Showing papers on "Graphite published in 2015"

Carbon Electrodes for K-Ion Batteries

Abstract: We for the first time report electrochemical potassium insertion in graphite in a nonaqueous electrolyte, which can exhibit a high reversible capacity of 273 mAh/g Ex situ XRD studies confirm that KC36, KC24, and KC8 sequentially form upon potassiation, whereas depotassiation recovers graphite through phase transformations in an opposite sequence Graphite shows moderate rate capability and relatively fast capacity fading To improve the performance of carbon K-ion anodes, we synthesized a nongraphitic soft carbon that exhibits cyclability and rate capability much superior to that of graphite This work may open up a new paradigm toward rechargeable K-ion batteries

A phosphorene–graphene hybrid material as a high-capacity anode for sodium-ion batteries

Abstract: Sodium-ion batteries have recently attracted significant attention as an alternative to lithium-ion batteries because sodium sources do not present the geopolitical issues that lithium sources might. Although recent reports on cathode materials for sodium-ion batteries have demonstrated performances comparable to their lithium-ion counterparts, the major scientific challenge for a competitive sodium-ion battery technology is to develop viable anode materials. Here we show that a hybrid material made out of a few phosphorene layers sandwiched between graphene layers shows a specific capacity of 2,440 mA h g−1 (calculated using the mass of phosphorus only) at a current density of 0.05 A g−1 and an 83% capacity retention after 100 cycles while operating between 0 and 1.5 V. Using in situ transmission electron microscopy and ex situ X-ray diffraction techniques, we explain the large capacity of our anode through a dual mechanism of intercalation of sodium ions along the x axis of the phosphorene layers followed by the formation of a Na3P alloy. The presence of graphene layers in the hybrid material works as a mechanical backbone and an electrical highway, ensuring that a suitable elastic buffer space accommodates the anisotropic expansion of phosphorene layers along the y and z axial directions for stable cycling operation. The sodiation–desodiation properties of few-layer phosphorene are mostly preserved by sandwiching the material between graphene layers, a behaviour that makes phosphorene–graphene hybrids a potentially suitable anode material for sodium-ion batteries.

Potassium Ion Batteries with Graphitic Materials

Abstract: Graphite intercalation compounds (GICs) have attracted tremendous attention due to their exceptional properties that can be finely tuned by controlling the intercalation species and concentrations. Here, we report for the first time that potassium (K) ions can electrochemically intercalate into graphitic materials, such as graphite and reduced graphene oxide (RGO) at ambient temperature and pressure. Our experiments reveal that graphite can deliver a reversible capacity of 207 mAh/g. Combining experiments with ab initio calculations, we propose a three-step staging process during the intercalation of K ions into graphite: C → KC24 (Stage III) → KC16 (Stage II) → KC8 (Stage I). Moreover, we find that K ions can also intercalate into RGO film with even higher reversible capacity (222 mAh/g). We also show that K ions intercalation can effectively increase the optical transparence of the RGO film from 29.0% to 84.3%. First-principles calculations suggest that this trend is attributed to a decreased absorbance...

Sodium Storage Behavior in Natural Graphite using Ether‐based Electrolyte Systems

Abstract: This work reports that natural graphite is capable of Na insertion and extraction with a remarkable reversibility using ether-based electrolytes. Natural graphite (the most well-known anode material for Li–ion batteries) has been barely studied as a suitable anode for Na rechargeable batteries due to the lack of Na intercalation capability. Herein, graphite is not only capable of Na intercalation but also exhibits outstanding performance as an anode for Na ion batteries. The graphite anode delivers a reversible capacity of ≈150 mAh g−1 with a cycle stability for 2500 cycles, and more than 75 mAh g−1 at 10 A g−1 despite its micrometer-size (≈100 μm). An Na storage mechanism in graphite, where Na+-solvent co-intercalation occurs combined with partial pseudocapacitive behaviors, is revealed in detail. It is demonstrated that the electrolyte solvent species significantly affect the electrochemical properties, not only rate capability but also redox potential. The feasibility of graphite in a Na full cell is also confirmed in conjunction with the Na1.5VPO4.8F0.7 cathode, delivering an energy of ≈120 Wh kg−1 while maintaining ≈70% of the initial capacity after 250 cycles. This exceptional behavior of natural graphite promises new avenues for the development of cost-effective and reliable Na ion batteries.

