Transformation of clinical waste into heteroatom-doped porous carbon materials towards a highly efficient energy storage device

Abstract

The persistent accumulation of plastic waste in the environment poses substantial ecological challenges over time. Thus, converting the plastic biowaste into valuable products is highly desirable. In this study, heteroatom-doped porous carbon materials derived from medical waste, specifically the outer layer of disposable masks (DM), have been investigated as promising electrode materials for energy storage applications. The synthesis of highly linked nanosheet architecture of porous carbon materials enriched with nitrogen and oxygen is demonstrated. The synthesized porous carbon material, pyrolyzed at 800 °C (DM-800), shows the highest specific capacitance. A solid-state device with DM-800 exhibited remarkable energy and power densities of 68.61 Wh kg⁻¹ and 15 kW kg⁻¹, respectively, at a current density of 15 A g⁻¹. Moreover, the fabricated devices have been demonstrated with both series and parallel connections to enhance the operating voltage, total capacitance, and current-handling capability. The device exhibited remarkable long-term cycling durability, maintaining its electrochemical performance even after 10 000 charge–discharge cycles. An empirical investigation further validates its effectiveness, as the device operated 38 red light-emitting diodes for nearly four minutes. This study highlights a straightforward approach for converting medical waste into high-performance porous carbon electrodes, offering a sustainable and innovative method for transforming biowaste into effective materials for energy storage applications.

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1. Introduction

Plastics are widely used across various sectors due to their cost-effectiveness, durability, hygiene, and versatility in shaping. According to the Plastics Europe 2023 report, global plastic production reached 390.7 million tons in 2021.^1–3 The COVID-19 pandemic has significantly increased the consumption of single-use plastics in recent years, driven by heightened demand for items such as disposable masks (DM) and test kits, thereby contributing substantially to waste accumulation.⁴ Prior to the COVID-19 pandemic, hospitals and laboratories generated approximately 300 million tonnes of plastic waste annually, a figure that surged to an estimated 630 million tonnes in 2020.⁵ However, only about 9% of plastic waste is recycled globally, primarily because the process is labour-intensive and cost-ineffective. By 2050, global plastic waste is projected to rise to an estimated 850 to 950 million metric tons, posing serious risks to public health and natural ecosystems.⁶ Improper handling of plastic waste can lead to severe environmental pollution and significant economic losses. Mechanical recycling is expensive and straightforward, but it is limited to single-component plastics.⁷ Chemical recycling, which involves breaking down plastics into their original monomers, offers high recycling potential but faces drawbacks such as complex processes, stringent conditions, and the emission of greenhouse gases and harmful substances.^8,9 Moreover, catalytic upcycling of plastics into valuable chemicals or fuels presents a promising approach to addressing plastic waste. However, due to the inherent chemical stability of plastics, existing catalytic methods often require extreme conditions to achieve significant conversion, such as high temperature and pressure. Such stringent requirements pose energy efficiency, safety, and environmental challenges, underscoring the need for innovative and milder catalytic systems.^10,11 Therefore, establishing an eco-friendly and efficient approach to transforming plastic waste into valuable products that support sustainable development is crucial.

Throughout the COVID-19 outbreak, 1.94 trillion DMs and gloves were utilised worldwide every month.¹² The massive production of DMs has led to a significant waste management issue.¹³ Moreover, the non-recyclable DMs are primarily composed of polypropylene non-woven fabric, which features a dense fibre structure and high porosity.¹⁴ As a result, exposing DMs to high-temperature carbonization or chemical degradation can serve a dual purpose, i.e., it effectively neutralises the viruses while also facilitating the creation of porous carbon materials.^2,15,16 Carbonising waste polymer materials is considered one of the most effective recycling methods. Recently, carbon materials have found extensive applications in energy storage, environmental remediation, and organic synthesis, primarily due to their high porosity, large surface area, excellent conductivity, and abundant surface functional groups. Nonetheless, the production process predominantly relies on precursors derived from fossil fuels, which present considerable disadvantages, including substantial energy consumption, increased production costs, and adverse environmental impacts. Thus, demand for the development of sustainable carbon materials is expanding. The high carbon content and low cost of plastic waste make it a compelling alternative. Materials such as polyvinyl chloride (PVC, 38.3% carbon), polystyrene (PS, 92.8%), polyethylene terephthalate (PET, 62.1%), polypropylene (PP, 85.0%), high-density polyethylene (HDPE, 85.0%), polyacrylonitrile (PAN, 67.9%), and phenol formaldehyde resin (PFR, 79.0%) have been recognized as promising precursors for the synthesis of high-value carbon-based materials.^13,17,18 A range of strategies have been established to create carbon materials derived from plastic waste, showcasing various morphologies such as carbon spheres,¹⁹ carbon nanotubes,²⁰ carbon nanosheets,²¹ hierarchically porous carbon,^22,23 porous carbon nanofibers,²⁴ and 3D carbon structures.²⁵ Two-dimensional carbon nanostructures composed of stacked graphene layers have garnered considerable attention for their remarkable properties, including a high surface area, well-developed porosity, abundant functional groups, and outstanding chemical and thermal stability.²⁶ Their distinct features make them well-suited for uses in adsorption,²⁷ energy storage,²⁸ organic transistors,²⁹ and the oxygen reduction reaction.³⁰ In contrast to graphene assemblies, carbon nanosheets exhibit diminished intersheet van der Waals forces, which reduce aggregation and restacking, thereby maintaining their unique properties. The benefits of carbon nanosheets position them as up-and-coming materials for environmental remediation and various advanced applications.

Herein, the synthesis of heteroatom-doped carbon from a medical waste material, i.e., a DM, is demonstrated. The synthesis process is facile and involves dissolving the outer layer of the DMs, followed by high-temperature carbonization to produce heteroatom-doped carbon nanosheets. In contrast to traditional methods that rely on supplementary chemical reagents, the present process incorporates nitrogen directly into the carbon framework from the precursor. The heteroatom-doped carbons are produced by simple carbonization without a porogen at 700, 800, and 850 °C, denoted as DM-700, DM-800, and DM-850, respectively. For comparison, the carbon material without carbonization (denoted as DMW) is also presented. The synthesized carbon nanomaterials are studied for use in supercapacitor applications. Among the synthesized samples, carbon nanosheets (DM-800) obtained by carbonization at 800 °C exhibit higher energy storage performance in both neutral and acidic media. The fabricated all-solid-state symmetric device using DM-800 displayed a significant energy density of 68.6 Wh kg⁻¹ and power density of 15 kW kg⁻¹ at a current density of 15 A g⁻¹. The series and parallel connections of the fabricated devices demonstrated a wider potential window and higher specific capacitance, respectively. This work emphasizes a straightforward method for converting medical waste into high-performance carbon electrodes for energy storage applications.

