Flexible Graphite Foils Research Articles

Anisotropy of Electrical and Thermal Conductivity in High-Density Graphite Foils.

Flexible graphite foils with varying thicknesses (S = 282 ± 5 μm, M = 494 ± 7 μm, L = 746 ± 8 μm) and an initial density of 0.70 g/cm3 were obtained using the nitrate method. The specific electrical and thermal conductivity of these foils were investigated. As the density increased from 0.70 g/cm3 to 1.75 g/cm3, the specific electrical conductivity increased from 69 to 192 kS/m and the thermal conductivity increased from 109 to 326 W/(m·K) due to the rolling of graphite foils. The study showed that conductivity and anisotropy depend on the shape, orientation, and contact area of thermally expanded graphite (TEG) mesoparticles (mesostructural factor), and the crystal structure of nanocrystallites (nanostructural factor). A proposed mesostructural model explained these increases, with denser foils showing elongated, narrowed TEG particles and larger contact areas, confirmed by electron microscopy results. For graphite foils 200 and 750 μm thick, increased density led to a larger coherent scattering region, likely due to the rotation of graphite mesoparticles under mechanical action, while thinner foils (<200 μm) with densities > 1.7 g/cm3 showed increased plastic deformation, indicated by a sharp reduction in the coherent scattering region size. This was also evident from the decrease in misorientation angles with increasing density. Rolling reduced nanocrystallite misorientation angles along the rolling direction compared to the transverse direction (TD) (for 1.75 g/cm3 density ΔMA = 1.2° (S), 2.6° (M), and 2.4° (L)), explaining the observed anisotropy in the electrical and mechanical properties of the rolled graphite foils. X-ray analysis confirmed the preferred nanocrystallite orientation and anisotropy coefficients (A) using Kearns parameters, which aligned well with experimental measurements (for L series foils calculated as: A0.70 = 1.05, A1.30 = 1.10, and A1.75 = 1.16). These calculated values corresponded well with the experimental measurements of specific electrical conductivity, where the anisotropy coefficient changed from 1.00 to 1.16 and mechanical properties varied from 0.98 to 1.13.

Improvement Synthesis of Graphene Oxide Yield in Two Steps of Intercalation and Oxidation of Flexible Graphite Foil by Electrochemical Exfoliation

Synthesis of high quality and quantity of graphene by cost-effective methods are highly desirable for various application. In electrochemical exfoliation, graphite foil has been used as carbon source for the synthesis of high yield of graphene flakes. Electrochemical exfoliation is one of the faster and cheaper method to synthesize graphene sheets. In this work, five different types of concentration of sulphuric acid were used for electrochemical exfoliation. The electrochemical cell design where graphite foil as anode and copper foil as cathode which were connected to DC power supply of 5V. To examine the morphology scanning electron microscope (SEM) was employed in which the sheet structures with large lateral dimension and thin graphene flakes. Furthermore, X-ray diffraction (XRD) revealed that exfoliated graphene samples showed a significant peak at about 2θ = 26º corresponded to graphite.

The Influence of Machining Conditions on the Orientation of Nanocrystallites and Anisotropy of Physical and Mechanical Properties of Flexible Graphite Foils.

