Facile one-pot synthesis of NiCo<Subscript>2</Subscript>O<Subscript>4</Subscript> hollow spheres with controllable number of shells for high-performance supercapacitors

In this work,single-and double-shelled NiCo2O4 hollow spheres have been synthesized in situ by a one-pot solvothermal method assisted by xylose,followed by heat treatment.Employed as supercapacitor electrode materials,the double-shelled NiCo2O4 hollow spheres exhibit a remarkable specific capacitance (1,204.4 F·g-1 at a current density of 2.0 A·g-1) and excellent cycling stability (103.6％ retention after 10,000 cycles at a current density of 10 A·g-1).Such outstanding electrochemical performance can be attributed to their unique internal morphology,which provides a higher surface area with a larger number of active sites available to interact with the electrolyte.The versatility of this method was demonstrated by applying it to other binary metal oxide materials,such as ZnCo2O4,ZnMn2O4,and CoMn2O4.The present study thus illustrates a simple and general strategy for the preparation of binary transition metal oxide hollow spheres with a controllable number of shells.This approach shows great promise for the development of next-generation high-performance electrochemical materials.     

Preparation of MnO<Subscript>2</Subscript>/graphene composite as electrode material for supercapacitors

MnO₂/graphene composite was synthesized by a facile and effective polymer-assisted chemical reduction method. The nanosized MnO₂ particles were homogeneously distributed on graphene nanosheets, which have been confirmed by scanning electron microscopy and transmission electron microscopy analysis. The capacitive properties of the MnO₂/graphene composite have been investigated by cyclic voltammetry(CV). MnO₂/graphene composite exhibited a high specific capacitance of 324 F g⁻¹ in 1 M Na₂SO₄ electrolyte. In addition, the MnO₂/graphene composite electrode shows excellent long-term cycle stability (only 3.2% decrease of the specific capacitance is observed after 1,000 CV cycles).     

Simple Hydrothermal Synthesis for Mesoporous Pebble-Like NiCo<Subscript>2</Subscript>O<Subscript>4</Subscript> Nanostructures and Their Attractive Magnetic Properties

The mesoporous pebble-like NiCo₂O₄ nanostructures were smoothly obtained via a simple hydrothermal route and the succeeding annealing treatment. Various techniques, including XRD, SEM, TEM, Raman spectra, FTIR, and MH curves have been used to study the obtained products. The obtained results exhibited that the morphology of spinel NiCo₂O₄ could be controlled by adjusting the content of cetyltrimethylammonium bromide (CTAB). The morphology of NiCo₂O₄ nanostructures transformed from a 300-nm popcorn-like to 30–40-nm pebble-like nanostructure by changing the content of CTAB. Compared with the popcorn-like NiCo₂O₄ nanostructures, the pebble-like samples showed effective blue shifting of Raman spectroscopy, which might be due to the nano-sized effect of NiCo₂O₄ particles. Finally, vibration sample magnetometer (VSM) measurements indicated the samples transformed from weak ferromagnetic (popcorn-like) to paramagnetic (pebble-like) due to small particle size.     

High-performance flexible supercapacitors based on C/Na<Subscript>2</Subscript>Ti<Subscript>5</Subscript>O<Subscript>11</Subscript> nanocomposite electrode materials

A new solution method to synthesize Na₂Ti₅O₁₁ with titanium powder is presented, and the C/Na₂Ti₅O₁₁ nanocomposite with high specific surface area and tunnel structure as the electrode material has excellent electrochemical performance. The single electrode composed of the C/Na₂Ti₅O₁₁ nanocomposite based on carbon fiber fabric (CFF) has the highest area capacitance of 1066 mF cm^?2 at a current density of 2 mA cm^?2, which is superior to other titanates and Na-ion materials for supercapacitors (SCs). By scan-rate dependence cyclic voltammetry analysis, the capacity value shows both capacitive and faradaic intercalation processes, and the intercalation process contributed 81.7% of the total charge storage at the scan rate of 5 mV s^?1. The flexible symmetric solid-state SCs (C/Na₂Ti₅O₁₁/CFF//C/Na₂Ti₅O₁₁/CFF) based on different C/Na₂Ti₅O₁₁ mass were fabricated, and 7 mg SCs show the best supercapacitive characteristics with an area capacitance of 309 mF cm^?2 and a specific capacitance of 441 F g^?1, it has a maximum energy density of 22 Wh kg^?1 and power density of 1286 W kg^?1. As for practical application, three SCs in series can power 100 green light-emitting diodes (LEDs) to light up for 18 min, which is much longer than our previous work by Wang et al. lighting 100 LEDs for 8 min. Thus, the C/Na₂Ti₅O₁₁ nanocomposite has promising potential application in energy storage devices.     

