Maximizing the electrochemical performance of supercapacitor electrodes from plastic waste

Journal of Energy Storage 72 (2023) 108660 Available online 17 August 2023 2352-152X/© 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Research Papers Maximizing the electrochemical performance of supercapacitor electrodes from plastic waste Ivan Dˇ edek a , b , Stanislav Bartusek c , Josef Jan Dvoˇ r´ aˇ cek c , Jan Neˇ cas d , Josef Petruˇ s e , Petr Jakubec a , * , Vojtˇ ech Kupka a , * , Michal Otyepka a , f , * a Czech Advanced Technology and Research Institute (CATRIN), Regional Centre of Advanced Technologies and Materials (RCPTM), Palacký University Olomouc, ˇ Slechtitelů 27, 783 71 Olomouc, Czech Republic b Department of Physical Chemistry, Faculty of Science, Palacký University, 17. listopadu 1192/12, 779 00 Olomouc, Czech Republic c Department of Chemistry and Physico-Chemical Processes, Faculty of Materials Science and Technology, Vˇ SB-Technical University of Ostrava, 17. Listopadu 2172/15, 708 00 Ostrava-Poruba, Czech Republic d Department of Mining Engineering and Safety, Faculty of Mining and Geology, Vˇ SBTechnical University of Ostrava, 17. Listopadu 2172/15, 708 00 Ostrava-Poruba, Czech Republic e Central European Institute of Technology, Brno University of Technology, Purkyˇ nova 656/123, 612 00 Brno, Czech Republic f IT4Innovations, Vˇ SB-Technical University of Ostrava, 17. Listopadu 2172/15, 708 00 Ostrava-Poruba, Czech Republic ABSTRACT The management of the increasing volume of plastic waste has become a key challenge for society. A promising strategy now consists in the transformation of plastic waste into high-value materials that can be utilized in energy storage devices such as batteries and supercapacitors. In this study, we demonstrate a two-step procedure, involving pyrolysis, followed by chemical activation that will convert common plastic waste into activated carbons (ACs). This technique makes ACs suitable for supercapacitor electrode materials. Further, the electrochemical performance of ACs is outstanding in terms of capacitance, energy density, and cycling stability. Besides the well-established parameters, including a specific surface area and micropore volume, we found that other critical factors such as polymer glass transition temperature, polymer-activating agent miscibility, activating agent (K 2 CO 3 ):AC ratio, and AC water dispersion stability also play a crucial role in determining the supercapacitors performance. Controlling these parameters, we obtained ACs as supercapacitor electrodes from a range of plastic waste materials with a competitive electrochemical performance. Specifically, the ACs exhibited a specific capacitance of 220 F g −1 (at a current density of 1 A g −1 ), energy and power densities of 61.1 Wh kg −1 and 36.9 kW kg −1 , respectively, and excellent cycling stability (95 % retention after 30,000 cycles). Our findings provide a pathway towards transforming plastic waste into valuable electrode materials for supercapacitors. 1. Introduction The ubiquitous plastic waste cause large-scale environmental pollution. Therefore, it is necessary to seek various methods for its recycling or transformation, instead of its incineration and landfilling. The worldwide estimation of plastic waste is 3 ×10 8 tons per year [1]. Recycling is one of the pathway to utilize plastic waste; however, globally, only 9 % of plastic waste is recycled, while 20 % is mismanaged [2]. Of the remaining 60–70 % of plastic waste, a large part is incinerated, and the rest is landfilled. Instead of incineration, transformation of plastics into valuable materials such as activated carbons (ACs) represents a viable strategy to sustainably handling plastic waste. Utilizing reused plastic in supercapacitor production can lower manufacturing costs and environmental burden compared to conventionally used feedstocks for active carbon [3,4]. This can lead to improved resource efficiency and a more sustainable approach to manufacturing supercapacitors [5]. Effective plastic waste management not only leads to cost savings in supercapacitor production but also alleviates the financial burden associated with the disposal of plastic waste as now the