Gi Dae Park

One‐Pot Synthesis of CoSex–rGO Composite Powders by Spray Pyrolysis and Their Application as Anode Material for Sodium‐Ion Batteries

A simple one-pot synthesis of metal selenide/reduced graphene oxide (rGO) composite powders for application as anode materials in sodium-ion batteries was developed. The detailed mechanism of formation of the CoSe(x)-rGO composite powders that were selected as the first target material in the spray pyrolysis process was studied. The crumple-structured CoSe(x)-rGO composite powders prepared by spray pyrolysis at 800 °C had a crystal structure consisting mainly of Co0.85 Se with a minor phase of CoSe2. The bare CoSe(x) powders prepared for comparison had a spherical shape and hollow structure. The discharge capacities of the CoSe(x)-rGO composite and bare CoSe(x) powders in the 50th cycle at a constant current density of 0.3 A g(-1) were 420 and 215 mA h g(-1), respectively, and their capacity retentions measured from the second cycle were 80 and 46%, respectively. The high structural stability of the CoSe(x)-rGO composite powders for repeated sodium-ion charge and discharge processes resulted in superior sodium-ion storage properties compared to those of the bare CoSe(x) powders.

MoSe2 Embedded CNT-Reduced Graphene Oxide Composite Microsphere with Superior Sodium Ion Storage and Electrocatalytic Hydrogen Evolution Performances

Highly porous MoSe2-reduced graphene oxide-carbon nanotube (MoSe2-rGO-CNT) powders were prepared by a spray pyrolysis process. The synergistic effect of CNTs and rGO resulted in powders containing ultrafine MoSe2 nanocrystals with a minimal degree of stacking. The initial discharge capacities of MoSe2-rGO-CNT, MoSe2-CNT, MoSe2-rGO, and bare MoSe2 powders for sodium ion storage were 501.6, 459.7, 460.2, and 364.0 mA h g-1, respectively, at 1.0 A g-1. The MoSe2-rGO-CNT composite powders had superior cycling and rate performances compared with the MoSe2-CNT, MoSe2-rGO composite, and bare MoSe2 powders. The electrocatalytic activity of MoSe2-rGO-CNT in the hydrogen evolution reaction (HER) was also compared with that of MoSe2-CNT, MoSe2-rGO, and bare MoSe2. MoSe2-rGO-CNT composite powders exhibited an overpotential of 0.24 V at a current density of 10 mA cm-2, which was less than that of MoSe2-CNT (0.26 V at 10 mA cm-2), MoSe2-rGO (0.32 V at 10 mA cm-2), and bare MoSe2 (0.33 V at 10 mA cm-2). Tafel slopes for the MoSe2-rGO-CNT, MoSe2-CNT, MoSe2-rGO, and bare MoSe2 powders were 53, 76, 86, and 115 mV dec-1, respectively. Because a large electrochemical surface area and ultrafine MoSe2 nanocrystals, the MoSe2-rGO-CNT composite possesses more active sites than the MoSe2-CNT, MoSe2-rGO composite, and bare MoSe2 powders with extensive stacking and large crystalline size, which provide greater catalytic HER activity.

Na-ion Storage Performances of FeSex and Fe2O3 Hollow Nanoparticles-Decorated Reduced Graphene Oxide Balls prepared by Nanoscale Kirkendall Diffusion Process

Uniquely structured FeSex-reduced graphene oxide (rGO) composite powders, in which hollow FeSex nanoparticles are uniformly distributed throughout the rGO matrix, were prepared by spray pyrolysis applying the nanoscale Kirkendall diffusion process. Iron oxide-rGO composite powders were transformed into FeSex-rGO composite powders by a two-step post-treatment process. Metallic Fe nanocrystals formed during the first-step post-treatment process were transformed into hollow FeSex nanoparticles during the selenization process. The FeSex-rGO composite powders had mixed crystal structures of FeSe and FeSe2 phases. A rGO content of 33% was estimated from the TG analysis of the FeSex-rGO composite powders. The FeSex-rGO composite powders had superior sodium-ion storage properties compared to those of the Fe2O3-rGO composite powders with similar morphological characteristics. The discharge capacities of the FeSex- and Fe2O3-rGO composite powders for the 200th cycle at a constant current density of 0.3 A g−1 were 434 and 174 mA h g−1, respectively. The FeSex-rGO composite powders had a high discharge capacity of 311 mA h g−1 for the 1000th cycle at a high current density of 1 A g−1.

