Rapid synthesis of homogeneous MnO2/multi-wall carbon nanotubes nanostructure and its electrochemical capacitive behavior

A homogeneous composite of MnO₂/multi-wall carbon nanotubes (MnO₂/MWCNTs) was rapidly and efficiently synthesized by a redox reaction of MnO₄⁻ and Mn²⁺ on the MWCNTs under ultrasonic irradiation. The structure and morphology of the obtained MnO₂ and MnO₂/MWCNTs composite were characterized by X-ray diffraction, Fourier transform infrared spectroscopy and transmission electron microscopy. Electrochemical investigation indicated that the maximum specific capacitance of the MnO₂/MWCNTs composite, measured by galvanostatic charge-discharge test, was 315 F g^− 1, compared to the pristine MnO₂ (192 F g^− 1) and MWCNTs electrode (25 F g^− 1), showing the synergistic effect of MWCNTs and MnO₂. The homogeneous hybrid nanostructure and the good conductivity of MWCNTs were considered to be responsible for its preferable electrochemical performances.     

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.     

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 performance of manganese oxide/carbon nanotubes composite as a cathode for rechargeable lithium battery with high power density

Manganese oxide/carbon nanotubes (MO/CNTs) composite was prepared by hydrothermally reducing KMnO₄ with CNTs, where the used CNTs are of dual role, i.e., they serve as reductant during reaction and the remaining CNTs act as conducting agent in the composite. This composite was characterized by X-ray diffraction and scanning electron microscopy techniques. In addition, the electrochemical performances of the composite were investigated, which suggested an excellent rate-capability of this material; e.g., it delivered a high discharge capacity as 131 mAh g^− 1 at a high current density of 4 A g^− 1 (20 C), and high capacity at low discharge current density, e.g., about 209 mAh g^− 1 at 0.2 C rate. Therefore, such a MO/CNTs composite is promising in high power application of lithium battery and electrochemical capacitor.     

Graphene nanosheets-poly(o-aminophenol) nanocomposite for supercapacitor applications

Graphene nanosheets-poly(o-aminophenol) (POAP/GNS) nanocomposite was fabricated on a platinum surface by potential cycling. Voltammograms of the POAP/GNS/Pt electrode showed an excellent capacitive behavior accompanied with a redox transition with a mid-peak potential of 295 mV. The POAP/GNS nanocomposite displayed a specific capacitance as high as 281.1 F g⁻¹ at 0.1 A g⁻¹ which is almost three times higher than that of pure graphene. The specific energy and power of the nanocomposite material were 25.0 Wh kg⁻¹ and 34.8 W kg⁻¹, respectively. The nanocomposite retained more than 99% of the initial capacitance after 1200 charge/discharge cycles.     

Electrochemical co-deposition and characterization of MnO2/SWNT composite for supercapacitor application

Manganese oxide/single-wall carbon nanotubes (MnO₂/SWNT) composite was co-deposited by the potentiostatic method on a graphite slice. Morphological and structural performances for MnO₂/SWNT composite were characterized by means of scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The wall surface of SWNT was wrapped by ramsdellite MnO₂ nanoparticles to fabricate MnO₂/SWNT coaxial nanotubes, which further interconnected other MnO₂ particles to form the porous MnO₂/SWNT composite. The electrochemical properties were examined by cyclic voltammograms, galvanostatic charge and discharge and electrochemical impedance spectrum. A high specific capacitance of 421 F g^?1 was obtained for overall MnO₂/SWNT composite electrode at the constant current density of 1 A g^?1 in 3 mol L^?1 KCl solution.     

Synthesis of MnO2/short multi-walled carbon nanotube nanocomposite for supercapacitors

Multi-walled carbon nanotubes (MWNTs) were selectively etched in molten nitrate to produce short MWNTs (s-MWNTs). MnO₂/s-MWNT nanocomposite was synthesized by a reduction of potassium permanganate under microwave irradiation. For comparative purpose, MnO₂/MWNT nanocomposite was also synthesized and investigated for its physical and electrochemical performance. Uniform and conformal MnO₂ coatings were more easily formed on the surfaces of individual s-MWNTs. MnO₂/s-MWNT nanocomposite estimated by cyclic voltammetry (CV) in 0.5 M Na₂SO₄ aqueous solution had the specific capacitance as high as 392.1 F g⁻¹ at 2 mV s⁻¹. This value was more than 48.9% larger than MnO₂/s-MWNT nanocomposite. In addition, MnO₂/s-MWNT nanocomposite was also examined by repeating the CV test at a scan rate of 50 mV s⁻¹, exhibiting an excellent cycling stability along with 99.2% specific capacitance retained after 1000 cycles. Therefore, MnO₂/s-MWNT nanocomposite is a promising electrode material in the supercapacitors.     

