Ultrasonic-assisted synthesis of magnetite/carbon nanocomposite for electrochemical supercapacitor

Iron oxides are considered as the promising pseudocapacitive materials for high-performance supercapacitors due to their high theoretical specific capacitance, low cost, environmental benignity, and natural abundance. In this work, we study capacitive behavior of different magnetite (Fe₃O₄) nanoparticles/carbon black (CB) composites ratios. These composites are synthesized by the coprecipitation method in the presence of ultrasonic waves. The structural and morphological characteristics of the magnetite/CB composites are investigated by X-ray diffraction and scanning electron microscopy, respectively. The electrochemical performance of magnetite/CB composite electrodes is tested by cyclic voltammetry and galvanostatic charge/discharge in a Na₂SO₄ electrolyte. The results indicate that the magnetite/CB electrodes show typical pseudo-capacitive behavior in Na₂SO₄ solution. Moreover, in comparison to the pure Fe₃O₄ (37 F g^?1) and carbon black (23 F g^?1), the as-prepared 45 % magnetite/CB nanocomposite electrode shows a higher specific capacitance (300 F g^?1). Additionally, the supercapacitor device of the magnetite/CB nanocomposite exhibits excellent long cycle life along with 98.5 % specific capacitance retained after 10,000 cycle tests.     

Thiourea aldehyde resin-based carbon/graphene composites for high-performance supercapacitors

Thiourea aldehyde resin-based heteroatom doping carbon and graphene composites (RHDC/GN) were prepared by an in situ polymerization and carbonization. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed that thiourea aldehyde resin deposited on lamellar GO flakes during the polymerization and RHDC/GN composites had a hierarchical structure. The specific capacitance of the RHDC/GN composites was high up to 355 F g^?1, much higher than that of the pure thiourea aldehyde resin-based heteroatom doping carbon (RHDC) with specific capacitance of 135 F g^?1 at a current density of 1.0 A g^?1 in 6-M KOH electrolyte. And the hetroatoms in RHDC/GN composites increase the specific capacitance, and GN enhances the conductivity of the electrodes which is beneficial to improving electrochemical cycling stability of the electrode significantly. The specific capacitance retains 90.97% after 5000 charge-discharge processes at 10 A g^?1, which provides potential as supercapacitors.     

Graphitized carbon from wastepaper as electrodes for high-performance electric double-layer capacitors

Graphitized carbon electrode material was prepared from wastepaper by graphitization in molten sodium metal. X-ray diffraction and Raman spectroscopy were used to investigate the structural change of resulted carbons, both of which well proved the formation of graphite structure. Graphitized carbons have surface area that is nearly 26 times larger than initial carbonized paper and exhibit better electrochemical performances. The electrochemical performances of graphitized carbons were investigated by cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge–discharge methods. The cyclic voltammetry results show a maximum specific capacitance of 194 F g^?1. Therefore, wastepaper can be a promising electrode material for high-performance electric double-layer capacitors (EDLCs).     

Facilely prepared polypyrrole-graphene oxide-sodium dodecylbenzene sulfonate nanocomposites by in situ emulsion polymerization for high-performance supercapacitor electrodes

The layered polypyrrole-graphene oxide-sodium dodecylbenzene sulfonate (PPyGO-SDBS) nanocomposites were facilely fabricated via an in situ emulsion polymerization method with the assistance of SDBS as dopant and stabilizer. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and electrochemical performance were employed to analyze the structure and the characteristics of the composites. The results showed that SDBS played an important role in improving the electrochemical performance of the PPyGO-SDBS, by dispersing the PPy between the layers of the GO. The obtained PPyGO-SDBS exhibited remarkable performance as an electrode material for supercapacitors, with a specific capacitance as high as 483 F g^?1 at a current density of 0.2 A g^?1 when the mass ratio of pyrrole to GO was 80:20. The attenuation of the specific capacitance was less than 20 % after 1,000 charge–discharge processes, supporting the idea that PPy inserted successfully into the GO interlayers. The excellent electrochemical performance seemed to arise from the synergistic effect between the PPy and the GO and the dispersion of the PPy induced by SDBS.     

