Recent advancements in biomass-origin carbon structures for next-generation supercapacitors

Summary

The urgent global need for efficient, clean, and sustainable energy storage technologies has underscored the importance of supercapacitors as vital components in future energy systems. Their high power density, excellent cycle life, and rapid charge-discharge capabilities make them particularly attractive for diverse sustainable applications. Among emerging materials, biomass-derived carbon has gained significant attention due to its renewability, hierarchical porosity, and tunable surface chemistry, especially when enhanced through heteroatom modification. These distinctive features improve electrochemical performance, positioning biocarbons as promising candidates for next-generation supercapacitor electrodes. This review highlights recent advances (2020-2025) in biocarbon-based supercapacitors, focusing on how structural engineering, porous architectures, and chemical doping influence charge retention, energy density, and conductivity. Beyond performance metrics, it emphasizes the environmental and circular-economy benefits of recycling biowastes for sustainable energy solutions. The paper concludes with reflections on current challenges, practical considerations for scaling, and future research directions to advance biomass-derived carbon electrodes.

Graphical abstract

Electrochemistry; Electrochemical energy storage; Materials science; Energy materials

Introduction

The increasing global energy consumption resulting from population growth, urban expansion, industrialization, transportation, and infrastructure development has led to a growing need for sustainable and energy-efficient storage devices.¹ Supercapacitors, or electrochemical capacitors, have attracted a great deal of interest because of their higher power density, rapid charging, and long cycle life. These characteristics make them appropriate for a wide range of applications related to electric mobility, grid control, wearable electronics, and renewable energy buffering. However, the upper potential of supercapacitors is constrained by the environmental cost and the use of conventional electrode materials. The majority of commercial electrodes employed at present are derived from fossil precursors, such as petroleum pitch, synthetic polymers, and energy-intensive carbon nanostructures, including graphene and carbon nanotubes.^2,3,4 These materials are generally low in sustainability, costly to manufacture, and challenging to scale up and environmentally.

Carbon materials derived from biomass are emerging as a new generation of sustainable electrode materials for supercapacitors to fill this gap.^{5,6,7,8,9,10,11} Biomass is globally distributed, renewable, and affordable and includes a wide range of residues and wastes, from crops such as rice husks, coconut shells, maize stalks, forest residues, food processing residues (bananas and oranges), and even invasive alien plants.^{8,12,13,14,15,16,17,18,19,20} These discarded raw materials are an appealing raw material for carbon sequestration through trash recovery technologies and circular economy initiatives. A number of thermochemical and chemical methods, including pyrolysis, hydrocarbonization, and activation techniques, are used to transform raw biomass into usable carbon material. All these processes yield porous carbons with the maximum physicochemical properties like great specific surface area, tunable pore size, and dense active surface area. Further performance improvement is through heteroatom doping, most commonly by nitrogen (N), sulfur (S), phosphorus (P), or oxygen (O), which enhances surface polarity, electrical conductivity, and pseudocapacitive behavior. Structural design strategies such as hierarchical porosity engineering and surface functionalization also impart higher ion diffusion kinetics, charge storage capacity, and overall electrochemical performance.

Biomass conversion to carbon materials not only meets the functional requirements of supercapacitor electrodes but also has other environmental and economic benefits.²¹ Synthesis is typically associated with lower energy demand and can be incorporated into scalable, low-cost production plans. These characteristics make them attractive alternatives for high-tech and distributed applications. Moreover, the conversion of biomass into carbon materials enables carbon neutrality to be achieved by carbon sequestration and the substitution of emissions from the production of fossil materials. However, there are some challenges. Biomass feedstocks are chemically and microstructurally heterogeneous, which is bound to affect the stability and quality of the resulting carbon materials. Synthesis conditions such as temperature, activation agents, and doping processes must be strictly controlled to ensure reproducibility and material homogeneity. Furthermore, the successful application of diverse biomass feedstocks has been recently reported in high-performance supercapacitor electrodes. Coconut shells are activated by KOH and converted into high-surface-area carbon materials with higher capacitance.¹³ It has been reported that nitrogen-doped hierarchical porous carbons derived from tobacco straw, garlic peel, and banana leaves have an increased specific capacity and flux due to synergistic doping and hierarchical pore structures.^{8,15,16,17,18,19,20,22} Composite electrodes through the incorporation of transition metals (Ni, Co, Mn) or conductive polymers into carbon matrices derived from biomass have further broadened the functional scope of these materials by coupling electrical double-layer capacitance with pseudocapacitive redox behavior. Furthermore, recent reviews have explored biomass-derived carbons (BCMs) for supercapacitors from different perspectives. For instance, Zhu et al. (2025)¹³ provided a broad survey of activation methods and biomass sources, emphasizing functional mechanisms but leaving questions about scalability and life cycle sustainability. In contrast, Yuan et al. (2024)²³ concentrated on heteroatom doping strategies, offering valuable insights into how nitrogen and oxygen functionalities enhance electrochemical performance, though their work did not fully connect these modifications to metal-doped and device-level outcomes across diverse electrolytes. Complementing these, Singh et al. (2025)²⁴ presented a case study on sugarcane bagasse-derived carbons, highlighting pore structure engineering and practical electrode fabrication, yet with limited discussion of circular-economy benefits. Against this backdrop, the present review aims to integrate performance metrics, sustainability considerations, and upscaling feasibility, thereby filling a critical gap by linking material design strategies with real-world energy storage applications. This work also presents a comprehensive overview of the latest advances in the synthesis, characterization, and use of BCMs as supercapacitor electrode materials. Emphasis is placed on the interdependent roles of feedstock selection, activation processes, transition metal and heteroatom doping, structural fine-tuning, and electrochemical properties. This research examines the potential of biomass as a sustainable energy storage solution and highlights its potential to reduce the environmental impact and support energy transition. It underlines the need to integrate biomass into wider energy strategies to improve sustainability and resilience and contributes to the growing literature on green energy technologies and energy systems for the future.

Supercapacitors: fundamentals and their application

Supercapacitors (also known as ultracapacitors or electrochemical capacitors) are energy storage devices that bridge the gap between traditional dielectric capacitors and rechargeable batteries by offering high power density, long cycle life, and rapid charge-discharge rates. Their ability to provide high energy density, a unique cycle life, and fast charging capability makes them essential in modern energy systems, particularly where the need for immediate power delivery and long lifetime is urgent.^{25,26,27,28,29,30} However, supercapacitors store energy via surface interactions rather than bulk electrochemical reactions, enabling faster reaction times and increased stability over many cycles.³¹ The basic operation of supercapacitors is based on two mechanisms: electric double-layer capacitance (EDLC) and pseudocapacitance.³² EDLCs store energy by the physical separation of charges at the electrode-electrolyte interface.³³ When an applied potential is introduced, ions from the electrolyte move to oppositely charged electrodes and form a very stable electrochemical double layer. This configuration permits energy storage and release at high rates without participating in faradaic (redox) reactions, thereby displaying excellent cycling stability and long device lifespan (Figure 1). EDLCs commonly utilize porous carbonaceous materials such as activated carbon, carbon nanotubes, and graphene because of their large surface area, conductivity, and chemical inertness.^25,26 Pseudocapacitors, on the other hand, work based upon fast and reversible faradaic reactions at or near the surface of the electrodes,³¹ which are electrochemical reactions between electrode materials and electrolyte species that involve electron transfer, resulting in much higher specific capacitance and energy density than EDLCs. Their standard pseudocapacitive materials are transition metal oxides such as MnO₂ and RuO₂, and conducting polymers such as polyaniline and polypyrrole.²⁶ While these systems possess a higher amount of stored energy, the power density and cycle life are slightly less than their EDLC counterparts.^32,34 Hybrid supercapacitors with both EDLC and pseudocapacitive modes designed for maximum energy and power densities are developed to achieve a versatile balance suitable for many emerging-edge applications. Through hydrothermal synthesis, nitridation, and electrochemical polymerization, a coral-like PPy/FeCoN/CuF hybrid supercapacitor was produced, displaying remarkable energy storage efficiency and long-term durability.³⁵ The device obtained a specific capacitance of 298.5 F/g at 0.5 A/g and preserved 85.73% of its capacitance after 10,000 cycles, emphasizing its promise as a high-performance energy storage medium. Also, the dual-phase electrode synthesized via hydrothermal techniques, consisting of NiCo₂O₄ with an added NiO phase, exhibited homogeneous spherical aggregates and delivered a high specific capacitance of 866 F/g with 85% retention after 5,000 cycles.³⁶

Figure 1.

