Green Synthesis of Biomass-Derived Activated Carbon from Wood Apple Shell for Enhanced Electrochemical Energy Storage

Abstract

In alignment with the recent sustainable development goals aimed at improving human life on Earth and minimizing carbon footprints, the utilization of natural and renewable resources for energy production presents a sustainable and forward-thinking approach to future energy needs. In this context, we report a biomass-waste-derived synthesis route for developing energy storage materials that support sustainable energy solutions. Specifically, wood apple shell (WAS), an underutilized agro-waste material, was employed as a sustainable carbon precursor for the fabrication of activated carbon electrodes suitable for electrochemical double-layer capacitors. Through chemical activation using sulfuric acid, a highly porous carbon structure was obtained. XRD analysis revealed broad (002) diffraction peaks, characteristic of an amorphous carbon framework. BET surface area analysis confirmed a high specific surface area of 590.65 m²/g, which enhances ion transport and accessibility within the electrode material. X-ray photoelectron spectroscopy and FT-IR analysis further indicated the presence of abundant oxygen-containing surface functional groups (such as CO and C–O), which are known to improve electrolyte wettability and facilitate efficient charge storage. Electrochemical performance was evaluated in KOH electrolytes with concentrations of 2, 4, and 6 M. Among these, the electrode demonstrated optimal performance in 4 M KOH, delivering a high energy density of 29 Wh/kg and a power density of 5512 W/kg. Furthermore, the electrode showed excellent cycling stability, retaining more than 96.8% of its initial capacitance after 10,000 charge–discharge cycles. To demonstrate the practical applicability of the fabricated device, we successfully powered a green LED, showcasing its real-world energy storage potential. These findings underscore the promise of WAS-derived activated carbon as a low-cost, eco-friendly electrode material for low-power electronic applications and next-generation sustainable energy storage technologies.

1. Introduction

Meeting the growing global demand for energy while preserving the Earth’s ecosystems is one of the major challenges of our time. Present-day global concerns include resource conservation, efficient waste management, environmental sustainability, the transition to renewable energy systems, and the reduction of greenhouse gas emissions. − The International Energy Agency predicts that by 2025, over one-third of the world’s electricity will be generated from renewable sources. Technologies such as artificial intelligence and developments in the carbon sector are expected to support this growth by enabling the formation of local and robust renewable energy supply chains. Despite these advancements, renewable energy sources are often limited by their dependence on environmental conditions, making energy storage systems essential for ensuring a reliable and continuous power supply.

Electrochemical energy storage technologies play a central role in addressing these challenges. Among them, electrochemical capacitors, commonly known as supercapacitors, have emerged as promising energy storage devices. They are valued for their fast charge–discharge capability, long cycle life, high power density, and low environmental impact. − These systems are widely used in fields such as electric transportation, consumer electronics, backup power, and grid stabilization. Supercapacitors are classified based on their charge storage mechanisms into two main types: electrochemical double-layer capacitors (EDLCs), which store energy through electrostatic ion adsorption at the electrode–electrolyte interface, and pseudocapacitors, which store energy via reversible faradic redox reactions involving electroactive species. ,

The theory of the electric double layer was first introduced by Helmholtz in 1853, laying the groundwork for understanding energy storage at electrode interfaces. EDLCs are particularly attractive due to their excellent cycle stability, safety, and immediate power delivery. The performance of EDLCs depends strongly on the properties of the electrode material, including its surface area, pore size distribution, electrical conductivity, and compatibility with electrolytes. , Activated carbon is widely regarded as the most suitable material for EDLC electrodes due to its high specific surface area, tunable pore structure, chemical stability, and electrical conductivity. ,

To support sustainability goals, researchers are increasingly turning to low-cost and renewable biomass resources to produce activated carbon. This approach also addresses the global issue of agro-waste accumulation. Biomass such as coconut shells, banana peels, lignin, cellulose, and marine algae are commonly used to produce carbon-rich materials suitable for electrochemical applications. Activation of these precursors can be achieved by physical or chemical means. Physical activation involves high-temperature treatment with agents such as steam or carbon dioxide, whereas chemical activation uses compounds like potassium hydroxide, phosphoric acid, sulfuric acid, and zinc chloride at lower temperatures to enhance porosity and functional group formation. ,

In this study, wood apple shell (WAS) is explored as a new and underutilized biomass precursor for the preparation of activated carbon. Wood apple, scientifically known as Limonia acidizsima, is an indigenous fruit widely found in arid and semiarid regions of India and Southeast Asia. − The fruit is traditionally valued for its nutritional and medicinal properties, yet its hard shell is often discarded as waste. The shell contains abundant lignocellulosic content and phenolic compounds, making it a potential candidate for carbon material synthesis. − Despite these favorable characteristics, WAS remains largely unexamined for electrochemical energy storage applications.

