High-performance supercapacitor of manganese oxide/reduced graphene oxide nanocomposite coated on flexible carbon fiber paper

Although supercapacitors have higher power density than batteries, they are still limited by low energy density and low capacity retention. Here we report a high-performance supercapacitor electrode of manganese oxide/reduced graphene oxide nanocomposite coated on flexible carbon fiber paper (MnO₂–rGO/CFP). MnO₂–rGO nanocomposite was produced using a colloidal mixing of rGO nanosheets and 1.8 ± 0.2 nm MnO₂ nanoparticles. MnO₂–rGO nanocomposite was coated on CFP using a spray-coating technique. MnO₂–rGO/CFP exhibited ultrahigh specific capacitance and stability. The specific capacitance of MnO₂–rGO/CFP determined by a galvanostatic charge–discharge method at 0.1 A g⁻¹ is about 393 F g⁻¹, which is 1.6-, 2.2-, 2.5-, and 7.4-fold higher than those of MnO₂–GO/CFP, MnO₂/CFP, rGO/CFP, and GO/CFP, respectively. The capacity retention of MnO₂–rGO/CFP is over 98.5% of the original capacitance after 2000 cycles. This electrode has comparatively 6%, 11%, 13%, and 18% higher stability than MnO₂–GO/CFP, MnO₂/CFP, rGO/CFP, and GO/CFP, respectively. It is believed that the ultrahigh performance of MnO₂–rGO/CFP is possibly due to high conductivity of rGO, high active surface area of tiny MnO₂, and high porosity between each MnO₂–rGO nanosheet coated on porous CFP. An as-fabricated all-solid-state prototype MnO₂–rGO/CFP supercapacitor (2 × 14 cm) can spin up a 3 V motor for about 6 min.     

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 dispersion of γ-MnO2 on well-aligned carbon nanotube arrays and its application in supercapacitors

The well-aligned carbon nanotube arrays (ACNTs) were used as supporting material and the γ-MnO₂/ACNT electrode with high dispersion of γ-MnO₂ has been prepared by electrochemically induced deposition method. The crystal structure and morphology of the γ-MnO₂/ACNT electrode were investigated by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The capacitive properties of γ-MnO₂/ACNT electrode were characterized by cyclic voltammetry and galvanostatic charge–discharge method. The specific capacitance of the γ-MnO₂/ACNT electrode is as high as 784 F g^− 1 based on γ-MnO₂ and 234 F g^− 1 based on γ-MnO₂/ACNT composites in 0.1 M Na₂SO₄ aqueous solution from 0 to 1 V when the charge–discharge current density is 1 mA cm^− 2. Additionally, the electrode shows excellent power characteristics, high electrochemical reversibility and excellent long-term charge–discharge cycle stability.     

Template synthesis of hollow carbon spheres anchored on carbon nanotubes for high rate performance supercapacitors

A carbon material consisting of hollow carbon spheres anchored on the surface of carbon nanotubes (CNT–HCS) has been synthesized by an easy chemical vapor deposition process using a CNT–MnO₂ hybrid as template. An electrode made of this material exhibits a maximum specific capacitance of 201.5 F g⁻¹ at 0.5 A g⁻¹ and excellent rate performance (69% retention ratio at 20 A g⁻¹). It has impressive cycling stability with 90% initial capacitance retained after 5000 cycles at 5 A g⁻¹ in 6 mol L⁻¹ KOH. Symmetric supercapacitors based on CNT–HCS achieve a maximum energy density of 11.3 W h kg⁻¹ and power density of 11.8 kW kg⁻¹ operated within a wide potential range of 0–1.6 V in 1.0 mol L⁻¹ Na₂SO₄ solution.     

A high-performance three-dimensional micro supercapacitor based on ripple-like ruthenium oxide–carbon nanotube composite films

A micro-supercapacitor with a three-dimensional configuration has been fabricated using an inductively coupled plasma etching technique. A ruthenium oxide–carbon nanotube (CNT) composite with a ripple-like morphology is successfully synthesized using a cathodic deposition technique while using silica-based three-dimensional microstructures as a template. The desired network of carbon nanotubes in the composite facilitates electrolyte penetration and proton exchange/diffusion. A single three dimensional microelectrode is studied using cyclic voltammetry, and a specific capacitance of 272 mF·cm⁻² is observed at 5 mV s⁻¹ in a neutral Na₂SO₄ solution. The accelerated cycle life is tested at 80 mV s⁻¹, and a satisfactory cyclability is observed. When placed on a chip, the symmetric cell exhibits good supercapacitor properties, the specific capacitance up to 37.23 mF cm⁻² and specific power density up to 19.04 mW cm⁻² were obtained at 50 mA cm⁻².     