costs of plastic waste management are more than $32 billion every year [6]. The ACs are exploited in diverse applications including gas purification [7], water treatment [8], sorption [9], catalysis [10], and energy storage [11]. In that context, their simple fabrication and enormous specific surface area (SSA) make them suitable candidates for EDLC (electric double layer capacitance) supercapacitors (SCs) [12–17]. Supercapacitors represent quickly evolving devices, which can be used for rapid energy storage [18–20]. The mechanism of EDLC SCs is simply based on electrostatic adsorption of ions, where the suitable pore size * Corresponding authors at: Czech Advanced Technology and Research Institute (CATRIN), Regional Centre of Advanced Technologies and Materials (RCPTM), Palacký University Olomouc, ˇ Slechtitelů 27, 783 71 Olomouc, Czech Republic. E-mail addresses: [email protected] (P. Jakubec), [email protected] (V. Kupka), [email protected] (M. Otyepka). Contents lists available at ScienceDirect Journal of Energy Storage journal homepage: www.elsevier.com/locate/est https://doi.org/10.1016/j.est.2023.108660 Received 30 May 2023; Received in revised form 24 July 2023; Accepted 7 August 2023 Journal of Energy Storage 72 (2023) 108660 2 (preferably micropores [21]) with the high SSA plays a main role in the high energy storage performance. As a result, such SCs display enormous cycling stability, fast charging and discharging ability, and enhanced power density [22]. To show the practical applicability of ACs derived from plastic waste, Elessawy et al. [23] and Mu et al. [24] selected, for instance, polyethylene terephthalate (PET) as the precursor for the preparation of AC. They obtained a highly porous material with high capacitance (405 F g −1 and 210 F g −1 ) and good cyclic stability (88 % and 90 %), respectively. An AC derived from polyethylene demonstrated an excellent cycling stability as the capacitance retention equaled to 97 % after 10,000 cycles measured at a current density of 2 A g −1 [25]. Chen et al. used a polystyrene foam as a precursor for the development of SC [12]. Plastic waste-derived ACs represent an interesting source of electrode materials [26–35]; however, little is known concerning the critical parameters that control the supercapacitive properties of plastic waste-derived ACs. This lack of knowledge hampers the possibility of efficiently utilizing and transforming plastic waste into high-value materials. We will describe the transformation of common plastic waste materials, such as polyethylene (PE), polypropylene (PP), polystyrene (PS), polyamide (PA), polyester fabric (PES), polyethylene terephthalate (PET), polyurethane (PU) and acrylonitrile-butadiene-styrene (ABS), into ACs with suitable electrochemical parameters towards supercapacitor applications. We demonstrate that besides the SSA and the porosity of ACs, polymer glass transition temperature (T g ), the miscibility of the polymer with K 2 CO 3 as an activating agent, and the stability of AC water dispersion represent critical parameters controlling the capacitive properties of ACs. Through controlling these critical parameters, we obtained the highest specific capacitance (C sp ) in the case of CPA, giving a value of C sp 521 F g −1 (at 1 A g −1 ). In a real device test, a CPA derived AC attained a C sp value of 220 F g −1 (at the same current density of 1 A g −1 ), alongside great cycling stability—keeping 95 % retention after 30,000 GCD cycles. The unraveled parameters should be considered an important metric in the screening of plastic waste materials from a capacitive perspective. 2. Materials and methods 2.1. Reagents Sulphuric acid (H 2 SO 4 ) was purchased from Lach-Ner (p.a. 96 %, Neratovice, Czech Republic). Polypropylene (PP), low-density polyethylene (LDPE) was obtained from Lanex a.s. (Bolatice, Czech Republic); polystyrene (PS) Krasten 154 was purchased from Synthos Kralupy a.s; acrylonitrile-butadiene-styrene (ABS) was obtained from Skarab s.r. o. (Czech Republic); polyamide (PA) was obtained from ELITE a.s. (Czech Republic); polyurethane (PU) was obtained from PUR izolace s.r. o. (Czech Republic); Potassium carbonate (K 2 CO 3 ) was purchased from MACH Chemicals (99.65 % purity). 