The composition of biomass is inherently diverse. For example, the residues of plant biomass are mainly composed of cellulose, hemicellulose and lignin, and their proportions vary (Figure 1 and Table 1), while animal biomass (such as meat) is rich in protein and grains are mainly composed of starch. Different chemical structures obviously lead to different chemical properties. These diverse chemical compositions are of great significance to the development of supercapacitor materials, because the components in biomass can be converted into different types of carbon materials, which will affect the electrochemical performance of supercapacitors.
Figure 1 Plant cell walls are mainly composed of cellulose, hemicellulose and lignin
Table 1 Structure and chemical composition of cellulose, hemicellulose and lignin in plant cell walls
1 The role of cellulose in supercapacitor materials
Cellulose is a macromolecular polysaccharide composed of D-glucose, which is insoluble in water and general organic solvents and is the main component of plant cell walls (for example, 90% and 50% of the cell wall structure of cotton and wood are contributed by cellulose). The base ring of cellulose macromolecule is a macromolecular polysaccharide composed of D-glucose with β-1,4 sugar bonds [Figure 2(a)], and its chemical composition contains 44.44% carbon, 6.17% hydrogen and 49.39% oxygen. Due to different sources, the number of glucose residues in cellulose molecules, that is, the degree of polymerization (DP), is in a wide range. In natural cellulose, the degree of polymerization can reach about 10,000; the degree of polymerization of regenerated cellulose is usually 200~800. In a sample, the degree of polymerization of each macromolecule can be different and has polydispersity.
Figure 2 (a) Structural formula of cellulose; (b) Intermolecular and intramolecular hydrogen bonds of cellulose
The chemical activity of cellulose molecules is mainly affected by the intramolecular or intermolecular hydrogen bonds formed between the hydrocarbon group on the C3 of the cellulose chain and the oxygen on the nearby glycoside ring [Figure 2(b)]. The formation of these hydrogen bonds increases the chemical stability and structural rigidity of cellulose molecules. Cellulose chains interact through intermolecular hydrogen bonds and van der Waals forces to form crystalline and non-crystalline regions (Figure 3). X-ray diffraction (XRD) studies have shown that cellulose macromolecules have a regular and orderly arrangement in their crystalline regions, which is stabilized by a strong network of intermolecular and intramolecular hydrogen bonds, and have a density of 1.588 g/cm3. In contrast, in the non-crystalline region, although hydrogen bonding also exists, the cellulose chains are arranged very irregularly and the distance between the molecular chains is large, so the density of the non-crystalline region is low, about 1.5 g/cm3. In general, the proportion of crystalline regions in cellulose is in the range of 30% to 80%. The non-crystalline region of cellulose is the most exposed part of the cellulose molecule, which is first attacked by external stimuli such as cellulase, resulting in hydrolysis of the cellulose molecule. When cellulose is completely hydrolyzed, D-glucose is produced, while partial hydrolysis forms cellobiose and polysaccharides with a lower degree of polymerization (degree of polymerization between 3 and 10).
Figure 3 Schematic diagram of the structure of cellulose before and after hydrolysis of the crystalline and non-crystalline regions
The changes in the hydrogen bond network and molecular orientation of cellulose produce many cellulose isomorphs, and the appearance of these isomorphs is closely related to the source of cellulose, purification method, treatment method and other factors. Cellulose has six interconvertible isomorphs, namely cellulose I, II, IIII, IIIII, IVI and IVII. The natural cellulose crystal form is cellulose I. The alternating coexistence of crystalline and non-crystalline regions of cellulose may affect the chemical accessibility of the hydroxyl functional groups involved in the reaction. In fact, the reactivity of cellulose depends on the reactivity of its primary and secondary hydroxyl groups, especially the primary hydroxyl groups, which are usually more reactive than the secondary hydroxyl groups due to lower steric hindrance.
In general, over the past two centuries, cellulose has become an important raw material for pulp and paper as well as chemical fiber industries due to its special physical and chemical properties. In addition, it currently plays an important role in various fields related to environmental protection, such as the purification of sewage containing heavy metal ions, radioactive elements and waste oil with cellulose-based composite materials. In the field of supercapacitor materials, one-dimensional fibers, two-dimensional films or papers, and three-dimensional hydrogels or aerogels assembled from cellulose molecules can be used as high-quality matrices to create high-performance green electrode materials, electrolytes, and diaphragm products.
