Lin Simpson

Nanoengineered Carbon Scaffolds for Hydrogen Storage

Single-walled carbon nanotube (SWCNT) fibers were engineered to become a scaffold for the storage of hydrogen. Carbon nanotube fibers were swollen in oleum (fuming sulfuric acid), and organic spacer groups were covalently linked between the nanotubes using diazonium functionalization chemistry to provide 3-dimensional (3-D) frameworks for the adsorption of hydrogen molecules. These 3-D nanoengineered fibers physisorb twice as much hydrogen per unit surface area as do typical macroporous carbon materials. These fiber-based systems can have high density, and combined with the outstanding thermal conductivity of carbon nanotubes, this points a way toward solving the volumetric and heat-transfer constraints that limit some other hydrogen-storage supports.

Solution-Phase Synthesis of Heteroatom-Substituted Carbon Scaffolds for Hydrogen Storage

This paper reports a bottom-up solution-phase process for the preparation of pristine and heteroatom (boron, phosphorus, or nitrogen)-substituted carbon scaffolds that show good surface areas and enhanced hydrogen adsorption capacities and binding energies. The synthesis method involves heating chlorine-containing small organic molecules with metallic sodium at reflux in high-boiling solvents. For heteroatom incorporation, heteroatomic electrophiles are added to the reaction mixture. Under the reaction conditions, micrometer-sized graphitic sheets assembled by 3-5 nm-sized domains of graphene nanoflakes are formed, and when they are heteroatom-substituted, the heteroatoms are uniformly distributed. The substituted carbon scaffolds enriched with heteroatoms (boron ∼7.3%, phosphorus ∼8.1%, and nitrogen ∼28.1%) had surface areas as high as 900 m(2) g(-1) and enhanced reversible hydrogen physisorption capacities relative to pristine carbon scaffolds or common carbonaceous materials. In addition, the binding energies of the substituted carbon scaffolds, as measured by adsorption isotherms, were 8.6, 8.3, and 5.6 kJ mol(-1) for the boron-, phosphorus-, and nitrogen-enriched carbon scaffolds, respectively.

Structural and Magnetic Studies of Two-Dimensional Solvent-Free Manganese(II) Complexes Prepared Via Ligand Exchange Reaction Under Solvothermal Conditions

Systematic investigation of the ligand exchange reactions between manganese(II) acetate and benzoic acid under solvothermal conditions led to the isolation of crystalline complexes {Mn5(OC(O)CH3)6(OC(O)C6H5)4}(infinity) ( 1) and {Mn5(OC(O)CH3)4(OC(O)C6H5)6}}(infinity) ( 2) in high (i.e., >90%) yields. The complexes are characterized structurally as 2-D honeycomb-like sheets comprised of edge-shared Mn 12 loops with some noteworthy differences as follows. First, buckling of the 2-D sheet in 1 is not observed for 2, presumably as a consequence of additional intersheet phenyl groups in the latter. Second, complex 1 is comprised of only six-coordinate MnII, while 2 has both pseudo-octahedral and distorted trigonal bipyramidal coordinate metal ions. Third, while complex 2 exhibits pi-stacking interactions with intersheet phenyl-phenyl contacts of 3.285 and 3.369 A, 1 exhibits no such bonding. Antiferromagnetic exchange is observed with Weiss constants (theta) of -28 and -56 K and Neel temperatures of 2.2 and 8.2 K for complexes 1 and 2, respectively. The paramagnetic transition at higher temperatures for complex 2 may be attributed to pi-pi exchange through phenyl groups in adjacent layers. Preliminary gas sorption studies (76 K) indidate preferential adsorption of H2 versus N2 for complex 1 only.

