Jooheon Kim

LnBaCo2O5 + δ Oxides as Cathodes for Intermediate-Temperature Solid Oxide Fuel Cells

LnBaCo 2 O 5+δ (Ln = Nd, Sm, Gd, and Y) oxides with a cation ordered perovskite structure have been investigated as cathode materials for intermediate-temperature solid oxide fuel cells (SOFCs). The oxygen content 5 + δ, thermal expansion coefficient (TEC), and electrical conductivity (metallic) decrease with decreasing size of the Ln 3+ ions from Ln = La to Y. While the decrease in TEC is due to the decreasing ionicity of the Ln-O bond, the decrease in electrical conductivity is due to the increasing oxide ion vacancies and a bending of the O-Co-O bonds. The power density of single-cell SOFCs fabricated with the LnBaCo 2 Ο 5+δ cathodes, La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 Ο 2.8 electrolyte, and Ni-Ce 0.9 Gd 0.1 Ο 1.95 cermet anode decrease with decreasing size of the Ln 3+ ions, partly due to a decreasing electrical conductivity. The LnBaCo 2 Ο 5+δ cathodes with an intermediate lanthanide ion such as Sm 3+ offer a trade-off between catalytic activity and TEC.

Characterization of GdBa1 − x Sr x Co2O5 + δ ( 0 ⩽ x ⩽ 1.0 ) Double Perovskites as Cathodes for Solid Oxide Fuel Cells

The effect of Sr2 + substitution for Ba 2+ on the crystal chemistry, oxygen content, thermal expansion, electrical conductivity, and catalytic activity for oxygen reduction reaction (ORR) of the double perovskite oxides GdBa 1-x Sr x Co 2 O 5+δ has been investigated for 0 ≤ x ≤ 1.0. The GdBa 1-x SrCo 2 O 5+δ system exhibits a structural change from orthorhombic (x = 0) to tetragonal (0.2 as x ≤ 0.6) to orthorhombic (x = 1) with increasing Sr content. The difference in ionic radii between (Ba 1-x Sr x ) 2+ and Gd 3+ plays a dominant role in determining the oxygen-content value in GdBa 1-x Sr X Co 2 O 5+δ , and the oxygen content and the oxidation state of cobalt increase with increasing Sr content. The electrical conductivity of the GdBa 1-x Sr x Co 2 O 5+δ system increases with Sr content due to an increasing oxygen content and a straightening of the O-Co-O bonds as evidenced by the structural change from orthorhombic to tetragonal. Sr substitution also improves the chemical stability of the GdBa 1-x Sr x Co 2 O 5+δ cathodes in contact with the Ce 0.9 Gd 0.1 O 1.95 and La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 2.8 electrolytes at 1100°C. The x = 0.2 and 0.6 samples with a tetragonal structure exhibit higher catalytic activity for ORR in single-cell solid oxide fuel cell than the x = 0 and 1.0 samples with an orthorhombic structure.

High Temperature Crystal Chemistry and Oxygen Permeation Properties of the Mixed Ionic–Electronic Conductors LnBaCo2O5 + δ ( Ln = Lanthanide )

The high temperature crystal chemistry and oxygen permeation properties of the cation-ordered LnBaCo 2 O 5+δ perovskite oxides [lanthanide (Ln) = Pr, Nd, and Sm] have been investigated in comparison with the cation-disordered La 0.5 Ba 0.5 CoO 3-δ perovskite. The LnBaCo 2 O 5+δ (Ln = Pr, Nd, and Sm) oxides exhibit a metal-insulator transition at T 350°C in air, as evidenced by an orthorhombic to tetragonal transition. At a given temperature, the oxygen permeation flux decreases from Ln = La to Nd to Sm due to the changes in crystal symmetry and lattice strain. The oxygen permeation mechanism in the Ln = Nd is bulk-diffusion-limited rather than surface-exchange-limited for membrane thickness L ≥ 1.1 mm.

