Hydrogen harvested from mixed waste plastics

According to Kevin Wyss, a Rice doctoral alumnus and lead author on a study published in Advanced Materials, the method also produces high-value graphene.

“If the produced graphene is sold at only five per cent of current market value ⎯ a 95 per cent off sale! ⎯ clean hydrogen could be produced for free,” Wyss said in a statement.

By comparison, ‘green’ hydrogen costs roughly $5 for just over two pounds. Though cheaper, most of the nearly 100 million tons of hydrogen used globally in 2022 was derived from fossil fuels, its production generating roughly 12 tons of carbon dioxide per ton of hydrogen.

“The main form of hydrogen used today is ‘grey’ hydrogen, which is produced through steam-methane reforming, a method that generates a lot of carbon dioxide” said James Tour, Rice’s T. T. and W. F. Chao Professor of Chemistry and a professor of materials science and nanoengineering. “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’re serious about reaching net zero emissions by 2050.”

The researchers exposed plastic waste samples to rapid flash Joule heating for about four seconds, bringing their temperature up to 3100 degrees Kelvin. The process vaporises the hydrogen present in plastics, leaving behind graphene.

“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,” Wyss said. “We wondered what they were, suspecting a mix of small hydrocarbons and hydrogen, but lacked the instrumentation to study their exact composition.”

Using funding from the US Army Corps of Engineers, the Tour lab acquired the necessary equipment to characterise the vaporised contents.

“We know that polyethylene, for example, is made of 86 per cent carbon and 14 per cent hydrogen, and we demonstrated that we are able to recover up to 68 per cent of that atomic hydrogen as gas with a 94 per cent purity,” Wyss said. “Developing the methods and expertise to characterise 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. 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.”