Non-emission hydrothermal low-temperature synthesis of carbon nanomaterials from poly (ethylene terephthalate) plastic waste for excellent supercapacitor applications

ABSTRACT Poly(ethylene terephthalate) (PET) has a wide range of applications that generate a lot of waste globally; thus, upcycling PET is important because it offers several industrial and economic advantages. This study describes a sustainable, emissions-free process for converting PET plastics into carbon nanomaterials (CNMs) named PT-nano powder. The thermal-hydrothermal method has employed the production of PT-nano powder above the glass transition temperature (Tg) of PET plastics. Under optimal conditions, PET plastics were efficiently converted into PT-nano powder with 86.6% crystallinity and an average particle size of 6.5 nm. The PT-nano powder was characterized for physical and chemical properties using different techniques, including UV-Vis, FTIR, Raman spectroscopy, XRD, FESEM, TEM, and proton NMR analysis. The characterization confirms the complete conversion of PET to solid fractions of carbon nanomaterial. The PT-nano powder was tested in supercapacitor performance application with electrochemical characterization. The symmetric fabrication showed a specific capacitance of 250.8 F/g, energy density of 34.83Wh/kg, and power density of 999.9W/kg with a current density of 0.5A/g. The device fabrication exhibited high cycle stability and high capacitance retention of 96.8% with a current density of 1.5A/g after 10000 cycles. GRAPHICAL ABSTRACT


