W.M. Liu

Flexible and conductive nanocomposite electrode based on graphene sheets and cotton cloth for supercapacitor

There is currently a strong demand for energy storage devices which are cheap, light weight, flexible, and possess high power and energy densities to meet the various requirements of modern gadgets. Herein, we prepare a flexible and easily processed electrode via a simple “brush-coating and drying” process using everyday cotton cloth as the platform and a stable graphene oxide (GO) suspension as the ink. After such a simple manufacturing operation followed by annealing at 300 °C in argon atmosphere, the as-obtained graphene sheets (GNSs)–cotton cloth (CC) composite fabric exhibits good electrical conductivity, outstanding flexibility, and strong adhesion between GNSs and cotton fibers. Using this GNSs–CC composite fabric as the electrode material and pure CC as the separator, a home-made supercapacitor was fabricated. The supercapacitor shows the specific capacitance of 81.7 F g−1 (two-electrode system) in aqueous electrolyte, which is one of the highest values for GNSs-based supercapacitors. Moreover, the supercapacitor also exhibits satisfactory capacitance in ionic-liquid/organic electrolyte. An all-fabric supercapacitor was also fabricated using pure CC as separator and GNSs–CC composite fabric as electrode and current collector. Such a conductive GNSs–CC composite fabric may provide new design opportunities for wearable electronics and energy storage applications.

Novel and high-performance asymmetric micro-supercapacitors based on graphene quantum dots and polyaniline nanofibers

In comparison with graphene sheets, graphene quantum dots (GQDs) exhibit novel chemical/physical properties including nanometer-size, abundant edge defects, good electrical conductivity, high mobility, chemical inertia, stable photoluminescence and better surface grafting, making them promising for fabricating various novel devices. In the present work, an asymmetric micro-supercapacitor, using GQDs as negative active material and polyaniline (PANI) nanofibers as positive active material, is built for the first time by a simple and controllable two-step electro-deposition on interdigital finger gold electrodes. Electrochemical measurements reveal that the as-made GQDs//PANI asymmetric micro-supercapacitor has a more excellent rate capability (up to 1000 V s(-1)) than previously reported electrode materials, as well as faster power response capability (with a very short relaxation time constant of 115.9 μs) and better cycling stability after 1500 cycles in aqueous electrolyte. On this basis, an all-solid-state GQDs//PANI asymmetric micro-supercapacitor is fabricated using H3PO4-polyvinyl alcohol gel as electrolyte, which also exhibits desirable electrochemical capacitive performances. These encouraging results presented here may open up new insight into GQDs with highly promising applications in high-performance energy-storage devices, and further expand the potential applications of GQDs beyond the energy-oriented application of GQDs discussed above.

Effects of concentration and temperature of EMIMBF4/acetonitrile electrolyte on the supercapacitive behavior of graphene nanosheets

Graphene nanosheets (GNSs)–ionic liquids (ILs) electrochemical system is of great interest as it shows excellent electrochemical properties for high performance supercapacitors. In this paper, the effects of concentration and temperature of ILs electrolyte on the electrochemical properties of a GNSs electrode are characterized by cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy measurements (EIS) in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4)/acetonitrile electrolyte. The results show that the internal resistance and the specific capacitance are strongly dependent on the variation of molar concentration of EMIMBF4, and the GNSs electrode exhibits high specific capacitance (128.2 F g−1) and a wide potential window (2.3 V) in 2.0 M EMIMBF4/acetonitrile electrolyte, indicating the excellent electrochemical performance. Moreover, the GNSs electrode has wide operating temperatures ranging from −20 °C to 60 °C with a potential window from −0.6 V to 1.5 V in the EMIMBF4/acetonitrile electrolyte. The result also reveals a weak dependence of the supercapacitive performance of the GNSs electrode on the temperature of the EMIMBF4/acetonitrile electrolyte. In addition, the specific capacitances have almost no decay after 1500 charge/discharge cycles in the above mentioned temperature region, demonstrating the good stability of the GNSs–ILs system in high-temperature and low-temperature environments.

