Platinum nanopaticles supported on stacked-cup carbon nanofibers as electrocatalysts for proton exchange membrane fuel cell

Inexpensive stacked-cup carbon nanofibers (SC-CNFs) supported Pt nanoparticles with a loading from 5 to 30 wt.% were prepared through a modified ethylene glycol method. XRD and TEM characterizations show that the average Pt particle sizes increase with increasing metal loading, and they can be controlled <5 nm with a uniform dispersion. A self-developed filtration process was employed to fabricate Pt/SC-CNFs film-based membrane electrode assembly (MEA), and the catalyst transfer efficiency can reach nearly 100% using a super-hydrophobic polycarbonate filter. The thickness of catalyst layer can be accurately controlled through altering Pt loadings of the catalyst and electrode, this is in good agreement with our theoretical calculation. For Pt/SC-CNFs-based-MEAs, Pt cathode loading was found more critical than Pt anode loading on proton exchange membrane fuel cell (PEMFC) performance. The Pt/SC-CNFs-based MEA with an optimized 50 wt.% Nafion content demonstrates higher PEMFC performance than the carbon black-based MEA with an optimized 30 wt.% Nafion content. SC-CNFs possess much larger length-to-diameter ratio than carbon black particles, it makes Pt/SC-CNFs more easily form continuously conductive networks in the Nafion matrix than carbon black. Therefore, the Pt/SC-CNFs-based MEA demonstrates higher Pt utilization than carbon black-based MEA, which implies possible reduction in Pt loading of MEA.     

Preparation, characterization and growth mechanism of platelet carbon nanofibers

The synthesis of platelet carbon nanofibers (PCNFs) on a silicon substrate using chemical vapor deposition method is reported. Scanning electron microscope, high-resolution transmission electron microscopy, and Raman spectroscopy were used to characterize the nanofibers. It is found that these platelet nanofibers are of the order of 10 μm long, and most have a nearly rectangular transverse section with several hundreds nm wide and several tens of nm thick. Structure analysis reveals that the carbon layers of platelet nanofibers are parallel to each other, and have a uniform (0 0 2) orientation that is perpendicular to the fiber axis. Many faults and nanodomain have been found in the nanofibers. It is suggested that the PCNF grow in tip growth mechanism by the precipitation of carbon from the side facet of catalyst flakes.     

Cone-stacked carbon nanofibers with cone angle increasing along the longitudinal axis

Using porous anodic aluminum oxide as template and petroleum pitch as precursor, a massive amount of uniform carbon nanofibers was obtained after thermal treatment. The diameter and length were 300 nm and 60 μm, respectively. The difference between these and the classic herringbone structure is that the angle between the graphenes and the fiber axis increases regularly along the axis instead of being fixed. TEM observations show that the nanofiber consists of stacked conical graphenes with cone angles that steadily increase from 60° to 180° along the fiber axis. This structure is the first to be produced without using catalytic CVD, and has not been reported using template procedures. The large deformation of the graphene planes at the tip of the nanofiber may produce interesting electronic applications.     

Evidence for atomically dispersed Pd in catalysts supported on carbon nanofibers

The state of palladium deposited on carbon nanofibers (CNFs) with stacked structure in 0.04–0.5 wt% concentration was studied by XRD, electron microscopy and EXAFS. Palladium was found to exist as single atoms attached to CNF in the samples with Pd concentration 0.2 wt% or less. Most probable location of Pd atoms according to EXAFS data was analysed using quantum-chemical calculation.     

