Lignocellulosic Biomass-Derived Carbon Electrodes for Flexible Supercapacitors: An Overview

With the increasing demand for high-performance electronic devices in smart textiles, various types of flexible/wearable electronic device (i.e., supercapacitors, batteries, fuel cells, etc.) have emerged regularly. As one of the most promising wearable devices, flexible supercapacitors from a variety of electrode materials have been developed. In particular, carbon materials from lignocellulosic biomass precursor have the characteristics of low cost, natural abundance, high specific surface area, excellent electrochemical stability, etc. Moreover, their chemical structures usually contain a large number of heteroatomic groups, which greatly contribute to the capacitive performance of the corresponding flexible supercapacitors. This review summarizes the working mechanism, configuration of flexible electrodes, conversion of lignocellulosic biomass-derived carbon electrodes, and their corresponding electrochemical properties in flexible/wearable supercapacitors. Technology challenges and future research trends will also be provided.


Introduction
With the rapid development of information, the demand for flexible/wearable electronic devices is increasing drastically. As a representative of wearable electronic devices by flexible textile manufacturing technology, smart textiles have drawn intense research interest. Smart textiles are defined as textile goods (i.e., woven, knitted or non-woven structures from fibers, filaments or yarns) that can interact with the environment/user [1]. The combination of conventional textiles and flexible wearable electronic devices (i.e., energy storage devices, sensors, etc.) is capable of accomplishing a spectrum of functions in various fields, such as health monitoring, medical care, sports, military, etc.
In this context, numerous efforts have been made in developing flexible and efficient energy storage systems (i.e., supercapacitors (SCs), batteries and fuel cells, etc.) for smart textiles. A good example is the supercapacitor, which is the main component of electrochemical (EC) energy storage devices. A supercapacitor, also known as ultracapacitor or electrochemical capacitor, has the characteristics of high power density, fast charge/discharge cycles, and good cycle performance (>100,000 cycles) [2][3][4][5]. Considerable research and development has focused on transforming a rigid capacitor into a flexible capacitor in recent years since the latter is lightweight, small and portable [6][7][8], and suitable for application in smart textiles. Two-dimensional (2D) planar structured flexible supercapacitors have been developed to be employed in 2D woven fabrics [9,10]. Three-dimensional (3D) carbon aerogels have also been applied in flexible SCs. To further integrate flexible SCs into fabric structure, it is of great importance to develop onedimensional (1D) fiber-shaped supercapacitors (FSSCs) with good mechanical strength for textile manufacturing (i.e., weaving, knitting, braiding, etc.), and excellent EC performance

Types and Working Mechanisms of Supercapacitors
In general, there are three types of supercapacitor: EDLCs, pseudocapacitor and asymmetric supercapacitor [32]. The energy storage mechanism of EDLC ( Figure 2a) is mainly based on the electrical double-layer theory, which involves the nanoscale charge separation, and the formation of electrochemical interface between an electrode and an electrolyte [33]. Since the physical charge transfer mechanism of EDLCs is non-faradaic and involves no chemical oxidation-reduction (redox) reaction, the life cycle of EDLCs is relatively long. Pseudocapacitors, also known as Faraday capacitors, involve faradaic redox reactions that mainly occur on the surface of the electrode materials ( Figure 2b). The chemical reactions contribute to the pseudocapacitor's higher energy density but lower life cycles in comparison with those of EDLCs [34]. The asymmetric supercapacitor ( Figure 2c) is a combination of EDLC and pseudocapacitor. Its working mechanism is that the electrolyte ions are absorbed by one electrode in the capacitor, forming an electrical double-layer structure for energy storage. The pseudocapacitive electrode materials (i.e., metal oxides, conductive polymers or metal-doped carbons) involve an electrochemical reaction for energy storage. Thus, asymmetric supercapacitors possess the advantages of both EDLCs and pseudocapacitors due to different energy storage mechanisms of the positive and negative electrodes [35]. Compared with the EDLCs, asymmetric supercapacitors have higher energy density, higher working voltage, and the superior cycle stability.

Types and Working Mechanisms of Supercapacitors
In general, there are three types of supercapacitor: EDLCs, pseudocapacitor and asymmetric supercapacitor [32]. The energy storage mechanism of EDLC ( Figure 2a) is mainly based on the electrical double-layer theory, which involves the nanoscale charge separation, and the formation of electrochemical interface between an electrode and an electrolyte [33]. Since the physical charge transfer mechanism of EDLCs is non-faradaic and involves no chemical oxidation-reduction (redox) reaction, the life cycle of EDLCs is relatively long. Pseudocapacitors, also known as Faraday capacitors, involve faradaic redox reactions that mainly occur on the surface of the electrode materials ( Figure 2b). The chemical reactions contribute to the pseudocapacitor's higher energy density but lower life cycles in comparison with those of EDLCs [34]. The asymmetric supercapacitor ( Figure 2c) is a combination of EDLC and pseudocapacitor. Its working mechanism is that the electrolyte ions are absorbed by one electrode in the capacitor, forming an electrical double-layer structure for energy storage. The pseudocapacitive electrode materials (i.e., metal oxides, conductive polymers or metal-doped carbons) involve an electrochemical reaction for energy storage. Thus, asymmetric supercapacitors possess the advantages of both EDLCs and pseudocapacitors due to different energy storage mechanisms of the positive and negative electrodes [35]. Compared with the EDLCs, asymmetric supercapacitors have higher energy density, higher working voltage, and the superior cycle stability. Reprinted from ref. [32], an open-access article.

Configuration of Flexible Electrodes in Supercapacitors
Compared with traditional rigid supercapacitors, flexible/wearable supercapacitors have shorter charge/discharge time, longer cycle life and other merits such as smaller size, lighter weight, and better flexibility [36][37][38][39]. The assembly of wearable supercapacitors typically involves three configurations, including 1D linear (fiber-shaped), 2D film-based planar and 3D aerogel interconnected porous structure. In this section, the configuration of flexible supercapacitors potentially for smart textiles applications based on 1D, 2D and 3D electrodes will be introduced.

Flexible One-Dimensional Fiber-Shaped Electrodes
One-dimensional (1D) fiber-shaped electrodes have good flexibility, small size, light weight, fast charge/discharge process, long service life and good stability [40,41]. More importantly, they can maintain stable EC performance during folding, stretching, compression and other deformation processes [42,43], thus showing broad application prospects in smart textiles.
Structurally, FSSCs can be assembled from fiber electrodes in parallel, twisted or coaxial form [44]. The parallel structure is obtained by placing two fiber electrodes in parallel and encapsulating them with a solid electrolyte (Figure 3a). The assembled FSSCs can be used together in series or in parallel to meet specific energy and power requirements of microelectronics. The FSSCs with twisted structure are fabricated by winding fiber electrodes together (Figure 3b). This process is quite similar to the twisting of yarns in textile manufacturing process. This twisted structure, with a high direct contact area between the fiber electrodes, is beneficial for the electrochemical reaction during the charge/discharge process. A coaxial structure (Figure 3c), comprising of a core fiber electrode, a separator or solid-state electrolyte, and an outer electrode layer, is achieved by the layer-by-layer method. Compared with the aforementioned two structures, this coaxial structure has a much larger effective electrode contact area, which is more conducive to the adsorption/desorption of charges on the electrode surface and the redox reaction. Moreover, the assembled device is more stable under deformation since it has good integrity as a whole.   One-dimensional (1D) FSSCs can be easily converted into energy storage yarns, which can be further integrated with traditional clothing [42,43] or fabricated into energy storage 2D fabrics (Figure 4a,b) [46] for smart textiles. For instance, Huang et al. [47] first loaded graphene onto cotton thread by the hydrothermal method, then grew manganese oxide (MnO 2 ) and polypyrrole (PPy) in sequence through electrodeposition, and finally assembled the linear yarn electrodes (Figure 4c-e) to achieve supercapacitors that were knitted into smart textiles (Figure 4f). Lucas et al. [48] produced an all-in-one graphene/MnO 2 composite FSSCs device. The core graphene fiber (GF) electrode was sequentially coated with a hierarchically nanostructured MnO 2 layer, gel electrolyte of poly (vinyl alcohol)/phosphoric acid (PVA/H 3 PO 4 ), and sheath GF electrode. The novel coaxial structure equipped the device with a high volumetric capacitance of 29.6 F/cm 3 at a scan rate of 2 mV/s, and remarkable capacitance retention of 93% after 1000 cycles, making it potentially woven into smart textiles. One-dimensional (1D) FSSCs can be easily converted into energy storage yarns, which can be further integrated with traditional clothing [42,43] or fabricated into energy storage 2D fabrics (Figure 4a,b) [46] for smart textiles. For instance, Huang et al. [47] first loaded graphene onto cotton thread by the hydrothermal method, then grew manganese oxide (MnO2) and polypyrrole (PPy) in sequence through electrodeposition, and finally assembled the linear yarn electrodes (Figure 4c-e) to achieve supercapacitors that were knitted into smart textiles (Figure 4f). Lucas et al. [48] produced an all-in-one graphene/MnO2 composite FSSCs device. The core graphene fiber (GF) electrode was sequentially coated with a hierarchically nanostructured MnO2 layer, gel electrolyte of poly (vinyl alcohol)/phosphoric acid (PVA/H3PO4), and sheath GF electrode. The novel coaxial structure equipped the device with a high volumetric capacitance of 29.6 F/cm 3 at a scan rate of 2 mV/s, and remarkable capacitance retention of 93% after 1000 cycles, making it potentially woven into smart textiles.