Electrochemical behavior of graphene nanosheets in alkylimidazolium tetrafluoroborate ionic liquid electrolytes: influences of organic solvents and the alkyl chains

In this study, the electrochemical properties of graphene nanosheets (GNSs) in alkylimidazolium tetrafluoroborate ionic liquids/organic solvent electrolytes are investigated by cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS). The organic solvents with different functional groups exhibit a significant influence on the electrochemical properties of the GNSs. From series of organic solvents, in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4)/N,N-dimethylformamide (DMF, C3H7NO) electrolyte the GNS electrode shows the best electrochemical performance. Furthermore, the effect of the alkyl chains of ionic liquids on the electrochemical properties of GNSs was also evaluated through the electrochemical tests. The electrochemical properties of GNS electrode in 1-methyl-3-methylimidazolium tetrafluoroborate (MMIMBF4)/DMF electrolyte are better than those in EMIMBF4/DMF and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4)/DMF electrolytes. This may be attributed to the difference in the length of the alkyl chain on the imidazole ring, which results in the structural change of the electrode/ionic liquid interface and thus affects the electrochemical performance of the GNS electrode.

Multilayer hybrid films consisting of alternating graphene and titanium dioxide for high-performance supercapacitors

Electrode materials with a three-dimensional (3D) network structure and high-conductivity structural scaffolds are indispensable requirements for the development of in-plane supercapacitors with a superior performance. Herein, the highly tunable thin films with oriented interpenetrating network structures are prepared by the layer-by-layer (LBL) self-assembly technique based on the alternate deposition of negatively charged graphene oxide (GO) and positively charged titanium dioxide (TiO2), followed by the thermal reduction under an argon atmosphere. The resulting films are characterized by UV visible absorption spectroscopy, atomic force microscopy (AFM), transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM) and Raman spectroscopy, which all support the formation of the ordered sandwich framework structures built by graphene nanosheets (GNS) and TiO2 nanoparticles. Importantly, the multilayer film electrode presents excellent electrochemical capacitance properties, which were also highly dependent upon the deposition sequence and the order of the structural components in the sandwiched film. The significantly improved capacitance of the [GNS/TiO2]15 film electrode is derived from the unique 3D nanostructure with separated graphene nanosheets, in which the electrochemical double layer formation and dynamic charge propagation could be especially efficient throughout the whole TiO2 bulk material by providing a smaller resistance and shorter diffusion pathways.

Supercapacitors based on graphene nanosheets using different non-aqueous electrolytes

In this study, the electrochemical properties of graphene nanosheet (GNS) electrodes are evaluated in depth by cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) techniques in [Et4N]BF4/acetonitrile electrolyte, [BPy]BF4/acetonitrile electrolyte, [BMIM]BF4/acetonitrile electrolyte and [P4,4,4,4]BF4/acetonitrile electrolyte, respectively. The electrochemical results exhibit that GNSs show good supercapacitive properties in these aforementioned four non-aqueous electrolytes, especially in [Et4N]BF4/acetonitrile electrolyte. It is also observed that the rate performance and the specific capacitance of GNS electrode increase in the order of [P4,4,4,4]BF4/acetonitrile < [BMIM]BF4/acetonitrile ≈ [BPy]BF4/acetonitrile < [Et4N]BF4/acetonitrile in these four non-aqueous electrolytes. The reasons are attributed to the difference of the relative ionic size and the discrepancy in the functional group among these four non-aqueous electrolytes, which result in the differences of equivalent series resistance, charge transfer resistance, and rate performance. In addition, the GNS electrode shows excellent stability in these four non-aqueous electrolytes after 1500 repeating charge–discharge cycles. These results may provide valuable information to explore new electrolytes and illustrate the exciting potential for high performance supercapacitors based on GNSs.

