Graphite Nanofibers Research Articles

Temperature-dependent thermal properties of a paraffin phase change material embedded with herringbone style graphite nanofibers

In many studies, carbon nanoparticles with high values of thermal conductivity (10–3000W/mK) have been embedded into phase change thermal energy storage materials (PCMs) in order to enhance their bulk thermal properties. While a great deal of work to date has focused on determining the effect of these nanoparticles on a PCM’s solid phase thermal properties, little is known about their effect on its liquid phase thermal properties. Thus, in this study, the effect of implanting randomly oriented herringbone style graphite nanofibers (HGNF, average diameter=100nm, average length=20μm) on the bulk thermal properties of an organic paraffin PCM (IGI 1230A, Tmelt=329.15K) in both the solid and liquid phase is quantified. The bulk thermal conductivity, volumetric heat capacity and thermal diffusivity of HGNF/PCM nanocomposites are obtained as a function of temperature and HGNF volume loading level. It is found that the property enhancement varies significantly depending on the material phase. In order to explain the difference between solid and liquid phase thermal properties, heat flow at the nanoparticle–PCM and nanoparticle–nanoparticle interfaces is examined as a function of HGNF loading level and temperature. To do this, the solid and liquid phase thermal boundary resistances (TBRs) between the nanoparticles and the surrounding PCM and/or between contacting nanoparticles are found. Results suggest that the TBR at the HGNF–PCM interface is nearly double the TBR across the HGNF–HGNF interface in both solid and liquid phases. However, both the HGNF–PCM and HGNF–HGNF TBRs are at least an order of magnitude lower when the PCM is in its solid phase versus when the PCM is in its liquid phase. Finally, the effect of nanofiber concentration on the PCM’s latent heat of fusion and melt temperature is investigated in order to determine the applicability of the HGNF/PCM nanocomposite in a wide variety of energy systems.

Synergistic effects of hybrid graphitic nanofillers on simultaneously enhanced wear and mechanical properties of polymer nanocomposites

In this study, hybrids of organosilane-functionalized graphitic nano-materials, nanofibers (GNFs, 1-D) and nanoplatelets (GNPs, 2-D) were applied to fabricate high-density polyethylene (HDPE) nanocomposites with simultaneously enhanced wear and mechanical properties. Organosilane as a surfactant was shown to effectively assist the nanofiller–polymer matrix interactions, filler dispersion and distribution, GNP interlayer sliding motion, as well as the establishment of a filler network. The ratio of weight percentage (wt%) of silane coated onto GNP relative to GNF (defined as factor R) was adjusted by the amount of surfactant. The results indicated that hybrid nanofillers consisting of the GNF and GNP with the highest wt% ratio of silane coating on GNP compared to that of GNF suggested a synergistic effect on the improvements in both wear resistance and storage modulus of the nanocomposites. Compared to the pure polymer, the wear resistance of the hybrid GNF–GNP composite with the highest factor R was improved by 89%; the storage modulus was increased: up to 70% at −70°C, and 83% at room temperature.

Electrochemical Characterization of Various Self-Assembled Nanocomposite Electrodes in E.coli-Catalyzed Mediatorless Microbial Fuel Cell

Microbial fuel cells (MFCs) is one of the promising green energy sources to produce electricity from organic wastes with the help of bacteria as catalyst. It converts the chemical energy in organic wastes into electricity by bioelectrochemical reactions. In spite of the promise of MFCs, their applications are limited by low power generation efficiency. Power limitations can be addressed by the use of better suited anodes, and to modify the electrode surface with nanostructures is one of the ways to improve the efficiency of the anode. In this study, We have demonstrated MFCs performance based on Escherichia coli (E.coli) by optimizing the electrode surface structure using various nanomaterials like reduced grapheme oxide(rGO), graphite nanofiber(GNF) and carbon nanotube(CNT). Each of the nanomaterials is doped with iron particles of 3~10nm size, and were formed into a nanocomposite. The synthesized nanocomposites was assembled on the anode electrode surface by directly applying a magnetic field. This conductive multilayer of nanocomposites coated on anode enhances the electron transfer between the bacteria and the anode. The electrochemical activities with each catalysts of such as Fe/rGO, Fe/GNF and Fe/CNT are investigated by cyclic voltammetry, Impedance spectra and polarization curve measurements.

