Iridium and Ruthenium Catalyzed Syntheses, Hydroborations, and Metathesis Reactions of Alkenyl-Decaboranes

The selective syntheses of new classes of 6,9-dialkenyl- and 6-alkenyl-decaboranes and 6-alkyl-9-alkenyl-decaboranes have been achieved via iridium and ruthenium catalyzed decaborane and 6-alkyl-decaborane alkyne-hydroborations. Reactions employing [Cp*IrCl₂]₂ and [RuCl₂(<i>p</i>-cymene)]₂ precatalysts gave β-E-alkenyl-decaboranes, while the corresponding reactions with [RuI₂(<i>p</i>-cymene)]₂ gave the α-alkenyl-decaborane isomers, with the differences in product selectivity suggesting quite different mechanistic steps for the catalysts. The alkenyl-decaboranes were easily converted to other useful derivatives, including coupled-cage and functionally substituted compounds, via iridium-catalyzed hydroborations and ruthenium-catalyzed homo and cross olefin-metathesis reactions

Electrochemical and Corrosion Stability of Nanostructured Silicon by Graphene Coatings: Toward High Power Porous Silicon Supercapacitors

We demonstrate the electrochemical stability of nanostructured silicon in corrosive aqueous, organic, and ionic liquid media enabled by conformal few-layered graphene heterogeneous interfaces. We demonstrate direct gas-phase few-layered graphene passivation (<i>d</i> = 0.35 nm) at temperatures that preserve the structural integrity of the nanostructured silicon. This passivation technique is transferrable both to silicon nanoparticles (Si-NPs) as well as to electrochemically etched porous silicon (P-Si) materials. For Si-NPs, we find the graphene-passivated silicon to withstand physical corrosion in NaOH aqueous conditions where unpassivated Si-NPs spontaneously dissolve. For P-Si, we demonstrate electrochemical stability with widely different electrolytes, including NaOH, enabling these materials for electrochemical supercapacitors. This leads us to develop high-power on-chip porous silicon supercapacitors capable of up to 10 Wh/kg and 65 kW/kg energy and power densities, respectively, and 5 Wh/kg energy density at 35 kW/kgcomparable to many of the best high-power carbon-based supercapacitors. As surface reactivity wholly dictates the utilization of nanoscale silicon in diverse applications across electronics, energy storage, biological systems, energy conversion, and sensing, we emphasize direct formation of few-layered graphene on nanostructured silicon as a means to form heterogeneous on-chip interfaces that can maintain stability in even the most reactive of environments

Engineered Porous Silicon Counter Electrodes for High Efficiency Dye-Sensitized Solar Cells

In this work, we demonstrate for the first time, the use of porous silicon (P-Si) as counter electrodes in dye-sensitized solar cells (DSSCs) with efficiencies (5.38%) comparable to that achieved with platinum counter electrodes (5.80%). To activate the P-Si for triiodide reduction, few layer carbon passivation is utilized to enable electrochemical stability of the silicon surface. Our results suggest porous silicon as a promising sustainable and manufacturable alternative to rare metals for electrochemical solar cells, following appropriate surface modification

Syntheses of Boron Nitride Nanotubes from Borazine and Decaborane Molecular Precursors by Catalytic Chemical Vapor Deposition with a Floating Nickel Catalyst

Multi- and double-walled boron nitride nanotubes (BNNTs) have been synthesized with the aid of a floating nickel catalyst via the catalytic chemical vapor deposition (CCVD) of either the amine-borane borazine (B₃N₃H₆) or the polyhedral-borane decaborane (B₁₀H₁₄) molecular precursors in ammonia atmospheres. Both sets of BNNTs were crystalline with highly ordered structures. The BNNTs grown at 1200 °C from borazine were mainly double-walled, with lengths up to 0.2 μm and ∼2 nm diameters. The BNNTs grown at 1200–1300 °C from decaborane were double- and multiwalled, with the double-walled nanotubes having ∼2 nm inner diameters and the multiwalled nanotubes (∼10 walls) having ∼4–5 nm inner diameters and ∼12–14 nm outer diameters. BNNTs grown from decaborane at 1300 °C were longer, averaging ∼0.6 μm, whereas those grown at 1200 °C had average lengths of ∼0.2 μm. The BNNTs were characterized using scanning and transmission electron microscopies (SEM and TEM), and electron energy loss spectroscopy (EELS). The floating catalyst method provides a catalytic and potentially scalable route to BNNTs with low defect density from safe and commercially available precursor compounds

