Jong Min Won

Electrochemical properties of yolk-shell structured ZnFe2O4 powders prepared by a simple spray drying process as anode material for lithium-ion battery.

ZnFe2O4 yolk–shell powders were prepared by applying a simple spray-drying process. Dextrin was used as a drying additive and carbon source material, and thus played a key role in the preparation of the powders. The combustion of precursor powders consisting of zinc and iron salts and dextrin obtained by a spray-drying process produced the yolk–shell-structured ZnFe2O4 powders even at a low post-treatment temperature of 350°C. The ZnFe2O4 powders prepared from the spray solution without dextrin had a filled and pockmarked structure. The initial discharge capacities of the ZnFe2O4 yolk–shell and filled powders post-treated at 450°C at a current density of 500 mA g−1 were 1226 and 993 mA h g−1, respectively, and the corresponding initial Coulombic efficiencies were 74 and 58%. The discharge capacities of the ZnFe2O4 powders with yolk–shell and filled structures post-treated at 450°C after 200 cycles were 862 and 332 mA h g−1, respectively. The ZnFe2O4 yolk–shell powders with high structural stability during cycling had superior electrochemical properties to those of the powders with filled structure.

Synthesis and electrochemical properties of spherical and hollow-structured NiO aggregates created by combining the Kirkendall effect and Ostwald ripening.

The Kirkendall effect and Ostwald ripening were successfully combined to prepare uniquely structured NiO aggregates. In particular, a NiO-C composite powder was first prepared using a one-pot spray pyrolysis, which was followed by a two-step post-treatment process. This resulted in the formation of micron-sized spherical and hollow-structured NiO aggregates through a synergetic effect that occurred between nanoscale Kirkendall diffusion and Ostwald ripening. The discharge capacity of the spherical and hollow-structured NiO aggregates at the 500(th) cycle was 1118 mA h g(-1) and their capacity retention, which was measured from the second cycle, was nearly 100%. However, the discharge capacities of the solid NiO aggregates and hollow NiO shells were 631 and 150 mA h g(-1), respectively, at the 500(th) cycle and their capacity retentions, which were measured from the second cycle, were 63 and 14%, respectively. As such, the spherical and hollow-structured NiO aggregates, which were formed through the synergetic effect of nanoscale Kirkendall diffusion and Ostwald ripening, have high structural stability during cycling and have excellent lithium storage properties.

Electrochemical Properties of Yolk–Shell‐Structured Zn–Fe–S Multicomponent Sulfide Materials with a 1:2 Zn/Fe Molar Ratio

Yolk-shell-structured Zn-Fe-S multicomponent sulfide materials with a 1:2 Zn/Fe molar ratio were prepared applying a sulfidation process to ZnFe2O4 yolk-shell powders. The Zn-Fe-S powders had mixed sphalerite (Zn,Fe)S and hexagonal FeS crystal structures. The discharge capacities of the Zn-Fe-S powders sulfidated at 350 °C at a constant current density of 500 mA g(-1) for the first, second, and fiftieth cycles were 1098, 912, and 913 mA h g(-1), respectively. The powders exhibited a high discharge capacity of 602 mA h g(-1) even at the high current density of 10 A g(-1). The synergistic effect of yolk-shell structure and multicomponent composition improved the electrochemical properties of Zn-Fe-S powders.

Selective NO 2 Sensors Using MoS 2 -MoO 2 Composite Yolk-shell Spheres

Abstract The gas sensing characteristic of MoS 2 -MoO 2 composite yolk-shell spheres were investigated. MoO 3 -carbon compositespheres were prepared by ultrasonic spray pyrolysis of aqueous droplets containing Mo-source and sucrose in nitrogen,which were converted into MoO 3 yolk-shell spheres by heat treatment at 400 o C in air. Subsequently, MoS 2 -MoO 2 compositeyolk-shell spheres were prepared by the partial sulfidation of MoO 3 . The MoS 2 -MoO 2 composite yolk-shell spheres showedrelatively low and irreversible gas sensing characteristics at < 200 o C. In contrast, the sensor showed high and reversibleresponse (S=resistance ratio) to 5 ppm NO 2 (S = 14.8) at 250 o C with low cross-responses (S = 1.17-2.13) to other interferencegases such as ethanol, CO, xylene, toluene, trimethylamine, NH 3 , H 2 , and HCHO. The MoS 2 -MoO 2 composite yolk-shellspheres can be used as reliable sensors to detect NO 2 in a selective manner.Keywords: Gas sensors, MoS 2 Yolk-shell Spheres, NO 2 Sensor, Selectivity