Biomass‐based materials for advanced supercapacitor: principles, progress, and perspectives

Abstract

Supercapacitors exhibit considerable potential as energy storage devices due to their high power density, fast charging and discharging abilities, long cycle life, and eco‐friendliness. With the increasing environmental concerns associated with synthetic compounds, the use of environment friendly biopolymers to replace conventional petroleum‐based materials has been widely studied. Biomass‐based materials are biodegradable, renewable, environment friendly and non‐toxic. The unique hierarchical nanostructure, excellent mechanical properties and hydrophilicity allow them to be used to create functional conductive materials with precisely controlled structures and different properties. In this review, the latest development of biomass‐based supercapacitor materials is reviewed and discussed. This paper describes the physical and chemical properties of various biopolymers and their impact on supercapacitors, as well as the classification and basic principles of supercapacitors. Then, a comprehensive discussion is presented on the utilization of biomass‐based materials in supercapacitors and their recent applications across a range of supercapacitor devices. Finally, an overview of the future prospects and challenges pertaining to the utilization of biomass‐based materials in supercapacitors is provided.

INTRODUCTION

With the continuous development of the global information industry, the constant advancements in digital products have generated a growing need for electrochemical energy storage devices that are efficient, practical, and dependable. Supercapacitors, being a novel energy storage device with excellent performance and extended lifespan, have played a pivotal role in advancing the global information industry. While considering the numerous problems, the world is currently facing, including resource scarcity, severe environmental pollution, and global warming, it is crucial to develop supercapacitor materials that are both low-cost and environment friendly.^[] The unique and precise structure of biomass materials allows them to be used as skeletons, binders, and, after carbonization, as support materials for supercapacitor electrodes.^[] In addition, the incorporation of biopolymers significantly contributes to the improvement of mechanical strength, ion mobility, and swelling characteristics of supercapacitor electrolytes.^[]

Biomass resources are abundant and include energy crops, agricultural waste, wood and wood waste, urban domestic waste, aquatic plants, and algae. They are considered as highly promising alternative materials for petroleum-based products with properties of biodegradability, renewability, environmental friendliness, and nontoxicity.^[] To date, biomass has been widely used to satisfy the growing command, for instance, food industry, biology and medicine, agriculture, energy storage, and other fields.^[] Most natural biopolymers are macromolecules with repetitive structures, such as chitosan, chitin, cellulose, and starch, etc.^[] Their delicate and complex nanostructures and morphology allow them to be used in supercapacitors. When exposed to high temperatures through thermochemical transformation, materials derived from biomass develop unique porous structures that exhibit enhanced electrical conductivity, rendering them suitable for use as carbon electrode materials. In addition, biomass materials can be used in combination with conductive components (conductive polymers, conductive carbon materials, two-dimensional (2D) conductive materials, metal nanoparticles, transition metal oxides, etc.) as structural guiding agents. Composites can merge the structural and performance benefits of an individual element while maximizing its physical or chemical characteristics.^[] Biopolymers have also been explored for the fabrication of functional supercapacitor electrolytes. The presence of hydrophilic groups within the structure of biopolymers imparts high wetting properties to polar solvents and facilitates preferential interaction with the anionic component of salt, thereby enhancing the solubility of the salt and the transport characteristics of the cation.^[] Figure  presents a concise timeline outlining the evolution and advancement of biomass-based materials for supercapacitors.

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There is currently a lack of a systematic overview of biopolymer-based supercapacitor materials. This paper presents an extensive overview of the research advancements concerning biopolymer-based materials in the realm of supercapacitors. The physical and chemical properties of biopolymers are discussed, along with the classification and basic principles of supercapacitors. Furthermore, the paper comprehensively examines recent specific applications of biomass-based materials, including electrode materials and electrolyte materials. Finally, this review addresses the existing challenges and explores prospective future directions within this field.

PROPERTIES OF BIOMASS-BASED MATERIALS

Biopolymers are classified into three categories based on their chemical structure and origin: polysaccharides (cellulose, chitosan/chitin, alginate, starch, etc.), proteins (collagen and gelatin, etc.), and microbial polymers (bacterial cellulose, polyhydroxyalkanoates, etc.).^[] Among them, biopolymers frequently used in supercapacitors include chitosan, chitin, cellulose, starch, alginate, lignin, etc. The structures of them are shown in Figure .

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Chitosan and chitin

Chitin, abundant in nature, is a prevalent biopolymer found extensively in the shells of lower plant fungi, crustaceans like shrimp and crabs, insects, and the cell walls of fungi.^[] The basic unit of chitosan is 2-acetamido-2-deoxy-β-d-glucose monomer, connected by a β-1,4-glycosidic bond.^[] Chitosan is produced from chitin by chemical or enzymatic deacetylation. In general, the basic unit of chitosan is a copolymer of D-glucosamine units and N-acetylglucosamine, connected by β-1,4-glycosidic bonds. Chitin and chitosan are classified according to the degree of N-deacetylation (DD), chitosan has DD of >50%. Due to the presence of amino groups that can undergo protonation at a pH of 6.5, chitosan demonstrates remarkable solubility in different acids, including acetic acid, hydrochloric acid, and formic acid.^[] The exceptional potential of its unique structure, high porosity, lightweight nature, and effective interaction with various polymers make it highly suitable for utilization in lightweight and flexible energy storage devices. In the realm of supercapacitors, both chitosan and chitin have emerged as promising contenders for the development of electrode and electrolyte materials in diverse applications.^[]

Cellulose and derivatives

Cellulose, a key component of plant cell walls, stands as the most widely distributed and abundant biopolymer in nature.^[] Comprising a linear polymer with glucose as its basic unit, cellulose is linked by α-(1-4)-glycosidic bonds. Notably, each glucose unit in cellulose possesses three alcohol hydroxyl groups at the C2, C3, and C6 positions, which facilitate both intramolecular and intermolecular hydrogen bonding within the cellulose molecule.^[] Individual cellulose chains interact through hydrogen bonding, leading to the formation of primary cellulose, also referred to as elementary cellulose. These elementary fibrils aggregate to create larger microfibrils, which, in turn, assemble into microfibril bundles, often called macrofibrils. Alongside microfibrils and macrofibrils, hemicellulose and lignin also play essential roles in the cell wall composition. When combined with other components such as proteins and inorganic compounds, these elements collectively shape the plant's structure. At the room temperature, cellulose is insoluble neither in water nor in general organic solvents, such as alcohol, ether, acetone, benzene, etc. It is also insoluble in dilute alkali solutions. The exceptional stability is a result of the strong hydrogen bonds formed between cellulose molecules, providing structural integrity and resistance to solvation under ambient conditions. Cellulose finds a myriad of applications in supercapacitors owing to its exceptional porosity, structural stability, remarkable flexibility, and its capacity to bond effectively with conductive materials.^[] In addition, cellulose derivatives are produced when the hydroxyl groups in cellulose macromolecules undergo esterification or etherification reactions with chemical reagents. Cellulose derivatives can be categorized into three major groups based on their structural characteristics: cellulose ethers, cellulose esters, and cellulose ether esters. In addition to these derivatives, nanocellulose stands out as another highly promising and noteworthy material.

