Lignin as a sustainable precursor for electrodes and electrolytes of emerging supercapacitors

Abstract

Lignin, an affordable renewable bioresource, is one of the most abundant naturally existing polymers, popular for its high carbon content and rich functional groups. Recently, advances in science and technology have unveiled the advantages of using lignin in the design of novel and emerging energy storage devices. In this review, we present the specific roles of lignin in the development of active materials for the electrodes and electrolytes of supercapacitors. The first section covers a brief introduction to lignin chemistry, functional groups, and classical isolation techniques. We then discuss the merits of lignin as an active material for electrodes and electrolytes featuring different material development approaches and novel device fabrication techniques. Towards emerging wearable electronics, we also highlight the mechanical and electrochemical performance of representative studies on traditional and flexible supercapacitors using lignin precursors. Beyond the lab, we conclude by summarizing the major challenges and prospective directions toward achieving the commercialization of lignin-based supercapacitors.

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Ridwan T. Ayinla

Ridwan T. Ayinla earned a BS in Physics (Summa Cum Laude) from the Ladoke Akintola University of Technology, Nigeria. He then pursued a master's degree at Universiti Teknologi Petronas, Malaysia. In 2021, he joined the Department of Chemistry at Mississippi State University (MSU) for another master's degree, focusing on single molecular electronics and scanning probe microscopy. Currently, he is a graduate researcher in MSU's Department of Sustainable Bioproducts under Professor El-Barbary Hassan, where his research explores electrochemical processes and imaging microscopy to characterize green batteries and supercapacitors from sustainable materials.

Islam Elsayed

Dr Islam Elsayed is an Assistant Research Professor at the Department of Sustainable Bio-products at Mississippi State University, with over 16 years of experience in teaching organic chemistry courses and conducting research. He earned his bachelor's degree in chemistry from Damietta University, where he also completed his Master's in Organic Chemistry in 2012. In 2019, he obtained his PhD from Mississippi State University in Sustainable Bio-products. His research focuses on sustainable bio-products and their applications, making significant contributions to advancements in this growing field.

El Barbary Hassan

Dr Hassan is a Professor in the Department of Sustainable Bioproducts at Mississippi State University. He earned his academic degrees in Chemistry from Ain Shams University, Egypt. His research focuses on exploring innovative concepts and strategies for the utilization of renewable biomass resources. He is especially interested in the research of lignocellulosic biomass conversion to liquid fuels, value-added chemicals, and the development of low-cost porous carbon materials for environmental applications and energy storage. Dr Hassan authored and co-authored over 90 peer-reviewed research articles, and he has also given over 60 talks and posters at scientific international and national conferences.

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1 Introduction

Industrialization, urbanization, and the escalating rise in the population have caused the world to rely heavily on fossil fuels (gas, oil, and coal) for energy generation. This heavy reliance has caused political and economic instability, destruction of natural habitats, environmental pollution, and depletion of fossil fuel reserves.^1–4 The quest to resolve these unprecedented crises prompted the United Nations (UN) and the International Energy Agency (IEA) to increase the campaign for harvesting low-cost, clean, and renewable energy.^4–7 To address the rapid increase in energy demand, various renewable and environmentally friendly energy alternatives, such as solar, wind, hydroelectric, etc., have demonstrated significant potential as substitutes for the dwindling fossil fuel reserves.^8,9 The energy supply chain requires energy harvesting, storage, and distribution. Thus, harnessing the full potential of renewable energy sources hinges on the advancement of sustainable energy storage technologies.^10,11 In addition to offering a sustainable method for energy storage and preventing energy waste, energy storage devices also facilitate inexpensive and easy distribution of energy to rural communities. According to the IEA, global energy investment experienced a notable increase of twelve percent in 2022, reaching a total of 1 trillion United States Dollars (USD), with a projected rise to 1.2 trillion USD by the end of 2023 (Fig. 1a).¹² The data also indicate a declining trend in fossil fuel investment alongside a rising trend in global investment in renewable energy and energy storage devices. Consequently, the advancement of sustainable energy storage devices has been recognized as complementary to the global goals for renewable and clean energy.

Fig. 1 (a) Energy investment chart from 2011–2023e.¹² Data extracted from “World Energy Investment” through the International Energy Agency (e – estimated). (b) Map depicting the worldwide lignin production capacity in 2017–2018.¹³ Copyright 2020. Reproduced with permission from Elsevier Inc.

Batteries, fuel cells, capacitors, and supercapacitors (SCs) are some of the widely researched emerging energy storage devices.^14–16 An ideal energy storage device is characterized by high specific energy density (measures the amount of energy that can be stored in a system), specific power density (measures how quickly a system can charge or discharge), cycling stability (measures the capacitance retention ability of the cell after certain charge–discharge cycles), and a wide operating temperature range (measures the temperature range in which the cell operates safely).¹⁷ Batteries and fuel cells exhibit enormous energy density and have found wide application in cars, medical devices, cell phones, power banks, uninterrupted power supplies (UPSs), sensors, laptops, etc.^18,19 However, several demerits such as low power density, short operational temperature range, long charge time, short life cycle, and end-use disposal pollution have raised public concerns about battery applications.^20,21 Conversely, SCs can store and dispense energy faster, which promotes their integration into modern electronic devices like televisions, transistor radios, sensors, actuators, etc. However, low specific energy density has limited their application as a bulk energy bank.²² SCs straddle batteries and capacitors in terms of their characteristic specific energy and power densities. This implies that SCs can simultaneously deliver an appreciable specific energy and power density.^23,24 A holistic comparison of batteries (Li-ion), capacitors, and SCs is presented in Table 1. In addition to the elevated power density of SCs, they are relatively cheap, exhibit a wider operational temperature range, and are environmentally friendly, versatile, and durable.^41–43 Therefore, the quest to improve the performance of SCs has received major attention within the scientific community in the last two decades. Structurally, SCs consist of an electrolyte solution sandwiched between two electrodes kept apart by a separator (charge permeable membrane) that allows charge diffusion but prevents physical contact between the two electrodes.⁴⁴ Mathematically, , this implies that the energy density (E) of any double-layer capacitor is proportional to the capacitance (C_s) of the electrodes and the square of the electrolyte's potential (V).⁴⁵ Therefore, the search for efficient electrode and electrolyte materials has garnered more momentum in the last few years. Specifically, some of the main research focus includes the search for low-cost, high surface area electrode materials that can improve capacitance, improve the ionic conductivity and charge mobility of electrolytes, engineer the electrode–electrolyte interface, improve the charge permeability and temperature tolerance of separators, and reengineer the structure of SCs.

Table 1 Parametric comparison of batteries, capacitors, and supercapacitors^a

Parameters	Fuel cells	Capacitors	Batteries	Supercapacitors
a NA – Not available.
Specific energy density (W h kg^−1)	600–1200 (ref. 25)	0.01–0.3 (ref. 26)	100–200 (ref. 27)	0.05–30 (ref. 25)
Specific power density (W kg^−1)	500–3000 (ref. 28)	10^4–10^7 (ref. 29)	150–315 (ref. 28)	About 10^5 (ref. 30)
Discharge time (t)	1 s–24 h (ref. 25)	10 s–12 h (ref. 31)	0.3–3 h (ref. 32)	0.3–30 s (ref. 32)
Cell voltage (v)	>750 (ref. 33)	6–800 (ref. 34)	3.6–3.7 (ref. 35)	2.3–2.75 (ref. 33)
Durability (years)	15 (ref. 30)	15 (ref. 30)	5–20 (ref. 30)	20+ (ref. 25)
Cycle durability (%)	20 000 (ref. 30)	NA	10^3–10^5 (ref. 35)	1M+ (ref. 36)
Capital cost ($ per kW h)	10^3–10^5 (ref. 13)	10 000+ (ref. 13)	600–2500 (ref. 37)	300–2000 (ref. 37)
Efficiency	90% (ref. 38)	99% (ref. 39)	85–90% (ref. 27)	97+% (ref. 40)

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Lignin is a complex, amorphous, and heterogeneous organic natural polymer present in the cell walls of plants.^46,47 As a result of the extensive utilization of plants as raw materials in major industries such as pulp and paper, LignoForce, LignoBoost, etc., lignin and lignin derivatives are extensively generated as by-products.^48,49 For instance, the pulp and paper industry alone is projected to produce approximately 70 million tons of lignin each year. However, it is surprising that less than 2% of this plentiful, cost-effective, and biodegradable lignin is utilized in making adhesives, concrete additives, and stabilizing agents.⁵⁰ The majority of lignin is either discarded as waste or burned as low-grade fuel. The lignin production capacity in each country is primarily influenced by the nations that operate lignosulfonate plants.^51,52Fig. 1b maps the lignin production capacity by country between 2017 and 2018.⁵³ The United States leads the lignin production evident from the successful collaboration of Borregaard LignoTech with Rayonier Materials, whose recent operation in northeast Florida produced about 100 000 tons per year. China ranks second globally in lignin production, boasting six major lignosulfonate producers. South Africa holds the third position on the global scale, benefiting from the presence of Borregaard LignoTech in Kwazulu Natal.⁵⁴

Harnessing the synergistic advantages of lignin, which encompass its cost-effectiveness, biodegradability, stiffness, high carbon content, excellent thermal stability, and adjustability, offers a compelling opportunity to produce value-added products such as electrode and electrolyte materials for SC applications.^55,56 Therefore, the use of lignin-derived products as active materials in energy storage devices has gained undivided attention from researchers. Lignin application in energy storage seeks to explore the unique physiochemical properties, such as high surface area, mechanical stability, high thermal stability, active sites for functionalization, tunable composite formation, high carbon content, abundance, and renewability, to develop active materials for supercapacitor design.⁵⁷

This review connects readers with the recent and representative scientific reports on the conversion of lignin into valuable products for supercapacitor applications. The first section introduces the recent trend in the world energy demand and the role of SCs as a complementary device for renewable energy systems. We then discussed the origin, linkages, and functional groups in native lignin, featuring the mechanism, techniques, and yield of different methods used in the isolation of lignin. In the third section, we present the application of lignin for supercapacitors highlighting lignin-based electrodes, electrolytes, and advances in lignin-based flexible SCs. Finally, we conclude and present the challenges and a prospective guide aimed at inspiring innovative ideas for integrated lignin-based SC design into practical and commercial applications.

