Junwu Xiao

Design Hierarchical Electrodes with Highly Conductive NiCo2S4 Nanotube Arrays Grown on Carbon Fiber Paper for High-Performance Pseudocapacitors

We report on the development of highly conductive NiCo2S4 single crystalline nanotube arrays grown on a flexible carbon fiber paper (CFP), which can serve not only as a good pseudocapacitive material but also as a three-dimensional (3D) conductive scaffold for loading additional electroactive materials. The resulting pseudocapacitive electrode is found to be superior to that based on the sibling NiCo2O4 nanorod arrays, which are currently used in supercapacitor research due to the much higher electrical conductivity of NiCo2S4. A series of electroactive metal oxide materials, including CoxNi1-x(OH)2, MnO2, and FeOOH, were deposited on the NiCo2S4 nanotube arrays by facile electrodeposition and their pseudocapacitive properties were explored. Remarkably, the as-formed CoxNi1-x(OH)2/NiCo2S4 nanotube array electrodes showed the highest discharge areal capacitance (2.86 F cm(-2) at 4 mA cm(-2)), good rate capability (still 2.41 F cm(-2) at 20 mA cm(-2)), and excellent cycling stability (∼ 4% loss after the repetitive 2000 cycles at a charge-discharge current density of 10 mA cm(-2)).

Surface Structure Dependent Electrocatalytic Activity of Co 3 O 4 Anchored on Graphene Sheets toward Oxygen Reduction Reaction

Catalytic activity is primarily a surface phenomenon, however, little is known about Co3O4 nanocrystals in terms of the relationship between the oxygen reduction reaction (ORR) catalytic activity and surface structure, especially when dispersed on a highly conducting support to improve the electrical conductivity and so to enhance the catalytic activity. Herein, we report a controllable synthesis of Co3O4 nanorods (NR), nanocubes (NC) and nano-octahedrons (OC) with the different exposed nanocrystalline surfaces ({110}, {100}, and {111}), uniformly anchored on graphene sheets, which has allowed us to investigate the effects of the surface structure on the ORR activity. Results show that the catalytically active sites for ORR should be the surface Co2+ ions, whereas the surface Co3+ ions catalyze CO oxidation, and the catalytic ability is closely related to the density of the catalytically active sites. These results underscore the importance of morphological control in the design of highly efficient ORR catalysts.

Facile and Green Synthesis of Palladium Nanoparticles-Graphene-Carbon Nanotube Material with High Catalytic Activity

We report a facile and green method to synthesize a new type of catalyst by coating Pd nanoparticles (NPs) on reduced graphene oxide (rGO)-carbon nanotube (CNT) nanocomposite. An rGO–CNT nanocomposite with three-dimensional microstructures was obtained by hydrothermal treatment of an aqueous dispersion of graphene oxide (GO) and CNTs. After the rGO–CNT composites have been dipped in K2PdCl4 solution, the spontaneous redox reaction between the GO–CNT and PdCl42− led to the formation of nanohybrid materials consisting rGO–CNT decorated with 4 nm Pd NPs, which exhibited excellent and stable catalytic activity: the reduction of 4-nitrophenol to 4-aminophenol using NaBH4 as a catalyst was completed in only 20 s at room temperature, even when the Pd content of the catalyst was 1.12 wt%. This method does not require rigorous conditions or toxic agents and thus is a rapid, efficient, and green approach to the fabrication of highly active catalysts.

Bio-inspired synthesis of NaCl-type CoxNi1−xO (0 ≤ x < 1) nanorods on reduced graphene oxide sheets and screening for asymmetric electrochemical capacitors

A bio-inspired approach has enabled the first synthesis of CoxNi1−xO (0 ≤ x < 1) nanorods on reduced graphene oxide (RGO) sheets. The key is the crystallization process from amorphous precursors in a disordered and hydrated state being able to take compositions arbitrarily different from that of the known stable mixed oxide NiCo2O4. This success has permitted further screening of the compositions for electrochemical capacitors. CoxNi1−xO/RGO nanocomposite electrodes achieve a peak specific capacitance when the Co/Ni molar ratio is close to 1. For example, Co0.45Ni0.55O/RGO nanocomposite electrode has exhibited a specific capacitance up to 823.0 F g−1 (based on the total active materials mass) and 909.4 F g−1 (based on the oxide mass) at 1 A g−1, which are among the highest for Co/Ni oxides. Also revealed was their superior cycling stability compared to the Co3O4/RGO and NiO/RGO nanocomposites, with a surprising increase of the specific capacity in the initial 100 cycles before flattening out. In addition, testing of (Co0.45Ni0.55O/RGO)//RGO asymmetric cells yielded an energy density up to 35.3 Wh kg−1 at a cell voltage of 1.5 V, much higher than those of the symmetric cells (Co0.45Ni0.55O/RGO)//(Co0.45Ni0.55O/RGO) (20.2 Wh kg−1) and RGO//RGO (4.5 Wh kg−1). Even at a high power density of 3614.0 W kg−1, the asymmetric cell could still maintain an energy density of 28.0 Wh kg−1. There was only a <4% loss of the initial specific capacitance after 1000 cycles of charge/discharge at 2 A g−1.

