Recent Progress of Biomass-Derived Carbon for Supercapacitors: A Review

Abstract

Carbon materials are very important for the commercial production of supercapacitors and they are crucial electrode materials. The porous carbon prepared with biomass materials as a precursor is of significance due to its sustainability, environmental friendliness, and low cost. Biomass-derived carbon (BDC) has been widely investigated and reported as the electrode of supercapacitors due to its abundant pores and high surface areas. In this work, the recent advancement of BDC for supercapacitors in the last three years is reviewed. The energy storage mechanism, synthesis techniques, and biomass classification of BDC are briefly summarized at the beginning of this work. Some new typical cases with different biomass resources as raw materials are addressed. Then, effective strategies to further improve the specific capacitance of BDC, including heteroatoms doping, designing composites, novel processes, enhancing graphitic degree, and unique preparation methods, are discussed in detail. Finally, the challenges and future perspectives of porous BDC for supercapacitors are outlined.

1. Introduction

With the rapid growth of the population and the exploration of energy-intensive technologies, the consumption of energy is rapidly increasing in the world. The excessive usage of traditional energy sources including natural gas, coal, and fossil fuels results in serious global climate changes and pollution. Thus, the application of renewable and clean energy, such as wind and solar power, is being driven by researchers and engineers [1]. Due to the intermittent nature of the renewables, novel devices to store energy with superior performance and excellent reliability are urgently needed to meet the growing energy requirement, including lithium and sodium batteries, aqueous Zn-ion batteries, fuel cells, and supercapacitors [2]. These devices are expected to have not only large energy densities but also high specific power [3]. Among these devices, supercapacitors (SCs), also known as electrochemical capacitors, have been intensively investigated because of their many advantages, with the distinctive features of fast charging and discharging rates, as well as long cycle life [4].

SCs have drawn much attention from researchers because of the increasing energy consumption in many fields from large- to small-scale devices such as electric vehicles, portable devices, etc. They have the capability to realize rapid charge–discharge cycles and bridge short-term power fluctuation [5]. The structure of SCs primarily comprises three components, i.e., electrodes, separators, and electrolytes [6]. The essential performance of SCs hinges on the basic characteristics of the electrode materials. SCs are easily classified into three types according to their different mechanisms of energy storage, i.e., pseudocapacitors, electric double-layer capacitors (EDLCs), and hybrid SCs [7]. Pseudocapacitors store energy through reversible charge transport between the electrode and electrolyte interfaces accompanied by redox chemical reactions [8], while EDLCs store charges by physical electrostatic ion adsorption, thus enabling quick charging/discharging speeds [9]. A hybrid capacitor is composed of two electrodes (for example, EDLC and pseudocapacitor) with different potential windows to obtain a wider operation voltage and a higher energy density than any single one. Thus, many attempts have been made to enhance the electrochemical properties of SCs through preparing or designing novel electrode materials.

Since the chief properties of SCs strongly depend on the advanced materials in electrodes, it is widely accepted that a porous structure with a high surface area and a good conductivity of electrode materials is beneficial for rapid charge transfer. Thus, a variety of carbon materials with rich resources, abundant pores, diverse structures, and good conductivity are popularly employed as electrode materials in SCs [10]. Meanwhile, an electrode material derived from biomass is of interest in the research regarding green and sustainable development concepts. Thus, biomass-derived carbon (BDC) is a new kind of environmentally friendly carbon material that has well-developed and easy controlled structures [11]. Compared with traditional high-cost precursors from industry products used to prepare carbon materials, the pyrolysis method using renewable biomass precursors as sources can obtain BDC for energy storage fields at a much lower cost, and the by-product gases can readily be used for power generation [12]. For preparing carbon materials for SCs, this is more sustainable than the conventional route [13].

Currently, a variety of biomass sources—including plants, animals, and microorganisms—have been reported as practical precursors of BDC for SC electrodes [14]. As per Web of Science, the trend of recently published papers on the topics of SCs and biomass-based carbon from 2016 to 2025 is exhibited in Figure 1. The data was obtained using the combined index topics of biomass, carbon, and supercapacitors. It is shown that SCs related to BDC have been a continuous area of interest, with ongoing research focusing on the electrolyte materials. An exponential increase in the reports for BDC in SC application is observed from 2016 to 2020. From 2021, the number of published papers reaches a stable value, about 600 papers per year. Meanwhile, there are also many comprehensive review papers related to BDC for energy storage [15,16,17,18,19]. Different from these review papers, the popular strategies to increase specific capacitance of BDC will be examined in this work. As a simple review, the main attention of this work will focus on the latest reports during the last three years, from 2023 to 2025. We hope to provide researchers with better understanding of the recent progress and challenges and uncover possible future research directions.

2. Energy Storage Mechanism of Supercapacitors

2.1. Pseudocapacitors

For SCs, the term “pseudocapacitance” is closely related to surface redox reactions and the capacitive effect can be easily observed by analyzing the cyclic voltammetric curves [20]. Previously, ruthenium oxide was the subject of in-depth research as a very promising electrode material for pseudocapacitors. It applies Faradaic electro-sorption, intercalation, and oxidation–reduction processes to realize charge storage. At the electrode surface and adjacent regions, the active electrode materials and the electrolytes undergo rapid and reversible redox reactions. Pseudocapacitors strongly rely on high-energy electrode materials that can be metal compounds (oxides, hydroxides, or sulfides) and conducting polymers. Although pseudocapacitors can usually demonstrate a significantly higher specific capacitance than EDLCs, the electrode of pseudocapacitors may suffer from volume changes during the rapid charge/discharge cycles, thus leading to reduced cycling life, poor rate capability, and mechanical instability.

2.2. EDLCs

Different from pseudocapacitors, EDLCs can accumulate charge at the interface between the electrolyte and electrode through physical adsorption of ions, thus forming an electric double layer, which involves fast and highly reversible physical processes [21]. The energy storage ability of EDLCs is affected by the atomic charge partition length and intrinsic shell area. Thus, it can make the SCs suitable for typical applications requiring high power density. Since there is no electrochemical reaction, EDLCs have a relatively lower safety risk than lithium-ion batteries. However, EDLCs have a much lower energy density than lithium-ion batteries.

For ELDCs, the ions in the electrolyte are attached to electrodes to generate an electric double layer on the surface during the charge process. When discharging, the stored charge is released when ions move back into the electrolyte. Thus, some features of EDLC electrode materials are very important, including pore structure (the size and distribution), specific surface area, chemical stability, and electrical conductivity. The high specific surface area and large pore volume are two critical parameters. Micropores can provide rich adsorption sites and contribute to the specific surface area. Mesopores and macropores reduce the diffusion distance and ion-transport resistance. Here, we choose surface area as one factor. If the specific surface area of a carbon electrode is larger, then more charge can be stored on it. During the past decades, researchers thought that one of the best methods to improve the specific capacitance of EDLCs was to enhance the specific surface area of electrode materials, and thus activated carbon (AC) was the most widely used electrode material for EDLCs due to its low price, rich pore structure, and high surface area.

