High-performance supercapacitor made from upcycled water bottles

Lots of single-use water bottles made from poly(ethylene terephthalate) (PET) end up in landfills, but there’s a growing interest in upcycling them instead. Researchers in Energy & Fuels report on new heat-based fabrication methods to transform PET into supercapacitor electrodes and separator films for upcycled energy storage devices. In demonstrations, an all-plastic supercapacitor made from discarded water bottles outperformed a similar design that used a traditional glass fiber separator.

“PET is used to produce over 500 billion single-use beverage bottles each year, which generates a significant amount of plastic waste and poses a major environmental challenge,” says lead researcher Yun Hang Hu. “PET-derived supercapacitors hold great potential for diverse applications in transportation and automotive systems, electronics and consumer devices, as well as industrial and specialized sectors.”

Converting waste plastics like PET into carbon-based materials, especially ones that are electrically conductive, is an attractive way to manufacture more cost-effective and sustainable energy storage devices like supercapacitors. These devices use highly conductive carbon electrodes to store and release a large amount of energy quickly and repeatedly. So, Hu and colleagues wanted to upcycle old water bottles into components for a type of supercapacitor called an electrical double-layer capacitor (EDLC). This device is characterized by two porous carbon-based electrodes separated by a thin, perforated film immersed in a liquid electrolyte.

Hu’s team developed two processes to turn used PET water bottles into components for the upcycled device:

• For the electrodes, the researchers cut the plastic bottles into tiny, couscous-sized grains. They added calcium hydroxide and heated the mixture to nearly 1300 degrees Fahrenheit (700 degrees Celsius) in a vacuum. This process converts the plastic into a porous, electrically conductive carbon powder. The researchers combined the carbon powder, carbon black and a polymer binder, and then dried it into thin layers.
• For the separator, the researchers flattened small plastic pieces about the size of postage stamps and poked holes in them using hot needles. The holes’ pattern optimized the passage of current through the electrolyte.

To assemble their PET-based supercapacitor, the researchers submerged two porous carbon electrodes in a liquid potassium hydroxide electrolyte and separated them with the perforated PET film. In demonstrations, the upcycled supercapacitor retained 79% of its capacitance (storage ability), while a similar device with a glass fiber separator retained 78%.

Hu and colleagues say this research introduces a potential strategy for transforming PET waste into supercapacitor components, “opening new opportunities for circular energy storage technologies.” In addition, they say the upcycled EDLC is less expensive to produce than devices made with glass fiber and is itself fully recyclable.

“With further optimization, PET-derived supercapacitors might realistically transition from laboratory prototypes to market-ready devices within the next five to 10 years,” says Hu, “especially as demand grows for sustainable, recyclable energy storage technologies.”