Hydrogen is seen as a possible replacement for fossil fuels, but the technologies utilized to produce it either emit too much CO2 or are too expensive. Rice University researchers have discovered a low-emissions method for harvesting hydrogen from plastic waste that could potentially more than pay for itself.
In this work, we 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. If the produced graphene is sold at only 5% of current market value—a 95% off sale! —clean hydrogen could be produced for free.
Kevin Wyss, Study Lead Author and Doctoral Student, Rice University
‘Green’ hydrogen, on the other hand, costs around $5 for slightly over two pounds and is created by splitting water into its two constituent atoms using renewable energy sources. Although it was more affordable, the majority of the over 100 million tons of hydrogen utilized globally in 2022 came from fossil fuels, with each ton of hydrogen produced generating around 12 tons of carbon dioxide.
The main form of hydrogen used today is ‘gray’ hydrogen, which is produced through steam-methane reforming, a method that generates a lot of carbon dioxide. Demand for hydrogen will likely skyrocket over the next few decades, so we can’t keep making it the same way we have up until now if we are serious about reaching net zero emissions by 2050.
James Tour, T. T. and W. F. Chao Professor, Chemistry, Rice University
For roughly four seconds, the researchers subjected plastic waste samples to fast flash Joule heating, raising their temperature to 3100 degrees Kelvin. The technique vaporizes the hydrogen in plastics, resulting in graphene, an extraordinarily light, durable substance composed of a single sheet of carbon atoms.
Wyss added, “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.”
The Tour lab obtained the requisite technology to characterize the vaporized contents with financing from the United States Army Corps of Engineers.
“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. Developing the methods and expertise to characterize and quantify all the gases, including hydrogen, produced by this method was a difficult but rewarding process for me. I am glad that techniques I learned and used in this work—specifically life-cycle assessment and gas chromatography—can be applied to other projects in our group,” Wyss further stated.
He concluded, “I hope that this work will allow for the production of clean hydrogen from waste plastics, possibly solving major environmental problems like plastic pollution and the greenhouse gas-intensive production of hydrogen by steam-methane reforming.”
The US 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) funded the study.
Wyss, K. M., et al. (2023) Synthesis of clean hydrogen gas from waste plastic at zero net cost. Advanced Materials. doi:10.1002/adma.202306763