NiO/CNTs derived from metal-organic frameworks as superior anode material for lithium-ion batteries

In this work, porous NiO microspheres interconnected by carbon nanotubes (NiO/CNTs) were successfully fabricated by the pyrolysis of nickel metal-organic framework precursors with CNTs and evaluated as anode materials for lithium-ion batteries (LIBs). The structures, morphologies, and electrochemical performances of the samples were characterized by X-ray diffraction, N₂ adsorption-desorption, field emission scanning electron microscopy, cyclic voltammetry, galvanostatic charge/discharge tests, and electrochemical impedance spectroscopy, respectively. The results show that the introduction of CNTs can improve the lithium-ion storage performance of NiO/CNT composites. Especially, NiO/CNTs-10 exhibits the highest reversible capacity of 812 mAh g^?1 at 100 mA g^?1 after 100 cycles. Even cycled at 2 A g^?1, it still maintains a stable capacity of 502 mAh g^?1 after 300 cycles. The excellent electrochemical performance of NiO/CNT composites should be attributed to the formation of 3D conductive network structure with porous NiO microspheres linked by CNTs, which benefits the electron transfer ability and the buffering of the volume expansion during the cycling process.     

KOH-activated nitrogen-doped graphene by means of thermal annealing for supercapacitor

KOH-activated nitrogen-doped graphene nanosheets (aNG) have been synthesized using thermal annealing method and applied in supercapacitor. The samples are characterized by transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and electrochemical techniques. Electrochemical results show that a capacitance of 132.4 F g^?1 at a charge/discharge current density of 0.1 A g^?1 is obtained for KOH-activated nitrogen-doped graphene which is nearly five times larger than that without KOH treatment. The present work demonstrates that KOH activation of thermally annealed nitrogen-doped graphene is a promising method for enhancing its application in energy storage system.     

Electrochemical exfoliation and in situ carboxylic functionalization of graphite in non-fluoro ionic liquid for supercapacitor application

Simultaneous electrochemical generation and functionalization of nano-sized graphite from graphite had been carried out in a non-fluoroanion-based ionic liquid, namely, triethylmethylammonium methylsulfate (TEMAMS) containing water and acetonitrile (AN) in different weight ratios. The oxygen-based functional groups attached with the exfoliated material had been identified using Fourier transform infrared spectroscopy (FTIR), and morphological changes were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A symmetrical supercapacitor was fabricated using the exfoliated nano-sized graphite, and the influence of surface functionalities on its performance was investigated using electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanostatic charge–discharge cycles (CC). The highest specific capacitance (C _sp) value of 140 F g^?1 at 0.25 A g^?1 was obtained in 1.0 M H₂SO₄, followed by aqueous TEMAMS (125 F g^?1), TEMAMS/acetonitrile (115 F g^?1), and TEMAMS (106 F g^?1) at 0.10 A g^?1.     

Synthesis of micro- and mesoporous carbon spheres for supercapacitor electrode

Micro- and mesoporous carbon spheres (MMCSs) are synthesized by the polymerization of colloidal silica-entrapped resorcinol/formaldehyde in the presence of ammonia as catalyst, followed by carbonization, sodium hydroxide (NaOH) etching to remove silica template, and potassium hydroxide (KOH) activation. The morphology and microstructure are characterized by scanning electron microscopy, transmission electron microscopy, and nitrogen adsorption–desorption. The results show that a typical sample (denoted as MMCS-3) unites the characteristics of regular spherical shape (uniform diameters of 500 nm), high specific surface area (1,620 m² g^?1), large pore volume (1.037 cm³ g^?1), and combined micropores and mesopores (11.0 nm), which endows MMCS-3 good electrochemical performance. MMCS-3 as supercapacitor electrode shows a specific capacitance of 314 F g^?1 under a current density of 0.5 A g^?1 and low internal resistance of 0.2 Ω in 6 M KOH aqueous solution. The electrochemical capacitance still retains 198 F g^?1 at a high current density of 10 A g^?1. After 500 cycle numbers of galvanostatic charge/discharge at 0.5 A g^?1, MMCS-3 electrode still remains the specific capacitance of 301 F g^?1 with the retention of 96 %. This study highlights the potential of well-designed MMCSs as electrodes for widespread supercapacitor applications.     