Working mechanisms of a supercapacitor supported with biomass carbon

Adopted with permission from.¹³

A supercapacitor consists of a structural sandwich of an electrolyte, a separator, and two electrodes,³⁷ where the electrolyte facilitates ion transport and prevents short-circuiting between the electrodes. Build-up of charge on the electrode surfaces occurs when it is in use, and depending on the mechanism (faradaic or physical), stored energy is maintained electrostatically or by surface-level redox reactions. This simple architecture enables supercapacitors to be efficient over extended charge-discharge cycles, typically more than 100,000 cycles with little performance degradation. Several performance parameters define the ability of supercapacitors. Specific capacitance typically ranges between 100 and 1,000 F/g depending on electrode design and composition. Energy density up to 25 Wh/kg, which, although lower than in lithium-ion batteries, is made up for in power densities of up to 10 kW/kg.^26,27 Cycle life is perhaps the most attractive feature, with some devices performing after hundreds of thousands of cycles. These values are significantly influenced by electrode material characteristics such as porosity, surface area, conductivity, and chemical stability.²⁵

Supercapacitors are also becoming more valuable in a variety of applications, including automotive systems and wearable electronics, grid-scale energy storage, and industrial power buffering.²³ In electric cars (electric vehicles), supercapacitors enable recovery and reuse of energy during regenerative braking, and also facilitate acceleration bursts by providing high power pulses. Both these features not only optimize energy efficiency but also reduce the load on battery systems, with the result that battery life spans get extended. In consumer goods, supercapacitors serve as buffers for sensors, handheld devices, and emergency backup power to provide instant recharge and a stable power supply.³⁸ Supercapacitors, in solar and wind farms, enhance grid stability and store or release energy abruptly in order to dampen oscillations and stabilize the grid. They are the best solution for dealing with non-continuous energy sources due to their high efficiency and short response time. Supercapacitors power machinery such as cranes, forklifts, and emergency lighting, which require a rapid energy supply. Therefore, their contribution to the integration of renewable energy is increasingly being valued.

In addition, BCM with hierarchical porosity boasts a tremendous surface area that can facilitate the adsorption of ions, thus increasing the value of the EDLC effect. However, with the addition of heteroatoms such as N, S, P, or O, extra redox activity is achieved, as carbon-based molecules now possess reactive centers. Taking note of the fact that pseudocapacitance involves fast chemical reactions, adding reactive centers can thus increase the pseudocapacitive component. With the absence of doping, the action takes place via the EDLC mechanism, whereby adsorption of ions takes place. Upon doping, the action occurs through a combination of EDLC, which provides high power, and the pseudocapacitive effect, which provides high energy. Nitrogen-based moieties increase conductivity, as well as facilitate electron transport. Heteroatom doping not only tunes the surface chemistry but also modulates the EDLC-pseudocapacitance coupling, enabling efficient adsorption of ions and their rapid redox, thus further increasing the material’s electrochemical properties. A comparison between undoped, as well as heteroatom-doped, BCM materials will serve as further evidence that this coupling changes with the material’s surface chemistry (Table 1). Undoped carbon materials, despite their rich hierarchically organized pores, exhibit dominant EDLC properties, where fast adsorption of ions within the electrode-electrolyte interface significantly enhances their power density as well as their charge-discharge rates. However, their primary weakness lies in their low energy densities due to the absent or insignificant faradic contributions. Compared with undoped carbon materials, heteroatom-doped carbon materials, particularly those with N, O, S, as well as P atoms, feature rich faradic sites, enabling rapid faradic reactions, thus greatly increasing the pseudocapacitive properties. Nitrogen-based, as well as other heteroatoms, increase the carbon material’s conductivity, as well as its ability for rapid electronic transport, allowing pyridinic, pyrrolic, as well as quaternary nitrogen atoms to feature effective pathways. Likewise, the added charge with oxidized carbon materials further enhances their pseudocapacitive properties.

Table 1.

Comparative analysis of the performance of supercapacitors using heteroatom-doped and undoped biomass-derived carbons

Parameter/mechanism	Undoped biomass carbon supercapacitor	Heteroatom-doped biomass carbon supercapacitor	Performance implications
Charge Storage13,25	Dominated by EDLC (physical ion adsorption)	Synergistic EDLC and pseudocapacitance (ion adsorption and faradaic reactions)	Doping adds a faradaic contribution, boosting energy density.
EDLC Contribution25	Ion adsorption on porous surfaces	Ion adsorption remains significant due to hierarchical porosity	Maintains high power density and rapid charge-discharge
Pseudocapacitance Contribution39,40	Minimal, limited to oxygen groups	Strong, via N, O, S, P redox-active sites	Enhances energy density through fast reversible reactions
Surface Chemistry41	Limited functional groups, weak redox activity	N, O, S, P doping introduces abundant redox-active sites and rich heteroatom functionalities (pyridinic N, quaternary N, oxygen groups)	Improves conductivity and redox activity, better pseudocapacitance and conductivity
Porosity25	Provides ion transport pathways	Hierarchical pores and doped sites act as diffusion reservoirs	Better ion accessibility and charge retention
Conductivity40	Moderate, restricted by carbon framework	Graphitic N and heteroatom defects improve electron transfer	Faster electron transfer, lower resistance
Cycle Stability	Stable but limited energy density	Stable with enhanced energy density and capacitance retention	Superior long-term performance
Overall Balance40,41,42	Primarily EDLC driven	Synergistic EDLC and pseudocapacitance	Achieves both high power and higher energy density, and is competitive with hybrid supercapacitors

Electrode materials for supercapacitors

Supercapacitors are becoming essential energy storage devices for contemporary applications, and their longevity, performance, and efficiency are all greatly influenced by the materials used for their electrodes.¹⁰ In contrast to batteries, supercapacitors accumulate energy through electrostatic charge storage or fast surface redox reactions, and thus the selection and design of electrode materials play an essential role in their functionality.⁴³ Thus, to ensure maximum energy density, power density, rate capability, and cycling stability, electrode material design and engineering are fundamental. To meet the increasing demand for compact, fast-charging, and green energy storage devices, recent efforts have been devoted to the design of novel, high-performance, and sustainable electrode materials.

