Lignin-Derived Quinone Redox Moieties for Bio-Based Supercapacitors

Because of their rapid charging and discharging, high power densities, and excellent cycling life stabilities, supercapacitors have great potential for use in electric vehicles, portable electronics, and for grid frequency modulation. The growing need for supercapacitors that are both efficient and ecologically friendly has generated curiosity in developing sustainable biomass-based electrode materials and electrolytes. Lignin, an aromatic polymer with remarkable electroactive redox characteristics and a large number of active functional groups, is one such candidate for use in renewable supercapacitors. Because its chemical structure features an abundance of quinone groups, lignin undergoes various surface redox processes, storing and releasing both electrons and protons. Accordingly, lignin and its derivatives have been tested as electroactive materials in supercapacitors. This review discusses recent examples of supercapacitors incorporating electrode materials and electrolytes derived from lignin, focusing on the pseudocapacitance provided by the quinone moieties, with the goal of encouraging the use of lignin as a raw material for high-value applications. Employing lignin and its derivatives as active materials in supercapacitor electrodes and as a redox additive in electrolytes has the potential to minimize environmental pollution and energy scarcity while also providing economic benefits.


Introduction
Our industrial development and the rising human population are necessitating a hunt for environmentally friendly and long-term energy sources [1,2]. Although it is possible to harvest energy from the wind, sun, and water, these renewable sources generate energy in an intermittent manner [3], making energy storage increasingly important [4]. Electrochemical supercapacitors are attractive energy storage systems because of their high power densities, long cycle lives, and high efficiencies [5], but they generally have energy densities lower than those of batteries and fuel cells [6]. Therefore, a great deal of work has been done to enhance the energy densities of supercapacitors while preserving their high power densities. Supercapacitors are classified in terms of their charge storage mechanisms, mainly into electrochemical double-layer capacitors (EDLCs), pseudocapacitors, and hybrid capacitors. In an EDLC, charge storage occurs through electrostatic adsorption and desorption of electrolyte ions at the interface between the electrode and electrolyte [7]. Pseudocapacitive charge storage involves rapid and reversible Faradaic charge transfer on or near the surfaces of the electrode materials [8]. The Faradaic redox reactions occur on the surfaces of the solid electrodes and in the bulk near those surfaces; therefore, the specific capacitances and energy densities of pseudocapacitors are much larger than those of EDLCs. The redox reactions, although rapid, are limited by the rates of diffusion and the degrees of penetration of the electrolyte (cf. rapid ionic adsorption in EDLCs). Moreover, similar to batteries, pseudocapacitors often lack stability during cycling because of the

Fundamentals of Lignin
Lignocellulose is an abundant bio-resource, consisting primarily of cellulose (40-50%), hemicellulose (20-30%), and lignin (25-35%) [12]. Lignin, a major component of vascular plants, is chemically intertwined with hemicellulose and wrapped around the exterior of cellulose fibers. Lignin is the most abundant non-carbohydrate component in lignocellulosic biomass, providing plant cell walls with strength and hydrophobicity while preserving polysaccharides from microbial decomposition [16]. As a result of the many structural units, linkages, and complicated connections between lignin and glycan in the cell wall, lignin is one of the most complex natural polymers in nature.
The lignin polymer contains three structural units: syringl (S), guaiacol (G), and p-hydroxyphenyl (H) moieties ( Figure 1) [13]. Because these different monolignols are connected in a three-dimensional network through multiple linkages, making complexities in structural elucidation structural analysis of lignin is much more difficult than that of other polymers [17,18]. The structure of lignin is more akin to a "chemical web" [19], lacking a clearly defined primary structure. The plant species (e.g., softwood, hardwood, or grass), its growth environment, and its various components (e.g., stalks, leaves, or seeds) all feature lignin at different levels of content and with different structural compositions, molecular weights, and interunit linkages. In general, coniferous species (softwoods) have lignin contents (25-35%) greater than those of deciduous species (hardwoods, 15-30%) [20]. Softwood lignin features predominantly G-type units (90-95 wt.%), whereas hardwood lignin features mostly G-and S-type units [15]; all three monolignol forms may be found in grass lignin. Furthermore, different pretreatment conditions can impact the amounts and types of monolignols, making the S:G:H ratio a helpful indicator during the lignin separation process. Lignin is divided into two types based on its origin: native or technical. Native lignin refers to the original structure of lignin in lignocellulose that has not been altered. Extraction of lignin from lignocellulose during the pulping processes changes its chemical structure and introduces inorganic contaminants; technical lignin is the name given to this kind of extracted lignin [22]. Kraft lignin (KL), organosolv lignin, enzymatic lignin, hydrolysis lignin, and pyrolytic lignin are common examples of technical lignins [23]. Because they undergo combinations of several reactions, including catalyzed biomass hydrolysis, condensation of lignin fragments, reduction of native lignin functional groups, and generation of new functional groups, technical lignins vary markedly from their native counterparts. Chemical pulping, which cleaves chemical bonds between lignins and carbohydrates at high temperatures and pressures, is currently used to produce the majority of technical lignins. The pulp and paper industry primarily uses Kraft, sulfite, soda, and organosolv pulping as a means of separating technical lignin, producing KL, lignosulfonate, soda lignin, and organosolv lignin, respectively [12], each having diverse structures, compositions, and characteristics.
Lignin can also be isolated in integrated lignocellulose biorefineries, using various biomass pretreatment methods (e.g., hydrothermal, dilute acid, and ammonia-based processes) [24]. In the biorefinery approach, the three basic components of lignocellulose (cellulose, hemicellulose, lignin) are extracted separately to achieve total utilization. Lignin is usually considered a byproduct in a biorefinery, due to its irregular structure. Among the various pretreatment methods in a biorefinery, physicochemical processes using steam explosion (SE) or mechanical grinding under high temperatures and pressures have been the most suitable for biomass fractionation, yielding high-purity lignin. Lignin recovery strategies, including lignin-first biomass fractionation procedures, have been progressing as a result of greater knowledge of the properties of lignin and its ever-increasing applications.
All pulping and biorefinery operations lead to the degradation of lignin. The reactions involved always result in a lower number of oxygenated aliphatic moieties, notably β-O-4 and aliphatic OH units; increases in the contents of phenolic OH, COOH, and saturated aliphatic moieties; and greater degrees of condensation (DCs), relative to those of the native lignins [13]. Nevertheless, considerable structural variations exist among the various technical lignins. The extent to which lignin degradation occurs is determined by the nature of the lignin isolation process and its severity.

