Myeongjin Kim

Redox Deposition of Birnessite-Type Manganese Oxide on Silicon Carbide Microspheres for Use as Supercapacitor Electrodes

Silicon carbide microsphere/birnessite-type MnOx (SiC/B-MnOx) composites were prepared by removal of a SiO2 layer with redox deposition of birnessite-type MnOx for supercapacitor electrode materials. The characterization studies showed that the birnessite-type MnOx in the composite was homogeneously deposited on the SiC surface. The capacitive properties of the as-prepared SiC/B-MnOx electrodes were measured in a three-electrode system using 1 M Na2SO4(aq) as the electrolyte. The SiC/B-MnOx(6) electrode, fabricated using a MnOx/SiC feeding ratio of 6:1, displayed a specific capacitance of 251.3 F g(-1) at 10 mV s(-1). Such excellent electrochemical performance is attributed to an increase in the electrical conductivity in the presence of silicon carbide, an increase in the effective interfacial area between MnOx and the electrolyte, and the contact area between MnOx and silicon carbide. The deposition of birnessite-type MnOx on a SiC surface may be a prospective fabrication technique for electrode materials for supercapacitors.

Superior electric double layer capacitors using micro- and mesoporous silicon carbide sphere

Three-dimensional silicon carbide-based frameworks with hierarchical micro and mesoporous structures (MMPSiC) are prepared by employing the template method and carbonization reaction via the aerosol-spray drying method. The mesopores are generated by the self-assembly of a structure-directing agent, while the micropores are derived from the partial evaporation of Si atoms during the carbonization process. MMPSiC has a unique three-dimensionally interconnected micro and mesoporous network; it also exhibits a faster ion-transport behavior and a larger utilization of the surface area of the electric double-layer capacitors. MMPSiC shows a high-charge storage capacity, with a specific capacitance of 253.7 F g−1 in 1 M Na2SO4 aqueous electrolyte at a scan rate of 5 mV s−1. In addition, a specific capacitance of 40.3 F g−1 is measured in the 3-ethyl-3-methylimidazolium bis(trifluorosulfonyl)imide ionic-liquid electrolyte at a scan rate of 5 mV s−1, with an energy density of 68.56 W h kg−1; and ∼98.4% specific capacitance being retained over 20 000 cycles. Such a high supercapacitor performance may arise from a synergistic effect ensured by the dual-pore system, which can provide a large accessible surface area for ion transport/charge storage by the mesopores and a continuous increase of charge accommodation by micropores. These encouraging results demonstrate the great potential of MMPSiC as high-performance electrode materials for supercapacitors.

Effect of Al2O3 coverage on SiC particles for electrically insulated polymer composites with high thermal conductivity

Al2O3-covered SiC/epoxy composites were prepared using a simple sol–gel method. The results of FE-SEM, TGA, and XPS indicated that the surfaces of the SiC particles had a large, dense, and homogenous distribution of Al2O3. It was found that the introduction of Al2O3 on the SiC surface improved the interfacial adhesion between the epoxy matrix and SiC particles; this resulted in an increase in the thermal conductivity of the composites since the thermal boundary resistance at the filler–matrix interface was decreased. In addition, Al2O3-covered SiC composites showed decreased electrical conductivity owing to decreased electron tunneling compared with raw SiC composites. Thus, the Al2O3-covered SiC composites prepared in the present work could prove to be desirable polymer composites to be used as thermal interface materials that are employed in the electronics industry.

Enhancement of thermal and mechanical properties of flexible graphene oxide/carbon nanotube hybrid films though direct covalent bonding

The thermal conductivity and mechanical properties of graphene oxide/multiwalled carbon nanotube (GO/MWCNT) hybrid films with and without covalent bonding were examined. Chlorinated GO and amino-functionalized MWCNT were bonded covalently to fabricate chemically bonded GO/MWCNT hybrid films. Mixtures of surface-modified GO and MWCNT were filtered and then subjected to hot-pressing to fabricate stacked films. Examination of these chemically bonded hybrid films revealed higher thermal conductivity than in physically bonded hybrid films, because of the synergetic interaction of functional groups in GO and MWCNT in the films. However, the addition of excess MWCNT to the films led to an increased phonon scattering density and a decreased thermal conductivity. The hybrid films fabricated by the optimized process endured about 20000 bending cycles without rupturing or losing their thermal conductivity. The mechanical properties showed enhanced performance after increased MWCNT loading at elevated temperature due to the reinforcement effect of the MWCNT between GO layers.

