Charles R. Martin

Carbon nanotubule membranes for electrochemical energy storage and production

Ensembles of aligned and monodisperse tubules of graphitic carbon can be prepared by a templating method that involves the chemical-vapour deposition of carbon within the pores of alumina membranes. Tubules with diameters as small as 20 nm have been prepared in this way,. The carbon comprising these tubules can be transformed from a disordered material to very highly ordered graphite. Here we show that template-synthesized carbon tubules can be fabricated as free-standing nanoporous carbon membranes, and that narrower, highly ordered graphitic carbon nanotubes can be prepared within the membranes tubules. Both the outer and the inner tubules are electrochemically active for intercalation of lithium ions, suggesting possible applications in lithium-ion batteries,. The membranes can also be filled with nanoparticles of electrocatalytic metals and alloys. Such catalyst-loaded membranes can be used to electrocatalyse O2 reduction and methanol oxidation, two reactions of importance to fuel-cell technology.

A general template-based method for the preparation of nanomaterials

This article reviews a general template-based approach for the preparation of nanomaterials. The method involves the synthesis of a desired material within the pores of a nanoporous membrane. We have termed this approach ‘template synthesis’ because the pores within these nanoporous membranes act as templates for the synthesis of nanostructures of the desired material. Because the pores within these membranes are cylindrical and of uniform diameter, monodisperse nanocylinders of the desired material are obtained. Depending on the chemistry of the pore wall and material, these nanocylinders may be either hollow (a tubule) or solid (a fibril or nanowire). This template process will be shown to be a very general approach in the fabrication of nanotubes and fibrils composed of a variety of materials including polymers, metals, semiconductors, carbons, and other materials.

The emerging field of nanotube biotechnology

Nanoparticles are being developed for a host of biomedical and biotechnological applications, including drug delivery, enzyme immobilization and DNA transfection. Spherical nanoparticles are typically used for such applications, which reflects the fact that spheres are easier to make than other shapes. Micro- and nanotubes — structures that resemble tiny drinking straws — are alternatives that might offer advantages over spherical nanoparticles for some applications. This article discusses four approaches for making micro- and nanotubes, and reviews the current status of efforts to develop biomedical and biotechnological applications of these tubular structures.

Temperature dependence of the electrode kinetics of oxygen reduction at the platinum/Nafion interface - A microelectrode investigation

Results of a study of the temperature dependence of the oxygen reduction kinetics at the Pt/Nafion interface are presented. This study was carried out in the temperature range of 30-80 C and at 5 atm of oxygen pressure. The results showed a linear increase of the Tafel slope with temperature in the low current density region, but the Tafel slope was found to be independent of temperature in the high current density region. The values of the activation energy for oxygen reduction at the platinum/Nafion interface are nearly the same as those obtained at the platinum/trifluoromethane sulfonic acid interface but less than values obtained at the Pt/H3PO4 and Pt/HClO4 interfaces. The diffusion coefficient of oxygen in Nafion increases with temperature while its solubility decreases with temperature. These temperatures also depend on the water content of the membrane.

Metal Nanotubule Membranes with Electrochemically Switchable Ion-Transport Selectivity

Membranes containing cylindrical metal nanotubules that span the complete thickness of the membrane are described. The inside radius of the nanotubules can be varied at will; nanotubule radii as small as 0.8 nanometer are reported. These membranes show selective ion transport analogous to that observed in ion-exchange polymers. Ion permselectivity occurs because excess charge density can be present on the inner walls of the metal tubules. The membranes reject ions with the same sign as the excess charge and transport ions of the opposite sign. Because the sign of the excess charge on the tubule can be changed potentiostatically, a metal nanotubule membrane can be either cation selective or anion selective, depending on the potential applied to the membrane.