2 Origin and isolation of lignin

Lignin, a complex organic polymer, originates from plant cell walls and serves as a crucial component in providing structural support.⁵⁸ Extracted primarily from biomass, lignin undergoes isolation to unlock its unique properties for various industrial applications. Understanding the origin and isolation methods of lignin is pivotal in harnessing its potential for sustainable materials and energy storage solutions.⁵⁹

2.1 Origin and linkages in lignin

Considering the complexity, competitive advantages, and the growing attention of lignin as a promising candidate for conversion into value-added products, a clear and comprehensive understanding of the origin and bonding nature of its complex structure is crucial for understanding its potential applications.^60–63 Lignin, the most abundant aromatic polymer found in many tracheophytes, is a three-dimensional amorphous macromolecule that links several methoxylated phenylpropanoid units through both ether and carbon–carbon bonds.⁶⁴ In vascular plants, lignin is chemically intertwined with hemicellulose, which wraps the cellulose fibrils. As an extended phenolic polymer, lignin helps the plant maintain structural rigidity, efficient water transportation, and resistance to decay pests and microorganisms (Fig. 2a).^65,67,68 Owing to the heterogeneous nature of vascular plants, the composition of lignin in plants depends on the type (Angiosperm, Gymnosperm, Gramineae, etc.), parts (stem, branch, leave, husk, etc.), species of different plants, defects (such as knots and reaction wood), and environmental growth factors.⁶⁶ In general, Gymnosperms have the highest composition of lignin with 25–30%, Angiosperms have an intermediate composition of about 20–25% lignin, and Gramineae have a characteristically low lignin composition of about 15–25%.⁶⁹ The chemical complexity of lignin is traceable to the heterogeneous nature of the monolignol linkages (ether and carbon–carbon).^69–71 Several ether (R-O-R) linkages such as β-O-4′, α-O-4′, and 4-O-5′ are present in lignin formation; however, β-O-4′ accounts for half of the interconnecting units of lignin monomers because of its low Gibbs free energy of formation (Fig. 2b).⁶⁷ During defragmentation of lignin, β-O-4′ is the most reactive and the favorite site for cleavage. The C–C linkages, including β-β′, β-5′, β-1′, and 5-5′, are present based on the lignin preparation processes. For example, β-β′ is popular in syringyl lignin due to the stability of the syringyl radicals during lignification. Zhang et al.⁷² reported that the β-1′ linkage is unstable in acidic media and is frequently observed in spirodienone structures, whereas the 5-5′ linkage is intrinsic to milled wood lignin and is indicative of biphenyl structure formation. The dehydrogenation polymerization of coniferyl alcohol forms phenylcoumaran units, where β-β′ dominates. The detailed structure of monolignols and the linkages highlighted in lignin are presented in Fig. 2c.^60,67 The phenyl–propane units, p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol are the foundational building units of the three types of monolignol structural units, hydroxyphenyl (H), guaiacyl (G), and syringyl (S), respectively.^73,74 The major difference in the monolignols is the number of methoxyl (–OCH₃) groups present in their benzene rings.⁷⁵ Extensive research has established that the percentage of H, G, and S monolignols in a lignin structure helps to identify the type of plant from which lignin is being sourced. The G-monolignols dominate softwood lignin with approximately 95 wt%, and the G and S-monolignols have a proportionate balance in hardwood, while the three H, G, and S monolignol units are present in grass lignin.^76,77

Fig. 2 (a) Structural arrangement and molecular representation of cellulose, hemicellulose, and lignin in biomass.^65,66 Adapted with permission from ref. 65 and 66. Copyright permission 2015 and 2017 Royal Society of Chemistry. (b) Ether and C–C linkages of lignin, and (c) basic building blocks of lignin (monolignol) and their corresponding structural polymers.⁶⁷ Adapted with permission from ref. 67. Copyright permission 2017 Royal Society of Chemistry.

Lignin exhibits an amphiphilic nature due to the presence of numerous functional groups, including hydroxyl, benzyl, methoxyl, ether, carboxyl, etc.^78–80 In lignin modifications, two types of hydroxyl groups (aliphatic and phenolic) stand out as the foremost functional groups involved in processes such as phenolation, esterification, hydroxypropylation, alkylation, etc.^81,82 The degree of hydroxylation is a crucial parameter that influences the extent of etherification and condensation, and reports have also shown that it also impacts solubility in some cases.⁸³ Methoxyl is a primary group that exists in monolignol units, and the number of methoxyl groups present in the ring helps to differentiate monolignol units. It is stable and requires high temperature (about 260 °C) and pressure (more than 5 MPa) for demethoxylation to occur.⁸⁴ In traditional lignin, the carboxylic functional group is found to have a relatively low concentration with a high tendency to increase after chemical modification.^85–87 The wide array of functional groups in lignin holds significant promise for modifying lignin, thereby facilitating its utilization in various fields, including material synthesis, chemical catalysis, and energy storage, among others.

2.2 Isolation of lignin

The isolation of lignin involves the separation of lignin from the other components of biomass, such as cellulose, hemicellulose, extractives, and pectin. This can be achieved through various methods, including chemical, physical, and biological processes. Some common techniques for lignin isolation include acid hydrolysis, steam explosion, organosolv extraction, and enzymatic treatment.^88,89

The choice of the isolation method depends on several factors such as the type of biomass, desired lignin purity, and intended application of the isolated lignin.^90–92 The lignin isolation methods significantly affect the functional groups that are present in lignin; therefore, a comprehensive understanding of the methods available for isolating lignin is critical to both the study of lignin and its effective utilization. Fig. 3 presents a summary of the three representative procedures (industrial (pulp and paper), laboratory protocol, and biorefinery) of extracting lignin from lignocellulosic biomass and their corresponding yields.⁵⁰ This section presents readers with an in-depth understanding of the isolation procedures, a comparative analysis of the yield of three approaches, and the chemical and structural changes noticeable in lignin as a result of specific isolation procedures.

Fig. 3 A comparative chart of the lignin extraction procedure from lignocellulosic biomass.³⁵ Copyright 2020. Reproduced with permission from Elsevier Inc.

2.2.1 Biorefinery process. The need to maximize the utilization of all major chemical components of biomass (cellulose, hemicellulose, and lignin) has motivated the isolation of lignin in the biorefinery process.⁹³ Refining biomass requires several key steps that help to reduce biomass recalcitrance to treatment, aid purity, catalyze enzymatic degradation, and assist downstream lignin isolation.^51,92,94,95 Some of these steps include biomass pretreatment (breaking down the lignocellulosic material for increased accessibility of lignin and other chemical components), fractionation (separating lignin from other chemical components), and purification (the process of removing impurities and residual chemicals from lignin).^91,96,97 Generally, lignin isolated from biorefinery processes is classified based on the pretreatment method (physical, biological, and chemical) and conditions employed during preparation. Here, we will focus on the chemical pretreatment process since it is the most used, has high yield and high purity, and is easily deployable.^98,99

Alkali-base pretreatment involves the use of basic solvents such as Na₂S, KOH, NH₃, NaOH, Ca(OH)₂, etc., to degrade biomass.⁶² The most commonly used alkaline is NaOH because of its solubility in water, low cost, and low environmental impact. Alkaline pretreatment has yielded approximately 30 to 60 wt% of lignin. In the process of alkali-based pretreatment, the ether linkages connecting monolignols undergo partial cleavage, leading to the formation of lignin oligomers, phenolic dimers, and monomers.^100,101 Ab Rahim et al.¹⁰² compared the yield of lignin in an alkaline (NaOH) and ionic liquid (1-butyl-3-methylimidazolium chloride) pretreatment of bamboo. They reported 42% yield for the alkaline pretreatment as against 11% in the ionic liquid pretreatment. Another commonly used alkaline in the pretreatment of biomass is ammonia. Some researchers prefer the use of ammonia due to its high volatility, which makes it easy to recover. It also offers selective degradation of lignin while preserving carbohydrates.^103,104 Conversely, acid-based pretreatment uses acids in the selective degradation of biomass. The commonly used acids are hydrochloric, sulfuric, and phosphoric acids.^99,105,106 The use of concentrated acid requires a low temperature (<110 °C), while diluted acids require a high temperature (>180 °C). The yield of the method depends on the biomass type, acid concentration, reaction time, and temperature.^94,107

Ionic liquids (ILs), introduced in 2007, have been widely considered as an emerging and promising solvent in lignin extraction. Its merits include low volatility and high thermal stability with a wide temperature range.^88,108 By default, most ILs are potent for dissolving carbohydrates and lignin; however, tuning the viscosity of ILs with water can induce selective dissolution and alter the degree of dissolution of lignocellulosic chemicals. The yield of lignin from IL pretreatment depends on several parameters such as the nature of the IL, treatment temperature, time, and nature of biomass.^109,110Table 2 summarizes the parametric performance and yield of different ILs in lignin extraction. We first present two different studies that investigated the effect of using the same ILs but different biomass at a uniform extraction time and temperature. Then, we present three studies that compared the yield of different ILs for the same biomass. In the former, Wang et al.¹¹¹ used 1-hexylpyridinium chloride ([Hpy]Cl) to extract popular and bamboo biomass at the same time and temperature. They reported a lignin yield of 61% and 51.7% for poplar and bamboo, respectively. In another study, Asim et al.¹¹² also extracted lignin from rice husks and wheat straw using pyridinium protic ILs under the same conditions. They reported a yield of 72% and 73% for rice husks and wheat straw, respectively. Using the same biomass, Penín et al.¹¹³ used different ILs (1-butyl-3-methylimidazolium hydrogen sulfate ([Bmim][HSO₄]) and triethylammonium hydrogen sulfate ([TEA][HSO₄]) to extract lignin from Eucalyptus nitens. They reported a low yield of lignin with [TEA][HSO₄] at 69% and [Bmim][HSO₄] at 87%, respectively. The lignin extracted from [Bmim][HSO₄] exhibited a low content of aliphatic –OH groups, indicating incomplete degradation. Hossain et al.¹¹⁵ used protic (1-ethyl imidazolium chloride) and aprotic (1-ethyl-imidazolium formate) ILs to extract Pinus radiata under the same conditions. Their result shows that the protic IL can fully dissolve Pinus radiata with improved enzymatic hydrolysability. Recently, researchers have identified that particle size significantly influences the degree of fractionation of biomass. Therefore, mechanical methods are explored to assist the chemical pretreatment measures. Physiochemical pretreatment seeks to explore the synergy of physical and chemical treatments in isolating lignin. Despite the advantages of ILs, challenges associated with the cost of ionic liquids, potential toxicity, and the need for effective recovery and recycling processes are areas of ongoing research.

