Preparation and characterization of nanostructured NiO/MnO2 composite electrode for electrochemical supercapacitors

Nanostructured nickel-manganese oxides composite was prepared by the sol-gel and the chemistry deposition combination new route. The surface morphology and structure of the composite were characterized by scanning electron microscope and X-ray diffraction. The as-synthesized NiO/MnO₂ samples exhibit higher surface area of 130-190 m² g⁻¹. Cyclic voltammetry and galvanostatic charge/discharge measurements were applied to investigate the electrochemical performance of the composite electrodes with different ratios of NiO/MnO₂. When the mass ratio of MnO₂ and NiO in composite material is 80:20, the specific capacitance value of NiO/MnO₂ calculated from the cyclic voltammetry curves is 453 F g⁻¹, for pure NiO and MnO₂ are 209, 330 F g⁻¹ in 6 mol L⁻¹ KOH electrolyte and at scan rate of 10 mV s⁻¹, respectively. The specific capacitance of NiO/MnO₂ electrode is much larger than that of each pristine component. Moreover, the composite electrodes showed high power density and stable electrochemical properties.     

Preparation and electrochemical properties of lamellar MnO2 for supercapacitors

Lamellar birnessite-type MnO₂ materials were prepared by changing the pH of the initial reaction system via hydrothermal synthesis. The interlayer spacing of MnO₂ with a layered structure increased gradually when the initial pH value varied from 12.43 to 2.81, while the MnO₂, composed of α-MnO₂ and γ-MnO₂, had a rod-like structure at pH 0.63. Electrochemical studies indicated that the specific capacitance of birnessite-type MnO₂ was much higher than that of rod-like MnO₂ at high discharge current densities due to the lamellar structure with fast intercalation/deintercalation of protons and high utilization of MnO₂. The initial specific capacitance of MnO₂ prepared at pH 2.81 was 242.1 F g⁻¹ at 2 mA cm⁻² in 2 mol L⁻¹ (NH₄)₂SO₄ aqueous electrolyte. The capacitance increased by about 8.1% of initial capacitance after 200 cycles at a current density of 100 mA cm⁻².     

Graphene sheets anchored with ZnO nanocrystals as electrode materials for electrochemical capacitors

An efficient approach consisted of modified Hummers' method, pulse microwave heating, and homogenizing dispersion has been demonstrated to prepare ZnO/graphene hybrid as electrode material for electrochemical capacitors (ECs). Highly-crystalline ZnO nanoparticles are anchored with graphene sheets, forming three-dimensional framework. The electrochemical properties of ZnO/graphene hybrids are characterized by cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge–discharge cycling. The EC equipped with ZnO/graphite hybrid (1:5 in w/w) exhibit the improved performance in terms of specific capacitance, high rate capability, and excellent cycling stability, as compared with blank graphene electrode. The maximum energy and power densities of ZnO/graphite capacitor can be obtained as high as 66 Wh kg⁻¹ and 15.2 kW kg⁻¹, respectively. The improved performance can be ascribed to the insertion of ZnO nanocrystals between individual graphite sheets, creating a conducting framework that provides more active sites for the formation of electric double layer. The unique framework allows the electrolyte ions to diffuse easily into the interior channels, leading to small inner resistance.     

Facile synthesis of α-MnO2 nanostructures for supercapacitors

Various α-MnO₂ nanostructures have been successfully synthesized by a simple hydrothermal method based on the redox reactions between the MnO₄⁻ and H₂O in mixture containing KMnO₄ and HNO₃. The effect of varying the hydrothermal time to synthesize MnO₂ nanostructures and the forming mechanism of α-MnO₂ nanorods were investigated by using XRD, SEM and TEM. The results revealed an evolvement of morphologies ranging from brushy spherical morphology to nanorods depending upon the hydrothermal time. The surface area of the synthesized nanomaterials varied from 89 to 119 m²/g. Electrochemical properties of the products were evaluated using cyclic voltammetry and galvanostatic charge–discharge studies, and the sample obtained by hydrothermal reaction for 6 h at 120 °C showed maximum capacitance with a value of 152 F/g. In addition, long cycle life and excellent stability of the material were also demonstrated.     

