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.     

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 graphene spheres self-assembled from graphene oxide sheets by a one-step hydrothermal process

We developed a one-step hydrothermal method to assemble graphene oxide (GO) sheets into hollow graphene spheres (HGSs), using only a GO/H₂SO₄ aqueous suspension as the starting material. Scanning electron microscope, focused ion beam scanning electron microscope and transmission electron microscope images show that the as-prepared HGSs vary from 1 to 3 μm in diameter and have a hollow interior structure. The as-prepared HGSs show a high capacitance of 207 F g⁻¹, as well as good rate capability and cycling stability when used as electrode materials for supercapacitors.     

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.     

Simultaneously morphology and phase controlled synthesis of cobalt manganese hydroxides/reduced graphene oxide for high performance supercapacitor electrodes

Cobalt manganese hydroxides with well-defined nanowire morphology (CoMn-HW) is scalable fabricated by adjusting solution contents, Mn/Co ratio and alkaline species. To further improve the conductivity of CoMn-HW, GO is introduced during fabrication process and reduced to rGO according to the high temperature and alkali atmosphere. By optimizing the adding mass of rGO, CoMn-HW/rGO with sandwiched like structure is successfully synthesized for supercapacitor electrode. The composite delivers a high specific capacitance of 784 F g⁻¹ at current density of 0.5 A g⁻¹, good rate capability (84.2% capacitance retention after current density increase 10 times). Moreover, an asymmetric supercapacitor with CoMn-HW/rGO10 as the positive electrode and active carbon as the negative electrode, is assembled and delivers a maximum energy density of 38.3 Wh kg⁻¹ and power density of 8000 W kg⁻¹, representing its potential in energy storage and conversion systems.     

Synthesis of electrochemically-reduced graphene oxide film with controllable size and thickness and its use in supercapacitor

An electrochemical synthesis method of reducing graphene oxide (GO) under constant potential is reported. Electrochemical technique offers control over reaction parameters such as the applied voltage, electrical current and reduction time; whereas the desired size and thickness of the film can be pre-determined by controlling the amount of precursor GO deposited on the electrode with defined shape and surface area. This synthesis technique produces high quality electrochemically reduced GO (ERGO) film with controllable size and thickness. Electrochemical symmetrical supercapacitors based on ERGO films achieved a specific capacitance of 128 F/g with an energy density of 17.8 Wh/kg operating within a potential window of 1.0 V in 1.0 M NaNO₃. The supercapacitor was shown to be stable, retaining ca. 86% of the original specific capacitance after 3500 charge–discharge cycles. The results indicate that this simple synthesis technique for providing graphene-like materials has great potential in various applications such as energy storage.     

Alkylated graphene nanosheets for supercapacitor electrodes: High performance and chain length effect

Single electrode materials capable of both electric double-layer and Faradic redox-based pseudo capacitance can be used for fabrication of high performance supercapacitors in an easy way and thus are highly desirable in the energy storage field. This contribution reports a new kind of such materials based on alkylated graphene materials (C_nrGO, n is the carbon number of their alkyl side chains) having different alkyl side chains (n = 4, 8, and 16). These materials were prepared via esterification of KOH-treated GO with the corresponding alkyl bromides in the presence of a phase transfer catalyst. More importantly, water was used as the reaction medium, and thus endowing the preparation method an eco-friend feature. The so-prepared graphene materials displayed chain length-dependent specific surface area and the population of residue CO functionalities, and thus affording vast differences in their supercapacitor behaviors. C₄rGO, the product having butyl side chains, showed the best supercapacitor performance with a capacitance up to 242.2 F g⁻¹ at a scan rate of 100 mV s⁻¹ and a good cycling stability.     

Holey graphene interpenetrating networks for boosting high-capacitive Co3O4 electrodes via an electrophoretic deposition process

A composite of Co₃O₄/holey graphene (Co₃O₄/HG) was prepared via a facile hydrothermal route, and was then processed into an electrode by an electrophoretic deposition process. Holey graphene (HG) wrapped Co₃O₄ to form a 3D skeleton network, thereby providing high electrical conductivity, and the holes in HG could further shorten the electrolyte ion diffusion pathway. Therefore, by adjusting the mass ratio of Co₃O₄ to HG, the Co₃O₄/HG composite afforded an enhanced capacitance of 2714 F g⁻¹ (at a current density of 1 A g⁻¹), which is 20 times higher than that of pure Co₃O₄. To further explore the practical applications of Co₃O₄/HG, a symmetric supercapacitor employing Co₃O₄/HG was fabricated. The supercapacitor functioned stably at potentials up to 1.2 V, with an enhanced energy density of 165 Wh kg⁻¹ and a high power density of 0.6 kW kg⁻¹ at 1 A g⁻¹.     

