Low-temperature modified-immersion molten ZnCl₂-carbonization and activation for continuous production of mesoporous carbon from plastic waste

Abstract

Plastic waste pollution has emerged as a critical global issue, threatening ecosystems, human health, and sustainable economic development. Molten salt carbonization and activation (MSCA) presents a promising pathway for converting plastic waste into valuable carbon materials. However, conventional MSCA approaches face challenges such as premature degradation of carbon sources before salt melting, undesirable side reactions that lower the specific surface area, and limited potential for industrial scalability. In this study, we introduce a prototype of a continuous MSCA system employing a modified-immersion zinc chloride (ZnCl₂) molten salt system at a relatively low temperature of 350 °C to convert common plastic waste including polypropylene (PP) bowls, polylactic acid (PLA) cups, and polyethylene terephthalate (PET) bottles into high-surface-area mesoporous carbon. The type of plastic waste and the ZnCl₂-to-PET mass ratio significantly influence the formation of mesoporous structures and the development of defect sites within the graphitic carbon matrix. Among all tested samples, MSW-PET-10 (derived from a 10 : 1 ZnCl₂-to-PET ratio) exhibited the highest specific surface area of 976 m² g⁻¹ and a pore volume of 0.95 cm³ g⁻¹. The resulting mesoporous carbon derived from plastic waste demonstrates excellent electrochemical performance as a supercapacitor electrode material, highlighting its potential for energy storage applications and sustainable waste management. Life cycle assessment (LCA) of the process revealed that its environmental footprint regarding this study can be reduced by about 37.3% compared to the conventional MSCA mixing method.

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Green foundation

1. This study presents a modified immersion molten-ZnCl₂ carbonization and activation process for the continuous production of mesoporous carbon from common plastic waste, including polypropylene (PP), polylactic acid (PLA), and polyethylene terephthalate (PET).

2. The process operates at a relatively low temperature of 350 °C and promotes intimate interfacial contact between the plastic waste and the molten salt phase, thereby minimizing undesirable side reactions. The resulting mesoporous carbon exhibits a high specific surface area of 976 m² g⁻¹ and demonstrates excellent electrochemical performance as a supercapacitor electrode material.

3. Future research could explore the utilization of mixed plastic waste as a feedstock and incorporation of eutectic salt systems in this modified immersion MSCA process.

1. Introduction

Plastic is extensively used in daily life due to its exceptional durability, flexibility, lightweight nature, and affordability. However, its resistance to biodegradation has resulted in a persistent accumulation of plastic waste.^1,2 The COVID-19 pandemic has significantly increased plastic consumption, driven by the heightened demand for personal protective equipment (PPE), such as masks, face shields, and gloves, along with the surge in users of food delivery and online shopping platforms. This has led to a substantial rise in single-use plastic waste, with studies reporting an estimated 17–40% increase globally during and after the pandemic.³ Direct incineration and landfills are the most common methods of plastic waste disposal. However, these approaches are environmentally harmful and pose significant risks to animals, human health, and ecosystems.^4–6 To mitigate this issue, various types of plastic waste have been repurposed as precursors for producing value-added products, including polyethylene terephthalate (PET),⁷ polypropylene (PP),^8,9 polyvinyl chloride (PVC),¹⁰ polystyrene (PS),¹¹ polyethylene,¹² and other plastic materials.^13–16

The conversion of plastic waste into value-added carbon materials is receiving significant attention due to its high carbon content and low impurity levels.^17–19 This approach not only addresses the critical issue of plastic waste pollution but also creates opportunities for synthesizing advanced carbon materials. Several types of carbon materials have been successfully derived from plastic waste such as carbon nanotubes,²⁰ carbon nanosheets,^21,22 carbon spheres,²³ graphene,^24,25 and porous carbon.^26–28 These carbon materials have broad applications across various fields including pollutant adsorption,^29–31 catalysis,^32,33 and energy storage,^34,35 owing to their high specific surface area. Several methods have been developed to convert plastic waste into porous carbon, such as anoxic pyrolysis,³⁶ physical activation,^37,38 chemical activation,³⁹ hydrothermal,⁴⁰ and template methods.⁴¹ However, these approaches often involve high energy consumption, lengthy processing times, and complex synthesis procedures.^18,36,42

Molten salt carbonization and activation (MSCA) has emerged as a promising approach for converting biomass, carbon-rich materials, and polymer waste into valuable carbon-based products.^43–45 This method offers several notable advantages, including short processing time, operational simplicity, cost-effectiveness, scalability, recyclability, and excellent thermal stability.^46–48 In the MSCA process, a molten salt acts as both a heat transfer medium and a catalyst, facilitating the breakdown of plastic waste into carbon-rich aromatic structures. Additionally, the molten salt promoted the formation of porous structures, thereby enhancing the surface area of the synthesized carbon materials.^45,49 Despite its potential, traditional MSCA processes were found to have decomposition reactions occurring in plastic precursors prior to salt melting, or complex reactions between the salt and plastic precursors during the early stages may hinder pore development and reduce the resulting surface area of the carbon product. Although many studies have demonstrated the effectiveness of the MSCA process at the laboratory scale, its use on the industrial scale is still limited.⁵⁰ Due to the systematic design, traditional MSCA processes can only be performed using batch systems, which pose challenges for scale-up. These systems often suffer from uneven heat distribution, making it difficult to control the product quality, while the discontinuity of the process results in lower production yields and higher operational costs.⁵¹

Continuous MSCA processes provide several advantages in process control, product quality, and scalability. These systems enable real-time control of key parameters such as heat input, salt-to-precursor feedstock ratio, and residence time. The continuous feeding of raw materials not only shortens processing time but also increases product volume, contributing to reduced production costs.⁵² Importantly, continuous systems can be designed to efficiently separate the salt from the carbon products for reuse. These advantages show the effective potential of continuous systems in MSCA processes as a viable approach for industrial-scale production of porous carbon materials from plastic waste.