Preparation and electrochemical characteristics of mesoporous carbon spheres for supercapacitors

Mesoporous carbon spheres serving as electrode materials for supercapacitors were synthesized by a facile polymerization-induced colloid aggregation method using melamines as a carbon precursor and commercial colloidal silica as a silica source for hard template. After the carbonization of as-formed resins-template composites at 1000 °C and the removal of the silica template by hydrofluoric acid, the resulting mesoporous carbon spheres with a diameter size of ∼5 μm, specific surface area (up to 1280 m²/g) and uniform pore size as large as 30 nm could be obtained. Due to the enriched nitrogen content and the large pore size of the mesoporous carbon spheres affecting the surface wettability, resistance, and ion diffusion process in the pores, the mesoporous carbon spheres showed a high specific capacitance of 196 F/g in 5 mol/l H₂SO₄ electrolytes at a discharge current density of 1 A/g.     

Preparation and electrochemical properties of nickel oxide as a supercapacitor electrode material

Hydrothermal synthesis has been introduced to fabricate NiO precursor at different temperatures, then nanostructured NiO with a distinct flake-like morphology was obtained via heating at low temperature. The NiO nanoflakes are 50-80 nm in width and 20 nm in thickness. The electrochemical capacitive characterization of the as-prepared NiO was studied in 2 M KOH electrolyte solution. The as-prepared NiO exhibits excellent cycle performance and keeps 91.6% initial capacity over 1000 charge-discharge cycles. Electrochemical impedance spectroscopy study reveals that the NiO electrode is controlled by the mass transfer limitation, and its internal resistance is 0.2 Ω. A specific capacitance approximate to 137.7 F g⁻¹ could be achieved at the current density of 0.2 A g⁻¹ in the potential window of 0-0.46 V in 2 M KOH electrolyte solution, due to higher surface area of NiO nanoflakes, which facilitates transport of electrolyte ions during rapid charge/discharge process. Due to higher surface area of NiO nanoflakes, which facilitates transport of electrolyte ions during rapid charge/discharge process.     

Preparation of flower-like CuO by a simple chemical precipitation method and their application as electrode materials for capacitor

A novel CuO electrode material with flower-like nanostructures was fabricated at a low temperature (80 °C) by a simple chemical precipitation method. Scanning electron microscopy (SEM) results showed that CuO with spherical and flower-like structure can be formed under a weak alkali (C₆H₁₂N₄), and CuO with sheets structure can be obtained under a strong alkali (NaOH). A possible growth mechanism of CuO nanocrystals was discussed. The flower-like CuO electrode exhibited a higher specific capacitance (133.6 Fg⁻¹) and an excellent cycle performance at a high current density of 10 mA/cm². Specific capacitance of flower-like CuO was 405.3% higher than globular CuO (26.44 Fg⁻¹) at 2 mA/cm².     

VO2(B) nanospheres: Hydrothermal synthesis and electrochemical properties

Monodisperse VO₂(B) nanospheres with an average size of 100 nm were synthesized by hydrothermal method. Experiments showed that the surfactant octadecyl-amine played an important role during the formation of the nanospheres, and possible mechanism was suggested. Moreover, the potential uses of VO₂ nanospheres were primarily probed as electrodes for lithium-ion batteries.     

Synthesis and electrochemical properties of LiV3O8 phase

Lithium vanadium oxide was synthesized by a new method in which LiOH, V₂O₅ and NH₄OH were used as the starting materials to synthesize a precursor containing Li and V, and then obtain the resulting product by calcining the precursor. The LiV₃O₈ compound prepared by this synthesis method gave a good charge-discharge and cycle performance. A specific capacity of 258 mAh/g is obtained in the range of 1.8−4.0 V in the first cycle and 247 mAh/g in the eighth cycle.     

Synthesis and electrochemical characteristics of Sn-Sb-Ni alloy composite anode for Li-ion rechargeable batteries

Micro-scaled Sn-Sb-Ni alloy composite was synthesized from oxides of Sn, Sb and Ni via carbothermal reduction. The phase composition and electrochemical properties of the Sn-Sb-Ni alloy composite anode material were studied. The prepared alloy composite electrode exhibits a high specific capacity and a good cycling stability. The lithiation capacity was 530 mAh g⁻¹ in the first cycle and maintained at 370-380 mAh g⁻¹ in the following cycles. The good electrochemical performance may be attributed to its relatively large particle size and multi-phase characteristics. The former reason leads to the lower surface impurity and thus the lower initial capacity loss, while the latter results in a stepwise lithiation/delithiation behavior and a smooth volume change of electrode in cycles. The Sn-Sb-Ni alloy composite material shows a good candidate anode material for the rechargeable lithium ion batteries.     