The novelty of this work lies in the selection and valorization of WASs for the first time as carbon precursors for EDLC electrodes. Concentrated sulfuric acid is used as an effective activating agent to promote dehydration, remove impurities, enhance surface functionality, and improve porosity. The resulting activated carbon exhibits a highly porous structure, improved surface area, and functional groups that contribute to the enhanced electrochemical performance. This study demonstrates a new route for converting agricultural waste into valuable energy storage materials, aligning with global sustainability and circular economy goals. It also expands the range of biomass-derived carbon sources for supercapacitor applications, offering a low-cost and eco-friendly alternative to traditional materials.

2. Experimental Section

The stepwise synthesis procedure for wood-apple-shell-derived activated carbon is schematically illustrated in Scheme . The material was prepared through a sequence of pretreatment, acid activation, and thermal carbonization steps, as described below.

1. Schematic Illustration of Activated Carbon Synthesis from WAS via Pretreatment, H₂SO₄ Impregnation, and Carbonization.

2.1. Synthesis of Activated Carbon

The initial phase of this study involved the synthesis of activated carbon derived from WASs. The shell of the fruit contains a significant quantity of lignocellulosic biomass, which can be transformed to activated carbon through a chemical activation process. Initially, the shell was utilized as the precursor and subjected to multiple washings with deionized water to remove the surface impurities. Subsequently, the shell was exposed to sunlight for 3 days, and the chopped pieces were dried in an oven at 80 °C for a duration of 12 h. Following the drying process, the shells were crushed and ground to achieve the desired particle size using a domestic mixer. The resulting pulverized powder was collected by being sieved through a mesh.

The activation of the sample was performed by using a chemical activation method with concentrated sulfuric acid as the activating agent. In this process, 48 g of WAS powder was immersed in 46 g (25 mL) of the activating agent for 48 h at room temperature. After the activation period, the sample underwent carbonization in a muffle furnace at a temperature of 400 °C for 2 h. The furnace was subsequently turned off and allowed to cool naturally after the activation phase. Finally, the resulting black powder was collected, ground using a mortar and pestle, and stored in an airtight container for further characterization. In this study, the production yield was found to be 58.33%, indicating a high conversion efficiency of the WAS precursor into carbon material [detailed calculation in the Supporting Information]. This relatively high yield confirms the suitability of the chosen activation and carbonization conditions for effective carbon recovery from the lignocellulosic biomass.

The process of treating WAS powder with concentrated sulfuric acid encompasses a variety of chemical interactions aimed primarily at eliminating impurities, introducing functional groups, and increasing the surface area. This activating agent functions as a dehydrating agent, facilitating the removal of water molecules from the organic constituents. In this context, the lignocellulosic structure present in biomass is transformed to elemental carbon. Concurrently, sulfuric acid introduces sulfonic acid (−SO₃H) and other oxygen-containing functional groups onto the carbon surface, thereby enhancing the material’s hydrophilicity and catalytic properties.

$${\mathrm{C}}_6{\mathrm{H}}_{10}{\mathrm{O}}_5+{\mathrm{H}}_2{\mathrm{SO}}_4\to 6\mathrm{C}+5{\mathrm{H}}_2\mathrm{O}$$ 1

$$\mathrm{C}+{\mathrm{H}}_2{\mathrm{SO}}_4\to \mathrm{C}-{\mathrm{SO}}_3\mathrm{H}+{\mathrm{H}}_2\mathrm{O}$$ 2

Unlike many previous reports that rely on aggressive acid–base combinations or high-temperature refluxing with corrosive agents, our method employs a simplified, room-temperature immersion process that minimizes chemical waste, avoids hazardous neutralization steps, and enables safe, scalable synthesis. Despite the use of a concentrated acid, the reaction conditions are mild and well-controlled, with no additional catalysts or oxidants involved. The resulting carbon demonstrated excellent porosity and electrochemical performance, establishing this approach as a more efficient and environmentally conscious alternative to traditional acid-based activation protocols.

