In situ co-deposition of nickel hexacyanoferrate nanocubes on the reduced graphene oxides for supercapacitors

A facile co-precipitation strategy is developed to prepare nickel hexacyanoferrate nanocubes (NiHCF NBs) supported on the reduced graphene oxide (rGO) in the presence of poly(diallyldimethylammonium chloride) (PDDA). The NiHCF NBs are uniformly deposited on the rGO by electrostatic interaction. Their size can be tuned from 10 nm to 85 nm by changing their content from 32.6% to 68.2%. Under the optimal condition, NiHCF/PDDA/rGO hybrids are composed of 51.4% NiHCF NBs with an average size of 38 nm. The specific capacitance of NiHCF/PDDA/rGO hybrids reaches up to 1320 F g⁻¹ at a discharge density of 0.2 A g⁻¹, more than twice that of the pure NiHCF, as well as slight capacitance decay by 15% at 0.2 A g⁻¹ and excellent cycling stability with 87.2% of its initial capacitance after 10,000 discharge/charge cycles. More importantly, NiHCF/PDDA/rGO hybrids exhibit an ultrahigh energy density of 58.7 Wh kg⁻¹ at the power density of 80 W kg⁻¹. The superior storage energy performance of NiHCF/PDDA/rGO hybrids, such as high specific capacitance, good rate capacity and long cycling stability, positions them as a promising candidate for supercapacitor materials.     

Amorphous CoS nanoparticle/reduced graphene oxide composite as high-performance anode material for sodium-ion batteries

Transition metal sulfides have been proved as promising candidates of anode materials for sodium-ion batteries (SIBs) due to their high sodium storage capacity, low cost and enhanced safety. In this study, the amorphous CoS nanoparticle/reduced graphene oxide (CoS/rGO) composite has been fabricated by a facile one-step electron beam radiation route to in situ decorate amorphous CoS nanoparticle on the rGO nanosheets. Benefiting from the small particle size (~2 nm), amorphous structure, and electronic conductive rGO nanosheets, the CoS/rGO nanocomposite exhibits high sodium storage capacity (440 mAh g⁻¹ at 100 mA g⁻¹), excellent cycling stability (277 mAh g⁻¹ after 100 cycles at 200 mA g⁻¹, 79.6% capacity retention) and high rate capability (149.5 mAh g⁻¹ at 2 A g⁻¹). The results provide a facile approach to fabricate promising amorphous and ultrafine metal sulfides for energy storage.     

Improved electrochemical performances of reduced graphene oxide based supercapacitor using redox additive electrolyte

Reduced graphene oxide (rGO) is prepared by simple and eco-friendly hydrothermal reduction method. X-ray photoelectron spectroscopy and Ultraviolet–visible analysis corroborated the reduction of graphene oxide into rGO in basic medium. The flexibility of the prepared rGO is inferred from transmission electron micrographs. Further, the identification of suitable electrolyte is carried out using different anions (SO₄²⁻, Cl⁻, OH⁻) and cations (K⁺, Na⁺) for the superior performance of the rGO based supercapacitors. The electrochemical performance revealed that K⁺ and OH⁻ ions are more active species in aqueous solutions. Subsequently, an effort was taken to improve the specific capacitance in the optimized 1 M KOH electrolyte by KI as redox additive at different concentrations (0.025, 0.05, 0.075 and 0.1 M). The calculated specific capacitance and energy density of rGO electrode in the optimized 1 M KOH + 0.05 M KI electrolyte is 500 F g⁻¹ and 44 Wh kg⁻¹, respectively. On the other hand, it exhibited the specific capacitance of 298 F g⁻¹ at 0.83 A g⁻¹ in non-aqueous polymer gel (PVA + KOH + KI) electrolyte. Finally, the charged aqueous device is utilized to glow the light emitting diode.     

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.     