2.2. Synthesis of activated carbon Granular polymers (PP, PE, PS, and ABS) and an activating agent K 2 CO 3 were grinded on a vibrating mill to obtain a fine powder. PA, PES, and PU were first melted (185 ◦C, 10 min), due to their lightweight structure, cooled down to laboratory temperature, and then converted to powder grinding on a vibrating mill. The polymer material was mixed with the K 2 CO 3 in a weight ratio 1:3. The polymer PA was chosen for its exclusive performance and was prepared with different K 2 CO 3 ratios (1:0.25; 1:0.5; 1:1; 1:3; 1:6), following the strategy that the ratio of the K 2 CO 3 plays a major role in the properties of the final form of carbons [36–41]. Pyrolytic-chemical activation was performed in a home-made stainless-steel autoclave with corundum liner at 800 ◦C for 2 h in N 2 (atmospheric pressure). When the activation was completed, the reactor was removed from the furnace and cooled down to laboratory temperature. The activated sample was neutralized with 1 M HCl to achieve pH 7, filtered, washed with distilled water, and then dried at 105 ◦C for 4 h. From 40 g of a plastic precursor, at least 2 g of ACs (yield around 5 %) was obtained. 2.3. Equipment Differential Scanning Calorimetry (DSC) was performed using the DSC Discovery (TA Instruments). Each sample (5–10 mg) was first cooled to −85 ◦C. The first heating was conducted at the temperature range from −85 to 200 ◦C at the rate 10 ◦C⋅min −1 . The temperature of 200 ◦C was held for 5 min to eliminate any thermal history; the sample was then cooled to −80 ◦C at 10 ◦C min −1 (the first cooling), and then heated to 200 ◦C at 10 ◦C min −1 (the second heating). The obtained data were evaluated using the TRIOS software to obtain glass transition temperature (T g ) and melting temperature (T m ). The specific surface area (SSA) and pore size analysis was performed by means of N 2 adsorption/desorption measurements at 77 K on a volumetric gas adsorption analyzer (Autosorb iQ XR, Anton-Paar Quanta Tec, USA) up to 0.965 relative pressure. Prior to the analysis, the sample was degassed under high vacuum (10 −7 Pa) at 130 ◦C for 12 h, while high purity (99.999 %) N 2 and He gases were used for the measurements. The SSA was evaluated using the Brunauer–Emmett–- Teller model with respect to Rouquerol criteria [42] for N 2 isotherm. The pore size distribution together with pore volume was evaluated by the QSDFT (quenched solid density functional theory). The samples were analyzed with the Scanning Electron Microscopy (SEM) using the JSM-7900F Jeol scanning electron microscope with accelerating voltage of 5 kV. The Energy Dispersive Spectrometry (EDS) was performed in the JSM-7900F scanning electron microscope on an accelerating voltage of 15 kV. A three-electrode setup was performed on the Metrohm Autolab PGSTAT128N instrument (Metrohm Autolab B.V., Netherlands). The setup comprised a glassy carbon electrode (GCE) (working electrode), a platinum wire electrode (counter electrode), and an Ag/AgCl (reference electrode). The GCE was modified by drop casting of a 10 μ l drop of a powder suspension (2 mg ml −1 ) and was allowed to dry at ambient laboratory temperature. A two-electrode setup was performed on a battery tester Bio-Logic instrument (Biologic Company, SeyssinetPariset, France, BCS-810) controlled with the BT-Lab software (version 1.64). The Electrochemical Impedance Spectroscopy (EIS) was recorded using 10 mV amplitude in the frequency range from 0.1 Hz to 100 kHz at an open circuit potential (OCP). Evaluation of the impedance spectra was performed deploying the NOVA software (version 1.11.2). A symmetrical two-electrode device was constructed to test the real capacitive performance. The AC was homogeneously dispersed in ultrapure water (7 mg ml −1 ) and sonicated for 60 min. 400 μ l of the dispersion was drop coated onto the surface of a gold disc current collector (diameter 18 mm) and dried at ambient temperature to achieve a mass loading of at least 2 mg cm −2 . After drying, both gold current collectors were placed in an insulator sleeve (El-Cell insulator sleeve) equipped with Whatman® glass microfiber filter paper as a separator (thickness of 260 μ m) to construct the full cell supercapacitor. The separator membrane was soaked with 100 μ l of 1 M H 2 SO 4 as a supporting electrolyte. Stainless steel plungers were used to press the electrodes, and the whole device was tightened and connected. All measurements were performed at room temperature (24 ±1 ◦C). The specific capacitance in the 3-electrode configuration (C s , F g −1 ) was calculated from galvanostatic charging|discharging (GCD) curves as follows (Eq. (1)): Cs=I×Δt m×ΔV(1) where C s is the gravimetric capacitance (F g −1 ), I is the discharge current (A), Δt is the discharge time (s), ΔV is the potential window, and m is the I. Dˇ edek et al. Journal of Energy Storage 72 (2023) 108660 3 mass of the deposited material on the surface of the working electrode (g). In the two-electrode setup, the specific capacitance of the cell (C s , F g −1 ) was calculated following the equation (Eq. (2)): Cs=4I×Δt m×ΔV(2) where C s represents the gravimetric capacitance (F g −1 ), I is the discharge current (A), Δt is the discharge time (s), ΔV is the potential window, and m is the total mass of active material on both electrodes. Multiplier of 4 adjusts the capacitance of the cell and the combined mass of two electrodes to the capacitance and mass of a single electrode [43,44]. The maximum of energy density and maximum of power density were calculated using the equations as follows: Energy density (Eq. (3)): E=1 2 C×ΔV2 4×3.6(3) Power density (Eq. (4)): P=E×3.6 Δt(4) where C S is the specific capacitance (F g −1 ), ΔV is the operating window, the dividing factor of 4 is used to normalize the results back to the values of the full-cell (two-electrode) system. The factor of 3.6 converts the energy from joules (the result of farads ×volts) into Wh kg −1 . Δt has the meaning of the discharging time (s). For the 2 electrode setup, specific capacitance was multiplied by 4 as a correction to one electrode. 3. Results and discussion 3.1. Preparation and structure characterization Overall, eight types of plastics were selected for the preparation of ACs, including commodity plastics such as PET (polyethylene terephthalate), PE (polyethylene), PP (polypropylene), PS (polystyrene), PU (polyurethane), engineering plastic ABS, and also woven materials such as polyester (PES) and polyamide (PA). PA was selected for preparation of AC using varying ratios of plastic and the K 2 CO 3 , in order to investigate the highest possible capacitance properties. The first step of the preparation of the ACs (Fig. 1) included the milling of the specific polymer together with the K 2 CO 3 . Polymers such as PS and PET pellets were processed easily, showing no difficulties during the manufacturing. Polymers including PE, PP, PU and ABS were difficult to process due to sintering when being milled. The sintering was attributed to glass transition temperature (T g ) of the particular polymer, where the polymer structure is in the rubbery state which makes the grinding complicated. In order to confirm this hypothesis, the polymers were analyzed by the DSC (Table 1). The phase transition analysis indicated that PE, PP, PU and ABS had the T g value below room temperature and the sintering of the particles during milling was caused by friction. The T g of PS and PET was 88 ◦C and 89 ◦C, respectively; therefore, the polymers behaved as brittle materials and were easy to grind. The other materials such as the PU foam, the PES cloth, and the PA cloth were difficult to mill due to their lightweight structure. In the first step, they were heated up to 185 ◦C in order to disrupt the porous structure and make the material more compact. After cooling down to the laboratory temperature, they were grinded the same way as the other plastics. Sintering during the milling process made the mixing with K 2 CO 3 more difficult. This negatively affected the specific capacitance of the activated material, as evidenced from the specific capacitance values obtained for the individual polymers (Table 1). On the other hand, there is one exception, the ABS. The AC from ABS was obtained in a form of fine powder with very high values of the SSA and high capacitance. The reason is that the K 2 CO 3 blends well with the ABS, which