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Hydrogen is seen as a promising alternative to fossil fuels, but the methods used to produce hydrogen generate too much carbon dioxide or are too expensive. Now, researchers at Rice University have found a way to extract hydrogen from plastic waste using a low-emissions method that could more than pay for itself.
“In this work, we converted waste plastic – including mixed waste plastic that does not require sorting by type or washing – into high-yield hydrogen gas and high-quality graphene,” said Kevin Wyss, lead author of a US National Science Foundation Graduate Research Fellowship Program supported study published in Advanced Materials. “If the graphene produced is sold at just 5% of its current market value – a 95% discount! – clean hydrogen could be produced for free.”
By comparison, “green” hydrogen, produced using renewable energy sources to split water into its two constituent elements, costs about $5 for just over two pounds. Although cheaper, most of the nearly 100 million tons of hydrogen used worldwide in 2022 came from fossil fuels, with production generating about 12 tons of carbon dioxide per ton of hydrogen.
“The main form of hydrogen in use today is ‘grey’ hydrogen, which is produced through steam methane reforming, a method that generates a lot of carbon dioxide,” says materials scientist James Tour. “Hydrogen demand is likely to skyrocket in the coming decades, so we can’t keep doing it the same way we’ve done so far if we’re serious about reaching net-zero emissions by 2050.”
The researchers exposed plastic waste samples to rapid Joule heating for about four seconds, raising the temperature to 3,100 degrees Kelvin. The process evaporates the hydrogen present in plastics, leaving behind graphene – an extremely light, durable material made up of a single layer of carbon atoms.
“When we first discovered flash-joule heating and applied it to upcycle waste plastic into graphene, we saw a lot of volatile gases being produced and shooting out of the reactor,” says Wyss. “We wondered what they were and suspected a mix of small hydrocarbons and hydrogen, but lacked the instruments to study their exact composition.”
“We know that polyethylene, for example, is made of 86% carbon and 14% hydrogen, and we have shown that we can recover up to 68% of that atomic hydrogen as a gas with a purity of 94%,” Wyss said.
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