Emerging Lignin-Based Materials in Electrochemical Energy Systems

Abstract

Lignin is a promising material due to its excellent properties. It is commonly used in electrochemical energy systems (including electrolytes, electrodes, diaphragms, and binders) due to its low price, sustainability and rich functional groups. However, lignin’s applications in energy storage systems have not been systematically reviewed in the current research. In this article, recent advances in the preparation and design of lignin-derived energy storage materials were reviewed. Starting with a brief overview of the basic chemistry of lignin and the separation process, progress in the preparation of lignin-based materials for lithium-ion batteries, supercapacitors, fuel cells, and solar cells were described, respectively. This review provides the basis for the application of lignin in the field of electrochemical energy systems. Also, the current bottleneck problems and perspectives of lignin-derived materials in improved energy storage device performance were presented for future developments.

This review summarizes research works related to lignin, a crucial biological resource that is widely used in energy storage systems.

1. Introduction

The key to replacing fossil fuels in electric vehicles and portable electronic devices to reduce energy shortages and environmental pollution lies in electrochemical energy systems, which typically rely on Faraday and non-Faraday reactions occurring at the electrolyte and electrode interfaces in rechargeable batteries, fuel cells, and supercapacitors. The power density and energy density, cycle numbers of electrochemical energy and rate performance mainly depend on the composition, structure, morphology and construction of the electrolyte and electrode materials in electrochemical energy storage systems [1,2,3,4]. Therefore, the development and design of high quality and inexpensive recyclable electrochemical energy materials is essential to expand their practical applications [5].

Lignin, a natural and abundant aromatic biopolymer, is composed of coniferyl alcohol, p-coumaryl alcohol and sinapyl alcohol monolignols with several absorbing properties such as high thermal stability, biodegradable ability, high carbon content, and oxidation prevention properties with abundant functional groups [6]. Industrial lignin is produced from the pulp production process with a yield of 70 million tons per year, while most of the lignin samples are burned for providing heat and energy in pulping industries, leading to environmental pollution and resource waste [7,8]. To achieve lignin valorization, research interests are shifting towards the development of lignin into valuable and functional products in the territory of electrochemical systems. Currently, the application of lignin-based materials in electrochemical energy systems such as supercapacitors, rechargeable batteries, fuel cells, and solar cells is attracting more and more attention.

The characteristics of lignin, such as its low price, high carbon content and good thermal stability have aroused the interests of researchers regarding the preparation of energy storage devices. However, the use of this promising material to prepare energy storage materials with a long service life, high energy density and good safety has become a major challenge in this field.

The purpose of this review is to highlight and summarize the recent advances in lignin-based electrochemical energy materials and their applications in electrochemical energy systems, with an emphasis on, for example, electrolytes, binders, electrodes, and diaphragms. Finally, the future challenges and their possible solutions are discussed.

2. Lignin and Industrial Lignin

2.1. Fundamental Structures of Lignin

Lignin is an amorphous phenolic polymer that accounts for 20–30% of the weight of a plant and consists of phenyl propane fragments, such as coniferyl alcohol (G), p-coumaryl alcohol (H) and sinapyl alcohol (S) (Figure 1a), collectively referred to as aromatic alcohol precursors [9].

The polymer network of lignin is generated through the random coupling of three different monolignols via C-O and C-C linkages. In plants, the biosynthesis of natural lignin is a very complex biochemical and physiological process [10]. The synthetic process has been studied for many years and is briefly explained as follows: Firstly, with L-phenylalanine as raw material, three aromatic alcohol precursors are synthesized via the catalysis of various enzymes [11,12]. Then, these three precursors are transferred to the cell wall with the help of glycosides [13]. Last, the precursors undergo dehydrogenative polymerization by laccase and peroxidase, leading to lignin synthesis on the cell wall [11]. Three kinds of monolignols (G, S and H units) [14,15,16] were finally synthesized from the aromatic alcohol precursors shown in Figure 1b. The synthesis process of lignin is the result of the synergistic action of several enzymes. At present, there is no natural lignin which can be synthesized in a laboratory.

Different contents and types of lignin monomers exist in different plants. For example, softwood lignin contains about 95% G structural units and a small quantity of H units [17]. However, hardwood lignin is mainly composed of S and G units with different proportions [18]. In grass lignin, all types of monomers can be observed [19]. Schematic representation structures of softwood and hardwood lignin are shown in Figure 1c,d [9]. As shown in these figures, β-O-4, 4-O-5, 5-5′ and β-β are main linkages in these units, in which β-O-4 is the most typical linkage (accounting for 40–60%) [20].

Figure 1. (a) The structure of lignin in lignocellulosic biomass; (b) three monolignols; schematic representation of a softwood lignin; (c) and a hardwood lignin (d) structure (From Refs. [9,15,21]).

Figure 1. (a) The structure of lignin in lignocellulosic biomass; (b) three monolignols; schematic representation of a softwood lignin; (c) and a hardwood lignin (d) structure (From Refs. [9,15,21]).

2.2. Industrial Lignin from the Pulping Process

Currently, industrial lignin is mainly classified as lignosulfonate, kraft lignin, soda lignin, and organosolv lignin according to different pulping processes. Figure 2a showed the preparation parameters of different types of industrial lignin. The physicochemical properties of four industrial lignin are summarized in

Table 1. Physicochemical properties of lignin produced in pulping process (From Refs. [15,22,23,24]).

	Kraft Lignin	Lignosulfonate	Soda Lignin	Organosolv Lignin
Extraction methods	NaOH, Na2S, 150–170 °C, 1–2 h	Sulfur dioxide (Na, Ca or Mg as counter ion), 125–150 °C, 1–5 h	NaOH or NaOH- anthraquinone, 140–170 °C, 1–2 h	Acetic acid/formic acid/water, 80–130 °C, 1–4 h
Isolation methods	Acid precipitation	Ultrafiltration	Acid precipitation	Antisolvent precipitation
Molecular weight (×103 g mol−1)	1.0–6.0	15–50	0.8–3.0	0.5–5.0
Polydispersity	2.5–3.5	6.0–8.0	2.5–3.5	1.5–4.0
Impurities (%)	Sulfur 1–3	Sulfur 3–8	Sulfur < 0.1%	ash < 10
Tg (°C)	140–150	130	140	90–110
Solubility	Organic solvents and alkali	Water	Alkali	Organic solvents

.