Application of Lignin in Supercapacitors
It is difficult to develop lignin-based electrodes for energy storage devices because of its inherent insulating properties. As a result, only a few reports have appeared describing the electrochemical characteristics of lignin. Milczarek et al. reported that lignosulfonate Lignin is divided into two types based on its origin: native or technical. Native lignin refers to the original structure of lignin in lignocellulose that has not been altered. Extraction of lignin from lignocellulose during the pulping processes changes its chemical structure and introduces inorganic contaminants; technical lignin is the name given to this kind of extracted lignin [22]. Kraft lignin (KL), organosolv lignin, enzymatic lignin, hydrolysis lignin, and pyrolytic lignin are common examples of technical lignins [23]. Because they undergo combinations of several reactions, including catalyzed biomass hydrolysis, condensation of lignin fragments, reduction of native lignin functional groups, and generation of new functional groups, technical lignins vary markedly from their native counterparts. Chemical pulping, which cleaves chemical bonds between lignins and carbohydrates at high temperatures and pressures, is currently used to produce the majority of technical lignins. The pulp and paper industry primarily uses Kraft, sulfite, soda, and organosolv pulping as a means of separating technical lignin, producing KL, lignosulfonate, soda lignin, and organosolv lignin, respectively [12], each having diverse structures, compositions, and characteristics.
Lignin can also be isolated in integrated lignocellulose biorefineries, using various biomass pretreatment methods (e.g., hydrothermal, dilute acid, and ammonia-based processes) [24]. In the biorefinery approach, the three basic components of lignocellulose (cellulose, hemicellulose, lignin) are extracted separately to achieve total utilization. Lignin is usually considered a byproduct in a biorefinery, due to its irregular structure. Among the various pretreatment methods in a biorefinery, physicochemical processes using steam explosion (SE) or mechanical grinding under high temperatures and pressures have been the most suitable for biomass fractionation, yielding high-purity lignin. Lignin recovery strategies, including lignin-first biomass fractionation procedures, have been progressing as a result of greater knowledge of the properties of lignin and its ever-increasing applications.
All pulping and biorefinery operations lead to the degradation of lignin. The reactions involved always result in a lower number of oxygenated aliphatic moieties, notably β-O-4 and aliphatic OH units; increases in the contents of phenolic OH, COOH, and saturated aliphatic moieties; and greater degrees of condensation (DCs), relative to those of the native lignins [13]. Nevertheless, considerable structural variations exist among the various technical lignins. The extent to which lignin degradation occurs is determined by the nature of the lignin isolation process and its severity.

Application of Lignin in Supercapacitors
It is difficult to develop lignin-based electrodes for energy storage devices because of its inherent insulating properties. As a result, only a few reports have appeared describing the electrochemical characteristics of lignin. Milczarek et al. reported that lignosulfonate could be electropolymerized on a glassy carbon (GC) electrode for energy storage applications [25]. From a sulfuric acid solution, they subjected lignosulfonate to oxidative electropolymerization on a GC electrode to fabricate a chemically modified electrode possessing anionic characteristics and redox activity. Repeated cycling of the lignosulfonate in an acidic electrolyte resulted in the formation of a new chemical species-quinone (Q) units-presumably through oxidation of the monolignols [26]. Because they include methoxyl (OMe) structures, the S and G monolignols can be oxidized to Q structures, but the third monolignol, H, cannot. Thus, lignin derivatives from plants having high contents of S and G units are better candidates for increasing the density of Q units in modified electrodes. To study the influence of the various lignin constituents and the function of lignin for charge storage, Liedel et al. used ball-milling and film-casting on a graphite sheet current collector to fabricate composite electrodes from Kraft lignin (KL) and conductive carbon (167 m 2 g −1 , C L ) with a 10% binder [27]. Cyclic voltammetry (CV) measurements ( Figure 2a) revealed considerable separation of the oxidative and reductive signals, as well as discrete redox signals and a rectangular region, demonstrating the influence of redox and electrochemical double-layer (EDL) charge storage, respectively. Figure 2a roughly displays the individual contributions to charge storage. The reversible redox couple near 0.35 and 0.55 V represents the Q/hydroquinone (QH 2 ) interconversions of various groups in lignin, with the shoulder near 0.35 V representing S groups and the peak near 0.55 V representing G groups. 31 P NMR spectroscopy ( Figure 2b) revealed that the content of S groups (0.48 mmol g −1 ) in KL was much lower than that of G groups (3.29 mmol g −1 ). could be electropolymerized on a glassy carbon (GC) electrode for energy storage applications [25]. From a sulfuric acid solution, they subjected lignosulfonate to oxidative electropolymerization on a GC electrode to fabricate a chemically modified electrode possessing anionic characteristics and redox activity. Repeated cycling of the lignosulfonate in an acidic electrolyte resulted in the formation of a new chemical species-quinone (Q) units-presumably through oxidation of the monolignols [26]. Because they include methoxyl (OMe) structures, the S and G monolignols can be oxidized to Q structures, but the third monolignol, H, cannot. Thus, lignin derivatives from plants having high contents of S and G units are better candidates for increasing the density of Q units in modified electrodes. To study the influence of the various lignin constituents and the function of lignin for charge storage, Liedel et al. used ball-milling and filmcasting on a graphite sheet current collector to fabricate composite electrodes from Kraft lignin (KL) and conductive carbon (167 m 2 g −1 , C L ) with a 10% binder [27]. Cyclic voltammetry (CV) measurements ( Figure 2a) revealed considerable separation of the oxidative and reductive signals, as well as discrete redox signals and a rectangular region, demonstrating the influence of redox and electrochemical double-layer (EDL) charge storage, respectively. Figure 2a roughly displays the individual contributions to charge storage. The reversible redox couple near 0.35 and 0.55 V represents the Q/hydroquinone (QH2) interconversions of various groups in lignin, with the shoulder near 0.35 V representing S groups and the peak near 0.55 V representing G groups. 31 P NMR spectroscopy ( Figure 2b) revealed that the content of S groups (0.48 mmol g −1 ) in KL was much lower than that of G groups (3.29 mmol g −1 ). Admassie et al. reported the electrochemical and supercapacitance performance of various lignin inside biopolymer-based electrodes [28]. They used 31 P NMR spectroscopy to investigate the chemical structures of several softwood and hardwood lignin derivatives from subtropical and tropical plants (eucalyptus, acacia, and African redwood), after phosphitylation of the biopolymers. The specific capacitance and charge capacity both rose significantly upon increasing the S-to-G ratio, suggesting that Q units produced by the S units were the major contributors to the charge storage effect in these biopolymer electrodes.
With improvements in commercial manufacturing, lignin is now widely available and inexpensive, thereby lowering device manufacture costs. The most prevalent element in lignin is carbon, accounting for more than 60% of its weight. Lignin is responsible for over 30% of the biosphere's organic carbon, suggesting its great potential as a carbon precursor. Studies of lignin in energy storage devices have increased considerably. Lignin Admassie et al. reported the electrochemical and supercapacitance performance of various lignin inside biopolymer-based electrodes [28]. They used 31 P NMR spectroscopy to investigate the chemical structures of several softwood and hardwood lignin derivatives from subtropical and tropical plants (eucalyptus, acacia, and African redwood), after phosphitylation of the biopolymers. The specific capacitance and charge capacity both rose significantly upon increasing the S-to-G ratio, suggesting that Q units produced by the S units were the major contributors to the charge storage effect in these biopolymer electrodes.
