Fabrication of functionalized nitrogen-doped graphene for supercapacitor electrodes

A unique and convenient one-step hydrothermal process for synthesizing functionalized nitrogen-doped graphene (FGN) via ethylenediamine, hydroquinone, and graphene oxide (GO) is described. The graphene sheets of FGN provide a large surface area for hydroquinone molecules to be anchored on, which can greatly enhance the contribution of pseudocapacitance. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, and electrochemical workstation are used to characterize the materials. The nitrogen content exhibited in FGN can be up to 9.83 at.%, and the as-produced graphene material shows an impressive specific capacitance of 364.6 F g^?1 at a scan rate of 10 mV s^?1, almost triple that of the graphene (GN)-based one (127.5 F g^?1). Furthermore, the FGN electrodes show excellent electrochemical cycle stability with 94.4 % of its initial capacitance retained after 500 charge/discharge cycles at the current density of 3 A g^?1.     

Self-assembled 3D Co3O4-graphene frameworks with high lithium storage performance

Three-dimensional Co₃O₄-graphene frameworks (3D-CGFs) are prepared with a one-pot hydrothermal method. Co₃O₄ particles are in situ anchored on graphene sheets, and the resulting composite self-assembles into 3D architecture during the hydrothermal treatment. Scanning electron microscope, transmission electron microscope, powder X-ray powder diffraction, and Raman spectroscopy are employed to characterize the sample. When tested as anode materials for lithium-ion batteries, 3D-CGFs demonstrate remarkable electrochemical lithium storage properties, such as large and stable reversible capacity (>530 mAh g^?1 at 500 mA g^?1 over 300 cycles), good capacity retention (88 % retention after 300 cycles at 500 mA g^?1 compared with the 4th cycle), excellent high-rate performance (515 mAh g^?1 at 1 A g^?1), making it a promising candidate for high-performance anode materials, especially for high-rate lithium-ion batteries.     

Supercapacitor performances of rich nitrogen-doped mesoporous graphene fabricated by a facile template-free copyrolysis process

Rich nitrogen-doped mesoporous graphene (NDMG) with a large specific surface area of 496.8 m² g^?1 and high electrical conductivity of 327.2 S cm^?1, and suitable pore size was synthesized by a facile co-thermal annealing of pre-prepared phenolic polymer and dicyandiamide. The NDMG has a high nitrogen content (7.9 wt%) and can act as promising electroactive materials for two-electrode symmetric supercapacitors. The NDMG cells displayed a high specific capacitance of ca. 316 F g^?1 at 0.5 A g^?1, which is much higher than that of the pristine graphene devices (ca. 123 F g^?1). Moreover, compared with the capacitance drop rate of pristine graphene devices (8.9 %), the specific capacitance of NDMG cells was decreased by only 3.2 % after 2000 cycles, exhibiting a good cycling performance and reversibility. In addition, the specific capacitance of the NDMG cells can reach 251 F g^?1 at 5.0 A g^?1, revealing an excellent rate capability and implying the ability to deliver a high energy density at a high power density. The good electrochemical performances of NDMG can be attributed to its high surface area, suitable mesopore size, and high electrical conductivity.     

Simple microwave synthesis and improved electrochemical performance of Nb-doped MnO<Subscript>2</Subscript>/reduced graphene oxide composite as anode material for lithium-ion batteries

Niobium-doped MnO₂/reduced graphene oxide (Nb-MnO₂/RGO) composite has been successfully synthesized via a simple microwave radiation method. The samples were systematically studied by X-ray diffraction (XRD), thermogravimetric analysis (TG), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), transmission electron microscope (TEM), and electrochemical measurements. As the anode material for lithium-ion batteries, the Nb-MnO₂/RGO (molar ratio of Mn/Nb?=?50:1) (NMG50) showed an outstanding reversible discharge capacity of 556.6 mAh g^?1 after 50 cycles with a capacity retention of 77% at a charge-discharge rate of 0.1 A g^?1 and the reversible discharge capacity can still retain 223.3 mAh g^?1 at a current of 1 A g^?1, which is much higher than those for Nb-MnO₂/RGO (molar ratio of Mn/Nb?=?10:1) (NMG10) and undoped MnO₂/RGO (MG). The improved electrochemical performance could be attributed to the proper amount of Nb doping, which could enhance both the conductivity and the structure stability of MnO₂.     