Influences of graphene oxide support on the electrochemical performances of graphene oxide-MnO2 nanocomposites

MnO₂ supported on graphene oxide (GO) made from different graphite materials has been synthesized and further investigated as electrode materials for supercapacitors. The structure and morphology of MnO₂-GO nanocomposites are characterized by X-ray diffraction, X-ray photoemission spectroscopy, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and Nitrogen adsorption-desorption. As demonstrated, the GO fabricated from commercial expanded graphite (denoted as GO(1)) possesses more functional groups and larger interplane gap compared to the GO from commercial graphite powder (denoted as GO(2)). The surface area and functionalities of GO have significant effects on the morphology and electrochemical activity of MnO₂, which lead to the fact that the loading amount of MnO₂ on GO(1) is much higher than that on GO(2). Elemental analysis performed via inductively coupled plasma optical emission spectroscopy confirmed higher amounts of MnO₂ loading on GO(1). As the electrode of supercapacitor, MnO₂-GO(1) nanocomposites show larger capacitance (307.7 F g^-1) and better electrochemical activity than MnO₂-GO(2) possibly due to the high loading, good uniformity, and homogeneous distribution of MnO₂ on GO(1) support.     

Simultaneous reduction,exfoliation, and nitrogen doping of graphene oxide via a hydrothermal reaction for energy storage electrode materials

Nitrogen (N)-doped graphene (NG) sheets were prepared using (NH₄)₂CO₃ and an aqueous dispersion of graphene oxide (GO) by an eco-friendly hydrothermal reaction. The in situ produced ammonia played an important role in the simultaneous nitrogen doping, the reduction and exfoliation of GO. The (NH₄)₂CO₃/GO mass ratio and reaction temperature were varied to investigate the effects on the N doping level. The elemental analysis determined from the X-ray photoelectron spectroscopy showed that the nitrogen content of the NG was about 10.1 at.% and the oxygen content decreased significantly due to the hydrothermal reduction of GO. The electrochemical performances of the NG sheets increased with increasing doped N content. The highest specific capacitance of 295 F g⁻¹ at a current density of 5 A g⁻¹ and the highest specific surface area of 412 m² g⁻¹ were observed with the sample processed at 130 °C. The retention of the specific capacitance was maintained at ∼89.8% after 5000 charge–discharge cycles. These results imply that NG sheets obtained by this simple eco-friendly approach are suitable for use in high performance energy storage electrode materials.     

Materials and electrode designs of high-performance NiCo2S4/Reduced graphene oxide for supercapacitors

NiCo₂S₄ is one of the most promising bimetallic sulfides for use in energy-storage systems, but more studies are needed to endow NiCo₂S₄ with a high electrochemical reaction capability and reversibility. In this work, we present rationally materials design of an optimal NiCo₂S₄ nanoparticle in a reduced graphene oxide (RGO) matrix as a NiCo₂S₄/RGO nanocomposite. Furthermore, we report the improvements in the materials technology, demonstrating the NiCo₂S₄/RGO nanocomposite electrode with an excellent specific capacitance of 963–700 F g⁻¹ at 1–15 A g⁻¹, high capacitance retention of 70%, and long cycle life of 3000 cycles. The practical application is showcased in an asymmetric supercapacitor with a high active-material loading. The NiCo₂S₄/RGO nanocomposite shows a high energy density of 31 Wh kg⁻¹ at a power density of 987 W kg⁻¹ and maintains an excellent density of 23 Wh kg⁻¹ at a high power density of 7418 W kg⁻¹. The outstanding electrochemical utilization and stability of the NiCo₂S₄/RGO nanocomposite confirm that our systematic optimization in the materials science and technology in terms of the active-material synthesis, the electrode development, and the device design/fabrication would benefit the future development of high-performance supercapacitors.     

Electrochemically synthesized graphene/polypyrrole composites and their use in supercapacitor

Composite films consisting of polypyrrole (PPy) and graphene oxide (GO) were electrochemically synthesized by electrooxidation of 0.1 M pyrrole in aqueous solution containing appropriate amounts of GO. Simultaneous chronoamperometric growth profiles and frequency changes on a quartz crystal microbalance showed that the anionic GO was incorporated in the growing GO/PPy composite to maintain its electrical neutrality. Subsequently, the GO was reduced electrochemically to form a reduced GO/PPy (RGO/PPy) composite by cyclic voltammetry. Specific capacitances estimated from galvanostatic discharge curves in 1 M H₂SO₄ at a current density of 1 A g^?1 indicated that values for the RGO/PPy composite were larger than those of a pristine PPy film and the GO/PPy composite. In the case of 6 mg mL^?1 GO for the preparation of GO/PPy, a high specific capacitance of 424 F g^?1 obtained at the electrochemically prepared RGO/PPy composite indicated its potential for use as an electrode material for supercapacitors.     