Solution Assembled Single-Walled Carbon Nanotube Foams: Superior Performance in Supercapacitors, Lithium-Ion, and Lithium–Air Batteries

We demonstrate a surfactant-free, solution processing route to form three-dimensional freestanding foams of pristine single-walled carbon nanotubes (SWCNTs) and explore the diverse electrochemical energy storage applications of these materials. This route utilizes SWCNT dispersions in organic <i>n</i>-methylpyrrolidone solvents and subsequent electrophoretic assembly onto a metal foam sacrificial template which can be dissolved to yield surfactant-free, binder-free freestanding SWCNT foams. We further provide a comparison between surfactant-free foams and conventional surfactant-based solvent processing routes and assess performance of these foams in supercapacitors, lithium-ion batteries, and lithium–air batteries. For pristine SWCNT foams, we measure up to 83 F/g specific capacitance in supercapacitors, specific capacity up to 2210 mAh/g for lithium-ion batteries with up to 50% energy efficiency, and specific discharge capacity up to 8275 mAh/g in lithium–air batteries. For lithium–air batteries, this corresponds to a total energy density of 21.2 and 3.3 kWh/kg for the active mass and total battery device, respectively, approaching the 12.7 kWh/kg target energy density of gasoline. In comparison, SWCNT foams prepared with surfactant exhibit poorer gravimetric behavior in all devices and compromised Faradaic storage that leads to depreciated amounts of usable, stored energy. This work demonstrates the broad promise of SWCNTs as lightweight and highly efficient energy storage materials but also emphasizes the importance of clean nanomanufacturing routes which are critical to achieve optimized performance with nanostructures

Uniform, Homogenous Coatings of Carbon Nanohorns on Arbitrary Substrates from Common Solvents

We demonstrate a facile technique to electrophoretically deposit homogenous assemblies of single-walled carbon nanohorns (CNHs) from common solvents such as acetone and water onto nearly any substrate including insulators, dielectrics, and three-dimensional metal foams, in many cases without the aid of surfactants. This enables the generation of pristine film-coatings formed on time scales as short as a few seconds and on three-dimensional templates that enable the formation of freestanding polymer-CNH supported materials. As electrophoretic deposition is usually only practical on conductive electrodes, we emphasize our observation of efficient deposition on nearly any material, including nonconductive substrates. The one-step versatility of deposition on these materials provides the capability to directly assemble CNH materials onto functional surfaces for a broad range of applications. In this manner, we utilized as-deposited CNH films as conductometric gas sensors exhibiting better sensitivity in comparison to equivalent single-walled carbon nanotube sensors. This gives a route toward scalable and inexpensive solution-based processing routes to manufacture functional nanocarbon materials for catalysis, energy, and sensing applications, among others

A Multifunctional Load-Bearing Solid-State Supercapacitor

A load-bearing, multifunctional material with the simultaneous capability to store energy and withstand static and dynamic mechanical stresses is demonstrated. This is produced using ion-conducting polymers infiltrated into nanoporous silicon that is etched directly into bulk conductive silicon. This device platform maintains energy densities near 10 W h/kg with Coulombic efficiency of 98% under exposure to over 300 kPa tensile stresses and 80 g vibratory accelerations, along with excellent performance in other shear, compression, and impact tests. This demonstrates performance feasibility as a structurally integrated energy storage material broadly applicable across renewable energy systems, transportation systems, and mobile electronics, among others