Graphene nanosheets as electrode material for electric double-layer capacitors

Graphene nanosheets (GNSs) with narrow mesopore distribution around 4 nm were mass-produced from natural graphite via the oxidation and rapid heating processes. The effects of oxidant addition on the morphology, structure and electrochemical performance of GNSs as electrode materials for electric double-layer capacitor (EDLC) were systematically investigated. The electrochemical properties of EDLC were influenced by the specific surface area, pore characteristics, layer stacking and oxygen-containing functional group contents of electrode materials. Deeper oxidation makes graphite possess both higher specific surface area and more graphene edges, which are favorable for the enhancement of capacitive performance of EDLC. The electrodes with freestanding graphene nanosheets prepared by coating method exhibited good rate capability and reversibility at high scan rates (to 250 mV s⁻¹) in electrochemical performances. GNS electrode with specific surface area of 524 m² g⁻¹ maintained a stable specific capacitance of 150 F g⁻¹ under specific current of 0.1 A g⁻¹ for 500 cycles of charge/discharge.     

RGO/KMn8O16 composite as supercapacitor electrode with high specific capacitance

Reduced graphene oxide/cryptomelane (RGO/KMn₈O₁₆) composites are successfully synthesized from α-MnO₂ nanorods and GO using a water-bathing precipitation method. The unique structure of KMn₈O₁₆ nanorods, with a length of 2–4 μm, dispersed on the surface of RGO leads to a much enhanced electrical conductivity and ionic transport, finally achieving composites with an improved electrochemical performance. Electrochemical measurement results show a specific capacitance of 222.3 F/g at a current density of 0.2 A/g, much higher than that of the original α-MnO₂. After 500 cycles at 2.0 A/g, the RGO/KMn₈O₁₆ composite electrode still retains 92.6% of its initial specific capacitance. The excellent electrochemical performance and durability observed for this composite electrode suggest its potential application for electrochemical capacitors.     

超级电容器电极材料研究最新进展

超级电容器是一种介于传统电容器与化学电源之间的新型储能元件,它具有充电时间短、循环寿命长、功率密度大、能量密度高、适用温度范围宽和经济环保等优势,目前在很多领域都受到广泛关注。本文概述了超级电容器电极材料的研究情况,包括碳基材料、金属氧化物材料及导电聚合物材料等。     

Fabrication of CuFe2O4@g-C3N4@GNPs nanocomposites as anode material for supercapacitor applications

The aim of this study was to synthesize CuFe₂O₄ together with g-C₃N₄ and GNPs in various combinations on the surface of Ni foam for use as anode materials in supercapacitors. The fabricated electrodes were investigated by XRD, FTIR, XPS, BET, SEM and TEM for content and by CV, GCD and EIS analysis for electrochemistry. The characterization results showed that CuFe₂O₄ was successfully synthesized together with g-C₃N₄ and GNPs in a nanosponge-like geometry. The highest value of specific capacitance was found to be 989 mF/cm² at 2 mA measurement in the triple combination. Moreover, the stability of this electrode was measured to be 70% after 1500 cycles at 16 mA, while the energy and power densities were calculated to be 27.8 mWh/cm² and 300 mW/cm², respectively. The EIS results show that the carbon-based component increased the Cs value by decreasing the charge transfer and diffusion resistances of the electrodes. Compared to its counterparts in the literature, its Cs value is quite high, but its stability is low, so it can be used in low-cycle applications.     

Effect of graphene nanosheets on electrochemical performance of Li4Ti5O12 in lithium-ion capacitors

In order to improve the electrochemical performance of lithium titanium oxide, Li₄Ti₅O₁₂ (LTO), for the use in the lithium-ion capacitors (LICs) application, LTO/graphene composites were synthesized through a solid state reaction. The composite exhibited an interwoven structure with LTO particles dispersed into graphene nanosheets network rather than an agglomerated state pristine LTO particles. It was found that there is an optimum percentage of graphene additives for the formation of pure LTO phase during the solid state synthesis of LTO/graphene composite. The effect of graphene nanosheets addition on electrochemical performance of LTO was investigated by a systemic characterization of galvanostatic cycling in lithium and lithium-ion cell configuration. The optimized composite exhibited a decreased polarization upon cycling and delivered a specific capacity of 173 mA h g⁻¹ at 0.1 C and a well maintained capacity of 65 mA h g⁻¹ even at 20 C. The energy density of 14 Wh kg⁻¹ at a power density of 2700 W kg⁻¹ was exhibited by a LIC full cell with a balanced mass ratio of anode to cathode along with a superior capacitance retention of 97% after 3000 cycles at a current density of 0.4 A g⁻¹. This boost in reversible capacity, rate capability and cycling performance was attributed to a synergistic effect of graphene nanosheets, which provided a short lithium ion diffusion path as well as facile electron conduction channels.     

