Joule Heating in Controlled Atmospheres to Process Nanocarbon/Transition Metal Oxide Composites and Electrodes.

Composites of nanocarbons and transition metal oxides combine excellent mechanical properties and high electrical conductivity with high capacitive active sites. These composites are promising for applications such as electrochemical energy conversion and storage, catalysis, and sensing. Here, we show that Joule heating can be used as a rapid out-of-oven thermal processing technique to crystallize the inorganic metal oxide matrix within a carbon nanotube fabric (CNTf) composite. We choose manganese oxide and vanadium oxide as model metal oxides and show that the Joule heating process is rapid and enables accurate control over the temperature and phase transitions. Next, we use thermogravimetric analysis and Joule heating experiments in controlled atmospheres to show that metal oxides can actually catalyze thermal degradation and reduce the thermal stability of the CNTs, which could limit processing of many oxides. We solve this by using a reducing hydrogen atmosphere to successfully extend the Joule processing window and thermal stability of the CNTf/metal oxide composite to ∼1000 °C.

Gas-to-nanotextile: high-performance materials from floating 1D nanoparticles.

Suspended in the gas phase, 1D inorganic nanoparticles (nanotubes and nanowires) grow to hundreds of microns in a second and can be thus directly assembled into freestanding network materials. The corresponding process continuously transforms gas precursors into aerosols into aerogels into macroscopic nanotextiles. By enabling the assembly of very high aspect ratio nanoparticles, this processing route has translated into high-performance structural materials, transparent conductors and battery anodes, amongst other embodiments. This paper reviews progress in the application of such manufacturing process to nanotubes and nanowires. It analyses 1D nanoparticle growth through floating catalyst chemical vapour deposition (FCCVD), in terms of reaction selectivity, scalability and its inherently ultra-fast growth rates (107-108 atoms per second) up to 1000 times faster than for substrate CVD. We summarise emerging descriptions of the formation of aerogels through percolation theory and multi-scale models for the collision and aggregation of 1D nanoparticles. The paper shows that macroscopic ensembles of 1D nanoparticles resemble textiles in their porous network structure, high flexibility and damage-tolerance. Their bulk properties depend strongly on inter-particle properties and are dominated by alignment and volume fraction. Selected examples of nanotextiles that surpass granular and monolithic materials include structural fibres with polymer-like toughness, transparent conductors, and slurry-free composite electrodes for energy storage.

Processing of Composite Electrodes of Carbon Nanotube Fabrics and Inorganic Matrices via Rapid Joule Heating.

Composites of nanocarbon network structures are interesting materials, combining mechanical properties and electrical conductivity superior to those of granular systems. Hence, they are envisaged to have applications as electrodes for energy storage and transfer. Here, we show a new processing route using Joule heating for a nanostructured network composite of carbon nanotube (CNT) fabrics and an inorganic phase (namely, MoS2), and then study the resulting structure and properties. To this end, first, a unidirectional fabric of conductive CNT bundles is electrochemically coated with MoS2. Afterward, the conformally coated inorganic phase is crystallized via heat generated by direct current passing through the CNT ensemble. The Joule heating process is rapid (maximum heating rate up to 31.7 °C/s), enables accurate temperature control, and takes only a few minutes. The resulting composite material combines a high electrical conductivity of up to 1.72 (±0.25) × 105 S/m, tensile modulus as high as 8.82 ± 5.5 GPa/SG, and an axial tensile strength up to 200 ± 58 MPa/SG. Both electrical and mechanical properties are orders of magnitude above those of wet-processed nanocomposites of similar composition. The extraordinary longitudinal properties stem from the network of interconnected and highly aligned CNT bundles. Conductivity and modulus follow approximately a rule of mixtures, similar to a continuous fiber composite, whereas strength scales almost quadratically with the mass fraction of the inorganic phase due to the inorganic constraining realignment of CNTs upon stretching. This processing route is applicable to a wide range of nanocarbon-based composites with inorganic phases, leading to composites with specific strength above steel and electrical conductivity beyond the threshold for electronic limitations in battery electrodes.

Highly Oriented Direct-Spun Carbon Nanotube Textiles Aligned by In Situ Radio-Frequency Fields.

Carbon nanotubes (CNTs) individually exhibit exceptional physical properties, surpassing state-of-the-art bulk materials, but are used commercially primarily as additives rather than as a standalone macroscopic product. This limited use of bulk CNT materials results from the inability to harness the superb nanoscale properties of individual CNTs into macroscopic materials. CNT alignment within a textile has been proven as a critical contributor to narrow this gap. Here, we report the development of an altered direct CNT spinning method based on the floating catalyst chemical vapor deposition process, which directly interacts with the self-assembly of the CNT bundles in the gas phase. The setup is designed to apply an AC electric field to continuously align the CNTs in situ during the formation of CNT bundles and subsequent aerogel. A mesoscale CNT model developed to simulate the alignment process has shed light on the need to employ AC rather than DC fields based on a CNT stiffening effect (z-pinch) induced by a Lorentz force. The AC-aligned synthesis enables a means to control CNT bundle diameters, which broadened from 16 to 25 nm. The resulting bulk CNT textiles demonstrated an increase in the specific electrical and tensile properties (up to 90 and 460%, respectively) without modifying the quantity or quality of the CNTs, as verified by thermogravimetric analysis and Raman spectroscopy, respectively. The enhanced properties were correlated to the degree of CNT alignment within the textile as quantified by small-angle X-ray scattering and scanning electron microscopy image analysis. Clear alignment (orientational order parameter = 0.5) was achieved relative to the pristine material (orientational order parameter = 0.19) at applied field intensities in the range of 0.5-1 kV cm-1 at a frequency of 13.56 MHz.

