Development of redox deposition of birnessite-type MnO2 on activated carbon as high-performance electrode for hybrid supercapacitors

Birnessite-type MnO₂/activated carbon nanocomposites have been synthesized by directly reducing KMnO₄ with activated carbon in an aqueous solution. It is found that the morphologies of MnO₂ grown on activated carbon can be tailored by varying the reaction ratio of activated carbon and KMnO₄. An asymmetric supercapacitor with high energy density was fabricated by using MnO₂/activated carbon (MnO₂/AC) nanocomposite as positive electrode and activated carbon as negative electrode in 1 M Na₂SO₄ aqueous electrolyte. The asymmetric supercapacitor can be cycled reversibly in the cell voltage of 0–2 V, and delivers a specific capacitance of 50.6 F g⁻¹ and a maximum energy density of 28.1 Wh kg⁻¹ (based on the total mass of active electrode materials of 9.4 mg), which is much higher than that of MnO₂/AC symmetric supercapacitor (9.7 Wh kg⁻¹).     

Preparation of porous SnO2 helical nanotubes and SnO2 sheets

We report a surfactant-free chemical solution route for synthesizing one-dimensional porous SnO₂ helical nanotubes templated by helical carbon nanotubes and two-dimensional SnO₂ sheets templated by graphite sheets. Transmission electron microscopy, X-ray diffraction, cyclic voltammetry, and galvanostatic discharge–charge analysis are used to characterize the SnO₂ samples. The unique nanostructure and morphology make them promising anode materials for lithium-ion batteries. Both the SnO₂ with the tubular structure and the sheet structure shows small initial irreversible capacity loss of 3.2% and 2.2%, respectively. The SnO₂ helical nanotubes show a specific discharge capacity of above 800 mAh g⁻¹ after 10 charge and discharge cycles, exceeding the theoretical capacity of 781 mAh g⁻¹ for SnO₂. The nanotubes remain a specific discharge capacity of 439 mAh g⁻¹ after 30 cycles, which is better than that of SnO₂ sheets (323 mAh g⁻¹).     

Solution synthesis and electrochemical capacitance performance of Mn3O4 polyhedral nanocrystals via thermolysis of a hydrogen-bonded polymer

Hausmannite Mn₃O₄ polyhedral nanocrystals have been successfully synthesized via a simple solution-based thermolysis route using a three-dimensional hydrogen-bonded polymer as precursor. The as-obtained product was characterized by means of powder X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). Possible formation mechanism of polyhedral nanocrystals was proposed based on the role of organic ligand dissociation from the polymer precursor at elevated temperature. The electrochemical capacitance performance of Mn₃O₄ electrode was investigated by cyclic voltammetry and galvanostatic charge/discharge measurements. A maximum specific capacitance of 178 F g⁻¹ was obtained for the nanocrystals in a potential range from −0.1 to 0.8 V vs. SCE in a 0.5 M sodium sulfate solution at a current density of 0.2 A g⁻¹.     

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.     

Synthesis and electrochemical properties of nanorod-shaped LiMn1.5Ni0.5O4 cathode materials for lithium-ion batteries

Nanorod-shaped LiMn_1.5Ni_0.5O₄ cathode powders were synthesized by a co-precipitation method with oxalic acid. Their structures and electrochemical properties were characterized by SEM, XRD and galvanostatic charge-discharge tests. The resulting nanorod-shaped LiMn_1.5Ni_0.5O₄ cathode active materials delivered a specific discharge capacity of 126 mAh g⁻¹ at 0.1 C rate. These active materials exhibited better capacity retention and higher rate performance than those of LiMn_1.5Ni_0.5O₄ cathode powders with irregular morphology.     