High-performance carbon electrodes for supercapacitors made from biomass have advanced significantly in recent years thanks to activation strategies and hierarchical porosity tailoring. Activation chemistry is instrumental in tuning the porosity, surface area, and electrochemical behavior of biocarbons. It has been discovered that chemical activation using activating agents like KOH, ZnCl₂, and H₃PO₄ is more efficient than physical methods for producing highly porous carbons with adjustable pore distributions. For example, two-step KOH activation resulted in a surface area of 4,048.2 m²/g and specific capacitance of 419 F/g with 91.2% capacitance retention after 20,000 cycles,⁴⁴ whereas ZnCl₂ and H₃PO₄ activation of Moringa oleifera biomass resulted in 201 F/g capacitance and 25 Wh/kg energy density.⁴⁵ Hybrid activation approaches, such as KHCO₃/KH₂PO₄ treatment of coconut shells, resulted in O/P co-doped porous carbon with 371.63 F/g and outstanding durability after 10,000 cycles.¹² While N- and S-co-doped carbons made from lignosulfonate and polyaniline demonstrated 360 F/g and 87% capacitance retention after 10,000 cycles,⁴⁶ squid ink-derived N-doped carbon made by electrospinning and KOH activation achieved 422.7 F/g and 48.01 Wh/kg when combined with ZnO.⁴⁷ Additionally, S. Yang et al. (2022) synthesized fungus bran-derived hierarchical porous carbon activated with KMnO₄ and showed 333 F/g and 6.09 Wh/kg in 6 M KOH electrolyte.⁴⁸ In another investigation, reed straw pyrolyzed with melamine yielded nitrogen-rich carbon (RSM-0.33-550) with 202.8 F/g and 96.3% capacitance retention after 5,000 cycles at 20 A/g using KOH electrolytes.⁴⁹ These findings illustrate the success of heteroatom doping and template-assisted activation for improving ion transportation, conductivity, and cycling stability. Research on ZnCl₂-activated lotus stem-based carbon at 600°C-1,000°C developed a balanced porous structure that facilitated ions to move freely and shortened diffusion routes. The material possesses a specific surface area of 1,103.0-1,316.7 m²/g and a high specific capacitance of 317.5 F/g at 5 mV/s, with excellent stability of 99.2% after 10,000 cycles, and good rate capability of 52.1%.⁵⁰ Activated carbon of Cerbera manghas fiber (CMF), carbonized at an optimum temperature of 600°C, exhibited a high specific surface area of 721.5 m²/g and specific capacitance of 221 F/g.⁹ CMF-600’s fibrous, mesoporous nature improved its performance in electrochemical tests. A review article by S. Lu, Fang et al. (2024)⁵¹ categorized activators by their working mechanism, cost, and environmental effect, providing recommendations for future advancement. These findings reflect the significance of optimizing activation parameters such as temperature, agent ratio, and heating rate to balance pore development and carbon yield. It further indicates how activation chemistry plays a crucial role in regulating pore structure, surface area, and doping features to enhance electrochemical storage, thereby enhancing the overall storage system’s performance. Figure 2 shows the use of heteroatom-doped acid-base activated biocarbon for supercapacitor applications. It shows the synthesis and electrochemical evaluation of porous biocarbons made from biomass treated with H₃PO₄ and KOH.

Figure 2.

Utilization of heteroatom-doped acid-base activated biocarbon for a supercapacitor

(A) Raw biomass and chemical activation pathways.

(B) SEM pictures of hierarchical porosity and microstructure, with zoomed-in views revealing the electrolyte diffusion reservoir and short ion transport pathways.

(C) A molecular diagram of heteroatom-doped carbon, highlighting the oxygen and nitrogen functions that contribute to pseudocapacitance.

(D) Ragone plot comparing energy density (Wh/kg) and power density (W/kg) across three electrolytes such as1M KOH, 1M H₂SO₄, and Na₂SO₄/GF and (E) pore size distribution curve (dV/dlog(w) versus pore width), with an inset bar chart comparing surface area or pore volume among samples. Adopted with permission.⁵²

Other sources of biomass have also shown great potential. Carbon from garlic peel co-doped with N, O, and activated by KHCO₃/melamine reached 2,808.65 m²/g and 396.25 F/g with retention of 92.5% following 10,000 cycles,²⁰ whereas a single-step mild pyrolysis process involving K₂CO₃, tobacco straw, and melamine yielded nitrogen-doped carbon with a surface area of 2,367 m²/g and 338 F/g, with 85.2% retention at high current densities.^8,19 Metal selenide-modified biomass carbon attained 840.6 F/g and 34.3 Wh/kg energy density. KOH-treated activated carbon from invasive Canada goldenrod exhibited up to 366 F/g and >86% capacity retention after 10,000 cycles. Devices incorporating the material had energy densities of 112.1 Wh/kg and power densities of 12.2 kW/kg.¹⁸ Hierarchically porous carbon derived from activated sludge and spent coffee grounds showed a surface area of 1,198 ± 60 m²/g and 98.4% retention after 20,000 cycles using sodium acetate as a green electrolyte.¹⁷ The supercapacitor exhibited 15.9 Wh/kg energy density and had the advantage of rapid surface kinetics. It was found that a water-based binder enhanced the capacitance by 76%, demonstrating a synergy between sustainability and performance. Interfacial regulation by chemical activation has been proven effective for enhancing the surface area and doping features. Nitric acid (APC-H800) and copper chloride (APC-C800) biomass-derived carbon activation led to specific capacitances of 384 and 423 F/g, respectively, with symmetric devices showing excellent cycling stability and energy densities up to 12.5 Wh/kg.⁵³ Celery-based biomass-activated carbon from celery leaves has a high specific capacitance of 753.0 F/g and excellent cycling stability after 5,000 cycles.¹⁶ It provided a high energy density of 17.36 Wh/kg at 250 W/kg. This is attributed to its high surface area and N/S heteroatom doping. Likewise, Samanea saman biomass carbon provides 434 F/g and 25.0 Wh/kg,⁵⁴ sulfur-doped silver birch wood carbon with 2,124 m²/g and 79.1 F/g,⁴² sorghum seed-derived carbon with KOH/melamine activation achieved 536.7 F/g and 97.6% retention,¹⁴ while corncob-derived carbon with KMnO₄ activation attained 691.05 F/g and 109.7% retention over −25 to +60°C.¹⁵ These investigations demonstrate the potential of agricultural and forestry wastes as high-performance electrode precursors, underlining the importance of activation chemistry, structure engineering, and sustainable sourcing in supercapacitor electrode technologies.

In biomass-derived carbon supercapacitors, nitrogen and sulfur doping have drawn the most attention, but less common dopants like boron and phosphorus are becoming more well known for their distinct contributions to electrochemical performance. Recent studies show that boron increases conductivity and facilitates charge storage by introducing electron-deficient sites into the carbon lattice.^55,56,57,58 For instance, boron-doped biomass carbon-based supercapacitors achieve a maximum specific capacitance of 539.5 F/g, energy density of 40.5 Wh/kg, power density of 7,500 W/kg, and coulombic efficiency of 104.4% with high cycling stability up to 5,000 cycles.⁵⁵ Taer et al. (2025)⁵⁶ produced an eco-friendly dual-atmosphere pyrolysis of self-boron/oxygen-doped sugarcane bagasse porous carbon with a hierarchical pore structure and high heteroatom content, yielding 151.6 F/g at 1 A/g, energy density of 21 Wh/kg, and power density of 167 W/kg. Likewise, Prosopis juliflora stems boron-doped activated carbon provided 307.14 F/g at 0.5 A/g, an energy density of 7.81 Wh/kg, a power density of 150 W/kg, and outstanding cycling stability with 98.6% retention after 1,000 cycles.⁵⁷ Boron/sulfur co-doped porous carbons made from various biomass sources showed synergistic effects beyond single doping, combining improved conductivity and increased redox activity to achieve 290 F/g at 0.5 A/g, an energy density of 16.65 Wh/kg, and exceptional stability over 10,000 cycles.⁵⁸ In contrast, phosphorus doping adds oxygenated and phosphate functional groups that enhance surface wettability and provide pseudocapacitance through reversible redox reactions.^59,60,61,62 Meng et al. (2023)⁶⁰ developed a cost-effective microwave irradiation method for producing phosphorus-doped hierarchical porous carbon from biomass. The method achieved a high specific capacitance of 297.1 F/g at 1 A/g, an energy density of 16.5 Wh/kg at 375 W/kg, and stable operation at 1.5 V with 94.9% retention after 30,000 cycles. Lin et al. (2020)⁶¹ used sawdust and phosphoric acid to create phosphorus-doped porous carbon, resulting in increased carbon output, surface area, and micropore volume compared with untreated samples. The improved electrode has specific capacitances of 292 F/g at 0.1 A/g and 169.4 F/g at 0.5 A/g, high cycling stability (98.3% retention after 5000 cycles), and an energy density of 10.6 Wh/kg at 224.8 W/kg, highlighting a sustainable pathway for converting waste biomass into high-performance supercapacitors. Moreover, the N-P double-doped carbon electrode from watermelon peel has a specific capacitance of 135.2 F/g at 1 A/g in a three-electrode system. This confirms its promise as a sustainable supercapacitor material.⁶² These studies show that, while single heteroatom doping improves conductivity or redox activity, multi-doped systems offer synergistic benefits, resulting in higher electrochemical performance and placing BCMs as sustainable, high-efficiency supercapacitor electrodes.