Mn<Subscript>3</Subscript>O<Subscript>4</Subscript> nanorods with secondary plate-like nanostructures; preparation,characterization and application as high performance electrode material in supercapacitors

Mn₃O₄ nanorods with secondary plate-like structures were prepared through precipitation from a 0.005 M manganese chloride bath, under the applying direct current mode (i = 2 mA cm^?2). The structural analysis through XRD and FTIR confirmed that the deposited nanopowder has pure monoclinic phase of Mn₃O₄. Further morphological assessment through SEM proved the product to have the Mn₃O₄ nanorods in large quantity, which constructed the secondary plate-like building blocks. Cyclic voltammetric and charge–discharge experiments on the product indicated the prepared Mn₃O₄ to possess high specific capacitance (SC) values of 298 F g^?1, as well as an outstandingly durable cycling stability (95.1 % of initial capacity after discharging 1000 cycles).     

Preparation and characterization of mesoporous Co<Subscript>3</Subscript>O<Subscript>4</Subscript> electrode material

Mesoporous electrode materials of spinel Co₃O₄ were synthesized by hydrothermal method using polyethylene glycol-6000 (PEG-6000) as dispersant and subsequent calcination at different temperatures in air. The samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), nitrogen adsorption and electrochemical measurements. The results showed that the dispersant PEG-6000 had a distinct effect to control the porosity, particle size and homogeneity of Co₃O₄; the calcination temperature had a significant influence on the crystal structure, surface area, porosity and morphology, and indeed electrochemical performance. The resultant Co₃O₄ sample calcinated at 350° C possessed a narrow mesopore distribution around 4 nm and exhibited excellent electrochemical performance. It had the specific capacitance as high as 348.7 F/g, increased by 21.5% over that of pure Co₃O₄ and showed good cyclical charge–discharge stability.     

Vertically-aligned Mn(OH)<Subscript>2</Subscript> nanosheet films for flexible all-solid-state electrochemical supercapacitors

The arrangement of the electrode materials is a significant contributor for constructing high performance supercapacitor. Here, vertically-aligned Mn(OH)₂ nanosheet thin films were synthesized by cathodic electrodeposition technique on flexible Au coated polyethylene terephthalate substrates. Morphologies, microstructures, chemical compositions and valence state of the nanosheet films were characterized systematically. It shows that the nanosheets arranged vertically to the substrate, forming a porous nanowall structures and creating large open framework, which greatly facilitate the adsorption or diffusion of electrolyte ions for faradaic redox reaction. Electrochemical tests of the films show the specific capacitance as high as 240.2 F g^?1 at 1.0 A g^?1. The films were employed to assemble symmetric all-solid-state supercapacitors with LiCl/PVA gel severed as solid electrolyte. The solid devices exhibit high volumetric capacitance of 39.3 mF?cm^?3 at the current density 0.3 mA cm^?3 with robust cycling stability. The superior performance is attributed to the vertically-aligned configuration.     

Synthesis of MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> nanopowders

A procedure has been developed for the synthesis of MgAl₂O₄ nanopowders with a characteristic particle size of 10–40 nm. Translucent hydrous xerogels have been synthesized as precursors to MgAl₂O₄. The synthesized magnesium aluminum spinel nanopowders are promising for the fabrication of optical ceramics.     

Mn<Subscript>3</Subscript>O<Subscript>4</Subscript>/worm-like mesoporous carbon synthesized via a microwave method for supercapacitors

A quick and facile microwave method has been employed to prepare Mn₃O₄/worm-like mesoporous carbon (Mn₃O₄–MC) composites. Structural and morphological characterizations of worm-like mesoporous carbon and Mn₃O₄–MC composites have been carried out using X-ray diffraction, transmission electron microscopy, N₂ adsorption–desorption, and electrochemical measurement. Cyclic voltammograms demonstrate that the Mn₃O₄–MC composites perform improved capacitive behavior at the range of −0.8~0.2 V (vs. Hg/HgO electrode) with reversibility. The Mn₃O₄–MC composite electrode possesses an enhanced specific capacitance of 266 F g⁻¹ at a sweep rate of 1 mV s⁻¹.     

Synthesis of carbon nanotube,graphene, CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript>, and NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript> polypyrrole nanocomposites and study their microwave absorption

In this experimental work, different conductive polymer nanocomposites were synthesized using polypyrrole as conductive polymer and CoFe₂O₄, NiFe₂O₄, CNT and graphene as fillers. X-ray diffraction pattern was used to study the crystallinity of the products and it was found CoFe₂O₄, NiFe₂O₄, CNT, and graphene were successfully embedded in the polymer matrix. To further approve the synthesis of the nanocomposites, energy dispersive X-ray spectroscopy was served. Surface groups of the synthesized nanocomposites were studied by Fourier transform infrared and Raman spectroscopy. The morphology of the products was examined by scanning electron microscopy and transmission electron microscopy. It was found the fillers were successfully embedded in the polymer matrix and they were in nanometer scales. To investigate the magnetic properties and conductivity of the polymer nanocomposites, alternating gradient force magnetometer and four-point probe were used, respectively. Finally, the microwave absorption properties of the polymer nanocomposites were studied and it was found the fillers have different effects on the polymer microwave absorption value.     