was confirmed by the EDS analysis of the milled sample (Fig. S1). To understand the results obtained from the capacitance measurements (Fig. 2c), the specific surface area (SSA), the total pore volume, and the micropore volume was analyzed by physisorption measurements. The steep increase in the adsorption isotherm at very low relative pressures p/p 0 indicates a microporous nature of the samples (Fig. 2a; Fig. S2). The micropore volume of the C-PP and C-PET was very low (≈0.04 cm 3 g −1 ) and corresponded with the low values of the capacitance (73 and 104 F g −1 , respectively). The other samples demonstrated both high SSA and the micropore volume (maximum C-PA, SSA =1616 m 2 g −1 , 0.54 cm 3 g −1 ), which was reflected by the high capacitance (maximum C-PA, C sp =323 F g −1 at 1 A g −1 ). The pore size distribution (QSDFT model) revealed a large amount of micropores, and also a high micropore volume within pores below 1 nm (Fig. 2b; Fig. S3). These micropores responsible for the high amount of the accommodated Fig. 1. Procedure workflow from polymer waste, via AC to supercapacitor device. Table 1 Analyzed glass transition temperature (T g ) and the melting point (T m ) of precursor polymers with the impact on the specific capacitance values (results obtained from the 3-electrode setup). *Note: T g of the PE was out of the temperature scale of the device and the value is taken from the literature [45]. Sample T g (◦C) T m (◦C) Specific capacitance of ACs (F g −1 ) PP −10 165 73 PET 89 243 104 PE* <−100* 112 157 PS 88 240 235 PU −49, 138 198 246 PES 56 219 296 ABS −49; 45; 97; 130 – 314 PA 56 262 323 I. Dˇ edek et al. Journal of Energy Storage 72 (2023) 108660 4 electrolyte ions and the immensely high capacitance [21,46–49]. C-PA 1:1 exhibited a micropore:total pore volume ratio 63 %, which predisposes it for the fabrication of the EDLC supercapacitor. C-PA 1:1 with best capacitance results (521 F g −1 ) had the specific surface area from BET equaled to 1974 m 2 g −1 with a total micropore volume of 0.59 cm 3 g −1 . An increase in the SSA of the electrode material led to an increase in the capacitance in the EDLC type supercapacitors. This trend is depicted in Fig. 2c and d, where the capacitance of the materials is compared to their SSA and the micropore volume. We observed even with the naked eye that the AC materials differed in their dispersibility in water, while some of them formed agglomerates and settled down very fast, the others formed stable dispersions. This could be explained by a different surface energy of the AC materials. However, for quantification of such a parameter, contact angle measurements are not reproducible due to the powder nature of the samples and inverse gas chromatography is not usable for characterizing the surface energy due to the microporous nature of the samples. Therefore, we identified the stability of the dispersion as an indirect parameter, which can be qualitatively evaluated by optical microscopy (Fig. S4; Fig. S5). The C-PS with SSA 750 m 2 g −1 without agglomerates showed similar values of capacitance as C-PU with SSA 1559 m 2 g −1 and agglomerated dispersion. Further, the C-PES had SSA 909 m 2 g −1 and formed very fine dispersion. Based on that observation, we concluded that the high surface area and the high micropore volume together with the ability to form fine dispersion were responsible for the capacitance. The sample with the highest value of capacitance (C-PA) was selected to determine the influence of the AC:K 2 CO 3 ratio on capacitance properties (Fig. 2d). With the decreasing amount of the K 2 CO 3 , the SSA grew up to C-PA:K 2 CO 3 ratio 1:0.5, giving a very high SSA of 2266 m 2 g −1 . However, this trend did not correlate with an increase in capacitance. Since a carbon with a lower ratio of K 2 CO 3 was proven ineffective in terms of capacitance, which could be explained by the poor ability to form stable dispersion, the AC was unstable and aggregated quickly, as visible in Fig. S4. Such behavior leads to low quality films on the surface of electrodes resulting in low values of capacitance. Although the samples with C-PA:K 2 CO 3 ratio 1:3 and 1:6 had lower SSA values then the other ones, they formed