Flexible Two-Dimensional Planar Electrodes
Flexible supercapacitors potentially used in smart textiles can be directly obtained from 2D planar electrodes, which form a sandwich structure with a solid electrolyte in

Flexible Two-Dimensional Planar Electrodes
Flexible supercapacitors potentially used in smart textiles can be directly obtained from 2D planar electrodes, which form a sandwich structure with a solid electrolyte in between. In general, 2D planar electrode, either in film or fabric form, is the most commonly used configuration for flexible supercapacitors.
Quite a few works in the literature deposited electroactive substances onto flexible current collectors to prepare 2D thin-film electrodes for flexible supercapacitors [49,50]. For instance, flexible supercapacitors were constructed by the intercalation of PVA/H 3 PO 4 gel electrolyte between two drop-casted reduced graphene oxide (rGO) thin-film electrodes on a tin-doped indium oxide (In 2 O 3 )-coated polyethylene terephthalate (PET) flexible substrate [51]. However, when the wearable devices were subjected to repeated body deformation, the delamination between the 2D electrode and current collector may have occurred. This can adversely affect the comfort of smart textiles and the energy storage performance of the device. To prevent this delamination issue, one strategy is to develop a self-supporting or binder-free flexible 2D thin-film electrode. This method can reduce the volume occupied by the current collector, increase the utilization rate of the active material, and finally increase the specific capacity of the electrode [52]. For instance, transition metal oxide (TMO) such as vanadium pentoxide (V 2 O 5 ) films were achieved via a thermal evaporation technique [53]. The 2D film-based symmetric device (V 2 O 5 /electrolyte/V 2 O 5 ) had a thickness of~540 nm, and its pseudocapacitive behavior contributed to a maximum areal specific capacitance of 9.7 mF/cm 2 at a scan rate of 10 mV/s. The promising performance remained even after being bent at 60 • and 120 • , indicating great potential in smart textile applications. Furthermore, the delamination between self-supporting flexible 2D film electrode and separator can be prevented by the formation of good interfacial adhesion between the two. A good example is that the wearable supercapacitor was constructed by utilizing 2D lignosulfonate/single-wall carbon nanotube/holey reduced graphene oxide (Lig/SWCNT/HrGO) film as the electrode/current collector and high-strength cellulose hydrogel as the separator. The strong interaction between lignosulfonate and cellulose effectively resulted in smaller contact resistance between different layers and prevent the delamination between the electrode and the separator [54].
Two-dimensional (2D) film is also achieved by electrospun polymeric nanofibers, which form a membrane or non-woven mat. Electrospun polymeric nanofibers can be further carbonized to yield freestanding, binder-free and current collector-free carbon electrodes with porous structure and tunable porosity [55]. These non-woven electrodes offer shorter diffusion distance, and effectively decrease charge transfer resistance at their interface with the electrolyte to ensure superior rate capability [55]. In particular, lignocellulose is widely employed as the precursor of electrospun carbon nanofiber electrode of flexible supercapacitors, which will be introduced in detail in Section 5 of this article.
To successfully promote the application of flexible supercapacitor in smart textiles, it is necessary to achieve 2D fabric-based electrodes with conventional textile manufacturing techniques such as weaving, knitting or non-woven processing. Direct coating of existing fabric with electroactive material is a simple and facile approach. For example, Zhai et al. [56] used electrodeposition and heat treatment methods to obtain MnO 2deposited carbon cloth as a positive electrode, and graphene-coated carbon cloth as a negative electrode. The assembled asymmetric fabric-based wearable supercapacitors were achieved by soaking two electrodes and a separator in electrolyte solution ( Figure 5). Norawish et al. [57] successfully fabricated a conductive cotton fabric electrode by screenprinting an ink containing conductive silver (Ag) and one of the electroactive materials-AC, graphene, and CNTs, onto the uncoated cotton fabric. Fabric-based symmetric supercapacitors were assembled by inserting a separator of uncoated cotton fabric containing an electrolyte into two coated conductive cotton cloths. It was found that the AC-based conductive cotton electrode displayed the highest areal specific capacitance of~3290 mF/cm 2 at a scan rate of 5 mV/s, as well as excellent long-term cyclic stability. However, the wash durability of the smart textiles from the coated fabric-based supercapacitors remains to be further explored. Wang et al. [58] carbonized weft-knitted Modal fabric, and assembled inherently stretchable and conductive fabric-based supercapacitors with PVA/H3PO4 gel as an electrolyte/separator. This fabric-based supercapacitor exhibited a maximum specific capacitance of 7.5 mF/cm 2 (1.2 F/g) at the scan rate of 10 mV/s, and a capacitance retention of 44% as the scan rate increased from 50 mV/s to 500 mV/s.

Flexible Three-Dimensional Areogel Electrodes
To obtain 3D interconnected aerogel electrodes, two routes are typically used. One route is that carbon-based framework filled with a high concentration of solvents (i.e., water) is directly converted into aerogel by different drying methods (i.e., freeze-drying, supercritical drying) to retain the 3D hierarchically porous structure [55,59]. Another strategy is to construct a 3D crosslinked structure of precursor materials before the carbonization process [60]. Both strategies result in highly porous structure. In general, hierarchical pores containing micro-(<2 nm), meso-(2-50 nm) and macropores (>50 nm) are preferred. Macro-/mesopores facilitate the fast ion transport by serving as ion buffering reservoirs and ion transport pathways, while micropores enhance the electrical double-layer capacitance by storing more electrolyte ions [61]. Thus, it is of great importance to optimize pore size to achieve high-performance supercapacitors.
Lignocellulosic biomass materials have been widely used in the fabrication of 3D carbon aerogel electrode-based supercapacitors. For instance, wood-derived carbon aerogel, which was obtained by freeze-casting, freeze-drying and carbonization, demonstrated good compressibility (Figure 6a) [62]. The lignin and cellulose nanofiber-containing ( Figure  6b) aerogel had a highly interconnected porous structure before ( Figure 6c) and after ( Figure  6d) carbonization, which was conducive to high EC performance. More details will be Moreover, a fabric-based electrode can be obtained from flexible bio-based fabrics through a simple carbonization process, which is a cost-effective and sustainable route. Wang et al. [58] carbonized weft-knitted Modal fabric, and assembled inherently stretchable and conductive fabric-based supercapacitors with PVA/H 3 PO 4 gel as an electrolyte/separator. This fabric-based supercapacitor exhibited a maximum specific capacitance of 7.5 mF/cm 2 (1.2 F/g) at the scan rate of 10 mV/s, and a capacitance retention of 44% as the scan rate increased from 50 mV/s to 500 mV/s.

Flexible Three-Dimensional Aerogel Electrodes
To obtain 3D interconnected aerogel electrodes, two routes are typically used. One route is that carbon-based framework filled with a high concentration of solvents (i.e., water) is directly converted into aerogel by different drying methods (i.e., freeze-drying, supercritical drying) to retain the 3D hierarchically porous structure [55,59]. Another strategy is to construct a 3D crosslinked structure of precursor materials before the carbonization process [60]. Both strategies result in highly porous structure. In general, hierarchical pores containing micro-(<2 nm), meso-(2-50 nm) and macropores (>50 nm) are preferred. Macro-/mesopores facilitate the fast ion transport by serving as ion buffering reservoirs and ion transport pathways, while micropores enhance the electrical double-layer capacitance by storing more electrolyte ions [61]. Thus, it is of great importance to optimize pore size to achieve high-performance supercapacitors.
Lignocellulosic biomass materials have been widely used in the fabrication of 3D carbon aerogel electrode-based supercapacitors. For instance, wood-derived carbon aerogel, which was obtained by freeze-casting, freeze-drying and carbonization, demonstrated good compressibility (Figure 6a) [62]. The lignin and cellulose nanofiber-containing (Figure 6b) aerogel had a highly interconnected porous structure before ( Figure 6c) and after (Figure 6d) carbonization, which was conducive to high EC performance. More details will be revealed in Section 5 regarding the EC performance of 3D carbon aerogel supercapacitors with electrodes derived from lignocellulose. Despite carbon aerogel's unique porous structure, its lower flexibility or mechanical properties in comparison with 1D fiber or 2D film/fabric can be an obstacle that hinders its wide application in flexible supercapacitors.