CO2 Adsorption capacity of activated N‐doping porous carbons prepared from graphite nanofibers/polypyrrole

ABSTRACTIn this study, N‐doping porous carbons (NPCs) with a 3D aperiodic hierarchical and layered structure were prepared by the sodium hydride (NaOH) activation of graphite nanofibers (GNFs)/polypyrrole (PPY) composites. The effects of the N groups and structural features on the CO2 adsorption capacity of NPCs were investigated by N2 full isotherms, XRD, SEM, and TEM. The CO2 adsorption capacity was measured by the CO2 isothermal adsorption at 25°C and 1 atm. It was found that GNFs served as a substrate and layered graphitic carbons were formed by the thermal annealing of PPY. The content of N groups and textural properties of NPCs were enhanced with increasing activation temperature, resulting in improved CO2 adsorption capacity. The CO2 adsorption isotherms showed that GPK‐600 exhibited the best CO2 adsorption capacity of 88.8 mg/g when the activation temperature was 600°C. The result indicates that the pore size and its distribution of NPCs lead to feasible contact CO2, and the presence of high N groups on the NPCs could have resulted in further stabilization of the surface effect. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40517.

Structural evolution in graphitization of nanofibers and mats from electrospun polyimide–mesophase pitch blends

The evolution of structure in multi-step thermal treatment of polyimide–mesophase pitch (PI–pitch) blend nanofiber mats obtained by an electrospinning process is described. The mats were thermally treated at a series of stages up to 3000°C. The structural transformation of the nanofiber mats consisted of three regimes. First regime corresponds to the removal of the majority of non-carbon elements and the formation of initial residual carbon. Second regime involves slow growth of the graphitic layers and slow improvement of their stacking order. Progressive graphitization occurs in regime three when the fibers become highly graphitic. The addition of pitch was found to give rise to overall enhanced graphitic order in the PI–pitch blend nanofibers as reflected in the smaller inter-layer spacing d002 approaching that of the perfect graphite crystal, and the larger crystal sizes, Lc and La, confirmed by XRD analysis, as well as the higher ratio of graphitic structure revealed by Raman spectroscopy. Development of highly localized oriented domains in these nanofibers were observed by dark field TEM. The addition of pitch led to enhancement of both electrical and thermal conductivity.

Hollow-tunneled graphitic carbon nanofibers through Ni-diffusion-induced graphitization as high-performance anode materials

Activated N-doped hollow-tunneled graphitic carbon nanofibers with a novel architecture are excellent anode materials for lithium ion batteries, displaying a superhigh reversible specific capacity and a remarkable volumetric capacity with outstanding rate capability and good cycling stability.

Fundamental Open Questions on Engineering of “Super” Hydrogen Sorption in Graphite Nanofibers: Relevance for Clean Energy Applications

Herein, some fundamental open questions on engineering of “super” hydrogen sorption (storage) in carbonaceous nanomaterials are considered, namely: 1) on thermodynamic stability and related characteristics of some hydrogenated graphene layers nanostructures: relevance to the hydrogen storage problem; 2) determination of thermodynamic characteristics of graphene hydrides; 3) a treatment and interpretation of some recent STM, STS, HREELS/LEED, PES, ARPS and Raman spectroscopy data on hydrogensorbtion with epitaxial graphenes; 4) on the physics of intercalation of hydrogen into surface graphene-like nanoblisters in pyrolytic graphite and epitaxial graphenes; 5) on the physics of the elastic and plastic deformation of graphene walls in hydrogenated graphite nanofibers; 6) on the physics of engineering of “super” hydrogen sorption (storage) in carbonaceous nanomaterials, in the light of analysis of the Rodriguez-Baker extraordinary data and some others. These fundamental open questions may be solved within several years.

