Research progress of biomass-derived carbon for the supercapacitors

In order to fulfil the requirements of various equipment in different fields for energy storage components, there is an increasing number of studies being conducted on the development of electrode materials for supercapacitors. At present, carbon materials used in electrode components in supercapacitors are mostly graphene, porous carbon, activated carbon, and carbon nanotubes. Due to the fast advancements in modern technology and science, which have stimulated the demand for sustainable and eco-friendly energy storage materials, biomass-derived carbon materials have gradually emerged in the public eye. The carbon source of biomass-derived carbon is usually a natural substance, which has advantages such as low cost, easy availability, and environmental friendliness. Due to the diversity of material structures, the required electrode materials can be designed and prepared according to performance requirements. At the same time, biomass-derived carbon is also capable of being combined alongside various electrode materials to create asymmetric capacitors, combining the advantages of the two to enhance capacitor electrochemical performance. This article first introduces different sources of biomass-derived carbon-based electrode materials and compares their electrochemical performance. Then, based on various research results, the factors that affect their electrochemical performance are discussed in detail. Then, the preparation methods of biomass-derived carbon electrode materials are introduced, and the specific requirements, advantages and disadvantages of different preparation methods are briefly analyzed. The application of biomass-derived carbon electrode materials in supercapacitors in combination with other materials is listed. Finally, a summary and outlook of the current research status are provided to provide a reference for the rational design of biomass carbon supercapacitors in the future.


Introduction
With social civilization's progress, people's energy demand is increasing.At the same time, the energy crisis and pollution of the environment due to the utilization of fossil fuels restrict the development of human society.Seeking renewable, low-cost, clean, and efficient clean energy has already become an unavoidable choice to ensure sustainable development of the economy and environment.With high cleanliness and renewable energy advantages, solar and wind power account for a large proportion of the energy structure.However, intermittent supply and other problems limit the use of solar and wind turbines.In order to meet human needs for energy use, new, cheap, and environmentally friendly energy storage and conversion equipment [1] must be developed.
In between batteries and capacitors, supercapacitors are an updated generation of energy-storing innovations.Their' benefits of quick charging and discharging, extended cycle life, large power density, exceptional coulomb efficiency, extensive temperature spectrum, and environmental protection have all been extensively researched by researchers [2][3][4].Figure 1 shows the contrast of various energy storage technologies' power and energy densities.At present, supercapacitors are able to be classified into three types on the basis of their charge storage mechanisms, and there are also significant differences in the electrode materials used for different types of supercapacitors.The first category of pseudo-capacitor, whose energy storage principle relies on rapid redox, is widely used as electrode materials, such as transition metal oxides (represented by RuO 2 and MnO 2 [5][6][7]), conductive polymers ( represented by polypyrrole, polyaniline, and polythiophene [8,9] ), and phosphate (represented by BiPO 4 [10,11]).Due to its highly reversible rapid surface or near-surface reactions, it has the advantage of high power density.
The second type of supercapacitor stores charge electrostatically by adsorbing ions on the electrode surface.The electrode materials used include carbon materials represented by graphene, activated carbon and biomassderived carbon [12][13][14][15].However, traditional activated carbon usually uses coal, petroleum coke and other nonrenewable resource as carbon sources, which will cause many adverse effects on the environment.As a new type of carbon compound, graphene is usually used as the precursor of supercapacitor electrodes because of its good conductivity, high strength and good thermal conductivity.It uses graphite as a carbon source, but naturally extracted graphite cannot meet the growing demand for use.As another way to obtain graphite, petroleum coke synthesis also has the problem of unsustainable traditional activated carbon.
In contrast, biomass-derived carbon has diverse sources, low production prices, and is environmentally friendly [16].Its porous structure provides more active sites, which are able to be effectively utilized as supercapacitor electrode materials.The preparation of biomass-derived carbon-based electrode materials using natural materials such as corn straw and peanut shells decreases the expenditure of producing carbon materials, as well as alleviates the environmental problems caused by excess biomass resources.Due to the structural diversity of biomass itself, biomass-derived carbon-based electrode materials used as carbon sources will also have different structures and electrochemical properties.Similarly, every step in the preparation process will have an impact on the electrode material's electrochemical performance.Choosing a suitable carbon source and reasonably controlling the preparation process is essential for enhancing the electrochemical performance of supercapacitor electrode materials.
In addition, biomass-derived carbon can be studied separately as electrode materials or can be utilized as a basis material to composite with different types of high-performance electrode materials.The main focus of improving the electrochemical performance of devices is to increase energy density while retaining their large power density as well as long cycle stability.According to a formula: E = 1/2 CV 2 , increasing their specific capacitance is important for improving energy density under a certain voltage window.This composite material integrates two different energy storage mechanisms, combines the advantages of large specific capacitance of pseudo-capacitor and high conductivity of double electric layer supercapacitor, and is able to be effectively used in energy storage devices' electrode materials.This type of composite material belongs to one branch of the third type of hybrid supercapacitors, that is, one or two of the electrodes is a composite material composed of double electric layer supercapacitor electrode material and pseudo-capacitor electrode material.Similarly, biomassderived carbon materials can be used to make electrodes of double electric layer supercapacitor and combine with another pseudo-capacitor electrode to form a supercapacitor, which belongs to another branch of hybrid supercapacitor, namely asymmetric supercapacitor.The cooperation of the two storage mechanisms can effectively boost supercapacitor energy density.materials, they can show good electrochemical performance.Wang et al [27] used wormwood as a carbon source and established graded porous carbon from wormwood leaves through simple carbonization and NaCl activation at different temperatures.The wormwood-based porous carbon (AARC-700) obtained by hightemperature sintering at 700 °C exhibits a large specific surface area of 1141.8 m 2 g −1 as well as the complex hierarchical structure including macropores, micropores, and mesopores, making it an efficient material (figure 5).These experimental results demonstrate that AARC-700, as an electrode in supercapacitors, has excellent electrochemical performance.The CV curve shows obvious electrolyte decomposition (figures 6(a), (b)), and the GCD curve indicates good charge-discharge performance of the device (figure 6(c)).In a system of dual electrodes with a 6 M KOH electrolyte, an area capacitance is equal to 348.4 F g −1 when the current density is 1 A g −1 .The symmetrical supercapacitor (AARC-700//ARC-700) with Na 2 SO 4 as the aqueous electrolyte achieves 32.06 Wh kg −1 of energy density with 2.0 kW kg −1 of power density, when the wide voltage window is 2.0 V.A high energy density of 16.67 Wh kg −1 can be attained even at a high power density of 40 kW kg −1 .(figures 6(d), (e)).Besides, this supercapacitor exhibits a 99.7% capacitance retention rate after 20000 consecutive charging/discharging tests with a high current density is equal to 5 A g −1 (figure 6(f)).
Nutshells have special and diverse pore structures, which affect their ability to transport and store electric charges.Based on this characteristic, Manimekala et al [28] studied the electrochemical performance of cashew nut shells as a carbon source used in supercapacitor electrode materials.Researchers utilized KOH to be the activator for preparing cashew shell-based porous carbon (CNSAC) from cashew shell biomass using the hydrothermal method.The CNSAC's morphology and microstructure were researched using the scanning electron microscope (SEM), and the surface of activated carbon showed a highly developed porous structure, similar to a three-dimensional honeycomb (figure 7).The cyclic voltammetry test detected the electrode's double-layer capacitance behavior.Through constant current charge-discharge (GCD) research, the calculated specific capacitance is equal to 247 F g −1 when the current density is is equal to 0.25 A g −1 .Dynamics studies have shown that the contribution of capacitance to diffusion components is more crucial.An asymmetric supercapacitor is prepared in the Swagelok battery, and its characteristics are analyzed.When the battery voltage range is 0-1 V, the specific capacitance calculated by GCD is 98 F g −1 with a current density of 0.25 A g −1 .After 20000 cycles, a capacitance retention rate of 97% is obtained in a two-electrodes system (figure 8).
Buckwheat, a plant resource with a world output of more than 1 million tons, is widely used as health products, such as Buckwheat tea, buckwheat pillow, but it is rarely used in other fields, especially in the energy field.Buckwheat contains 70% globulin and 10% starch, as well as trace elements and dietary fibre [29,30].A certain amount of starch is beneficial for obtaining rich carbon structures during high-temperature carbonization, and a rich protein content is beneficial for establishing n-doped structures in the carbon matrix [31,32].Therefore, Ou et al [33] prepared porous carbon using buckwheat kernels as precursors by the one-step activation method.Obtained buckwheat core-based porous carbon (BCPC-3) exhibits a large specific surface area is equal to 805.91 m 2 g −1 and a high pore volume is equal to 0.60 cm 3 g −1 .The materials were characterized using SEM.For starters, when buckwheat core powder is mixed with KOH in a 1:1 ratio, the substance comprises A great number of macropores and mesopores (figure 9(a)).When the ratio is increased to 1:3, BCPC-3 forms an interlinked porous foam-like carbon skeleton (figure 9   components effectively boost the active materials' rate capacity and capacitance during the charging and discharging process [34].The BCPC-3's detailed structural characteristics were further elucidated through TEM.Meanwhile, in figure 9(h), we can discover that numerous bright and dark plaques were uniformly distributed in BCPC-3, which were carbon materials' microporous structures after organic component pyrolysis and KOH activation.In addition, The measured lattice stripe spacing in figure 9(j) is consistent with the estimated x-ray diffraction (XRD) spectrum values.The consequence may be attributed to the combined effect of KOH activation and low carbonization temperature, which increases the materials' degree of graphitization but does not entirely form A graphite structure.Due to the different proportions of KOH, the prepared materials also exhibit different shapes and pore structures.This three-dimensional layered porous structure is composed of pores with different diameters, which can improve the active materials' electrochemical performance.Especially The AARC-0//AARC-700 power density and energy density graphs (e) as well as previously published carbon-based SCs.The AARC-0//AARC-700 ′s cyclic stability image (f) with a current density is equal to 5 A g −1 .Reproduced from [27], with permission from Springer Nature.a large number of micropores' presence enhances the capacitance of the double-layer [35].When it acts on a 6 M KOH three-electrode system, this material provides the high capacitance is equal to 330 F g −1 (0.5 A g −1 of current density), especially, which shows that the capacitance can reach 140 F g −1 even when the current density increases to 100 A g −1 .