A Novel DNA Biosensor Based on a Pencil Graphite Electrode Modified with Polypyrrole/Functionalized Multiwalled Carbon Nanotubes for Determination of 6-Mercaptopurine Anticancer Drug

Abstract: A novel and sensitive biosensor employing immobilized DNA on a pencil graphite electrode modified with polypyrrole/functionalized multiwalled carbon nanotubes for the determination of 6-mercaptopurine (6-MP) is presented. In the first step, we modified the pencil graphite surface with polypyrrole and functionalized multiwalled carbon nanotubes (MWCNT/COOH). The developed electrode was characterized by scanning electron microscopy, atomic force microscopy, reflection–absorption infrared spectroscopy, X-ray photoelectron spectroscopy, and electrochemical impedance spectroscopy. In the other step, we used decreases in the oxidation responses of guanine and adenine as a sign of the interaction of 6-MP with salmon sperm double-stranded DNA using differential pulse voltammetry. The signal of guanine oxidation was linear with respect to the 6-MP concentration in the range of 0.2–100 μmol L–1 with a detection limit of 0.08 μmol L–1. The modified electrode was utilized for the determination of 6-MP in real samples.

Sodium intercalation chemistry in graphite

Abstract: The insertion of guest species in graphite is the key feature utilized in applications ranging from energy storage and liquid purification to the synthesis of graphene. Recently, it was discovered that solvated-Na-ion intercalation can occur in graphite even though the insertion of Na ions alone is thermodynamically impossible; this phenomenon enables graphite to function as a promising anode for Na-ion batteries. In an effort to understand this unusual behavior, we investigate the solvated-Na-ion intercalation mechanism using in operando X-ray diffraction analysis, electrochemical titration, real-time optical observation, and density functional theory (DFT) calculations. The ultrafast intercalation is demonstrated in real time using millimeter-sized highly ordered pyrolytic graphite, in which instantaneous insertion of solvated-Na-ions occurs (in less than 2 s). The formation of various stagings with solvated-Na-ions in graphite is observed and precisely quantified for the first time. The atomistic configuration of the solvated-Na-ions in graphite is proposed based on the experimental results and DFT calculations. The correlation between the properties of various solvents and the Na ion co-intercalation further suggests a strategy to tune the electrochemical performance of graphite electrodes in Na rechargeable batteries.

Ultrahigh-throughput exfoliation of graphite into pristine ‘single-layer’ graphene using microwaves and molecularly engineered ionic liquids

Abstract: Graphene has shown much promise as an organic electronic material but, despite recent achievements in the production of few-layer graphene, the quantitative exfoliation of graphite into pristine single-layer graphene has remained one of the main challenges in developing practical devices. Recently, reduced graphene oxide has been recognized as a non-feasible alternative to graphene owing to variable defect types and levels, and attention is turning towards reliable methods for the high-throughput exfoliation of graphite. Here we report that microwave irradiation of graphite suspended in molecularly engineered oligomeric ionic liquids allows for ultrahigh-efficiency exfoliation (93% yield) with a high selectivity (95%) towards ‘single-layer’ graphene (that is, with thicknesses <1 nm) in a short processing time (30 minutes). The isolated graphene sheets show negligible structural deterioration. They are also readily redispersible in oligomeric ionic liquids up to ~100 mg ml–1, and form physical gels in which an anisotropic orientation of graphene sheets, once induced by a magnetic field, is maintained. Graphene possesses numerous interesting properties yet the preparation of pristine sheets has remained challenging, hindering practical applications. Now, a rapid, highly efficient step has been devised that uses microwave irradiation in oligomeric ionic liquids to exfoliate graphite into pristine ‘single layer’ sheets (<1 nm thick). A concentrated dispersion of the resulting material behaves as a physical gel.

