Encapsulation of manganese oxides nanocrystals in electrospun carbon nanofibers as free-standing electrode for supercapacitors

Flexible composites with manganese oxides (MnO_x) nanocrystals encapsulated in electropun carbon nanofibers were successfully fabricated via a simple and practical combination of electrospinning and carbonization process. The as-formed MnO_x/carbon nanofibers composites have a rough surface with MnO_x nanoparticles well embedded in the carbon nanofibers backbones. When used as electrodes for supercapacitor, the resulting MnO_x/carbon nanofiber composites exhibit good electrochemical performance with a specific capacitance of 174.8 F g⁻¹ at 2 mV s⁻¹ in 0.5 M Na₂SO₄ electrolyte, a good rate capability at high current density and long-term cycling stability. It is expected that such freestanding composites could be promising electrodes for high-performance supercapacitors.     

Enhanced ions and electrons transmission enables high-performance KxMnO@C cathode for hybrid supercapacitors

Manganese oxides have been regarded as one of the most promising electrode materials for energy storage systems. Especially, they can be used as battery-type electrodes in hybrid supercapacitors to achieve high energy density and power density at the same time. In such an application, the redox reaction on the battery-type electrodes needs to speed up to match the fast charging-discharging process of the counter capacitive electrodes. Herein, we intercalated K⁺ ions into MnO₂ to enlarge the interlayer space as channels for ion diffusion, and coated the particles with carbon layer to achieve fast charging/discharging ability. The obtained K_xMnO@C particles delivered a high specific capacitance of 1039 F g^?1 in 5 M LiTFSI aqueous electrolyte. Coupled with activated carbon anode, the hybrid supercapacitor showed outstanding energy and power density.     

Benign enzymatic synthesis of multiwalled carbon nanotube composites uniformly coated with polypyrrole for supercapacitors

BACKGROUND: Recently, various composites of carbon nanomaterials and conducting polymers have been actively investigated as potential electrode materials for supercapacitors which can store and deliver large amounts of electrical energy promptly. Harsh chemical or complex electrodeposition methods have been studied to prepare such composites. In this report, the mild and simple enzymatic catalysis of horseradish peroxidase (EC 1.11.1.7) in aqueous solutions (pH 4.0) was utilized for the first time to prepare composites of multiwalled carbon nanotubes and polypyrrole. RESULTS: Electron micrographs show that in situ enzymatic reaction by horseradish peroxidase enables the uniform coating of multiwalled carbon nanotubes with polypyrrole without containing the polymer aggregates. The specific capacitance of the composites (46.2 F g^?1) measured with a two‐electrode cell containing an electrolyte of 1 mol L^?1 NaNO₃ increased more than four‐fold compared with that obtained with bare multiwalled carbon nanotubes (10.8 F g^?1). CONCLUSIONS: Horseradish peroxidase‐catalyzed in situ synthesis of the composites of multiwalled carbon nanotubes and polypyrrole requires neither the derivatization of multiwalled carbon nanotubes and/or pyrrole monomers nor the post‐doping of the synthesized composites to enhance the capacitance of the composites. © 2012 Society of Chemical Industry     

MnO2 depositing on the surface of hollow porous carbon microspheres for supercapacitor application

A supercapacitor electrode material was synthesized by using hollow carbon spheres prepared via high temperature sintering of dopamine hydrochloride and subsequent coating with MnO₂. SEM, TEM and analysis of energy pattern were used to characterize the structure, morphology and elemental composition of the material, which proved that the material had a good hollow structure and uniform surface morphology, and that MnO₂ was successfully coated on the surface of the carbon material. Electrochemical characterization using charge-discharge cycles at constant current and other methods show that the prepared materials have good specific capacitance and cycle stability, and have a specific capacitance of 198 F.g^?1 at a current of 1 A·g^?1. When the charge and discharge cycle is carried out at 10 A·g^?1 for 5000 cycles, the capacitance remains stable at more than 180 F·g^?1.     