Synthesis of graphene/nickel oxide composite with improved electrochemical performance in capacitors

A novel reduced graphene oxide/NiO nanosheet composite (r-GO/NiO) (ca. 75 % NiO in weight) was synthesized by a facile two-step method, where the NiO nanosheets were decorated with some voids. The composite was characterized by using X-ray diffraction, transmission electron microscopy, thermal gravimetric analysis, and Raman spectroscopy. The electrochemical properties of the composite were investigated by cyclic voltammetry, galvanostatic charge, and discharge measurements. The results show that the r-GO/NiO composite exhibits a stable average specific capacitance of ca. 1,139 F g^?1 (at 0.5 A g^?1) during 1,000 charge–discharge cycles, suggesting that the r-GO/NiO composite is a potential supercapacitor material. The main correlation between the electrochemical performance and the structure of the materials was studied, and the formation process of the composite was also discussed.     

Influence of graphene coating on supercapacitive behavior of sandwich-like N- and O-enriched porous carbon/graphene composites in aqueous and organic electrolytes

Polyacrylonitrile nanofiber cloth coated with graphene oxide was carbonized and activated to fabricate nitrogen- and oxygen-enriched porous carbon/graphene (NAC@Gr) sandwich-like composites. The influence of graphene coating on the microstructure, surface composition, and supercapacitive performance of the as-prepared composites was investigated. The results indicated that significantly enhanced energy storage capability can be achieved due to the high specific surface area, optimized pore structure, and surface functionality. The composites show both high gravimetric and volumetric specific capacitances, for example, 380 F g^?1 (178 F cm^?3) at 0.1 A g^?1 in 6 M KOH and 228 F g^?1 (125 F cm^?3) at 1 A g^?1 in 1 M TEABF₄/AN electrolyte. The assembled symmetric supercapacitors exhibit high energy density, high power density, excellent cycling stability, and high-rate performance.     

N-doped graphene/Bi nanocomposite with excellent electrochemical properties for lithium–ion batteries

N-doped graphene/Bi nanocomposite was prepared via a two-step method, combining the gas/liquid interface reaction with the rapid heat treatment method. The as-prepared sample was characterized by X-ray diffraction (XRD), field-emission scanning electron microscope (FESEM), X-ray photoelectron spectroscopy (XPS), and elemental analyzer. The XRD, FESEM, XPS, and elemental analysis results confirm the successful synthesis of N-doped graphene/Bi nanocomposite. As a result, the prepared N-doped graphene/Bi nanocomposite as an anode material for lithium-ion batteries delivers excellent electrochemical performance. A high lithium storage capacity of about 522 mAh g^?1 in the voltage range of 0.01–3.5 V is obtained. After 50 cycles at different current densities from 50 to 1000 mA g^?1, the specific capacity can still remain 386 mAh g^?1. Even at the high current density of 1000 mA g^?1, the N-doped graphene/Bi nanocomposite can still deliver a specific capacity of 218 mAh g^?1. The excellent electrochemical performance of the N-doped graphene/Bi nanocomposite is supposed to benefit from the high electronic conductivity of nitrogen-doped graphene and the synergistic effect of bismuth nanoparticles and nitrogen-doped graphene.     

Synthesis and characterization of nitrogen-doped graphene hollow spheres as electrode material for supercapacitors

Recently, the rapid development of graphene industry in the world, especially in China, provides more opportunities for the further extension of the application field of graphene-based materials. Graphene has also been considered as a promising candidate for use in supercapacitors. Here, nitrogen-doped graphene hollow spheres (NGHS) have been successfully synthesized by using industrialized and pre-processed graphene oxide (GO) as raw material, SiO₂ spheres as hard templates, and urea as reducing-doping agents. The results demonstrate that the content and pretreatment of GO sheets have important effect on the uniform spherical morphologies of the obtained samples. Industrialized GO and low-cost urea are used to prepare graphene hollow spheres, which can be a promising route to achieve mass production of NGHS. The obtained NGHS have a cavity of about 270 nm, specific surface area of 402.9 m² g^?1, ultrathin porous shells of 2.8 nm, and nitrogen content of 6.9 at.%. As electrode material for supercapacitors, the NGHS exhibit a specific capacitance of 159 F g^?1 at a current density of 1 A g^?1 in 6 M KOH aqueous electrolyte. Moreover, the NGHS exhibit superior cycling stability with 99.24% capacitive retention after 5000 charge/discharge cycles at a current density of 5 A g^?1.     

