The effect of Ag loading on supercapacitor performance of graphene based nanocomposites

In the present study, we prepared reduced graphene oxide (rGO) decorated with Ag nanoparticles by a one pot, simultaneous reduction method. The effect of AgNO₃ amount on the chemical, morphological and electrochemical properties of binary rGO-Ag nanocomposite for supercapacitor application was investigated. The chemical and morphological characterization of prepared rGO-Ag nanocomposites was realized with field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), ultraviolet-visible spectroscopy (UV-Vis), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). For supercapacitor application, electrochemical performance of the nanocomposites was investigated with cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) techniques. As a result of their excellent conductivity and spacer role which prevent aggregation of rGO nanosheets and maintain the electroactive surface area, Ag nanoparticles significantly enhance the electrochemical performance of the nanocomposite. The rGO-Ag nanoparticle nanocomposite exhibited a maximum specific capacitance of 34.2?mF?cm^?2 at 0.6?A?cm^?2 current density. The nanocomposite electrode also has excellent rate capability and cycle life. The capacitance retention of rGO-Ag electrode is 98% after 1000 charge-discharge cycle. The results showed that rGO-Ag nanocomposite is a building block for ternary or other multicomponent nanocomposites.     

High-performance ammonia gas sensor based on bimetallic oxide Zn2SnO4 decorated with reduced graphene oxide

The coupling effect and synergistic effect between the two metal elements of the bimetallic oxide make it has unique electrical characteristics and gas-sensitive properties, but it has the limitation of low conductivity. In this paper, the bimetallic oxide Zn₂SnO₄ was decorated with reduced graphene oxide (rGO) to increase its electrical conductivity and promote charge transfer during gas adsorption, which enhances the response and shortens the response time of the bimetallic oxide gas sensor. The high-performance ammonia sensor based on Zn₂SnO₄/rGO nanocomposite material was prepared by environmentally friendly hydrothermal method and spin coating technology. The structure and properties of composite materials were analyzed by X-ray diffraction (XRD), field emission scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The ammonia sensing performance of Zn₂SnO₄/rGO nanocomposite sensor was tested at room temperature, including the dynamic response, response/recovery time, selectivity, repeatability, long-term stability. It showed a good sensing response to ammonia (22.94 for 100 ppm), and a fast response/recovery time (20 s/27 s). Finally, the response mechanism of Zn₂SnO₄/rGO nanocomposite sensor is explained. The enhanced ammonia sensing properties of Zn₂SnO₄/rGO nanocomposite sensor were ascribed to the synergistic effect and p–n heterojunction between Zn₂SnO₄ and rGO.

     

Nontopotactic Reaction in Highly Reversible Sodium Storage of Ultrathin Co9Se8/rGO Hybrid Nanosheets

Transition metal chalcogenide with tailored nanosheet architectures with reduced graphene oxide (rGO) for high performance electrochemical sodium ion batteries (SIBs) are presented. Via one‐step oriented attachment growth, a facile synthesis of Co₉Se₈ nanosheets anchored on rGO matrix nanocomposites is demonstrated. As effective anode materials of SIBs, Co₉Se₈/rGO nanocomposites can deliver a highly reversible capacity of 406 mA h g^?1 at a current density of 50 mA g^?1 with long cycle stability. It can also deliver a high specific capacity of 295 mA h g^?1 at a high current density of 5 A g^?1 indicating its high rate capability. Furthermore, ex situ transmission electron microscopy observations provide insight into the reaction path of nontopotactic conversion in the hybrid anode, revealing the highly reversible conversion directly between the hybrid Co₉Se₈/rGO and Co nanoparticles/Na₂Se matrix during the sodiation/desodiation process. In addition, it is experimentally demonstrated that rGO plays significant roles in both controllable growth and electrochemical conversion processes, which can not only modulate the morphology of the product but also tune the sodium storage performance. The investigation on hybrid Co₉Se₈/rGO nanosheets as SIBs anode may shed light on designing new metal chalcogenide materials for high energy storage system.     