Table 2 Parametric comparison of the yield of lignin isolation using ionic liquids

Ionic liquid	Biomass	Time (h)	Temperature (°C)	Lignin yield (%)	Ref.
[Hpy]Cl	Poplar	0.5	100	61.0	111
[Hpy]Cl	Bamboo	0.5	100	51.7	111
[Py][H2PO4·H3PO4]	Rice husks	2.0	100	72.0	112
[Py][H2PO4·H3PO4]	Wheat straw	2.0	100	73.0	112
[Bmim][HSO4]	Eucalyptus nitens	0.5	170	87.0	113
[TEA][HSO4]	Eucalyptus nitens	0.8	170	69.0	113
[DIPEA][O]	Coffee husk	4.0	120	54.2	114
[DIPEA][Ac]	Coffee husk	4.0	120	71.2	114
[Eim][OAc]	Pinus radiata	18.0	115	17.0	115
[Eim][HCOO]	Pinus radiata	18.0	115	27.0	115
[C2mim][ABS]	Bagasse	2	180	78	116
[C2mim][ABS]	Bagasse	2	190	118	116
[C2mim][ABS]	Bagasse	1.5	190	97	116
[Emim]Ac	Maple	1.5	130	63	116
[Emim]Ac	Maple	24	80	51	117
[Mmim][MeSO4]	Maple	24	80	9	117
[Bmim][CF3SO3]	Maple	24	80	6	117

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2.2.2 Laboratory protocol. Lignin extraction in the lab is mostly relevant for the structural profiling and characterization of lignin; therefore, the extraction of lignin at the lab scale prioritizes purity and maintaining the structural integrity of lignin.¹¹⁸ The three main protocols for isolating lignin in the lab are milled wood lignin, cellulolytic enzyme lignin, and enzymatic acidolysis lignin.

Milled wood lignin (MWL), first demonstrated in the lab in 1957, is essentially useful for harvesting crude lignin from biomass in its native form. It operates by reacting neutral solvents and water with milled wood under reflux in an extraction system.¹¹⁹ Most of the reagents used in the reflux and extraction process are neutral to avoid major structural changes. This method is popular for its exact representation of native lignin, low cost, and environmental friendliness. However, it has been widely criticized because it is laborious and time-consuming, with a characteristic yield lower than 40%.⁵⁰ More importantly, lignin extracted using milled wood isolation has carbohydrates in trace amounts due to some unbreakable lignin-carbohydrate bonds.¹²⁰ Researchers mimic the natural enzymatic degradation of cellulose to develop cellulase, an enzyme that is capable of hydrolyzing cellulose in biomass in a process called cellulolytic enzyme lignin (CEL). This method is technically an extension of the MWL process, where finely milled biomass is pretreated with a cellulolytic enzyme to hydrolyze carbohydrates (cellulose and hemicellulose).^121,122

Lignin is then precipitated from the residue using neutral solvents such as dioxane/water. A cycle of this method takes approximately four days and leads to the generation of protein and fragments of carbohydrates as impurities. Combining the processes of MWL, CEL, and mild acidolysis of biomass presents an improvement in the purity (approximately 90%) and yield (up to 70%) of lignin isolation.^123,124 In addition to the CEL protocol, the biomass is treated with a low concentration of acid, which enhances the degradation of carbohydrates while preserving lignin in its native form.

Enzymatic mild acidolysis lignin (EMAL) represents an enhanced lignin derivative, attainable through optimization of the CEL method with the addition of a small quantity of hydrochloric acid.¹²¹ This modification enables the extraction of lignin with yields of up to 70 wt% and purity levels approaching 90% from woody plant sources.¹²⁵ EMAL is adaptable to several other pretreatment processes and works with any type of biomass. A quest to unravel the lignin isolation process of EMAL spurred Guerra et al.^126,127 to investigate and compare the mechanism and efficiencies of MWL, CEL, and EMAL in softwood and hardwood. They observed major structural depolymerization of native lignin during vibratory milling through the cleavage of the β-aryl ether linkage. Alternatively, ball milling with low mechanical activity preserved the native lignin macromolecules; however, the yield is low. Under standard conditions, the yield of EMAL was reported to be two times higher than that of CEL and four times higher than that of MWL. Finally, they also confirmed that the purity of lignin is higher in hardwoods than in softwoods for the three methods. Overall, the major challenge with extracting lignin in the native form is the art of preserving the structural integrity of lignin and preventing the loss of easily cleaved motifs.⁹⁰

2.2.3 Biomass pulping. Lignin isolation in the laboratory allows for small-scale lignin studies. In contrast, industries demand isolation procedures that are efficient, economical, and fast and can be implemented on a large scale.^77,128 The pulping process fairly meets most of these requirements; thus, it is the most ubiquitous industrial-scale approach for isolating technical lignin. Conventionally, pulping can be carried out using four different routes: soda, kraft, sulfite, and organosolv pulping. The major differences in the processes are in the type of chemical reagents used in the biomass treatment and the chemical constituent of the lignin end products.¹²⁹

Kraft pulping is the most popular method used for extracting lignin in applied industries. Approximately 50 million tons of lignin are generated annually as waste in the pulp and paper industries all over the world.^130,131 Demuner et al.¹³² reported that 98% of the total lignin generated as waste is being incinerated to turn energy turbines and less than 2% is being converted to lignosulfonate. “Kraft” – a German word for “strength” – symbolizes the high strength of the pulp generated from kraft pulping. Alkaline solutions (such as potassium hydroxide, sodium hydroxide, sodium sulfide, etc.) are used to cleave the ether linkages of α and β with monolignols.^133,134 The monolignols are then dissolved in basic solutions to form a deep, brownish-colored solution called black liquor. Finally, the black liquor is then acidified to form kraft lignin. This process efficiently removes lignin from the wood fibers, resulting in a relatively pure form of lignin with minimal contamination from the other components of the wood. This high purity makes kraft lignin suitable for a wide range of applications, including as a precursor for carbon fibers, dispersants in concrete, and additives in adhesives and resins. Additionally, kraft lignin's purity allows for easier chemical modification and processing, further expanding its potential for modified derivatives.^135,136

Soda pulping is one of the oldest methods of isolating lignin from biomass. It involves the treatment of wood or non-woody biomass with soda in the presence of anthraquinone as a catalyst. The anthraquinone helps to speed up the dissolution of lignin and delay the depolymerization of carbohydrates.^137,138 Soda pulping produces pure lignin with a low molecular weight. Unlike kraft and lignosulfonate pulping, soda lignin is free from sulfur, the purest, and the most ideal for native-structural lignin characterization and modification.¹³⁹

Organosolv pulping uses a mixture of organic solvents (such as cresol, methanol, acetic acid, phenol, tetrahydro-furfuryl alcohol, ethanol, propanol, butanol, etc.) with a controlled amount of water in the presence of a catalyst to dissolve lignin from lignocellulosic biomass.¹⁴⁰ After the dissolution of lignin, the organic solvent can be easily recovered making the method a cheap alternative. Compared to kraft pulping, it also reduces the generation of sulfur.¹⁴¹ The major factors that determine the efficiency of organosolv pulping are the type of biomass, choice of organic solvent, concentration, temperature, and time.¹⁰⁷ Therefore, optimizing these parameters is key to achieving high yield. The organosolv method produces uniform molecular weight lignin due to the excellent dissolution power of the solvents.¹⁴² El Mansouri and Salvadó characterized technical lignin extracted using kraft, lignosulfonate, soda, and ethanol as organosol solvents.^87,143 Their results show that soda, kraft, and organosolv produce lignin with high purity. They used elemental analysis to show excessive sulfur and ash content in the lignosulfonate method. They also used UV-Vis spectroscopy and Mannich reactivity to test the modification tendencies of lignin from each of the methods. The results show that the kraft and soda pulping are more liable to chemical modification. Overall, the choice of the pulping method depends on factors such as the type of wood or plant material, the desired properties of the lignin product, and environmental considerations. Advances in lignin valorization and the development of more sustainable pulping processes are areas of ongoing research in the pulp and paper industry.

3 Lignin as the active material of supercapacitors

Chemical complexity and advances in green synthesis protocols have offered lignin the advantage of multiple roles as an electrode, electrolyte, and binder material in supercapacitors. This flexibility further underscores the versatility of lignin, offering a sustainable and multifaceted solution for energy storage designs. Harnessing lignin for these key functions aligns with the pursuit of eco-friendly and efficient supercapacitor technologies. This section will focus on the recent and relevant scientific efforts that contribute to the design of supercapacitors from lignin-based precursors.

3.1 Lignin-based electrode material for supercapacitors

Lignin has been largely explored as a starting material for the active material of electrodes in supercapacitor development.^60,144,145 Lignin in its crude form cannot be used to fabricate electrodes; therefore, concerted efforts have been deployed to convert, modify, and improve the properties of lignin to accommodate efficient physiochemical and electrochemical activities that translate to high-performance supercapacitors.¹⁴⁶ Our attention is mainly focused on electrochemical double-layer capacitors, and we categorize the conversion of lignin into carbonaceous compounds into three categories: activated carbon, template carbon, and composite carbon for more technical and insightful discussion.

