Katherine A. Pettigrew

Electroless deposition of conformal nanoscale iron oxide on carbon nanoarchitectures for electrochemical charge storage.

We describe a simple self-limiting electroless deposition process whereby conformal, nanoscale iron oxide (FeO(x)) coatings are generated at the interior and exterior surfaces of macroscopically thick ( approximately 90 microm) carbon nanofoam paper substrates via redox reaction with aqueous K(2)FeO(4). The resulting FeO(x)-carbon nanofoams are characterized as device-ready electrode structures for aqueous electrochemical capacitors and they demonstrate a 3-to-7 fold increase in charge-storage capacity relative to the native carbon nanofoam when cycled in a mild aqueous electrolyte (2.5 M Li(2)SO(4)), yielding mass-, volume-, and footprint-normalized capacitances of 84 F g(-1), 121 F cm(-3), and 0.85 F cm(-2), respectively, even at modest FeO(x) loadings (27 wt %). The additional charge-storage capacity arises from faradaic pseudocapacitance of the FeO(x) coating, delivering specific capacitance >300 F g(-1) normalized to the content of FeO(x) as FeOOH, as verified by electrochemical measurements and in situ X-ray absorption spectroscopy. The additional capacitance is electrochemically addressable within tens of seconds, a time scale of relevance for high-rate electrochemical charge storage. We also demonstrate that the addition of borate to buffer the Li(2)SO(4) electrolyte effectively suppresses the electrochemical dissolution of the FeO(x) coating, resulting in <20% capacitance fade over 1000 consecutive cycles.

Electroless Deposition of Nanoscale MnO2 on Ultraporous Carbon Nanoarchitectures: Correlation of Evolving Pore-Solid Structure and Electrochemical Performance

The self-limiting redox reaction of carbon nanofoam substrates with permanganate at room temperature in neutral-pH solutions produces conformal nanoscale MnO 2 deposits throughout the macroscopic thickness (∼0.17 mm) of the nanofoam structure. The nanoscale MnO 2 morphology ranges from ∼10 nm layered ribbons and rods for a 4 h deposition to ∼20 nm polycrystalline nanoparticles that form at long deposition times (20 h). The through-connected pore network of the carbon nanofoam is maintained at all deposition times (5 min to 20 h), although the average pore size shifts to smaller values and the cumulative pore volume decreases as the MnO 2 coatings grow and thicken within the nanofoam structure. The electrochemical capacitance of the resulting hybrid electrode structure is dominated by the pseudocapacitance of the MnO 2 and increases with MnO 2 loading (a function of the exposure time in permanganate), particularly at low charge-discharge rates and at ac frequencies <0.1 Hz. The significant enhancement in mass-, volume-, and footprint-normalized capacitance at high MnO 2 mass loadings is accompanied by a modest increase in the Warburg resistance that develops as the pore size and void volume of the nanofoam substrate are reduced by internal MnO 2 deposition.

Solution reduction synthesis of surface stabilized silicon nanoparticles

This paper describes the preparation of air and moisture stable octanol derivatized crystalline silicon nanoparticles by room temperature sodium naphthalenide reduction of silicon halides.

Electrochemical Li-ion storage in defect spinel iron oxides: the critical role of cation vacancies

Rechargeable lithium-ion batteries are the preferred power source for consumer electronic devices, but the cost and toxicity of many cathode materials limit their scale-up. Worldwide research efforts are addressing this concern by transitioning from conventional Co- and Ni-based intercalation hosts towards Fe- and Mn-based alternatives. The unfavorable energetics of the Fe2+/3+redox couple and limited Li-insertion capacities render the use of iron oxides impractical. We address this limitation with the defect spinel ferrite γ-Fe2O3 as a model structure for Li-ion insertion by replacing a fraction of the Fe3+ sites with highly oxidized Mo6+ to generate cation vacancies that shift the onset of Li-ion insertion to more positive potentials as well as increase capacity. In the present study, native and Mo-substituted iron oxides are synthesized via base-catalyzed precipitation in aqueous media, yielding nanocrystalline spinel materials that also exhibit short-range disorder characteristic of a proton-stabilized structure. The Mo-substituted ferrite reported herein is estimated to have ∼3× as many cation vacancies as γ-Fe2O3 with a corresponding increase in the Li-ion capacity to >100 mA h g−1 between 4.1 and 2.0 V vs.Li/Li+. This dual enhancement in capacity and insertion potential will enable these and related defect spinel ferrites to be explored as positive electrode materials for lithium batteries, while retaining the cost advantages of a material whose metal composition is still predominately iron based.