Nanocellulose can be categorized into three groups based on its size, morphological properties and processing techniques, namely, nanocellulose crystal (CNC), cellulose nanofibril (CNF), and bacterial cellulose (BC).^[] CNC, also known as nanocellulose (NCC) or cellulose nanowhisker (CNW), typically have diameters ranging from 10 to 30 nm and lengths below 500 nm. The majority of CNC exhibit a crystalline structure and possess a rod-like shape.^[] CNF, also referred to as nanofibrillated cellulose (NFC) or microfibrillated cellulose (MFC), are characterized by elementary fibril diameters ranging from 5 to 20 nm, lengths between 2 and 10 μm, and aggregate diameters ranging from 20 to 50 nm. CNC is commonly obtained through acid hydrolysis methods from various cellulosic materials. CNF is extracted through mechanical approaches, which may or may not involve enzymatic and chemical pretreatment.^[] BC is synthesized in the form of a pellicle of the desired shape and size by some bacteria, such as Komagataeibacter, Agrobacterium, Achromobacter, and Alcaligenes.^[] It is often necessary for bacterial development to create BC in a liquid media that contains carbon, nitrogen, and other nutrient sources.^[] The application of acid hydrolysis or mechanical fibrillation to BC can lead to the formation of BC nanocrystals (BCNC) or BC nanofibers (BCNF), respectively.^[] The sustainable and environment friendly nature of nanocellulose and its derivatives, along with the simplicity of the manufacturing process, establish cellulose energy storage devices as a promising candidate for the future of environmentally conscious and sustainable electronics.^[]

Alginate

Alginic acid, often known as “alginate”, is a common natural polysaccharide and an anionic, water-soluble biopolymer, which is normally extracted from brown algae (Phaeophyceae). Alginate is a block copolymer consisting of (1,4)-linked β-D-mannuronate (M) and α-L-guluronate (G) residues. The blocks are made up of alternating M and G residues (GMGMGM), consecutive G residues (GGGGGG), and consecutive M residues (MMMMMM). Alginates derived from different sources exhibit variations in M and G contents, as well as block lengths, resulting in the production of over 200 distinct types of alginates. The G-blocks of alginate engage in intermolecular crosslinking with divalent cations, while the M-blocks govern the overall conformation of the polymer chain. Alginate can encapsulate other species when the pH is low because an increase in viscosity causes the alginate to mildly gel. For example, in the presence of Ca²⁺, some of the H⁺ and Na⁺ ions in alginate can be replaced, resulting in the formation of a calcium alginate gel. Moreover, the molecule contains highly reactive carboxylic acid groups that can be modified to create functional materials for a wide range of applications.^[]

Other biomass materials

Agarose is a polysaccharide derived from red algae and serves as an uncharged, neutral component of agar. The main component of agarose is polygalactose, comprising approximately 70% agarose and 30% branched agarose. Agarose possesses a linear structure composed of D-galactose and 3,6-anhydrogalactose units, interconnected by alternating β-1,4 and α-1,3 linkages to form repeating disaccharide units. The branched agarose chain splits from the β-1,3 bond to form another chain.^[] An important characteristic of agarose is its capability to form a semi-flexible water-filled gel through self-assembly and cross-linking, facilitated by the formation of hydrogen bonds with water. It has abundant interconnected submicron pores (pore size of 400–500 nm) and high water retention (>90% water). High water retention and sub-micron pores can provide a high charge carrier concentration. Agarose contains multiple ether groups (–OH) and hydrophilic groups (–O–) that readily interact with aqueous electrolytes, contributing to its high ionic conductivity and the abundance of ion migration sites.^[] Agarose gel stands as a soft, low-cost, and biocompatible medium with excellent ionic conductivity, making it a promising material for a variety of energy storage devices.^[] Starch is a complex carbohydrate composed of glucose molecules that undergo polymerization, resulting in a high molecular structure. Among the biomass, starch is the most economical and renewable natural biopolymer, sourced from a wide range of renewable plant resources. There are two types of starch, amylose and amylopectin. Amylopectin consists of polymer chains composed of up to 10,000 D-glucose units connected by α-1,4-glycosidic bonds. Branching occurs every 24–30 units through α-1,6-glycosidic bonds. Amylopectin contributes mainly to the peripheral crystalline organization of the starch granule. In contrast, amylose is characterized by continuous, unbranched chains that form helical structures consisting of up to 4000 D-glucose units. The glucose units are also connected by α-1,4-glycosidic bonds.^[] Starch is cheap, easy to obtain, environment friendly, renewable, etc. Starch is extensively utilized in the production of porous carbon materials for supercapacitor electrodes through carbonization. In addition, starch also has been used to develop electrolytes for supercapacitors.^[] Lignin, a three-dimensional biopolymer, consists of interconnected benzene propane units linked by both ether and carbon–carbon bonds. It is typically found in woody tissues. It serves as the primary component of the secondary wall, contributing to the cell wall's hardening through the formation of an interwoven network. Lignin is predominantly found between the cellulose fibers and functions as a compressive agent. Lignin accounts for approximately 25% of the composition in woody plants, making it the second most abundant organic matter on Earth. Since lignin is often interconnected with cellulose and hemicellulose in nature, forming a lignin–carbohydrate complex, there is no way to separate the original lignin with completely undamaged structure.^[] Due to its many benefits, lignin is now used extensively in the production of supercapacitors and rechargeable batteries. It is notable that carbon, which makes up more than 60% of lignin by weight, is the primary component. Lignin serves as a source of approximately 30% by weight of organic carbon in the biosphere, offering numerous potential applications as a carbon precursor. Lignin-based carbon aerogels are highly sought-after materials for supercapacitors due to their exceptional characteristics, including low density, high specific surface area, and a distinctive three-dimensional network structure.^[]

Biomass materials all have the advantage of being environment friendly and sustainable. Biomass materials all have environmental and sustainability advantages. Chitosan, cellulose, starch, and lignin on can all serve as good carbon precursors due to their high carbon content.^[] Among them, the large number of oxygen-containing functional groups on cellulose makes it easier to complex with electroactive materials and change their physicochemical properties, thus making it an excellent binder and matrix.^[] Alginate is water soluble, easily modified by metal ions, and is an effective carbon source.^[] By undergoing crosslinking with water and self-assembling through hydrogen bonds, agarose forms flexible hydrogels that possess plentiful pores.^[] The unique properties of biomass materials make them promising for use in supercapacitors.

WORKING MECHANISM OF SUPERCAPACITOR

Batteries and fuel cells have much higher energy density than supercapacitors, but limited power density, as shown in Figure . Conventional capacitors exhibit high power density, thanks to the rapid movement of electrons in the conducting pole plates, allowing them to efficiently distribute high energy density. Figure  clearly indicates that supercapacitors will bridge the gap between the two types of devices mentioned above. They are expected to exhibit high power density like fuel cells and batteries while maintaining a higher energy density.^[] A supercapacitor is composed of several key parts, including a positive electrode, a negative electrode, an electrolyte, a non-conductive separator (which prevents a short circuit between the two electrodes), and two current collectors that connect the electrodes to an external circuit.^[]

