Vertically-aligned Mn(OH)<Subscript>2</Subscript> nanosheet films for flexible all-solid-state electrochemical supercapacitors

The arrangement of the electrode materials is a significant contributor for constructing high performance supercapacitor. Here, vertically-aligned Mn(OH)₂ nanosheet thin films were synthesized by cathodic electrodeposition technique on flexible Au coated polyethylene terephthalate substrates. Morphologies, microstructures, chemical compositions and valence state of the nanosheet films were characterized systematically. It shows that the nanosheets arranged vertically to the substrate, forming a porous nanowall structures and creating large open framework, which greatly facilitate the adsorption or diffusion of electrolyte ions for faradaic redox reaction. Electrochemical tests of the films show the specific capacitance as high as 240.2 F g^?1 at 1.0 A g^?1. The films were employed to assemble symmetric all-solid-state supercapacitors with LiCl/PVA gel severed as solid electrolyte. The solid devices exhibit high volumetric capacitance of 39.3 mF?cm^?3 at the current density 0.3 mA cm^?3 with robust cycling stability. The superior performance is attributed to the vertically-aligned configuration.     

Binder-free polyaniline interconnected metal hexacyanoferrates nanocomposites (Metal = Ni,Co) on carbon fibers for flexible supercapacitors

Improvement of the electrical conductivity, specific capacitance and binder-free polyaniline (PANI) interconnected with metal(II) hexacyanoferrate(III) (MHCF) nanocomposites (M?=?Ni, Co) on flexible carbon fibers (CF) were designed in our present research goal. PANI/MHCF/CF nanocomposites were prepared by one-step co-polymerization method. Electrochemical studies like cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy were analyzed. Under the optimized conditions, the nanocomposites demonstrated remarkable electrochemical performances as supercapacitor electrode with outstanding specific capacitances of ~725 F g^?1 at a current density of 1 A g^?1, and retained ~325 F g^?1 even at a high current density of 20 A g^?1 in 0.5 M H₂SO₄?+?0.5 M Na₂SO₄ solution. The excellent cycling stability with capacitance retention of 80% after 1000 cycles may be a potential electrode material for future supercapacitor when its cycling stability and rate performance are addressed.     

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.     

Facial synthesis of MnO<Subscript>2</Subscript>/three dimensional graphene composite and its application in supercapacitors

MnO₂ nanoparticle/three dimensional graphene composite (MnO₂/3DG) was synthesized by a hydrothermal template-free method and subsequent ultrasonic treatment in KMnO₄ solution. The MnCO₃/3DG particles can be detected after the hydrothermal process, which may be produced through the reaction between Mn²⁺ and \({\text{C}}{{\text{O}}_{\text{3}}}^{{\text{2}} - }\) due to the decarboxylation of GO under the hydrothermal condition. The final product MnO₂/3DG displayed high specific capacitance (324 F g^??1 at 0.4 A g^?1) and good cycle stability (91.1% capacitance retention after 5000 cycles). Furthermore, the asymmetric supercapacitor assembled with MnO₂/3DG and activated carbon (AC) exhibits an energy density of 33.78 Wh kg^?1 at the powder density of 380 W kg^?1. The excellent supercapacitance of the MnO₂/3DG composite may be due to the high pseudocapacitance of the dispersed MnO₂ nanoparticles and the conductive graphene with three dimensional porous microstructure.     

The preparation of novel hollow tetragonal starlike polyaniline and its electrochemical performance

Novel hollow tetragonal starlike polyaniline (HTS-PANI) doped with citric acid has been successfully synthesized by hydrothermal method for the first time. Scanning electron microscopy, transmission electron microscopy, UV–visible spectroscopy, Fourier transmission infrared spectroscopy, and X-ray diffraction were employed to analysis the morphology and structure of the obtained PANI. The results show that the HTS-PANI is in semi redox state and highly crystallized, accompanied with good thermal stability. According to the galvanostatic charge–discharge analysis, the specific capacitance of the sample is up to 460 F g^?1 at a current density of 0.2 A g^?1 in 1 M KCl electrolyte, and retains about 58 % after 1,000 charge–discharge processes at a current density of 5 A g^?1.     