Effect of Fe on the synthesis and electrochemical performance of nanostructured MnO2

Different MnO₂ nanostructures were synthesized in stoichiometric KMnO_4/MnSO₄ aqueous solutions in the absence/presence of Fe³⁺ at temperature ranging from 30 °C to 180 °C. The phase structures, morphologies and electrochemical properties of the as-prepared MnO₂ products were investigated using X-ray powder diffraction, scanning electron microscope, N₂ physical adsorption and cyclic voltammetry techniques. The results showed that the presence of Fe³⁺ addition had a significant effect on the phase structural evolution, morphological features and electrochemical properties of the MnO₂ products. Fe³⁺ was found to greatly prevent the epitaxial growth and crystallization of MnO₂ nucleus, which in turn influenced textual characteristics. The electrochemical performance of the nanostructured MnO₂ products had a complex relationship with the phase structures, specific surface area as well as pore characteristics. MnO₂ prepared in the presence of Fe³⁺ (KMF-MnO₂) showed relatively higher specific capacitance compared to that of MnO₂ prepared in the absence of Fe³⁺ (KM-MnO₂). Maximum capacitance of 214 F g⁻¹ was obtained for KMF-MnO₂ prepared at 30 °C at a scan rate of 2 mV s⁻¹ in 0.1 M Na₂SO₄ electrolyte.     

Microwave homogeneous synthesis of porous nanowire Co3O4 arrays with high capacity and rate capability for lithium ion batteries

In this paper, an efficient microwave-assisted homogeneous synthesis approach by urea hydrolysis is used to synthesize cobalt-basic-carbonate compounds. The dimensions and morphology of the synthesized precursor compounds are tailored by changes in the incorporated anions (CO₃²⁻ and OH⁻) under different conditions of temperature and time under microwave irradiation. The wire-like cobalt-basic-carbonate compound self-assembles into one-dimensional porous arrays of Co₃O₄ nanowires constructed of interconnected Co₃O₄ nanocrystals along the [1 1 0] axis after thermal decomposition at 350 °C. The textural characteristics of the Co₃O₄ products have strong positive effects on their electrochemical properties as electrode materials in lithium-ion batteries. The obtained porous nanowire Co₃O₄ arrays exhibit excellent capacity retention and rate capability at higher current rates, and their reversible capacity of 600 mAh g⁻¹ can be maintained after 100 cycles at the high current rate of 400 mA g⁻¹.     

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.     

Electrochemical capacitors of flower-like and nanowire structured MnO2 by a sonochemical method

Nanostructured manganese dioxide, MnO₂, was synthesized by a sonochemical method. Nanostructured MnO₂ had the shapes of flower-like and nanowires by changing the pH in the aqueous solution, as observed via scanning electron microscopy and transmission electron microscopy. The electrochemical capacitance was studied by cyclic voltammetry. A maximum specific capacitance of 300 Fg⁻¹ was obtained for the nanowires in a potential range from 0.1 to 0.9 V vs. SCE in 1 M sodium sulfate solution at a scan rate of 5 mV s⁻¹. These materials can be useful to increase the specific capacitance by wetting behavior of electrolytes from their structural properties.     

Enhanced reduction properties of mesostructured Ce0.5Zr0.5O2 solid solutions

Mesostructured Ce_0.5Zr_0.5O₂ solid solutions (M-CZ) were hydrothermally synthesized using Gemini surfactant as the template. X-ray diffraction (XRD), small-angle X-ray diffraction (SAXRD), N₂ adsorption–desorption isotherms and high-resolution transmission electronic microscopy (HRTEM) were adopted to characterize the samples. The product had a surface area of 123.5 m² g⁻¹ with maximum oxygen storage capacity (OSC) of 0.58 mol O₂/mol Ce. Oxygen anions in the M-CZ can be repeatedly released and resumed during the redox recycles. Reduction of Ce⁴⁺ to Ce³⁺ or lower valence and Zr⁴⁺ to Zr³⁺ were fulfilled with an obvious color change during the temperature programmed reduction (TPR) process, while the crystal structure of the product remained unchanged even after severe reduction. The mesostructure of the product can improve the reductive ability of Ce⁴⁺ and Zr⁴⁺ cations, which was beneficial to the enhancement of OSC.     