One-pot synthesis of carbon nanotube–graphene hybrids via syngas production

Multi-walled carbon nanotubes (MWCNTs) are limited by entanglement, and it is rather difficult to prevent graphene stacking in a polymer composite. These two challenges can be addressed by developing a MWCNT–graphene hybrid where MWCNTs and graphene are born as twins. We in this study employed a syngas production method using microwave irradiation for a one-pot synthesis of porous, crumpled and loose MWCNT–graphene hybrids, investigated the substrate compositions, measured their performance as electrodes for energy storage devices, and proposed the synthesis mechanisms. A number of hybrids, including MWCNT–graphene, MWCNT–cup-stacked CNT and MWCNT–graphitic nanofiber, were synthesized on Cr–Ni, Fe–Ni and Ni–CeO2 substrates, respectively. TEM analysis shows that the two challenges have been markedly addressed in the hybrids. They performed better in terms of capacitive properties than commercial MWCNTs.

SWNT and MWNT from a Polymeric Electrospun Nanofiber Precursor

ABSTRACTCarbon nanotubes (CNT) are expected to revolutionize a range of technologies because of their unique mechanical and electrical properties. Using nanotubes in structural materials holds significant promise due to their extremely high modulus and tensile strength, however their cost, production rate and integration into a fiber form severely limit the current structural application opportunities. The high cost of CNT is tied to their slow, batch synthesis using vapor phase, vacuum processes. We report the investigation of the formation of carbon nanotubes from a polymeric precursor using an electrospinning production process. Electrospinning generates nanofibers at velocities up to 10 m/s from a single nozzle without a vacuum requirement, with the potential to generate CNT appropriate from structural and electrical applications. Our CNT formation concept is based upon Reactive Empirical Bond order calculations that show carbon nanofibers have a thermodynamic preference for the cylindrical graphite conformation. Simulations suggest that for small diameter carbon fibers, less than about 60 nm, the single wall and multi wall nanotubes (SWNT and MWNT) phases are thermodynamically favored relative to an amorphous or planar graphitic nanofiber structure. We have developed a novel process using continuous electrospun polyacrylonitrile (PAN) nanofibers as precursors to continuous SWNT and MWNT. The process for converting PAN nanofibers to SWNT's and MWNT's follows the process for typical carbon fiber manufacture. The PAN nanofibers, of 10 to 100 nm in diameter, are crosslinked by heating in air and then decomposed to carbon via simple pyrolysis in inert atmosphere. The pyrolyzed carbon nanofibers are then annealed to form the more energetically favorable SWNT or MWNT phase, depending upon the precursor diameter. We will discuss the process and characterization data.

Nitrogen functionalized graphite nanofibers/Ir nanoparticles for enhanced oxygen reduction reaction in polymer electrolyte fuel cells (PEFCs)

Nitrogen functionalization of graphite nanofibers (N-GNF) was performed using hexa methyl tetra amine (HMTA) as the nitrogen source and used as a support material for metal nanoparticle deposition. The successful incorporation of nitrogen was confirmed using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy analysis. Iridium (Ir) nanoparticles with a particle size of ∼2.2 nm were deposited onto N-GNF by a simple ethanol reduction method. The oxygen reduction reaction (ORR) activity of N-GNF and the ameliorating effect of ORR on Ir deposited N-GNF (Ir/N-GNF) were studied by various physicochemical and electrochemical methods. The enhancement of ORR activity for Ir/N-GNF was evidenced by high onset potentials and mass activities. The presence of nitrogen in the Ir/N-GNF catalyst facilitates quick desorption of the –OH species from the Ir surface and accelerates the electrochemical reaction of Ir particles which in turn enhances the ORR activity. The electrochemical stability of the Ir/N-GNF was investigated by repeated potential cycling up to 2500 cycles and was found to have excellent stability for ORR activity. The PEFC with Ir/N-GNF catalyst delivers a peak power density of 450 mW cm−2 at a load current density of 1577 mA cm−2, while the PEFC with Ir/GNF catalyst delivers a peak power density of only 259 mW cm−2 at a load current density of 1040 mA cm−2 under identical operation conditions.