2 Application of hemicellulose in supercapacitor materials
Hemicellulose is one of the main components of plant cell walls. It is a heteropolymer composed of several different types of monosaccharides, which are pentoses (xylan and arabinose) and hexoses (glucose, galactose, mannose, and rhamnose), with an average molecular weight of <30,000. Hemicellulose xylan accounts for 50% of the total amount in wood tissue. It is bound to the surface of cellulose microfibrils and interconnected. These fibers form a hard, fine, interconnected network. In nature, hemicellulose is amorphous and has adhesive properties. When hemicellulose is dehydrated, it has a strong tendency to toughen. Hemicellulose undergoes thermal decomposition between 180 and 350°C, producing condensable gases and various ketones/aldehydes/acids and flavor furans. In the development of supercapacitor materials, hemicellulose has been studied as a potential precursor of carbon-based materials due to its structural properties.
2.1 Xylan
Xylan is a polysaccharide with structural functions in plant cell walls. The main chain of xylan is similar to the main pin of cellulose, but it is composed of D-xylose as a monomer unit and contains trace amounts of L-arabinose. The characteristic of xylan is that the D-xylose unit on the main chain is connected to the branch through a β14 bond (Figure 4). The branches on xylan mainly include L-arabinose, D-glucuronic acid, acetyl ester or 4-0 methyl-D-glucuronic acid connected to the D-xylose unit. Their abundance and composition in plant cells depend on the origin of the plant.
Figure 4 Structural formula of xylan and derivatives
2.2 Mannan
Hemicellulose is mainly composed of mannan. Mannan derivatives include galactomannan, glucomannan, glucuronic acid, mannan, etc. (Figure 5). Mannans are composed of mannose units linked by β-(1→4) bonds, while galactomannans are composed of galactose units linked by β-(1→6) bonds. The main chain of glucomannan is composed of glucose and mannose units (the hydroxyl group of mannose can also be acetylated), linked by β-(1→4) bonds in a ratio of 1:3, and with galactose residues as branches. Glucuronic acid and mannan, which are dominant in the cell wall, are composed of mannose and glucuronic acid residues, which are linked by -(1→4) and β-(1→2) bonds on the main chain, respectively.
Figure 5 Chemical structure of mannan and its derivatives
2.3 Galactan and arabinogalactan
The backbone of galactan is composed of galactose units linked by β(1-4) bonds, and galactose residues are also attached as branches on O6. Arabinogalactan is a neutral polysaccharide composed of arabinose and galactose, with a skeleton composed of galactose and branches composed of glucose and arabinose. The molar ratio of galactose to arabinose is 6:1. Arabinogalactan is found in large quantities in the xylem of conifers, especially in larch (Larix), where it can reach 25%. The ratio of L-arabinose to D-galactose is generally 1:5 to 1:66. Due to their unique chemical structure and functionality, these polysaccharides are also used to study how to improve the conductivity and energy storage capacity of supercapacitor materials, thus providing new possibilities for the development of green supercapacitor materials.
3 Application of lignin in supercapacitor materials
Lignin is a complex class of organic polymers that are important structural materials in the supporting tissues of vascular plants and some algae. Lignin is essential for the formation of cell walls, especially in wood and bark, because they can impart rigidity and are not easily rotten. The content of lignin varies depending on the plant species and age of the tree (for example, in very hard woods such as ebony, the content can reach 25%~30% or even 50%). The lignin content of annual plants is about 10%~12%. The elemental composition of lignin is about 61%~65% carbon, 5%~6% hydrogen, and the rest is oxygen.
Chemically, lignin is a complex amorphous aromatic polymer, a complex phenolic polymer formed by three alcohol monomers (p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol). Due to different monomers, lignin can be divided into 3 types (Figure 6): syringyl lignin (S lignin) formed by the polymerization of syringyl propane structural monomers, guaiacyllignin (G lignin) formed by the polymerization of guaiacyllignin structural monomers, and p-hydroxy-phenyllignin (H lignin) formed by the polymerization of p-hydroxyphenyl propane structural monomers. Gymnosperms are mainly guaiacyl lignin (G), dicots mainly contain guaiacyl-syringyl lignin (G-S), and monocots contain guaiacyl-syringyl-p-hydroxyphenyl lignin (G-S-H). In addition, due to the presence of many polar groups and hydrocarbon groups in the lignin structure, lignin molecules can form strong intramolecular and intermolecular hydrogen bonds, which in turn leads to the fact that lignin is generally insoluble in other solvents except alkaline solutions. Through condensation reactions, lignin can form soluble and insoluble lignin, the latter forming a precipitate.