How to get between the sheets: a review of recent works on the electrochemical exfoliation of graphene materials from bulk graphite

Abstract: Since the beginning of the ‘graphene era’ post-2004, there has been significant interest in developing a high purity, high yield, and scalable fabrication route toward graphene materials for both primary research purposes and industrial production. One suitable approach to graphene production lies in the realm of electrochemical exfoliation, in which a potential difference is applied between a graphite anode/cathode in the presence of an electrolyte-containing medium. Herein we review various works on the electrochemical fabrication of graphene materials specifically through the use of electrochemical intercalation and exfoliation of a graphite source electrode, focusing on the quality and purity of products formed. We categorise the most significant works in terms of anodic and cathodic control, highlighting the merits of the respective approaches, as well as indicating the challenges associated with both procedures.

Raman Spectra of Carbon-Based Materials (from Graphite to Carbon Black) and of Some Silicone Composites

Abstract: Carbon-based nanomaterials have emerged as a subject of enormous scientific attention due to their outstanding mechanical, electrical and thermal properties. Incorporated in a polymeric matrix, they are expected to significantly improve physical properties of the host medium at extremely small filler content. In this work, we report a characterization of various carbonaceous materials by Raman spectroscopy that has become a key technique for the analysis of different types of sp2 nanostructures, including one-dimensional carbon nanotubes, two-dimensional graphene and the effect of disorder in their structures. The dispersion behavior of the D and G’ Raman bands, that is, their shift to higher frequencies with increasing laser excitation energy, is used to assess the interfacial properties between the filler and the surrounding polymer in the composites.

Measurement of the cleavage energy of graphite

Abstract: The basal plane cleavage energy (CE) of graphite is a key material parameter for understanding many of the unusual properties of graphite, graphene and carbon nanotubes. Nonetheless, a wide range of values for the CE has been reported and no consensus has yet emerged. Here we report the first direct, accurate experimental measurement of the CE of graphite using a novel method based on the self-retraction phenomenon in graphite. The measured value, 0.37±0.01 J m(-2) for the incommensurate state of bicrystal graphite, is nearly invariant with respect to temperature (22 °C≤T≤198 °C) and bicrystal twist angle, and insensitive to impurities from the atmosphere. The CE for the ideal ABAB graphite stacking, 0.39±0.02 J m(-2), is calculated based on a combination of the measured CE and a theoretical calculation. These experimental measurements are also ideal for use in evaluating the efficacy of competing theoretical approaches.

Electrochemical exfoliation of graphite and production of functional graphene

Abstract: Graphite, being a conductive material, has traditionally been used as an electrode in batteries and other electrochemical devices. In addition to its function as an inert electrode, electrochemical methods have been employed to form graphite intercalation compounds (GICs) and, more recently, to exfoliate graphite into few-layered graphene. The electrochemical methods are attractive as they eliminate the use of chemical oxidants as the driving force for intercalation or exfoliation, and an electromotive force is controllable for tunable GICs. More importantly, the extensive capabilities of electrochemical functionalization and modification enable the facile synthesis of functional graphene and its value-added nanohybrids. This review examines recent progress in electrochemical exfoliation of graphene from graphite. Attention is given not only to the production of pure/pristine graphene sheets, but also to the production of functionalized graphene.

Continuous Carbon Nanotube-Ultrathin Graphite Hybrid Foams for Increased Thermal Conductivity and Suppressed Subcooling in Composite Phase Change Materials