3.1.1 Lignin AC electrodes. Activated carbon (AC), known for its high surface area, hierarchical porosity, conductivity, tunability, thermal, and chemical stability, has garnered significant attention from researchers.^147–149 One of the most important parameters that affect the physiochemical properties of AC is the chemistry of the starting material. Lignin as a sustainable material has been widely considered as a starting material for use as AC.^150–152 The conversion of lignin into carbonaceous compounds requires a systematic cascade of thermochemical processes such as carbonization, activation, and/or modification. Physical and chemical methods are the two main established routes commonly used to prepare activated carbon. In physical activation, lignin-based precursors are heated in the presence of an inert environment using an oxidizing gas, typically steam (H₂O) or carbon dioxide (CO₂), at temperatures ranging from 600 to 1200 °C.¹⁴⁴ This process removes volatile components and creates pores within the carbon structure, increasing its surface area. The resulting activated carbon has a predominantly microporous structure and is suitable for several applications.¹⁵³ Chemical activation involves impregnating carbonaceous materials with a chemical activating agent, such as potassium hydroxide (KOH), phosphoric acid (H₃PO₄), zinc chloride (ZnCl₂), potassium carbonate (K₂CO₃), etc.^154–156 The impregnated material is then heated to temperatures typically between 400 and 900 °C. During heating, the chemical activating agent reacts with the carbonaceous material, leading to the development of pores and increasing the surface area of the activated carbon. Chemical activation often results in activated carbon with a combination of micropores and mesopores, offering enhanced physiochemical properties.¹⁵⁷ Activating carbon from biomass does not have a specific route/order; therefore, the quest to optimize the properties of activated carbon from the lignin precursor has spurred researchers to try several conditions and activation processes. Numerous cross-linking and polycondensation reactions occur when lignin interacts with chemical activators, liberating carbon, hydrogen, and oxygen atoms from the lignin and generating a plethora of multi-scale pores.^158,159Table 3 presents an overview of the recent studies on lignin-based activated carbon, including the starting materials, activation parameters, and their corresponding physiochemical and electrochemical performance. Notably, chemical activation exhibits superior pore-forming capabilities and operates at lower temperatures compared to physical activation, establishing it as the primary method for producing lignin-based activated carbons with significant specific surface areas. Lim et al.¹⁴⁷ separated lignin from larch raw materials using the Klason and organosolv methods (Fig. 4a). They used Klason, and organosol lignin as precursors to prepare activated carbon for use as the electrode material of supercapacitors. They used KOH as a chemical agent to convert lignin into activated carbon at 900 °C. The Klason and organosolv lignin-based AC exhibited superior carbonization yield and carbon content, while all other activated samples exhibited comparable physical and electrochemical properties, pore structures, and surface areas. Their cyclic voltammogram showed a quasi-rectangular-shaped behavior, which symbolizes double-layer charge formation on the parallel electrodes. The lignin-based double-layer capacitor also showed 131 F g⁻¹ at 1 mA cm⁻² with 99% of its initial capacitance retained after 10 000 cycles at 10 mA cm⁻¹ (Fig. 4b). These findings indicate a high yield, favorable cycling stability, and elevated capacitance for activated carbon prepared from lignin-based materials. Importantly, chemical activation agents significantly influence the specific surface area (SSA) and pore structure of lignin-based carbon materials. Zapata and coworkers obtained kraft lignin from pulp to prepare a series of activated carbon materials at constant temperature using different activation agents (KOH, K₂CO₃, and H₃PO₄) at 1 : 1 and 1 : 2 activation ratios.¹⁸⁹ Activated carbon from KOH showed a surface area, specific capacitance, energy density, and power density of 1515 m² g⁻¹, 236 F g⁻¹, 18.47 W h kg⁻¹, and 49.48 W kg⁻¹. Their work showed the specific role of activation agents in lignin-based activated carbon for energy storage systems.

Table 3 Summary of lignin-derived activated carbon for supercapacitor applications^a

Starting materials	Activation parameters	Physiochemical properties	C s (F g^−1)	Cycling stability	E d (W h kg^−1)	P d (W kg^−1)	Electrolyte	Ref.
			I d (A g^−1)
a CA – Chemical activation, PA – physical activation, Temp – temperature, IR – impregnation ratio, NA – not available, SBET – surface area, Vt – pore volume, Cs – specific capacitance, Id – current density, Ed – energy density, Pd – power density, CS – cycling stability, and NA – not available.
Larch	CA: KOH	S BET: 2304 m^2 g^−1	131	99.0% after 10 000 cycles	33.9	4477.1	TEABF4 in ACN	147
	IR – 1 : 2.5
	Temp: 900 °C	V t: 0.94 cm^3 g^−1	NA
	Time: 1 h
Kraft lignin	CA: KOH	S BET: 1609 m^2 g^−1	122	90.0% after 5000 cycles	3.2	209.1	1 M H2SO4	160
	IR – 1 : 3
	Temp: 800 °C	V t: 0.98 cm^3 g^−1	1.0
	Time: 1 h
Enzymatic hydrolysis lignin	CA: KOH	S BET: 202 m^2 g^−1	1264	NA	32.05	193.9	6 M KOH	161
	IR – 1 : 2
	Temp: 800 °C	V t: NA	1.0
	Time: 3 h
Alkali lignin	CA: CO2/N2	S BET: 2149.5 m^2 g^−1	300	98.2% after 10 000 cycles	19.15	250	6 M KOH	162
	IR – 1 : 2
	Temp: 800 °C	PV: 0.88 m^3 g^−1	0.5
	Time: 2 h
Corn straw	PA: N2	S BET: 1831 m^2 g^−1	428	96.7% after 10 000 cycles	66.18	312	6 M KOH	163
	Temp: 900 °C
	Time: 2 h	PV: 1.52 cm^3 g^−1	1.0
Enzymatic hydrolysis lignin	CA: NA	S BET: 1504 m^2 g^−1	324	99.7% after 5000 cycles	17.9	50 400	6 M KOH	164
	Temp: 800 °C
	Time: 3 h	V t: NA	0.5
Lignin	CA: K2FeO4	S BET: 2351.6 m^2 g^−1	236	94% after 20 000 cycles	23.6	360	6 M KOH	165
	IR – 1 : 2
	Temp: 800 °C	V t: 1.44 cm^3 g^−1	0.5
	Time: 2 h
Calcium lignosulphonate	CA: KOH	S BET: 1771 m^2 g^−1	147	78% after 5000 cycles	3.3	25	6 M KOH	166
	IR – 2 : 3
	Temp: 800 °C	V t: 1.17 cm^3 g^−1	0.25
	Time: 3 h
Sodium lignosulphonate	CA: NaNO3	S BET: 2453.3 m^2 g^−1	166	93.1% after 10 000 cycles	59	59	EMIBF4	167
	IR – 1 : 0.5
	Temp: 800 °C	V t: 1.933 cm^3 g^−1	0.5
	Time: 1 h
Mason-pine kraft	CA: KOH	S BET: 3742 m^2 g^−1	504.7	98.8% after 10 000 cycles	18.1	16 000	1 M Na2SO4	168
	IR – 1 : 3
	Temp: 900 °C	V t: 1.881 cm^3 g^−1	0.2
	Time: 3 h
Lignin nanoparticles	PA: N2	S BET: 3577.3 m^2 g^−1	248.8	NA	34.6	250	6 M KOH	169
	Temp: 800 °C
	Time: 3 h	V t: 1.97 cm^3 g^−1	0.5
Lignin	CA: K3C6H5O7	S BET: 3174 m^2 g^−1	241	95% after 10 000 cycles	30	377.6	6 M KOH	170
	IR – 1 : 3
	Temp: 900 °C	V t: 2.796 cm^3 g^−1	1.0
	Time: 3 h
Enzymatic hydrolysis lignin	PA: N2	S BET: 617.7 m^2 g^−1	232	97.2% after 10 000 cycles	NA	NA	6 M KOH	171
	Temp: 800 °C
	Time: 2 h	V t: NA	0.5
Lignosulfonate	CA: KOH	S BET: 1372.8 m^2 g^−1	340	94.5% after 5000 cycles	9.7	250	3 M KOH	172
	IR – 1 : 3
	Temp: 550 °C	V t: NA	0.5
	Time: 2 h
Apricot shell lignin	CA: H3PO4	S BET: 1474.8 m^2 g^−1	169.05	NA	NA	NA	6 M KOH	173
	IR – 1 : 3
	Temp: 550 °C	V t: 0.612 cm^3 g^−1	0.5
	Time: 2 h
Lignin porous carbon	CA: KOH	S BET: 2738 m^2 g^−1	173	99.9% after 2000 cycles	55.5	23.0	PVA/LiCl	174
	IR – 1 : 4
	Temp: 800 °C	V t: 2.02 cm^3 g^−1	0.5
	Time: 20 min
Lignin carbon	CA: Mg(NO3)2	S BET: 1140 m^2 g^−1	248	97% after 1000 cycles	NA	NA	6 M KOH	175
	IR – 1 : 2
	Temp: 800 °C	V t: 0.627 cm^3 g^−1	0.2
	Time: 1 h
Oxidized lignin	CA: KOH	S BET: 3094 m^2 g^−1	352.9	88.46% after 50 000 cycles	9.5	25.0	6 M KOH	176
	IR – 1 : 3
	Temp: 800 °C	V t: 1.97 cm^3 g^−1	0.5
	Time: 2 h
Lignin carbon	CA: KOH	S BET: 2287 m^2 g^−1	334	95% after 10 000 cycles	NA	NA	6 M KOH	177
	IR – 1 : 3
	Temp: 800 °C	V t: 1.11 cm^3 g^−1	1
	Time: 2 h
Kraft lignin	PA: N2	S BET: 1307 m^2 g^−1	244.5	91.6% after 10 000 cycles	8.5	100	6 M KOH	178
	Temp: 900 °C
	Time: 1 h	V t: NA	0.2
Sodium lignosulfonate	PA: N2	S BET: 903 m^2 g^−1	247	104% after 10 000 cycles	8.4 W h L^−1	5.58.1 W L^−1	7 M KOH	179
	IR – NA
	Temp: 700 °C	V t: 0.53 cm^3 g^−1	0.05
	Time: 4 h
Feedstock	CA: KOH	S BET: 3033 m^2 g^−1	214.03	90.21% after 10 000 cycles	NA	NA	6 M KOH	180
	IR – 1 : 2
	Temp: 900 °C	V t: Na	0.5
	Time: 3 h
LMCNFs@SnO2	CA: KOH	S BET: 659 m^2 g^−1	406	95% after 10 000 cycles	11.5	451	6 M KOH	181
	IR – 1 : 1
	Temp: 900 °C	PV: 0.56 cm^3 g^−1	0.5
	Time: 3 h
Biomass	CA: ZnCl2	S BET: 2592 m^2 g^−1	384	96.96% after 10 000 cycles	10.48	NA	6 M KOH	182
	IR – NA
	Temp: 900 °C	V t: 1.29 cm^3 g^−1	40
	Time: 1.5 h
Eucalyptus kraft lignin	PA: N2/CO2	S BET: NA	155	94% after 6000 cycles	4	5200	6 M KOH	183
	IR – NA
	Temp: 800 °C	V t: NA	0.1
	Time: 1 h
Lignin carbon	CA: KOH	S BET: 606 m^2 g^−1	117.9	98.2% after 75 000 cycles	0.618	148	Lignin/6 M KOH	184
	IR: NA
	Temp: 1100 °C	V t: 0.44 cm^3 g^−1	0.5
	Time: 2 h
Poplar lignin carbon	CA: H3PO4	SBET: 837.4 m^2 g^−1	346.6	NA	31.5	40	6 M KOH	185
	IR – 1 : 2.5
	Temp: 800 °C	V t: 0.48 cm^3 g^−1	0.1
	Time: 1 h
Lignin porous carbon	CA: H3PO4	S BET: 1867 cm^2 g^−1	440	95.1% after 3000 cycles	10.4	300	6 M KOH	159
	IR – 1 : 3
	Temp: 800 °C	V t: 0.99 cm^3 g^−1	0.5
	Time: 1 h
AILCFN/Ni–Co–S	CA: KOH	S BET: 736.14 m^2 g^−1	1140	84.7% after 3000 cycles	30.8	800	6 M KOH	186
	IR – 2 : 3
	Temp: 1000 °C	V t: 0.31 ml g^−1	10
	Time: 1 h