Hydrothermal synthesis of Co3O4 with different morphologies and the improvement of lithium storage properties

A simple hydrothermal treatment was developed to synthesize Co₃O₄ powders with different morphologies in mass production by using hexamethylenetetramine (HMT, C₆H₁₂N₄) as a precipitator. By changing the initial HMT concentrations, the prepared Co₃O₄ powders were readily regulated in its morphologies, which varied from microsphere to urchin-like hollow microsphere, and finally to collapsed porous structure. Moreover, the four Co₃O₄ powders with different HMT concentrations had been applied in the negative electrode materials for lithium ion batteries, which exhibited different electrochemical properties. The present research demonstrated that morphology was one of the crucial factors that affected the electrochemical properties of electrodes. The capacity retention of sample with an original Co(NO₃)₂:HMT mole ratio of 1:1 is almost above 94% from the 5th cycle at different current densities of 40 and 60 mA g⁻¹, exhibiting the better long-life stability and favorable electrochemical behaviors due to its higher specific surface area (97.1 m² g⁻¹) and the uniform urchin-like hollow structure.     

Solvothermal synthesis of sheet-like Co3O4 and Ag/Co3O4 nanocomposites and their electrocatalysis performances

Precursors of Co₃O₄ and Ag/Co₃O₄ composites with sheet-like shape were synthesized with assistance of ethylene glycol via a solvothermal process. The final samples were obtained by calcining each precursor at 400 °C. The as-prepared samples were identified and characterized by thermogravimetric analysis (TG) and differential thermal gravimetric (DTG) analysis, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and field emission scanning electron microscopy (FE-SEM). The Co₃O₄ and Ag/Co₃O₄ composite nanosheets were used as electrocatalysts modified on a glassy carbon electrode for p-nitrophenol and H₂O₂ reduction respectively in a basic solution. The electrocatalytic results showed that p-nitrophenol could be reduced by pure Co₃O₄ at a large peak current but a rather higher peak potential, and could be reduced effectively by Ag/Co₃O₄ composites at lower potential. Ag/Co₃O₄ composites with 6% Ag displayed the highest electrocatalytic activity for H₂O₂ reduction at the largest peak current and a lower peak potential. The reduction peak potentials of H₂O₂ all reduced a great deal using Ag/Co₃O₄ composite.     

A novel single-step synthesis of N-doped TiO2 via a sonochemical method

A novel single-step synthetic method for the preparation of anatase N-doped TiO₂ nanocrystalline at low temperature has been devoleped. The N-doped anatase TiO₂ nanoparticles were synthesized by sonication of the solution of tetraisopropyl titanium and urea in water and isopropyl alcohol at 80 °C for 150 min. The as-prepared sample was characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and UV–vis absorption spectrum. The product structure depends on the reaction temperature and reaction time. The photocatalytic activity of the as-prepared photocatalyst was evaluated via the photodegradation of an azo dye direct sky blue 5B. The results show that the N-doped TiO₂ nanocrystalline prepared via sonication exhibit an excellent photocatalytic activity under UV light and simulated sunlight.     

The electrocatalytic application of RuO2 in direct borohydride fuel cells

A high electrocatalytic activity of RuO₂ has been found for oxygen reduction reaction (ORR) in the cathode of direct borohydride fuel cells (DBFCs). The electron transfer number n during the ORR changes from 3.58 to 3.86 and the percentage of the intermediate product H₂O₂ decreases from 20.8% to 7.2% correspondingly when the disk potential scans negatively from −0.39 V to −0.8 V versus Hg/HgO. Peak power densities of 425 mW cm⁻² has been obtained at 60 °C, when RuO₂ has been used as a cathodic catalyst in DBFCs. RuO₂ displays low sensitivity to the BH₄⁻ oxidation in DBFCs. Moreover, RuO₂, as a cathodic catalyst, demonstrates a superb stability during a 200-h durability test. The identical X-ray diffraction (XRD) patterns of the RuO₂ before and after the durability test also prove its stability.     