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).     

Cathode materials based on carbon nanotubes for high-energy-density lithium–sulfur batteries

The rational integration of conductive nanocarbon scaffolds and insulative sulfur is an efficient method to build composite cathodes for high-energy-density lithium–sulfur batteries. The full demonstration of the high-energy-density electrodes is a key issue towards full utilization of sulfur in a lithium–sulfur cell. Herein, carbon nanotubes (CNTs) that possess robust mechanical properties, excellent electrical conductivities, and hierarchical porous structures were employed to fabricate carbon/sulfur composite cathode. A family of electrodes with areal sulfur loading densities ranging from 0.32 to 4.77 mg cm⁻² were fabricated to reveal the relationship between sulfur loading density and their electrochemical behavior. At a low sulfur loading amount of 0.32 mg cm⁻², a high sulfur utilization of 77% can be achieved for the initial discharge capacity of 1288 mAh g_S⁻¹, while the specific capacity based on the whole electrode was quite low as 84 mAh g_{C/S+binder+Al}⁻¹ at 0.2 C. Moderate increase in the areal sulfur loading to 2.02 mg cm⁻² greatly improved the initial discharge capacity based on the whole electrode (280 mAh g_{C/S+binder+Al}⁻¹) without the sacrifice of sulfur utilization. When sulfur loading amount further increased to 3.77 mg cm⁻², a high initial areal discharge capacity of 3.21 mAh cm⁻² (864 mAh g_S⁻¹) was achieved on the composite cathode.     

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.     

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.     

Enhanced rate capability of nanostructured three-dimensional graphene/Ni3S2 composite for supercapacitor electrode

Three-dimensional graphene/Ni₃S₂ (3DG/Ni₃S₂) composite electrodes were produced by a facile two-step synthesis route involving chemical vapor deposition (CVD) growth of graphene foam and in situ hydrothermal synthesis of Ni₃S₂. The porous structure of the prepared 3DG is ideal for use as a scaffold for fabricating monolithic composite electrodes. The relative content of Ni₃S₂ initially increased and then decreased with increasing hydrothermal reaction time. The basal surface of the electrode was completely covered after 6 h of hydrothermal reaction. The size of the Ni₃S₂ microspheres also increased with increasing hydrothermal reaction time. The composite electrodes exhibited good specific capacitance (11.529 F cm⁻² at 2 mA cm⁻², i.e., 2611.9 F g⁻¹ at 5 mV s⁻¹) and cyclability (retention of 88.97% capacitance after 1000 charge/discharge cycles at 20 mA cm⁻²). These results are attributed to the fact that the uniform distribution of the Ni₃S₂ microspheres increased the specific surface area of the electrode and facilitated electron transfer and ion diffusion. The 3D multiplexed and highly conductive pathways provided by the defect-free graphene foam also ensured rapid charge transfer and conduction to improve the rate capability of the supercapacitors.     

Design and synthesis of three-dimensional needle-like CoNi2S4/CNT/graphene nanocomposite with improved electrochemical properties

In this paper, we described a simple two–step method for preparing needle-like CoNi₂S₄/CNT/graphene nanocomposite with robust connection among its ternary components. The prepared CoNi₂S₄/CNT/graphene nanocomposite has been thoroughly characterized by spectroscopic (Fourier-transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy), X-ray diffraction and thermogravimetric analysis. Microscopy techniques (scanning electron microscopy–energy dispersive spectroscopy and transmission electron microscopy) were employed to probe the morphological structures. The electrochemical properties of the as-prepared 3D architectures were investigated with three and two-electrode systems. In addition to its high specific capacitance (710 F g⁻¹ at 20 A g⁻¹), after charging–discharging for 2000 cycles, the electrode still maintained the capacity retention of about 82%. When used as the active electrode material for supercapacitors, the fabricated CoNi₂S₄–g–CNT nanostructure exhibited excellent specific capacitance and good rate capability, making it a promising candidate for next-generation supercapacitors.     