AC materials are popularly made from carbon-rich organic materials via heat treatment in an oxygen-free atmosphere or thermal pyrolysis (carbonization), followed by an activation process to generate abundant porosity [22]. To carbonize biomass materials, pyrolysis carbonization and the hydrothermal method are commonly used. Pyrolysis is performed under an atmosphere with very limited oxygen at high temperatures. The hydrothermal method generates a partially carbonized product, named hydrochar, which has abundant oxygen-containing groups, a low degree of condensation, and poorly developed porosity with a small specific surface area. The activation process can make bio-char exhibit a larger specific surface area and more developed pores, where environmentally friendly physical activation by steam, air, or CO₂ and complicated chemical activation by alkalis, salts, and acids are usually involved. Among various kinds of activation reagents, KOH is the most widely used. However, the combined carbonization and activation (one-pot) process could also be carried out, as reported elsewhere [23]. AC is amorphous carbon with worse conductivity when compared with other nanocarbon-based materials, such as graphene, carbon nanotubes, and templated carbon. These carbon nanomaterials are prepared via complex processes with high costs. However, AC precursors can be natural renewable resources, such as wood, plants, fossil fuels and their derivatives such as pitch or coal, or synthetic polymers. Thus, AC is the best commercially available electrode materials for EDLCs due to its low cost, renewable origins, natural porosity, and high surface area [24].

Certain key characteristics of AC must be paid close attention because they essentially determine the electrochemical properties, such as specific surface area, pore size and distribution, stability, electrical conductivity, and compatibility with the electrolyte. AC is usually chemically stable with good conductivity and it is a very popular choice for EDLC electrodes. The future development of carbon-based SC electrodes will increasingly depend on sustainability and renewability goals. Compared with the synthetic and non-renewable precursors, biomass-based AC can offer more sustainable and scalable options for SC electrodes at much lower prices.

3. Various Sources of Biomass-Based Carbon

Biomass precursors for AC chiefly include plants, animal bones, and excreta, as well as microorganisms, where most biomass can be employed to obtain porous carbon materials because of their intrinsically high carbon content.

Plants, predominantly consisting of cellulose, lignin, and hemicellulose, are often applied to prepare AC because they have evolved through thousands of years with plenty of resources in the world. Cellulose, lignin, and sucrose could be mixed and ground to simulate ternary biomass carbonaceous precursors for the synthesis of porous AC, as reported by Xue et al. [25]. Plant-based biomass is particularly attractive due to its abundant reserves, rapid regeneration, and diverse sources. Animal bones and excreta have also been taken as biomass precursors for AC by researchers. Using them as carbon precursors can result in layered porous carbon materials with guest atomic doping and can also protect the environment. Microorganisms have also obtained much attention as a precursor for the fabrication of porous carbon due to their sub-cellular structures. Among these three types of precursors, using plants as AC precursors is favored because of their ample sources and short growth periods. The biomass residues from plants including leaves, nut shells, and nuts are also sustainable and enriched with lignin and cellulose. The advantages and shortcomings of the three types of precursors have been summarized [26], as shown in

Table 1. Comparison of biomass sources for BDC.

Precursor	Advantage	Shortcoming
Plants	(1) Easy to obtain.	(1) High content of impurities, such as silicon.
	(2) BDC with special structures.	(2) Complex preparation process.
Animal bones and excreta	(1) Full utilization of pollutants.	(1) Single preparation process.
	(2) Rich in heteroatoms.	(2) Abundant by-products.
Microorganisms	(1) Easy to re-generate.	(1) Long synthesis process.
	(2) Rich in polysaccharide chains.	(2) High cost.

.

4. Carbon Electrodes Prepared from Different Biomass Sources

To achieve a high specific capacitance for EDLCs, one of the important criteria is the use of carbon-based electrode materials with both high specific surface areas and high conductivity. The common strategy to convert biomass to porous AC usually involves carbonization followed by various activation processes. Thus, the function of the activator is to generate additional pores in the carbon and improve the specific surface area via increasing the activation temperature, leading to a rise in specific surface area [27]. Some important conditions in the activation process have been investigated by researchers. For instance, Hegde et al. [28] found that KOH-activated Tectona grandis under different temperatures (650–850 °C) produced porous carbon and that the sample activated at 850 °C exhibited the highest surface area as well as the largest pore volume and a specific capacitance of 572 F g⁻¹ at 0.5 A g⁻¹ in 6 M KOH among the samples. Other biomass solids such as spent coffee grounds [29], thistle [30], cotton stalks [31], peanut shells [32], mangifera indica leaf [33], hemp hurd sticks [34], abutilon theophrasti [35], banana leaves [36], pistachio shells [37], walnut green seedcases [38], cynomorium songaricum [39], grapevine [40], cotton meal [41], almond shells [42], semen ziziphi spinosae [43], waste bamboo fibers [44], ceiba speciosa flowers [45], tea leaves [46], zanthoxylum armatum seeds [47], camphor leaves [48], sugarcane bagasse [49], and sawdust [50] have also recently been reported for the preparation of carbon electrodes.

4.1. Plant-Derived Carbon Electrodes

Plants have their own advantages for producing porous carbons, including abundant resources, unique cell structures, short growth periods, and low costs. Thus, they are the most suitable biomass precursors. They are organic materials made up of elements such as carbon, oxygen, hydrogen, nitrogen, etc., and have substantial amounts of cellulose, hemicellulose, and lignin, where these substances create tissues with complex three-dimensional (3D) channel structures. Their fruits, flowers, and shells are also potential precursors. Meanwhile, different plants have different distributions of mineral elements and microstructure networks.

O and N co-doped carbon originating from eucalyptus bark could be prepared by a combined process of heteroatom doping assisted by molten salt (KCl/LiCl) and KOH activation at 600 °C [51]. The optimized electrode exhibited a high surface area of 1719.15 m² g⁻¹ and a high specific capacitance of 483.5 F g⁻¹ at 0.5 A g⁻¹ in 1 M H₂SO₄ as the electrolyte. The effects of three different chemical activators (KOH, K₂CO₃, and CuCl₂) and ratios of C/KOH on the pore volume and electrochemical performance of carbon synthesized using camellia seed shells were researched by Yang et al. [52], where KOH was the most effective for generating porous structures. It provided an excellent case for the high-value application of agricultural waste. Dong et al. [53] also prepared porous carbon by activating discarded sunflower plates using various alkaline activators (KOH, K₂CO₃, and KCl) at different temperatures. The effect of KOH was also the most potent for SC electrodes, though it was corrosive and had some adverse impacts on the environment and the equipment.

Some gramineous straws have been employed to fabricate porous carbon electrodes for SCs because there are several hundred million tons of various plant straws available across the world every year. Recently, Liu et al. [54] reported that all the key components of flexible SCs including the electrode, separator, and electrolyte could be all made from the eulaliopsis binata (EB) straw, as schematically shown in Figure 2. By ingeniously utilizing the structures and composition features of different parts of EB, the reported method realized the multiple utilization of EB biomass for SCs. The symmetrical SCs assembled with EB-based carbon electrodes and separators had a high specific capacitance (356 F g⁻¹ at 0.5 A g⁻¹) and excellent rate capability in the KOH electrolyte. This work shed novel light on the full utilization of biomass for SCs. However, its preparation process is somewhat complicated.

Fully utilizing the chemical compatibility between ramie chain and phenolic chain, a self-crosslinked approach to prepare high-yield carbon using ramie as a precursor was reported by Wang et al. [55]. The ramie embedded with KOH molecules underwent condensation reaction with the phenolic hydroxyl group in the phenolic resin, keeping the ramie structure well. The presence of ammonium groups accelerated the self-crosslinked reaction to produce large aggregation chains and form a crosslinked structure. Thus, the above process could enhance the carbon yield to above 31%, much higher than the pure-ramie-based carbon (3.56%) and pure-phenolic-derived carbon (15.22%). The carbon electrode had a low specific capacitance of 39.03 F g⁻¹ at 40 A g⁻¹ in 1 M organic electrolyte. Moso bamboo could be treated by a combined process of hydrothermal carbonization and activation to produce 2D carbon which had crosslinked carbon nanosheets, hierarchical porous structures, and highly electrochemically active oxygenic groups [56]. The rich oxygenic groups in carbon were beneficial for enhancing energy storage capability because the oxygenic groups had a positive effect on the absorption of potassium ions in the electrolyte. It showed high specific capacitance values of 409.5 and 308.33 F g⁻¹ at 0.5 and 20 A g⁻¹, respectively.