With improvements in commercial manufacturing, lignin is now widely available and inexpensive, thereby lowering device manufacture costs. The most prevalent element in lignin is carbon, accounting for more than 60% of its weight. Lignin is responsible for over 30% of the biosphere's organic carbon, suggesting its great potential as a carbon precursor. Studies of lignin in energy storage devices have increased considerably. Lignin and lignin-derived materials have several properties that make them attractive components for the manufacture of rechargeable batteries and supercapacitors. Many of the functional groups in lignin (particularly the benzyl and phenolic groups) function as active reaction sites, allowing the storage of ions in the case of supercapacitors. The various forms of lignin possess different functionalities, leading to a range of properties for energy storage. Lignosulfonates, for example, have an abundance of sulfur atoms and have been employed in rechargeable batteries and supercapacitors as sulfur-doped agents. Nevertheless, lignin cannot be used directly for electrodes because it is inherently an insulator [29]. One of the best strategies for using lignin is to develop composites with other conductive materials (e.g., conducting polymers) to increase its electrochemical performance as an electrode material.  [30]. The aromatic units on the lignin species and RGO sheets experienced strong π-π and hydrophobic interactions, forging strong connections in the RGO-lignin (RL) composites. The lignin units separated as discrete nanocrystals and were firmly confined on the surfaces of the RGOs (Figure 3a), a favorable arrangement for electrochemical processes that demand uniformly distributed and discrete electroactive nanomaterials having large surface areas and good interactions with conductive substrates. The noncovalent interactions between the lignin and the RGO sheets did not disrupt the intrinsic structure of the RGO, allowing the transfer of charge from the electron-withdrawing groups of lignin (i.e., sulfonate and carboxylate groups) to the RGO. The CV of the hybrid electrode indicated a reversible redox wave at 0.52 V. XPS spectra recorded before and after charging (Figure 3b,c) revealing the change in the chemical circumstance of the Q moieties, arising from specific interactions with protons generated through the redox reactions of the renewable hybrid electrodes (RHEs). Equation (1) represents the reversible Faradaic reaction occurring in the RL composites:

Lignin-Based
(Lignin-QH 2 )@RGO ⇔ (Lignin-Q)@RGO + 2H + + 2e − where the Q/QH 2 transformation of the RGO-bound lignin could store and release two electrons and protons while charging and discharging, respectively, in aqueous acidic solutions. Thus, two electrons and two protons were stored in a structure containing 6 C atoms and 2 O atoms, giving a charge density of 2 Faraday/108 g, equal to 1787 C g −1 or 496 mA h g −1 [31]. Ye et al. demonstrated that lignin could be employed as a green reductant when preparing RGO through the deoxygenation of GO sheets, and then as a morphological guiding agent in the construction of three-dimensional (3D) RGOderived composites [32]. Because π-π interactions with lignin hindered the aggregation of the RGO sheets, the RGO solution generated in the presence of lignin was more stable than those obtained using common reductants (e.g., ascorbic acid) (Figure 3d,e). The RL composite produced through hydrothermal carbonization had a large specific surface area, high conductivity, high specific capacitance (190 F g −1 at 0.5 A g −1 ), and excellent cycling stability (86.5% retention of capacitance after 10,000 continuous cycles of galvanostatic charging/discharging).
To maximize the benefits of lignin while preventing graphene sheets from aggregating, Yang et al. developed a simple one-step hydrothermal technique for fabricating lignin/graphene hydrogels [29]. Here, the lignin provided strong pseudocapacitance while preventing re-stacking of the graphene sheets. When used directly as a binder-free supercapacitor electrode, the 3D porous composite hydrogel exhibited high specific capacitance (549.5 F g −1 ) at a current density of 1 A g −1 , as well as excellent rate capacity and cycling stability. The synergistic effect of pseudocapacitance (resulting from the reversible redox reactions of lignin) and electrical double-layer capacitance (resulting from the large specific surface area of the graphene hydrogel) contributed to the high specific capacitance of the lignin/graphene hydrogels. To overcome the low cycling efficiency and high selfdischarging rate of lignin/graphene composite-based electrodes, Geng et al. developed a reconfigurable and hierarchical graphene cage-imitating the prey-trapping of a Venus flytrap-to capture lignin [33]. The reconfigurable graphene trapped the lignin within To maximize the benefits of lignin while preventing graphene sheets from aggregating, Yang et al. developed a simple one-step hydrothermal technique for fabricating lignin/graphene hydrogels [29]. Here, the lignin provided strong pseudocapacitance while preventing re-stacking of the graphene sheets. When used directly as a binder-free supercapacitor electrode, the 3D porous composite hydrogel exhibited high specific capacitance (549.5 F g −1 ) at a current density of 1 A g −1 , as well as excellent rate capacity and cycling stability. The synergistic effect of pseudocapacitance (resulting from the reversible redox reactions of lignin) and electrical double-layer capacitance (resulting from the large specific surface area of the graphene hydrogel) contributed to the high specific capacitance of the lignin/graphene hydrogels. To overcome the low cycling efficiency and high self-discharging rate of lignin/graphene composite-based electrodes, Geng et al. developed a reconfigurable and hierarchical graphene cage-imitating the prey-trapping of a Venus flytrap-to capture lignin [33]. The reconfigurable graphene trapped the lignin within the electrode and prevented its dissolution, while also acting as a 3D current collector and providing channels for efficient electron transport throughout the electrochemical process. With its unique bio-inspired design, the graphene-lignin cathode exhibited high capacitance (211 F g −1 at 1.0 A g −1 ) and excellent rate performance and cycle stability (88% retention of capacity after 15,000  [34]. The LS-GHs were sufficiently mechanically sturdy for handling and cutting into slices. Because of the reversibility of the redox-based charge transfer of the Q groups in the lignin, this supercapacitor performs as well as, or better than, previously reported pseudocapacitive supercapacitors based on transition metals. An integrated flexible device featuring the LS-GH (electrodes) and H 2 SO 4 -polyvinyl alcohol (PVA) gel (electrolyte) exhibited outstanding capacitive performance (408 F g −1 at 1 A g −1 ; 75.4% retention of capacitance at 20 A g −1 ; 84.0% retention of capacitance after 10,000 cycles). The Q/QH 2 structure in the conductive network allowed the storage and release of two electrons and protons during the reversible charge/discharge process. The resulting status of the oxygen atoms in the LS-GH network varied before and after charging, as evidenced through XPS analysis is shown in Figure 4b. After charging, there was observable decrease in the content of C=O groups. groups in the lignin, this supercapacitor performs as well as, or better than, previously reported pseudocapacitive supercapacitors based on transition metals. An integrated flexible device featuring the LS-GH (electrodes) and H2SO4-polyvinyl alcohol (PVA) gel (electrolyte) exhibited outstanding capacitive performance (408 F g −1 at 1 A g −1 ; 75.4% retention of capacitance at 20 A g −1 ; 84.0% retention of capacitance after 10,000 cycles). The Q/QH2 structure in the conductive network allowed the storage and release of two electrons and protons during the reversible charge/discharge process. The resulting status of the oxygen atoms in the LS-GH network varied before and after charging, as evidenced through XPS analysis is shown in Figure 4b. After charging, there was observable decrease in the content of C=O groups. Liu et al. employed a solvent-free mechanical milling process to mix graphite and lignosulfonate for the scalable production of additive-free lignosulfonate/graphene electrodes [36]. They sheared the graphite into microscopic particles with sizes ranging from 50 to 2000 nm. Graphene structures with few layers were generated during ballmilling. The conductivity and discharge capacity of the hybrid electrodes were 290 S m −1 and 35 mA h g −1 , respectively. Because these electrodes exhibited a high self-discharge rate, resulting in losses of both energy density and power density, it was critical to explore the self-discharge process. Therefore, in a subsequent study, Liu et al. employed self- Liu et al. employed a solvent-free mechanical milling process to mix graphite and lignosulfonate for the scalable production of additive-free lignosulfonate/graphene electrodes [36]. They sheared the graphite into microscopic particles with sizes ranging from 50 to 2000 nm. Graphene structures with few layers were generated during ball-milling. The conductivity and discharge capacity of the hybrid electrodes were 290 S m −1 and 35 mA h g −1 , respectively. Because these electrodes exhibited a high self-discharge rate, resulting in losses of both energy density and power density, it was critical to explore the self-discharge process. Therefore, in a subsequent study, Liu et al. employed self-discharge measurements and models to better understand the mechanism of the self-discharging of the lignosulfonate/graphene electrodes, concluding that it occurred through a combination of activation control and diffusion control, depending on the charging potential [37].