Introduction
The use of plastics in our daily life has increased tremendously because they have become part of most of the materials we use.Life without plastics is unimaginable due to their wide application and integration in materials such as body implants, domestic utensils, building materials, textiles, food packaging, and soft drinks (1).The increased use of plastic materials is due to their low cost of production and high demand for single-use applications, leading to the massive production of different polymers.Most of the common plastic materials on the market are single-use low-cost plastics that have no value for reuse, including plastic bottles, food packaging, soft drinks containers, and others.These plastics pervade daily usage in our life as carriers/packages (2), implants (3), textiles and clothing (4), and electronic components (5,6), fuels (7), among others.
The escalating demand for plastic products is causing plastic litter management challenges due to their singleuse practices, poor recycling policies, and slow environmental degradation, affecting soil and water quality (8).
There is an urgent need to address the challenge of plastic waste management to minimize plastic waste littering.The recent accumulation of plastic garbage and its detrimental effects on the environment and public health is becoming more visible.Unlike organic garbage, this strewn plastic can take hundreds to thousands of years to disintegrate in nature.The strewn plastic debris clogs drain shortens the lifespan of animals when consumed, contaminates water bodies when dumped into rivers, lakes, and oceans, and causes respiratory problems when burned.Oceans are amassing plastic in miles-wide spinning gyres.Plastic can break down into tiny particles known as microplastics that are nearly impossible to recover, disrupt food chains, and harm natural environments when exposed to UV light from the sun and other sources (9).Proper waste management can help reduce plastic waste, reduce environmental pollution effects, and enhance the recycling of new materials.
The consumption and production of plastic products are still rising with industrialization.For example, in 2015, factories produced approximately 6.5 billion metric tons (MT) of plastic (10).It was estimated that 9% of 6.5 billion MT was recycled, 12% incinerated, and 79% littered and discarded in a different environment, including landfills (4,10).It is estimated that approximately 12 billion metric tons of plastic waste will be dumped in landfills and water bodies by 2050 if recycling is not improved (11).This is because the global use of plastic materials is in high demand, which may cause a worldwide plastic waste crisis (10).Plastic waste can be recycled into secondary raw materials used to produce the same product.Using modern methods, plastic waste can also be upcycled into tertiary materials that can be used to manufacture more advanced products than the previous ones (12).Once introduced into the environment, plastics can disintegrate to form microplastics, another growing plastic pollution problem.Microplastics are potentially high-risk emerging pollutants in both soil and aquatic environments resulting from the disintegration of plastic waste by different conditions.The degradation of plastic waste mainly occurs through some combination of photochemical, thermal, mechanical, and even biological processes (8).Studies show that microplastics are finding their way into the atmosphere, contaminating the air we breathe and several water sources (13).
Most of the soft drink containers, food covers, and water bottles produced globally are fabricated from poly(ethylene terephthalate) (PET) polymers, with an estimated demand of 3.5% annually in the packaging industry, as mentioned earlier (14).The petrochemicalbased plastics like polyethene (PE), Polypropylene (PP), Polystyrene (PS), Polyvinyl chloride (PVC), Polyurethene (PUR), Polybutylene terephthalate (PBT), and Nylons have different properties.Hence the society of plastic industries (SPI) assigned PET a code □ for easy and visual identification from other plastic types.Polyethene terephthalate, abbreviated as PET / PETE, is a semicrystalline polymer, lightweight, transparent, with low permeability to oxygen, carbon dioxide, water, and thermoplastic material known for high strength (4).The PET polymer material is synthesized either by condensation polymerization reaction between terephthalic acid (TA) and ethylene glycol (EG) or by the transesterification reaction method of EG with dimethyl terephthalate (DMT), as shown in Scheme 1 (2).
A promising approach in plastic litter management is up-cycling, which involves the conversion of plastic waste into valuable chemical products in commercial demand.The technology of up-cycling of PET is advancing, and some promising methods have been reported for this process.Different approaches have been employed to up-cycle PET into primary or secondary Scheme 1. Polymerization reaction to produce poly (ethylene terephthalate) (PET).
products that can be utilized in manufacturing new polymers and other products (15,16).However, these methods have been fraught with a lot of challenges.The main PET recycling challenge is the reduction in the property performance of the recycled polymer.Literature shows that; the mechanical strain property of recycled PET reduces to 0.7% for the polymer formed from recycled monomer compared to 42% for the virgin polymer (17).This reduction in performance limits the recycling ability for thermo-mechanical applications.To improve plastic recycling performance, tertiary methods such as chemical recycling have been promoted as the new focus in recent work (15,18).
The tertiary chemical methods reverse the polymerization process to monomer units and other carbon-based materials.The yield of the precursor monomer of EG and TA has been studied using different techniques.The pyrolysis method of PET yields the monomers of EG and TA.Al-Sabagh et al. (18) reported pyrolysis and hydrolysis of PET at 450°C to yield oligomers and TA.The pyrolysis of PET also leads to liquid and gaseous products via depolymerization (7,15).The use of catalytic degradation with different materials such as zeolites, commercial catalysts, and metal oxides, among others, yields several products that broadly include TA, EG, and other products, as shown in Scheme 2 (4).
The depolymerization of PET with steam at elevated temperatures in the presence of catalysts is one of the most usual hydrolysis methods in PET recycling (Scheme 2).The hydrolysis method is categorized into three routes: alkaline, neutral, and acidic hydrolysis, which has been studied in incredible detail (19).The alkaline hydrolysis has a good output yield, but the main drawback is the use of high temperatures (>200°C ) and also requires a more extended reaction (3−5 hrs).The neutral hydrolysis yields relatively lower purity monomers than alkaline ones and requires high temperatures between 200-300°C with elevated Scheme 2. General depolymerization reactions of PET to different monomers (2, 4) with modifications.
pressure (1-4 MPa).However, this process is less corrosive than the acid and alkaline methods since it only uses steam in the presence of a catalyst (19).Acidic hydrolysis utilizes concentrated acids such as sulphuric, nitric, and phosphoric acid.The method gives a high yield of the EG, but there is a challenge in separating it from the highly acidic media.In addition, disposal of the acidic solution also is a challenge to the environment (4).
Different methods are used in industries to recycle PET and other plastic waste into monomers, oligomers, and polyols for different applications.Most recycling processes focus on monomers that can be reused for the same products, as shown in Table 1.
Several items can be made from waste plastic, but carbon is the most valuable.Waste plastic is a good carbon source for products with carbon added to them due to its high carbon concentration (20,21).Numerous published data show that various carbon-based compounds, including carbon microspheres, carbon nanotubes, carbon nanofibers, 2D graphene-based materials, fullerene graphite, and composite materials, may be produced from plastics (22).Activated carbon (AC), graphite, graphene, fullerene, and carbon nanotubes are materials produced from PET carbon sources, as shown in Table 1.Due to their unique qualities and possible usage in various applications, carbon nanotubes (CNTs) are the ones that interest scientists from PET carbon.The waste PET converted into valuable carbon materials has enormous properties like specific surface area, stable physicochemical qualities, and high electrical conductivity.They are used in energy storage devices, drawing attention from researchers.
Our goal was to discover an easy method to make recycled PET materials electrically conductive while allowing the functionality of carbon atoms to be changed to create pseudo-capacitance (PC) in the final carbonaceous material.We outline a direct carbonization and functionalization procedure that can be used to up-cycle PET and create electrical conductivity for an electrode for a supercapacitor.This allowed carbonaceous material to display PC and electrochemical double-layer capacitor (EDLC) properties during carbonization, producing high-performance supercapacitors.The study describes an emission-free sustainable method to produce carbon nanomaterials for application in energy storage.The materials upcycled from PET can be scalable with several economic and environmental advantages for application in batteries, supercapacitors, and hybrid devices.The prepared carbon nanomaterials can also be improved by doping or making composite for advanced applications.