Electrophoretic deposition of MnO2-coated carbon nanotubes on a graphite sheet as a flexible electrode for supercapacitors

A flexible electrode was prepared by microwave heating deposition of manganese oxide (MnO₂) on carbon nanotubes (CNTs) followed by electrophoretic deposition of the MnO₂-coated CNTs on a flexible graphite sheet (FGS). The prepared MnO₂-coated CNTs were characterized by scanning and transmission electron microscopy, and X-ray diffraction. A uniformly thin nano-scale MnO₂ coating was formed on the surface of the CNTs. The MnO₂-coated CNTs–FGS electrode showed highly capacitive behaviour in the 0.5 M Na₂SO₄ aqueous solution, with a specific capacitance of 442.9 F/g based on MnO₂ at 2 mV/s. It exhibited an excellent cycling stability with no more than 1.1% capacitance loss after 1000 cycles at 50 mV/s.     

Electrochemical energy storage performance of heterostructured SnO2@MnO2 nanoflakes

In this work, we report synthesis of SnO₂@MnO₂ nanoflakes grown on nickel foam through a facile two-step hydrothermal route. The as-obtained products are characterized by series of techniques such as scanning electron microscopy (SEM), X-ray diffraction spectroscopy (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The as-obtained SnO₂@MnO₂ nanoflakes are directly used as supercapacitor electrode materials. The results show that the electrode possesses a high discharge areal capacitance of 1231.6 mF cm⁻² at 1 mA cm⁻² and benign cycling stability with 67.2% of initial areal capacitance retention when the current density is 10 mA cm⁻² after 6000 cycles. Moreover, the heterostructured electrode shows 41.1% retention of the initial capacitance when the current densities change from 1 to 10 mA cm⁻², which reveals good rate capability. SnO₂@MnO₂ nanoflakes products which possess excellent electrochemical properties might be used as potential electrode materials for supercapacitor applications.     

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.     

Facile synthesis of Sm2S3 diffused nanoflakes and their pseudocapactive behavior

The Sm₂S₃ thin films with diffused nanoflakes morphology are prepared by an environment-friendly facile chemical synthesis method and used in electrochemical supercapacitors. The structural, elemental and surface morphological characterization are carried out using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM) and wettability techniques. The FESEM images show tree root like distribution of flakes with average flake width of about 80 nm. The film surface is lyophilic with propylene carbonate contact angle of 21°. The supercapacitive measurements are carried out through cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS) techniques. The Sm₂S₃ film electrode exhibited a highest specific capacitance (C_s) of 213 Fg⁻¹ at 5 mVs⁻¹ scan rate in LiClO₄-propylene carbonate electrolyte. Asymmetric nature of charge–discharge curves confirmed pseudocapacitive behavior of electrode with energy and power densities of 39.39 Whkg⁻¹ and 4.33 kWkg⁻¹, respectively. An equivalent series resistance of 0.44 Ωcm⁻² indicated negligible ohmic losses in charge storage. An electrochemical stability of 81.47% is retained after 1000 cycles indicating that Sm₂S₃ is a promising candidate for supercapacitor application.     

Improved activity of a graphene–TiO2 hybrid electrode in an electrochemical supercapacitor

We present a simple and fast approach for the synthesis of a graphene–TiO₂ hybrid nanostructure using a microwave-assisted technique. The microstructure, composition, and morphology were characterized by X-ray diffraction, Fourier-transform infrared spectroscopy, Raman microscopy, X-ray photoelectron spectroscopy, and field-emission scanning electron microscopy. The electrochemical properties were evaluated using cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge–discharge tests. Structural analysis revealed a homogeneous distribution of nanosized TiO₂ particles on graphene nanosheets. The material exhibited a high specific capacitance of 165 F g⁻¹ at a scan rate of 5 mV s⁻¹ in 1 M Na₂SO₄ electrolyte solution. Theenhanced supercapacitance property of these materials could be ascribed to the increased conductivity of TiO₂ and better utilization of graphene. Moreover, the material exhibited long-term cycle stability, retaining ∼90% specific capacitance after 5000 cycles, which suggests that it has potential as an electrode material for high-performance electrochemical supercapacitors.     

