Doh Won Jung

Development of High Performance Ceria/Bismuth Oxide Bilayered Electrolyte SOFCs for Lower Temperature Operation

This study examines the development of lower temperature solid oxide fuel cells (SOFCs) and the incremental improvement in performance obtained from a wide range of techniques, from pressed anodes to tape-cast anodes, from gadolinia-doped ceria (GDC) single-layer electrolytes to erbium-stabilized bismuth oxide (ESB)/GDC bilayer, and from La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ -GDC composite cathodes to optimized Bi 2 Ru 2 O 7 -ESB composites. GDC single-layer electrolyte-based SOFCs were prepared from four different fabrications and exhibit maximum power densities ranging from 0.338 to 1.03 W/cm 2 at 650°C. At each fabrication stage, an ESB layer was applied to form a bilayer electrolyte. ESB was deposited by a range of techniques including colloidal deposition and pulsed laser deposition. The result confirms that depending on a fabrication route, the bilayer electrolyte can reduce the total area specific resistance (ASR) 33-49% and increase the maximum power density 44-93%. By using a combination of the materials and fabrication routes, a maximum power density of 1.95 W/cm 2 and 0.079 Ω cm 2 total cell ASR was achieved at 650°C for a bilayer cell.

Metallic conduction induced by direct anion site doping in layered SnSe2.

The emergence of metallic conduction in layered dichalcogenide semiconductor materials by chemical doping is one of key issues for two-dimensional (2D) materials engineering. At present, doping methods for layered dichalcogenide materials have been limited to an ion intercalation between layer units or electrostatic carrier doping by electrical bias owing to the absence of appropriate substitutional dopant for increasing the carrier concentration. Here, we report the occurrence of metallic conduction in the layered dichalcogenide of SnSe2 by the direct Se-site doping with Cl as a shallow electron donor. The total carrier concentration up to ~1020 cm−3 is achieved by Cl substitutional doping, resulting in the improved conductivity value of ~170 S·cm−1 from ~1.7 S·cm−1 for non-doped SnSe2. When the carrier concentration exceeds ~1019 cm−3, the conduction mechanism is changed from hopping to degenerate conduction, exhibiting metal-insulator transition behavior. Detailed band structure calculation reveals that the hybridized s-p orbital from Sn 5s and Se 4p states is responsible for the degenerate metallic conduction in electron-doped SnSe2.

Stabilization of high-cobalt-content perovskites for use as cathodes in solid oxide fuel cells

B-site modification of the high-cobalt-content perovskite Ba0.5Sr0.5Co0.8Fe0.2O3−δ reduces its thermal expansion coefficient and stabilizes its structure, allowing its long-term operation without degradation of the area-specific resistance, and thus, maintaining the high electrode performance.

Mechanistic Understanding of Cr Poisoning on La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF)

The effects of Cr poisoning on the catalytic activity of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) and the poisoning mechanism was investigated. AC impedance spectroscopy and temperature programmed isotopic exchange (TPX) were consistent in proving poisoning mainly degraded the surface catalytic activity as a passive layer evolved on the surface or near-surface region. The formation of small particles was clearly observed in scanning electron microscopy when LSCF was heat treated in the presence of Crofer22APU. Low angle X-ray diffraction identified these were SrCrO4. As a result of Sr diffusion to form SrCrO4 particles, it was found that a Sr-deficient phase was formed and a phase transition from rhombohedral to cubic perovskite occurred in the near surface region. While previous Cr poisoning studies on LSCF mostly emphasized the effects of SrCrO4 formation, the Sr deficient surface layer also significantly degrades the catalytic activity of LSCF due to reduction of oxygen vacancies and surface Co/Fe concentration.

Understanding the structural, electrical, and optical properties of monolayer h-phase RuO2 nanosheets: A combined experimental and computational study

The structural, electrical, and optical properties of monolayer ruthenium oxide (RuO2) nanosheets (NSs) fabricated by chemical exfoliation of a layered three-dimensional form of K-intercalated RuO2 are studied systematically via experimental and computational methods. Monolayer RuO2 NS is identified as having a distorted h-MX2 structure. This is the first observation of a RuO2 NS structure that is unlike the t-MX2 structure of the RuO2 layers in the parent material and does not have hexagonal symmetry. The distorted h-MX2 RuO2 NSs are shown to have optical transparency superior to that of graphene, thereby predicting the feasibility of applying RuO2 NSs to flexible transparent electrodes. In addition, it is demonstrated that the semiconducting band structures of RuO2 NSs can be manipulated to be semi-metallic by adjusting the crystal structure, which is related to band-gap engineering. This finding indicates that RuO2 NSs can be used in a variety of applications, such as flexible transparent electrodes, atomic-layer devices, and optoelectronic devices.Two-dimensional materials: A more transparent way to get in contactNanosheets of ruthenium oxide could make excellent transparent electrical contacts, show researchers from Korea. Graphene is the wonder material of the last decade owing to its amazing electrical, mechanical and thermal properties. Scientists are thus keen to fabricate single layers of atoms other than carbon. Now, Dong-Su Ko, Jung-Hwa Kim and Jong Wook Roh from the Samsung Advanced Institute of Technology and co-workers have combined experiments and theory to fully characterize this unusual two-dimensional material. They created their nanosheets by exfoliating a three-dimensional block of ruthenium oxide. Transmission electron microscopy, X-ray diffraction experiments and first-principles calculations showed that the two-dimensional material has a distorted atomic arrangement, which makes it a semiconductor rather than a metal like its parent material. Furthermore, ruthenium oxide nanosheets are more transparent than graphene, making them useful for flexible transparent electrodes.As a new two-dimensional (2D) material, monolayer ruthenium oxide (RuO2) nanosheets (NSs) have distorted h-MX2 type crystal structures that lead to semiconducting properties and good optical transmittance. This study suggests that monolayer RuO2 can be useful in applications of flexible optoelectronics.