Straightforward synthesis of hierarchical Co3O4@CoWO4/rGO core–shell arrays on Ni as hybrid electrodes for asymmetric supercapacitors

Hierarchical Co₃O₄@CoWO₄/rGO core/shell nanoneedles arrays are successfully grown on 3D nickel foam using a simple, effective method. By virtue of its unique structure, Co₃O₄@CoWO₄/rGO demonstrates an enhanced specific capacitance of 386 F g⁻¹ at 0.5 A g⁻¹ current density. It can be used as an integrated, additive-free electrode for supercapacitors that boasts excellent performance. As illustration, we assemble an asymmetric supercapacitor (ASC) using the as-prepared Co₃O₄@CoWO₄/rGO as the positive electrode and activated carbon as the negative electrode. The optimized ASC displays a maximum energy density of 19.99 Wh kg⁻¹ at a power density of 321 W kg⁻¹. Furthermore, the ASC also presents a remarkably long cycle life along with 88.8% specific capacitance retention after 5000 cycles.     

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

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

Graphitic carbon nitride nanosheet-assisted preparation of N-enriched mesoporous carbon nanofibers with improved capacitive performance

N-enriched mesoporous carbon nanofibers (NMCNFs) were prepared by an electrospinning technique using graphitic carbon nitride (g-C₃N₄) nanosheets both as sacrificial template and N-doping source. The resultant NMCNF film has a high N-doping level of 8.6 wt% and a high specific surface area of 554 m² g⁻¹. When directly used as the electrode material for supercapacitor, the free-standing NMPCNF film shows a significantly improved capacitive performance including a higher specific capacitance (220 F g⁻¹ at 0.2 A g⁻¹) and a better rate capability (∼70% retention at 20 A g⁻¹) than those of microporous carbon nanofiber film prepared using the same process without using g-C₃N₄ nanosheets (145 F g⁻¹ at 0.2 A g⁻¹ and ∼45% retention at 20 A g⁻¹). Moreover, the NMCNFs show superior stability with only a ∼3% decrease of its initial capacitance after 1000 cycles at a high current density of 10 A g⁻¹. More significantly, the energy density of a symmetrical supercapacitor (SC) based on the NMPCNF film can reach 12.5 Wh kg⁻¹ at a power density of 72 W kg⁻¹.     

Nanodiamond particles/reduced graphene oxide composites as efficient supercapacitor electrodes

The paper reports on the preparation of reduced graphene oxide (rGO) modified with nanodiamond particles composites by a simple solution phase and their use as efficient electrode in electrochemical supercapacitors. The technique relies on heating aqueous solutions of graphene oxide (GO) and nanodiamond particles (NDs) at different ratios at 100 °C for 48 h. The morphological properties, chemical composition and electrochemical behavior of the resulting rGO/NDs nanocomposites were investigated using UV/vis spectrometry, Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, transmission electron microscopy (TEM) and electrochemical means. The electrochemical performance, including the capacitive behavior of the rGO/NDs composites were investigated by cyclic voltammetry and galvanostatic charge/discharge curves at 1 and 2 A g⁻¹ in 1 M H₂SO₄. The rGO/ND matrix with 10/1 ratio displayed the best performance with a specific capacitance of 186 ± 10 F g⁻¹ and excellent cycling stability.     

Graphene-wrapped polyaniline nanofibers as electrode materials for organic supercapacitors

Graphene-wrapped polyaniline nanofibers were prepared by assembly of negatively charged graphene oxide with positively charged aqueous dispersible polyaniline nanofibers in an aqueous dispersion, followed by the reduction of the graphene oxide. The hybrid material with a graphene oxide loading of 9.1 wt.% displayed a high specific capacitance of over 250 F g⁻¹ in a 1 M Et₄N⁺·BF₄⁻/propylene carbonate electrolyte, a 39.7% increase compared with pristine polyaniline nanofibers. A significant improvement in long-term cycle life was also realized. The hybrid exhibited an initial specific capacitance of 236 F g⁻¹, which remained as high as 173.3 F g⁻¹ over 1000 cycles, or a 26.3% decrease, much better than that for pure polyaniline nanofibers. An asymmetric supercapacitor based on this hybrid material and activated carbon was assembled. An energy density of 19.5 W h kg⁻¹ at a power density of 738.95 W kg⁻¹ was obtained for the cell under an operating voltage window of 2 V.     