Animal-derived carbon
Animal skin or its metabolites contain proteins, carbohydrates, and lipids [36], and its tissue structure contains elements such as N, P, and S, such as fish scales [37], pig bones [38], and waste crab shells [39].They can be carbonized to form porous carbon and used in electrode materials in supercapacitors, demonstrating good   (d-g) elemental mapping and (h-j) HRTEM graphics for the BCPC-3.Reproduced from [33], with permission from Springer Nature.electrochemical performance.Niu et al [40] used fish skin as raw material and converted it into water-soluble gelatin through KOH treatment.Meanwhile, N, O, and S doped microporous carbon nanosheets (MPCNS) were prepared by carbonization using potassium compounds derived from KOH as templates and activators (figure 10 shows this process).The comparatively moderate pyrolysis temperature (600 °C) retains a substantial quantity of S, O, and N heteroatoms from fish skin, which can provide pseudo capacitance capability for electrode materials, resulting in better pore volume, specific surface area and compaction density of biomassderived porous carbon.At the same time, the microporous nanosheet structure can help in making the electrode surface fully touch with the electrolyte, and this carbon prepared has excellent volumetric and gravimetric performance as a supercapacitor electrode in water electrolyte.
Wang et al [41] selected pig skin rich in collagen as an origin of carbon and used KOH to be an activator to manufacture porous carbon doped with nitrogen and oxygen, which was used in supercapacitor electrodes.By adjusting the activation temperature and altering the electrochemical properties of pig skin charcoal, surface chemical properties and adjustable pore structure were obtained.At 600 °C, the capacitance of PSAC-600 (pig skin-based carbon obtained from high-temperature sintering at 600 °C) is 547 F g −1 , when the current density is 2 A g −1 .The capacitance retention rate after 10000 cycles is about 93%, and the cycle stability is excellent.
Chitin is the second richest Biopolymer after cellulose, which has many good characteristics, including wide availability, sustainability and internal network nanostructures.In addition, it is important that chitin contains a large amount of nitrogen, and direct pyrolysis can evenly blend heteroatoms into it, thereby obtaining highquality nitrogen-doped biomass-derived carbon.Therefore, chitin has great potential as a superior-quality origin of carbon for electrode materials in supercapacitors.Hao et al [42] prepared N-doped nanofiber carbon using N-rich chitin as raw material through pre-carbonization and high-temperature carbonization.During the carbonization process, the N atom in the amide group of chitin gradually transforms into a nitrogen-containing functional group.It is doped in the form of pyrrole N and pyridine N in carbon material, thereby improving the electrochemical energy storage performance.These dopants provide enough active sites to allow fast electron/ ion transport inside an electrode.
Zhao et al [43] used ant powder to prepare S-O-N co-doped hierarchical porous carbon (HPCs) through a one-step method.The resulting material has a substantial specific surface area (2650 m 2 g −1 ) as well as a typical three-dimensional (3D) framework composed of macroscopic, mesoscopic, and microporous structures with appropriate distribution of pore size and doped with elements like O and N. Applying HPCs as working electrodes in the 6 M KOH system of three-electrodes, a quite high specific capacitance approximately 576 F g −1 is obtained when the current density is 1.0 A g −1 .And its cycle stability is excellent.An asymmetric supercapacitor prepared with the 1-ethyl-3-methylimidazolium Tetrafluoroborate (EMIMBF 4 ) electrolyte provides an energy density is equal to 67 Wh kg −1 even at the power density of up to 18000 W kg −1 .
Figure 10.The process of preparing for supercapacitor electrode materials using fish skin for carbon source and KOH as activator.