Abstract: Continuous ultrathin graphite foams (UGFs) have been actively researched recently to obtain composite materials with increased thermal conductivities. However, the large pore size of these graphitic foams has resulted in large thermal resistance values for heat conduction from inside the pore to the high thermal conductivity graphitic struts. Here, we demonstrate that the effective thermal conductivity of these UGF composites can be increased further by growing long CNT networks directly from the graphite struts of UGFs into the pore space. When erythritol, a phase change material for thermal energy storage, is used to fill the pores of UGF-CNT hybrids, the thermal conductivity of the UGF-CNT/erythritol composite was found to increase by as much as a factor of 1.8 compared to that of a UGF/erythritol composite, whereas breaking the UGF-CNT bonding in the hybrid composite resulted in a drop in the effective room-temperature thermal conductivity from about 4.1 ± 0.3 W m(-1) K(-1) to about 2.9 ± 0.2 W m(-1) K(-1) for the same UGF and CNT loadings of about 1.8 and 0.8 wt %, respectively. Moreover, we discovered that the hybrid structure strongly suppresses subcooling of erythritol due to the heterogeneous nucleation of erythritol at interfaces with the graphitic structures.

Development of a ReaxFF Potential for Carbon Condensed Phases and Its Application to the Thermal Fragmentation of a Large Fullerene

Abstract: In this article, we report the development of a ReaxFF reactive potential that can accurately describe the chemistry and dynamics of carbon condensed phases. Density functional theory (DFT)-based calculations were performed to obtain the equation of state for graphite and diamond and the formation energies of defects in graphene and amorphous phases from fullerenes. The DFT data were used to reparametrize ReaxFFCHO, resulting in a new potential called ReaxFFC-2013. ReaxFFC-2013 accurately predicts the atomization energy of graphite and closely reproduces the DFT-based energy difference between graphite and diamond, and the barrier for transition from graphite to diamond. ReaxFFC-2013 also accurately predicts the DFT-based energy barrier for Stone–Wales transformation in a C60(Ih) fullerene through the concerted rotation of a C2 unit. Later, MD simulations of a C180 fullerene using ReaxFFC-2013 suggested that the thermal fragmentation of these giant fullerenes is an exponential function of time. An Arrheni...

3D Si/C Fiber Paper Electrodes Fabricated Using a Combined Electrospray/Electrospinning Technique for Li-Ion Batteries

Abstract: Although the theoretical capacity of silicon is ten times higher than that of graphite, the overall electrode capacity of Si anodes is still low due to the low Si loading and heavy metal current collector. Here, a novel flexible 3D Si/C fiber paper electrode synthesized by simultaneously electrospraying nano-Si-PAN (polyacrylonitrile) clusters and electrospinning PAN fibers followed by carbonization is reported. The combined technology allows uniform incorporation of Si nanoparticles into a carbon textile matrix to form a nano-Si/carbon composite fiber paper. The flexible 3D Si/C fiber paper electrode demonstrate a very high overall capacity of ≈1600 mAh g-1 with capacity loss less than 0.079% per cycle for 600 cycles and excellent rate capability. The exceptional performance is attributed to the unique architecture of the flexible 3D Si/C fiber paper, i.e., the resilient and conductive carbon fiber network matrix, carbon-coated Si nanoparticle clusters, strong adhesion between carbon fibers and Si nanoparticle clusters, and uniform distribution of Si/C clusters in the carbon fiber frame. The scalable and facile synthesis method, good mechanical properties, and excellent electrochemical performance at a high Si loading make the flexible 3D Si/C fiber paper batteries extremely attractive for plug-in electric vehicles, flexible electronics, space exploration, and military applications.

Graphene nanosheets, carbon nanotubes, graphite, and activated carbon as anode materials for sodium-ion batteries

Abstract: The electrochemical sodium-ion storage properties of graphene nanosheets (GNSs), carbon nanotubes (CNTs), mesocarbon microbeads (MCMBs), and activated carbon (AC) are investigated. An irreversible oxidation occurs for the AC electrode during desodiation, limiting its use in sodium-ion batteries. The MCMB electrode shows a negligible capacity (∼2 mA h g−1), since the graphitic structure has a low surface area and is thus not capable of storing a sufficient amount of Na+. In contrast, the CNT and GNS electrodes exhibit reversible capacities of 82 and 220 mA h g−1, respectively, at a charge–discharge rate of 30 mA g−1. The high electro-adsorption/desorption area, large number of Na+ entrance/exit sites, and a large d-spacing of GNSs contribute to their superior Na+ storage capacity. At a high rate of 5 A g−1, the GNS electrode still delivers a capacity of as high as 105 mA h g−1, indicating great high-power ability. The charge storage mechanism of the electrode is examined using an ex situ X-ray diffraction technique.