In this study, we present a prototype of a continuous system MSCA process at low temperature for synthesizing mesoporous carbon materials from common plastic waste including PP, PLA, and PET using zinc chloride (ZnCl₂) molten salt. The synthesis takes place under air atmospheric conditions at a relatively low temperature of 350 °C. Furthermore, the impact of the mass ratio between ZnCl₂ and PET bottles was systematically investigated. The resulting mesoporous carbon was subsequently utilized as an active material for fabricating electrodes intended for supercapacitor applications.

2. Experimental

2.1. Modified-immersion ZnCl₂ MSCA

The plastic waste was cut down into small pieces of around 1 cm² and then washed with an isopropyl alcohol solution (IPA, Chemex Industry, Thailand) in an ultrasonic bath and dried at 110 °C for 30 min. Synthetic carbon from the modified ZnCl₂-based molten salt carbonization process was labeled MSW-Plastic-X, where MSW denotes molten salt waste, the plastic type is PP, PLA, or PET, and X is the ZnCl₂-to-plastic mass ratio. MSW-PP-X, MSW-PLA-X, and MSW-PET-X were prepared by heating zinc chloride (ZnCl₂, 98%, Qrec, New Zealand) up to 350 °C under an air atmosphere. Subsequently, plastic pieces were added to the zinc chloride molten salt, stirred with a glass rod to obtain a black slurry, and kept at this temperature for 30 min. After slowly cooling down to ambient temperature, the obtained carbon product was washed with 0.1 M hydrochloric acid (HCl, 36%, Kemaus, Australia) and deionized water until a neutral pH value was reached, followed by vacuum filtration and drying at 110 °C for 120 min.

2.2. Materials characterization

The crystal structures were characterized by X-ray diffractometry (XRD) using a Siemens D-500 with Cu Kα radiation in the 2θ range between 5° and 80°. The morphology and the surface elemental distribution of samples were characterized by scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDX) using a Hitachi S-3400N. Raman spectroscopy was conducted using a Thermo Scientific DXR3 Raman microscope with a 523 nm wavelength laser as the excitation source. Nitrogen adsorption/desorption isotherms of the samples were obtained using a Micromeritics 3Flex surface characterization analyser. The Brunauer–Emmett–Teller (BET) method was used to calculate the specific surface area, and the pore distribution was calculated using the Barrett–Joyner–Halenda (BJH) method.

2.3. LCA methodology

To evaluate the environmental impacts of PET-derived mesoporous carbon, a life cycle assessment (LCA) was conducted in strict accordance with the ISO 14040 and ISO 14044 standards. The assessment comprised four sequential phases: goal and scope definition; life cycle inventory analysis; life cycle impact assessment; and interpretation, each following the principles and requirements outlined in ISO 14040⁵³ and ISO 14044,⁵⁴ thereby ensuring a consistent, transparent, and reproducible evaluation of the PETAC production process.

2.4. Fabrication of supercapacitor electrodes

The synthetic carbon (MSW-PET-10) or commercial carbon (TF-B520) electrodes were prepared by mixing the respective carbon material with polyvinylidene fluoride (PVDF, Canrud, China) and carbon black (CB, conductive acetylene black for a Li-ion battery, MTI Corporation, batch number 130502) at a mass ratio of 80 : 10 : 10 with N-methyl-2-pyrrolidone (NMP, Acros Organic, Belgium, batch number A0433323) as a solvent to form a homogeneous black slurry. For comparison purposes, the blank electrode was prepared by mixing carbon black and PVDF at a mass ratio of 90 : 10 with NMP as the solvent. All resulting slurries were coated onto a graphite sheet substrate using a doctor blade with a gap of 120 μm and dried in a vacuum oven at 70 °C overnight. The working electrodes had an area of 2 × 2 cm² with an active material loading of approximately 1 mg cm⁻².

2.5. Electrochemical performance testing

Electrochemical performance of the active electrode material was evaluated by cyclic voltammetry (CV) at different scanning rates between −1.0 and 0.0 V, and galvanostatic charge and discharge (GCD) tests were performed between 0.1 and 5 A g⁻¹ with a typical three-electrode system using a potentiostat (Multi Autolab/M204, Metrohm) with Pt as a counter electrode, Ag/AgCl as the reference electrode, and 6 M potassium hydroxide (KOH) as the electrolyte. A symmetric two-electrode device was assembled using two identical as-prepared carbon electrodes.