Mesoporous N-containing carbon nanosheets towards high-performance electrochemical capacitors

We developed a direct carbonization strategy to efficiently fabricate mesoporous N-containing carbon nanosheets (N-CNSs) by using polyaniline nanosheets as a carbon precursor. Physicochemical characterizations revealed that the as-synthesized N-CNSs with 5.9 wt.% N species possessed a well-developed mesoporous architecture with large specific surface area of 352 m² g⁻¹, high mesoporous volume of 0.32 cm³ g⁻¹, and average pore size of ∼5.2 nm. When further utilized as an electrode for electrochemical capacitors, the mesoporous N-CNSs delivered a large specific capacitance of 239 F g⁻¹ at 0.5 A g⁻¹, and even 197 F g⁻¹ at a high current load of 8 A g⁻¹, indicating its good rate behavior. Furthermore, the capacitance degradation of ∼4% over continuous 5000 charge–discharge cycles at 6 A g⁻¹ further verified its good electrochemical stability at high rates for long-term electrochemical capacitors application.     

Facile synthesis of sandwich-like polyaniline/boron-doped graphene nano hybrid for supercapacitors

The physicochemical property of chemically prepared graphene can be significantly changed due to the incorporating of heteroatoms into graphene. In this article, boron-doped graphene sheets are used as carbon substrates instead of graphene for loading polyaniline by in situ polymerization. Compared with the individual component and polyaniline/non-doped graphene, the sandwich-like polyaniline/boron-doped graphene exhibits remarkably enhanced electrochemical specific capacitance in both acid and alkaline electrolytes. In a three-electrode configuration, the hybrid has a specific capacitance about 406 F g⁻¹ in 1 M H₂SO₄ and 318 F g⁻¹ in 6 M KOH at 1 mV s⁻¹. In the two-electrode system of a symmetric supercapacitor, this hybrid achieves a specific capacitance about 241 and 189 F g⁻¹ at 0.5 A g⁻¹ with a specific energy density around 19.9 and 30.1 Wh kg⁻¹, in the acid and alkaline electrolytes, respectively. The as-obtained polyaniline/boron-doped graphene hybrid shows good rate performance. Notably, the obtained electrode materials exhibit long cycle stability in both acid and alkaline electrolytes (∼100% and 83% after 5000 cycles, respectively). The improved electrochemical performance of the hybrid is mainly attributed to the introduction of additional p-type carriers in carbon systems by boron-doping and the well combination of pseudocapacitive conducting polyaniline.     

Functionalized three-dimensional graphene networks for high performance supercapacitors

Three-dimensional (3D) thermal reduced graphene network (TRGN) deposition on Ni foam without any conductive agents and polymer binders was successfully synthesized by dipping Ni foam into graphene oxide (GO) suspension and subsequent thermal reduction process. The direct and close contact between thermal reduced graphene and Ni foam is beneficial to the enhanced conductivity of the electrode, as well as the improvement of ion diffusion/transport into the electrode. Additionally, low-temperature reduction of GO possesses a large amount of stable oxygen-containing groups that can provide high pseudocapacitance. As a result, the TRGN electrode delivers a high specific capacitance of 442.8 F g⁻¹ at 2 mV s⁻¹ in 6 mol L⁻¹ KOH. Moreover, symmetric supercapacitor based on TRGN exhibits a maximum energy density of 30.4 Wh kg⁻¹ based on the total mass of the two electrodes in 1 mol L⁻¹ Na₂SO₄ electrolyte, as well as excellent cycling stability with 118% of its initial capacitance after 5000 cycles.     