Conversion of PET into PT-NANO materials
The littered PET plastic water bottles were collected, cleaned, and cut into small pieces of 1 cm with manual cutters.The threaded pieces were heated in the oil bath at 100°C for 45 min above the PET glass transition temperature (85-95°C) to attain brittle properties before quenching the materials with an ice bath.The process has zero emission of greenhouse gases since the heating did not reach its melting point, which may vaporize the volatile materials.The brittle solid PET quenched material was then ground into powder, as illustrated in Figure 1, to produce the powder herein referred to PT-Tg (PTTg) material.The PTTg powder was then activated in a sealed hydrothermal synthesis at 200 °C for 18 h in an autoclave fitted with a Teflon liner in a tetrahydrofuran (THF)/water solvent system (50 v/v).The activated material, PT-NANO, was removed from the autoclave, filtered under a vacuum, and then dried for 15 h at 70 °C in an oven.The dried white material was grounded into a fine powder and labeled as PT-NANO.This makes the whole procedure a green synthesis because there was no greenhouse gas emission throughout the conversion process.

Material characterization techniques of PT-Tg and PT-NANO
Using different analytical techniques, the PT-Tg and PT-NANO materials were characterized for their physicalchemical properties.The material's surface functional groups were determined using Fourier transform infrared (FTIR) spectroscopy (Bruker α-Alpha P, Ettlingen Germany) through the scan range between 400-4000 cm −1 .The transition bonds in the materials were analyzed using a UV-Vis spectrophotometer (UV-1800 SHIMADZU Tokyo, Japan).The morphology of the materials and the elemental composition were characterized by field emission scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (FESEM-EDX, Gemini SEM 500 M/S Carl ZEISS-EDAX Z2 Analyser AMETEK, Bangalore India).Transmission electron microscopy (TEM) was employed for the surface morphology analysis and the in-depth mapping with Thermo-Fisher scientific Tecna12.The surface composition and structure of the materials were further characterized with Renishaw inVia Raman microscope (Renishaw plc Watton-under Edge Gloucestershire United Kingdom) with an excitation wavelength of 514 nm, exposure time of 10 s, accumulative scan of 10, laser power of 1% and objective of X20.The material structure of the PT-NANO powder sample was examined by X-Ray Diffraction (Rigaku Smartlab Autosampler, RIGAKU Corp., Tokyo, Japan) using a Cu kα radiation with the JCPDS-ICDD database (23).The nuclear magnetic resonance (NMR) spectrometer (AVANCE III -400, Bruker-F-67166 Wissembourg, France) was used to determine the 1H proton structure of the materials.
The PT-NANO sample was dissolved in deuterated chloroform (CDCl 3 ), and spectra were referenced to CDCl 3 .About 1.0 mg of analysis sample in 0.6 mL of solvent was used for 1H NMR.PTTg and PT-NANO carbon-based nanomaterials' degradation processes were investigated by DSC utilizing a TA instruments Q100 thermal analyzer and a heating rate of 10 C/min.All measurements were made in a 50 mL/min air environment.The samples were heated between 25 and 1000 °C.

Electrode preparation and electrochemical studies of PT-NANO for supercapacitor application
The working electrodes for the electrochemical experiments were fabricated using graphite paper as the current collector and PT-NANO as the active material.
The working electrode constituted a composition of PT-NANO (80%), polyvinylidene difluoride (10%), and carbon black (10%) by weights (23).The different constituents of the electrode were made into a paste using N-methyl-2-pyrrolidone solvent and then coated with graphite paper covering an area of 1.0 cm 2 .The coated electrodes were dried at 70 °C in a vacuum oven for 15 h.The electrochemical studies for the materials were performed with a CH instrument system (CH 6600E) testing module using a three-electrode setup with PT-NANO as the working electrode, platinum as the counter electrode, and Ag + /AgCl as a reference electrode with mass loading of the active material of 0.0016 g.The galvanostatic charge-discharge (GCD) with potential limitations was carried out under constant current cycling of 0.5, 1.0, 1.5, 2.0 and 2.5A/g current density.The cyclic voltammetry (CV) was conducted with the potential window range of 0 to -1 V with scan rates of 5 and 10 mV/s.The electrochemical impedance spectroscopy (EIS) was carried out within 100 kHz to 10 mHz with a perturbation amplitude of 10 mV.Using the Swagelok system, the two electrode tests for device assembly in a symmetrical set-up.The assembled devices were tested with the BIO-LOGIC B805 electrochemical testing system, the Galvanostatic charge-discharge (GCD) at a constant current density of 0.5, 1.0, 1.5 and 2.0 A/g with potential limits, cyclic voltammetry (CV) with a different scan rate of 10, 20, 50, and 100 mV/s, at a potential window range of 0-1.0 V.The electrochemical impedance spectroscopy (EIS) was conducted in the frequency range of 10 kHz to 10 mHz with a perturbation amplitude of 10 mV.All were tested with freshly prepared 6M KOH as the electrolyte.As shown in the results, the charge-discharge cycling stability test for the device was carried out at a constant current density.The charge-discharge cycling stability was performed at a current density of 1.5A/g for 10000 cycles for the symmetric device stability.The self-discharge was carried out following the stability analysis.This was done by employing the 0.25 A/g current density to charge the gadget up to its maximum voltage.The charge was kept for no more than five minutes at its maximum before being allowed to selfdischarge in the open circuit.(24,25).