Cathodic electrosynthesis of CuFe<Subscript>2</Subscript>O<Subscript>4</Subscript>/CuO composite nanostructures for high performance supercapacitor applications

In this work, CuFe₂O₄/CuO nanocomposites have been synthesized by galvanostatic cathodic electrodeposition. The obtained nanocomposites were characterized by field emission scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, Fourier Transform Infrared, and Brunauer–Emmett–Teller surface area analysis. The electrochemical properties of CuFe₂O₄/CuO nanocomposites were evaluated by cyclic voltammetry, galvanostatic charge–discharge cycling, and electrochemical impedance spectroscopy in 1.0 M KOH. The CuFe₂O₄/CuO nanocomposites have shown the high specific capacitance of 322.49 F g^?1 at the scan rate of 1 mV s^?1. After 5000 cycles, 92% of this specific capacitance was retained. Although the prepared nanocomposite has shown a mediocre specific capacitance compared to other metal oxide-based materials, the low cost of the starting materials and the ease of preparation make this nanocomposite a good candidate for supercapacitor applications.     

Facile one-step hydrothermal synthesis of reduced graphene oxide/Co3O4 composites for supercapacitors

This paper reports a facile one-step hydrothermal treatment of graphene oxide (GO) and cobalt acetate (Co(Ac)₂) for preparing reduced GO (rGO)/Co₃O₄ composites which were used as electrode materials for supercapacitors containing electrolytes of 2 M KOH aqueous solution. The morphologies and structures of rGO/Co₃O₄ composites were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman spectrum, and N₂ adsorption–desorption isotherms. The electrochemical performances of two-electrode supercapacitors were evaluated by cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance spectroscopy techniques. During the hydrothermal reaction, GO was reduced and 10–30 nm-sized Co₃O₄ nanoparticles were in situ grown onto the rGO sheets simultaneously. The effects of mass ratios of GO and Co(Ac)₂ on the performances of supercapacitors were investigated. In comparison with pure Co₃O₄-based supercapacitor, supercapacitors based on rGO/Co₃O₄ composites show better performances because both the specific surface areas and the electrical conductivities of electrode materials were increased by the introduction of rGO. When the mass ratio of GO and Co(Ac)₂ is 1:2, rGO/Co₃O₄ composite electrode exhibits the highest capacitance of 263.0 F/g at a constant current density of 0.2 A/g in a two-electrode supercapacitor. In addition, the supercapacitor shows high rate capability and long cyclic durability.     

Ultrathin Co3O4 nanosheets highly dispersed on reduced graphene oxide: An efficient bi-functional catalyst for Li-O2 battery

Ultrathin Co₃O₄ nanosheets grown on the reduced graphene oxide (Co₃O₄/rGO) was synthesized by a simple hydrothermal method and was investigated as a cathode in a Li-O₂ battery. Benefited from the synergistic effect between Co₃O₄ and rGO, the hybrid exhibits a high initial capacity of 10,528 mAh g^?1 along with a high coulombic efficiency (84.4%) at 100 mA g^?1. In addition, the batteries show an enhanced cycling stability and after 113 cycles, the cut-off discharge voltage remains above 2.5 V. The outstanding performance is intimately related to the high surface area of rGO, which not only provide carbon skeleton for the uniform distribution of Co₃O₄ nanosheets but also facilitate the reversible formation and decomposition of insoluble Li₂O₂. The results of electrochemical tests confirm that the Co₃O₄/rGO hybrid is a promising candidate for the Li-O₂ batteries.     