The galvanostatic charging/discharging profiles were used to calculate the specific capacitance of the synthetic carbon product using the following equation:⁵⁵

C = I × Δt/(m × ΔV) (1)

where C is the specific capacitance (F g⁻¹), I is the discharge current, Δt is the discharge time, m is the mass of mesoporous carbon on the electrode, and ΔV is the potential change during the GCD test.

3. Results and discussion

The traditional MSCA process consists of two routes, including mixing and immersion processes, as illustrated in Fig. 1(a) and (b). However, both face challenges such as poor reproducibility, limited salt recovery, and high operating costs. The mixing MSCA process involves blending carbon precursors with a salt, followed by heating the mixture in air or inert gases to the desired temperature, converting the precursor into carbonaceous products. The excess salt is then removed from the resulting carbon products by washing with water or diluted acid. However, the mixing route may experience issues, such as decomposition reactions in the carbon precursors before the salt melts, complex interactions between the salt and the carbon precursors before melting and the need for longer reaction times.⁴⁶ To address these limitations, the MSCA immersion process has been used by immersing the carbon precursors in a molten salt to convert the precursors into carbonaceous products and then washing with water or diluted acid to remove the impurities from carbon products. Nevertheless, reproducibility, salt recovery, low production yields, and high operational costs are still unresolved as previous MSCA processes are still operated under a batch system, which is the main issue in large-scale production. The synthetic conditions and properties of the porous carbon obtained from different methods are shown in Table 1.

Fig. 1 Schematic illustration of the MSCA process for porous carbon synthesis: (a) mixing MSCA process, (b) traditional immersion MSCA process, and (c) modified immersion MSCA process in this work.

Table 1 Comparison of carbon synthesis conditions from different methods

Activation methods	Carbon sources	Conditions					Carbon products			Ref.
		Activating agent	Ratio	Temperature (°C)	Holding time (min)	Atmosphere	Structure	Carbon yield (%)	S BET (m^2 g^−1)
Anoxic pyrolysis	PET bottles	—	—	750	30	—	Porous carbon	—	282	62
Stream activation	PET bottles	Steam	—	900	60	—	Porous carbon	6	1061	63
CO2 activation	PET bottles	CO2	—	940	300	—	Porous carbon	20	1830	63
Chemical activation	PET bottles	KOH	2 : 1	800	60	N2	Porous carbon	—	979	64
Template method	PET bottles	K2CO3	4 : 1	700	120	N2	Porous carbon	34	607	41
Impregnation	PET bottles	ZnCl2	1 : 1	500	120	N2	Mesoporous carbon	13	700	65
	PET bottles	ZnCl2	1 : 1	400 followed by 800	60, 60	N2	Porous carbon	25	682	66

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Activation methods	Carbon sources	Conditions					Carbon products			Ref.
		Salt types	Salts : carbon sources	Temperature (°C)	Holding time (min)	Atmosphere	Structure	Carbon yield (%)	S BET (m^2 g^−1)
Mixing MSCA	PET bottles	ZnCl2	6 : 1	1300	60	Air	Porous carbon	—	879	67
	PET bottles	NaCl	5 : 1	1300	0	Air	Porous carbon	12	522	68
	PET bottles	NaCl–KCl	1.5 : 1	800	10	Air	Porous carbon	32	535	69
	PET	ZnCl2–NaCl	2 : 1	280 followed by 550	10, 8	Air	Porous carbon	27	776	70
	PET bottles	ZnCl2–NaCl	2 : 1	280 followed by 550	10, 8	—	Porous carbon	22	646	49
Immersing MSCA	Coffee beans	CaCl2	25 : 1	850	60	Ar	Porous carbon	—	550	71
	Bamboo shells	Na2CO3–K2CO3	100 : 1	850	60	Ar	Porous carbon	—	843	72
	Soyabean straw	KOH	—	800	60	Ar	Porous carbon	—	1615	73
Modified-immersion MSCA	PP bowls	ZnCl2	10 : 1	350	30	Air	Mesoporous carbon	2–14	417–976	This work
	PLA cups
	PET bottles

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Scaling up the MSCA process is a promising approach but research in this area remains limited.⁵⁰ The continuous MSCA process represents a scalable and industrially feasible approach, owing to its ease of operation, product quality control, high production capacity, and cost-effectiveness. In the proposed model, the process begins with the heating of a salt, which flows through the reactor along with the introduction of size-reduced plastic waste. Upon contact between plastic waste and the molten salt, carbonization and activation reactions occur, resulting in high-quality porous carbon products after the salt separation process. The continuous MSCA process begins with plastic size reduction, where various types of plastic waste are cut into small pieces to increase the surface area and promote effective heat and mass transfer during the reaction. These plastic fragments are then introduced into a molten salt reactor operating at 350 °C containing ZnCl₂ delivered from a salt melting unit at a salt-to-plastic ratio of 10 : 1. Inside the reactor, the molten salt acts as a thermal medium and a catalyst for carbonization and activation of the plastic waste into carbon. The carbon–salt mixture is then directed to a separation unit, where the residual salt is removed using diluted acid and water, followed by filtration. The resulting carbons are then dried in a drying unit to remove residual moisture, resulting in porous carbon. For enhanced sustainability, ZnCl₂ after the reaction can be recovered from the aqueous leachate by an evaporation step, enabling the reuse of most of the molten salt with minimal addition of fresh reagents.^56–58 This method allows for the recovery of more than 75% of the ZnCl₂ after the first washing cycle, thereby improving the overall efficiency of molten salt utilization. The conceptual model of the modified immersion for a continuous MSCA process is shown in Fig. 1(c).