The physical and mechanical properties and structural condition of flexible graphite foils produced by processing natural graphite with nitric acid, hydrolysis, thermal expansion of graphite and subsequent rolling were studied. The processes of obtaining materials and changing their characteristics has been thoroughly described and demonstrated. The structural transformations of graphite in the manufacture of foils were studied by X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM). A decrease in the average size of the coherent scattering regions (CSR) of nanocrystallites was revealed during the transition from natural graphite to thermally expanded graphite from 57.3 nm to 20.5 nm at a temperature of 900 °C. The rolling pressure ranged from 0.05 MPa to 72.5 MPa. The thickness of the flexible graphite foils varied from 0.11 mm to 0.75 mm, the density-from 0.70 to 1.75 g/cm3. It was shown that with an increase in density within these limits, the compressibility of the graphite foil decreased from 65% to 9%, the recoverability increased from 5% to 60%, and the resiliency decreased from 10% to 6%, which is explained by the structural features of nanocrystallites. The properties' anisotropy of graphite foils was studied. The tensile strength increased with increasing density from 3.0 MPa (ρ = 0.7 g/cm3) to 14.0 MPa (ρ = 1.75 g/cm3) both in the rolling direction L and across T. At the same time, the anisotropy of physical and mechanical properties increased with an increase in density along L and T to 12% with absolute values of 14.0 MPa against 12.5 MPa at a thickness of 200 μm. Expressed anisotropy was observed along L and T when studying the misorientation angles of nanocrystallites: at ρ = 0.7 g/cm3, it was from 13.4° to 14.4° (up to 5% at the same thickness); at ρ = 1.3 g/cm3-from 11.0° to 12.8° (up to 7%); at ρ = 1.75 g/cm3-from 10.9° to 12.4° (up to 11%). It was found that in graphite foils, there was an increase in the coherent scattering regions in nanocrystallites with an increase in density from 24.8 nm to 49.6 nm. The observed effect can be explained by the coagulation of nanocrystallites by enhancing the Van der Waals interaction between the surface planes of coaxial nanocrystallites, which is accompanied by an increase in microstrains. The results obtained can help discover the mechanism of deformation of porous graphite foils. The obtained results can help discover the deformation mechanism of porous graphite foils. We assume that this will help predict the material behavior under industrial operating conditions of products based flexible graphite foils.

High-performance expanded graphite from flake graphite by microwave-assisted chemical intercalation process

Herein, a facile preparation method with characteristics of energy-saving, efficient and environmentally friendly is proposed. Firstly, GICs (Graphite Intercalation Compounds) were synthesized by microwave assisted hydrothermal method using flake graphite (FG) as raw material, H2SO4, and K2S2O8 as oxidative intercalations by microwave-assisted oxidation for 5 min, and then EG was prepared from obtained GICs by microwave irradiation for 40 s under the microwave power of 800 W, and the effect of different process conditions on EG expansion volume (EV) was studied. The results show that EG with a good expansion can be rapidly prepared by microwave irradiation, and the EV value can reach 455 mL /g under the optimal conditions. In particular, the prepared EG has a high graphitization degree with less structural damage, exihibiting good industrial application prospects. The mechanism of K2S2O8 oxidation and graphite expansion under microwave irradiation was analyzed. In adition, flexible graphite foil papers prepared with the obtained EG exhibit good electrical conductivity and thermal properties. The as prepared EG material shows an efficient adsorption of hydraulic oil and vegetable oil and excellent flame retardant performance. This preparation method is green, low cost, and safe, and can be used for the efficient batch preparation of EG.

Treatment of NiMoO4/nanographite nanocomposite electrodes using flexible graphite substrate for aqueous hybrid supercapacitors.

The cycling performance of supercapacitors sometimes becomes limited when electrode materials slough off during frequent charge–discharge cycles, due to weak bonding between the active material and the current collector. In this work, a flexible graphite foil substrate was successfully used as the current collector for supercapacitor electrodes. Graphite foil substrates were treated in different ways with different acid concentrations and temperatures before being coated with an active material (NiMoO4/nanographite). The electrode treated with HNO3 (65%) and H2SO4 (95%) in a 1:1 ratio at 24°C gave better electrochemical performance than did electrodes treated in other ways. This electrode had capacitances of 441 and 184 Fg–1 at current densities of 0.5 and 10 Ag-1, respectively, with a good rate capability over the current densities of the other treated electrodes. SEM observation of the electrodes revealed that NiMoO4 with a morphology of nanorods 100–120 nm long was properly accommodated on the graphite surface during the charge–discharge process. It also showed that treatment with high-concentration acid created an appropriately porous and rough surface on the graphite, enhancing the adhesion of NiMoO4/nanographite and boosting the electrochemical performance.

