Far-infrared reduced graphene oxide as high performance electrodes for supercapacitors

We report a novel far-infrared (FIR) thermal reduction process to effectively reduce graphene oxide films for supercapacitor electrode applications. The binder-free graphene oxide films used in this study were produced by electro-spray deposition of a graphene oxide colloidal solution onto stainless steel current collectors. The reduction of graphene oxide was performed using a commercial FIR convection oven that is ubiquitous in homes for cooking and heating food. The reduction process incorporated a simple, one-step FIR irradiation carried out in ambient air. Further, the FIR irradiation process was completed in ∼3 min, wherein neither special atmosphere nor high temperature was employed, resulting in an economic, efficient and simplified processing technique. The as-produced FIR graphene electrode gave a specific capacitance of ∼320 F/g at a current density of ∼0.2 A/g with less than 94% loss in specific capacitance over 10,000 charge/discharge cycles. This is one of the best specific capacitances reported for all-carbon electrodes without any additives. Even at ultrafast charge/discharge rates (current densities as high as ∼100 A/g), the FIR graphene electrode still delivered specific capacitances in excess of 90 F/g. The measured energy and power densities of the FIR supercapacitors were found to be ∼3–6 times higher than commercial (activated carbon) supercapacitor devices. This excellent electrochemical performance of the FIR graphene coupled with its ease of production (in air at low temperatures) using a commercial home-use FIR convection oven indicates the significant potential of this concept for large-scale commercial electrochemical supercapacitor applications.     

Electrochemically reduced graphene oxide sheets for use in high performance supercapacitors

Graphene oxide (GO) was reduced by a rapid, effective and eco-friendly electrochemical method of repetitive cathodic cyclic potential cycling, without using any reducing reagents. The electrochemically reduced graphene oxide (ERGO) was characterized by UV–vis, EIS and zeta-potential measurements. Most of the oxygen functional groups in ERGO were successfully removed resulting in smaller charge transfer resistance. However, some electrochemically stable residuals still remained, enabling ERGO to facilitate electrolyte penetration and pseudocapacitance. Since ERGO was readily stabilized by cathodic potential cycling, it exhibited an outstanding stability in cycle life, nearly with no capacitive loss from the second cycle on. A specific capacitance of 223.6 F g⁻¹ was achieved at 5 mV s⁻¹, which makes the ERGO a competitive material for electrochemical energy storage.     

Preparation of scale-like nickel cobaltite nanosheets assembled on nitrogen-doped reduced graphene oxide for high-performance supercapacitors

Ultrathin scale-like nickel cobaltite (NiCo₂O₄) nanosheets supported on nitrogen-doped reduced graphene oxide (N-rGO) are successfully synthesized through a facile co-precipitation of Ni²⁺ and Co²⁺ in the presence of sodium citrate and hexamethylenetetramine and subsequent calcination treatment. The composition and morphology of NiCo₂O₄ nanosheets@nitrogen-doped reduced graphene oxide (denoted as NiCo₂O₄ NSs@N-rGO) were characterized by Scanning electron microscope, Transmission electron microscope, X-ray diffraction, Raman spectra, X-ray photoelectron spectroscopy, Brunauer–Emmett–Teller and thermogravimetric analysis. The thickness of NiCo₂O₄ nanosheets anchored on the reduced graphene oxide is around 4 nm. The capacitance of NiCo₂O₄ NSs@N-rGO is evaluated by cyclic voltammogram and galvanostatic charge/discharge with the result that the NiCo₂O₄ NSs@N-rGO could deliver a specific capacitance of 1540 F g⁻¹ after 1000 cycles at 10 A g⁻¹.     

Polydopamine-assisted formation of Co3O4-nanocube-anchored reduced graphene oxide composite for high-performance supercapacitors

Tailoring transition-metal oxide nanoparticles with two-dimensional carbon has become a favorite way to improve their electrochemical performance. In this study, a composite of reduced graphene oxide was anchored by Co₃O₄ nanocubes and easily prepared with the assistance of polydopamine (PDA), using a combination of hydrothermal reaction and pyrolysis (Co₃O₄@PDA-rGO). Polydopamine, which possesses abundant catechol and amine groups, could be easily grafted onto graphene oxide to reduce the aggregation of graphene particles. Furthermore, PDA provided active sites, i.e., catechol and amine groups, which coordinated with Co²⁺, enabling enrichment of metal ions on the surface of graphene. After the pyrolysis of Co²⁺-containing PDA-grafted graphene at 400 °C, the Co²⁺ ions were converted into Co₃O₄ nanocubes, while the PDA carbonized to form N-doped porous carbon on the surface of graphene. The resulting product, Co₃O₄@PDA-rGO, demonstrated extraordinary supercapacitive behavior with good cycling stability owing to its unique porous structure as well as the intimate contact between Co₃O₄ and the carbon matrix.     