Multiphase and Double-Layer NiFe2O4@NiO-Hollow-Nanosphere-Decorated Reduced Graphene Oxide Composite Powders Prepared by Spray Pyrolysis Applying Nanoscale Kirkendall Diffusion

Multicomponent metal oxide hollow-nanosphere decorated reduced graphene oxide (rGO) composite powders are prepared by spray pyrolysis with nanoscale Kirkendall diffusion. The double-layer NiFe2O4@NiO-hollow-nanosphere decorated rGO composite powders are prepared using the first target material. The NiFe-alloy-nanopowder decorated rGO powders are prepared as an intermediate product by post-treatment under the reducing atmosphere of the NiFe2O4/NiO-decorated rGO composite powders obtained by spray pyrolysis. The different diffusion rates of Ni (83 pm for Ni(2+)) and Fe (76 pm for Fe(2+), 65 pm for Fe(3+)) cations with different radii during nanoscale Kirkendall diffusion result in multiphase and double-layer NiFe2O4@NiO hollow nanospheres. The mean size of the hollow NiFe2O4@NiO nanospheres decorated uniformly within crumpled rGO is 14 nm. The first discharge capacities of the nanosphere-decorated rGO composite powders with filled NiFe2O4/NiO and hollow NiFe2O4@NiO at a current density of 1 A g(-1) are 1168 and 1319 mA h g(-1), respectively. Their discharge capacities for the 100th cycle are 597 and 951 mA h g(-1), respectively. The discharge capacity of the NiFe2O4@NiO-hollow-nanosphere-decorated rGO composite powders at the high current density of 4 A g(-1) for the 400th cycle is 789 mA h g(-1).

Effect of esterification reaction of citric acid and ethylene glycol on the formation of multi-shelled cobalt oxide powders with superior electrochemical properties

In this study, for the first time, polymeric precursors have been used in the preparation of yolk-shell powders using a large-scale spray drying process. An esterification reaction between the carboxyl group of citric acid and the hydroxyl group of ethylene glycol inside the droplet produced organic polymers during the drying process of the droplet. During the spray drying process, the polymeric precursors enabled the formation of multi-shell cobalt oxide yolk-shell powders with superior electrochemical properties. The maximum number of shells of the particles in the yolk-shell powders post-treated at 300, 400, and 500 °C were six, five, and four, respectively. The initial discharge capacities of the cobalt oxide yolk-shell powders post-treated at 300, 400, and 500 °C were 1,188, 1,331, and 1,110 mAh·g−1, and their initial charge capacities were 868, 1,005, and 798 mAh·g−1, respectively. The discharge capacities of the powders post-treated at 300, 400, and 500 °C after 100 cycles were 815, 958, and 670 mAh·g−1, respectively, and their corresponding capacity retentions measured after the first cycles were 92%, 93%, and 82%, respectively. The pure phase Co3O4 yolk-shell powders post-treated at 400 °C had low charge transfer resistance and high lithium-ion diffusion rate.

Kilogram-Scale Synthesis of Pd-Loaded Quintuple-Shelled Co3O4 Microreactors and Their Application to Ultrasensitive and Ultraselective Detection of Methylbenzenes

We report the kilogram-scale, simple, and cost-effective synthesis of Pd-loaded quintuple-shelled Co3O4 microreactors by spray drying of aqueous droplets containing cobalt nitrate, palladium nitrate, citric acid, and ethylene glycol and subsequent heat treatment. Highly viscous gel spheres containing Co and Pd salts were successfully converted into multi thin-shelled Co3O4 reactors uniformly loaded with Pd catalysts by the sequential combustion of carbon and decomposition of the metal salts from the outer to the inner regions during one-step heat treatment. The responses (resistance ratio) of the Pd-loaded quintuple-shelled Co3O4 microreactors to 5 ppm toluene and p-xylene were 30.8 and 64.2, respectively, and the selectivity values to toluene and p-xylene against ethanol interference (response ratio) were 14.5 and 30.1, respectively. The unprecedented high response and selectivity were attributed to the effective dissociation of less reactive methylbenzenes into more active smaller species assisted both by catalytic Co3O4 and Pd during the prolonged retention within the microreactors. Kilogram-scale preparation of noble metal-loaded multishelled microreactors and their unique gas-sensing characteristics based on a novel microreactor concept can pave a new way to design of high-performance gas sensors for practical applications.

