Functional Porous Carbon/Nickel Oxide Nanocomposites as Binder‐Free Electrodes for Supercapacitors

High‐surface‐area, guava‐leaf‐derived, heteroatom‐containing activated carbon (GHAC) materials were synthesized by means of a facile chemical activation method with KOH as activating agent and exploited as catalyst supports to disperse nickel oxide (NiO) nanocrystals (average size (2.0±0.1) nm) through a hydrothermal process. The textural and structural properties of these GHAC/NiO nanocomposites were characterized by various physicochemical techniques, namely, field‐emission SEM, high‐resolution TEM, elemental analysis, X‐ray diffraction, X‐ray photoelectron spectroscopy, thermogravimetric analysis, and Raman spectroscopy. The as‐synthesized GHAC/NiO nanocomposites were employed as binder‐free electrodes, which exhibited high specific capacitance (up to 461 F g^?1 at a current density of 2.3 A g^?1) and remarkable cycling stability, which may be attributed to the unique properties of GHAC and excellent electrochemical activity of the highly dispersed NiO nanocrystals.     

Electrophoretic Deposition of Binder‐Free MnO2/Graphene Films for Lithium‐Ion Batteries

Binder‐free, nano‐sized needlelike MnO₂ ‐submillimeter‐sized reduced graphene oxide (nMnO₂‐srGO ) hybrid films with abundant porous structures were fabricated through electrophoretic deposition and subsequent thermal annealing at 500 °C for 2 h. The as‐prepared hybrid films exhibit a unique hierarchical morphology, in which nMnO₂ with a diameter of 20—50 nm and a length of 300—500 nm is randomly anchored on both sides of srGO . When evaluated as binder‐free anodes for lithium‐ion half‐cell, the nMnO₂‐srGO composites with a content of 76.9 wt% MnO₂ deliver a high capacity of approximately 1652.2 mA •h•g⁻¹ at a current density of 0.1 A•g⁻¹ after 200 cycles. The high capacity remains at 616.8 mA •h•g⁻¹ (ca. 65.1% capacity retention) at a current density as high as 4 A•g⁻¹. The excellent electrochemical performance indicates that the nMnO₂‐srGO hybrid films could be a promising anode material for lithium ion batteries (LIBs ).     

Hybrid Fibers Made of Molybdenum Disulfide,Reduced Graphene Oxide,and Multi‐Walled Carbon Nanotubes for Solid‐State,Flexible, Asymmetric Supercapacitors

One of challenges existing in fiber‐based supercapacitors is how to achieve high energy density without compromising their rate stability. Owing to their unique physical, electronic, and electrochemical properties, two‐dimensional (2D) nanomaterials, e.g., molybdenum disulfide (MoS₂) and graphene, have attracted increasing research interest and been utilized as electrode materials in energy‐related applications. Herein, by incorporating MoS₂ and reduced graphene oxide (rGO) nanosheets into a well‐aligned multi‐walled carbon nanotube (MWCNT) sheet followed by twisting, MoS₂‐rGO/MWCNT and rGO/MWCNT fibers are fabricated, which can be used as the anode and cathode, respectively, for solid‐state, flexible, asymmetric supercapacitors. This fiber‐based asymmetric supercapacitor can operate in a wide potential window of 1.4 V with high Coulombic efficiency, good rate and cycling stability, and improved energy density.     

Binder Free Modification of Glassy Carbon Electrode by Employing Reduced Graphene Oxide/ZnO Composite for Voltammetric Determination of Certain Nitroaromatics

Reduced Graphene oxide/ZnO nanoflowers ( rGO/ZnO‐NFs ) composite has been synthesized in‐situ using asymmetric Zn complex ( 1 ) as a single‐source molecular precursor (SSMP) with GO at 150 °C. The rGO/ZnO‐NFs composite was characterized by PXRD, UV‐vis, SEM, EDX mapping, TEM and SAED pattern to confirm its purity and morphology. The rGO/ZnO‐NFs composite shows uniform distribution of nanoflowers on graphene sheets. The modified glassy carbon electrode ( GCE ) was fabricated by drop wise layering of the rGO/ZnO‐NFs composite at the surface of the GCE without using binder. The binder free modified electrode ( GCE‐rGO/ZnO ) was explored for detection of nitroaromatics such as p‐nitro‐phenol ( p ‐NP ), 2,4‐dinitrophenol ( 2,4‐DNP ), 2,4‐dinitrotoluene ( 2,4‐DNT ) and 2,4,6‐trinitrophenol ( 2,4,6‐TNP ). The fabricated sensor showed remarkable response for the both toxicants and explosives. The LOD, sensitivity and linear range for the studied toxicants and explosives were found to be in a good range: p ‐NP= 0.93 μM, 240 μA mM⁻¹ cm⁻² and 0.2–0.9 mM; 2,4‐DNP= 6.2 μM, 203 μA mM⁻¹ cm⁻² and 0.1–0.9 mM; 2,4‐DNT= 10 μM, 371 μA mM⁻¹ cm⁻² and 0.2–0.9 mM; 2,4,6‐TNP= 16 μM, 514 μA mM⁻¹ cm⁻² and 0.2–0.9 mM, respectively.     