3. Results and Discussion

The X-ray diffraction profile of activated carbon, illustrated in Figure a, displays a broad peak at 2θ = 25.46°, which corresponds to a d-spacing of 3.498 Å, indicative of the (002) plane of amorphous carbon. This finding provides evidence of a turbostratic graphitic structure. Furthermore, a less intense peak at 2θ = 43.4° corresponds to the (100) plane, implying a limited degree of graphitization. The d-spacing values, as referenced in the JCPDS card, typically range from 3.34 to 3.375 Å. The minor variation in the d-spacing observed in this investigation, in comparison to the JCPDS values, can be ascribed to the acid treatment, which introduces functional groups such as −COOH and −OH, leading to steric effects that cause a slight expansion of interlayer spacings. The activated carbon produced via concentrated sulfuric acid treatment is primarily amorphous, demonstrating minimal graphitic crystallinity due to the lack of high-temperature graphitization. The prominent and broad peak in the low-angle region (25.46°) signifies a substantial presence of micropores within the sample. The average crystallite size, determined using the Scherrer equation, is calculated to be 3.1 nm, further corroborating the existence of micropores that enhance the surface area.

1.

(a) X-ray diffraction pattern of activated carbon derived from WAS. (b) FT-IR spectrum of WAS-derived activated carbon.

The WAS comprises structural biopolymers such as cellulose, hemicellulose, lignin, and pectin, along with secondary metabolites including psoralene, xanthotoxin, 2,6-dimethylbenzoquinone, osthenol, and various amino acids. FT-IR spectroscopy (Figure b, Table S1) was employed to identify the functional groups present in WAS powder. A broad, intense band at 3425 cm^–1 corresponds to O–H stretching vibrations from hydroxyl groups in cellulose, lignin, phenols, and carboxylic acids. Peaks at 2922 and 2852 cm^–1 are attributed to asymmetric and symmetric −CH stretching of aliphatic methylene chains. A prominent absorption in the range of 1635–1642 cm^–1 is assigned to the CO stretching of conjugated ketones and quinones, along with CC stretching in aromatic rings. The shift of this band to lower wavenumbers (red shift) from the typical ∼1700 cm^–1 range indicates π-conjugation with adjacent CC systems, which reduces the bond order and hence the vibrational frequency of the CO group. The band at 1417 cm^–1 confirms the presence of organic sulfate groups, while the peak at 1382 cm^–1 corresponds to aliphatic SO stretching in the sp²-hybridized carbon frameworks. Weak bands at 1116, 1032, and 450 cm^–1 arise from overlapping C–O and SO stretching vibrations, indicating sulfates, sulfoxides, and thiols. A moderately intense band at 780 cm^–1 is attributed to out-of-plane C–H bending vibrations. These spectral features confirm the successful incorporation of oxygenated and sulfur-containing functional groupshydroxyl, carbonyl, and sulfatethrough sulfuric acid activation, thereby enhancing the surface chemistry of the material for electrochemical energy storage applications.

The FESEM images in Figure a,b illustrate the surface morphology of WAS-derived carbon subjected to thermal pretreatment at 400 °C. The heat treatment promotes the decomposition of volatile lignocellulosic componentsprimarily cellulose, hemicellulose, and ligninas also reported by Njewa et al. This thermal decomposition results in the formation of a heterogeneous and porous carbon texture. A high density of micro- and mesopores is observed, contributing to enhanced ion accessibility and rapid charge accumulationboth critical for EDLC performance.

2.

(a and b) FESEM micrographs of WAS-derived activated carbon at different magnifications; TEM analysis of WAS-derived activated carbon: (c) 100 and (d) 200 nm images, corresponding (e) high-resolution TEM and (f) SAED pattern, (g) EDAX spectrum of WAS-derived activated carbon, and (h) EDAX analysis of TEM-analyzed WAS-derived activated carbon.

Elemental analysis via EDAX (Figure g) confirms the predominance of carbon (C), along with oxygen (O) and trace levels of sulfur (S). The presence of oxygenated functional groups improves the electrode’s surface wettability, aiding electrolyte diffusion. Sulfur, introduced via the activation process, may enhance electrical conductivity and contribute to EDLC behavior.

Transmission electron microscopy (TEM) images (Figure c–f) provide additional insights into the internal structure, confirming the presence of well-distributed micro- and mesopores formed through acid etching. These features significantly enhance the surface area and ion transport properties. Regions of lighter contrast in the TEM micrographs suggest areas enriched with oxygen-containing functionalities, which promote electrolyte affinity and ion diffusion kinetics.