Carbon nanotubes (CNTs) interact through π-stacking with polymers containing aromatic moieties. Milczarek et al. prepared multi-walled CNTs surface-functionalized with KL and studied their electrochemical characteristics (Figure 5a) [31]. The ligninmodified nanotubes readily dispersed in an organic solvent (DMSO) and aqueous ammonia (0.1 M), yielding black solutions; such solutions did not form with unmodified CNTs. Atomic force microscopy (AFM) of the CNT/KL nanocomposite revealed debundling of the CNTs, as well as densely packed globular structures on the surfaces of the individual CNTs ( Figure 5b). The redox behavior of the CNT-supported biomolecules was persistent and reversible, due to the KL-derived Q units (Figure 5c). The mass-specific capacitances (C m ) of the unmodified and KL-functionalized CNTs, determined from their CV curves, were 75 and 143 F g −1 , respectively. Figure 5d displays the anodic differential pulse voltammogram of the composite, deconvoluted into four Gaussian curves representing oxidation processes occurring at different redox centers. The complex character of the lignin suggests that these curves originated from Q units experiencing different chemical environments. Peng et al. assembled a high-performance, biomass-based, flexible, solid-state supercapacitor featuring a lignin/single-walled CNT (SWCNT HNO3 ) hydrogel as flexible electrodes and a biocompatible cellulose/Li 2 SO 4 gel prepared using a simple phase-inversion method as an electrolyte separator [38]. The supercapacitor exhibited good specific capacitance (292 F g −1 at 0.5 A g −1 ), superior rate capability, and a remarkable energy density (17.1 W h kg −1 at a power density of 324 W kg −1 ). Moreover, the pressure-sensitive characteristics of the biomass-based, compressible supercapacitor were outstanding, with its shape being completely recoverable (i.e., without plastic deformation) after performing a compression cycle.
with KL and studied their electrochemical characteristics (Figure 5a) [31]. The ligninmodified nanotubes readily dispersed in an organic solvent (DMSO) and aqueous ammonia (0.1 M), yielding black solutions; such solutions did not form with unmodified CNTs. Atomic force microscopy (AFM) of the CNT/KL nanocomposite revealed debundling of the CNTs, as well as densely packed globular structures on the surfaces of the individual CNTs (Figure 5b). The redox behavior of the CNT-supported biomolecules was persistent and reversible, due to the KL-derived Q units (Figure 5c). The mass-specific capacitances (Cm) of the unmodified and KL-functionalized CNTs, determined from their CV curves, were 75 and 143 F g −1 , respectively. Figure 5d displays the anodic differential pulse voltammogram of the composite, deconvoluted into four Gaussian curves representing oxidation processes occurring at different redox centers. The complex character of the lignin suggests that these curves originated from Q units experiencing different chemical environments. Peng et al. assembled a high-performance, biomassbased, flexible, solid-state supercapacitor featuring a lignin/single-walled CNT (SWCNTHNO3) hydrogel as flexible electrodes and a biocompatible cellulose/Li2SO4 gel prepared using a simple phase-inversion method as an electrolyte separator [38]. The supercapacitor exhibited good specific capacitance (292 F g −1 at 0.5 A g −1 ), superior rate capability, and a remarkable energy density (17.1 W h kg −1 at a power density of 324 W kg −1 ). Moreover, the pressure-sensitive characteristics of the biomass-based, compressible supercapacitor were outstanding, with its shape being completely recoverable (i.e., without plastic deformation) after performing a compression cycle.  Using a simple ultrasonic-assisted deposition procedure, Zhou et al. prepared KLmodified, HNO 3 -treated active carbon (KL/TAC) and examined its potential as a supercapacitor electrode material [39]. The combined effects of the treatment with HNO 3 and the coupling of KL enhanced the electrochemical properties of activated carbon (AC), with the as-prepared KL/TAC material exhibiting a specific capacitance (293 F g −1 ) significantly higher than those of the HNO 3 -treated AC (TAC, 120 F g −1 ) and the KL-modified AC (KL/AC, 118 F g −1 ). In a subsequent study, Zhou et al. used a functional group modification strategy to convert the OMe groups in KL to phenolic OH groups, using H 2 O 2 as the oxidant and Fe 2 O 3 as the catalyst, thereby facilitating the formation of additional QH 2 units and leading to higher pseudocapacitances [35]. This mild H 2 O 2 system allowed the variable oxidation of KL. The highest abundance of phenolic OH groups was present in the oxidized KL (OKL) sample subjected to 4 h of oxidation. Excessive oxidation (>5 h) resulted in the transfer of phenolic OH groups to unwanted products. Compared with the raw KL, the OKL materials had a maximum increase of 25.6% in their content of phenolic OH groups. Composite electrode materials were formed from the TAC mixed with the OKL. The OKL/TAC composite electrode exhibited a specific capacitance of 390 F g −1 at 0.5 A g −1 -21.9% higher than that of the KL/TAC system (322 F g −1 ) and double that of the single-phase TAC system (132 F g −1 ). Zhou et al. further improved the electrochemical performance of the lignin-based composite by coupling the OKL with a hierarchical porous nitrogen-doped carbon (NC) [40]. The OKL/NC electrode exhibited good electrochemical properties arising from the synergistic effects of rapid electron transfer, efficient ion diffusion, and a large electroactive surface area. In an acidic solution, the capacitance of the OKL/NC composite (412 F g −1 at 1 A g −1 ) was double that of the NC composite (154 F g −1 ).