Lignosulfonates are produced by the sulfite pulping process in acidic solutions. The product is called lignosulfonate because of the formation of many sulfonate groups on the side chain. Simplified reaction diagram in the process of sulfite pulping is shown in Figure 2b [25]. Lignosulfonates are characterized by good water solubility, high ash content, and high molecular weight [26], which have been used in many areas, such as adhesives, newspaper production, surfactants, batteries, and dispersants [27].

Soda lignin is extracted from the soda anthraquinone or soda pulping process [20], which was generally was used for herbaceous plants [28]. As shown in Figure 2c, during soda pulping, hydroxide anions attack and hydrolyze the α-ether bonds to produce low molecular weight soluble fragments [25]. Among various industrial lignin, soda lignin is a good choice for chemical modification [22,29] with a high purity and relatively similar structure to natural lignin. So far, a variety of reagents and polymers, such as hexamethylene diisocyanate [30] polyurethane [31] and ionic liquids [32] have been used for soda lignin modification.

The kraft extraction process, also called the sulfate process, is widely used for lignin extraction in many pulp industries due to its high performance. The chemical reaction of the kraft process was described in Figure 2d [25]. In the kraft pulping process, lignin generates via cleavage of ether bonds attached to the α or β position of lignin monomers [12], resulting in a low molecular weight of lignin samples in black liquor [26]. The kraft lignin is obtained by extracting from the black liquor.

Organosolv lignin is extracted from lignocellulose via organic solvents (alcohols, ketones and diols) mixed with water. The resulting organosolv lignin showed high purity and well-preserved β-O-4 linkages. Moreover, with the absence of sulfur contaminants and the advantages of low ash content and the recovery of organic solvent via reduced pressure from liquor fraction [33], organosolv lignin is widely utilized in paints, pharmaceutical industry, varnishes and adhesives [22,34].

Figure 2. (a) Preparation of industrial lignin by various pulping methods. (b) Schematic isolation reaction in sulfite process. (c) Hydrolysis of α-ether bonds in soda process. (d) Kraft reactions of β-aryl bonds in phenolic and nonphenolic phenylpropane units (From Refs. [6,15,25,34]).

Figure 2. (a) Preparation of industrial lignin by various pulping methods. (b) Schematic isolation reaction in sulfite process. (c) Hydrolysis of α-ether bonds in soda process. (d) Kraft reactions of β-aryl bonds in phenolic and nonphenolic phenylpropane units (From Refs. [6,15,25,34]).

3. Lignin in Lithium Batteries (LIBs)

Recently, NiMH batteries, NiCd batteries, lithium batteries (LIBs) [35] and lead-acid batteries are the main secondary batteries. Among them, LIBs have attracted increasing attention because of their high energy density and rechargeability. LIBs are mainly consisting of positive electrodes, negative electrodes, electrolytes and diaphragms, in which the charges are stored by inserting and extracting LIBs between the negative and positive electrodes. Although LIBs are the most promising energy devices, they still need to overcome some challenges, such as safety, cost-effectiveness, and environmental friendliness.

In this regard, considerable efforts have been taken to explore new electrolytes and electrodes to make environmentally friendly LIBs. Lignin, as the green and natural aromatic biopolymer, is regarded as a potential feedstock for producing valuable and functional materials in LIBs. For example, lignin-based electrodes in LIBs have shown relatively higher energy density in comparison to commercial graphite electrodes [36]. The lignin-based gel electrolyte also displays excellent ion-exchange performance. In addition, the use of lignin instead of traditional binders also promotes the improvement of battery energy density [37,38].

3.1. Lignin-Based Electrodes

To achieve LIBs with high energy and power density, it is crucial to find suitable electrode materials. Ideal electrode materials should provide good cycling life, high capacity, and easy diffusion of lithium ions. At present, graphite is usually used as the anode of commercial LIBs, and its theoretical capacity is 372 mA h g⁻¹. For improving the theoretical current capacity, a series of carbon materials including resin carbon, phenolic resins and organic polymer pyrolytic carbon [36] with larger pores and wider layer spacing have been extensively investigated. However, these carbon materials have the disadvantages of serious irreversibility and high cost, which limits their large-scale application in LIBs.

Lignin is a promising renewable carbon precursor and the second richest biopolymer on the earth. The aromatic hydrocarbon ring and macromolecular structure of lignin make high carbon content (over 50 wt.%). In addition, the high electrical conductivity of lignin-based carbon promotes fast lithium storage kinetics and high reversible Li⁺ storage capacity. Recently, lignin-derived electrodes with good cycle performance and higher capacity have been studied by many researchers [39,40,41,42,43].

The theoretical capacity of lignin-based porous carbon for LIBs is 1.5~2 times in comparison to that of graphite, and its unique hierarchical porous structure results in a significant effect on the transport and storage of Li ions.

Table 2. List of applications of different industrial lignin-based electrodes for LIBs.

Lignin Type	Lignin-Based Materials	SSA (m2 g−1)	Rate (A g−1)	Specific Capacitance (mA h g−1)	Ref.
KL	Carbon fibers	-	0.1 C	335	[40]
SL	Hollow mesoporous spheres	-	1	756	[39]
AL	3D hierarchical porous carbon	-	0.2	470	[44]
AL	N-doped carbon nanospheres	419.2	0.06	225	[42]
AL	3D porous carbon	167.5	0.2	469	[43]
AL	N, P- codoped porous carbon	675.4	1	1463.8	[45]
LS	Hierarchical mesoporous carbon nanospheres	462.8	0.1	520	[46]
OSL	Carbon nanofibers	381	2	200	[47]
OSL	Carbon nanofibers	-	0.015	193	[48]

and

Table 3. Specific capacity of composite lignin-based electrodes for LIBs.

Lignin-Based Materials	Porogen	SSA (m2 g−1)	Rate (A g−1)	Specific Capacitance (mA h g−1)	Ref.
AL/MgO/G	Mg (NO3)2·6H2O	628.09	2 C	1064.7	[36]
SL/Fe3O4	FeCl3·6H2O and Fe (NO3)3·9H2O	-	1	750	[49]
AL/SiNPs	Self-assembly	-	9	800	[50]
AL/Si	Mixing mixture	-	0.3	880	[51]
LS/NiO	Ni (OH)2	851.8	0.1	863	[52]
OSL/PEO	urea	381	2	200	[47]
EHL/CNTs	K2CO3	740	1	240	[53]

summarize the performance of various industrial lignin and lignin composites as electrodes for LIBs, respectively.

