Na Rae Kim

Citrus-Peel-Derived, Nanoporous Carbon Nanosheets Containing Redox-Active Heteroatoms for Sodium-Ion Storage

Advanced design of nanostructured functional carbon materials for use in sustainable energy storage systems suffers from complex fabrication procedures and the use of special methods and/or expensive precursors, limiting their practical applications. In this study, nanoporous carbon nanosheets (NP-CNSs) containing numerous redox-active heteroatoms (C/O and C/N ratios of 5.5 and 34.3, respectively) were fabricated from citrus peels by simply heating the peels in the presence of potassium ions. The NP-CNSs had a 2D-like morphology with a high aspect ratio of >100, high specific surface area of 1167 m(2) g(-1), and a large amount of nanopores between 1 and 5 nm. The NP-CNSs also had an electrical conductivity of 2.6 × 10(1) s cm(-1), which is approximately 50 times higher than that of reduced graphene oxide. These unique material properties resulted in superior electrochemical performance with a high specific capacity of 140 mAh g(-1) in the cathodic potential range. In addition, symmetric full-cell devices based on the NP-CNSs showed excellent cyclic performance over 100,000 repetitive cycles.

Long-Lasting Nb2O5-Based Nanocomposite Materials for Li-Ion Storage

Advanced nanostructured hybrid materials can help us overcome the electrochemical performance limitations of current energy storage devices. In this study, three-dimensional porous carbon nanowebs (3D-CNWs) with numerous included orthorhombic Nb2O5 (T-Nb2O5) nanoparticles were fabricated using a microbe-derived nanostructure. The 3D-CNW/T-Nb2O5 nanocomposites showed an exceptionally stable long-term cycling performance over 70 000 cycles, a high reversible capacity of ∼125 mA h g-1, and fast Li-ion storage kinetics in a coin-type two-electrode system using Li metal. In addition, energy storage devices based on the above nanocomposites achieved a high specific energy of ∼80 W h kg-1 together with a high specific power of ∼5300 W kg-1 and outstanding cycling performance with ∼80% capacitance retention after 35 000 cycles.

Pyroprotein‐Based Electronic Textiles with High Stability

Thermally reducible pyroprotein-based electronic textiles (e-textiles) are fabricated using graphene oxide and a pyroprotein such as cocoon silk and spider web without any chemical agents. The electrical conductivity of the e-textile is 11.63 S cm-1 , which is maintained even in bending, washing, and temperature variation.

Critical role of silk fibroin secondary structure on the dielectric performances of organic thin-film transistors

Silk fibroin (SF) is being considered as an emerging class of dielectric material in electronics, such as organic thin-film transistors (OTFTs). SF has several advantageous properties, including high transparency, flexibility, and solution-processibility. In this study, we investigated the effects of processing solvent and post-treatment on the surface properties and structural development of SF films, which were used in the gate-dielectric of OTFTs. The SF films cast from aqueous solution exhibited an amorphous structure with random coil conformations, leading to poor dielectric properties that were inadequate for developing OTFTs. In contrast, the use of formic acid solution and sequential methanol vapor treatment after film casting induced a smoother SF surface. Moreover, its crystallinity also improved dramatically. Furthermore, the SF films cast from formic acid solution and treated with methanol vapor were capable of growth of highly ordered organic semiconductor thin films deposited on these films. As a result, pentacene TFTs fabricated with the SF films showed high performance and stable operation with nearly zero turn-on voltage, negligible hysteresis, and high bias stability. This indicates a strong correlation between the structure of the SF film and dielectric performances of OTFTs. We believe that our study would be useful in designing silk materials, which are highly compatible with the human interface and other biological environments.

Dispersion stability of chemically reduced graphene oxide nanoribbons in organic solvents

In this study, the dispersion stability of graphene oxide nanoribbons (GONRs) and chemically reduced GONRs (CR-GONRs) in various organic solvents was investigated. Homogeneous colloidal suspensions of GONRs were obtained in hydrophilic solvents such as water, N,N-dimethylformamide (DMF), and N-methyl-2-pyrrolidone (NMP). After chemical reduction, amphiphilic dispersion behaviour of CR-GONRs was observed over a wide range of solvents including water, methanol, ethanol, DMF, NMP, acetone, styrene, and xylene. In contrast, the dispersion stability of CR-GONRs was poor in ambiguously hydrophilic or hydrophobic solvents such as tetrahydrofuran, methyl methacrylate, and chloroform.

Waste coffee grounds-derived nanoporous carbon nanosheets for supercapacitors

The development of nanostructured functional materials derived from biomass and/or waste is of growing importance for creating sustainable energy-storage systems. In this study, nanoporous carbonaceous materials containing numerous heteroatoms were fabricated from waste coffee grounds using a top-down process via simple heating with KOH. The nanoporous carbon nanosheets exhibited notable material properties such as high specific surface area (1960.1 m2 g–1), numerous redox-active heteroatoms (16.1 at% oxygen, 2.7 at% nitrogen, and 1.6 at% sulfur), and high aspect ratios (>100). These unique properties led to good electrochemical performance as supercapacitor electrodes. A specific capacitance of ~438.5 F g–1 was achieved at a scan rate of 2 mV s–1, and a capacitance of 176 F g–1 was maintained at a fast scan rate of 100 mV s–1. Furthermore, cyclic stability was achieved for over 2000 cycles.

Tin Sulfide‐Based Nanohybrid for High‐Performance Anode of Sodium‐Ion Batteries

Nanohybrid anode materials for Na-ion batteries (NIBs) based on conversion and/or alloying reactions can provide significantly improved energy and power characteristics, while suffering from low Coulombic efficiency and unfavorable voltage properties. An NIB paper-type nanohybrid anode (PNA) based on tin sulfide nanoparticles and acid-treated multiwalled carbon nanotubes is reported. In 1 m NaPF6 dissolved in diethylene glycol dimethyl ether as an electrolyte, the above PNA shows a high reversible capacity of ≈1200 mAh g-1 and a large voltage plateau corresponding to a capacity of ≈550 mAh g-1 in the low-voltage region of ≈0.1 V versus Na+ /Na, exhibiting high rate capabilities at a current rate of 1 A g-1 and good cycling performance over 250 cycles. In addition, the PNA exhibits a high first Coulombic efficiency of ≈90%, achieving values above 99% during subsequent cycles. Furthermore, the feasibility of PNA usage is demonstrated by full-cell tests with a reported cathode, which results in high specific energy and power values of ≈256 Wh kg-1 and 471 W kg-1 , respectively, with stable cycling.