Tribological behavior of gradient modified layer on 2024 aluminum alloy modified by plasma-based ion implantation

The N-pre-implanted 2024 aluminum alloy was implanted with Ti and N, or implanted with Ti, and then with Ti and N by plasma-based ion implantation (PBII) to form two gradient layers, respectively. The composition depth profiles of the gradient layers were characterized by X-ray photoelectron spectroscopy. A series of ball-on-disk wear experiments have been carried out in ambient air, to investigate the tribological behavior of the gradient layer against steel ball under dry and un-lubricated conditions, employing various applied loads and a constant sliding speed. The results revealed that tribological properties of the gradient layers were improved markedly in contrast with those of the unmodified sample, and strongly dependent on their composition depth profiles. The gradient layer implanted with Ti, and then with Ti and N was much thicker and contains higher N, thus it corresponded to higher hardness which slowly decrease from surface to substrate and the optimal tribological properties including higher load carrying capacity. As load was increasing, the tribological properties decreased, and the adhesive degree increased since the gradient layer became thinner rapidly. Of course, more proper gradient layers will be obtained as the qualified candidates in some particular engineering applications by optimizing PBII parameters.

The structure and tribological properties of gradient layers prepared by plasma-based ion implantation on 2024 Al alloy

Using plasma-based ion implantation, two types of gradient layers have been prepared on 2024 Al alloy. One is prepared by N-implantation then C-deposition, the other adds an interlayer composed of a Ti layer and a Ti–N layer between N-implantation and C-deposition. C-deposition is carried out at various implanting voltages or C2H2/H2 ratios. The composition depth profiles of these layers were characterized by x-ray photoelectron spectroscopy. The structure, morphologies and microstructure of the C layers were studied using Raman spectroscopy, atomic force microscope and transmission electron microscope, respectively. The surface hardness was measured with a Knoop tester and a mechanical property microprobe. The dry ball-on-disc wear tests were performed in ambient air. The gradient layer without interlayer is composed of an N-implanted layer rich in AlN and a diamond-like carbon (DLC) layer (film), and the two layers are connected with a C–Al transition layer containing Al4C3. The Ti layer rich in α -Ti and the N-implanted layer are connected by a Ti–Al transition layer containing TiAl3, while the Ti–N layer rich in TiN and the DLC film are connected by a C–Ti transition layer containing TiC, TiCN, etc. Thus, the gradient layer with interlayers has optimized the gradient structure. DLC films are compact and amorphous, contain high sp3/sp2 ratios and depend on the implanting voltage and the C2H2/H2 ratio. Similarly, these gradient layers exhibit significant improvement in morphologies, surface hardness and tribological properties; the interlayer, the implanting voltage and the C2H2/H2 ratio all have prominent effects on these properties.

Facile Approach to Preparation of Nitrogen-doped Graphene and Its Supercapacitive Performance

Nitrogen-doped graphene nanosheets (N-GNSs) with good morphologies were successfully synthesized by a facile solvothermal method. The result of X-ray photoelectron spectroscope (XPS) revealed that most oxygen con- taining functional groups on the surface of graphene oxide (GO) were removed during solvothermal reduction, and ni- trogen atoms from dimethylformamide (DMF) were effectively doped into graphene structure in the form of pyrrolic-N and graphite-N. As an electro-active material, the N-GNSs exhibited superior capacitive behavior in 2 mol/L KOH electrolyte with a high specific capacitance of 181.3 F/g at the current density of 0.5 A/g. Also, it displayed good electrochemical stability with 92.5% of the initial capacitance over consecutive 2000 cycles. Therefore, the N-GNSs can be an attractive candidate as electrode materials for supercapacitors.

1.54 µm Er3+ electroluminescence from an erbium-compound-doped organic light emitting diode with a p-type silicon anode

By doping an erbium complex, erbium (III) 2,4-pentanedionate (Er(acac)(3)), into the ALQ layer, we fabricate a series of infrared emission organic light emitting diodes (OLED) with structures of p-Si/SiO2/NPB/ALQ/ALQ:Er(acac)(3)/ALQ/Sm/Au, where p-Si is the anode and Sm/Au is the cathode. The 1.54 mu m emission from Er3+ is observed. The impact of doping level of Er(acac)(3) in ALQ on 1.54 mu m electroluminescence (EL) intensity is studied, and the best mass ratio of Er(acac)(3) to ALQ is found at 1:60. A competitive EL mechanism from the ALQ and Er(acac)(3) is found and the Er3+ ions excitations are attributed to energy transfer from the ligands to Er ions.