The selected area electron diffraction (SAED) pattern (Figure f) reveals diffuse rings corresponding to the (002) and (100) planes, indicative of a partially disordered graphitic structure. This amorphous character is typical of activated carbons and is favorable for ion diffusion pathways.

Energy-dispersive X-ray analysis (Figure h) further validates the elemental composition, detecting carbon, oxygen, sulfur, and calcium (Ca). The sulfur signal, particularly between 2.0 and 2.5 keV, is associated with the activating agent. Sulfur incorporation may intercalate between graphene layers, contributing to structural disorder. The interlayer spacing (d-spacing) measured from high-resolution TEM (Figure e) is 0.3512 nm, slightly greater than that of crystalline graphite (0.335 nm, JCPDS No. 75-1621). This expanded spacing may facilitate ion intercalation and improve electrolyte diffusion, enhancing the electrochemical performance of the carbon material.

X-ray photoelectron spectroscopy (XPS) was employed to investigate the surface elemental composition, chemical states, and electronic environment of activated WAS-derived carbon. The wide-scan XPS spectrum (Figure a) confirms the presence of carbon (74.76%), oxygen (18.00%), nitrogen (3.85%), and sulfur (3.39%), indicating successful heteroatom incorporation during the activation process. The high-resolution C 1s spectrum (Figure b) is deconvoluted into two distinct peaks: the main peak at 285.05 eV corresponds to C–C and C–H bonds in sp³-hybridized carbon, typically attributed to adventitious hydrocarbons or residual solvents. A secondary peak at 287.28 eV is assigned to O–CO bonds, consistent with carboxyl or ester groups introduced during chemical treatment. The N 1s spectrum exhibits a peak centered at 399.68 eV, which is characteristic of nitrogen species such as pyridinic, pyrrolic, or amino groups. The presence of nitrogen can be attributed to trace organic nitrogen-containing compounds (e.g., amino acids) that are inherently present in the WAS precursor. These nitrogen functionalities are known to enhance the electronic conductivity and introduce EDLC behavior, thereby improving the overall electrochemical performance. The O 1s spectrum (Figure c) shows peaks at 531.44 and 533.40 eV, corresponding to carbonyl (CO), carboxylic (COOH), and ether (C–O–C) oxygen species. These groups originate from the precursor oxygen content, oxidative activation (e.g., H₂SO₄ treatment), and air exposure. They participate in faradic reactions such as

$$>\mathrm{COOH}\leftrightarrow \mathrm{COO}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$ 3

$$>\mathrm{C}=\mathrm{O}+{\mathrm{e}}^{-}\leftrightarrow >\mathrm{C}-{\mathrm{O}}^{-}$$ 4

3.

XPS spectra of WAS-derived activated carbon: (a) survey scan, (b) deconvoluted C 1s peak, and (c) deconvoluted O 1s peak. BET surface area analysis of WAS-derived activated carbon: (d) N₂ adsorption–desorption isotherm.

The S 2p spectrum further confirms the presence of thiophenic sulfur species (C–S–C), formed via sulfonation reactions between sulfuric acid and the carbon matrix. A peak at 165.2 eV is attributed to organic sulfur, particularly sulfoxide (R–SO–R′) groups. These sulfur functionalities not only increase electrical conductivity but also introduce surface polarity, promoting electrolyte wettability and improving ion transport. Collectively, the copresence of O, N, and S heteroatoms of the atoms enhances the physicochemical and electrochemical properties of the material. These surface functionalities significantly improve double-layer charge storage via increased wettability, electrolyte penetration, and ion-accessible active sites, thereby optimizing the overall capacitive performance of the WAS-derived carbon electrode.

The textural characteristics of the activated WAS carbon were evaluated using N₂ adsorption–desorption isotherms at 77 K, following IUPAC guidelines. As shown in Figure d, the isotherm corresponds to a type IV curve with a pronounced H2-type hysteresis loop, which is characteristic of mesoporous materials with ink-bottle-shaped pores. This observation confirms the coexistence of both micro- and mesopores within the carbon matrix.

The Brunauer–Emmett–Teller (BET) surface area was calculated to be 590 m²/g, and the total pore volume was 0.623 cm³/g. Although this surface area is relatively lower than that of some conventionally activated carbons (typically exceeding 1000 m²/g), it is consistent with prior reports on sulfuric acid-activated biomass carbons, such as the findings of M. Sivachidambaram et al. The moderate surface area may be attributed to the low carbonization temperature (400 °C) and the use of concentrated H₂SO₄, which, while facilitating functionalization and pore development, may not induce the extensive microporosity achieved by alkali activation at higher temperatures. Additionally, the introduction of oxygen- and sulfur-containing functional groups may lead to partial pore blockage or structural collapse, thereby limiting the accessible surface area.