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revealed in Section 5 regarding the EC performance of 3D carbon aerogel supercapacitors with electrodes derived from lignocellulose. Despite carbon aerogel's unique porous structure, its lower flexibility or mechanical properties in comparison with 1D fiber or 2D film/fabric can be an obstacle that hinders its wide application in flexible supercapacitors.

Fabrication of Lignocellulose-Based Carbon
Electrode material is one of the key factors determining the EC performance of supercapacitors. Among various electrode materials (i.e., CNTs, AC, metal oxides, carbon fibers, conductive polymers, etc.) (Figure 7), carbon materials (graphene, CNTs and AC) [32,63,64] are widely used in EDLCs and asymmetric supercapacitors [65] mainly due to their desirable chemical and physical properties such as tunable porous structure, large SSA, excellent electrical conductivity, and good chemical/thermal stability [66].
One of the challenges to achieve carbon materials of good quality is to select a suitable

Fabrication of Lignocellulose-Based Carbon
Electrode material is one of the key factors determining the EC performance of supercapacitors. Among various electrode materials (i.e., CNTs, AC, metal oxides, carbon fibers, conductive polymers, etc.) (Figure 7), carbon materials (graphene, CNTs and AC) [32,63,64] are widely used in EDLCs and asymmetric supercapacitors [65] mainly due to their desir-able chemical and physical properties such as tunable porous structure, large SSA, excellent electrical conductivity, and good chemical/thermal stability [66]. Carbon aerogels, activated carbons, carbon fibers, carbon nanotubes and graphene are mainly for EDLCs; metal oxide and conducting polymers are used for pseudocapacitors; carbon materials/conducting polymers or carbon materials/metal oxides are for asymmetric capacitors.
Lignocellulosic biomass consists of three main biopolymers, which are carbohydrates (cellulose and hemicellulose) and lignin ( Figure 8). In the cell wall, the primary component is cellulose, which forms highly oriented and long fibrils. The amorphous, highly branched hemicellulose are attached to the adjacent cellulose fibrils by non-covalent crosslinks. Lignin is an amorphous, three-dimensional molecule linked to carbohydrates by hydrogen and covalent bonds. Typically, cellulose, lignin or raw lignocellulosic biomass are converted into carbon electrodes for flexible supercapacitors by carbonization, activation and possible surface modification. The SSA, porosity and surface chemistry can be tuned to yield flexible supercapacitors with good EC performance. These fabrication processes will briefly be introduced in this section. Figure 7. The diagram illustrating typical electrode materials for different types of supercapacitor. Carbon aerogels, activated carbons, carbon fibers, carbon nanotubes and graphene are mainly for EDLCs; metal oxide and conducting polymers are used for pseudocapacitors; carbon materials/conducting polymers or carbon materials/metal oxides are for asymmetric capacitors.
One of the challenges to achieve carbon materials of good quality is to select a suitable source. The carbon source affects the structure or performance of the resulting carbon, including heteroatom content, nanostructure, and SSA [67,68]. Commercial AC, CNTs and rGO are the most commonly used carbon materials for supercapacitors. However, high production cost, potential environmental hazard, and complex and unsustainable preparation methods may hinder their wide application in flexible supercapacitors. Therefore, it is of great significance to develop biomass-derived carbon electrodes with long-term sustainability and low cost to achieve flexible supercapacitors with excellent EC performance.
Lignocellulosic biomass consists of three main biopolymers, which are carbohydrates (cellulose and hemicellulose) and lignin ( Figure 8). In the cell wall, the primary component is cellulose, which forms highly oriented and long fibrils. The amorphous, highly branched hemicellulose are attached to the adjacent cellulose fibrils by non-covalent crosslinks. Lignin is an amorphous, three-dimensional molecule linked to carbohydrates by hydrogen and covalent bonds. Typically, cellulose, lignin or raw lignocellulosic biomass are converted into carbon electrodes for flexible supercapacitors by carbonization, activation and possible surface modification. The SSA, porosity and surface chemistry can be tuned to yield flexible supercapacitors with good EC performance. These fabrication processes will briefly be introduced in this section.

Carbonization
Different methods have been developed to prepare biochar derived from lignocellulosic biomass. Carbonization of lignocellulosic biomass can be undertaken beforehand or combined with activation process. The thermochemical carbonization process, including pyrolysis and hydrothermal carbonization, was the earliest method used for the fabrication of biochar from biomass [71][72][73]. Pyrolysis occurs in the temperature range of 400−850 °C in an anoxic environment or an inert atmosphere to allow thermochemical decomposition of biomass for biochar. The properties of the resulting biochar mainly depend on the parameters such as heating rate, reaction temperature, catalyst and biomass type, etc. Hydrothermal carbonization is defined as the physical and chemical transformation of biomass in an aqueous environment with a low temperature (180−250 °C) and a saturated pressure (2−10 MPa) in a confined space. Dehydration, polymerization and carbonization occur in this process. The chemical structure, porosity and composition of the biochar can be tuned by carbonization parameters such as pressure, reaction time, catalyst and temperature, etc. However, the biochar or carbon material obtained by solely carbonization has limited SSA and porosity (Table 1), which are not preferred in the enhancement of the electrode's EC performance [17]. Therefore, an activation process is always employed to increase the SSA and porosity of biochar [71].

Carbonization
Different methods have been developed to prepare biochar derived from lignocellulosic biomass. Carbonization of lignocellulosic biomass can be undertaken beforehand or combined with activation process. The thermochemical carbonization process, including pyrolysis and hydrothermal carbonization, was the earliest method used for the fabrication of biochar from biomass [71][72][73]. Pyrolysis occurs in the temperature range of 400-850 • C in an anoxic environment or an inert atmosphere to allow thermochemical decomposition of biomass for biochar. The properties of the resulting biochar mainly depend on the parameters such as heating rate, reaction temperature, catalyst and biomass type, etc. Hydrothermal carbonization is defined as the physical and chemical transformation of biomass in an aqueous environment with a low temperature (180-250 • C) and a saturated pressure (2-10 MPa) in a confined space. Dehydration, polymerization and carbonization occur in this process. The chemical structure, porosity and composition of the biochar can be tuned by carbonization parameters such as pressure, reaction time, catalyst and temperature, etc. However, the biochar or carbon material obtained by solely carbonization has limited SSA and porosity (Table 1), which are not preferred in the enhancement of the electrode's EC performance [17]. Therefore, an activation process is always employed to increase the SSA and porosity of biochar [71].

Activation
Activation is an essential step to transforming lignocellulose-derived carbon into highly porous carbon materials for further use in flexible supercapacitors. It can be categorized into physical activation, chemical activation, and other methods such as physicochemical activation, microwave-assisted activation, and template-assisted activation, which will be introduced in this section. Table 2 summarizes the pore characteristics of lignocellulose-derived carbons based on different activation methods and activators.