Synthesis and structure of carbon belts made of carbon nanofibers supported on carbon foams

Two-dimensional carbon belts (CBs) made of carbon nanofibers (CNFs) supported on a carbon foam (CFoam) substrate have been synthesized by a procedure involving carbonization of polyamic acid (PAA)/Ni(NO₃)₂ solution impregnated polyurethane foam in flowing H₂ at 700 °C and catalytic chemical vapor deposition (CCVD) using C₂H₄ as a carbon source and SO₂ as a promoter. The CBs, which are hundreds of micrometers in length, several micrometers in width and tens of nanometers in thickness, are made of CNFs with a low degree of graphitization that array with an orientation roughly parallel to the longitudinal axis of the CBs. The results show that the mass ratio of Ni to PAA, a H₂ atmosphere in carbonization and SO₂ in CCVD process are the three key factors governing the growth of the CBs.     

Preparation of high purity helical carbon nanofibers by the catalytic decomposition of acetylene and their growth mechanism

Helical nanofibers were synthesized at 271 °C using acetylene gas as the reactant and copper nanocrystals, produced by decomposition of copper tartrate, as the catalyst. Their chemical structure was confirmed to be organic compounds containing CC groups using characterization by FT-IR, ¹H NMR and elemental analyses. The morphologies of the catalyst before, during and after the fiber growth were observed by SEM and TEM, and the results reveal that the shape of the copper nanoparticles changes from quasi-spherical to polyhedral during the adsorption of acetylene. Chemisorption of acetylene on different planes of the copper crystal was simulated by density functional theory calculations under the generalized gradient approximation. The adsorption energy on the (1 1 1), (1 0 0) and (1 1 0) planes of Cu was investigated to address the interaction modes between acetylene and Cu surfaces. The adsorbability is in the order (1 0 0) > (1 1 1) > (1 1 0). Based on the experimental and theoretical evidence, a growth mechanism of coordination polymerization and asymmetric growth on distinctive crystal planes was proposed to interpret the structural and morphological variations of the nanofibers.     

Helical carbon nanofibers with a symmetric growth mode

Helical carbon nanofibers with a symmetric growth mode were synthesized by the decomposition of acetylene with a copper catalyst. There were always only two helical fibers symmetrically grown over a single copper nanocrystal. The two helical fibers had opposite helical senses, but had identical cycle number, coil diameter, coil length, coil pitch, cross section, and fiber diameter. The irregular tips and helical reversals of the two helical fibers further revealed the symmetric growth mode. This mirror-symmetric growth mode was induced by the shape changes in copper nanocrystals during catalyzing the decomposition of acetylene. Upon contacting the initial copper nanocrystals with irregular shapes, acetylene began to decompose to form two straight fibers (the irregular tips). At the same time, shape changes in copper nanocrystals began. Once they changed from an irregular to a regular faceted shape, the two straight fibers ceased to grow and two regular helical nanofibers with opposite helical senses began to grow. If the regular faceted nanocrystals continue to change shapes during fiber growth, the two helical fibers possibly changed helical senses at the same time, resulting in helical reversals. The shape changes were caused by the changes in surface energy resulting from the acetylene-adsorption on the copper nanocrystals.     

Temperature effect on the formation of catalysts for growth of carbon nanofibers

Turbostratic carbon nanofibers (CNFs), platelet graphite nanofibers (PGNFs) and tubular graphite nanofibers (TGNFs, also called multi-walled carbon nanotubes) were synthesized using thermal decomposition from a mixture of poly(ethylene glycol) and NiCl₂. A detailed study found that the synthesis temperature dramatically affected the morphology and topography of the catalysts, which play an important role in the synthesis of the various CNFs. At the temperature of 600 °C, irregular shape nanocatalysts with very rough surfaces were formed for the synthesis of turbostratic CNFs. Cubic-like nanocatalysts were formed at 750 °C for PGNFs and truncated cone-like nanocatalysts were formed at 850 °C for TGNFs. The surface roughness and the shape of the catalysts determined the stacking order of the graphene layers so that different types of CNF were formed. The growth direction of the graphene layers was from the Ni(1 1 1) plane for PGNFs and from the Ni(1 1 0) plane for TGNFs. Characterizations and field emission properties of these materials were also studied and compared.     