4.2. Carbon from Animal Bones and Excreta

In addition to other biomass precursors, animal bones and excreta have also been reported on by many researchers. Animal bone is a naturally formed composite chiefly composed of inorganic minerals and organic materials. The organic substance is the source of carbon and nitrogen, leading to it being rich in many heteroatoms. Recently, a honeycomb-like hierarchical porous carbon material was prepared by a self-template coupled dual-hydroxide (NaOH/KOH) activation strategy using mantis shrimp shell as the precursor, as reported by Wei et al. [57]. The activation conditions were optimized, and the optimal carbon had a high specific surface area of up to 2465 m² g⁻¹ and a specific capacitance of 300.3 F g⁻¹ at 0.05 A g⁻¹. There were plenty of nitrogen species and oxygen-containing groups, which contributed to additional pseudocapacitance. A carbon material with a high surface area was made from waste marine sponge using the pyrolysis technique, where the high specific surface area of 519.82 m² g⁻¹ and oxygen/nitrogen groups on the carbon surface contributed to ion storage and pseudocapacitive behavior, showing a specific capacitance of 287.33 F g⁻¹ at 0.5 A g⁻¹ [58].

Silk is a fine, soft, lustrous fiber produced by a silkworm, and it is also reported as a precursor of carbon. An immiscible liquid-mediated approach was reported to increase the ionic conductivity of nitrogen-doped porous carbon electrodes derived from silk, where silk was deconstructed in LiBr solution to prepare silk protein solution and NaCl as a template during the followed freeze-drying process [59]. The introduction of immiscible organic liquid into the carbon promoted the ion transport speed in the inner pores of the electrodes. A composite of FeO_x quantum-dot-modified carbon derived from silk fibroin was also prepared by hydrothermal synthesis, where FeO_x dots were evenly anchored with the carbon material as a conductive carrier, which resulted in an excellent rate performance and cycle performance [60]. Figure 3a shows the spherical morphology with smooth edges and internal pores. The lattice stripe spacing in Figure 3b is attributed to the (210) plane of Fe₃O₄. The structure enhances the conductivity and specific surface area.

Electrospun carbon nanofibers using a single and pure polymer usually have a solid structure and low specific surface areas, thus leading to rather poor electrochemical behavior. Cuttlefish ink as a main structural unit of self-supporting fibers was reported [61]. The carbon nanofibers were prepared by electrospinning and carbonization of the mixture of polyacrylonitrile, cuttlefish ink, and polymethyl methacrylate. Cuttlefish ink could significantly increase the pore structure and make it flexible, constructing necklace-like carbon nanofibers. With hollow channels, a high specific surface area, and a large nitrogen content, the flexible electrode of carbon nanofibers had a specific capacitance of 364.8 F g⁻¹ at 0.5 A g⁻¹.

4.3. Microorganism-Originated Carbon

Microorganisms can be classified into bacteria, fungus, and algae, consisting of proteins, carbohydrates, lipids, and nucleic and amino acids. Fungal biomass as a raw material to produce carbon has attracted the attention of several research groups due to the rapid growth of fungi and the large amount of chitin in its structure [62]. Meanwhile, fungal biomass is applied in the biosynthesis of nanoparticles and the process is also called “green” synthesis. Bazzana et al. [62] reported a sustainable electrode material for SCs by anchoring Ag nanoparticles onto carbon derived from fungal biomass. Compared with plants, microorganisms are less popularly used as porous carbon precursors. In various precursors of microorganism, mushrooms and algae have been popularly reported in the recent years.

N and S co-doped carbon prepared from needle mushrooms through a one-step fast pyrolysis process was reported by Guo et al. [63]. The fast pyrolysis led to a fast release of volatile gases, resulting in the coalescence of some micropores into the more-developed pore structure. Porous carbon with abundant mesopores was prepared by using waste Ganoderma as a precursor, which was then selected as a supporting material to in situ grow NiMoO₄/CoMoO₄ nano-flower balls on its surface via the hydrothermal method to obtain a composite [64]. The composite electrode had a high specific capacitance of 802 F g⁻¹ at 1 A g⁻¹.

Enteromorpha prolifera has adverse effects on marine ecological balance. It is a common seaweed whose large-scale growth has been observed under conditions of water eutrophication. Li et al. [65] activated it using different activators, including KOH, ZnCl₂, and H₃PO₄, and systematically evaluated the resultant carbon properties. The KOH-activated carbon exhibited better electrochemical performance than the others, with a specific capacitance of 256 F g⁻¹ at 1 A g⁻¹. Li et al. [66] reported a strategy to synthesize high-energy quasi-solid-state SCs, where the electrode materials, binder, and electrolyte were entirely derived from sodium alginate, as schematically shown in Figure 4. The N-doped carbon with rich hierarchical pores and a high nitrogen content could be synthesized by the direct in situ carbonization of Ca²⁺-crosslinked alginate hydrogel and urea. Sodium alginate (SA) with abundant hydrophilic groups as a binder could improve water wettability and reduce resistance. The interpenetrating alginate and polyacrylamide (PAM) network of the ZnSO₄ electrolyte enhanced the mechanical strength, electrolyte retention rate, and ionic conductivity. The supercapacitor delivered a specific capacitance of 180 F g⁻¹ at 0.25 A g⁻¹. This work highlights the high-value utilization of sodium alginate for high-energy SCs.

The abundance of heteroatoms in microorganisms implies that they are suitable for the preparation of heteroatom-doped porous carbon. A natural heteroatom doping strategy was reported, using bamboo and spiral algae as carbon sources, and spiral algae as a natural heteroatom (N, O, S) dopant [67]. N/O/S co-doped carbon was prepared via co-pyrolysis of spiral algae and bamboo, which was a simple synthesis route. After activation by potassium acetate-coupling natural heteroatom doping, the resultant porous carbon had a capacitance of 320.4 F g⁻¹ (0.5 A g⁻¹). Spiral algae were also used as the raw material through a strategy combining the one-step carbonization method, hard template technique, and activation approach to prepare multi-heteroatom self-doped carbon [68]. Self-sourced nitrogen-doped carbon was also synthesized using algae-derived bio-oil distillation residue as the precursor [69].