Waste-derived carbon
One important aspect of green and sustainable development is waste treatment and reuse.Using waste to be the precursor in the manufacture of electrode materials not only ensures the electrochemical performance of energy storage devices, but also is crucial for sustainable economic and environmental development.Lignosulfonate is a common by-product in the papermaking industry and is often discarded as waste.The preparation of supercapacitor electrode materials using lignosulfonate as a carbon source not only effectively utilizes byproducts but also improves the specific capacitance of energy storage components.Bai et al [44] prepared ordered electrochemical energy storage carbon with interconnected pores using lignin sulfonates as precursors.The lignosulfonate's unique molecular characteristics and structure ensure the availability of good-quality biomass-derived carbon with adjustable pore shape and enhanced physical characteristics.When applied in symmetrical supercapacitors, the obtained graded porous carbon exhibits extraordinary energy storage capacity, reaching up to 289 F g −1 higher than capacitance when the current density is 0.5 A g −1 .Its energy density is 40 Wh kg −1 at a power density is 900 W kg −1 .
Solid leather waste is a common waste in our daily life.If a large amount of leather waste is not properly managed, it will pollute our environment and waste a large amount of resources.It is crucial to convert leather waste into usable resources.As a result of its high content of carbon, leather waste is able to be carbonized into porous carbon using appropriate methods.Liu et al [45] prepared nitrogen-doped biomass-derived carbon using plant tannin shavings as precursors through the KOH activation carbonization method.By changing the activation conditions of the product, the nitrogen content and pore structure can be effectively changed.When using nitrogen-doped porous carbon as the supercapacitor electrode material, its specific capacitance value per hour is up to 421 F g −1 when the current density is 1.0 A g −1 .It has excellent nitrogen content and a significant specific surface area.After 5000 cycles in the test of charge-discharge, the capacitance retention rate of the solidstate supercapacitor using porous carbon electrodes remains at 83% (figure 11).
Plastic is one of the most commonly used materials in our lives.However, plastic waste is difficult to degrade, and if handled improperly, it poses significant environmental hazards.Because of the high degree of carbon content, plastics are able to be employed as the carbon source in the preparation of biomass-derived carbon.Waste is genuinely turned into treasure when plastics react with templates and activators to create plasticderived carbon with exceptional electrochemical characteristics.Waste polystyrene foam was used by Zhang et al [46] to build a rich porous structure carbon, and 3D mesh structure biomass-derived porous carbon could be successfully produced through the Friedel Crafts process.The instrument has the specific capacitance of approximately 208 F g −1 with a current density is equal to 1 A g −1 .It has 22.5 Wh kg −1 of excellent energy density, at 1024 W kg −1 of power density.Capacitance retention rate is equal to 94.3% at a current density is equal to 5 A g −1 after 5000 charging and discharging cycles of the electrode material.
In addition to factory waste, household waste can also serve as electrode materials' carbon source.Compared to factory waste, domestic waste carbon sources have advantages such as environmental sustainability, non-toxic and harmless properties, and a wide range of sources.When utilized as supercapacitor electrode materials, biomass-derived carbon can exhibit different electrochemical properties, including specific capacitance and specific surface area, due to its diverse internal structure.Zhou et al [47] produced activated biomass-derived carbon from bagasse utilizing KOH carbonization and activation processes.Then, a nanosheet/activated carbon (AC) composite material (MnO 2 /AC) made of ultra-thin manganese oxide (MnO 2 ) was prepared using hydrothermal technology.The supercapacitors' cathode and anode materials are AC and MnO 2 /AC composite materials, respectively.When the current density is 1 A g −1 , AC shows 89 F g −1 of specific capacitance and exhibits good electrochemical performance.After 5000 cycles, the capacitance still has 89% cycle stability.Lan et al [48] used the walnut shell as a precursor and prepared biomass-derived carbon through KOH activation.After various activation processes, the specific surface area of carbon generated from walnut shells is 1016.4m 2 g −1 .SEM images exhibit that each experimental sample has a relatively rich pore structure, which is due to the gasification of some volatile substances during the activation process (figure 12).The AC-650 (porous carbon obtained by high-temperature sintering at 650 °C) electrode shows the high specific capacitance is equal to 169.2 F g −1 under a 6 M KOH electrolyte when the current density is 0.5 A g −1 .The structural characteristics and electrochemical properties of various biomass-based derived carbons listed in this chapter are shown in table 1.

Factors affecting the electrochemical performance of biomass-derived carbon
3.1.Specific surface area and pore structure The double electric layer supercapacitor' working principle is that through electrostatic action, the charge on the electrode surface adsorbs ions with opposing charges in the electrolyte to its surface, forming an interface layer between the electrolyte and electrode with the same amount of charge on the electrode surface charge and opposite symbols, which is close to the side of electrolyte.When a field of electricity is provided to both ends of a supercapacitor, the electrolyte's negative and positive ions flow towards the positive and negative electrodes, resulting in a potential difference.At this time, a double-layer is formed.After the electric field is removed, the double-layer is stable due to the mutual attraction of charges on the electrode and charges with opposite symbols in the electrolyte, and the voltage of capacitor remains stable.After joining the supercapacitor's two poles, the charges loaded on the electrodes move under the right and left voltage, generating an external circuit's current.Then, its electrolyte turns electrically neutral once again.Ideally, the capacitance value of the supercapacitor is proportional to its electrode materials' specific surface area, scilicet, with the increase of specific surface area, its capacitance value also increases.Relevant research shows that under the same electrolyte conditions, porous carbon materials with larger specific surface areas have better capacitive properties.Heteroatom doping can usually be used to boost the materials' specific surface area.Bian et al [49] synthesized biomass-derived carbon composites co-doped with O, P, N, and S in one step using bean worms as precursors and KOH as activators.A complete rectangular block structure with a uniform and smooth surface was discovered through SEM, as shown in figure 13.In the block structure, there are also some open large pores and some broken particles.TEM images show that there are many nanopores on the carbon wall of the bean insect-based porous carbon (BWPC 1/3 ) with a bean insect content of 1/3, which may increase the pore capacity and specific surface area.Because of the co-doping of several heteroatoms, the sample's specific surface area increases to 1967.1 m 2 g −1 .Its specific capacitance reaches a maximum value of 371.8 F g −1 when the current density is equal to 0.1 A g −1 .
Zheng et al [50] prepared carbon co-doped N and P materials using bagasse and NH 4 H 2 PO 4 as the main materials using the hydrothermal activation method.Compared with the original carbon material, The P and N co-doped sheet porous carbon materials' specific surface area increased from 1307.21 m 2 g −1 to 2118.59 m 2 g −1 , significantly improving its electrochemical performance.
However, researchers found that the porous carbon materials' specific surface area is only positively correlated with specific capacitance within a certain range.When the charge reaches the active materials' surface, theoretically, charging and discharging are completed.However, many porous carbon materials' structure leads to long electrolyte ion transport distances, limited migration, and limited channel usage.Therefore, the porous carbon' effective specific surface area is not is equal to its specific surface area [51][52][53][54].Qu et al [55] found through testing and comparing the electrochemical performance and physical structure of various carbon materials that the specific capacitance value of the materials does not go up with the boost of specific surface area.On the contrary, some activated carbon materials have a relatively small specific surface area.Still, the specific capacitance value of capacitors prepared from this material is higher than other materials with a large specific surface area.
Carbon materials are classified internationally based on their pore size.Pores smaller than 2 nm are classified as microporous carbon materials, pores between 2 nm and 50 nm are classified as mesoporous carbon materials and pores larger than 50 nm are classified as macroporous carbon materials.The consensus is that the micropores that provide charge storage space are proportional to the specific surface area of the material.Mesopores provide a vast space for the electrolyte ions' transport and shuttle, improve the mobility of charges, and increase adsorption efficiency.Macropores can lead to a reduction in specific surface area, which is only the electrolytes' storage 'warehouse' and the active channel of internal carbon particles [56][57][58].The research results of Salitra et al [59][60][61] indicate that only when the electrode materials' pore size matches the electrolyte ions' size can the materials' specific capacitance be maximized.Although many studies have shown that the larger the proportion of mesopores inside a material, the easier it is to form a double-layer and the better its capacitive performance [55], this does not mean that micropores below 2 nm cannot have superior performance.As shown by the research results of Chimiola et al [62] shows that the material can also have a high specific capacitance when the aperture is less than 1 nm.