Hollow ZnSnO3 cubes@carbon/reduced graphene oxide ternary composite as anode of lithium ion batteries with enhanced electrochemical performance

The ternary composite, carbon coated hollow ZnSnO₃ (ZS@C) cubes encapsulated in reduced graphene oxide sheets (ZS@C/rGO), was synthesized via low-temperature coprecipitation and colloid electrostatic self-assembly. The uniform carbon-coating layer not only plays a role in buffering the volume change of ZnSnO₃ cubes in the charging/discharging processes, but also forms three-dimensional network with the cooperation of graphene to maintain the structural integrity and improve the electrical conductivity. The results show that the reduced graphene oxide sheets encapsulated ZS@C microcubes with a typical core-shell structure of ~700 nm in size exhibit an improved electrochemical performance compared with bare ZS@C microcubes. The ZS@C/rGO electrode delivered an initial discharge capacity of 1984 mA h g⁻¹ at a current density of 0.1 A g⁻¹ and maintained a capacity of 1040 mA h g⁻¹ after 45 cycles. High specific capacity and superior cycle stability indicate that the ZS@C/rGO composite has a great potential for the application of lithium-ion anode material.     

Tubular graphene nanoribbons with attached manganese oxide nanoparticles for use as electrodes in high-performance supercapacitors

Graphene nanoribbons (GNRs) with tubular shaped thin graphene layers were prepared by partially longitudinal unzipping of vapor-grown carbon nanofibers (VGCFs) using a simple solution-based oxidative process. The GNR sample has a similar layered structure to graphene oxide (GO), which could be readily dispersed in isopropyl alcohol to facilitate electrophoretic deposition (EPD). GO could be converted to graphene after heat treatment at 300 °C. The multilayer GNR electrode pillared with open-ended graphene tubes showed a higher capacitance than graphene flake and pristine VGCF electrodes, primarily due to the significantly increased surface area accessible to electrolyte ions. A GNR electrode with attached MnO₂ nanoparticles was prepared by EPD method in the presence of hydrated manganese nitrate. The specific capacitance of GNR electrode with attached MnO₂ could reach 266 F g⁻¹, much higher than that of GNR electrode (88 F g⁻¹) at a discharge current of 1 A g⁻¹. The hydrophilic MnO₂ nanoparticles attached to GNRs could act as a redox center and nanospacer to allow the storage of extra capacitance.     

Synthesis of reduced graphene oxide/tungsten trioxide nanocomposite electrode for high electrochemical performance

A facile and well-controllable reduced graphene oxide/tungsten trioxide (rGO/WO₃) nanocomposite electrode was successfully synthesized via an electrostatic assembly route at 350 rpm for 24 h. In this study, hexagonal-phase WO₃ (h-WO₃) nanofiber was well distributed on rGO sheets by applying optimal processing parameters. The as-synthesized rGO/WO₃ nanocomposite electrode was compared with pure h-WO₃ electrode. A maximum specific capacitance of 85.7 F g⁻¹ at a current density of 0.7 A g⁻¹ was obtained for the rGO/WO₃ nanocomposite electrode, which showed better electrochemical performance than the WO₃ electrode. The incorporation of WO₃ into rGO could prevent the restacking of rGO and provide favourable surface adsorption sites for intercalation/de-intercalation reactions. The impedance studies demonstrated that the rGO/WO₃ nanocomposite electrode exhibited lower resistance because of the superior conductivity of rGO that improved ion diffusion into the electrode. To evaluate the contribution of WO₃ to the rGO/WO₃ nanocomposite, the influence of mass loading of WO₃ on the capacitance was investigated.     

Impregnation assisted synthesis of 3D nitrogen-doped porous carbon with high capacitance

Three-dimensional (3D) porous carbons with controlled mesopore and micropore structures were prepared through a simple and low-cost ultrasonic and impregnation assisted method from waste air-laid paper. The ammonia management was used to dope the 3D porous carbons with different types of nitrogen heteroatoms in a way that replaced carbon atoms. The N₂ adsorption–desorption characterization suggested that the nitrogen-doped carbons have a high surface area of 1470 m² g⁻¹ with the average pore diameter of 4.2 nm, which are conducive to form electric double layer under high current density. The resulting 3D carbon exhibited a higher capacitance at 296 F g⁻¹ in comparison with the nitrogen-free one at 252 F g⁻¹ in 6 M KOH electrolyte. Moreover, a high power density ca. 0.313 kW kg⁻¹ and energy density ca. 34.3 Wh kg⁻¹ were achieved in the ionic liquid ([EMIm]BF₄). The findings will open a new avenue to use waste materials for high-performance energy-storage devices.     