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Based on their charge storage mechanisms, supercapacitors can be categorized into three fundamental types: electrical double layer capacitors (EDLC) (Figure ), pseudocapacitors (PC) (Figure ), and battery hybrid supercapacitors (BHS) (Figure ). Typically, EDLCs primarily rely on carbon-based electrode materials like activated carbon, graphene, and carbon aerogel nanostructured carbon to accumulate charge through the reversible adsorption/desorption of ions at the interface between the electrode and the electrolyte.^[] As Faraday reactions are not involved in the energy storage process, the charging/discharging mechanism of EDLCs displays similarities to the dielectric behavior observed in conventional capacitors.^[] Second, PC, also known as redox capacitors, store energy through a rapid and reversible Faraday reaction occurring on the surface of the active material.^[] In addition, the pseudocapacitance (of induced electricity) occurs on the surface of the electrode where electron transfer (redox reaction) occurs, similar to the charge and discharge of a battery. Therefore, the PCs have more similarities to a battery than a battery. This hybrid supercapacitor leverages the characteristics of both EDLC and PC, yielding enhanced performance with high energy density and power supply ability.^[]

BIOMASS-BASED MATERIALS FOR SUPERCAPACITOR

Biomass-based materials for supercapacitor electrodes

Biomass-based materials combined with active conductive materials

Composites based on biomass and conducting polymers are recognized as promising materials for supercapacitors. Biomass materials can be combined with electrically conductive components to prepare one-dimensional composites (e.g., fibers, threads, etc.), two-dimensional composites (e.g., paper, film, etc.), and three-dimensional composites (e.g., aerogels, hydrogels, foams, etc.) by means of blending, vacuum filtration, in-situ polymerization, electrodeposition, etc.^[] The addition of fillers, such as metal nanoparticles, clay, and carbon compounds formed as composites, that is, heterogeneous materials made of organic-inorganic components, considerably increased the performance of chitosan.^[] The abundance of oxygen-containing functional groups on nanocellulose facilitates easy compound formation with electroactive materials (e.g., graphene, transition metal–disulfide carbon, MXene), allowing for modification of their physicochemical properties.^[] Furthermore, polyaniline (PANI)/sodium alginate (SA) composites with remarkable electrochemical properties were synthesized by conducting in situ polymerization of aniline within a SA solution.^[] The combination of biomolecules and conducting polymers in composites presents numerous advantages, including enhanced electrical conductivity, extensive hydrophilic surfaces, abundant hierarchical pore structures, and rapid rates of the electron–ion transport.^[] Lignin sulfonates can be used as a promising pseudocapacitive material. It is a lignin derivative derived from the traditional sulfite pulping process.^[] Lignin sulfonate has a complex composition (side chains containing phenolic hydroxyl, methoxy, carboxyl, and other functional groups). The material possesses an abundance of phenolic hydroxyl groups, which can be transformed into a redox-active hydrazine/hydroquinone (Q/QH₂) structure. This distinctive structure contributes to the material's high theoretical specific capacitance, particularly in the positive potential range. In addition, the P–π conjugated structure of lignosulfonate (LS) confers a strong chemical reactivity to α and β carbons, especially under acidic conditions. Pure LSs exhibit almost no electrical conductivity. In order to fully utilize their exceptional theoretical specific capacitance, LSs should be integrated with electrode materials that demonstrate excellent electronic and ionic conductivity.^[] Biomass materials undergo changes in structure, composition, and properties after carbonization. Carbonization regulates their specific surface area and porosity, making them electrically conductive and allowing them to act as a conductive carbon skeleton. However, biomass materials compounded with conductive components retain their original microstructure and are mostly used as support structures or adhesives.^[] Electrochemical performance of supercapacitors based on non-carbonized biomass electrode materials are summarized and compared in Table .

TABLE 1 Electrochemical performance of supercapacitors based on non-carbonized biomass electrode materials.

Electrode	Electrolyte	Capacitance (supercapacitor)	Power density (energy density)	Cycling stability (cycles)	Refs.
3D printed MXene/CNF pectinate electrodes	PVA H2SO4	2.02 F cm−2 (1 mA cm−2)	0.299 mW cm−2 (101 μWh cm−2)	85% (5,000)	[]
CNF/MXene/CNT aerogel	PVA H2SO4	849.2 mF cm−2 (0.8 mA cm−2)	21.2 μW cm−2 (240 μWh cm−2)	88% (10,000)	[]
CNF/Rgo/Bio-AC aerogel	PVA Na2SO4	812.2 mF cm−2 (1 mA cm−2)	18,000 mW cm−2 (0.365 mWh cm−2)	99% (5000)	[]
Lig/HrGO/SWCNT film	Cellulose H2SO4	4,110 mF cm−2 (5 mA cm−2)	2500 μW cm−2 (285.4 μWh cm−2)	86.4% (10,000)	[]
LS/MXene/rGO aerogel	3 M H2SO4	652 mF cm−2 (2 mV s−1)	4.9 mW cm−2 (142 μWh cm−2)	96.3% (10,000)	[]
CNF/PEDOT:PSS nanopaper	1 M H2SO4	854.4 mF cm−2 (5 mV s−1)	0.22 mW cm−2 (30.86 μWh cm−2)	95.8% (10,000)	[]
BC/PEDOT/CNT fiber	PVA H3PO4	175.1 F g−1 (5 mV s−1)	120.1 W kg−1 (4.0 Wh kg−1)	85% (5,000)	[]
SA/PEDOT:PSS/CNT/PAM hydrogel	PAM SA	128 mF cm−2 (1 mA cm−2)	0.2 mW cm−2 (3.6 μWh cm−2)	78% (5,000)	[]
TOCNF/CNT/PANI hydrogel	CNF PVA	226.8 F g−1 (0.4 A g−1)	–	90% (10 cutting/healing cycles)	[]
Cellulose/dopamine textile	PVA LiCl	1208.4 mF cm−2 (1 mA cm−2)	–	94% (4,000)	[]
CNF/Co3O4/acetylene black film	–	–	799.97 W kg−1 (18.75 Wh kg−1)	–	[]