Graphitization degree
Conductivity is another key indicator of double-layer supercapacitors.Porous carbon electrode materials have good conductivity, low internal resistance, fast charge transfer speed, good charge-discharge performance, and high hole utilization rate.For carbon materials, the conductivity is closely associated with factors such as the carbon content on the internal pore walls, the shape of micropores, the material's ability to adsorb electrolyte ions, the material's location, and the contact area between materials [63,64].At present, a large amount of research work is aimed at finding porous carbon materials with excellent conductivity and studying how to decrease the materials' internal resistance.Generally speaking, the degree of graphitization can reflect conductivity, which has an enormous impact on the porous carbon materials' conductivity.High temperature carbonization and transition element catalysis are two technologies for improving the porous carbon materials' graphitization degree.Carbon materials' graphitization at high temperatures (over 2000 °C) is called the process of carbonization at high temperatures.However, the produced porous carbon composite materials have low porosity, are prone to pore collapse and have a small specific surface area, which limits the electrolyte ions' transport [65].Transition metals can be used as catalysts.Since the electrons outside the atomic nucleus are in a saturated state, and the entry of carbon electrons will not lead to a change in the electronic energy level, Amorphous carbon can be dissolved to form solid solution, reducing the activation energy required for graphitization in the precipitation process, playing a role in catalytic graphitization, thus improving the graphitization degree of Carbon compounds at low temperatures.
Tan et al [66] generated superior three-dimensional graphite porous biomass-derived carbon using dandelion flower stems as original materials and K 2 FeO 4 as the activator.Raman' s results showed that the ID/ IG value of HGBPC (dandelion flower stem biomass-derived carbon) (0.906) was lower than that of the original carbon (0.954).TEM results show obvious local microcrystals, which improved the material's conductivity.The prepared HGPBC has a 780.4 m 2 g −1 specific surface area, a 309 F g −1 specific capacitance, with 0.5 A g −1 of specific current density.14.22 Wh kg −1 of energy is present, when the power density is 218.8W kg −1 .By raising the porous biomass-derived carbon's graphitization level, the porous carbon materials' conductivity might be greatly increased.It is possible to maintain outstanding specific capacitance under large current density as well as supply high-performance electrode materials for ultra-fast charging supercapacitors by reducing the ohmic impedance of devices under superior current density.By utilizing the catalytic activation procedure, Yang et al [67] created graphite-like porous carbon (LGPC) that has a high degree of graphitization as well as a sizable specific surface area about uniform carbon sheet (100±20 nm) stacked layer by layer (figure 14 shows the preparation method for it).This LGPC electrode has a superior specific capacitance (0.5 A g −1 ) under the current density of 337.0 F g −1 in a three-electrode system.And for the 0.5-20 A g −1 of current densities, the rate is faster (85.6%).This work provides a simple and novel way to utilize coal-based heavy products and develop graphitized porous carbon effectively.(c, d), BWPC 1/2 (e, f), and (g-i) BWPC 1/3 TEM images.Reproduced from [49] with permission from the Royal Society of Chemistry.

Surface functional groups
The surfaces of the vast majority of carbon materials have a substantial number of functional groups attached to them, which not only affect the material's ability to adsorb electrolytes but also directly affect the size of the material's double-layer capacitance.In charge-discharge experiments, at a certain potential, many carbon materials exhibit symmetrical redox peaks on their CV curves.This is the pseudo capacitance caused by the functional groups adhered to the materials' surface [68].According to extensive scientific research, adding heteroatoms or functional groups such as sulfur, nitrogen, phosphorus, and oxygen to porous carbon materials can increase pseudocapacitance, thereby increasing the carbon-based supercapacitors' specific capacitance.In order to prepare P-doped layered porous carbon, Wang et al [69] used basswood as carbon source, phytic acid as the dopant to treat biomass.The significant rise in specific capacity of supercapacitors is due to the existence of these six negatively charged phosphate groups in phytic acid, which provide sufficient cross-linking sites to support the significant doping of P on carbon substrates.This large-scale doping produces more defects and active sites, and greatly improves the supercapacitors' specific capacity.On the other hand, the functional groups' introduction also enhances the adsorption wettability and stability between the active substance's surface and the electrolyte.It accelerates the ion mobility of the electrolyte.
However, improper or excessive introduction of functional groups can increase the equivalent internal resistance and reduce its conductivity.At the same time, the generation of pseudocapacitance is a chemical reaction that can easily generate leakage current during the operation of supercapacitors, thereby reducing their service life.

Preparation method of biomass-derived carbon electrode material for supercapacitors 4.1. Direct carbonization
Biomass-derived carbon preparation usually requires the addition of additional reagents to assist in the carbonization of it, but there is an exception to this method.Some studies have shown that biomass does not require additional activators, and the material is directly carbonized at high temperatures to acquire biomassderived carbon with a huge specific surface area.This method is called self-activation [70].The reaction mechanism of self-activated carbonization is able to be divided into four procedures: (1) below 120 °C, mainly the loss of free water and bound water of biomass materials, (2) 220 °C-315 °C, carbonization and degradation of Hemicellulose, (3) 315 °C-400 °C, carbonization and degradation of cellulose, (4) 500 °C-1000 °C, carbonization of biomass materials.The self-activation method is more environmentally friendly and economical than other carbonization methods.The disadvantage is that self-activated carbonization requires higher temperatures and has lower yields.The preparation of biomass-derived carbon using the direct carbonization method (porous organic materials are suitable for direct carbonization) usually requires pyrolysis in an inert atmosphere and preparation of appropriate carbon precursors.Sometimes, a small amount of air (or oxygen) can be introduced according to experimental needs to reduce the carbonization temperature [71].
Researchers have used direct carbonization to manufacture various biomass-derived carbon with excellent electrochemical performance.
Bommier et al [72] prepared cellulose porous carbon that has 2508 m 2 of specific surface area by direct carbonization at 1100 °C and argon (Ar) atmosphere for two hours.The carbon materials' specific capacitance reaches 132 F g −1 , when the current density is equal to 1 A g −1 .Kurosaki et al [73] prepared three-dimensional biomass-derived carbon materials by pre-flash heating and post low-temperature treatment of sawdust, resulting in high porosity and specific surface area.Liu et al [74] proposed a method of direct pyrolysis a series of amino acids to produce O, N co-doped carbon nanosheets (figure 15).The prepared carbon nanosheets contain 1.70% (atomic fraction) of high-quality nitrogen (N) and 18.76% and oxygen (O).The carbon nanosheet's capacitance is 287 F cm −3 , when the current density is 0.5 A g −1 .The capacitance retention rate is 96.1%, showing significant cycling stability, after 10000 cycles.
In summary, although the direct carbonization method is simple to operate, it is limited by the porosity, morphology, and physicochemical properties of the biomass material itself.It is necessary to control the pyrolysis conditions such as carbonization temperature, holding time, and heating rate reasonably so as to fully use the advantages of biomass material itself.

