Breakthrough Recycling Turns Waste PET into Hydrogen

Abstract

Polyethylene terephthalate (PET) is a primary target for chemical plastic recycling due to its widespread use and relatively weak ester bonds in its structure. However, conventional PET depolymerization methods-such as alkaline hydrolysis, glycolysis, and methanolysis-are energy-intensive and require complex separation steps, which increase both costs and environmental impact. This study introduces a polyoxometalate-based recycling process to address these limitations. Under mild conditions (100 °C and low pressure in aqueous solution), polyoxometalates catalyze the depolymerization of PET via acid hydrolysis, producing high-purity terephthalic acid (TPA) and ethylene glycol (EG) as solid and liquid products, respectively. EG is further oxidized by polyoxometalates to yield valuable compounds such as glycolic acid and formic acid, while simultaneously storing electrons. Under optimized conditions, EG oxidation achieves high selectivity (∼85%) toward formic acid. The stored electrons can be utilized for low-energy hydrogen production (125 mA cm−2 at 1.2 V) or electricity generation (12.5 mW cm−2 at 0.05 V). Crucially, our techno-economic analysis reveals that this approach, which combines revenue from high-purity TPA and valorized co-products, is cost-competitive and has the potential to supply TPA at a price lower than that of the virgin material. This work presents a technically robust and economically viable pathway toward a circular economy for plastic waste.

Despite polyethylene terephthalate (PET) being one of the most widely recycled plastics, only about 20% of used PET bottles are actually recovered as high-quality raw materials. The majority are transformed into lower-grade fibers or fillers before eventually being discarded. Addressing this gap, researchers at UNIST have developed a novel chemical recycling process that not only restores PET to its original high-grade form but also produces valuable chemicals and clean hydrogen, all under mild conditions.

Led by Professors Jungki Ryu and Tae Hoon Oh from the School of Energy and Chemical Engineering, the team introduced a multifunctional catalyst that enables PET depolymerization at just 100°C-significantly milder than conventional methods that operate above 200°C. The process involves mixing shredded PET with water, a solvent (DMSO), and the catalyst, then heating the mixture. This causes the plastic to break down into terephthalic acid (TPA), a high-quality raw material for PET production, and ethylene glycol (EG), a valuable chemical.

A key advantage of this approach is its simplicity: after filtration, high-purity TPA can be recovered for reuse. Additionally, the process produces formic acid (FA), a high-value chemical, while the catalyst can be recycled multiple times. Interestingly, the catalyst also functions as an electron reservoir, storing electrons during PET breakdown. These stored electrons can be harnessed to produce hydrogen efficiently at low voltage or generate electricity through fuel cells, offering an integrated solution for energy recovery.

Experimental results demonstrated that hydrogen could be generated at a voltage 25% lower than typical water electrolysis-at just 1.2 volts-and a fuel cell system produced 12.5 milliwatts per square centimeter. Economic analysis further revealed that the minimum selling price of the recycled TPA is estimated at just $0.81 per kilogram, making it up to 46% cheaper than traditional chemical recycling methods and more affordable than virgin TPA derived from crude oil.

Professor Ryu stated, "Our process shows that high-quality recycled PET and clean hydrogen can be produced cost-effectively under mild conditions. This not only supports a circular economy for plastics but also offers a sustainable pathway for hydrogen energy production."

Supported by the Ministry of Science and ICT (MSIT) and the InnoCORE program of hydro*studio at UNIST, this research has been featured as the back cover of the January 2026 issue of Green Chemistry. Additional support was provided by the Basic Science Research Program, the Regional Leading Research Center (RLRC), and the Engineering Research Center of Excellence Program, all funded by the National Research Foundation (NRF) of Korea. The project also received backing from the Regional Innovation System & Education (RISE) through the Ulsan RISE Center, funded by the Ministry of Education (MOE) and Ulsan Metropolitan City.

Journal Reference

Hyeonmyeong Oh, Ye Chan Lee, Inhui Lee, et al., "Mild hydrolysis of PET and electrochemical energy recovery via multifunctional polyoxometalate catalysts," Green Chem., (2026).