4.4. Other Biomass-Based Carbon

In addition to three above-mentioned types of biomass precursors, other biomass-based porous carbon materials have also been reported as SC electrodes. Some biomass waste materials are highly advantageous for the preparation of porous carbon due to their abundant, cost-effective, and renewable nature. They may contain a lot of heteroatoms, including nitrogen (N), phosphorous (P), sulfur (S), oxygen (O), etc., in their natural structure, which can result in a self-doping process. For instance, distiller grains are a by-product generated during the process of liquor production. Han et al. [70] reported that distiller grain powder was carbonized and activated by KOH to generate N,O self-doped AC. A large number of pores were formed by the direct activation, which could provide a lot of active sites for the adsorption of the electrolyte. The carbon material exhibited a high specific capacitance of 345.2 F g⁻¹ at 1 A g⁻¹. A porous carbon material with multiple heteroatom dopants was prepared using rapeseed meals via hydrothermal treatment and activation at a high temperature [71]. It exhibited a high specific surface area of 3291 m² g⁻¹ and was doped by nitrogen (1.05 at.%), oxygen (7.4 at.%), and phosphorus (0.31 at.%), exhibiting a specific capacitance of 416 F g⁻¹ (at 1 A g⁻¹). The carbon was self-doped with heteroatoms because of the abundant components in it. Spent mushroom substrates were also employed to prepare porous carbon for SCs via pre-carbonization and activation processes [72]. Rose petals were selected and activated by K₂CO₃ to prepare porous carbon with a specific capacitance of 320.8 F g⁻¹ at 1 A g⁻¹ [73].

A solid-phase activation method was introduced by Geng et al. [74] to synthesize O-functionalized porous carbon with embedded graphene-like structures using D(+)-glucose as a precursor. The porous carbon had a very high surface area (3572 m² g⁻¹) with rich oxygenated functional groups on the surface when it was treated by KOH at 500 °C, where the activation agent introduced oxygen functional groups on the carbon surface. The optimized carbon showed a high specific capacitance of 305 F g⁻¹ and 207 F g⁻¹ at 0.5 A g⁻¹ and at 10 A g ⁻¹, respectively.

5. Strategies to Improve the Specific Capacitance of Biomass-Based Carbon

The limited energy density of SCs still significantly hinders their commercial viability. Despite their numerous benefits, the specific capacitance of carbon electrodes prepared from biomass must be further increased in order to meet the future demands of SCs. There are some strategies that have been developed by researchers to further enhance the specific capacitance of biomass-derived carbon. In the following section, we will introduce the recently developed strategies to enhance the energy storage ability related to BDC electrodes.

5.1. Doping with Heteroatoms

Although SCs made of carbon electrodes have been commercialized, their low energy density has limited their widespread application to a large extent. Heteroatom doping has proved effective in improving the specific capacitance of carbon-based electrodes for SCs, because the incorporation of heteroatoms (such as N, S, P, etc.) into BDC has been proven to improve the surface wettability and the electrode–electrolyte interaction.

Nitrogen is one of the most popularly used heteroatoms to dope BDC in order to obtain an enhanced specific capacitance. N-doping in the carbon skeleton is readily carried out owing to the comparable covalent radius of nitrogen (0.74 Å) and carbon (0.77 Å). Nitrogen atoms do not alter the fundamental structure of carbon materials significantly. The electronegativity of nitrogen atoms guides the conduction of induced current within the carbon skeleton, thereby enhancing the stability between particles. N-doped BDC from red beetroot was prepared by heating pre-carbonized powder, KOH, and urea at 800 °C in a nitrogen atmosphere [75]. Compared with the materials without N-doping and KOH activation, the N-doped BDC had a larger surface area with a sponge-like structure and a higher specific capacitance of 492 F g⁻¹ at 1 A g⁻¹ in 1 M Na₂SO₄ as the electrolyte. Similarly, N-doped porous carbon from marula nutshell was obtained by KOH treatment and with melamine as the source of nitrogen through ex situ doping [76]. It displayed a specific capacitance of 350 F g⁻¹ in 6 M KOH and 248 F g⁻¹ in 2.5 M KNO₃. It was reported that KOH treatment could change the states of nitrogen atoms in the carbon materials. Via one-step KOH activation of melamine and houttuynia cordata, nitrogen-doped BDC could be synthesized and the etching effect from KOH promoted the change of N-Q (quaternary nitrogen) to N-6 (pyridinic N) and N-5 (pyrrolic N) [77]. The carbon electrode had a high specific surface area of 2491.5 m² g ⁻¹ with a specific capacitance of 220.54 F g⁻¹ at 0.5 A g⁻¹. The N-doped carbon prepared from hazelnut shell as the electrode material had a specific capacitance of 334 F g⁻¹ at 0.5 A g⁻¹ [78]. The authors claimed that these nitrogen-containing groups on the carbon surface endowed it with a high pseudocapacitance, in addition to EDLC contribution.

More recently, Chen et al. [79] compared four different nitrogen sources and investigated various nitrogen species in order to determine the most suitable nitrogen source for biomass carbon, where Job’s tears straw was a biomass precursor. The results showed that melamine was the most suitable for biomass-based SCs with a high surface nitrogen content of 4.54%, and the pyrrole nitrogen contributed greatly to the capacitor performance. Numerical calculation showed that the large amount of pyrrole nitrogen provided by melamine played a critical role in the adsorption of electrolyte ions, thus delivering a specific capacitance of 349.3 F g⁻¹ at 0.5 A g⁻¹. The relationship between the content of pyrrolic nitrogen and the electrochemical performance of hydrothermal carbon by utilizing grapevine as the carbon source was explored by Li et al. [80], where coffee residue, rich in heteroatoms, was chosen as the doping agent. Figure 5 shows an overview of the experimental process, in which the hydrochar from grapevine and coffee is treated by KOH in an argon atmosphere to obtain N-doped carbon. They found that excessive N-doping led to the conversion of pyrrolic N into graphitic N, reducing the effect of heteroatom doping.

In addition to nitrogen, other heteroatoms including P and S have also been applied to dope carbon. The introduction of heteroatoms (P) and oxygen surface functionalities into the cellulose allowed for the tuning of the electrochemical performance of SCs where honeydew peel was a source of carbon [81]. A high phosphorous doping degree enabled the transportation of ions at higher current rates. P-doping on carbon could also be achieved by hydrothermal treatment with orthophosphoric acid and carbon from coconut husk [82]. Meanwhile, S-doped carbon electrode was also synthesized [83].

In addition to single-atom doping, the compatibility between carbon and electrolytes with significant synergistic effects can be achieved by polyatomic doping. Thus, multi-atom doping has also been reported in recently published papers. B,N co-doped porous carbon composites prepared with wheat-straw-based cellulose nanofibers had desirable contents of B and N dopants, showing a specific capacitance of 433.4 F g⁻¹ at 1 A g⁻¹ [84]. N- and P-doped BDC originating from sugarcane bagasse synthesized via a hydrothermal approach with subsequent KOH activation had a typical hierarchically porous structure, a large interlayer spacing, a high surface area, and a high specific capacitance of 356.4 F g⁻¹ at 1 A g⁻¹ where melamine and NaH₂PO₄ were dopants [85]. N,O-doped BDC originating from garlic peels was fabricated by pre-carbonization and activation with KHCO₃/melamine [86]. High contents of O and N enhanced the wettability of the materials and introduced additional pseudocapacitance. It obtained a specific capacitance of 396.25 F g⁻¹ at 1 A g⁻¹ with good rate capability. N,O co-doped BDC originating from rapeseed stalks also exhibited a high specific surface area (3085 m² g⁻¹) and a specific capacitance of 354.7 F g⁻¹ at 1 A g⁻¹ [87], where N was the naturally present heteroatom in the biomass. N,S-doped paper-fiber carbon foam was synthesized via a foaming technique and thiourea doping [88]. The optimized material had a specific capacitance of 245.24 F g⁻¹ at 0.1 A g⁻¹. Co-doping of carbon materials by P and N for their application as electrodes in SCs was carried out through conventional activation by H₃PO₄, using chitosan of different molecular weights (low, medium, and high) as a N-containing natural precursor [89]. N,S co-doped carbon was carried out via carbonizing the waste fibers of sugarcane bagasse in the presence of H₂S and N₂H₄ as sources of sulfur and nitrogen, respectively, with subsequent liquid-phase and gas-phase activation [90]. A very large surface area of 2455.6 m² g⁻¹ and a high specific capacitance of 405.67 F g⁻¹ at 0.2 A g⁻¹ were both achieved for the material.

