Ray H. Baughman

Carbon Nanotubes: Present and Future Commercial Applications

Exploiting Carbon Nanotubes Individual defect-free carbon nanotubes can have exceptional mechanical, thermal, and electrical properties, which has led to speculation on a wide range of potential applications. However, challenges in growing large quantities of pure nanotubes, and for some applications tubes of only one type, have limited their widespread use. De Volder et al. (p. 535) review the efforts that have been made to scale up carbon nanotube production and discuss a number of applications where enhanced materials have made use of carbon nanotubes. Worldwide commercial interest in carbon nanotubes (CNTs) is reflected in a production capacity that presently exceeds several thousand tons per year. Currently, bulk CNT powders are incorporated in diverse commercial products ranging from rechargeable batteries, automotive parts, and sporting goods to boat hulls and water filters. Advances in CNT synthesis, purification, and chemical modification are enabling integration of CNTs in thin-film electronics and large-area coatings. Although not yet providing compelling mechanical strength or electrical or thermal conductivities for many applications, CNT yarns and sheets already have promising performance for applications including supercapacitors, actuators, and lightweight electromagnetic shields.

Super-tough carbon-nanotube fibres

The energy needed to rupture a fibre (its toughness) is five times higher for spider silk than for the same mass of steel wire, which has inspired efforts to produce spider silk commercially. Here we spin 100-metre-long carbon-nanotube composite fibres that are tougher than any natural or synthetic organic fibre described so far, and use these to make fibre supercapacitors that are suitable for weaving into textiles.

Conducting polymer artificial muscles

Abstract The application of conducting polymers for the direct conversion of electrical energy to mechanical energy in electromechanical actuators is analyzed using theoretical and experimental results. Basic principles of operation, predicted performance advantages and disadvantages, comparisons with natural muscle, evaluations of initial device demonstrations, and methods for improving device performance are provided. The very high predicted work densities per cycle, force generation capabilities, and power densities provide major advantages compared with piezoelectric polymers – as do the low operation voltages. These advantages are countered by cycle life and energy conversion efficiency limitations, as well as the need to use microelectrodes in order to achieve very high rates. Hydrostatic devices and extensional devices that provide either in-phase or out-of-phase electrode deformations are considered. Special types of conducting polymer actuators are also proposed, including photo-powered, chemically powered and self-powered actuators, which provide novel methods for assembling complex microstructures. Novel methods are described for actuator fabrication, such as ‘redox poling’, wherein anode, cathode and separating electrolyte layers are generated from a film in a single redox step. New actuator compositions are also proposed for obtaining improved performance, such as conjugated carbon phases having conjugation in either two or three dimensions. Finally, conducting polymer actuators based on double-layer charging are proposed which are predicted to provide increased energy efficiency and cycle life compared with the faradaic actuators.

Structure‐property predictions for new planar forms of carbon: Layered phases containing sp2 and sp atoms

Structure, thermodynamics, and electronic properties are predicted for a new low energy phase of carbon which contains planar sheets equally occupied by sp2 and sp carbon atoms. The isolated planar sheets have the same planar symmetry as do the layers in graphite (p6m) and can be formally viewed as resulting from the replacement of one‐third of the carbon–carbon bonds in graphite by –C 3/4 C– linkages. This material, called graphyne, is predicted to have a crystalline state formation energy of 12.4 kcal/mol carbon, which appears to be much lower than for any carbon phase which contains acetylenic groups as a major structural component. Based on the major structural reorganization required for graphitization and the observed high temperature stability of known model compounds, high temperature stability is predicted for graphyne. While graphyne will have similar mechanical properties as graphite, it is predicted to be a large bandgap semiconductor (Eg=1.2 eV) rather than a metal or semimetal. Based on this...

Direct electron transfer of glucose oxidase on carbon nanotubes

In this report, exploitation of the unique properties of single-walled carbon nanotubes (SWNT) leads to the achievement of direct electron transfer with the redox active centres of adsorbed oxidoreductase enzymes. Flavin adenine dinucleotide (FAD), the redox active prosthetic group of flavoenzymes that catalyses important biological redox reactions and the flavoenzyme glucose oxidase (GOx), were both found to spontaneously adsorb onto carbon nanotube bundles. Both FAD and GOx were found to spontaneously adsorb to unannealed carbon nanotubes that were cast onto glassy carbon electrodes and to display quasi-reversible one-electron transfer. Similarly, GOx was found to spontaneously adsorb to annealed, single-walled carbon nanotube paper and to display quasi-reversible one-electron transfer. In particular, GOx immobilized in this way was shown, in the presence of glucose, to maintain its substrate-specific enzyme activity. It is believed that the tubular fibrils become positioned within tunnelling distance of the cofactors with little consequence to denaturation. The combination of SWNT with redox active enzymes would appear to offer an excellent and convenient platform for a fundamental understanding of biological redox reactions as well as the development of reagentless biosensors and nanobiosensors.