3D Graphene Frameworks/Co3O4 Composites Electrode for High‐Performance Supercapacitor and Enzymeless Glucose Detection

3D graphene frameworks/Co₃O₄ composites are produced by the thermal explosion method, in which the generation of Co₃O₄ nanoparticles, reduction of graphene oxide, and creation of 3D frameworks are simultaneously completed. The process prevents the agglomeration of Co₃O₄ particles effectively, resulting in monodispersed Co₃O₄ nanoparticles scattered on the 3D graphene frameworks evenly. The prepared 3D graphene frameworks/Co₃O₄ composites used as electrodes for supercapacitor display a definite improvement on electrochemical performance with high specific capacitance (≈1765 F g^?1 at a current density of 1 A g^?1), good rate performance (≈1266 F g^?1 at a current density of 20 A g^?1), and excellent stability (≈93% maintenance of specific capacitance at a constant current density of 10 A g^?1 after 5000 cycles). In addition, the composites are also employed as nonenzymatic sensors for the electrochemical detection of glucose, which exhibit high sensitivity (122.16 µA mM ^?1 cm^?2) and noteworthy lower detection limit (157 × 10^?9 M , S/N = 3). Therefore, the authors expect that the 3D graphene frameworks/Co₃O₄ composites described here would possess potential applications as the electrode materials in supercapacitors and nonenzymatic detection of glucose.     

Green gelatin-assisted: Synthesis of Co3O4NPs@rGO nanopowder for highly efficient magnetically separable methylene orange dye degradation

GO and Co(NO₃)₂ were respectively used as rGO and Co₃O₄ precursors for preparing magnetically separable Co₃O₄NPs attached Co₃O₄NPs@rGO nanocomposites by a straightforward sol–gel technique. To characterize the nanocomposite materials, FESEM, EDX, elemental mapping, XRD, FTIR, Raman spectroscopy, UV–vis, VSM and BET were employed. When exposed to UV rays, the nanocomposite showed extraordinary photocatalytic degradation of MO dye. According to the measurements of photocatalytic activity, the highly efficient photocatalytic efficiency of the nanocomposite could be attributed to preventing electron-hole recombination by highly effective electron transfer between rGO and semiconductor NPs. The nanocomposite succeeded in the efficient degradation of MO dye, even after five photocatalytic cycles.     

Reduced graphene oxide enhanced magnetic nanocomposites for removal of carbamazepine

Ferrite, as a kind of common magnetic adsorbent, always tend to reuniting and lead to the poor performance for removing contaminant in aqueous. Meanwhile, reduced graphene oxide (rGO), a high-efficiency adsorbent used for water treatment, is hydrophobic and easy to stack because of the Van der Waals force, resulting in the low adsorption capacity. Herein, we prepare the magnetic CoFe₂O₄/rGO nanocomposites to solve the reuniting of CoFe₂O₄ and stacking of rGO simultaneously. The rGO nanosheets can improve the dispersion of CoFe₂O₄ nanoparticles. As sorbent, the adsorption behaviors of carbamazepine on the nanocomposites can be fitted well by the Langmuir and pseudo-second-order kinetic models. In addition, the CoFe₂O₄/rGO nanocomposites show enhanced adsorption performance than that of the pure CoFe₂O₄ nanoparticles, and the loading of rGO can affect their adsorption performance. The adsorption of carbamazepine on CoFe₂O₄/rGO is exothermic and mainly controlled by π–π interaction and hydrogen bond interaction. Furthermore, the CoFe₂O₄/rGO can be used to remove other organic pollutants simultaneously. Finally, the nanocomposites can be collected by external magnet, and the regenerated nanocomposites still showed a high adsorption capacity retention (90%) after five cycles.     