Controllable synthesis of monodisperse Mn3O4 and Mn2O3 nanostructures via a solvothermal route

A facile one-step solvothermal route was developed to synthesize monodisperse Mn₃O₄ and Mn₂O₃ nanostructures with the introduction of poly(vinyl-pyrrolidone)/stearic acid (PVP/SA) mixture. H₂O₂ played a key role in the determination of the products Mn₃O₄ and Mn₂O₃. The synthesis parameters for nanostructured MnO_x such as surfactant and reaction time were investigated, along with their influences on morphology and composition. The morphology evolution of the Mn₃O₄ and Mn₂O₃ reveals that the nanostructures formed via two distinct mechanisms of nucleation and growth of nanocrystals.     

Morphologies and electrochemical properties of 0.6Li2MnO3·0.4LiCoO2 composite cathode powders prepared by spray pyrolysis

Nanosized 0.6Li₂MnO₃·0.4LiCoO₂ composite cathode powders are prepared by spray pyrolysis. The micron-sized composite powders are converted into nanosized powders by a simple milling process. The mean sizes of the composite powders measured from the TEM images increase from 20 to 170 nm when the post-treatment temperatures increase from 650 to 900 °C. The Brunauer–Emmett–Teller surface areas of the composite powders post-treated at 650 and 900 °C are 24 and 3 m² g⁻¹, respectively. The XRD patterns indicate that the layered composite powders post-treated at 800 and 900 °C have high crystallinity and low cation mixing. The mean crystallite sizes of the powders, measured from the (003) peak widths of the XRD patterns using Scherrer's equation, are 35 and 56 nm at post-treatment temperatures of 800 and 900 °C, respectively. The initial discharge capacities of the 0.6Li₂MnO₃·0.4LiCoO₂ composite are 262, 267, 264, and 263 mAh g⁻¹ when the post-treat temperatures of the powders are 650, 700, 800, and 900 °C, respectively. The discharge capacity of the composite powders post-treated at 900 °C abruptly decreases from 263 to 214 mAh g⁻¹ by the seventh cycle and then slowly decreases to 198 mAh g⁻¹ with increasing cycle number, up to 30.     

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.     

Controllable synthesis of hierarchical nanostructures of CaWO4 and SrWO4 via a facile low-temperature route

CaWO₄ and SrWO₄ nanostructures have been synthesized via a simple microemulsion-mediated route. With careful control of the fundamental experimental parameters including the concentration of reactants, the reaction time and the temperature, the products with different morphologies of dumbbell, coral, rod and dendrite have been obtained, respectively. The possible formation mechanism of these unique morphologies has been proposed based on surfactant self-assembly under different experimental conditions. The as-synthesized CaWO₄ samples with various morphologies exhibit different photoluminescence properties. X-ray powder diffraction, transmission electron microscopy, field-emission scanning electron microscopy, and luminescence spectroscopy were used to characterize these products.     

Synthesis, characterization, and hydrophobic properties of Bi2S3 hierarchical nanostructures

Novel Bi₂S₃ hierarchical nanostructures self-assembled by nanorods are successfully synthesized in mild benzyl alcohol system under hydrothermal conditions. The hierarchical nanostructures exhibit a flower-like shape. X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED) were used to characterize the as-synthesized samples. Meanwhile, the effect of various experimental parameters including the concentration of reagents and reaction time on final product has been investigated. In our experiment, PVP plays an important role for the formation of the hierarchical nanostructures and the possible mechanism was proposed. In addition, Bi₂S₃ film prepared from the flower-like hierarchical nanostructures exhibits good hydrophobic properties, which may bring nontrivial functionalities and may have some promising applications in the future.     

Preparation of Fe-doped TiO2 nanotube arrays and their photocatalytic activities under visible light

Fe-doped TiO₂ nanotube arrays have been prepared by the template-based liquid phase deposition method. Their morphologies, structures and optical properties were investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and UV-vis absorption spectroscopy. Their photocatalytic activities were evaluated by the degradation of methylene blue under visible light. The UV-vis absorption spectra of the Fe-doped TiO₂ nanotube arrays showed a red shift and an enhancement of the absorption in the visible region compared to the undoped sample. The Fe-doped TiO₂ nanotube arrays exhibited good photocatalytic activities under visible light irradiation, and the optimum dopant amount was found to be 5.9 at% in our experiments.