Conducting Polymer/Lignin Composites
Conducting polymers are suitable hosts for preparing hybrid materials with lignin because of their high electrical conductivities and processabilities, affordability, and electrochemical reversibility [41,42]. Rapid charge transfer occurs in the polymeric chains, with associated counter-ion intercalation and deintercalation during the redox process. Lignin derivates, on the other hand, serve as scalable biopolymer sources to generate Q moieties. The Q/QH 2 redox reaction has great potential to store charge in lignin/conducting polymer systems. Lignin provides the Q groups in these systems, while the conducting polymer provides electrochemical accessibility to these functional groups [43]. The electrochemical polymerization of pyrrole to polypyrrole (PPy) is feasible in solutions of lignosulfonate and other lignin derivatives [44][45][46]. By combining lignin derivatives with PPy, Milczarek et al. generated an interpenetrating composite appropriate for use as a cathode [44]. Ajjan et al. electropolymerized pyrrole in the presence of a water-soluble lignin derivative (dopant) to prepare PPy/lignin composite electrodes (Figure 6a), then used in situ FTIR spectroelectrochemistry to determine their charge storage mechanism [47]. The formation of Q units, and their reversible redox reactions, occurred during charging/discharging of the electrode materials. A C=O absorption near 1705 cm −1 was linked to the creation of Q units during oxidation, whereas that near 1045 cm −1 was linked to QH 2 units. The cyclic voltammogram of the PPy/lignin system exhibited quasi-reversible redox signals corresponding to Q units with a formal potential (E 0 ) of 0.57 V (vs. Ag/AgCl) in acid solution; these moieties were derived from the monolignol units present in the lignin (Figure 6b). In contrast to the PPy/lignin system, the PPy/ClO 4 blend provided a capacitive-like rectangular voltammogram without any pronounced peaks, implying that only rapid insertion/withdrawal of the ClO 4 − anion occurred into/from the polymer. Because it provided an additional Faradaic reaction, the presence of lignin enhanced the total capacity of this system greatly.
Admassie et al. added phosphomolybdic acid (HMA, H 3 PMo 12 O 40 ·nH 2 O) to the PPy/lignin composite to improve its specific capacitance even further [48]. The inclusion of HMA boosted the specific capacitance of the PPy/lignin composite from 477 to 682 F g −1 (at 1 A g −1 ) and greatly enhanced the charge storage capacity (from 69 to 128 mA h g −1 ). Notably, the biopolymer and the inorganic acid dopant both required an acidic electrolyte medium (0.1 M HNO 3 ) to ensure that protonation occurred during the reduction process as well as for the development of the reversible redox activities of each dopant. Zhang et al. used in situ polymerization of pyrrole in a lignin/pyrrole solution to develop an integrated, mechanically stiff, all-in-wood supercapacitor featuring a lignin/PPy hydrogel embedded in the wood [49]. Taking advantage of the high pseudocapacitance of lignin, strong interactions between lignin and wood, and the hierarchical porous structure of the wood, featuring vertical channels, the as-prepared integrated lignin/PPy-wood supercapacitor displayed a high areal capacitance (1062 mF cm −2 ), a high energy density (47.2 µW h cm −2 ), and favorable cyclic performance. Peng et al. used oxidative polymerization of pyrrole in the presence of lignin to fabricate a lignin/PPy hydrogel (LP 54 ) [50]. To enhance its mechanical strength and electrochemical performance, they introduced functionalized porous carbon nanospheres (FPCS; prepared through the hydrothermal treatment of porous carbon nanospheres) and pyrrole into the prepared LP 54 system prior to oxidative polymerization of pyrrole, thereby obtaining an FPCS/lignin/PPy hydrogel (FPCSLP 54 , Figure 7). Because of its porous structure and functionalization with pyrrole, the FPCS was anchored and distributed uniformly within the lignin/PPy framework. Although the LP 54 sample exhibited high specific capacitance (409 F g −1 at 0.5 A g −1 ), that of the FPCSLP 54 hydrogel was even higher (538 F g −1 ) when measured at the same current density. Symmetric flexible supercapacitors constructed using the FPCSLP 54 hydrogel (electrodes) and a biodegradable cellulose hydrogel (electrolyte separator) exhibited mechanical flexibility and excellent electrochemical properties. Admassie et al. added phosphomolybdic acid (HMA, H3PMo12O40·nH2O) to the PPy/lignin composite to improve its specific capacitance even further [48]. The inclusion of HMA boosted the specific capacitance of the PPy/lignin composite from 477 to 682 F g −1 (at 1 A g −1 ) and greatly enhanced the charge storage capacity (from 69 to 128 mA h g −1 ). Notably, the biopolymer and the inorganic acid dopant both required an acidic electrolyte medium (0.1 M HNO3) to ensure that protonation occurred during the reduction process as well as for the development of the reversible redox activities of each dopant. Zhang et al. used in situ polymerization of pyrrole in a lignin/pyrrole solution to develop an integrated, mechanically stiff, all-in-wood supercapacitor featuring a lignin/PPy hydrogel embedded in the wood [49]. Taking advantage of the high pseudocapacitance of lignin, strong interactions between lignin and wood, and the hierarchical porous structure of the wood, featuring vertical channels, the as-prepared integrated lignin/PPy-wood supercapacitor displayed a high areal capacitance (1062 mF cm −2 ), a high energy density (47.2 µW h cm −2 ), and favorable cyclic performance. Peng et al. used oxidative polymerization of pyrrole in the presence of lignin to fabricate a lignin/PPy hydrogel (LP54) [50]. To enhance its mechanical strength and electrochemical performance, they introduced functionalized porous carbon nanospheres (FPCS; prepared through the hydrothermal treatment of porous carbon nanospheres) and pyrrole into the prepared LP54 system prior to oxidative polymerization of pyrrole, thereby obtaining an FPCS/lignin/PPy hydrogel (FPCSLP54, Figure 7). Because of its porous structure and functionalization with pyrrole, the FPCS was anchored and distributed uniformly within the lignin/PPy framework. Although the LP54 sample exhibited high specific capacitance (409 F g −1 at 0.5 A g −1 ), that of the FPCSLP54 hydrogel was even higher (538 F g −1 ) when measured at the same current density. Symmetric flexible supercapacitors constructed using the FPCSLP54 hydrogel (electrodes) and a biodegradable cellulose hydrogel (electrolyte separator) exhibited mechanical flexibility and excellent electrochemical properties.   