A chemist has created an ‘upcycling’ method of turning the disposable plastic bags into carbon nanotubes. The research is published in The Journal of Environmental Monitoring.
Vilas Ganpat Pol at Argonne National Lab developed the bag-to-nanotube technique and converts high or low-density polyethylene (HDPE and LDPE) into valuable multiwalled carbon nanotubes.
The nanotubes have even been used to make lithium-ion batteries.
Pol made the nanotubes by cooking 1-gram pieces of HDPE or LDPE at 700 C for 2 hours in the presence of a cobalt acetate catalyst and then letting the mixture cool gradually.
Above 600 C, the chemical bonds within the plastic completely break down and multiwalled carbon nanotubes grow on the surface of the catalytic particles.
A lot of catalyst is needed to get good results – about one-fifth of the weight of the plastic being converted – and it cannot easily be recovered afterward.
However, Pol said that this is still one of the cheapest and environmentally friendly ways yet found to grow nanotubes.
“Other methods generally require a vacuum to avoid oxygen interaction with the catalyst as well as with the system. In my new reaction there is no vacuum – the formation of oxide is inhibited due to the presence of a continuous reducing hydrocarbon atmosphere at 700 C,” he said.
Individual pieces of the catalyst become trapped inside forests of newly grown nanotubes. However, Pol has shown the nanotubes can be used as is without further processing to cut them free.
“I have used the as-prepared cobalt-encapsulated nanotubes as an anode material for lithium-ion batteries and they work fantastically. The specific capacity of my carbon nanotubes is higher than commercial nanotubes,” he said.
He thinks that might be down to slight imperfections in the usually-regular structure of the nanotubes, created by the reducing atmosphere during fabrication.
Pol said that the cobalt impurities also make the nanotubes suitable for use in lithium-air batteries, because the cobalt is converted to cobalt oxides that perform as catalysts to help the reactions of ions in the battery that let current flow.
He has patented the use of the cobalt-containing nanotubes in both lithium-ion and lithium-air batteries.
“The cobalt is not an impurity, it is an asset,” he said.