Electrocatalytic Upcycling of PET Waste to Chemical Products and Fuels Coupled With Cathodic Reactions

Abstract

Plastic waste continues to be a major environmental challenge, worsened by energy‐intensive conventional recycling methods that require highly pure feedstocks. In this review, emerging electrochemical upcycling technologies are critically examined, focusing on the electro‐oxidation transformation of polyethylene terephthalate (PET) into valuable chemical products. Key reaction pathways and target products are outlined to clarify the selective electrochemical reforming of PET. The key focus is on the integration of anodic PET oxidation with complementary cathodic reactions, including water, carbon dioxide, and nitrate reduction reactions, to demonstrate how integrated systems can simultaneously transform plastic waste into valuable products and generate sustainable fuels. Furthermore, a techno‐economic analysis is provided to highlight the essential factors that will influence the transition towards industrial‐scale implementation. This review is intended to advance new research approaches and innovative strategies for improving electrochemical PET recycling techniques, supporting broader goals in circular economy growth and sustainable chemical manufacturing.

This review offers a comprehensive overview of recent progress in electrocatalytic upcycling of polyethylene terephthalate (PET) waste, with a key focus on reaction mechanisms, catalyst design, and system integration. It highlights strategies for integrating anodic PET oxidation with complementary cathodic reactions, including water, carbon dioxide, and nitrate reduction reactions to simultaneously produce valuable chemical products and green fuels. Future directions for scaling up electrochemical PET upcycling are discussed, addressing key challenges toward industrial‐scale implementation of these emerging technologies.

1. Introduction

Plastic waste has emerged as one of the most pressing global environmental challenges, driven by the massive consumption of durable, non‐biodegradable polymeric materials.^{[ 1} , ^{2 ]} Among these, polyethylene terephthalate (PET) is the fourth most produced thermoplastic worldwide, following polyethylene, polypropylene, and polystyrene.^{[ 3} , ⁴ , ^{5 ]} PET is synthesized through the polycondenzation process of ethylene glycol (EG) with either dimethyl terephthalate (DMT) or terephthalic acid (TPA), resulting in a polymer with remarkable mechanical strength, high chemical resistance, low thermal expansion, and excellent dimensional stability.^{[ 6} , ⁷ , ⁸ , ^{9 ]} This unique combination of properties makes PET an ideal material for various applications spanning from food and beverage packaging to textiles and medical devices, and electronics.^{[ 10} , ¹¹ , ¹² , ^{13 ]} However, the high stability of PET, while favorable for its applications, on the other hand it highly resistant to environmental degradation. Consequently, PET waste accumulates persistently, contributing to the global plastic pollution crisis.^{[ 14} , ^{15 ]}

Since the 1950s, more than 8.3 billion tonnes of non‐degradable PET have been produced, yet only about 11% have been effectively recycled.^{[ 16 ]} PET degradation is exceedingly slow, with estimated lifetimes exceeding 450 years under natural conditions.^{[ 17} , ^{18 ]} Alarmingly, it is estimated that nearly one million PET bottles are sold globally every minute, with approximately 90% ending up discarded or improperly managed, making the environmental problem worse. These facts make it clear that we need better, more long‐lasting ways to deal with PET waste soonest possible.^{[ 19} , ^{20 ]}

Conventional recycling methods, such as mechanical and chemical recycling, face significant challenges.^{[ 21} , ^{22 ]} Mechanical recycling implies the physical processing of PET by melting and extrusion to produce new products from recycled PET materials.^{[ 23} , ^{24 ]} While cost‐effective, repeated processing degrades polymer properties, limiting the quality and applications of recycled materials. Chemical recycling techniques, including pyrolysis,^{[ 25 ]} hydrogenolysis,^{[ 26 ]} solvolysis,^{[ 27 ]} and catalytic oxidation^{[ 28 ]} facilitate PET depolymerization into monomers and valuable chemical building blocks.^{[ 29} , ³⁰ , ³¹ , ^{32 ]} However, these processes often require harsh reaction conditions such as elevated temperatures, high pressures, and aggressive chemical environments, raising concerns over scalability, energy consumption, and environmental footprint.^{[ 33} , ^{34 ]} In contrast, functional upcycling offers a viable alternative by transforming PET waste into value‐added chemical products with enhanced usefulness.^{[ 35} , ^{36 ]} In this context, electrocatalytic upcycling^{[ 37} , ³⁸ , ^{39 ]} has emerged as an effective approach, utilizing renewable electricity to facilitate chemical changes under mild conditions.^{[ 40} , ⁴¹ , ⁴² , ^{43 ]} This approach has the potential to address the fundamental limitations of conventional recycling methods, contributing toward a more sustainable and circular plastics economy.^{[ 44} , ^{45 ]}

Electrocatalytic PET upcycling typically starts with hydrolytic depolymerization, producing TPA and EG as primary intermediates. The electrochemical oxidation of EG at the anode can subsequently yield valuable products such as formate, formic acid, or glycolic acid, while TPA can be purified and reused for new PET production.^{[ 46} , ⁴⁷ , ⁴⁸ , ^{49 ]} More importantly, the anodic PET oxidation reaction can be synergistically coupled with cathodic reactions, including hydrogen evolution reaction (HER), carbon dioxide reduction reaction (CO₂RR), and nitrate reduction reaction (NO₃RR). This enables co‐production of green fuels such as hydrogen and ammonia, while mitigating CO₂ reduction and valorizing multiple waste streams simultaneously.^{[ 50} , ⁵¹ , ⁵² , ^{53 ]} These integrated co‐electrolysis systems not only replace the energy‐intensive oxygen evolution reaction (OER) with a more favorable oxidation process,^{[ 54} , ^{55 ]} but also increase overall energy efficiency and economic value.^{[ 56} , ^{57 ]} Translating these fundamental electrochemical advancements into practical solutions involves more than just optimization at the reaction level. The performance, selectivity, and long‐term stability of the overall system are fundamentally reliant on the design of efficient, cost‐effective electrocatalyst materials and scalable reactor architectures. Integrating anodic PET oxidation with compatible cathodic reactions under realistic operating conditions remains a central engineering challenge.^{[ 58} , ^{59 ]} Moreover, when powered by renewable electricity, such systems offer a dual benefit, reducing dependence on fossil‐derived feedstocks and enabling carbon‐neutral recycling strategies aligned with global decarbonization goals.^{[ 57} , ⁵⁸ , ^{59 ]} Recent research has focused on developing advanced catalysts, including transition metal‐based oxides,^{[ 60 ]} hydroxides, oxyhydroxides^{[ 61} , ^{62 ]} carbon‐based materials,^{[ 63} , ⁶⁴ , ^{65 ]} and single‐atom catalysts,^{[ 66} , ^{67 ]} to enhance the selectivity, efficiency, and operational stability.^{[ 68 ]} Meanwhile, addressing system‐level challenges such as scalability, feedstock variability, and cost‐performance trade‐offs remains essential for commercial deployment.^{[ 69} , ⁷⁰ , ⁷¹ , ^{72 ]}

In this review, we provide a comprehensive and critical review of the current status and future potential of electrocatalytic PET upcycling. We begin with mechanistic insights into the electrochemical oxidation of PET‐derived intermediates, followed by a discussion on recent advancements in coupling PET oxidation with cathodic reactions such as HER, NO₃RR, and CO₂RR. Finally, we evaluate the techno‐economic analysis and environmental feasibility of these integrated systems and highlight the key scientific and engineering challenges that must be addressed to accelerate their path toward industrial implementation.

2. PET Waste as Feedstock for Value‐Added Chemicals

Global PET waste production has reached alarming levels, yet recycling accounts for only a small fraction of this waste.^{[ 73 ]} Traditional waste management follows a linear system based on the “take‐make‐dispose” model, which produces major environmental pollution.^{[ 74 ]} PET recycling methods are generally categorized into mechanical, thermal, and chemical recycling processes, as discussed in the following section:

2.1. PET Recycling Methods

2.1.1. Mechanical Recycling

The most widely used recycling technique is mechanical recycling, involving the collection, sorting, and cleaning of plastic trash, which is then recycled into new products.^{[ 75} , ^{76 ]} This technique has the potential to produce new products that are more energy‐efficient and cost‐effective, as it is based on thermoplastics such as Polypropylene, Polyethylene, and Polyethylene terephthalate as shown in Figure 1. Mechanical recycling has plenty of challenges, including recurring property degradation across multiple recycling cycles, mixed and contaminated waste collection, and difficulties in sorting various plastic materials.^{[ 77 ]}

Figure 1.

Generalized illustration of the flow diagram of plastic bottle recycling.^{[ 26 ]} Adapted from the Green Chem., 2022,24, 8899–9002, No permission required (Open Access).