Delafossite CuCrO2 nanoparticles as possible electrode material for electrochemical supercapacitor

Delafossites are popularly known materials for thermoelectric and electrochemical device applications due to their layered structural features. In this paper, delafossite CuCrO₂ nanoparticles (NPs) have been synthesized using a simple chemical procedure and are investigated as a supercapacitor material. To determine the phases of delafossite CuCrO₂ NPs, the morphological and phase formation experiments were conducted using diffraction patterns and microscopic analysis. The cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) studies were performed to evaluate the supercapacitative behavior of delafossite CuCrO₂ NPs. As prepared delafossite CuCrO₂ NPs based electrode showed an outstanding electrochemical property as compared to annealed delafossite CuCrO₂ NPs at 300–500 °C. A good specific capacitance of ～464.7 Fg^-1 at 0.01 Vs^-1 was found for the fabricated supercapacitor using non-annealed delafossite CuCrO₂ NPs based electrode, which was further validated by GCD results. The electrochemical supercapacitor fabricated with both non-annealed and annealed delafossite CuCrO₂ NPs displayed considerably the outstanding cycling stability by maintaining up to ～88% after 5000 cycles. This work sets the pace for a new and efficient method of preparing delafossite CuCrO₂ for high-performance electrochemical supercapacitors.     

Synthesis of graphene/vitamin C template-controlled polyaniline nanotubes composite for high performance supercapacitor electrode

A graphene nanosheet/polyaniline nanotube (GPNT) composite is prepared for the first time by in-situ chemical oxidative polymerization of aniline using vitamin C as a structure directing agent. The vitamin C molecules lead to the synthesis of polyaniline (PANI) nanotubes through the development of rod-like assembly by H-bonding in an aqueous medium. The initially synthesized graphene oxide/polyaniline nanotubes composite is reduced to graphene using hydrazine monohydrate followed by re-oxidation and protonation of the PANI to produce the GPNT nanocomposite. This novel composite showed a high specific capacitance of 534.37 F/g and an excellent energy density of 74.27 Wh/kg at a constant current of 0.5 mA. Besides, the GPNT composite exhibited excellent cycle life with 91.4% specific capacitance retained after 500 charge-discharge cycles. The excellent performance is due to the synergistic combination of graphene which provides good electrical conductivity and mechanical stability, and PANI nanofiber which deals with good redox activity.     

Nanostructured manganese dioxides: Synthesis and properties as supercapacitor electrode materials

Low-cost layered manganese oxides with the rancieite structural type were prepared by reduction of KMnO₄ or NaMnO₄ in acidic aqueous medium, followed or not by successive proton- and alkali-ion-exchange reactions. Samples were characterized by X-ray diffraction, energy dispersive X-ray analysis, BET surface area measurements, thermal analyses and X-ray photoelectron spectroscopy. As a result of successive exchange steps, compounds with high surface area (reaching 200 m² g⁻¹) can be obtained, and in the case of syntheses made with KMnO_4, the α-MnO₂ phase is formed. Capacitive properties of the synthesized materials were studied using potentiodynamic cycling in K₂SO₄. Correlations between the electrochemical and the physicochemical properties of the samples were investigated. The interesting conclusion is that the morphology and the size of the particles influence directly the capacitance, and that among the samples presenting the best morphology, the compounds derived from K-containing rancieite-type compounds (and containing α-MnO₂) present a better cycleability.     