Super-capacitive performance depending on different crystal structures of MnO2 in graphene/MnO2 composites for supercapacitors

In the present study, we synthesize nanoneedle structures of MnO₂/graphene nanocomposites (N-RGO/MnO₂) and birnessite-type MnO₂/graphene nanocomposites (B-RGO/MnO₂). The morphologies and microstructures of as-prepared composites are characterized by X-ray diffractometry, field-emission scanning electron microscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis. Our characterizations indicate that nanoneedle structures of MnO₂ and birnessite-type MnO₂ are successfully formed on graphene surfaces. Capacitive properties of the N-RGO/MnO₂ and B-RGO/MnO₂ electrodes are measured using cyclic voltammetry, galvanostatic charge/discharge tests, and electrochemical impedance spectroscopy in a three-electrode experimental setup using a 1 M Na₂SO₄ aqueous solution as the electrolyte. The N-RGO/MnO₂ electrode displays a specific capacitance as high as 327.5 F g^?1 at 10 mV s^?1, which is higher than that of a B-RGO/MnO₂ electrode (248.5 F g^?1). It is believed that the nanoneedle structure of MnO₂ shows excellent electrochemical properties than birnessite-type MnO₂ for the electrode materials for supercapacitors.     

Facile fabrication of activated carbonized horseweed-based biomaterials and their application in supercapacitors

Carbonized horseweed was prepared for the first time using KOH as activating agent and employed as an electrode material. Varying the KOH/C weight ratio had a dramatic effect on the electrochemical capacitance of this electrode material. The obtained results showed that the sample prepared using a KOH/C weight ratio of 5/1 exhibited the highest specific surface area of 1469 m² g^?1, with average pore diameter of 3.18 nm. Further, this sample also exhibited the highest specific capacitance (184.2 F g^?1) at a current density of 0.4 A g^?1 in 6 M KOH electrolyte. In addition, the sample retained 97.6 % of its initial specific capacitance even after 1000 cycles, owing to the formation of a microporous/mesoporous structure by the activation process, the structure which provided suitable sites for charge transport and electrolyte diffusion. Thus, activated microporous carbon materials derived from horseweed could be effective as electrode materials in supercapacitors.     

Building an interpenetrating network of Ni(OH)<Subscript>2</Subscript>/reduced graphene oxide composite by a sol–gel method

Herein, a facile sol–gel strategy for building the ordered interpenetrating network of Ni(OH)₂ and reduced graphene oxide (rGO) was proposed. In this strategy, rGO nanosheets were homogeneously fixed inside composite utilizing the pores of Ni(OH)₂ gel as template, forming rGO-interpenetrated gel network. It was found that the rGO nanosheets could effectively reduce the internal resistant of composites and provide mechanical support for the gel network of Ni(OH)₂. Therefore, the composite presented high electrochemical performance, especially high-rate performance, due to the interpenetrating of rGO nanosheets plus the supplementary role of acetylene black. It had high specific capacitance of 2163 F g^?1 at low current density of 2.9 A g^?1 and 733 F g^?1 at high current density of 86.8 A g^?1.     

Supercapacitive performance of chemically synthesized polypyrrole thin films: effect of monomer to oxidant ratio

Polypyrrole (PPY) thin films with different PPY monomer to ammonium peroxidisulphate (APS) oxidant molar ratios have been synthesized using simple and inexpensive chemical oxidative polymerization method. An interrelation between the monomer to oxidant molar ratio, morphology and supercapacitive performance of PPY thin films is studied. Initial polymerization conditions strongly affect the morphology and electrical properties of PPY thin films. Thermo-gravimetric and differential scanning calorimetric curves show the thermal stability of PPY up to 483 K. The supercapacitive performance of PPY films is studied using cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy techniques. In the present work, PPY films deposited with 0.1:0.2 monomer to oxidant molar ratio (pyrrole:APS) show maximum specific capacitance of 754 F g^?1 in 1 M H₂SO₄ electrolyte at the scan rate 5 mV s^?1 in potential window of ?0.4 to +0.6 V/SCE.     