Hydrolytic dehydrogenation of ammonia borane catalyzed by poly(amidoamine) dendrimers-modified reduced graphene oxide nanosheets supported Ag0.3Co0.7 nanoparticles

Poly(amidoamine) dendrimers-modified reduced graphene oxide nanosheets (PAMAM/rGO) composite was selected as a carrier of heterogeneous Ag_0.3Co_0.7 nanoparticles in order to obtain an excellent catalyst for ammonia borane (AB) hydrolysis. During the synthetic processes, GO could easily assembled with PAMAM by the electrostatic and hydrogen-bonding interactions. Structural characterization revealed that Ag_0.3Co_0.7 bimetallic nanoparticles with uniform size distribution of 5 nm are well dispersed on PAMAM/rGO composite architecture. Ag_0.3Co_0.7@PAMAM/rGO was found to be a highly active and reusable catalyst in hydrogen generation from the hydrolysis of AB with a turnover frequency value (TOF) of 19.79 mol_H2 min^–1 mol_M^–1 at 25.0 ± 0.1 °C and retained 75.4% of their initial activity with a complete release of hydrogen in five runs. The relatively high TOF value and low apparent activation energy (34.21 kJ mol^–1) make these Ag_0.3Co_0.7@PAMAM/rGO NPs as a high-efficient catalyst for catalytic dehydrogenation of AB facilitating the development of practically applicable energy storage materials.     

Facile synthesis of poly(Safranine T)/Reduced graphene oxide nanocomposite for supercapacitors with wide potential window in aqueous neutral electrolyte

A facile method is introduced for incorporating reduced graphene oxide (rGO) into poly(safranine T) (PST) films. First, ST-functionalized GO (ST/GO) was obtained via the absorption of ST on GO in pH 7.0 phosphate buffer solution. Then rGO/PST composite was synthesized by the electropolymerization of ST and the subsequent electrochemical reduction of GO. The as-prepared PST/rGO composite films are characterized using scanning electron microscope, X-ray diffraction, and Fourier transform–infrared spectroscopy. PST/rGO composites possess a microporous structure, which creates enormous amount of pores, and therefore provides larger interfaces for charge carrier. The properties of electrochemical capacitance for PST/rGO composites have also been investigated with cyclic voltammetry (CV) and galvanostatic charge–discharge measurements. The experimental results manifest that the PST/rGO composite showed high capacitance (293.2 F g^?1) at 20-mV s^?1 CV scan and an excellent cycling stability (8.3% drop after 1000 cycles) in 0.1 M Na₂SO₄ electrolyte.     

Co2SnO4/activated carbon composite electrode for supercapacitor

A new kind of Co₂SnO₄-based electrode materials for supercapacitor was synthesized by co-precipitation method. The microstructure and surface morphology of Co₂SnO₄ were characterized by X-ray diffraction and scanning electron microscopy, respectively. Cyclic voltammetry, chronopotentiometry and electrochemical impedance spectroscopy were employed for the determination of specific capacitance and the equivalent series resistance of Co₂SnO₄/activated carbon composite electrode in KCl solution. It was shown that the composite electrode with 25 wt% Co₂SnO₄ had excellent specific capacitance up to 285.3 F g⁻¹ at the current density of 5 mA cm⁻². In addition, the composite electrode exhibited excellent long-term stability and, after 1000 cycles, 70.6% of initial capacitance was retained. Regarding the low cost, easy preparation, steady performance and environment friendliness, Co₂SnO₄/activated carbon composite electrode could have potentially promising application for supercapacitor.     

A promising mechanical ball-milling method to synthesize carbon-coated Co<Subscript>9</Subscript>S<Subscript>8</Subscript> nanoparticles as high-performance electrode for supercapacitor

A Co₉S₈/C nanocomposite has been prepared using a solid-state reaction followed by a facile mechanical ball-milling treatment with sucrose as the carbon source. The phases, morphology, and detailed structures of Co₉S₈/C nanocomposite are well characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. Our experimental results show that not only a process of particle size reduction, the ball-milling treatment also promotes the carbon and Co₉S₈ combining with each other more effectively to form an ultrafine nanocomposite. When used as an electrode material in supercapacitor, Co₉S₈/C nanocomposite exhibits a high initial specific capacitance of 756.2 F g^?1 at 1 A g^?1 and excellent cycling stability with 73.4% retention after 2000 cycles. Its outstanding electrochemical properties are mainly attributed to the nanosize of particles and amorphous carbon layer coating on its surface.     