Flower-like MoS<Subscript>2</Subscript> supported on three-dimensional graphene aerogels as high-performance anode materials for sodium-ion batteries

Flower-like MoS₂ supported on three-dimensional graphene aerogel (MoS₂/GA) composite has been prepared by a facile hydrothermal method followed by subsequent heat-treatment process. Each of MoS₂ microflowers is surrounded by the three-dimensional graphene nanosheets. The MoS₂/GA composite is applied as an anode material of sodium-ion batteries (SIBs) and it exhibits high initial discharge/charge capacities of 562.7 and 460 mAh g^?1 at a current density of 0.1 A g^?1 and good cycling performance (348.6 mAh g^?1 after 30 cycles at 0.1 A g^?1). The good Na⁺ storage properties of the MoS₂/GA composite could be attributed to the unique structure which flower-like MoS₂ are homogeneously and tightly decorated on the surface of three-dimensional graphene aerogel. Our results demonstrate that as-prepared MoS₂/GA composite has a great potential prospect as anodes for SIBs.     

Construction of 3D polypyrrole/CoS/graphene composite electrode with enhanced pseudocapacitive performance

Herein, 3D graphene/nickel foam (GE/NF) composite matrix was successfully fabricated by using NF as template through a self-catalytic thermal chemical vapor deposition process. By using the prepared GE/NF as substrate, CoS nanosheets were deposited via a facial one-step electrochemical deposition method. Owing to the advantage of GE in boosting the electrical contact between the electroactive host material and current collector, the as-prepared 3D CoS/GE/NF electrode demonstrated a superior capacitance value of 2308 F g^?1 at 1 A g^?1 and a high rate capability of 70.49% at 20 A g^?1. After depositing the polypyrrole (PPY) film on 3D CoS/GE/NF electrode, the electrochemical performance of CoS was further greatly improved and delivered an extremely high capacitance value of 3450 F g^?1 at 1 A g^?1, with good rate capability (62.61% at 20 A g^?1) and improved cycling stability. The enhanced electrochemical performance of PPY/CoS/GE/NF electrode is closely related to the advantage of PPY film in increasing the electrical conductivity and reinforcing the integrity of electrode.     

Three-dimensional nitrogen-doped graphene/sulfur composite for lithium-sulfur battery

A three-dimensional nitrogen-doped graphene/sulfur composite (NGS3) was synthesized by a simple hydrothermal method using urea as the nitrogen source and subsequent thermal treatment. The structure and electrochemical performance of the prepared nitrogen-doped graphene/sulfur composite (NGS3) were confirmed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), Energy dispersive spectroscopy mapping (EDS), and galvanostatic charge/discharge measurements. SEM and EDS mapping show that NGS3 exhibits a porous structure with uniform distribution of sulfur. Compared with the graphene/sulfur composite (NGS1), NGS3 delivers an outstanding rate capability with 1501, 1278, 1136, and 1024 mAh g^?1 at 200, 400, 800, and 1000 mA g^?1, respectively, and the cycle stability of NGS3 is also wonderful, a reversible discharge capacity of 1330 mAh g^?1 is obtained after 80 cycles under the current rate of 200 mA g^?1. The wonderful electrochemical performance could be attributed to the special three-dimensional conductive structure with the help of nitrogen atom.     

Enhanced lithium storage performance of a self-assembled hierarchical porous Co<Subscript>3</Subscript>O<Subscript>4</Subscript>/VGCF hybrid high-capacity anode material for lithium-ion batteries

A Co₃O₄/vapor-grown carbon fiber (VGCF) hybrid material is prepared by a facile approach, namely, via liquid-phase carbonate precipitation followed by thermal decomposition of the precipitate at 380 °C for 2 h in argon gas flow. The material is characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Brunauer-Emmett-Teller specific surface area analysis, and carbon elemental analysis. The Co₃O₄ in the hybrid material exhibits the morphology of porous submicron secondary particles which are self assembled from enormous cubic-phase crystalline Co₃O₄ nanograins. The electrochemical performance of the hybrid as a high-capacity conversion-type anode material for lithium-ion batteries is investigated by cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic discharge/charge methods. The hybrid material demonstrates high specific capacity, good rate capability, and good long-term cyclability, which are far superior to those of the pristine Co₃O₄ material prepared under similar conditions. For example, the reversible charge capacities of the hybrid can reach 1100–1150 mAh g^?1 at a lower current density of 0.1 or 0.2 A g^?1 and remain 600 mAh g^?1 at the high current density of 5 A g^?1. After 300 cycles at 0.5 A g^?1, a high charge capacity of 850 mAh g^?1 is retained. The enhanced electrochemical performance is attributed to the incorporated VGCFs as well as the porous structure and the smaller nanograins of the Co₃O₄ active material.     