Synthesis and electrochemical performances of a novel two-dimensional nanocomposite: polyaniline-coated laponite nanosheets

A novel two-dimensional nanocomposite, polyaniline-coated laponite (polyaniline/laponite) nanosheets, has been prepared by in situ oxidative polymerization of aniline on the surface of laponite nanosheets. These sheets present a loosely stacked structure with the formation of a great number of pores, which it can provide a larger electrode/electrolyte contact surface area, shorten the path for ions transport in the active material, and alleviate the expansion and contraction of the electrode material during the charge/discharge processes, leading to an improved electrochemical performance. As an active material for supercapacitors, the specific charge/discharge capacitance of polyaniline/laponite nanosheets is 375 and 330 F g^?1 (based on the total working electrode mass) at a current density of 0.5 A g^?1, respectively, with a coulombic efficiency of 88 % which is higher than that of pure polyaniline (28 %). Moreover, polyaniline/laponite nanosheets also show a good rate capability with a growth of current density from 0.5 to 30 A g^?1, a specific discharge capacitance of 266 F g^?1 remained at 30 A g^?1. This work suggests a strategy to improve the electrochemical performances of polyaniline.     

Assessment of supercapacitive performance of europium tungstate nanoparticles prepared via hydrothermal method

Nano-sized europium tungstate particles were prepared by reacting europium nitrate hexahydrate and sodium tungstate solutions, and the structures, morphology and optical properties of the product were evaluated by XRD, FT-IR, SEM, and UV–Visible techniques. The Eu₂(WO₄)₃ nanoparticles were evaluated as potential materials for constructing supercapacitor electrodes using the results of cyclic voltammetry, galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy and the electrodes were found to have a specific capacitance (SC) value of 347 F g^?1 in a 2.0 M H₂SO₄ electrolyte at a scan rate of 2 mV s^?1. The electrodes were further studied at the GCD at a current density of 1 A g^?1, and the SC of the building material was found to be 282 F g^?1. The cycling durability of the electrodes was also found to be excellent. After 4000 cycles the SC values of the electrodes were found to reach 129%. The preparation method and the resulting nano-particles, were hence found to be promising for high performance energy applications.     

One pot synthesis of a highly water-dispersible hybrid glucose carbides and reduced graphene oxide material with superior electrical capacitance

In this paper, reduced graphene oxide with glucose carbides (RGO-GC) was synthesized in a simple one pot synthesis via a hydrothermal approach. Graphene oxide (GO) dispersion was dissolved together with glucose in water, and the mixture was heated to 180 °C in an autoclave. After hydrothermal treatment, a thin GO-like glucose carbides (GC) film grows in situ on the RGO surface. Differing from RGO, the obtained RGO-GC not only has a much better dispersion in water, but also can be used as a support and a green reductant to fabricate Ag-decorated RGO-GC. Moreover, the maximum specific capacitance of the RGO-GC reaches as high as 247 F g^?1 at a charging/discharging current density of 0.1 A g^?1 in 1 M H₂SO₄ solution. After 1000 charging/discharging cycles, the specific capacitance still retains 95 % of its initial specific capacitance. The greatly improved electrochemical performance is attributed to the increased surface wettability, more effective ionic diffusion, and a higher conductivity arising from the hydrophilic GO-like GC grown on the RGO surfaces.     

Samarium-doped Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoparticles with improved magnetic and supercapacitive performance: a novel preparation strategy and characterization