The feasibility of operating the process under continuous conditions was demonstrated using a prototype screw-extruder reactor with simultaneous feeding of ZnCl₂ and plastic waste at 2.75 g min⁻¹ (SI Fig. S1a–c). Inside the reactor, the mixture was retained for approximately 10 min before exiting as a solid product. The product was then washed with dilute HCl and DI water to remove residual ZnCl₂, followed by drying. This demonstration provides proof-of-concept for the applicability of the approach in continuous systems.

3.1. Comparison of methods for the synthesis of plastic waste-derived porous carbon

Table 1 summarizes the available methods for the conversion of plastic waste and biomass into mesoporous carbon products, including anoxic pyrolysis and chemical and physical activation in comparison with the MSCA process. Although the carbon materials produced by these conventional methods typically exhibit high specific surface areas, they often require high temperatures, extended reaction times, and complex synthesis procedures.^42,50 In contrast, the MSCA process presents a more attractive alternative, offering rapid carbonization, tunable product structure and porosity, lower energy consumption, compatibility with a wide range of precursors, and ease of scalability.^43,46 The conventional MSCA process is generally constrained to batch operation, which presents challenges for large-scale industrial application. Most prior studies have conducted salt-assisted carbonization by pre-mixing plastics with salts prior to heat treatment, whereas there are reports of immersing plastic waste in molten salt baths during the gasification process. In contrast, a few reports describe immersing plastic waste directly into molten salt baths during gasification.⁵⁹ While immersion MSCA methods have been investigated using biomass as the carbon source, in the context of plastic waste, a molten salt has typically been employed solely as a heating medium for pyrolysis.^60,61

In this work, a modified immersion MSCA approach was developed, in which plastic waste was instantly combined with a molten salt rather than subjecting it to the traditional prolonged immersion. This method enabled the rapid synthesis of mesoporous carbon with a high specific surface area of 976 m² g⁻¹ at 350 °C within 30 min. This approach facilitates the production of high-quality porous carbon rapidly, at low temperatures, and without the need for additional chemicals, highlighting its strong potential for continuous operation.

ZnCl₂ is a well-established chemical agent used in the synthesis of porous carbon from biomass,^74,75 coal,^76,77 and plastic waste⁶⁵via the chemical activation process. The mechanism of porous carbon synthesis in this MSCA process is illustrated in Fig. 2. Upon direct immersion of plastic waste into the molten salt, ZnCl₂ acts both as a heat transfer medium to melt the plastic and as a sealant between the plastic and air, preventing undesirable thermal decomposition products.⁴⁵ The decomposition of oxygen-containing plastic begins with random scission at the ester linkage, where the C–O bond breaks through a β-hydrogen transfer mechanism, producing intermediate chain fragments.⁷⁰ ZnCl₂ further promotes dehydration and decarboxylation while eliminating weak bonds in the intermediate fragments to facilitate the formation of stable crosslinking structures.^49,70 These crosslinking structures then undergo cyclization and aromatization, ultimately leading to the formation of a graphitic carbon structure.^49,70 In the case of non-oxygen-containing plastics, the process begins with the breaking down of C–H bonds and dehydrogenation of the polymer backbone, promoting aromatization into a laddered structure that can carbonize into a graphitic carbon structure without completely degrading into volatile hydrocarbons.⁷⁸ Moreover, an optimal ZnCl₂ content serves as a template, preventing structural collapse and promoting high porosity of the final carbon product.^43,45

Fig. 2 Mesoporous carbon synthesis mechanism via the MSCA process in this work.

3.2. Effect of plastic type on the characteristics of porous carbon

Various plastic waste, including PP bowls, PLA cups, and PET bottles, were utilized as carbon sources in our synthetic process to produce carbon products under comparable conditions. The structures of the synthesized carbon were characterized by Raman spectroscopy, XRD, and SEM-EDX, as shown in Fig. 3(a–c). The Raman spectra of MSW-PET, MSW-PLA, and MSW-PP exhibited two main peaks at approximately 1348–1354 cm⁻¹ and 1583–1591 cm⁻¹, corresponding to the D band, which presents structural disorder or defects in graphite, and the G band representing the in-plane vibrations of sp² bonded carbon atoms in a graphite layer, respectively.^79–81 The intensity ratio of the D and G bands (I_D/I_G) serves as an indicator of the defect density in graphene planes. Higher I_D/I_G values reflect increased defects in graphitic carbon materials.^1,67,68 The I_D/I_G ratios for MSW-PET, MSW-PLA, and MSW-PP were found to be 0.75, 0.78, and 0.70, respectively. XRD analysis revealed characteristic broad peaks at 2θ degrees of 26°–30° and 40°–42°, corresponding to the (002) and (101) planes of the graphitic carbon structure, respectively.^1,67,68 The morphology of the carbon products was examined through SEM images, which demonstrated micrometer-sized particles with rough and irregular surfaces. Additionally, EDX analysis from SEM images reveals an ultra-low residual content of ZnCl₂, demonstrating the effectiveness of our process in separating high-quality porous carbon products from the molten salt matrix by washing with diluted acid and deionized water.