Hierarchical porous onion-shaped LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> as ultrahigh-rate cathode material for lithium ion batteries

Spinel LiMn₂O₄ is a widely utilized cathode material for Li-ion batteries. However, its applications are limited by its poor energy density and power density. Herein, a novel hierarchical porous onion-like LiMn₂O₄(LMO) was prepared to shorten the Li⁺ diffusion pathway with the presence of uniform pores and nanosized primary particles. The growth mechanism of the porous onion-like LiMn₂O₄ was analyzed to control the morphology and the crystal structure so that it forms a polyhedral crystal structure with reduced Mn dissolution. In addition, graphene was added to the cathode (LiMn₂O₄/graphene) to enhance the electronic conductivity. The synthesized LiMn₂O₄/graphene exhibited an ultrahigh-rate performance of 110.4 mAh·g^–1 at 50 C and an outstanding energy density at a high power density, maintaining 379.4 Wh·kg^–1 at 25,293 W·kg^–1. Besides, it shows durable stability, with only 0.02% decrease in the capacity per cycle at 10 C. Furthermore, the (LiMn₂O₄/graphene)/graphite full-cell exhibited a high discharge capacity. This work provides a promising method for the preparation of outstanding, integrated cathodes for potential applications in lithium ion batteries.

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Synthesis and characterization of Cd-doped ZnMn<Subscript>2</Subscript>O<Subscript>4</Subscript> microspheres as supercapacitor electrodes

In this paper, we demonstrate the effects of Cd-doping ZnMn₂O₄ on structural and electrochemical performance. Cd-doped ZnMn₂O₄ spheres with diameters of about 2 μm were successfully synthesized by a facile hydrothermal method at 200 °C for 18 h. The fabricated Cd-doped ZnMn₂O₄ samples were characterized by X-ray diffraction, scanning electron microscopy, Brunauer Emmett Teller surface area analyzer and X-ray photoelectron spectroscopy. The electrochemical performance was investigated by cyclic voltammetry and electrochemical impedance spectrometry. The experimental results show that the synthesized spherical Cd-doped ZnMn₂O₄ exhibit far better rate capability and cyclic stability than that of pure spinel porous ZnMn₂O₄ microspheres. The result of cyclic voltammetry measurement indicates that the obtained Cd-doped ZnMn₂O₄ microspheres exhibited the high specific capacitance of 364 Fg^?1 at 2 mV/s.     

Er-Doped LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript>

LiMn_2-x Er_xO₄ (x ≤ 0.02) materials were synthesized by a rheological phase reaction method. The thermal behavior of the materials was examined by thermogravimetric and differential scanning calorimetry. X-ray diffraction showed that the samples (x ≤ 0.02 ) exhibited the same phase as the pure spinel. The lattice parameter of the Er-doped spinel was smaller than that of the undoped one and decreased with increasing doping level. Cyclic voltammograms showed two reversible processes corresponding to the typical response of spinel LiMn₂O₄ and revealed an insertion-extraction reaction occurring at two stages in the 4-V region. The electrochemical performances of the samples were studied and displayed a better reversibility and cyclability.__________From Neorganicheskie Materialy, Vol. 41, No. 6, 2005, pp. 740–743.Original English Text Copyright © 2005 by Haowen Liu, Li Song, Kelli Zhang.This article was submitted by the authors in English.     

Structural stability of In<Subscript>2</Subscript>O<Subscript>3</Subscript> films as sensor materials

A structural stability of In₂O₃ films in gas sensors was studied in conditions of intensive exploitation of sensor device at elevated temperatures. Structural changes of In₂O₃ films as well as a surface electromigration of In atoms were observed. The degradation effects are caused by simultaneous influence on In₂O₃ films of elevated temperatures and conditions of the working device. It was found that a structural degradation of In₂O₃ films in a sensor device could be suppressed using thin substrates. Fabrication of sensors with uniform In₂O₃ films led to improvement of their operational parameters.     