Sm³⁺-doped magnetite (Fe₃O₄) nanoparticles were synthesized through a one-pot facile electrochemical method. In this method, products were electrodeposited on a stainless steel (316L) cathode from an additive-free 0.005 M Fe(NO₃)₃/FeCl₂/SmCl₃ aqueous electrolyte. The structural characterizations through X-ray diffraction, field-emission electron microscopy, and energy-dispersive X-ray indicated that the deposited material has Sm³⁺-doped magnetite particles with average size of 20 nm. Magnetic analysis by VSM revealed the superparamagnetic nature of the prepared nanoparticles (Ms = 41.89 emu g^?1, Mr = 0.12 emu g^?1, and H _Ci = 2.24 G). The supercapacitive capability evaluation of the prepared magnetite nanoparticles through cyclic voltammetry and galvanostat charge–discharge showed that these materials are capable to deliver specific capacitances as high as 207 F g^?1 (at 0.5 A g^?1) and 145 F g^?1 (at 2 A g^?1), and capacity retentions of 94.5 and 84.6% after 2000 cycling at 0.5 and 1 A g^?1, respectively. The results proved the suitability of the electrosynthesized nanoparticles for use in supercapacitors. Furthermore, this work provides a facile electrochemical route for the synthesis of lanthanide-doped magnetite nanoparticles.     

Facile synthesis of papillae-like polyaniline nanocones on graphene nanosheets for ultracapacitors

Papillae-like polyaniline (PANI) nanocones arrays growing on graphsene nanosheets (GNs) were synthesized in mass at low cost by in situ polymerization with the assistant of ethanol. Scanning electron microscopy and transmission electron microscopy images show that papillae-like PANI nanocones arrays are located uniformly on flexible two-dimensional GNs. Electrochemical properties are tested by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy. The electrochemical performances of GNs/PANI hybrid are better than those of bare GNs or PANI. GNs/PANI electrode delivered a maximum specific capacitance of 372 F g^?1 at a current density of 0.1 A g^?1 in 1.0 M Na₂SO₄ aqueous solution. And the composite exhibit an excellent cycle life with ~80% specific capacitance retention over 3000 cycles at 1 A g^?1. The GNs/PANI nanocomposites will be one of the most promising flexible electrode materials for high-performance ultracapacitors.     

Synthesis and characterization of polypyrrole nanotubes/multi-walled carbon nanotubes composites with superior electrochemical performance

Novel polypyrrole nanotubes/multi-walled carbon nanotubes (PPyNTs/MWCNTs) composites have been successfully synthesized via in situ chemical oxidation polymerization with methyl orange as soft template. Scanning electron microscopy and transmission electron microscopy images revealed that MWCNTs intertwined with the PPyNTs and PPyNTs/MWCNTs composites formed in water–ethanol solution. The obtained composites exhibited perfect electrochemical characteristic compared with PPyNTs and MWCNTs owing to the synergetic effect and the specific capacitance of the composites was strongly influenced by the mass ratio of pyrrole to MWCNTs. According to the galvanostatic charge/discharge analysis, the specific capacitance of PPyNTs/MWCNTs composites is up to 352 F g^?1 at a current density of 0.2 A g^?1 in 1 M KCl solution, much higher than that of the PPyNTs (178 F g^?1) and MWCNT (46 F g^?1), suggesting its potential application in supercapacitors.     

Preparation of amorphous cobalt/carbon nanofibers composite as binder-free and conductive-free electrode materials for high supercapacitor

One dimensional carbon nanofibers embedded amorphous cobalt oxide with high electrochemical performances were successfully prepared by electrospinning Co(NO₃)₂ in PAN/DMF solution followed by a high-temperature heat treatment process. The different molar ratio of AN/Co(NO₃)₂ were synthesis. The optimized Co/CNFs(30), in which the molar ratio of AN/Co(NO₃)₂ was 30/1, exhibited a specific capacitance of 1096 F g^?1 at 1 A g^?1 and almost no decay in specific capacitance after cycling 2500 times at 5 A g^?1. The Co/CNFs were characterized by scanning electron microscopy, X-ray diffraction, Raman, transmission electron microscopy, X-ray photoelectron spectroscopy, thermal-gravity-analysis and the N₂ adsorption–desorption. The result showed that cobalt element was successfully dispersed in the carbon nanofibers with an amorphous state.     