Li et al. introduced the lignin-derived porous carbon from rice husk as an anode material for LIBs [43]. The porous carbon obtained from soda lignin was isolated from black liquor by addicting acid to it. Then, the porous carbon was fabricated by one-step and two-step methods at different temperatures with a ZnCl₂ activator, respectively. The carbons obtained at 500 °C with one-step method showed a unique porous structure, which can decrease the diffusion distance of lithium ions and enhance the connection between electrode and electrolyte. It shows high specific capacity (469 mA h g⁻¹, after 100 cycles) and excellent cycle stability, possessing much higher capacity than graphite.

To further increase the capacitance of lignin-based porous carbon, Liu et al. [45] attempted to adjust the microstructure and the surface chemical state of lignin-derived porous carbon. They prepared the precursor lignin/Zn@g-C₃N₄ by anchoring ultrathin g-C₃N₄ nanosheets in the lignin structure. The g-C₃N₄ was used as the nitrogen source and soft template, while lecithin was added to provide phosphorus (Figure 3a). The porous carbon (N, P@C) prepared by two-step carbonization presents a high degree of graphitization (I_D/I_G = 1.02), a large specific surface area (S_BET = 675.4 cm² g⁻¹), and mesoporous dominant pores (average pore size of 6.898 nm). N, P@C exhibits an excellent multiplicative capacity of 261 mA h g⁻¹ at 10 A g⁻¹, and a satisfactory cycling stability of 1463.8 mA h g⁻¹ at 1 A g⁻¹ after 500 cycles as an anode material for LIBs. Due to the synergistic effect of N and P heteroatoms, N, P@C contributed the LIBs with electrochemical performance superior to most other biomass-derived carbon-based anodes [54,55,56].

Carbon nanofibers (CNFs) are considered to have a high performance rate because of their one-dimensional morphology, which provides high conductivity and a large surface area. To construct high-performance LIBs, Wang et al. [36] added lignin to carbon nanofibers using a gas-electric hybrid technique, and the prepared anodes were flexible and self-supporting to be used directly in LIBs without any binder (Figure 3b). At a high current density of 2 C, the specific capacity of the first cycle is 1135.4 mA h g⁻¹, and the specific capacity after 100 cycles is 1064.7 mA h g⁻¹, showing stable cycling performance, excellent discharge specific capacity, and high-rate performance.

Porous lignin-derived carbon (PLC) prepared by activation is expected to be applicated in energy storage because of its high surface area. However, pure PLC with low graphitization, large microporous volume, and poor structural stability have become major obstacles for further applications as electrode materials of LIBs. Therefore, Table 3 summarizes the preparation methods and electrochemical properties of various composite lignin-based electrodes.

As shown in Table 3 and Figure 3c, highly dispersed lignin/carbon nanotubes composites (lignin/CNTs) were prepared by Xi et al. [53] via hydrophobic self-assembly method based on π-π interactions. In Figure 3d,e, the rate performance and galvanostatic charging-discharging (GCD) curves of PLC/CNTs-5:5 and CNTs showed that the capacity increased by 24% and the efficiency of the cell constructed by PLC/CNTs-5:5 was improved by 25% than the cell constructed by CNTs. Then, PLC/CNTs samples were obtained after carbonization with a layered structure (as shown in Figure 3f,g) supported by carbon nanotubes. The introduction of carbon nanotubes improves the structural stability and conductivity of PLCs, especially the microstructure, providing active sites and transport channels for lithium ions. Chen et al. [57] formed an excellent composite electrode SiO_x/C from lignin and high-capacity silicon nanoparticles. The composite anode exhibited good rate performance and excellent cycling stability. After 250 cycles, the capacity retention was nearly 99% maintained at 900 mA h g⁻¹ at 200 mA g⁻¹.

The above researches show that several preparation methods have made lignin-derived carbon possess adjustable and targeted microstructure. To obtain high-performance devices, lignin acts as a flexible precursor that can be adapted to any structure, as expected, while it can also be complexed with other materials. Lignin has an ecologically friendly and controllable structure, which makes it a highly competitive green carbon material that can be produced on a large scale. Moreover, lignin-derived porous carbon materials with strengthened conductivity networks and abundant ion diffusion pathways, are usually considered to be a promising substitute for graphite electrodes.

Figure 3. (a) Synthesis of lignin/Zn@g-C3N4 and N, P@C. (b) Preparation route and electrochemical performance of LCNF/G. (c) The fabrication processes of PLC/CNTs. The GCD curves (d) and rate performance (e) of CNTs, PLC and PLC/CNTs-5:5. (f) The SEM image and local high resolution SEM image of PLC/CNTs-5:5 (layer structure shown as yellow arrows). (g) The TEM of PLC/CNTs-5:5 (PLC is tightly wrapped around carbon nanotubes shown as yellow arrows). (From Refs. [36,45,53]).

Figure 3. (a) Synthesis of lignin/Zn@g-C3N4 and N, P@C. (b) Preparation route and electrochemical performance of LCNF/G. (c) The fabrication processes of PLC/CNTs. The GCD curves (d) and rate performance (e) of CNTs, PLC and PLC/CNTs-5:5. (f) The SEM image and local high resolution SEM image of PLC/CNTs-5:5 (layer structure shown as yellow arrows). (g) The TEM of PLC/CNTs-5:5 (PLC is tightly wrapped around carbon nanotubes shown as yellow arrows). (From Refs. [36,45,53]).

3.2. Lignin-Based Gel Electrolytes

Liquid electrolytes in LIBs may raise several serious issues, such as explosion, flame and leakage [58]. Gel polymer electrolytes (GPEs) can isolate the anode and cathode of the cells to inhibit short circuits, and also accelerate rapid transfer of charge. To date, although various polymers such as polyacrylonitrile (PAN) [59], polyethylene oxide (PEO) [37], plexiglass (PMMA) [60], polyvinylidene fluoride (PVDF) [61], polyvinyl acetate (PVA) [62] and polyvinylpyrrolidone (PVP) [63] have been used as the primary matrices in GPEs, their non-biodegradable and non-renewable weaknesses of fossil sources cause serious environmental pollution and resources waste. Exploring new green and environmentally friendly lignin matrices of GPEs for LIBs will achieve green and environmentally friendly LIBs.

