Exfoliated Graphite Nanofibers for Hydrogen Storage

Exfoliation of graphite nanofibers (GNF) expands the interplanar spacing of the GNF which leads to increased hydrogen storage. The exfoliation technique plays a large role in the resulting GNF microstructure, surface area, and hydrogen storage properties. Variations in preparation conditions expand the GNF lattice from 3.4 Å to over 500 Å. The BET surface area of the exfoliated GNF increases as much as 10-fold to 555 m/g. Increased surface area correlates with low temperature physisorption of hydrogen at 77K with an observed uptake of 1.2% at 77K and 20 bar. Conversely, observed dislocations in the graphitic structure correlate with a fourteen-fold increased ambient temperature adsorption at 20 bar. These results suggest that selective exfoliation of a nanocarbon is a means by which to control the relative binding energy of the hydrogen interaction with the carbon structure and thus vary the operative adsorption temperature. Introduction Despite early hydrogen storage claims in carbon materials that were largely irreproducible, reports continue to emerge showing hydrogen uptake ranging from 3-17%. An explanation emerging from these reports is that post-synthesis treatments modify the carbon structure and enhance hydrogen adsorption. Nuclear diffraction has shown that hydrogen may become chemically bound to graphitic carbon during certain preparations [1], terminal carbons have been suggested to act as catalytic entities to dissociate hydrogen [2], and electron micrographs have suggested that simple exposure to hydrogen may expand the graphite lattice of certain carbon nanostructures [3]. In an effort to test the emerging hypothesis that defects, dislocations, and/or terminal carbons lead to increased hydrogen storage via hydrogen intercalation into the graphite lattice, we have worked to develop methods that expand the graphite lattice a priori in nano-carbonaceous materials. Unlike exfoliation of single-wall nanotubes, intended to separate bundles into individual tubes, our exfoliation method targets intra-particle spacings. Our method is derived from well-established techniques to exfoliate graphite, in which intercalation of graphite is followed by a thermal shock to expand the graphitic layers. Herringbone GNF provide an interesting candidate for carbon exfoliation, with their slit-pore geometry, nano-scale dimensions, high aspect ratio, and graphitic layers that terminate along the fiber axis. Herringbone graphite nanofibers (GNF) provide an interesting candidate for carbon exfoliation, with their slit-pore geometry, nanoscale dimensions, high aspect ratio, and graphitic layers that terminate along the fiber axis. It was previously thought that the high aspect ratio and nanoscale dimensions of the GNF would preclude exfoliation. However, our preliminary data shows that we have successfully exfoliated herringbone GNF, with a resulting nanoscale structure that is previously unreported. Experimental Highly ordered, herring-bone graphite nanofibers were purchased from Catalytic Materials, Ltd; with a metal content of less than 1% as reported by the manufacturer; these GNF had a low baseline hydrogen uptake and were not activated prior to us. Based on preliminary studies with graphite exfoliation, the primary exfoliation method used in this study was a 50/50 mixture of nitric and sulfuric acids followed by thermal shocking at 700 °C (EGNF700). A portion of this sample was subjected to an additional high temperature treatment by heating the sample under flowing Argon at 1000 oC for 36 hours (EGNF-1000). Characterization. Materials were characterized using standard BET methods with nitrogen at 77K (Quantachrome Autosorb I) and helium densitometry measurements (Hiden IGA-003). Scanning electron microscopy (SEM) (Philips XL20) and TEM (JEOL 2010 and JEOL 2010F) were used to characterize the microstructure of the material. Total ash content was determined by temperature programmed oxidation on a low-pressure Perkin Elmer Thermogravimetric analyzer 7 (TGA). Hydrogen Uptake. A high-pressure thermo-gravimetric analyzer (Hiden Isochema IGA-003) was used to evaluate hydrogen uptake at pressures up to 20 bar. The IGA provided a highly sensitive (+1 μg) measurement with precise temperature and pressure control for automated measurements of adsorption and desorption isotherms. All samples were subjected to an in situ degas at 150 °C, unless otherwise stated. Hydrogen uptake measurements were normalized to sample mass after pretreatment, with buoyancy corrections determined from density measurements with helium. With a typical sample size of 50 mg, the error in the hydrogen due to instrumental limitations is +0.02 wt% absolute. Select samples were chosen for quality checks to ensure the reproducibility of the measurements. Results and Discussion The exfoliated GNF retains the overall nanosized dimensions of the original GNF, with the exfoliation temperature determining the degree of induced defects, lattice expansion, and resulting microstructure. Transmission electron microscopy (TEM) confirmed the herringbone structure of the lattice spacing and a 3.4 Å lattice spacing (Figure 1). The EGNF-700 fibers had dislocations in the graphitic structure and a 4% increase in graphitic lattice spacing to 3.5 Å (Figure 2). The EGNF-1000 fibers were significantly expanded along the fiber axis, with regular intervals of graphitic and amorphous regions ranging from 0.5 to >50 nm in width (Figure 3). The surface area of the starting material was increased from 47 m/g to 67 m/g for EGNF-700 and to 555 m/g for the ENGF-1000 (Table 1). Table 1. Characterization of GNF before and after Exfoliation GNF EGNF-700 ENGF-1000