Making the most of a scarce platinum-group metal: conductive ruthenia nanoskins on insulating silica paper.

Subambient thermal decomposition of ruthenium tetroxide from nonaqueous solution onto porous SiO(2) substrates creates 2-3 nm thick coatings of RuO(2) that cover the convex silica walls comprising the open, porous structure. The physical properties of the resultant self-wired nanoscale ruthenia significantly differ depending on the nature of the porous support. Previously reported RuO(2)-modified SiO(2) aerogels display electron conductivity of 5 x 10(-4) S cm(-1) (as normalized to the geometric factor of the insulating substrate, not the conducting ruthenia phase), whereas RuO(2)-modified silica filter paper at approximately 5 wt % RuO(2) exhibits approximately 0.5 S cm(-1). Electron conduction through the ruthenia phase as examined from -160 to 260 degrees C requires minimal activation energy, only 8 meV, from 20 to 260 degrees C. The RuO(2)(SiO(2)) fiber membranes are electrically addressable, capable of supporting fast electron-transfer reactions, express an electrochemical surface area of approximately 90 m(2) g(-1) RuO(2), and exhibit energy storage in which 90% of the total electron-proton charge is stored at the outer surface of the ruthenia phase. The electrochemical capacitive response indicates that the nanocrystalline RuO(2) coating can be considered to be a single-unit-thick layer of the conductive oxide, as physically stabilized by the supporting silica fiber.

Nickel ferrite aerogels with monodisperse nanoscale building blocks--the importance of processing temperature and atmosphere.

Using two-step (air/argon) thermal processing, sol-gel-derived nickel-iron oxide aerogels are transformed into monodisperse, networked nanocrystalline magnetic oxides of NiFe(2)O(4) with particle diameters that can be ripened with increasing temperature under argon to 4.6, 6.4, and 8.8 nm. Processing in air alone yields poorly crystalline materials; heating in argon alone leads to single phase, but diversiform, polydisperse NiFe(2)O(4), which hampers interpretation of the magnetic properties of the nanoarchitectures. The two-step method yields an improved model system to study magnetic effects as a function of size on the nanoscale while maintaining the particles within the size regime of single domain magnets, as networked building blocks, not agglomerates, and without stabilizing ligands capping the surface.

Metal-catalyzed graphitic nanostructures as sorbents for vapor-phase ammonia

Activated carbons have long been used as substrates for the filtration of vapor-phase molecules, often with metal salts or oxides added to improve their sorption capacities for specific agents, but their real-world performance and applicability may be hindered by such factors as long-term stability and complex processing. En route to a new class of carbon-based sorbents, we have developed solid-state synthetic methods to produce bulk carbonaceous solids based on the pyrolysis of thermoset solids containing low concentrations (<1 wt%) of metal precursors (based on either Ni, Fe, or Co) that decompose in situ to catalyze the formation of a complex graphitic nanostructure. Selective combustion of residual amorphous carbon from the pyrolyzed solid generates a mesoporous network that facilitates diffusional transport of gas-phase molecules to the interior surfaces of the solid, and also converts the entrained metals to their respective metal oxide forms. We examine the ammonia-sorption properties of a series of these graphitic nanostructured compositions, and demonstrate that ammonia uptake is primarily determined by the type of residual metal oxide, with the Co-containing carbonaceous solid providing the best ammonia-sorption capacity (1.76 mol kg−1). Thermal reduction of the Co-containing material drastically decreases its ammonia-sorption capacity, showing that the oxide form (in this case Co3O4) of the entrained metal nanoparticles is most active for ammonia filtration. The effects of the carbon–oxygen functionalities on the nanostructured graphitic surfaces for ammonia sorption are also discussed.

Achieving electrochemical capacitor functionality from nanoscale LiMn2O4 coatings on 3-D carbon nanoarchitectures

Conformal nanoscale coatings of Na+-birnessite manganese oxide (MnOx) produced via redox reaction between aqueous permanganate (NaMnO4·H2O) and the carbon surfaces of fiber-paper-supported carbon nanofoams are converted to LiMn2O4 spinel through topotactic exchange of Na+ for Li+ in the as-deposited lamellar birnessite, followed by mild thermal treatments to complete the transformation to LiMn2O4. The evolution of the birnessite-to-spinel conversion is verified with X-ray diffraction, solid-state nuclear magnetic resonance, X-ray absorption spectroscopy, electron microscopy, cyclic voltammetry, and electrochemical impedance spectroscopy. The mild conditions used to convert birnessite to spinel ensure that the conformal nanoscale nature of the oxide coating is retained throughout the macroscopically thick (170 μm) carbon nanofoam substrate during the conversion process. The architecture of the LiMn2O4–carbon nanofoam facilitates rapid ion/electron transport, enabling the LiMn2O4 to insert and extract Li+ from aqueous electrolytes at scan rates as high as 25 mV s−1, and with a relaxation time of 37 s as derived from electrochemical impedance. This architectural expression of nanoscale LiMn2O4 delivers full theoretical capacity (148 mA h g−1) at 2 mV s−1.

