There are huge differences in the types and quantities of different biomasses, and their composition characteristics are also significantly different. Therefore, there is no unified classification method for the time being, but in the supercapacitor field, biomass can be roughly classified according to its own characteristics and purpose of use.

According to origin, function and end product, biomass can be classified in the following two ways: ① Classification according to the form in nature (based on ecology or vegetation type); ② Classification according to the purpose of use of biomass as raw material.

The former is the most commonly used classification method, which can divide biomass into different categories (as shown in Table 1): ① Woody biomass; ② Herbaceous biomass; ③ Aquatic biomass; ④ Animal waste biomass; ⑤ Mixed biomass. The typical chemical composition of different types of biomass is shown in Table 2.

Table 1 Classification of biomass according to the form in nature (based on ecology or vegetation type) Biomass type

Table 2 The proportion range of chemical components of different types of biomass (mass percentage)

1 Application of woody biomass in the supercapacitor field

Woody biomass is currently the most important biomass renewable energy in the world. Woody biomass mainly comes from the biomass produced during the growth and production of woody plants. Woody plants refer to strong plants whose roots and stems form a large amount of xylem due to thickening growth, and whose cell walls are mostly lignified. Wood is the most typical representative of woody biomass, which can be converted into energy through direct combustion (or gasification) or a variety of conversion processes (Figure 1). In the supercapacitor field, wood has abundant parallel pipeline structures along its growth direction (Figure 2). These pipeline structures can not only provide channels for the rapid diffusion of electrolyte ions, but also serve as microreactors to achieve confined growth of electrochemically active materials to create high-performance, self-supporting wood-based composite energy storage materials. In addition, wood with high ion permeability and electrical insulation can also be cut into a specific thickness according to actual needs to be used as a diaphragm between the positive and negative electrodes of supercapacitors. Chen et al. prepared an all-wood asymmetric supercapacitor, as shown in Figure 3. The supercapacitor uses activated charcoal as the negative electrode, wooden film as the diaphragm, and MnO2/charcoal as the positive electrode. These three components are all from natural wood, have a unique anisotropic structure, and have many open channels along the growth direction for direct ion transmission. The electrode material not only has a large load capacity, but also has low deformability, so the all-wood supercapacitor exhibits a fairly high energy/power density and cycle stability. All-wood materials are inexpensive and biodegradable, and are a green and renewable energy storage device.

Figure 1 Wood biomass energy conversion under thermal and chemical action

Figure 2 Wood microstructure

(a) Scanning electron microscope images of wood cross section and radial section; (b)~(c) Scanning electron microscope images of wood cross section at different magnifications;

(d)~(e) Scanning electron microscope images of wood tangential section at different magnifications

Figure 3 Schematic diagram of all-wood asymmetric supercapacitor structure

Supercapacitor device assembled with activated charcoal as anode, activated charcoal and MnO2 composite as cathode, and thin wood film as diaphragm

2 Application of herbaceous biomass in the  supercapacitor field

Herbaceous biomass mainly comes from biomass produced during the growth and production of herbaceous plants. Herbaceous plants refer to plants with underdeveloped xylem in the stem, few lignified cells, and weak support. Generally speaking, herbaceous biomass resources mainly come from the following two categories, namely agricultural residues and energy crops.

(1) Agricultural residues are by-products of agriculture, fiber or food industries. These by-products are collected for different end applications, such as animal feed. In many cases, these residues are not fully utilized, and their availability and potential application as energy are not systematic.

(2) Energy crops are mainly developed by the bioenergy sector. Regarding the definition of energy crops, domestic literature has a variety of expressions from different perspectives. For example, any crop that can be effectively converted into an energy substitute can be called an energy crop; or energy crops refer to a variety of cultivated plants for the purpose of providing fuel raw materials or providing fuel oil. Typical energy crops include sorghum, sugarcane, etc.

Crop straw is a typical biomass produced by herbaceous plants during their growth. More than half of the products of crop photosynthesis are present in the straw. The straw is rich in nitrogen, phosphorus, potassium, calcium, magnesium and organic matter, and is a renewable biomass resource with multiple uses. In the supercapacitor field, the rich nitrogen organic matter in straw can promote the self-activation of straw when pyrolyzed in an inert atmosphere to obtain nitrogen-doped straw-derived carbon. These ammonia components can introduce the Faraday pseudocapacitive energy storage mechanism through redox reaction when the activated carbon itself undergoes a double-layer energy storage mechanism, thereby improving the overall electrochemical activity through synergistic effects. In addition to this simple carbonization utilization method, conventional plant biomass such as straw can also obtain nanoscale cellulose, lignin and other components through physical, chemical and biological disassembly techniques, and achieve orderly assembly from micro to macro through hydrogen bonding, click chemical reactions, positive and negative charge effects, etc., thereby obtaining one-dimensional, two-dimensional and three-dimensional regenerative matrix materials with larger specific surface area and more developed pore structure. These regenerative matrix materials often have more obvious performance advantages in the electrochemical reaction process. These contents will be introduced in detail in subsequent chapters.

3 Aquatic biomass

Aquatic biomass mainly refers to biomass produced by aquatic plants during growth and production and processing. Plants that can grow in water are collectively referred to as aquatic plants. According to the lifestyle of aquatic plants, they are generally divided into the following categories: emergent plants, floating leaf plants, submerged plants and floating plants and wet plants.