2.1.2. Thermal Recycling

Thermal recycling refers to processes that recover energy or chemicals from PET waste through heat‐based methods. This includes incineration, which combusts plastics to generate heat and electricity, and pyrolysis, which thermally decomposes plastics in the absence of oxygen to produce fuels and basic chemical compounds.^{[ 78 ]} The combustion technique reduces landfill waste but generates toxic atmospheric emissions and hazardous contaminants, creating substantial environment risks. Pyrolysis has not been widely adopted due to its significant energy requirements and complex equipment required for decomposition process, with the significant production of liquid fuels, syngas, and carbon waste.^{[ 26} , ^{79 ]}

2.1.3. Chemical Recycling

Chemical recycling is a feedstock recycling method that depolymerizes PET polymers into their original monomers and valuable compounds using hydrolysis, solvolysis, gasification, and catalytic depolymerization processes.^{[ 79 ]} Chemical recycling efficiently generates materials of comparable quality to virgin plastics from recycled PET polymers. In addition, chemical recycling is expensive and requires complex processes and facility infrastructure, due to which it faces industrial challenges.^{[ 80 ]}

Despite improvements in recycling technologies, large‐scale plastic recycling initiatives are still hampered by limited consumer participation, financial constraints, and ineffective sorting techniques. To enhance recycling efficiency and mitigate PET pollution, it is imperative to implement a circular economy model that incorporates enhanced sifting technologies, improved collection systems, and advancements in electrocatalytic upcycling.

2.2. Electrocatalysis as a Tool for Breaking Down Complex PET Polymers into Monomers and Aromatic Compounds

Recent developments in chemical recycling offer sustainable ways to break down PET into its basic chemical components, such as monomers and aromatic compounds, so they may be used again in industry.^{[ 82 ]} These advancements are essential for sustainable plastic waste management, reducing dependency on petrochemical feedstocks and promoting a circular economy.

Depolymerization is one of the most efficient ways to recover PET monomers like TPA and EG among the various chemical recycling procedures.^{[ 5 ]} Chemical depolymerization techniques mainly emphasize their potential to transform PET waste into valuable raw materials. This approach addresses the worldwide plastic waste problem and ensures that recovered monomers can be reintroduced into the polymer production cycle. Theoretical insights have also demonstrated that PET degradation via glycolysis, employing aromatic solvents to facilitate the conversion of PET into bis(2‐hydroxyethyl) terephthalate (BHET), is an essential intermediary for PET repolymerization.^{[ 83 ]}

Beyond glycolysis, hydrothermal liquefaction (HTL) has attracted increasing interest as another approach for PET breakdown. HTL is known as an effective approach for synthesis of both monomers and important aromatic molecules. Recent advancements have shown that the controlled thermal degradation of PET waste can produce a wide spectrum of hydrocarbons suitable for multiple industrial applications.^{[ 23 ]} For example, one of the studies in the field of HTL has demonstrated that an integrated plant design model can effectively transform PET waste into alkyl aromatics, which are the main precursors in the chemical and pharmaceutical sectors. These investigations provide scalable substitutes for conventional incineration and landfill disposal, therefore highlighting the industrial potential of chemical PET recycling.^{[ 57 ]}

Enzymatic pathways are another emerging method for PET depolymerization, investigate the possibilities of enzymes such as PETase and MHETase, has shown the extraordinary efficiency in breaking down PET under moderate reaction conditions. Enzymatic depolymerization is considered an appealing, environmentally acceptable alternative to chemical methods, as it generates fewer toxic byproducts and requires lower energy input. Although bio‐based PET recycling has significant potential for sustainable plastic waste management even though its industrial applications are still in its early stages.^{[ 81} , ^{82 ]}

Chemo‐lytic depolymerization, including hydrolysis, glycolysis, and enzymatic degradation, has also been considered an efficient recycling technique.^{[ 83} , ^{84 ]} Recent developments in this field underscore the significance of matching different PET materials with specific depolymerization pathways.^{[ 85} , ^{86 ]} For instance, innovative glycolysis catalysts have been introduced to improve monomer recovery, particularly for polyester textiles, which have previously faced recycling challenges. The role of green catalysts, such as diatomaceous earth, has been demonstrated to enhance the hydrolysis efficiency of PET into TPA and EG. Alternative applications for recycled PET have been proposed, for converting post‐consumer PET into thermoset alkyd coatings, has shown the improved life‐cycle performance for industrial coatings and adhesives.^{[ 87 ]}

A significant advantage of PET chemical recycling is its ability to contribute to fostering a circular economy. By helping PET to be selectively broken down into high‐purity monomers that underlie back‐to‐monomer recycling minimizes the dependence on virgin petroleum‐based plastics.^{[ 88 ]} The objective of this approach is to close the plastic life cycle loop, which is consistent with the global sustainability agenda. However, more technological advances and industrial scalability are needed to fully realize its potential in conventional waste management systems.

2.3. Progress in Upcycling of PET Electrocatalysis

The electrocatalytic process has recently become significant as an eco‐friendly and effective method for transforming complex polymeric materials into useful chemical compounds as shown in Figure 2. Unlike conventional recycling techniques, electrocatalysis operates under mild reaction conditions and utilizes renewable energy to selectively break molecular bonds, making it a more sustainable option.^{[ 56} , ^{94 ]} PET conversion is among the most thoroughly investigated uses in plastic upcycling. PET can be depolymerized into its monomeric components, such as TPA and EG, which can then transform into high‐value compounds by electrochemical oxidation. For example, formate and glycolate are produced by EG oxidation both of which have extensive usage in the industrial, agricultural, and pharmaceutical industry.^{[ 95} , ^{96 ]}

Figure 2.

Schematic illustration of chemical catalytic upcycling of PET plastics waste into materials, fuels, and chemicals.

Since electrolysis is conducted in liquid electrolytes, pretreatment of solid‐state PET offers a significant challenge in the processing of PET hydrolysate.^{[ 97 ]} PET hydrolyzed under acidic, alkaline, or neutral conditions effectively generates TPA and EG monomers without the introduction of catalytic impurities, thereby providing a suitable reaction environment for electrocatalysis. In this context, hydroxide (OH⁻) ions serve a dual function in electrocatalytic oxidation by promoting the production of reactive oxygen species (•OH) and establishing an ideal reaction environment for oxidative reaction. As a result, PET alkaline hydrolysate is considered an optimal electrolyte for the electrocatalytic valorization of PET.^{[ 23} , ^{81 ]} However, the ambient‐temperature PET hydrolyses usually proceeds slowly and insufficiently; therefore, hydrothermal and solvothermal methods are used to accelerate the depolymerization.^{[ 98} , ^{99 ]} Additionally, microwave and sonication‐assisted techniques have been explored to enhance polymer chain breakdown. In alkaline conditions, TPA and EG can be produced by hydrolyzing commercial PET bottles, which are generally processed into powder or flakes. However, the complete hydrolysis is not always achieved due to the limited mass transfer, inadequate reaction conditions, and the presence of additives or contaminants in post‐consumer plastics.^{[ 94} , ^{100 ]} Quantitative data correlating PET precursors to monomer yields is limited, highlighting the necessity for a standardized pretreatment strategy. Establishing a precise ratio diagram to correlate PET precursor concentrations to lysate compositions may facilitate the precise quantification of TPA and EG, thus enhancing the overall efficiency of electrocatalytic processes.^{[ 101} , ^{102 ]}

Table 1 summarizes key factors in electrochemical PET upcycling, including pretreatment methods, electrolysis conditions, electrochemical performance metrics, and post‐treatment of the resulting products. As the field progresses, further research into catalyst design, reaction kinetics, and process scalability will be crucial in enhancing the feasibility of electrocatalytic plastic upcycling as a sustainable waste management strategy.

Table 1.

Summary of PET electrocatalytic upcycling.