Flexible and Binder‐Free Electrodes of Sb/rGO and Na3V2(PO4)3/rGO Nanocomposites for Sodium‐Ion Batteries

Flexible power sources have shown great promise in next‐generation bendable, implantable, and wearable electronic systems. Here, flexible and binder‐free electrodes of Na₃V₂(PO₄)₃/reduced graphene oxide (NVP/rGO) and Sb/rGO nanocomposites for sodium‐ion batteries are reported. The Sb/rGO and NVP/rGO paper electrodes with high flexibility and tailorability can be easily fabricated. Sb and NVP nanoparticles are embedded homogenously in the interconnected framework of rGO nanosheets, which provides structurally stable hosts for Na‐ion intercalation and deintercalation. The NVP/rGO paper‐like cathode delivers a reversible capacity of 113 mAh g^?1 at 100 mA g^?1 and high capacity retention of ≈96.6% after 120 cycles. The Sb/rGO paper‐like anode gives a highly reversible capacity of 612 mAh g^?1 at 100 mA g^?1, an excellent rate capacity up to 30 C, and a good cycle performance. Moreover, the sodium‐ion full cell of NVP/rGO//Sb/rGO has been fabricated, delivering a highly reversible capacity of ≈400 mAh g^?1 at a current density of 100 mA g^?1 after 100 charge/discharge cycles. This work may provide promising electrode candidates for developing next‐generation energy‐storage devices with high capacity and long cycle life.     

Synthesis of ternary rGO–ZnO–Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanocomposites and their application for visible light photocatalytic degradation of dyes

The rGO–ZnO–Fe₃O₄ nanocomposites were synthesized by a simple, economical and environmentally benign solvothermal method. The morphology, structure and photocatalytic activity of rGO–ZnO–Fe₃O₄ nanocomposites were characterized, and the effects of the preparation conditions were analyzed. The 25 mg of rGO–ZnO–Fe₃O₄ nanocomposites performed efficient photocatalytic activity for the degradation of 25 mL of 4.79 mg/L Rhodamine B under both ultraviolet and visible light irradiation. The nanocomposites could be easily retrieved from the wastewater by extra magnetic field due to the presence of magnetic Fe₃O₄ nanoparticles. After the degradation experiment was carried out five times, the degradation ratio is still close to 90% after irradiation after 5 cyclic, indicating that the rGO–ZnO–Fe₃O₄ nanocomposites have high stability for reuse. Therefore, the prepared rGO–ZnO–Fe₃O₄ nanocomposites may be a promising potential material for wastewater treatment.     

Preparation and characteristics of polyimide/CaMoO<Subscript>4</Subscript> nanocomposites with enhanced dielectric performance and thermal stability

The polyimide/CaMoO₄ nanocomposites were successfully prepared by ex-situ polymerization. (3-Aminopropyl) triethoxysilane (KH550) was used as coupling agent to disperse the CaMoO₄ nanoparticles in polyimide matrix homogeneously. To characterize the structure and properties of the obtained nanocomposites, Scanning electron microscope, Fourier transform infrared spectroscopy, X-ray diffraction, dielectric properties and thermal stability were studied. It was observed that the CaMoO₄ nanoparticles were dispersed homogeneously in the polyimide matrix without obvious aggregation. The results show that nano-sized CaMoO₄ will improve dielectric constant and decrease dielectric loss of the nanocomposites in the relatively high-frequency region (>10 kHz). And the nanocomposite with excellent dielectric properties of ε?=?3.04, δ?=?8.0?×?10^?3 was obtained after doping 2.5 wt% nano-sized CaMoO₄ into polyimides. Dielectric loss of the nanocomposites is reduced in low frequency domain (<10 kHz) and enhanced in high frequency domain (>10 kHz). In addition, the thermal stability of the nanocomposites was enhanced from 544 to 651?°C compared to pure polyimide.     