Preparation of manganese dioxide/multiwalled carbon nanotubes hybrid hollow microspheres via layer-by-layer assembly for supercapacitor

Manganese dioxide/multiwalled carbon nanotubes hybrid hollow microspheres were prepared via layer-by-layer assembly technique by alternately adsorbing aminated manganese dioxide (AMnO₂) and carboxylated multiwalled carbon nanotubes (CMWCNTs) using polystyrene sulfonate microspheres as sacrificial templates. The structures and morphologies of the hybrid hollow microspheres were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, transmission electron microscopy, and field emission scanning electron microscope. Capacitive properties of the prepared hybrid hollow microspheres were investigated by cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance spectroscopy in a three-electrode experimental setup using 1.0 mol L^?1 Na₂SO₄ solution as electrolyte. The result showed that the specific capacitance increased with the increase in the content of AMnO₂ and CMWCNTs, which reached 169 F g^?1 at a current density of 0.5 A g^?1 when AMnO₂ and CMWCNTs alternately adsorbed for ten times. And the capacitance retention was about 82 % after 800 times of cyclic voltammetry tests at a scan rate of 80 mV s^?1.     

Electrochemical preparation and energy storage properties of nanoporous Co(OH)<Subscript>2</Subscript> via pulse current deposition

Nanoporous Co(OH)₂ films are electrochemically deposited on Ni foams by pulse current deposition for supercapacitor application. The pore size and density are controlled by reaction conditions including frequency modulation and reaction time. The morphology of the films is monitored by SEM, and the chemical composition and crystal structure are confirmed by XPS and XRD, respectively. The electrochemical performance of the Co(OH)₂ film is characterized by cyclic voltammetry and charge–discharge tests. The charge-transfer resistances of the electrodes are examined by electrochemical impedance spectroscopy. The Co(OH)₂ film exhibits an excellent specific capacity of 1681 F g^?1 at a current density of 2 A g^?1 in a potential range of ?0.1 to 0.4 V from the charge/discharge test; this specific capacity is much higher than that obtained by direct current deposition (623 F g^?1 at the same condition) due to the highly porous structure.     

Electrochemical performance of Sn-doped δ-MnO<Subscript>2</Subscript> hollow nanoparticles for supercapacitors

Sn-doped δ-MnO₂ (Sn-MnO₂) hollow nanoparticles have been synthesized via chemical process at room temperature. Many characterizations have been carried out to fully identify the intrinsic information of the as-prepared samples and investigate their electrochemical properties. The results indicate that the morphologies of the samples can be adjusted by changing the concentration of Sn while the capacitance of Sn-MnO₂ nanoparticles increased corresponded with that of the undoped δ-MnO₂ nanoparticles. The specific capacitance of Sn(1 at.%)-MnO₂ is up to 258.2 F g^??1 at a current density of 0.1 A g^??1. What’s more, over 90% of the initial specific capacitance still remains after 1000 cycles at a current density of 2.0 A g^??1, displaying excellent cycling stability.     

Nitrogen-doped multiwalled carbon nanotubes decorated with copper(I) oxide nanoparticles with enhanced capacitive properties

We report on the enhanced capacitive properties of a copper(I) oxide nanoparticle (Cu₂O NP)-decorated multiwalled carbon nanotube (MWCNT) forest with nitrogen (N) doping. A careful in situ solid-state dewetting and plasma doping method was developed that ensured homogeneous decoration and contamination-free Cu₂O NPs with N doping on the nanotube sidewalls. The morphology and structure of the hybrid materials were characterised by scanning electron microscopy, transmission electron microscopy, energy-dispersive spectroscopy, Raman spectroscopy and X-ray photoemission spectroscopy. The electrochemical performance of the hybrid materials was investigated by cyclic voltammetry and galvanostatic charge/discharge tests in a 0.1 M Na₂SO₄ electrolyte. The electrochemical tests demonstrated that the Cu₂O NP/N-MWCNT electrode exhibits a specific capacitance up to 132.2 F g^?1 at a current density of 2.5 A g^?1, which is 30% higher than that of the pure MWCNT electrode. Furthermore, the electrode could retain the specific capacitance at 85% stability over 1000 cycles. These observations along with the simple assembly method for the hybrid materials suggest that the Cu₂O NP/N-MWCNT could be a promising electrode for supercapacitor applications.