PET pretreatment	Catalyst	Electrolyte	Faraday Efficiency of Anodic product	Cell Voltage	Stability Test	Post‐treatment Product	Refs.
1.5 g PET was dispersed in 20 mL of 6 M KOH, reacted at 60 °C for 24 h.	Pt/Ni(OH)2	1 M KOH +0.1 M EG	Formate (>90%)	1.44 at 1000 mAcm−2	500 h of continuous operation with no significant degradation.	The electrolyte was acidified for TPA with further condenzation and crystallization for KDF.	[66]
8 g PET was reacted in 200 mL of 4 M KOH at 80 °C in an oil bath for 48 h, then diluted to 1 M KOH to obtain PET hydrolysate.	TbCo‐MOF/NF	1 M KOH +0.5 M EG	Formate (97.3%)	1.55 V of cell voltage to reach at 10 mA cm−2	60 h continuous operation with minimal degradation.	TPA powder further crystalized to KDF.	[45]
A clean PET bottle was cut into threads, washed, dried, and 0.5 g was reacted in 30 mL of 2 M KOH at 180 °C for 2 h in a teflon‐lined autoclave, then cooled, filtered, and used as an electrolyte.	CuCoO@rGO	1 M KOH +0.1 M EG	Formate (85.7%)	1.9 V at 10 mA cm−2	22 h.	Formic acid, KDF, and TPA	[89]
4 g plastic bottle cut into pieces was pretreated in 100 mL of 4 M KOH solution at 80 °C for 70 h.	Nitrogen‐doped Ni3P–NiMoO4/NF	Direct PET Hydrolysate	Formate (92.2%)	197 mV decrease of voltage for reaching the current density 100 mA cm− 2	10 mA cm−2 2 is quite stable for 20 h.	N/A	[71]
PET of 10 g was added to 200 mL of 1 M NaOH solution and heated to 80 °C for 24 h with continuous stirring.	Pd‐CuCo2O4/NF	1 M NaOH + 0.5 M NaCl + 1 M EG	Glycolic acid (96.1%)	1.15 V versus RHE at 600 mA cm−2	100 h at an industrial grade current of 1.6 A.	TPA separation.	[50]
2.0 g dried powder was dispersed in 100 mL of 2 M KOH aqueous solution in a flask. Then, the flask was sealed with a rubber stopper and heated on a hotplate at 60 °C with stirring (1500 rpm) for a specific time (48 h).	Co3O4/NF	1 M KOH with different concentration of EG.	TPA and KDF (89% of FE).	1.2 V versus NHE.	Maintained up to 15 h with electrolyte refresh every 3 h.	TPA and KDF.	[88]
PET plastic waste is being hydrolysed into PET hydrolysate.	Ru‐CoOOH/NF	Direct PET Hydrolysate	Formate FE of 96.53%	1.53 V at 50 mA cm−2	10 mA cm−2 for 45 h	Formic acid and KDF.	[21]
16 g PET powder (300 mesh) was dispersed in a reagent bottle containing 200 mL of 4 M KOH aqueous solution, heated in an oil bath at 80 °C for 70 h.	PdNi Bimetallenes	1 M KOH + 1 M EG	Formic Acid	1.64 V at 10 mA cm−2.	10 mA cm−2 for 13 h	Formic acid and TPA.	[58]
PET, 6.3 g dried powder (PETTMCB‐ 102, 300 mesh, Dupont) was dispersed in 100 mL of 2 M KOH aqueous solution in a flask.	CoNi0.25P/NF	1 M KOH + 0.3 M EG	Formate (>80% selectivity)	1.8 V at 500 mA cm−2	1.5 V versus RHE for 33 h.	KDF and TPA.	[40]
PET (3.5 g), DI water (120 g), and KOH (6.732 g) were placed in a quartz‐lined autoclave. The reactor was heat treated at 160 °C under magnetic stirring for 60 min.	NiCo2O4/C	1 M KOH + 1 M EG	Formate over 90% FE	1.42 V vs RHE) under a current density of 50 mA cm−2	1.42V versus RHE for 24h in PET hydrolysate.	After filtering TPA obtained by the acidification of electrolysate, the filtrate containing FA and KCl is condensed and crystallized to separate products.	[90]
100 g of PET plastic powder was placed in 200 mL of 10 M KOH aqueous solution with continuous stirring at 90 °C for 32 h.	PdCu metallene/NF	1 M KOH + 1 M EG	Glycolic acid (FE: 92.12%)	0.67 V at 100 mAcm−2	N/A	N/A	[91]
0.48 g of PET plastic bottle pieces (0.5 ×0. 5 cm) were introduced into a 12 ml solution containing DMF and water (2:1) and heated up to 180 °C for 15 h in a 25 ml stainless steel autoclave.	RuxMn1.2Co0.8Oy/CFP	0.5 M H2SO4	N/A	214 mV decrease of voltage for reaching the current density 10 mA cm− 2	500 h at 10 mA cm−2 with minimal degradation	N/A	[92]
The PET plastic bottle was cut into small pieces, and 10.0 g of the PET pieces were placed in 6 M KOH and depolymerized at 90 °C for 12 h.	F/CNTs/CoNi/CNCs/ NF	Direct PET Hydrolysate	Formic Acid F.E 90.7%	250 mV decrease of voltage for reaching the current density 10 mA cm− 2	10 h at 10 mA cm−2 with minimal degradation	N/A	[93]

3. Mechanistic Insights into PET Upcycling Coupled with Cathodic Reaction

Prior to electro‐reforming, PET undergoes hydrolysis, producing a hydrolysate that contains TPA and EG. Different modified hydrolysis techniques have been investigated, including neutral, enzymatic, acid, and alkaline hydrolysis.^{[ 103} , ^{104 ]} Alkaline hydrolysis has garnered significant interest owing to its cost‐effectiveness and the high purity of TPA. The results of PET hydrolysis are significantly affected by the type, origin, and particle size of PET. Moreover, essential reaction parameters such as temperature, KOH concentration, PET loading, stirring speed, and reaction duration significantly influence the selectivity and efficiency of the hydrolysis process.^{[ 105 ]}

3.1. Pathway 1: EG to Formate or Formic Acid (FA)

Formic acid is a chemical intermediate that is utilized in a variety of industrial processes, including leather tanning and textile dyeing, and as a preservative and antibacterial agent in livestock.^{[ 106} , ^{107 ]} Formate salts, including sodium and potassium formate, serve as de‐icing agents and reducing agents in chemical synthesis. The selectivity of formic acid and formate production is critical in catalytic and electrochemical processes because it influences efficiency, yield, and applicability in energy storage, fuel cells, and sustainable chemical manufacturing.

In this section, we describe the mechanistic pathways through which PET‐derived EG can be electro‐oxidized to FA and formate. The electrooxidation process begins with the adsorption of EG onto the catalyst surface and proceeds through a series of intermediates, ultimately resulting in the formation of formate as the final product. As shown in Figure 3a, the key intermediates involved in this process include *EG, *glycolaldehyde, *glyoxal, *glycolic acid, and *formate. ^{[ 108 ]} One proposed pathway involves the oxidation of EG to glycolaldehyde, followed by oxidative cleavage of the C‐C bond to produce formaldehyde and formic acid. This process follows the Cannizzaro reaction, which occurs in alkaline environments and leads to the formation of formaldehyde and methanol, both of which can subsequently undergo further oxidation to form formic acid, as shown in Figure 3c.^{[ 109 ]}

Figure 3.

a) Proposed reaction mechanism of the EGOR to form formate over a CuCo₂O₄ NWA/NF electrode under alkaline conditions. b) Proposed reaction pathway to produce formate in alkaline solution via EGOR. Adapted with the permission.^{[ 108 ]} Copyright 2022, The Royal Society of Chemistry. c) Free energy diagram for formate production and optimized configurations of the EGOR surface species on the Co₂CuO₄ catalyst. d) ¹H NMR spectra of products before and after electrolysis. Adapted with the permission.^{[ 109 ]} Copyright 2022, The Royal Society of Chemistry.

An alternate mechanistic route has been proposed based on density functional theory (DFT) calculations, as shown in Figure 3b, where the structures and energetics of each intermediate during the EGOR were optimized. The corresponding free energy diagram provides critical insight into the reaction pathways. The transition from *glycolaldehyde to *glycolic acid has a calculated energy barrier of 1.30 eV, which is 0.22 eV higher than that for the formation of *glyoxal (1.08 eV). These results suggest that *glyoxal formation is more thermodynamically favorable than the *glycolic acid pathway. This theoretical predication aligns well with experimental findings, particularly NMR analysis shown in Figure 3d, which confirms *glyoxal is the main reaction intermediary, which is consistent with experimental data showing that *glyoxal is the dominant intermediate during EGOR. The experimental results indicate that EG primarily oxidizes via the *glyoxal intermediate, achieving formate yield of 83.5%.^{[ 108 ]}

In‐situ FTIR studies using Co/Ni‐C catalysts under alkaline conditions have provided valuable insights into the reaction pathways of EGOR. In the FTIR spectrum shown in Figure 4a, a distinct absorption peak at 1439 cm⁻¹ corresponds to the O─H bending vibration of methanol, while a peak at 1740 cm⁻¹ is attributed to the formation of aldehyde species. These findings indicate that glycolaldehyde acts as a key reaction intermediate during the oxidation process.^{[ 109 ]} Complementary computation investigations, illustrated in Figure 4b, further elucidate the EGOR mechanism. These studies reveal that O─H bond cleavage in EG leads to the formation of *OCH₂─CH₂O intermediate via an exothermic reaction, whereas the generation of OCH─H₂CHO intermediate involves sequential O─H and C─H bond breaking and is endothermic. This thermodynamic contrast implies that the *OCH₂─CH₂O intermediate is more favorably produced on CuO surfaces. Moreover, the subsequent conversion of OCH₂─CH₂O to the OCH─HCO intermediate is also exothermic, while the final transformation to formic acid is endothermic, highlighting the stepwise energetic landscape of EGOR pathways.^{[ 36 ]} The combined insights from FTIR and DFT studies emphasize the complexity and selectivity of EGOR and the critical influence of catalyst surface interactions on product distribution. The prominent role of *glyoxal as a key intermediate emphasizes its importance in achieving high selectivity toward formate during PET electro‐upcycling. Despite these mechanistic advancements in EGOR pathways, most insights have been derived from controlled model systems using pure EG solutions. In real PET upcycling scenarios, the presence of impurities, additives, stabilizers, and degradation byproducts introduces significant challenges by influencing reaction routes, catalyst stability, and product selectivity. Recent studies have begun to address these challenges. For example, polyoxometalate‐based ‐based catalysts have demonstrated strong activity towards PET depolymerization while simultaneously facilitating EG oxidation to formic acid.^{[ 106 ]} Notably, this catalyst system exhibits continuous self‐regeneration behavior under real PET feedstock conditions, demonstrating their practical viability. Similarly, MoCo–Ni(OH)₂/NF electrocatalysts have demonstrated their ability to preserve catalytic performance and maintain structural integrity during prolonged electrolysis with real PET‐derived feedstocks.^{[ 110 ]} These recent advancements highlight the importance of impurity‐tolerant catalyst design, employing operando characterization techniques, and long‐term stability assessment as essential steps toward translating laboratory‐scale mechanistic understanding into practical, scalable PET upcycling technologies.