Synthesis of Ru/multiwalled carbon nanotubes by microemulsion for electrochemical supercapacitor

An efficient way to decorate multiwalled carbon nanotubes with Ru had been developed. In this method, Ru nanoparticles were prepared by water-in-oil reverse microemulsion, and the produced Ru anchored on MWCNTs. Transmission electron microscopy (TEM) result showed that RuO₂ nanoparticles had the uniform size distribution after electrochemical oxidation. Energy dispersive X-rays (EDX) spectra elucidated the presence of ruthenium oxide in the as-prepared composites after electrochemical oxidation. Cyclic voltammetry result demonstrated that a specific capacitance of deposited ruthenium oxide electrode was significantly greater than that of the pristine MWCNTs electrode in the same medium.     

Preparation of graphene/TiO2 anode materials for lithium-ion batteries by a novel precipitation method

This paper reports a large-scale production route for graphene/TiO₂ nanocomposites using water-based in situ precipitation method. In this method, freshly prepared graphene oxides/TiO₂ obtained by precipitating Ti(SO₄)₂ with NH₃H₂O was subjected to heat treatment in the presence of N₂, which resulted in the formation of graphene/TiO₂ nanocomposites. Graphene/TiO₂ composites prepared by our method were found to be suitable as anode materials for lithium ion batteries because of its stable cycling performance and high capacity.     

Preparation, characterization, and electrochemical application of mesoporous copper oxide

Mesoporous CuO was successfully synthesized via thermal decomposition of CuC₂O₄ precursors. These products had ring-like morphology, which was made up of nanoparticles with the average diameter of 40 nm. The electrochemical experiments showed that the mesoporous CuO decreased the overvoltage of the electrode and increased electron transference in the measurement of dopamine.     

Microwave synthesis of nanocrystalline Sb2S3 and its electrochemical properties

Nanocrystalline antimony trisulfide (Sb₂S₃) was successfully synthesized via microwave irradiation by the reaction of antimony trichloride (SbCl₃) and thiourea (CS(NH₂)₂) with PVP as the surfactant. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and high resolution TEM (HRTEM). XRD results show that the as-prepared sample is orthorhombic-phase Sb₂S₃. TEM image of the as-prepared Sb₂S₃ shows the rod-like structure. HRTEM image indicates that rodbundles of Sb₂S₃ consists of a number nanorods with the diameter ranging from 30 nm to 50 nm. Detailed HRTEM image demonstrates the preferential direction growth of the Sb₂S₃ nanorods. The electrochemical properties of Sb₂S₃ were primarily investigated by constant current charge/discharge cycling tests in lithium hexafluorophosphate (LiPF₆) solution. The possible electrochemical reaction mechanism was explained. The results indicate that the nanocrystalline Sb₂S₃ shows potential application in the field of the electrode materials.     

Synthesis and electrochemical behavior of nanostructured cauliflower-shape Co-Ni/Co-Ni oxides composites

Nanostructured Co-Ni/Co-Ni oxides were electrochemically deposited onto stainless steel electrode by electrochemical method and characterized for their structural and supercapacitive properties. The SEM images indicated that the obtained Co-Ni/Co-Ni oxides had cauliflower-type nanostructure. The X-ray diffraction pattern showed the formation of Co₃O₄, NiO, Co and Ni. The EDX elemental mapping images indicated that Ni, Co and O are distributed uniformly. The deposited Co-Ni/Co-Ni oxides showed good supercapacitive characteristics with a specific capacitance of 331 F/g at 1 mA/cm² current density in 1 M KOH electrolyte. A mechanism of the formation of cauliflower-shape Co-Ni/Co-Ni oxides was proposed. A variety of promising applications in the fields such as energy storage devices and sensors can be envisioned from Co-Ni/Co-Ni oxides.     