Lignin, as a natural biopolymer, is biodegradable and biocompatible compared to synthetic polymers. GPEs with pure lignin-based LIBs were obtained by evaporative drying of lignin suspensions from the result of Gong et al. [58]. The resulting membranes exhibited an excellent porous structure and bending properties with a high conductivity of up to 230 wt.% liquid electrolyte uptakes, which is superior to conventional polymer-based GPEs (less than 100 wt.%). The excellent performance is ascribed to the existence of strong hydrogen bonding between a large number of phenolic hydroxyl units in lignin and the lithium ions, which generates more lithium ion charge carriers. It is promising that the lithium ion mobility number is as high as 0.85, whereas, for GPEs based on conventional polymers, the number usually ranges from 0.20 to 0.70. Thus, gel electrolytes prepared from lignin show a considerable potential for applications in the fast charge and discharge of LIBs.

Due to the poor mechanical strength of pure lignin film, Wang et al. [64] prepared a self-supporting and robust lignin-based composite film loaded with polymers through a simple blending solution casting process, which further improved the comprehensive performance of lignin-based GPEs. The film exhibited a tensile strength of 4 MPa, which was more than 10 times that of pure lignin film. After fully absorbing the organic electrolyte, the sample exhibited excellent electrochemical properties such as a wide electrochemical window (4.7 V), high conductivity (6.3 × 10⁻⁴ S cm⁻¹, 30 °C), and an excellent lithium ion transfer number (0.63). In addition, the compatibility and stability of the interface between the electrode and electrolyte membrane, as well as the effective prevention of lignin-based electrolyte membrane on lithium dendrite, have been confirmed.

The preparation of lignin electrolyte membrane is simple, and its comprehensive properties such as thermal stability, liquid electrolyte absorption, ionic conductivity and lithium ions migration number are outstanding, which indicates that lignin is a potential candidate material for GPEs.

3.3. Lignin as Separators for LIBs

The separator is an important component between the anode and cathode and provides a microchannel for lithium ions migration. The mechanical properties of the diaphragm are critical to the performance and safety of the batteries. Currently, although polyolefins are the most popular separators such as polyethylene (PE) and polypropylene (PP), polyolefin films show many weaknesses such as low porosity (about 40%), insufficient electrolyte wetting, and poor thermal stability. Lignin is an ideal material to prepare membranes due to its high biocompatibility and stable aromatic rings.

Zhao et al. prepared lignin/acrylonitrile (PAN) composite fiber nonwoven membranes (L-PAN) with different lignin contents by electrostatic spinning [38]. The introduction of lignin significantly improved thermal stability. Compared with the significant shrink of PP film, there is no significant change on the lignin/PVA film after heating at 150 °C for 15 min, which showed that PAN 3:7 achieved the best electrochemical performance. After 50 cycles, the capacity retention was 95% at 0.2 C. Satisfactory electrolyte affinity and high porosity of L-PAN can further improve the electrochemical properties such as rate, cycling performance and ionic conductivity (1.24 × 10⁻³ S cm⁻¹) of LIBs. Md-Jamal et al. [65] prepared lignin/polyvinyl alcohol (PVA) nanofiber membranes as separators for LIBs by blending lignin with polyvinyl alcohol (PVA) using an electrostatic spinning method. With a porous, highly interpenetrating network structure, it demonstrates high electrolyte absorption, excellent compatibility, and excellent wettability. The film also shows high thermal stability and flame retardancy. Polyacrylonitrile (PAN) is the most widely used host polymer, which forms a strong backbone to be a separator. Moreover, the introduction of lignin can further improve the electrolyte porosity and wettability of the diaphragm, thereby improving the electrochemical performance of LIBs, such as C-rate performance, ionic conductivity and cycling performance.

3.4. Lignin-Based Binders for LIBs

Currently, the most universally utilized binder, PVDF, is difficult to biodegrade. PVDF is not easily recyclable, expensive, and uses volatile organic compounds in its processing [66]. In addition, the reaction between lithium and PVDF is exothermic and may cause thermal runaway and self-heating. Lignin itself is a cross-linked biopolymer with adhesive properties that can be used as an environmentally friendly and low-cost binder to replace the PVDF polymer in rechargeable battery electrodes.

Roblesa et al. [66] used lignin extracted from three different pulping liquors (sulfate pulping, organic pulping, and soda pulping) to prepare a binder for cathodes of lithium batteries, respectively, and compared them with conventional PVDF. The results of the electrochemical performance tests are shown in Figure 4b. It was found that the samples had higher capacitance retention after long cycles as compared to the electrodes with PVDF as the binder. This result indicated that lignin could significantly improve the performance of LIBs at high current rates. In addition, they found that the cycling performance of different obtained lignin is close to that of PVDF (as shown in Figure 4a) and all can be environmentally friendly alternatives to PVDF binders.

For high-voltage LIBs, when the cathode voltage exceeds 4.3 V, the carbon electrolyte is prone to oxidation and decomposition, resulting in a rapid decrease in capacity. It is generally believed that free radical reactions are responsible for their decomposition. However, commercially available PVDF binders cannot effectively scavenge free radicals to avoid electrolyte decomposition at the electrolyte–electrode interface. Inspired by the radical scavenging performance of phenolic structures, Ma et al. [67] investigated lignin as a binder added to a high-voltage cell, which consists of LiNi_0.5Mn_1.5O₄ as the cathode and 1 M LiPF₆ in ethylene carbonate/dimethyl carbonate as the electrolyte. A schematic representation of the membrane formation process based on PVDF adhesive and lignin adhesive was shown in Figure 4c. They found that the free radical capture energy (3.37 eV) on lignin was stronger compared to PVDF (0.29 eV), as shown in Figure 4d furtherly demonstrated the effective free radical removal ability of the lignin binder. Figure 4e explains the process of free radical removal of PVDF and lignin, respectively. Capacitance retention of the cell assembled with lignin binder was 94.1% after 1000 cycles, significantly higher than that of cell with PVDF binder (capacitance retention was 46.2%).

Lignin-based binder can also stabilize the electrolyte solvent by building a good multidimensional interface in the whole battery, which can solve the interface problem of high-voltage electrodes. Moreover, biomass-based lignin adhesives can use water as solvent to fabricate electrodes, thus achieving environmentally friendly production.