Figure 4.

a) In situ infrared spectra of EGOR at potential applied from 1.10 to 1.40 V. Adapted with the permission.^{[ 109 ]} Copyright 2022, The Royal Society of Chemistry. b) Free energy diagram presents different pathways of the EGOR reaction with CuO catalyst. Adapted with the permission.^{[ 36 ]} Copyright 2022, The Royal Society of Chemistry.

3.2. Pathway 2: EG to Glycolic Acid (GA)

Glycolic acid, which exists in the form of glycolate under basic conditions, is a widely used building block in various industry, particularly in the production of biodegradable plastics. Traditionally, GA has been synthesized from coal‐derived feedstock, raising concerns over the depletion of non‐renewable resources. Consequently, more environmentally friendly manufacturing techniques, including electrochemical approaches are in increasing demand. Despite major progress in understanding the essential chemical processes, the selective oxidation of EG to GA is still hindered by significant challenge. This section explores the possible reaction pathways to produce GA from EG through electrochemical process.^{[ 87 ]}

One proposed mechanism for the conversion of EG to GA, utilizing a Pd−Ni(OH)₂ based catalyst, involves a cascade oxidation pathway as follows:

$$\begin{array}{cc}& \mathrm{HOC}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_2\mathrm{OH}\to \mathrm{HOC}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_2\mathrm{OH}\to \mathrm{HOC}{\mathrm{H}}_2\hfill \\ & -\mathrm{C}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{RO}\ast \right)\to \mathrm{HOC}{\mathrm{H}}_2-\mathrm{CHO}\ast \to \mathrm{HOC}{\mathrm{H}}_2\hfill \\ & -\mathrm{CO}\ast \to \mathrm{HOC}{\mathrm{H}}_2-\mathrm{COOH}\ast \to \mathrm{HOC}{\mathrm{H}}_2-\mathrm{COOH}\left(\mathrm{GA}\right).\ast \ast \hfill \end{array}$$ (1)

As illustrated in Figure 5a,b, EG first generates the reactive oxygenated intermediates before finally producing glycolic acid through consecutive oxidation processes. Due to the downshifted d‐band centre of Pd and the oxophilic nature character of Ni, the produced HOCH₂−COOH* readily desorbs from the active Pd sites and moves to adjacent Ni sites. This mechanism generates selective GA by essentially preventing over‐oxidation or C−C bond cleavage.^{[ 68 ]}

Figure 5.

a) Calculated adsorption energies of Glycolic acid on the surfaces of Pd and Pd‐Ni(OH)₂, inset shows the most energetically favourable adsorption configurations of GA* on Pd‐ Ni(OH)₂. b) The optimized configurations of the EGOR process on Pd‐Ni(OH)₂. Adapted with the permission.^{[ 68 ]} Copyright 2023 John Wiley & Sons – Books. c) Calculated free energy diagrams for GA production on Pt/𝛾‐NiOOH and Pt. Adapted with the permission.^{[ 111 ]} Adapted with the permission of John Wiley & Sons – Books.

In another study, the same oxidation sequence by using a Pt catalyst, proceeding through the intermediate:

$$\begin{array}{cc}& \ast \mathrm{HOC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{OH}\to \ast \mathrm{OC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{OH}\to \ast \mathrm{CHOC}{\mathrm{H}}_2\mathrm{OH}\hfill \\ & \to \ast \mathrm{COC}{\mathrm{H}}_2\mathrm{OH}\to \mathrm{COOHC}{\mathrm{H}}_2\mathrm{OH}\hfill \end{array}$$ (2)

On the Pt/γ‐NiOOH catalyst, a new intermediary, CHOHCH₂OH is observed instead of OCH₂CH₂OH as shown in Figure 5c. The shift is probably caused by the synergistic interaction changing the chemical route between Pt and γ‐NiOOH.^{[ 111 ]} These studies highlight that the composition of the catalyst is essential in directing the selective electro‐oxidation of EG to GA. Based on the referenced studies Pd─Ni(OH)₂ and Pt/γ‐NiOOH exhibit potential selectivity for GA by stabilising intermediates and inhibiting C─C bond cleavage, providing useful insights for the development of efficient electrochemical routes. These findings offer significant insights into the preferred intermediates and the mechanistic preference, guiding the systematic design of more efficient electrocatalytic systems for EGOR.

While the fundamental reaction pathways and intermediates involved in EG electrooxidation are becoming increasingly understood, achieving high catalyst selectivity remains a main challenge. As the field continues to evolve, ongoing efforts are directed towards developing catalysts that can selectively direct the reaction towards specific, high‐value products with minimal byproduct formation. Notably, a few bimetallic catalysts based on Pd and Pt have shown promising dual‐function capabilities, efficiently producing both glycolic acid and formate from EG electrooxidation.^{[ 112} , ^{113 ]}

4. Integrated Co‐Electrolysis for Green Fuels and Value‐Added Chemicals

The anodic oxidation of PET hydrolysate can be coupled with several cathodic processes, such as hydrogen evolution reaction (HER),^{[ 121 ]} CO₂ reduction reaction (CO₂RR),^{[ 122 ]} and nitrate reduction reaction (NO₃RR).^{[ 123 ]} These integrated systems offer a sustainable approach for plastic upcycling, and their importance is highlighted in terms of anodic products, faradaic efficiency, cell voltage, and stability tests in Table 2. Replacing the sluggish OER, which has a thermodynamic threshold of 1.23 V versus RHE, with the EGOR significantly reduces power consumption and enhances the overall economic value of the system. As illustrated in Figure 6a,b, the polarization curves clearly demonstrate that EGOR requires lower overpotentials and delivers higher current densities compared to OER. This performance advantage is primarily attributed to the inherently faster kinetics of EGOR, in contrast to the multistep and energy‐intensive pathway of OER. Moreover, this co‐electrolysis method not only facilitates the valorization of PET waste but also enables the co‐production of value‐added products. Glycolate and formate, derived from PET oxidation, act as redox mediators, facilitating potential alignment and lowering energy demands. However, their long‐term stability remains a concern, as these intermediates are prone to further oxidation, and their use introduces added system complexity. Addressing these limitations will be critical for scaling mediator‐assisted co‐electrolysis.

Table 2.

Summary of integrated co‐electrolysis for green fuels and value‐added chemicals.

Coupling Reaction	Catalyst	Anodic Product and its Faraday Efficiency	Cell Type	EGOR Voltage	PET Voltage	ECSA (Cdl)	Stability Test
HER	Co and Cl co‐doped Ni3S2 [ 114 ]	Formate >92%	H‐type electrochemical cell (MEA flow electrolyser with AEM membrane)	1.34 V versus RHE for 100 mA cm−2	1.51V at 100 mA cm−2	4.31 mF cm−2 (ECSA = 107 cm2)	86% retention after 12 h of continuous electrolysis
	B and Co co‐doped Ni3S2 [ 115 ]	>93% for EG‐to‐formate conversion	H‐type electrochemical cell (MEA flow electrolyser with AEM membrane)	1.341V versus RHE at 100 mA cm−2	1.491V at100 mA cm−2	5.52 mF cm−2 (ECSA = 138 cm2)	86% retention after 12h, stable operation in 48h test
	Co2P‐Ni2P/CC[ 46 ]	Formate 90.6%	Membrane‐electrode assembly (MEA) flow electrolyser	1.30V versus RHE at 100 mA cm−2	1.473V at100 mA cm−2	49.5 mF cm−2	Stable for 70 h at 100 mA cm−2
	Pd‐NiTe/NF[ 116 ]	Formate 95.6%	H‐type electrochemical cell with MEA configuration	1.35 V versus RHE at 100 mA cm−2	1.6 V at 10 mA cm−2 for overall HER/PET electrolysis	13.4 mF cm−2	Stable for 12 h at 100 mA cm−2, minimal degradation over 6 cycles
	PdSb Bimetallene[ 117 ]	Glycolic Acid 93.4%	Membrane‐electrode assembly (MEA) flow electrolyser	0.71 V at 10 mA cm−2	1.5 V at 6 h for full conversion of EG to GA.	(ECSA = 39.3 m2 g−1)	Stable for 12 h, minimal degradation over time, long‐term performance retention
	CoFe‐P/NF[ 42 ]	Formate 90%	Two‐electrode electrochemical cell	1.5 V at 100 mA cm−2	1.52 V at 20 mA cm−2	N/A	Stable for 12 h at 100 mA cm−2, good durability with minimal degradation after 5 repeated oxidation cycles
	Porous Fe–Ni2P Nanosheets [ 12 ]	Formate	Two‐electrode electrochemical cell	1.326 V at 100 mA cm−2	1.39 V at 10 mA cm−2	16.79 mF cm−2 (ECSA = 419.75 cm2)	Stable for 30 h, minimal performance degradation over time
NO3RR	mPd3Au/NF[ 49 ]	Glycolic Acid 95.32%	Two‐electrode Membrane Electrode Assembly (MEA) flow cell	0.4 V (onset), >1.3 V (full oxidation)	N/A	N/A	Stable for multiple cycles, high FE retention after 6 cycles
	NiCo2O4/N‐doped Carbon Nanotubes[ 118 ]	Formic Acid 98%	Photovoltaic‐driven Electrochemical System	N/A	1.45 V vs RHE	N/A	Stable for multiple cycles, high efficiency retained under 30 mW cm−2 simulated sunlight
	PdCu Mesoporous Nanocavities[ 52 ]	Glycolic Acid Formic Acid >90%	Two‐electrode system for NO3‐to‐NH3 and PET upcycling	0.8268 V (for GA), 1.6268 V (for FA)	1.5 V at 100 mA cm−2	4.61 mF cm−2	Stable over 10 cycles, minimal performance decay in NH3 and GA production
	Ru‐Co(OH)2 (Cathode) and Ru‐CoOOH (Anode)[ 21 ]	Formate 96.53%	Two‐electrode coelectrolysis system	1.33 V at 50 mA cm−2	1.53 V at 50 mA cm−2	Ru‐Co(OH)2: 6.66 mF cm−2 Ru‐CoOOH: 7.14 mF cm−2	Stable for 10 cycles, minimal performance decay
	Pd‐modified Ni/CeO2 [ 30 ]	Dimethyl 1,4‐cyclohexanedicarboxylate (DMCD) 86.5%	One‐pot reaction system (Methanolysis and hydrogenation)	N/A	N/A	N/A	Catalyst retained performance over multiple cycles; reactivated by hydrogen reduction
CO2RR	NiOOH/Ni3Bi2S2 [ 119 ]	Formate >90%	Zero‐gap Membrane Electrode Assembly (MEA) electrolyser	1.5 V at 50 mA cm−2	N/A	4.85 mF cm−2	Stable for over 30 h with minimal performance degradation
	Oxygen‐vacancy‐rich Ni(OH)2‐VO (Anode) and Bi/Bi2O3 heterostructure (Cathode)[ 120 ]	Formic Acid 86%	Two‐electrode solar‐powered flow reactor	N/A	1.6 V at 300 mA cm−2	Ni(OH)2‐VO: 1.56 mF cm−2 Bi/Bi2O3: N/A	Stable operation with a record FE of 181% for HCOOH at 100 mA cm−2 over extended periods
	NiCo2O4 (Anode) and SnO2 (Cathode)[ 95 ]	Formic Acid 90%	Two‐electrode integrated electrolyser	1.45 V at 50 mA cm−2	1.55 V at 20 mA cm−2	N/A	Stable for 20 h