Hydrothermal synthesis and activation of graphene-incorporated nitrogen-rich carbon composite for high-performance supercapacitors

Graphene-incorporated nitrogen-rich carbon composite with nitrogen content of ca. 10 wt.% has been synthesized by an effective yet simple hydrothermal reaction of glucosamine in the presence of graphene oxide (GO). The nitrogen content of carbon composite is nearly twice as high as that of hydrothermal carbon without graphene. GO is favorable for the high nitrogen doping in the carbon composite by the reaction between the glucosamine-released ammonia and GO. The hydrothermal carbon composite is further activated by KOH, and graphene in the activated carbon composite demonstrates a positive effect of increasing specific surface area, pore volume and electrical conductivity, resulting in superior electrochemical performance. The activated carbon composite with higher specific surface area and micropore volume possesses higher specific capacitance with a value of 300 F g⁻¹ at 0.1 A g⁻¹ in 6 M KOH aqueous solution in the two electrode cell. Larger mesopore volume and higher conductivity of the activated carbon composite will provide fast ion and electron transfer, thus leading to higher rate capacity with a capacitance retention of 76% at 8 A g⁻¹ in comparison to the activated hydrothermal carbon without graphene.     

Easy synthesis of porous graphene nanosheets and their use in supercapacitors

We report the easy synthesis of porous graphene nanosheets (PGNs) using the etching of graphene sheets by MnO₂. An electrode made from PGNs exhibits a specific capacitance of 154 F g^?1 at 500 mV s^?1 in 6 M KOH compared to a value of 67 F g^?1 for graphene nanosheets, and a low capacitance loss of 12% after 5000 cycles. Interestingly, PGN electrode material shows an excellent rate capability due to its open layered and mesopore structures that facilitate the efficient access of electrolytes to the electrode material and shorten the ion diffusion pathway through the porous sheets. This approach offers the potential for cost-effective, environmentally friendly and large-scale production of PGNs.     

Ag nanoparticles anchored NiO/GO composites for enhanced capacitive performance

Hierarchical nickel oxide/graphene oxide (NiO/GO) and nickel oxide/graphene oxide/silver (NiO/GO/Ag) heterostructures were sucessfully fabricated as high-performance supercapacitors electrode materials by using a hydrothermal process and a photoreduction process. The experimental results showed that the NiO/GO/Ag heterostructure electrodes showed better electrochemical performance than those of NiO/GO and bare NiO nanosheets. The NiO/GO/Ag electrode exhibited a higher specific capacitance of 229 F g⁻¹ at a current density of 1 A g⁻¹, higher than that of 161 F g⁻¹ for NiO/GO composites. Furthermore, NiO/GO/Ag electrode also showed good rate capability (still 200 F g⁻¹ at 6 A g⁻¹) and cycling stability (24% loss after 2000 repetitive cycles at a scan rate of 20 mV s⁻¹). The enhanced capacitive performance of the NiO/GO/Ag composites was mainly attributed to the introduction of Ag nanoparticles, which increased the electrical conductivities of the composites, and promoted the electron transfer between the active components. This study suggested that NiO/GO/Ag composites were a promising class of electrode materials for high performance energy storage applications.     

Three-dimensional hybrid materials of fish scale-like polyaniline nanosheet arrays on graphene oxide and carbon nanotube for high-performance ultracapacitors

Three-dimensional (3D) hybrid materials composed of 2D fish scale-like polyaniline (PANI) nanosheet arrays on graphene oxide sheets and carbon nanotubes were synthesized by a one-step process using a simplified template-free polymerization method. PANI nanosheet growth is proposed to be accomplished through electrostatic interaction, hydrogen bonding, and π–π stacking interaction. Such a material exhibits specific capacitances of 589 and 413 F g⁻¹ at 0.2 and 5 A g⁻¹, respectively, compared to pristine PANI of 397 and 180 F g⁻¹. After 1000 cycles, the composite still retains 81% of its initial capacitance, while PANI retains only 48%.