Despite the moderate surface area, the mesoporous structure, as evidenced by the Barrett–Joyner–Halenda pore size distribution in the inset of Figure d, shows a dominant pore size range between 30 and 60 nm. This mesoporosity is beneficial for enhancing ion transport and electrolyte accessibility, contributing to the improved electrochemical performance in EDLCs.

4. Electrochemical Analysis

Figure a–c presents the cyclic voltammograms (CVs) of activated carbon derived from WAS, recorded in a potential window of −1.0 to 0.1 V at scan rates ranging from 5 to 100 mV s^–1, using aqueous KOH electrolytes of varying concentrations (2, 4, and 6 M). All CV curves exhibit a quasi-rectangular profile, a hallmark of EDLC behavior, indicative of fast, reversible charge–discharge processes and efficient ion transport within the porous carbon framework.

4.

CVs of WAS-derived activated carbon at various scan rates for (a) 2 M, (b) 4 M, and (c) 6 M KOH electrolytes. (d) Comparison of specific capacitance.

The influence of KOH concentration on capacitive performance is evident from the increased current response with increasing molarity, attributed to enhanced ionic conductivity and improved electrolyte–electrode interactions. Notably, the CV profile at 6 M KOH maintains its rectangular shape across all scan rates, suggesting stable EDLC behavior and efficient ion accessibility even under high ionic strength conditions. The gravimetric capacitance, which indicates the capacitance per unit mass of the material, was calculated using the following formula:

$$C_{\mathrm{g}}=\frac{A}{mk\mathrm{\Delta }V}\mathrm{F}/\mathrm{g}$$ 5

where C _g (F/g), A (cm²), m (g), k (mV/s), and ΔV (V) are the gravimetric capacitance, area of the curve, mass of the active material, scan rate, and potential window, respectively.

Furthermore, the volumetric capacitance was determined to evaluate the capacitance per unit volume of the electrode, an essential factor for applications with limited space, such as micro energy storage devices. The volumetric capacitance (C _v) is calculated based on the material’s density and its gravimetric capacitance using the following formula:

$$C_{\mathrm{v}}=\rho C_{\mathrm{s}}\mathrm{F}/{\mathrm{cm}}^3$$ 6

where ρ is the electrode density (in g/cm³).

At a scan rate of 5 mV s^–1, the specific capacitance values obtained were 396, 383, and 417 F·g^–1 for 2, 4, and 6 M KOH, respectively. The corresponding volumetric capacitances were 537, 519, and 565 F·cm^–3, respectively. This nonlinear trend reflects a complex interplay among ion concentration, mobility, and accessibility within the porous matrix. The slight drop in capacitance at 4 M may be due to ion crowding or limited mobility, while the increase at 6 M can be attributed to improved electrolyte conductivity and enhanced wettability, facilitating more efficient electric double-layer formation, especially in meso- and microporous domains.

At low scan rates (e.g., 5 mV s^–1), ions have sufficient time to diffuse deeply into the porous structure, leading to full utilization of the internal surface area and minimal distortion of the CV shape due to the reduced internal resistance (IR) drop. As the scan rate increases, the capacitance gradually declines due to limited ion penetration and sluggish transport within narrower pores. Despite this, the WAS-derived electrode retains a near-rectangular CV shape even at 100 mV s^–1, demonstrating excellent rate capability and a fast charge–discharge response, which are desirable for high-power supercapacitor applications.

As illustrated in Table and Figure d, both gravimetric and volumetric capacitances exhibit decreasing trends with increasing scan rate. In 6 M KOH, the gravimetric capacitance dropped from 417 to 120 F·g^–1 and the volumetric capacitance dropped from 565 to 162 F·cm^–3 as the scan rate increased from 5 to 100 mV s^–1. This is consistent with reduced ion accessibility and limited diffusion into micropores at higher scan rates.

1. CV Analyses Mediated Gravimetric (C _g) and Volumetric (C _v) Capacitance of the WAS-Derived Activated Carbon Electrode at Different Scan Rates and KOH Concentrations.

