Tubular graphene nanoribbons with attached manganese oxide nanoparticles for use as electrodes in high-performance supercapacitors

Graphene nanoribbons (GNRs) with tubular shaped thin graphene layers were prepared by partially longitudinal unzipping of vapor-grown carbon nanofibers (VGCFs) using a simple solution-based oxidative process. The GNR sample has a similar layered structure to graphene oxide (GO), which could be readily dispersed in isopropyl alcohol to facilitate electrophoretic deposition (EPD). GO could be converted to graphene after heat treatment at 300 °C. The multilayer GNR electrode pillared with open-ended graphene tubes showed a higher capacitance than graphene flake and pristine VGCF electrodes, primarily due to the significantly increased surface area accessible to electrolyte ions. A GNR electrode with attached MnO₂ nanoparticles was prepared by EPD method in the presence of hydrated manganese nitrate. The specific capacitance of GNR electrode with attached MnO₂ could reach 266 F g⁻¹, much higher than that of GNR electrode (88 F g⁻¹) at a discharge current of 1 A g⁻¹. The hydrophilic MnO₂ nanoparticles attached to GNRs could act as a redox center and nanospacer to allow the storage of extra capacitance.     

High performance wire-shaped supercapacitive electrodes based onactivated carbon fibers core/manganese dioxide shell structures

Micro/nano hierarchical structures with uniformly patterned nanostructures shell and activated internal core are promising for boosting electrochemical performance. Here we report the fabrication of wire-shaped supercapacitive electrodes with manganese dioxide (MnO₂) nanostructures shell integrated onto activated carbon fiber (ACF) core. The ACF core is doped with nitrogen heteroatom and shows good conductivity and hydrophilicity, which endow fast ion and electron transport and high accessibility of electrolyte. The MnO₂ nanostructures shell integrated on the ACF core by electrodeposition method together provide significant pseudocapacitive contribution associated with fast faradaic reactions. The electrochemical performance of the fabricated electrodes was evaluated by cyclic voltammetry, galvanostatic charging/discharging and electrochemical impedance spectroscopy techniques. The integrated wire-shaped electrodes, which boost the synergetic effect of MnO₂ nanostructures and ACF, have excellent current collecting capabilities thus resulting high electrochemical performance (with the specific capacitance of 26.64 mF cm⁻¹ at the current density of 0.1 mA cm⁻¹ and 96% capacitance retention after 8000 charging/discharging cycles at the current density of 1 mA cm⁻¹).     

A high-performance asymmetric supercapacitor based on carbon and carbon–MnO2 nanofiber electrodes

An asymmetric supercapacitor with high energy and power densities has been fabricated using MnO₂/carbon nanofiber composites as positive electrode and activated carbon nanofibers as negative electrode in Na₂SO₄ aqueous electrolyte. Both electrode materials are freestanding in nature without any conductive additives or binders and exhibit outstanding electrochemical performances. The as-assembled asymmetric supercapacitor with optimal mass ratio can be operated reversibly over a wide voltage range of 0–2.0 V, and presents a maximum energy density of 30.6 Wh kg⁻¹, which is much higher than those of symmetric supercapacitors. Moreover, the supercapacitor exhibits excellent rate capability (high power density of 20.8 kW kg⁻¹ at 8.7 Wh kg⁻¹) and long-term cycling stability with only 6% loss of its initial capacitance after 5000 cycles. These attractive results make these freestanding materials promising for applications in aqueous electrolyte-based asymmetric supercapacitors with high energy and power densities delivery.     

Fabrication of manganese oxide/three-dimensional reduced graphene oxide composites as the supercapacitors by a reverse microemulsion method

Manganese oxide (MnO₂)/three-dimensional (3D) reduced graphene oxide (RGO) composites were prepared by a reverse microemulsion (water/oil) method. MnO₂ nanoparticles (3–20 nm in diameter) with different morphologies were produced and dispersed homogeneously on the macropore surfaces of the 3D RGO. Scanning electron microscopy and transmission electron microscopy were applied to characterize the microstructure of the composites. The MnO₂/3D RGO composites, which were annealed at 150 °C, displayed a significantly high specific capacitance of 709.8 F g⁻¹ at 0.2 A g⁻¹. After 1000 cycles, the capacitance retention was measured to be 97.6%, which indicates an excellent long-term stability of the MnO₂/3D RGO composites.     

