A new technology has been developed that converts mixed waste plastics, which have not been recycled because separation and sorting are difficult, into high-purity hydrogen. It operates at lower temperatures and pressures than existing methods and can also cut carbon emissions, drawing attention as a technology that can address both waste-plastic treatment and clean-energy production at the same time.
A research team led by Kim U-jae, a professor in the Department of Chemical and Materials Engineering at Ewha Womans University, said it developed an "alkaline heat treatment" process that converts mixed plastic waste into high-purity hydrogen. The findings were published on the 7th in the Proceedings of the National Academy of Sciences (PNAS).
Currently, only about 9% of plastic waste worldwide is actually recycled. Most of the rest is landfilled or incinerated. According to the research team, about 79% of plastic waste is landfilled and about 12% is incinerated.
One reason the recycling rate is low is that waste plastics are typically discarded as a mix of various types, reducing recycling efficiency. Polyethylene terephthalate (PET), widely used for beverage bottles; polyethylene (PE), used for plastic bags and packaging; and polypropylene (PP), widely used for household goods and containers, are all plastics with different properties. When they are mixed, they must be sorted by type before recycling, a process that is complex and costly.
To solve this problem, the research team not only treated discarded plastics but also found a way to reuse them as a hydrogen resource. The newly developed alkaline heat treatment process breaks down plastics using alkaline substances such as sodium hydroxide and obtains hydrogen in the process. Sodium hydroxide, commonly called "caustic soda," is a strongly alkaline substance that helps break the rigid chemical structure of plastics.
Technologies to convert mixed waste plastics into hydrogen already existed. A representative method is gasification, which decomposes waste at very high temperatures and pressures to obtain gaseous fuel. But gasification has limitations, including high energy consumption, stringent process conditions, and significant carbon emissions during treatment.
In particular, PE and PP have stable chemical structures and do not decompose easily. To address this, the research team added a thermal oxidation pretreatment step. Before full processing, the plastics were oxidized to make them more reactive. The team then adjusted the thermal oxidation conditions and the ratio of sodium hydroxide to plastic to boost hydrogen production efficiency.
As a result, the research team confirmed that high-purity hydrogen can be produced not only from individual plastics but also from mixed waste plastics. The team noted that, unlike conventional gasification, there is no need to meticulously sort plastics one by one, enhancing practicality.
Hydrogen is considered a clean fuel that emits little carbon dioxide when burned or used for energy. However, if producing hydrogen requires a lot of energy or emits carbon, its environmental benefits can diminish. This technology is meaningful in that it uses discarded plastics as feedstock while producing hydrogen under relatively low-energy conditions.
The research team said, "Compared with conventional gasification, it could reduce energy burdens and carbon emissions, so it may be used as a sustainable recycling technology," adding, "the alkaline heat treatment process could become a new upcycling pathway that converts plastic waste into clean hydrogen fuel."
References
PNAS (2026), DOI: https://doi.org/10.1073/pnas.2537552123