2. Experimental section

2.1. Chemicals

The disposable masks (DMs) used in the present study were purchased from Hariom Polypacks Limited, India. Sulphuric acid (H₂SO₄), nitric acid (HNO₃), polyvinylidene difluoride (PVDF), N-methyl pyrrolidone (NMP), polyvinyl alcohol (PVA), and sodium sulfate (Na₂SO₄) were purchased from SRL India. The graphite sheets were purchased from Nikunj Enterprise India. All reagents were of analytical grade.

2.2. Synthesis of activated carbon from DMs

The outer layers of the DMs, composed of polypropylene, serve as an excellent carbon source. The two exterior layers of a DM were utilized for the synthesis process. The outer layers were cut into small pieces, weighed 3 g, and dipped in a 100 mL beaker containing a concentrated acid solution (80 mL) of H₂SO₄ and HNO₃ in a 3 : 1 ratio. The beaker is then positioned on a hot plate and refluxed at 90 °C for 8 h with a stirring speed of 400 rpm. After reflux, the acid solution is removed from the beaker, leaving a black mass. The obtained sample was washed with deionized water until its pH reached neutrality. After drying overnight, the obtained samples were subjected to pyrolysis at 700, 800, and 850 °C for 2 h in an argon atmosphere, as shown in Scheme 1, and are designated as DM-700, DM-800, and DM-850, respectively. The resulting products were the heteroatom-doped porous carbons.

Scheme 1 Schematic representation of the synthesis of carbon materials from medical biowaste, i.e., outer layers of the disposable masks.

2.3. Characterization

The surface morphology of the synthesized products was examined using field-emission scanning electron microscopy (FESEM, Gemini 500, Zeiss). The crystal structure of the synthesized products was determined using X-ray diffraction (XRD) with an Empyrean Panalytical diffractometer equipped with a PIXcel3D detector and a copper Kα radiation source (λ = 0.15 nm) over the range of 5–90°. High-resolution transmission electron microscopy (HRTEM) (JEM-ARM300F2, JEOL) operating at 300 kV was employed to examine the microstructure. High-angle annular dark-field scanning TEM (HAADF-STEM) with energy-dispersive spectroscopy (EDS) mapping was performed to map elemental distributions in the synthesized carbon materials. Electron energy loss spectroscopy (EELS) was performed to determine the carbon bonding in a selected sample. X-ray photoelectron spectroscopy (XPS) with an Al Kα source (PHI-5000 VersaProbe II, ULVAC-PHI Inc.) was used to obtain the surface chemical composition and bonding.

2.4. Electrochemical measurements

The electrochemical analysis of the synthesized carbon materials was performed using a conventional three-electrode system, with a saturated calomel electrode (SCE) as the reference electrode, a Pt wire serving as the counter electrode, and the fabricated electrode acting as the working electrode. The electrochemical energy storage performance of the synthesized porous carbon materials was assessed using a symmetric solid-state device. The performance of the synthesized materials for energy storage was evaluated using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) with a CHI 7091E electrochemical workstation (CH Instrument, Inc., USA). To prepare the electrode, a mixture of synthesized porous carbon material, polyvinylidene fluoride (PVDF), and N-methyl-2-pyrrolidone (NMP) was taken in a mortar. This mixture was then meticulously ground in a mortar and pestle using NMP as the solvent to achieve a uniform slurry. The slurry was applied to a thoroughly cleaned 1 × 1 cm² graphite sheet substrate with an active porous carbon material loading of 1 mg each, then dried at 80 °C overnight. To understand the electrochemical behaviour of the synthesized materials and bare graphite sheet, CV measurements were performed under identical conditions for comparison (Fig. S1). The bare graphite sheet showed negligible specific capacitance (C_s) compared to the synthesized carbon material DM-800 in both electrolytes. For comparison, the areal capacitance was measured.³¹ The calculated areal capacitances of DM-800 and bare graphite sheet were 336 F cm⁻² and 0.078 F cm⁻², respectively, at a scan rate of 10 mV s⁻¹ in a neutral electrolyte. Similarly, in an acidic electrolyte, the areal capacitances of 401 and 1.18 F cm⁻² were calculated with DM-800 and bare graphite, respectively, at the same scan rate. Given the negligible areal capacitance from the bare graphite sheet, the reported capacitance in the present work is from the synthesized carbon materials. All electrochemical measurements were performed in both neutral and acidic media for energy storage applications. The C_s of the synthesized porous carbon materials was estimated from CV and GCD through eqn (1) and (2), respectively, for the three-electrode and two-electrode systems.³²

The area enclosed by the CV curve in the provided equations signifies the C_s, where m represents the mass of the active electrode materials, ΔV denotes the potential window (V₂ − V₁), and ν indicates the scan rate. In eqn (2), I/m refers to the applied current density, and Δt represents the discharge time. EIS of the active material on a graphite sheet was performed in both electrolytes. The analysis covered a frequency range of 0.1–10⁵ Hz, starting at an initial voltage of 0 V vs. RHE.

2.5. Fabrication of a solid-state symmetric device

A solid-state symmetric device was fabricated using synthesized porous carbon materials as both the cathode and anode, and polymethyl methacrylate (PMMA) gel as the electrolyte. A Whatman filter paper saturated with a PMMA-Na₂SO₄ gel functioned as the separator and electrolyte medium. The energy (ED) and power density (PD) of the fabricated device were calculated from the GCD plots using the following equations.where C_s is the specific capacitance (F g⁻¹), V is the potential window, and Δt is the discharge time.

3. Results and discussion

3.1. Structural properties

XRD is used to determine the structural ordering, the extent of graphitization, and the crystalline/amorphous nature of the synthesized carbon materials. The XRD patterns of the synthesized carbon materials are shown in Fig. 1a, which exhibits diffraction peaks at 26.3° and 43.1°, corresponding to the characteristic reflections of graphitic carbon for the (002) and (101) planes, respectively (JCPDS card no. 41-1487).^33,34 The full-width at half maximum (FWHM) of the (002) plane is measured to be the largest for DMW (7.221°) and decreases for the pyrolyzed carbon materials. The smallest FWHM (3.076°) is measured for DM-800, indicating maximum graphitization among the derived-DMs (3.076–3.796°). Moreover, the XRD pattern of DMW exhibits the (002) plane at a lower angle (23.6°), which is believed to be due to the presence of functional groups within the layers. This suggests a disordered carbon structure and the less graphitic nature of DMW.

Fig. 1 (a) XRD patterns and (b) Raman spectra of the synthesized carbon materials, i.e., DMW, DM-700, DM-800, and DM-850, from disposable masks.