Hydrothermal carbonization
Hydrothermal carbonization is one of the most widely used processes for processing carbon generated from biomass.Many researchers have successfully prepared biomass-derived carbon for supercapacitor electrode materials using this method and achieved significant results.Carbonization of hydrothermal is the term for a production of carbonaceous materials in closed spaces and solvent media at low temperatures (typically< 250 °C).Its carbon source (mainly natural biomass resources) has several different sources, mild reaction conditions, and low energy consumption.As the hydrothermal process is impled in an aqueous solution, it is not necessary to perform preliminary drying treatment on the raw material.It also avoids environmental pollution caused by organic solvents in the course of the reaction.Additionally, hydrothermal carbonization makes it simple to dole out elemental doping and modify the surface of carbon compounds as compared to other carbon manufacturing techniques.The time, applied pressure, solvent medium, and temperature are the key determinants of the produced material's shape, structure, and surface chemical state [75].In hydrothermal carbonization, biomass materials are decomposed into monomers with small shapes, and some complex reactions such as polymerization, dehydration and aromatization will occur.At 200 degrees Celsius, an increase in applied temperature will increase the porosity of the material.However, the specific surface area and porosity about a material diminish by rising temperature as a degree of aromatization increases [76].Hydrothermal carbonization can greatly increase carbon electrode materials' specific capacitance obtained from biomass as compared to a direct carbonization technique.A variety of saccharin carbon particles were created by Xia et al [77] utilizing the hydrothermal carbonization technique.The particles exhibited nearly spherical morphology or spherical, and the size of monodisperse particles was adjustable, according to SEM pictures about these products of sucrose, glucose, and b-cyclodextrin following hydrothermal processing (figures 16(a), (c), (e)) as well as carbonization (figures 16(b), (d), (f)).This BET-specific surface area about these carbon spheres are at 400-500 m 2 g −1 , and their micropore ratio is around 84%, showing that they have a lot of micropores.The volumetric capacitance of sucrose-based carbon spheres in a 30% KOH electrolyte may reaches 170 F cm −3 , as well as 164 F g −1 of specific capacitance.
During the hydrothermal reaction process, some heterofunctional groups are introduced while causing morphological changes.When using hydrothermal carbonization to prepare electrode materials, reasonable control of their heating temperature and doping reagents can effectively improve their pseudocapacitance performance, thereby improving the supercapacitors' electrochemical performance.Cakici et al [78] used green hydrothermal method to functionalize coral-like MnO 2 nanostructures in carbon fiber fabric (CFF), improving the hybrid composite materials' pseudocapacitance performance.These CFF/MnO 2 composite materials serve as ideal flexible electrodes in supercapacitor of high-performance electrochemical.This coral-like MnO 2 nanostructure can support the matrix and serve as a reducing agent under hydrothermal conditions, making it an ideal template.Under the condition of 1.0 mol L −1 Na 2 SO 4 electrolyte, the electrode material's specific capacitance is 463 F g −1 , at 1 A g −1 of current density.Its capacitance retention rate is up to 99.7% throughout the entire 5000 cycles.Zhao et al [79] utilized the hydrothermal approach to in situ grow and deposit hollow carbon nanofibers (MnO 2 /HCNFs) δ-A novel layered hollow nanostructure has been synthesized using MnO 2 nanosheets.Because of its peculiar hollow structure, the combined asymmetric supercapacitor (ACS) achieves 293.6 F g −1 of specific capacitance, when the current density is 0.5 A g −1 , which is higher than the non-hollow structure at a Na 2 SO 4 electrolyte of 1.0 mol L −1 .
In summary, it can be concluded that hydrothermal approach has become a significant way for preparing biomass-derived carbon materials due to its characteristics of green environmental protection, convenient operation, and mild reaction conditions.However, due to the low operating temperature of this hydrothermal carbonization approach, the carbonization degree of the acquired material is limited, resulting in poor conductivity.In addition, carbon materials obtained in this way typically have a lower porosity and specific surface area require the addition of templates or surfactants to modify the materials' specific surface area.

Activation carbonization 4.3.1. Physical activation
In addition to hydrothermal carbonization, activated carbonization is the most commonly used technology for preparing carbon from biomass.The two main types of activated carbonization are chemical activation and physical activation.Pyrolysis in an inert environment, adding the necessary reagents for activation, is the main method for preparing biomass-derived carbon sources for physical activation.Method of physical activation, additionally referred to as gas activation method, common physical activators include water vapor, carbon dioxide, etc.The mechanism is that volatile components and impurities are removed by high-temperature carbonization in an inert gas environment, and then H 2 O, CO 2 , NH 3 and other gases react with carbon atoms or other atoms to generate a large number of gases such as CO and H 2 .Under the activation of gases, the materials become relaxed, thus forming a porous structure.Physical activation is more direct and ecologically friendlier than chemical one, but usually requires higher temperatures and it is difficult to accurately control the activation reaction process.Usually, higher temperatures and longer operating times can achieve more significant porosity, while excessive pore development may affect pore distribution.Carbon electrode materials with good electrochemical performance made from biomass can be obtained by carefully managing the heating duration and temperature.Yu et al [80] utilized the unique biological structure of cattail biomass and prepared porous carbon using CO 2 activation method.The morphology and composition of cattail Biochar and cattail fiber were studied by SEM.The surface of the original camphor fiber is clean and flawless, with almost no breakage.After activation and carbonization, the skeleton of this ribbon fiber was retained, but the shape of this fiber changed, becoming wavy and uneven, and the width decreased.This may be resulting from the pyrolysis of Biopolymer at high temperature, which leads to the generation of carbon skeleton (figures 17(a), (b), (d), (e), (g), (h)).The oxygen reduction peaks in the carbon and CAC spectra further explain the functional groups' decomposition in cattail fibers with oxygen atoms (figures 17(c), (f), (i)).Activated carbon exhibits a specific capacitance is equal to 126.5 F g −1 at 0.5 A g −1 of current density.