---

Fig. 4 (a) Cascade of kraft and organosolv lignin extraction protocols and supercapacitor design; (b) comparison of specific capacitance vs. current density of a Klason and organosolv lignin-derived supercapacitor.¹⁴⁸ Copyright 2022. Reproduced with permission from Elsevier Inc. (c) The preparation scheme of a lignin-based nanosphere and (d) specific capacitance vs. current density of the lignin-based carbon nanosphere at 700 and 900 °C.¹⁸⁷ Copyright 2022. Reproduced with permission from Elsevier Inc. (e) Synthesis illustration of lignin hierarchical porous carbon through hydrothermal carbonization and activation, and (f) scanning electron micrograph of lignin hierarchical carbon.¹⁸⁸ Adapted with permission from ref. 188. Copyright permission 2017 Royal Society of Chemistry.

In another experiment, Wu et al.¹⁹⁰ activated alkaline lignin to prepare activated carbon in a one-step activation process using ZnCl₂, KOH, and K₂CO₃. Micropores dominate the activated carbon prepared using the three chemical agents, whereas those activated with ZnCl₂ and KOH contain a few mesopores. ZnCl₂-AC exhibited the lowest surface area (866 m² g⁻¹) because it acts as a dehydration and dehydroxylation agent, while KOH and K₂CO₃ serve as oxidizing agents in the activation process with 1191 m² g⁻¹ and 1585 m² g⁻¹ surface areas, respectively. Researchers often attempt to proportionately connect the numerical value of the surface area and porosity to the electrochemical performance of activated carbon. Wang et al.¹⁷³ attempted to establish a relationship between the pore structure and electrochemical performance of activated carbon prepared from H₃PO₄-activated apricot shell lignin. They reported a surface area of 1474 m² g⁻¹ with a high specific capacitance of 169.05 F g⁻¹ at 0.5 A g⁻¹. Interestingly, they used statistical tools to evaluate the effect of a range of pore volumes on mass capacitance at low current density and predicted that in a three-electrode system with 6 M KOH electrolyte, pore volume in the range of 0.85 to 1.93 nm could be used to estimate mass-specific capacitance at low current density. Lignin possesses a loose structure that facilitates the formation of pores during pyrolysis. A high specific surface area is crucial for enhancing the performance of lignin-based activated carbons, but an appropriate pore structure is essential for maximizing the utilization of this surface area in supercapacitor applications. When micropores dominate, the ionic mobility of the electrolytes is restricted to the accessible sites. However, mesopores and macropores present wider openings for ions during charge–discharge cycles. Interestingly, a size-controlled lignin-based spherical nanosphere (LNS) was self-assembled, stabilized, and carbonized at different temperatures for supercapacitor applications (Fig. 4c).¹⁸⁷ Their findings indicated the successful construction of monodispersed, ordered, and regular carbon nanospheres. By adjusting the initial concentration of lignin from 0.5 to 2 mg m L⁻¹, the size of the LCNS could be tuned, ranging from 256 to 416 nm. By adjusting the size and microstructure, the as-prepared LCNS yielded a specific surface area ranging from 652 to 736 m² g⁻¹. Upon integration into an electrochemical capacitor, the LNS electrode material demonstrated a high specific capacitance of 147 F g⁻¹ (Fig. 4d). The controllable capacitance performance suggests that the as-prepared LCNS holds great promise as a candidate material for energy storage. Therefore, more attention should be paid to engineering the pore structures of activated carbon in supercapacitor applications. In this context, considerable efforts have been dedicated to constructing high-performance hierarchical carbons through tailored pyrolysis and activation approaches using lignin as a carbon source. Guo et al.¹⁸⁸ developed a hierarchical porous carbon with a three-dimensional structure from lignin through enzymatic hydrolysis, employing hydrothermal carbonization, and subsequently activating it with KOH as an activation agent at different activation ratios (Fig. 4e). The sample with the highest impregnation ratio showed a well-developed 3D network of pores (Fig. 4f) with a high specific surface area (SSA) of 1660 m² g⁻¹ and achieved an outstanding specific capacitance of 420 F g⁻¹ at 0.1 A g⁻¹ in a 6 M KOH electrolyte. Notably, they developed supercapacitors using 6 M KOH and pristine EMIM TFSI ionic liquid electrolytes. They reported an energy and power density of 10 W h kg⁻¹ at a power density of 50 W kg⁻¹ in KOH and 46.8 W h kg⁻¹ in an ionic liquid. Therefore, 3D hierarchical lignin-based carbons present an opportunity to bridge the energy gap between traditional capacitors and batteries. Zhang et al.¹⁶³ also developed activated carbon from corn straw lignin using the green bacterial activation method. Bacterial activation produces lignin with a low molecular weight, which enhances carbonization due to the breakage of major linkages. Their result highlights an improved specific capacitance of 428 F g⁻¹ at 1 A g⁻¹ in 6 M KOH electrolyte. Furthermore, the symmetric supercapacitor demonstrated an exceptional energy density of 66.18 W h kg⁻¹ at a power density of 312 W kg⁻¹ in the ionic liquid system. In general, activated carbon utilized for supercapacitors should possess a well-developed pore structure comprising micropores, mesopores, and macropores to facilitate efficient electrolyte penetration and ion diffusion.

3.1.2 Lignin template carbon electrodes. Beyond the primitive and traditional activated carbon preparation, templating is a clever approach deployed by researchers to engineer the pore structure of lignin. We have established the relevance of material porosity and matching with the ionic size of the electrolyte to the electrochemical performance of the device; therefore, templates are being used to build a uniform network of pores in carbon.^191–193 Depending on the template material, there are two types of templates (hard and soft). Hard template carbon also known as exotemplate or hard-casting requires incorporating thermally stable, uniform-sized solid materials such as polymer, silica, zinc, magnesium, and calcium-based templates into the matrix of lignin before carbonization.¹⁹⁴ Thermal stability is a major requirement for hard templates, which makes them resistant to decomposition during carbonization. The templates are then removed either physically or chemically to create a network of perforated and uniform spongy matrices.¹⁹⁵ Song and coworkers also developed mesoporous carbon from lignin extracted from wheat straw alkaline pulping combining nano-dimension MgO and Pluronic F127 as templates (Fig. 5a).¹⁹⁸ They observed that the mass ratio of lignin to MgO significantly affects the porosity of carbon and showed a well-developed network of pores with an average pore diameter of 9 nm after removing the template (Fig. 5b). The template carbon also showed an improved electrochemical response of 126 F g⁻¹ at 0.5 A g⁻¹ current density. The uniformity in the pore distribution provides evidence that using a “hard template” is effective for pore engineering in carbon design. Wang et al.¹⁹⁹ used KCl as a salt template to develop hierarchical porous carbon from nitrogen-doped lignin. They reported a wide range of pores (micropores, mesopores, and macropores) with a surface area of 1012.5 m² g⁻¹. Salt template carbon was used to fabricate electrodes, which showed superior electrochemical properties with 245 F g⁻¹ at 0.5 A g⁻¹ current density and cyclic stability with 95.7% capacitance retention after 5000 cycles. The outcome of their experiment showed good promise for high electrochemical performance in template-based lignin supercapacitor design.

Fig. 5 (a) Step-by-step flow of the synthesis of lignin-based mesoporous carbon using a dual-template approach; (b) differential pore size distribution of representative lignin-derived mesoporous carbon.¹⁹⁶ Adapted with permission from ref. 196. Copyright permission 2017 Royal Society of Chemistry. (c) Schematic flow of the development of hierarchical lignin-derived porous carbon.¹⁹⁶ Adapted with permission from ref. 196. Copyright permission 2020 Royal Society of Chemistry.

