Carbon based nanostructures for electrochemical energy storage

Nowadays, the decreasing availability of fossil fuels has made the demand for efficient storage and usage of energy urgent than ever. Recently, two types of electrochemical energy storage devices have caught extensive attention, namely lithium ion batteries (LIBs) and electrochemical capacitors (ECs). LIBs are now serving as the most important power sources for mobile electronics, while ECs are rising rapidly as an important complement to LIBs, because of their capability to deliver much higher power density as well as longer life time. However, almost all of the advanced materials for anodes in LIBs or electrodes in ECs face the problems of structural failure and hence severe energy density fading, which have dramatically hindered the further development of these systems. In this project, fabricating these materials into nanostructures and combining them with advanced carbon materials, such as carbon nanotubes (CNTs) and ultrathin graphite (UG), are employed as an effective solution. The obtained carbon based nanostructures have been successfully demonstrated as promising candidates for high performance anode materials in LIBs and/or electrode materials for ECs. The work starts with the growth of CNT network on stainless steel (SS) substrate by chemical vapor deposition (CVD). Molybdenum disulfide (MoS2), a typical type of TMDs, is then decorated onto the CNT network through hydrothermal reaction and annealing treatment. The rationally designed MoS2/CNT composite can be directly tested as an anode material in LIBs, without the necessity for any binders. Encouragingly, a high and reversible capacity over 1300 mAh•g−1 is retained for 50 cycles at the current density of 200 mA•g−1, while excellent rate performance is also delivered because of the enhanced structural stability and reaction kinetics in the material. Secondly, vertical aligned arrays of CNTs are fabricated through plasma enhanced CVD (PECVD). The CNT array can be a good template for Si, which is of the highest theoretical capacity among alloy-type anodes in LIBs. By introducing rough Ni layers in between the CNT cores and the Si shells, enhanced adherence, accelerated charge transport and more effective strain relaxation can be achieved. The core-shell Si/Ni/CNT anode manages to provide a high capacity of over 2500 mAh•g−1 with a low fading rate of merely 0.2% per cycle over 110 cycles. The CNT arrays are also used to support NiCo2O4, a novel type of TMOs, with advantages in electrical conductivity and ionic reactivity. The coating of NiCo2O4 is realized by a facile electrochemical deposition method followed by subsequent annealing in air. The NiCo2O4/CNT arrays obtained demonstrate outstanding performances as anodes in LIBs, including a high capacity (1147.6 mAh•g−1 at 100 mA•g−1), excellent cycling retention (no capacity fading over 200 cycles) and outstanding rate capability (712.9 mAh•g−1 at 1000 mA•g−1). The applications of NiCo2O4/CNT core-shell structures are not limited to LIBs but also suitable for ECs. When tested in a three-electrode configuration, the as-prepared NiCo2O4/CNT structures exhibit a specific capacitance of 695 F•g−1 at the current density of 1 A•g−1 and 576 F•g−1 at 20 A•g−1. Hence, the capacitance of the NiCo2O4/CNT electrode remains 91% of its initial value after 1500 cycles at the current density of 4 A•g−1. Furthermore, this structure is enhanced by using the substrate of 3D UG-coated Ni foams, resulting in the architecture of NiCo2O4/CNT/UG on Ni foams. The fabrication processes consist of sequential CVD growth of 3D UG on Ni foam and CNT network on the surface of UG, respectively and hydrothermal and annealing preparation of NiCo2O4 nanorods. This architecture brings advantages including huge surface area, enhanced active material loading and facile electrolyte access. A high capacitance of 973.0 F•g−1 at the current density of 1 A•g−1, outstanding rate performance (capacitance of 715.9 F•g−1 at 15 A•g−1) and also stable cycling at 5 A•g−1 have been successfully demonstrated.DOCTOR OF PHILOSOPHY (EEE

A three dimensional vertically aligned multiwall carbon nanotube/NiCo2O4 core/shell structure for novel high-performance supercapacitors

