Dandan Wang

Comparison of the Electrochemical Performance of NiMoO4 Nanorods and Hierarchical Nanospheres for Supercapacitor Applications

Much attention has been paid to exploring electrode materials with enhanced supercapacitor performance as well as relatively low cost and environmental friendliness. In this work, NiMoO4 nanospheres and nanorods were synthesized by facile hydrothermal methods. The hierarchical NiMoO4 nanospheres were about 2.5 μm in diameter and assembled from thin mesoporous nanosheets with a thickness of about 10-20 nm. The NiMoO4 nanorods were about 80 nm in diameter and about 300 nm to 1 μm in length. Their electrochemical properties were investigated for use as electrode materials for supercapacitors (SCs). The NiMoO4 nanospheres exhibited a higher specific capacitance and better cycling stability and rate capability, which were attributed to their large surface area and high electrical conductivity. The specific capacitances were 974.4, 920.8, 875.5, 859.1, and 821.4 F/g at current densities of 1, 2, 4, 6, and 10 A/g, respectively. Remarkably, the energy density was able to reach 20.1 Wh/kg at a power density of 2100 W/kg. After 2000 cycles, the NiMoO4 nanospheres still displayed a high specific capacitance of about 631.8 F/g at a current density of 5 A/g. These results implied that the hierarchical NiMoO4 nanospheres could be a promising candidate for use as high-performance SCs.

Three-Dimensional Co3O4@NiMoO4 Core/Shell Nanowire Arrays on Ni Foam for Electrochemical Energy Storage

In this work, we report a facile two-step hydrothermal method to synthesize the unique three-dimensional Co3O4@NiMoO4 core/shell nanowire arrays (NWAs) on Ni foam for the first time. The Co3O4 nanowires are fully covered by ultrathin mesoporous NiMoO4 nanosheets. When evaluated as a binder-free electrode for supercapacitors in a 2 M KOH aqueous solution, the Co3O4@NiMoO4 hybrid electrode exhibits a greatly enhanced areal capacitance of 5.69 F cm(-2) at a high current density of 30 mA cm(-2), nearly 5 times that of the pristine Co3O4 electrode (1.10 F cm(-2)). The energy density of the hybrid electrode is 56.9 W h kg(-1) at a high power density of 5000 W kg(-1). In addition, the Co3O4@NiMoO4 hybrid electrode also exhibits good rate capability and cycling stability, which would hold great promise for electrochemical energy storage.

High-performance supercapacitor electrode based on the unique ZnO@Co3O4 core/shell heterostructures on nickel foam

Currently, tremendous attention has been paid to the rational design and synthesis of unique core/shell heterostructures for high-performance supercapacitors. In this work, the unique ZnO@Co3O4 core/shell heterostructures on nickel foam are successfully synthesized through a facile and cost-effective hydrothermal method combined with a short post annealing treatment. Mesoporous Co3O4 nanowires are multidirectional growing on the rhombus-like ZnO nanorods. In addition, the growth mechanism for such unique core/shell heterostructures is also proposed. Supercapacitor electrodes based on the ZnO@Co3O4 and Co3O4 heterostructures on nickel foam are thoroughly characterized. The ZnO@Co3O4 electrode exhibits high capacitance of 1.72 F cm(-2) (857.7 F g(-1)) at a current density of 1 A g(-1), which is higher than that of the Co3O4 electrode. Impressively, the capacitance of the ZnO@Co3O4 electrode increases gradually from 1.29 to 1.66 F cm(-2) (830.8 F g(-1)) after 6000 cycles at a high current density of 6 A g(-1), indicating good long-term cycling stability. These results indicate the unique ZnO@Co3O4 electrode would be a promising electrode for high-performance supercapacitor applications.