Fig. 1b shows the Raman spectra of DM-derived carbons with two distinct peaks, i.e., the D band at ∼1350 cm⁻¹ associated with sp³ defect sites indicating disordered carbon, and the G band at around 1590 cm⁻¹, corresponding to ordered graphitic carbon with sp² hybridization.³⁵ An increase in pyrolysis temperature leads to an increase in graphitic carbon, as confirmed by the decrease in I_D/I_G band intensity.³⁶ A reduced I_D/I_G ratio in the DM-derived carbon materials indicates a significant presence of graphitic sp² carbon relative to disordered sp³ carbon, resulting in enhanced electronic conductivity, which is advantageous for energy storage.³⁷ Moreover, the presence of a 2D band at 2850 cm⁻¹ indicates the existence of graphene layers.³⁸ The presence of the 2D band in Raman spectra of the synthesized carbon materials indicates improved graphitization or more ordered sp² domains, enhancing electronic conductivity.

3.2. Growth mechanism of carbon from DM

The outermost layer of a DM is made of nonwoven polypropylene fabric, a type of plastic material recognized for its effectiveness in filtering droplets and particulate matter. To enhance its performance, this layer is frequently treated with a hydrophobic (water-repellent) material derived from specific polymers or chemical agents, which inhibits moisture absorption and penetration. Polypropylene is unstable above 200 °C but is resistant to most chemicals and solvents. However, it reacts with strong inorganic acids, such as fuming nitric acid and concentrated sulfuric acid, at a certain temperature. Keeping this in mind, the outer layer of the DM was first cut into small pieces and kept in a round-bottom flask. Then, H₂SO₄ was added to the round-bottom flask and stirred continuously at 90 °C. The H₂SO₄ interacts with polypropylene through a complex process that involves surface sulfonation, oxidation, and thermal decomposition occurring at 90 °C.^39,40 Rather than merely dissolving the polymer, the acid treatment chemically alters it by introducing sulfonic groups, breaking down the polymer chains, and potentially causing carbonization, accompanied by the release of SO₂ gas. The reaction exhibits autocatalytic behaviour, suggesting that the products formed can act as catalysts to enhance and sustain the reaction.⁴¹ This reaction modifies the surface of hydrophobic polypropylene, transforming it into a hydrophilic surface with carbon chain groups that create ion-exchange sites. Then an oxidising agent, i.e., concentrated HNO₃, was added to the reaction. Oxidation at the carbon sites leads to cleavage of polymer chains, a process known as chain scission, thereby reducing the molecular weight of polypropylene.⁴² This degradation process incorporates a range of functional groups containing oxygen and nitrogen into the polymer structure, such as carboxylic acids, alcohols, and nitro groups. After reflux, the product from DM was washed with DI water to neutralise the acidic surface, and finally pyrolyzed at different temperatures in an argon atmosphere to obtain the desired products, i.e., DM-700, DM-800, and DM-850.

3.3. Morphology

Fig. 2 displays FESEM images of the synthesized samples at different magnifications. DMW displays a block with a sheet-like morphology (Fig. 2a and a1). During the initial pre-carbonization step, the polymer chains partially decompose, allowing the removal of volatile impurities and weakly bound surface groups. This treatment preserves the essential active components of the DM materials and improves their interaction with acid. Fig. 2b and b1 show FESEM images of DM-700 with the stacked sheet-like structure. Upon increasing the pyrolysis temperature to 800 °C, the sheets are found to be well-separated and distinct for DM-800, as shown in Fig. 2c and c1, forming a defined space between the sheets. The pyrolysis at 800 °C is believed to act as an exfoliation process, allowing the controlled separation of well-defined sheets. This is due to the production and release of gases such as CO, CO₂, and NO₂ during pyrolysis. However, an increase in pyrolysis temperature to 850 °C resulted in a crumpled morphology with non-uniform, micron-sized particles, lacking any trace of sheet morphology. This reveals that an optimum pyrolysis temperature (800 °C) produces well-defined sheet morphology. The elemental analysis of the synthesized materials reveals the presence of C, N, and O (Fig. 2c2 and S2a–c), indicating the incorporation of heteroatoms into the carbon skeleton during chemical activation. The atomic percentages of N and O are found to decrease with an increase in pyrolysis temperature, and consequently, the carbon percentage increases. Fig. 3a shows a TEM image of DM-800, revealing its internal morphology and structural features, confirming the presence of two-dimensional layers. The multilayer morphology of DM-800 led to higher capacitive performance than other samples (discussed later).⁴² Furthermore, the curved graphitic layer is apparent in Fig. 3b, revealing that a substantial amount of graphitized carbon is present.⁴³ In contrast to traditional activated carbon, which generally fails to form graphene sheets even after thermal treatment at temperatures exceeding 3000 °C, DM-800 exhibits a sheet morphology with significant graphitization at a notably lower temperature of 800 °C.¹ This phenomenon likely arises from the thinning of pore walls as plastic-based polymers expand into bubbles, which aids in the formation of graphitized carbon layers.⁴² As a result, DM-800 incorporates a significant quantity of graphene within the thin membranes of its macropores. Moreover, the fringe spacing of 0.237 nm is measured using the line profile in Fig. 3b, which suggests that this value is likely associated with the (101) planes of the graphitic structure. Moreover, the lattice fringe spacing of 0.47 nm (inset), as shown in Fig. 3b, is associated with the (002) planes of graphite.

Fig. 2 (a–d) Low-magnification and (a1–d1) high-magnification FESEM images of the DM-derived carbon materials, i.e., (a and a1) DMW, (b and b1) DM-700, (c and c1) DM-800, and (d and d1) DM-850. (c2) Energy dispersive X-ray spectrum of DM-800.

Fig. 3 (a) TEM image, (b) HRTEM image, (c) carbon K-edge EELS spectra of DM-800 collected at different spots, (d) HAADF-STEM image, the inset displays FFT of the STEM image, (e and f) the mask applied on FFT, (g–i) HAADF-STEM EDX elemental mapping of (h) C, (i) N, and (j) O.

EELS was employed to examine the surface electronic structure and graphitization of DM-800. As illustrated in Fig. 3c, the carbon K-edge spectra reveal a prominent peak around 292 eV and a shoulder peak at 285 eV, which correspond to the characteristic 1s → σ* and 1s → π* transitions, respectively, of the sp² carbon.⁴⁴ The observation of an increment in the 1s → π* intensity of the DM-800 material indicates a shift from sp³ to sp² carbon in the active material framework. Additionally, Fig. 3c illustrates the sp² carbon fraction assessed through EELS, employing highly oriented pyrolytic graphite as a benchmark. The HAADF-STEM image of a nanosheet (DM-800) was acquired as shown in Fig. 3d. The micrograph was further processed with GATAN software to produce fast Fourier transforms (FFTs) and inverse FFTs. The FFT pattern (inset of Fig. 3d), obtained from the STEM image (Fig. 3d), displays a truncated hexagonal symmetry, thereby affirming the crystalline characteristic of the material and suggesting the existence of lattice defects. Fig. 3e and f show the inverted FFT images of DM-800, which exhibit a defective region along with a distinct hexagonal lattice of a graphitic structure.¹ To understand the distribution of the elements of DM-800, the elemental mapping was performed using STEM analysis. Fig. 3g–j shows the distribution of C, N, and O in the carbon framework of the synthesized carbon material DM-800.