PT-Tg and PT-NANO materials characterization
The results for the UV-Vis spectrophotometric analysis of the PT-Tg and PT-NANO materials are shown in the spectra presented in Figures 2 (a & b).The observed spectra show absorption for the two materials in the region between 200-300 nm.The two samples exhibited a broad peak at 238 nm and another at 238 nm wavelength corresponding to the π-π* transition for the aromatic C-C bond in the material structures ( 23), (26).Another peak appeared at 282 nm wavelength for both materials.This peak can be attributed to the n-π* transition for the C = O bond as an attachment to the aromatic ring with the carboxylic functional group (27)(28)(29).Considering the similarity in peaks from PT-Tg and PT-NANO materials, this indicates that both materials have similar vibrations, which suggest little change in structure and phase arrangement after activation.The bond transition shows no evidence of EG, TA or DMT, which confirms that the PET was converted entirely to carbon-based materials with the response of only π-π* and n-π*, mainly for carbon structure.
The FTIR spectra for PT-NANO and PT-Tg materials are shown in Figures 2 (c & d), respectively.The materials spectra were recorded in the range of 4000-400 cm −1 wavenumber.The fingerprint region (<1500 cm 1 ) exhibited similar bond stretching for both materials as their material structure framework.The strong intensity of the peaks in the fingerprint region is at positions of 726, 1124, and 1236 cm −1 wavelength.The peak at 726 cm −1 suggests the interaction of the polar ester groups and the benzene ring in the structure flamework of PET, indicating that it's the same starting as the principle for the fingerprint region.The peak at 1124 and 1236 cm −1 is attributed to the stretching of the terephthalate group of -OOC-C 6 H 4 -COO-, which implies that some of the aromatic rings and the carboxylic groups are not distorted as earlier predicted with the UV-Vis analysis.The PT-Tg material showed a split peak at 2857 and 2922 cm −1 with medium intensity, with no peak for PT-NANO material, indicating the stretch of the C-O group and response to the deformation of the C-H group and wagging and bending vibrational modes of the ethylene glycol segment (30).The diagnostic region (≥ 1500 cm −1 ) revealed one common stretch at 1721cm −1 which confirms the presence of C = O of the carboxylic acid in both materials.The PT-Tg material exhibited a symmetrical stretch with a split of C-H at 2925 cm −1 for alkane, which did not appear in the FTIR spectra for the PT-NANO materials.This indicates a shift and asymmetrical arrangement of C-H bonds in the PT-NANO, which was caused by an alteration in the grain direction and phase of the material after structure modification which caused the removal of volatiles and small molecules from the material matrix, hence increasing its thermal stability (23,24,30).
The Raman spectrum of PT-NANO material shown in Figure 3a shows peaks at 1000 to 2000cm −1 .The peaks at 1128.9 cm  (31).This is attributed to symmetric structural stretching created by 1,4-para disubstituted benzene ring in the structure (32).The peak at 1831.6 cm −1 indicates the vibration stretching of the C = O bond for the carbonyl group from different fractions of the degraded polymer to smaller solid units.The allocation of 1390.1 and 1591.2 cm −1 peak positions are assigned to the Dband and G-band, respectively, representing the structure's arrangement and state.This is commonly reported in oxidated materials like graphene oxide and multiwall carbon nanotubes (MWCNT) (32) and microfibre PET (10).Also, the D-band peak indicates defects and outof-plane vibration because of disorder and defects in the layers of carbon structure by the sp 3 bonding.The G-band peak in the spectrum is attributed to a highly arranged and ordered carbon structure that comes from the in-plane vibration mode with the sp 2 aromatic bonds of C = C (17,18).The ratio of the D-band to the Gband (I D /I G ) is used to measure the degree of alignment and graphitization within the carbon structure.This is GREEN CHEMISTRY LETTERS AND REVIEWS commonly known as R-value (the relative intensity ratio) and is highly considered for the graphite edge plane to the standard graphite plane, where a smaller ratio value corresponds to a highly graphitic cluster of sp 2 bonds.In this study, the R-value of 0.82 in the PT-NANO sample indicates the presence of a highly arranged carbon domain (33).The R-value of the PT-NANO material is close to that of 0.89 reported for microfibre PET (10) and RGO (34,35).
The XRD diffractogram of PT-NANO material revealed several peaks, as shown in Figure 3b.The PT-NANO material exhibited more peaks than the characteristic peaks for a standard PET (32,36).These peaks are also exhibited in the PT-NANO material, including those from crystalline PET.The characteristic peaks show some shifts that indicate changes in the structure orientation after exposing it to high pressure at 180 °C in the hydrothermal process.The crystallinity of the sample was determined by Farrow and Preston's method, and the degree of crystallinity was calculated using Equation 1.
Where X is the degree of crystallinity, C c is the integrated area of the curve corresponding to the crystalline phase, and C A is the integrated area corresponding to the amorphous phase.Thermal-hydrothermal processed PT-NANO material exhibited a crystallinity of 86.6%, evaluated in Equation 1.The crystallinity is usually induced by heating above the glass transition temperature (Tg) and before melting Temperature (Tm), which is accompanied by molecular orientation.This can push the crystallinity to a higher percentage with low free energy due to the non-uniform molecular weight.(37).The crystallinity observed for PT-NANO showed an increase compared to the reported value for PET waste (38), PET fiber (39), and PET polymer composite (36).The crystallinity indicates that heating the PET material above the Tg caused molecular orientation within the structure.The material's crystallinity was also driven by the intrinsic and extrinsic heating factors of the synthesis process.This caused a breakdown of the PET polymer to create narrow molecular weights, linear chain structure, and variation in the high molecular weight of the total material.The extrinsic factors like crystallization temperature due to heating in the synthesis process and mode of extension at high pressure in the autoclave help unfold under stress.When the temperature is above Tg, randomly coiled and entangled polymer chains start to unfold, align, and straighten.The Scherrer equation was used to determine the size of the crystallite of the PT-NANO, as shown in Equation 2 (38).