Chemical activation
Another way to activate carbonization is through chemical activation.Chemical activation is mainly achieved by introducing a certain amount of activator into the carbon source.Usually, the carbon source is stirred with an activator or ultrasound to evenly disperse, and then pyrolysis is carried out to obtain materials of biomassderived carbon.Chemical activation has the following advantages over physical activation: (1) mild reaction conditions, (2) high carbon yield, (3) high porosity, (4) well-developed micropores, and narrow distribution range.However, carbonized materials typically must be treated with an acid solution to remove remaining metal oxides and prevent more significant or agglomerated metal oxides from clogging pores.Therefore, chemical activators have stronger corrosiveness to equipment, which to some extent increases the application cost.Common activators can be divided into alkaline activators (KOH, NaOH, etc.), acid activators (phosphoric acid, sulfuric acid, etc.), and salt activators (ZnCl 2 , etc.) based on their acid-base properties [81][82][83][84].Among them, KOH and ZnCl 2 are the preferred methods for preparing biomass electrode materials.
KOH is the most common alkaline activator, which is easily mixed with carbon precursor materials and hightemperature carbonization for obtaining carbon materials with developed pore structure and high specific surface area.KOH's activation reaction mechanism is able to be roughly divided into the following steps [85,86] : (1) At 600 °C, KOH decomposes to generate K 2 O and K 2 CO 3 , (2) the escape of water vapor or CO gas during the reaction process will promote the formation of pores, (3) during the reaction process, excess metal K or K 2 CO 3 will remain in the carbon framework as a template, and high specific surface area carbon materials will be obtained through further acid washing.
Chen et al [87] used biomass waste soybean meal as raw material and then carbonized it for two hours in a tube furnace at 750 °C.The carbonized product was combined with KOH activator at a 1:4 mass proportion, and then soaked in a mixture of deionized water and ethanol to filter to obtain honeycomb shaped N, O self-doped porous carbon.Based on research using XPS (x-ray photoelectron spectroscopy), the porous carbon's O, N, and C contents are 8.04 at%, 2.20 at%, and 89.93 at%, respectively.Honeycomb materials with N and O co-doping exhibit larger specific surface area and larger pore volume, reaching 2690.3 m 2 g −1 and 1.34 cm 3 g −1 , because of the graded porosity framework and in situ doping of O and N. Fu et al [88] prepared nitrogen-doped graded activated carbon by carbonizing pomelo peel at 600 °C and activating it with KOH.The volume capacitance is 219.3F cm −3 of the supercapacitor, and the weight capacitance is 208.7 F g −1 , using an electrolyte of 1 mol L −1 H 2 SO 4 .Its retention rate of the capacitor is up to 96.2% after 10000 cycles under the condition of a current density is 5 A g −1 .Cooper et al [89] used potassium hydroxide to activate cellular porous carbon (HCP) to prepare three separate categories of biomass-derived carbon materials.The three materials have various activation temperatures and specific surface areas, about 2682 m 2 g −1 , 4334 m 2 g −1 , and 3105 m 2 g −1 , respectively.Compared with the initial porous organic material precursor, the specific surface area significantly increased (figure 18 shows the material preparation process).
As a common salt activator, ZnCl 2 also serves as a template, and its activation mechanism can be roughly divided into the following points: (1) ZnO consumes carbon atoms and plays a role in pore formation, (2) the escape of gases such as Cl 2 and CO 2 generated during high-temperature processes can also create pores, (3) at high temperatures, Zn will corrode the materials' interior and surface, leaving pores, (4) the incompletely reacted ZnOCl and ZnO serve as templates to fix them in the material, and retain some of the pore structure after acid washing.Sun et al [90] prepared chitosan-derived carbon materials with good electrochemical properties using chitosan as a carbon source and ZnCl 2 as an activator.
However, traditional templates such as ZnCl 2 and KOH generate uncontrolled and irregular pore shapes and sizes, making it even more difficult to generate regular planar pores.Therefore, it is necessary to seek a chemical reagent that can induce the formation of regular planar pores in the carbon skeleton while achieving the prosperity of the pore structure and graphitization enhancement of carbon materials.Qi et al [91] used carbonrich and nitrogen-rich conjugated polyimides as precursors and K 2 FeO 4 as multifunctional carbonization reagents to prepare nitrogen self-doped honeycomb-like porous carbon nanoframeworks (NHPC) with abundant planar pores for ultra stable energy storage.The preparation process is shown in figure 19.They found that through K 2 FeO 4 assisted activation, different metal compounds were produced in carbon materials at different pyrolysis temperatures.When the temperature rises to 500 °C, it mainly decomposes to form a mixture of K 2 CO 3 and Fe 2 O 3 .When the temperature reaches 600 °C or higher (700 °C), it mainly decomposes to form a mixture of K 2 CO 3 and Fe 3 N, while the initial Fe 2 O 3 disappears.This is because Fe 2 O 3 is further reduced in situ by carbon to Fe nanoparticles, and then immediately reacts with the nitrogen reduction products generated by PI thermal decomposition to generate iron nitride (Fe 3 N) [92].The K 2 CO 3 , Fe 2 O 3 , and Fe 3 N generated by its thermal decomposition can serve as activators, catalysts, and pore forming templates during high-temperature pyrolysis, respectively, forming well-developed pore structures [93].Previous studies have found that the activation of K 2 CO 3 involves the reduction of K 2 CO 3 to metallic potassium/potassium compounds (K/K 2 O) and expanding gases (CO and CO 2 ) during the activation process.Therefore, gasification expansion can promote the formation of honeycomb-like porous carbon nanoframeworks, while potassium and iron compounds penetrate the internal structure of the carbon matrix, enhancing existing pores and generating new ones.In addition, non volatile metal compounds (Fe 2 O 3 and Fe 3 N) can be embedded into the carbon framework as in situ self generated templates at high temperatures.After removing the metal compounds through acid  etching, a honeycomb-like porous carbon nanoframework with rich planar pores can be obtained.The symmetrical supercapacitor based on NHPC electrode has excellent electrochemical performance.
In summary, although chemical reagents can act as activators and templates, issues such as the release of corrosive gases during pyrolysis and the need for corrosive solvents during pickling still need improvement.

Carbonization of formwork
To make the pore structure orderly, scientists have developed a second kind of preparation method-template method, in recent years.According to the template classification, both hard formwork procedures and soft formwork methods can be used.

Hard formwork method
Usually, hard templates need to have good pore structure and heat resistance, while ensuring pore structure and preventing skeleton collapse during high-temperature carbonization.The hard template method usually consists of four steps: first, select or synthesize some hard particles with rich pore structures (rigid templates), such as MCM-48, Al 2 O 3 , zeolite, SiO 2 , etc.The second step is to add surfactants.The third step is to add precursor materials (carbon sources) and directly carbonize them.Finally, the composite template was subjected to strong acid and alkali corrosion to obtain ordered and uniform porous carbon materials.The carbon materials prepared by the method of hard template carbonization can perfectly inherit the template morphology.Sucrose was served as the carbon supply, as well as nano-calcium carbonate utilized as the template by Xu et al [94] and combined in various ratios, then activated for two hours at 800 °C to create mesopore-rich porous carbon composites.The resulting materials have 892 m 2 g −1 specific surface area under 6 mol L −1 current density.This substance's electrochemical capabilities are examined using a potassium hydroxide electrolyte device.This material's specific electric capacity is 155 F g −1 and 106 F g −1 for 50 mA g −1 as well as 20,000 mA g −1 current densities, respectively.As a result, it displayed excellent electrochemical performance with high current densities.
Many researchers have combined template carbonization with activated carbonization to create biomassderived carbon electrode materials in an effort for increasing the supercapacitor electrode materials' electrochemical performance.Celine et al [95] utilized calcium carbonate nanoparticles as the rigid template and potassium oxalate as the activator.With biomass-derived substances (microalgae, glucose, soybean powder, and glucosamine) as the precursors, numerous functional porous carbon materials with a hierarchical structure were created by a one-step carbonization process, with nitrogen doping ratios up to 2-3 wt% and specific surface areas upward to 2400 m 2 -3050 m 2 .This material's pore structure is able to flexibly controlled via altering a precursor.During the process of carbonization for the template, niacin served as an activator by Jiang et al [96] to made exceptional specific surface area NPC (nitrogen-doped layered porous carbon).The nitrogen concentration and the end product's porosity are significantly influenced by a mass proportion of KOH as well as Mg(OH) 2 to the precursor of carbon.large hole volume (4.0 cm 3 g −1 ), acceptable nitrogen concentration (2.2%), and Wonderful specific surface area (2608 m 2 g −1 ) are all present in this sample (figure 20).Additionally, a capacitor composed of this material might achieves 282 F g −1 excellent specific capacitance with the current density is 1 A g −1 .This device performs exceptionally well in terms of magnification, and at 20 A g −1 of current density, reaches 64% of capacitance.This device still kept a 95% retention ratio, during testing at a temperature of 25 °C, when the current density is 5 A g −1 (figure 21).Although the hard template is extremely stable during the reaction process, the demolding process is complex and costly, and its economic practicality needs to be improved.