Synergistic toughening of graphene oxide-molybdenum disulfide-thermoplastic polyurethane ternary artificial nacre.

Abstract: Inspired by the ternary structure of natural nacre, robust ternary artificial nacre is constructed through synergistic toughening of graphene oxide (GO) and molybdenum disulfide (MoS2) nanosheets via a vacuum-assisted filtration self-assembly process. The synergistic toughening effect from high mechanical properties of GO and lubrication of MoS2 nanosheets is successfully demonstrated. Meanwhile, the artificial nacre shows high electrical conductivity. This approach for constructing robust artificial nacre by synergistic effect from GO and MoS2 provides a creative opportunity for designing and fabricating integrated artificial nacre in the near future, and this kind of ternary artificial nacre has great potential applications in aerospace, flexible supercapacitor electrodes, artificial muscle, and tissue engineering.

Study of reduced graphene oxide preparation by hummers’ method and related characterization

Abstract: As a novel two-dimensional carbon material, graphene has fine potential applications in the fields of electron transfer agent and supercapacitor material for its excellent electronic and optical property However, the challenge is to synthesize graphene in a bulk quantity In this paper, graphite oxide was prepared from natural flake graphite by Hummers' method through liquid oxidization, and the reduced graphene oxide was obtained by chemical reduction of graphene oxide using NH3ċH2O aqueous solution and hydrazine hydrate The raw material graphite, graphite oxide, and reduced graphene oxide were characterized by X-ray diffraction (XRD), attenuated total reflectance-infrared spectroscopy (ATR-IR), and field emission scanning electron microscope (SEM) The results indicated that the distance spacing of graphite oxide was longer than that of graphite and the crystal structure of graphite was changed The flake graphite was oxidized to graphite oxide and lots of oxygen-containing groups were found in the graphite oxide In the morphologies of samples, fold structure was found on both the surface and the edge of reduced graphene oxide

In situ fabrication of three-dimensional, ultrathin graphite/carbon nanotube/NiO composite as binder-free electrode for high-performance energy storage

Abstract: Metal oxides have attracted considerable attention as promising electrode materials for energy storage, but the use of metal oxides for electrodes still faces challenges such as attaining high capacity, good cycle stability, and high-rated performance. Therefore, rational design of electrode architectures and assembling metal oxides into desired structures to further enhance electrochemical performance is necessary. Here, novel 3D electrode architectures consisting of 3D ultra-thin graphite film (UGF)/carbon nanotubes (CNTs) uniformly covered by NiO nanosheets are successfully constructed by a chemical vapor deposition and subsequent electrodeposition, which are directly used as bind-free electrodes for supercapacitors and Li-ion batteries. In such composite structures, 3D UGF/CNTs serve as substrates for NiO nanosheet decoration, and act as spacers to stabilize the composite structure, making the active surfaces of NiO nanosheets accessible for electrolyte penetration and accommodating volume changes during charge/discharge processes. As expected, 3D UGF/CNTs/NiO as electrode material for supercapacitors showed high specific capacitance (750.8 F g−1 at current density of 1 A g−1), superior rate performance (capacitance of 575.6 F g−1 at 10 A g−1) and excellent cycle stability (no decay after 3000 cycles). Moreover, the 3D CNTs/UGF/NiO composite also exhibited enhanced lithium storage properties as anode materials for Li-ion batteries.

