Synthesis of Iron Sulfide Nanocrystals Encapsulated in Highly Porous Carbon-Coated CNT Microsphere as Anode Materials for Sodium-Ion Batteries.

Highly porous carbon materials with a rationally designed pore structure can be utilized as reservoirs for metal or nonmetal components. The use of small-sized metal or metal compound nanoparticles, completely encapsulated by carbon materials, has attracted significant attention as an effective approach to enhancing sodium ion storage properties. These materials have the ability to mitigate structural collapse caused by volume expansion during the charging process, enable short ion transport length, and prevent polysulfide elution. In this study, a concept of highly porous carbon-coated carbon nanotube (CNT) porous microspheres, which serve as excellent reservoir materials is suggested and a porous microsphere is developed by encapsulating iron sulfide nanocrystals within the highly porous carbon-coated CNTs using a sulfidation process. Furthermore, various sulfidation processes to determine the optimal method for achieving complete encapsulation are investigated by comparing the morphologies of diverse iron sulfide-carbon composites. The fully encapsulated structure, combined with the porous carbon, provides ample space to accommodate the significant volume changes during cycling. As a result, the porous iron sulfide-carbon-CNT composite microspheres exhibited outstanding cycling stability (293 mA h g-1 over 600 cycles at 1 A g-1 ) and remarkable rate capability (100 mA h g-1 at 5 A g-1 ).

Controllable Synthesis of Carbon Yolk-Shell Microsphere and Application of Metal Compound-Carbon Yolk-Shell as Effective Anode Material for Alkali-Ion Batteries.

Recently, nanostructured carbon materials, such as hollow-, yolk-, and core-shell-configuration, have attracted attention in various fields owing to their unique physical and chemical properties. Among them, yolk-shell structured carbon is considered as a noteworthy material for energy storage due to its fast electron transfer, structural robustness, and plentiful active reaction sites. However, the difficulty of the synthesis for controllable carbon yolk-shell has been raised as a limitation. In this study, novel synthesis strategy of nanostructured carbon yolk-shell microspheres that enable to control morphology and size of the yolk part is proposed for the first time. To apply in the appropriate field, cobalt compounds-carbon yolk-shell composites are applied as the anode of alkali-ion batteries and exhibit superior electrochemical performances to those of core-shell structures owing to their unique structural merits. Co3 O4 -C hollow yolk-shell as a lithium-ion battery anode exhibits a long cycling lifetime (619 mA h g-1 for 400 cycles at 2 A g-1 ) and excellent rate capability (286 mA h g-1 at 10 A g-1 ). The discharge capacities of CoSe2 -C hollow yolk-shell as sodium- and potassium-ion battery anodes at the 200th cycle are 311 mA h g-1 at 0.5 A g-1 and 268 mA h g-1 at 0.2 A g-1 , respectively.

Morphological and Electrochemical Properties of ZnMn2O4 Nanopowders and Their Aggregated Microspheres Prepared by Simple Spray Drying Process.

Phase-pure ZnMn2O4 nanopowders and their aggregated microsphere powders for use as anode material in lithium-ion batteries were obtained by a simple spray drying process using zinc and manganese salts as precursors, followed by citric acid post-annealing at different temperatures. X-ray diffraction (XRD) analysis indicated that phase-pure ZnMn2O4 powders were obtained even at a low post-annealing temperature of 400 °C. The post-annealed powders were transformed into nanopowders by simple milling process, using agate mortar. The mean particle sizes of the ZnMn2O4 powders post-treated at 600 and 800 °C were found to be 43 and 85 nm, respectively, as determined by TEM observation. To provide practical utilization, the nanopowders were transformed into aggregated microspheres consisting of ZnMn2O4 nanoparticles by a second spray drying process. Based on the systematic analysis, the optimum post-annealing temperature required to obtain ZnMn2O4 nanopowders with high capacity and good cycle performance was found to be 800 °C. Moreover, aggregated ZnMn2O4 microsphere showed improved cycle stability. The discharge capacities of the aggregated microsphere consisting of ZnMn2O4 nanoparticles post-treated at 800 °C were 1235, 821, and 687 mA h g-1 for the 1st, 2nd, and 100th cycles at a high current density of 2.0 A g-1, respectively. The capacity retention measured after the second cycle was 84%.