Physical Activation
In a physical activation process, the biochar is mainly activated by water (H 2 O) steam and carbon dioxide (CO 2 ) in the temperature range of 700-1100 • C after carbonization [26,[78][79][80]. The chemical reactions between biochar and H 2 O or CO 2 in a physical activation process are as expressed in the following Equations (1)-(3) [81,82]: The activation agents promote high SSA and high total pore volume by controlled active carbon atom burn-off and elimination of volatile substances [26]. The porosity or the quality of the activated lignocellulose-derived carbon relies on different activation parameters such as temperature, time, gas flow rate, etc. [26,28]. Moreover, an appropriate activation agent is required for biomass to achieve high surface area or porosity due to the difference in chemical structure and composite component of precursor [83]. For instance, pine nutshell activated by H 2 O steam at 800 • C had SSA of 956 m 2 /g, total pore volume of 0.62 cm 3 /g and a mesopore ratio of 37.1% [84]. CO 2 was employed as an activator of willow at 800 • C to yield lignocellulose-based carbon with SSA of 739 m 2 /g and total pore volume of 0.37 cm 3 /g [85]. However, it is quite hard to achieve SSA > 2000 m 2 /g with physical activation. Thus, an alternative activation method such as chemical activation can be used to obtain lignocellulose-derived carbon with abundant micropores and high SSA.
Among different chemical activation agents, KOH is the most widely used activation agent since it demonstrates the best activation effect to enhance the SSA up to >2000 m 2 /g [92][93][94]. The mechanism of KOH activation involves the etching of carbon skeleton for the expansion or creation of pores caused by the reactions at high temperature (>750 • C), as shown in Equations (4)-(7) [75,95]: Different types of lignocellulose show various pore characteristics after chemical activation with KOH [94,[96][97][98]. For example, walnut shell activated by KOH had extremely high SSA of 3577 m 2 /g and large pore volume of 2.19 cm 3 /g [96]. Lignocellulose-based carbon from tobacco rods by hydrothermal carbonization and KOH activation also possessed high SSA of 2115 m 2 /g and pore volume of 1.22 cm 3 /g [94].
NaOH is another commonly used activation agent for lignocellulose-derived AC. It has less corrosiveness, lower weight, and lower cost [99], in comparison with the aforementioned KOH. For instance, wheat bran-derived carbon by NaOH activation at 800 • C demonstrated substantial SSA (2543 m 2 /g) and high total pore volume (1.68 cm 3 /g) [100]. The biochar from peanut shell activated by NaOH had superior SSA of 2764 m 2 /g and large total pore volume (1.31 cm 3 /g) [101].
As a mild salt activation agent, ZnCl 2 is also a frequently used activation agent to yield biomass-derived porous carbon. The possible activation mechanism of ZnCl 2 is based on the catalytic dihydroxylation and dehydration during pyrolysis process. In this process, H and O atoms are released as H 2 O, creating a micropore-rich structure in biochar [28]. Moreover, ZnCl 2 acts as a framework for carbon deposition in the activation process, and leaves voids in carbon skeleton for more pores after acid washing [28]. For example, waste tea activated by ZnCl 2 at 450 • C displayed good SSA of 1308 m 2 /g and total pore volume of 0.81 cm 3 /g.
In comparison with physical activation, chemical activation has shorter reaction time, lower reaction temperature, and yields activated biochar with higher SSA and total pore volume. However, it is necessary to wash the activation carbon to remove chemical reagent residues and ash content after chemical activation. Thus, this process has some drawbacks such as high cost, equipment corrosion and non-recyclable chemicals.

Other Activation Methods
Other activation methods such as physicochemical activation, microwave-assisted activation, and template-assisted self-activation have been employed in the activation of lignocellulose-derived carbon materials (Table 2).
Physicochemical activation is a combination of physical and chemical activation to yield AC with good porosity by partially replacing hazardous chemicals with steam [28]. For instance, peanut shell activated by bimetallic activation with CO 2 at 800 • C yielded biochar with SSA of 1428 m 2 /g, total pore volume of 0.55 cm 3 /g, and a large proportion of micropores (73.91%) [102]. Lignin activated by KOH/H 2 O with microwave heating showed high SSA (2482 m 2 /g) and total pore volume (1.45 cm 3 /g) [103]. Microwave-assisted activation transforms the microwave energy with dipole rotation and ionic conduction within the particles [104] to yield biomass-based AC. Lignin activated by KOH with microwave had significant SSA of 3065 m 2 /g and pore volume of 0.54 cm 3 /g [105]. Microwave-assisted activation of coconut shell by CO 2 also showed good pore characteristics with SSA of 2288 m 2 /g and pore volume of 1.3 cm 3 /g [106].
Template-assisted self-activation converts biomass into AC with no additional activators. The biomass precursor's inherent inorganic salts or metallic ions could etch the carbon skeleton or react with pyrolysis gases (i.e., CO 2 , H 2 O) generated in the carbonization process [107,108]. This method combines the template and self-activation agents together without post-activation [109]. For example, cotton containing magnesium oxide (MgO) and ZnCl 2 was converted into AC with MgO as template and ZnCl 2 as activator. The resulting biochar exhibited satisfactory SSA of 1990 m 2 /g and pore volume of 1.23 cm 3 /g [110]. In summary, lignocellulose-derived AC materials typically have the following common characteristics: (a) large SSA to supply sufficient active sites for electrolyte ions storage [114,115]; (b) well-developed porous structure to facilitate fast diffusion of electrolyte ions into pores at high current loads [116,117]; (c) a 3D interconnected porous framework to ensure fast electrolyte ion transfer [118,119]; (d) a heteroatom-rich structure to improve surface wettability and offer additional pseudocapacitance [120]. Thus, they are suitable and promising electrode materials to fabricate flexible supercapacitors with high EC performance.

Surface Modification
The EC performance of carbon materials for supercapacitors is mainly dictated by the architecture of the porous structure [17] and the surface chemistry. Three-dimensional (3D) hierarchically porous structure with different portions of micro-, meso-and macropores can be achieved by proper selection and control of the activation as mentioned above. However, the surface chemistry of AC is not affected by the chosen activation process [121].
The surface modification of biochar generally changes the electron accepting/donating characteristics, and the chemical structures. The EC performance is influenced by the introduction of heteroatoms (i.e., O, H, N, S, P, etc.). The acidic or basic characteristic of the AC surface is determined by the surface functional groups and the delocalized electron of the carbon structure [122]. Acidic surface groups are related to oxygen-containing functionalities, such as carboxylic, lactonic, and phenolic groups, which are unstable and can be obtained by the exposure of carbon to oxygen at high temperature or by the reaction of carbon with oxidation agents at room temperature [123]. A basic surface feature originates from the delocalized π-electrons of carbon structure or basic surface functionalities such as nitrogen-containing groups [124]. Thus, one efficient way to increase the number of basic surface groups of AC is to either remove/neutralize the acidic functional groups, or to replace acidic groups with basic groups. To properly tailor the surface chemistry of the activated biochar, surface modification is achieved by thermal or chemical treatments.
Thermal treatment refers to the heat treatment of AC at high temperature (700-1000 • C) in hydrogen (H 2 ) or inert atmosphere via conventional or microwave heating. The main purpose of thermal treatment is to selectively decompose or remove some oxygen-containing surface functionalities to achieve basic characteristics [125]. Thermal treatment is completed in H 2 or an inert atmosphere (i.e., nitrogen (N 2 ) or helium (He)) to yield oxygen-free surface [26,124]. H 2 is very effective in the removal or decomposition of the oxygen functionalities, the stabilization of the carbon surface by deactivating the active sites via the formation of carbon-hydrogen (C-H) bonds, and the gasification of unstable and reactive carbon atoms [124].
Chemical surface modification of AC can be achieved by the addition of chemical agents (i.e., nitric acid (HNO 3 ), hydrogen peroxide(H 2 O 2 ), sulfuric acid (H 2 SO 4 ) [126,127], ammonium peroxydisulfate, melamine [128] and ammonia (NH 3 ) [129], etc.) to introduce acidic (oxygen-containing) or basic (nitrogen-containing) functional groups. For instance, carbon atoms on the AC surface can be partially oxidized by HNO 3 to yield oxygen-containing functional groups such as hydroxyl (-OH), and carboxyl (-COOH) groups [126]. These functional groups further participate in redox reactions to offer extra pseudocapacitance. They also boost the polarity and hydrophilicity of AC, which contribute to the enhanced EC performance of the AC-based electrode. However, excessive oxygen-containing surface groups can hinder the organic electrolyte diffusion into the pores and deteriorate the capacitive performance of the AC electrode [126]. Nitrogen-rich (graphitic-, pyridinic-, pyrrolic-and pyridinic-N) AC showed better EC performance than the oxygen-rich one. In general, pyridinic-N and graphitic-N facilitate the acceleration of the electron transfer, while electrochemically active and electron-rich nitrogens (pyridinic-N and pyrrolic-N) provide pseudocapacitance effects [128,130,131].
In summary, heteroatoms determine the chemical properties of AC, and influence the EC performance of the assembled supercapacitors [132]. In particular, heteroatoms provide additional pseudocapacitance, increase electrical conductivity, promote charge transfer and alter surface wettability to facilitate the electrode/electrolyte interface reactions. Lignocellulose-derived AC inherits the heteroatoms from the precursor materials in the chemical structure after controlled carbonization. Also, these atoms can be introduced into AC by heteroatom-containing dopants during the activation process [17].

