and this porous structure of S-AC was well maintained after S-AC/Alg bead preparation (1215.4 m2/g). S-AC/Alg beads had an excellent MB adsorption capacity (502.5 mg/g) with stable regeneration stability, and 90.1% of the original removal efficiency was maintained after 5 cycles of the repeated adsorption-desorption process. These findings reveal that S-AC/Alg composite beads can be used as low-cost adsorbents for the removal of contaminants in aqueous solutions.

Surface-Modified Cellulose Nanocrystal-incorporated Poly(butylene succinate) Nanocomposites

In this work, surface acetylation of cellulose nanocrystals was performed to improve their interfacial adhesion with hydrophobic polymer matrix and to restore their thermal stability by removing the sulfate groups. The morphological, chemical, and thermal characteristics of the surface-modified cellulose nanocrystals (ACNs) were confirmed by field emission-transmission electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy. Furthermore, poly(butylene succinate) (PBS)/ACNs nanocomposites were also prepared via melt-mixing process, and the reinforcing effects of ACNs on the thermal, mechanical, and biodegradable properties of the nanocomposites were investigated. The Young’s modulus and tensile strength of the PBS/ACN nanocomposites increased from 115.36 and 33.67 MPa for the neat PBS to 130.55 MPa and 39.97 MPa, respectively. The thermal stability and biodegradability of the nanocomposites also increased with increasing ACN content.