The centrifugally constructed and thermally activated three-dimensional graphene toward a binder-free highly performed anode of the lithium-ion battery

A binder-free three-dimensional porous interconnected graphene (a-3DrGO@NF) was centrifugally constructed and KOH-activated at 800 °C, leading a mechanically strong and pore-developed anode candidate for lithium ion batteries (LIBs). The unique approach of the integration of the mechanical construction and thermal activation demonstrated favorable frameworks to facilitate the stable and fast migrations of both ion and electron during the galvanostatic charge/discharge process, thus significantly improving its durability and electrochemical performance compared to those without the activated and thermal treatment. The a-3DrGO@NF LIBs showed a highly reversible capacity of 1250 mAh g^?1 at a current density of 0.1 A g^?1 after 50 cycles without degradation relative to the first cycle. More importantly, the a-3DrGO@NF LIBs exhibited excellent large current discharge property and cyclic stability of 965 mAh g^?1 in its first cycle and 545 mAh g^?1 after 150 cycles at a current density of 4 A g^?1. Furthermore, it can be quickly charged and discharged in a very short time of 92 s together with high-rate capability of 256 mAh g^?1 after 200 cycles at 10 A g^?1. At both lower and higher its current density as to 10 A g^?1, the coulombic efficiency was close to 100% and showed the reliability of a-3DrGO@NF LIBs.     

A novel synthesis of size-controllable mesoporous NiMoO<Subscript>4</Subscript> nanospheres for supercapacitor applications

A novel hydrothermal emulsion method is proposed to synthesize mesoporous NiMoO₄ nanosphere electrode material. The size of sphere-shaped NiMoO₄ nanostructure is controlled by the mass ratio of water and oil phases. Nickel acetate tetrahydrate and ammonium heptamolybdate were used as nickel and molybdate precursors, respectively. The resultant mesoporous NiMoO₄ nanospheres were characterized by X-ray diffraction, N₂ adsorption and desorption, scanning electron microscopy, and transmission electron microscopy. The electrochemical performances were evaluated by cyclic voltammetry (CV), cyclic chronopotentiometry (CP), and electrochemical impedance spectroscopy (EIS) in 6 M KOH solution. The typical mesoporous NiMoO₄ nanospheres exhibit the large specific surface area of 113 m² g^?1 and high specific capacitance of 1443 F g^?1 at 1 A g^?1, an outstanding cyclic stability with a capacitance retention of 90 % after 3000 cycles of charge-discharge at a current density of 10 A g^?1, and a low resistance.     

Hydrothermal synthesis of carbon- and reduced graphene oxide-supported CoMoO4 nanorods for supercapacitor

Hybrid CoMoO₄ nanorods with carbon (C) and graphene oxide (rGO) are successfully synthesized via one-step hydrothermal process. Hybrid α-CoMoO₄ nanorods have shown excellent electrochemical performances compared to pristine CoMoO₄ in alkaline electrolyte. Specifically, CoMoO₄/C nanorod exhibits a maximum specific capacitance of 451.6 F g^?1 at the current density of 1 A g^?1, whereas CoMoO₄/rGO shows high specific capacitance of 336.1 F g^?1 at the same current density. Both the hybrid nanorods show good rate capability even at high current density of 20 A g^?1 and long-term cyclic stability. The observed electrochemical features of the hybrid CoMoO₄ nanostructure could be attributed to the presence of highly conductive carbonaceous material on unique one-dimensional nanorod microstructure which enhances the electrical conductivity of the nanorods thereby allowing faster electrolyte ion diffusion during the redox process.     

Graphene-Co/CoO shaddock peel-derived carbon foam hybrid as anode materials for lithium-ion batteries

A novel graphene (G)-Co/CoO shaddock peel-derived carbon foam (SPDCF) hybrid was fabricated as anode materials for lithium-ion batteries. The preparation of G-Co/CoO SPDCF was according to the following two steps. Firstly, the dried shaddock peels were immersed into the mixture of Co(NO₃)₂/graphene oxide for about 12 h. Then, the shaddock peels were taken out and heated at 800 °C for 2 h under N₂ atmosphere. The strategy is simple, low-cost, and environmentally friendly because the shaddock peel is abundant and renewable. The obtained G-Co/CoO SPDCF hybrid were carefully characterized by SEM, EDS, XPS, XRD, TGA, BET, TEM, and electrochemical techniques. The results showed that the carbonized shaddock peels had hierarchical porous nanoflakes structures and graphene was uniformly dispersed into the SPDCF. The nanosized Co/CoO was formed on the G-SPDCF. The resulted G-Co/CoO SPDCF hybrid could maintain a high capacity of 600 mA h g^?1 at 0.2 A g^?1 after 80 cycles, which was much higher than that of commercial graphite (372 mA h g^?1). The enhanced performance might be ascribed to the existence of lots of uniform Co/CoO and the hierarchical G-SPDCF alleviating the mechanical stress during the process of lithiation/delithiation.     