Electrochemical (EC) Performance of Lignocellulose-Derived Carbon-Based Flexible Supercapacitors
The heteroatoms that originate from the chemical structure of lignocellulosic biomass precursor materials, together with 3D hierarchically porous structure in lignocellulosederived AC, often result in the enhancement of capacitance performance of supercapacitors. In this section, EC performance of flexible supercapacitors with cellulose-, lignin-and readily available lignocellulose-based carbon electrodes will be discussed.

Flexible Supercapcitors from Cellulose-Derived Carbon
Cellulose plays an indispensable role in the preparation of active electrode materials of flexible supercapacitors due to good flexibility, light weight, low cost and sustainability [133,134]. Table 3 summarizes different aspects of cellulose-based carbon for flexible supercapacitors including raw material based on cellulose or its derivatives, thermal treatment condition, pore characteristics and EC performance, which will be introduced in this section.
One-dimensional (1D) cellulose fiber-based carbon electrode can be achieved by wetspinning followed by carbonization. However, literature in this area is quite limited, which is possibly due to the high cost of cellulose fiber spinning. Nitrogen-doped, cellulose nanofiber-based flexible carbon mesh was prepared by the computerized numerical control of wet-spinning, and the carbonization of cellulose/graphene oxide (GO)/silk fibroin (mass ratio of 55/40/5) mixture [135]. The resulting carbon mesh had wrinkled morphology of the carbon fiber structural unit, which was oriented along the axial direction. After nitrogen-doping, the interconnected mash structure facilitated the electron movement, increased the transport path of electrons, offered extra pseudocapacitance and large SSA, thereby significantly enhancing the EC performance of the carbon mesh electrode-based flexible symmetric supercapacitor: maximum areal specific capacitance of~4.5 mF/cm 2 at a current density of 2.5 µA/m 2 , and excellent folding endurance with capacitance retention up to 152% after 50,000 folding times.
Carbon nanofibers obtained by carbonization of partially hydrolyzed cellulose acetate nanofibers showed great potential as electrode materials to fabricate flexible supercapacitors with high specific capacitance of~241.4 F/g at 1 A/g [136]. The significant EC performance is ascribed to the inter-bonded fibrous structure and porous nanofiber surface. The relatively high SSA, appropriate pore distribution, good electric conductivity, short ion diffusion path, and fast charge transfer rate within the electrode during charge/discharge also contribute to the EC performance. Carbonized cellulose paper composing of carbon fibers was synthesized via nickel (Ni) decoration and subsequent carbonization. The freestanding Ni-decorated carbon paper demonstrated high specific capacitance of 268 F/g at 0.2 A/g [137]. Thus, flexible, highly conductive 2D cellulose-based electrodes with good EC performance are promising for the fabrication of high-performance flexible supercapacitors.
Cellulose fibers have been widely used in the manufacturing of textile fabrics. As mentioned in Section 3.2, inherently stretchable and conductive fabric-based supercapacitors were fabricated from cellulose based Modal raw material by weft-knitting technology, followed by carbonization [58]. The assembled stretchable supercapacitor demonstrated stable EDLC behavior and unique mechanical-electrical properties ( Figure 9). The EC performance was quite mediocre and it could be optimized by employing higher carbonization temperature to decrease the internal resistance of carbonized fabric or by increasing the SSA in the carbonization process.
A/g. Its highly porous and 3D interconnected structure results in better conductivity and ion transportation [146]. Moreover, all-nanofiber asymmetric supercapacitors with nanocellulose-derived hierarchically porous carbon anode (SSA of 2,046 m 2 /g), a mesoporous nanocellulose membrane separator, and a nickel cobaltite (NiCo2O4) cathode with nanocellulose carbon as the support matrix were fabricated. The supercapacitors with 3D porous structure had capacitance of 64.83 F/g at 0.25 A/g, and energy density of 23.05 Wh/kg at 213 W/kg [147]. Carbon aerogels based on wood-derived cellulose nanofibers have been synthesized by pyrolysis with p-toluene sulfonic acid (TsOH) catalyst. The electrode had an electrically conductive 3D nanofibrous network structure, and a large SSA (689 m 2 /g), thereby a high specific capacitance of 140 F/g at 0.5 A/g [148]. Nitrogen-doped carbon nanofibers/graphene aerogels were prepared from cellulose nanofibers/graphene oxide (GO)/ethylenediamine mixture by freeze-drying and carbonization. The device exhibited a capacitance of 289 F/g at 5 mV/s and a capacitance retention of 90.5% [149]. It is believed that the porous structure of the carbon aerogel with high porosity offers short ion transfer pathways during charge/discharge. In addition, the good electrical conductivity and wettability due to nitrogen-doping facilitate the electron transport and electrolyte permeation, further contributing to the enhanced EC performance [149]. Elastic carbon aerogel with cellulose nanofibers Carboxymethyl cellulose (CMC) was coated on cotton fabric, and later a designed pattern of carbonized CMC was obtained with laser direct writing process. Metal oxide (M x O y )/carbonized CMC electrodes on the substrate fabrics were separated by a layer of PVA/H 3 PO 4 electrolyte to form supercapacitors with a capacitance of 1.095 mF/cm 2 , high flexibility and mechanical durability [138]. However, the durability of coated material, and the adhesion between active electrode and substrate fabric remain to be further investigated. To achieve good adhesion, cellulose textile fabric integrated with polydopamine was obtained by in situ polymerization of dopamine on the fabric [139]. The carbonized fabric surface was covered by interconnected polydopamine nanoparticles that formed a 3D porous network. The porous and interconnected structure offered high SSA (347.6 m 2 /g) and facilitated fast ion transportation. As a result, the all-solid-state supercapacitor based on the flexible fabric electrode had excellent capacitance of 1208 mF/cm 2 , as well as excellent flexibility.
Cellulose-based 3D carbon aerogels are also promising electrode materials for flexible supercapacitors due to their versatile properties (i.e., sustainability, high porosity, good flexibility, large SSA, high electrical conductivity and 3D porous structure) [140][141][142][143]. For example, they could be prepared by dissolving microcrystalline cellulose in NaOH aqueous solution, followed by gelation, regeneration, and supercritical CO 2 drying [144]. After pyrolysis and subsequent CO 2 activation, cellulose-based activated carbon aerogels exhibited a high SSA of 1873 m 2 /g and a large pore volume of 2.65 cm 3 /g. The flexible supercapacitors possessed promising performance: specific capacitance values of 302 F/g and 205 F/g at 0.5 A/g and 20 A/g, respectively, and capacitance retention of 92% after 4000 cycles of charge/discharge. Similarly, Zhou et al. [145] synthesized hierarchically porous cellulose-based carbon aerogel by dissolving-gelling and freeze-drying processes. After carbonization and CO 2 activation, 3D carbon aerogel with abundant macro-, mesoand micropores displayed high specific capacitance of 328 F/g at 0.5 A/g, and 96% of capacitance retention after 5000 cycles. As mentioned, hierarchical porous structures with micro-, meso-and macropores are effective to achieve fast ion transport, shorten ion transport pathways, and store more electrolyte ions to enhance the electrical double-layer capacitance [61]. KOH-activated carbon aerogel from sodium CMC had a capacitance of 152.6 F/g at 0.5 A/g. Its highly porous and 3D interconnected structure results in better conductivity and ion transportation [146]. Moreover, all-nanofiber asymmetric supercapacitors with nanocellulose-derived hierarchically porous carbon anode (SSA of 2046 m 2 /g), a mesoporous nanocellulose membrane separator, and a nickel cobaltite (NiCo 2 O 4 ) cathode with nanocellulose carbon as the support matrix were fabricated. The supercapacitors with 3D porous structure had capacitance of 64.83 F/g at 0.25 A/g, and energy density of 23.05 Wh/kg at 213 W/kg [147].
Carbon aerogels based on wood-derived cellulose nanofibers have been synthesized by pyrolysis with p-toluene sulfonic acid (TsOH) catalyst. The electrode had an electrically conductive 3D nanofibrous network structure, and a large SSA (689 m 2 /g), thereby a high specific capacitance of 140 F/g at 0.5 A/g [148]. Nitrogen-doped carbon nanofibers/graphene aerogels were prepared from cellulose nanofibers/graphene oxide (GO)/ethylenediamine mixture by freeze-drying and carbonization. The device exhibited a capacitance of 289 F/g at 5 mV/s and a capacitance retention of 90.5% [149]. It is believed that the porous structure of the carbon aerogel with high porosity offers short ion transfer pathways during charge/discharge. In addition, the good electrical conductivity and wettability due to nitrogen-doping facilitate the electron transport and electrolyte permeation, further contributing to the enhanced EC performance [149]. Elastic carbon aerogel with cellulose nanofibers and lignin was fabricated as a flexible electrode by freezing-casting, vacuum freeze-drying, and carbonization [62]. The highly entangled flexible cellulose nanofibers easily formed an interconnected framework, whereas lignin provided sufficient mechanical stiffness. The flexible and free-standing all-solid-state symmetric supercapacitor was assembled with two cellulose/lignin aerogel electrodes, a PVA/H 2 SO 4 gel electrolyte and a cellulose paper separator in a sandwich structure. The supercapacitor had good mechanical flexibility and mediocre EC performance (specific capacitance of 62.4 F/g at 0.5 A/g, rate capability of 58.5% from 0.5 A/g to 4.0 A/g and energy density of 8.6 Wh/kg at power density of 250 W/kg) ( Table 3). Despite high SSA (1124 m 2 /g), the tracheid-like structure of the carbon aerogels ( Figure 6) demonstrated dominant large pores, and insufficient microand mesopores, which resulted in mediocre EC performance.
Apart from the aforementioned cellulose products used for carbon electrodes of flexible supercapacitors, commonly used carbon precursors of flexible supercapacitors include cellulose derivatives such as cellulose esters, cellulose ethers and cellulose ether esters. These are functional polymers obtained by the esterification or etherification of cellulose hydroxyl functional groups with chemical reagents [150]. Cellulose derivatives are mainly employed as substrates for flexible supercapacitors. Studies of active materials for flexible/wearable energy storage devices based on these cellulose derivatives are still at an initial stage. For instance, isophorone diisocyanate (IPDI) was used as a chemical modifier to effectively connect lignin and cellulose acetate by covalent bond (Figure 10) for better fiber spinnability [151]. The biomass-based carbon fibers had enhanced mechanical properties (maximum tensile strength of 49 MPa, Young's modulus of 3 GPa), as well as good EC performance (later presented in Section 5.2, Table 4). Cellulose acetate was mixed with phosphatized lignin to fabricate an electrospun biomass-derived carbon electrode [152,153]. The optimal specific capacitance of biomass-derived carbon nanofibers as a three-electrode system was 363.1 F/g. At a power density of 800 W/kg, the biomass-based supercapacitor device delivered an energy density of 31.2 Wh/kg. The electrodes exhibited great potential for a flexible free-standing supercapacitor with capacitance retention of 98% after 10,000 cycles. It is believed that molecular spatial conformation of lignin plays a crucial role in improving the properties of biomass-based carbon electrodes. Thus, it is important to understand the relationship between the structure/functionality of lignocellulose, the lignocellulose-derived carbon electrode and the EC performance of supercapacitor.