Highly flexible supercapacitors with manganese oxide nanosheet/carbon cloth electrode

We report a simple and cost-effective synthesis of hierarchically porous structure composed of Birnessite-type manganese dioxide (MnO₂) nanosheets on flexible carbon cloth (CC) via anodic electrodeposition technique. Petal-shaped MnO₂, having sheet thickness of a few nm and typical width of 100 nm, with a strong adhesion on CC is observed. This hierarchically porous MnO₂–CC hybrid structure dose exhibit not only excellent capacitance properties, such as up to 425 F g⁻¹ in specific capacitance, but also high crack resistance owing to its efficient release of bending stress, as observed by cyclic voltammetry and galvanostatic charge/discharge measurements under different curvature of bending configurations. Furthermore, flexible supercapacitors based on this kind of MnO₂ nanosheet/CC electrode showed significantly improved stability in capacitive performance over 3000 cycles under the bending test, which is highly promising for future applications in flexible energy storage device.     

One-step electrosynthesis of MnO2/rGO nanocomposite and its enhanced electrochemical performance

We present a facile one-step electrochemical approach to generate MnO₂/rGO nanocomposite from a mixture of Mn₃O₄ and graphene oxide (GO). The electrochemical conversion of Mn₃O₄ into MnO₂ through potential cycling is expedited in the presence of GO while the GO is reduced into reduced graphene oxide (rGO). The MnO₂ nanoparticles are evenly distributed on the rGO nanosheets and act as the spacer to prevent rGO nanosheets from restacking. This unique structure provides high electroactive surface area (1173?m² g^?1) that improves ions diffusion within the MnO₂/rGO structure. As a result, the MnO₂/rGO nanocomposite exhibits high specific capacitance of 473?F?g^?1 at 0.25?A?g^?1, which is remarkably higher (3 times) than the Mn₃O₄/GO prior conversion. In addition, the electrosynthesized nanocomposite shows higher conductivity and excellent potential cycling stability of 95% at 2000 cycles.     

Electrochemical fabrication of polyaniline/MnO2/graphite felt as free‐standing,flexible electrode for supercapacitors

Polyaniline/MnO₂/graphite felt (PMGF) composite, which can be used as a novel free‐standing, flexible electrode for supercapacitors, was fabricated via a facile electrochemical method. Polyaniline/graphite felt (PANI/GF) electrode was prepared by electropolymerization of PANI onto the GF. Subsequently, manganese dioxide (MnO₂) was electrodeposited on the surface of the PANI/GF electrode to prepare PMGF electrode. The microstructure and morphology of the as‐prepared samples were characterized by Fourier transform infrared spectra, X‐ray diffraction, scanning electron microscopy, and transmission electron microscopy. Specific surface area was examined using N₂ adsorption/desorption test. Cyclic voltammogram, chronopotentiometry techniques and electrochemical impedance spectroscopy were introduced to investigate the electrochemical performance of the composites. The PMGF electrode exhibited specific capacitance as high as about 630 F g⁻¹ at the current density of 0.5 A g⁻¹, which is much higher than that of PANI/MnO₂ composites reported previously. The high specific capacitance of PMGF may be attributed to the fact that the porous GF is a good conductive matrix for the dispersion of PANI/MnO₂ and it can facilitate easy access of electrolytes to the electrode, which results in enhancement of the electrochemical performance of the composite. Moreover, the specific capacitance of PMGF is much larger than that of MnO₂/GF (MGF), which may be ascribed to the participant of PANI, which contributes additional pseudocapacitance and electron transport path. POLYM. COMPOS., 34:819–824, 2013. © 2013 Society of Plastics Engineers     

Fabrication of a fibrous MnO2@MXene/CNT electrode for high-performance flexible supercapacitor