Reduced graphene oxide for Li–air batteries: The effect of oxidation time and reduction conditions for graphene oxide

Reduced graphene oxide (rGO) has shown great promise as an air-cathode for Li–air batteries with high capacity. In this article we demonstrate how the oxidation time of graphene oxide (GO) affects the ratio of different functional groups and how trends of these in GO are extended to chemically and thermally reduced GO. We investigate how differences in functional groups and synthesis may affect the performance of Li–O₂ batteries. The oxidation timescale of the GO was varied between 30 min and 3 days before reduction. Powder X-ray diffraction, micro-Raman, FE-SEM, BET analysis, and XPS were used to characterize the GO’s and rGO’s. Selected samples of GO and rGO were analyzed by solid state ¹³C MAS NMR. These methods highlighted the difference between the two types of rGO’s, and XPS indicated how the chemical trends in GO are extended to rGO. A comparison between XPS and ¹³C MAS NMR showed that both techniques can enhance the structural understanding of rGO. Different rGO cathodes were tested in Li–O₂ batteries which revealed a difference in overpotentials and discharge capacities for the different rGO’s. We report the highest Li–O₂ battery discharge capacity recorded of approximately 60,000 mAh/g_carbon achieved with a thermally reduced GO cathode.     

Self-assembled flower-like FeS2/graphene aerogel composite with enhanced electrochemical properties

Graphene aerogel (GA) supported flower-like ferrous disulfide (FeS₂) composite was synthesis by a two-step self-assembly method using eco-friendly and low-cost precursors. The formation of well-crystallized pyrite FeS₂ and the reduction of graphene oxide (GO) was demonstrated by X-ray diffraction, Fourier transform infrared spectroscopy and Raman spectroscopy. According to the scanning electron microscopy images, the flower-like FeS₂ distributes uniformly on the inter-linking GA networks. The electrochemical tests indicate that the as-prepared GA-FeS₂ exhibits enhanced specific capacitance (313.6 F/g at the current density of 0.5 A/g), which is almost twice as high as that of bare FeS₂ (163.5 F/g). It is noticed that this composite also has excellent cyclability (88.2% retention after 2000 cycles at 10 A/g) and low transfer resistance. A symmetric supercapacitor device with wide potential range was assembled using GA-FeS₂, while its energy density could reach 22.86 Wh/kg. The excellent specific capacitance, good rate capability, and high energy density make it a promising candidate for next generation supercapacitors.     

Synthesis of polypyrrole-reduced graphene oxide composites by in-situ photopolymerization and its application as a supercapacitor electrode

A highly conductive polypyrrole (PPy)-reduced graphene oxide (RGO) composite with an electrical conductivity of 610 S m⁻¹ was successfully synthesized by the in-situ photopolymerization of pyrrole in a graphene oxide suspension. Graphene oxide (GO) played the role of an electron acceptor and was reduced as it accepted electrons. The reduction of GO was confirmed by the increase in the C/O ratio of RGO with the UV irradiation time as well as the high electrical conductivity of PPy-RGO composite. Through the thermogravimetric analysis, it has been found that the PPy-RGO composite exhibited high thermal stability compared to the GO and PPy. This material was used as an electrode in a supercapacitor cell and showed excellent performance for electrical energy storage. The composite exhibited a specific capacitance of 376 F g⁻¹ at a scan rate of 25 mV s⁻¹.     

Bifunctional ternary manganese oxide/vanadium oxide/reduced graphene oxide as electrochromic asymmetric supercapacitor

A bifunctional ternary manganese oxide/vanadium oxide/reduced graphene oxide (MnO₂/V₂O₅/rGO) was developed for asymmetric electrochromic supercapacitor (EC-SC) application. The elemental mapping revealed uniformly distributed MnO₂, V₂O₅ and rGO, depicting homogenous synthesis of the hybrid composite. The phase composition, vibration modes and valance state of the ternary composite were analyzed via X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis, respectively. Interestingly, the as-prepared MnO₂/V₂O₅/rGO composite disclosed tremendous C_sp of 1403.5 F/g, which was higher compared to MnO₂/V₂O₅ (801.1 F/g), V₂O₅ (613.1 F/g), MnO₂ (126.7 F/g) and rGO (60.7 F/g). MnO₂/V₂O₅/rGO that appeared in dark green switched its visual color to orange at the charged state, confirming the electrochromic property. The bifunctional manganese oxide/vanadium oxide/reduced graphene oxide//copper-based metal-organic framework/reduced graphene oxide (MnO₂/V₂O₅/rGO//MrGO) asymmetrical EC-SC device revealed outstanding cycling stability (90.3% charge retention over 5000 cycles), tremendous specific capacitance (652.7 F/g) and maximum specific energy (60.4 Wh/kg). MnO₂/V₂O₅/rGO//MrGO asymmetrical EC-SC device demonstrated reversible color changes from dark green to orange at the discharged and charged states, respectively. The significantly great electrochromic and supercapacitive performance revealed that MnO₂/V₂O₅/rGO//MrGO is an outstanding electroactive candidate for the next generation of electrochromic supercapacitors.     