Figure 3 shows the sources and transformation processes of BCMs for supercapacitor applications. Animal, plant, and microbial biomass is carbonized and activated before being doped with heteroatoms (N, O, S, P, B), which increases surface area, pore structure, and conductivity. These alterations provide functional groups and customized porosity, resulting in high-performance supercapacitors. Lignocellulosic biomass, like coconut shell or rice husk, has extremely porous carbons that promote EDLC. Protein-rich sources, like soybean waste or animal leftovers, offer nitrogen functionalities that increase pseudocapacitance. Sulfur-rich precursors like garlic peel and marine algae provide redox-active sites, increasing energy density.

Figure 3.

Source of biocarbons and their application

Adopted with permissions.⁶³

Furthermore, H. Yang et al. (2024)⁶⁴ synthesized carbon from sugarcane bagasse combined with beef bone powder-based hydroxyapatite via hydrothermal carbonization and KOH activation. The carbon provided a symmetric supercapacitor electrode test with a high specific capacitance of 397.9 F/g at 0.5 A/g in 6 M KOH and an energy density of 23.2 Wh/kg at 874 W/kg using 1M Na₂SO₄ electrolyte. In another related advance, L. Yang et al. (2021)⁶⁵ prepared activated carbon from Sapindus mukorossi peel through carbonization and KOH activation and achieved a high specific surface area of 1,254.5 m²/g. Electrochemical tests exhibit a specific capacitance of 314.5 F/g at 1 A/g, good cycle stability with only 4.2% loss after 5,000 cycles, and good rate capability with 80.4% retention. A symmetrical supercapacitor electrode delivers an energy density of 6.68 Wh/kg at 0.1 A/g and a power density of 3,250 W/kg at 5 A/g. Ramulus mori biomass produced porous carbon with 796 F/g and 59 Wh/kg energy density, retaining 93.6% after 10,000 cycles.⁶⁶ The assembled symmetrical supercapacitor also delivers a high energy density of 59 Wh/kg at a power density of 1.2 kW/kg. Hierarchically porous carbon (HPC) from rubber wood is fabricated using a CaCO₃ hard template with a 3D network, exhibiting efficient ion transport and a high surface area of 1,604.9 m²/g.⁶⁷ When applied in a symmetric supercapacitor structure, the HPC electrode delivered a specific capacitance of 113.3 F/g at 0.5 A/g current density over 5,000 charge-discharge cycles with a 1 M TEA-BF₄/propylene carbonate electrolyte. Besides, hybrid lithium-ion capacitors that are a combination of HPC and Li₄Ti₅O₁₂ have an ultimate energy density of 113.3 Wh/kg at a power density of 281 W/kg and retained 92.8% of the initial capacitance for 3,000 cycles at 0.75 A/g. Guan et al. (2020)⁶⁸ also convert biomass straws into lamellar porous carbon via hydrothermal treatment and KOH activation. It presented a high specific capacitance of 250 F/g at 1 A/g and superior cycling stability for as many as 18,000 cycles. These findings celebrate the potential of biomass valorization and hybridization strategies toward the design of future energy storage devices, bridging the gap between sustainability and cost-effectiveness in the applications of supercapacitors.

Multi-heteroatom doping and hierarchical structuring have also been shown to enhance the electrochemical properties of biomass-derived carbons. Nitrogen-doped carbons, for example, show enhanced conductivity and pseudocapacitance, as demonstrated by products synthesized from Nelumbo nucifera seeds.⁶⁹ The resulting product achieved up to 379.2 F/g and retained 65.9% capacitance at 50 A/g with stable cycling over 10,000. Similarly, Oyinkanola et al. (2023) synthesized porous nitrogen-rich carbon from spirulina-extracted impregnated castor shell.⁷⁰ The carbon product also achieved a 1,527 m²/g surface area and 333 F/g capacitance with only 0.3% loss after 10,000 cycles. Tobacco straw one-step activation using nano-ZnO achieved 1,293.2 m²/g and 220.7 F/g while retaining 94.83% after 3,000 cycles.⁷¹ These findings demonstrate the flexibility of nitrogen doping and activation techniques to enhance electrochemical performance. Also, Xiao et al. (2024)⁷² prepared rapeseed meal-derived carbon doped with (N, O, P, S) through hydrothermal carbonization and activation. The as-prepared material had 3,291 m²/g surface area, specific capacitance of 416 F/g at 1 A/g, and retained 92% after 10,000 cycles. Its asymmetric device delivered 195.94 Wh/kg at 1,125 W/kg, confirmed by density functional theory, revealing improved electron density and ion diffusion. Similarly, L. Yang et al. (2020)⁷³ synthesized multi-doped carbons from laver, soybean milk, and silver-doped soybean milk and achieved 119.8%, 96.0%, and 131.1% specific capacitance retention values, respectively, after 40,000-50,000 cycles, with energy densities of 6.2-9.5 Wh/kg. Walnut peel-derived carbon from a dual-porogen strategy recorded 557.9 F/g and 12.44 Wh/kg, with improvement in pore expansion and doping.⁷⁴ These findings highlight the potential of multifunctional systems while emphasizing the importance of activation chemistry in influencing pore design and doping properties.The synergistic effects of multi-element doping and hierarchical porosity result in enhanced energy storage capability.Furthermore, the development of sustainable electrode systems serves as a bridge between traditional capacitors and batteries, enabling next-generation energy storage solutions for electronics, mobility, and smart grids..