Cathodic electrodeposition of Ag-doped manganese dioxide films for electrodes of electrochemical supercapacitors

Cathodic electrodeposition method has been developed for the fabrication of Ag-doped MnO₂ films from the KMnO₄ aqueous solutions containing AgNO₃ for the application in electrodes of electrochemical supercapacitors (ES). The films were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), cyclic voltammetry (CV) and impedance spectroscopy. The Ag-doped MnO₂ films showed improved capacitive behaviour and lower electrical resistance compared to pure MnO₂ films. The highest specific capacitance (SC) of 770 F g^− 1 was obtained at a scan rate of 2 mV s^− 1 in the 0.5 M Na₂SO₄ electrolyte.     

Preparation and characteristics of nanostructured MnO2/MWCNTs using microwave irradiation method

Ball-nanostructured MnO₂/MWCNTs composite was successfully prepared by microwave irradiation. The surface morphology and structures of the composite were examined by scanning electron microscope and X-ray diffraction. Multi-walled carbon nanotubes play a role as sustainment to inhibit MnO₂ nanoplates from collapsing into nanorods. The electrochemical studies indicated that the composite had ideal capacitive performance and high specific capacitances of 298 F g^− 1, 213 F g^− 1 and 198 F g^− 1 at the current density of 2 mA·cm^− 2, 10 mA·cm^− 2 and 20 mA·cm^− 2, respectively. The formation mechanism of nanostructured MnO₂/MWCNTs and the electrochemical behaviour of composites were discussed in detail.     

Rapid sonochemical synthesis of mesoporous MnO2 for supercapacitor applications

Mesoporous MnO₂ samples with average pore-size in the range of 2–20 nm are synthesized in sonochemical method from KMnO₄ by using a tri-block copolymer, namely, poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (P123) as a soft template as well as a reducing agent. The MnO₂ samples are found to be poorly crystalline. On increasing the amplitude of sonication, a change in the morphology of MnO₂ from nanoparticles to nanorods and also change in porosity are observed. A high BET surface area of 245 m² g⁻¹ is achieved for MnO₂ sample. The MnO₂ samples are subjected to electrochemical capacitance studies by cyclic voltammetry (CV) and galvanostatic charge–discharge cycling in 0.1 M aqueous Ca(NO₃)₂ electrolyte. A maximum specific capacitance (SC) of 265 F g⁻¹ is obtained for the MnO₂ sample synthesized in sonochemical method using an amplitude of 30 μm. The MnO₂ samples also possess good electrochemical stability due to their favourable porous structure and high surface area.     

Microporous carbon derived from polyaniline base as anode material for lithium ion secondary battery

Microporous carbon with large surface area was prepared from polyaniline base using K₂CO₃ as an activating agent. The physicochemical properties of the carbon were characterized by scanning electron microscope, X-ray diffraction, Brunauer-Emmett-Teller, elemental analyses and X-ray photoelectron spectroscopy measurement. The electrochemical properties of the microporous carbon as anode material in lithium ion secondary battery were evaluated. The first discharge capacity of the microporous carbon was 1108 mAh g⁻¹, whose first charge capacity was 624 mAh g⁻¹, with a coulombic efficiency of 56.3%. After 20 cycling tests, the microporous carbon retains a reversible capacity of 603 mAh g⁻¹ at a current density of 100 mA g⁻¹. These results clearly demonstrated the potential role of microporous carbon as anode for high capacity lithium ion secondary battery.     

Microwave synthesis and electrochemical characterization of mesoporous carbon@Bi2O3 composites

An efficient and quick microwave method has been employed to prepare worm-like mesoporous carbon@Bi₂O₃ composites for the first time. As-prepared products have been characterized by X-ray diffraction, N₂ adsorption-desorption, scanning electron microscopy, transmission electron microscopy and inductive coupled plasma atomic emission spectroscopy. The electrochemical measurement shows the worm-like mesoporous carbon@Bi₂O₃ composites exhibits excellent capacitance performance and the maximum specific capacitance reaches 386 F g⁻¹, three times more than the pure worm-like mesoporous carbon.     

Fabrication and capacitance of Ni-Fe LDHs/MnO2 layered nanocomposite via an exfoliation/reassembling process

Ni²⁺-Fe³⁺ layered double hydroxides (LDHs)/MnO₂ layered nanocomposite has been fabricated by using both layer-by-layer self-assembly method and flocculated technology, based on electrostatic interaction of positively charged Ni²⁺-Fe³⁺ LDHs nanosheets and negatively charged MnO₂ nanosheets. Ultraviolet-visible spectroscopy is used to probe the dynamic growth of the multilayer film, exhibiting progressive enhancement of optical absorption due to the assembly of Ni²⁺-Fe³⁺ LDHs nanosheets and MnO₂ nanosheets. The assembled Ni²⁺-Fe³⁺ LDHs/MnO₂ nanocomposite has been characterized by XRD, SEM and TEM. The electrochemical property of the synthesized Ni²⁺-Fe³⁺ LDHs/MnO₂ layered nanocomposite has been studied using cyclic voltammetry in a mild aqueous electrolyte. The Ni²⁺-Fe³⁺ LDHs/MnO₂ nanocomposite exhibits a relative good capacitive behavior in a neutral electrolyte system, and its initial capacitance value is 104 F g⁻¹.     