Flexible Supercapcitors from Lignin-Derived Carbon
Lignin-derived carbon fibers possess high SSA, short diffusion paths, and a rich porous structure that facilitate faster electron conduction and more efficient electrolyte infiltration in flexible supercapacitors. Thus, lignin is a promising carbon precursor to yield microporous, oxygen-functionalized activated carbon fibers (ACFs) [154] or carbon nanofibers (CNFs) electrodes with good EC performance [155][156][157][158][159][160]. Electrospun Kraft lignin-based ACFs were prepared by low temperature stabilization (250 °C), carbonization (900 °C for 1 h in N2), and CO2 activation (800 °C for 1 h). ACFs electrode had specific gravimetric capacitance of 155 F/g at 0.1 A/g, excellent rate capability with 113 F/g at 250 A/g and good capacitance retention of 94% after 6000 cycles [155]. Similarly, lignin-based ACFs were fabricated with hexamine in spinning solution acting as crosslinker to accelerate the thermostabilization process [156]. The resulting ACFs electrode possessed good capacitance (133.3 F/g at a current density of 1 A/g), and high energy density and power density (42 Wh/kg and 91 kW/kg, respectively) due to large SSA (2185 m 2 /g) and porous structure. Moreover, the activation of electrospun lignin-based mats with H3PO4 in the presence of oxygen gas (O2) boosted SSA up to 2340 m 2 /g. The resulting ACFs electrode had an optimal specific capacitance of 240 F/g in 1 M H2SO4, and the assembled symmetric supercapacitors had specific capacitance of 48 F/g at 2 A/g, energy density of 8.4 Wh/kg, maximum power density of 47 kW/kg [157]. Hardwood and softwood lignins were used to develop different porosities after crosslinking and degradation upon high temperature treatment due to the difference in side-chain structure, which in turn influenced the EC performance of ligninbased CNFs in flexible supercapacitors. Hardwood lignin showed both a higher degree of crosslinking and degradation, hence developing higher SSA (~1600 m 2 /g) than that of softwood lignin (~1300 m 2 /g) Thus, the best performance of a free-standing electrode in supercapacitors was observed by hardwood lignin stabilized at 310 °C with specific capacitance of 164 F/g at 0.1 A/g and 119 F/g at 250 A/g, respectively, and a capacitance retention of >90% after 10,000 cycles [158]. In summary, the aforementioned reports have shown that cellulose-derived carbon is a promising material towards green and high-performance flexible SCs with facile preparation process and hierarchical porous structures.