Recently developed architectures and hybrid devices are continuously improving the performance of supercapacitors based on biomass (Figure 4). A pseudocapacitive carbon matrix from cellulosic biomass that is flexible achieved 1,024 F/g and 32 Wh/kg energy density with performance being retained under mechanical stress.⁷⁶ A graphitic carbon and Rhein composite from lignin and a redox-active plant molecule exhibited 250.2 F/g and stable cycling in RH-saturated electrolytes.⁷⁷ These multi-component materials demonstrate the potential for combining redox-active materials with porous carbon matrices to bridge the gap between capacitors and batteries. A chitosan binder and SSP-900 biochar freestanding electrode formed a 3D porous structure with 199.2 F/g and 4.37 F/cm² areal capacitance, retaining as much as 98% after 10,000 cycles.⁷⁸ Distillers' grains are converted to N, S self-doped hierarchical porous carbon of surface area 3,642.87 m²/g, with 347 F/g capacity at 1 A/g and 11.29 Wh/kg energy density in a symmetric cell.⁶ Roselle flower biomass activated with KOH demonstrated 1,699.96 m²/g and 216 F/g capacitance with a retention of 90.4% for 5,000 cycles.⁷⁹ In addition, hydrothermal synthesis and KOH activation have enabled the synthesis of manganese oxide/biomass carbon composites from soybean straw with 482 F/g and 14.5 Wh/kg energy density.⁸⁰ In contrast, O, N co-doped carbons, synthesized from black fungus and Hericium erinaceus by KOH activation, exhibited specific capacitances of 209.3 F/g and 238.6 F/g, respectively, with superior rate performance and cycling stability.²¹ N, S co-doped activated carbon-impregnated polyaniline nanorods registered 304 F/g, 23.6 Wh/kg energy density, 395 W/kg power density, and 90.2% retention upon 5,000 cycles.⁸¹ Oxygen-doped carbon from reed prepared via baking-mediated carbonization delivered 93% capacitance retention after 10,000 cycles and achieved 336.4 F/g and 18.66 Wh/kg.⁸² Biomimetic layered carbon from konjac glucomannan and cysteine achieved 351.7 F/g and 95.57% retention at 10,000 cycles.⁸³ Chitosan-grafted SSP-900 biochar fabricated a free-standing electrode with 199.2 F/g and 4.37 F/cm² areal capacitance, with a 98% retention after 10,000 cycles.⁷⁸ Nanostructured porous S, N-doped carbons synthesized by pyrolysis and hydrothermal methods have shown improved ion adsorption, charge transport, and catalysis.⁸⁴ Wang et al. (2024)⁸⁵ prepared a KOH-activated ultra-thick carbon electrode derived from paulownia wood by ball-milling and achieved 257.5 F/g after 5,000 cycles and 2.55 mWh/cm³ energy density, and KOH-activated palm oil stick waste-derived electrodes showed great encapsulation density for portable devices.⁸⁶ Liu et al. (2024)⁸⁷ synthesized a carbon-based flexible supercapacitor using Eulaliopsis binata with cuticle-derived fiber as a separator, pith-derived carbon sponge as electrodes, and gel electrolyte from sodium salts. The as-prepared supercapacitor demonstrated excellent mechanical flexibility and energy density due to N/O/S co-doping and hierarchical porosity. In the same manner, graphene-like porous carbons activated using different ratios of KOH from waste orange peel exhibited a wide voltage window (2.3 V) as well as competitive energy densities in aqueous electrolytes.⁸⁸ Further, argan husk, date seeds (ACds), and olive stones (ACos)-sourced carbons activated with phosphoric acid exhibited 138.26 F/g, 50.41 F/g, and 34.61 F/g, respectively.⁷ The approaches reflect the growing interest in marrying sustainability with novel device architecture. In addition, parali biomass-carbon induced LEDs and motors with 54 Wh/kg energy density, demonstrating practical usability.⁸⁹ These approaches target the optimization of binder chemistry, structural robustness, and scalable fabrication in advancements of supercapacitor technology.

Figure 4.

Electrode and electrolytes materials for energy storage

Adopted with permission from^75.

Composite hybrid materials provide additional advantages for performance improvement (Figure 5). Carbonization and CoZn-ZIF-8 hybridization increased biomass carbon’s ion diffusion and mass transport, resulting in 757.8 F/g.⁹¹ Lignosulfonate/polyaniline-derived N, S-doped carbon achieved 333.5 F/g with the ability to retain over 95% after 5,000 cycles.⁹² Its 8.33 Wh/kg energy density and 62.5 W/kg power density identify it as a low-cost, high-efficiency supercapacitor electrode. Hydrothermal and KOH activation treatments have resulted in high-capacity biocarbons with excellent rate capability and energy density.⁹³ The reviews of Shah et al. (2023)⁹⁴ and Rawat et al. (2023)⁹⁵ emphasize the significance of activation protocols, pore structure, and heteroatom doping for achieving maximum charge storage. Mn/Cu bimetal-decorated carbon from peanut shell recorded 632 F/g at 90% capacity retention at 10,000 cycles with 15.62 Wh/kg energy density and stable cycling toward powering LEDs,⁹⁶ while water hyacinth-based carbon activated via KOH and HNO₃ recorded 374 F/g with 87.3% capacity retention at 5,000 cycles.⁹⁷ Its symmetrical device recorded 330 F/g, a testament to its suitability for application in energy storage. Zhao et al. (2023) prepared Abutilon theophrasti stem-derived carbon activated with N and O at high-temperature carbonization and KOH-assisted activation.⁹⁸ The optimized product material exhibited an ultrahigh surface area of 3,783.1 m²/g and 365.1 F/g capacitance, retaining 97% capacitance after 6,000 cycles, and incorporation of EDLC with pseudocapacitance is evidenced to be of immense potential through strategic doping and activation. Le et al. (2021) also prepared porous carbon electrode material from cornstalk waste by simple carbonization and activation.⁹⁹ The material demonstrated an energy density of 15.3 Wh/kg, a specific surface area of 408 m²/g, capacitance of 125 F/g, and retained 88% after 2,000 cycles in aqueous electrolyte. These findings reinforce the value of combining biomass valorization with tailored activation and doping strategies to produce efficient, sustainable supercapacitor electrodes. As the field progresses, integrating biomass-derived carbons with pseudocapacitive materials like metal oxides,¹⁰⁰ MXenes,¹⁰⁰ and metal-organic framework (MOFs) will be key to bridging the gap between sustainability and commercial viability in supercapacitor applications.

Figure 5.

Diagram of a hybrid supercapacitors system

Adopted with permission from.⁹⁰

Supercapacitors have various charge-storage methods and performance profiles. EDLCs, primarily based on activated carbons or BCMs, offer great power density and cycling stability but suffer from limited energy density. Pseudocapacitors, employing transition-metal oxides or conducting polymers, give increased energy density through quick surface redox reactions, although they often confront issues in long-term stability. Hybrid supercapacitors combine these advantages by merging EDLC-type carbons with pseudocapacitive materials, producing a synergistic balance of high capacitance, higher energy density, and durable cycle performance (Figure 5).

Table 2 illustrates how hybrid supercapacitors, which combine the benefits of both systems and consistently outperform both EDLCs and pseudocapacitors, are positioned as promising next-generation energy storage devices that can meet high power and high energy demands. EDLCs based on BCMs are excellent in terms of cost, scalability, and long-term cycling stability, even though their energy density is still rather low. However, even though pseudocapacitors achieve higher capacitance and energy density via rapid redox reactions, their durability is often limited. Hybrid systems effectively bridge this gap by combining carbon frameworks with redox-active materials, offering enhanced energy density and strong cycle retention.

Table 2.

Performance comparison of hybrid supercapacitor versus pure biomass carbon electrodes

Feature/Metric	Pure biomass-based carbon electrodes (EDLCs)	Hybrid supercapacitors (carbon and redox-active materials)
Charge storage mechanism101	Electrostatic ion adsorption (non-faradaic)	Synergistic EDLC and pseudocapacitance
Specific capacitance25,35,36	100-300 F/g	500-900 F/g
Energy density41,102	5-15 Wh/kg	20-40 Wh/kg
Power density102	1,000–5,000 W/kg	2,000–8,000 W/kg
Cycle stability35,103	Excellent (>90% after 10,000 cycles)	High (85–92% after 10,000 cycles)
Material cost & scalability101	Low cost, highly scalable	Moderate cost, scalable with optimized synthesis
Key advantage101	Durability and affordability	Balanced high energy, high power, and good durability