Yolk-Shell-Structured Nanospheres with Goat Pupil-Like S-Doped SnSe Yolk and Hollow Carbon-Shell Configuration as Anode Material for Sodium-Ion Storage.

Rationally nanostructured electrode materials exhibit excellent sodium-ion storage performance. In particular, yolk-shell configurations of metal chalcogenide@void@C are introduced in various synthetic strategies for use as superior anode materials. Herein, yolk-shell-structured nanospheres, with goat pupil-like configuration of S-doped SnSe yolks and hollow carbon shells, are synthesized by salt-infiltration and a simple post-treatment procedure. Impressively, the co-infiltration of thiourea and selenium oxide enables the doping of sulfur into SnSe (SnSeS) and carbon shells, as well as the formation of a goat pupil-like yolk-shell architecture. High-reactivity thiourea-derived H2 S gas forms nanocrystals inside the carbon nanospheres. The nanocrystals act as seeds for the crystal growth of SnSeS through Ostwald ripening. The unique yolk-shell structure and composition with a heterointerface provide not only structural stability but also fast electrode reaction kinetics during repeated cycling. The SnSeS@C electrode shows an excellent cycle life (186 mA h g-1 for 1000 cycles at 0.5 A g-1 ) and rate capability (112 mA h g-1 at 5.0 A g-1 ).

Conversion Reaction Mechanism of Ultrafine Bimetallic Co-Fe Selenides Embedded in Hollow Mesoporous Carbon Nanospheres and Their Excellent K-Ion Storage Performance.

Potassium-ion batteries (KIBs) are considered as promising alternatives to lithium-ion batteries owing to the abundance and affordability of potassium. However, the development of suitable electrode materials that can stably store large-sized K ions remains a challenge. This study proposes a facile impregnation method for synthesizing ultrafine cobalt-iron bimetallic selenides embedded in hollow mesoporous carbon nanospheres (HMCSs) as superior anodes for KIBs. This involves loading metal precursors into HMCS templates using a repeated "drop and drying" process followed by selenization at various temperatures, facilitating not only the preparation of bimetallic selenide/carbon composites but also controlling their structures. HMCSs serve as structural skeletons, conductive templates, and vehicles to restrain the overgrowth of bimetallic selenide particles during thermal treatment. Various analysis strategies are employed to investigate the charge-discharge mechanism of the new bimetallic selenide anodes. This unique-structured composite exhibits a high discharge capacity (485 mA h g-1 at 0.1 A g-1 after 200 cycles) and enhanced rate capability (272 mA h g-1 at 2.0 A g-1 ) as a promising anode material for KIBs. Furthermore, the electrochemical properties of various nanostructures, from hollow to frog egg-like structures, obtained by adjusting the selenization temperature, are compared.

Amorphous iron oxide-selenite composite microspheres with a yolk-shell structure as highly efficient anode materials for lithium-ion batteries.

Yolk-shell structured transition metal compounds have intrinsic structural advantages as anode materials and have been synthesized in a highly crystalline form. Thus, development of a synthesis process for a yolk-shell structure with an amorphous state, that displays high structural stability and fast ionic diffusion, is a notable research subject, with wide applications in fields such as energy storage. Herein, a novel approach for synthesizing amorphous materials with a yolk-shell structure using several facile phase transformation processes is presented. Crystalline iron oxide microspheres with a yolk-shell structure were formed by oxidation of the spray-dried product at 400 °C. Using the pitch/tetrahydrofuran solution infiltration method, pitch-infiltrated iron oxide was selenized at 350 °C to form a crystalline iron selenide-C composite. The following partial oxidation process at 375 °C produced a yolk-shell structured amorphous iron oxide-selenite composite. The amorphous characteristics, the yolk-shell structure, and the formation of a heterostructured interface from iron selenite during the initial cycle contributed to high electrochemical kinetic properties and excellent cycling performance of the iron oxide-selenite composite. The amorphous iron oxide-iron selenite yolk-shell microspheres exhibited enhanced reversible capacities, cycling stability, and remarkable electrochemical kinetic properties when compared to crystalline iron oxide.

Enhanced Li-ion storage performance of novel tube-in-tube structured nanofibers with hollow metal oxide nanospheres covered with a graphitic carbon layer.

