Fabric batteries for 'wearable electronics'

STANFORD University researchers have discovered a way to turn cotton fabrics into supercapacitors, energy storage devices similar to a battery. A team led by materials science professor Yi Cui, incorporated single-walled carbon nanotubes into textiles using a technique usually used for dyeing. To create the nanotube dye, carbon nanotubes are dispersed in water with sodium dodecylbenzenesulphonate as a surfactant. The fabric sample is dipped into this mixture before being dried at 120�C for ten minutes.

This treatment gives the fabric a conductivity of 5 S/cm. Simple mechanical pressing of the fabric increases this to 125 S/cm, creating a highly-conductive material. Conductivity can be increased still further by increasing the number of dipping and drying cycles. Stretching the fabric also increases the conductance, which the researchers believe is due to better mechanical contacts between the fibres within the fabric.

Crucially, the fabric retains its texture and structure following treatment. The nanotubes bond easily to the fabric due to strong van der Waals forces and hydrogen bonding, and their flexibility means they can conform to the shape of the fabric fibres. Cotton samples washed in water and wrung out show no decrease in conductivity, with no carbon nanotubes present in the water. This demonstrable high adhesion is vital for high-speed roll-to-roll production processes, and to ensure to stability of energy storage.

Cotton proved to be around 2-3 times better than man-made fibres for energy storage as the porous nature of cotton fibres allows for better ion transport to the nanotubes, which act as the supercapacitor. Such fabric supercapacitors show very good cycling stability. 130,000 cycles caused only 2% variation in capacitance. The capacitance can be increased by electrodepositing pseudocapacitors, such as manganese oxide and ruthenium oxide, onto the carbon nanotubes. Again, the porous structure of cotton facilitates this.