Electrophoretic deposition (EPD) of hydrous ruthenium oxides with PTFE and their supercapacitor performances

The effect of PTFE addition was investigated for the electrophoretic deposition (EPD) of hydrous ruthenium oxide electrodes. Mechanical stability of electrode layers, together with deposition yield, was enhanced by using hydrous ruthenium oxide/PTFE dispersions. High supercapacitor performance was obtained for the electrodes prepared with 2% PTFE and 10% water. When PTFE content was higher, the rate capability became poor with low electronic conductivity; higher water content than 10% resulted in non-uniform depositions with poor cycleability and power capability. When electrodes were heat treated at 200 °C for 10 h, the specific energy was as high as 17.6 Wh/kg based on single electrode (at 200 W/kg); while utilizable energy was lower with heat treatment time of 1 and 50 h, due to the high resistance and gradual crystallization, respectively. With PTFE addition and heat treatment at 200 °C for 10 h, the specific capacitance was increased by 31% (460 → 599 F/g at ca. 0.6 mg/cm²) at 10 mV/s, and the deposition weight was increased up to 1.7 mg/cm² with initial capacitance of 350 F/g.     

Reduced chemically modified graphene oxide for supercapacitor electrode

An efficient active material for supercapacitor electrodes is prepared by reacting potassium hydroxide (KOH) with graphene oxide followed by chemical reduction with hydrazine. The electrochemical performance of KOH treated graphene oxide reduced for 24 h (reduced chemically modified graphene oxide, RCMGO-24) exhibits a specific capacitance of 253 F g^-1 at 0.2 A g^-1 in 2 M H₂SO₄ compared to a value of 141 F g^-1 for graphene oxide reduced for 24 h (RGO-24), and good cyclic stability up to 3,000 cycles. Interestingly, RCMGO-24 demonstrated a higher specific capacitance and excellent cycle stability due to its residual oxygen functional groups that accelerate the faradaic reactions and aid in faster wetting. This non-annealed strategy offers the potential for simple and cost-effective preparation of an active material for a supercapacitor electrode.     

High-performance binder-free supercapacitor electrode by direct growth of cobalt-manganese composite oxide nansostructures on nickel foam

A facile approach composed of hydrothermal process and annealing treatment is proposed to directly grow cobalt-manganese composite oxide ((Co,Mn)₃O₄) nanostructures on three-dimensional (3D) conductive nickel (Ni) foam for a supercapacitor electrode. The as-fabricated porous electrode exhibits excellent rate capability and high specific capacitance of 840.2 F g^-1 at the current density of 10 A g^-1, and the electrode also shows excellent cycling performance, which retains 102% of its initial discharge capacitance after 7,000 cycles. The fabricated binder-free hierarchical composite electrode with superior electrochemical performance is a promising candidate for high-performance supercapacitors.     

Electrochemical evaluation of binary Ni2V2O7 nanorods as pseudocapacitor electrode material

Pyrochlore structured nickel vanadate nanorods had been prepared by simple co-precipitation method. It was examined for pseudocapacitor electrode material. Morphological, optical and structural aspects of synthesized materials had been studied using a high-resolution transmission electron microscopy, UV–Visible absorption spectroscopy and powder X-ray diffraction analysis, respectively. The functional groups, stretching and bending vibrations were traced by Fourier transform infrared spectroscopy and the formation of nickel vanadate nanorods was confirmed by the binding energy analysis through X-ray photoelectron spectroscopic studies. The rod-shaped nanostructures of pyro nickel vanadate were confirmed by the scanning electron microscopy and HR-TEM analysis. Electrochemical techniques such as cyclic voltammetry, chronopotentiometry and electrochemical impedance spectroscopy techniques were used to analyse the supercapacitive behaviour of the prepared nanorods. Pyro nickel vanadate nanorods possesses excellent electrochemical stability up to 3000 cycles and the performance retention of about 94.1% was achieved even after 3000 repetitive charge-discharge cycles.     