3.4. Surface area

The surface area of the active electrode materials plays an important role in the electrochemical reactions. The specific surface area and porosity of the synthesized carbon materials derived from DMs are evaluated from N₂ adsorption–desorption isotherms. The isotherms (Fig. S3a) demonstrate a hysteresis loop characteristic of a type IV isotherm. This loop indicates capillary condensation, resulting from the varying adsorption and desorption mechanisms within micropores. This can be further classified as an H4-type hysteresis loop, typically associated with materials that have a restricted pore-size distribution. The manifestation of low-pressure hysteresis in the graph substantiates the existence of micropores. The specific surface areas were determined to be 170.5, 96.2, 805.8, and 476 m² g⁻¹ for DMW, DM-700, DM-800, and DM-850, respectively, by the standard BET method. Among the synthesized carbon materials, DM-800 has the highest specific surface area compared to other carbon materials, suggesting an optimal pyrolysis temperature. The specific surface area of 805.8 m² g⁻¹ for DM-800 is found to be significantly higher than that obtained through pyrolysis alone, without any specific activation or supplementary treatments. This figure exceeds numerous reports on synthetic carbon, in which activating reagents such as KOH and K₂CO₃ are typically employed to generate pores and enhance the surface area.⁴⁵ The pore-size distribution of DM-derived carbon (Fig. S3b) obtained from N₂ adsorption showcases both micropores and mesopores. The pore-size distributions, illustrated as an inset in Fig. S3b, demonstrate the presence of micropores and mesopores in DM-800, whereas only mesopores were observed in the case of other synthesized carbon materials (DMW, DM-700, and DM-850), resulting in the highest surface area of DM-800. The integration of a substantial surface area with a highly porous structure, featuring both micropores and mesopores, shows significant potential for applications in supercapacitors, as elaborated later. Micropores, due to their high surface area-to-volume ratio, play a crucial role in influencing adsorption and desorption behavior through diffusion when present in substantial amounts. In addition, mesopores contribute to the total surface area and improve the accessibility of adsorbates by providing wider transport pathways that connect to the micropores.⁴⁶

3.5. Surface composition

XPS is used to investigate the bonding characteristics and determine the elemental composition of DM-800. The survey spectrum (Fig. S4) shows the C 1s (284.8 eV), O 1s (532.3 eV), and N 1s (400.2 eV) peaks, indicating their presence on the surface.⁴⁷ The deconvoluted C 1s spectrum (Fig. 4a) displays four distinct peaks at 284.6, 285.7, 288.4, and 290.3 eV.⁴⁸ The prominent peak at 284.6 eV is ascribed to C–C/C=C, indicating the existence of graphitic sp² hybridized carbon alongside sp³ hybridized carbon within the synthesized DM-800 material. A peak at 285.7 eV corresponds to the C–O species, indicating the presence of oxygen-containing functional groups, such as hydroxyl or ether groups. The peak at 288.4 eV is attributed to O–C=O, indicating the presence of carbonyl or carboxyl groups. The oxygen-containing functional groups enhance the surface polarity and improve electrolyte wettability, enabling more efficient ion diffusion throughout the porous structure. These functional groups can also contribute additional pseudocapacitance through faradaic interactions, thereby increasing the overall capacitance. A higher binding energy peak at 290.3 eV corresponds to CO₃²⁻ groups, indicating partial surface oxidation or the formation of carbonate species during synthesis. Overall, the C 1s peak for DM-800 reveals the presence of graphitic carbon alongside various oxygen-containing functional groups. The N 1s spectrum (Fig. 4b) displays three distinct peaks at 398.8 eV, 400.4 eV, and 402.5 eV, corresponding to pyridinic N, pyrrolic N, and graphitic N. Earlier investigations suggest that pyridinic N and pyrrolic N play a role in defect formation, providing pseudocapacitive reactive sites that increase specific capacitance. In contrast, oxidized N enhances electrical conductivity and diminishes internal resistance. The presence of graphitic nitrogen in DM-800 suggests graphitization. This form of nitrogen with the limited graphite lattice serves as an electron donor, facilitating accelerated electron transport across the carbon structure.^49–51 The O 1s spectrum (Fig. 4c) exhibits peaks at 531.8 and 532.9 eV, which correspond to C–O/C=O and C–O/C–OH, respectively. The oxygen-containing functional groups promote better ion diffusion and transport in the electrolyte, enhancing pseudocapacitive behavior, improved electrical conductivity, and excellent surface wettability.^52,53 The involvement of nitrogen and oxygen functional groups in the redox reactions contributes to the enhancement of electrode energy storage capabilities.^54,55

Fig. 4 High-resolution XPS (a) C 1s, (b) N 1s, and (c) O 1s spectra of DM-800.

3.6. Electrochemical analysis

3.6.1. Electrochemical measurement in a three-electrode setup. The electrochemical properties of the as-synthesized DM-derived carbon materials were evaluated in two electrolytes, i.e., 0.5 M H₂SO₄ and 0.5 M Na₂SO₄, utilizing a three-electrode setup comprising a standard SCE as the reference electrode, Pt foil as the counter electrode, and an active material-coated graphite sheet as the working electrode. A schematic representation of the electrochemical setup is presented in Fig. 5a. Fig. 5b and d present the CV profiles at a sweep rate of 5 mV s⁻¹ using acidic and neutral electrolytes, respectively. The CV curves exhibited a quasi-rectangular shape in both electrolytes, signifying characteristic electric double-layer capacitance (EDLC) and favorable electrochemical reversibility.⁵⁶ The specific capacitances (C_s) of 164, 290, 431, and 365 F g⁻¹ are evaluated at 5 mV s⁻¹ in 0.5 M H₂SO₄ electrolyte for DMW, DM-700, DM-800, and DM-850 samples, respectively. The C_s is found to be slightly lower for all samples in a neutral electrolyte (0.5 M Na₂SO₄), i.e., 135, 130, 372, and 249 F g⁻¹ for DMW, DM-700, DM-800, and DM-850, respectively, under identical conditions. Among the samples, DM-800 delivered the highest performance in both electrolytes, attributed to its hierarchical porous structure, characterized by a two-dimensional shape, a higher surface area, and surface functional groups. Moreover, minor redox peaks are observed in the CV curves obtained with both electrolytes, indicating the presence of pseudocapacitance due to nitrogen and oxygen-containing functional groups.⁵⁷Fig. 5c and e show the C_s values estimated at different scan rates for DMW, DM-700, DM-800, and DM-850 in 0.5 M Na₂SO₄ (Fig. S5a–d) and 0.5 M H₂SO₄ (Fig. S6a–d) electrolytes, respectively. The C_s value is maximum at the lowest scan rate due to the increased duration taken by electrolyte ions to reach the internal surfaces and pores of the electrode, facilitating deeper charge penetration and enhanced use of the active materials. Conversely, the C_s values decreased with an increase in scan rate due to the shorter electrochemical interaction of the electrolyte ions and diffusion at the electrode's outer surface, rendering a significant portion of the inner active materials inactive.^58,59

