Jolts of electricity can convert discarded plastic into hydrogen gas and graphene, potentially offering a way to manage a growing waste problem while also producing an environmentally-friendly fuel (Adv. Mater. 2023, DOI: 10.1002/adma.202306763). The team behind the work, led by James M. Tour at Rice University, calculates that the commercial value of the graphene should more than offset the costs of the process, essentially offering a free source of hydrogen gas.
“This is a solved problem now, to get hydrogen for free—it just has to be scaled up in industry,” Tour says.
“I think it’s a really important advance, it’s very exciting,” says Peter Edwards at the University of Oxford, who has developed a microwave-based method for extracting hydrogen from plastics, and was not involved in the research. “It’s showing again that plastics waste is actually a resource.”
The United States recycles less than 10% of its plastic waste. The remainder, often containing intractable mixtures of different plastics, is incinerated, sent to landfills, or discarded as litter. Although pyrolysis can convert mixed plastic waste into useful hydrocarbons, this energy intensive approach has also raised environmental concerns.
Meanwhile, there is huge demand for hydrogen as a clean-burning fuel and chemical feedstock. Yet more than 95% of global hydrogen production comes from steam-methane reforming, which generates 11 kg of carbon dioxide per kg of H 2 , and using renewable sources of electricity to produce ‘green’ hydrogen relies on expensive metal catalysts such as platinum.
Tour says his process can tackle both of these challenges at once, because plastics are such a rich source of hydrogen atoms. The method relies on flash Joule heating, in which a burst of electrical current superheats samples until they decompose. Tour’s team has previously used this to recycle graphite battery anodes, and to produce graphene from petroleum waste, coal and plastic. Although they didn’t know it at the time, their previous experiments on plastics also released hydrogen, says Tour.
In the latest work, the researchers ground up polyethylene and added small amounts of a conductive additive such as carbon black. Then they loaded the material into a small quartz tube with a hollow electrode that allowed gas to escape. Several jolts of current, each less than 3 s long, heated the sample to about 2800 °C. This converted up to 93% of the hydrogen atoms present in the polymer into hydrogen gas with a purity of 87%. Using mixed plastic waste also produced hydrogen, albeit at somewhat lower yields and purity.
The carbon atoms left behind formed tiny flakes of graphene, several atoms thick, which agglomerated into disordered stacks. Tour says this material is suitable for use as an additive to strengthen composite materials.
The process joins a growing list of methods to mine hydrogen from plastic waste. “It’s a hot topic,” says Taylor Uekert, a circular economy researcher at the National Renewable Energy Laboratory in Colorado.
Uekert previously worked on a photochemical process to extract hydrogen from waste plastic. Another option uses pyrolysis to turn polymers into small hydrocarbons that can be reformed into H2, CO and CO2. In a third approach, Edwards’s microwave-based technique liberates hydrogen directly, leaving carbon behind as nanotubes.
But Tour points out that flash Joule heating requires no catalyst, and uses less energy than other ways to extract hydrogen from waste plastic. If it runs on renewable electricity, it also produces very little CO2. His team calculates that even if the graphene they produce were sold at just $3 per kg—about 5% of its current market price—that income would more than pay for the operational costs.
Still, Uekert says that mechanical recycling tends to be a very energy-efficient way to turn plastic waste into fresh plastic. From an environmental point of view, mining hydrogen from plastic waste might be best applied to waste that is unsuitable for recycling. “You need to be using the heavily contaminated plastic waste that no one wants to buy,” she says. She adds that the team should carry out a more comprehensive life-cycle analysis of how the process would operate in a pilot-scale plant, including the energy used to purify the hydrogen and the toxicity of any materials involved.
The company Tour cofounded, Universal Matter, plans to operate a demonstration plant in Ontario that will produce roughly 1 metric ton of graphene per day, from waste materials and biomass, by the end of this year. Tour expects they will subsequently try to capture hydrogen released from the process, and insists that harnessing the flammable gas will be no more dangerous than steam-methane reforming. “Once the engineers get hold of this, I don’t think this is going to be a problem at all,” he says.