stable dispersions and their capacitance was still very high (350 F g −1 and 300 F g −1 , respectively). The morphology of the C-PA sample and other porous activated carbons was also analyzed by SEM to confirm the results obtained from previous measurements (Fig. 3 and Fig. S6). The carbons showed crumpled sheet morphology with lateral size in micrometer range. The magnified images (0.5–10 μ m) of the C-PA material with the highest capacitance results are depicted in Fig. 3. Higher magnification illustrates the presence of individual sheets, as visible in Fig. 3d. Interconnected sheets form spongy network, which implies the formation of highly electrochemically active SSA for rapid electrolyte ions accommodation. 3.2. Electrochemical characterization All AC samples were characterized first using the three-electrode setup and a 0–1 V window for rapid evaluation of the electrochemical performance. Sulphuric acid was used as an electrolyte because it yields excellent capacitive results especially in connection with carbon-based materials [50]. The recorded cyclic voltammograms (CVs) at a constant scan rate of 100 mV s −1 exhibited a distorted quasi rectangular shape, corresponding to the non-perfect EDLC behavior (Fig. 4a). A minor peak observed at around 0.4 V was caused by the presence of oxygen, which bore functional groups in ACs [51]. As discussed earlier, the highest value of capacitance was obtained for the C-PA. Such behavior originated in the higher SSA and a large micropore volume, where a higher amount of electrolyte ions was attached. On top of that, the stable C-PA dispersion formed a neat film layer on the GCE, thus contributing to the overall performance. To confirm the results from CV, we adopted the galvanostatic charging/discharging (GCD) method at a constant current density of 1 A Fig. 2. (a) Adsorption|desorption isotherm of C-PA 1:1. (b) DFT pore size distribution. (c) Comparison of ACs prepared from various polymers, ratio 1:3. Specific capacitance evaluated in the 3-electrode setup at a current density of 1 A g −1 in 1 M H 2 SO 4 , (d) Comparison of C-PA prepared with different ratio between polyamide:K 2 CO 3 . I. Dˇ edek et al. Journal of Energy Storage 72 (2023) 108660 5 g −1 (Fig. 4b). In all cases, small deviation from the ideal triangular shape behavior occurred, which fully correlated with the CV results. The C-PA showed the longest discharging time among the carbon samples, hence the highest capacitance (Fig. 4b,c). At such current density, ions have enough time to interact with the material and completely utilize the porous structure [47,49,52,53]. To shed more light on the electrochemical performance of the C-PA, we used the electrochemical impedance spectroscopy (EIS). The impedance spectra of the C-PA in the form of the Nyquist plot and the related circuit for data evaluation are depicted in Fig. S7c, d. The Nyquist plot displays no semicircle in the high frequency region, indicating a small value of charge transfer resistance (R ct =1.7 Ω). The missing semicircle also corresponds to the non-active faradaic reactions. In the region of middle frequencies, there is a 40◦line, which points towards the capacitive behavior of the activated carbon. A sharp increase in the low frequencies indicates good accessibility of the porous structure for ions, which shows good predispositions for an EDLC supercapacitor [54]. Compared with others, the C-PU, C-PES, C-ABS, and C-PA materials delivered a specific capacitance of 246 F g −1 , 296 F g −1 , 314 F g −1 and 323 F g −1 respectively. The highest specific capacitance of the C-PA among all the carbon samples is attributed not only to the very high specific surface area and the micro-pore volume, but also to the ability to form stable dispersion, which eventually benefits the adsorption of the higher amount of the electrolyte ions [55]. Due to its best performance, the C-PA sample was prepared also with different K 2 CO 3 ratios to clarify the activating agent effect (Fig. 4d, e). The maximum performance of 521 F g −1 (at 1 A g −1 ) was reached when the ratio of C-PA:K 2 CO 3 was 1:1. Increasing or decreasing the ratio worsened the sample performance, as shown in GCD test (Fig. 4d). This is