As shining stars of 2-dimensional materials, transition metal carbides (MXene) and transition metal oxides have attracted much interest in various energy fields due to their excellent conductive and electrochemical properties. However, big challenge still remains in the accessibility of high-performance fibrous electrodes for flexible supercapacitors. In this paper, MnO₂ nanorods are loaded on MXene sheets to obtain MnO₂@MXene composites by a facile hydrothermal method, which are subsequently coated on carbon nanotube fibers (CNTFs). With a fine control on morphology, the resulting MnO₂@MXene/CNTF electrode exhibits a high specific capacitance of 181.8 F/g at 1 A/g, a capacitance retention of 91% after 5000 charge-discharge cycles, as well as superb flexibility, i.e., neglected capacitance loss at a bending angle of 180°. The as-fabricated flexible composite fiber opens a new door for transition metal carbides and transition metal oxides with great potential in flexible electronics.     

Anode electrodeposition of 3D mesoporous Fe2O3 nanosheets on carbon fabric for flexible solid-state asymmetric supercapacitor

Novel nanostructured Fe₂O₃ with a network of 3D mesoporous nanosheets was synthesized by depositing on carbon fabric (Fe₂O₃@CF) for use as an anode using a potentially low-cost electrodeposition technique. The electrode with freestanding binder-free Fe₂O₃@CF of high surface area displayed an exceptional specific capacitance of 394.2?F?g^?1. Moreover, a flexible solid-state asymmetric supercapacitor (ASC) was fabricated with a negative electrode based on Fe₂O₃@CF and a positive electrode based on MnO₂@CF in the presence of PVA-LiCl as gel electrolyte. The above ASC exhibited a high operating potential up to 1.8?V, a favorable specific capacitance of 93.5?F?g^?1 (2.92?F?cm^?3), long-term stability (91.3% retention of initial value over 5000 cycles), and remarkable mechanical stability and flexibility, suggesting its potential application for wearable electronics.     

Aloe peel-derived honeycomb-like bio-based carbon with controllable morphology and its superior electrochemical properties for new energy devices

In this study, aloe peel-derived honeycomb-like porous carbons (AP-HC) are controllably prepared by combining simple hydrothermal carbonization with chemical activation. A morphology transformation from the spherical structure (AP-SC) to the honeycomb-like structure (AP-HC) is achieved for biomass-derived carbon materials and is accompanied by an increase in the specific surface area from 13 to 1286?m² g^?1. The AP-HC as a counter electrode (CE) for dye-sensitized solar cells (DSSCs) exhibits remarkable catalytic activity for I₃^- ion reduction and a high power conversion efficiency (PCE) of 6.92% that matches the Pt-based DSSC's performance (7.19%). As a working electrode in supercapacitors (SCs), a high specific capacitance of 264?F?g^?1 at 0.5?A?g^?1 is achieved in a three-electrode system. Additionally, a high retention rate of ～77.45% (ranging from 0.5 to 30.0?A?g^?1) and superior cycling performance (91% capacitance retention after 5000 cycles) are also demonstrated. This study provides an efficient strategy for fabricating morphology-controllable porous bio-based carbon with higher specific surface area (1286?m² g^?1) that exhibits significant potential for new energy devices.     

CoO/rGO composite prepared by a facile direct-flame approach for high-power supercapacitors

In this study, CoO nanoparticles (NPs) measuring approximately 20?nm in size are successfully grown on reduced graphene oxide (rGO) layers through a facile direct-flame approach. The obtained CoO/rGO nanocomposites are applied as electrode materials and show a high specific capacitance, reaching 1615.0?F?g^?1 at a current of 1?A?g^?1 (737.5?F?g^?1 at 50?A?g^?1), and good cycling stability (88.12% retention after more than 15,000 cycles at 5?A?g^?1), which are outstanding characteristics compared with those of recently reported pseudosupercapacitors. Furthermore, an asymmetric supercapacitor (ASC) produced using CoO/rGO as a positive electrode material and activated graphene (AG) as a negative electrode achieves a high cell voltage of 1.6?V and delivers a maximum energy density of 62.46?Wh?kg^?1 at a power density of 1600?W?kg^?1. The fabrication technique is facile and represents a promising means of obtaining metal oxide/graphene composites for high-performance supercapacitors.     