The importance of combining disorder with order for Li-ion insertion into cryogenically prepared nanoscopic ruthenia

Cryogenically prepared RuO2 (cryo-RuO2), a material known for its ability to “self-wire” into continuous, nanoscopic electronic pathways, is proposed as an electrode for Li-ion microbatteries with three-dimensionally interpenetrated components. We determined processing guidelines that optimize Li-ion uptake in cryo-RuO2 powders by varying the solid-state structure of cryo-RuO2 with thermal processing at 50–250 °C in flowing O2(g) or Ar(g). The highly disordered structure of as-prepared cryo-RuO2 is transformed to rutile RuO2 at 200 °C in O2(g), resulting in a 60% loss of Li-ion capacity (as-prepared: 214 mA h g−1; rutile: 84 mA h g−1). In contrast, thermal processing in Ar(g) preserves structural disorder in the cryo-RuO2, even up to 250 °C. The highest Li-ion capacity occurs for the treatment that mixes order (crystallinity) with disorder: >250 mA h g−1 for cryo-RuO2 heated in oxygen to 50 °C. This study provides processing guidelines to achieve fabrication of 3-D microbattery architectures containing a nanoscopic RuO2 electrode component.

Surface Passivated Air and Moisture Stable Mixed Zirconium Aluminum Metal-Hydride Nanoparticles

Synthesis of surface passivated Zr-Al mixed metal-hydride nanoparticles was accomplished via a multi-step process. The initial reaction to produce the zirconium aluminum hydride was via decomposition of zirconium tetrahydroaluminate (Zr(AlH4)4) while exposed to ultrasound produced by a bench-top ultrasonic cleaning bath. The particles were surface passivated using carbohydrates and were shown to be stable in air and partially stable in water. TEM imaging suggests the existence of smaller particles made of a Zr-Al alloy that range in size from 1.8 nm to 7.9 nm in diameter and are interspersed with larger particles that range from tens to hundreds of nanometers in diameter. It was also shown that the carbohydrate-derived coating of the nanoparticles is present as an aluminum alkoxide gel surrounding the core particles. INTRODUCTION Metal-hydride nanoparticles have been sought after, for one, due to their potential utility as hydrogen storage materials. There have been several patents issued, as well as several publications in recent years for metal-hydride nanoparticle materials. The authors of a recent review on nano-engineered hydrogen storage materials asserted that nanoparticle materials decrease the active particle size increasing surface area and grain boundaries, in turn improving the adsorption and desorption kinetics, decreasing the enthalpy of formation and thereby decreasing the release temperature of metal hydrides. A theoretical investigation by Clark et al. of various Zr-Al materials showed such materials as potentially having good hydrogen storage properties. It has recently been reported that NaAlH4 acquires significantly improved hydrogen adsorption and desorption properties when doped with Ti or Zr. Larger scale synthesis of air and moisture sensitive metal nanoparticles is a challenge, and poses an obstacle to investigating the physical properties of these materials via traditional techniques that would expose them to air. To obtain materials that do not oxidize when handled in air while still retaining certain useful properties, the surface of the nanoparticles can be protected by a passivating agent. In order to keep the intrinsic properties of the unpassivated material it is important to maximize the active metal content of the material while minimizing the amount of passivator present on the nanoparticle surface. We report a homogeneous solution-based method used to produce well-defined passivated air and moisture stable Zr-Al partial hydride nanoparticle materials on gram scale. Herein we report the initial characterization of these particles by Al magic angle spinning (MAS) NMR, SEM, TEM, oxygen bomb calorimetry, and microanalysis. EXPERIMENT General. All air and moisture sensitive manipulations were performed in a Vacuum Atmospheres glove box under an atmosphere of helium or via traditional Schlenk technique under an atmosphere of nitrogen. Dry diethyl ether (Et2O) was purchased from Aldrich packaged under Mater. Res. Soc. Symp. Proc. Vol. 1056