2D conductive materials (e.g., graphene, transition metal carbon disulfide, transition metal carbide, etc.) have large specific surface area and excellent electrochemical properties, but are prone to stacking, have poor mechanical properties and low capacity. Therefore, biomass materials are usually combined with them to provide support structures and spacers. A lot of research has been conducted on the composite of 2D materials with biomass materials for supercapacitor electrodes. MXene, which has recently joined the ranks of 2D materials, has shown excellent performance in energy storage applications.^[] Zhou et al. prepared low solids, viscoelastic, 3D printing MXene inks by using CNF rheological modifiers (Figure ). The high aspect ratio of CNF allows for excellent tunable rheological properties when dispersed in water. Additionally, it can be used as a spacer to be inserted between MXene lamellae to prevent MXene buildup. Structures printed with this ink have a very small shrinkage after freeze-drying, which increases his specific surface area and thus further improves the electrochemical properties. The supercapacitor printed with this ink has an area capacitance of 2.02 F cm⁻² and a capacitance retention of 85% after 5000 cycles.^[] More recently, Xu et al. prepared CNF/CNT/MXene composite aerogels using CNF as a spacer. The electrostatic repulsion arising from the interaction between CNF and MXene successfully hindered the re-stacking of MXene nanosheets during the process of aerogel formation. In addition, the CNF and CNT “mortar” entangled with the tubular structure of MXene “bricks” can produce good interfacial interactions, which together promote the electron transfer efficiency. When employed as a supercapacitor electrode material, the aerogel showcases a notable electrical conductivity of 2400 S m⁻¹ and achieves a specific capacitance of 849.2 mF cm⁻².^[] Furthermore, graphene, another 2D material, has garnered extensive attention for its utilization in electrode applications. Chen et al. fabricated composite aerogels composed of biomass carbon, reduced graphene oxide (rGO), and CNF for electrode applications. By integrating biomass carbon particles, the re-stacking of rGO nanosheets was effectively hindered, leading to the attainment of a high specific surface area and excellent electrical conductivity. The incorporation of negatively charged cellulose nanofibers facilitated the formation of a reinforcement network to connect the biomass carbon particles with the rGO sheets. The aerogel was used as a supercapacitor electrode with a capacitive performance of 812.2 mF cm⁻² and retained 99% capacitance after 5000 cycles (Figure ).^[] To achieve a sufficiently high theoretical specific capacitance, LSs are frequently combined with graphene and should be utilized in conjunction with other electrode materials that possess exceptional electronic and ionic conductivity. Peng et al. fabricated flexible LS/SWCNT/porous reduced graphene oxide (Lig/SWCNT/HrGO) films using a filtration process. Lig provided pseudocapacitance and it adsorbed on the surfaces of SWCNT and HrGO through hydrogen bonding and π–π interactions, which ensured a stable dispersion of the solution, resulting in a dense film with a multilayer structure. The film exhibited an area capacitance of 4110 mF cm⁻² and an initial capacitance retention of 86.4% (after 10,000 cycles) after assembly into a supercapacitor.^[] Ma et al. prepared MXene/LS/reduced graphene oxide (MLSG) aerogels and LS-functionalized reduced graphene oxide (LSG) aerogels based on LS as supercapacitor electrodes. The LS-modified graphene has larger layer spacing, which efficiently prevents the stacking of graphene. The pseudo-capacitance property of LS at positive potentials in conjunction with MXene facilitates the broadening of the working potential. The specific capacitance of the assembled supercapacitor is 80 F g⁻¹ (652 mF cm⁻²), and the capacitance retention rate is 96.3% after 10,000 charge/discharge cycles.^[]

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Combining conductive polymers with biomass materials allows the preparation of high-quality electrode materials with desirable electrical conductivity, high specific surface area and fast electron-ion transport rates, but it has relatively low cycling stability. PEDOT:PSS exhibits excellent dispersibility in water and certain organic media, which contributes to its good solution processability. In addition, PEDOT:PSS has favorable properties including high electrical conductivity, excellent optical/electrical characteristics, and ease of processability. These qualities make it an ideal material for fabricating flexible supercapacitors when combined with biomass materials as electrodes.^[] For example, Du et al. fabricated high-performance PEDOT:PSS/CNF nanopaper (PEDOT:PSS/CNP) by employing CNF as the fundamental building blocks (Figure ). The assembled supercapacitor demonstrated exceptional cycling stability, retaining 95.8% of its capacitance after 10,000 charge/discharge cycles, along with a specific capacitance of 854.4 mF cm⁻². PEDOT:PSS/CNP has excellent mechanical properties and extraordinary electrochemical properties. When a PEDOT:PSS/CNP strip is connected to a circuit, the LED can be lit even in a twisted state (Figure ).^[] Liang et al. prepared a self-stretching hybrid helical fiber composed of PEDOT, BC, and carbon nanotube (CNT). In this hybrid structure, dissolved BC acted as the bonding substrate, while undissolved BCNF and CNT served as the backbone. PEDOT played the role of reinforcing material. Supercapacitors assembled with these hybrid fibers have a specific capacitance of 175.1 F g⁻¹ and maintain a capacitance of more than 85% after 5,000 cycles.^[] Zeng et al. fabricated an all-hydrogel flexible supercapacitor. Polyacrylamide (PMA)/SA/CNT/PEDOT:PSS was used as electrodes. Then, Na₂SO₄ electrolyte salt and potassium ferricyanide [K₃Fe(CN)₆/K₄Fe(CN)₆] REDOX pairs were added to the PAM/SA matrix as electrolytes (Figure ). The physical entanglement of the long-chain SA and intermolecular chain interactions increase the resistance to flow of the solution and act as a support. This makes the hydrogel super tough, stretchable, compressible, and recoverable (Figure ). The assembled period had an area capacitance (232 mF cm⁻² at 5 mV s⁻¹ and 128 mF cm⁻² at 1 mA cm⁻²) and the area capacitance was maintained at 78% of the initial value.^[] Wang et al. fabricated solid-state symmetric one-piece supercapacitors by employing hydrogel electrodes and electrolyte. By introducing CNF into a poly(vinyl alcohol)–borax (PVA) hydrogel matrix, a self-supporting, moldable hydrogel electrolyte with remarkable self-healing properties was synthesized. PVA hydrogel matrix was compounded with 2,2,6,6-tetramethylpiperidine-1-oxo (TEMPO)-cellulose nanofibers (TOCNF), CNT, and PANI to synthesize a novel electro-conductive hydrogel as an electrode (Figure ). CNTs offer exceptional electrical conductivity, TOCNFs function as dispersants to facilitate the formation of stable suspensions by CNTs, and PANI enhances electrochemical properties by creating “core–shell” structural composites. CNT and PANI serve as electrochemically active materials within electrodes. TOCNF is a type of biodispersant used to enhance the dispersion of CNT in water. The TOCNF–CNT nanohybrid materials underwent in situ oxidative polymerization to coat them with PANI, resulting in the formation of “core-shell” TOCNF-CNT@PANI composites. After undergoing 10 cut/heal cycles, the supercapacitor device maintains a capacitance retention of 90% (Figure ) and exhibits no cracking at the healing interface when stretched to 300% strain (Figure ). Furthermore, the device demonstrates 85% capacitance retention after 1000 bending cycles.^[]

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In addition, some metal oxides and biomolecules can also be combined with biomass materials to prepare supercapacitor electrodes. For example, Chen et al. prepared a new scheme to enhance the performance of flexible textile electrodes. The deposition of an interfacial layer of dopamine (DA) onto a cellulose fabric (Cell) substrate through in situ polymerization is a clever and innovative approach. The Cell acts as a substrate to provide support for the conductive actives. The electrode has a high capacitance (1208.4 mF cm⁻²) and retains 84% of its capacitance after long charge/discharge cycles.^[] Xiao et al. fabricated layer-by-layer assembled CNF/porous Co₃O₄ polyhedral (called CNF/PCP or NPC) films by the aqueous papermaking method. CNF serves as the foundation of the electrode, leveraging its hydrophilicity and solubility to establish effective contact between the active component and electrolyte. This contact facilitates the diffusion of electrolyte ions. In addition, it prevents the aggregation of active substances, further improving the electrochemical properties of the film. The electrode has a specific capacitance of 594.8 mF cm⁻² and a capacitance retention of 64% (after 2,000 cycles).^[]

Carbonized biomass-based material as active electrode

Research on biomass materials as carbon material precursors for supercapacitor electrodes has been widely reported. During the carbonization process, stable carbon is preserved, while other volatile and unstable components are removed. Importantly, the formation of sp² carbon atoms during the carbonization process significantly contributes to improving the electrical conductivity of the material. The micro/nanostructure of the biomaterial can be maintained after the calcination process by selecting the appropriate carbonization treatment parameters and biomass source, or using the metal salt system.^[] Activation (both physical and chemical activation) was used as an adjunct strategy to carbonization to control the internal pore size and the distribution of functional groups on the surface. Besides activation, hard templates, such as mesoporous silica and metal oxides, can also be employed in combination with natural biomaterials to precisely control the porosity of carbon materials.^[] Electrochemical performance of supercapacitors based on carbonized biomass electrode materials are summarized and compared in Table .