Figure 4. (a) The 5th and 50th discharge curves of four electrodes at 100 mA g⁻¹. (b) Capacitance retention of four electrodes at C/4 rate after 50 cycles. (c) Schematic representation of the membrane forming process based on lignin and PVDF binder. (d) Potential energy map of the reaction of the free radicals with the lignin/PVDF binder. (e) Diagram of the free radical removal process for the lignin and PVDF binders (From Refs. [66,67]).

Figure 4. (a) The 5th and 50th discharge curves of four electrodes at 100 mA g⁻¹. (b) Capacitance retention of four electrodes at C/4 rate after 50 cycles. (c) Schematic representation of the membrane forming process based on lignin and PVDF binder. (d) Potential energy map of the reaction of the free radicals with the lignin/PVDF binder. (e) Diagram of the free radical removal process for the lignin and PVDF binders (From Refs. [66,67]).

4. Lignin in Supercapacitors

Supercapacitor is another kind of electric energy storage equipment with long cycle life (about millions of cycles), high energy and power density, and good reversibility [68]. It mainly consists of electrodes, electrolytes, collectors, and a diaphragm [69]. According to the mechanism of energy storage, supercapacitors are usually divided into double-layer capacitors (EDLCs) and pseudo-capacitors (PCs). The preparation of high-performance supercapacitors with lignin is a promising method. In this part, we review the recent research progress and application of lignin in supercapacitor electrode and electrolyte materials.

4.1. Lignin-Based Electrodes for Supercapacitors

The electrode material, the main energy storage component, has a decisive influence on the electrochemical properties of supercapacitors. At present, the commercial activated carbon mainly uses coconut shells or petroleum coke as precursors, and generally has a microporous structure, which has the disadvantages of low capacitance and high price. Due to the lack of fossil resources, tremendous efforts have been paid to make porous carbon using biomass using conventional methods, such as silk cocoon [70], willow catkin [71], tobacco [72], and moringa oleifera branches [73] for supercapacitors. The porous carbon of these biomass has a large surface area and is mainly contributed by micropores, while micropores always hinder the entry of electrolyte ions into the internal pores, leading to capacity decay at a high current density. In addition, with high ash content, these biomass precursors are not suitable to produce electrode materials for supercapacitors.

In comparison, lignin has a high carbon content (about 60%) and an inherent aromatic structure, which is a promising precursor for the fabrication of carbon materials [15]. The porous carbon prepared from lignin has a hierarchical pore structure that is conducive to ion transport and provides an efficient way of obtaining high-energy density. In recent years, the fabrication of carbon materials (e.g., activated carbon, carbon fiber) for supercapacitors from lignin has attracted extensive attention. According to the different structure and preparation processes, we divided the lignin-derived porous carbon into three types: template carbon, activated carbon and composite carbon. Among them, activated carbon and template carbon are mainly used for double-layer supercapacitors, while composite carbon is mainly used for pseudocapacitors. In this section, the applications of lignin-derived carbon electrodes in different supercapacitors are described in detail.

Table 4. Specific capacity values of electrodes for EDLCs based on lignin.

Materials	Porogen	Electrolyte	Rate (A g−1)	Specific Capacitance (F g−1)	Ref.
AL	Freeze drying	1 M H2SO4	0.5	281	[74]
AL	KOH	6 M KOH	0.2	286.7	[75]
AL	F127, MgO	6 M KOH	0.2	186.3	[76]
KL	KOH	1 M H2SO4	0.63	196.5	[77]
LS	KOH	6 M KOH	0.5	305	[78]
OL	KOH	1 M TEABF4	1 mA cm−2	131	[79]
OL	Self-assembly	6 M KOH	-	90 F cm−3	[80]
Commercial AC	-	6 M KOH	1	139.35	[81]

and

Table 5. Specific capacitance of composite lignin-based electrodes for PCs.

Materials	Porogen	Electrolyte	Rate (A g−1)	Specific Capacitance (F g−1)	Ref.
KL/PANI		6 M KOH	0.5	141.3	[82]
KL/Fe2O3	-	1 M H2SO4	0.5	390	[83]
KL/aniline	KOH	6 M KOH	1	333	[84]
KL/CNT	-	1 M H2SO4	2.5	177	[85]
AL/PPy	-	0.5 M H2SO4	0.5 mA g−1	444	[86]
LS/PANI	-	1 M H2SO4	10	377.2	[87]
LS/PEDOT	-	0.1 M HClO4/acetonitrile	1	170.4	[88]
LS/PANI/GO	-	6 M KOH	0.5	266.7	[89]
OL/Fe(acac)3	-	1 M Na2SO3	0.5	121	[90]
EHL/urea	KOH	6 M KOH	0.5	318	[91]
EHL/PANI	-	0.5 M H2SO4	0.29	229	[92]

summarize the various lignin-derived electrodes used in EDLCs and PCs, including the types of activators and the corresponding electrochemical properties.

4.1.1. Lignin-Derived Electrodes for Double-Layer Capacitors

Lignin-based porous carbon can be easily obtained by activation and carbonization. It has received much attention as electrode materials for supercapacitors owing to its excellent electrical conductivity, high surface area, and high chemical stability. So far, lignin-based activated carbons have been prepared mainly by physical and chemical activation methods [6]. The effects of three different activators (H₃PO₄, K₂CO₃ and KOH) on the activated carbon preparation process from kraft lignin were discussed by Zapata et al. [74]. They found that all porous carbons exhibited a microporous-dominated structure when they choose different kinds of activators. However, the samples that used KOH as an activator required a relatively lower carbonization temperature and showed the highest specific surface area (1515 m² g⁻¹) and outstanding capacity retention (93% remained after 2000 cycles). They concluded that KOH is an effective activator for the preparation of supercapacitor electrodes from kraft lignin due to its satisfactory electrochemical response and high specific surface area.

Lim et al. [77] successfully converted lignin into porous carbon using a two-step activation method by selecting KOH as the activator. The specific surface area of the obtained porous carbon was as high as 2304 m² g⁻¹, and the specific capacitance was as high as 131 F g⁻¹ at a current density of 1 mA cm⁻². The capacitance retention was 91% after 10,000 cycles at a current density of 30 mA cm⁻². The assembled device has excellent rate performance, providing a high energy density and power density of 33.9 W h/kg and 4.48k W/kg, respectively.