Hydrothermally synthesized RuO2/Carbon nanofibers composites for use in high-rate supercapacitor electrodes

A conventional hydrothermal deposition process is used to graft ruthenium oxide (RuO₂) nanoparticles onto carbon nanofibers (CNFs). The obtained RuO₂ nanoparticles have an average diameter of 2 nm and are homogenously distributed on the CNF surfaces. Supercapacitors are fabricated using the resulting RuO₂ grafted CNFs nanocomposite as the electrodes. The existence of CNFs leads to reduced contact resistance among the RuO₂ nanoparticles and provides a network for fast electron transport, which then contributes to enhanced electrochemical performance. The enhancement is proportional to the RuO₂ content and can be as high as 638% at a high sweep rate of 200 mV s⁻¹, at which a capacitance is 155 F g⁻¹. Stability of the RuO₂-grafted CNF capacitor is also demonstrated by subjecting the capacitor to a potential sweep at 500 mV s⁻¹ for 1000 cycles. Furthermore, the RuO₂ grafted CNF capacitor exhibits a very short relaxation time of 0.17 s, which is desirable for high rate charge and discharge.     

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.     

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.     

Electrochemical investigation of TiO2/carbon nanotubes nanocomposite as anode materials for lithium-ion batteries

Mechanically blended composite of nanosized TiO₂ and carbon nanotubes (CNTs) was investigated as potential anode materials for Li-ion batteries. It was found that the TiO₂/CNTs nanocomposite exhibits an improved cycling stability and higher reversible capacity than CNTs. The reversible capacity of the TiO₂/CNTs composite reaches 168 mAh g^− 1 at the first cycle and remains almost constant during long-term cycling. The electrochemical results show that the TiO₂ nanoparticles in the composite not only restrain the formation of surface film, but also make a contribution to the overall reversible capacity.     

Structure and electrochemical performance of ZnO/CNT composite as anode material for lithium-ion batteries

Metal oxides are well-known potential alternatives to graphite as anode materials of lithium-ion batteries, and they can deliver much higher reversible capacities than graphite even at high current densities. In this study, hexagonal disk-shaped ZnO are synthesized by a facile solution reaction of ZnCl₂ and its composite is prepared in the presence of carbon nanotubes (CNTs). The as prepared ZnO/CNT composite has been characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, fourier transform-infrared spectroscopy and Rutherford backscattering spectroscopy. Electrochemical characterization by cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic discharge/charge tests demonstrate that the conversion reactions in ZnO and ZnO/CNT electrodes enable reversible capacity of 478 and 602 mAh g^?1, respectively for up to 50 cycles. Our investigation highlights the importance of anchoring of small ZnO particles on CNTs for maximum utilization of electrochemically active ZnO and CNTs for energy storage application in lithium-ion batteries.     

Preparation and electrochemical performance of LiFePO4/C composite with network connections of nano-carbon wires

LiFePO₄/C composite with network connections of nano-carbon wires was successfully prepared by using polyvinyl alcohol as carbon source. The composite was characterized by X-ray diffraction and transmission electron microscopic, and its electrochemical performance was investigated by galvanostatic charge and discharge tests. The experimental results show that LiFePO₄ grains are tightly connected by the network of nano-carbon wires. Moreover LiFePO₄/C composite exhibits high capacity of 168 mAh g⁻¹ applied 15 mA g⁻¹ current density (C/10), excellent cyclic ability and rate capability. When 1500 mA g⁻¹ current density (10C) was applied, the high discharge capacity of 129 mAh g⁻¹ has been obtained at room temperature.     

Synthesis and super capacitance of goethite/reduced graphene oxide for supercapacitors

We report a one-step fabrication of α-iron oxyhydroxide/reduced graphene oxide (α-FeOOH/rGO) composites, in which the ferrous sulfate (FeSO₄·7H₂O) are used as the iron raw and reducing agent to grow goethite (α-FeOOH) and reduce graphite oxide (GO) to rGO in the same time. The morphology, composition and microstructure of the as-obtained samples are systematically characterized by thermogravimetric (TG) analysis, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and FT-IR. Moreover, their electrochemical properties are investigated using cyclic voltammetry and galvanostatic charge/discharge techniques. The specific capacitance of 452 F g⁻¹ is obtained at a specific current of 1 A g⁻¹ when the mass ratio of α-FeOOH to rGO is up to 80.3:19.7. In addition, the α-FeOOH/rGO composite electrodes exhibit the excellent rate capability (more than 79% retention at 10 A g⁻¹ relative to 1 A g⁻¹) and well cycling stability (13% capacitance decay after 1000 cycles). These results suggest the importance and great potential of α-FeOOH/rGO composites in the applications of high-performance energy-storage.