Where D is the crystallite size, K is Scherrer constant (≈0.94), λ is the wavelength of Cu-Alpha strip (1.5406 Å), β is the full width at half maximum (FWHM) in radian, θ is Bragg's angle.
The PT-NANO material shows a crystalline dimension of ≈ 6.582 nm (equation 2), indicating the primary reason for the dense crystalline crystals as exhibited in FESEM and TEM analysis (Figures 4 and 5).The dimension result is higher in crystal size than reported, with an average of 4.97 nm (38).The higher crystalline dimension indicates a higher binding power of the particles when under pressure and creates pathways for the mobility of the ions from the electrolyte solution during electrochemical conduction.
Results of the high magnification field emission scanning electron microscopy (FESEM) and EDX elemental composition analysis of the PT-NANO are shown in Figure 4.The agglomeration of the PT-NANO exhibited reduced particle size of the nucleating material (Figure 4a) at a more profound magnification of 20 nm.The material shows highly charged particles (Figure 4b) scattered within the entire material surface.The EDX analysis in Figure 4c revealed the elemental composition of PT-NANO to be 72.8%carbon, 27.18% oxygen, and 0.02% sulphur.The detector also identified traces of silica and nitrogen that could not be quantified (40).
The HRTEM micrograph analysis of PT-NANO at different magnifications is shown in Figure 5.The analysis revealed the internal structure of the material.This exhibit displays a stacked epitaxy arrangement of different transparent grains on top of each other.Figure 5a shows the formation of multi-shell grains within graphene sheets.The experimental HRTEM images prove that PET can be converted into carbon nanostructure materials.The HRTEM pictures of the PT-NANO samples shown in Figure 5a reveal a mixture of thick deposits of graphite with multiple layers of graphene surrounding them and thick carbon nanomaterials of graphene.Figure 5b exhibited the creation of a few carbon sheets of graphene as rippling, entangled layers that are reasonably transparent, as well as thick carbon walls.Transparent nanosheets generated in carbon nanostructures make it simple to identify the carbon nanomaterial of graphene structures.Figures 5b show the PT-NANO material structure with different grains and increased porosity and transparency.The transparency of the grain confirms the crystallinity exhibited by the XRD analysis.
To further investigate the structure of synthesized PT-NANO concerning PET, 1H NMR analysis was performed, and the results are shown in Figure 6.The PT-NANO exhibited the chemical shift at 8.03 ppm, the characteristic absorption band on the benzene ring for aromatic-CH (Ar-CH) (d).The chemical shift at 4.63 ppm corresponds to the proton for COO-CH 2 -CH 2 -OCO (e).The chemical shift at 7.26 ppm is related to the proton of CHCl 3 .The 1 H resonance at 3.49 ppm is assigned to proton (b, c) in the ethoxy segment (36).
The spectrum did not show any peak for the EG (at 5.0 and 3.7pmm), and there was no peak formation for DMT (two peaks at 4.0 and 8.1 ppm).This means there is no monomer formation in the solid PT-NANO structure.The thermogravimetric analysis (TGA) was conducted on each material to ascertain the ideal temperature for thermal deterioration.At temperatures of 74 and 229°C , the PTTg materials exhibit different degradation behavior, with hydrocarbon weight rapidly losing mass, as shown in Figure 7.The material displayed a two-step decomposition process, with the first stage beginning at a lower temperature and the second beginning at a higher temperature.This could be because of volatile contaminants, like the additional filler used in manufacturing plastic (41).The PTTg material's highest deterioration was accomplished between 356 and 420°C . PTTg demonstrated a two-step breakdown and maintained 14% of its mass up to 1000°C.Under controlled circumstances, the PT-NANO material demonstrated a one-stage breakdown.With an increase in temperature, the carbon-carbon bond that causes the single-step decomposition supports the random scission mechanism (42).Compared to the PTTg sample, PT-NANO degradation began at a higher temperature (354°C).At temperatures between 354 and 698°C, the PT-NANO displayed a steady weight reduction that reduced the material by 60 percent.Tertiary carbon, a component of PT-NANO carbon production, facilitates the creation of carbocation throughout the material's thermal deterioration (43).PTTg showed a loss of more than 60% by 356°C, but PT-NANO showed a loss of less than 10%, and it took PT-NANO until 556°C to experience a loss of 60%.According to EDX examination, the PT-NANO material retained 23 percent at 1000°C, which the presence of silica and sulphur may have caused.
According to several research, PET needs higher temperatures to degrade than other plastics (41,44).As a result of the ester link's random scission during PET breakdown, oligomers are created (42).Perhaps some volatile contaminants, such as diethylene glycol, caused the initial breakdown of PTTg.According to published research, the presence of these volatile contaminants accelerates polymer degradation, which only occurs in the PTTg sample.The mesoporous structure of the two materials may cause a discrepancy in TGA curves between them (45).