One-dimensional (1D) nanofibers constructed with structurally stable nano-architecture and highly conductive carbon components can be employed to develop enhanced anodic materials for lithium-ion batteries. However, achieving an intricate combination of well-designed 1D-nanostructural materials and conductive carbon components for excellent lithium-ion storage capacity is a key challenge. In this study, novel and unique tube-in-tube structured nanofibers consisting of hollow metal oxide (CoFe2O4) nanospheres covered with a graphitic carbon (GC) layer were feasibly and successfully synthesized. A facile pitch solution infiltration method was applied to provide electrical conductivity in the tube-in-tube structure. Generally, mesophase pitch with liquid characteristics uniformly infiltrates the porous nanocrystals and transforms into graphitic layers around metallic CoFe2 alloys during the reduction process. The oxidation process that follows produces the hollow CoFe2O4 nanosphere by the nanoscale Kirkendall effect and the GC layer by selective decomposition of amorphous carbon layers. Hollow CoFe2O4 nanospheres comprising tube-in-tube structured nanofibers and GC layers are formed by pitch-derived carbon; these have improved structural stability and electrical conductivity resulting in excellent cycling and characteristics. Tube-in-tube structured nanofibers consisting of hollow CoFe2O4@GC nanospheres showed excellent long-cycle performance (682 mA h g-1 for the 1400th cycle at a high current density of 3.0 A g-1) and excellent rate capability (355 mA h g-1) even at an extremely high current density of 50 A g-1.

Structural combination of polar hollow microspheres and hierarchical N-doped carbon nanotubes for high-performance Li-S batteries.

Hierarchical structured materials constructed with conductive carbon materials have been extensively studied as S host materials for Li-S batteries. However, their outwardly developed hierarchical structures, which do not contain structures or materials to inhibit polysulfide dissolution, lead to the dissipation of dissolved polysulfides and poor dispersion properties during the slurry-making process, which results in non-uniformity in the cathodes. Herein, an assembly of polar materials (hollow structured SiO2 microspheres) and electrically conductive hierarchical N-doped bamboo-like carbon nanotubes (b-NCNTs) is designed as an efficient S host material for minimizing the dissolution of polysulfides during Li-S battery operations. Highly aligned and packed b-NCNTs are grown in hollow structured SiO2 microspheres. The SiO2 layer coated on the surface of the hollow CoFe2O4 microspheres plays a key role in the synthesis of easily dispersible hierarchical b-NCNTs microspheres (b-NCNTs@SiO2). The S-loaded b-NCNTs@SiO2 electrodes show better cycling stability than S-loaded b-NCNTs electrodes. The polysulfide trapping of the polar SiO2 layer and the well-developed b-NCNTs minimize the dissolution of polysulfides during cycling. In addition, the introduction of electronegative N atoms into the b-NCNTs lattice enhances their polysulfide trapping ability. The S-loaded b-NCNTs@SiO2 electrodes exhibit stable discharge capacities of >771 mA h g-1 over 195 cycles at a current density of 0.5 C and a high reversible capacity of 486 mA h g-1 even at a high current density of 5.0 C.

Investigation of Binary Metal (Ni, Co) Selenite as Li-Ion Battery Anode Materials and Their Conversion Reaction Mechanism with Li Ions.

Highly efficient anode materials with novel compositions for Li-ion batteries are actively being researched. Multicomponent metal selenite is a promising candidate, capable of improving their electrochemical performance through the formation of metal oxide and selenide heterostructure nanocrystals during the first cycle. Here, the binary nickel-cobalt selenite derived from Ni-Co Prussian blue analogs (PBA) is chosen as the first target material: the Ni-Co PBA are selenized and partially oxidized in sequence, yielding (NiCo)SeO3 phase with a small amount of metal selenate. The conversion mechanism of (NiCo)SeO3 for Li-ion storage is studied by cyclic voltammetry, in situ X-ray diffraction, ex situ X-ray photoelectron spectroscopy, in situ electrochemical impedance spectroscopy, and ex situ transmission electron microscopy. The reversible reaction mechanism of (NiCo)SeO3 with the Li ions is described by the reaction: NiO + CoO + xSeO2 + (1 - x)Se + (4x + 6)Li+ + (4x + 6)e- ↔ Ni + Co + (2x + 2)Li2 O + Li2 Se. To enhance electrochemical properties, polydopamine-derived carbon is uniformly coated on (NiCo)SeO3 , resulting in excellent cycling and rate performances for Li-ion storage. The discharge capacity of C-coated (NiCo)SeO3 is 680 mAh g-1 for the 1500th cycle when cycled at a current density of 5 A g-1 .