Carbon nanofiber-based counter electrodes for low cost dye-sensitized solar cells

Carbon materials represent an attractive alternative to platinum in dye-sensitized solar cells (DSSC) counter electrodes to contribute to an efficient conversion of solar energy into electricity. The use of highly graphitic carbon nanofibers (CNFs) is investigated by analyzing the effect of the filament diameter, surface area and graphitization degree on the DSSC cathode performance. To this purpose, transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy and physisorption analysis are used to characterize the main properties of the CNFs. The behavior of CNFs as counter electrodes in DSSC is investigated by polarization experiments and electrochemical impedance spectroscopy. Among the different materials, the CNF characterized by the highest surface area (183 m2 g−1), thinnest filament size (24 nm) and highest density of surface defects shows the best performance in terms of efficiency, open circuit potential and short circuit current density. Further investigation of the electrode thickness together with series and charge transfer resistance cross-analysis evidences the key role played by the surface area and surface graphitization to obtain a suitable performance. Compared to literature, so-obtained CNFs represent an interesting alternative to manufacture low cost DSSC cathodes.

Electrophoretic deposition of different carbon nanoscale reinforcements on carbon fiber fabrics and mechanical properties of the resulting hybrid multi-scale epoxy composites

Three types of carbon nanoscale reinforcements (CNRs) including the shortened electrospun carbon nanofibers (ECNFs, with diameters and lengths of ∼200 nm and ∼15 µm, respectively), carbon nanofibers (CNFs), and graphite nanofibers (GNFs) were electrophoretically deposited on carbon fiber (CF) fabrics for the fabrication of hybrid multi-scale epoxy composites. The results indicated that the electrophoretic deposition (EPD) of CNRs onto CF fabrics led to substantial improvements on mechanical properties of hybrid multi-scale epoxy composites; in particular, the hybrid multi-scale epoxy composite containing surface-functionalized ECNFs (with amino groups) exhibited the highest mechanical properties. The study also indicated that some agglomerates of CNRs (particularly GNFs) could form during the EPD process, which would decrease mechanical properties of the resulting composites. Additionally, the reinforcement mechanisms were investigated, and the results suggested that continuous (or long) ECNFs would outperform short ECNFs on the reinforcement of resin-rich interlaminar regions in the composites. POLYM. COMPOS., 35:1229–1237, 2014. © 2013 Society of Plastics Engineers

Nano-epoxy resins containing electrospun carbon nanofibers and the resulting hybrid multi-scale composites

For the first time, electrospun carbon nanofibers (ECNFs, with diameters and lengths of ∼200nm and ∼15μm, respectively) were explored for the preparation of nano-epoxy resins; and the prepared resins were further investigated for the fabrication of hybrid multi-scale composites with woven fabrics of conventional carbon fibers via the technique of vacuum assisted resin transfer molding (VARTM). For comparison, vapor growth carbon nanofibers (VGCNFs) and graphite carbon nanofibers (GCNFs) were also studied for making nano-epoxy resins and hybrid multi-scale composites. Unlike VGCNFs and GCNFs that are prepared by bottom-up methods, ECNFs are produced through a top-down approach; hence, ECNFs are more cost-effective than VGCNFs and GCNFs. The results indicated that the incorporation of a small mass fraction (e.g., 0.1% and 0.3%) of ECNFs into epoxy resin would result in substantial improvements on impact absorption energy, inter-laminar shear strength, and flexural properties for both nano-epoxy resins and hybrid multi-scale composites. In general, the reinforcement effect of ECNFs was similar to that of VGCNFs, while it was higher than that of GCNFs.