with the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) through oxidative chemical and electrochemical polymerizations of EDOT in the presence of lignosulfonate [51]. Although the polymerization of EDOT is simple, poor solubility makes the processing of PEDOT difficult. Polymerizing EDOT in the presence of a watersoluble polyelectrolyte can overcome this problem. Polystyrenesulfonate (PSS) is most typically used, providing a PEDOT:PSS dispersion in water, with PSS serving as both the dopant and dispersing agent. Lignosulfonate, which has sulfonate groups that function similarly to those of PSS, also acts as a dopant and dispersion agent. Indeed, lignosulfonate could disperse EDOT in an aqueous solution, resulting in a PEDOT/lignosulfonate composite after polymerization. The PEDOT/lignosulfonate composite synthesized through chemical and electrochemical polymerizations (Figure 8) were ionic complexes in which the PEDOT polymer and the lignin sulfonate anions interacted to form an interpenetrating polymer network. The specific capacitance of the lignosulfonate-doped PEDOT (170.4 F g −1 ) was double that of the reference PEDOT electrode (80.4 F g −1 ). Improved charge storage qualities have been achieved when combining redox-active biopolymers with conducting polymers, but their performances have been studied mostly at the electrode level. Navarro-Suárez et al. assembled full-cell supercapacitors featuring positive electrodes made of natural lignin (lignin/PEDOT) and negative electrodes prepared from partially reduced graphite oxide (prGrO) [52]. Combining lignin with PEDOT enhanced its redox characteristics. The asymmetric device had a high cell Improved charge storage qualities have been achieved when combining redox-active biopolymers with conducting polymers, but their performances have been studied mostly at the electrode level. Navarro-Suárez et al. assembled full-cell supercapacitors featuring positive electrodes made of natural lignin (lignin/PEDOT) and negative electrodes prepared from partially reduced graphite oxide (prGrO) [52]. Combining lignin with PEDOT enhanced its redox characteristics. The asymmetric device had a high cell capacitance (34.6 F g −1 at 0.1 A g −1 ). Using PEDOT/lignin as the base layer, and subsequent deposition of polyaminoanthraquinone (PAAQ), Ajjan et al. prepared a hybrid PEDOT/lignin/PAAQ electrode through two-step galvanostatic electropolymerization, then examined its electrochemical properties as a supercapacitor [43]. Hybrid electrode materials constructed from electroactive and conducting components result in supercapacitors possessing intrinsically high specific power and enhanced energy density. The asymmetric supercapacitor fabricated using PEDOT/lignin/PAAQ (positive electrode) and PEDOT/PAAQ (negative electrode) exhibited superior specific capacitance (74 F g −1 ) as a result of the synergistic effects of its two electrodes.
Wang et al. performed in situ oxidative polymerization of aniline with reticulated lignin, yielding polyaniline (PANI)/lignin composites possessing interpenetrating fibrous networks (Figure 9). [53]. In acidic solutions, aniline mostly forms cations, according to the polymerization mechanism. Because of its aromatic OH groups and CH 2 OH groups, lignin can be negatively charged in slightly acidic solutions. When combined, aniline cations can adsorb electrostatically onto the surface of lignin. Upon the introduction of ammonium persulfate (APS), aniline cations attached to the surface of reticulated lignin, which underwent oxidation to aniline cationic radicals, which reacted through radical polymerization to form PANI/lignin composites. These composites outperformed PANI itself as electrodes for supercapacitors, with respect to electrochemical capacitance, rate capability, and cycle stability. At 30 A g −1 , the PANI/lignin composite provided a specific capacitance of 284.4 F g −1 and a capacitance retention of 58.6% when the current density was increased from 0.5 to 30 A g −1 ; for PANI these values were 123.1 F g −1 and 29.2%, respectively. Furthermore, at 1.0 A g −1 , the capacity retention of the PANI/lignin symmetrical capacitor was 67.4% after 5000 cycles, outperforming PANI (17.8%). polymerization to form PANI/lignin composites. These composites outperformed PANI itself as electrodes for supercapacitors, with respect to electrochemical capacitance, rate capability, and cycle stability. At 30 A g −1 , the PANI/lignin composite provided a specific capacitance of 284.4 F g −1 and a capacitance retention of 58.6% when the current density was increased from 0.5 to 30 A g −1 ; for PANI these values were 123.1 F g −1 and 29.2%, respectively. Furthermore, at 1.0 A g −1 , the capacity retention of the PANI/lignin symmetrical capacitor was 67.4% after 5000 cycles, outperforming PANI (17.8%).  The types of counter-anions used to compensate for the positive charges of the conjugated polymer chains during synthesis can greatly impact the morphological and structural properties, as well as the electrochemical behavior, of the conducting polymers. Small anionic dopants (e.g., Cl − , ClO 4 − , p-toluenesulfonate, and dodecylbenzenesulfonate) are prone to migrate out of the electrodes, whereas the use of large (bio)polymers as dopants can result in confinement of the redox moieties within the matrix of the conductive polymer. Dianat et al. employed an improved method for the preparation of a nanocomposite featuring an interpenetrating PANI/lignin network on an electrode of electrochemically etched carbon fiber [54]. They obtained a uniform PANI/lignin film after applying a pattern of potential pulses to regulate the nucleation kinetics and growth behavior ( Figure 10). This PANI/lignin nanocomposite outperformed the best-known conducting polymer/lignin supercapacitors by delivering a specific capacitance of 1200 F g −1 at 1 A g −1 , based on rapid H + insertion/de-insertion kinetics, rather than sluggish SO 4 2− doping/dedoping processes. The PANI and H 2 Q/Q couples responsible for the charge storage mechanism in PANI/lignin nanocomposite electrodes can be represented as follows: with the overall reaction expressed as with the overall reaction expressed as PANI . Lignin-Q 4H 4e -⇄ PANI . Lignin-H Q (5) Because the SO3 − dopants on lignosulfonate cannot migrate from the polymer, charges must be balanced through insertion of H + cations into the polymer. The transport of small H + cations in and out of the PANI/lignin matrix is more efficient than that of bulkier SO4 2− anions, which mediate charge compensation during the redox reactions of pure PANI electrodes. A symmetric PANI/lignin||PANI/lignin device offered high specific energy (21.2 W h kg −1 ) and exceptional specific power (26.0 kW kg −1 ), as well as remarkable cycle stability and flexibility.
Tanguy et al. used faradaic reactions of Q groups in natural lignin, covalently attached to high-strength cellulose nanofibrils, to construct mechanically strong and flexible thin film electrodes displaying good energy storage capability [55]. To fabricate Because the SO 3 − dopants on lignosulfonate cannot migrate from the polymer, charges must be balanced through insertion of H + cations into the polymer. The transport of small H + cations in and out of the PANI/lignin matrix is more efficient than that of bulkier SO 4 2− anions, which mediate charge compensation during the redox reactions of pure PANI electrodes. A symmetric PANI/lignin||PANI/lignin device offered high specific energy (21.2 W h kg −1 ) and exceptional specific power (26.0 kW kg −1 ), as well as remarkable cycle stability and flexibility.