	C g (F/g)			C v (F/cm3)
	KOH concentration
scan rate (mV/s)	2 M	4 M	6 M	2 M	4 M	6 M
5	396	383	417	537	519	565
10	334	341	358	453	467	485
20	266	277	280	361	376	380
30	232	235	229	309	318	310
40	228	205	195	315	277	265
50	186	182	173	252	247	235
60	173	166	157	234	224	212
70	162	151	145	219	205	197
80	152	139	135	207	189	183
90	145	130	127	196	176	172
100	138	123	120	186	167	162

The electrochemical performance of activated carbon derived from WAS was further investigated through galvanostatic charge–discharge (GCD) analysis in a potential window of −1.0 to 0.1 V at current densities ranging from 2 to 10 A g^–1, using 2, 4, and 6 M KOH electrolytes (Figure a–d). All GCD curves display highly symmetric, near-isosceles triangular shapes, signifying excellent electrochemical reversibility, capacitive behavior, and low IR. The consistent shape across different current densities highlights the stability and rapid charge propagation of the WAS-based electrode material.

5.

(a–c) GCD profiles of WAS-derived activated carbon at different current densities for (a) 2 M, (b) 4 M, and (c) 6 M KOH. (d) Current density versus specific capacitance plot; and (e) Ragone plots.

As expected, increasing the current density leads to a reduction in the discharge time due to faster charge extraction. Nonetheless, the nearly unaltered triangular shape at higher current densities confirms efficient ion transport within the porous carbon framework, which is a critical parameter for high-power supercapacitor applications. The GCD-mediated capacitance was calculated using the following formula:

$$C_{\mathrm{s}}=\frac{I\mathrm{\Delta }t}{m\mathrm{\Delta }V}\mathrm{F}/\mathrm{g}$$ 7

where C _s (F/g), I (A), Δt (s), m (g), and ΔV (V) are specific capacitance, discharge current, discharge time, mass of the active electrode material, and potential window during discharge, excluding the IR drop if significant, respectively. At 2 A g^–1, the specific capacitances were 209 F·g^–1 (2 M KOH), 173 F·g^–1 (4 M), and 235 F·g^–1 (6 M), confirming that the electrolyte concentration significantly affects charge storage behavior. The lower capacitance at 4 M may result from ion crowding or partial pore blocking, while the superior performance at 6 M is attributed to enhanced ionic conductivity and improved electrode–electrolyte interactions. These observations align closely with CV results, reinforcing the capacitive nature of the material.

The rate capability was evaluated across current densities of 2–10 A g^–1. All electrolytes showed a decreasing trend in specific capacitance with increasing current density, a behavior typical of diffusion-limited systems. In 6 M KOH, the C _s decreased from 235 to 100 F·g^–1, while in 4 and 2 M KOH, the values declined from 173 to 82 F·g^–1 and 209 to 91 F·g^–1, respectively. This decline is due to the insufficient time for electrolyte ions to penetrate deep into micropores at high current rates, limiting surface utilization. Nevertheless, 6 M KOH consistently provided the highest capacitance across all current densities, confirming its superior electrochemical environment.

To assess the energy storage characteristics, Ragone plots were constructed (Figure e), illustrating the trade-off between the energy density (E, Wh kg^–1) and power density (P, W·kg^–1). These values were calculated based on galvanostatic discharge curves using established equations reported in the literature. At 2 A g^–1, the energy densities were 40 Wh·kg^–1 (6 M), 35 Wh·kg^–1 (2 M), and 29 Wh·kg^–1 (4 M). The higher energy density in 6 M KOH reflects improved ion accessibility and double-layer formation in meso- and microporous domains. As the current density increased, the energy density declined due to the shortened discharge time and limited ion diffusion. Conversely, the power density increased, with all systems achieving a peak of ∼5500 W·kg^–1 at 10 A g^–1, illustrating the materials’ ability to support high-rate operations. The values above are calculated using the respective formulas and are displayed in Table .

2. GCD Analyses Mediated Parameters of the WAS-Derived Activated Carbon Electrode at Different Current Densities and KOH Concentrations.