Rational Design and Synthesis of Extremely Efficient Macroporous CoSe2–CNT Composite Microspheres for Hydrogen Evolution Reaction

Uniquely structured CoSe2 -carbon nanotube (CNT) composite microspheres with optimized morphology for the hydrogen-evolution reaction (HER) are prepared by spray pyrolysis and subsequent selenization. The ultrafine CoSe2 nanocrystals uniformly decorate the entire macroporous CNT backbone in CoSe2 -CNT composite microspheres. The macroporous CNT backbone strongly improves the electrocatalytic activity of CoSe2 by improving the electrical conductivity and minimizing the growth of CoSe2 nanocrystals during the synthesis process. In addition, the macroporous structure resulting from the CNT backbone improves the electrocatalytic activity of the CoSe2 -CNT microspheres by increasing the removal rate of generated H2 and minimizing the polarization of the electrode during HER. The CoSe2 -CNT composite microspheres demonstrate excellent catalytic activity for HER in an acidic medium (10 mA cm-2 at an overpotential of ≈174 mV). The bare CoSe2 powders exhibit moderate HER activity, with an overpotential of 226 mV at 10 mA cm-2 . The Tafel slopes for the CoSe2 -CNT composite and bare CoSe2 powders are 37.8 and 58.9 mV dec-1 , respectively. The CoSe2 -CNT composite microspheres have a slightly larger Tafel slope than that of commercial carbon-supported platinum nanoparticles, which is 30.2 mV dec-1 .

One‐Pot Method for Synthesizing Spherical‐Like Metal Sulfide–Reduced Graphene Oxide Composite Powders with Superior Electrochemical Properties for Lithium‐Ion Batteries

A facile, one-pot method for synthesizing spherical-like metal sulfide-reduced graphene oxide (RGO) composite powders by spray pyrolysis is reported. The direct sulfidation of ZnO nanocrystals decorated on spherical-like RGO powders resulted in ZnS-RGO composite powders. ZnS nanocrystals with a size below 20 nm were uniformly dispersed on spherical-like RGO balls. The discharge capacities of the ZnS-RGO, ZnO-RGO, bare ZnS, and bare ZnO powders at a current density of 1000 mA g(-1) after 300 cycles were 628, 476, 230, and 168 mA h g(-1), respectively, and the corresponding capacity retentions measured after the first cycles were 93, 70, 40, and 21 %, respectively. The discharge capacity of the ZnS-RGO composite powders at a high current density of 4000 mA g(-1) after 700 cycles was 437 mA h g(-1). The structural stability of the highly conductive ZnS-RGO composite powders with ultrafine crystals during cycling resulted in excellent electrochemical properties.

Superior Lithium-Ion Storage Properties of Mesoporous CuO-Reduced Graphene Oxide Composite Powder Prepared by a Two-Step Spray-Drying Process.

Mesoporous CuO-reduced graphene oxide (rGO) composite powders were prepared by using a two-step spray-drying process. In the first step, hollow CuO powders were prepared from a spray solution of copper nitrate trihydrate with citric acid and were wet milled to obtain a colloidal spray solution. In the second step, spray drying of the colloidal solution that contained dispersed GO nanosheets produced mesoporous CuO-rGO composite powders with particle sizes of several microns. Thermal reduction of GO nanosheets to rGO nanosheets occurred during post-treatment at 300 °C. Initial discharge capacities of the hollow CuO, bare CuO aggregate, and CuO-rGO composite powders at a current density of 2 A g(-1) were 838, 1145, and 1238 mA h g(-1) , respectively. Their discharge capacities after 200 cycles were 259, 380, and 676 mA h g(-1) , respectively, and their corresponding capacity retentions measured from the second cycle were 67, 48, and 76 %, respectively. The mesoporous CuO-rGO composite powders have high structural stability and high conductivity because of the rGO nanosheets, and display good cycling and rate performances.

Characteristics of precursor powders of a nickel-rich cathode material prepared by a spray drying process using water-soluble metal salts

The electrochemical properties of LiNi0.8Co0.15Al0.05O2 as a cathode material prepared by a simple spray drying process are investigated. Citric acid, which is used as the chelating agent, enables the production of the lithium-containing precursor powders with uniform compositions from the water-soluble metal salts by a spray drying process. The post-treatment of the precursor powders under an atmosphere of oxygen results in a cathode material in the form of a powder with good electrochemical properties. The composition of the powders is determined by inductively coupled plasma analysis to be Li1.05Ni0.81Co0.15Al0.04O2. The discharge capacities of the powders post-treated at 750 °C after the 1st and 100th cycles are 188 and 178 mA h g−1, respectively. The simple spray drying process is successfully applied to the preparation of precursors of the cathode material with complex compositions. The electrochemical properties of the micron sized LiNi0.8Co0.15Al0.05O2 aggregates with a filled morphology prepared from the precursor powders obtained by the spray drying process are also investigated. The aggregated powders obtained by the second spray drying process show a discharge capacity of 161 mA h g−1 after 100 cycles.