Fig. 5 (a) Schematic representation of the three-electrode configuration, (b) CVs at a scan rate of 5 mV s⁻¹ and (c) C_sversus scan rates in 0.5 M H₂SO₄, (d) CVs at a scan rate 5 mV s⁻¹ and (e) C_sversus scan rates in 0.5 M Na₂SO₄, (f) GCD at 1 A g⁻¹ and (g) C_sversus current densities in an acidic electrolyte, (h) GCD at 1 A g⁻¹ and (i) C_sversus current densities of the as-synthesized electrode materials in 0.5 M Na₂SO₄ electrolyte.

The GCD of the as-synthesized carbon materials is further obtained at 1 A g⁻¹ in both acidic and neutral electrolytes, as shown in Fig. 5f and h, respectively. The GCD profiles exhibit nearly symmetrical isosceles triangles with minimal voltage drop, indicating high reversibility in the charge–discharge processes. The symmetrical isosceles triangles indicate the synergistic effect of the double-layer and pseudocapacitance behaviour.⁶⁰ The minor distortions in the curves were due to the pseudocapacitive effect, which exhibits redox behaviour. Fig. 5g and i show the C_s values for DMW, DM-700, DM-800, and DM-850 using the GCD curves at different current densities in acidic and neutral electrolytes, respectively. The synthesized DM-derived carbon materials, i.e., DMW, DM-700, DM-800, and DM-850, demonstrated C_s values of 77 (and 93), 168 (and 114), 282 (and 340), and 220 (and 171) F g⁻¹ at 1 A g⁻¹ in the acidic electrolyte (and neutral electrolyte). DM-800 exhibits an extended discharge duration compared to the other synthesized materials, consistent with its larger CV curve area, which suggests enhanced capacitive performance in both electrolytes. To evaluate the rate capacity, the GCD curves of the synthesized DM-derived carbon materials were further examined at variable current densities (Fig. S5a1–d1 in the neutral electrolyte and Fig. S6a1–d1 in the acidic electrolyte). Fig. 5f and h show the GCD curves of the as-synthesized electrode materials and their corresponding C_s values in acidic (Fig. 5g) and neutral (Fig. 5i) electrolytes, respectively. The C_s values of 168 and 120 F g⁻¹ at a higher current density of 10 A g⁻¹ are estimated to be for DM-800 in acidic and neutral electrolytes, respectively (Fig. 5g and i). The GCD curves exhibited a significant level of symmetry across a broad current density range of 1 to 10 A g⁻¹, indicating exceptional reversibility of the charge–discharge process with both electrolytes. The analysis of electrokinetic phenomena is typically performed to assess supercapacitor performance, as it provides insights into charge transfer dynamics, ion transport, and the interfacial behavior of electrode materials. Applying Dunn's Power law to electrokinetic measurements makes it possible to effectively differentiate between surface-controlled capacitive processes and diffusion-controlled battery-like mechanisms.^61,62 The overall charge accumulated in the electrode material is made up of two primary elements: a diffusion-limited faradaic contribution linked to bulk redox reactions and a surface-related component that encompasses rapid faradaic charge transfer as well as non-faradaic double-layer capacitance (EDLC), resulting from the electrostatic adsorption and desorption of ions. Dunn's methodology (eqn (5) and (6)) links the response to the scan rate in CV, facilitating a deeper understanding of the underlying charge-storage mechanism.⁶¹

i = aν^b (5)

log i = log a + b log ν (6)

The exponent b is crucial in defining the characteristics of the charge-storage mechanism. The b of ∼0.5 suggests a diffusion-controlled process, which is a characteristic behaviour similar to that of batteries. A b value ∼1 indicates a mechanism that is controlled by the surface and is capacitive in nature. The b value reflects the charge storage mechanism established by plotting the logarithm of peak current (i_p) against the logarithm of scan rate (ν) (Fig. 6b) through CV at different scan rates (Fig. 6a, S7a–c and a1–c1). The calculated b values of the synthesized samples in acidic and neutral electrolytes are presented in Table 1 from the CV curves (Fig. S7 and S8). This suggests that both capacitive and diffusion-controlled mechanisms contribute to proton transport across the synthesized materials in both electrolytes (Table 2).

Fig. 6 (a) CV curves at different scan rates, (b) logarithmic plot of peak current vs. scan rates, (c) capacitive and diffusive contribution plot at a constant scan rate of 60 mV s⁻¹, and (d) bar diagram of C_s at different scan rates of DM-800 in 0.5 M Na₂SO₄.

Table 1 Calculated b values of the synthesized carbonaceous materials in acidic and neutral electrolytes

Electrolyte	b Values
	DMW		DM-700		DM-800		DM-850
0.5 M H2SO4	0.68	0.76	0.71	0.74	0.82	0.84	0.69	0.68
0.5 M Na2SO4	0.77	0.79	0.62	0.54	0.87	0.88	0.78	0.79

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Table 2 Calculated capacitive contribution (CC) and diffusive contribution (DC) of the samples in acidic and neutral electrolytes at a sweep rate of 60 mV s^−1a

Electrolyte	DMW		DM-700		DM-800		DM-850
	CC	DC	CC	DC	CC	DC	CC	DC
a CC = % capacitive contribution, DC = % diffusive contribution.
0.5 M H2SO4	67	33	79	22	82	18	35	65
0.5 M Na2SO4	87	13	33	67	83	17	74	26

