Material and energy intensity of fullerene production.

Fullerenes are increasingly being used in medical, environmental, and electronic applications due to their unique structural and electronic properties. However, the energy and environmental impacts associated with their commercial-scale production have not yet been fully investigated. In this work, the life cycle embodied energy of C(60) and C(70) fullerenes has been quantified from cradle-to-gate, including the relative contributions from synthesis, separation, purification, and functionalization processes, representing a more comprehensive scope than used in previous fullerene life cycle studies. Comparison of two prevalent production methods (plasma and pyrolysis) has shown that pyrolysis of 1,4-tetrahydronaphthalene emerges as the method with the lowest embodied energy (12.7 GJ/kg of C(60)). In comparison, plasma methods require a large amount of electricity, resulting in a factor of 7-10× higher embodied energy in the fullerene product. In many practical applications, fullerenes are required at a purity >98% by weight, which necessitates multiple purification steps and increases embodied energy by at least a factor of 5, depending on the desired purity. For applications such as organic solar cells, the purified fullerenes need to be chemically modified to [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM), thus increasing the embodied energy to 64.7 GJ/kg C(60)-PCBM for the specified pyrolysis, purification, and functionalization conditions. Such synthesis and processing effects are even more significant for the embodied energy of larger fullerenes, such as C(70), which are produced in smaller quantities and are more difficult to purify. Overall, the inventory analysis shows that the embodied energy of all fullerenes are an order of magnitude higher than most bulk chemicals, and, therefore, traditional cutoff rules by weight during life cycle assessment of fullerene-based products should be avoided.

Enhanced capacity and rate capability of carbon nanotube based anodes with titanium contacts for lithium ion batteries.

Carbon nanotubes are being considered for adoption in lithium ion batteries as both a current collector support for high-capacity active materials (replacing traditional metal foils) and as free-standing electrodes where they simultaneously store lithium ions. The necessity to establish good electrical contact to these novel electrode designs is critical for success. In this work, application of nickel and titanium as both separable and thin film electrical contacts to free-standing single-wall carbon nanotube (SWCNT) electrodes is shown to dramatically enhance both the reversible lithium ion capacity and rate capability in comparison with stainless steel. Scanning electron microscopy showed that evaporation of Ni and Ti can effectively coat the SWCNT bundles in a bulk electrode which is capable of providing an improved electrical contact. A thin film of titanium emerged as the preferred electrical contact promoting the highest capacity ever measured for a SWCNT free-standing electrode of 1250 mAh/g. In addition, the titanium contacting approach demonstrated a 5-fold improvement in lithium ion capacity at extraction rates greater than 1C for a high-energy density Ge-SWCNT electrode. The overall performance improvement with Ti contacts is attributed to a lower contact resistance, nanoscale "wetting" of SWCNT bundles to improve contact uniformity, and effective electron coupling between Ti and SWCNTs due to work function-energy level alignment. The experimental results provide the basis for a Ragone analysis (power vs energy parameters), whereby Ge-SWCNT-Ti anodes paired with a LiFePO(4) cathode can lead to a 60% improvement over conventional graphite anodes in both power and energy density for a complete battery.

Multi-walled carbon nanotube paper anodes for lithium ion batteries.

The lithium ion capacity has been measured for multi-walled carbon nanotubes (MWCNTs) synthesized by injection chemical vapor deposition (CVD) using a cyclopentadienyl iron dicarbonyl dimer catalyst. The high quality of the as-synthesized MWCNTs has enabled free-standing electrodes to be fabricated independent of polymeric binder or copper support. Galvanostatic cycling of these electrodes demonstrates excellent reversibility and coulombic efficiency (> 97% after cycle 3) using propylene carbonate based electrolytes, with no evidence for material degradation. A reversible capacity exceeding 225 mAh/g was measured after 20 cycles when using the electrolyte combination of (1:1:1 v/v) ethylene carbonate (EC):propylene carbonate (PC):diethyl carbonate (DEC) at a constant current of 74 mA/g (equivalent of C/5 for LiC6). Modification of the catalyst solvent during synthesis from xylenes to pyridine improved the lithium ion capacity in the resulting MWCNT paper to 340 mAh/g. In addition, this MWCNT paper showed a stable reversible capacity after 10 cycles, exceeding 225 mAh/g when cycled at an equivalent 1C rate. Therefore, the use of a nitrogen source during synthesis can lead to improved lithium ion capacity in novel MWCNT anodes.

Effects of carrier gas dynamics on single wall carbon nanotube chiral distributions during laser vaporization synthesis.

We report on the utility of modifying the carrier gas dynamics during laser vaporization synthesis to alter the single wall carbon nanotube (SWNT) chiral distribution. SWNTs produced from an Alexandrite laser using conventional Ni/Co catalysts demonstrate marked differences in chiral distributions due to effects of helium gas and reactor chamber pressure, in comparison to conventional subambient pressures and argon gas. Optical absorption and Raman spectroscopies confirm that the SWNT diameter distribution decreases under higher pressure and with helium gas as opposed to argon. Fluorescence mapping of the raw soots in sodium dodecylbenzene sulfonate (SDBS)-D2O was used to estimate the relative (n, m)-SWNT content of the semiconducting types. A predominance of type II structures for each synthesis condition was observed. The distribution of SWNT chiral angles was observed to shift away from near-armchair configurations under higher pressure and with helium gas. These results illustrate the importance of gas type and pressure on the condensation/cooling rate, which allows for synthesis of specific SWNT chiral distributions.

Purity assessment of single-wall carbon nanotubes, using optical absorption spectroscopy.

A demand currently exists for a method of assessing the purity of single-wall carbon nanotubes (SWNTs), which will allow for meaningful material comparisons. An established metric and protocol will enable accurate and reproducible purity claims to be substantiated. In the present work, the ability to accurately quantify the mass fraction of SWNTs in the carbonaceous portion of a given sample is demonstrated, using optical absorption spectroscopy on both laser and arc discharge-generated SWNT-N,N-dimethylacetamide (DMA) dispersions. Verification of purity assessment protocols is based upon constructed sample sets comprising designed mass fractions of purified SWNTs and representative carbonaceous synthesis byproducts. Application of a previously reported method based on a ratio of the areal absorbance from linear subtractions of the second interband electronic transitions of semiconducting SWNTs ((S)E(22)) has shown a severe overestimation of SWNT purity (average error >24%). Instead, the development of a nonlinear pi-plasmon model, which considers overlap of electronic transitions and peak broadening, has dramatically improved the purity assessment accuracy (average error <7%), derived from a strong correlation to the constructed sample sets. This approach has enabled corroboration of rapid assessment procedures, such as absorbance peak maxima ratio and Beer's law analysis, directed at purification monitoring and synthesis sample screening. Specifically, a simple protocol for purity assessment of laser and arc-discharge SWNTs has been established that can be extended to other synthetic types (i.e. CVD, HiPco, etc.) and diameter distributions.