Increased charge transfer of PVDF-HFP based electrolyte by addition of graphite nanofiber and its application in dye-sensitized solar cells

The PEO and PVDF-HFP mixtures were used as polymer electrolytes in solid-state dye-sensitized solar cells (DSSCs). Correlation between the ionic conductivity and cell performance by varying the composition of polymer electrolytes was investigated to elucidate the importance of the ionic conductivity in determining the charge transfer and energy conversion efficiency of solid-state DSSCs. In this work, for increasing the ionic conductivity and charge transfer, GNF was added to the polymer electrolyte. The ionic conductivity of polymer electrolyte containing GNF (0.005g) is 8.67×10−4Scm−1 and pristine polymer electrolyte is 3.81×10−4Scm−1. The charge transfer of GNF (0.005g) added DSSCs is faster than the other samples, the electron transport time is 1.53ms and electron life time is 27.20ms. The increase of current density with the polymer electrolyte containing GNF (0.005g) can be possibly attributed to the direct contact between dye/TiO2 and I−/I3− that will improve the charge transportation. The highest energy conversion efficiency of 4.60% is obtained for polymer electrolyte containing GNF (0.005g).

Surface chemical composition of chromatographically fractionated graphite nanofiber-derived carbon dots

Carbon dots (C-dots) were synthesized from graphite nanofiber starting material by oxidizing in strong acid (i.e., top-down synthesis). The resulting product was heterogeneous with regard to size and surface functionality. After separating the mixture into fractions by means of anion-exchange (AE) high-performance liquid chromatography (HPLC), fractions with unique photoluminescent properties were selected. We examined the surface of these unique C-dot fractions by means of X-ray photoelectron spectroscopy and infrared spectroscopy and identified the surface functional groups on the C-dot fractions that exhibited higher photoluminescence. Although some surface functionalities were common to all fractions (e.g., carboxylic groups), others were different (e.g., aliphatic CH bonding). Among the luminescent nanoparticles examined, those with the higher luminescence showed less oxygen content. This strongly suggests that in the mixture of the as-synthesized C-dots, all nanoparticles did not have identical surface functionality, and they contributed differently to the observed photoluminescence.

Catalytic activity of Pt anchored onto graphite nanofiber-poly (3,4-ethylenedioxythiophene) composite toward oxygen reduction reaction in polymer electrolyte fuel cells

The potential of graphite nanofiber (GNF)–Poly(3,4-ethylenedioxythiophene) (PEDOT) composite is explored as a catalyst support for polymer electrolyte fuel cells (PEFCs). Due to electron accepting nature of GNF and electron donating nature of PEDOT, the monomer EDOT adsorbs on the surface of GNF due to strong electrostatic π–π interaction. Pt nanoparticles are impregnated on GNF–PEDOT composite by ethylene glycol reduction method and their effects on electro catalytic activity for oxygen reduction reaction (ORR) are systemically studied. Pt particles supported on GNF–PEDOT with catalyst loading of 0.2mgcm−2 exhibit a peak power density of 537mWcm−2 at a load current density of 1120mAcm−2, while it was only 338mWcm−2 at a load current density of 720mAcm−2 in case of Pt particles supported on pristine GNF. The superior behavior of GNF–PEDOT supported Pt catalyst could be exclusively credited to the high graphitic nature of GNF and their mild functionalization with PEDOT increasing uniform dispersion of Pt. Indeed, the non-destructive functionalization of GNF with conducting polymer, such as PEDOT, makes them promising catalyst-supports for PEFCs.

Surface-modified graphite nanomaterials for improved reinforcement efficiency in cementitious paste