Fig. 3 The properties of carbon products obtained from different plastic waste types are characterized by Raman spectroscopy, XRD, and SEM-EDX: (a) MSW-PET-10, (b) MSW-PLA-10, and (c) MSW-PP-10.

The specific surface area and porosity characteristics were further analyzed using the nitrogen adsorption–desorption method, and the results are shown in Fig. 4(a). The nitrogen adsorption–desorption isotherms for MSW-PET, MSW-PLA, and MSW-PP samples exhibited type IV isotherms with H4-type hysteresis loops, characterized by sharp capillary condensation at high relative pressures. This behavior confirms the presence of mesoporous structures,⁸² consistent with the pore size distribution profiles calculated from the adsorption branches of the isotherms using the BJH model, as illustrated in Fig. 4(b). The influence of plastic types on the carbon yield was evident in the results. Carbon products from PET bottles and PLA cups exhibited higher yield percentages of 15.0 and 9.2%, respectively, compared to only 2% for PP bowls. This disparity can be attributed to the presence of oxygen functional groups in PET and PLA, which promote the dehydration reaction, enhance cross-linking, and facilitate stable aromatization during carbonization, thereby increasing carbon yield.^49,70 In contrast, the lack of oxygen functional groups in PP plastic waste leads to predominant decomposition rather than carbonization, resulting in significantly lower carbon yield. This observation aligns with previous studies, which reported that plastic waste precursors lacking functional groups, such as PP, primarily generate hydrogen, carbon monoxide, and carbon dioxide gases rather than solid carbon products.⁷⁸

Fig. 4 (a) Nitrogen sorption isotherms, and (b) BJH pore size distribution plot of MSW-PET-10, MSW-PLA-10, and MSW-PP-10.

3.3. Effect of ZnCl₂ to PET bottle mass ratio on porous carbon characteristics

The effect of ZnCl₂ to PET bottle mass ratio on the morphology and structure of the carbon products was analyzed, as shown in Fig. 5(a–d). The resulting carbons were labeled MSW-PET-X, where X denotes the mass ratio of ZnCl₂ to PET bottles. The SEM images revealed the morphology of the synthesized carbons obtained from different ZnCl₂-to-PET mass ratios. Increasing the ZnCl₂ content did not significantly change the morphology of the carbon products; they exhibited a smooth surface with certain areas exhibiting a rough appearance. Larger pores were observed in MSW-PET-12 and MSW-PET-14. The XRD patterns of the products showed broad peaks at 2θ degrees of 25°–27° and 40°–42°. Upon comparing the characteristics of each carbon product, it was found that an increase in ZnCl₂ content resulted in broader and lower intensity peaks, indicating a rise in the amorphous carbon structure of the as-synthesized carbon products.⁶⁸ The Raman spectra of MSW-PET-8, MSW-PET-10, MSW-PET-12, and MSW-PET-14 exhibited an intense D peak at ∼1348–1360 cm⁻¹ and a G band at ∼1583–1590 cm⁻¹. The I_D/I_G values of MSW-PET-8, MSW-PET-10, MSW-PET-12, and MSW-PET-14 were calculated to be 0.88, 0.75, 0.81, and 0.82, respectively, as shown in Table 2. The porosity characteristics of the carbon products obtained from different ZnCl₂-to-PET mass ratios are presented in Fig. 6. All the products exhibited a type IV isotherm and a H4-type hysteresis loop with sharp capillary condensation at high relative pressures, indicating mesoporous structures.⁸² The increase in the mass ratio of ZnCl₂ to PET bottles from 8 to 10 resulted in an increase in the BET surface area from 605 to 976 m² g⁻¹, when the ratio exceeded 10, the surface area decreased. The results indicate that insufficient ZnCl₂ content leads to an insufficient activation agent, resulting in limited pore formation, template formation, and incomplete carbonization, which in turn lowers porosity and product yield. Conversely, upon increasing the ZnCl₂ content, it can act as a sealant and restrict contact with O₂, thereby helping to suppress the decomposition of PET into byproduct gases and promoting dehydration and cross-linking effects that can increase the carbon yield.^46,83,84

Fig. 5 The properties of carbon products obtained from different plastic waste types are characterized by Raman spectroscopy, XRD, and SEM-EDX: (a) MSW-PET-8, (b) MSW-PET-10, (c) MSW-PET-12, and (d) MSW-PET-14.

Fig. 6 (a) Nitrogen adsorption/desorption isotherms, (b) pore size distribution calculated using the BJH method of carbon products from different mass ratios of ZnCl₂ to PET bottles.