A controlled process of atomic-scale material design via temperature-mediated grain refinement of NiCo2O4 rods for capacitive energy storage

We report on a controlled process of atomic-scale material design based on calcination-temperature-induced structural variation and its influence on capacitive energy storage characteristics of spinel-structured NiCo 2 O 4 rods (NCOR). Precisely, morphology-tuned NCOR were grown through a facile solvothermal process followed by a controlled calcination. The meticulous thermal-kinetics study revealed that the activation energy of 269.53 kJ/mol could transform the hydroxides into oxides completely. The precipitate was annealed at variable temperatures (350, 450 and 550 °C) to incorporate structural changes with variation in size of the crystallites. NCOR-450 consisting of fine-sized nanocrystallites coated on a flexible graphite foil exhibited a maximum gravimetric specific capacitance of 326.1 F/g at a constant current density of 0.5 A/g. The comparatively smaller crystallites lead to attractive capacitances as the temperature-induced grain-growth-correlated electrical properties influence the electrochemical properties significantly. • Shape-controlled 1-D NiCo 2 O 4 nanorods were grown via template-free method. • The activation energy of 269.53 kJ/mol could transform hydroxide into complex bimetallic oxide. • Calcination temperature strongly influences the size of nanocrystallites by controlling the atom-scale material design. • Smaller the crystallite size better the electrical conductivity and electrochemical performance. • Custom-designed graphite foil is explored as current collector.

PANI/Au/Fe3O4 nanocomposite materials for high performance energy storage

In this work new energy storage components were prepared depositing films made of polyaniline (PANI) modified with gold/magnetite nanoparticles on flexible graphite foils. Three types of composite materials termed PANI/Fe3O4, PANI/Au/Fe3O4 and PANI/Au/Fe3O4@Yne (where @Yne is a propynylcarbamate group) were obtained by electrosynthesis. Galvanostatic charge-discharge (CD) and impedance tests (EIS) were performed to verify their efficiency in charge storage properties: for the gold-containing electrodes PANI/Au/Fe3O4 and PANI/Au/Fe3O4@Yne areal capacities values of 45.6 and 46.5 mAh cm−2 were found in 0.5 M H2SO4 + 0.1 M LiClO4 electrolyte solution at a current density of 0.5 mA cm−2. These values are twofold higher than those found for PANI/Fe3O4 electrodes and fourfold greater than those for PANI alone (11.0 mAh cm−2). In turn PANI/Au/Fe3O4 and PANI/Au/Fe3O4@Yne were employed to assemble gel-state symmetric devices. CD, EIS and longtime resistance tests were made on the new devices that displayed an areal capacities of 100.0 mAh cm−2 for PANI/Au/Fe3O4 and 73.6 mAh cm−2 PANI/Au/Fe3O4@Yne respectively. To our knowledge this is the first time that AuNP–modified magnetite nanoparticles are used in energy storage devices preparation.

Graphitization of graphene oxide films under pressure

Lightweight, flexible graphite foils that are chemically inert, high-temperature resistant, and highly electrically and thermally conductive can be used as component materials in numerous applications. “Graphenic” foils can be prepared by thermally transforming graphene oxide films. For this transformation, it is desirable to maintain a densely packed film structure at high heating rates as well as to lower the graphitizing temperatures. In this work, we discuss the pressure-assisted thermal decomposition of graphene oxide films by hot pressing at different temperatures (i.e., 300 °C, 1000 °C, or 2000 °C). The films pressed at 1000 °C or 2000 °C were subsequently heated at 2750 °C to achieve a higher degree of graphitization. The combination of heating and pressing promotes the simultaneous thermal decomposition and graphitic transformation of G-O films. Films pressed at 2000 °C as well as films further graphitized at 2750 °C show high chemical purity, uniformity, and retain their flexibility. For films pressed at 2000 °C and then further heated at 2750 °C, the mechanical performances outperform the reported values of the “graphite” foils prepared by calendering exfoliated graphite flakes; the electrical conductivity is ∼3.1 × 105 S/m and the in-plane thermal conductivity is ∼1.2 × 103 W/(m·K).

