In 2023, Rice researchers flashed mixed waste plastic for about 4 seconds at 3,100 K; the process recovered up to 68% of its hydrogen at purities reaching 94% and produced graphene

Although hydrogen is known as the fuel of the future, currently the majority of hydrogen is being produced through a technology that only makes the problem even worse. The U.S. Department of Energy's overview of hydrogen production from natural gas reforming puts the figure at more than 95% of hydrogen produced worldwide today being made through steam methane reforming, a process responsible for large amounts of carbon dioxide emissions. At the same time, another technology, which can produce hydrogen by using water electrolysis and renewable energy, is much too expensive to compete. The inability of the hydrogen industry to balance between the demand for this fuel and the expenses connected with its production is one of the reasons why this industry is looking for some other alternatives.

Chemists at Rice University have developed an innovative solution to this contradiction, using an unexpected raw material: plastic waste found in landfills. Instead of considering this waste simply as something that needs to be disposed of, the scientists turned this plastic into raw material, obtaining hydrogen gas and graphene from it and light hydrocarbons as by-products, without having to clean or sort it.

Splitting plastic into hydrogen and graphene at 3,100 Kelvin


In 2023, researchers in the Tour Lab at Rice University exposed samples of mixed waste plastic material that did not need to be sorted by type or washed beforehand to a technique called flash Joule heating. The plastic was blended with a conductive material, packed into a tube, and hit with a high-voltage electrical pulse for about four seconds, according to a Rice University news release describing the work. During that brief window, the temperature inside the reactor shot up to roughly 3,100 Kelvin, or about 2,827°C. At that heat, the hydrogen locked inside the plastic vaporizes almost instantly, while the carbon atoms left behind rearrange themselves into graphene, a single-layer sheet of carbon known for its strength and conductivity. This work was published in the journal Advanced Materials, in a paper titled 'Synthesis of Clean Hydrogen Gas from Waste Plastic at Zero Net Cost', a title that reflects the paper's economic model rather than a guaranteed market outcome. The process recovered up to about 68% of the hydrogen locked inside common plastics, with purities reaching 94%, figures the research team confirmed through direct measurement rather than estimation.

How this compares with grey and green hydrogen production

To comprehend the importance of this finding, it may be helpful to consider the methods of hydrogen production currently available. The world's demand for hydrogen gas is more than 90 million tonnes per year, and according to the International Energy Agency's Global Hydrogen Review 2022, more than 95% of current worldwide production takes place through steam methane reforming, which results in 9 to 12 tonnes of carbon dioxide per tonne of hydrogen produced directly. Although electrolysis of water ('green' hydrogen) does not produce such emissions, it costs about two to three times as much, a gap that, per the IEA's World Energy Investment 2020 report, has so far kept it from replacing fossil-fuel-produced hydrogen.

In this regard, flash Joule heating solves both of the problems, at least as concerns the reaction itself. Since the plastic provides its own carbon and hydrogen, the procedure requires no metal catalyst and produces no direct carbon dioxide emissions when breaking down polyolefin-based plastics like polyethylene and polypropylene, since the resulting carbon becomes part of the graphene. That figure covers only the reaction itself, though: once the full life cycle is considered, including the emissions from generating the electricity that powers the device, Scientific American's coverage of the study reports that researchers assess the emissions reduction at 39% to 84% compared with other hydrogen production methods, lower than 'zero,' but still a substantial cut. Polyethylene, the most widespread plastic type in use, is 86% carbon and 14% hydrogen by weight and it is the ability to extract most of the hydrogen from polyethylene as gas that makes the reaction so important.

Selling graphene below market price to make hydrogen free

The economics of all of this hinges upon the leftover graphene. Rice University's announcement of the findings quotes senior author James Tour and first author Kevin Wyss, who note that selling the graphene at just 5% of its market value, a modeled economic threshold rather than a guaranteed sale price, would be enough to cover the total cost of producing the hydrogen, effectively letting the plastic pay for itself. Tour further noted that because hydrogen demand is anticipated to soar in the years ahead, it would be economically untenable to continue using processes with high levels of emissions if the world is to reach net zero by 2050.

The scale of the world's plastic waste problem

However, none of this would carry much weight if plastic waste were a problem that could be ignored, which it is not. A widely cited study in the journal Science Advances, 'Production, Use, and Fate of All Plastics Ever Made,' found that by 2015 the world had cumulatively produced 6,300 million metric tons of plastic waste, of which only 9% had been recycled and 12% had been incinerated. The remaining 79% was sitting in landfills or in the environment, though the study notes some of it may have degraded or been exported rather than remaining in place. On the trend at the time of the study, the researchers project that by 2050, around 12,000 million metric tons of plastic waste will have accumulated in landfills or the environment, a trajectory that, a decade on, still has years left to run. Those figures make a process that turns plastic waste into hydrogen and graphene especially appealing: it avoids the expensive sorting and cleaning that make plastic recycling costly, and for the reaction itself, if not across its full life cycle, it adds no additional carbon dioxide.

Moving from lab-scale results to industrial production

Flash Joule heating has not been scaled past laboratory scale yet. Moving a chemical reaction from grams per day to industrial volumes is always a challenge, and here that challenge is compounded by market dependence: the process's economics rest on graphene continuing to command a strong market price, so its viability outside the lab will follow that market as much as the chemistry itself.