Table 2 The properties of carbon products obtained from the modified immersive MSCA

Sample	Carbon source	ZnCl2/plastic waste mass ratio	% yield	I D/IG	BET surface area (m^2 g^−1)	Pore volume (cm^3 g)	Avg. pore size (nm)
MSW-PLA-10	PLA cups	10	9.11	0.78	657	0.68	4.1
MSW-PP-10	PP bowls	10	2.01	0.70	417	0.40	3.8
MSW-PET-10	PET bottles	10	14.1	0.75	976	0.95	3.9
MSW-PET-8	PET bottles	8	11.19	0.88	605	0.71	4.7
MSW-PET-12	PET bottles	12	11.43	0.81	541	0.68	5.1
MSW-PET-14	PET bottles	14	12.58	0.82	684	0.83	4.8

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Nevertheless, excessive ZnCl₂ leads to overactivation or agglomeration of salts, promoting the formation of macropores and blocking the development of a micropore network, which ultimately lowers the BET surface area and overall yield.^49,85 Therefore, the optimal mass ratio between ZnCl₂ and PET bottles achieves both a high surface area and a high product yield.

3.4. Life cycle impact assessment

The environmental impacts of the modified immersion MSCA for PET-derived mesoporous carbon were assessed via LCA conducted in accordance with ISO 14040 and 14044. The method encompasses 18 midpoint impact categories;⁸⁶ six were prioritized in this study due to their significant relevance: global warming (GW), marine eutrophication (ME), terrestrial ecotoxicity (TET), freshwater ecotoxicity (FET), marine ecotoxicity (MET), and human carcinogenic toxicity (HCT). Material and energy consumption data for processing 1 kg of PET waste were collected experimentally and are presented in Table 3. From the LCA results (Table 4), the production of porous carbon based on the modified immersion MSCA process generated greenhouse gas emissions corresponding to a global warming potential (GWP) of 4.77 kg CO₂-eq per kg PET waste whereas that based on the conventional mixing MSCA generated 7.61 kg CO₂-eq per kg PET waste, primarily attributable to electricity consumption in the carbonization step. Therefore, with the new approach, the global warming potential could be reduced by 37.3%. Normalized results using the ReCiPe 2016 method revealed that human carcinogenic toxicity potential (HTPc) exhibited the highest impact, followed by freshwater ecotoxicity potential (FETP).

Table 3 The data for material and energy consumption for processing 1 kg PET waste

LCA inventory		Unit	Modified immersion	Conventional mixing
Product	Activated carbon	kg	0.14	0.08
Energy consumption		kWh	4.01	7.01
	PET waste	kg	1	1
	IPA	kg	0.08	0.08
	Zinc chloride	kg	10	10
	Deionized water	L	200	200

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Table 4 Environmental impacts of producing mesoporous carbon from PET waste using modified immersion and conventional mixing MSCA processes

Impact category	Unit	Modified immersion	Conventional mixing	Normalized
				Modified	Conventional
Global warming potential (GWP)	kg CO2 eq.	4.77	7.61	0.000597	0.000951
Stratospheric ozone depletion potential (ODP)	kg CFC11 eq.	8.04 × 10^−7	1.51 × 10^−6	1.34 × 10^−5	2.52 × 10^−5
Ionizing radiation potential (IRP)	kBq Co-60 eq.	0.0194	0.0255	4.04 × 10^−5	5.31 × 10^−5
Ozone formation potential for human health (HOFP)	kg NOx eq.	0.00487	0.00895	0.000237	0.000435
Ozone formation potential for terrestrial ecosystems (EOFP)	kg NOx eq.	0.0051	0.00934	0.000287	0.000526
Fine particulate matter formation potential (PMFP)	kg PM2.5 eq.	0.003	0.00548	0.000117	0.000214
Terrestrial acidification potential (TAP)	kg SO2 eq.	0.00749	0.014	0.000183	0.000341
Freshwater eutrophication potential (FEP)	kg P eq	0.00214	0.00415	0.00329	0.0064
Marine eutrophication potential (MEP)	kg N eq	0.000166	0.000325	3.65 × 10^−5	7.05 × 10^−5
Terrestrial ecotoxicity potential (TETP)	kg 1,4-DCB	9.94	17.4	0.000654	0.00115
Freshwater ecotoxicity potential (FETP)	kg 1,4-DCB	0.183	0.353	0.00725	0.014
Marine ecotoxicity potential (METP)	kg 1,4-DCB	0.242	0.466	0.00556	0.0107
Human carcinogenic toxicity potential (HTPc)	kg 1,4-DCB	0.302	0.544	0.0293	0.0528
Human non-carcinogenic toxicity potential (HTPnc)	kg 1,4-DCB	3.56	6.8	0.000114	0.000218
Land use potential (LUP)	m^2 a crop eq.	0.0277	0.0478	4.48 × 10^−6	7.75 × 10^−6
Mineral resource scarcity potential (MRSP)	kg Cu eq.	0.0052	0.00935	4.33 × 10^−8	7.79 × 10^−8
Fossil resource scarcity potential (FRSP)	kg oil eq.	0.922	1.7	0.00094	0.00174
Water consumption potential (WCP)	m^3	0.0117	0.0216	4.4 × 10^−5	8.1 × 10^−5

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3.5. Applications for electrochemical energy storage