The effect of carbon counter electrodes on fully printable mesoscopic perovskite solar cells

Abstract: Mesoporous graphite/carbon black counter electrodes (CEs) using flaky graphite with different sizes were applied in hole-conductor-free mesoscopic perovskite solar cells by a screen-printing technique. Conductivity measurements, current–voltage characteristics, and impedance spectroscopy measurements were carried out to study the influence of CEs on the photovoltaic performance of devices. The results indicated that graphite, which acted as the conductor in carbon counter electrodes (CCEs), could significantly affect the square resistance of CCEs, thus resulting in differences in fill factor and power conversion efficiency (PCE) of the devices. Based on the optimized CCEs with a thickness of 9 μm, PCEs exceeding 11% could be achieved for the fully printable hole-conductor-free mesoscopic perovskite solar cells due to the low square resistance and large pore size of graphite based CCEs. The abundant availability, low cost and excellent properties of such carbon material based CEs offer a wide prospect for their further applications in perovskite solar cells.

Ab initio study of sodium intercalation into disordered carbon

Abstract: Graphite, a predominantly chosen anode material for commercial lithium ion batteries (LIBs), has been reported to have negligible intercalation capacity as an anode for sodium ion batteries (NIBs). Disordered carbon exhibits high Na intercalation capacity and emerges as a leading candidate for NIB applications. However, the mechanism of Na+ ion insertion into disordered carbon is still controversial. Here, we propose an ab initio model for disordered carbon and investigate the intercalation mechanism of Na into the layered domains. Our ab initio calculations reveal that a larger interlayer distance and the presence of defects can effectively overcome the van der Waals interaction between graphene sheets and help Na intercalation to form NaC8. The calculation results clarify the mechanism of the Na intercalation and account for the presence of sloping and flat regions of charge–discharge curves in disordered carbon reported in numerous experiments. This reveals new prospects for helping Na intercalation into graphite.

Layered phosphorus-like GeP5: a promising anode candidate with high initial coulombic efficiency and large capacity for lithium ion batteries

Abstract: In this work, we for the first time investigate GeP5 as an anode material for lithium ion batteries (LIBs). Using a facile high energy mechanical ball milling (HEMM) method, we successfully synthesize pure GeP5 and GeP5/C nanocomposite at ambient temperature and pressure. According to XRD Rietveld refinement and first principle calculations, GeP5 possesses a two-dimensional layered structure similar to that of black P and graphite, and a high conductivity that is 10 000 and 10 times that of black P and graphite, respectively. Serving as novel anode materials, both GeP5 and its carbon composite deliver an unprecedented high reversible capacity of ca. 2300 mA h g−1, combined with a high initial coulombic efficiency of ca. 95%. Ex situ XRD and CV tests demonstrate that GeP5 undergoes conversion and alloying type lithium storage mechanism and that its capacity is co-contributed to by both the Ge and P components. In addition, GeP5/C exhibits superior cycle stability and excellent high-rate performance with a capacity of 2127 mA h g−1 at 5 A g−1. These properties suggest the promising application of these anode materials in next-generation high-energy and high-power LIBs.

Characterizing Graphitic Carbon with X-ray Photoelectron Spectroscopy: A Step-by-Step Approach

Abstract: X‐ray photoelectron spectroscopy (XPS) is a widely used technique for characterizing the chemical and electronic properties of highly ordered carbon nanostructures, such as carbon nanotubes and graphene. However, the analysis of XPS data—in particular the C 1s region—can be complex, impeding a straightforward evaluation of the data. In this work, an overview of extrinsic and intrinsic effects that influence the C 1s XPS spectra—for example, photon broadening or carbon–catalyst interaction—of various graphitic samples is presented. Controlled manipulation of such samples is performed by annealing, sputtering, and oxygen functionalization to identify different CC bonding states and assess the impact of the manipulations on spectral line shapes and their binding energy positions. With high‐resolution XPS and XPS depth profiling, the spectral components arising from disordered carbon and surface‐defect states can be distinguished from aromatic sp2 carbon. These findings illustrate that both spectral line shapes and binding energy components must be considered in the analysis of potentially defective surfaces of carbon materials. The sp2 peak, characteristic of aromatic carbon, features a strong asymmetry that changes with the curvature of the sample surface and, thus, cannot be neglected in spectral analysis. The applied deconvolution strategy may provide a simple guideline to obtaining high‐quality fits to experimental data on the basis of a careful evaluation of experimental conditions, sample properties, and the limits of the fit procedure.