The quest for heterogeneous porosity spurred Xi et al.¹⁹⁶ to prepare large-scale three-dimensional hierarchical porous carbon using gas to exfoliate ZnCO₃ as an environmentally friendly template activator (Fig. 5c). They reported a mixture of micropores and mesopores as a result of the gas release of ZnCO₃ and the in situ generation of ZnO nanoparticles of size 10–20 nm. The highly porous carbon retained 99% of its capacitance after 10 000 cycles in a symmetric battery design. This work opened a new route to the simultaneous development of uniformly spaced mesopores in the matrix of carbon and the preparation of nanosized particles. The resulting mesoporous carbon depends on the nature of the template. However, removing the hard templates from the lignin-based carbon matrix is challenging, laborious, and time-consuming. To solve these problems, researchers tried the use of soft materials as templates.

Using soft templates is another reliable method for creating ordered mesopores in the matrix of lignin-derived carbon.²⁰⁰ Instead of hard templates, this approach uses ionic liquids or gels as templates which assemble into precursors through non-covalent interactions like van der Waals force, hydrogen bonding, and π stacking.¹⁹⁴ Therefore, the interactions between the templates are weak, and removing the template after carbonization is unnecessary. Herou et al.¹⁹⁷ prepared an ordered mesoporous carbon from hardwood-based organosolv lignin using phloroglucinol and glyoxal as green soft templates, as shown in Fig. 6a. They prepared template carbon from lignin only, lignin and glyoxal, and lignin, glyoxal, and phloroglucinol in equal proportions and reported superior electrochemical performance from the lignin, glyoxal, and phloroglucinol (PLG at 50 wt%) because they realized more mesopores from these templates, which translates to better electrochemical performance. Compared to the phloroglucinol and lignin (PL) only, they reported an enhanced volumetric capacitance of 90 F cm⁻¹ in KOH electrolyte.

Fig. 6 (a) Schematics of the cascade of self-assembly of phloroglucinol-lignin-glyoxal in acetone (left), self-assembly achieved after evaporation of solvent showing hexagonal symmetry (middle), and highly ordered hexagonal mesoporous carbon after the decomposition of the F127 template (right).¹⁹⁷ Adapted with permission from ref. 197. Copyright permission 2018 Royal Society of Chemistry. (b) Synthesis flow of nitrogen-doped lignin-based hierarchical porous carbon (N-LHPC) using a metal activator, (c) pore size distribution of N-LHPC, and (d) rate capability performance for LC, N-LC, LCM, and N-LHPC.²⁰² Copyright 2023. Reproduced with permission from Elsevier Inc.

In another study, Li et al.²⁰² used the one-pot approach to develop a nitrogen-doped hierarchical porous carbon from lignin using a soft template. They first prepared a complex solution of lignin and melamine before freeze-drying, and the dry powder was then homogeneously crosslinked with Zn/Mg before carbonization to convert the heterogeneous material into carbon (Fig. 6b). The pore distribution of the nitrogen-doped carbon with mesopores dominating and few macropores is shown in Fig. 6c. The electrochemical performance of the nitrogen-doped hierarchical carbon was compared with that of carbon prepared from pristine lignin and metal doped lignin. The resulting carbon was then washed to remove excess impurities from the carbon. The melamine introduced nitrogen heteroatoms in the porous carbon matrix and the N-doped caron was tested electrochemically. The nitrogen-doped carbon showed superior performance with 235.75 F g⁻¹ specific capacitance at 0.5 A g⁻¹ and it retained 68.89% at 20 A g⁻¹ (Fig. 6d). Their results showed that doping can be incorporated into the soft template during hierarchical carbon preparation. In the existing literature, no defined standard has compared the two methods under the same conditions. However, porous carbon developed from hard templates appears to produce more defined pores with a higher surface area. On the other hand, soft templates offer less defined pores but provide researchers with a less cumbersome route to efficiently tailor pores in carbon. In contrast to the direct activated carbon preparation method, templating provides a unique route to creating a highly ordered network of pores in the matrix of the lignin carbon structure.

3.1.3 Lignin composite electrodes. Composite formation refers to the process of combining two or more distinct components to create a new material with unique and superior properties from those of the individual starting materials.^146,201 Apart from developing a low-cost new material, compositing also increases our ability to improve material properties, and attain structural and chemical uniformity, and recyclability.⁴⁷ To enhance the inherent properties of lignin, researchers have explored combining lignin with various materials to create composites with improved physicochemical and electrochemical characteristics. Heteroatoms, polymers, and carbonaceous compounds are widely explored due to their cost-effectiveness, availability, and chemistry.^203–205 Introducing heteroatoms (atoms of elements different from carbon or hydrogen) has proved to be a potent means to influence the default charge state of surfaces that attract ions through electrostatic interactions. Wang et al.²⁰⁶ introduced a bidirectional strategy to create well-developed pores in lignin-based carbon for supercapacitor applications. Their idea introduced activation mediators (ammonium oxalate monohydrate and potassium carbonate) with distinct pore-forming functionalities to form lignin-based supramolecules through electrostatic interactions (Fig. 7a). Subsequently, hierarchical pore formation and heteroatom (N and O) doping occur concurrently during the carbonization process. The O-1s spectra of the obtained porous carbon showed three distinct peaks at 531.8, 532.9, and 533.5 eV (Fig. 7b), corresponding to quinone groups or keto oxygen (C=O), C–OH/C–O–C, and –COOH, respectively. Meanwhile, the N-1s spectra exhibit four peaks at 398.8 (pyridinic N, N-6), 400.2 (pyrrolic N, N-5), 401.6 (graphitic N, N-Q), and 403 eV (N–O), respectively (Fig. 7c). These oxygen- and nitrogen-containing groups potentially serve as active sites, enhancing wettability and electrolyte compatibility by altering the polarity of the graphite carbon. The resulting carbon materials exhibit a specific surface area of 1502.3 m² g⁻¹ at high heteroatom content up to 16.2 at%. Leveraging these exceptional physicochemical properties, the porous lignin-derived carbons demonstrate a remarkable affinity for ions in the electrolyte, achieving an ultrahigh capacitance of 380.5 F g⁻¹ at 0.2 A g⁻¹, along with exceptional cycling stability (100% capacitance after 10 000 cycles). Qi et al.²⁰⁸ introduced heteroatoms onto the surface of lignin carbon to achieve a high surface area (3342 m² g⁻¹), mesoporosity (80%), and wettability using urea. The oxygen group translates to high wettability while urea-formaldehyde resin served as a complimentary activation mediator and nitrogen dopant. The material was used to fabricate the electrode of supercapacitors with a high specific capacitance (305 F g⁻¹). This study showed the role of formaldehyde in the pursuit of developing mesoporous materials. Li et al.²⁰³ subjected enzymatic hydrolysis lignin extracted from corn stalk to multiple heteroatom doping to benefit from the synergistic effect of multiple heteroatoms. The outcomes indicated that the fractionation process effectively yielded three lignin fractions characterized by progressively increasing the molecular weight and specific surface area. The three lignin fractions were carbonized and activated by co-doping nitrogen, phosphorus, and sulfur. The sample with the highest molecular weight and the highest level of dopant heteroatoms shows the highest surface area (2022.4 m² g⁻¹) and thermal stability. The material also exhibited improved electrochemical properties with 96.5% stability after 10 000 cycles and 337.3 F g⁻¹ capacitance at 0.5 A g⁻¹ current density. The study proposes a straightforward approach to enhance the electrochemical properties of heteroatom-doped carbon materials by employing lignin fractionation.

Fig. 7 (a) Schematic diagram of the development of hierarchical lignin porous carbon: high-resolution XPS spectra of (b) N 1s and (c) O 1s.²⁰⁶ Copyright 2023. Reproduced with permission from Elsevier Inc.

Lignin possesses abundant phenol groups, which can store energy through a reversible faradaic redox reaction. Nevertheless, the insulating characteristics of lignin pose a challenge to effectively utilizing these redox functional groups.⁶⁰ Adopting a strategy similar to metal oxide/carbon composite electrodes, researchers have explored the synthesis of composite electrodes by combining lignin with conductive polymers to address this limitation. Du et al.²⁰⁷ extracted lignin from representative hardwood (poplar: containing mostly coniferyl and sinapyl units), softwood (pine: containing mostly coniferyl units), and grass (corn stalk: containing coniferyl, sinapyl, and p-hydroxyphenyl units) to develop composites. Taking advantage of the unique functional groups in each of the sourced lignin materials, they compared the electrochemical performance of the composite blends of each material with polyacrylonitrile. Their results showed that the poplar-derived lignin showed the highest tensile strength, surface area, and specific capacitance of 35.32 MPa, 1062.5 m² g⁻¹, and 349.2 F g⁻¹, respectively. Their results prioritize the use of hardwood in forming composites for electrodes of supercapacitors. Li et al.¹⁴⁶ synthesized a composite of lignosulfonate and polyaniline by in situ chemical oxidation. The optimized mass loading of 0.1 g lignosulfonate showed a higher specific capacitance of 553.7 F g⁻¹ at 1 A g⁻¹ compared to the pure polyaniline electrodes with 450 F g⁻¹. Their research further establishes the tunability of alloyed products for enhanced electrochemical performance. In another experiment, Xuan et al.²⁰⁹ also prepared a composite of lignin, polyacrylonitrile (PAN), and polymethyl methacrylate (PMMA) as the electrode material of supercapacitors. After several mass ratio optimization experiments, their results showed that the composite with the three materials (lignin, PAN, and PMMA) showed the highest specific capacitance (233 F g⁻¹ at 0.5 A g⁻¹) with exceptional cycling stability with 96.8% capacitance retention after 50 000 cycles. This method demonstrates the synergy of different materials in enhancing the electrochemical performance of lignin in aqueous electrolyte. Table 4 summarizes the recent trend in the performance of lignin-based composites as electrode materials for supercapacitors.