Morphology-conserved transformation: synthesis of hierarchical mesoporous nanostructures of Mn2O3 and the nanostructural effects on Li-ion insertion/deinsertion properties

By means of morphology-conserved transformation, we have synthesized hierarchically structured Mn2O3 nanomaterials with different morphologies and pore structures. The key step of this method consists of the formation of a precursor containing the target materials interlaced with the judiciously chosen polyol-based organic molecules, which are subsequently knocked out to generate the final nanomaterials. In the present work, two kinds of precursor morphologies, oval-shaped and straw-sheaf-shaped, have been selectively prepared by hydrothermal treatment of different functional polyol molecules (oval-shape with fructose and straw-sheaf-shape with β-cyclodextrin) and potassium permanganate. Thermal decomposition of the precursors resulted in the formation of mesoporous Mn2O3 maintaining the original morphologies, as revealed by extensive characterization. These novel hierarchical nanostructures with different pore sizes/structures prompted us to examine their potential as anode materials for lithium ion batteries (LIBs). The electrochemical results with reference to LIBs show that both of our mesoporous Mn2O3 nanomaterials deliver high reversible capacities and excellent cycling stabilities at a current density of 200 mA g−1 compared to the commercial Mn2O3 nanoparticles. Moreover, the straw-sheaf-shaped Mn2O3 exhibits a higher specific capacity and a better cycling performance than the oval-shaped one, due to the relatively higher surface area and the peculiar nanostrip structure resulting in the reduced length for lithium ion diffusion. Morphology-conserved transformation yields two kinds of hierarchical mesoporous Mn2O3 nanomaterials with high capacities and cycling stabilities for lithium ion batteries.

Scalable Synthesis of Freestanding Sandwich-structured Graphene/Polyaniline/Graphene Nanocomposite Paper for Flexible All-Solid-State Supercapacitor

We reported a scalable and modular method to prepare a new type of sandwich-structured graphene-based nanohybrid paper and explore its practical application as high-performance electrode in flexible supercapacitor. The freestanding and flexible graphene paper was firstly fabricated by highly reproducible printing technique and bubbling delamination method, by which the area and thickness of the graphene paper can be freely adjusted in a wide range. The as-prepared graphene paper possesses a collection of unique properties of highly electrical conductivity (340 S cm−1), light weight (1 mg cm−2) and excellent mechanical properties. In order to improve its supercapacitive properties, we have prepared a unique sandwich-structured graphene/polyaniline/graphene paper by in situ electropolymerization of porous polyaniline nanomaterials on graphene paper, followed by wrapping an ultrathin graphene layer on its surface. This unique design strategy not only circumvents the low energy storage capacity resulting from the double-layer capacitor of graphene paper, but also enhances the rate performance and cycling stability of porous polyaniline. The as-obtained all-solid-state symmetric supercapacitor exhibits high energy density, high power density, excellent cycling stability and exceptional mechanical flexibility, demonstrative of its extensive potential applications for flexible energy-related devices and wearable electronics.

Solid-State Thin-Film Supercapacitors with Ultrafast Charge/Discharge Based on N-Doped-Carbon-Tubes/Au-Nanoparticles-Doped-MnO2 Nanocomposites

Although carbonaceous materials possess long cycle stability and high power density, their low-energy density greatly limits their applications. On the contrary, metal oxides are promising pseudocapacitive electrode materials for supercapacitors due to their high-energy density. Nevertheless, poor electrical conductivity of metal oxides constitutes a primary challenge that significantly limits their energy storage capacity. Here, an advanced integrated electrode for high-performance pseudocapacitors has been designed by growing N-doped-carbon-tubes/Au-nanoparticles-doped-MnO2 (NCTs/ANPDM) nanocomposite on carbon fabric. The excellent electrical conductivity and well-ordered tunnels of NCTs together with Au nanoparticles of the electrode cause low internal resistance, good ionic contact, and thus enhance redox reactions for high specific capacitance of pure MnO2 in aqueous electrolyte, even at high scan rates. A prototype solid-state thin-film symmetric supercapacitor (SSC) device based on NCTs/ANPDM exhibits large energy density (51 Wh/kg) and superior cycling performance (93% after 5000 cycles). In addition, the asymmetric supercapacitor (ASC) device assembled from NCTs/ANPDM and Fe2O3 nanorods demonstrates ultrafast charge/discharge (10 V/s), which is among the best reported for solid-state thin-film supercapacitors with both electrodes made of metal oxide electroactive materials. Moreover, its superior charge/discharge behavior is comparable to electrical double layer type supercapacitors. The ASC device also shows superior cycling performance (97% after 5000 cycles). The NCTs/ANPDM nanomaterial demonstrates great potential as a power source for energy storage devices.