Soft formwork method
There are a few variations between the hard template method and the soft template method, because of it usually selects some macromolecular polymers as structural guides (soft templates).Then, use the carbon source (phenolic resin, hydroquinone, m-neneneba phloroglucinol formaldehyde resin, etc.) to produce porous carbon, and remove the soft template through direct carbonization.Compared with the complex hard Template method pattern, it reduces the damage to the material itself when removing the template, but the template needs to have good thermal stability, otherwise, the template will decompose at high temperature.Therefore, the selection of templates and heating conditions has become very important.This has also attracted many scientists to explore the application of soft Template method pattern in the field of supercapacitors.
Tu et al [97] used lignin micelles as soft templates for preparing catalysts.Then, L-CNTs (lignin-based porous carbon) were prepared using the gaseous byproducts of lignin pyrolysis.The research results demonstrate the key effects of catalyst synthesis temperature, catalyst precursor concentration, and catalyst preparation procedure on the shape and structure of L-CNT.L-CNT has a spectral length of approximately 1-6 m and a diameter range of 10-110 nm.The synthesized L-CNT exhibits excellent graphitization and thermal stability, with many multi-walled carbon nanotubes with a crystal layer spacing of approximately 0.35 nm.He et al [98] used coal tar pitch as the main resource, melamine as a soft template, and KOH is used as an activator in the preparation of porous honeycomb carbon (HPC).The specific surface area of HPC is 2038 m 2 g −1 .They have a specific capacitance is equal to 221 F g −1 with a current density of 0.05 A g −1 in 6 M KOH electrolyte.After 5000 cycles, 95.3% of the capacitance was maintained.
5. Biomass-derived carbon composite electrode material applied to supercapacitors 5.1.Biomass-derived carbon/activated carbon composite electrode material Double-layer capacitor is a novel form of energy storage equipment based on the interface double-layer theory discovered by Helmholtz [99].Due to its unique electrode structure and proximity to the charge layer, the twolayer capacitor's capacitance is very high.Carbon materials are commonly employed as Supercapacitor electrode materials.Research on the electrode materials' preparation depending on a mix of biomass-derived carbon and activated carbon has also emerged endlessly.
By reducing the electrolyte ions' diffusion resistance in pores, improving surface modification as well as wettability, nitrogen loading can increase the biomass-derived carbon electrode materials' specific surface area and energy density.In order to manufacture nitrogen-rich carbonaceous materials for electrochemical energy storage applications, Iqbal et al [100] used egg protein as a nitrogen source.Mix the untreated egg protein with the aqueous dispersion of activated carbon, and then heat the compound in a polytetrafluoroethylene-coated Autoclave to 220 °C for about 12 h to produce the combination of egg-derived protein and activated carbon (AC/EDP).Then, the synthesized composite material was subjected to KOH chemical activation and thermal activation at 500 °C-700 °C.Nitrogen-containing activated carbon has mesoporous and microporous structures, and BET analysis shows that it has 1660 m 2 g −1 of specific surface area.HRTEM and scanner are utilized to examine the complex's morphology.A large amount of pyrrole, pyridine, and quaternary nitrogen were detected in AC/EDP using x-ray photoelectron spectroscopy, which can effectively increase the energy storage devices' electrochemical performance.This capacitance measured at 0.2 A g −1 of current density is up to 263 F g −1 .The values of power density and energy density are 7920 W kg −1 and 32 Wh kg −1 .After ten thousand cycles, its capacitance retention rate reached 98%, indicating that AC/EDP has good cycling stability.
In summary, although biomass-derived carbon/activated carbon supercapacitor electrode materials have excellent cycling stability and large specific capacitance, achieving higher energy density while maintaining high power density still poses some difficulties.

Biomass-derived carbon/metal compound composite electrode material
Supercapacitor has several advantages long cycle life, fast charging and discharging, and environmental friendliness.Metal oxide is a commonly used electrode material for Faraday capacitors, which has the advantages of higher energy density and larger capacitance [101].On account of the high cost and environmental harm of ruthenium and other precious metals [102], transition metals that are easily available, inexpensive, and environmentally friendly are commonly used as metal oxide materials for electrodes.However, due to the poor conductivity of metal oxides, it is difficult to fully utilize their specific capacitance during repeated charging and discharging processes [103], and there are drawbacks such as underdeveloped pores, poor cycling stability, and short service life.Therefore, in order to manufacture a supercapacitor with excellent cycle stability, large specific capacitance and excellent cycle stability, It's essential to combine biomass-derived carbon with large specific surface area, high conductivity as well as good cycle stability to coordinate the interests of various components in a combination of biomass-derived carbon and transition metal oxides [104].
By utilizing the adsorption capacity of carbon precursors derived from biomass, metal oxide precursors are fixed on the carbon precursors.Tightly combining metal oxide precursors with biomass-derived carbon precursors to obtain composite materials.This composite material combines the Faraday capacitance characteristics of metal oxides with the biomass-derived carbon's double-layer capacitance characteristics, which can significantly improve the supercapacitors' specific capacitance and energy density.Jiang et al [105] used hemp stalks to produce compounds using biomass-derived.They prepared Fe 2 O 3 /porous carbon nanocomposites (Fe 2 O 3 /HAC) using Fe 2 O 3 /HAC as the matrix material through effective and simple hydrothermal processes.The research results indicate that porous carbon made from hemp straw and Fe 2 O 3 can solve the problem of metal oxide agglomeration.In addition, contacting additional components with the electrolyte will shorten the ions' diffusion path, thereby significantly improving the electrode material's electrochemical performance.Zhang et al [106] synthesized manganese dioxide/biomass-derived carbon nanocomposites by hydrothermal method.They used the carbon precursors from the feces of silkworms and activated them with a mixture of ZnCl 2 and FeCl 3 chemicals in an argon atmosphere.Biomass carbon surfaces are in situ anchored by thin, flower-shaped MnO 2 nanowires.This superior specific capacitance about electrode material of dioxide/biomass-derived carbon nanocomposite is 238 F g −1 , with 0.5 A g −1 of current density, t.This deterioration rate is only 7% following 2000 cycles.The electrochemical performance of a biomass-derived carbon powder (SWK), which was prepared using ZnCl 2 as an activator (SWK-Z), and the biomass-derived carbon powder prepared using ZnCl 2 , FeCl 3 as a mixed activator (SWK-ZF) are shown in figure 22.
For improving the electrode materials' conductivity, many researchers have attempted to prepare supercapacitor electrode materials by combining bimetallic materials and biomass-derived carbon.The results indicate that this new method can successfully increase the supercapacitors' electrochemical performance.Zhan et al [107] prepared Mn/Cu bimetallic decorative biomass porous carbon using agricultural waste (peanut shell) as raw material (AC@MnCu-X).This carbon material combines the metal compounds' pseudocapacitance characteristics with the biomass-derived carbon materials' double-layer capacitance characteristics.The obtained carbon sample is successfully modified with Mn/Cu nanoband structure in the carbon matrix, and exhibits large surface area and unique microporous properties.CuO and MnO 2 constitute the majority of Mn/ Cu nanostructures, and CuMn 2 O 4 is an excellent pseudo-capacitor electrode material.As AC@MnCu-32 for electrode materials, a system of three-electrodes generates 632 F g −1 of specific capacitance with 1 A g −1 of current density, as well as has an excellent cycling life of about 90% after 10000 cycles.This sample has also been successfully used as an electrode in asymmetric devices, successfully driving light-emitting diodes (LEDs).Between asymmetric capacitors' three various types, the capacitor of AC@MnCu-32 //AC electrochemical performance is the most excellent, it has 15.62 Wh kg −1 of energy density with 625 W kg −1 of power density, and a long service life.
To sum up, biomass-derived carbon/metal compound materials have below advantages: (1) carbon materials' large specific surface area can load more Transition metal oxides, improve the active site, and obtain pseudo capacitance contributions, (2) enhance charge transfer capabilities and ion diffusion through control the morphology as well as structure of carbon materials, (3) using carbon materials to counteract irreversible damage to the electrode material structure during the redox process increases the electrode material's lifespan.In addition, this type of composite carbon material can also be applied to asymmetric supercapacitors.The combination of two or more materials can effectively improve the capacitor's energy density.However, transition metals and conductive polymers commonly used in pseudocapacitors are non-renewable resources, so whether they can be quantitatively produced still needs further research.