Emergent plants: lotus, reed, cattail, shiso, water plantain, reed, calamus, cattail, black three-edged, water candle, oriental water lily, arrowhead, etc.

Floating leaf plants: spring pondweed, bamboo leaf pondweed, water lily, duckweed, water chestnut, water chestnut, fluorescent fruit, Victoria amazonica, etc.

Submerged plants: silk leaf pondweed, thread leaf pondweed, water cabbage, sea cabbage, sea calamus, Vallisneria, goldfish, water plantain, spike foxtail, black algae, etc.

Floating plants: duckweed, purple back duckweed, water hyacinth, big thin and other plants.

Wet plants: canna, pickerel grass, loosestrife, water lily, aquatic iris, red Polygonum, Pennisetum, cattail and other plants suitable for growing by the water.

In addition to these higher plants, algae, as a lower plant, is also a typical aquatic plant and can contribute a large amount of aquatic biomass every year. In the supercapacitor field, certain components in aquatic plants and algae have unique conductive properties and structural characteristics, which makes them have potential application value in the preparation of high-performance energy storage materials. my country is a major producer of algae, mainly through aquaculture. The main varieties include kelp, river lily, kelp and laver. According to the “2020 China Fisheries Statistical Yearbook”, the total output of algae in 2019 was 2.56 million tons. Algae mainly include macroalgae and microalgae:

(1) Macroalgae are multicellular organisms that can quickly reach 60m in length and are mainly used in food production and hydrocolloid extraction.

(2) Microalgae are microorganisms that can be subdivided into the following subclasses: diatoms, green algae and golden algae. Diatoms are a type of single-celled plant with chloroplasts, often composed of several or many individual cells connected into various groups. They are one of the main components of aquatic microbial flora and one of the largest sources of biomass on Earth. Green algae are particularly abundant in freshwater resources. Although oil can be produced from these algae, the main product obtained is starch. Golden algae are similar to diatoms and mainly produce oils and carbohydrates.

In nature, there are about 30,000 species of algae, which are mainly distributed in freshwater or seawater and are divided into two types: freshwater algae and marine algae. The biggest advantage of using these simple organisms is that they can convert sunlight, water and carbon dioxide into a variety of metabolites and chemicals, ultimately forming algal biomass.

Currently, aquatic biomass is considered to be an ideal raw material for the production of third-generation biodiesel because it does not compete with food crops and produces more biomass per hectare than terrestrial crops. However, there are still considerable economic and technical challenges to overcome before this biofuel can be effectively commercialized. In addition, it should be noted that there are many parameters that affect the performance of aquatic biomass, such as solar radiation levels, CO2 and O2 concentrations, temperature, pH, salinity and nutrients. In the supercapacitor field, similar to herbaceous plants, high-temperature pyrolysis to carbonize and nano-disassembly and reorganization to prepare advanced matrix materials or composites are two main ways to apply algae biomass.

4 Animal waste biomass

The most common sources of animal waste biomass are hair, skin, flesh, bones, shells, excrement, etc. of various animals. These wastes are often recycled and used as fertilizers on agricultural land, but with the introduction of stricter environmental regulations regarding pollution, health and odor, this application has been significantly restricted. Anaerobic digestion is the most convenient way to convert these wastes into useful products. For example, biogas, as a bioenergy product of the process, can be used for turbines and internal combustion engines to generate electricity, or directly used as fuel for rural cooking or heating. In addition, for crustaceans such as shrimps, crabs, and lobsters, their shells are an important source of the chemical component chitin. Chitin is a polysaccharide substance extracted from the shells of crustaceans with a chemical formula of (C8H13O5N)n. Chitosan has a wide range of applications. In industry, it can be used as fabrics, clothing, dyes, paper, and water treatment. In agriculture, it can be used as pesticides and plant antiviral agents. In fisheries, it can be used as fish feed. It can be used as cosmetics, hair protectors, moisturizers, etc. In medical supplies, it can be used as contact lenses, artificial skin, sutures, artificial dialysis membranes, and artificial blood vessels. In the  supercapacitor field, chitosan nanofibers are also a very promising nanomaterial. Chitosan is a biopolymer with reserves second only to cellulose in nature. Its product after deacetylation is chitosan. Chitosan is an intrinsic nitrogen-containing biomass with good biocompatibility and biological activity. Therefore, it is considered to be a functional biopolymer with more application potential than cellulose. Due to the overlapping and entanglement between chitosan molecular chains, it is easy to form a planar or film-like polymer, and after carbonization, a layered carbon material can be obtained, which enables its derivative materials to achieve simultaneous regulation of structure and surface chemistry (Figure 4). Gong et al. used chitin biomass as raw material, freeze-drying technology and potassium ferrate as pore-forming agent/catalyst to achieve the “one-step” preparation of nitrogen-doped biomass graphitized hierarchical porous carbon materials. The optimized graphitized hierarchical pore structure and the modification of nitrogen atoms make it have excellent capacitance performance and energy-power output characteristics as an electrode material for supercapacitors. The symmetrical capacitor of the water system can output an energy density of 10.2Wh/kg at a power density of 100 W/kg, and the energy density of the symmetrical capacitor with ionic liquid as electrolyte can reach 63.3 Wh/kg (power density of 300 W/kg).

Figure 4 Using chitin biomass as raw material, freeze-drying technology and potassium ferrate as pore-forming agent/catalyst, the “one-step” preparation of nitrogen-doped biomass graphitized hierarchical porous carbon materials was achieved