B/N/P-doped carbon was synthesized via the hydrothermal method and KOH activation using orange peel as a raw material for the SC electrodes [91], where (NH₄)₂HPO₄ and boric acid were used as dopants. It had high specific capacitances of 318.8 F g⁻¹ and 289 F g⁻¹ at 1 A g⁻¹ and 5 A g⁻¹, respectively, with only 6.4% loss after 10,000 cycles. The above results from recent papers indicate that heteroatom doping can enhance the performance of BDC. The electrochemical properties and doping agents of heteroatoms are summarized in

Table 2. Electrochemical behavior of biomass-derived carbon doped with various heteroatoms.

Biomass Precursor	Doping Atom	Doping Agent	Specific Capacitance * (F g−1)	Specific Current (A g−1)	Electrolyte	Ref.
Red beetroot	N	Urea	492	1	1 M Na2SO4	[75]
Marula nutshell	N	Melamine	350	1	6 M KOH	[76]
Houttuynia cordata	N	Melamine	220.5	0.5	6 M KOH	[77]
Hazelnut shell	N	Polypyrrole	334	0.5	6 M KOH	[78]
Job’s tears straw	N	Melamine	349.3	0.5	6 M KOH	[79]
Grapevine	N	Coffee residue	551.25	0.5	6 M KOH	[80]
Honeydew peel	P, O	H3PO4	486	0.6	1 M H2SO4	[81]
Coconut husk	P	Orthophosphoric acid	480.06	1	6 M KOH	[82]
New Zealand slash	S	H2SO4	148	0.5	6 M KOH	[83]
Wheat straw	B, N	Boron nitride	433.4	1	6 M KOH	[84]
Sugarcane bagasse	N, P	Melamine and NaH2PO4	356.4	1	6 M KOH	[85]
Garlic peels	N, O	Melamine	396.25	1	6 M KOH	[86]
Rapeseed stalk	N, O	Self-doping	354.7	1	6 M KOH	[87]
Paper fiber	N, S	Thiourea	245.24	0.1	6 M KOH	[88]
Sugar mills	N, S	Hydrazine and H2S	405.67	0.2	6 M KOH	[90]
Orange peel	B, N and P	(NH4)2HPO4 and boric acid	318.8	1	6 M KOH	[91]

* The values were measured in three-electrode configurations.

. The values of specific capacitance are distributed in a wide range, even though some reports have the same electrolyte and current. This can be attributed to the different carbon sources, structures, and doping contents, as well as various mass loadings in the electrode. It can be seen that most BDC electrodes display high specific capacitances of more than 200 F g⁻¹. However, different dopants are involved during the preparation process. Some dopants can be completely decomposed into gases at elevated temperatures, such as urea, melamine, etc. However, some will leave residuals after doping. Thus, an additional purification process is necessary, which will inevitably increase the final production cost.

To evaluate the practical applications of the optimized BDC materials, symmetric supercapacitors are usually assembled. Energy and power density are two meaningful parameters to illustrate the energy storage performance. The energy density and power density of the SCs can be easily measured in two-cell systems. The electrochemical properties of symmetric SCs with the same two BDC electrodes prepared using different biomass are displayed in

Table 3. Properties and electrochemical performance of symmetric SCs using BDC electrodes doped with different heteroatoms.

Biomass Precursor	Doping Atom	Cell Voltage (V)	Specific Power (W kg−1)	Specific Energy * (Wh kg−1)	Electrolyte	Ref.
Red beetroot	N	2	500	12	1 M Na2SO4	[75]
Marula nutshell	N	2	448.7	17.2	2.5 M KNO3	[76]
Houttuynia cordata	N	1.3	162.33	9.82	6 M KOH	[77]
Houttuynia cordata	N	1.5	187.34	8.1	1 M Na2SO4	[78]
Hazelnut shell	N	1.2	300.4	11.4	6 M KOH	[78]
Job’s tears straw	N	1.4	169.9	20.3	6 M KOH	[79]
Grapevine	N	1.0	500	49.52	6 M KOH	[80]
Coconut husk	P	1.5	375	12.5	6 M KOH	[82]
New Zealand slash	S	1.0	250	15.3	6 M KOH	[83]
Sugarcane bagasse	N, P	1.0	251.9	6.5	6 M KOH	[85]
Garlic peels	N, O	1.0	309.2	9.07	6 M KOH	[86]
Rapeseed stalk	N, O	1.2	150.01	17.05	6 M KOH	[87]
Paper fiber	N, S	1.0	500	13.24	6 M KOH	[88]
Orange peel	B, N and P	1.0	499.7	8.9	6 M KOH	[91]

* The values were measured in two-electrode configurations.

. It can be seen that the energy density is mostly less than 20 Wh kg⁻¹. The work voltages of the symmetric SCs are from 1.0 to 2.0 V in aqueous electrolytes. This means that further research to improve the energy density is necessary.

5.2. Designing Novel Composites

BDC with a 3D interconnected porous structure can provide a continuous channel and shorten the diffusion path length for electrolyte ions. Though it has high cycling stability and excellent rate capability, it suffers from a relatively small energy density and low specific capacitance. To enhance specific capacitance, an effective method is to deposit a pseudocapacitive substance into the carbon electrodes. Thus, a composite consisting of BDC and guest materials is expected to have improved energy storage capacity. The guest substance contributes to the high specific capacitance, and the carbon skeleton prevents aggregation and ensures the rapid transfer of electrolyte ions. Recently, various novel composites have been prepared for SC electrodes, such as NiCo-layered double hydroxide (LDH) grown on peanut-shell-based carbon [92], NiCo₂S₄/carbon derived from Zanthoxylum bungeanum branches [93], MnCO₃/mango peel BDC [94], NiCo-LDH/MnO₂@carbon from wood [95], NiO/carbon from sugarcane bagasse [96], NiO/carbon from putranjiva seeds [97], and NiO/walnut-shell-based carbon [98].

In addition to the above-mentioned metal oxides and hydroxides, metal sulfides have been chosen to prepare composites with BDC. An activated hollow tubular biomass carbon (BC) derived from typha orientalis was employed as a conductive substrate for the uniform growth of NiCo-LDH nanosheets [99]. After a partial in situ sulfidation process, CoNi₂S₄ could be embedded in the interlayer of NiCo-LDH nanosheets. The synthesis process and the structure features of the trinary composite are schematically shown in Figure 6. The unique heterostructure enhanced ion adsorption capacity, leading to a significant improvement in ion/electron reaction kinetics, and the robust BC framework prevented electrode degradation. The combination of NiCo-LDH/BC and CoNi₂S₄ resulted in the formation of p-n heterojunctions, showing a refined energy-band structure with reduced bandgaps. The island-like CoNi₂S₄ particles could also generate more active edges. These typical advantages enabled the composite electrode to achieve a high specific capacity (1655.75 C g⁻¹ at 1 A g⁻¹). A hybrid capacitor assembled with it and AC had a large energy density of 95.57 Wh kg⁻¹ at 866.61 W kg⁻¹.