One-pot synthesis of core–shell-structured tin oxide–carbon composite powders by spray pyrolysis for use as anode materials in Li-ion batteries

Core–shell-structured tin oxide–carbon composite powders with mixed SnO₂ and SnO tetragonal crystals are prepared by one-pot spray pyrolysis from a spray solution with tin oxalate and polyvinylpyrrolidone (PVP). The aggregate, made up of SnO_x nanocrystals (several tens of nanometers), is uniformly coated with an amorphous carbon layer. The initial discharge capacities of the bare SnO₂ and SnO_x–carbon composite powders at a current density of 1 A g⁻¹ are 1473 and 1667 mA h g⁻¹, respectively; their discharge capacities after 500 cycles are 78 and 1033 mA h g⁻¹, respectively. The SnO_x–carbon composite powders maintain their spherical morphology even after 500 cycles. On the other hand, the bare SnO₂ powder breaks into several pieces after cycling. The structural stability of the SnO_x–carbon composite powders results in a low charge transfer resistance and high lithium ion diffusion rate even after 500 cycles at a high current density of 2 A g⁻¹. Therefore, the SnO_x–carbon composite powders have superior electrochemical properties compared with those of the bare SnO₂ powders with a fine size.     

Decoration of carbon cloth by manganese oxides for flexible asymmetric supercapacitors

Here we describe the production of carbon cloth coated with MnO₂ nanosheets or MnOOH nanorods through a normal temperature reaction or a hydrothermal approach, respectively. Of note, the electrochemical performance of MnO₂-coated carbon cloth was better (429.2 F g⁻¹) than that of MnOOH-coated carbon cloth. When the MnO₂-coated carbon cloth is introduced as the positive electrode and the Fe₂O₃-coated carbon cloth as the negative electrode, a flexible asymmetric supercapacitor was obtained with an energy density of 22.8 Wh kg⁻¹ and a power density of 159.4 W kg⁻¹. Therefore, such a hierarchical MnO₂-coated carbon cloth nanocomposite is a promising high-performance electrode for flexible supercapacitors.     

Low-temperature catalytic oxidation of chlorobenzene over MnOX/TiO2–CNTs nano-composites prepared by wet synthesis methods

Catalytic oxidation of chlorobenzene (CB) was studied over MnO_X/TiO₂–CNTs (carbon nanotube) nano-composites prepared by the solvothermal and sol–gel methods. Microstructures and redox properties of these composites were characterized by X-ray diffraction, electron microscopy, and temperature-programmed reduction. The catalytic activity for CB oxidation was promoted with the introduction of CNTs into MnO_X/TiO₂, and CB oxidation efficiencies of 90% and almost 100% could be obtained at 150 °C and 300 °C, respectively, under a gas hour space velocity of 36,000 h^− 1.The high catalytic performance could be attributed to the good dispersion of active component and the selective adsorption of CB by CNTs.     

Electrodeposited hybrid films of polyaniline and manganese oxide in nanofibrous structures for electrochemical supercapacitor

Hybrid films of polyaniline (PANI) and manganese oxide (MnO_x) were obtained through potentiodynamic deposition from solutions of aniline and MnSO₄ at pH 5.6. The hybrid films demonstrated characteristic redox behaviors of PANI in acidic aqueous solution. Characterization of the hybrid films by XRD indicated the amorphous nature of MnO_x in the films in which manganese existed in oxidation states of +2, +3 and +4, based on XPS measurement. Hybrid film of PANI and MnO_x, PM120 obtained from the solution of 0.1 M aniline and 120 mM Mn²⁺ displayed a well opened nanofibrous structure which showed an 44% increase in specific capacitance from that of PANI (408 F g^?1) to 588 F g^?1, measured at 1.0 mA cm^?2 in 1 M NaNO₃ (pH 1). The hybrid film kept more than 90% of its capacitance after 1000 charging-discharging cycles, with a coulombic efficiency of 98%. The specific capacitance of a symmetric capacitor using PM120 as the electrodes is 112 F g^?1.     

Influence of Ce addition on the structural,magnetic, and magnetocaloric properties in La0.7−xCexSr0.3MnO3 (0≤x≤0.3) ceramic compound