Polymer artificial muscles

The various types of natural muscle are incredible material systems that enable the production of large deformations by repetitive molecular motions. Polymer artificial muscle technologies are being developed that produce similar strains and higher stresses using electrostatic forces, electrostriction, ion insertion, and molecular conformational changes. Materials used include elastomers, conducting polymers, ionically conducting polymers, and carbon nanotubes. The mechanisms, performance, and remaining challenges associated with these technologies are described. Initial applications are being developed, but further work by the materials community should help make these technologies applicable in a wide range of devices where muscle-like motion is desirable.

Giant-Stroke, Superelastic Carbon Nanotube Aerogel Muscles

Improved electrically powered artificial muscles are needed for generating force, moving objects, and accomplishing work. Carbon nanotube aerogel sheets are the sole component of new artificial muscles that provide giant elongations and elongation rates of 220% and (3.7 × 104)% per second, respectively, at operating temperatures from 80 to 1900 kelvin. These solid-state–fabricated sheets are enthalpic rubbers having gaslike density and specific strength in one direction higher than those of steel plate. Actuation decreases nanotube aerogel density and can be permanently frozen for such device applications as transparent electrodes. Poissons ratios reach 15, a factor of 30 higher than for conventional rubbers. These giant Poissons ratios explain the observed opposite sign of width and length actuation and result in rare properties: negative linear compressibility and stretch densification.

Negative Poisson's ratios as a common feature of cubic metals

Poissons ratio is, for specified directions, the ratio of a lateral contraction to the longitudinal extension during the stretching of a material. Although a negative Poissons ratio (that is, a lateral extension in response to stretching) is not forbidden by thermodynamics, this property is generally believed to be rare in crystalline solids. In contrast to this belief, 69% of the cubic elemental metals have a negative Poissons ratio when stretched along the [110] direction. For these metals, we find that correlations exist between the work function and the extremal values of Poissons ratio for this stretch direction, which we explain using a simple electron-gas model. Moreover, these negative Poissons ratios permit the existence, in the orthogonal lateral direction, of positive Poissons ratios up to the stability limit of 2 for cubic crystals. Such metals having negative Poissons ratios may find application as electrodes that amplify the response of piezoelectric sensors.

Ultrafast charge and discharge biscrolled yarn supercapacitors for textiles and microdevices

Flexible, wearable, implantable and easily reconfigurable supercapacitors delivering high energy and power densities are needed for electronic devices. Here we demonstrate weavable, sewable, knottable and braidable yarns that function as high performance electrodes of redox supercapacitors. A novel technology, gradient biscrolling, provides fast-ion-transport yarn in which hundreds of layers of conducting-polymer-infiltrated carbon nanotube sheet are scrolled into ~20 μm diameter yarn. Plying the biscrolled yarn with a metal wire current collector increases power generation capabilities. The volumetric capacitance is high (up to ~179 F cm(-3)) and the discharge current of the plied yarn supercapacitor linearly increases with voltage scan rate up to ~80 V s(-1) and ~20 V s(-1) for liquid and solid electrolytes, respectively. The exceptionally high energy and power densities for the complete supercapacitor, and high cycle life that little depends on winding or sewing (92%, 99% after 10,000 cycles, respectively) are important for the applications in electronic textiles.

Superior Rechargeability and Efficiency of Lithium–Oxygen Batteries: Hierarchical Air Electrode Architecture Combined with a Soluble Catalyst†

The lithium-oxygen battery has the potential to deliver extremely high energy densities; however, the practical use of Li-O2 batteries has been restricted because of their poor cyclability and low energy efficiency. In this work, we report a novel Li-O2 battery with high reversibility and good energy efficiency using a soluble catalyst combined with a hierarchical nanoporous air electrode. Through the porous three-dimensional network of the air electrode, not only lithium ions and oxygen but also soluble catalysts can be rapidly transported, enabling ultra-efficient electrode reactions and significantly enhanced catalytic activity. The novel Li-O2 battery, combining an ideal air electrode and a soluble catalyst, can deliver a high reversible capacity (1000 mAh g(-1) ) up to 900 cycles with reduced polarization (about 0.25 V).