Table 4 Summary of lignin-based composites and the electrochemical performance of their corresponding supercapacitor^a

Starting material	S BET (m^2 g^−1)	C s (F g^−1)	Cycling stability	E d (W h kg^−1)	P d (W kg^−1)	Electrolyte	Ref.
		I d (A g^−1)
a S BET – surface area, Cs – specific capacitance, Id – current density, Ed – energy density, Pd – power density, CS – cycling stability, NA – not available, LRGO – lignin reduced graphene oxide, CNF – carbon nanofiber, PAN – polyacrylonitrile, MOF – metal–organic framework, PVA – polyvinyl alcohol, LPC – lignin porous carbon, and PANI – polyaniline.
Lignosulfonate/PANI	32.10	553.7	68.01% after 5000 cycles	7.50	5000	1 M H2SO4	146
		1
Lignin-graphene hydrogel	512	408	84.0% after 10 000 cycles	13.8	500	H2SO4–PVA	210
		1
Lignin/PAN carbon	2042.8	428.9	97.1% after 10 000 cycles	37.1	400	6 M KOH	211
		1
Lignin/PEDOT/anthraquinone	NA	74	80% after 10 000 cycles	8.2	700	0.1 M HCLO4	212
		1
PEDOT/lignin	NA	170.4	83% after 1000 cycles	NA	NA	HClO4/H2O: acetonitrile	213
		0.2
Lignin/PVA hydrogel	790.5	102.3	106.1% after 5000 cycles	250	2.1	EMIMBF4	151
		0.5
Lignin/PAN carbon fiber/graphene	2439	267	96.7% after 5000 cycles	9.3	493	6 M KOH	214
		1
PAN/sodium lignosulfonate	2041	325.7	90.1% after 10 000 cycles	9.88	NA	6 M KOH	215
		0.5
Polyaniline@carbon	NA	200.3	75% after 6000 cycles	17.80	43.45	Lignin gel	216
		0.1
NiCO2S4/HPC	202	1264.2	68.14% after 5000 cycles	32.05	193.9	6 M KOH	161
		1
δ-MnO2/LPC	552	198	98% after 5000 cycles	3.82	125	1 M Na2SO4	217
		1
Lignin/Zn-PTCDA-MOF	731.09	229.6	90.3% after 10 000 cycles	5.1	0.5	6 M KOH	218
		2
LRGO carbon	444.29	330	100% after 10 000 cycles	11.3	254	1 M H2SO4	219
		1
PAN: lignin	2370	128	75% after 1000 cycles	59	15 000	Pyr14TFSI:PC:EC	220
		1
PAN/pitch/lignin	1194	165	94% after 3000 cycles	22	400	6 M KOH	221
		0.01
Lignin/PAN-CNF/NiCo2O4	NA	1757	138% after 5000 cycles	47.8	799.53	2 M KOH	222
		0.02
Lignosulfonate/zinc oxalate	1069	320	93.5% after 10 000 cycles	9.75	6157.9	PVA/KOH	223
		1.0
Lignin-derived carbon/zinc oxide	NA	193	94.2% after 10 000 cycles	6.7	197.7	PVA/KOH	224
		0.5
ARS/PGLS	1727.7	469.5	99.7% after 2000 cycles	9.45	100.06	6 M KOH	225
		0.5

---

Graphene and its derivatives known for their high electrical conductivity, high surface area, and chemical stability have also attracted unwavering attention as preferred materials for lignin-based composites. The main challenge with using graphene is the strong van der Waals force that causes inadvertent π–π stacking into multiple layers.^50,60,226 However, the propane-phenolic group of lignin is used to inhibit the π–π stacking when forming composites. Ye and coworkers²²⁷ developed a porous composite of lignin and reduced graphene oxide with a high surface area (1804 m² g⁻¹) using hydrothermal carbonization as shown in Fig. 8a. Lignin was used to weaken the van der Waals interaction between the adjacent layers of reduced graphene oxide. The material delivered a high specific capacitance of 190 F g⁻¹ at 0.5 A g⁻¹ in 6 M KOH. This work showed the role of lignin in enhancing the performance of reduced graphene by preventing agglomeration. Apart from preventing π–π stacking, graphene also serves as a structural support in the preparation of coaxial fiber electrodes for supercapacitor applications. Hu et al.²²⁸ developed a cost-effective method to prepare lignin-based carbon/graphene fiber using coaxial wet spinning. The resulting fiber electrodes exhibit a remarkable specific capacitance of 260.48 mF cm⁻² at a current density of 0.1 mA cm⁻², surpassing that of pristine graphene fiber fabricated from conventional wet spinning by a factor of 11. This work demonstrated the efficacy of graphene as a structural support and opened a new route for the exploration of lignin in coaxial supercapacitor development.

Fig. 8 (a) Demonstration of lignin-aided chemical reduction of graphene oxide (GO) and hydrothermal carbonization, leading to the design of a lignin-reduced graphene oxide (RGO) composite for utilization as an electrode in supercapacitors.²¹⁸ Copyright 2017. Reproduced with permission from Elsevier Inc. (b) Schematic flow of the preparation of hollow particle-based carbon nanofibers (HCNFs).²¹⁸ Copyright 2023 reproduced with permission from Elsevier Inc.

In another experiment, Yu et al.²⁰¹ used a three-dimensional (3D) carbon composite prepared from a blend of carbon dots and lignin-based activated carbon for supercapacitor electrodes. The composite material showed improved electrochemical performance with a specific capacitance of 301.7 F g⁻¹ as compared to pristine activated carbon, which exhibits 125.8 F g⁻¹.

Compositing carbon dots and activated carbon for electrodes of supercapacitors presents a promising pathway for employing renewable porous carbon materials in advanced energy storage devices. 1000 °C showed the best result with a specific capacitance of 229 F g⁻¹ at 2 A g⁻¹ current density. The two-electrode system generated an energy density of 5.1 W h kg⁻¹ at 0.5 kW kg⁻¹ power density. Their research shows the ingenious application of a MOF as a porogen in the design of well-developed heterogeneous pores in lignin-based nanofibers. A systematic approach was deployed by Zhou and coworkers to design a 1-dimensional particle-based carbon nanofiber from an alloy of softwood kraft lignin-based carbon and a Zn-based metal–organic framework (MOF).²¹⁸ Micropores dominate the initially prepared lignin-based carbon and Zn-MOF was used as a porogen to create abundant mesopores in the cavity of the carbon nanofibers. They reported that the highly porous material enhanced the ionic mobility of electrolytes during charge–discharge when the material was electrospun as an electrode material (Fig. 8b). The material carbonized and stabilized at 1000 °C showed the best result with a specific capacitance of 229 F g⁻¹ at 2 A g⁻¹ current density. The two-electrode system generated an energy density of 5.1 W h kg⁻¹ at 0.5 kW kg⁻¹ power density. Their research shows the ingenious application of a MOF as a porogen in the design of well-developed heterogeneous pores in lignin-based nanofibers.

3.2 Lignin-based electrolyte for supercapacitors

Electrolytes play a crucial role in enabling capacitive devices to achieve high power and energy density, extended cycling life, and enhanced safety. Ionic mobility and conductivity serve as critical parameters for assessing the effectiveness of electrolytes. Aqueous electrolytes typically exhibit higher conductivity compared to non-aqueous and solid electrolytes due to their lower dynamic viscosity. The conductivity (σ) of an electrolyte species (i) is proportionally linked to its ionic mobility (μ_i), the concentration of charge carriers (n_i), the elementary charge (e), and the magnitude of the valence of the mobile ion charges (z_i).^229–232

Therefore, extending the operational potential window and consequently the energy density of capacitive cells depends on the ability of researchers to simultaneously push the limits of the parameters beyond their present realities. Recently, a few researchers have considered lignin as a sustainable material in the design of electrolytes for supercapacitor applications. Liu et al.²³³ developed a lignin-based hybrid double-crosslinked hydrogel with an excellent compression fracture of 4.74 MPa and ionic conductivity (0.08 S cm⁻¹). They used the lignin hydrogel electrolyte with a carbon cloth electrode and reported a specific capacitance of 190 F g⁻¹ at 0.5 A g⁻¹. This research introduces lignin as a promising candidate for potential use in compression-resistant and foldable energy storage devices. In another study, Wang et al.²³⁴ used lignin as an additive to induce adhesion and cryogenic tolerance in a bio-based hydrogel electrolyte. The electrolyte exhibited 0.06 S cm⁻¹ ionic conductivity, and the supercapacitor fabricated using the lignin-based hydrogel can retain 76.2% of its capacitance at −40 °C. Their result showed that lignin favors supercapacitor operation under cryogenic conditions. Mondal et al.²³⁵ introduced a facile crosslink of Fe³⁺ ions into the matrix of a composite of lignin and poly(ethylene glycol) to prepare a hydrogel electrolyte. Taking advantage of the high Fe³⁺ concentration, the hydrogel achieved a remarkably high ionic conductivity (6.69 S m⁻¹), high specific capacitance (301.8 F g⁻¹), energy density (26.73 W h kg⁻¹), and power density (2.38 kW kg⁻¹) with 94.1% retention after 10 000 cycles. Their result supports the use of lignin as a host for metals in electrolyte designs. Huang and coworkers also used lignin as a thickening agent in the synthesis of a lignin-based gel polymer electrolyte via a deep eutectic solvent.²³¹ They reported a remarkably high specific capacitance of 181.5 F g⁻¹ at 1 A g⁻¹ current density and the device can retain 80.2% capacitance after 2000 cycles. Presently, lignin is predominantly investigated as an additive within electrolyte formulations. In conclusion, the use of lignin as an additive in electrolyte formulations for supercapacitors holds significant promise for advancing sustainable energy storage technologies. Its renewable nature, along with its potential to enhance electrolyte stability and conductivity, underscores its importance in driving towards more environmentally friendly energy solutions. As research continues to explore and optimize lignin's role in electrolyte formulations, it stands poised to contribute significantly to the evolution of supercapacitor technology.