Growth of vertically aligned carbon nanotubes on carbon fiber: thermal and electrochemical treatments

Composite electrodes of vertically aligned carbon nanotubes (VACNT) were synthesized on carbon fiber (CF) substrate by pyrolysis of camphor/ferrocene using a SiO₂ interlayer as a barrier against metal diffusion into the substrate. Two treatments were used to remove iron from CF/VACNT structure: thermal annealing at high temperature under inert atmosphere and electrochemical oxidation in H₂SO₄ solution. The composites were characterized by scanning electron microscopy and Raman scattering spectroscopy. Besides, the electrochemical behavior of CF/VACNT was analyzed by cyclic voltammetry and charge/discharge tests. CF/VACNT composite submitted to the electrochemical oxidation showed the best electrochemical performance, with high specific capacitance, which makes it very attractive as electrode for supercapacitors.     

Co-electrodeposition of MnO2/graphene oxide coating on carbon paper from phosphate buffer and the capacitive properties

MnO₂/graphene oxide sheet composite (MnO₂/GOS) has been co-electrodeposited on the thermally treated carbon paper (TTCP) in phosphate buffer solution containing GOS and KMnO₄. The resulted samples have been characterized by scanning and transmission electron microscopy, Raman, X-ray diffraction, and X-ray photoelectron energy spectroscopy. The results show that the synthesized MnO₂ may be δ-MnO₂ and the morphology of MnO₂/GOS is very different from that of MnO₂, indicating that the introduction of GOS in electrolyte can influence the morphology during the deposition. The capacitive properties of the samples are investigated by using cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy. The specific capacitance of MnO₂ for MnO₂/GOS can reach about 829 F g^?1 at discharged current density of 1.0 A g^?1 in 1 M Na₂SO₄ aqueous solution, which is larger than that of MnO₂ deposited on TTCP. The composite of MnO₂/GOS also exhibits excellent cyclic stability with a decrease of 18.5 % specific capacitance after 1,500 cycles.     

Mechanochemically prepared polyaniline and graphene-based nanocomposites as electrodes of supercapacitors

Conductive nanocomposites based on polyaniline and graphene (PAni/Gr) were prepared by cheap and efficient mechanochemical method. The uniform distribution of Gr nanoparticles in the polymer matrix and the ordering of the polymer chains due to the action of mechanical shear stresses, which were established by TEM, stipulated high specific capacitance about 920 F g^?1 in ??0.2–1.0 V vs. Ag/AgCl potential range. PAni/Gr-based electrodes are able to provide the specific capacitance of ~?750 F g^?1 at 2 A g^?1 in symmetric supercapacitors (SSC) and stably cycle at the operating voltage V?=?0.65 V for 10,000 charge-discharge cycles with 96% capacitance retention, whereas the increasing of V leads to the loss of stability as a result of the cathode degradation. PAni/Gr-based SSC possessed improved self-discharge showed high rate capability, and the specific power of such SSC could reach ~?10 kW kg^?1 at the specific energy of ~?18 W h kg^?1.     

Chemical and electrochemical treatment effects on the morphology,structure, and electrochemical performance of carbon fiber with different graphitization indexes

The association of capacitive charging of the double-layer and a faradic redox reaction is desirable on carbon fiber (CF) when oxygen functional groups or other heteroatoms are present on its surface enhancing its capacitive properties. In this work, a systematic study of carbon fiber produced at three different heat treatment temperatures (HTT) of 1000, 1500, and 2000 °C was performed upon two approaches: middle (chemical) and severe (electrochemical) oxidative treatments. Morphological, structural, and surface chemical changes were investigated by field emission gun-scanning electron microscopy, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. Electrochemical responses were analyzed by galvanostatic charge/discharge, electrochemical impedance spectroscopy, and cyclic voltammetry. Raman results showed that the electrochemical oxidation promoted structural variation on CF samples independently of their HTT. Concerning the specific capacitance, the results indicated that chemical treatment was more effective for CF1000 than those for CF1500 and CF2000. This behavior may be attributed to higher amount of oxygen on its surface as well as its lower structural ordering. Otherwise, for CF1000, the electrochemical treatment increased its resistivity. However, for CF1500 and CF2000, which present higher graphitization levels and less heteroatom contents, greater capacitance values were observed after their electrochemical oxidative treatment.     