Chemically Exfoliated SnSe Nanosheets and Their SnSe/Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) Composite Films for Polymer Based Thermoelectric Applications

UNLABELLED Tin selenide (SnSe) nanosheets (NSs) are prepared by hydrothermal lithium-intercalation and a subsequent exfoliation process from a SnSe ingot. Conducting polymer poly(3,4-ethylenedioxythiohene):poly(styrenesulfonate) ( PEDOT PSS)-based thermoelectric composites are fabricated with varying SnSe NSs content, and the thermoelectric properties of the composites are examined at 300 K. The exfoliated SnSe particles show thin two-dimensional sheet-like structures that are evenly distributed into the PEDOT PSS matrix. The significantly enhanced power factor (S(2)·σ) of the SnSe NS/PEDOT:PSS composites with increasing SnSe NSs content can be explained by the potential difference at the interface between the SnSe and PEDOT PSS. The fabricated SnSe NS/PEDOT:PSS composites show a maximum figure of merit (ZT) of 0.32 at a SnSe NSs loading of 20 wt %. The mixing of inorganic nanoparticles with the conducting polymer matrix forms products with extremely low thermal conductivities, which is a promising strategy for the realization of polymer based efficient thermoelectric applications.

The thermal conductivity of embedded nano-aluminum nitride-doped multi-walled carbon nanotubes in epoxy composites containing micro-aluminum nitride particles.

Amino-functionalized nano-aluminum nitride (nano-AlN) particles were doped onto the surfaces of chlorinated multi-walled carbon nanotubes (MWCNTs) to act as fillers in thermally conducting composites. These synthesized materials were embedded in epoxy resin. Then, the untreated micro-aluminum nitride (micro-AlN) particles were added to this resin, whereby the composites filled with nano-AlN-doped MWCNTs (0, 0.5, 1, 1.5, 2 wt%) and micro-AlN (25.2, 44.1, 57.4 vol%) were fabricated. As a result, the thermal diffusivity and conductivity of all composites continuously improved with increasing nano-AlN-doped MWCNT content and micro-AlN filler loading. The thermal conductivity reached its maximum, which was 31.27 times that of the epoxy alone, when 2 wt% nano-AlN-doped MWCNTs and 57.4 vol% micro-AlN were added to the epoxy resin. This result is due to the high aspect ratio of the MWCNTs and the surface polarity of the doped nano-AlN and micro-AlN particles, resulting in the improved thermal properties of the epoxy composite.

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.

Effect of homogeneous Al(OH)3 covered MWCNT addition on the thermal conductivity of Al2O3/epoxy-terminated poly(dimethylsiloxane) composites

Aluminum hydroxide covered multiwalled carbon nanotubes (A-MWCNTs) were synthesized as a conducting additive to alumina-epoxy-terminated poly(dimethylsiloxane). The measured diffusivity and calculated conductivity exhibited dissimilar behavior between several Al2O3 concentrations as a function of A-MWCNT loading, which correlated with the interface density and interconnectivity of the structures. The fabricated heterostructured A-MWCNT did not have a significant effect on the thermal conductivity of the composite because of phonon scattering at the interface. A small amount of A-MWCNT was feasible for establishment of a heat conductive percolating network with the greatest enhancement of thermal conductivity and diffusivity at an A-MWCNT loading of 1.0 and 2.0 wt%. Continuously increasing thermal transport properties were observed with the 49.1 vol.% Al2O3 loading which derived from a lower interface density nanowire and polymer matrix with enhanced interconnectivity.

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.

Dispersion, hybrid interconnection and heat dissipation properties of functionalized carbon nanotubes in epoxy composites for electrically conductive adhesives (ECAs)

Different types of MWNTs/epoxy composites were prepared with diglycidyl ether of bisphenol F (DGEBF) and bisphenol A (DGEBA) used as epoxy resins. MWNTs were functionalized to enhance the properties of epoxy composites by treatment with strong acids (acid-treated MWNTs, a-MWNTs) followed by m-phenylenediamine grafting (amine grafted MWNTs, m-MWNTs). Raw, a-, and m-MWNTs were dispersed in DGEBF or DGEBA to a concentration of 1 wt.%. X-ray photoelectron spectroscopy and thermogravimetric analysis verified the effectiveness of acid treatment and confirmed the amine-functionalization of the MWNTs. Scanning electron microscopy of the fracture surface of the epoxy matrix showed that chemical functionalization improves compatibility between the epoxy and MWNTs. Good dispersion of MWNTs leads to the improvement in coalescence and pull strength in the quad flat package (QFP) test. Further, the thermal conductivity of MWNTs/epoxy composites was higher than that of pure epoxy resins. In particular, the m-MWNT/epoxy composite has the best heat dissipation properties, due to the formation of an effective network for heat flow.

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.