Polypyrrole/carbon composite electrode for high-power electrochemical capacitors

Nano-thin polypyrrole (PPy) layers with thickness from ∼5 nm to several 10s nm were deposited on vapor grown carbon fibers (VGCF) by an in situ chemical polymerization. Using different concentrations of the pyrrole could control the thicknesses of deposited PPy layers. Surface morphology and thickness of the deposited PPy layers were confirmed by means of scanning electron microscopy and scanning transmission emission microscopy. Pseudo-capacitive behavior of the deposited PPy layers on VGCF investigated by means of cyclic voltammetry. Then, the PPy/VGCF composites were mixed with activated carbons (AC) at various mixing ratios. For the PPy/VGCF/AC composite electrodes, characteristics of specific capacitance and power capability were examined by half-cell tests. As results of this study, it was investigated that nano-thin PPy layer below ∼10 nm deposited on VGCF had high pseudo-capacitance and fast reversibility. Its specific capacitance per averaged weight of active material (PPy) was obtained as ∼588 F g⁻¹ at 30 mV s⁻¹ and maintained as ∼550 F g⁻¹ at 200 mV s⁻¹ of scan rate. Also, from the mixing 60 wt.% of the PPy/VGCF with 25 wt.% of AC, the PPy/VGCF/AC composite electrode exhibited higher power capability maintaining the specific capacitance per active materials of PPy and AC as ∼300 F g⁻¹ at 200 mV s⁻¹ in 6 M KOH.     

A facile synthesis of ZnMn2O4/Mn2O3 composite nanostructures for supercapacitor applications

This article reports the preparation of new ZnMn₂O₄/Mn₂O₃ composites by ultrasonication and microwave irradiation assisted hydrothermal method. The composite formation (ZnMn₂O₄/Mn₂O₃) is confirmed by powder X-ray diffraction, X-ray photoelectron spectroscopy and micro-Raman spectroscopy. The four ‘petals’ flower and six ‘petals’ flower-shaped morphology of composite material are confirmed through scanning electron microscopic analysis. The synthesis method directly influences the morphology of the composites. Further, nanosized composite formation is confirmed through transmission electron microscopic analysis. The prepared composite materials' electrochemical properties are studied using a three-electrode system and it shows a pseudocapacitance nature. The high specific capacitance of 380 F/g @ 0.5 A/g is found for the composite material in galvanostatic charge-discharge studies. The prepared ZnMn₂O₄/Mn₂O₃ composite shows an excellent cycling stability of 92% of the initial specific capacitance at current density of 3 A/g after 2000 cycles. Electrochemical studies of the composite reveal that the ZnMn₂O₄/Mn₂O₃ composite can be used for supercapacitor applications.     

MOF-derived PPy/carbon-coated copper sulfide ceramic nanocomposite as high-performance electrode for supercapacitor

To obtain a battery-type ceramic electrode material for supercapacitors, we used a metal-organic framework (HKUST-1) as a template to prepare ceramic material Cu₉S₈@C. Firstly, we adopted the calcination–vulcanization method to synthesize Cu₉S₈@C. Then we deposited it onto a carbon fiber cloth and employed it. Moreover, polypyrrole PPy/Cu₉S₈@C-CC nanocomposite electrodes were prepared via electrochemical deposition. By means of high-temperature calcination and vulcanization, the copper atoms of HKUST-1 were successfully transformed into Cu₉S₈ nanoparticles, and the organic ligand was carbonized into amorphous carbon in Cu₉S₈@C. The results showed that the PPy/Cu₉S₈@C-CC electrode presented a specific capacitance of 270.72F/g at a scan speed of 10 mV/s in a 1 M KCL aqueous solution. This value was much higher than that of Cu₉S₈@C-CC and Cu₉S₈-CC electrodes, as confirmed by the results of electrochemical test. At a scan rate of 10 mV/s, the capacitance retention rate for PPy/Cu₉S₈@C-CC after 3000 cycles was 80.36%, indicating its superior cycle characteristics. Moreover, PPy/Cu₉S₈@C-CC exhibited good frequency response. These results indicate that PPy/Cu₉S₈@C is an ideal electrode material for energy storage and conversion applications.     

Highly dispersed hydrous ruthenium oxide in poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonic acid) for supercapacitor electrode

Hydrous RuO₂ particles were electrochemically loaded into poly(3,4-ethylenedioxythiophene) doped poly(styrene sulfonic acid), PEDOT-PSS, matrix by employing various potential cycles in cyclic voltammetry and to fabricate the PEDOT-PSS-RuO₂·xH₂O electrode. The amount of hydrous RuO₂ particles loaded into the PEDOT-PSS matrix was easily controlled by varying the number of potential cycles. Scanning electron microscopy photographs reveal a uniform dispersion of hydrous RuO₂ particles in the porous structure of PEDOT-PSS matrix. Raman spectrum confirms the incorporation of hydrous RuO₂ into PEDOT-PSS matrix. Chronopotentiometry and cyclic voltammetry were employed in 0.5 M H₂SO₄ to evaluate the capacitor properties. Specific capacitance values were determined by chronoamperometry. An increasing trend in specific capacitance with loaded amount of hydrous RuO₂ particles in PEDOT-PSS was noticed. A maximum specific capacitance of 653 F/g was achieved.     