Simultaneous reduction,exfoliation, and nitrogen doping of graphene oxide via a hydrothermal reaction for energy storage electrode materials

Nitrogen (N)-doped graphene (NG) sheets were prepared using (NH₄)₂CO₃ and an aqueous dispersion of graphene oxide (GO) by an eco-friendly hydrothermal reaction. The in situ produced ammonia played an important role in the simultaneous nitrogen doping, the reduction and exfoliation of GO. The (NH₄)₂CO₃/GO mass ratio and reaction temperature were varied to investigate the effects on the N doping level. The elemental analysis determined from the X-ray photoelectron spectroscopy showed that the nitrogen content of the NG was about 10.1 at.% and the oxygen content decreased significantly due to the hydrothermal reduction of GO. The electrochemical performances of the NG sheets increased with increasing doped N content. The highest specific capacitance of 295 F g⁻¹ at a current density of 5 A g⁻¹ and the highest specific surface area of 412 m² g⁻¹ were observed with the sample processed at 130 °C. The retention of the specific capacitance was maintained at ∼89.8% after 5000 charge–discharge cycles. These results imply that NG sheets obtained by this simple eco-friendly approach are suitable for use in high performance energy storage electrode materials.     

The additive-free electrode based on the layered MnO2 nanoflowers/reduced,graphene oxide film for high performance supercapacitor

The MnO₂ nanoflowers/reduced graphene oxide composite is coated on a nickel foam substrate (denoted as MnO₂ NF/RGO @ Ni foam) via the layer by layer (LBL) self-assembly technology without any polymer additive, following the soft chemical reduction. The layered MnO₂ NF/RGO composite is uniformly anchored on the Ni foam skeleton to form the 3D porous framework, and the interlayers have access to lots of ions channels to improve the electron transfer and diffusion. This special construction of 3D porous structure is beneficial to the enhancement of electrochemical property. The specific capacitance is up to 246 F g⁻¹ under the current density of 0.5 A g⁻¹. After 1000 cycles, it can retain about 93%, exhibiting excellent cycle stability. The electrochemical impedance spectroscopy measurements confirm that MnO₂ NF/RGO @ Ni foam electrode has lower R_ESR and R_CT values when compared to MnO₂ @ Ni foam and RGO @ Ni foam. This study opens a new door to the preparation of composite electrodes for high performance supercapacitor.     

A green strategy for the synthesis of graphene supported Mn3O4 nanocomposites from graphitized coal and their supercapacitor application

A by-product free strategy based on modified Hummers method was proposed to synthesize graphene/Mn₃O₄ composites without any additional manganese source. Coal-derived graphite (CDG) was used as carbon source instead of conventional natural graphite flakes and MnSO₄ produced from the modified Hummers was in situ transformed into Mn₃O₄ by precipitation in air. After reduction with hydrazine, the reduced coal-derived graphene oxide/Mn₃O₄ (RCDGO/Mn₃O₄) was obtained and employed as the electrode material for the supercapacitors. In addition, K₂SO₄ produced from the modified Hummers was used as electrolyte, as a result, residual-free was achieved during the whole process, and the atom utilization was calculated as high as about 97%. A maximum specific capacitance of 260 F g⁻¹ was achieved for RCDGO/Mn₃O₄ composite with 86% Mn₃O₄ in saturated K₂SO₄ electrolyte solution based on the synergetic effects between coal-derived graphene and attached Mn₃O₄ nanoparticles. Its specific energy density reached 8.7 Wh kg⁻¹ at a current density of 50 mA g⁻¹ when used as a symmetrical supercapacitor. The good capacitance retention (92–94%) was also observed after 1000 continuous cycles of galvanostatic charge–discharge.     

High-performance supercapacitor based on V2O5/carbon nanotubes-super activated carbon ternary composite

A ternary composite of V₂O₅/carbon nanotubes/super activated carbon (V₂O₅/CNTs–SAC) was prepared by a simple hydrothermal method and used as a supercapacitor electrode material. The electrochemical performance of the electrode was analyzed using cyclic voltammetry, galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy, which were performed in 2 M NaNO₃ as the electrolyte. The V₂O₅/CNTs–SAC nanocomposite exhibited a specific capacitance as high as 357.5 F g⁻¹ at a current density of 10 A g⁻¹, which is much higher than that of either bare V₂O₅ nanosheets or a V₂O₅/CNTs composite. Furthermore, the capacitance increased to 128.7% of the initial value after 200 cycles, with 99.5% of the maximum value being retained after 1000 cycles. These results demonstrated that the V₂O₅/CNTs–SAC ternary composite is suitable for use as an electrode material for supercapacitors.