Three dimensional (3D) vertically aligned structures have attracted tremendous attention from scientists in many fields due to their unique properties. In this work, we have built the 3D vertically aligned carbon nanotube (CNT)/NiCo2O4 core/shell nanoarchitecture via a facile electrochemical deposition method followed by subsequent annealing in air. The morphology and structure have been in-depth characterized by SEM, TEM, XRD and Raman spectroscopy. Impressively, when used as the electrode material in a 6 M KOH electrolyte, the vertically aligned CNT/NiCo2O4 core/shell structures exhibit excellent supercapacitive performances, including high specific capacitance, excellent rate capability and good cycle stability. This is due to the unique 3D vertically aligned CNT/NiCo2O4 core/shell structures, which support high electron conductivity, large surface area of NiCo2O4 and fast ion/electron transport in the electrode and at the electrolyte–electrode interface. Furthermore, the synthesis strategy presented here can be easily extended to fabricate other metal oxides with a controlled core/shell structure, which may be a promising facile strategy for high performance supercapacitors, and even advanced Li-ion batteries.Published versio

Fabrication of Patterned Graphene FETs Array

AbstractA new approach of fabrication of back-gated graphene FETs array based on the nano-wall channel between the source and the drain were investigated. Patterned metal film on three-dimentional nano-wall structure prepared by photo lithography was used as the source and the drain to bond the graphite foil. With SU-8 process in MEMS and lift off processes, metal is sputtered exactly onto the SU-8 patterns and forms electrodes.The potential transistor design relying only on a single sheet could be achieved by placing the graphene sheet film on the nano-wall channel between the source and the drain. Chemically derived graphene samples are then transferred onto 300nm SiO2/highly doped Si which serves as the back gate.A gas sensing region is expected to be present because the graphene sheet segment has great adsorption capacity of gas with the help of the nano-wall structure betwee the source and the drain strips. This method has offered potential convenience for future research on graphene properties, such as the anomalously quantized Hall effects, large charge carrier mobility and so on, and demonstrated a great potential application of novel structured FETs based on graphene

Mesoporous NiCo2O4 nano-needles supported by 3D interconnected carbon network on Ni foam for electrochemical energy storage

In this work, a three dimensional (3D) interconnected carbon network consisting of ultrathin graphite (UG) and carbon nanotubes (CNTs) on Ni foam is fabricated and employed as a novel type of substrate for mesoporous NiCo 2 O 4 nano-needles. The successfully synthesized NiCo 2 O 4 nano-needles/CNTs/UG on Ni foam has many advantages including facile electrolyte access and direct conducting pathways towards current collectors, which enable it to be a promising electrode material in battery-like electrochemical energy storage. Encouragingly, a high capacity of 135.1 mAh/g at the current density of 1 A/g, superior rate performance and also stable cycling for 1200 cycles at the current density of 5 A/g have been demonstrated in this novel material

High-Performance Microsupercapacitors Based on Two-Dimensional Graphene/Manganese Dioxide/Silver Nanowire Ternary Hybrid Film

Microsupercapacitors (MSCs), as one type of significant power source or energy storage unit in microelectronic devices, have attracted more and more attention. However, how to reasonably design electrode structures and exploit the active materials to endow the MSCs with excellent performances in a limited surface area still remains a challenge. Here, a reduced graphene oxide (RGO)/manganese dioxide (MnO₂)/silver nanowire (AgNW) ternary hybrid film (RGMA ternary hybrid film) is successfully fabricated by a facile vacuum filtration and subsequent thermal reduction, and is used directly as a binder-free electrode for MSCs. Additionally, a flexible, transparent, all-solid state RMGA-MSC is also built, and its electrochemical performance in an ionic liquid gel electrolyte are investigated in depth. Notably, the RGMA-MSCs display superior electrochemical properties, including exceptionally high rate capability (up to 50000 mV·s^–1), high frequency response (very short corresponding time constant τ₀ = 0.14 ms), and excellent cycle stability (90.3% of the initial capacitance after 6000 cycles in ionic liquid gel electrolyte). Importantly, the electrochemical performance of RGMA-MSCs shows a strong dependence on the geometric parameters including the interspace between adjacent fingers and the width of the finger of MSCs. These encouraging results may not only provide important references for the design and fabrication of high-performance MSCs, but also make the RGMA ternary hybrid film promising for the next generation film lithium ion batteries and other energy storage devices