Carbon nanofibers have become another common activated carbon due to their unique three-dimensional mesh structure, abundant accessible porous structure, high specific surface, high electrical conductivity, and low cost. Lignin-derived carbon nanofibers can be prepared by an efficient and facile electrostatic spinning method. Du et al. used lignin extracted from different plants (poplar, pine and corn stover) as raw materials with ethanol as a solvent [82]. As shown in Figure 5a, the lignin-containing different monolignols were mixed with polyacrylonitrile (PAN) as the precursor and spun into nanofiber yarns by electrostatic spinning method, and then carbonized at high temperature to prepare carbon nanofibers. They found that carbon nanofibers with excellent physical and electrochemical properties were prepared by mixing the poplar lignin and PAN in a 5:5 ratio (conclude from Figure 5b). Their tensile strength was up to 35.32 MPa and also had a high specific surface area of 1062.5 m² g⁻¹. As shown in Figure 5c,d, the assembled symmetrical supercapacitor in the symmetrical electrode device also shows an excellent energy density of 39.6 W h/kg at a power density of 5 k W/kg and a capacitance retention of 90.52% after 5000 cycles, indicating excellent cycling stability.

Template is considered as another effective method to design structures such as pore size, porosity of carbon to improve their electrochemical performance [6]. Herou et al. [80] increased the volumetric energy density by adjusting the mesoporous structure to fit the size of electrolyte ions. They adopted the evaporation-induced self-assembly (EISA) soft template method to use F127 as a soft template to produce biomass-based ordered mesoporous carbon from a complex equilibrium of lignin, phloroglucin and glyoxal. The preparation process is shown in Figure 5e. It was found that the mesopore diameter of the porous carbon prepared from 50 wt.% lignin and 50 wt.% phloroglucin decreased to 4 nm. The presence of narrower mesopores was sufficient to limit the diffusion layer to accelerate the adsorption kinetics. And the volumetric energy density of the prepared porous carbon material was 3 W h L⁻¹ at 1 k W L⁻¹, which was better than that of the material prepared from pure phloroglucin. However, most templates suffer to operate on a large scale because of the high cost. Liu et al. [79] prepared two-dimensional lignin flakes by freeze-drying the lignin dispersion in liquid nitrogen, in which ice crystals played the role of laminar templates (Figure 5f). The symmetric supercapacitor assembled from the prepared porous carbon showed an energy density of 14.3 W h/kg at a power density of 28,611 W/kg. Therefore, it is greatly significant to choose materials with low cost and convenient operation as templates to prepare supercapacitors.

As EDLC electrodes, lignin-derived porous carbon has significant electrochemical properties, such as high energy densities, fast charging rates, and a long cycle life.

Figure 5. (a) The formation process of lignin-based carbon nanofibers. (b) GCD curves of LCNFs-PRL (5:5) at different current densities from 1 to 10 A/g. (c) Capacitance retention and the cycle number and (d) Ragone plots of symmetric supercapacitors. (e) Schematic diagram of the preparation of ordered mesoporous carbon. (f) The preparation process of carbonized lignin sheets (From Refs. [79,80,82]).

Figure 5. (a) The formation process of lignin-based carbon nanofibers. (b) GCD curves of LCNFs-PRL (5:5) at different current densities from 1 to 10 A/g. (c) Capacitance retention and the cycle number and (d) Ragone plots of symmetric supercapacitors. (e) Schematic diagram of the preparation of ordered mesoporous carbon. (f) The preparation process of carbonized lignin sheets (From Refs. [79,80,82]).

4.1.2. Lignin-Based Electrodes for Pseudocapacitors (PCs)

Pseudocapacitors are one of the supercapacitors in which fast redox reactions occur on the surface and inside of electrodes to provide pseudocapacitors with higher capacity and energy density than EDLCs. Materials used for pseudocapacitor electrodes are mainly transition metal oxides (Fe₃O₄, MnO₂ and RuO₂) and conductive polymers such as poly(3-4-ethylenedioxythiophene) (PEDOT), polypyrrole (PPy), polyaniline (PANI), and polythiophene. Transition metal oxides are relatively high cost and have lower porosity. Conductive polymers are prone to swelling during charging and discharging, which leads to safety problems. Therefore, they are all unsuitable for large-scale applications. To improve the properties of electrochemical supercapacitors, lignin-based electrodes have been widely studied.

The quinone groups on structure of lignin and its derivatives can accompany with the storage and exchange of electrons and protons during the oxidation and reduction process [83]. Zhou et al. further oxidized the methoxyl of lignosulfonate to phenolic hydroxyl groups by functional modification. It could form hydroquinones with the adjacent phenolic hydroxyl to provide additional pseudocapacitance [83]. The phenolic hydroxyl groups were increased by 25.6% and the capacitance was increased by 21.9% (from 322 to 390 F g⁻¹ at 0.5 A g⁻¹) compared to the material without modification. The doping of heteroatoms in activated carbon can initial redox reactions. Also, it can effectively improve the wettability of the electrolyte and electrodes. Due to the low nitrogen content of lignin, many efforts have been made on introducing nitrogen atoms into the porous carbon skeleton in recent years. For instance, Wang [84] prepared nitrogen-doped rod-shaped porous carbon from aniline-modified lignin. The obtained porous carbon had a high specific capacitance of 336 F g⁻¹ and excellent cyclic stability. As shown in Figure 6a, Shi et al. synthesized three-dimensional oxygen, nitrogen and sulfur co-doped porous carbon (ONS-HPC) from aminating enzymatic lignin (EHL) by the Mannich reaction, then followed by KOH activating [91]. The energy density of the asymmetric supercapacitor devices constructed by ONS-HPC was 16.7 W h/kg at a power density of 249 W/kg. They also provided the explicit process of the N-O-S group providing pseudocapacitance in 6M KOH electrolyte, as shown in the following equations:

–C- N -C ─ + H₂O + e⁻ ↔ –C-NH-C– + OH⁻

(1)

–C- S -C ─ + H₂O + e⁻ ↔ –C-SO₂-C– + 3OH⁻

(2)

–C =O + H₂O + 2e⁻ ↔ –CH-O– + OH⁻

(3)

The combination of conducting polymers or metal oxides with carbon to prepare high-performance materials in energy storage devices has also attracted more research interest. Hollow carbon nanofibers (HCNFs, Figure 6b) decorated with iron oxide particles (as shown in Figure 6c) were prepared by Yu et al. [90] with a coaxial electrospinning technique. The samples were prepared with iron (III) acetylacetonate as the coaxial material, vinyl acetate lignin as the shell, and polystyrene-acrylonitrile solution as the core. As shown in Figure 6d,e, the specific capacitance of the sample was 121 F g⁻¹ at 0.5 A g⁻¹ and was maintained at 90% after 1000 cycles in a 1 M sodium sulfite solution.

