Researchers have used a low-emission method to extract hydrogen and graphene from waste plastics. Not only does this solve environmental problems such as plastic pollution and greenhouse gas production, but the value of the graphene by-product could offset the cost of producing hydrogen, they say. Hydrogen can be used to power cars, generate electricity and heat homes and businesses. Hydrogen contains more energy per unit weight than fossil fuels, which is important from an environmental perspective because the main cause of global greenhouse gas emissions is the carbon dioxide released by burning fossil fuels.
More than 95% of hydrogen currently sold is synthesized through steam methane reforming, producing 11 tons (12 tons) of carbon dioxide per ton of hydrogen, the vast majority of which is gray hydrogen. By comparison, "green hydrogen" produced by using renewable energy sources such as solar, wind or water to separate water into its elements is expensive, costing about $5 per two pounds (about one kilogram) of hydrogen.
Rice University researchers have now developed a way to harvest valuable hydrogen and graphene from waste plastic using a low-emission, catalyst-free method that has the potential to pay for itself.
"In this work, we convert waste plastics, including mixed waste plastics that do not need to be sorted by type or cleaned, into high-yield hydrogen and high-value graphene," said Kevin Wyss, first author of the study. "If the graphene produced were sold for just 5% of the current market value - a 95% discount! - clean hydrogen could be produced for free."
In the steam-methane reforming process, high-temperature steam (1292°F to 1832°F/700°C to 1000°C) is used to produce hydrogen from methane sources such as natural gas. Methane reacts with steam under the action of a catalyst to produce hydrogen, carbon monoxide and carbon dioxide.
James Tour, one of the study's corresponding authors, said: "The main form of hydrogen currently in use is 'grey' hydrogen, which is produced by steam-methane reforming, a method that produces large amounts of carbon dioxide. Demand for hydrogen is likely to surge in the coming decades, so if we really want to achieve net-zero emissions by 2050, we can no longer make hydrogen using the methods to date."
Waste plastic persists in the environment for long periods of time, threatening wildlife and spreading toxins to animals and humans. In the current study, the researchers exposed waste plastic to rapid flash Joule heating for about 4 seconds. When the temperature rises to 3,100 Kelvin, the hydrogen in the plastic evaporates, leaving behind graphene, a lightweight and durable material composed of a single layer of carbon atoms. Graphene can be used in areas such as electronics, energy storage, sensors, coatings, composite materials and biomedical devices, just to name a few of its applications.
"When we first discovered flash Joule heating and applied it to upcycle waste plastics into graphene, we observed large amounts of volatile gases being produced and ejected from the reactor," Wyss said. "We wanted to know what they were, suspecting a mixture of small hydrocarbons and hydrogen, but lacked the instrumentation to study their exact composition."
With funding from the U.S. Army Corps of Engineers, the researchers obtained the equipment needed to analyze the gasification contents, and later discovered that their suspicions were correct: The process produced hydrogen.
"For example, we know that polyethylene is composed of 86 percent carbon and 14 percent hydrogen, and we have shown that we can recover up to 68 percent of the atomic hydrogen in it as a gas with a purity of 94 percent. For me, developing the methods and expertise to characterize and quantify all the gases produced by this method, including hydrogen, has been a difficult but rewarding process," Wyss said.
The researchers say their method produces fewer emissions than other hydrogen production methods, based on a life cycle assessment. Life cycle assessment is a technique used to analyze the overall environmental impact and resource requirements associated with production methods.
Compared to other hydrogen production methods from waste plastic or biomass deconstruction, the flash hydrogen production process provides improvements in both cumulative energy requirements (33-95% energy reduction) and greenhouse gas emissions (65-89% emission reduction). The researchers say a benefit of their flash-Joule heating process is that waste plastics do not need to be cleaned or separated, allowing the scrap material to be used to produce clean hydrogen at a negative cost. They plan to further understand the flash Joule heating mechanism to improve its scalability and optimize hydrogen production.