Tanguy et al. used faradaic reactions of Q groups in natural lignin, covalently attached to high-strength cellulose nanofibrils, to construct mechanically strong and flexible thin film electrodes displaying good energy storage capability [55]. To fabricate the flexible electrodes, they grew PANI on flexible films made of lignin-containing cellulose nanofibrils (LCNFs) and RGO nanosheets at various loadings. The LCNF/RGO/PANI electrode containing 20 wt.% RGO nanosheets displayed specific capacitances (475 F g −1 at 10 mV s −1 ; 733 F g −1 at 1 mV s −1 ) up to 68% greater than those of comparable electrodes constructed without lignin. The electrochemically active LCNFs performed multiple functions, providing the composite electrode with mechanical strength and flexibility, while improving the overall energy storage. The presence of the lignin and Q/QH 2 units on the surface of the LC-NFs presumably resulted in additional π-π interactions with the PANI and RGO nanosheets in the films, helping to increase the specific capacitance of the produced electrodes.

Metal Oxide/Lignin Composites
Because of their attractive structural and electrochemical characteristics, metal oxides (e.g., MnO 2 , CoO, V 2 O 5 , RuO 2 , NiO, and CuO) and their composites have been applied widely for supercapacitors [56][57][58]. The ability to attain relatively high pseudocapacitive performance, arising from their many valence state transitions, is key to their electrochemical characteristics [59]. Combining lignin with metal oxides can improve the electrochemical characteristics of electrodes [60][61][62][63][64]. Incorporating lignin into metal oxides for supercapacitor applications has proven challenging, due to difficulties in regulating the resulting electrochemical properties, with significant impacts on the cycle life and performance of the devices. As a result, only a few studies have looked at mixing metal oxides with lignin to take advantage of their individually attractive properties. Jha et al. fabricated an anode by depositing NiWO 4 nanoparticles (NPs), coated with bio-derived alkali lignin, onto an aluminum substrate for use as a supercapacitor [63]. To determine the proportions of these components which provides the greatest electrochemical performance, they examined a variety of lignin:NiWO 4 :poly(vinylidene difluoride) (PVDF) compositions: 80:10:10, containing mostly lignin; 45:45:10, having equal levels of lignin and NiWO 4 ; and 10:80:10, featuring mostly NiWO 4 NPs. A larger fraction of NiWO 4 NPs in the supercapacitor, relative to lignin, resulted in better specific capacitance and retention. A supercapacitor fabricated using an Al/lignin-NiWO 4 anode, an Al/AC cathode, and PVA/H 3 PO 4 as the gel electrolyte had a specific capacitance of 17.01 mF cm −2 at 0.13 A g −1 , good stability (84% capacitance retention after 2000 cycles), a maximum energy density of 2 W h cm -2 , and a maximum power density of 100 W cm −2 . Jha et al. also prepared NiCoWO 4 -decorated lignin electrodes for flexible supercapacitors and investigated the effect of Ni as a secondary metal on pseudocapacitance of NiCoWO 4 [60]. Electrochemical testing of the lignin/NiCoWO 4 /AC-and lignin/CoWO 4 /AC-based supercapacitors revealed that the specific capacitance of the bimetallic tungstate (NiCoWO 4 )-containing supercapacitor (862.26 mF cm −2 ) was 141 times higher than that of the monometallic tungstate (CoWO 4 )-containing supercapacitor. Because of the synergistic effect of the bimetallic tungstate NPs encapsulated in the lignin, the lignin/NiCoWO 4 supercapacitor also possessed an exceptionally high power density (854.76 kW kg −1 ) and energy density (5.75 W h kg −1 ). Moreover, Jha et al. also studied the electrochemical performance of carbonized lignin/NiCoWO 4 on a graphene cathode, to identify the effects of smaller fragments of lignin, and partially oxidized lignin, on the capacitance performance. The specific capacitance of the carbonized lignin/NiCoWO 4 was inferior to that of the noncarbonized lignin/NiCoWO 4 . During carbonization, the long carbon chains of lignin were broken down into smaller fragments. To some extent, oxidation occurred. As a result of the higher impedance to charge transfer resulting from fragmentation of the lignin chains, and related irregularity in the chain structure and arrangement, the carbonized lignin exhibited decreased capacity to store charge.
Jha et al. used a similar approach to fabricate Al/lignin/MnO 2 -based anodes for supercapacitors [61]. They decorated alkali lignin with MnO 2 particles and then coated it onto an Al substrate to make the composite electrode. To build the supercapacitor, an Al/lignin/MnO 2 anode and an Al/AC cathode were sandwiched together with a polymer gel-type electrolyte (PVA/H 3 PO 4 ). Lignin/MnO 2 blends of various ratios were used to optimize the electrode performance. The assembled supercapacitor had a high specific capacitance (379 mF cm −2 at 40 mA g −1 ), good cycling stability (80% retention of capacitance), and maximum energy and power densities of 6 W h kg −1 and 355 W kg −1 , respectively. Later, Jha et al. deposited MnO 2 hydrothermally onto AC and lignin to create an asymmetric supercapacitor [62]. The supercapacitor, which featured an anode of Al/AC/lignin-MnO 2 , a cathode of Al/AC, and a gel electrolyte of PVA/H 3 PO 4 , exhibited a specific capacitance of 5.52 mF cm −2 at 6.01 mA g −1 . Mehta et al. employed microwave irradiation to create micro-MnO 2 particles, which they subsequently deposited hydrothermally onto Al/lignin and Al/AC/lignin substrates [64]. The specific capacitance of the AC/lignin-MnO 2 -based supercapacitor was greater than that of that based on lignin-MnO 2 , because of the high surface area and porosity of AC.
Lignin has been examined as an electrolyte additive to allow reversible electron saving/extraction in Q-type moieties. Through faradaic reactions occurring at porous electrode/electrolyte interfaces, lignin can efficiently boost the capacitance of supercapacitors. Lota et al. studied the effect of addition of a lignosulfonate in the electrolyte on the electrochemical performance of supercapacitors [83]. The presence of the lignosulfonate as an electrolyte additive improved the total capacitance of the supercapacitor by up to 33%, according to measurements of two-electrode cells. Three-electrode experiments revealed that adding lignosulfonate to the electrolyte resulted in a reversible redox system developing on the positive electrode, arising from deposition of a thin lignosulfonate-derived film with substantial redox activity, attributed to Q-type units.
The use of lignin and its derivatives to make gel electrolytes for flexible energy storage devices has also been investigated. Park et al. used base-catalyzed ring-opening polymerization (ROP) and chemical cross-linking to create lignin hydrogel electrolytes [85]. Because of their mechanical and dimensional stabilities and the large amount of water they could uptake as a result of their cross-linking chemistry, these cross-linked lignin hydrogel electrolytes were mechanically robust (swelling ratio: 523%) and displayed high ionic conductivity (10.35 mS cm −1 ). A renewable flexible supercapacitor incorporating the chemically cross-linked lignin hydrogel electrolyte and electrospun lignin/polyacrylonitrilederived nanofiber electrodes exhibited high capacitance, flexibility, and durability when bent at various angles, and retained 95% capacitance over 10,000 cycles. The maximum specific capacitance of the supercapacitor electrode incorporating the lignin hydrogel electrolyte (129.23 F g −1 ) was greater than those of PVA/KOH gel electrolytes at 0.5 A g −1 (68.48 F g −1 for 1.0 M KOH; 104.09 F g −1 for 3.3 M KOH).