Ultrasonic-assisted pyrolyzation fabrication of reduced SnO<Subscript>2–<Emphasis Type="Italic">x</Emphasis></Subscript>/g-C<Subscript>3</Subscript>N<Subscript>4</Subscript> heterojunctions: Enhance photoelectrochemical and photocatalytic activity under visible LED light irradiation

Novel SnO_2–x /g-C₃N₄ heterojunction nanocomposites composed of reduced SnO_2–x nanoparticles and exfoliated g-C₃N₄ nanosheets were prepared by a convenient one-step pyrolysis method. The structural, morphological, and optical properties of the as-prepared nanocomposites were characterized in detail, indicating that the aggregation of g-C₃N₄ nanosheets was prevented by small, well-dispersed SnO_2–x nanoparticles. The ultraviolet–visible spectroscopy absorption bands of the nanocomposites were shifted to a longer wavelength region than those exhibited by pure SnO₂ or g-C₃N₄. The charge transfer and recombination processes occurring in the nanocomposites were investigated using linear scan voltammetry and electrochemical impedance spectroscopy. Under 30-W visible-light-emitting diode irradiation, the heterojunction containing 27.4 wt.% SnO_2–x exhibited the highest photocurrent density of 0.0468 mA·cm^–2, which is 33.43 and 5.64 times larger than that of pure SnO₂ and g-C₃N₄, respectively. The photocatalytic activity of the heterojunction material was investigated by degrading rhodamine B under irradiation from the same light source. Kinetic study revealed a promising degradation rate constant of 0.0226 min^?1 for the heterojunction containing 27.4 wt.% SnO_2–x , which is 32.28 and 5.79 times higher than that of pure SnO₂ and g-C₃N₄, respectively. The enhanced photoelectrochemical and photocatalytic performances of the nanocomposite may be due to its appropriate SnO_2–x content and the compact structure of the junction between the SnO_2–x nanoparticles and the g-C₃N₄ nanosheets, which inhibits the recombination of photogenerated electrons and holes.

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Building an interpenetrating network of Ni(OH)<Subscript>2</Subscript>/reduced graphene oxide composite by a sol–gel method

Herein, a facile sol–gel strategy for building the ordered interpenetrating network of Ni(OH)₂ and reduced graphene oxide (rGO) was proposed. In this strategy, rGO nanosheets were homogeneously fixed inside composite utilizing the pores of Ni(OH)₂ gel as template, forming rGO-interpenetrated gel network. It was found that the rGO nanosheets could effectively reduce the internal resistant of composites and provide mechanical support for the gel network of Ni(OH)₂. Therefore, the composite presented high electrochemical performance, especially high-rate performance, due to the interpenetrating of rGO nanosheets plus the supplementary role of acetylene black. It had high specific capacitance of 2163 F g^?1 at low current density of 2.9 A g^?1 and 733 F g^?1 at high current density of 86.8 A g^?1.     

Co2(OH)3Cl nanoparticles as new-type electrode material with high electrochemical performance for application in supercapacitor

In this work, we report the preparation of Co₂(OH)₃Cl nanoparticles with average size of ～20 nm and well-defined cubic shape at room temperature by an epoxide precipitation route. It was found that the as-prepared Co₂(OH)₃Cl nanoparticles could be used as a promising new electrode material for application in redox supercapacitors due to its high electrochemical performance. It presented superior specific capacitance of 783 F g^?1 at low current density of 2.8 A g^?1, while it had a high value of 604 F g^?1 at high current density of 56.6 A g^?1, proving its excellent high rate performance. Its 75% capacitance retention after 10,000 cycles of charge–discharge demonstrated its long-life span. According to characterization results, the possible mechanism for the electrochemical process that Co₂(OH)₃Cl nanoparticles underwent was proposed as a process of Co₂(OH)₃Cl → β-Co(OH)₂ → CoOOH ? Co₃O₄.