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Among the synthesized samples, DM-800 exhibited the highest b values, indicating a more significant involvement of the capacitive process, thereby improving charge storage performance. This is believed to be due to the higher surface area, well-structured pathways, and the presence of an optimal amount of heteroatoms in the carbon skeleton, which promote effective ion diffusion and charge storage. This is reflected in its enhanced C_s in both electrolytes. The capacitive response is a result of surface charge accumulation, whereas the diffusion-controlled response pertains to the transport of electroactive species within the electrolyte. The combined effects of these contributions are revealed in the electrochemical behaviour, with the total current representing the aggregate of capacitive (k₁ν) and diffusive-controlled components, utilizing Dunn's model (eqn (7)) to assess their individual impacts.⁶³

i(V) = k₁ν + k₂ν^1/2 (7)

At a scan rate of 60 mV s⁻¹, the capacitive contribution of DM-800 is determined to be 83% (Fig. 6c), suggesting that charge storage was predominantly influenced by surface capacitance rather than a diffusion-controlled mechanism in the neutral electrolyte. This behavior differs from that of other samples, which exhibited reduced capacitive contributions (Fig. S7a2, b2 and c2). The capacitive mechanism corresponds with insights gained from eqn (7), indicating more reversible and rapid electrochemical reactions that improve overall charge storage efficiency. Conversely, diminished capacitive contributions indicate a strong dependence on slower ion diffusion and intercalation processes. At all higher scan rates, DM-800 exhibits higher capacitive performance than other synthesized porous carbon materials in the neutral electrolyte (Fig. 6d, S7a3, b3 and c3). Electrokinetic measurements of all samples are also carried out in an acidic electrolyte and show the same trend: DM-800 displays the largest capacitive contribution among other samples (Fig. S8a2–d3). Moreover, DM samples exhibit higher capacitive performance at higher scan rates in both neutral (Fig. S7a2–c3) and acidic (Fig. S8a2–d3) electrolytes.

To gain further insights into the charge-storage activity of the best electrode material (DM-800), SEM, EDS, and XPS measurements were performed after charging DM-800 in a 0.5 M Na₂SO₄ electrolyte. The SEM image confirms that the nanosheets are intact after galvanostatic charging (Fig. S9a). Moreover, the elemental mapping of DM-800 reveals the presence of C, N, O, Na, and S (Fig. S9b–g and h), confirming the incorporation of Na²⁺ and SO²⁻ into the carbon skeleton of the electrode material during the charge storage process. Furthermore, XPS confirms the presence of the aforementioned elements on the surface of DM-800 (Fig. S10a). The survey XPS spectrum (Fig. S10a) confirms the presence of C, N, O, Na, and S on the surface of DM-800 after galvanostatic charging. The C 1s spectra (Fig. S10b) display four distinct peaks associated with C–C/C=C (284.9 eV), C–O (286.0 eV), O–C=O (288.8 eV), and CO₃²⁻ (290.7 eV) bondings.⁴⁸ A small shift in the binding energy of the electrode after charging is believed to be due to the electrolyte ion's incorporation into the carbon matrix. The O 1s spectrum (Fig. S10c) of the electrode after charging reveals three peaks at 531.8, 533.3, and 535.1 eV corresponding to the C–OH and C–O–C functional groups, and adsorbed water, respectively. The N 1s spectrum (Fig. S10d) shows three distinct peaks at 398.8 eV, 400.4 eV, and 403.4 eV, which are attributed to pyridinic, pyrrolic, and graphitic nitrogen, respectively. A peak at ∼1072.0 eV (Fig. S10e) corresponds to Na 1s, suggesting the incorporation of Na ions into the carbon electrode.⁶⁴ The S 2p XPS spectrum (Fig. S10f) shows peaks at 167.6 and 168.9 eV, corresponding to the S atom of SO²⁻ from the Na₂SO₄ electrolyte.⁶⁵ This suggests that electrolyte ions are inserted into the carbon material during charging. The elemental mapping of the electrode material (DM-800) after charging in an acidic electrolyte also shows the presence of C, N, O, and S (Fig. S11a–f). The EDX spectrum (Fig. S11g) further confirms the presence of the above-mentioned elements in the carbon after charging in an acidic electrolyte. This reveals the incorporation of ions into the active material during charging.

Electrochemical impedance spectroscopy (EIS) was performed over a frequency range of 0.1 Hz to 106 Hz at open circuit potential to investigate charge transport between the electrode and the electrolyte. The Nyquist plots (Fig. S5e) of the synthesized carbon materials in the neutral electrolyte, accompanied by an inset of the equivalent circuit model, were obtained. The internal resistance between the electrode and the electrolyte plays a crucial role in influencing electrical conductivity. The EIS spectra show an intercept on the X-axis corresponding to the bulk solution resistance (R_s), indicating the smallest R_s for DM-800. The R_s values obtained from the EIS data for DMW, DM-700, DM-800, and DM-850 are measured to be 8.30, 6.36, 4.95, and 10.51 Ω, respectively. Furthermore, the charge transfer resistances (R_ct) for the active carbon materials DMW, DM-700, DM-800, and DM-850 are estimated to be 5.61 Ω, 5.98 Ω, 4.37 Ω, and 6.23 Ω, respectively, from the circuit diagram. The smallest R_ct for DM-800 correlates with the higher supercapacitor performance.