Synthesis Process of CoSeO3 Microspheres for Unordinary Li-ion Storage Performances and Mechanism of Their Conversion Reaction with Li ions.

Multicomponent materials with various double cations have been studied as anode materials of lithium-ion batteries (LIBs). Heterostructures formed by coupling different-bandgap nanocrystals enhance the surface reaction kinetics and facilitate charge transport because of the internal electric field at the heterointerface. Accordingly, metal selenites can be considered efficient anode materials of LIBs because they transform into metal selenide and oxide nanocrystals in the first cycle. However, few studies have reported synthesis of uniquely structured metal selenite microspheres. Herein, synthesis of high-porosity CoSeO3 microspheres is reported. Through one-pot oxidation at 400 °C, CoSex -C microspheres formed by spray pyrolysis transform into CoSeO3 microspheres showing unordinary cycling and rate performances. The conversion mechanism of CoSeO3 microspheres for lithium-ion storage is systematically studied by cyclic voltammetry, in situ X-ray diffraction and electrochemical impedance spectroscopy, and transmission electron microscopy. The reversible reaction mechanism of the CoSeO3 phase from the second cycle onward is evaluated as CoO + xSeO2 + (1 - x)Se + 4(x + 1)Li+ + 4( x + 1)e- ↔ Co + (2x + 1)Li2 O + Li2 Se. The CoSeO3 microspheres show a high reversible capacity of 709 mA h g-1 for the 1400th cycle at a current density of 3 A g-1 and a high reversible capacity of 526 mA h g-1 even at an extremely high current density of 30 A g-1 .

Yolk-shell-structured microspheres composed of N-doped-carbon-coated NiMoO4 hollow nanospheres as superior performance anode materials for lithium-ion batteries.

Novel yolk-shell-structured microspheres consisting of N-doped-carbon-coated metal-oxide hollow nanospheres are designed as efficient anode materials for lithium-ion batteries and synthesized via a spray pyrolysis process. A NiMoO4 yolk-shell architecture formed via spray pyrolysis is transformed into equally structured NiSe2-MoSe2 composite microspheres. Because of the complementary effect between the Ni and Mo components that prevents severe crystal growth during selenization, NiSe2-MoSe2 nanocrystals are uniformly distributed over the yolk-shell structure. Then, the yolk-shell-structured NiSe2-MoSe2 microspheres are oxidized, which yields microspheres composed of NiMoO4 hollow nanospheres by nanoscale Kirkendall diffusion. Uniform coating with polydopamine and a subsequent carbonization process produce uniquely structured microspheres consisting of N-doped-carbon-coated NiMoO4 hollow nanospheres. The discharge capacity of the yolk-shell-structured NiMoO4-C composite microspheres for the 500th cycle at a current density of 3.0 A g-1 is 862 mA h g-1. In addition, the NiMoO4-C composite microspheres show a high reversible capacity of 757 mA h g-1 even at an extremely high current density of 10 A g-1. The synergetic effect between the hollow nanospheres comprising the yolk-shell structure and the N-doped carbon coating layer results in the excellent lithium-ion storage performance of the NiMoO4-C composite microspheres.

Design and synthesis of interconnected hierarchically porous anatase titanium dioxide nanofibers as high-rate and long-cycle-life anodes for lithium-ion batteries.

We suggest an efficient and simple synthetic strategy to prepare interconnected hierarchically porous anatase TiO2 (IHP-A-TiO2) nanofibers by two synergetic effects: phase separation between polymers and relative humidity control during electrospinning. The macro channels formed by polystyrene decomposition were interconnected by numerous mesopores that were formed by evaporation of infiltrated water vapor in the structure. The resulting IHP-A-TiO2 nanofibers showed better Li+ ion storage performances than the TiO2 materials reported in the literature. The discharge capacity of IHP-A-TiO2 nanofibers for the 3000th cycle at 1.0 A g-1 and corresponding coulombic efficiency from the 20th cycle onward were 142 mA h g-1 and >99.0%, respectively. Well-interconnected, ultrafine TiO2 nanocrystals within the nanofiber showed structural stability during cycling and facilitated facile charge transfer at the electrode-electrolyte interface.

