Plastic bottles can be recycled into energy-storing supercapacitors

Supercapacitors are charged like a battery but release their energy more rapidly – and some of their components can now be built from old plastic bottles.

Plastic bottles can be upcycled into parts for supercapacitors, which store energy like batteries but release it much faster.

We throw away billions of plastic bottles each year, but they could be turned into useful electronics components instead
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Plastic pollution is prevalent in our environment, from litter in parks to garbage patches in the oceans. Disposable drink bottles are a significant part of the problem. According to some sources, 38 billion of the bottles are sent to landfill each year in the US alone. Shengnian Wang at Louisiana Tech University and his colleagues wanted to turn some of them into energy storage devices called supercapacitors instead.

A supercapacitor can be charged like a battery but it releases energy – or discharges – more quickly. In addition, it often functions well for many more charging and discharging cycles than batteries do.

The researchers developed a chemical procedure that rearranges the carbon atoms in the clear polyethylene plastic used in bottles into a supercapacitor component.

First, they cut plastic bottles into pieces and reacted them with water, nitric acid and ethanol at a high temperature and pressure for 8 hours. They then placed the resulting mixture into a centrifuge device that spins the solution at high speed, before finally drying it in an oven. This left them with thin sheets of carbon speckled with many carbon nanoparticles, each of which had a diameter of less than 2 nanometres. Their small size gives the carbon nanoparticles unusual electrical properties and makes them what physicists call carbon “quantum dots”.

When it comes to making energy storage devices with carbon components, each shape – sheets and dots – has drawbacks. But using the two in combination makes storage devices more effective, said Wang in a presentation at the American Chemical Society fall meeting in San Francisco, California, last week.

He and his colleagues tested whether this “ball-sheet carbon structure” makes for a working supercapacitor electrode. They found that the supercapacitor charged and discharged as expected and, after charging, it could power a small red LED light. Additionally, charging and discharging it 12,000 times decreased its capacity to store energy by only 2 per cent.

In further experiments, the researchers built ball-sheet carbon structures using a more pure and less processed plastic, but when used in supercapacitors, these components had a similar performance level to those made from the old drink bottles. As such, Wang said that their method may offer a way to upcycle plastic waste directly, with no intermediate processing steps.

However, Xiaojun Ren at the University of New South Wales in Australia, who was not involved in the research, points out that supercapacitors built using the upcycled plastic did not always release all of the energy they had gained during charging. He says this means there is still room to further improve the performance of the supercapacitors.