3.7. Solid-state symmetric supercapacitor study

The solid-state symmetric supercapacitor device (SSD) is fabricated using DM-800, the best-performing electrode for supercapacitors as discussed earlier in both neutral and acidic electrolytes. The device (Fig. S12a) was assembled using DM-800 as the active electrode material and a PMMA-Na₂SO₄ gel as the electrolyte. The PMMA-Na₂SO₄ gel electrolyte was chosen due to its high dielectric constant, low volatility, excellent electrochemical stability, and cost-effectiveness. The positively charged Na⁺ ions in the electrolyte migrate toward the negative electrode while sulfate ions (SO₄²⁻) move to the positive electrode. This leads to a potential difference between the electrodes, facilitating the charging process. The working potential window is another critical parameter for an assembled solid-state supercapacitor device. To determine the working potential window of the fabricated symmetric supercapacitor DM-800, CV measurements were performed at different potential windows ranging from 1.0 V to 2.0 V. All CV curves (Fig. S12b) exhibit quasi-rectangular behaviour within this potential range. To understand the electrochemical performance of the fabricated device, CV measurements were performed at different scan rates from 5 to 100 mV s⁻¹, which show a quasi-rectangular shape (Fig. 7a), suggesting EDLC and pseudocapacitive behaviour along with electrochemical reversibility.^66,67 From the CV measurement, the fabricated SSD of DM-800 exhibits C_s values of 192 and 67 F g⁻¹ at 5 and 100 mV s⁻¹, respectively, with a potential window of 2.0 V (Fig. S12c). The GCD measurement of the SSD was conducted with a potential window of 2.0 V at current densities ranging from 1 to 15 A g⁻¹ (Fig. 7b). The C_s values of 123 F g⁻¹ and 27 F g⁻¹ were estimated at the current densities of 1 A g⁻¹ and 15 A g⁻¹, respectively (Fig. S12d). The C_s retention of the fabricated SSD is found to be quite high even while using a high applied current density. The energy density (ED) and power density (PD) are the most critical parameters for an energy storage device for practical applications. The fabricated SSD with DM-800 exhibits a maximum ED of 68.6 Wh kg⁻¹ at a current density of 1 A g⁻¹ and a PD of 14.58 kW kg⁻¹ at the current density of 15 A g⁻¹ (Fig. 7c), taking the total mass of the active materials in the electrodes. The obtained performance is comparable to or higher than those reported with similar materials. Reddy et al. reported an ED of 22.9 Wh kg⁻¹ and a PD of 0.502 kW kg⁻¹ at the current density of 0.5 A g⁻¹ using the heteroatom-doped porous carbon materials, synthesized from cow urine.⁶⁸ Similarly, Wang et al. reported an ED of 61.19 Wh kg⁻¹ at a current density of 0.5 A g⁻¹ with nitrogen-doped graphite-like porous carbon materials, synthesized from wood waste.⁶⁹ The EIS study of the device was performed using Nyquist plots (Fig. 7d). The inset of Fig. 7d shows an equivalent circuit used to fit the Nyquist plots. Using an equivalent circuit, the solution resistance (R_s) and the charge transfer resistance (R_ct) of the fabricated device are measured to be 2.12 and 0.13 Ω, respectively. This suggests facile charge transfer and remarkable ion diffusion at the electrode–electrolyte interface. The device's stability is vital in supercapacitor applications, ensuring reliable energy storage performance throughout extended cycling and repeated use. The cycling stability of the device is assessed over 10 000 continuous GCD cycles at 15 A g⁻¹, as illustrated in Fig. 7e. The device demonstrates exceptional durability, retaining 98% of its initial capacitance, underscoring its potential for real-world electrochemical applications. Additionally, Fig. 7e illustrates the relationship between capacitance retention and coulombic efficiency as a function of the cycle number. The inset of Fig. 7e displays the initial and final 10 GCD cycles. To examine the slight capacity loss noted during cycling, post-mortem analysis of the electrode material (DM-800) is performed. The morphology of the electrode material exhibited a negligible change after the cyclic process (Fig. S13a). The elemental mapping of DM-800 exhibits the presence of C, N, and O (Fig. S13b–f). A slight capacitance reduction of approximately 2% is believed to be due to partial blockage of the ion pathway after 10 000 GCD cycles. This is indicated by an increase in R_ct (0.4 Ω) as well as R_s (2.5 Ω) (Fig. 7d). The excellent cycle stability may be attributed to the presence of optimized heteroatoms (N, O) in the carbon skeleton, lower charge transfer resistance, and a high surface area and pore volume. The enhanced stability contributes significantly to the reliability and longevity of devices, positioning supercapacitors as a viable option for sustainable energy systems. To understand the real-world application of the fabricated SSD device, the device was charged from low to high current densities (5 to 50 A g⁻¹), but discharged at a constant low current density (0.25 A g⁻¹), as shown in Fig. 7f. ED (and C_s) values were measured to be 109 Wh kg⁻¹ (196 F g⁻¹) and 35 Wh kg⁻¹ (and 63 F g⁻¹) at 5 A g⁻¹ and 50 A g⁻¹ current density, respectively, at a fixed discharge current rate of 0.25 A g⁻¹ (Fig. S12e). This demonstrates that the device can charge rapidly and provides a controlled power output, making it suitable for use as a dependable power backup. The fabricated SSD with a PMMA-Na₂SO₄ gel electrolyte is used to power two electronic components, such as LEDs and a motor fan, showcasing its practical application in a real-world scenario. Thirty-eight red LEDs (1.5 V each), connected in parallel, are illuminated for up to 1.5 min (Fig. 7g) after 1 min of full charge. Additionally, a 1 V motor-driven fan operated continuously for approximately 1.5 minutes (Fig. S12f) when the device was fully charged. Furthermore, three devices using DM-800 were fabricated, and their supercapacitor performance was evaluated using GCD measurements (Fig. S14, SI) to confirm reproducibility. Negligible changes in C_s, ED, and PD have been observed in the three fabricated devices. In particular, the device's C_s is ± 3 F g⁻¹ at a current density of 1 A g⁻¹. Similarly, the ED and PD of the fabricated SSD device using DM-800 are ±1.66 Wh kg⁻¹ and ±0.025 kW kg⁻¹, respectively, under identical conditions. The fabricated SSA using DM-800 as the electrode material demonstrates excellent C_s, ED, PD, and durability compared to recent reports on porous carbon materials, as listed in a comparison table (Table S1, SI).

Fig. 7 (a) CVs at different scan rates, (b) GCD curves at different current densities, (c) Ragone plot, (d) EIS spectra and the inset shows an equivalent circuit, (e) durability test up to 10 000 GCD cycles, (f) GCD curves at different charging rates with a constant discharging rate of 0.25 A g⁻¹, and (g) practical demonstration of the fabricated solid-state symmetric device using PMMA-Na₂SO₄ gel electrolyte.

3.8. Series and parallel connected SSD for supercapacitor application

The symmetric SSDs, fabricated with DM-800 as the electrode and PMMA-Na₂SO₄ as the gel electrolyte, are connected in series and parallel to evaluate their performance. In particular, the series and parallel connections of the device can increase the operating voltage window and specific capacitance, respectively. For example, the two devices in series connection (SSD-2S, Fig. S15a) exhibit a maximum working potential window of 3.5 V, which is higher than that of a single device. The CVs (Fig. S15b) and GCDs (Fig. S15c) of SSD-2S are used to measure the C_s as shown in Fig. S15d and e, respectively. The GCD measurement with the device SSD-2S exhibits C_s of 27 and 11 F g⁻¹ at 8 and 15 A g⁻¹, respectively, with an ED of 18.25 Wh kg⁻¹ at a PD of 26.25 kW kg⁻¹ (Fig. S15f). Furthermore, the R_s and R_ct of the fabricated device are measured to be 5.3 and 1.8 Ω, respectively, using the Nyquist plot (Fig. S15g). Similarly, the three devices connected in series are denoted as SSD-3S, which exhibits a working potential window of 6.0 V, as shown in the CVs at varied scan rates ranging from 10 to 100 mV s⁻¹ (Fig. S16a) and GCDs at varied current densities (Fig. S16b). The C_s values are calculated to be 77 and 43 F g⁻¹ at a scan rate of 10 mV s⁻¹ and a current density of 8 A g⁻¹, from the CV and GCD plots, respectively. SSD-3S achieves a maximum PD of 45 kW kg⁻¹ at a current density of 15 A g⁻¹ (Fig. S16c). Moreover, the device exhibits excellent rate capability and coulombic efficiency even after 10 000 continuous GCD cycles (Fig. S16d), with a low R_s (9.06 Ω) and R_ct (2.5 Ω) (Fig. S16e). To obtain a detailed analysis of SSD-3S, the device was charged at different current densities (8–40 A g⁻¹) and discharged at a lower and fixed current density of 0.25 A g⁻¹ (Fig. S16f). The device delivers a maximum ED of 158 Wh kg⁻¹ at a charging rate of 40 A g⁻¹ and a discharging rate of 0.25 A g⁻¹ (Fig. S16g). Additionally, a 2 V motor-driven fan functioned continuously for approximately 8 minutes after charging for 60 s at 2 V (SI video), underscoring its applicability in practical energy storage and power supply scenarios.