Figure 6 Structural formula of lignin
The softening temperature of dry lignin is between 127 and 129 ° C, and the softening temperature increases with the increase of lignin molecular weight. In addition, the water in the lignin molecule acts as a plasticizer, causing the softening temperature to decrease with the increase of water content. In general, given that the annual production of lignin is as high as about 300 billion tons and continues to grow at a rate of about 20 billion tons/a, it is crucial to develop lignin value-added technology. In the past decade, the pulp and paper industry alone has produced about 50 million tons of lignin. However, only a small part of the industry’s lignin is used to produce various high-value-added products, while the rest is mostly burned as low-value fuel. It is worth mentioning that lignin has a higher calorific value than cellulose. High-value-added products derived from lignin can be divided into three categories: materials, chemicals and biofuels (Table 2). These products have shown broad application potential in fields such as supercapacitor materials, especially in the development of new environmentally friendly energy storage materials. Its rich aromatic structure makes lignin a potential carbon source material.
Table 2 High-value-added products derived from lignin
4 Starch
Starch is the main reserve material for carbohydrates in vegetable tubers and seeds. It exists in the form of granules (different plant species have different forms), each granule contains millions of branched starch molecules and a large number of amylose molecules. In nature, starch has two forms: amylose soluble in hot water accounts for about 25%~27%, while branched starch insoluble in water accounts for about 73%~75%. Amylopectin (without amylose) can be separated from “waxy” corn starch, while amylose (without amylopectin) is obtained by hydrolyzing amylopectin with pullulanase. The structural formula of starch is (C6H1oOs)n, where the degree of polymerization n can vary from a few hundred to a few thousand, indicating the number of residues of α-D-glucose units (4C. conformation) that are linked together to form the polymer. Therefore, starch biomass can be used as a high-quality carbon source and has become a hot area for studying supercapacitor materials. In recent years, starch-based supercapacitor activated carbon has received widespread attention from researchers. In April 2021, the Shanxi Coal Chemistry Research Institute of the Chinese Academy of Sciences released the results of “Mass Preparation Technology of Starch-Based Supercapacitor Activated Carbon”, which solved the problems of continuous activation, deep purification and surface deoxygenation in the pilot preparation process of starch-based capacitor carbon, and is expected to achieve the localization of supercapacitor activated carbon. In addition, Zhai et al. studied the preparation of green and sustainable carbon aerogels using starch as raw material, and used potassium chloride and potassium carbonate as templates and activators, making the specific surface area of starch-based carbon aerogels as high as 2367m2/g and having a 3D network structure. When the starch-based carbon aerogel is used as an electrode material for supercapacitors, the specific capacitance can reach 292.3 F/g and 245 F/g at current densities of 1.0 A/g and 10A/g, respectively.
4.1 Amylose
Amylose is a polysaccharide chain in which D-glucose groups are connected by α-(1.4) glycosyl bonds [Figure 7(a)]. There are about 200 glucose groups in the molecule, the molecular weight is 1×105~2×105, the degree of polymerization is about 990, and the spatial conformation is curled into a spiral shape, with 6 glucose groups in each turn. Amylose exists in three forms: A, B and V. Among them, both forms A and B have a rigid left-handed spiral morphology. The only difference between the two forms is the different stacking methods of starch spirals. Single-helix amylose forms hydrogen bonds between the O2 and O6 atoms on the outer surface of the helix. V-type amylose is produced by the co-crystallization process of compounds including iodine, alcohol, fatty acids and dimethyl sulfoxide. V-type amylose undergoes dehydration condensation between the aligned chains to release water molecules, allowing the aligned chains to form a double-chain hydrophobic structure that is resistant to amylase, which is why amylose can form resistant starch.