Figure 6.

The typical polarization curves for HER, OER, and EGOR, η_I, and η_II represent the overpotential required to achieve a current density of 10 mA cm⁻². a) LSV polarization curve of HER∥OER b) LSV polarization curve of HER∥EGOR.

In a common HER‐coupled system, this approach provides a low‐energy alternative to the oxygen evolution reaction (OER), producing green hydrogen (H₂) as a clean energy carrier, while simultaneously converting waste streams into useful chemicals. The oxidation of PET hydrolysate, alongside CO₂RR, enables the electrochemical conversion of CO₂ into useful products such as formate, methanol, or syngas, hence advancing carbon capture and utilization strategies. Furthermore, the integration of NO₃RR with PET oxidation enhances the electrosynthesis of ammonia (NH₃) or other nitrogenous compounds, offering an energy‐efficient route for nitrogen fixation. This novel paired electrolysis method improves energy efficiency, reduces environmental pollution, and fosters circular economy concepts by converting plastic waste and greenhouse gases into valuable fuels and chemicals.^{[ 119} , ^{120 ]}

4.1. Upcycling of PET Coupled with H2 Production

Recent developments in the electrocatalytic upcycling of PET, coupled with hydrogen production, have been investigated using various transition and noble metals.^{[ 125} , ^{126 ]} For instance, a recent study on an optimization of electronic structure with a dual doping of (Co, Cl‐Ni₃S₂) catalyst in a membrane‐electrode assembly (MEA) electrolyser, shown in Figure 7a, achieved a substantial hydrogen generation rate of 50.26 mmol h⁻¹ at 1.7 V in 2 M KOH, with a formate FE above 92%. The post‐treatment process facilitated the extraction of pure TPA and KDF, thereby affirming the viability of PET recycling. ^{[ 114 ]}

Figure 7.

a) The membrane electrode assembly (MEA) setup for PET hydrolysate oxidation reaction.Adapted with the permission.^{[ 114 ]} b) LSV curves over the TbCo‐MOF/NF in 0.5 м EG and 1 M KOH. Reproduced with the permission.^{[ 45 ]} Copyright 2024 Elsevier Science & Technology Journals. c) LSV curves of Co₂P‐Ni₂P/CC in 1 M KOH solution with/without 0.5 м EG.Adapted with the permission.^{[ 46 ]} Copyright 2024 John Wiley & Sons – Books. d) Application of a cost‐free solar to hydrogen energy conversion using a solar battery with an operating potential of 2 V. Adapted with the permission.^{[ 127 ]} Copyright 2024 Elsevier Science & Technology Journals.

Similarly, a metal‐organic framework based on a terbium‐doped cobalt‐based (TbCo‐MOF/NF) catalyst demonstrated superior performance by providing the abundant Co‐based active side in a two‐electrode system, achieving an EGOR at 1.32 V versus RHE at 100 mA cm⁻² as shown in Figure 7b, while the HER occurred at an overpotential of 161 mV. This system showed a remarkable FE of 97.3% for formate synthesis, hence underlining the potential of MOF‐based catalysts for PET electrolysis.^{[ 45 ]} In another study, a nitrogen‐doped Ni₃P–NiMoO₄ (N–Ni₃P–NiMoO₄) heterostructure catalyst was developed, which demonstrated a notable decrease of 195 mV in cell voltage relative to conventional water splitting together with a remarkable FE of 92.2% for formate production, thereby improving the energy efficiency in PET‐assisted electrolysis. This scheme of heterostructure provides the synergistic effect that enhances the electronic properties and gives lower cell voltages for HER and high formate selectivity.^{[ 71 ]}

Further studies on a B and Co co‐doped Ni₃S₂ (B,Co‐NiS) catalyst in a MEA electrolyser demonstrated a formate production rate of 15.24 mmol h⁻¹ with EGOR at 1.341 V versus RHE at 100 mA cm⁻². This system exhibited over 70 times higher hydrogen production efficiency than traditional electrolysis, highlighting the effect of hetero‐atom doping on catalytic performance.^{[ 115 ]} Similarly, a heterointerface catalyst based on Co₂P–Ni₂P in a flow‐cell electrolyser achieved EGOR at 1.300 V versus RHE, as shown in Figure 7c, and HER at −0.112 V versus RHE, with the stability test over 70 h. The flow‐cell configuration offered benefits in mass transport and reaction scalability, though challenges such as electrode stability and product separation efficiency remain significant.^{[ 46 ]} The development of trimetallic systems, such as CoFeNi catalyst synthesised using saltwater corrosion engineering, reported EGOR at 1.25 V (10 mA cm⁻²) and 1.38 V (100 mA cm⁻²) versus RHE, with over 100% hydrogen generating efficiency during 105 h of operation using a solar battery at an operating voltage of 2V as shown in Figure 7d. This work proposes that trimetallic catalysts, especially in seawater‐based electrolysis systems, provide enhanced catalytic activity and durability.^{[ 127 ]}

Another study focused on a trimetallic system based on a self‐supported amorphous cobalt iron phosphide (CoFe‐P/NF) electrode, fabricated through electrodepositing onto nickel foam. This electrode enables bifunctional catalysis for HER and selective PET hydrolysate oxidation into formate, as shown in Figure 8e, with a productivity of 0.1 mmol cm⁻² h⁻¹. This system runs with a low overpotential of 168 mV at 100 mA cm⁻² for HER, obtaining 90% FE for formate production at 1.5 V, reaching 20 mA cm⁻² at just 1.52 V allows the electrolyser to consume 84% less electricity compared to conventional water electrolysis. The amorphous structure of CoFe‐P and its surface oxide layer increases the catalytic activity, consequently, 12 h of continuous operation without minimal degradation. This system provides a practical route for sustainable electrochemical PET upcycling and energy‐efficient H₂ generation, as it co‐produces hydrogen and formate while lowering the energy demands and maximizing resource utilization.^{[ 42 ]}

Figure 8.

a) Schematic illustration of the production of H₂ and GA by chlorine‐free seawater splitting coupling EGOR at low potentials. Adapted with the permission.^{[ 50 ]} Adapted with the permission of John Wiley & Sons – Books. b) HER polarization curves of different samples and corresponding. c) LSV curves over the Pd‐NiTe/NF in PET hydrolysate and 1 M KOH. Adapted with the permission.^{[ 116 ]} Reproduced with permission from, Applied Catalysis B: Environmental 340 (2024) 123 236, Copyright 2024 Elsevier Science & Technology Journals. d) Diagram of simultaneous electrolysis of hydrogen production and PET upcycling. REproduced with the permission.^{[ 117 ]} Copyright 2024 Elsevier Science & Technology Journals. e) Electricity consumptions of the CoFe‐P/NF//CoFe‐P/NF electrolyser in 1 M KOH and PET hydrolysate electrolytes. Adapted with the permission.^{[ 42 ]} Copyright 2024 Elsevier Science & Technology Journals.

A promising method to simultaneously reduce energy consumption in hydrogen generation and transform plastic waste into useful chemicals is the coupling of seawater electrolysis with PET plastic upcycling.^{[ 85 ]} The high energy demands and chlorine corrosion associated with the conventional alkaline saltwater electrolysers (ASEs) make alternative anodic processes, including EGOR, particularly appealing.