We report improvement in the magnetocaloric properties of Ce-doped lanthanum manganites. Polycrystalline La_0.7−xCe_xSr_0.30MnO₃ (0≤x≥0.3) samples were prepared using the conventional solid-state reaction method with phase purity and structure confirmed using X-ray diffraction. Temperature dependent magnetization measurements and Arrott analysis reveal second order ferromagnetic transition in parent sample and as well as in doped sample with Curie temperature decreasing progressively with increasing Ce-concentration from ~370 K for x=0.0 to 310 K for x=0.30. Magnetic entropy change (ΔS_M) was calculated by applying the thermodynamic Maxwell equation to a series of isothermal field dependent magnetization curves. A large ΔS_M associated with the ferromagnetic–paramagnetic transition in La_0.7−xCe_xSr_0.30MnO₃ samples has been observed. The value of ΔS_M was found to increase with Ce-doping up to x=0.15 and the highest value of 2.12 J kg⁻¹ K⁻¹ (at ΔH=2 T) was observed for La_0.55Ce_0.15Sr_0.30MnO₃ sample near the Curie temperature of 356 K. Also, improved relative cooling power of ~122 J kg⁻¹ was observed for the same sample. Due to the large magnetic entropy change and high Curie temperature, the La_0.55Ce_0.15Sr_0.30MnO₃ sample is suggested to be used as potential magnetic refrigerants for magnetic refrigeration technology above room temperature.     

Ionic conductivity of Mn2+ doped dense tin pyrophosphate electrolytes synthesized by a new co-precipitation method

Mn²⁺-doped Sn_1−xMn_xP₂O₇ (x = 0–0.2) are synthesized by a new co-precipitation method using tin(II)oxalate as tin(IV) precursor, which gives pure tin pyrophosphate at 300 °C, as all the reaction by-products are vaporizable at <150 °C. The dopant Mn²⁺ acts as a sintering aid and leads to dense Sn_1−xMn_xP₂O₇ samples on sintering at 1100 °C. Though conductivity of Sn_1−xMn_xP₂O₇ samples in the ambient atmosphere is 10⁻⁹–10⁻⁶ S cm⁻¹ in 300–550 °C range, it increases significantly in humidified (water vapor pressure, pH₂O = 0.12 atm) atmosphere and reaches >10⁻³ S cm⁻¹ in 100–200 °C range. The maximum conductivity is shown by Sn_0.88Mn_0.12P₂O₇ with 9.79 × 10⁻⁶ S cm⁻¹ at 550 °C in ambient air and 2.29 × 10⁻³ S cm⁻¹ at 190 °C in humidified air. It is observed that the humidification of Sn_1−xMn_xP₂O₇ samples is a slow process and its rate increases at higher temperature. The stability of Sn_1−xMn_xP₂O₇ samples is analyzed.     

Carbon-coated SiO/ZrO2 composites as anode materials for lithium-ion batteries

A combination of high-energy ball milling and constant pressure chemical vapor deposition was used to prepare carbon-coated SiO/ZrO₂ composites. It was found that the as-prepared composites were composed of amorphous carbon, amorphous SiO, and paracryslalline ZrO₂. The electrochemical analysis results revealed excellent electrochemical performances for the composites, including a high initial discharge capacity (1737 mA h g⁻¹), a remarkable cyclic stability (reversible capacity of 721 mA h g⁻¹ at 800 mA g⁻¹, after 100 cycles), and a good rate capability (870 mA h g⁻¹ at 800 mA g⁻¹). These features demonstrate that these composites are promising alternative candidates for high-efficiency electrode materials of Li-ion batteries.     

Electrospun ZnO–SnO2 composite nanofibers with enhanced electrochemical performance as lithium-ion anodes

ZnO–SnO₂ composite nanofibers with different structures were synthesized by a simple electrospinning approach with subsequent calcination at three different temperatures using polyacrylonitrile as the polymer precursor. The electrochemical performance of the composites for use as anode materials in lithium-ion batteries were investigated. It was found that the ZnO–SnO₂ composite nanofibers calcined at 700 °C showed excellent lithium storage properties in terms of cycling stability and rate capability, compared to those calcined at 800 and 900 °C, respectively. ZnO–SnO₂ composite nanofibers calcined at 700 °C not only delivered high initial discharge and charge capacities of 1450 and 1101 mAh g⁻¹, respectively, with a 75.9% coulombic efficiency, but also maintained a high reversible capacity of 560 mAh g⁻¹ at a current density of 0.1 A g⁻¹ after 100 cycles. Additionally, a high reversible capacity of 591 mAh g⁻¹ was obtained when the current density returned to 0.1 A g⁻¹ after 50 cycling at a high current density of 2 A g⁻¹. The superior electrochemical performance of ZnO–SnO₂ composite nanofibers can be attributed to the unique nanofibrous structure, the smaller particle size and smaller fiber diameter as well as the porous structure and synergistic effect between ZnO and SnO₂.     