磺化石墨烯/活性炭复合电极的制备及其不对称电容器脱盐

在还原剂NaBH₄存在下, 采用对氨基苯磺酸重氮盐与氧化石墨(GO)表面共价键合制备磺化石墨烯(GP-SO₃H). 傅里叶变换红外光谱(FTIR)证明磺酸基团在石墨烯表面接枝. 采用扫描电子显微镜(SEM)研究了磺化石墨烯的表面形貌. 以磺化石墨烯为添加剂, 制备了磺化石墨烯/活性炭(GP-SO₃H/AC)复合电极. 循环伏安及阻抗分析结果表明, 该复合电极的电容特性及导电性有明显改善. 以活性炭电极为对电极组装了不对称电容器(GP-SO₃H/AC|AC), 研究了该不对称电容器的电化学脱盐性能. 与对称电容器(AC|AC)相比, 不对称电容器中由于电极内磺酸基团对反离子的屏蔽作用, 电容器的电流效率达到89.4%以上, 脱盐量提高2.4倍, 单个循环脱盐量达到10.87 mg/g.     

Electrochemical Properties of Tin Oxide Flake/Reduced Graphene Oxide/Carbon Composite Powders as Anode Materials for Lithium‐Ion Batteries

Hierarchically structured tin oxide/reduced graphene oxide (RGO)/carbon composite powders are prepared through a one‐pot spray pyrolysis process. SnO nanoflakes of several hundred nanometers in diameter and a few nanometers in thickness are uniformly distributed over the micrometer‐sized spherical powder particles. The initial discharge and charge capacities of the tin oxide/RGO/carbon composite powders at a current density of 1000 mA g^?1 are 1543 and 1060 mA h g^?1, respectively. The discharge capacity of the tin oxide/RGO/carbon composite powders after 175 cycles is 844 mA h g^?1, and the capacity retention measured from the second cycle is 80 %. The transformation during cycling of SnO nanoflakes, uniformly dispersed in the tin oxide/RGO/carbon composite powder, into ultrafine nanocrystals results in hollow nanovoids that act as buffers for the large volume changes that occur during cycling, thereby improving the cycling and rate performances of the tin oxide/RGO/carbon composite powders.     

Electrochemical Detection of Alloxan on Reduced Graphene Oxide Modified Glassy Carbon Electrode

Alloxan is a toxic reagent that strongly induces the diabetes by destroying insulin‐producing β‐cells in the pancreas of living organisms. The reduction product of alloxan is dialuric acid, which is responsible for the intracellular generation of ROS to enhance the stress in living cells to cause kidney disease or diabetic nephropathy. Herein, we studied for the first time the electrochemical properties of alloxan on reduced graphene oxide modified glassy carbon electrode (rGO/GCE) in 0.1 M phosphate buffer solution (PBS) at pH 7. The obtained results were compared with graphene oxide modified GCE (GO/GCE) and bare GCE surfaces. The modified rGO/GCE showed well defined redox couple with 10 fold increase in both reduction as well as oxidation peak current for alloxan than that of GO/GCE and bare GCE. Differential pulse voltammetry (DPV) technique shows the linear increase in both oxidation and reduction peak current of alloxan in the range of 30 μM to 3 mM with LOD of 1.2 μM. An amperometric signal of alloxan is also increases with respect to each addition of 50 μM of alloxan on rGO/GCE at constant potential of ?0.05 V. The linear range of alloxan is observed between 50 μM to 750 μM (S/N=3). This kind of rGO/GCE surface is more suitable platform or sensor matrix for estimating unknown concentration of alloxan molecule in the real biological systems.     

