ENERGY

Carbon nanotubes have the intrinsic characteristics desired for material used as electrodes in batteries, capacitors, and fuel cells. These characteristics include tremendously high surface area (~1000 m2/g), good electrical conductivity, excellent chemical stability in acidic environments, and very importantly, their linear geometry makes their surface highly accessible to the electrolyte. Conceptually, batteries, capacitors, and fuel cells all have the structure shown below.


Carbon nanotubes serve as material for the electrodes (anode, cathode or both) in batteries, capacitors and fuel cells. Research has shown that carbon nanotubes have the highest reversible capacity of any carbon material for use in lithium-ion batteries [B. Gao, Chem. Phys. Lett. 327, 69 (2000); Frackowiak et al, carbon 40, 1775 (2002)]. Addition of carbon nanotubes to the carbon black traditionally used as lithium ion battery electrode material increases the conductivity of the electrode, improves the electrolyte penetration into the electrodes and enables the electrodes to maintain their mechanical integrity during the charge-discharge cycles as they absorb and desorb lithium ions. All these, taken together, provide for more efficient, longer lasting Li-ion batteries [Endo et al. Phil. Trans. Roy. Soc. Lond. A 362 2223 (2004). In addition, carbon nanotubes are outstanding materials for supercapacitor electrodes [Baughman et al Science 297, 787 (2002)] and capacitors with carbon nanotube electrodes can provide capacitances exceeding 100 Farads/gram.

Carbon nanotubes also have applications in fuel cell electrodes. In a fuel cell, the electrodes must have excellent conductivity, high durability in an acidic environment, high surface area, and they must be able to support and tightly hold precious-metal catalyst particles having diameters of 2-3 nanometers. The chemical and mechanical properties of carbon nanotubes are ideal for this application. Research at Unidym and elsewhere has shown that one can deposit nanometer-scale platinum particles on carbon nanotube ropes and that these make an extremely effective electrode for many different kinds of fuel cells. They provide electrodes with excellent mechanical integrity and their durability in fuel cells has now been demonstrated to be superior to that of carbon blacks traditionally used for fuel-cell electrodes. In addition, using the high-surface-area carbon nanotubes to support the precious metal catalyst particles provides electrodes that use the catalyst more effectively and therefore require less precious metal to achieve the same amount of electrical output from the fuel cell. A free-standing fuel cell electrode manufactured at Unidym is shown below.