	C s (F/g)			energy density (Wh/kg)			power density (W/kg)
	KOH concentration
current density (A/g)	2 M	4 M	6 M	2 M	4 M	6 M	2 M	4 M	6 M
2	209	173	235	35	29	40	1099	1102	1102
3	175	153	205	29	26	35	1654	1652	1654
4	156	145	182	26	24	31	2195	2193	2202
5	155	118	168	26	19	28	2758	2745	2747
6	120	120	153	20	20	26	3300	3300	3305
7	115	108	140	19	18	24	3865	3843	3850
8	102	95	124	17	16	21	4408	4421	4412
9	90	90	115	15	15	19	4950	4950	4967
10	91	82	100	15	14	17	5506	5512	5500

These results emphasize the typical energy–power trade-off in supercapacitors. The 6 M KOH system delivers high energy at low currents but shows a declining performance at higher rates. The 2 M system provides moderate energy and good power delivery, while 4 M KOH offers a balanced performance, making it suitable for applications requiring both energy storage and a fast response. The previously reported specific capacitance values of various activated carbon for supercapacitor electrode materials are displayed in Table .

3. Specific Capacitance of Different Activated Carbon Materials in Various Electrolytes: Comparison of the Present Study with Reported Values.

activated carbon	electrolyte	scan rate/current density	specific capacitance (F/g)	references
areca nutshell	0.1 M Na2SO4	5 mV/s	64
poplar wood	2 M KOH	5 mV/s	234
cow dung	6 M KOH	5 mV/s	347
UF resins	6 M KOH	1 A/g	224
activated carbon AOW	6 M KOH	10 A/g	92
rice husk	6 M KOH	2 A/g	233
sunflower stalk	6 M KOH	0.5 A/g	263
pomelo peels	2 M KOH	1 A/g	356
human hair	6 M KOH	0.5 A/g	445
Mangiferaindica leaves	6 M KOH	0.5 A/g	521.65
wood apple shell	2 M KOH	5 mV/s	396	present study
	4 M KOH	5 mV/s	383
	6 M KOH	5 mV/s	417
	2 M KOH	2 A/g	209
	4 M KOH	2 A/g	173
	6 M KOH	10 A/g	100

Electrochemical impedance spectroscopy was conducted to evaluate the charge-transfer behavior and ion diffusion kinetics of WAS-derived carbon electrodes in 2, 4, and 6 M KOH electrolytes. The corresponding Nyquist plots are shown in Figure a–c, and the extracted impedance values are listed in Table .

6.

(a–c) Nyquist plots, (d and e) Bode plots, (f) capacitance retention over 10,000 charge–discharge cycles at 2, 4, and 6 M, and (g) illumination of a green LED using WAS-derived activated carbon as the electrode material.

4. EIS-Mediated Parameters of the WAS-Derived Activated Carbon Electrode at Different KOH Concentrations.

WAS sample	series resistance R s (Ω)	charge-transfer resistance R ct (Ω)	Warburg impedance (Ω)	time constant (τ0) (× 10–3 s)	C dl (μF)	τr (s)
2 M	8.27	42.96	26.1	6.1	14.19	1.21
4 M	10.14	58.06	38.32	4.15	71.76	1.46
6 M	20.54	29.24	33.8	4.7	16.07	1

All spectra exhibit a semicircle in the high-frequency region, associated with charge-transfer resistance (R _ct), followed by a linear Warburg-type tail in the low-frequency region, indicating diffusive ion transport and capacitive behavior, characteristic of EDLCs.

Among the three systems, the 4 M KOH electrode displays the largest semicircle, corresponding to the highest R _ct value of 58.06 Ω, suggesting a slower interfacial electron transfer. The 2 M KOH electrode, despite exhibiting a smaller arc, shows a higher R _ct of 42 Ω than the 6 M sample. Interestingly, the 6 M KOH electrode demonstrates a lower R _ct value of 29.24 Ω, indicating more efficient charge transfer despite a higher electrolyte viscosity. This apparent anomaly may be attributed to differences in electrode surface wetting, double-layer structure, or model fitting accuracy.

The transition into the Warburg region is smoother in 6 M KOH, reflecting a relatively unhindered ion transport. These findings underscore that while viscosity increases with KOH concentration, the associated rise in ionic strength and enhanced wettability can offset diffusion resistance.

The frequency-dependent capacitance and phase angle responses are presented in Figure d,e, respectively. At low frequencies (0.01–1 Hz), all electrodes show high capacitance values, consistent with the EDLC behavior, where ions have sufficient time to form a well-developed electric double layer within micropores and mesopores.

Notably, the 2 M KOH electrode exhibits the most negative capacitance (∼−0.025 F), possibly due to minimal ion crowding and superior pore accessibility. The 4 M KOH system offers a more uniform capacitance profile across frequencies, suggesting an optimal balance between ionic conductivity and double-layer formation. In contrast, the 6 M KOH electrode shows lower overall capacitance, likely due to viscosity-induced ion aggregation, which hinders full pore utilization.