The electrochemical properties of BCMs are, therefore, determined by the interplay between charge storage and ion transport kinetics, which are significantly affected by structural engineering and heteroatom doping. Fundamentally, EDLC occurs due to the adsorption of ions on the carbon electrode surface. Micropores with sizes smaller than 2 nm are very important in EDLC, which significantly increases the accessible surface area, thereby increasing EDLC.¹⁰⁴ Yet, EDLC cannot alone account for the high capacitance observed in heteroatom-doped carbons. In such materials, pseudocapacitance is also very important due to the role of heteroatoms such as N, B, P, and S, which generate redox active sites to generate pseudocapacity.¹⁰⁵ This is, therefore, a synergy between EDLC and pseudocapacity, which gives carbons their high performance. Notably, when multiple heteroatoms are used, their synergies improve conductivity, and between P and sulfur, which improves reversible redox.¹⁰⁶

The ion transport rate kinetics are also very important. Although the presence of micropores is beneficial due to their high surface areas, it can also impede ion transport if not supported by the presence of macropores. Mesopores with sizes ranging from 2 to 50 nm function as ion buffer reservoirs, which assist mainly in reducing diffusion restrictions and achieving high charge/discharge cycle speeds.¹⁰⁷ Macropores, on the other hand, with sizes above 50 nm, work as ion highways, which increase the entry ability of the electrolyte and rate capabilities.¹⁰⁸ Hence, hierarchical structure architectures incorporating micro-, meso-, and macropores are very important to achieve high energy density and high power density at the electrode, which is very important for supercapacitor devices.¹⁰⁴ This structural engineering ensures that electrodes can deliver both high energy density and high power density, which are crucial for practical supercapacitor applications.

Heteroatom doping is another example of the mechanistic complexity of such materials. Boron doping creates electron-deficient areas, which improve conductivity and charge transfer ability.⁵⁵ Phosphorus doping incorporates phosphate and oxygen functionalities, which improve electrode wettability and allow for reversible redox reactions.⁵⁸ Nitrogen doping donates lone-pair electrons, which increase electron density and conductivity. Sulfur doping increases interlayer distances, which enhance ion accessibility and diffusion paths.⁸⁴ All of these dopants have unique roles to play in understanding the collective role of each toward achieving superior performance by dual-doped carbons.

Model-based interpretations are further supported by theoretical grounds. Transmission line models (TLMs) are ubiquitous to explain the role of pore connectivity and tortuosity on ion transport resistance in porous carbons. Abouelamaiem et al. (2018)¹⁰⁹ proved the possibility of correlating electrochemical impedance with hierarchical porosity of biomass carbons using truncated TLMs. Recently, Zhang et al. (2023)¹¹⁰ illustrated impedance models and optimization to relate hierarchical porosity to supercapacitor performance. Density functional theory (DFT) further suggests the binding energies of ions on heteroatoms, establishing the role of doping on enhanced pseudocapacitance. Data on co-doped carbons by nitrogen, phosphorus, and sulfur indicate changes to the electronic structure to generate active adsorption areas, promoting bifunctional charge applications.⁶¹ Similarly, Ghosh et al. (2020)¹¹¹ discussed DFT-analyzed heteroatoms on nanocarbons to improve supercapacitor performance. On the other hand, electrochemical impedance spectroscopy (EIS) enables the analysis of charge transfer resistance and ion diffusion, which were found to establish a correlation between structure and performance. Azizpour et al. (2025)¹¹² successfully applied finite element simulations and EIS techniques on carbon supercapacitor devices to demonstrate impedance modeling on charge transfer kinetics.

Generally, the content of biomass precursors determines the activation approach and the electrochemical characteristics of the generated carbons. Agricultural wastes include lignocellulosic structures that give thermal stability while resisting pore growth. Strong chemical activation is necessary to break down the stiff frameworks of these materials, resulting in carbons with high surface areas and hierarchical porosity, which promote double-layer capacitance and long-term cycling stability. Forestry residues and woody biomass contain lignin, which carbonizes into strong aromatic networks that make good backbones. Activation using cross-linking or dehydration agents results in carbons with balanced micro- and mesopores, improving ion transport and power density. Food industry and aquatic wastes, such as peels, fungus, and marine by-products, are inherently heteroatom-rich and contain nitrogen, sulfur, phosphorus, and oxygen. Activation techniques for these precursors focus on conserving or improving native heteroatoms, resulting in carbons with high capacitance due to pseudocapacitive redox processes rather than surface area. Engineered hybrids and composites blend biomass with polymers or metal oxides to create multifunctional carbons with double-layer capacitance and robust pseudocapacitance, resulting in superior energy density and cycling stability. Engineered hybrids/composites also include pre-processed biomass or composites including redox-active elements (lignosulfonate/polyaniline, biomass, and MOFs). Their synthesis includes specialized techniques such as polymerization, hydrothermal treatment, or planned precursor pyrolysis, resulting in materials that combine EDLC with extreme pseudocapacitance to attain the greatest known capacitances. Overall, cellulose promotes micropore production, while hemicellulose contributes to mesopores through volatile release, lignin offers aromatic stability and conductivity, and mineral components serve as natural templates or dopants. This classification highlights that, while single heteroatom doping improves conductivity or redox activity, multi-heteroatom doping with tailored pore architectures consistently improves capacitance, energy density, and cycling stability, making biomass-derived carbons scalable and sustainable supercapacitor electrodes (Table 3).

Table 3.