Electrochemical performance of PT-NANO material
The electrochemical investigation of the PT-NANO material was carried out with the galvanostatic charge-discharge (GCD) analysis.This was implemented to determine the material's specific capacitance using a varying specific current at a fixed window as shown in Figure 7a.The triangular symmetrical V-shaped curves were observed at the specific current.The symmetrical behavior exhibited domination of double-layer and some reduction and oxidation (redox) pseudo-capacitance on the discharge curves.The displayed GCD curves in Figure 7a revealed a material-specific capacitance using Equation 3 with varying specific current (10,23,25).
Where: i is the charge/discharge current (A), m is the active mass of one electrode (g), dt is the change in the discharge time (s), and dV is the working potential window range (V).
The PT-NANO material showed good performance in the electrochemical system with the highest specific capacitance of 277.8 F/g with a current density of 0.5A/g.The charge-discharge performance exhibited a quasi-triangular V-shape with a varying current density as a function of the typical ideal behavior of the electrochemical double-layer capacitor (Figure 8a).The performance gradually decreased with the increasing current density, as shown in Figure 8c.The electrochemical performance of the PT-NANO material is higher than the recently reported performance by Mirjalili et al. (Mirjalili et al. 2020) of 32.6 F/g under 2.5 mA/cm 2 for the mechanically processed PET up-cycling method and other reported as shown in Table 2.The CV curves for the PT-NANO materials shown in Figure 8b exhibited a quasi-rectangular behavior as an indication for both double-layer capacitance with surface parasitic reactions.The quasi-rectangular shapes of different scanning rates indicated slightly low diffusional restriction with the materials at higher potential.At lower potential, some restriction in the electrolytic ions caused interaction which was also exhibited throughout the scanning rates (23).The exponential distortion at the higher potential may be due to high polarization that shows an overcharge load to the material.This causes a parasitic side reaction between the electrode material and the electrolytic ions (46).The material structure of PT-NANO exhibited different directional grain arrangements with pathways (Figure 5) which enhance the electrolytic ion mobility within the structure to improve the non-faradaic reactions.The shapes are slightly similar to CV previously reported with microfiber PET (10) and reduced graphene oxide (24,47) and acid-activated biomass (23).
With the symmetric device assembly, the capacitance of the ideal EDLC of the GCD curves was calculated using equation 4 and the charge-discharge curves (Figure 9b) (24,48).
I is applied current (A), Δt is the discharge time (s), ΔV is the change in cell potential (V), The specific discharge capacitance Cs (F/g) of a single electrode was calculated using equation 5, and the energy density (E), power density (P) and the specific capacitance Cp from CV were calculated using equations 6, 7, and 8, respectively (49,50), Where m el is the total mass of electrodes (g), E is the energy density (Wh/kg), P is the power density (W/kg), 'A' is the area inside the CV curve (AV), 's' scan rate of CV (V/s).
With a charge-discharge specific capacitance (Cs) of 250.5F/g, an energy density of 34.83Wh/kg, and a power density of 999.9W/kg at a current density of 0.5 A/g, the PT-NANO symmetric device had the highest energy and power densities Shown in Figure 9b.According to Figure 10c, the trend of the material's specific capacitance decreased as current density increased.A specific capacitance of 22.4F/g produced by a current density of 2.0A/g had the highest power density of 3998.57W/kg and the lowest energy density of 3.11Wh/kg.The maximum specific capacitance Cp was  detected from the CV curves (Figure 9a) at a scan rate of 5 of 457.05F/g.As seen in Figure 10b, the Cp performance continued to decline as scan rates increased.A material's outstanding performance in supercapacitors is enhanced by the morphological structure of a hierarchically porous structure, as shown in Figure 5.The primary benefits of this approach are using a carbon precursor made from inexpensive waste PET beverage bottles, (ii) a straightforward synthetic process that helps mitigate environmental issues, and (iii) the high performance of PT-NANO in electrical double-layer supercapacitors.
The carbon-based nanomaterial's morphology, crystal structure, and surface functional groups were adjusted by controlling the synthetic parameters, such as thermal degradation temperature and pressure.The open porous structure and superior electrical conductivity of the 3D porous architecture allow for ion transit while maintaining high specific capacitance, stability, and rate performance.The hydrothermal synthesis approach alters the morphology and structure of produced materials.It improves ion propagation and diffusion according to cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests.The internal resistance (IR) (Figure 9b) in the device may be from an additive effect on the cell components and electrolytes.
The current results are compared well with the reported literature (Table 2).PET carbon via carbonization supplemented with KOH activation was reported by (48).The outstanding capacitance of 169 F/g in 6M KOH solution and 135 F/g in 1 M Na 2 SO 4 solution, as well as a minimum resistance of 11.4 ohm and a maximum stability of 90.6 percent even after 5000 cycles in an aqueous electrolyte, are all due to the asprepared porous carbon nanosheets (PCNS), which displayed excellent physicochemical properties.Additionally, PCNS exhibit an impressive rate capability with a high specific capacitance of 121 F/g, a high energy density of 30.6 Wh/kg at 0.2 A/g in an organic electrolyte (1 M TEATFB/PC), and a high current density value of 10 A/g.The capacitance displayed by the material was 95 F/ g. ( 48).Elessawy et al. used a different approach to address PET waste by creating 3D N-doped graphene nanosheets using a scalable, eco-friendly, straightforward method.With a specific capacitance of 405 F/g at 1 A/g and maximum power and energy density values of 558.5 W/kg and 68.1 W h/kg, respectively, in 6 M KOH solution for the ideal sample, the produced materials exhibit exceptional performance.The produced nanosheets of graphene samples exhibit cyclic stability by maintaining 87.7 percent capacitance even after 5000 cycles at a current density of 4 A/g with prolonged charge/discharge cycles (51)  The Nyquist plot performance of the PT-NANO material as the electrode is shown in Figure 9c before and after cycle stability.The in the device exhibited almost a straight line towards the vertical axis with the equivalence series resistance (ESR) at the high-frequency zone before cycling.The device shows an ESR of 6.8Ω from the real frequency region.The straight-line behavior at high and low frequencies indicates the electrochemical double layer capacitor (EDLC) mechanism of the material electrode and electrolytic ions interaction with no semicircle at the high frequency.This also indicates that the charge storage happens at the electrodeelectrolyte interface.The lower value of the ESR may be responsible for the dissipation of the stored energy with the material and cannot limit the total power and energy efficiency performance.Since the material did not exhibit any semicircle at high frequency, the porosity of the material is suitable for EDLC storage for the physical process without any charge transfer.After stability, the behavior changed at a low frequency, possibly due to some material degradation after a long continuous current load and high potential parasitic reactions.This caused the more extended interaction to a lower frequency that was more than before (23,46,54).Figure 9e displays the experimental EIS Nyquist curve, the fitting data, and circuit inset of the PT-NANO prior cycle stability.The circuit displays W (Warburg) in parallel with real capacitance (Q1) and ESR in series with charge transfer resistance (R CT ) at high frequencies.At low frequencies, a mass capacitance (C3) is parallel to a leakage resistance (R L ).R CT is a representation of the parasitic reactions at the electrode/electrolyte interface.The experimental numbers ESR = 6.8Ω and R CT = 0.3Ω and the obtained values for ESR = 6.9Ω and R CT = 0.2Ω from the fitting are comparable, showing successful fitting of the Nyquist plot.Fast ion transport and charge-transfer kinetics, which define favorable features for capacitive materials, are revealed by the small R CT = 0.3Ω.
The PT-NANO material's cycle stability was conducted with a constant current density of 1.5A/g for 10000 cycles.The material retained a specific capacitance of 96/8% after 10000 cycles, as shown in Figure 9d.The material exhibited a stable capacitance throughout the charge-discharge process without any significant disturbance.This means that there is low interaction between the material and the electrolytic ions and the creation of uniform intercalation and no trapping of ions in the material pores, which may delay the electronic mobility within the process (23,55,40).The cycle stability of the current work is highly comparable with the reported work, as highlighted in Table 2.
Another crucial method for assessing the supercapacitor's life is self-discharge.This was done on the same device following the cycle stability test.The devices were charged to 1.0 V, held there for 5 min, and then allowed to discharge themselves in an open circuit.The functionality of the assembled gadget made from PT-NANO material is depicted in Figure 10a.The material demonstrated higher performance for supercapacitor application material after 3.0 h, with a voltage loss of 80% (Figure 10a).