Evaluation of standard durability tests towards the qualification process for the glass-zeolite ceramic waste form

Glass-bonded zeolite is being developed as a potential ceramic waste form for the disposition of radionuclides associated with the Department of Energy`s (DOE`s) spent nuclear fuel conditioning activities. The utility of several standard durability tests was evaluated as a first step in developing methods and criteria that can be applied towards the process of qualifying this material for acceptance into the DOE Civilian Radioactive Waste Management System. The effects of pH, leachant composition, and sample surface-area-to leachant-volume ratios on the durability test results are discussed, in an attempt to investigate the release mechanisms and other physical and chemical parameters that are important for the acceptance criteria, including the establishment of appropriate test methodologies required for product consistency measurements.

Atomic Layer Deposition of Platinum onto Functionalized Aligned MWNT Arrays for Fuel Cell Application

High aspect ratio materials, such as carbon nanotubes (CNTs), provide unique opportunities and advantages as catalyst support materials in fuel cells. In particular, CNTs are highly conductive and corrosion resistant; properties which represent limitations for current carbon supports. While most advanced catalysts research focuses on the production of small nanoparticles to increase the percent of surface accessible Pt; here, we specifically attempt to conformally coat Pt in thin layers onto CNT arrays. We present our work on modifying CNT surfaces inside high-density, surface-bound aligned CNT arrays (aspect ratio ~1:750) with non-toxic gas phase chemistries. The number of nucleation sites and the onset of growth of Pt by ALD can be tuned by using Ar plasma, O2 plasma and chemical functionalization. This, in turn, affects the uniformity of the Pt ALD coating down the length of the tubes within the CNT array.

Humidity-resistant high-conductivity amorphous-InZnO transparent conductors

Sputtered, amorphous mixed metal transparent conductive oxides, TCOs, are of increasing interest. The TCOs have excellent opto-electronic properties and smoothness (RRMS ≪ 0.5 nm) obtained for films deposited at 50–150 °C.1 In the case of amorphous InZnO (a-InZnO) films grown from a ceramic target with 20 atomic % ZnO in In2O3, conductivities σ ≥ 2500 S/cm are common.2–5 This project specifically centers on the combined materials phase space of oxygen stoichiometry and metals composition (In:Zn ratio) and their effect on the environmental stability and water permeability of the resultant transparent films. Amorphous IZO films deposited from a fixed composition target with a range of oxygen concentrations allowed for a comparison of the relative stability of various composition and conductivity. In the initial testing within an 85/85 chamber, the more conductive a-InZnO films with ≫ 1000 S/cm, did not show any change in conductivity or transparency after 1000 hrs. In contrast a-InZnO films of comparable thickness and ≪0.01 S/cm while remaining transparent would improve in conductivity anywhere from 10% to over 2 orders of magnitude. These results establish the possibility that a-InZnO layers may be a viable replacement to traditional resistive and conductive ZnO layers and may find application as a transparent, non-organic barrier layer.

Novel transparent conducting barriers for photovoltaics

NREL has leveraged its expertise in multifunctional thin-film technologies to develop enabling and inexpensive transparent conductive coatings for energy applications (Figure 1). The design of these films provides an unique complement of performance characteristics including enhanced durability, flexibility, and impermeable and self-healing barriers to environmental contaminants (e.g., water and oxygen). This is especially needed for environmentally sensitive thin-film and organic photovoltaic technologies. In general, the majority of transparent conducting films used in industry today involves 0.5 to 2 micrometers of relatively expensive indium tin oxide (ITO) and/or doped zinc oxide coatings that have 60%–80% transmission in the visible region and resistances from 10 to 500 ohms/sq. However, to reduce materials/processing costs and maintain a high level of conductivity/transparency while enhancing barrier and durability performance, we have developed nanoscale film composites of ultra-thin TCOs (focusing on lower-cost materials that will include doped ZnO) and atomic layer-deposited materials. These composite structures will be a disruptive technology providing the transparency, conductivity, structural integrity (adhesion and fracture toughness), and impermeability needed for the most demanding applications, with processing that is scalable and adaptable for inexpensive, low-temperature manufacturing. Specifically, NREL has demonstrated transparent conducting films with the potential to reduce water/oxygen vapor transport rates by more than five orders of magnitude compared to typical organic materials. In addition, these films have resistances of ∼5 ohms/sq. and transmissions over 90%. These transparent conducting barrier films also have significantly enhanced cyclic loading durability and strain tolerance. Finally, despite these substantial improvements, NRELs transparent conducting barrier layers can be integrated with PV devices for two orders of magnitude less cost than ITO. NRELs ultimate goal is to improve the technology and provide water vapor transport rates (WVTRs) lower than 10−6 g/m2-day—the barrier protection needed to enable organic, some thin-film, and 3rd generation photovoltaics.