Surfactant-free synthesis and electrochemical properties of chrysanthemum-like InVO4 hierarchical microstructures

Novel chrysanthemum-like hierarchical microstructures of orthorhombic InVO₄ were synthesized via a hydrothermal route without assistance of any template or organic additive. The chrysanthemum-like InVO₄ microstructures are built up of numerous nanobelts radially aligned around the spherical surface. Based on the structural feature of orthorhombic InVO₄ and the key role of the pH value, a probable mechanism of the etching-splitting growth process induced by H⁺ ions was proposed to explain the formation of InVO₄ microstructures. Furthermore, the chrysanthemum-like InVO₄ sample shows a high discharge capacity of 608.6 mAh g⁻¹ and acceptable capacity retention when used as an electrode material in lithium ion batteries. The pure orthorhombic phase and unique porous morphology play basic roles in the structural requirement to serve as transport paths for lithium ion.     

Synthesis, structure, and electrochemical characteristics of LiNi1/3Co1/3Mn1/3O2 prepared by thermal polymerization

A thermal polymerization route was adopted to synthesize layered LiNi_1/3Co_1/3Mn_1/3O₂ materials. After annealing the polymer gel containing metal salts at different temperatures from 850 to 1000 °C for different time between 6 and 25 h, powders of pure α-NaFeO₂ phase were obtained. The crystal structure, morphology and electrochemical properties of the products were investigated by XRD, SEM, electrochemical cell cycling and AC impedance spectroscopy. It is found that the powder annealed at 950 °C for 15 h shows the best electrochemical property with the first specific discharge capacity of 188 mAh/g at C/10 and 87% retention after 100 cycles. It exhibits good rate capability with the specific capacity of 169 mAh/g at 1 C and 110 mAh/g at 6 C. Adopting a slowly cooling procedure during the powder annealing can improve the electrochemical performance of the LiNi_1/3Co_1/3Mn_1/3O₂ powder.     

Preparation and electrochemical properties of multiwalled carbon nanotubes-nickel oxide porous composite for supercapacitors

Porous nickel oxide/multiwalled carbon nanotubes (NiO/MWNTs) composite material was synthesized using sodium dodecyl phenyl sulfate as a soft template and urea as hydrolysis-controlling agent. Scanning electron microscopy (SEM) results show that the as-prepared nickel oxide nanoflakes aggregate to form a submicron ball shape with a porous structure, and the MWNTs with entangled and cross-linked morphology are well dispersed in the porous nickel oxide. The composite shows an excellent cycle performance at a high current of 2 A g⁻¹ and keeps a capacitance retention of about 89% over 200 charge/discharge cycles. A specific capacitance approximate to 206 F g⁻¹ has been achieved with NiO/MWNTs (10 wt.%) in 2 M KOH electrolyte. The electrical conductivity and the active sites for redox reaction of nickel oxide are significantly improved due to the connection of nickel nanoflakes by the long entangled MWNTs.     

One-pot syntheses of Li3V2(PO4)3/C cathode material for lithium ion batteries via ascorbic acid reduction approach

Monoclinic Li₃V₂(PO₄)₃/C composite synthesized by ascorbic acid reduction method is examined as a cathode material for Li-ion batteries. Transmission electron microscopy (TEM) images show that the nano-size particles are obtained. The reversible capacity of Li₃V₂(PO₄)₃/C prepared with LiOH and H₃PO₄ is 141.2 mAh g⁻¹ after 100 cycles at 1C discharge rate between 3 V and 4.8 V, and the retention rates of discharge capacity is 93.4%. Ascorbic acid plays not only as reduction reagent, but also as carbon sources. This strategy shortens the time of solid state reaction and facilitates the procedure of synthesis. Effects of different precursors materials on the performance of the Li₃V₂(PO₄)₃/C are investigated.     

Formation of hollow NiO single crystals and Ag/NiO flowers

In this work, the Ni(OH)₂ core-shell nanostructures were fabricated by a hydrothermal method without using any templates. After the core-shell sample was annealed, the novel hollow NiO nanostructures were obtained. The Ostwald ripening process was proposed to elucidate the formation of the samples. Moreover, Ag/NiO flowers were prepared by impregnating hollow NiO particles with Ag(NO₃)₃ solution. The samples were characterized by TEM, HREM, SAED, XRD, BET, SEM and EDS methods. Most importantly, Ag/NiO flowers and hollow NiO nanostructures exhibited the excellent catalytic activities for CO oxidation.