Conclusion
We designed a non-emission sustainable method for converting PET to carbon nanomaterials in the research study.The PET bottle's trash was gathered and later upcycled into PT-NANO material with no greenhouse gas emissions.To align the material structure of the powder at its Tg temperature and add the brittle property, the PET polymer was submerged in heated white oil at 95°C for 45 min.The brittle materials were crushed into powder and coded as PT-Tg.The PT-Tg powder was heated in a hydrothermal autoclave batch reactor with tetrahydrofuran as the solvent at 200°C for 18 h and produced powder coded as a PT-NANO sample.The produced sample was characterized by different techniques: FESEM-EDX, TEM, XRD, Raman, 1H NMR, FTIR, TGA, and UV-VIS for its physical and chemical properties.The PT-NANO material exhibited a crystalline structure, as demonstrated by an XRD analysis with 86.6 percent crystallinity.According to an EDX examination, the material underwent additional processing in a hydrothermal synthesis to enhance the carbon content to 72.3%.Because of the electrochemical application that led to the testing of the material for supercapacitor energy storage, the property of enhanced carbon is advantageous.Under a current density of 0.5A/g, the PT-NANO displayed a specific capacitance of 250.8F/g, an energy density of 34.83Wh/kg and a power density of 999.9W/kg.The material exhibited an outstanding retention of 96.8%, possibly due to the open, transparent grains in the material structure.TEM's sheet-like structure verified the non-faradaic surface-controlled kinetics' storage mechanism.The method can be adopted for recycling PET waste in carbon-based materials for energy storage applications and could reduce the emissions of greenhouse gases in support of the upcycling of PET plastics.

