The hype around hydrogen keeps growing. The main form of hydrogen used today is grey hydrogen, which is produced through steam-methane reforming, a carbon dioxide-intensive method. Green hydrogen, produced using renewable energy to split water into hydrogen and oxygen, has the potential to drastically reduce emissions in industries such as steelmaking, refining, and chemical production. It is also hailed as a solution for powering fuel cell vehicles, energy storage, and even heat our homes, although studies have shown that it is unsuitable for the latter application. At $4 to $6 per kilogram, two to three times more than grey hydrogen, however, producing green hydrogen is expensive.
Now, scientists from Rice University in the United States have developed a method to produce clean hydrogen from waste plastics that could pay for itself. The key to the economic viability is the by-product of the process, high purity graphene. The researchers claim that if the graphene by-product was sold at just 5% of its current value, the hydrogen production would have negative cost.
The new method doesn’t require waste plastic to be sorted by type or washed. The researchers exposed plastic waste samples to rapid flash Joule heating for about four seconds, bringing their temperature up to 3100 degrees Kelvin. Flash Joule heating is an innovative approach that enables the rapid heating of materials to exceptionally high temperatures within milliseconds. It works by passing a current thorough a resistive material, which rapidly converts the electricity to heat. The process vaporizes 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,” said Kevin Wyss, a Rice doctoral alumnus and lead author of the study. “We wondered what they were, suspecting a mix of small hydrocarbons and hydrogen, but lacked the instrumentation to study their exact composition.”
After acquiring the required equipment with funding from the United States Army Corps of Engineers, the team demonstrated that up to 68% of atomic hydrogen can be recovered from polyethylene, which is made of 86% carbon and 14% hydrogen.
The process is scalable and produces no carbon dioxide when deconstructing polyolefins. It produces hydrogen in purities up to 94% at high mass yields. Life-cycle assessment showed a 39% to 84% reduction in emissions compared to other hydrogen production methods, suggesting the flash hydrogen process to be an economically viable, clean production route.
The scientists shared their findings in “Synthesis of Clean Hydrogen Gas from Waste Plastic at Zero Net Cost,” recently published in Advanced Materials.