Synthesis of manganese (IV) oxide at activated carbon on reduced graphene oxide sheets via laser irradiation technique for organic binder-free electrodes in flexible supercapacitors

We report a novel, green, scalable technique to synthesize binder-free, high-purity conductive composite comprising activated carbon (AC), manganese dioxide nanorods (MnO₂), and reduced graphene oxide sheets (rGO) for flexible supercapacitors with outstanding electrochemical performance. UV pulsed laser irradiation of GO-based composite dispersion (AC/GO or MnO₂@AC/GO) in ethanol aqueous medium was used to induce a photocatalytic reduction of GO and simultaneous anchor AC particles or AC loaded MnO₂ nanorods (MnO₂@AC) on the reduced GO sheets (rGO) at room temperature and atmospheric pressure. rGO sheets serve as a large surface area, conductive binder to enhance the ion adsorption, electrical conductivity, and mechanical flexibility of supercapacitor electrodes. This laser-induced photocatalytic reduction method was used to prepare two different rGO-based colloidal composites AC/rGO (CG) and MnO₂@AC/rGO (MCG). The prepared rGO-based colloidal composites were used to fabricate symmetric supercapacitors (CG//CG and MCG//MCG) and asymmetric supercapacitors (MCG//CG) in which MCG is the positive electrode and CG is the negative one. All prepared rGO-based supercapacitors demonstrated significant improvement in their electrochemical performance compared with rGO-free AC based supercapacitors. The enhancement in the electrochemical properties of rGO-based supercapacitors could be attributed to the intrinsic characteristics of rGO, such as high surface area, excellent electrical conductivity, and super mechanical flexibility. Our approach is a one-step, scalable, cost-effective synthesis technique to produce all binder-free AC/rGO based composites for flexible energy-storage devices.     

High performance supercapacitors based on reduced graphene oxide in aqueous and ionic liquid electrolytes

Partially reduced graphene oxide (RGO) has been fabricated using hydrobromic acid. Since hydrobromic acid is a weak reductant, some oxygen functional groups which are relatively stable for electrochemical systems remain in RGO. Therefore, RGO can be re-dispersed in water and 2–3 layers of graphene can be observed by transmission electron microscopy, showing excellent affinity with water. RGO facilitates the penetration of aqueous electrolyte and introduces pseudocapacitive effects. Moreover, its capacitive nature is enhanced after cycling measurements. It is concluded that the increase of capacitance is due to the reduction of the oxygen functional groups by the cyclic voltammetry and electrochemical impedance spectroscopy analysis. The electrochemical properties in the ionic liquid electrolyte, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIPF₆), are also investigated. At a current density of 0.2 A g⁻¹, the maximum capacitance values of 348 and 158 F g⁻¹ are obtained in 1 M H₂SO₄ and BMIPF₆, respectively.     

Facile synthesis of reduced graphene oxide/CeO2 nanocomposites and their application in supercapacitors

The combination of the attractive properties of graphene with excellent characteristics of other functional nanomaterials has become a popular pathway for achieving applications in multiple fields. Herein, reduced graphene oxide (RGO)/CeO₂ nanocomposites with enhanced capacitive performance were designed and synthesized by a facile two-step approach with a self-assembly method followed by thermal treatment. The structure, morphology and composition of the resulting RGO/CeO₂ nanocomposites were systematically investigated. The presence of RGO can prevent the aggregation and control the structures of the CeO₂ nanocrystals in the annealing process. The nanocomposites as electrode materials for supercapacitor exhibited an enhanced capacitive performance due to the synergic effect between RGO nanosheets and CeO₂ nanocrystals. The excellent capacitive performance of the RGO/CeO₂ nanocomposites offer great promise for supercapacitor applications.     

Rapid and controllable synthesis of nitrogen doped reduced graphene oxide using microwave-assisted hydrothermal reaction for high power-density supercapacitors

Nitrogen doped reduced graphene oxide (N-RGO) is synthesized using microwave-assisted hydrothermal (MAHA) reaction. The proper configurations of nitrogen atoms in graphene sheets considerably increase the intrinsic electrical properties of N-RGO resultantly improving its capacitance and other kinetic properties in supercapacitor. Here, under the controlled MAHA reaction, we adjusted the ratio of nitrogen configurations (pyridinic-N, pyrrolic-N and quaternary-N) for the most optimum supercapacitor performances of N-RGOs in the shortest time ever reported, and clarified that its enhanced electrical conductivity and supercapacitor performances are attributed to its enlarged concentration of quaternary-N. With this MAHA reaction, we present a supercapacitor based on N-RGO, which is capable of displaying the promising electrochemical properties.     