TABLE 2 Electrochemical performance of supercapacitors based on carbonized biomass electrode materials.

Electrodes	Electrolyte	Capacitance (supercapacitor)	Power density (energy density)	Cycling stability (cycles)	Refs.
Chitosan/Mxene powder	–	286.28 F g−1 (3 A g−1)	2343.75 W kg−1 (21.47 Wh kg−1)	98% (10,000)	[]
Chitosan/GQDs powder	–	256 F cm−3 (1 A g−1)	–	–	[]
Chitosan aerogel	1 M H2SO4	168 F g−1 (0.5 A g−1)	25 W kg−1 (6.36 Wh kg−1)	–	[]
Chitosan/CuO/Cu nanoflowers	–	246 F g−1 (0.5 A g−1)	374.5 W kg−1 (76.87 Wh kg−1)	82.43 % (10,000)	[]
Chitin-based porous carbon	PVA/KOH	69.6 F g−1 (0.25 A g−1)	– (9.67 Wh Kg−1)	90.2% (10000)	[]
Chitin-based porous carbon	6 M KOH	221 F g−1 (0.5 A g−1)	0.19 kW kg−1 (15.41 Wh kg−1)	96% (5,000)	[]
Lignin-based porous carbon	6 M KOH	274.02 F g−1 (0.1 A g−1)	25.0 W kg−1 (9.5 Wh kg−1)	88.46% (5,000)	[]
Methyl cellulose-based carbon nanosheets	BC/H2SO4	241 F g−1 (1 A g−1)	– (43 Wh kg−1)	97.8 % (20,000)	[]
CNF/CNT/RGO aerogel	PVA/H2SO4	109.4 mF cm−2 (0.4 mA cm−2)	649.8 mW cm−2 (12.0 mWh cm−2)	85% (10,000)	[]
CNF/RGO aerogel	PVA/H2SO4	31.2 mF cm−2 (0.3 mA cm−2)	–	82% (5,000)	[]
Calcium alginate/Mxene film	3 M H2SO4	92.8 F g−1 (0.1 A g−1)	– (27.2 Wh L−1)	95% (10,000)	[]
3D printed agarose/Mxene electrodes	PVA/H2SO4	40 mF cm−2 (0.2 mA cm−2)	50 mW cm−2 (1.39 μWh cm−2)	97.2% (5,000)	[]
Starch-based porous carbon	6 M KOH	49.77 F g−1 (1 A g−1)	649.68 W kg−1 (11.68 Wh kg−1)	90% (20,000)	[]
Starch/PAN porous carbon nanofibers	6 M KOH	80 F g−1 (0.25 A g−1)	125 kW kg−1 (12 Wh kg−1)	99.9% (10,000)	[]
Deoxyagar-based activated carbon	21 m LiTFSI	224.3 F g−1 (0.05 mA cm−2)	0.7 kW kg−1 (308.3 Wh kg−1)	89% (8,000)	[]

Carbonization of chitosan, chitin, and lignin enables N self-doped carbon materials, and the introduction of N elements can contribute to the pseudocapacitance effect and make carbon materials with semiconductor properties. This effectively improves the electrical conductivity of the materials. For instance, Pu et al. prepared nitrogen-doped MXene electrodes using chitosan as a nitrogen source. After high-temperature calcination, chitosan was transformed into elemental nitrogen and amorphous carbon, which retained the complete MXene structure and realized the doping of nitrogen atoms in MXene. After assembling it into a supercapacitor, it has a specific capacitance of 286.28 F g⁻¹ (154.59 C g⁻¹), and the efficiency is always maintained above 98% (after 10,000 charge/discharge cycles).^[] Jiang et al. prepared activated carbon electrodes using a mixture of chitosan and graphene quantum dots (GQD) as a precursor. The strong electrostatic attraction between the positive charge carried by chitosan and the negative charge carried by GQDs made led to the densification of the carbon precursor at the molecular level. This makes the electrostatically densified activated carbon obtained after carbonization also have high density and high specific surface area. The sample exhibited a specific capacitance of 341 F g⁻¹/256 F cm⁻³ and 70% capacitance retention for in the three-electrode system.^[] Wu et al. obtained carbon aerogels with different morphologies and heteroatom doping directly from chitosan by the freeze-drying carbonization/activation process. Chitosan contains N, which allows stable N self-doping. In addition, chitosan molecules interact with each other to form larger 2D layered polymers, which can easily form three-dimensional carbon aerogels after carbonization. The researchers prepared unactivated, KOH-activated CNS (CNSK-800) and phosphoric acid-activated (CNSP-800) aerogels, respectively, and their specific capacitances were 171, 318, and 416 F g⁻¹, respectively; and their capacitance retention after 10,000 cycles was 86.17%, 95.51%, and 93.72%, respectively.^[] Xi et al. developed a novel CuO/Cu@C composite through a combination of mechanical mixing, freeze-drying, and carbonization methods. They converted chitosan into a nitrogen-doped porous carbon substrate and then constructed CuO/Cu nanoflowers on this basis. In addition, the electrode exhibited a specific capacitance of 2479 F g⁻¹ and a cycling stability of 82.43% (after 10,000 cycles).^[] Wang et al. constructed heteroatom-doped multistage porous carbon with chitin as a nitrogen-containing carbon precursor and KMnO₄ as an activator and template precursor, assisted by amino and hydroxyl groups. The presence of pyridine N and pyrrolidine N in the carbon framework can generate some pseudocapacitance, which improves the specific capacitance of the carbon material, while quaternary N improves the electrical conductivity. The specific capacitance of the electrode reaches 412.5 F g⁻¹, and the cycling stability remains 99.6% only after 10,000 cycles.^[] Luo et al. prepared fibrous O/N self-doped and graded porous carbon using chitin as carbon source (Figure ). The special fibrous structure of chitin provides a fast transport pathway for electrolyte ions. CuCl₂⋅2H₂O, as a mild activator, generates abundant pores through REDOX reactions while preserving the natural fiber and O/N-rich structure of chitin through melting protection. The optimized supercapacitor based on PC demonstrates a notable energy density of 15.41 Wh kg⁻¹ at 0.19 kW kg⁻¹ and maintains 76% of its energy density when the power density is increased by a factor of ten.^[] In addition to the ability to realize the self-doping of elemental N, the large number of oxygen-containing functional groups in lignin contributes to the improvement of capacitance and effectively improves the wettability of carbon materials. Wan et al. pretreated lignocellulosic PHP (phosphoric acid plus hydrogen peroxide) to obtain oxidized lignin, and then successfully prepared hierarchical porous carbon 3D structures through one-step carbonization. Furthermore, the oxygen-containing functional groups present in lignin serve as intrinsic physical activators, enhancing the specific surface area and pore size distribution of carbon materials, thus facilitating the transportation of electrolyte ions. As the electrode of supercapacitor, it shows the excellent energy density of 6.73–9.5 Wh kg⁻¹, and the maximum specific capacitance of 352.85 F g⁻¹ at 0.5 A g⁻¹ current density.^[] Thongsai et al. prepared hybrid fibers of lignin extracted from palm kernel shell (PKS) (PKs-Lignin) and polyacrylonitrile (PAN) by electrostatic spinning for the production of carbon fibers. The phenolic ring in lignin induces electron transfer around the electrodes, thereby increasing the capacitance of the electrode material. The electrode has a specific capacitance of 148 F g⁻¹ and 90% cycling stability when assembled into a supercapacitor.^[] Guo et al. prepared lignin carbon aerogel/nickel (LCAN) ultra-thick cubic electrodes under ZnCl₂ conditions. ZnCl₂ is an ideal pore-forming agent, dehydrating agent, activator, and hard template for hierarchical porous carbon aerogels. The binary network structure of LCAN contributes significantly to the enhancement of the electrochemical performance. The area-specific capacitance of the electrode was 26.6 F cm⁻².^[] Additionally, by employing methyl cellulose as a precursor and utilizing iron nitrate and boric acid as a dual template and source of nitrogen/boron, Li et al. successfully achieved the simultaneous double-doping of N and B in porous carbon nanosheets. The BC-based gel polymer was used as electrolytes, which achieved high-performance all-cellulose-based supercapacitors. The device can achieve a specific capacitance of 572 F g⁻¹ with 97.8% capacity retention after 20,000 cycles.^[]