Comparative Study on Electroanalysis of Fenthion Using Silver Amalgam Film Electrode and Glassy Carbon Electrode Modified with Reduced Graphene Oxide

Oxidation and reduction processes of the insecticide fenthion was comparatively investigated at a reduced graphene oxide modified glassy carbon electrode (RGO‐GCE) and a cyclic renewable silver amalgam film electrode (Hg(Ag)FE) using square wave stripping voltammetry (SWSV). The influence of pH and SW parameters was investigated. The linear concentration ranges were found to be 1 × 10⁻⁶ – 2 × 10⁻⁵ and 1 × 10⁻⁷ – 2 × 10⁻⁵ mol L⁻¹ for Hg(Ag)FE and RGO‐GCE, respectively. The detection and quantification limits were calculated as 1.3 × 10⁻⁷ and 4.5 × 10⁻⁷ mol L⁻¹ for Hg(Ag)FE and 7.6 × 10⁻⁹ and 2.5 × 10⁻⁸ mol L⁻¹ for RGO‐GCE. Both of the developed electroanalytical methods offer rapid and simple detection of fenthion and were used on spiked tap and river water and apple juice samples. Scanning electron microscopy was used for RGO‐GCE surface characterization.     

Self‐Assembled Platinum Nanoflowers on Polydopamine‐Coated Reduced Graphene Oxide for Methanol Oxidation and Oxygen Reduction Reactions

The morphology‐ and size‐controlled synthesis of branched Pt nanostructures on graphene is highly favorable for enhancing the electrocatalytic activity and stability of Pt. Herein, a facile approach is developed for the efficient synthesis of well‐dispersed Pt nanoflowers (PtNFs) on the surface of polydopamine (PDA)‐modified reduced graphene oxide (PDRGO), denoted as PtNFs/PDRGO, in high yield. The synthesis was performed by a simple heating treatment of an aqueous solution that contained K₂PtCl₄ and PDA‐modified graphene oxide (GO) without the need for any additional reducing agent, seed, surfactant, or organic solvent. The coated PDA serves not only as a reducing agent, but also as cross‐linker to anchor and stabilize PtNFs on the PDRGO support. The as‐prepared PtNFs/PDRGO hybrid, with spatially and locally separated PtNFs on PDRGO, exhibits superior electrocatalytic activity and stability toward both methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) in alkaline solutions.     

Reduced and Surface‐Modified Graphene Oxide with Nonlinear Resistivity

Field‐grading materials (FGMs) are used to reduce the probability for electrical breakdowns in critical regions of electrical components and are therefore of great importance. Usually, FGMs are heavily filled (40 vol.%) with semi‐conducting or conducting particles. Here, polymer‐grafted reduced graphene oxide (rGO) is used as a filler to accomplish percolated networks at very low filling ratios (<2 vol.%) in a semi‐crystalline polymer matrix: poly(ethylene‐co‐butyl acrylate) (EBA). Various simulation models are used to predict the percolation threshold and the flake‐to‐flake distances, to complement the experimental results. A substantial increase in thermal stability of rGO is observed after surface modification, either by silanization or subsequent polymerizations. The non‐linear DC resistivity of neat and silanized rGO and its trapping of charge‐carriers in semi‐crystalline EBA are demonstrated for the first time. It is shown that the polymer‐grafted rGO improve the dispersibility in the EBA‐matrix and that the graft length controls the inter‐flake distances (i.e. charge‐carrier hopping distances). By the appropriate selection of graft lengths, both highly resistive materials at 10 kV mm^‐1 and FGMs with a large and distinct drop in resistivity (six decades) are obtained, followed by saturation. The nonlinear drop in resistivity is attributed to narrow inter‐flake distance distributions of grafted rGO.     

Reduced Graphene Oxide|Carbon Ceramic Electrode Modified with CdS‐Hemoglobin as a Sensitive Hydrogen Peroxide Biosensor

A sensitive hydrogen peroxide (H₂O₂) biosensor was developed based on a reduced graphene oxide|carbon ceramic electrode (RGO|CCE) modified with cadmium sulfide‐hemoglobin (CdS‐Hb). The electron transfer kinetics of Hb were promoted due to the synergetic function of RGO and CdS nanoparticles. The transfer coefficient (α) and the heterogeneous electron transfer rate constant (k_s) were calculated to be 0.54 and 2.6 s^?1, respectively, indicating a great facilitation achieved in the electron transfer between Hb and the electrode surface. The biosensor showed a good linear response to the reduction of H₂O₂ over the concentration range of 2–240 µM with a detection limit of 0.24 µM (S/N=3) and a sensitivity of 1.056 µA µM^?1 cm^?2. The high surface coverage of the CdS‐Hb modified RGO|CCE (1.04×10^?8 mol cm^?2) and a smaller value of the apparent Michaelis? Menten constant (0.24 mM) confirmed excellent loading of Hb and high affinity of the biosensor for hydrogen peroxide.     