Lithium-ion storage performances of sunflower-like and nano-sized hollow SnO2 spheres by spray pyrolysis and the nanoscale Kirkendall effect.

Nanostructured metal selenides with a variety of morphologies are crucial for fabricating porous, hollow metal-oxide nanomaterials by nanoscale Kirkendall diffusion. Herein, SnSe-SnO2 composite powders and SnSe nanospheres were synthesized via one-pot spray pyrolysis by optimizing the concentration of the Se precursor in the spray solution; these were then used to fabricate sunflower-like SnO2 and hollow SnO2 nanospheres, respectively, via nanoscale Kirkendall diffusion. Post-treatment of the SnSe-decorated SnO2 under air produced sunflower-like SnO2, in which ray and disk florets consisting of hollow nanoplates and dense nanospheres, respectively, were present. The mean diameter of the homogeneous hollow SnO2 nanospheres was 150 nm. The hollow morphology shortens the diffusion length, increasing the contact area between the electrolyte and voids and buffering large volume changes during repeated cycling. As anode materials for lithium-ion batteries, the hollow SnO2 nanospheres showed excellent cycling and rate performances. The discharge capacity of the hollow SnO2 nanospheres, after 500 cycles from 0.001 V to 3.0 V, was 1043 mA h g-1, at a current density of 3.0 A g-1. The hollow SnO2 nanospheres showed a high reversible capacity of 638 mA h g-1, even at current density as high as 10 A g-1.

Three-dimensional porous microspheres comprising hollow Fe2O3 nanorods/CNT building blocks with superior electrochemical performance for lithium ion batteries.

It is highly desirable to develop anode materials with rational architectures for lithium ion batteries to achieve high-performance electrochemical properties. In this study, three-dimensional porous composite microspheres comprising hollow Fe2O3 nanorods/carbon nanotube (CNT) building blocks are successfully constructed by direct deposition and further thermal transformation of beta-FeOOH nanorods on CNT porous microspheres. The CNT porous microsphere, which is prepared by a spray pyrolysis, provides ample sites for the direct growth of beta-FeOOH nanorods. During the further oxidation process, the beta-FeOOH nanorods are transformed into hollow Fe2O3 nanorods as a result of dehydroxylation and lattice shrinkage, resulting in the formation of hollow Fe2O3 nanorods/CNT porous microspheres. Such a hierarchical structure of composite microspheres not only facilitates electrolyte accessibility but also offers conductive networks for electrons during electrochemical reactions. Accordingly, the electrodes exhibit a high discharge capacity of 1307 mA h g-1 after 300 cycles at a current density of 1 A g-1; this is associated with an excellent capacity retention of 84%, which is calculated from the initial cycle. In addition, the composite delivers a discharge capacity of 703 mA h g-1 at a current density of 15 A g-1.

Multiroom-structured multicomponent metal selenide-graphitic carbon-carbon nanotube hybrid microspheres as efficient anode materials for sodium-ion batteries.

Novel three-dimensional (3D) multiroom-structured multicomponent metal (NiFe) selenide-graphitic carbon (GC)-carbon nanotube (CNT) hybrid microspheres were prepared by spray pyrolysis and a subsequent selenization process. Phase segregation and the decomposition of dextrin resulted in multiroom-structured microspheres with uniformly distributed empty nanovoids in the spray pyrolysis process. Metal nanocrystals of iron and nickel components that formed as intermediate products during the selenization process transformed amorphous carbon into GC by acting as nanocatalysts. GC layers surrounding (NiFe)Sex nanocrystals and CNTs uniformly skeletonized in multiroom-structured microspheres improved the electrical conductivity and structural stability. Due to the synergistic effect of the unique structure and conductivity of the carbon components, the multiroom-structured (NiFe)Sex-GC-CNT hybrid microsphere showed excellent cycling and rate performance for sodium-ion storage. The discharge capacities of (NiFe)Sex-GC, (NiFe)Sex-CNT, and (NiFe)Sex-GC-CNT for the 100th cycle at a current density of 0.3 A g-1 were 369, 284, and 455 mA h g-1, respectively, and the respective capacity retentions measured from the second cycle were 74.1, 56.1, and 92.2%.