Synergistic interaction between pseudocapacitive Fe3O4 nanoparticles and highly porous silicon carbide for high-performance electrodes as electrochemical supercapacitors

Composites of micro- and mesoporous SiC flakes (SiCF) and ferroferric oxide (Fe3O4), SiCF/Fe3O4, were prepared via the chemical deposition of Fe3O4 on SiCF by the chemical reduction of an Fe precursor. The SiCF/Fe3O4 electrodes were fabricated at different Fe3O4 feeding ratios to determine the optimal Fe3O4 content that can maintain a high total surface area of SiCF/Fe3O4 composites as well as cause a vigorous redox reaction, thereby maximizing the synergistic effect between the electric double-layer capacitive effects of SiCF and the pseudo-capacitive effects of Fe3O4. The SiCF/Fe3O4 electrode fabricated with a Fe3O4/SiCF feeding ratio of 1.5:1 (SiCF/Fe3O4(1.5)) exhibited the highest charge storage capacity, showing a specific capacitance of 423.2 F g-1 at a scan rate of 5 mV s-1 with a rate performance of 81.8% from 5 to 500 mV s-1 in an aqueous 1 M KOH electrolyte. The outstanding capacitive performance of the SiCF/Fe3O4(1.5) electrode could be attributed to the harmonious synergistic effect between the electric double-layer capacitive contribution of the SiCF and the pseudocapacitive contribution of the Fe3O4 nanoparticles introduced on the SiCF surface. These encouraging results demonstrate that the SiCF/Fe3O4(1.5) electrode is a promising high-performance electrode material for use in supercapacitors.

Synergistic interaction between embedded Co3O4 nanowires and graphene papers for high performance capacitor electrodes

Graphene/Co3O4 nanowire composite films were successfully synthesized using a simple, three-step treatment, and the effect of the Co3O4 nanowire content on the electrochemical properties of the composite films was studied. The one-dimensional Co3O4 nanowires were homogeneously embedded and dispersed between prepared graphene papers, forming a layered graphene/Co3O4 nanowire hybrid structure. These composite films exhibited better electrochemical properties than previously reported ones, such as graphene/CNT, where carbon spheres existed in the graphene composites, which were fabricated using the same method but without the Co3O4 nanowires. The addition of a small amount of Co3O4—typically 8 : 1 by weight (reduced graphene oxide (RGO) : Co3O4)—to form thick RGO/Co3O4 sandwiches in the form of papers resulted in an excellent specific charge capacity of 278.936 C g−1 at a scan rate of 5 mV s−1. These results indicate the potential of the composite for the development of highly capacitive energy storage devices for practical applications.

Highly efficient bifunctional catalytic activity of bismuth rhodium oxide pyrochlore through tuning the covalent character for rechargeable aqueous Na–air batteries

Na–air batteries have received significant attention as possible candidates for alternative battery systems due to their high specific energy density (1683 W h kg−1). However, the undesirable sluggish kinetics of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) limit the practicality of the production of rechargeable Na–air batteries. Recently, pyrochlore oxides (A2B2O7) have received great attention as effective bifunctional electrocatalysts. However, the comprehensive understanding of catalytic activity with the development of new pyrochlore catalysts is unsatisfactory due to the limited use of B-site cations. Here, we report the use of a novel nanocrystalline bismuth rhodium oxide (Bi2Rh2O6.8) crystal with a pyrochlore structure as a bifunctional electrocatalyst. Moreover, the surface of Bi2Rh2O6.8 was modified via phosphate-ion functionalization (P-Bi2Rh2O6.8) to enhance its surface chemical reactivity, resulting in fast and efficient charge transfer with high ORR and OER activities. During electrocatalysis, the functionalized H2PO4− ion can not only significantly enhance the surface reactivity for a fast and efficient electron/charge-transfer reaction but also facilitate the oxidation of Bi and Rh ions and boost the electron donation by improving the electron transport. Finally, the first successful translation of the bifunctional electrocatalytic activities of P-Bi2Rh2O6.8 to a practical device, an aqueous Na–air battery, was demonstrated.

Dependence of packing fraction and surface area of the particles in the composites made by the combination of aluminum oxide and nitride for improving the thermal conductivity

Aluminum oxide and aluminum nitride with different sizes were used alone or in combination to prepare thermally conductive polymer composites. Particle size can have an influence on the thermal conductivity of composites at the same volume loading, so the composites examined in this study were categorized into two systems. One included composites filled with large-sized aluminum nitride and small-sized aluminum oxide particles. The other included composites filled with large-sized aluminum oxide and small-sized aluminum nitride. The use of these hybrid fillers was found to be effective in increasing the thermal conductivity of the composite, which was probably due to the enhanced connectivity offered by the structuring filler. At total filler content above 53.5 vol.%, the maximum values of both thermal conductivities in the two systems were 3.402 W/mK and 2.842 W/mK, respectively, when the volume ratio of large particles to small particles was 7∶3. This result was represented when the composite was filled with the maximum packing density and the minimum surface area at the same volume content. As such, the proposed thermal model predicted thermal conductivity in good agreement with experimental values.