The additive-free electrode based on the layered MnO2 nanoflowers/reduced,graphene oxide film for high performance supercapacitor

The MnO₂ nanoflowers/reduced graphene oxide composite is coated on a nickel foam substrate (denoted as MnO₂ NF/RGO @ Ni foam) via the layer by layer (LBL) self-assembly technology without any polymer additive, following the soft chemical reduction. The layered MnO₂ NF/RGO composite is uniformly anchored on the Ni foam skeleton to form the 3D porous framework, and the interlayers have access to lots of ions channels to improve the electron transfer and diffusion. This special construction of 3D porous structure is beneficial to the enhancement of electrochemical property. The specific capacitance is up to 246 F g⁻¹ under the current density of 0.5 A g⁻¹. After 1000 cycles, it can retain about 93%, exhibiting excellent cycle stability. The electrochemical impedance spectroscopy measurements confirm that MnO₂ NF/RGO @ Ni foam electrode has lower R_ESR and R_CT values when compared to MnO₂ @ Ni foam and RGO @ Ni foam. This study opens a new door to the preparation of composite electrodes for high performance supercapacitor.     

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.     

Hydrogen-assisted pulsed KrF-laser irradiation for the in situ photoreduction of graphene oxide films

We report on the use of pulsed KrF-laser irradiation for the in situ reduction of graphene oxide (GO) films under both vacuum and partial hydrogen pressure. By exposing GO films to 500 pulses of a KrF-laser, at a fluence of 10 mJ/cm², their sheet resistance (R_s) is dramatically reduced from highly insulating (∼10¹⁰ Ω/sq) to conductive values of ∼3 kΩ/sq. By increasing the laser fluence, from 10 to 75 mJ/cm², we were able to identify an optimal fluence around 35 mJ/cm² that leads to highly conductive films with R_s values as low as 250 Ω/sq and 190 Ω/sq, under vacuum (10⁻⁵ Torr) and 50 mTorr of H₂, respectively. Raman spectroscopy analyses confirmed the effective reduction of the KrF-laser irradiated GO films through the progressive recovery of the characteristic 2D band of graphene. Furthermore, systematic Fourier-transform infrared spectroscopy analysis has revealed that KrF-laser induced reduction of GO preferentially occurs through photodissociation and removal of carboxyl (COOH) and alcohol (OH) groups. A direct correlation is established between the electrical resistance of photoreduced GO films and their COOH and OH bond densities. The KrF-laser induced reduction of GO films is found to be more efficient under H₂ background than under vacuum. It is concluded that our KrF-laser reduced GO films mainly consist of turbostratic graphite built from randomly organized few-layers-graphene building blocks, which contains some residual oxygen atoms and defects. Finally, by monitoring the KrF-laser fluence, it is shown that reduced GO films combining optical transmission as high as ∼80% along with sheet resistance as low as ∼500 Ω/sq can be achieved with this room-temperature and on-substrate process. This makes the laser-based reduction process developed here particularly attractive for photovoltaic hybrid devices using silicon substrates.     

Green preparation of reduced graphene oxide using a natural reducing agent

A simple and efficient method was introduced for the high-conversion preparation of graphene oxide (GO) from large graphite flakes (average flake size=100 μm) using a simplified Hummer׳s method. Natural reducing agents such as lemon juice and vinegar were compared with hydrazine (N₂H₄) as potential reducing agents. Graphene was prepared by chemical reduction of GO because this method was low cost and could be used for large-scale graphene production. This one-pot graphene preparation was performed at room temperature. Different degrees of oxidation of graphite flakes were obtained by stirring graphite in a mixture of sulfuric acid and potassium permanganate at different oxidation times, and highly exfoliated GO sheets were produced. GO was subsequently reduced effectively by lemon juice, a new, green, and potential reducing agent with pH 2.3. This reduced GO exhibited a high electrical conductance of 24.6 μS attributed to its higher C/O ratio (≈8:2) compared with other samples.