Restacking-inhibited nitrogen-incorporated mesoporous reduced graphene oxides for high energy supercapacitors

Graphene is considered a promising active electrode material due to a large surface area, high electronic conductivity, and chemical and mechanical stabilities for supercapacitor (SC) applications. However, the current bottleneck is the fabrication of restacking-inhibited graphene on an electrode level which otherwise loses the capability to achieve the aforementioned properties. Herein, we demonstrate the synthesis of restacking-inhibited nitrogen (N)-incorporated mesoporous graphene for high energy SCs. The melamine-formaldehyde acts as a restacking inhibitor by forming a bonding with reduced graphene oxide (RGO) through a condensation reaction and as an N precursor to be decomposed to create open pores and N sources upon heat treatment. The d-spacing increases up to 0.352 nm and the surface area is as high as 698 m² g^?1 with high mesoporosity, confirming restacking inhibition by N incorporation decomposed by melamine-formaldehyde. The restacking-inhibited RGO-based SC cells in organic electrolyte show the specific capacitance of 25.8 F g^?1, the energy density of 21.8 kW kg^?1 and 85% of capacitance retention for 5000 cycles, which are better than those of pristine RGO-based cells. These improved SC performances are attributed to the fast ion transport through a mesoporous channel in crumpled structure and the doping effect of N incorporation. This work provides a simple yet effective chemical approach to fabricate restacking-inhibited RGO electrodes for improved SC performances.     

A flexible UV nanosensor based on reduced graphene oxide decorated ZnO nanostructures

A low-cost, compatible with flexible electronics, high performance UV sensor has been achieved from a reduced graphene oxide (RGO) decorated hydrangea-like ZnO film on a PDMS substrate. The hydrangea-like ZnO UV sensor has the best UV sensing performance among devices made of three kinds of ZnO nanostructures synthesized by a hydrothermal method, and demonstrated a dramatic enhancement in on/off ratio and photoresponse current by introducing an appropriate weight ratio of RGO. The on/off ratio of the 0.05% RGO/ZnO sensor increases almost one order of magnitude compared to that of a pristine hydrangea-like ZnO UV sensor. While for the 5% RGO decorated ZnO sensor, the photoresponse current reaches as high as ～1 μA and exceeds 700 times that of a ZnO UV sensor. These results indicate that RGO is an appropriate material to enhance the performance of ZnO nanostructure UV sensors based on its unique features, especially the high optical transparency and excellent electronic conductivity. Our findings will make RGO/ZnO nanohybrids extraordinarily promising in optoelectronics, flexible electronics and sensor applications.     

Fabrication of manganese oxide/three-dimensional reduced graphene oxide composites as the supercapacitors by a reverse microemulsion method

Manganese oxide (MnO₂)/three-dimensional (3D) reduced graphene oxide (RGO) composites were prepared by a reverse microemulsion (water/oil) method. MnO₂ nanoparticles (3–20 nm in diameter) with different morphologies were produced and dispersed homogeneously on the macropore surfaces of the 3D RGO. Scanning electron microscopy and transmission electron microscopy were applied to characterize the microstructure of the composites. The MnO₂/3D RGO composites, which were annealed at 150 °C, displayed a significantly high specific capacitance of 709.8 F g⁻¹ at 0.2 A g⁻¹. After 1000 cycles, the capacitance retention was measured to be 97.6%, which indicates an excellent long-term stability of the MnO₂/3D RGO composites.     

Spray dryer processed graphene oxide/reduced graphene oxide for high-performance supercapacitor

This study deals with the utility of mini spray dryer process to improve the dispersibility, of graphene oxide(GO) and its application for high-performance supercapacitor. Initially, the neutral solution of GO was obtained using the modified Hummer's method. After this, the prepared GO solution was processed by mini spray dryer to obtain a more purified, lighter, and dispersed form of GO which is named as spray dryer processed GO (SPGO). The SPGO thus obtained showed excellent dispersibility behavior with various solvents, which is not found in case of conventional oven drying. Furthermore, utility of SPGO and its reduced form (r-SPGO) for supercapacitor applications have been investigated. Results obtained from the cyclic voltammetry(CV) analysis, impedance, and charge-discharge behavior of supercapacitor fabricated using r-SPGO shows enhanced features. Therefore, the simple spray dried GO and its reduced form, that is, r-SPGO can be utilized as a potential candidate for the supercapacitor application. Herein, as synthesized SPGO exhibited the specific capacitance of 12.07 and 37.6 F/g with PVA-H₃PO₄ and 1 mol/L H₃PO₄, respectively, at a scan rate of 5 mV/s. On the other hand, reduced form of SPGO, that is, r-SPGO showed the specific capacitance of 27.16 and 230 F/g with PVA-H₃PO₄ and 1 mol/L H₃PO₄, respectively.     