Metal oxides are limited in commercialization because of their high cost and environmental problems. Conductive polymers are considered to be one of the most promising electrode materials for pseudocapacitors because of their simple preparation, low cost and high capacitance. Among them, polyaniline (PANI) is the subject of current research owing to its high theoretical specific capacity (2000 F g⁻¹) compared to other conductive polymers. However, the structure of PANI is easily destroyed during charging and discharging. To solve this problem, Hu et al. proposed combining carbon with PANI to produce composite electrodes with enhanced stability [92]. They tried to use lignin carbon nanofibers as a substrate to make PANI electrodeposit in situ and evaluated them as electrodes of pseudocapacitors (Figure 6f). They found that the sample obtained with a mass ratio of lignin to PAN of 4 had a higher specific surface area. After assembling the solid-state supercapacitor, as shown in Figure 6g,h, the specific capacitance of the device can reach 229 F g⁻¹, and energy density is 11.13 W h/kg at a power density of 0.08 k W/kg. The cycling stability is superior to pure PANI as the electrode.

Lignin shows promise in the preparation of high-performance composite electrodes for pseudocapacitors due to its rich functional groups and can be easily modified for use in composite with other polymers. In addition to the capacitance of EDLCs, pseudocapacitors constructed from lignin also have active elements to arouse redox reactions for faradic capacitance.

Figure 6. (a) The principle of lignin being chemically modified and the preparation process diagram. (b) SEM images of HCNFs. (c) EDX mapping of Fe element of HCNFs-25. (d) N₂ adsorption/desorption isotherm of HCNFs. (e) GCD curves of HCNFs at 0.5 A g⁻¹. (f) Manufacturing process of LCNF/PANI hybrid electrodes. (g) Specific capacitances of LCNF/PANI supercapacitors at different current densities. (h) Cyclic stability of LCNF/PANI supercapacitors (From Refs. [90,91,92]).

Figure 6. (a) The principle of lignin being chemically modified and the preparation process diagram. (b) SEM images of HCNFs. (c) EDX mapping of Fe element of HCNFs-25. (d) N₂ adsorption/desorption isotherm of HCNFs. (e) GCD curves of HCNFs at 0.5 A g⁻¹. (f) Manufacturing process of LCNF/PANI hybrid electrodes. (g) Specific capacitances of LCNF/PANI supercapacitors at different current densities. (h) Cyclic stability of LCNF/PANI supercapacitors (From Refs. [90,91,92]).

4.2. Lignin-Based Hydrogel Electrolytes

Hydrogel electrolytes are important in the construction of flexible supercapacitors as they can prevent the leakage of liquid electrolytes and provide high ionic conductivity. Although many hydrogel electrolytes have been used to build supercapacitors and show excellent electrochemical performance [93,94,95], the main materials used in hydrogel electrolytes come from fossils, which thus place a constraint on the sustainability of electronics. Few studies have been reported on the application of lignin as electrolytes of supercapacitors. Currently, the most reported lignin-based electrolytes are mainly hydrogels for the construction of solid-state supercapacitors. Park et al. first synthesized lignin hydrogel electrolytes by alkali-catalyzed cross-linking reactions and ring-opening polymerization [96]. These cross-linked lignin-based hydrogel electrolytes have high ionic conductivity (10.35 mS cm⁻¹) and mechanical stability (523% solubility ratio). The constructed flexible supercapacitors maintain high capacitance at various bending angles, showing excellent power and energy densities of 2.63 k W/kg and 4.49 W h/kg, respectively.

When the lignin content in the hydrogel is high, the mechanical strength of the hydrogel prepared using the single chemical cross-linking method is poor. In addition, when they are subjected to large stress, strain, or compression, their strength and toughness are limited. Therefore, they cannot meet the mechanical properties of flexible energy storage devices. According to the principle of different solubility of lignin in acidic and alkaline solutions, Liu et al. [97] synthesized a lignin hydrogel by the chemical–physical double cross linkage method. The compressive mechanical strength of the prepared hydrogel (4.74 MPa) is 40 times higher than that of a single chemically cross-linked hydrogel, and has excellent compression properties for cyclic loading and unloading. The ionic conductivity of the lignin-based hydrogel electrolyte is superior (0.08 S cm⁻¹) compared to that of pure H₂SO₄ solution. The supercapacitor constructed from this hydrogel has an excellent energy density of 15.24 W h/kg at a power density of 95 W/kg. The capacitance retention is close to 100% after 500 cycles under 180° bending. The use of renewable lignin as electrolyte material for supercapacitors is of great importance for sustainable chemistry and energy storage.

Lignin is a strongly cross-linked aromatic heteropolymer that includes a large number of phenolic, carbonyl and phenolic groups. Therefore, these different chemical functions of lignin make it a promising candidate for the preparation of high-performance electrolytes. Currently, there are just few routes involving converting lignin into electrolytes for supercapacitors, and more simple preparation methods need to be further explored.

5. Lignin in Fuel Cells

The conversion of complex compounds in biomass into sustainable fuels is a rapidly growing area of research. Biomass is an alternative to fossil resources and can be used to produce fuels. In addition, starch, triglycerides and lignocellulose are the main raw materials from biomass that can be used for fuel production. Lignocellulose is a rich and inexpensive inedible source that can be used in place of expensive starches. Many fuel cells, such as microbial fuel cells (MFCs), direct carbon fuel cells (DCFCs) and direct methanol fuel cells (DMFCs) have been reported to achieve high performance by using lignin as a raw material. In this section, we describe the recent knowledge about lignin-based materials for fuel cells.