Biomimetic mineralization of CaCO3 on a phospholipid monolayer: from an amorphous calcium carbonate precursor to calcite via vaterite.

A phospholipid monolayer, approximately half the bilayer structure of a biological membrane, can be regarded as an ideal model for investigating biomineralization on biological membranes. In this work on the biomimetic mineralization of CaCO(3) under a phospholipid monolayer, we show the initial heterogeneous nucleation of amorphous calcium carbonate precursor (ACC) nanoparticles at the air-water interface, their subsequent transformation into the metastable vaterite phase instead of the most thermodynamically stable calcite phase, and the ultimate phase transformation to calcite. Furthermore, the spontaneity of the transformation from vaterite to calcite was found to be closely related to the surface tension; high surface pressure could inhibit the process, highlighting the determinant of surface energy. To understand better the mechanisms for ACC formation and the transformation from ACC to vaterite and to calcite, in situ Brewster angle microscopy (BAM), ex situ scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and X-ray diffraction analysis were employed. This work has clarified the crystallization process of calcium carbonate under phospholipid monolayers and therefore may further our understanding of the biomineralization processes induced by cellular membranes.

Hierarchical porous Ni/NiO core–shells with superior conductivity for electrochemical pseudo-capacitors and glucose sensors

Although the NiO nanostructures potentially hold outstanding electrochemical activity in theory, dual enhancements in both electrical conductivity and electrolyte transport are two challenging issues for designing high performance electrodes. In this work, hierarchical porous Ni/NiO core–shells are synthesized. The interconnected Ni skeletons with favorable electrical conductivity are uniformly covered by the continuous NiO scarfskins that hold both high energy storage capacity and efficient catalysis. The hierarchical porous Ni/NiO electrode exhibits superior pseudo-capacitive performance evidenced by an areal capacitance up to 255 mF cm−2. Meanwhile, this conductive electrode also exhibits electrocatalytic activity for glucose oxidation with a sensitivity of 4.49 mA mM−1 cm−2 and a reliable detection limit of 10 μM. On the other hand, hierarchical porosities enhance the effective transport of electrolytes and ions within the interconnected porous channels, making dramatic contributions to a superior storage stability of 4000 cycles and prompt an amperometric response time of 1.5 s. These concepts of the hierarchical metal–metal oxide core–shell open an avenue to design high-performance materials for energy storage and electrochemical catalysis.

Mesoporous Mn3O4–CoO core–shell spheres wrapped by carbon nanotubes: a high performance catalyst for the oxygen reduction reaction and CO oxidation

The controllable synthesis of transition metal oxide nanomaterials has attracted considerable attention for the replacement of the current precious metal catalysts. Herein, we have developed a facile method to successfully synthesize Mn3O4–CoO core–shell mesoporous spheres, which are wrapped by carbon nanotubes (CNT), and investigated the catalytic activity for the oxygen reduction reaction (ORR) and CO oxidation for the first time. The ORR process on the Mn3O4–CoO/CNT catalysts was via a complete oxygen reduction process (4e−), and the catalytic activity was far better than for the Mn3O4/CNT and CoO/CNT catalysts. The durability even out-performed the commercial Pt/C catalysts. As compared with the Mn3O4/CNT and CoO/CNT catalysts, the Mn3O4–CoO/CNT catalysts also exhibited better catalytic activity for CO oxidation. The initial and complete conversion temperatures for the Mn3O4–CoO/CNT catalysts can decrease to 30 and 120 °C, respectively. The good catalytic activity for the ORR and CO oxidation is due to the high specific surface area (138.9 m2 g−1) provided which gives many catalytically active sites, mesoporous structure (15 to 120 nm) favoured for molecule accessibility to the active surface of the nanocrystals and mass transport, and the synergistic catalytic effect of Mn3O4 and CoO catalytically active sites.