Electrochemical reduction of graphene oxide and its electrochemical capacitive performance

A convenient method for the production of graphene is developed using the electrochemical reduction of graphite oxide (GO) in solution without assembling it onto the electrode. The samples were examined by X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and Raman spectroscopy. The results show that the number of oxygen functional groups can be significantly decreased. The electrochemical capacitance of the prepared graphene after 8 h of reduction is 158.5 F g^?1 at 0.5 A g^?1, much higher than that of GO and carbon nanotubes. The mechanism for this reaction is also proposed in this paper.     

Nanostructured ferrite/graphene/polyaniline using for supercapacitor to enhance the capacitive behavior

Graphene nanosheets, polyaniline (PANI), and nanocrystallites of transition metal ferrite {Fe₃O₄ (Mag), NiFe₂O₄ (NiF), and CoFe₂O₄ (CoF)} have been prepared and characterized via XRD, FTIR, SEM, TEM, UV–vis spectroscopy, cyclic voltammetry, galvanostatic charge discharges, and impedance spectroscopy. Electrochemical measurements showed that supercapacitances of hybrid electrodes made of the ternary materials are higher than that of hybrid electrode made of binary or single material. The ternary hybrid CoF/graphene (G)/PANI electrode exhibits a highest specific capacitance reaching 1123 Fg^?1, an energy density of 240 Wh kg^?1 at 1 A g^?1, and a power density of 2680 Wkg^?1 at 1 A g^?1 and outstanding cycling performance, with 98.2% capacitance retained over 2000 cycles. The extraordinary electrochemical performance of the ternary CoF/G/PANI hybrid can be attributed to the synergistic effects of the individual components. The PANI conducting polymer enhances an electron transport. The Ferrite nanoparticles prevent the restocking of the carbon sheets and provide Faradaic processes to increase the total capacitance.     

In situ synthesis of crosslinked-polyaniline nano-pillar arrays/reduced graphene oxide nanocomposites for supercapacitors

Crosslinked-polyaniline (CPA) nano-pillar arrays adsorbed on the surface of reduced graphene oxide (RGO) sheets were synthesized by in situ solution polymerization through two steps of reduction. The electrochemical analyses demonstrated that the befittingly reduced CPA/RGO composite exhibited high performance as electrode materials for supercapacitors. The CPA/RGO composite showed very high specific capacitance of 1532 F g^?1 at a scan rate of 10 mV s^?1 or 694 F g^?1 at a current density of 2 A g^?1 in 1 M H₂SO₄ electrolyte, as well as great energy density of 61.4 W h kg^?1 at a current density of 2 A g^?1. The electrode material also had decent power density of 4 kW kg^?1 at a current density of 10 A g^?1, and good cycling stability of 92.5 % capacitance retained after 500 cycles of cyclic voltammetry at 500 mV s^?1. The neat microstructures and super electrochemical properties suggest the potential use of the composites in supercapacitors.     

Activated carbon/graphene composites with high-rate performance as electrode materials for electrochemical capacitors

Glucose-derived activated carbon (GAC)/reduced graphene oxide (RGO) composites are prepared by pre-carbonization of the precursors (aqueous mixture of glucose and graphene oxide) and KOH activation of the pyrolysis products. The effect of the mass ratio of graphene oxide (GO) in the precursor on the electrochemical performance of GAC/RGO composites as electrode materials for electrochemical capacitors is investigated. It is found that the thermally reduced graphene oxide sheets serves as a wrinkled carrier to support the activated carbon particles after activation. The pore size distribution and surface area are depended on the mass ratio of GO. Besides, the rate capability of GAC is improved by the introduction of GO in the precursor. The highest specific capacitance of 334 F g^?1 is achieved for the GAC/RGO composite prepared from the precursor with a GO mass ratio of 3 %.     