Flexible Supercapcitors from Lignin-Derived Carbon
Lignin-derived carbon fibers possess high SSA, short diffusion paths, and a rich porous structure that facilitate faster electron conduction and more efficient electrolyte infiltration in flexible supercapacitors. Thus, lignin is a promising carbon precursor to yield microporous, oxygen-functionalized activated carbon fibers (ACFs) [154] or carbon nanofibers (CNFs) electrodes with good EC performance [155][156][157][158][159][160]. Electrospun Kraft lignin-based ACFs were prepared by low temperature stabilization (250 • C), carbonization (900 • C for 1 h in N 2 ), and CO 2 activation (800 • C for 1 h). ACFs electrode had specific gravimetric capacitance of 155 F/g at 0.1 A/g, excellent rate capability with 113 F/g at 250 A/g and good capacitance retention of 94% after 6000 cycles [155]. Similarly, ligninbased ACFs were fabricated with hexamine in spinning solution acting as crosslinker to accelerate the thermostabilization process [156]. The resulting ACFs electrode possessed good capacitance (133.3 F/g at a current density of 1 A/g), and high energy density and power density (42 Wh/kg and 91 kW/kg, respectively) due to large SSA (2185 m 2 /g) and porous structure. Moreover, the activation of electrospun lignin-based mats with H 3 PO 4 in the presence of oxygen gas (O 2 ) boosted SSA up to 2340 m 2 /g. The resulting ACFs electrode had an optimal specific capacitance of 240 F/g in 1 M H 2 SO 4 , and the assembled symmetric supercapacitors had specific capacitance of 48 F/g at 2 A/g, energy density of 8.4 Wh/kg, maximum power density of 47 kW/kg [157]. Hardwood and softwood lignins were used to develop different porosities after crosslinking and degradation upon high temperature treatment due to the difference in side-chain structure, which in turn influenced the EC performance of lignin-based CNFs in flexible supercapacitors. Hardwood lignin showed both a higher degree of crosslinking and degradation, hence developing higher SSA (~1600 m 2 /g) than that of softwood lignin (~1300 m 2 /g) Thus, the best performance of a free-standing electrode in supercapacitors was observed by hardwood lignin stabilized at 310 • C with specific capacitance of 164 F/g at 0.1 A/g and 119 F/g at 250 A/g, respectively, and a capacitance retention of >90% after 10,000 cycles [158].   Lignin/PMMA (1/9, 3/7, 5/5, 7/3 and 9/1) in DMF as core; 9 wt.% SnCl 2 ·2H 2 O/PVP in DMF as shell [190] Stabilization: 300 • C, However, it is typically quite difficult to use lignin as an independent raw material to yield biomass-derived carbon fibers primarily due to its low molecular weight, 3D amorphous molecular structure and poor flexibility. Alternatively, lignin-based carbon fibers were fabricated by blending it with other polymers for good spinnability, thermal stability and excellent flexibility. The commonly used blending polymers include polyacrylonitrile (PAN), PVA, and polyethylene oxide (PEO), etc., which are summarized in Table 4.
One of the most common binder polymers for lignin-based CNFs is PAN. It has good spinnability, linear structure, high molecular weight, which contribute to good mechanical properties of the resulting fiber mats. Lignin/PAN-based CNFs [161,162,169,170] have been used as flexible supercapacitor electrodes since lignin is effective in increasing SSA, and lowering the average fiber diameter [161]. To further enhance the EC performance, metal compounds, including metal oxides (i.e., MnO 2 [191], NiCo 2 O 4 [163], zinc oxide (ZnO) [166]), metal sulfide [164], metal chloride (i.e., manganese chloride (MnCl 2 ) [165]) were employed as additives in lignin/PAN CNFs electrodes to offer pseudocapacitance.
Another effective way is to introduce heteroatom into lignin/PAN-based CNFs. With the addition of nitrogen/sulfur co-doped graphene [167] or air plasma treatment [168] of CNFs, the increased heteroatom content contributed to enhanced EC performance (344.6 F/g at 1 A/g [168]) due to faradaic reactions.
PVA is also frequently blended with lignin to yield electrospun CNFs with good spinnability for a flexible supercapacitor electrode. Electrospun CNFs from lignin/PVA with various mass ratios have been achieved with large SSA [171][172][173][174]. The decomposition of PVA in high temperature treatment for pore-generation was stressed [174]. It was found that at 75/25 (w/w) lignin/PVA, phase separation occurred in as-spun fibers. Later in the carbonization process, PVA with abundant oxygen atoms, served as a sacrificial material to create microporosity. Apart from the activation for high SSA and porosity [173,174], metal oxide such as MnO 2 has also been used as an additive in lignin/PVA CNFs electrodes to tune the fiber mats' morphology and provide extra pseudocapacitance [175,176]. Interestingly, twisted electrospun lignin/PVA CNFs-based yarns were developed to increase the mechanical and electrical properties [192]. The highest number of twists (78 turn/cm) efficiently decreased the yarn diameter down to 127 µm, and increased the electrical conductivity (21.9 S/cm), and mechanical properties (tensile strength of 0.5 GPa, Modulus of 26 GPa). However, the electrochemical capacitance decreased from 7.73 F/g of the mat to <1 F/g of the highly twisted yarn system. Further investigation of twisted yarn in smart textiles as energy storage will be of great interest since it is possible that yarns are strong enough to endure a textile manufacturing process.
Lignin/PEO was electrospun to yield interconnected fiber structures, which were beneficial for the electron transport and conductive property of the carbon networks. High SSA and porosity after carbonization and activation [178,179,181,182] contribute to the fast ion transfer. Moreover, iron oxide incorporated into electrospun lignin/PEO offered extra pseudocapacitance [180]. These film-based electrodes exhibited an excellent specific capacitance as shown in Table 4.
Lignin-based 2D film flexible electrode has also been fabricated for supercapacitor applications from lignin-derived porous carbon/reduced graphene oxide (rGO) film [194,195]. The optimal heat-treatment temperature was 250 • C. The assembled flexible supercapacitor achieved areal specific capacitance of 324.5 mF/cm 2 at 0.2 mA/cm 2 , and 91.8% capacitance retention after 1000 charge/discharge cycles [195]. Furthermore, MnO 2 has been deposited on lignin-derived porous carbon/rGO film [194]. The highly porous ligninderived porous carbon promoted SSA and electrolyte ions transport. Also, rGO sheets interconnecting with the bio-based carbon provided a conductive bridge for ion transport. The deposited MnO 2 nanoflakes interconnected with each other to form a porous structure with significantly increased SSA. Thus, the composite electrode had a good EC performance with maximum specific capacitance of 1136 mF/cm 2 at a current density of 1 mA/cm 2 . The carbon electrode from molybdenum disulfide (MoS 2 )-decorated lignin/PAN film by laser direct writing had enhanced areal capacitances up to 16 mF/cm 2 (2.2 F/cm 3 ) and at 10 mV/s [196]. Chemically modified lignin with sulfonic groups was transformed into porous lignin-based carbon with large SSA (3149 m 2 /g) after carbonization and chemical activation [197]. The heteroatoms and the graphitic structure had a profound impact on the EC performance. Thus, the flexible symmetric supercapacitor assembled by this electrode demonstrated capacitance of~140 F/g at 0.5 A/g, high energy density of 5.41 Wh/kg at power density of 0.5 kW/kg.
Lignin-derived carbon aerogels were obtained by ultrafast freezing of lignin/KOH solution droplets, followed by freeze-drying and in situ activation during carbonization at 900 • C [198]. The resulting carbon aerogels had hierarchically porous structure (corresponding to a high SSA of 1681.6 m 2 /g) that facilitated ion diffusion and charge transport. Moreover, the abundant oxygenated groups provided additional pseudocapacitance that boosted the EC performance of supercapacitor electrodes: a high specific capacitance of 189 F/g at 1 A/g, a high energy density of 26.25 Wh/kg at a power density of 1000 W/kg, and a capacitance retention of 97.4% after 10,000 cycles.
Three-dimensional (3D) lignin/cellulose-derived carbon aerogel electrode was pressed into film to fabricate symmetrical flexible supercapacitors for the application of wearable and portable energy storage devices ( Figure 11) [199]. The devices revealed capacitance behavior and a stable power delivery at different bending angles (Figure 11c,d): a superior energy density of 102 Wh/kg at power density of 175 W/kg, and good capacitance retention (>99%). Kraft and soda lignins with cellulose nanofibers were converted to carbon aerogel. It was found that Kraft lignin-based carbon aerogel had better specific capacitance of 163 F/g and energy density of 5.67 Wh/kg at a power density of 50 W/kg when assembled into a two-electrode symmetrical supercapacitor due to higher SSA and hierarchically porous structure [200].
pacitance of ~140 F/g at 0.5 A/g, high energy density of 5.41 Wh/kg at power density of 0.5 kW/kg. Lignin-derived carbon aerogels were obtained by ultrafast freezing of lignin/KOH solution droplets, followed by freeze-drying and in situ activation during carbonization at 900 °C [198]. The resulting carbon aerogels had hierarchically porous structure (corresponding to a high SSA of 1681.6 m 2 /g) that facilitated ion diffusion and charge transport. Moreover, the abundant oxygenated groups provided additional pseudocapacitance that boosted the EC performance of supercapacitor electrodes: a high specific capacitance of 189 F/g at 1 A/g, a high energy density of 26.25 Wh/kg at a power density of 1000 W/kg, and a capacitance retention of 97.4% after 10,000 cycles.
Three-dimensional (3D) lignin/cellulose-derived carbon aerogel electrode was pressed into film to fabricate symmetrical flexible supercapacitors for the application of wearable and portable energy storage devices ( Figure 11) [199]. The devices revealed capacitance behavior and a stable power delivery at different bending angles (Figure 11c,d): a superior energy density of 102 Wh/kg at power density of 175 W/kg, and good capacitance retention (>99%). Kraft and soda lignins with cellulose nanofibers were converted to carbon aerogel. It was found that Kraft lignin-based carbon aerogel had better specific capacitance of 163 F/g and energy density of 5.67 Wh/kg at a power density of 50 W/kg when assembled into a two-electrode symmetrical supercapacitor due to higher SSA and hierarchically porous structure [200].
In summary, lignin-based carbon materials in fiber, nanofiber membrane and carbon aerogel configuration for high-performance flexible SC electrodes are potential high-value products for smart textiles.

Flexible Supercapcitors from Raw Lignocellulose-Derived Carbon
Flexible supercapacitors assembled from raw lignocellulose-derived carbon electrodes have indicated a facile way to fabricate low-cost wearable electronics for smart textiles. A large number of research articles regarding raw lignocellulose-derived AC electrodes with In summary, lignin-based carbon materials in fiber, nanofiber membrane and carbon aerogel configuration for high-performance flexible SC electrodes are potential high-value products for smart textiles.