In this context, Pd has emerged as an efficient material for electrolysis due to its strong adsorption capabilities and tuneable electronic properties, enhancing both hydrogen production and the selective oxidation of PET‐upcycling. A significant development in this domain with an integration of seawater electrolyser using Pd─CuCo₂O₄ as the anode for EGOR and CoP as the cathode for HER, as shown Figure 8a. The operational voltage at 1.15 V versus RHE to drive 600 mA cm⁻², this system runs at 1.15 V instead of traditional ASEs, which requires 4.79 kWh m⁻² H₂, therefore drastically lowering the electricity usage to 2.45 kWh m⁻². By means of a synergistic interaction, Pd and CuCo₂O₄ enhance catalytic activity and lower chlorine corrosion, therefore ensuring long‐term operational stability at 1.6 A for almost 100 h. The electron‐deficient state of Pd (caused by electron transfer to CuCo₂O₄) greatly improves EG adsorption, when compared to pure Pd or CuCo₂O₄ alone. DFT also reveals EG adsorption energy on Pd–CuCo₂O₄ (−1.35 eV) is stronger than on Pd (−0.92 eV), while the downshifted d‐band centre in Pd helps glycolic acid desorption and prevents over‐oxidation.^{[ 50 ]}

Further advancements in Pd‐based electrolysis involve the development of a two‐electrode electrochemical system using Pd‐NiTe nanoarrays on Ni foam (Pd‐NiTe/NF) as both anodic and cathodic catalysts for PET hydrolysate oxidation and HER, as shown Figure 8b,c. This system runs at a low overpotential of 0.019 V at 10 mA cm⁻² for HER and 1.35 V at 100 mA cm⁻² for PET oxidation, achieving a remarkable FE of 95.6% for formate generation and 98.6% for hydrogen generation. The long‐term stability, catalytic activity, and charge transfer of the system are all enhanced by the robust interfacial electronic interactions between Pd and NiTe, maintaining efficacy for up to 12 h at 100 mA cm⁻². The ideal cell voltage of 1.6 V, which is 200 mV lower than that of conventional water splitting, further highlights its energy‐saving capacity.^{[ 87 ]}

Another study based on PdSb bimetallene with a defect‐rich, crimped structure, utilizing p‐d orbital hybridization to enhance electronic interactions and catalytic activity. Operating at an ultra‐low cell voltage of 0.71 V at 10 mA cm⁻², which is 1.21 V lower than traditional OER‐based electrolysis (1.92 V), integrated into a MEA flow electrolyser, the PdSb bimetallene as the anode with Pt/C as the cathode. Using PET hydrolysate (0.72 V at 10 mA cm⁻²), the system achieved a FE of 93.4% for GA with 92% selectivity, thus reducing energy input. The p‐d orbital hybridization between Pd and Sb enhances the d‐band centre alignment, therefore enabling selective PET‐derived EG oxidation into GA with 12 h continuous operational stability.^{[ 117 ]}

A coherent relationship emerges between catalyst structure, electrochemical performance, and system‐level integration in coupled PET electro‐upcycling. Structural strategies such as doping with elements like Co, Cl, B, N, and Tb can change the electronic properties of the catalyst, increase active site density, and reduce EGOR onset potentials, thereby enhancing overall faradaic efficiency.^{[ 45} , ^{114 ]} Additionally, amorphous architectures and heterostructures enhance the charge transfer and intermediate adsorption through tailored surface properties.^{[ 110} , ^{128 ]} These catalytic benefits are further enhanced when integrated with appropriate reactor configurations. For example, membrane electrode assemblies (MEAs) promote efficient product separation and hydrogen recovery, flow‐cell systems enhance mass transport and scalability, and solar‐integrated or self‐supported architectures enable low‐carbon or carbon‐negative, sustainable operation. Together, these findings highlight the importance of co‐optimizing catalyst design and system architecture to achieve high‐efficiency, scalable, and sustainable PET valorization.

4.2. Upcycling of PET Coupled with Ammonia Production

By integrating nitrate reduction with PET upcycling, ammonia and other value‐added compounds can be simultaneously generated through electrocatalytic co‐conversion. A mesoporous Pd₃Au/Ni foam (mPd₃Au/NF) catalyst was demonstrated in a two‐electrode MEA system, as shown in Figure 9a. The catalyst achieved a high ammonia production rate of 184.25 µmol h⁻¹ cm⁻² with a FE of 97.28%, along with the formation of GA as depicted in Figure 9b with FE of 95.32%.^{[ 49 ]} Another research proposed a solar‐driven electro‐reforming strategy by utilizing NiCo₂O₄/N‐doped CNTs as the anode and CuO nanowires as the cathode for coupling of PET hydrolysate oxidation with nitrate reduction, achieving 98% FE for formic acid and 90% nitrate conversion under simulated sunlight (30 mW cm⁻²).^{[ 118 ]} A recent study based on a bimetallic PdCu mesoporous nanocavities (MCs) as a bifunctional catalyst to selectively oxidise PET‐derived EG into GA and FA with efficiencies above 90%, together with the recovery of purified TPA. The overall reaction flow is illustrated in Figure 9c, this integrated system enables nitrate electroreduction, with the production of ammonia with a F.E of 96.6%. In comparison to traditional (−) NITRR || OER (+) coupling system, the onset potential of (−) NITRR || PETHOR (+) system is remarkably low, indicating its high performance in the two‐electrode coupling system. Specifically, (−) NITRR || PETHOR (+) coupling system requires only 0.92, 1.14, and 1.78 V at the current densities of 10, 20, and 50 mA cm⁻², respectively, which are much lower than (−) NITRR || OER (+) (1.77 V@_{10 mA cm−2}, 2.01 V@_{20 mA cm−2}, and 2.52 V@_{50 mA cm−2}).^{[ 52 ]}

Figure 9.

a) Schematic illustration of a two‐electrode MEA flow electrolytic cell for the co‐production of NH₃ and GA. b) F.E. of NH₃ and F.E. of GA values at various voltages. Adapted with the permission.^{[ 49 ]} Copyright 2024 John Wiley & Sons‐Journals. c) A scheme illustrating the reaction pathways of PET upcycling. Adapted with the permission.^{[ 52 ]}

A MOF catalyst incorporated with Co and Ru was synthesized, which transforms in situ during electrolysis into Ru‐Co(OH)₂ at the cathode and Ru‐CoOOH at the anode. This system demonstrated a 94.3% FE for the production of ammonia and 96.53% FE for PET‐derived formate production at a low cell voltage of 1.53 V, as shown in Figure 10a,b.^{[ 21 ]} These studies, however, highlight the promise of electrocatalytic co‐conversion systems for sustainable waste upcycling, offering effective methods for ammonia synthesis and the valorization of plastic waste into high‐value chemicals, therefore enabling scalable green chemical production.

Figure 10.

a) Schematic illustration of the electrocatalytic system for NO₃RR and PET hydrolysate upcycling. b) LSV curves (with 95% iR‐compensation) of the CoRu‐MOF/NF pre‐catalyst as cathode and anode in (−)NO₃RR∥PET hydrolysate oxidation (+) and (−)NO₃RR∥OER(+).^{[ 21 ]} Copyright 2023, American Chemical Society.

4.3. Upcycling of PET Coupled with CO₂ Reduction

Another promising approach for converting waste into valuable chemicals while addressing environmental challenges implies the electrocatalytic upcycling of PET plastic waste coupled with CO₂RR. By substituting PET hydrolysate oxidation for the energy‐intensive oxygen evolution reaction (OER), this integrated approach not only mitigates plastic pollution and CO₂ emissions but also improves the efficiency of electrochemical conversion, hence reducing overall energy consumption.^{[ 129} , ^{130 ]}

Recent studies on a two‐electrode electrolyser achieved a remarkable total FE of 155% at 1.9 V using NiCo₂O₄ as an anodic and SnO₂ as a cathodic catalyst, significantly reducing energy input compared to conventional CO₂ electrolysis.^{[ 95 ]} Another study presented a zero‐gap MEA electrolyser as shown in Figure 11a, based on a sulphide‐reconstructed engineering approach with Ni₃Bi₂S₂/NiOOH at the anode and Bi (1.5 S) at the cathode. This system achieved FEs of 96% and 97% at the anode and cathode, respectively, with catalytic stability exceeding for over 30 h.^{[ 125 ]}

Figure 11.

a) Schematic illustration of the preparation of Bi (1.5 S) and NiOOH/ Ni₃Bi₂S₂ and MEA electrolyser for POR//CO₂RR. Adapted with the permission.^{[ 93 ]} Copyright 2024, Elsevier Science & Technology Journals. b) LSV curves for the SnO2||NiCo2O4 electrolytic cell with and without the existence of 0.1 м PET hydrolysate in the anode cell. c) Faradaic efficiencies of NiCo₂O₄/CFP for PET hydrolysate oxidation and SnO₂/CC for the CO₂RR to produce formic acid at different applied cell voltages. Reproduced with the permission.^{[ 95 ]} Copyright 2022, American Chemical Society. d) Schematic diagram of an integrated flow cell powered by solar energy. Adapted with the permission.^{[ 120 ]} Copyright 2023, American Chemical Society.