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In the carbonization composition, CNF, calcium alginate, and agarose can act as a spacer to prevent the buildup of 2D conductive materials. For example, Liu et al. developed ultra-low density and superhydrophilic carbon aerogels by combining CNF, CNT, and rGO. Hydroxyl groups within cellulose can form hydrogen bonds with the numerous oxygen-containing groups on the surface of rGO, while the carboxyl groups on CNF create electrostatic repulsion that aids in the dispersion of CNTs. The homogeneous dispersion of the solution facilitates the formation of homogeneous aerogel structures in the subsequent bi-directional freezing and carbonization (Figure ). The aerogel exhibited an area capacitance of 109.4 mF cm⁻² and a cyclic compression performance of 88% (after 5000 cycles at 50% compressive strain) in supercapacitors.^[] Liu et al. proposed a CNF/rGO carbon aerogel with a three-dimensional porous structure (Figure ). The CNF was interspersed between the graphite oxide skeleton to prevent its stacking. And its porosity and hydrophilicity promoted the diffusion of electrolyte ions. The prepared carbon aerogel has excellent compressibility (80% strain) and elasticity (96% stress retention after 2000 compression cycles at 30% strain). After assembling it into a super multiplayer, it has an area-specific capacitance of 31.2 mF cm⁻² and a retention rate of 82% (after 5,000 cycles).^[] Zhang et al. prepared flexible electrodes with large ion-accessible active surfaces and high densities by gelation and carbonization of calcium alginate in MXene nanosheets (Figure ). During the gelation process, SA reacted with calcium ions between the MXene sheets to form a cross-linked calcium alginate hydrogel. The capillary forces induced during evaporation drying give the MXene/calcium alginate hydrogel film a high density. Subsequently, calcium alginate-derived carbon dots were embedded into the MXene nanosheets during the carbonization process. This embedding increases the layer spacing and facilitates electrolytic diffusion within the MXene film. Supercapacitors assembled from this film exhibited a volume specific capacitance of 1244.6 F cm⁻³ and 93.5% cycling stability (after 30,000 cycles).^[] Sangili et al. synthesized carbon-coated Ti₃C₂T_x MXene (Ti₃C₂T_x@C) by the hydrothermal method by using agarose as a carbon source. After the hydrothermal treatment, the phenolic carbon layer in situ covers the Ti₃C₂T_x to form Ti₃C₂T_x@C to protect the Ti atoms from oxidation by water and O₂, which in turn avoids the re-stacking of the Ti₃C₂T_x@C lamellae to come, and effectively enhances its electrochemical performance. The capacitance retention of this aerogel assembled into supercapacitor was 97.2% after 5,000 GCD cycles.^[]

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Starch and deoxyagar exhibit a substantial carbon content, rendering them highly promising as carbon precursors. Zhao et al. chose gelatinized starch (α-starch) as the carbon source and obtained interconnected porous activated carbon. After assembling a supercapacitor with this activated carbon electrode, the specific capacitance reached 49.77 F g⁻¹ and retained 90% capacitance after 20,000 cycles.^[] Zhai et al. prepared a green sustainable carbon aerogel using starch as raw material and KCl and K₂CO₃ as templates and activators. During the gelation process, starch forms a network structure, where KCl and K₂CO₃ are interspersed. This unique arrangement facilitates the formation of a three-dimensional network structure resembling ant tunnels, thereby creating pathways for the transport of electrolytes. The specific capacitance of this electrode was 292.3 F g⁻¹ at a current density of 1 A g⁻¹.^[] Wang et al. prepared porous carbon nanofibers using high amylose starch as a sacrificial polymer template. The carbon nanofibers exhibit a distinctive micro/mesoporous structure, characterized by a significant proportion of micropores and an impressive specific surface area of up to 1204 m² g⁻¹. The specific capacitance of the nanofiber electrode is 1.0 A g⁻¹ and 344 F g⁻¹. The cycle durability of the corresponding device is 99.9% after 10,000 cycles.^[] Zhang et al. introduced an innovative approach by utilizing calcined deoxyagar to produce an activated carbon material, which proves advantageous in enhancing the performance of hybrid lithium-ion supercapacitors. The activated carbon derived from deoxyagar exhibits a pore size that closely matches the size of electrolyte ions, providing favorable conditions for the storage of electrolyte ions. Furthermore, the combination of a higher specific surface area and the disorderliness of deoxyagar-based activated carbon enhances the double-layer capacitance. The hybrid lithium-ion supercapacitor assembled with this electrode had a specific capacitance of 224.3 F g⁻¹ and 89% cycling stability.^[]

Biomass-based materials for supercapacitor electrolytes

The inclusion of hydrophilic functional groups, such as –OH, –COOH, –NH₂, and CONH₂, within biopolymers provides them with excellent wetting capabilities toward polar solvents. Additionally, these functional groups exhibit a preference for interacting with salt anions, leading to improved solubility in saline environments and enhanced properties for cationic transport. Biopolymer-based hydrogels can be modified by grafting, incorporation of inorganic fillers, and blending.^[] The incorporation of polymer monomers through blending or co-embedding polymerization offers a means to further enhance the ionic conductivity of biopolymer-based hydrogels. This approach effectively reduces the crystallinity of biopolymers, resulting in enhanced ionic conductivity. Moreover, the incorporation of polymer compounds provides a substantial enhancement to the toughness and ductility of biopolymer hydrogels, thereby offering further improvements to their mechanical properties.^[] The electrochemical performance of supercapacitors based on biomass electrolyte materials is summarized and compared in Table .

TABLE 3 Electrochemical performance of supercapacitors based on biomass electrolyte materials.

