Victor L. Pushparaj

Flexible energy storage devices based on nanocomposite paper

There is strong recent interest in ultrathin, flexible, safe energy storage devices to meet the various design and power needs of modern gadgets. To build such fully flexible and robust electrochemical devices, multiple components with specific electrochemical and interfacial properties need to be integrated into single units. Here we show that these basic components, the electrode, separator, and electrolyte, can all be integrated into single contiguous nanocomposite units that can serve as building blocks for a variety of thin mechanically flexible energy storage devices. Nanoporous cellulose paper embedded with aligned carbon nanotube electrode and electrolyte constitutes the basic unit. The units are used to build various flexible supercapacitor, battery, hybrid, and dual-storage battery-in-supercapacitor devices. The thin freestanding nanocomposite paper devices offer complete mechanical flexibility during operation. The supercapacitors operate with electrolytes including aqueous solvents, room temperature ionic liquids, and bioelectrolytes and over record temperature ranges. These easy-to-assemble integrated nanocomposite energy-storage systems could provide unprecedented design ingenuity for a variety of devices operating over a wide range of temperature and environmental conditions.

Continuous Carbon Nanotube Reinforced Composites

Carbon nanotubes are considered short fibers, and polymer composites with nanotube fillers are always analogues of random, short fiber composites. The real structural carbon fiber composites, on the other hand, always contain carbon fiber reinforcements where fibers run continuously through the composite matrix. With the recent optimization in aligned nanotube growth, samples of nanotubes in macroscopic lengths have become available, and this allows the creation of composites that are similar to the continuous fiber composites with individual nanotubes running continuously through the composite body. This allows the proper utilization of the extreme high modulus and strength predicted for nanotubes in structural composites. Here, we fabricate such continuous nanotube polymer composites with continuous nanotube reinforcements and report that under compressive loadings, the nanotube composites can generate more than an order of magnitude improvement in the longitudinal modulus (up to 3,300%) as well as damping capability (up to 2,100%). It is also observed that composites with a random distribution of nanotubes of same length and similar filler fraction provide three times less effective reinforcement in composites.

Three-Dimensionally Engineered Porous Silicon Electrodes for Li Ion Batteries

The ultimate goal of Li ion battery design should consist of fully accessible metallic current collectors, possibly of nanoscale dimensions, intimately in contact with high capacity stable electrode materials. Here we engineer three-dimensional porous nickel based current collector coated conformally with layers of silicon, which typically suffers from poor cycle life, to form high-capacity electrodes. These binder/conductive additive free silicon electrodes show excellent electrode adhesion resulting in superior cyclic stability and rate capability. The nickel current collector design also allows for an increase in silicon loading per unit area leading to high areal discharge capacities of up to 0.8 mAh/cm(2) without significant loss in rate capability. An excellent electrode utilization (∼85%) and improved cyclic stability for the metal/silicon system is attributed to reduced internal stresses/fracture upon electrode expansion during cycling and shorter ionic/electronic diffusion pathways that help in improving the rate capability of thicker silicon layers.

Effects of compressive strains on electrical conductivities of a macroscale carbon nanotube block

A macroscopic block (∼9mm3) of aligned carbon nanotubes (CNTs) was grown by chemical vapor deposition and its simultaneous electrical conductivity and compressive strain responses were measured parallel and perpendicular to the CNT alignment. The block exhibits elastic moduli of 0.9 and 1.6MPa for compressive strain of <20% in parallel and perpendicular configurations, respectively. The electrical conductivity increases with increasing compressive strain in both configurations. The reversible electrical conductivity and compressive strain responses of block is attributed to elastic bending of CNTs. These excellent properties of CNT block can be used in compressive strain sensing applications.

Contact transfer of aligned carbon nanotube arrays onto conducting substrates

The authors demonstrate the fabrication of different architectures of carbon nanotubes on conducting substrates via contact transfer of nanotubes using low temperature solders. Lithographically patterned multiwalled carbon nanotube arrays grown on silica substrates by chemical vapor deposition methods are transferred onto solder coated substrates. Both negative and positive patterns can be obtained by changing the printing parameters. Good wetting and electrical contacts are confirmed by measuring their field emission properties. This method can be used to construct nanotube structures of different shapes and dimensions over large areas on substrates of choice and could be a feasible process to integrate nanotubes into various devices.

G-Quadruplex Guanosine Gels and Single Walled Carbon Nanotubes

Solubilization of single walled carbon nanotubes (SWNTs) in aqueous gel phases formed by reversible, G-quadruplex self-assembly of guanosine monophosphate (GMP) alone or with guanosine (Guo) is described. Unlike other media and methods for aqueous solubilization of SWNTs, the guanosine gels (“G-gels”) are found to readily disperse high (>mg/mL) concentrations of individual rather than bundled SWNTs. SWNT dispersions in GMP alone precipitate in several hours and re-form upon shaking; however, dispersions in the binary GMP/Guo gels are indefinitely stable. Increasing GMP or KCl concentration in the binary gels increased the relative abundance of large diameter and semi-conducting SWNTs. Different gel compositions also displayed different selectivities toward SWNTs of different chiralities. These results indicate a strong connection between the self-assembled G-gels and the dimensions and structures of the SWNTs that they solubilize.

Continuous Carbon Nanotube-PDMS Composites

Carbon nanotubes are considered short fibers and the nanotube reinforced composites are always analogues of randomly distributed short fiber composites. In contrast, the real structural fibrous composites often contain fiber reinforcements where fibers run continuously through the matrix material. With the recent advance in nanotube growth, vertical arrays of nanotubes in macroscopic lengths have become available and this allows the fabrication of continuous nano-composites that are similar to the continuous fiber composites utilizing the nanotube arrays as the continuous reinforcement in the composites. This provides a chance to take full advantage of the extreme high modulus and strength for the nanotubes in structural composites. Here, this study fabricates continuous nanotube reinforced polydimethylsiloxane (PDMS) composites and shows that under compressive loadings such continuous nanotube composites can generate dramatic increase in the longitudinal modulus and also significantly enhanced damping capability.Copyright

Flexible Energy Storage Devices Using Nanomaterials

Publisher Summary This chapter discusses the flexible energy storage devices using nanomaterials. Energy storage systems are critical components, especially when the harvested renewable energy is to be stored in remote locations. Types of energy storage system include batteries, supercapacitors, and hydrogen storage. Traditional energy storage devices are not fully compatible with flexible electronics devices such as smart cards and electronic paper. Hence, advances in flexible energy storage devices are important to meet this demand. The research and development of new and innovative nanomaterials is progressing well to address the requirements of the flexible electronics industry. The flexible energy storage device is still in its infancy and hence there is still plenty of room available in the materials exploratory domain; for instance, making a flexible, mechanically robust device of metal nanowires/nanoparticles for the flexible electronics market.