Direct Electrochemistry of Glucose Oxidase at Reduced Graphene Oxide and β‐Cyclodextrin Composite Modified Electrode and Application for Glucose Biosensing

A simple glucose biosensor has been developed based on direct electrochemistry of glucose oxidase (GOx) immobilized on the reduced graphene oxide (RGO) and β‐cyclodextrin (CD) composite. A well‐defined redox couple of GOx appears with a formal potential of ～?0.459 V at RGO/CD composite. A heterogeneous electron transfer rate constant (K_s) has been calculated for GOx at RGO/CD as 3.8 s^?1. The fabricated biosensor displays a wide response to glucose in the linear concentrations range from 50 µM to 3.0 mM. The sensitivity and limit of detection of the biosensor is estimated as 59.74 µA mM^?1 cm^?2 and 12 µM, respectively.     

Electrochemically Reduced Graphene Oxide as Screen‐printed Electrode Modifier for Fenamiphos Determination

In this study, electrochemically reduced graphene oxide (ERGO) was used for the preparation of a screen‐printed modified electrode and applied for the voltammetric determination of fenamiphos (FNP) in tomato samples. Graphene oxide (GO) used for sensor construction was prepared according to an improved Hummers method and characterized by XRD, TEM, and FTIR, which confirmed the nanomaterial obtention. The ERGO formation was carried out from the electrodeposition by cyclic voltammetry, at 50 mV s^?1 in the potential range of 0.0 to ?1.5 V, during 50 cycles. ERGO‐SPE was used in the evaluation of the voltammetric behavior of FNP. The ERGO‐SPE proposed presented excellent electrochemical performance towards FNP oxidation, promoting an enhance on the anodic peak current and a decrease of peak potential. Under optimized conditions, it was possible to construct an analytical curve, using square wave voltammetry, with a linear region of 0.25 to 25.0 μM, with calculated limits of detection and quantification of 0.067 and 0.22 μM. From this, it was possible to analyze FNP in fortified tomato samples at three concentration levels, which showed recoveries values varying between 82 and 102 %. The ERGO‐SPE device proved useful in determining FNP, where the effect of the electrodeposition of the GO promoted a significant increase in the employability of the printed electrode.     

Partially Reduced Graphite Oxide as an Electrode Material for Electrochemical Double‐Layer Capacitors

Partially reduced graphite oxide was prepared from graphite oxide by using synthetic graphite as precursor. The reduction of graphite oxide with a layer distance of 0.57 nm resulted in a reduction of the layer distance depending on the degree of reduction. Simultaneously the amount of oxygen functionalities in the graphite oxide was reduced, which was corroborated by elemental analysis and EDX. The electrochemical activation of the partially reduced graphite oxide was investigated for tetraethylammonium tetrafluoroborate in acetonitrile and in propylene carbonate. The activation potential depends significantly on the degree of reduction, that is, on the graphene‐layer distance and on the solvent used. The activation potential decreased with increasing layer distance for both positive and negative activation. The resulting capacitance after activation was found to be affected by the layer distance, the oxygen functionalities and the used electrolyte. For a layer distance of 0.43 nm and with acetonitrile as the solvent, a differential capacitance of 220 Fg^?1 was achieved for the discharge of the positive electrode near the open‐circuit potential and 195 Fg^?1 in a symmetric full‐cell assembly.     

Three‐dimensional Nitrogen‐Doped Reduced Graphene Oxide/Carbon Nanotube Composite Catalysts for Vanadium Flow Batteries

The development of vanadium redox flow battery is limited by the sluggish kinetics of the reaction, especially the cathodic VO₂⁺/VO²⁺ redox couples. Therefore, it is vital to develop new electrocatalysts with enhanced activity to improve the battery performance. Herein, we synthesized the hydrogel precursor by a facile hydrothermal method. After the following carbonization, nitrogen‐doped reduced graphene oxide/carbon nanotube composite was obtained. By virtue of the large surface area and good conductivity, which are ensured by the unique hybrid structure, as well as the proper nitrogen doping, the as‐prepared composite presents enhanced catalytic performance toward the VO₂⁺/VO²⁺ redox reaction. We also demonstrated the composite with carbon nanotube loading of 2 mg/mL exhibits the highest activity and remarkable stability in aqueous solution due to the strong synergy between reduced graphene oxide and carbon nanotubes, indicating that this composite might show promising applications in vanadium redox flow battery.