A double-crosslinking strategy based on two networks has improved the mechanical characteristics of hydrogels. The first network, which was rigid and brittle, acted as a sacrificial bond, efficiently dissipating energy; the second network, which was soft and ductile, kept the hydrogel intact during deformation. Liu et al. reported a new double-crosslinked lignin hydrogel electrolyte possessing excellent compressibility ( Figure 11) [86]. Because of its high phenol content, they chose enzymatically hydrolyzed lignin (M w = 3000 -5000 Da) to create a highly chemically crosslinked network. First, a chemically crosslinked lignin hydrogel (SC lignin hydrogel) was synthesized using base-catalyzed ROP and crosslinking; simply soaking it in acid converted the SC lignin hydrogel into a highly mechanically stable hybrid double-crosslinked lignin hydrogel (DC lignin hydrogel). Immersion of the SC lignin hydrogel in 1 M H 2 SO 4 protonated any free phenol and carboxyl groups within the lignin, resulting in hydrophobic interactions (physical crosslinking) between lignin chains. The resulting DC lignin hydrogel has a compressive mechanical strength of 4.74 MPa, 40 times higher than that of the SC lignin hydrogel, as well as outstanding cyclic loading/unloading compression performance. Furthermore, the ionic conductivity of this type of DC lignin hydrogel electrolyte was extremely high (0.08 S cm −1 ), equivalent to that of pure H 2 SO 4 . Therefore, when Liu et al. constructed a supercapacitor incorporating this synthetic DC lignin hydrogel as the electrolyte, the device had a remarkable specific capacitance (190 F g −1 ), an excellent rate capability, and a high energy density (15.24 W h kg −1 )-performance characteristics similar to, and in some cases greater than, those of other flexible supercapacitors.
well as outstanding cyclic loading/unloading compression performance. Furthermore, the ionic conductivity of this type of DC lignin hydrogel electrolyte was extremely high (0.08 S cm −1 ), equivalent to that of pure H2SO4. Therefore, when Liu et al. constructed a supercapacitor incorporating this synthetic DC lignin hydrogel as the electrolyte, the device had a remarkable specific capacitance (190 F g −1 ), an excellent rate capability, and a high energy density (15.24 W h kg −1 )-performance characteristics similar to, and in some cases greater than, those of other flexible supercapacitors.  In general, increases in specific capacitance by approximately 2-4 times have been observed for carbon-based supercapacitors when adding these organic redox additives to the electrolyte [73,87]. Notably, migration between the electrodes of a redox-active species in the electrolyte often resulted, however, in much more rapid self-discharging of the supercapacitors when using a QH 2 -based redox-active electrolyte [87].

Summary
Lignin, a renewable bioresource, is a very attractive material for use in high-performance supercapacitors because of its low cost and environmentally benign character. This review summarizes recent developments in the fabrication of lignin-based electrode materials and electrolytes applied in supercapacitors. The phenol groups in lignin and its derivatives can form redox-active Q/QH 2 structures. As summarized in Table 1, early studies revealed that lignin and its derivatives could function as effective supercapacitor electrodes when combined with electrically conducting substances. Lignin may also be used as a redoxactive additive in supercapacitor electrolytes. The inclusion of lignin in the electrode materials and electrolytes of supercapacitors not only enhances their performance but also lowers their cost and toxicity, resulting in greener supercapacitor devices.  Table 1 shows that only a few lignin sources, such as kraft lignin and lignosulfonate, have been studied for use in supercapacitors. As a result, it is essential to conduct in-depth study on lignin products from diverse plant species in different biorefinery processes for applications in supercapacitors.

Challenges and Future Perspectives
It has been successfully demonstrated in prior studies that lignin and its derivatives may be used as redox electrolytes and electrode materials for supercapacitors. However, much more research will be necessary to realize lignin-based electrode materials for practical energy storage applications. Here we provide a summary of the current difficulties, and potential prospects, of lignin-based electrode materials and electrolytes.

•
Lignin is a chemical extracted from biomass. During biomass pulping, cellulose recovery has had priority over lignin recovery. As a result, instead of favoring the production of lignin, pulping procedures are tailored to extract high-quality cellulose. The result is a somewhat impure lignin byproduct that contains contaminants from upstream processes. This impure lignin of high calorific value has generally been burned to generate heat and power in the pulp and paper industries. Therefore, improved lignin-first recovery strategies for the extraction and application of highpurity lignin will be critical for future developments. • Technical lignins exhibit chemical and structural heterogeneity depending primarily on the plant source and the nature of the extraction method. Therefore, it will be necessary to develop efficient methodologies for the fractionation of lignin to address the technical issues associated with lignin heterogeneity, in terms of chemical compositions and molecular weight distributions. Moreover, a detailed and methodical research of the various lignin types is required to understand how differing chemical structures may change the electrochemical activity of lignin.

•
Since lignin is electrically insulating, accessing the redox active Q-groups requires conductive materials. Lignin has been coupled with a number of expensive materials, such as conductive polymers, metal oxides, and carbon materials, to form hybrid materials for supercapacitors. Considering the cost reduction for the large-scale production of supercapacitors, finding compatible and cost-effective conductive materials to synergistically interact with lignin to create hybrid electrodes is highly desirable. • Lignin-based pseudocapacitive materials are susceptible to rapid self-discharge. A high self-discharge rate results in rapid loss of energy and power density. To reduce the self-discharging rate, it is crucial to investigate the self-discharge mechanisms of lignin-based hybrid electrodes.

•
Because of the relatively fixed amounts of QH 2 species in lignin, increasing the pseudocapacitance as a result of the natural lignins themselves, undergoing Q/QH 2 redox cycling, remains difficult. Therefore, achieving greater pseudocapacitances would ideally occur through the development of functionalized modification techniques to selectively cleave ether bonds between pairs of propylphenyl units, thereby generating additional phenolic OH and QH 2 groups. • It is necessary to conduct a more thorough techno-economic evaluation of novel ligninderived products to pinpoint process bottlenecks and guide future research paths. Performance-to-price considerations are always crucial for the practical application of lignin in supercapacitors.
Although significant improvements have been made in the performance of ligninderived materials, as well as in our understanding of the mechanisms responsible for their behavior in supercapacitors, many obstacles will need to be overcome before lignin can be used commercially in supercapacitors. We hope that continued studies by the scientific community will provide additional intriguing breakthroughs and-eventually-viable, high-value, lignin-derived, sustainable supercapacitors.