Flexible Supercapcitors from Raw Lignocellulose-Derived Carbon
Flexible supercapacitors assembled from raw lignocellulose-derived carbon electrodes have indicated a facile way to fabricate low-cost wearable electronics for smart textiles. A large number of research articles regarding raw lignocellulose-derived AC electrodes with considerable EC performance have been reported [24,201]. Thus, it is possible to convert lignocellulosic raw materials into carbon electrodes of flexible supercapacitor with intrinsic porous structure; however, only a few works in the literature have focused on this aspect. Table 5 concisely summarizes the fabrication and EC performance of raw lignocellulose-derived carbon electrode for flexible supercapacitor applications.
AC derived from straw possessed nitrogen atoms and high SSA up to 1923 m 2 /g [203], which were beneficial for better EC performance. The resulting AC/GO film had a dense structure of 1.23 g/cm 3 and good conductivity. The assembled flexible solid-state supercapacitor device had a volumetric capacitance of 326 F/cm 3 at 0.5 A/g. Cane stalk [204] or sugarcane bagasse [205], porous lettuce slice [206], corncob sponge [207], soybean pod [208], kenaf stem [209], jute [210,211], fruit shell [212], and cotton [213] have also been directly converted to carbon film electrodes for flexible supercapacitors, which are summarized in Table 5. They all demonstrated good pore characteristics, which correspond to the excellent EC performance of flexible supercapacitors.
Moreover, the surface chemistry of heteroatom-doping has been applied to enhance the EC performance of raw lignocellulose-derived flexible supercapacitor carbon electrodes. For example, nitrogen-doped AC cloth was achieved by ammonia treatment of commercial cotton fabrics at 1000 • C [214]. The resulting flexible supercapacitors assembled by this hierarchically porous carbon cloth had a high specific capacitance (up to 215.9 F/g at 1 A/g), high rate capability (89% from 1 A/g to 200 A/g), and excellent cycling stability (98% capacitance retention over 20,000 cycles). Oxygen/nitrogen-doped porous carbon based on cotton were prepared by the immersion of cotton in diammonium hydrogen phosphate (DAP), subsequent activation at 800 • C and oxidation in 300-400 • C [215]. High SSA (1022 m 2 /g) and relatively high pore volume (0.53 cm 3 /g) were obtained, and optimal electrodes oxidized at 350 • C had high specific capacitance of 292 F/g at a current density of 0.5 A/g in a three-electrode system. The symmetrical supercapacitor had a high stability with 87% of capacitance retained after 5000 cycles at 5 A/g, as well as a high volumetric energy density (18 Wh/kg at 250 W/kg).
In summary, flexible carbon electrodes from raw lignocellulosic biomass with high SSA, and hierarchically porous structures are promising low-cost bio-based electrodes for flexible supercapacitors. Carbonization, activation and heteroatom-doping greatly influence the surface chemistry and microstructures of electrodes, and enhance the EC performance of the resulting flexible supercapacitors.

Supercapacitors from Lignocellulose-Based Graphite
A high level of graphitization of lignocellulosic biomass-derived carbon is preferred to achieve high electrical conductivity, low internal resistance of electrode materials and fast charge transfer, and thereby enhanced EC performance. Lignocellulosic graphite can be obtained by high temperature treatment [216], which increases sp 2 carbon atoms in the heating process. However, excessively high temperatures may cause the collapse of pores, which reduces SSA, pore volume, and surface functional groups, negatively affecting the EC performance of the electrodes [28].
The preferred method to obtain highly graphitized carbon from lignocellulose is catalytic graphitization during activation with different catalysts such as ferric chloride (FeCl 3 ) [217,218], ferric nitrate (Fe(NO 3 ) 3 ) [219,220], potassium ferrate (K 2 FeO 4 ) [221,222], potassium carbonate (K 2 CO 3 ) [223], and nickel chloride (NiCl 2 ) [224,225], etc. Table 6 summarizes some examples of supercapacitors based on lignocellulose-derived graphitic carbon including the raw material, thermal treatment, catalyst, structural characteristics and EC performance, etc. For instance, graphitic bamboo char was achieved at 800 • C, with K 2 FeO 4 acting both as activator and graphitization catalyst. The resulting electrode had a porous structure with SSA of 1732 m 2 /g, and good electrical conductivity of 4.7 S/cm. These characteristics significantly contributed to the enhanced EC performance [221]. Similarly, graphite from coconut shells was obtained with K 2 CO 3 acting as activator/graphitization catalyst simultaneously at 900 • C [223]. The resulting graphene-like sheets had high SSA (1506 m 2 /g), and excellent electrical conductivity (32.14 S/cm) due to high degree of graphitization with an ordered structure. Potassium in the catalyst catalyzes the crystallization of amorphous carbon, and forms charge transfer complexes with aromatic hydrocarbons to achieve catalytic graphitization with increased sp 2 hybridized valences.
Graphitic carbons with a hierarchical porous structure often have elevated ion/electrons conductivity, and thereby excellent EC performance. For example, hierarchically porous graphitic carbon derived from rice straw with interconnected pores had significant SSA of 3333 m 2 /g, abundant micro-/mesopores contributing to high specific capacitance (400 F/g), as well as good cycling stability [226]. Porous graphitic carbon from willow catkins was obtained at 900 • C with potassium hexacyanoferrate (II) (K 4 Fe(CN) 6 ) as activator/graphitization catalyst. The resulting material had a high graphitization degree (I D /I G 0.82) and SSA (1067 m 2 /g), which were much better than those of willow catkins-derived carbon without K 4 Fe(CN) 6 (I D /I G~0 .87 and 31.54 m 2 /g, respectively) [227].
To further increase the EC performance of lignocellulose-derived graphitic carbon, heteroatom-doping can be used. For instance, one-step carbonization and graphitization of lotus leaf by FeCl 3 , together with heteroatom-doping resulted in supercapacitors with high specific capacitance of 385 F/g at 0.5 A/g [218]. This was mainly due to the high SSA, high level of graphitization (I D /I G~1 .04), good electrical conductivity (8.7 S/cm) and extra psuedocapacitance by heteroatom-doping. Similarly, nitrogen-doped porous graphite carbon from peach gum [225] obtained at 700 • C by NiCl 2 had a high specific capacitance of 426 F/g at 0.5 A/g.

Future Direction
Due to the inherent structural properties of lignocellulose-based materials, it is possible to utilize a simple, scalable heat-treatment process to obtain bio-based carbon/graphite electrodes for flexible/wearable supercapacitors with different configurations. Even though lignocellulose is quite cost effective, a flexible fiber/yarn-shaped carbon electrode with high SSA, pore volume and sufficient mechanical performance for textile manufacturing process has rarely been demonstrated. Future advances towards lignocellulose-derived carbon electrode for flexible supercapacitors are listed below.

1.
A green manufacturing process and product safety of flexible supercapacitors should be addressed. As mentioned, physical and chemical activations of lignocellulosederived carbon are common methods for the preparation of porous carbon electrodes with preferably microporous structures. However, energy conservation and hazards should be considered. Chemical activation reagent, such as KOH, reacts with the carbon matrix at high temperatures and generates a large number of volatile gases, as well as metallic potassium. This may cause hazard corrosion issues. Also, due to the corrosive nature of most of the electrolytes (i.e., H 2 SO 4 , KOH) used in flexible supercapacitors, strict packaging is required before the final application of flexible supercapacitors in smart textiles. This may cause discomfort, laundering difficulties and electrolyte leaking problems in smart textile applications. Thus, greener activation strategies and environmentally friendly electrolyte should be developed to promote scalable production of porous biomass-derived carbon for supercapacitor applications.

2.
Flexible lignocellulose-derived carbon electrode is mainly achieved by film, electrospun mats, aerogel or existing cloth. It is still rare that inherently functional, continuous lignocellulose-derived carbon fibers/yarn are employed in supercapacitor applications possibly due to the difficulty of fiber spinning/yarn formation. Typically, continuous fibers are achieved by textile fiber spinning techniques, which prefer dense fiber structure with no pores. This contradicts the need for the formation of hierarchically porous electrode with high EC performance. Thus, fiber spinning/yarn formation processes or surface chemistry should be more systematically investigated to form continuous fiber/yarn with sufficient pores and mechanical strength.

3.
The enhancement of the mechanical performance of flexible supercapacitors is still an issue to be addressed. The vulnerability to withstand an intense textile manufacturing process such as weaving or knitting is still an obstacle that hinders the wide application of flexible supercapacitors in practical smart textiles. To solve this problem, the mechanical performance of fiber-, yarn-or fabric-based lignocellulose-derived carbon/graphite electrodes should be significantly investigated and enhanced. A comprehensive understanding of the structure-property relationships of lignocellulose, lignocellulose-derived carbon/graphite and the resulting fiber-, yarn-or fabric-based flexible supercapacitors is required. In particular, the revelation of the relationships of a material's molecular arrangement, micro-to macro-structure and mechanical properties, will be beneficial for an expanded application of biopolymers in flexible energy storage systems.
With the development of a green manufacturing process, along with progress of inherently functional continuous fiber/yarn formation, and enhancement of the mechanical performance of lignocellulosic carbon/graphite electrode, it is very promising that lignocellulosic stock will be a renewable resource for the manufacture of low-cost flexible supercapacitor carbon electrodes for smart textile applications.

Data Availability Statement:
No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest:
The authors declare no conflict of interest.