Summary
Biomass-derived carbon, as a low-cost and environmentally friendly electrode material, has broad development and application prospects.These materials have made some progress in the field of supercapacitors, but there are still the following problems: (1) How to maintain a large specific surface area while reasonably configuring pore size distribution and pore size.An ideal carbon material for supercapacitors should have multiple types of pore sizes simultaneously.How to achieve the simultaneous existence of mesopores, macropores, and micropores is currently the research focus of the material of biomass-derived carbon.For example, using activation methods, we can produce materials with many nanopores smaller than 2 nm as well as a large specific surface area.However, the electrolyte ions' diffusion and transfer efficiency will significantly decrease due to a significant increase in macroscopic voids, resulting in a low energy density of supercapacitors, which is very harmful in practical applications.
(2) How to balance the advantages and disadvantages of doped biomass-derived carbon.Although doping with certain heteroatoms can enhance the carbon-based electrode materials' electrochemical performance, it can also reduce the materials' conductivity with sulfur and oxygen containing functional groups.Therefore, when selecting the type, quantity, and method of doping heteroatoms, different needs should be considered.
(3) In addition to external doping, the self-doping of biomass also has a significant impact on the preparation of carbon materials.If the content of inorganic substances (ash) in biomass is too high, although the posttreatment of these inorganic substances may create additional pores, the removal of inorganic substances may involve the use of strong acids, alkalis, and toxic substances.Meanwhile, ash content is one of the factors that causes poor electrochemical performance and low stability of supercapacitors.Ash content promotes the diffusion of ions from the double-layer to the solution, accelerates the disintegration of the double-layer, increases leakage current and self-discharge, reduces the stability of the double-layer, hinders the contact between the activator and carbon atoms, inhibits activation, and affects the development and changes of pores.However, the higher the carbon content of the carbon source, the corresponding increase in ash content.The high temperature and oxidation atmosphere during the preparation process can promote the production of ash, and some reagents used may also promote the production of ash.Therefore, it is important to find low ash carbon sources, control the temperature during the preparation process, and use chemical reagents.
(4) Biomass-derived can be combined with materials with pseudocapacitance.However, factors such as the type of pseudocapacitive material used and the method used to prepare the composite material can affect the internal interface and overall performance of the composite material.How to achieve the most reasonable electrode material structure and minimize pollution is currently the research focus.Therefore, a key area for the biomass-derived carbon materials future development is the design and research of composite materials for this material.
Looking forward to the future, we should further focus on developing biological energy storage devices, fully utilizing the structural diversity of biomass, providing new ideas for sustainable, clean, and green development through biomass resources' rational utilization, and achieving waste biomass resources' secondary utilization.

Figure 1 .
Figure 1.Structure overview of this article.

Figure 3 .
Figure 3. CV curve (a) of symmetrical supercapacitor devices.Supercapacitor GCD image (b) for symmetrical devices at a series of current densities.Supercapacitor device's specific capacitance image (c) for current densities' range is 1 to 20 A g −1 .EIS picture about this gadget (d).The device's cycling stability performance image (e).Reproduced from [19], with permission from Springer Nature.
(b)).Subsequently, by further increasing the ratio to 1:5, the microstructure of BCPC-5 collapsed and produced a large number of material fragments (figure 9(c)), indicating the destruction of the interconnection structure.The energy element mapping (figures 9(d)-(g)) clearly indicates that N, C, and O elements are uniformly distributed in BCPC-3.These

Figure 4 .
Figure 4. CWC' SEM images (a-b) with diferent magnifcations.Vertical-sectional wood-derived carbon in SEM image (c).Pit channel shown in SEM picture (d) on the surface.Mesopore image (e) from an HR-TEM.Reproduced from [23], with permission from Springer Nature.

Figure 6 .
Figure 6.The electrical system performance about the AARC-700 are used in dual electronic cells with 1 M Na 2 SO 4 electrolyte: CV curves (a) under various potential voltages using a 20 mV s −1 scanning rate.Symmetric supercapacitors' CV curves (b).Device's GCD curve (c).Device's specific capacitance and current density are shown in image (d).The AARC-0//AARC-700 power density and energy density graphs (e) as well as previously published carbon-based SCs.The AARC-0//AARC-700 ′s cyclic stability image (f) with a current density is equal to 5 A g −1 .Reproduced from[27], with permission from Springer Nature.

Figure 8 .
Figure 8.(a) The supercapacitor with CNSAC as the electrode showed a capacitance retention rate of over 95% through 20000 cycles of charging and discharging tests, the voltage range during the cycle is 0-1 V. EIS images (b) of the device before and after 20000 charge-discharge cycles of testing.Reproduced from [28], with permission from Springer Nature.

Figure 11 .
Figure 11.CV curve (a).GCD curve (b) about the device visually compares specific capacitors at five current densities.(c) Cyclic stability.Power density and energy density diagram (d) about the device, illustrated by a photo of an LED illuminated by three supercapacitors connected in series.Reproduced from [45], with permission from Springer Nature.

Figure 15 (
Figure 15 (a) Principle flowchart of direct pyrolysis of various amino acids to synthesize this carbon nano-sheets.(b) SEM tests about carbon nano-sheets and (c) capacitance comparison of various carbon nano-sheets.Reprinted from [74], Copyright (2017), with permission from Elsevier.

Figure 17 .
Figure 17.SEM pictures of unblemished cattail fibers in (a) low and (b) high magnification (c) Cattail fibers with their original EDS spectra.(d) Low-magnification and (e) high-magnification SEM pictures of carbon.carbon's EDS spectrum (f).SEM pictures of CAC at low and high magnifications are shown in (g) and (h), respectively.Activated carbon's (i) EDS spectrum.Reprinted from [80], Copyright (2017), with permission from Elsevier.

Figure 18 .
Figure 18.The process of carbonization that follows the creation of the hypercrosslinked polymers[89].

Figure 20 .
Figure 20.FESEM pictures (a, b) of NPC.TEM and HRTEM pictures (c, d) of NPC.(e) Analysis of elemental spectra of NPC samples.Reproduced from[96], with permission from Springer Nature.

Figure 21 .
Figure 21.(A) three-electrode system with NPC electrodes was tested.Test chart for cyclic voltammetry (a).The device's cycling stability test curve (c), constant current charging and discharging test curve (b).Comparison chart between the results about 5000 charging and discharging cycles (d), with an enlarged Nyquist diagram in the example.Reproduced from [96], with permission from Springer Nature.