A composite composed of 3D N-doped carbon and nickel-iron sulfide nanoparticles was fabricated by a simple pyrolysis process, as reported by Sun et al. [100], with chitosan as a carbon precursor. The composite exhibited excellent charge storage ability owning to the integration of binary nickel-iron sulfides and 3D hierarchical porous structures, having a specific capacitance of 2812.4 F g⁻¹ at 0.5 A g⁻¹. Small nickel-iron sulfide particles in the carbon matrix could provide rich reaction sites for reduction and oxidation reactions to store more energy.

In addition to pseudocapacitive materials, some carbon materials were also selected to make composites using BDC as a component. A lignin depolymerization product was mixed with phenol and formaldehyde to generate a lignin-based phenolic resin [101]. The carbon from lignin-based phenolic resin exhibited a 3D linkage structure and a graphitized structure that improved charge mobility, and it showed a specific capacitance of 108.2 F g⁻¹ at 1 A g⁻¹. Carbon aerogel was prepared with cellulose fibers, graphene oxide, and polyimide nanofibers, and it showed excellent compressibility, flexibility, and cycling stability as SC electrodes [102]. The biowaste of litchi seed was used to synthesize 3D activated carbon by carbonization and activation [103]. Then, a 3D composite could be prepared as a negative electrode by integrating 2D reduced graphene oxide and 1D carbon nanotubes with the 3D activated carbon as a support matrix. The new 3D composite had a specific capacitance of 320 F g⁻¹ at 1 A g⁻¹ using 6 M KOH as the electrolyte. Similarly, 3D activated carbon was also used as a substrate to grow zinc-cobalt sulfide nanoparticles as a positive electrode [103]. Finally, asymmetric capacitors based on the above two electrodes exhibited promising a electrochemical performance, with enhanced energy density, high cycling stability, and high capacitance. More recently, Cheng et al. [104] applied jute fibers as a precursor and fabricated AC via the combined methods of hydrothermal synthesis and activation. They discovered that graphene nanosheets spontaneously formed on the edge or surface of AC particles during the process by KOH activation, as exhibited in Figure 7. The composite displayed a specific capacitance of 220 F g⁻¹ at 0.5 A g⁻¹. In Figure 7a,b, it can be seen that graphene deposits on the edge or surface of BDC. A high-resolution TEM (HRTEM) image, presented in Figure 7c, confirms the existence of layered graphene and clearly exhibits graphite interplanar spacing (about 0.34 nm). The corresponding electron diffraction pattern in Figure 7d displays (002) and (100) planes from graphite. KOH can aid in the generation of graphene, in addition to micropores.

Some conductive polymers have also been coupled with BDC to synthesize composite SC electrodes. For example, Lv et al. [105] reported a binary composite in which pseudocapacitive polypyrrole was deposited on the hierarchical BDC originating from Chinese fir to improve its energy density and specific capacitance. The weight-specific capacitance of the optimal electrode was 374.1 F g⁻¹ at 1 A g⁻¹. Thus, some guest materials including pseudocapacitive, carbon-based, and conductive polymeric materials have been incorporated with biomass carbon in order to improve its conductivity and guarantee rapid transfer of electrolyte ions. Due to the obvious difference of guest materials and biomass precursors, the electrochemical performance, typically the specific capacitance, of the resultant composites varies to a large extent.

5.3. Innovative Processes

The production of AC from biomass usually involves two sequential steps, i.e., carbonization and activation. Carbonization is the thermal decomposition of biomass precursors at high temperatures under oxygen-free conditions, chiefly including pyrolysis, hydrothermal carbonization, and microwave-assisted carbonization, with pyrolysis being the most widely applied. The activation process can be frequently categorized into two approaches, chemical and physical activation. He et al. [106] employed four different methods to prepare BDC to systematically investigate the effects of preparation processes (one-step/two-step) using walnut shells as the raw material. The influences of the synthesis process, heating rate, and carbonization temperature on the product yield, electrochemical performance, and economic benefits were investigated. In addition to the above traditional conditions, some novel processes have also been developed by researchers to improve the electrochemical behavior. The chief purpose of these new processes is to modify the surface functional groups and porosity of carbon, because the BDC surface plays a significant role in determining its specific capacitance. To improve the surface area of carbon, it is crucial to ensure the presence of a well-developed porous structure. BDC can inherit the precursor structure by tuning the functional surface during the carbonization process, resulting in the product delivering high capacitance and long-term stability.

KOH is a common activation agent in producing porous carbon due to its good ability to generate a substantial specific surface area. It is more effective than both H₃PO₄ and ZnCl₂ to activate pre-carbonized peanut shell [107]. Physically activated carbon through gaseous CO₂ activation at 850 °C was reported by Ahmad et al. [108], with date seed biomass as the precursor, where the activated carbon exhibited a low specific surface area of 659.56 m² g⁻¹. Although physical activation has environmental benefits, its efficiency is lower than that of the chemical method.

The surface modification of BDC has been reported by using chemicals and air oxidization. The acid-functionalized carbon made from the inner skin of arachis hypogaea significantly outperformed the non-functionalized one, showing a fourfold increase in specific capacitance [109]. Two different modification methods were applied by using camphor tree as the carbon precursor [110]. Through chemical activation by nitric acid, nitrogen functional groups were generated on the surface of carbon. After treatment by copper chloride, the specific surface of carbon could be enlarged. The electrochemical measurements indicated that the carbon after the above two treatments had a high specific capacitance. Mesoporous BDC was prepared using coconut fibers by H₃PO₄-assisted hydrothermal treatment combined with a low-ratio KOH hydrochar activation [111]. Hydrothermal treatment could increase the number of micropores of the carbon by hydrolyzing the β-glycosidic linkage of the aryl ether bond of lignin and hemicelluloses. The following KOH activation would develop a number of mesopores and oxygen groups. The carbon electrode yielded a specific capacitance of 315.5 F g⁻¹ at 1 A g⁻¹. A coordinated regulation method consisting of carbonization at 450 °C, ZnCl₂ activation at 400–800 °C, and air oxidation at 200–350 °C could efficiently improve the pore structure and electrochemical behavior of BDC from bamboo. The additional air oxidation on the basis of 600 °C activation showed the most outstanding optimization effect, which could be further strengthened by the increased oxidation temperature in air, as reported by Qin et al. [112]. Air oxidation following 600 °C activation could improve the mesoporous rate and simultaneously introduce more oxygen-containing groups, including carboxyl (–COOH) and carbonyl (C=O) groups, as evidenced by XPS analysis. The optimized sample had a specific capacitance of 256 F g⁻¹ at 1 A g⁻¹.

In addition to surface modification, some novel treatments of fresh biomass via chemical method can also enhance the chemical performance and activation effect. Recently, Li et al. [113] treated wheat straw with a Fenton reagent of Fe³⁺ and H₂O₂ to obtain a crumbly and porous 3D structure. The presence of iron ions appeared to promote more hydroxyl radicals in the H₂O₂ solution, which enhanced the oxidation of the hemicellulose, cellulose, and lignin in wheat straw, and destroyed the network structure of the original macromolecules. After carbonization and activation, a hierarchical porous material was achieved, with a high specific capacitance of 425.2 F g⁻¹ at 1 A g⁻¹ and a high specific surface area of 3440 m² g⁻¹. Similarly, Yang et al. [114] also synthesized porous carbon by pretreating biomass with Fe³⁺/H₂O₂ Fenton-like reagent using agricultural waste biomass. The hydroxyl radicals could oxidize the biomass by increasing the porosity and enhancing the followed activation efficiency. The prepared carbon electrode had a specific capacitance of 349 F g⁻¹ at 0.5 A g⁻¹.