Shape-controlled porous carbon from calcium citrate precursor and their intriguing application in lithium-ion batteries

Porous carbon nanosheets (PCNSs), porous carbon nanofibers (PCNFs), and flowerlike porous carbon microspheres (FPCMs) were successfully synthesized through a carbonization method combined with a simple acid pickling treatment using calcium citrate as the precursor. The as-prepared products show uniform morphologies, in which the FPCMs are self-assembled from PCNSs. As anodes of lithium-ion (Li-ion) batteries, these carbon materials deliver a stable reversible capacity above 515 mAh g^?1 after 50 cycles at 100 mA g^?1. Compared with PCNSs and PCNFs, FPCMs demonstrate preferable rate capability (378 mAh g^?1 at 1 A g^?1) and cyclability (643 mAh g^?1 at 100 mA g^?1). These results suggest that an appropriate select of morphology and structure will significantly improve the lithium storage capacity. The study also indicates that the novel shape-controlled porous carbon materials have potential applications as electrode materials in electronic devices.     

Hierarchical Co<Subscript>3</Subscript>O<Subscript>4</Subscript>@C hollow microspheres with high capacity as an anode material for lithium-ion batteries

Three-dimensional hierarchical Co₃O₄@C hollow microspheres (Co₃O₄@C HSs) are successfully fabricated by a facile and scalable method. The Co₃O₄@C HSs are composed of numerous Co₃O₄ nanoparticles uniformly coated by a thin layer of carbon. Due to its stable 3D hierarchical hollow structure and uniform carbon coating, the Co₃O₄@C HSs exhibit excellent electrochemical performance as an anode material for lithium-ion batteries (LIBs). The Co₃O₄@C HSs electrode delivers a high reversible specific capacity, excellent cycling stability (1672 mAh g^?1 after 100 cycles at 0.2 A g^?1 and 842.7 mAh g^?1 after 600 cycles at 1 A g^?1), and prominent rate performance (580.9 mAh g^?1 at 5 A g^?1). The excellent electrochemical performance makes this 3D hierarchical Co₃O₄@C HS a potential candidate for the anode materials of the next-generation LIBs. In addition, this simple synthetic strategy should also be applicable for synthesizing other 3D hierarchical metal oxide/C composites for energy storage and conversion.     

Cobalt imidazoledicarboxylate coordination complex microspheres: stable intercalation materials for lithium and sodium-ion batteries

A simple and versatile method for preparation of cobalt 4,5-imidazoledicarboxylate microspheres is developed. The cobalt 4,5-imidazoledicarboxylate complex is a kind of stable intercalation materials for lithium- and sodium-ion batteries. When tested as an anode material for lithium-ion batteries, the coordination complex microspheres based composite electrode delivers a second discharge capacity of 595.4 mAh g^?1 at a current density of 1 Ah g^?1. A reversible capacity of 416.1 mAh g^?1 remained after 143 cycles, while a reversible capacity of 259.9 mAh g^?1 remained after 500 cycles at a current density of 1.5 A g^?1. In addition, it can also serve as stable anode materials for sodium-ion batteries. Research based on the topics would shed some light on the discovery of new alternative intercalation materials to graphite.     

Novel synthesis and characterization of ZnCo<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoflakes grown on nickel foam as efficient electrode materials for electrochemical supercapacitors

ZnCo₂O₄ nanoflakes were directly grown on Ni foam via a two-step facile strategy, involving cathodic electrolytic electrodeposition (ELD) method and followed by a thermal annealing treatment step. The results of physical characterizations exhibit that the mesoporous ZnCo₂O₄ nanoflakes have large electroactive surface areas (138.8 m² g^?1) and acceptable physical stability with the Ni foam, providing fast electron and ion transport sites. The ZnCo₂O₄ nanoflakes on Ni foam were directly used as integrated electrodes for supercapacitors and their electrochemical properties were measured in 2 M KOH aqueous solution. The ZnCo₂O₄ nanoflake electrode exhibits a high capacitance of 1781.7 F g^?1 at a current density of 5 A g^?1 and good rate capability (62% capacity retention at 50 A g^?1). Also, an excellent cycling ability at various current densities from 5 to 50 A g^?1 was obtained and 92% of the initial capacitance maintained after 4000 cycles. The results demonstrate that the proposed synthesis route is cost-effective and facile and can be developed for preparation of electrode materials in other electrochemical supercapacitors.