Figure 7 (a) The structural formula of amylose and (b) the structural formula of amylopectin
4.2 Amylopectin
Amylopectin is the main component of starch and is one of the largest molecules found in nature. It is composed of a linear chain of α-D-pyranose units connected by α-(1→4) bonds, while the branches are connected by α-(1→6) bonds [Figure 7(b)]. The molecule contains 300~400 glucose groups, the molecular weight is greater than 2×107, the degree of polymerization is about 7200, and each branch is also curled into a spiral shape. Amylopectin is composed of three different types of chains. The number of “external” unbranched chains (A chains) is greater than that of internal branches (B chains). The third chain (C chain) is the only chain that contains a reducing group. The average length of the A chain is 13 to 23 residues, while the average length of the B chain is higher, about 23 to 25 residues. Studies have shown that the molecular weight of amylopectin varies from 107 to 108 depending on its source. Buléon et al. estimated that the number of amylopectin chains in a single starch granule is 5.4×107. The most important chemical reaction of amylopectin is hydrolysis. During seed germination, the α-(1→4) bond is dissolved by α- and β-amylase, followed by the production of dextrin [a small segment containing (1→6) bonds]. Dextrin is then enzymatically hydrolyzed by dextrinase to complete the digestion of starch to glucose. In addition, amylose turns blue when exposed to iodine, amylopectin turns purple-red when exposed to iodine, and dextrin turns blue-purple, purple, orange, etc. when exposed to iodine. These color reactions are highly sensitive and can be used as quantitative and qualitative methods for identifying starch, and can also be used to analyze the content of iodine. It is used in the textile industry to measure the completeness of desizing of cloth.
5 Other trace components
5.1 Other organic matter
In addition to high-content organic matter such as cellulose, hemicellulose, lignin, and starch, studies have shown that different biomass may also contain different trace organic components, which may affect the processing process. For example, seaweed contains 2% to 40% lipids, 6% to 71% protein (Spirulina has a protein content of up to 60% to 70%), and up to 6% nucleic acids. Some rice straws and flax contain 2% to 4% acetyl. There are 1% to 4% uronic acid in pine, eucalyptus, and sorghum grass. Pine, reed, spruce, birch, and corn contain up to 10% protein. In plant biomass materials, in addition to carbon, hydrogen, oxygen, and other basic elements, there are also a few other organic components, such as pigments, waxes, alkaloids, and mushroomene. These trace organic components may affect the properties of the final product during the conversion and processing of biomass. For example, some organic matter may have an effect on the performance and stability of supercapacitor materials.
5.2 Inorganics
Generally speaking, biomass also contains trace amounts of inorganics, and their content varies depending on the type of raw material. Common inorganic elements include calcium, sodium, potassium, magnesium, phosphorus, silicon, aluminum, and iron. In addition to crystalline substances, there are also many natural non-crystalline phases in inorganic substances. Usually, the most common solid residues present in biomass are water-soluble residues, such as chlorides, sulfates, oxalates, nitrates, carbonates, etc. In some pretreatment processes, biomass is dried at 105°C and then further pyrolyzed at a furnace temperature of 750°C. In this case, elements such as carbon, hydrogen, oxygen, nitrogen, and sulfur evaporate in the form of gaseous compounds due to their chemical properties, while pyrolysis residues containing metal elements mostly exist in the form of solid oxides. The content and type of inorganics have an important influence on the preparation and performance of supercapacitor materials, because some metal elements can play a catalytic role in the synthesis of supercapacitor materials and improve their electrochemical properties.
5.3 Fluid substances
Fluid substances are liquid substances contained in biomass, which contain a variety of anions and cations. The water content of biomass is usually between 10% and 60%, but in some special biomass (such as seaweed, water hyacinth, etc.), its water content can even be as high as 80% to 90%. The content of fluid substances depends on the initial composition of the biomass. Taking fast-growing crops as an example, their water content is high. In addition, for mature plant cells, the central large vacuole occupies most of the space, and the vacuole is filled with cell sap, so the vacuole is the main liquid environment in plant cells. There are sugars, proteins, inorganic salts and anthocyanins in the cell sap. Anthocyanins show different colors in different pH environments, which makes the leaves, petals and fruits have different colors. The leaves of purple duck grass are purple, which is that the color of anthocyanins contained in the vacuole of the mesophyll cells masks the color of chlorophyll. In the preparation of supercapacitor materials, the presence and treatment of fluid substances may also affect the properties and performance of the final product.