Three-dimensional carbon-based endogenous-exogenous MoO2 composites as high-performance negative electrode in asymmetric supercapacitors and efficient electrocatalyst for oxygen evolution reaction

It is not an easy way to design composite electrodes with a high concentration of the constituent. This study cleverly exploited the phase transformation of molybdenum oxide to synthesize three-dimensional carbon-based endogenous-exogenous MoO₂ composites (EEC) by a two-step process. MC-15 exhibited the most outstanding electrochemical performance among EEC, with a specific capacitance up to 411.1 F g^?1 in Na₂SO₄, due to the design of MoO₂, which could be highly loaded with three-dimensional carbon. In addition, the electrode capacitance remains up to 94.1% after 5000 cycles, attributed to the synergy effect of three-dimensional carbon and molybdenum dioxide by providing an abundance of active sites for MoO₂ and overcoming its stacking. In this way, the electrochemical properties of the EEC electrode are not compromised by the volume expansion during the electrochemical process. The energy density of the asymmetric supercapacitor using this material as the negative electrode and MnO₂@CC is 14 W h kg^?1 at a power density of 802 W kg^?1, showing a significant increase in energy density over the asymmetric supercapacitor with a conventional negative electrode (activated carbon, energy density of 3.36 W h kg^?1 and power density of 700 W kg^?1). Its specific capacitance remained 84.9% after 2500 cycles. In addition, an overpotential of only 348 mV was required to drive oxygen evolution in alkaline electrolytes with a Tafel slope as low as 88.7 mV dec^?1; the 20 h stability test retains almost 100%. The results show that the design optimization of the negative electrode material provides a simple and effective strategy to increase the energy density of supercapacitors, and EEC electrode materials are a great candidate to be utilized in supercapacitors with excellent performance as well as electrolytic water.     

Graphite oxide/polypyrrole composite electrodes for achieving high energy density supercapacitors

Fabrication and characterization of high energy density supercapacitor based on graphite oxide/polypyrrole (GO/PPy) composites is reported. Improvement in charge storage has been obtained by exfoliation of graphite oxide sheets via intercalation of polypyrrole. The formation of composite has been shown by the analysis of X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, and Fourier transfer of infrared spectroscopy data. Scanning electron and transmission electron microscopy clearly show sheet-like layered structure of graphite oxide surrounded by polypyrrole. Supercapacitors fabricated using this composite system result in a reduced equivalent series resistance value ~1.85 Ω. Such low value can be attributed to the intercalation of conducting polypyrrole into the graphite sheets. A specific capacitance of ~181 F g^?1 in 1 M Na₂SO₄ aqueous electrolyte with a corresponding specific energy density of ~56.5 Wh kg^?1 could be achieved. These values make GO-based materials suitable for their use as electrodes in high performance supercapacitors.     

Flexible electrodes based on polypyrrole/manganese dioxide/polypropylene fibrous membrane composite for supercapacitor

The composites of polypyrrole/manganese dioxide/polypropylene fibrous films (PPy/MnO₂/PPF) have been prepared in situ through chemical oxidation polymerization by using the mixture of FeCl₃·6H₂O and MnO₂ adsorbed on PPF as oxidant in the atmosphere of pyrrole vapor at room temperature. The morphologies and structures of the composites are investigated by using scanning electron microscope and X-ray diffraction spectroscopy. The properties of the capacitor cells assembled by the composites of PPy/MnO₂/PPF are evaluated by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy methods. The results reveal that the morphologies, conductivities and capacitance performance of the composites are influenced strongly by the content of MnO₂ in the solution of oxidant. The capacitors assembled by PPy/MnO₂/PPF exhibit the property of quick charge/discharge, and the highest specific capacitance of about 110 F g⁻¹ is obtained when the PPy/MnO₂ content in the composite is about 17.4%.     