Atomic layer deposition for aligned growth of and conformal deposition onto double and triple walled carbon nanotubes

We present our work on the growth and functionalization of carbon nanotubes (CNTs). A significant challenge in the growth of aligned single, double and triple walled nanotubes is in the deposition of a controlled thickness catalyst layer. Conventional techniques using line of sight deposition such as sputtering and evaporation produce uniform catalyst layers only when extreme care is taken in the placement of flat substrates. Growth of aligned low wall number carbon nanotubes on contoured, complex geometry, or large surface area substrates is simply not technically feasible through these techniques. Using iron atomic layer deposition (ALD) with ferrocene and oxygen precursors for catalyst deposition circumvents the line of sight problems and allows for uniform coverage across almost all substrates. Furthermore the ALD technique allows for extremely accurate and reproducible thickness depositions. Using these ALD catalyst layers reproducible aligned arrays consisting of primarily double and triple wall CNTS can be fabricated. Conformal coatings onto high aspect ratio surfaces are particularly challenging. The walls of single carbon nanotubes in a nanotube array are inaccessible by line of sight techniques. ALD circumvents this problem by relying on a gas-surface reaction to initiate growth. Generally, growth of ALD films on CNTs results in beading of the deposited materials around CNT defects. This is particularly true of high surface energy materials. The number of nucleation sites and the onset of growth of Pt by ALD can be tuned by use of Ar plasma, O2 plasma and chemical functionalization.

The Influence of Surfaces and Deposition Processes on Pt Structure and Properties

Transparent conductive oxides (TCOs), In-Zn-O (IZO) and Ga-Zn-O (GZO), on glass are used as model substrates to study the effect of surface treatments and deposition processes on Pt growth and nanostructure. The TCO type and surface treatments appear to affect Pt nucleation and growth. Ar and O2 plasma surface treatments significantly lowered the contact angle of water measured on TCOs compared to trimethylaluminum surface treatment and samples without surface treatment. Annealing TCO samples in oxygen resulted in lower IZO conductivity and higher contact angle; while annealing in vacuum or hydrogen resulted in increased carbon on the surface, which appears to be related to higher water contact angles and higher conductivity. Higher amounts of zinc and carbon (probably due to contamination from the annealing chamber) on the IZO surface seem to correlate with lower water contact angles and lower conductivity.

Remote power systems for sensors on the northern border

The National Renewable Energy Laboratory (NREL) is working with the Department of Homeland Security (DHS) [1] to field sensors that accurately track different types of transportation across the northern border of the U.S.. To do this, the sensors require remote power so that they can be placed in the most advantageous geographical locations, often where no grid power is available. This enables the sensors to detect and track aircraft/vehicles despite natural features (e.g., mountains, ridges, valleys, trees) that often prevent standard methods (e.g., monostatic radar or visual observers) from detecting them. Without grid power, portable power systems were used to provide between 80 and 300 W continuously, even in bitter cold and when buried under feet of snow/ice. NREL provides details about the design, installation, and lessons learned from long-term deployment of a second-generation of novel power systems that used adjustable-angle photovoltaics (PV), lithium ion batteries, and fuel cells that provide power to achieve 100% up-time.