Heterostructure integrated with a solar‐powered flow reactor, as shown in Figure 11d, using oxygen‐vacancy‐rich Ni(OH)₂‐VO as the anode and Bi/Bi₂O₃ as the cathode, achieved a FE of 86% for formic acid production. This system demonstrated at remarkably low potentials of 1.6 V (300 mA cm⁻²) for PET oxidation and ‐1.4 V (272 mA cm⁻²) for CO₂ reduction, highlights the role of oxygen vacancies in enhancing the generation of Ni³⁺ species and accelerating the C─C cleavage, which lowers the reaction barrier for the formation of glyoxal and formate intermediates.^{[ 120 ]}

Pairing anodic PET oxidation with cathodic CO₂RR or NO₃RR faces operational mismatches arising from differences in kinetics, potential windows, and current density demands. Selective cathodic reactions such as CO₂RR and NO₃RR typically compete with HER due to their occurrence within similar potential ranges and reaction conditions. Achieving elevated faradaic efficiencies for desired products such as formate and ammonia requires careful catalyst design. Such misalignments can compromise product selectivity and energy efficiency, particularly under fluctuating renewable inputs where dynamic load balancing is required. In addition, impurities in PET hydrolysates may poison cathodic catalysts, complicating process control. Hetero‐atom doping (such as Co, Cl, or B), alloying (such as PdCu), and creating structures with abundant oxygen vacancies (such as Ni(OH)₂‐VO) are methods to modify intermediate adsorption energies and inhibit hydrogen adsorption, hence enhancing selectivity against hydrogen evolution. To hinder HER and enhance selectivity of CO₂RR products, heteroatom‐doped Cu catalysts incorporating elements such as phosphorus (P) or boron (B) can significantly alter the adsorption energies of critical intermediates (*CO, *COOH)^{[ 131 ]}. PdCu bimetallic alloy catalysts modify their electronic structure to minimise hydrogen spillover effects, significantly reducing HER and enhancing nitrate reduction selectivity ^{[ 21 ]}. Additionally, vacancy‐induced electronic localization in oxygen‐vacancy‐rich Ni(OH)₂ (Ni(OH)₂‐VO) complexes alters intermediate adsorption properties, inhibiting HER and enhancing product selectivity during CO₂RR and NO₃RR ^{[ 120 ]}. Integrated catalyst design, coupled with reactor engineering, system‐level optimization, and feedstock pretreatment, can mitigate operational mismatches. Together, these strategies enhance sustainability and efficiency, paving the way for industrial‐scale electrocatalytic PET upcycling.

5. Techno‐Economic Analysis and Environmental Impact

The rapid increase in global plastic production and waste generation poses significant environmental and economic challenges.^{[ 132} , ^{133 ]} According to the report of OECD's 2024, plastics production and waste are predicted to rise by 70% between 2020 and 2040, reaching 736 million tonnes yearly. This rise is expected to outpace global population growth, particularly in developing economies across Asia, Sub‐Saharan Africa, and Latin America, as well as in OECD countries. The plastic life cycle, from production to disposal, is a major contributor to greenhouse gas emissions, with an estimated 2.8 gigatonnes of CO₂ equivalent emissions by 2040. Despite global efforts, only 9% of plastic waste was effectively recovered in 2019, while nearly 70% was either incinerated or sent to landfills, and over 20% directly entered the environment.^{[ 134 ]} These alarming statistics highlight the urgency of implementing comprehensive worldwide regulations, which could reduce plastic pollution by up to 96% by 2040. Nonetheless, beyond regulatory measures, innovative technological solutions are necessary to create a sustainable and circular plastics economy.^{[ 135} , ^{136 ]}

Life‐cycle analysis (LCA) is a critical tool for evaluating the environmental impact of plastic across their entire lifespan, from production and use to end‐of‐life disposal. Most plastics are derived from fossil‐based feedstocks through energy‐intensive polymerization methods, contributing significantly to global carbon emissions.^{[ 6 ]} During their use phase, plastics are employed across a wide range of industries, including construction, healthcare, and packaging. However, their post‐use disposal phase presents a major environmental challenge. Poor recycling rates and inefficient waste management practices have led to the persistent accumulation of plastic debris, causing long‐term ecological damage.^{[ 137 ]} One of the most alarming consequences of inadequate disposal is the gradual accumulation of micro‐ and nano‐plastics in aquatic environments, which poses long‐term risks to marine diversity and food safety. To address this issue, research efforts are increasingly focused on advanced degradation and separation strategies, including advanced oxidation processes, electrochemical treatments, and biological degradation techniques.^{[ 138 ]} Traditional recycling techniques, including mechanical recycling,^{[ 139 ]} thermochemical depolymerization,^{[ 140 ]} catalytic oxidation,^{[ 141 ]} photocatalysis,^{[ 124} , ^{142 ]} and enzymatic hydrolysis, often face limitations related to energy consumption, product downcycling, and feedstock purity.^{[ 143 ]} In contrast, electrochemical reforming has emerged as a promising alternative that enables the selective conversion of plastic waste into value‐added chemicals under mild, energy‐efficient conditions.^{[ 144} , ^{145 ]} This approach not only reduces energy input but also offers greater integration potential with renewable electricity, positioning it as a sustainable pillar in the future of circular plastic economies.

5.1. Market Analysis of PET Upcycled Products

Electrocatalytic upcycling of plastic has attracted significant attention as a sustainable solution for the conversion of plastic waste into high‐value compounds and fuels. Currently, most of the studies are centred on PET plastic waste, which undergoes the alkaline hydrolysis to produce TPA and EG^{[ 100} , ¹⁴⁶ , ^{147 ]} Subsequently, TPA can be recovered and utilized as a feedstock for fresh PET manufacture.^{[ 148} , ¹⁴⁹ , ^{150 ]} The global market of EG was projected to produce an output of around 24.95 million tonnes by the end of 2021, with the market consumption reaching 25.61 million tonnes, indicating a state of equilibrium between supply and demand. However, the demand for EG, as an important component used in the production of PET, is expected to decline, mainly due to a growing global trend towards biodegradable plastics. Though, EG holds a potential for electrochemical valorization into formate, formic acid, or glycolic acid. Therefore, the electrochemical valorization of EG offers a possibility to produce value‐added compounds, rather than relying on a direct physical recovery.^{[ 6} , ⁶⁹ , ^{151 ]}

The global market of formic acid is growing rapidly because of its use in medicine, leather, textiles, agriculture, and rubber.^{[ 152 ]} The market is expected to grow from 750 thousand tonnes in 2022 and is expected to raise at a compound annual growth rate (CAGR) of 4.5% up to 2035.^{[ 153 ]} Similarly, the global market for GA is also expanding due to its extensive use in cosmetics and industrial applications.^{[ 69 ]} The market, valued at US$2.1‐30.9 per kg, is reaching approximately US$ 734.6 million by 2030, anticipated to expand at a CAGR of 7.1% from 2024 to 2030. Formate and its salts, such as sodium and potassium formate, are widely used in various industrial applications, including high‐pressure, high‐temperature (HTPT) oil wells as drilling fluids, which help to achieve stability and efficiency in extreme weather conditions. The most common use as de‐icing agents in airports and local municipalities to maintain a safe surface during the winters. The market value of KDF with the CAGR of 2.8% between 2021 and 2026, it is anticipated to reach $779.5 million by 2026, while sodium formate market is also anticipated to expand at a CAGR of 4.9%, rising from $507.3 million in 2023 to $709.1 million by 2030.^{[ 154} , ^{155 ]}

Comprehensive techno‐economic analyses (TEAs) have been conducted to assess the feasibility of coupling system such as EGOR with cathodic processes such as HER for co‐producing H₂ and value‐added chemicals. Capital expenditure (CAPEX) for comparable electrochemical systems, including electrolyzers, power supplies, and supporting infrastructure is estimated at $35‐$60 per tonne of PET processed.^{[ 156 ]} In parallel, feedstock pre‐treatment, comprising collection, sorting, washing, and alkaline hydrolysis of PET waste, is anticipated to add an extra $150–200 per tonne.^{[ 157 ]} As illustrated in Figure 12a, operation at 300 mA/cm² and an applied potential of 1.7 V requires approximately 4.3–4.5 kWh per kg of PET, translating to an energy cost of $0.43–$0.45/kg, assuming an industrial electricity price of $0.10 per kWh. Product diversification can further enhance economic returns, for example, upgrading formate into KDF, recovering TPA, and capturing H₂ is projected to yield a combined product value of $700‐900 per tonne. According to the plant‐gate levelized cost analysis as shown in Figure 12b, the projected net income exceeds $350 per tonne of PET upcycled, provided catalyst stability and formate selectivity remain above 80% at commercially viable current densities (>300 mA cm⁻²). These findings highlight the industrial viability of electrochemical PET upcycling and emphasize the importance of further advances in catalyst durability and system integration.^{[ 40 ]} Another study on TEA calculations shows that coupling of EGOR with CO₂RR for formic acid synthesis can generate a net income of $557 per tonne of PET. This highlights the possibility of using both PET and CO₂ waste streams to generate profitable value‐added compounds. Beyond PET‐derived EG, the electrooxidation of monomers from other plastics, such as polybutylene terephthalate (PBT), polythene furanoate (PEF), and polythene (PE), has shown potential in producing useful compounds, including succinic acid.^{[ 108 ]} Economic assessments indicate that the profitability of electrochemical PET upcycling is more influenced by the current density rather than by FE for formate production. Operating at current densities above 320 mA cm⁻² with FE exceeding 80% significantly enhances economic returns. Large‐scale production at 800–1000 mA cm⁻² ensures high FE and catalyst stability over extended operational periods. The Pt₁/Ni(OH)₂ catalyzed process has demonstrated revenue generation potential of $588–700 per tonne of PET, making it a viable commercial approach that surpasses previous EG electrooxidation methods. This economic feasibility is particularly important considering the growing demand for green chemistry solutions and sustainable waste management practices.^{[ 66 ]} Another reported study with the coupled reaction of EGOR into formic acid at a cell voltage of 1.90 V with CO₂RR integration, as illustrated in Figure 12c. Their reported TEAs estimated that electro‐reforming of per tonne of PET waste yields a net profit of approximately ≈$557 with the production of TPA, FA, and Na₂SO₄. Based on a 6‐electron EGOR at 85% FE, the electricity requirement is ≈1870 kWh t⁻¹ PET, corresponding to ≈$262 t⁻¹ at $0.07 kWh⁻¹. From one tonne of PET as raw material, it can generate approximately 864.5 kg of TPA (at $1260 per tonne), 1152.9 kg of FA (at $1000 per tonne), and 4530 kg of Na2SO4 (at $200 per tonne).^{[ 95 ]}

Figure 12.