The synthesis of porous carbon via the MSCA process has garnered significant attention for applications in various fields such as catalysis, adsorption, and energy storage.^29–35 In this work, the carbon product derived from PET bottles (MSW-PET-10) was employed as an active material in the fabrication of supercapacitor electrodes. Its performance was compared with those of blank CB (carbon black) and TF-B520 (commercial activated carbon). Fig. 7a presents the cyclic voltammetry (CV) curves of all electrodes, which were recorded at a scan rate of 20 mV s⁻¹ over a potential window of −1.0 to 0.0 V. The CV curves exhibit a rectangular-like shape characteristic of electric double-layer capacitance behavior in a 6 M KOH electrolyte. Fig. 7b exhibits the CV curves of the MSW-PET-10 electrode within the scan rate range of 2 to 200 mV s⁻¹. As the scan rate increases, the plateau current rises accordingly, while the quasi-rectangular shape of the CV loops is largely maintained with minimal distortion up to 50 mV s⁻¹. This behavior evidences the electrode's excellent high-rate capability and electrochemical stability.^9,87,88 At higher scan rates (100–200 mV s⁻¹), the shape of the CV curve is slightly distorted due to the limited ion diffusion and increased internal resistance.⁸⁹ The galvanostatic charge and discharge (GCD) curves of blank carbon black, MSW-PET-10, and TF-B520 electrodes measured at a current density of 0.1 A g⁻¹ are shown in Fig. 7c. The GCD profiles are not perfectly linear but show slight distortion, suggesting the presence of pseudocapacitive behavior due to the incorporation of oxygen-containing groups.⁹⁰ The specific capacitance of electrodes, calculated from the GCD curves, is shown in Fig. 7d; the MSW-PET-10 electrode shows the highest specific capacitance of 265 F g⁻¹ at a current density of 0.1 A g⁻¹, surpassing those of TF-B520 and blank carbon black, which deliver 149 F g⁻¹ and 46 F g⁻¹, respectively. As the current density increases, the specific capacitance of MSW-PET-10 gradually decreases to 62, 48, 44, 45, and 50 F g⁻¹ at current densities of 0.5, 1.0, 2.0, 3.0 and 5.0 A g⁻¹, respectively, and tends to stabilize at higher current densities. In contrast, TF-B520 demonstrates a continuously declining capacitance across the entire current range. The CV curves of blank CB and TF-B520 at scan rates ranging from 2 to 200 mV s⁻¹, along with their GCD profiles at various current densities from 0.1 to 5 A g⁻¹, are shown in Fig. S2.

Fig. 7 Comparison of synthetic mesoporous carbon (MSW-PET-10), blank carbon black (blank CB), and commercial activated carbon (TF-B520) in a three-electrode system using 6 M KOH electrolyte. (a) CV curves at a scan rate of 20 mV s⁻¹; (b) CV curves of MSW-PET-10 at various scan rates; (c) GCD curves at a current density of 0.1 A g⁻¹; and (d) specific capacitances at different current densities.

To further evaluate the practical application potential of MSW-PET-10 compared with commercial activated carbon (TF-B520), we prepared a symmetric supercapacitor using 6 M KOH as an electrolyte. Fig. 8a shows the CV curves of MSW-PET-10 and TF-B520 recorded at a scan rate of 20 mV s⁻¹. The CV curves of both electrodes exhibit a nearly rectangular shape from −1.0 to 0.0 V, indicating EDLC behavior. The CV curves of the MSW-PET-10 symmetric supercapacitor at different scan rates, ranging from 2 to 200 mV s⁻¹, are shown in Fig. 8b. The CV curves maintain a rectangular-like shape with no significant current polarization within the voltage range of −1.0 to 0.0 V, demonstrating excellent capacitive performance and rapid, efficient charge transfer at high scan rates. The GCD curves of MSW-PET-10 and TF-B520 at a current density of 0.1 A g⁻¹ are shown in Fig. 8c. The GCD profiles are nearly rectangular-shaped GCD profiles. MSW-PET-10 exhibits a longer discharging time than TF-B520, suggesting a higher energy storage capability. Fig. 8d shows the GCD profiles of MSW-PET-10 at various current densities ranging from 0.1 to 5 A g⁻¹. It is observed that all charge–discharge curves are almost triangular-shaped. This is also a typical characteristic of the ideal electric double-layer charge storage process, which is consistent with the result of CV curves. At lower current densities, the prolonged discharge time indicates enhanced ion–electrode interactions, thereby contributing to higher capacitance.⁹¹ The specific capacitance values of the MSW-PET-10 and TF-B520 symmetric supercapacitors, calculated from eqn (1), are shown in Fig. 8e. MSW-PET-10 exhibited a specific capacitance of 57 F g⁻¹, which is higher than that of TF-B520 (40 F g⁻¹) at a current density of 0.1 A g⁻¹, indicating that the PET waste-derived MSW-PET-10 possesses superior charge-storage capability compared to the commercial TF-B520 under low current density conditions. However, with increasing current density, the specific capacitance of MSW-PET-10 decreased to 29, 24, 27, and 30 F g⁻¹ at 1, 2, 3, and 5 A g⁻¹, respectively, which are lower than those of TF-B520 (39, 40, 39, and 30 F g⁻¹ at the corresponding current densities), likely due to an insufficient time for electrolyte ions to diffuse into all accessible pores at high current densities.⁸⁹ The stability of MSW-PET-10 and TF-B520 is illustrated in Fig. 8f. The capacitance of MSW-PET-10 demonstrates impressive capacitance retention (∼96%) after 3000 charge–discharge cycles at a current density of 5 A g⁻¹, which is close to that of the commercial activated carbon (TF-B520), indicating good durability and stability for practical supercapacitor applications. A comparison with the literature data on the electrochemical capacitance of plastic-derived carbon is presented in Table 5, highlighting the excellent electrochemical capacitive performance of the as-prepared carbon in this work.