Some novel methods are also reported to modulate pore structures of BDC. The liquefaction process can degrade high-molecular solids into low-molecular liquid products with active groups. A solid–liquid state conversion was realized under the liquefaction condition of the green organic solvent polyethylene glycol (PEG400) using soybean straw as a raw material and further in situ doping of the N and S liquid phase under hydrothermal reaction [115], improving the active sites of soybean straw liquefaction products. The pore structure of carbon was controlled with non-toxic potassium citrate. The carbon had a specific capacitance of 220 F g⁻¹ at 0.5 A g⁻¹. The hydrogel-controlled carbonization of glucose was applied to produce AC with controllable pore structures and high surface areas, where polyacrylamide acted as a pore-forming agent, and pore structures could be controlled by the content of polyacrylamide [116]. The obtained carbon had a specific capacitance of 441 F g⁻¹ at 0.25 A g⁻¹. Apple-waste-derived carbon was prepared by a two-step method including carbonization and chemical activation with KMnO₄, where KMnO₄ not only served as an activator for porosity but also enhanced electrochemical performance by forming abundant oxygen-rich groups [117].

Jiang et al. [118] prepared N,O co-doped hierarchical AC material via a one-step carbonization and activation treatment of Chinese yam biomass with ZnCl₂ as an activator, using NH₄Cl and dopamine from Chinese yam as the nitrogen sources. By changing the aggregation degree of zinc-containing hydrolysates in biomass and the subsequent release of large amounts of gas, the carbon had rich interconnected micropores and controllable small-sized mesopores. As schematically shown in Figure 8a–d, the aggregated zinc-containing substances grow larger with increasing ZnCl₂ content and act as an activator, etching carbon atoms to form a uniform mesoporous structure. TEM images in Figure 8e–j show the creation of many mesopores and micropores in the carbon matrix. Via the precise control of pore structures and surface areas, the electrode could achieve fast ion transport and enhance energy storage capability. Owing to micropore-dominant pores and high N and O contents, a high gravimetric specific capacitance of 414 F g⁻¹ at 1 A g⁻¹ was achieved. Recently, Narayanan et al. [119] reported that the chemical treatment of KOH-activated carbon from grape marc with ZnCl₂ would lead to the expansion of micropores into mesopores.

To summarize,

Table 4. Electrochemical properties of symmetric SCs using BDC electrodes via novel processes.

Biomass Precursor	Modification	Cell Voltage (V)	Specific Power (W kg−1)	Specific Energy * (Wh kg−1)	Electrolyte	Ref.
Peanut shell	KOH activation	1.3	319.97	22.2	6 M KOH	[107]
S kin of Arachis H ypogaea	Acid functionalization	1.3	325	23.17	3 M KOH	[109]
Camphor tree grains	C opper chloride activation	1.0	250	12.5	6 M KOH	[110]
Coconut fiber	H3PO4-assisted hydrothermal	1.0	504.1	9.2	6 M KOH	[111]
Bamboo	Air oxidation	1.8	225	12.54	1 M Na2SO4	[112]
Agricultural waste	Fenton-like reagent treatment	1.8	450	26.2	1 M Na2SO4	[114]
Soybean straw	Potassium citrate activation	1.8	400	11.24	1 M KOH	[115]
Chinese yam	ZnCl2 (activating agent) and NH4Cl	1.6	102.9	22.9	1 M Na2SO4	[118]

* The data were measured in two-electrode configurations.

shows the biomass source, novel modification process, and electrochemical properties of BDC prepared from different sources. The energy density is ranged from about 10 to 23 Wh kg⁻¹. The variety of energy density is attributed to many factors including different source types, surface functional group, pore structure, and surface area. The novel processes to modify BDC are also effective in improving the energy storage ability.

5.4. Improving Graphitic Degree

Generally, fresh plant precursors, consisting of cellulose, hemicellulose, and lignin, play an important role in the polymorphism of BDC because they essentially influence the graphitization degree of carbon. Cellulose-derived carbon with parallel hydrogen bond networks has remarkable crystallinity for the improvement of electrical conductivity. The existence of hemicellulose in biomass strongly hinders the lateral growth of carbon crystals, and the presence of lignin extensively forms amorphous carbon. Thus, BDC usually exhibits an amorphous nature. Both the low graphitization degree and content in carbon will reduce its conductivity. It is still crucial for conductive BDC to rationally develop highly graphitized microcrystals because they will form excellent conductive networks for increasing conductivity. The creation of a graphitic framework in BDC is frequently used to enhance the conductivity and electrochemical performance. For example, annealing the carbon derived from pumpkin skin under H₂ atmosphere could improve the specific surface area and create a graphitic-like structure with mesopores [120]. This graphitization of porous carbon enhanced ion transport and helped to enhance the conductivity.

Regulating cellulose content in biomass can modulate the graphitization degree of BDC. Wang et al. [121] applied a simple capillary evaporation strategy to prepare high-density conductive carbon with ramie as the precursor. This chemically stripped ramie had a high content of cellulose (95.37%), improving the intrinsic properties including electrical conductivity, surface area, pore volume, and compacted density, as illustrated in Figure 9. Figure 9a,b schematically show the strategy of preparation and dense cellulose molecules, where hydroxyl and superoxide anions produced by peroxyformic acid solution can depolymerize and dissolve lignin and hemicellulose, improving the cellulose content. A typical SEM (Figure 9c) and a TEM image (Figure 9d) display the obviously ordered structure after carbonization, proving the reorganization of carbon atoms during pyrolysis. A capacitor prepared using the highly conductive carbon had an energy density of 9.01 Wh L⁻¹ at 25.87 kW L⁻¹, improving the electrochemical stability and volume performance. Similarly, cellulose nanofibrils linked with lignin and acted as a skeleton of lignin/cellulose aerogels, which could regulate the graphitization degree and manage the specific surface area and pore structure. In a two-electrode system, the optimal electrode displayed a specific capacitance of 178 F g⁻¹ at 0.2 A g⁻¹ [122].

Catalytic graphitization using transition metals is still a powerful approach to enhance the degree of graphitization at low temperatures. However, a catalyst must be present in the carbon matrix, which is eventually removed by subsequent purification. The use of boron as a catalyst could greatly enhance the graphitization process at relatively low temperatures, thereby improving the electrochemical behavior of carbon electrodes. This was first report with boron as a catalyst [123]. Highly graphitized BDC using pure boron as a catalyst and logging residues from pine tree as a carbon source were prepared [123]. Compared with the sample without boron, the boron-treated sample exhibited far more graphitic layers in the structure, delivering a specific capacitance of 144 F g⁻¹ at 1 A g⁻¹. Similarly, K₂FeO₄ as a bifunctional catalytic activator significantly contributed to enhance the graphitization degree and the generation of micropores with peanut shells as raw material [124]. The optimal carbon had a specific capacitance of 339.25 F g⁻¹ at 1 A g⁻¹. The increasing conductivity of graphitic carbon is expected to reduce the resistance of carbon electrodes, typically suitable for the application of high current densities. However, a too-high graphitization degree will significantly decrease the specific surface area and pore volume with a hydrophobic surface.