Technoeconomic Analysis of PET upcycling into value‐added products. a) PET plastic waste into HCOOH, PTA, H₂, and K₂SO₄. b) PET upcycling to feedstock chemicals and H₂ fuel w.r.t different current densities.^{[ 40 ]} No permission required (Open Access). c) PET upcycling to feedstock chemicals, formic acid, PTA, and Na₂SO₄.^{[ 95 ]} Copyright 2022, American Chemical Society.

TEA provides a comprehensive overview of market trends and energy cost; however, it is crucial to examine the scalability challenges that may impact industrial usage. At elevated current densities (>300 mA cm⁻²), throughput may increase and energy expenses per unit of product may decrease. This intensification can accelerate catalyst degradation and lead to electrode delamination, hence reducing faradaic efficiency and selectivity of the product. The durability of the membranes during prolonged electrolysis is a significant concern.^{[ 158 ]} Commercial anion exchange membranes (AEMs) typically begin to deteriorate, accumulate contaminants, and exhibit mechanical failures after 500 to 1000 h of operation. The expense of membrane replacement ($150–$300/m²) and the frequency of such replacements significantly contribute to the long‐term operational costs.^{[ 159 ]} To address these issues, we must advance catalyst stability and membrane technology. Future research should concentrate on developing more robust electrode materials and durable membranes to facilitate economically viable large‐scale electrochemical PET upcycling processes.

The integration of PET upcycling to produce value‐added chemicals such as glycolic acid, potassium formate, and sodium formate underscores the potential of electrochemical recycling for providing both environmental and economic benefits. The market for sustainable chemical products manufactured from plastic waste is expected to expand as green chemistry, waste valorization, and circular economy practices gain traction. This offers promising opportunities for both environmental and industrial advancement. However, the commercialization of this technology suffers from the fundamental challenge of scaling up the electrochemical process while maintaining cost‐effectiveness and efficiency. The primary challenges include the purification of reaction intermediates, the high cost and stability of catalysts, and the durability of membranes and electrolysers. Moreover, ensuring a stable and high‐quality feedstock from post‐consumer PET waste poses practical and financial limitations. Real‐world PET waste often contains colorants, multilayer packaging, and blended polymers, which can interfere with oxidation performance, reduce catalyst stability, and lower product selectivity. Such impurities may cause catalyst fouling and uneven depolymerization, presenting barriers to scalability. Addressing these issues by improving catalyst design, optimization methods, and supply chain integration can help to make the electrochemical upcycling more feasible in large‐scale opportunities. Beyond technical advances, external drivers will also help to shape the feasibility of PET electro‐upcycling. For example, regulatory incentives, carbon pricing, and extended producer responsibility (EPR) frameworks can enhance competitiveness, strengthen recycling infrastructure, and accelerate the adoption of PET electro‐upcycling technologies.

6. Conclusion and Future Perspective

Electrochemical upcycling of PET offers a viable and practical way to simultaneously address the global plastic waste crisis and support the transition toward a circular, low‐carbon, or carbon‐negative economy. In this review, we have highlighted recent advancements in catalyst design and development, reactor engineering, computational modelling, and in situ characterization techniques that collectively advance the progress of PET valorization technologies.

Innovative catalyst designs such as bifunctional systems, single‐atom catalysts, and multi‐metallic heterostructure systems have significantly improved reaction selectivity, operational stability, and energy efficiency. Meanwhile, advanced reactor designs, including membrane‐free electrolyzers, flow cells, and solar‐integrated systems, are enabling more scalable and modular deployment. Computational tools such as machine learning algorithms and DFT modelling are accelerating advanced catalytic materials discovery, while in situ spectroscopy techniques, on the other hand, are shedding light on electrochemical reaction intermediates and pathways in real‐time.

Despite these advances, numerous key challenges remain. Most studies still rely on pure PET feedstocks, which do not reflect the heterogeneity of real‐world waste streams. Future efforts must prioritize the development of contaminant‐tolerant catalyst materials, standardized pre‐treatment protocols, and robust separation and purification strategies. Long‐term catalyst stability under industrial conditions, especially in complex electrochemical environments, must also be addressed.

Looking ahead, the successful industrial translation of coupled PET electro‐upcycling will require an integrated approach that combines catalyst design innovation, system‐level co‐design, and technoeconomic modelling and optimization. Special emphasis should be given on pairing PET oxidation with complementary cathodic reactions, specifically CO₂RR and NO₃RR to maximize product value and energy efficiency. Finally establishing benchmarking standards for current density, Faradaic efficiency, energy consumption and scalability will be crucial to guide future research and commercialization implementation.

We are confident that by advancing both the fundamental and practical deployment of coupled PET upcycling technologies, this field has the potential to redefine how society manages plastic waste, while co‐producing value‐added chemicals, transforming environmental liability into a sustainable and high economic value chemical resource.

Conflict of Interest

The authors declare no conflict of interest.

Biographies

Chaudhry Muhammad Furqan is a doctoral researcher at the University of New South Wales (UNSW), Sydney, working under the supervision of Dr. Mahesh Suryawanshi. His research focuses on the electrocatalytic upcycling of polyethylene terephthalate (PET) waste, coupled with the simultaneous production of hydrogen production, offering a promising pathway for energy‐efficient plastic valorization and clean energy generation. Furqan holds a master's degree in electronic engineering with a specialization in nanotechnology from the Hong Kong University of Science and Technology (HKUST).

Xinyao Guo is a doctoral researcher in the School of Photovoltaic and Renewable Energy Engineering at the University of New South Wales, Sydney, under the supervision of Dr. Mahesh Suryawanshi. Her research focuses on scalable electrochemical catalysts for solar‐assisted hydrogen production. Ms. Guo holds UNSW Tuition Fee Scholarship and CSIRO Top‐up scholarships and earned her B.S. from Monash University and Central South University.

Muhammad Humza Javed is a doctoral researcher at the University of Calgary, Canada. He holds an MPhil in Mechanical Engineering with a focus on nanotechnology from the Hong Kong University of Science and Technology, and a BSc (Hons) in Nanoscience & Nanotechnology from Preston University. Humza has also been awarded the UNESCO‐UNISA Fellowship under the African Chair in Nanoscience and Nanotechnology at iThemba LABS, South Africa. His research spans thermal management composites, TENG systems, and the synthesis of advanced nanomaterials.

Rishabh Mishra is a postdoctoral research fellow in the School of Photovoltaic and Renewable Energy Engineering (SPREE) at the University of New South Wales, Australia. His research focuses on investigating strongly correlated quantum materials and semiconductor interfaces for photovoltaic applications, utilizing ultrafast spectroscopic techniques.

Meysam Amini is a doctoral researcher in the School of Materials Science and Engineering at the University of New South Wales (UNSW), Sydney, under the supervision of Prof. Sean Li. His research focuses on the epitaxial growth of free‐standing oxide semiconductor membranes, lithography, and high‐performance electronic device fabrication. Meysam earned his master's degree in Materials Science and Engineering with a specialization in Corrosion and Protection of Materials from Shiraz University of Technology, where he graduated as the top‐ranked student.

Mahesh P. Suryawanshi is a Senior lecturer and ARC DECRA Fellow at the School of Photovoltaic and Renewable Energy Engineering, UNSW Sydney. He has received multiple prestigious awards, including a Doctoral Exchange Scholarship (2012), the Brain‐Korea (BK‐21) Postdoctoral Fellowship (2016–2019), the Early Career Researcher Award (2019–2021) from the Ministry of Spain and the European Commission, and the Australian Research Council's Discovery Early Career Researcher Award (DECRA) (2021–2024). Since 2021, he leads a “Materials Innovation Lab for Sustainable Energy Futures” that focuses on the nanoscale design and development of functional materials, exploring their structure‐optoelectronic properties through experiments and modelling for solar energy conversion and catalysis applications.

Furqan C. M., Guo X., Javed M. H., Mishra R., Amini M., and Suryawanshi M. P., “Electrocatalytic Upcycling of PET Waste to Chemical Products and Fuels Coupled With Cathodic Reactions.” Small 21, no. 43 (2025): e06556. 10.1002/smll.202506556