Polyaniline/MnO2 composite with high performance as supercapacitor electrode via pulse electrodeposition

A method of pulse electrodeposition was proposed to synthesize polyaniline (PANI)/MnO₂ composite in aniline, H₂SO₄, and MnSO₄ aqueous solution. The PANI/MnO₂ composite has rod‐like structure and MnO₂ particles are distributed on PANI uniformly. To evaluate the performance of the as‐prepared materials as supercapacitor electrodes, cyclic voltammetry, galvanostatic charge–discharge measurements, and electrochemical impedance spectroscopy were performed. The PANI/MnO₂ composite shows a higher specific capacitance (810 F g⁻¹) than pure PANI (662 F g⁻¹) at a current density of 0.5 A g⁻¹. The cycle life of the composite was also excellent. After 1,000 cycles, it maintained 86.3% of its initial capacitance. POLYM. COMPOS., 36:113–120, 2015. © 2014 Society of Plastics Engineers     

Facile preparation of Ni-doped MnCO3 materials with controlled morphology for high-performance supercapacitor electrodes

Doping homogeneous elements and conducting morphological adjustment as commonly-used modification methods are both effective to promote the electrochemical properties of electrode materials. In this work, nickel-doped manganese carbonate with 3D flower-like structure was synthesized by a one-step hydrothermal method, and the corresponding growth mechanism was investigated. The electrochemical characteristics of as-fabricated electrode materials were measured, among which 3D self-assembled Ni_0.2Mn_0.8CO₃ nanoflower with large surface area exhibited superior areal capacitance of 583.5?F?g^?1 at 1?A?g^?1 (fourfold more than MnCO₃ microcubes), excellent electrical conductivity as well as satisfactory cycling stability (84.78% capacitance retention after 2000 cycles at 2?A?g^?1). In addition, the asymmetric supercapacitor assembled with Ni_0.2Mn_0.8CO₃ as cathode and commercial activated carbon as anode displayed a high energy density of 24.1?Wh?kg^?1 at the power density of 0.74?kW?kg^?1 and showed a desirable cycle life. In summary, the unique 3D flower-like Ni_0.2Mn_0.8CO₃ nanomaterial could be regarded as a promising electrode material for high-performance supercapacitors.     

Manganese dioxide nanosheets decorated on MXene (Ti3C2Tx) with enhanced performance for asymmetric supercapacitors

The incorporation of nanosized pseudocapacitive materials and structure design are general strategies to enhance the electrochemical performance of MXene-based materials. Herein, the decoration of manganese dioxide (MnO₂) nanosheets on MXene (Ti₃C₂T_x) surfaces was prepared by a facile liquid phase coprecipitation method. Ti₃C₂T_x is initially modified by polydopamine (PDA) coating to ensure the homogeneous distribution of MnO₂ nanosheets and tight and close connections between MnO₂ and the Ti₃C₂T_x backbone. Due to the obtained three-dimensional (3D) nanostructure, facilitating electron transport within the electrode and promoting electrolyte ion accessibility, the δ-MnO₂@Ti₃C₂T_x-0.06 electrode yields superior electrochemical performances, such as a rather large areal capacity of 1233.1 mF cm^?2 and high specific capacitance of 337.6 F g^?1 at 2 mV s^?1, as well as high cyclic stability for 10000 cycles. Furthermore, δ-MnO₂@Ti₃C₂T_x-0.06 composites are employed as positive electrodes, and activated carbon (AC) materials act as negative electrodes with an aqueous electrolyte of 1 M Na₂SO₄ to assemble asymmetric supercapacitors. The prototype device is reversible at cell voltages from 0 to 1.8 V, and manifests a maximum energy density of 31.4 Wh kg^?1 and a maximum power density of 2700 W kg^?1. These encouraging results show enormous possibilities for energy storage applications.     