5.5. Unique Preparation Methods

The popular method to prepare BDC for SCs involves carbonization followed by activation. Environmentally friendly and high-efficiency preparation methods are also becoming a focus. Thus, some one-step methods to prepare BDC have been developed more recently. With broad bean shells as a source, N,O self-doped BDC could be prepared via a one-pot activation method using a mixed salt of potassium citrate and potassium carbonate with two-stage programmed pyrolysis [125]. Due to inherent heteroatoms in the biomass, the BDC material achieved a specific capacitance of 376.1 F g⁻¹ at 1 A g⁻¹. Zhang et al. [126] also used a one-step method to prepare nitrogen and phosphorus co-doped BDC from chitosan, with phosphoric acid as activator, phosphorus source, and crosslinking agent, where chitosan was the carbon matrix. The resultant carbon had a specific capacitance of 350 F g⁻¹ at 1 A g⁻¹. Bamboo-waste-derived N,P co-doped carbon could be fabricated through a facile one-step approach, where K₂CO₃, urea, Na₂HPO₄, and bamboo cellulose were heated at 600 °C for 2 h [127]. This was used as the positive electrode of zinc-ion hybrid capacitors.

There are also some unique methods that have been developed by researchers. However, these methods are strongly dependent on the special features of the precursor. For the biomass materials with low ignition points and flammable features, such as cotton, dandelion, and catkin, hierarchical porous carbon could be rapidly prepared using them as precursors by flame burning carbonization in air with the assistance of molten salt (K₂CO₃-KCl) as a fire retardant [128]. The salts played the roles of activator and templates. The obtained carbon had a high specific surface area, abundant heteroatom doping, and a high specific capacitance of 367 F g⁻¹. Meanwhile, a new electrolyte of K₂CO₃-glycol-arginine was developed to show a large voltage window in a wide temperature range from −25 to 100 °C. Wang et al. [129] applied environmentally friendly activation for the synthesis of free-standing carbon electrodes with basswood. An ultra-thick carbon electrode (1200 μm) with a high specific surface area of 766 m² g⁻¹ and a multiscale pore structure could be successfully fabricated. SEM images demonstrated the preservation of the natural structure of wood and its diffuse-porous nature. The free-standing electrode had a specific capacitance of 211 F g⁻¹ at 1 mA cm⁻² in 6 M KOH electrolyte.

Meanwhile, a new heating approach with rapid rates has been used to treat biomass materials. Wang et al. [130] reported the rapid production of N,O co-doped carbon via electromagnetic pyrolysis using enteromorpha as a carbon source. Compared with the conventional pyrolysis, the ultra-fast heating and quenching rates, coupled with the use of a closed high-pressure environment in electromagnetic pyrolysis, could improve the degree of interaction between carbon precursors and the activator (KOH). The resultant carbon had porous structures with high surface areas and high-level heteroatom doping.

6. Future Directions and Conclusions

In this review paper, we delved into the recent research related to porous carbon from biomass for SCs, especially from the last three years. This work highlights the significant role of BDC for energy storage applications. Biomass materials are very good and renewable resources for preparing porous carbon because biomass is the most popular renewable resource in the world. The full application of biomass can boost the economy and effectively reduce environmental pollution.

Among the key factors influencing BDC electrochemical performance, both specific surface area and pore structure play critical roles. Thus, advanced BDC can be synthesized by chemical activation with some typical chemicals such as KOH and ZnCl₂ to adjust surface area and graphitization degree. However, it is hard to control the pore geometry, size, and connection network. The corrosive features under elevated temperatures are harmful to the environment, equipment, and workers. Thus, physical activation is more suitable for large-scale production with negligible environmental impacts, though its activation efficiency is not as prominent as that of the chemical method.

Considering the diversity of resources, BDC from animal bones and excreta is rich in heteroatoms and by-products, and thus additional complicated processes are necessary to purify it. Microorganisms are not popularly used as raw materials of BDC due to their high cost and limited reserve. Thus, plants and their derivatives are more suitable for commercialization because they have rapid growth rates and a wealth of pipeline structures along the growth direction, guaranteeing abundant supply, low cost, porous features, and high performance. Agricultural residue and food industrial waste as the source of BDC are cheap and abundant. Primary research on the relationship between biomass composition and the electrochemical behavior of the resultant carbon is needed. It is a reasonable case to carry out this study with lignin-, cellulose-, and hemicellulose-derived carbon. However, the composition content varies between different plant organs. Some plants may contain small amounts of inorganic compounds (such as Mg, K, Ca, Fe, Mn, and Si) during growth, and it is hard to remove certain components, such as SiO₂, via the conventional purification method. Meanwhile, the mineral content is varied and depends on the biomass organs or species and the soil. Thus, the specific capacitance of the BDC will be reduced due to the presence of inert impurities. To obtain advanced BDC materials, a purification process is inevitably needed.

The methods to improve the specific capacitance of BDC have been well developed. We summarized five strategies in this review paper. Heteroatom doping in the BDC can improve its wettability and pseudocapacitive behavior. However, some heteroatom sources can be self-sourced and some are additionally added. The doping content is not easily controlled, typically for the former case, because of the diverse chemical compositions. At the same time, the doping content of BDC is closely related to carbonization or the activation temperature, reaction time, and activator type. Better control of doping conditions is expected to achieve good electrochemical properties. The dopants without any residuals after treatment are especially welcome.

Though the composites of BDC serving as a structural scaffold and guest pseudocapacitive materials, including metal oxides, metal sulfides, and conducting polymers, can have larger specific capacitances than single BDC, the chiefly electrochemical shortcomings of pseudocapacitive materials still exist.

These new processes developed to prepare or treat BDC are mainly intended to control the pore structure in BDC particles and functional groups on the surface. Researchers are interested in the effects of pore size, specific surface area, and surface chemistry on the electrochemical behavior of BDC. Investigating the diffusion process of ions in hierarchical pore structures is still of significance. These functional groups on the carbon surface can endow some pseudocapacitance; however, this contribution is not so prominent.

A high graphitization degree will lead to undeveloped pore structures and reduced specific surface area, as well as a hydrophobic surface. Thus, there should be a balance between the pore structure and graphitization degree, aiming to maximize the electrochemical properties. Compared with thermal treatment under high temperatures, catalytic graphitization using a transition metal as catalyst is a more efficient way to synthesize BDC with a certain graphitization degree. In addition, modifying the mass content of cellulose in the biomass before carbonization can also modulate the graphitization degree of BDC.

Although the two-step process including pyrolysis and activation has been accepted by researchers, some unique preparation processes have also been explored, with the main aim of improving the efficiency. Some new methods have been designed, and they are strongly dependent on the special features of the biomass precursor. Among the five strategies, heteroatom doping and novel processes are more suitable for industrialization. In addition to improving the specific capacitance, the energy density of SCs can also be improved by widening the operation voltage by using ionic liquid electrolytes and assembling asymmetric capacitors.

Considerable technical and economic challenges must be overcome for BDC in SC application. These reported BDC electrode materials with good supercapacitive performance are mostly synthesized at the lab scale. There is an urgent need to carry out scale-up research to support the further development of the technology. During large-scale production of BDC, biomass with cheap, abundant, and stable sources like agriculture waste and food industrial by-products (e.g., coconut shells, bamboo, and straw) is very promising. However, the homogeneity of the resultant BDC must be paid much attention because of the variation in the chemical compositions originating from the growth conditions and regions. Future advancements related to BDC for SCs closely depend on improving processing technologies and cost-effective production methods.

In summary, the pore structure, surface groups, specific surface area, heteroatom doping, and graphitization degree of BDC play very important roles in determining its electrochemical behavior. Synergistic approaches that combine theoretical models with experimental results to guide the research of BDC with improved structural and electrochemical performance are urgently required.