Construction of unique NiCo2O4 nanowire@CoMoO4 nanoplate core/shell arrays on Ni foam for high areal capacitance supercapacitors

In this work, we present a facile two-step hydrothermal method with a successive annealing treatment to integrate two ternary metal oxides (NiCo2O4 and CoMoO4) into unique core/shell nanowire arrays (NWAs) on Ni foam as advanced binder-free electrodes for the first time. In addition, a possible growth mechanism for the growth of CoMoO4 nanoplates (NPs) on NiCo2O4 nanowires (NWs) is put forward based on the time-dependent experiments. When investigated as binder-free electrodes for supercapacitors (SCs), such unique NiCo2O4@CoMoO4 core/shell hybrid electrodes exhibit ultrahigh areal capacitances, which are several times larger than the pristine NiCo2O4 electrode. The remarkable electrochemical performance is attributed to the rational combination of two electroactive materials and the reasonable array configuration.

Rational synthesis of ZnMn2O4 porous spheres and graphene nanocomposite with enhanced performance for lithium-ion batteries

Currently, tremendous efforts have been focused on developing high-performance anode materials for lithium-ion batteries (LIBs). In this work, we develop an effective strategy to synthesize a nanocomposite of ZnMn2O4 porous spheres and graphene (ZMO-G PSs) as an advanced anode material for high-performance LIBs. SEM and TEM characterizations reveal that the ZMO PSs are tightly wrapped by graphene sheets and uniformly distributed within the graphene matrix. Owing to the rational combination of the merits of both ZMO PSs and graphene, the as-prepared ZMO-G PSs nanocomposite exhibits significant enhanced LIB performance with high reversible capacity, long cycle life and good rate performance. Remarkably, the nanocomposite exhibits a high reversible capacity of 926.4 mA h g−1 after 100 cycles at a current density of 200 mA g−1, which is much higher than that of pure ZMO PSs (493.3 mA h g−1). Moreover, a high capacity of 560.8 mA h g−1 can be retained at a high current density of 1200 mA g−1. These electrochemical results suggest the ZMO-G PSs nanocomposite could be a promising anode material in energy storage applications for high-performance LIBs.

High performance and negative temperature coefficient of low temperature hydrogen gas sensors using palladium decorated tungsten oxide

Gas sensors based on a noble metal–semiconductor are widely used, and they show a positive temperature response to hydrogen below 400 °C. In this study, a catalytically activated hydrogen sensor is obtained based on Pd decorated WO3 nanoplates constructed by a solvothermal method. An insight into the role of Pd catalyst in the outstanding performance is provided by comparing the sensing properties of this sensor with those of a traditional one made from the same pristine WO3. The pure WO3 sensor exhibits poor selectivity and low sensitivity to hydrogen. In contrast, the observed response of the as-produced sensor is up to 843 at a low operating temperature of 80 °C; the response value is even greater than that of WO3 sensors at high temperatures (250–400 °C). In addition, the Pd-loaded WO3 sensors show excellent selectivity towards H2 in comparison to other common gases (CH4, C3H6O, C2H6, C3H8O and NH3). The significantly improved performance is thoroughly explained in terms of the adsorption–desorption mechanism and chemical kinetics theories. Furthermore, an interfacial model demonstrated in this report indicates that the interfacial barrier between WO3 nanoparticles can be a novel effect for excellent gas sensing performance.

High-temperature humidity sensors based on WO3–SnO2 composite hollow nanospheres

Three kinds of humidity sensors were fabricated from WO3–SnO2 composite hollow nanospheres (WO3–SnO2 HNS), WO3 nanoparticles (WO3 NPs) and SnO2 nanoparticles (SnO2 NPs). WO3–SnO2 HNS were prepared by a facile hydrothermal process with a diameter and thickness of about 550 nm and 30 nm, respectively. Temperature-dependent properties of as-prepared humidity sensors were investigated at various values of relative humidity and temperature. It was found that the WO3–SnO2 HNS humidity sensor showed good performance at 80 °C. The response time, recovery time and sensitivity were evaluated while switching the humidity between 35% RH and 98% RH. The response time decreased from 289 to 29 s, the recovery time reduced from 22 to 8 s, and the sensitivity changed from 16.2 to 11.4 as the work temperature was raised from 24 to 80 °C. An opposite humidity sensing phenomenon was observed between WO3 NPs and SnO2 NPs at a high temperature, which might explain the temperature-dependent properties of the WO3–SnO2 HNS humidity sensor. This work could stimulate a right approach to design practical humidity sensors with high sensitivity, long stability and fast response.

