Rice University researchers in the Tour Lab have converted waste plastics—including mixed waste plastics that don’t have to be sorted by type or washed—into high-yield hydrogen gas and high-value graphene. A paper on their work is published in Advanced Materials.

Hydrogen gas (H₂) is the primary storable fuel for pollution-free energy production, with over 90 million tonnes used globally per year. More than 95% of H₂ is synthesized through metal-catalyzed steam methane reforming that produces 11 tonnes of CCO₂O2 per tonne H2. “Green H₂H2” from water electrolysis using renewable energy evolves no CO₂, but costs 2–3x more, making it presently economically unviable.

Here we report catalyst-free conversion of waste plastic into clean H₂ along with high purity graphene. The scalable procedure evolves no CO₂ when deconstructing polyolefins and produces H₂ in purities up to 94% at high mass yields. Sale of graphene byproduct at just 5% of its current value yields H₂ production at negative cost. Life-cycle assessment demonstrates a 39–84% reduction in emissions compared to other H₂ production methods, suggesting the flash H2 process to be an economically viable, clean H₂ production route.

—Wyss et al.

Scanning electron microscope (SEM) image of layered stacks of nano-scale flash graphene sheets formed from waste plastic. (Image courtesy of Kevin Wyss/Tour lab)

The researchers exposed plastic waste samples to rapid flash Joule heating (FJH) (earlier post) for about four seconds, bringing their temperature up to 3100 Kelvin (2827 ˚C). The process vaporizes the hydrogen present in plastics, leaving behind graphene—an extremely light, durable material made up of a single layer of carbon atoms.

The Tour Lab introduced the flash Joule heating process in 2020 to produce graphene from any solid carbon source.

When we first discovered flash Joule heating and applied it to upcycle waste plastic into graphene, we observed a lot of volatile gases being produced and shooting out of the reactor. We wondered what they were, suspecting a mix of small hydrocarbons and hydrogen, but lacked the instrumentation to study their exact composition.

—Kevin Wyss, lead author

Using funding from the United States Army Corps of Engineers, the Tour lab acquired the necessary equipment to characterize the vaporized contents.

We know that polyethylene, for example, is made of 86% carbon and 14% hydrogen, and we demonstrated that we are able to recover up to 68% of that atomic hydrogen as gas with a 94% purity.

—Kevin Wyss

The research was supported by the United States Army Engineer Research and Development Center (W912HZ-21-2-0050), the Air Force Office of Scientific Research (FA9550-22-1-0526), the National Science Foundation and the Office of Naval Research (N00014-22-1-2788).

Resources

• Wyss, K. M., Silva, K. J., Bets, K. V., Algozeeb, W. A., Kittrell, C., Teng, C. H., Choi, C. H., Chen, W., Beckham, J. L., Yakobson, B. I., Tour, J. M., (2023) Synthesis of Clean Hydrogen Gas from Waste Plastic at Zero Net Cost. Adv. Mater. doi: 10.1002/adma.202306763