Solid source growth of Si oxide nanowires promoted by carbon nanotubes

We report a method to promote solid source growth of Si oxide nanowires (SiONWs) by using an array of vertically aligned carbon nanotubes (CNTs). It starts with the fabrication of CNT array by plasma enhanced chemical vapor deposition (PECVD) on Si wafers, followed by growth of SiONWs. Herein, CNTs serve as a scaffold, which helps the dispersion of catalysts for SiONWs and also provides space for hydrogen which boosts the diffusion of Si atoms and hence formation of SiONWs. As the result, a three dimensional (3D) hybrid network of densely packed SiONWs and CNTs can be produced rapidly.Accepted versio

Thermal conductivity characterization of three dimensional carbon nanotube network using freestanding sensor-based 3ω technique

A novel three-dimensional (3D) carbon nanotube (CNT) network, composed of vertically aligned CNT array (primary CNT) bridged with randomly oriented secondary CNT, is synthesized in this work. We report the first data for the thermal properties of this new structure using freestanding sensor-based 3ω technique. Introducing freestanding sensor to conventional 3ω system enables the nondestructive characterization for samples with rough surfaces. The thermal conductivities of CNT films, as well as the contact resistance between the sensor and sample surfaces, are extracted numerically by a finite-element thermal model. The thermal conductivities of 3D CNT network under different array densities range from 9.3 to 19.8 W/mK. It is found that at lower CNT array density of 5.6 × 108/cm2, the growth of secondary CNT enhances the thermal conductivity of primary CNT array by 55.9%. This significant improvement in thermal conductivity can be attributed to the additional thermal pathway provided by the secondary CNTs in the primary CNT forest. However as the density of primary CNT array increases beyond 7.2 × 108/cm2, the growth of secondary CNTs on primary CNT forest reduces its thermal conductivity. This reduction in thermal conductivity can possibly be caused by the excessive thermal resistance from the CNT-CNT connection points within 3D CNT network.MOE (Min. of Education, S’pore

Thermal conductivity enhancement of carbon@ carbon nanotube arrays and bonded carbon nanotube network

Carbon nanotubes (CNTs) are long considered as a promising material for thermal applications. However, problems such as low volume CNT fraction abhorrent to practical applications have been raising the demand for novel architecture of this material. Here we demonstrate two fabrication methods, in which a self-assembly method for fabricating covalent-bonded CNT network (3D CNT) and another method for covalent-bonded C to CNTs (C@CNT) network, and presented both as a potential method to enhance thermal conductivity of CNT arrays. We utilized pulsed photothermal reflectance technique and using new four-layer heat conduction model based on the transmission-line theory to measure thermal conductivity of the samples. The 3D CNT with thermal conductivity of 21 W mK(-1) and C@CNT with thermal conductivity of 26 W mK(-1) turn out to be an excellent candidate for thermal interface material as the thermal conductivity increased by 40% and 70% respectively as compared to conventional CNT arrays. The improvement is attributed to the efficient thermal routines constructed between CNTs and secondary CNTs in 3D CNT and between C layer and CNTs in C@CNT. The other factor to improve thermal conductivity of the samples is decreasing air volume fraction in CNT arrays. Our fabrication methods provide a simple method but effective way to fabricate 3D CNT and C@CNT and extend the possibility of CNTs towards TIM application

Carbon nanostructures dedicated to millimeter-wave to THz interconnects

This paper focuses on the use of CNTs for new mm-to-THz interconnects for nanopackaging. To successfully integrate CNT to be in line with nanoelectronics trends, new growth processes and modeling approaches are proposed. Several experimental works are presented such as millimeter-wave flip-chip bonding. In addition, novel THz 3-D wireless interconnect, based on CNT monopole antennas, working at 200 and 300 GHz are designed, simulated, and fabricated