explained by the ability to form stable dispersion. The samples with the highest capacitance tend to form stable dispersions, which was electrochemically proven to be very useful [56]. The C-PA samples, even with the high SSA form unstable dispersion, aggregate quickly, and, from the electrochemical point of view, result in far lower capacitance values. The best performing samples of ACs derived from C-PA, C-ABS, CPES, and C-PU were further characterized by the 2-electrode symmetrical setup, which is close to a real supercapacitor device. The highest current response belongs to the C-PA sample, indicating the best capacitive performance of this material compared to the others (Fig. 5a). Such results are also mirrored in the GCD experiments where they showed a similar trend (Fig. 5b). Fig. 5b shows the capacitance response of the selected polymers acquired from the GCD results. A symmetric cell with the C-PA showed a specific capacitance of 220 F g −1 at current density 1 A g −1 , followed by C-PU with 132 F g −1 , C-ABS with 176 F g −1 and C-PES with 152 F g −1 all recorded at 1 A g −1 (Fig. 5c). Selected ACs showed an excellent cyclic stability (Fig. 5f; Fig. S9), where ACs held near 80 % capacity retention after 50,000 cycles at the current density 5 A g −1 . In comparison with previously published results (Table S3), our C-PA derivative exhibited significantly better results in terms of specific capacitance, energy, and power density. Last but not least, excellent cyclic stability retention (≈95 % at 30000 cycles) in H 2 SO 4 was significantly higher compared to materials prepared via similar carbonization methodology (PS [12,57,58], PE [16,25] or PET [24,29]). To further explore the C-PA sample applicability, we employed an ionic liquid [EMIM][BF 4 ] as an electrolyte to boost the capacitive performance in terms of energy and power density. Ionic liquids are group of electrolytes commonly used for achieving higher capacitance and energy density, as published previously [59–62]. 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF 4 ]) was chosen as the Fig. 3. (a-d) SEM images of C-PA:K 2 CO 3 1:3 sample, showing porous morphology, with (c, d) clearly visible thin sheets of carbon connected together. I. Dˇ edek et al. Journal of Energy Storage 72 (2023) 108660 6 electrolyte with an achievable high potential window and a suitable molecule size that matched the pore width of our ACs [63]. The stable potential window was explored by the CV technique in the range of 0–3.7 V. The CV curves showed a semi-rectangular shape, without visible peaks. The GCD at various current densities showed high energy density and power density, as can be seen in Fig. 6a. The Ragone plot (Fig. 6b) compares relevant publications with ACs obtained from plastic waste. In our case, the maximum achieved energy and power density was found to be 61.1 Wh kg −1 and 36.9 kW kg −1 , respectively. Such results significantly outperform previously published results and suggest that the C-PA material can serve as an effective alternative to commercially available carbons. The C-PA was characterized in detail in this work due to its best properties. Moreover, other promising ACs, specifically C-PU, C-PES, CABS also showed excellent properties in terms of capacitance and cycling stability. Separated plastic waste (PU from furniture and car industry), waste from the textile materials (polyamide cloth, polyester cloth), and also ABS as a material often used for commercial 3D printing can be exploited as a raw material for the preparation of ACs for supercapacitor electrodes with excellent performance. 