One-step hydrothermal synthesis of ZnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nano-octahedrons as a high capacity anode material for Li-ion batteries

Binary transition metal oxides are considered as promising anode materials for lithium-ion batteries (LIB), because they can effectively overcome the drawbacks of simple oxides. Here, a one-step hydrothermal method is described for the synthesis of regular ZnFe₂O₄ octahedrons about 200 nm in size at a low temperature without further annealing being required. The ZnFe₂O₄ octahedrons were characterized by powder X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. The electrochemical performance of the ZnFe₂O₄ octahedrons was examined in terms of cyclic voltammetry and discharge/charge profiles. The ZnFe₂O₄ octahedrons exhibit a high capacity of 910 mA·h/g at 60 mA/g between 0.01 and 3.0 V after 80 cycles. They also deliver a reversible specific capacity of 730 mA·h/g even after 300 cycles at 1000 mA/g, a much better performance than those in previous reports. A set of reactions involved in the discharge/charge processes are proposed on the basis of ex situ high-resolution transmission electron microscopy (HRTEM) images and selected area electron diffraction (SAED) patterns of the electrode materials. The insights obtained will be of benefit in the design of future anode materials for lithium ion batteries.      

Synthesis and microwave absorption properties of sandwich-type CNTs/Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>/RGO composite with Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> as a bridge

A novel sandwich-type CNTs/Fe₃O₄/RGO composite with Fe₃O₄ as a bridge was successfully prepared through a simple solvent-thermal and ultrasonic method. The structure and morphology of the composite have been characterized by Fourier-transform infrared spectroscopy, X-ray diffraction and scanning electron microscopy. This new structure can effectively prevent the agglomeration of GO and the combination of CNTs/Fe₃O₄ and RGO shows a strong reflection loss (R_L) (?50 dB) at 8.7 GHz with absorber thickness of 2.5 mm. Moreover, compared with CNTs/Fe₃O₄/GO composite, it is found that the thermal treating process is beneficial to enhance the microwave absorption properties, which may be attributed to high conductivity of RGO. On this basis, the microwave absorbing mechanism is systematically discussed. All the data show that the CNTs/Fe₃O₄/RGO composite exhibits excellent microwave absorption properties with light density and is expected to have potential applications in microwave absorption.     

Synthesis and structure of the CsSmP<Subscript>4</Subscript>O<Subscript>12</Subscript> cyclotetraphosphate (cubic CsNdP<Subscript>4</Subscript>O<Subscript>12</Subscript> structure)

CsSmP₄O₁₂ crystals have been prepared at 300°C in molten polyphosphoric acids containing Cs, Mg, and Sm cations, and their crystal structure has been determined: sp. gr. I \(\bar 4\) 3d, a = 15.1225(8) Å, Z = 12, CsNdP₄O₁₂ structure.     

Electrical conductivity of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-Tm<Subscript>2</Subscript>O<Subscript>3</Subscript>-Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> solid solutions

New solid solutions, Bi_2?x?y Tm _x Nb _y O_3+δ, with tetragonal and cubic structures have been synthesized in the Bi₂O₃-Tm₂O₃-Nb₂O₅ system, and their electrical conductivity has been measured at temperatures from 670 to 1020 K. The 1020-K conductivity of the tetragonal solid solution Bi_1.8Tm_0.15Nb_0.05O_3+δ is comparable to that of Bi_1.75Tm_0.25O₃, the best conductor in the Bi₂O₃-Tm₂O₃ system.     

Characterization and electrical properties of semiconducting Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-K<Subscript>2</Subscript>B<Subscript>4</Subscript>O<Subscript>7</Subscript> glasses

Semiconducting glasses of the Fe₂O₃-Bi₂O₃-K₂B₄O₇ system were prepared by the press-quenching method and their dc conductivity in the temperature range 223–393 K was measured. The glass transition temperature values (T_g) of the present glasses were larger than those of tellurite glasses. This indicates a higher thermal stability of the glass in the present system. The density for these glasses was consistent with the ionic size, atomic weight and amount of different elements in the glasses. Mössbauer results revealed that the relative fraction of Fe increases with increasing Fe₂O₃ content. Electrical conductivity showed a similar composition dependency as the fraction of Fe. The glasses had conductivities ranging from 10 to 10 Scm at temperatures from 223 to 393 K. Electrical conduction of the glasses was confirmed to be due to non-adiabatic small polaron hopping and the conduction was primarily determined by hopping carrier mobility.     

Reactions of V<Subscript>2</Subscript>O<Subscript>5</Subscript>, Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>, and Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> with AlN

Reactions of vanadium, niobium, and tantalum pentoxides with aluminum nitride have been studied using X-ray diffraction. At temperatures from 1000 to 1600°C, we have identified various V, Nb, and Ta nitrides. The composition of the niobium and tantalum nitrides depends on the reaction temperature. The tendency toward nitride formation becomes stronger in the order V₂O₅ < Ta₂O₅ < Nb₂O₅.