The conversion of lignin or lignosulfonate to produce electricity can be achieved using DMFCs, MFCs, DCFCs, etc. Lima et al. [98] were the first to use direct carbon fuel cells to convert lignosulfonate and sulfate lignin into electricity directly. In addition, lignin-based porous carbon was also involved in the electrode construction of the device. Shewa et al. [99] produced electricity from lignosulfonate pretreated with photocatalysis in a single-chamber microbial fuel cell. The current density and power density of 868 mA m⁻² and 248 m W m⁻² were obtained, respectively. However, these approaches usually require some external treatment of the raw materials in advance, which complicates the process and reduces energy efficiency. Liu et al. [100] developed a high-performance liquid-phase catalytic fuel cell (LCFC) with heteropoly acid as the catalyst and electrolyte, as shown in Figure 7a. Under the catalysis of heteropoly acid (H₃PW₁₁MoO₄₀), the raw biomass materials can undergo oxidation reaction in the cell after being preheated at 100 °C. Compared with many previous studies, this method can realize the direct electrical conversion of lignin without any complicated pretreatment of lignin. Zhao et al. [101] also reported a new direct biomass fuel cell with heteropoly acids as the electron carrier and photocatalyst (Figure 7b). This fuel cell has been improved based on the previous work. The efficient conversion of lignin into electrical energy at low temperatures was achieved by using heteropoly acid as an electron carrier for anodes. The use of lignin in fuel cells is a feasible and promising research direction while related researches are still in the initial stage.

6. Lignin in Solar Cells

Solar cells are devices that can convert solar energy into electrical energy. Poly(3,4-ethylenedioxythiophene): polystyrene sulfonic acid (PEDOT: PSS) is typically used as a hole extraction layer (HEL) or hole injection layer in common organic optoelectronic products [102]. However, the relatively low work function (4.8–5.1 eV) imposes limitations on the extraction or injection of cavities from adjacent functional layers. In recent years, considerable research [103,104] has been carried out to improve the application performance of PEDOT films. Biomass-derived materials are recognized as the most promising candidates for their multiple reaction sites and compatibilities with aromatic substances.

Lignin is the most plentiful natural aromatic macromolecule on earth [105]. It has a branching, heterogeneous, three-dimensional network structure consisting of phenylpropane units connected by conjugate and conjugative block bonds. It was found that the chemical structure of lignin contained phenol units that had hole transport properties and could be used as a conjugated polymer in solar cells. Using lignosulfonate (LS) as raw material, Li et al. [102] developed a 3,4-ethylenedioxythiophene/lignosulfonate (PEDOT/LS) composite film with the high-power function, good uniformity, and excellent water resistance. The solar cell has a high-power conversion efficiency of 12.9%. The cells prepared with PEDOT/LS films exhibit more durable stability in N₂ and air under unencapsulated conditions.

In addition to synthesizing hole transport materials, lignin can also show superior performance as electron transport materials (ETMs) in carrier transport layers. Hu et al. [106] successfully prepared efficient isotropic electron transport interface layers using dimethyl kraft lignin (D_MeKL) and perylene diimide (PDIN) as shown in Figure 8a–e. The transport interface layer takes advantage of the special 3D mesh structure of D_MeKL and the high conductivity of PDIN, which can significantly improve the electron collection and transfer capability in the cross-section and vertical section. The relatively higher power conversion efficiency of organic solar cells using D_MeKL/PDIN as an interlayer can reach 16.0%, which is higher than the original PDIN interlayer (15.4%).

An alternative approach to develop organic solar cells is the use of lignin-based activated carbon as electrodes. Ma et al. [107] synthesized freestanding, mechanically flexible nanofibers that were obtained from alkali lignin by electrospinning carbon nanofibers (ECNFs). The SEM and TEM images of ECNFs and ECNFs–Pt nano-felts are shown in Figure 8f,g. A small amount of Pt nanoparticles was deposited on the surface of the nanofibers to fabricate electrodes for dye-sensitized solar cells (DSSCs). The energy conversion efficiency can reach 6.94%, which is comparable to the DSSCs consisting of conventional Pt as the counter electrode (7.29%). At present, the application of lignin as an anode dopant in photovoltaic devices is rarely studied, which needs to be further studied.

Figure 8. (a) Device structure. (b) Typical linkages of conjugate and conjugate-blocked in D_MeKL. (c) Intramolecular conformational locks formed in D_MeKL by hydrogen bonds. (d) Secondary bonds assist in the extraction and transport of electrons. (e) Preparation of electron transfer 3D network by D_MeKL/PDIN/Y6. (f) SEM images showing typical shapes of ECNFs and ECNFs–Pt nano-felts. (g) TEM images of ECNFs–Pt nano-felts (From Refs. [106,107]).

Figure 8. (a) Device structure. (b) Typical linkages of conjugate and conjugate-blocked in D_MeKL. (c) Intramolecular conformational locks formed in D_MeKL by hydrogen bonds. (d) Secondary bonds assist in the extraction and transport of electrons. (e) Preparation of electron transfer 3D network by D_MeKL/PDIN/Y6. (f) SEM images showing typical shapes of ECNFs and ECNFs–Pt nano-felts. (g) TEM images of ECNFs–Pt nano-felts (From Refs. [106,107]).

7. Conclusions and Perspectives

Many studies have shown that electrodes, electrolytes, diaphragms and other components in electrochemical energy systems can be manufactured successfully from a diverse range of lignin. In addition, lignin, as a by-product of the pulping process, can be simply precipitated by adding carbon dioxide, and then the separated precursors can be dried and prepared into precursors. This extraction process has the advantage of being a green, low cost, simple operation. It is therefore easy to realize industrialization. However, the actual application of lignin-derived high value materials is unfortunately still challenging. Future research is required to realize the practical applications of lignin in energy storage devices. The current challenges and future prospects of lignin-based energy storage materials are summarized below.

(1) Unlike cellulose, lignin has no definite molecular structure. Different molecular weights and chemical structures of lignin from different raw materials and different growth sites. It is necessary to promote research on the specific impacts of chemical structure, molecular weight, and the many other properties of lignin in lignin-based materials.

(2) It is well known that the incorporation of heteroatoms, such as oxygen, sulfur and nitrogen into the carbon skeleton can improve the specific capacitance. At present, the doping work mainly focuses on the selection of heteroatomic types and sources. However, the combination of one kind of heteroatom with carbon skeleton is divided into many types, such as nitrogen atoms are divided into pyridine nitrogen, pyrrole nitrogen and so on. Therefore, more efforts are needed to explore about the capacitor contribution efficiency of different doping types prepared via different conditions.

(3) The structure and porosity of lignin-based porous carbon have a huge impact on the performance of energy storage devices. At present, it is widely recognized that the hierarchical porous structure has better electrochemistry performance, but few researchers have studied the storage and transmission efficiency of different diameter of pores. Therefore, a promising future research direction is to rationally design the pore size and 3D structure of porous carbon according to different electrolytes.