Rational synthesis of metal–organic framework composites, hollow structures and their derived porous mixed metal oxide hollow structures

Metal–organic frameworks (MOFs) have attracted considerable attention for their important applications. Recently, significant efforts have been devoted to constructing hollow MOF structures and integrating MOFs with other functional materials to further enhance the inherent performance and endow them with new functions. However, it still remains a big challenge to synthesize well-defined MOF composites and MOF hollow structures, especially those with structural and compositional complexity. In this work, we demonstrate a chemical transformation method to synthesize unique MOF composites, as well as hollow and complex hollow MOF structures. This method could intelligently overcome the difficulty that is caused by the possible lattice mismatch between MOFs and other functional materials. Impressively, a series of unique MOF@MOF core–shell structures, MOF composites, MOF hollow and complex hollow structures are successfully synthesized. Moreover, porous mixed metal oxide hollow and complex structures are obtained by annealing the corresponding MOF hollow and complex hollow structures in air. We anticipate that these unique MOF composites, hollow structures and the derived porous mixed metal oxide hollow structures could find promising applications in many other fields. As an example, NiFe oxide cube-in-box complex hollow nanostructures are evaluated as anode materials for lithium-ion batteries (LIBs), which exhibit enhanced electrochemical performance compared to the simple nanobox counterparts.

A nanocomposite of tin dioxide octahedral nanocrystals exposed to high-energy facets anchored onto graphene sheets for high performance lithium-ion batteries

The synthesis of nanocrystals with high-energy facets is an important and challenging research topic. In this work, we develop a facile hydrothermal method to synthesize a nanocomposite of SnO2 octahedral nanocrystals (ONCs) exposed to high-energy {332} facets on graphene sheets (GS) as an advanced anode material for high performance lithium-ion batteries (LIBs). Electrochemical characterization of SnO2 ONCs/GS nanocomposite shows that it exhibits much enhanced Li-battery performance compared with a nanocomposite of SnO2 nanoparticles (NPs) exposed to stable facets on GS. The as-prepared SnO2 ONCs/GS nanocomposite has a reversible discharge capacity of as high as 844 mA h g−1 after 50 cycles at a current density of 100 mA h g−1. Even at a higher current density of 5000 mA g−1, the discharge capacity of the SnO2 ONCs/GS nanocomposite is still as high as approximately 555 mA h g−1, indicating good rate capability. These excellent results are attributed to the exposure of SnO2 ONCs to high-energy facets, and the rational growth of the SnO2 ONCs on GS. It is believed that the SnO2 ONCs/GS nanocomposite hold great promise for applications in high performance LIBs.

Ethanol-sensing performance of tin dioxide octahedral nanocrystals with exposed high-energy {111} and {332} facets

Tin dioxide octahedral nanocrystals with exposed high-energy {111} and {332} facets were hydrothermally synthesized and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and selected-area electron diffraction (SAED). Gas sensors were fabricated from the prepared SnO2 nanocrystals and applied to ethanol-sensing tests. Octahedral SnO2 {332} exhibited a maximum response of 2200 under an ethanol concentration of 800 ppm at 250 °C with a response time of 1.5 s and a recovery time of 32.5 s, whereas SnO2 {111} exhibited a maximum response of 179 at 360 °C with a response time of 9.5 s and a recovery time of 6.7 s. The sensing mechanisms responsible for SnO2 nanocrystals to ethanol vapor are discussed.