Feasibility of bamboo-based activated carbons for an electrochemical supercapacitor electrode

This study focused on the preparation and electrochemical properties of bamboo-based activated carbons (ACs) through carbonization and subsequent activation with steam and non-aqueous electrolyte solutions. The specific surface areas and the capacitances of samples ranged from 445 to 1,025 m²/g and from 5 to 60 F/g, respectively, depending on the activation conditions. The sample activated at 900 ‡C for 60 min under our experimental conditions exhibited the highest capacitance and the largest specific surface area.     

Effect of SiC whiskers and graphene nanosheets on the mechanical properties of ZrB2-SiCw-Graphene ceramic composites

Ultrahigh temperature ZrB₂-SiC_w-Graphene ceramic composites are fabricated by hot pressing ZrB₂-SiC_w-Graphene oxide powders at 1950 °C and 30 MPa for 1 h. The microstructures of the composites are characterized by Scanning electron microscopy, Raman spectroscopy and X-ray diffraction. The results show that multilayer graphene nanosheets are achieved by thermal reduction of graphene oxide during sintering process. Compared with monolithic ZrB₂ materials, flexural strength and fracture toughness are both improved due to the synergistic effect of SiC whisker and graphene nanosheets. The toughening mechanisms mainly are the combination of SiC whisker and graphene nanosheets crack bridging, pulling out.     

Influence of multi-walled carbon nanotubes on the electrochemical performance of graphene nanocomposites for supercapacitor electrodes

In this work, multi-walled carbon nanotube (MWNT) bonded graphene (M-GR) composites were prepared using the chemical reduction of graphite oxide (GO) and acid treated MWNTs with different ratios. The M-GR/polyaniline (PANI) nanocomposites (M-GR/PANI) were prepared using oxidation polymerization. The effect of the M-GR ratio on the electrochemical performances of the M-GR/PANI was investigated. It was found that the substrate 2D graphene was coated with 1D MWNTs by chemical reduction and the M-GR was further coated with PANI, leading to increased electrical properties by the π–π interaction between the M-GR and PANI. In addition, the electrochemical performances, such as the current density, charge–discharge, and specific capacitance of the M-GR/PANI were higher than those of graphene/PANI and the highest specific capacitance (1118 F/g) of the composites was obtained at a scan rate of 0.1 A/g for the PANI containing a 0.5 M-GR ratio compared to 191 F/g for the graphene/PANI. The dispersion of the MWNTs onto the graphene surface and the ratio of M-GR had a pronounced effect on the electrochemical performance of the PANI-based composites, which was attributed to the highly conductive pathway created by the M-GR incorporated in the PANI-based composites and the synergistic effect between M-GR and PANI.     

Nanocomposites of manganese oxides and carbon nanotubes for aqueous supercapacitor stacks

Symmetrical supercapacitors and their serially connected two-cell stacks via a bipolar electrode were constructed with nanocomposites of manganese oxides and carbon nanotubes (MnO_x/CNTs) as the electrode materials. Nanocomposites with different contents of MnO_x were synthesised through the redox reaction between KMnO₄ and CNTs in aqueous solutions. The nanocomposites were characterised by scanning and transmission electron microscopy, BET nitrogen adsorption and X-ray diffraction before being examined in a three-electrode cell with a novel trenched graphite disc electrode by electrochemical means, including cyclic voltammetry, galvanostatic charging-discharging, and electrochemical impedance spectroscopy. The nanocomposites demonstrated capacitive behaviour in the potential range of 0-0.85 V (vs Ag/AgCl) in aqueous KCl electrolytes with less than 9% capacitance decrease after 9000 charging-discharging cycles. Symmetrical supercapacitors of identical positive and negative MnO_x/CNTs electrodes showed capacitive performance in good agreement with the individual electrodes (e.g. 0.90 V, 0.53 F, 1.3 cm²). The bipolarly connected two-cell stacks of the symmetrical cells exhibited characteristics in accordance with expectation, including a doubled stack voltage and reduced internal resistance per cell.