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.     

Scalable preparation of three-dimensional porous structures of reduced graphene oxide/cellulose composites and their application in supercapacitors

Controlling the assembled structures of graphene has recently attracted enormous attention due to intriguing properties of the resultant structures. In this study, three-dimensional (3D) porous structures of reduced graphene oxide (RGO) with various ratios of RGO to cellulose have been fabricated by a scalable, but simple and efficient, approach that consists of ball milling assisted chemical reduction of GO, template shaping, coagulating, and lyophilization. The efficient mechanical shearing of ball milling and the hydrogen bond interactions between RGO and cellulose molecules contribute to the formation of a homogeneous RGO/cellulose hydrogel, improved thermal stability of the resultant composites, and enhanced crystallinity of the cellulose in the composites. The coagulation effect of cellulose maintains the RGO sheets in the 3D structures of cellulose; on the other hand, the RGO sheets facilitate the preservation of the 3D structures during freeze-drying, leading to the formation of 3D porous structures of RGO/cellulose composites. Benefiting from the continuous RGO network in the composites, the 3D porous structures of RGO(70)/cellulose(100) (GO:cellulose = 70:100 in weight) show an electrical conductivity of 15.28 S m⁻¹. Moreover, the 3D porous structures show potential application in supercapacitors due to the fact that they provide high specific surface area and fast charge propagation.     

Templated patterning of graphene oxide using self-assembled monolayers

A cost-effective and efficient route for graphene oxide (GO) patterning through site-selective deposition of GO onto a photopatterned self-assembled monolayer (SAM) of octadecylphosphonic acid on the surface of native oxide of titanium was demonstrated. In this process, SAMs on titanium oxide was first patterned via ultraviolet light exposure through a mask, subsequent drop-casting of GO aqueous dispersion on the substrate resulted in the site-selective deposition of GO on the hydrophilic areas. The formation of GO patterns is evidenced by Raman mapping. Atomic force microscopy studies indicate that the thickness of patterned GO can be controlled by adjusting the concentration of the GO dispersion. Scanning electron microscopy, and inverted fluorescence microscopy of large area GO patterns demonstrate this process is scalable and that the fluorescent property of nanoscale GO sheets is still reserved after deposition.     

Two-beam-laser interference mediated reduction,patterning and nanostructuring of graphene oxide for the production of a flexible humidity sensing device

Two-beam-laser interference was used for the simultaneous reduction, patterning and nanostructuring of graphene oxide on flexible polyethylene terephthalate substrates for the production of a high performance humidity sensing device. Hierarchical graphene nanostructures were formed after laser interference treatment of graphene oxide, which holds great promise for gaseous molecular adsorption, and thereby significantly increases their sensing performance. By tuning the laser power, the content of oxygen functional groups, could be changed within a certain range, which contributes not only controllable conductivity but also tunable response/recovery time of the humidity sensor due to the interaction between water molecules and oxygen functional groups on the graphene oxide sheets. The laser interference processing of graphene oxide films is a mask-free, surfactant-free and large-area approach to the production of hierarchical graphene micro-nanostructures, and thus shows great potential for fabrication of future graphene-based microdevices.     

Covalently-grafted polyaniline on graphene oxide sheets for high performance electrochemical supercapacitors

A simple route to achieve covalently-grafted polyaniline (PANI)/graphene oxide (GO) nanocomposites has been developed. The synthesized composites showed a uniform hierarchical morphology of the PANI thin film and short rod-like nanostructures that had densely grown on the GO sheets, in contrast to the nonuniform morphology of noncovalently-grafted PANI/GO. Compared to pure PANI and noncovalently-grafted PANI/GO composites, the covalently-grafted PANI/GO composites possessed a much larger specific surface area and pore volume, which increased the accessible surface area for the redox reaction and allowed faster ion diffusion. This unique hierarchical morphology maximized the synergistic effect between PANI and GO, resulting in excellent electrochemical performance (capacitance 442 F/g of PANI/GO (6:1) vs. 226 F/g of pure PANI) and improved cycling stability (83% @ 2000 cycles of PANI/GO (6:1) vs. 54.3% @ 1000 cycles of pure PANI). The enhanced electrochemical performance demonstrates the advantage of the PANI/GO composites prepared via this covalent grafting method.