Synthesis of dandelion-like V2O3/C composite with bicontinuous 3D hierarchical structures as an anode for high performance lithium ion batteries

The dandelion-like V₂O₃/C composite was synthesized by a simple and facile template-free solvothermal method followed by a suitable thermal treatment. The dandelion-like V₂O₃/C composite is constructed by bicontinuous 3D hierarchical structures, which are formed by interconnected nanoparticles and interconnected pores, respectively. Moreover, the surface of interconnected nanoparticles is uniformly coated with an ultrathin carbon layer. Upon evaluation as an anode material for LIBs, the as-synthesized product shows superior electrochemical performance. Under the current density of 0.1?A?g^?1, the specific discharge capacity of V₂O₃/C composite is 737?mA?h?g^?1 after 100 cycles. Moreover, after 1000 cycles at a high current density of 2?A?g^?1, the sample exhibits a discharge capacity of 315?mA?h?g^?1 which is 94% of the first-cycle discharge capacity. This excellent electrochemical performance can be ascribed to its unique hierarchical structure with 3D interconnected nanopores and uniform carbon coating.     

Barnacle-like manganese oxide decorated porous carbon nanofibers for high-performance asymmetric supercapacitors

In this study, barnacle-like manganese oxide (MnO₂) decorated porous carbon nanofibers (PCNF) were synthesized using electrospinning and the chemical precipitation method for high-performance asymmetric supercapacitors. The porous structure of PCNF was acquired using poly(styrene-co-acrylonitrile) in the electrospinning solution. In order to obtain the optimized barnacle-like MnO₂ on PCNF (MnO₂-PCNF), the barnacle-like MnO₂ was synthesized using different synthetic times (namely, 1.5, 3.0, and 7.0 min) of the chemical precipitation. Among them, the optimized MnO₂-PCNF for 3.0 min exhibited the well-dispersed MnO₂ on the PCNF with the nano-size of 190–218 nm. The optimized MnO₂-PCNF showed the superior specific capacitance of 209.8 F g^?1 at 10 mV s^?1 and the excellent high-rate performance of 160.3 F g^?1 at 200 mV s^?1 with the capacitance retention of 98.7% at 100 mV s^?1 for 300 cycles. In addition, electrochemical performances of asymmetric cell (constructed activated carbon and MnO₂-PCNF) showed the high specific capacitance of 60.6 F g^?1 at the current density of 0.5 A g^?1, high-rate capacitance of 30.0 F g^?1 at the current density of 10 A g^?1, and the excellent energy density of 30.3–15.0 Wh kg^?1 in the power density range from 270 to 9000 W kg^?1. The enhanced electrochemical performance can be explained by the synergistic effects of barnacle-like MnO₂ nanoparticles with a high active area related to high specific capacitance and well-dispersed MnO₂ with a short ion diffusion length related to the excellent high-rate performance.     

Coaxial carbon nanofibers/MnO2 nanocomposites as freestanding electrodes for high-performance electrochemical capacitors

Carbon nanofibers (CNFs)/MnO₂ nanocomposites were prepared as freestanding electrodes using in situ redox deposition and electrospinning. The electrospun CNFs substrates with porosity and interconnectivity enabled the uniform incorporation of birnessite-type MnO₂ deposits on each fiber, thus showing unique and conformal coaxial nanostructure. CNFs not only provided considerable specific surface area for high mass loading of MnO₂ but also offered reliable electrical conductivity to ensure the full utilization of MnO₂ coatings. The effect of MnO₂ loading on the electrochemical performances was investigated by cyclic voltammetry (CV), impedance measurements and Galvonostatic charging/discharging technique. The results showed that an ultrathin MnO₂ deposits were indispensable to achieve better electrochemical performance. The maximum specific capacitance (based on pristine MnO₂) attained to 557 F/g at a current density of 1 A/g in 0.1 M Na₂SO₄ electrolyte when the mass loading reached 0.33 mg/cm². This freestanding electrode also exhibited good rate capability (power density of 13.5 kW/kg and energy density of 20.9 Wh/kg at 30 A/g) and long-term cycling stability (retaining 94% of its initial capacitance after 1500 cycles). These characteristics suggested that such freestanding CNFs/MnO₂ nanocomposites are promising for high-performance supercapacitors.