3.3 Lignin-derived solid-state and flexible supercapacitors

The introduction of lignin-based flexible supercapacitors heralds a new era in sustainable energy storage technology. Leveraging lignin, a renewable and abundant natural polymer, offers the potential to address the growing demand for flexible energy storage solutions while reducing reliance on non-renewable resources.^236,237 These flexible supercapacitors hold promise for diverse applications, from wearable electronics to flexible displays, where traditional rigid energy storage devices may be impractical. Harnessing lignin's unique properties has been widely explored for use in electrodes, electrolytes, or both in the design of flexible supercapacitors. The design of flexible supercapacitors can be achieved through the gelation of thermochemically stable electrolytes towards mitigating the leakage problem that is characteristic of liquid electrolytes. Gong et al.²³⁸ made a successful attempt at developing an environmentally friendly lignin-based hydrogel electrolyte using distilled water, lignin, and liquid electrolyte. The lignin-based hydrogel is thermally stable with 3.72 mS cm⁻¹ ionic conductivity at room temperature. This approach shows that lignin can serve as a gelling agent while maintaining ionic conductivity and thermal and mechanical stability. Mahmood et al.²³⁹ used direct laser writing to fabricate a symmetrical and flexible supercapacitor by transferring the composite of laser-induced graphene (LIG) from kraft lignin and polyethylene oxide on polydimethylsiloxane (PDMS) (Fig. 9a). Two lignin-based flexible electrodes were used to sandwich H₂SO₄/poly(vinyl alcohol) and tested with bending radii of 0.725 cm (BR1) and 1.1825 cm (BR2). The SEM image of the LIG/PDMS composite electrode material showed a heterogeneous interconnected network of pores that aids the ionic mobility of the gel electrolyte (Fig. 9b).

Fig. 9 Flexible supercapacitor development and mechanical and electrochemical testing: (a) schematic illustration of the development of an all lignin-based supercapacitor. (b) SEM image of the LIG/PDMS. (c) Specific areal and volumetric capacitance for (d) a charge–discharge stability test at 0.02 mA cm⁻² for BR1 (0.725 cm) and BR2 (1.1825 cm); inset shows the charge–discharge plot for the first ten cycles for BR1 and BR2.²³⁹ Reproduced with permission from ref. 239. Copyright 2020 American Chemical Society. (e) Schematic flow of the use of lignin as the active material of the electrode and electrolyte of a flexible supercapacitor, and (f) Ragone plot comparing the energy and power densities of different flexible supercapacitors.²³⁰ Adapted with permission from ref. 230. Copyright permission 2019 Royal Society of Chemistry.

Results from the real volumetric capacitance showed that the capacitance of the flexible cell was preserved upon low mechanical bending (BR1 = 0.725 cm) and there is a significant reduction in the capacitance when the cell is subjected to high bending (BR2 = 1.1825 cm), as shown in Fig. 9c. Lastly, stability tests were conducted by monitoring the charge–discharge cycle of the flexible cell for 1000 cycles (Fig. 9d). The results showed that the charge–discharge cycles were preserved at low and high bending radii for the first ten cycles, while the last ten cycles showed a slight delay in the discharge branch of the high bending radii as shown in the Fig. 9d inset. The suggested approach to creating flexible supercapacitors, utilizing LIG derived from lignin, shows considerable promise in employing an economical and renewable material for crafting portable and wearable electronic devices. In another study, Peng and coworkers used electroactive lignin as a low-cost and renewable source to induce pressure sensitivity in the design of wearable and flexible supercapacitors.²⁴⁰ They prepared a bio-based composite of lignosulfonate and a single-walled carbon nanotube hydrogel as a pressure-sensitive electrode with a cellulose hydrogel playing a hybrid role as an electrode and separator. They recorded a high specific capacitance of 292 F g⁻¹ at 0.5 A g⁻¹ and a power density of 324 W kg⁻¹ at 17.1 W h kg⁻¹ energy density. Their report also showed a stable electrochemical performance after 1000 bending cycles. The exceptional combination of flexibility, chemical stability, and electrochemical performance positions biomass-based flexible supercapacitors as leading contenders for integration into wearable electronic devices. In the pursuit of renewable and exceptionally efficient energy storage solutions, all-lignin-based flexible supercapacitors were crafted through the fusion of chemically cross-linked lignin hydrogel electrolytes with electrospun lignin/polyacrylonitrile nanofiber electrodes as shown in Fig. 9e.²³⁰ The prepared lignin hydrogel electrolytes exhibit notable characteristics, featuring high ionic conductivity and high mechanical integrity. The lignin-based carbon/polyacrylonitrile (PAN) composite electrode also demonstrates outstanding charge storage capability and kinetics, attributed to its interconnected porous channels. The fabricated devices exhibit a remarkable capacitance of 129.23 F g⁻¹ with an impressive capacitance retention of 95% over 10 000 cycles. Furthermore, these devices showcase exceptional flexibility and durability, maintaining their performance even under various bending angles. Additionally, these renewable flexible supercapacitors achieve a maximum energy density of 4.49 W h kg⁻¹ and a power density of 2.63 kW kg⁻¹, as shown in Fig. 9f. Thus, the utilization of renewable lignin-based materials to construct environmentally friendly and biocompatible flexible supercapacitors offers a novel avenue for advancing sustainable energy storage systems.

4 Conclusion and prospective outlook

This review presents a comprehensive review of the recent and representative advances in the use of lignin as a sustainable bioresource in the fabrication of supercapacitors. It has been confirmed that lignin can serve as a precursor for creating effective porous carbons (such as activated carbons, templated carbons, and carbon fibers), lignin-based nanofibers, and metal and polymer-based composites, which hold promise as the active material of several key components (such as electrodes, electrolytes, and binders) in bio-based supercapacitor design. Despite the significant contributions made thus far, research into lignin-based supercapacitors remains relatively underexplored. This section unveils novel and strategic developmental and complementary ideas for readers and prospective researchers whose aim is to push the limits of lignin-based supercapacitors beyond the laboratory.

4.1 Source–property relationship

Natural sources and extraction methods of agricultural biomaterials have been shown to significantly influence the physicochemical properties of their derivatives. Lignin extracted from grass, softwood, and hardwood has major differences in terms of bonding, functional groups, and structures at the molecular level. Also, the extraction routes of lignin have been shown to induce major alteration to the default chemistry of lignin. These major differences in the source and the method-induced alteration have presented lignin users with different forms of inhomogeneous and impure lignin. There is no basis for tracing the physiochemical properties of lignin to specific natural sources and methods of extraction. Therefore, additional efforts should be directed toward the development of facile and feasible methods to achieve stable and uniform lignin. This focus is crucial for understanding and optimizing the properties of lignin-based materials to meet the physiochemical requirements of commercial supercapacitors.

4.2 Controlled material engineering

It has been established that the electrochemical performance of supercapacitors largely depends on the structural properties of the active materials. Most lignin-based materials are prepared using conventional methods that rely on chemical pollutants and operate at extreme temperatures. Reports have also shown that during carbonization at high temperatures, the abundant oxygen atoms present in lignin are lost. These oxygen atoms are advantageous for enhancing the adsorption of electrolyte ions and facilitating redox reactions in supercapacitors. Regrettably, these processes predominantly yield carbon materials characterized by micropores, with limited to no presence of meso- and macropores. The absence of large pores hinders the free mobility of ions, which forms the basis of charge storage in a double-layer capacitor. Hence, it is crucial to develop optimal carbonization and activation processes that can simultaneously produce a well-developed pore structure and surface area while retaining an appropriate number of oxygen atoms.

4.3 Functional chemistry

Lignin-based materials are yet to fully capitalize on the flexibility and advantages presented by the abundance of functional chemistry. For example, low conductivity has challenged the exploration of lignin derivatives for electrolytes in supercapacitor applications. However, there are several known established approaches in which the conductivity of lignin can be improved via doping and functionalization with metals, conductive polymers, ionic liquids, etc. Another promising and emerging technology whose exploration has garnered significant attention is the development of lignin-based flexible supercapacitors. The major requirement for developing a standard material for flexible and solid-state supercapacitors is the electrochemical property retention ability of the material when exposed to stress (bending, stretching, and pressing). We recommend that prospective researchers take advantage of functional chemistry to improve the mechanical properties of lignin-based materials without compromising their electrochemical properties. This area has great promise in speedy integration into wearable electronics.

4.4 Characterization

A review of the literature in this field indicates that more than 65 percent of reports focus on highlighting the improved performance and beneficial impacts of lignin-based materials through capacitance, power, and energy densities. However, there is a notable absence of discussion concerning the relationships between the structure, properties, performance, and the long-term deterioration potential, which gives rise to concerns regarding long-term stability issues. For example, electrolyte ions with sub-nanometer (less than 1 nm in size) sizes and smaller than solvated ions can impact capacitance and energy density. Consequently, discussions regarding the charge storage mechanisms for nanoporous electrode materials should be explained using classical ion dynamic theory. These approaches underscore the importance of developing mechanistic models and simulations to elucidate mobile ion diffusion during charge–discharge. Also, utilizing appropriate in situ techniques and suitable characterization methods is essential for monitoring ion confinement within nanopores. To the best of our knowledge, no studies have yet explored the in situ charge formation and diffusion of confined ions on the surface and within the nanopores of lignin-derived electrodes during charge–discharge processes. Thus, integrated techniques are imperative to comprehensively understanding three key performance indicators: ion diffusivity, ion migration, and the velocity of ion migration, in real-time.

In summary, significant progress has been made in recent decades, although numerous challenges need to be addressed before realizing the commercialization of lignin in supercapacitor applications. Substantial advancements have been achieved in both improving performance and understanding the mechanisms involved in utilizing lignin-derived materials. We believe that continued research efforts within the scientific community will yield more exciting results, ultimately paving the way for the development of practical, high-value lignin-derived materials for supercapacitor applications.

Data availability

No primary research results, software or code have been included and no new data were generated or analyzed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This material is based on work supported by the Department of Agriculture (ARS) under Agreement No. 58-0204-2-142. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture. This publication is also a contribution of the Forest and Wildlife Research Center, Mississippi State University. This manuscript is publication #SB1128 of the Sustainable Bioproducts, Mississippi State University.

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