Graphite nanomaterials can play multi-faceted roles towards enhancing the mechanical, physical and functional attributes of cementitious materials. Graphite nanoplatelets and carbon nanofibers, when compared with carbon nanotubes, offer desired mechanical and physical characteristics at reduced cost. However thorough dispersion of nanomaterials in the cementitious matrix is critical for effective use of their distinct geometric and engineering properties towards development of higher-performance cementitious nanocomposites. The dispersion and interfacial interaction of nanomaterials in the aqueous medium of cementitious matrix can benefit from proper surface treatment of nanomaterials. The surface modification techniques employed in this study emphasize introduction of hydrophilic groups on graphite nanomaterials to facilitate their dispersion in aqueous media. These include: (i) polymer wrapping of oxidized carbon nanofiber; and (ii) covalent attachment of functional groups. The effects of these surface modifications on the performance characteristics of cementitious nanocomposite were evaluated. It was found that wrapping of oxidized graphite nanomaterials with PAA, at polymer-to-nanofiber weight ratio of 10%, was particularly beneficial. With the addition of 0.13wt.% (0.81vol.%) of nanomaterials (with respect to anhydrous cementitious materials), up to 73% gain in the flexural strength of cementitious matrix was realized. Test results also indicated that oxidized of graphite nanoplatelets markedly lowered (by up to 50%) the moisture sorptivity.

Supercapacitors based on 3D network of activated carbon nanowhiskers wrapped-on graphitized electrospun nanofibers

Due to their cycling stability and high power density, the supercapacitors bridge the power/energy gap between traditional dielectric capacitors and batteries/fuel cells. Electrode materials are key components for making high performance supercapacitors. An activated carbon nanowhiskers (ACNWs) wrapped-on graphitized electrospun nanofiber (GENF) network (ACNWs/GENFN) with 3D porous structure is prepared as a new type of binder-free electrode material for supercapacitors. The supercapacitor based on the ACNWs/GENFN composite material displays an excellent performance with a specific capacitance of 176.5 F g−1 at current density of 0.5 A g−1, an ultrahigh power density of 252.8 kW kg−1 at current density of 800 A g−1 and an outstanding cycling stability of no capacitance loss after 10,000 charge/discharge cycles.

Engineering interfaces in carbon nanostructured mats for the creation of energy efficient thermal interface materials

One of the few remaining opportunities to increase heat dissipation in IC circuitry is to substantially decrease the thermal interface resistance between solid–solid contacts from source to sink. In this study, heterogeneous nanostructured mats (1–100μm thick, randomly oriented networks of nanostructures) are synthesized for use as thermal interface materials (TIMs). Recent studies suggest that mats composed entirely of carbon nanotubes (CNTs) or graphite nanofibers (GNFs) can act as thermal insulators due to significant phonon scattering at interfaces. In this work, graphene nanoplatelets (xGnPs) with high surface areas are included in CNT and GNF mats in order to increase the contact area between nanostructures and mitigate phonon scattering. Results indicate that an increase in contact area between nanostructures increases the thermal conductance across nanostructure networks by nearly an order of magnitude. Additionally, a study of the surface topography of CNT and GNF mats using atomic force microscopy (AFM) indicates that they are able to conform well to the asperities between rough, mating surfaces. Thus, an increase in contact area between CNT junctions not only produces a thermally conductive network, but also increases the reliability of a CNT mat TIM by avoiding common issues associated with the use of wetting agents.

Investigation of Space Charge Distribution of Low-Density Polyethylene/GO-GNF (Graphene Oxide from Graphite Nanofiber) Nanocomposite for HVDC Application

This paper reported a research on space charge distribution in low-density polyethylene (LDPE) nanocomposites with different types of graphene and graphene oxide (GO) at low filler content (0.05 wt%) under high DC electric field. Effect of addition of graphene oxide or graphene, its dispersion in LDPE polymer matrix on the ability to suppress space charge generation will be investigated and compared with MgO/LDPE nanocomposite at the same filler concentration. At an applied electric field of 80 kV/mm, a positive packet-like charge was observed in both neat LDPE, MgO/LDPE, and graphene/LDPE nanocomposites, whereas only little homogenous space charge was observed in GO/LDPE nanocomposites, especially with GO synthesized from graphite nano fiber (GNF) which is only -100 nm in diameter. Our research also suggests that dispersion of graphene oxide particles on the polymer matrix plays a significant role to the performance of nanocomposites on suppressing packet-like space charge. From these results, it is expected that nano-sized GO synthesized from GNF can be a promising filler material to LDPE composite for HVDC applications.