Enhanced electrochemical performance of nano-MnO2 modified by Ni(OH)2 as electrode material for supercapacitor

Ni(OH)₂ was compounded to MnO₂ in an easy liquid phase process to improve the diffusion process of the electrode. The as-prepared materials were a mixture of amorphous and nanocrystalline with aggregated nanoparticles forming slit-shaped pore structures. The composite has higher specific surface area and smaller pore volume compared with pristine MnO₂. Electrochemical properties of the electrodes were carried out with cyclic voltammetry (CV), galvanostatic charge–discharge tests, and electrochemical impedance spectroscopy (EIS). The MnO₂/Ni(OH)₂ composites exhibited enhanced electrochemical properties than that of pristine MnO₂. Remarkably, the composite which contains 3 % Ni(OH)₂ exerted the best discharged specific of 408 F g^?1 under 0.2 A g^?1, much higher than 247 F g^?1 of pristine MnO₂ at the same current density. Better rate capability and cycling stability were also realized by the same composite in comparison.     

石墨烯/聚苯胺复合材料的制备及其电化学性能

以苯胺和氧化石墨烯(GO)为原料, 采用电化学方法制备了石墨烯/聚苯胺(GP)复合材料. 利用X射线衍射(XRD)、扫描电镜(SEM)、拉曼(Raman)光谱、X射线光电子能谱分析(XPS)对其结构、微观形貌进行了表征,并对复合材料电化学性能进行了测试. 结果表明, 复合材料保持了石墨烯的基本形貌, 聚苯胺颗粒均匀地分散在石墨烯表面, 复合材料在500 mA·g^-1的电流密度下比电容达到352 F·g^-1, 1000 mA·g^-1下比电容为315 F·g^-1, 经过1000 次的充放电循环后容量保持率达到90%, 远大于石墨烯和聚苯胺单体的比电容. 复合材料放电效率高, 电解质离子易于在电极中扩散和迁移.     

Hybrid materials utilizing polyelectrolyte-derivatized carbon nanotubes and vanadium-mixed addenda heteropolytungstate for efficient electrochemical charging and electrocatalysis

Preparation and electrochemical behavior of new hybrid materials composed of multi-walled carbon nanotubes (CNTs) that were derivatized with poly(diallyldimethylammonium) chloride and modified with vanadium-mixed addenda Dawson-type heteropolytungstate, [P₂W₁₇VO₆₂]^8?, is described here. These nanostructured composite systems exhibited fast dynamics of charge propagation. They were characterized by the transport (effectively diffusional) kinetic parameter of approximately 8?×?10^?8 cm^?2 s^?1/2 and the specific capacitance parameter of 82 F g^?1 (at the charging/discharging current of 200 mA g^?1). The latter parameter for bare CNTs was found to be only 50 F g^?1 under analogous conditions. These observations were based on the results of galvanostatic charging–discharging, cyclic voltammetric, and AC impedance spectroscopic measurements. The improved capacitance properties were attributed to the systems’ pseudocapacitive features originating from the fast redox transitions of the [P₂W₁₇VO₆₂]^8? polyanions. In addition to the fast redox conduction, the proposed organic–inorganic hybrid materials exhibited interesting electrocatalytic activity toward reduction of bromate in the broad concentration range (sensitivity, 0.24 mA cm^?2 mmol^?1 dm³).     

Facile low-temperature polyol process for LiFePO4 nanoplate and carbon nanotube composite

Crystalline LiFePO₄ nanoplates were incorporated with 5 wt.% multi-walled carbon nanotubes (CNTs) via a facile low temperature polyol process, in one single step without any post heat treatment. The CNTs were embedded into the LiFePO₄ particles to form a network to enhance the electrochemical performance of LiFePO₄ electrode for lithium-ion battery applications. The structural and morphological characters of the LiFePO₄–CNT composites were investigated by X-ray diffraction, Fourier Transform infrared spectroscopy, Raman spectroscopy, scanning electron microscopy and transmission electron microscopy. The electrochemical properties were analyzed by cyclic voltammetry, electrochemical impedance spectroscopy and charge/discharge tests. Primary results showed that well crystallized olivine-type structure without any impurity phases was developed, and the LiFePO₄–CNT composites exhibited good electrochemical performance, with a reversible specific capacity of 155 mAh g⁻¹ at the current rate of 10 mA g⁻¹, and a capacity retention ratio close to 100% after 100 cycles.