Furthermore, three SSDs were connected in parallel (Fig. 8a, SSD-3P) to investigate their electrochemical properties. The CV profiles (Fig. 8b) at varying scan rates indicate that the parallel connection of multiple devices results in a larger enclosed area, indicating an increase in total capacitance. At scan rates of 10 and 100 mV s⁻¹, the SSD-3P delivers C_s of 323 and 112 F g⁻¹, respectively (Fig. 8b and S17a). The C_s value of SSD-3P is almost 3 times that of a single SSD. Similarly, the GCD plots obtained at a current density of 2 A g⁻¹ indicate that the C_s capacitance (285 F g⁻¹) nearly triples for the three parallel-connected devices (Fig. 8c). At a high current density of 15 A g⁻¹, the device SSD-3P exhibits a C_s value of 80 F g⁻¹ (Fig. 8c and S17b). The fabricated device (SSD-3P) shows a maximum ED of 158 Wh kg⁻¹ at a current density of 2 A g⁻¹ and a PD of 16 kW kg⁻¹ at a current density of 15 A g⁻¹ (Fig. 8d). To determine the chemical stability of the fabricated device, i.e., SSD-3P, the device was tested for 10 000 continuous GCD cycles (Fig. 8e). The fabricated SSD-3P retains 94% capacitance after 10 000 continuous GCD cycles. Moreover, the coulombic efficiency remains excellent even after 10 000 continuous charge–discharge cycles (Fig. 8e). The charge transfer resistance of the parallel-connected devices (SSD-3P) was measured from the Nyquist plot. SSD-3P demonstrates series and charge transfer resistances of 4.6 and 0.35 Ω, respectively (Fig. 8f), which are lower than those of the series-connected device, i.e., SSD-3S. Moreover, the SSD-3P device exhibits R_s and R_ct values of 5.6 Ω and 1.1 Ω, respectively, after 10 000 GCD cycles (Fig. 8f). The device was further tested by performing measurements at different current densities, ranging from 6 to 25 A g⁻¹, with a fixed discharge rate of 0.5 A g⁻¹ (Fig. 8g). The device exhibits C_s values of 432 and 159 F g⁻¹ at current densities of 6 and 25 A g⁻¹, respectively, with a fixed discharge rate of 0.5 A g⁻¹ (Fig. 8g and S17c). The ED is measured to be 240 Wh kg⁻¹ at a charged current density of 6 A g⁻¹ and a discharge current density of 0.5 A g⁻¹ (Fig. S17d). The fabricated SSD-3P device demonstrated its practical use by effectively powering multiple electronic components, including LEDs and a motor fan. Interestingly, 38 red LEDs stayed lit for as long as 4 min (Fig. 8h), underscoring their applicability in practical energy storage and power supply scenarios.

Fig. 8 (a) Schematic representation of three SSD devices connected in a parallel manner, (b) CVs at different scan rates, (c) GCD curves at different current densities, (d) Ragone plot, (e) durability test, (f) EIS spectra and the inset shows an equivalent circuit, (g) GCD curves at different charging rates (A g⁻¹) with a fixed discharge rate of 0.5 A g⁻¹, (h) practical demonstration of the fabricated SSD-3P device to light LEDs.

4. Conclusions

This study focused on synthesizing a valuable material from the outer layer of a DM, a waste material. The outer polypropylene layer of the DM decomposes through direct chemical activation. The preactivated precursor is pyrolyzed at different temperatures in an argon environment to produce a heteroatom-doped carbon material. The product obtained at 800 °C (DM-800) with a thin sheet-like morphology exhibits the highest electrochemical properties for energy storage. DM-800 delivers C_s values of 282 and 340 F g⁻¹ at a current density of 1 A g⁻¹ in acidic and neutral electrolytes, respectively. Subsequent evaluation in a symmetric solid-state two-electrode device using a PMMA-Na₂SO₄ gel electrolyte revealed sustained superior performance, with ED and PD of 68.61 Wh kg⁻¹ and 15 kW kg⁻¹, respectively, at a current density of 15 A g⁻¹. Moreover, the fabricated SSDs are tested in both series and parallel connections. The series connection exhibits a larger potential window, and the parallel connection offers a higher capacitance. The fabricated device exhibits outstanding durability, with excellent C_s retention up to 10 000 continuous GCD cycles. This study presents a straightforward, template-free approach for converting medical waste into a high-performance carbon electrode, offering a sustainable and innovative method for transforming biowaste into effective materials for energy storage applications.

Author contributions

S. R. and S. D. contributed equally to the work, and D. P. supervised the project. S. D. performed the initial experiments. S. R. repeated the synthesis and performed the detailed supercapacitor measurement. S. R. and D. P. wrote the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: CVs of the synthesized porous carbon materials and a bare graphite sheet; EDX spectra; N₂ adsorption–desorption isotherms; survey XPS spectrum; GCD curves; EIS spectra; FESEM images, elemental mappings, survey and high-resolution XPS spectra of DM-800 after galvanic charging in neutral and acidic electrolyte; schematic representation of the fabricated SSD; FESEM images and elemental mapping of DM-800 after cycle stability; schematic representation fabricated SSDs in series connection and their electrochemical applications; CV curves of the three fabricated SSDs in series (SSD-3S) and parallel (SSD-3P) connection, and their electrochemical applications; Comparison table of energy storage performance of recently reported porous carbon materials in the symmetric devices. See DOI: https://doi.org/10.1039/d5ta10382g.

Acknowledgements

S. R. acknowledges the CSIR, India, for a research associate fellowship (Award No. 324-1409-10770/2K23/1). This work was supported by DST, Govt of India, through the grant DST/TMD/MES/2K18/26(G). The DST-FIST facility at the Materials Science Centre and the DST-SATHI facility at IIT Kharagpur are acknowledged for FESEM and HRTEM, respectively.

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Footnote

† Equally contributed.

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