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.     

Room temperature synthesis of Li-doped MnO2 and its electrochemical activity

A new method was developed to synthesize a uniform round-shaped Li-doped MnO₂ by ozonation of acidic MnSO₄ in the presence of Li⁺ ions. X-ray diffraction study showed that the prepared compound was of a hexagonal-closed-packed (hcp) crystal structure with an average crystallite size of ~ 103 nm. The merging of 221/240 and 061/002 lines suggested a high microtwinning defect. Chemical composition has been explained in terms of Ruetschi's cation vacancy model. The electrochemical properties were studied by recording discharge profile in 9 M KOH at 1 mA/0.1 g and 100 Ω/0.5 g. The discharge proceeded with homogenous solid-state reduction into MnOOH followed by subsequent formation of Mn(OH)₂ via dissolution-precipitation mechanism.     

The mechanism of specific capacitance improvement of supercapacitors based on MnO2 at an elevated operating temperature

Amorphous nanostructured MnO₂ film was anodically deposited onto economical duplex stainless steel substrate. The obtained MnO₂ film was characterized by X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy for microstructural, morphological, and compositional studies. The capacitive behavior was systematically investigated by cyclic voltammetry, charge-discharge cycling and electrochemical impedance spectroscopy (EIS) in 1 M Na₂SO₄ electrolyte at different operating temperatures ranging from 20 to 60 °C. The specific capacitance (SC) was improved with an increase of operating temperature, and the highest SC of 398 F/g was achieved at a scan rate of 10 mV/s and operating temperature of 60 °C. The mechanism of SC improvement at elevated operating temperature was investigated using EIS. With an increase of operating temperature, the conductivity of electrolyte was improved, and the charge-transfer resistance (R_ct) was decreased. The temperature dependence of 1/R_ct follows an Arrhenius equation. The MnO₂ film was electrochemically activated at 60 °C due to the formation of Na_yMnO₂ after discharging.     

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.     

Hydrothermal synthesis of carbon nanotube/cubic Fe3O4 nanocomposite for enhanced performance supercapacitor electrode material

Carbon nanotube/Fe₃O₄ (CNT/Fe₃O₄) nanocomposite with well-dispersed Fe₃O₄ nano-cubes inlaid on the surfaces of carbon nanotubes, was synthesized through an easy and efficient hydrothermal method. The electrochemical behaviors of the nanocomposite were analyzed by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronopotentiometry in 6 M KOH electrolyte. Results demonstrated that CNT as the supporting material could significantly improve the supercapacitor (SC) performance of the CNT/Fe₃O₄ composite. Comparing with pure Fe₃O₄, the resulting composite exhibited improved specific capacitances of 117.2 F/g at 10 mA/cm² (3 times than that of pure Fe₃O₄), excellent cyclic stability and a maximum energy density of 16.2 Wh/kg. The much improved electrochemical performances could be attributed to the good conductivity of CNTs as well as the anchored Fe₃O₄ particles on the CNTs.     

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.     

Facile synthesis of tremella-like MnO2 and its application as supercapacitor electrodes

In this work, three kinds of ultrathin tremella-like MnO₂ have been simply synthesized by decomposing KMnO₄ under mild hydrothermal conditions. When applied as electrode materials, they all exhibited excellent electrochemical performance. The as-prepared MnO₂ samples were characterized by means of XRD, SEM, TEM and XPS. Additionally, the relationship of the crystalline nature with the electrochemical performance was investigated. Among the three samples, the product with the poorest crystallinity had the highest capacitance of 220 F/g at a current density of 0.1 A/g. It is thought that the ultrathin MnO₂ nanostructures can serve as promising electrode materials for supercapacitors.     

Mn2O3/carbon aerogel microbead composites synthesized by in situ coating method for supercapacitors

A series of Mn₂O₃/carbon aerogel microbead (Mn₂O₃/CAMB) composites for supercapacitor electrodes have been synthesized by in situ encapsulation method. The structure and morphology of Mn₂O₃/CAMB are characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectrum and scanning electron microscopy (SEM). Electrochemical performances of the synthesized composites are evaluated by cyclic voltammetry and galvanostatic charge/discharge measurement. All the composites with different Mn₂O₃ contents show higher specific capacitance than pure CAMB due to the pseudo-capacitance of the Mn₂O₃ particles dispersed on the surface of CAMB. The highest specific capacitance is up to 368.01 F g⁻¹ when 10 wt% Mn₂O₃ is coated on the surface of CAMB. Besides, 10%-Mn₂O₃/CAMB supercapacitor exhibits excellent cyclic stability, the specific capacitance still retains 90% of initial capacitance over 5000 cycles.     

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 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.     

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.     