4. Conclusion In response to the global demand for long-lasting sustainable electronics and plastic waste recycling, we successfully developed a highly porous AC material from common plastic waste, ideal for supercapacitor electrodes. By pyrolyzing and chemically activating various plastic waste types (PE, PP, PES, PS, PA, PET, ABS, PU), we obtained high capacitive ACs with excellent performance. Our investigation revealed that specific surface area (SSA), microporosity, polymer glass transition temperature, compatibility of the polymer with the activating agent (K 2 CO 3 ), and AC dispersion stability are crucial parameters affecting the final supercapacitor properties. Carefully selecting the sample (C-PA) with the best capacitive properties, we further optimized the material by preparing AC with different K 2 CO 3 ratios. Remarkably, the C-PA sample with a 1:1 ratio exhibited outstanding results, achieving a highest capacitance of 220 F g −1 (at current density 1 A g −1 ) and demonstrating excellent cyclic stability (95 % retention after 30,000 cycles) in an aqueous electrolyte. Additionally, experiments with an ionic liquid electrolyte depicted a significant enhancement in energy and power density for C-PA. Our findings highlight the critical interplay of specific surface area, micropore volume, dispersion quality, pore size distribution matching the electrolyte, K 2 CO 3 :AC ratio, and the precursor's glass transition temperature in obtaining high-performance capacitive electrode materials. This careful evaluation allowed us to successfully develop novel sustainable materials, capable of producing supercapacitors with superior electrochemical properties, including both capacitance and stability. In conclusion, our research presents a promising pathway towards sustainable and efficient energy storage systems, contributing to the global efforts for a greener future. Fig. 4. AC samples measured in the 3-electrode setup. (a) CV response of ACs depicting quasi-rectangular shape. Recorded at the scan rate of 100 mV s −1 . (b) GCD response of ACs at the current density of 1 A g −1 . (c) Difference of specific capacitance on ACs made from various polymer materials. (d) GCD response of C-PA with variable C-PA:K 2 CO 3 ratios. (e) Difference of specific capacitance of C-PA with variable C-PA:K 2 CO 3 ratios. I. Dˇ edek et al. Journal of Energy Storage 72 (2023) 108660 7 CRediT authorship contribution statement Ivan Dˇ edek: Methodology, Investigation, Formal analysis, Visualization, Writing – original draft, Writing – review & editing. Stanislav Bartusek: Methodology, Investigation, Writing – original draft. Josef Jan Dvoˇ r´ aˇ cek: Methodology, Investigation, Writing – original draft. Jan Neˇ cas: Writing – original draft. Josef Petruˇ s: Investigation, Writing – original draft. Petr Jakubec: Conceptualization, Writing – original draft, Writing – review & editing, Supervision, Project administration. Vojtˇ ech Kupka: Conceptualization, Investigation, Writing – original draft, Writing – review & editing, Supervision, Project administration. Michal Otyepka: Conceptualization, Writing – original draft, Writing – Fig. 5. (a) CV quasi-rectangular shape of C-PU, C-ABS, C-PES and C-PA recorded at 100 mV s −1 . (b) GCD response recorded at 1 A g −1 of best performing ACs (C-PU, C-ABS, C-PES and C-PA) in 2-electrode setup. (c) Comparison of specific capacitances of C-PU, C-PES, C-ABS, C-PA. Additionally, C-PA was tested at different C-PA: K 2 CO 3 ratios, where the GCD is shown in (d) and compared in terms of specific capacitances in (e). (f) The cyclic stability test of C-PA 1:1 for 50,000 cycles at 5 A g −1 . Fig. 6. (a) Energy density (orange) and power density (green) plot of C-PA 1:1 on various current density for 2-electrode setup in [EMIM][BF 4 ]. (b) Ragone plot comparing current published research vs our results. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) I. Dˇ edek et al. Journal of Energy Storage 72 (2023) 108660 8 review & editing, Supervision, Funding acquisition. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability Data will be made available on request via ZENODO service. Acknowledgement We acknowledge the Research Infrastructure NanoEnviCz, supported by the Ministry of Education, Youth and Sports of the Czech Republic under Project No. LM2023066. We also acknowledge the support from ERDF/ESF “Nano4Future” (No. CZ.02.1.01/0.0/0.0/16_019/0000754) and support from the Internal Student Grant Agency of the Palacký University in Olomouc, Czech Republic (IGA_PrF_2023_018). The authors gratefully thank to Jiˇ rí Hoˇ sek, Eirini Ioannou (SEM, EDS) and Jan Pauswang (electrochemical testing). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.est.2023.108660. References [1] H. Ritchie, M. 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