Fig. 8 Comparison of the electrochemical performance of MSW-PET-10 and TF-B520 in a symmetric two-electrode system using 6 M KOH as the electrolyte: (a) CV curves at a scan rate of 20 mV s⁻¹; (b) CV curves of MSW-PET-10 at various scan rates ranging from 2 to 200 mV s⁻¹; (c) GCD curves at a current density of 0.1 A g⁻¹; (d) GCD curves of MSW-PET-10 at different current densities (0.1–5 A g⁻¹); (e) specific capacitances of MSW-PET-10 and TF-B520 at different current densities; and (f) capacitance retention of MSW-PET-10 and TF-B520.

Table 5 Comparison of the electrochemical performances of various plastic waste-derived carbon materials

Synthesis methods	Carbon source	Electrolyte	System	Current density (A g^−1)	Specific capacitance (F g^−1)	Ref.
Chemical activation	PET	1 M TEABF4/ACN	Two-electrode	0.1	25	39
	Resole-type phenolic resin	6 M KOH	Three-electrode	0.2	256	92
Solvothermal method	PET	6 M KOH	Three-electrode	1	111	93
Template method	Mixed plastic	6 M KOH	Two-electrode	0.2	207	94
	PET	6 M KOH	Three-electrode	0.1	169	21
	PET	6 M KOH	Three-electrode	0.5	332	41
Molten salt carbonization	PE	6 M KOH	Three-electrode	1	225	95
	PP	6 M KOH	Three-electrode	1	346	96
	PE	6 M KOH	Three-electrode	0.2	244	97
	Polyolefin	1 M TEABF4/AN	Two-electrode	0.5	110	95
	PET	6 M KOH	Two-electrode	0.1	57	This work
	PET	6 M KOH	Three-electrode	0.1	265

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4. Conclusions

In this study, we present a prototype of a continuous system for producing mesoporous carbon materials from commonly available plastic waste, such as PET bottles, PLA cups, and PP bowls, using ZnCl₂ as both an activation agent and a porogen in a modified immersion MSCA process. The process operates under an air atmosphere at 350 °C. The mesoporous carbons derived from PET bottles and PLA cups exhibit higher product yields compared to those from PP cups, primarily due to the presence of oxygen functional groups in their structure. These groups help facilitate dehydration reactions, cross-linking formation, and enhance the aromatization of carbon products during the carbonization step. The mass ratio of ZnCl₂ to PET bottles significantly influences the development of defective sites within the graphitic carbon structure. Moreover, the optimal mass ratio of ZnCl₂ to PET bottles plays a crucial role in the formation of mesoporous structures. The MSW-PET-10 sample, obtained at a ZnCl₂ to PET bottle mass ratio of 1 : 10, exhibits a mesoporous structure with the highest specific surface area of 976 m² g⁻¹ and a pore volume of 0.95 cm³ g⁻¹. This material was utilized as an active material in supercapacitor electrodes. MSW-PET-10 demonstrates a high specific capacitance of 265 F g⁻¹ in a three-electrode system at a current density of 0.1 A g⁻¹ with a 6 M KOH electrolyte. This approach not only provides a straightforward operational framework for industrial-scale production but also demonstrates enhanced effectiveness in generating high-quality porous carbon materials. This is attributed to the intrinsic characteristics of the MSCA process, which requires extensive interfacial contact between the plastic waste and the molten salt phase while minimizing undesirable side reactions. The modified immersion MSCA process for porous carbon production also reduces GWP by 37.3% compared to the conventional MSCA mixing method.

Author contributions

Theerawat Waiyaka: writing – original draft, investigation, formal analysis, and visualization. Rungkiat Nganglumpoon: conceptualization, methodology, formal analysis, and writing – original draft. Chonticha Rajrujithong: investigation. Weerachon Tolek: investigation. Pui Vun Chai: writing – review & editing. Masayuki Shirai: writing – review & editing. Anuchit Wuttitrairat: investigation. Paisan Kittisupakorn: investigation, writing – review & editing, and resources. Joongjai Panpranot: supervision, validation, writing – review & editing, resources, and funding acquisition.

Conflicts of interest

There are no conflicts to declare.

Data availability

All datasets analyzed in this study, including characterization data, are provided within this article and are available from the corresponding authors upon reasonable request.

Supplementary information is available. See DOI: https://doi.org/10.1039/d5gc02465j.

Acknowledgements

The R4Ent scholarship for T. W. from Chulalongkorn University and the Second Century Fund (C2F) is acknowledged. The first author would like to thank the NSRF via the Program Management Unit for Human Resources & Institutional Development, Research and Innovation [grant number B16F650003] and CrystalLyte Co., Ltd for the financial support of this research.

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Footnote

† Equal contribution.

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