Controlling reaction paths for ultra-fast growth of inorganic nanowires floating in the gas phase.

Synthesis of inorganic nanowires/nanotubes suspended in the gas through floating catalyst chemical vapour deposition (FCCVD) produces exceptional growth rates of 5-1000 micron per second, several orders of magnitude faster than conventional substrate processes. It leads to nanowire lengths >100 microns and thus to the possibility of direct assembly into freestanding macroscopic networks as a continuous process. This work studies the different reaction paths controlling conversion and selectivity in FCCVD applied to the synthesis of silicon nanowires (SiNWs) from silane, grown through an aerosol of gold catalyst nanoparticles. There are two main competing reactions: catalysed growth of SiNWs and non-catalysed formation of amorphous Si nanoparticles. The mass fraction of the two populations can be precisely determined by XRD and Raman spectroscopy, enabling high-throughput screening of reaction parameter space. The experimental data and accompanying analytical model show that selectivity is kinetically controlled by the ratio of precursor/hydrogen carrier gas, through its inhibition of the pyrolisis of silane into silylene. In contrast, the rate of SiNW growth is largely unaffected by hydrogen and not limited by precursor availability. These results provide a framework to describe the kinetics of nanomaterials growth by FCCVD.

Synthesis of NiMnO3/C nano-composite electrode materials for electrochemical capacitors.

Demand for high-performance energy storage materials has motivated research activities to develop nano-engineered composites that benefit from both high-rate and high-capacitance materials. Herein, NiMnO3 (NMO) nanoparticles have been synthesized through a facile co-precipitation method. As-prepared NMO samples are then employed for the synthesis of nano-composites with graphite (Gr) and reduced graphene oxide (RGO). Various samples, including pure NMO, NMO-graphite blend, as well as NMO/Gr and NMO/RGO nano-composites have been electrochemically investigated as active materials in supercapacitors. The NMO/RGO sample exhibited a high specific capacitance of 285 F g(-1) at a current density of 1 A g(-1), much higher than the other samples (237 F g(-1) for NMO/Gr, 170 F g(-1) for NMO-Gr and 70 F g(-1) for NMO). Moreover, the NMO/RGO nano-composite has shown excellent cycle stability with a 93.5% capacitance retention over 1000 cycles at 2 A g(-1) and still delivered around 87% of its initial capacitance after cycling for 4000 cycles. An NMO/RGO composite was assessed in practical applications by assembling NMO/RGO//NMO/RGO symmetric devices, exhibiting high specific energy (27.3 Wh kg(-1)), high specific power (7.5 kW kg(-1)), and good cycle stability over a broad working voltage of 1.5 V. All the obtained results demonstrate the promise of NMO/RGO nano-composite as a high-performance electrode material for supercapacitors.

Facile synthesis of nanostructured CuCo2O4 as a novel electrode material for high-rate supercapacitors.

CuCo2O4 nanostructures were synthesized through a facile solution combustion method. Electrochemical investigations demonstrate a novel electrode material for supercapacitors with remarkable performance including high-rate capability, high-power density (22.11 kW kg(-1)) and desirable cycling stability at different current densities.

Morphologically controlled preparation of CuO nanostructures under ultrasound irradiation and their evaluation as pseudocapacitor materials.

Various morphologies of copper oxide (CuO) nanostructures have been synthesized by controlling the reaction parameters in a sonochemical assisted method without using any templates or surfactants. The effect of reaction parameters including molar ratio of the reactants, reaction temperature, ultrasound exposure time, and annealing temperature on the composition and morphology of the product(s) has been investigated. The prepared samples have been characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDAX), and thermogravimetric analysis (TGA). It has been found that Cu2(OH)3NO3 nanoplatelets are achieved in mild conditions which can be then converted to various morphologies of CuO nanostructures by either using high concentrations of OH(-) (formation of nanorods), prolonging sonication irradiation (nanoparticles), or thermal treatment (nanospheres). Application of the prepared CuO nanostructures was evaluated as supercapacitive material in 1 M Na2SO4 solution using cyclic voltammetry (CV) in different potential scan rates ranging from 5 to 100 mV s(-1). The specific capacitance has been calculated using CV curves. It has been found that the pseudocapacitor performance of CuO can be tuned via employing morphologically controlled samples. Accordingly, the prolonged sonicated sample (nanoparticles) showed the high specific capacitance of 158 F.g(-1).