Advanced supercapacitor prototype using nanostructured double-sided MnO2/CNT electrodes on flexible graphite foil

Development of advanced supercapacitors which provide greater energy density while retaining high power density can revolutionize energy storage solutions for both civilian and military applications. Herein, we present fabrication and characterization of nanostructured supercapacitor electrodes using manganese dioxide and carbon nanotubes (MnO2/CNTs) on flexible graphite foil substrate for integration into a supercapacitor prototype cell, without any additives or binders. Low-cost, thermal chemical vapor deposition process in a tube furnace was used to synthesize CNTs, acting as the current conductors, directly on both sides of the flexible, conducting graphite foil current collector. Thin-film MnO2 deposition on the CNTs was achieved by electrochemical technique using in situ reduction of potassium permanganate (KMnO4), without any supporting electrolyte, which provides excellent bonding between the two for enhanced stability. Eight electrodes were assembled in a stack with polypropylene separators and packaged in a pouch cell with organic electrolyte (1 M tetraethylammonium tetrafluoroborate in acetonitrile) yielding a total capacitance of nearly 2.8 F at 2.5 V and a corresponding specific capacitance of 388 F g−1 was calculated for MnO2. High cell capacitance and a low DC resistance yield a maximum specific power value of 36.1 kW kg−1 and a maximum specific energy of 48.5 Wh kg−1 at 2.5 V when considering the total mass including that of CNTs and MnO2. Cycling data showed nearly 100% capacitance retention over 3000 charge discharge cycles. To the best of our knowledge, a supercapacitor prototype cell in this configuration has not been fabricated and/or reported previously. Schematic representation of the prototype supercapacitor cell consisting of a stack of eight electrodes with polypropylene separators. On the right is a blow-up representation of a double side electrode with MnO2/CNT on both sides of the graphite foil substrate. The equivalent circuit diagram can be seen on the left, consisting of seven capacitors in parallel

Usage of flexible graphite foil for the protection of collector ars of aluminium electrolysers

Usage of flexible graphite foil for the protection of collector ars of aluminium electrolysers

All-Carbon Electrode Consisting of Carbon Nanotubes on Graphite Foil for Flexible Electrochemical Applications.

We demonstrate the fabrication of an all-carbon electrode by plasma-enhanced chemical vapor deposition for use in flexible electrochemical applications. The electrode is composed of vertically aligned carbon nanotubes that are grown directly on a flexible graphite foil. Being all-carbon, the simple fabrication process and the excellent electrochemical characteristics present an approach through which high-performance, highly-stable and cost-effective electrochemical applications can be achieved.

Influence of chemical treatment on the electrical conductivity and thermopower of expanded graphite foils

AbstractWe investigated the interaction of chemical materials with flexible graphite foils which were fabricated from the expanded graphite (EG) flakes by mechanical rolling and compressing. A variety of performed experiments demonstrated that the electrical conductivity and thermoelectric power (TEP) of the graphite foils can be modified by chemical treatments. In particular, the “as prepared” p‐type graphite foil was successfully transferred into an n‐type doped material upon a treatment with amine containing compounds. Generally, the acceptor‐like chemicals increasing the concentration of the electric charge carriers enhanced the conductivity of the graphite foils, thereby, showing a decrease in the TEP reflected by the Seebeck coefficient, whereas the donor molecules significantly affected the conductivity and changed the sign and value of the TEP. Thermal and electrical insulating polymers, such as PVDF, PMMA, PVA, PS or PC, filling the inter‐lamellar spacing reduced the conductivity of the foil due to increase of the layer‐to‐layer resistance. They also blocked heat flow in the foil, and consequently increased the Seebeck coefficient. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Improvement of the oxidation stability and the mechanical properties of flexible graphite foil by boron oxide impregnation

Improvement of the oxidation stability and the mechanical properties of flexible graphite foil by boron oxide impregnation

Improvement of the oxidation stability and the mechanical properties of flexible graphite foil by boron oxide impregnation

Flexible graphite foil produced by rolling expanded graphite impregnated with boron oxide was analyzed by laser mass spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and thermogravimetry. It was shown that the modification of the graphite foil by boron oxide increases the onset temperature of oxidation by ∼ 150 °C. Impregnation of less than 2 mass% boron oxide also increased the tensile strength of the materials. The observed improvement was attributed to the blocking of active sites by boron oxide, which is probably chemically bonded to the edges of graphene sheets in expanded graphite particles.