Highly efficient synthesis of graphene/MnO2 hybrids and their application for ultrafast oxidative decomposition of methylene blue

A highly efficient method has been reported to fabricate the reduced graphene oxide/MnO₂ (RGO/MnO₂) hybrid materials, a kind of catalysts for oxidative decomposition of methylene blue (MB). The pristine suspension of graphene oxide/manganese sulfate (GO/MnSO₄) produced by the modified Hummers method is in situ transformed into GO/MnO₂ composites in combination with KMnO₄, and then further into RGO/MnO₂ composites by means of glucose-reduction. It is found that MnO₂ nanoparticles with the size of 20–30 nm are uniformly distributed in the structure of RGO. A series of composites with different mass ratios of RGO to MnO₂ has been proved superior catalytic activities, much higher than that of the bare MnO₂ for decomposition of MB dye in the presence of H₂O₂. Typically, 50 mL of MB (50 mg L⁻¹) can be completely decolorized and nearly 66% mineralized at 50 °C in 5 min with 10 mg of the RGO/MnO₂ hybrid. According to the adsorption–oxidation–desorption mechanism, the high activity of RGO/MnO₂ composites for decomposition of MB is closely related to the positive synergistic effect of RGO and MnO₂ with the assistance of H₂O₂.     

Large relative cooling power of Bi-doped La0.8−xBixSr0.08(Ca0.55Ba0.45)0.12MnO3 (x=0.0, 0.1 and 0.3) perovskites: Magnetic and magnetocaloric properties

We report magnetic and magnetocaloric properties of La_0.8−xBi_xSr_0.08(Ca_0.55Ba_0.45)_0.12MnO₃ (x=0.0, 0.1 and 0.3) perovskite manganites. Polycrystalline samples have been synthesized in air by the sol gel method at a sintering temperature of 1150 °C. Powder X-ray diffraction data show that samples are phase pure and their cell parameters slightly decrease with increase in Bi content. Scanning electron micrographs show that the average particle size increases with increase in Bi content. The temperature dependent magnetization measurements show that the Curie temperatures decrease from 315 K (x=0.0) to 140 K (x=0.3) with increase in Bi content. The isothermal magnetization data is used to estimate the magnetic entropy changes (−$\Delta S_M$) and their calculated values are 1.12 J kg⁻¹ K⁻¹, 1.96 J kg⁻¹ K⁻¹ and 1.62 J kg⁻¹ K⁻¹ for x=0.0, 0.1 and 0.3 samples respectively under an applied magnetic field of 2.0 T. The corresponding values of relative cooling power (RCP) are 90 J kg⁻¹, 180 J kg⁻¹, 136 J kg⁻¹ for x=0.0, 0.1 and 0.3 samples respectively. These results of magnetocaloric effect in our samples suggest that they are promising materials for the magnetic refrigeration applications.     

High performance MnO2 nanoflower supercapacitor electrode by electrochemical recycling of spent batteries

MnO₂ nanoflower is prepared by electrochemical conversion of Mn₃O₄ obtained by heat treatment of spent zinc‒carbon batteries cathode powder. The heat treated and converted powders were characterized by TGA, XRD, FTIR, FESEM and TEM techniques. XRD analyses show formation of Mn₃O₄ and MnO₂ phases for the heat treated and converted powders, respectively. FESEM images indicate the formation of porous nanoflower structure of MnO_2, while, condensed aggregated particles are obtained for Mn₃O₄. The energy band gap of MnO₂ is obtained from UV‒Vis spectra to be 2.4 eV. The electrochemical properties are investigated using cyclic voltammetry, galvanostatic charge‒discharge and electrochemical impedance techniques using three-electrode system. The specific capacitance of MnO₂ nanoflower (309 F g⁻¹ at 0.1 A g⁻¹) is around six times higher than those obtained from the heat treated one (54 F g⁻¹ at 0.1 A g⁻¹). Moreover, it has high capacitance retention up to 93% over 1650 cycles. Impedance spectra of MnO₂ nanoflower show very small resistances and high electrochemical active surface area (340 m² g⁻¹). The present work demonstrates a novel electrochemical approach to recycle spent zinc-carbon batteries into high value supercapacitor electrode.     

Controllable synthesis of porous CxNy nanofibers with enhanced electromagnetic wave absorption property

Porous C_xN_y nanofibers are controllably synthesized by a simple two-step method. The prepared samples possess uniform micropores and a chemical composition of C_0.73 N_0.27 with a surface area of 329 m² g⁻¹. The obtained C_xN_y nanofibers exhibit remarkable electromagnetic (EM) wave absorption properties when compared with conventional one-dimensional carbon materials. The minimum reflection loss (RL) reaches −36 dB at 2.7 GHz when the ratio of the C_xN_y absorbent added in paraffin matrix is only 1:3. The bandwidth of the RL below −10 dB covers 7.7 GHz (8.1–15.8 GHz) at the sample thickness of 2.5 mm. A possible EM wave loss mechanism was proposed in detail. The multiple reflection and dielectric loss could govern the excellent EM absorption leading the product to a probable application in stealth materials.     