NiCo2S4 nanoparticles grown on reduced graphene oxides for high-performance asymmetric supercapacitors

The nanostructures of reduced graphene oxide (rGO)/NiCo₂S₄ are prepared using the simple hydrothermal method and the thermal treatment process, which could provide good conductivity and ideal specific surface area. The rGO/NiCo₂S₄ electrode shows a maximum specific capacitance of 1059 F g⁻¹, excellent rate capability, and good cycle life. Furthermore, the three dimensional structures of rGO/MnO (3D rGO/MnO) are also synthesized by the hydrothermal method and the thermal treatment process, which have the high specific surface area and good conductivity. The rGO/MnO electrode exhibits a maximum specific capacitance of 469 F g⁻¹. A rGO/MnO//rGO/NiCo₂S₄ asymmetric supercapacitors (ASC) is assembled using 2 M KOH solution as electrolyte, rGO/NiCo₂S₄ as positive electrode and rGO/MnO as negative electrode. The rGO/MnO//rGO/NiCo₂S₄ ASC shows an energy density of 38.8 Wh kg⁻¹ at a power density of 0.4 kW kg⁻¹ and a good cycle life, which provides a possibility toward actual application in energy-storage systems.     

Graphene/MnO2 hybrid nanosheets as high performance electrode materials for supercapacitors

Graphene/MnO₂ hybrid nanosheets were prepared by incorporating graphene and MnO₂ nanosheets in ethylene glycol. Scanning electron microscopy and transmission electron microscopy analyses confirmed nanosheet morphology of the hybrid materials. Graphene/MnO₂ hybrid nanosheets with different ratios were investigated as electrode materials for supercapacitors by cyclic voltammetry (CV) and galvanostatic charge–discharge in 1 M Na₂SO₄ electrolyte. We found that the graphene/MnO₂ hybrid nanosheets with a weight ratio of 1:4 (graphene:MnO₂) delivered the highest specific capacitance of 320 F g⁻¹. Graphene/MnO₂ hybrid nanosheets also exhibited good capacitance retention on 2000 cycles.     

Self-template route to MnO2 hollow structures for supercapacitors

Birnessite-type MnO₂ hierarchical hollow structures were prepared through a self-template route, by the direct reaction between the aqueous solution of KMnO₄ and solid MnCO₃ precursor crystals, and followed by the removal of MnCO₃ core with HCl. Field emission scanning microscopy (FESEM) images indicate that the shells of hierarchical hollow structures consist of the interconnected sheets with a thickness of about 30 nm, and transmission electron microscopy (TEM) images show that the thickness of the shells can be adjusted over a range from 50 to 80 nm by changing the molar ratio of MnCO₃/KMnO₄. The electrochemical properties of the as-prepared MnO₂ were characterized by cyclic voltammetry (CV) and galvanostatic charge-discharge tests in 1 M Na₂SO₄ solution. The sample obtained at a higher MnCO₃/KMnO₄ molar ratio (i.e., 50:1) shows a relatively higher specific capacitance of 169 F g^− 1 than 111 F g^− 1 of the sample obtained under a lower molar ratio of 25:1 at the current density of 250 mA g^− 1.     

Improved electrochemical capacitive characteristics by controlling the carbon nanotube morphology and electrolyte solution concentration

Carbon nanotubes (CNTs) have been used as the electrochemical double layer in capacitor (EDLC) electrodes. CNTs were synthesized using thermal chemical vapor deposition (CVD) at a growth temperature of 750 °C by flowing C₂H₂. The surface morphology of the synthesized CNTs could be controlled with or without Al film deposition between the stainless (SUS) sheet and Fe catalyst film. Electrochemical measurements were performed in a three-electrode arrangement. H₂SO₄ with different concentrations was used as the electrolyte solution. The relation between the specific capacitance and the surface morphology of the CNTs and the electrolyte concentration were investigated. The results showed that the electrode formed using vertically aligned CNTs with higher electrolyte concentration exhibited higher specific capacitance.     

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.     

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.     

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.     

聚噻吩/纳米MnO2复合材料的制备表征及电化学性能

纳米二氧化锰（MnO₂）作为超级电容材料已被广泛研究。为了改善其充放电性能,采用原位化学氧化聚合法制备聚噻吩/纳米MnO₂ （PTh/MnO₂）复合材料,对纳米MnO₂进行性能改性。通过改变聚噻吩在PTh/MnO₂复合材料中的掺杂量,制备出一系列的复合材料。采用傅里叶转换红外光谱（FIIR）、X射线衍射仪（XRD）、场发射扫描电子显微镜（FE-SEM）和透射电子显微镜（TEM）对PTh/MnO₂复合材料的化学性能、晶体结构以及表面形貌等进行了详细考察。接着采用CT001A型电池测试系统对以PTh/MnO₂复合材料做负极所制得的密封扣式电池进行了充放电性能测试。结果表明,MnO₂和聚噻吩在不同的PTh/MnO₂复合材料中形貌各异。当聚噻吩含量为8wt%~10wt%时,MnO₂在PTh/MnO₂复合材料中分布最为均匀;当聚噻吩含量较高时,MnO₂的形貌受到严重影响,其原来的管状结构接近消失。聚噻吩含量的不同,同样也影响了电池的充放电性能。当聚噻吩的含量为20wt%时,在循环20次后,电池的平衡容量为最高,可达700 mAh/g。这明显高于以纳米MnO₂为负极时的电池容量。由此可见,聚噻吩对纳米MnO₂的充放电性能具有明显的增强作用。该研究为PTh/MnO₂复合材料作为电池负极材料的使用提供了实验基础。     

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.