Performance comparison of biomass-derived carbon electrodes for supercapacitors

Biomass Category	Specific Precursor	Activation/Doping Method	Specific Capacitance (F/g)	(Electrolyte, Current Density/Scan Rate)	Cycle Stability/Retention	Energy (Wh/kg), Power Density (W/kg), Surface Area (m2/g)	Reference
Agricultural Waste	Coconut shell	KOH activation	419	0.5 A/g	91.2%, 20,000 cycles	30.3 W h/kg, 250 W/kg 4048 m2/g	Li et al.44
	Coconut shell	KHCO3/KH2PO4 (O/P co-doping)	371.63	0.5 A/g	96.23%, 10,000 cycles	9.15 Wh/kg, 500 W/kg, 892.06 m2/g	Dong et al.12
	Reed straw & melamine	Pyrolysis (N-rich)	202.8	6M KOH, 1A/g	96.3%, 5000 cycles	547.1 m2/g,	Liao et al.49
	Tobacco straw & melamine	K2CO3, mild pyrolysis (N-doped)	338	1 M Na2SO4, 1 A/g	85.2%, 5000 cycles	14.78 Wh/kg, 400 W/kg, 2367 m2/g	Zhang and Li19
	Tobacco straw	Activation with nano-ZnO	220.7	6 M KOH, 1 A/g	94.83%, 3000 cycles@5A/g	1293.2 m2/g	Jiang et al.71
	Sorghum seed	KOH/melamine activation	536.7	6 M KOH, 0.5A/g	97.6%,15000 cycles	13.8 Wh/kg, 298W/kg, 2132.1 m2/g	Feng et al.14
	Corncob	KMnO4 activation	691.05	0.5 A/g	109.7%,10000 cycles @ 10 A/g	16.47 Wh/kg, 15360 W/kg,	Sun et al.15
	Sugarcane bagasse	Dual-atm. pyrolysis (self-B/O doped)	151.6	1 A/g	Not specified	21 Wh/kg, 167 W/kg, 563.72 m2/g	Taer et al.56
	Sugarcane bagasse and bone powder	Hydrothermal and KOH activation	397.9	6M KOH, 1 M Na2SO4 0.5 A/g	Not specified	23.2 Wh/kg, 874 W/kg, 3103.14 m2/g	Yang et al.64
	Soybean straw	Hydrothermal and KOH (MnO2 composite)	482	0.5 A/g	Not specified	14.5 Wh/kg	Li et al.80
	Peanut shell	Mn/Cu bimetal-decorated	632	1A/g	90%, 10,000 cycles	15.62 Wh/kg, 625 W/kg	Zhan et al.96
	Cornstalk waste	Carbonization & activation	125/62	Aqueous/organic electrolyte	88%, 2000 cycles	15.3 Wh/kg; 408 m2/g	Le et al.99
	Abutilon theophrasti stem	KOH activation (N, O-doped)	365.1/74.81	6 M KOH, 1A/g/0.5A/g	97%, 6000 cycles	51.83 Wh/kg, 375 W/kg, 3783.1 m2/g	Zhao et al.98
	Walnut peel	Dual-porogen strategy	557.9/291	1A/g/30A/g	Not specified	12.44 Wh/kg, 5679.62 W/kg	Xu et al.74
	Ramulus mori biomass	Not specified	796	1A/g	93.6%, 10,000 cycles@10A/g	59 Wh/kg, 1.2 kW/kg	Xu et al.66
	Biomass straws	Hydrothermal + KOH activation	250	1 A/g	Stable, 18,000 cycles	Not specified	Guan et al.68
Forestry Residue	Moringa oleifera	ZnCl2 and H3PO4 activation	201	1 M H2SO4, 1 A/g, 1 mV/s	Not specified	25 Wh/kg, 122W/kg	Taer et al.45
	Lotus stem	ZnCl2 activation	317.5/272.9	5 mV/s 1 A/g	99.2%, 10,000 cycles	1103.0–1316.7 m2/g	Shrestha et al.50
	Prosopis juliflora stems	Boron-doped activation	307.14	0.5 A/g	98.6%, 1000 cycles	7.81 Wh/kg; 150 W/kg	Devarajan and Arumugam57
	Sawdust	H3PO4 activation (P-doped)	292/169.4	1 M H2SO4, 0.1 A/g/0.5A/g	98.3%, 5000 cycles	10.6 Wh/kg, 224.8 W/kg	Lin et al.61
	Sapindus mukorossi Peel	Carbonization and KOH activation	314.5	1 A/g	95.8%, 5000 cycles	6.68 Wh/kg; 3250 W/kg, 1254.5 m2/g	Yang et al.65
	Rubber wood	CaCO3 template (3D HPC)	113.3	1M TEA-BF4/PC, 0.5 A/g	92.8%, 3000 cycles @0.75 A/g,	113.3 Wh/kg, 281 W/kg, 1604.9 m2/g	Cho et al.67
	Silver birch wood	S-doping	79.1	1 A/g.	85.3%, 10,000 cycles	2124 m2/g	Simoes dos Reis et al.42
	Castor shell and spirulina	N-rich carbon synthesis	333	50 A/g	99.7%, 10,000 cycles	1527 m2/g	Oyinkanola et al.70
	Invasive Canada goldenrod	KOH treatment	320	6 M KOH, 0.1 A/g	>86%, 10,000 cycles	112.1 Wh/kg, 12.2 kW/kg, 2,526.1 m2/g	Tu et al.18
	Paulownia wood	KOH activation, ball-milling	257.5	Not specified	87.75%, 5000 cycles	2.55 mWh/cm3, 3308 mW/cm3,	Wang et al.85
Food Industry Waste	Garlic peel + melamine	KHCO3 activation (N, O co-doped)	396.25	1 A/g	92.5%, 10,000 cycles	9.07 Wh/kg, 309.2 W/kg, 2808.65 m2/g	Liu et al.20
	Spent coffee grounds & sludge	Not specified	Not specified	Sodium acetate electrolyte 10A/g	98.4%, 20,000 cycles	1198 ± 60 m2/g, 115.9 Wh/kg	Jafari and Botte17
	Celery leaves	Activation (N/S doping)	753.0	0.5A/g	96.7%, 5,000 cycles	17.36 Wh/kg; 250 W/kg	Wu et al.16
	Watermelon peel	N-P double doping	135.2	1 A/g	Not specified	Not specified	Yang et al.62
	Distillers' grains	N, S self-doped	347	6 M KOH, 1 A/g	Not specified	11.29 Wh/kg, 250 W/kg 3642.87 m2/g;	Yu et al.6
	Waste orange peel	KOH activation (graphene-like)	Not specified	1M Na2SO4 &12M NaNO3 (water-in-salt), aqueous (2.3 V window)	Not specified	1150 m2/g	Karaman et al.88
Aquatic and Other	Squid ink	Electrospinning & KOH (N-doped)and ZnO	422.7	1A/g	Not specified	48.01 Wh/kg, 1124.43 W/kg, 636.68 m2/g	Gao et al.47
	Fungus bran	KMnO4 activation	333	6 M KOH, 0.5 A/g	Not specified	6.09 Wh/kg, 250 W/kg	Yang et al.48
	Black fungus/Hericium erinaceus	KOH activation (O, N co-doped)	209.3/238.6	6.0 MKOH, 1A/g	Not specified	Not specified	Zhong et al.21
	Nelumbo nucifera seeds	N-doping	379.2	1A/g	10,000 cycles	1059.6 to 2489.6 m2/g	Shrestha et al.69
	Rapeseed meal	Hydrothermal and activation (N,O,P,S)	416	1 A/g	92%, 10,000 cycles	195.94 Wh/kg, 3291 m2/g;	Lu et al.72
	Water hyacinth	KOH and HNO3 activation	374/105	1A/g/20A/g	87.3%, 5000 cycles	11.46 Wh/kg	Lu et al.97
	Roselle flower	KOH activation	216	6 M KOH, 1A/g, 10-100 mV/s	90.4%, 5000 cycles	1699.96 m2/g	Yan et al.79
Hybrid Materials	Lignosulfonate/polyaniline	N, S-doped carbon	333.5	0.5 A/g	95%, 5000 cycles	8.33 Wh/kg; 62.5 W/kg	Li et al.92
	BCM and CoZn-ZIF-8	Carbonization and hybridization	757.8	5 mV/s	Not specified	Not specified	Zhang et al.91
	Coral-like PPy/FeCoN/CuF	Hydrothermal, nitridation, polymerization	298.5	0.5 A/g	85.73%, 10,000 cycles	Not specified	Saadh et al.35
	NiCo2O4/NiO (dual-phase	Hydrothermal synthesis	866	2 M KOH, 5 m V/s	85%, 5000 cycles	Not specified	Ramachandran et al.36

Major bottlenecks and future research directions

Current limitations

Biomass-derived porous carbons have attracted extensive research attention as green and high-performance electrode materials for supercapacitors based on their tunable porosity, high specific surface area, and intrinsic heteroatom content. These features provide for the efficient adsorption/desorption of ions and rapid charge transfer, which are of prime importance for providing high power density. Biomass feedstocks such as crop residues, including peanut shells, cornstalks, Abutilon stem, water hyacinths, and cereal straw are also convertible to active porous carbons by controlled carbonization and chemical activation. These treatments tend to create hierarchical pore networks (micro-, meso-, and macropores) to enable favorable ion transport and storage. Yet, despite the favorable characteristics of BCM-based electrodes, some intrinsic and technical limitations restrict their widespread application in high-performance energy storage devices. One of the most significant challenges is their relatively low energy density compared with electrochemical batteries. Since supercapacitors store energy primarily via electrostatic double-layer capacitance or fast surface redox, supercapacitor energy density is naturally limited by capacitance and operating voltage window. This limitation makes them inappropriate for applications in need of sustained energy delivery over extended timescales, such as electric power propulsion systems or utility-scale renewable energy storage, where batteries now dominate.