Enhancement of pyroelectric properties of lead-free 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 ceramics by La doping

Lead-free 0.94NBT-0.06BT-xLa ceramics at x = 0.0–1.0 (%) were synthesized by a conventional solid-state route. XRD shows that the compositions are at a morphotropic phase boundary where rhombohedral and tetragonal phases coexist. With increasing La³⁺ content pyroelectric coefficient (p) and figures of merits greatly increase; however, the depolarization temperature (T_d) decreases. p is 7.24 × 10⁻⁴C m⁻² °C⁻¹ at RT at x = 0.5% and 105.4 × 10⁻⁴C.m^−2 °C⁻¹ at T_d at x = 0.2%. F_i and F_v show improvements at RT from 1.12 (x = 0%) to 2.65 (x10 ⁻¹⁰ m v⁻¹) (x = 0.5%) and from 0.021 to 0.048 (m².C⁻¹) respectively. F_i and F_v show a huge increase to 37.6 × 10⁻¹⁰ m v⁻¹ and 0.56 m² C⁻¹ respectively at T_d at x = 0.2%. F_C shows values of 2.10, 2.89, and 2.98 (x10⁻⁹C cm⁻² °C⁻¹) at RT at 33, 100 and 1000 (Hz) respectively. Giant pyroelectric properties make NBT-0.06BT-xLa at x = 0.2% and 0.5% promising materials for many pyroelectric applications.     

Preparation and characterization of manganese oxide/CNT composites as supercapacitive materials

Manganese oxide was synthesized and dispersed on carbon nanotube (CNT) matrix by thermally decomposing manganese nitrates. CNTs used in this paper were grown directly on graphite disk by chemical vapor deposition technique. The capacitive behavior of manganese oxide/CNT composites was investigated by cyclic voltammetry and galvanostatic charge–discharge method in 1 M Na₂SO₄ aqueous solutions. When the loading mass of MnO₂ is 36.9 μg cm^− 2, the specific capacitance of manganese oxide/CNT composite (based on MnO₂) at the charge–discharge current density of 1 mA cm^− 2 equals 568 F g^− 1. Additionally, excellent charge–discharge cycle stability (ca. 88% value of specific capacitance remained after 2500 charge–discharge cycles) and power characteristics of the manganese oxide/CNT composite electrode can be observed. The effect of loading mass of MnO₂ on specific capacitance of the electrode has also been investigated.     

Fabrication of highly insulating foam glass made from CRT panel glass

We prepared low-density foam glasses from cathode-ray-tube panel glass using carbon and MnO₂ as the foaming agents. We investigated the influence of the carbon and MnO₂ concentrations, the glass-powder preparation and the foaming conditions on the density and homogeneity of the pore structure and the dependence of the thermal conductivity on the foam density. The results show that the moderate foaming effect of the carbon is greatly improved by the addition of MnO₂. A density as low as 131 kg m⁻³ can be achieved with fine glass powder. The foam density has a slight dependence on the carbon and MnO₂ concentrations, but it is mainly affected by the foaming temperature and the time. The thermal conductivity of the foam-glass samples is lower than that of commercial foam glasses with the same density. The lowest value was determined to be 42 mW m⁻¹ K⁻¹ for a foam glass with a density of 131 kg m⁻³. A further improvement in the closed porosity could potentially decrease the thermal conductivity even further, and thus our approach has great potential in terms of a thermal insulation material.     

Graphene-wrapped polyaniline nanofibers as electrode materials for organic supercapacitors

Graphene-wrapped polyaniline nanofibers were prepared by assembly of negatively charged graphene oxide with positively charged aqueous dispersible polyaniline nanofibers in an aqueous dispersion, followed by the reduction of the graphene oxide. The hybrid material with a graphene oxide loading of 9.1 wt.% displayed a high specific capacitance of over 250 F g⁻¹ in a 1 M Et₄N⁺·BF₄⁻/propylene carbonate electrolyte, a 39.7% increase compared with pristine polyaniline nanofibers. A significant improvement in long-term cycle life was also realized. The hybrid exhibited an initial specific capacitance of 236 F g⁻¹, which remained as high as 173.3 F g⁻¹ over 1000 cycles, or a 26.3% decrease, much better than that for pure polyaniline nanofibers. An asymmetric supercapacitor based on this hybrid material and activated carbon was assembled. An energy density of 19.5 W h kg⁻¹ at a power density of 738.95 W kg⁻¹ was obtained for the cell under an operating voltage window of 2 V.