Ion exchange and transport of neurotransmitters in nation films on conventional and microelectrode surfaces

Abstract Very small Nafion-coated voltammetric electrodes are very advantageous for in vivo detection of catecholamine neurotransmitters. In this study their quantitative characteristics with regard to partitioning of the catecholamines into the Nafion film have been studied. The properties of these electrodes have been compared with larger graphite electrodes covered with Nafion film.

A High-Rate, High-Capacity, Nanostructured Sn-Based Anode Prepared Using Sol-Gel Template Synthesis

Li-ion battery anodes derived from oxides of tin have recently received considerable interest because in principle they can store over twice as much Li as graphite. However, large volume changes occur when Li is inserted and removed from these Sn-based materials, and this causes internal damage to the electrode, resulting in loss of capacity and rechargability. We describe here a new nanostructured SnO 2 -based electrode that has extraordinary rate capabilities, can deliver very high capacities (e.g., >700 mAh g -1 at 8 C rate), and still retain the ability to be discharged and recharged through as many as 1400 cycles. These electrodes, prepared via the template method, consist of monodisperse 110 nm diam SnO 2 nanofibers protruding from a current-collector surface like the bristles of a brush.

A High-Rate, Nanocomposite LiFePO4 ∕ Carbon Cathode

We describe here a new type of template-prepared nanostructured LiFePO4 electrode, a nanocomposite consisting of monodispersed nanofibers of the LiFePO4 electrode material mixed with an electronically conductive carbon matrix. This unique nanocomposite morphology allows these electrodes to deliver high capacity, even when discharged at the extreme rates necessary for many pulse-power applications. For example, this nanocomposite electrode delivers almost 100% of its theoretical discharge capacity at the high discharge rate of 3 C, and 36% of its theoretical capacity at the enormous discharge rate of 65 C. This new nanocomposite electrode shows such excellent rate capabilities because the nanofiber morphology mitigates the problem of slow Li+-transport in the solid state, and the conductive carbon matrix overcomes the inherently poor electronic conductivity of LiFePO4.

Rate Capabilities of Nanostructured LiMn2 O 4 Electrodes in Aqueous Electrolyte

Nanostructured LiMn 2 O 4 electrodes consisting of LiMn 2 O 4 nanotubules that protrude from a current collector surface like the bristles of a brush were prepared using the template method. The rate capabilities of these nanostructured electrodes were investigated at the 4 V (vs. Li/Li + ) potential plateau in aqueous LiNO 3 electrolyte. Rate capability improved with decreasing wall thickness of the tubules which formed the electrode. This result is in agreement with our prior investigations of template-synthesized electrode materials which showed that rate capabilities improve with decreasing distance for Li + transport in the solid state. The rate capabilities of electrodes prepared from the smallest-wall-thickness tubules are extraordinary; these electrodes can be cycled at C rates as high as 109 C. In addition, these investigations suggest that the poor cycling performance observed in prior studies of this electrode/electrolyte system results from unwanted oxidation of water during the charging process. By controlling the charge rate and the dimensions of the nanotubules making up the template-synthesized cathodes, this unwanted side reaction can he eliminated and good cycle life is observed. These data show that the nanostructured electrodes offer a unique advantage to this particular electrode/electrolyte system.

A High‐Rate, High‐Capacity, Nanostructured Tin Oxide Electrode

Recently, battery anodes derived from oxides of tin have been of considerable interest because they can, in principle, store more than twice as much as graphite. However, large volume changes occur when is inserted and removed from these materials, and this causes internal damage to the electrode resulting in loss of capacity and rechargeability. We describe here a nanostructured electrode that has extraordinary rate capabilities, can deliver very high capacities , and still retain the ability to be discharged and recharged for as many as 800 cycles. These electrodes, prepared via the template method, consist of monodisperse nanofibers protruding from a current‐collector surface like the bristles of a brush. The dramatically improved rate and cycling performance are related to the small size of the nanofibers that make up the electrode and the small domain size of the grains within the nanofibers. ©2000 The Electrochemical Society