Furthermore, biomass-derived carbon electrodes often have larger voltage dips during discharge, ranging from 0.15 to 0.30 V. This behavior has been described in multiple studies and is mostly due to irregular pore architectures and reduced electrical conductivity, both of which increase internal resistance.^25,102 Conventional activated carbons, on the other hand, have significantly smaller voltage drops, typically in the range of 0.05–0.15 V, due to better pore distribution and enhanced conductivity.⁴¹ Advanced nanocarbon materials, such as carbon nanotubes and graphene, perform even better, with voltage dips of 0.02–0.08 V, owing to their highly organized structures and outstanding electron transport capabilities.³⁵ These comparisons show that, while biomass-based carbons are sustainable and cost-effective, their higher voltage losses remain a barrier. To improve the efficiency and reduce voltage drop in biomass-based supercapacitor devices, structural optimization, heteroatom doping, and hybridization with pseudocapacitive materials are crucial. In addition, while biomass feedstocks are abundant and inexpensive when in raw material form, their conversion to high-performance carbons typically involves sequential processes of controlled pyrolysis, template-inclusive activation, and high-temperature heteroatom doping using chemical reagents, followed by post-processing operations. These contribute to higher energy input and material costs, which make industrial upscaling economically challenging. Special activating agents such as KOH, ZnCl₂, and H₃PO₄, though effective pore-generating agents, are responsible for environmental as well as cost problems due to their corrosive nature and disposal. Also, the heterogeneity in composition of the biomass due to variations in growth conditions, species, and pre-treatment can lead to non-uniform electrochemical properties, and thus, precise process optimization is necessary for each type of precursor.

Activating BCMs with agents like KOH, ZnCl₂, and H₃PO₄ has the potential to produce cost-effective, sustainable, and high-performance electrode materials for supercapacitors. KOH activation creates microporous structures with large surface areas, often exceeding 2,000 m²/g, enhancing ion adsorption and specific capacitance. However, excessive microporosity can hinder ion transport at high current densities, limiting rate performance.^113,114 ZnCl₂ activation enhances mesopore formation through dehydration and cross-linking, improving ion transport paths, reducing internal resistance, and lowering voltage drop during discharge. This leads to improved rate capability and power density.¹¹³ H₃PO₄ activation introduces oxygenated and phosphate functional groups that improve surface wettability and contribute pseudocapacitance via redox reactions, leading to enhanced energy density and cycling stability. However, excessive functionalization can reduce conductivity.⁵⁹ Despite these advantages, there are also obstacles, such as excessive voltage drops during discharge, environmental concerns from corrosive activators, and scaling limitations. To overcome limitations and improve efficiency, strategies such as dual activation, heteroatom doping (N, S, B), hybridization with pseudocapacitive materials (e.g., MnO₂, conducting polymers), and greener activation agents (steam, CO₂, bio-derived acids) are being investigated. Despite such limitations, recent advances highlight that BCMs, given their renewability, resource abundance, and tunability of their structure, remain viable contenders for sustainable supercapacitor electrodes, especially alongside improvements in nanomaterials engineering, asymmetric device structure, and the development of solid-state electrolytes with the ability to widen voltage windows and improve stability.^13,115,116

Direction for future research

Future research in biomass-based supercapacitors will increasingly focus on improved structural engineering and compositional modification strategies to overcome existing limits while maximizing the intrinsic benefits of biomass precursors. One of the more effective ways is heteroatom doping, in which atoms such as N, S, P, and B are inserted into the carbon chain. For example, nitrogen doping may induce additional pseudo-capacitating behavior by reversible redox reactions in heteroatom sites, while simultaneously increasing the electrical conductivity and surface water permeability. Doping with sulfur and phosphorus may widen the spacing between layers, improving the availability of ions, while incorporation of boron may modify the structure of the electronic band and introduce gaps favorable to charge storage. If heteroatom doping is integrated with micro- and mesoporous construction, the resulting synergy effect can dramatically increase the energy and energy density of BCM electrodes. At the same time, the hierarchical pore structure engineering combining micropores for charge storage with meso- and macropores for rapid ion diffusion remains the main focus of the design. A flawless synergy between pore arrangement, surface functionality, and conductivity will be crucial to the introduction of next-generation bio-based electrodes with enhanced electrochemical performance and minimization of environmental pressures. These include the employment of green activation processes with non-poisonous chemicals (steam, CO₂, organic acids) and the integration of one-step carbonization processes for reducing processing time and energy input. Agricultural and industrial biomass wastes, such as rice husks, banana peel, wood dust, and algae, can be exploited as low-cost raw materials with intrinsic heteroatom content and structural variety. In addition, hybrid electrode designs combining biomass-based carbons with complementary materials such as transition metal oxide, MXenes, or conductive polymers offer a path to multi-functional systems combining the high energy capacity of supercapacitors with the higher energy density of batteries. Also, scaling up from laboratory achievement to commercial viability remains a significant problem. Considerations include optimizing reactor design for uniform doping and pore formation at larger volumes, developing cost models that account for energy input, chemical usage, and waste management, and integrating with existing industrial waste-processing chains to leverage biomass feedstocks more efficiently. Addressing engineering and economic concerns is crucial for moving biomass-derived, heteroatom-doped carbons from promising prototypes to scalable, sustainable supercapacitor technology. Another focus of research is the development of flexible, wearable, and self-healing supercapacitors, especially for portable electronics, energy mobility devices, and smart grid applications. Subsequent submissions are likely to leverage the identification of materials by artificial intelligence (AI) to optimize the selection of precursors, process conditions, and structural properties in an anticipatory fashion. Thus, to effectively bridge the performance gap between batteries and supercapacitors, an integrated approach is needed that couples material innovation with device architecture optimization and environmentally friendly and scalable manufacturing processes so that biomass carbon-based supercapacitors become an economically viable and green energy storage technology.

Conclusion

Biomass-derived porous carbons have become a versatile and sustainable category of electrode materials for next-generation supercapacitors, bridging the gap between superior energy storage capabilities and environmental sustainability. Their natural advantages, such as customizable pore structures, high specific surface area, and heteroatom content, facilitate easy ion transport, fast charge-discharge cycles, and structural flexibility suitable for various device designs. By effectively utilizing biological and agricultural waste residues, biomass-based carbons support the promotion of a circular economy while providing a scalable solution for green energy storage technology. Over the last decade, significant advances have been made in optimizing precursor selection, activation methods, pore engineering, and heteroatom doping, leading to notable improvements in energy density, power density, and long-term cycling stability. Nonetheless, several challenges still impede the full commercialization of biomass-carbon supercapacitors. These include lower energy density compared with batteries, performance variability stemming from biomass differences, and production costs associated with certain activation processes and manufacturing conditions. Addressing these issues requires an interdisciplinary approach that combines green and cost-effective synthesis methods, large-scale manufacturing techniques, hybrid material structures, and high-performance device designs based on double-layer and pseudocapacitive mechanisms. Looking ahead, integrating biomass carbon technology with emerging paradigms such as flexible and wearable devices, hybrid supercapacitors, and artificial intelligence-driven material design holds great potential to expand their applications. Through continuous innovation in materials science, manufacturing processes, and green engineering, biomass-derived carbons could become foundational materials in the global transition toward clean, efficient, and sustainable energy storage. Although laboratory results are promising, scaling up biomass carbon materials for supercapacitors presents multiple challenges. Therefore, reactor designs must be optimized for uniform activation and pore development at industrial scales, and cost models should account for chemical use, energy consumption, and waste management. Combining carbon production with existing industrial waste processing can lower feedstock costs and enhance sustainability but requires coordination and regulatory approval. Overcoming these scale-up challenges is essential to transforming current scientific breakthroughs into commercially viable energy storage solutions.

Acknowledgments

We would like to thank Bahir Dar Energy Center, Bahir Dar Institute of Technology, of Bahir Dar University, Ethiopia for their assistance in completing the endeavor effectively.

Author contributions

M.A.A., conceptualization, drafting, and editing of this manuscript. A.A.A., reviewing and editing of the manuscripts.

Declaration of interests

The author states that I have no known conflicting financial interests or personal ties that might have influenced the work presented in this study.