Design and Synthesis of Spherical Multicomponent Aggregates Composed of Core-Shell, Yolk-Shell, and Hollow Nanospheres and Their Lithium-Ion Storage Performances.

Micrometer-sized spherical aggregates of Sn and Co components containing core-shell, yolk-shell, hollow nanospheres are synthesized by applying nanoscale Kirkendall diffusion in the large-scale spray drying process. The Sn2 Co3 -Co3 SnC0.7 -C composite microspheres uniformly dispersed with Sn2 Co3 -Co3 SnC0.7 mixed nanocrystals are formed by the first-step reduction of spray-dried precursor powders at 900 °C. The second-step oxidation process transforms the Sn2 Co3 -Co3 SnC0.7 -C composite into the porous microsphere composed of Sn-Sn2 Co3 @CoSnO3 -Co3 O4 core-shell, Sn-Sn2 Co3 @CoSnO3 -Co3 O4 yolk-shell, and CoSnO3 -Co3 O4 hollow nanospheres at 300, 400, and 500 °C, respectively. The discharge capacity of the microspheres with Sn-Sn2 Co3 @CoSnO3 -Co3 O4 core-shell, Sn-Sn2 Co3 @CoSnO3 -Co3 O4 yolk-shell, and CoSnO3 -Co3 O4 hollow nanospheres for the 200th cycle at a current density of 1 A g-1 is 1265, 987, and 569 mA h g-1 , respectively. The ultrafine primary nanoparticles with a core-shell structure improve the structural stability of the porous-structured microspheres during repeated lithium insertion and desertion processes. The porous Sn-Sn2 Co3 @CoSnO3 -Co3 O4 microspheres with core-shell primary nanoparticles show excellent cycling and rate performances as anode materials for lithium-ion batteries.

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 .

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.

Superior Na-ion storage properties of high aspect ratio SnSe nanoplates prepared by a spray pyrolysis process.

SnSe nanoplates with thin and uniform morphology are prepared by one-pot spray pyrolysis, and are examined as anode materials for Na-ion batteries. During the spray pyrolysis process, metallic Se and Sn are prepared from SeO2 and SnO2, respectively, under a reducing atmosphere. Metallic Sn and metalloid Se, with melting points of 232 and 221 °C, respectively, form a melted Sn-Se mixture, which reacts exothermally to form SnSe nanocrystals. Several of these nanocrystals are grown simultaneously forming a micron-sized powder. Complete elimination of the excess amount of metalloid Se, by forming H2Se gas, results in aggregation-free SnSe nanoplates. The aspect ratio of these nanoplates is as high as 11.3. The discharge capacities for the SnSe nanoplates, prepared from spray solutions containing 100, 400, and 800% of the stoichiometric SeO2 content needed to form SnSe, are 407, 558, and 211 mA h g(-1), respectively, after 50 cycles at a constant current density of 0.3 A g(-1); their capacity retentions calculated from the second cycle onwards are 77, 100, and 60%, respectively. The phase pure SnSe nanoplates with a high aspect ratio show good cycling and rate performances for Na-ion storage.

Na-ion Storage Performances of FeSe(x) and Fe2O3 Hollow Nanoparticles-Decorated Reduced Graphene Oxide Balls prepared by Nanoscale Kirkendall Diffusion Process.

Uniquely structured FeSe(x)-reduced graphene oxide (rGO) composite powders, in which hollow FeSe(x) 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 FeSe(x)-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 FeSe(x) nanoparticles during the selenization process. The FeSe(x)-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 FeSe(x)-rGO composite powders. The FeSe(x)-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 FeSe(x)- and Fe2O3-rGO composite powders for the 200(th) cycle at a constant current density of 0.3 A g(-1) were 434 and 174 mA h g(-1), respectively. The FeSe(x)-rGO composite powders had a high discharge capacity of 311 mA h g(-1) for the 1000(th) cycle at a high current density of 1 A g(-1).