Electrolyte	Electrode	Ionic conductivity	Mechanical properties	Voltage window	Refs.
Carboxylated chitosan membrane	Activated carbon	7.82 × 10−2 S cm−1	–	0.9 V	[]
Carboxylated chitosan hydrogel	Activated carbon	–	–	1.8 V	[]
BC/PAM hydrogel	RGO PANI PMFT	125 mS cm−1	330 kPa	0.8 V	[]
Sodium alginate hydrogel	Activated carbon	2.05 mS cm−1	0.24 kPa	1.4 V	[]
Agar-based gel-like electrolyte	Carbon	–	–	1.6 V	[]
Starch/PVA hydrogel	Activated carbon	1.40 ± 0.04 S m−1	1.04 ± 0.03 MPa	1.0 V	[]
Starch/PVA hydrogel	Activated carbon	1.00 ± 0.04 S m−1	0.53 ± 0.03 MPa	1.0 V	[]
Chitosan/PVA/ionic liquid/CNT hydrogel	Activated carbon	–	–	1.6 V	[]
Chitin/ionic liquid hydrogel	MXene carbon black chitin	8.5 ± 0.4 mS cm−1	–	0.8 V	[]
Microcrystalline cellulose/ ionic liquid ionogel	Activated carbon	22.4 mS cm−1	1.26 MPa	3 V	[]
Agar/PVA hydrogel	Activated carbon	43.6 mS cm−1	1012.3 kPa	1.0 V	[]
Sodium alginate/SPMA-Zn:ZnSO4/polymethylacrylic acid hydrogel	Activated carbon Zn	7.35 S m−1	0.058 MPa	2.2V	[]
Alginate/ PEDOT:PSS hydrogel	Activated carbon	13.7 × 10−3 S cm−1	–	0.8 V	[]
Lignin/PAA/NiCl2 hydrogel	Carbon cloth PANI	6.85 S⋅m−1	617 kPa	0.9 V	[]

Aqueous gel polymer electrolytes

Aqueous gel polymer electrolytes (AGEs) consist of a main polymer matrix (PVA, PMMS, PAA, and PEO), plasticizer water and electrolytic salts, which can be strong acids (H₂SO₄ and H₃PO₃), strong bases (KOH) or neutral (LiCl, Na₂SO₄).^[] Zhang et al. prepared a noval carboxylated chitosan-based alkaline-tolerant hydrogel electrolyte membranes. They copolymerized and cross-linked the graft between acrylic acid, N,N′-methylenebisacrylamide and potassium persulfate, and strongly absorbed the KOH electrolyte. Supercapacitors based on this alkaline-resistant hydrogel electrolyte film have good energy density and power density in the potential window range from 0 to 0.9 V. This polymer gel electrolyte remains functional in both acidic and neutral electrolytes.^[] Lin et al. prepared carboxylated chitosan-based hydrogel electrolytes for symmetric pseudo-solid-state supercapacitors. A pair of cationic and anionic groups on the N-acetyl-D-glucosamine unit in carboxylated chitosan would confer unique zwitterionic properties, allowing the electrolyte to bind more tightly to water molecules. Supercapacitors based on this hydrogel electrolyte have significant specific energy density (32.6 Wh kg⁻¹) and specific power density (900 W kg⁻¹), while retaining more than 98% of their capacitance after 3000 cycles.^[] Li et al. prepared an all-solid supercapacitor with BCNF reinforced PAM as the hydrogel electrolyte and rGO/PANI/poly(diallyldimethylammonium chloride)-modified fiber textile (PMFT) as the electrodes. The incorporation of BC results in the BC/PAM composite possessing high mechanical strength without sacrificing flexibility, while also enhancing the stability of ion transport channels. In addition, the hydrophilicity of the BC skeleton plays an important role in improving the water retention capacity of the hydrogel. This all-solid-state supercapacitor has ionic conductivity of 125 mS cm⁻¹ and a tensile strength of 330 kPa.^[] Yang et al. reported a novel temperature-sensitive supercapacitor electrolyte (SPA), who used a natural flame retardant biopolymer SA as a matrix and acrylic acid (AA) and acrylamide (AM) as functional monomers. The electrolyte obtained thermal responsiveness by adjusting the content of AA and AM. The SPA hydrogel demonstrates excellent flexibility, with a remarkable strain capacity of 1578% and stress resistance of 0.24 MPa. Additionally, the hydrogel exhibits favorable electrical conductivity. The supercapacitor built upon this hydrogel showcases a specific capacity of 27.7 F g⁻¹ and operates within a voltage range of 1.4 V.^[] Menzel et al. proposed a K₂SO₄-based agar base electrolyte for the preparation of symmetrical carbon/carbon supercapacitors. They investigated the role of gel electrolytes on hydrogen adsorption phenomena and the performance of electrochemical capacitors. The incorporation of agar decreased ion mobility while enhancing hydrogen adsorption efficiency, consequently restricting both diffusion and kinetically driven self-discharge mechanisms. The gel electrolyte showed the stable performance at 1.6 V.^[]

Organic gel electrolytes

In general, organic gel electrolyte (OGE) consists of a physical mixture of high molecular weight polymers PMMA and polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF–HFP) with conductive salts dissolved in a non-aqueous solvent. This OGE overcomes the formation of low interfaces and improves the ionic conductivity.^[] He et al. prepared starch/PVA/dimethylsulfoxide/CaCl₂ (SPDC) organohydrogels. The interaction between hydroxyl groups during the freezing process led to the formation of a physical cross-linked network, where the starch chains and PVA served as the backbone of the hydrogel. The addition of dimethyl sulfoxide (DMSO) and CaCl₂ enhances the mechanical properties and extends the working temperature range of starch/PVA hydrogels (Figure ). SPDC organohydrogels have excellent strength, toughness, and recyclability, remain flexible even at temperatures of −20°C, and can be effortlessly extruded, bent, and twisted (Figure ). The SPDC-based supercapacitor exhibits a high surface capacitance of 156.50 mF cm⁻² at a current density of 1 mA cm⁻² and a capacitance retention of 82.23% after 8,000 charge/discharge cycles.^[] Lu et al. developed a starch/PVA/glycerol/CaCl₂ organo-hydrogel. The presence of numerous hydrogen bonds within the starch and PVA molecules facilitated the direct formation of intramolecular and intermolecular hydrogen bonds. This phenomenon effectively improved the stability of the network structure and promoted the mutual solubility between starch and PVA. The addition of glycerol and CaCl₂ improved the mechanical, thermal, and electrical conductivity of the starch/ PVA hydrogel, as well as the compatibility between starch and PVA (Figure ). As a result, the generated organic hydrogels had good mechanical flexibility, electrical conductivity, and frost resistance (Figure ).^[]

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Ionic-liquid based gel polymer electrolytes