A graphite foil electrode covered with electrochemically exfoliated graphene nanosheets

We demonstrate a highly efficient and large area synthesis of 2-D graphene nanosheets on the surface of flexible graphite foils by electrochemical exfoliation of graphite in an effective electrolyte, poly(sodium-4-styrenesulfonate) solution. A constant current of 150 mA/cm was applied to the vertically aligned graphite (anode) and copper (cathode) sheet in the PSS electrolyte solution during a preset time for electrolytic surface exfoliation of the graphite sheet; uniform expansion of the graphite foil was observed. This expanded foil was characterized using scanning electron microscopy, confocal laser scanning microscopy, and high-resolution transmission electron microscopy. Furthermore, we demonstrate the ability of this high surface area foil, covered with uniform graphene, to enable improved electrolyte permeability and Li ion transfer, thereby enhancing electrochemical performance of Li ion battery electrodes.

Properties of carbon-carbon composites based on exfoliated graphite

We report the first study of the mechanical and electrical properties of carbon-carbon composites based on flexible graphite foils modified with pyrolytic carbon. Our results demonstrate that slight densification with pyrolytic carbon enhances the chemical and thermal stability of the graphite foils, increases their Young’s modulus by a factor of 2.5, and reduces their resistivity by 25%. Decomposition of the polymeric carbon precursors poly(hydridocarbyne) and poly(naphthylhydridocarbyne) in an exfoliated graphite matrix increases the tensile strength of the composite by a factor of 2 compared to the as-prepared graphite foil.

Preparation and characterization of graphene oxide paper

Free-standing paper-like or foil-like materials are an integral part of our technological society. Their uses include protective layers, chemical filters, components of electrical batteries or supercapacitors, adhesive layers, electronic or optoelectronic components, and molecular storage. Inorganic 'paper-like' materials based on nanoscale components such as exfoliated vermiculite or mica platelets have been intensively studied and commercialized as protective coatings, high-temperature binders, dielectric barriers and gas-impermeable membranes. Carbon-based flexible graphite foils composed of stacked platelets of expanded graphite have long been used in packing and gasketing applications because of their chemical resistivity against most media, superior sealability over a wide temperature range, and impermeability to fluids. The discovery of carbon nanotubes brought about bucky paper, which displays excellent mechanical and electrical properties that make it potentially suitable for fuel cell and structural composite applications. Here we report the preparation and characterization of graphene oxide paper, a free-standing carbon-based membrane material made by flow-directed assembly of individual graphene oxide sheets. This new material outperforms many other paper-like materials in stiffness and strength. Its combination of macroscopic flexibility and stiffness is a result of a unique interlocking-tile arrangement of the nanoscale graphene oxide sheets.

Current Carriers Energy Spectrum of Sulfur Acid-Graphite and Graphite Foils

The present work means to study the infrared spectra and to determinate energy spectra parameters of current carriers for graphite intercalation compounds of low stages with sulfur acid in the presence of different oxidizers, where oxidizers are K2Cr2O7, KMnO4, Ce(SO4)24(NH4)2SO4·2H2O, (NH4)2S2O8, and for low density graphite materials (flexible graphite foils) produced on their base.

Physical and Chemical Properties of Flexible Graphite Foils

The tensile strength, electrical resistivity, compressibility, elastic recovery, and Young's modulus of foils were studied as a function of the production conditions of graphite foils (the foaming temperature of oxidized graphite, the fraction consumption of graphite, the nature of graphite intercalation compounds and their stage number).

Temperature-induced ruby fluorescence shifts up to a pressure of 15 GPa in an externally heated diamond anvil cell

The temperature-induced shifts (100-600 degrees C) in ruby lines at high pressure up to 15 GPa have been investigated. A heating assemblage, designed and fabricated with flexible graphite foils, is used to provide homogeneous temperature throughout the pr