Nanowire battery is ‘world’s smallest’

Led by Professor Pulickel Ajayan, the team built centimetre scale arrays containing thousands of nanowire devices, each about 150nm wide. According to the researchers, each nanowire is a completely discrete battery, consisting of all the usual elements: anode, cathode, and electrolyte. Prof Ajayan's team has been inching towards single nanowire devices for years. The researchers first reported the creation of three dimensional nanobatteries last December. In that project, they encased vertical arrays of nickel tin nanowires in PMMA, a widely used polymer known as Plexiglas, which served as an electrolyte and insulator. They grew the nanowires via electrodeposition in an anodised alumina template atop a copper substrate. They then widened the template's pores with a simple chemical etching technique that created a gap between the wires and the alumina and drop coated PMMA to encase the wires in a smooth, consistent sheath. A chemical wash removed the template and left an array of electrolyte encased nanowires. In that battery, the encased nickel tin was the anode, but the cathode had to be attached on the outside. According to Ajayan, the new process tucks the cathode inside the nanowires. The researchers used the polymer polyethylene oxide as the gel like electrolyte to store lithium ions. It also served as an electrical insulator between the nanowires. After much trial and error, the researchers settled on an easily synthesised polymer known as polyaniline (PANI) as their cathode. Drop coating the widened alumina pores with PEO coated the insides, encased the anodes and left tubes at the top into which PANI cathodes could also be drop coated. An aluminum current collector placed on top of the array completed the circuit. "The idea here was to fabricate nanowire energy storage devices with ultrathin separation between the electrodes," said Arava Leela Mohana Reddy, a research scientist at Rice. "This affected the electrochemical behavior of the device. Our devices could be a very useful tool to probe nanoscale phenomena." According to Reddy, the device showed good capacity and was also scalable. The researcher maintained, however, that there was still a lot to be done to optimise the device in terms of performance. "Optimisation of the polymer separator and its thickness and an exploration of different electrode systems could lead to improvements," he concluded.