Phase angle analysis further confirmed the capacitive nature of the electrodes. An ideal EDLC exhibits a phase angle near −90°, while real porous systems often range from −40° to −60°. The 4 M KOH sample reaches a maximum phase angle of −48°, closest to the ideal EDLC behavior. The 2 and 6 M KOH systems follow with phase angles of −39° and −44°, respectively, the former suggesting stronger diffusion limitations.

It is noteworthy that, although the 4 M KOH system exhibited a comparatively lower gravimetric capacitance (383 F/g at 5 mV s^–1) than both 2 and 6 M KOH in the CV analysis, it was identified as electrochemically optimal based on impedance characteristics. This apparent discrepancy can be explained by the complementary yet distinct nature of CV and EIS techniques. While CV primarily evaluates charge storage capability under dynamic potential sweeps, EIS probes charge-transfer kinetics, electrolyte resistance, and ion diffusion efficiency at the electrode–electrolyte interface. In the case of 4 M KOH, the moderate ionic concentration likely offered an ideal balance between ion availability and mobility. Unlike the 2 M system, which may have limited ion density, or the 6 M system, which suffers from increased viscosity and potential ion crowding, the 4 M KOH electrolyte provided a stable double-layer structure, low Warburg impedance, and near-ideal capacitive phase angle (−48°), as evident in the Bode plot. This reflects enhanced wetting, uniform ion transport, and efficient double-layer formation, which collectively result in optimized capacitive response over a broad frequency range, despite the modest specific capacitance measured via CV. Therefore, 4 M KOH emerges as a balanced and electrochemically stable system for EDLC applications.

The long-term cycling stability of the WAS-derived carbon electrode was evaluated over 10,000 consecutive charge–discharge cycles at a constant current density. As shown in Figure f, the electrode retained 96.07, 96.8, and 95.16% of its initial capacitance in 2, 4, and 6 M KOH electrolytes, respectively. These exceptional retention values confirm the remarkable electrochemical durability of the material, underscoring its stability and reliability for long-term supercapacitor operation.

To further demonstrate its practical applicability, a green LED was successfully powered by using the prepared WAS-derived carbon electrode (Figure g). The bright and sustained illumination of the LED affirms the electrode’s efficient energy storage and delivery capability, validating its potential for use in real-world energy storage and portable electronic applications.

5. Summary and Conclusions

In this study, activated carbon was successfully synthesized from a WAS via chemical activation and evaluated as a sustainable electrode material for EDLCs. The acid-treated carbon exhibited a high BET surface area of 590 m²/g, as determined by N₂ adsorption–desorption isotherms, and displayed a predominantly amorphous structure with abundant oxygen-containing functional groups, as confirmed by XRD and XPS analyses. These features contributed to enhanced surface polarity and improved electrolyte accessibility.

Electrochemical characterization using 2, 4, and 6 M KOH electrolytes revealed typical EDLC behavior with rectangular CV profiles and symmetric GCD curves. Among the tested systems, the 4 M KOH electrolyte delivered the most balanced performance, offering moderate specific capacitance, low charge-transfer resistance, and an ideal capacitive phase angle, as supported by the Nyquist and Bode plots. Despite this, the highest capacitance was observed in 6 M KOH, indicating the influence of the ion concentration and transport dynamics on performance.

The WAS-derived electrode demonstrated outstanding cycling stability, retaining over 96.8% of its initial capacitance after 10,000 cycles across all electrolytes. Its practical applicability was further validated by successfully powering a green LED, affirming its potential for real-world energy storage applications.

Overall, this work highlights the promise of WAS-derived activated carbon as a cost-effective, eco-friendly, and high-performance electrode material for next-generation EDLCs, particularly in low-voltage, high-rate energy storage systems.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c05285.

• Materials and methods, material characterization, carbon yield percentage, electrochemical characterization, electrode preparation, calculation of working electrode thickness, Table S1 (PDF)

N.S.: Conceptualization, methodology, investigation, writingoriginal draft; M.S.: material preparation, data curation, formal analysis; A.S.K.: characterization, validation, visualization; and K.P.J.: writingreview and editing, supervision.

The authors declare no competing financial interest.

This article published ASAP on August 29, 2025. In the Electrochemical Analysis (CV and GCD), the potential window value has been updated to –1.0 to 0.1 V. The corrected version reposted on September 2, 2025.