Ionic liquid-based gel electrolytes (ILGE) display superior characteristics compared to water-based and organic electrolytes, including high ionic conductivity, non-volatility, non-flammability, and an extended potential window. Consequently, these properties render them suitable for incorporation into flexible and stretchable energy storage devices.^[] Li et al. developed hydrogel electrolytes using quaternary ammonium side chain chitosan and polyvinyl alcohol as the main chain components. Chitosan acts as a super backbone for gel electrolytes with high-density hydrogen bonding cross-linked to polyvinyl alcohol, exhibiting excellent mechanical properties. Furthermore, incorporating polyvinyl alcohol as the main chain component in the hydrogel enables the resulting material to exhibit excellent sensing performance when subjected to external forces, thereby endowing the assembled supercapacitors with sensing capabilities. The device based on this hydrogel electrolyte has a mass capacitance of 43.15 F g⁻¹ and an area capacitance of 258.89 mF cm⁻². After 5,000 charge/discharge cycles, the capacitance of the device decreases by only 7.5%.^[] More recently, Kasprzak et al. designed an all-solid-state supercapacitor based on chitin. They used chitin and ionic liquids to prepare a gel electrolyte. Chitin provides support for the gel electrolyte, which also acts as an adhesive to integrate the supercapacitor components together. The electrolyte has an ionic conductivity of 8.5 ± 0.4 mS cm⁻¹.^[] Rana et al. synthesized a sustainable cellulose-based dual-network ionic gel electrolyte. Cellulose was first dissolved with an ionic mixture to form a cellulose network. Then, hydroxyethyl methacrylate (HEMA) monomer was polymerized on it to form a dual network ionic gel. The electrolyte has a large voltage window of 3 V and an ionic conductivity of up to 22.4 mS cm⁻¹ at 120°C.^[] Peng et al. prepared a new physically cross-linked self-healing dual-network hydrogel electrolyte (PVA/Agar– EMIMBF₄–Li₂SO₄). By incorporating agar, the hydrogel achieved improved tensile properties and excellent flexibility. Furthermore, the addition of Li₂SO₄ and ionic liquid (EMIMBF₄) enhanced its ionic conductivity and expanded its working temperature range, respectively. The dual-network hydrogel exhibited notable self-healing capability, with a healing efficiency surpassing 80% of the initial state after undergoing five self-healing cycles.^[] In another study, Huang et al. also prepared a self-healing ionic conductive hydrogel electrolyte that was based on SPMA–Zn:ZnSO₄, SA and polymethacrylic acid (Figure ). Among these, SPMA-Zn demonstrated outstanding flexibility (Figure ) and remarkable self-healing capability, allowing it to regain its electrochemical properties after undergoing multiple mechanical damages. Moreover, it can readily regenerate the hydrogel electrolyte following pulverization while retaining its stable electrochemical properties. The hydrogel-based supercapacitor has a voltage window of 0–2.2 V, an energy density of 164.13 Wh kg⁻¹ and a power density of 1283.44 Wh kg⁻¹.^[]

Redox-active solid electrolytes

Redox-active solid electrolytes pertain to the inclusion or addition of redox-active components within the electrolyte with the objective of optimizing capacitance and consequently boosting the energy density of the device.^[] Using H₂SO₄ as a polymerization initiator and ion source, Badawi et al. developed an intelligent self-healing hydrogel electrolyte based on alginate/poly (3,4-ethylenedioxythiophene):polystyrene sulfonic acid (A/PEDOT:PSS) (A/P:P). Alginate serves as the foundation of the electrolyte, while the incorporation of PEDOT:PSS imparts flexibility and self-healing properties to the electrolyte. This hydrogel has an ionic conductivity of 13.7 × 10⁻³ S cm⁻¹ at 25°C.^[] Mondal et al. prepared polyacrylic acid (PAA) (SL-g-PAA-Ni) hydrogels containing sulfonated lignin (SL) under high concentration of nickel chloride (NiCl₂). The presence of phenolic hydroxyl and methoxyl groups on the SL enabled the formation of a hydroquinone/quinone redox system with Ni²⁺/Ni³⁺ pairs, thereby promoting the polymerization reaction. In addition, the presence of NiCl₂ imparts antifreeze properties to the hydrogel, allowing it to maintain its properties similar to room temperature at 20°C temperature. Due to the successful coordination of Ni²⁺ ions with catechol groups and carboxyl groups, coupled with the abundant hydrogen bonding ability, the obtained hydrogels exhibit good mechanical properties. The hydrogels obtained a high ionic conductivity (6.85 S m⁻¹) due to the high concentration of Ni²⁺ provided.^[]

CHALLENGES AND PERSPECTIVES

In conclusion, there is a necessity to innovate energy storage devices that demonstrate exceptional performance, environmental sustainability, and affordability. The latest advancements in the application of biomass-based materials for flexible supercapacitor have been comprehensively summarized. This summary encompasses the physical and chemical properties of biopolymers, their impact on supercapacitors, the working mechanism of supercapacitors, and the application of biomass materials in supercapacitors. Numerous endeavors have been dedicated to create advanced biomass-based materials, employing highly efficient fabrication strategies to achieve rational structures and enhanced electrochemical performances.^[] In all, the exquisite and complicated hierarchical nanostructures and morphologies of biopolymers demonstrate the significant application in flexible supercapacitor electrodes. The abundance of hydrophilic groups, such as –OH, –COOH, –NH₂, and CONH₂ in the biopolymer structure contributes to their enhanced salinity solubility and cation transport properties, making them highly promising for flexible electrolytes. Despite significant breakthroughs in recent years regarding biomass-based supercapacitor composites, there are still a number of challenges that need to be addressed before these supercapacitor devices can be widely used in a variety of fields (Figure ).

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First, one of the main limitations for the application of biomass-based materials in supercapacitors is the current lack of feasible techniques for mature and large-scale fabrication of these materials. For example, chitosan is made from crab shells, and its processing requires high-temperature treatment with strong acid and alkali, which generates a large amount of wastewater and energy consumption, making it difficult to achieve large-scale production. Nanocellulose requires complex and time-consuming processing steps (e.g., freeze-drying) to prepare functional self-supporting structures such as thin films/nanopaper, fibers, and aerogels. The process of converting biomass to carbon material is costly in terms of both the chemicals used and the heat treatment involved. In most cases, the utilization of high-temperature carbonization/pyrolysis methods remains inevitable in various biomass conversion processes, leading to significant energy consumption.

Second, biomass materials inherently exhibit poor conductivity, necessitating their combination with conductive active materials or their carbonization to serve as carbon precursors for supercapacitor electrodes. However, the presence of polar functional groups in certain biomass materials leads to the formation of hydrogen bonds with water molecules, causing their aqueous solutions to adopt a gelled state. The poor mobility of the gels leads to poor processability during electrode preparation and negatively affects the electrode performance. The impact of pore size, specific surface area, and surface chemistry of biomass-derived activated carbon on the electrochemical performance of biomass-based energy storage devices remains largely unexplored. On the other hand, it is difficult to quantify sustainability in terms of precursors, processing steps (chemicals, processing steps, energy consumption), CO₂ emissions, harmful releases, and product recycling.

Third, there are relatively few researches on the synergistic effects of combining biomass materials with various conductive components.

For example, the combination of active substances with nanocellulose will destroy the hydrogen bonding of nanocellulose and reduce the mechanical properties of the composite. Therefore, in the future work, more attention should be paid to the selection and ratio of composite materials, and explore its internal mechanism. In addition, the current function of supercapacitors is relatively simple, the future should be developed here based on intelligent, wearable, flexible, and other functions in one of the supercapacitors.

Fourth, compatibility between electrolyte and electrode materials has a great impact on the performance of supercapacitors. Some studies mentioned in the paper use biomass-based electrodes with conventional electrolytes, or biomass-based electrolytes with conventional electrodes, or both electrolytes and electrodes are biomass-based materials. However, the compatibility between electrode and electrolyte is less described. Therefore, establishing an in situ monitoring technique to assess the interfacial charge distribution and structural evolution is necessary to predict the compatibility between electrodes and electrolytes, ensuring they are well matched with each other.

In the future, efforts aim to minimize the reliance on fossil energy, adopt eco-friendly preparation methods, and produce cost-effective, high-performance, and durable supercapacitor materials. There is no doubt that the development of biomass materials for energy storage devices has great potential. The development of new material synthesis strategies, functional energy storage devices such as flexible wearable supercapacitors, temperature-sensitive or light-sensitive supercapacitors may be the future direction of development.

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (32301530), Tianjin Excellent Special Commissioner for Agricultural Science and Technology Project (22ZYCGSN00350) and Tianjin Enterprise Technology Commissioner Project (22YDTPJC00930).

CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interests.

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