A model for the structure and growth of carbon nanofibers synthesized by the CVD method using nickel as a catalyst

Carbon nanofibers were synthesized at 450-800 °C by the catalytic CVD method using alumina plate-supported nickel as catalyst and acetylene as carbon source. It was found that Ni/alumina catalyst exhibited a large catalytic effect on the growth of carbon nanofibers at the temperatures between 550 and 700 °C. TEM observation revealed that most of the carbon nanofibers synthesized at 550 °C had a coil-like shape, and many thick platelet nanofibers were found in the product at 700 °C. A growth model was proposed to explain the structural diversity of the carbon nanofibers. Although the carbon nanofibers showed low crystallinity, they can be easily graphitized at 2500 °C.     

Super growth of vertically-aligned carbon nanofibers and their field emission properties

We demonstrate a very efficient synthesis of vertically-aligned ultra-long carbon nanofibers (CNFs) with sharp tip ends using thermal chemical vapor deposition. Millimeter-scale CNFs with a diameter of less than 50 nm are readily grown on palladium thin film deposited Al₂O₃ substrate, which activate the conical stacking of graphitic platelets. The field emission performance of the as-grown CNFs is better than that of previous CNFs due to their extremely high aspect ratio and sharp tip angle. The CNF array gives the turn-on electric field of 0.9 V/μm, the maximum emission current density of 6.3 mA/cm² at 2 V/μm, and the field enhancement factor of 2585.     

Direct synthesis and characterization of a nonwoven structure comprised of carbon nanofibers

A unique type of nonwoven carbon material has been developed which is flexible, resilient, and produced at modest temperature and near ambient pressure using catalytic deposition. This material is comprised entirely of nanoscale carbon fibers, which are extensively interlaced to create a coherent, bulk material. The structure and basic mechanical and electrical properties of this material were investigated through cyclic compression and in situ resistance measurement. The material was highly elastic and capable of being repeatedly compressed without disintegration. The mechanical response varied with density, and the density was controlled by the amount of catalyst used. The material exhibited a high electrical resistivity, which varied nonlinearly with compression.     

Role of cone angle on the mechanical behavior of cup-stacked carbon nanofibers studied by atomistic simulations

Classical molecular dynamics simulations are used to study the effects of cone angle on mechanical properties and failure mechanisms in thermally-treated cup-stacked CNFs. We find a 22-fold reduction in elastic modulus and 4-fold decrease in tensile strength of cup-stacked CNFs with a wide range of cone angles between 19.2° and 180°. Our results show significant elastic stiffening for intermediate angles between 38.9° and 112.9°, as well as a minimum in tensile strength at a critical cone angle, due to the competition between weak van-der-Waals forces between layers and strong strengthening mechanisms from surface bonds introduced during thermal treatment. Different failure modes in CNFs subjected to tensile deformation are also predicted as a function of cone angle. This study constitutes an important step toward understanding the origin of strength dispersions observed experimentally in CNFs, and suggests that the design of high-strength CNFs can be optimized structurally by appropriately tuning the cone angle.     

Room-temperature growth of ion-induced carbon nanofibers: Effects of ion species

Graphite surfaces were bombarded with Ne⁺, Ar⁺ and Xe⁺ ions at 450 eV–1 keV to induce the carbon nanofiber (CNF) growth at room temperature, and the dependence of size and numerical density of ion-induced CNFs on the ion species and ion energy was investigated in detail. The ion-sputtered surfaces were covered with densely distributed conical protrusions and aligned CNFs grew on the tips, except for the low-energy Xe⁺-sputtered surfaces. Longer CNFs grew by lighter-mass-ion irradiation, and finer CNFs formed by heavier-mass-ion bombardment. In addition, the higher the ion energy, the longer the length of the ion-induced CNFs. Because the size and numerical density were controllable by the ion-irradiation parameters, ion-induced CNFs were believed to be quite promising for myriad of applications such as high-resolution scanning probe microscope cantilevers, bio-cell manipulators and field emission source operating at low voltage.