Figure 1 .
Figure 1.Synthesis of PT-Tg NANO from PET plastic water bottles.
−1 can be attributed to the ester C-O-C and the ethylene glycol C-C bond in the structure.The peak at 1285.1 cm −1 only indicates the stretching of the C-O-C bond.The bending of C-C-H and O-C-H stretching is depicted at 1390.1 as the D-peak band and the ring mode at 1591.2 as the G-peak band

Figure 4 .
Figure 4. FESEM-EDX analysis of PT-NANO material; (a) with 1μm, (b) with 20 nm of highly charged flake particle, (c) the material composition from EDX analysis and (d) the carbon mapping.

Figure 5 .
Figure 5. TEM morphology of PT-NANO with the material with different magnification.

Figure 8 .
Figure 8.The 3-electrode electrochemical performance plots (a) GCD at varying specific currents, (b) CV at different scanning rates, and (c) the specific capacitance at different specific currents for PT-NANO material.

Figure 9 .
Figure 9.The symmetric electrochemical performance plots for PT-NANO material (a) GCD at varying specific currents, (b) CV at different scanning rates, (c) The EIS Nyquist plot before and after stability, (d) material cycle stability at constant charge/discharge with a current density of 1.5A/g for 10000 cycles, and (e) EIS Nyquist plot for PT-NANO with its fitting before cycle stability with an insert circuit.

Figure 10 .
Figure 10.The symmetric electrochemical performance plots for PT-NANO material (a) self-discharge, (b) specific capacitance from CV curves at different scan rates, (c) specific capacitance from CD curves at different current densities, (d) Ragone plot from the chargedischarge performance.

Table 1 .
The depolymerization/conversion and up-cycling methods of PET for different products.

Table 2 .
Electrochemical performance of carbon-based materials derived from PET plastic waste.
. Using MgO/Coacetate as a catalyst, Mu et al. carbonized PET plastic in a single step to produce a high yield of 36.4 wt percent of 3D porous carbon.In a supercapacitor, the PCS-MnO 2 −2 composite displayed outstanding capaci-