Lignin Structure Research Articles

Mild three-stage alkali-oxygen treatment preserving the native macromolecular structure of lignin for effective disassembling of tobacco stalk

Tobacco stalks, as one of the annual economic crops rich in biomacromolecules such as cellulose and hemicellulose, are more difficult to decompose into cellulose fibers due to their high degree of lignification compared to other ordinary straw feedstocks, resulting in their underutilization. In this study, we developed a mild three-stage alkali‑oxygen (AO) process to efficiently deconstruct the tobacco stalk cell walls. The process, involving alkaline dosages of 15 %, 10 %, and 3 % at each stage, effectively dissociated the cell walls and yielded cellulose fibers with high brightness (42.0 % ISO). The organics in the spent liquor, including lignin, hemicellulose, and small-molecular extracts, were isolated through acid/ethanol precipitation and organic solvent extraction. Lignin characterization by 2D HSQC NMR indicated that the majority of native β-aryl ether linkages were preserved after AO treatment, making it suitable for producing chemicals or biofuels via depolymerization. Additionally, the small-molecular extracts contained numerous depolymerized products from lignin and carbohydrates, as well as bioactive compounds derived from the tobacco stalk. Overall, this mild, efficient, and eco-friendly process offers a promising approach for the valorization of tobacco stalks and similar biomass resources.

Structural optimization of the NiFe2O4 spinel catalyst aimed at efficient electrocatalytic C-O bond cleavage of lignin

The intricate structure of lignin poses significant challenges to its high-value utilization. Electrocatalytic conversion of lignin into valuable aromatic compounds using spinel catalysts under mild conditions is a promising approach. The electrochemical conversion of lignin and its model compounds was conducted using Ni-Fe spinel catalysts, achieving selective cleavage of the C-O bond in lignin model with 72 % phenol selectivity and 82 % conversion. Additionally, efficient electrochemical cleavage of the C-O bond was achieved using alkali lignin. Structural characterization and theoretical calculations revealed that the Ni-Fe spinel catalyst exhibited a uniform electron distribution, enhancing the adsorption of the C-O bond and lowering the activation free energy associated with C-O bond cleavage. Moreover, the heterogeneous interface of NiFe2O4 further accelerated the reaction kinetics. This study represents a significant breakthrough in active site identification and mechanistic analysis, facilitating the selective conversion of lignin and providing an efficient method for lignin depolymerization.

FeCl3-Promoted Photocatalytic Cleavage of Cα-Cβ Bond in Lignin and Lignin Model to Benzoic Acid.

Because of the complex structure and inherent inert chemical activity of lignin, it is still challenging to depolymerize lignin to obtain valuable chemicals efficiently. Here, we present an FeCl3-promoted photocatalytic depolymerization strategy to realize the Cα-Cβ oxidative cleavage of lignin model compounds at room temperature. The method generates benzoic acid and phenol compounds in high yields. In addition, the method is effective for the depolymerization of organosolv lignin by cleavage of the products of Cα-Cβ bonds and affords the corresponding products. This strategy provides a method of using an economical photocatalyst to depolymerize lignin and provides a reference for the industrial depolymerization of lignin.

Capability lignin from Acacia crassicarpa black liquor as an environmentally benign antibacterial agent to produce antibacterial and hydrophobic textiles

Recently, the growing health awareness of society on the utilization of fabrics has led to an increasing demand for natural-based antibacterial textiles. Lignin, a generous polyphenol compound in nature, is capable of preventing bacterial growth; in particular, it dwells bacteria closely together on human skin, such as Staphylococcus epidermidis, Bacillus subtilis, Propionibacterium acnes, and Staphylococcus aureus. However, the antibacterial properties of lignin are limited by factors such as the lignin concentration, source, and type of bacteria. This study aimed to evaluate the potency of lignin as an antibacterial agent for textiles. Moreover, the thermal properties and wettability of the textile after lignin coating were also investigated. This study showed that lignin isolation methods significantly contributed to the inhibition of bacterial growth in the clear zone diameter. In addition, the lignin structure, lignin concentration, and type of bacteria had notably different antibacterial effects. SEM images showed that lignin was successfully coated on the fiber, and the antibacterial textile was successfully fabricated with clear zones in the range of 0.1–0.5 cm against four different bacteria. Lignin did not significantly improve the thermal stability of the textile, as proven by the TGA results. After the HDTMS coating by dispersion method, the wettability of the lignin-textile improved to that of the hydrophobic material, with a contact angle greater than 119.05° with excellent antibacterial properties (clear zone of 0.1–0.43 cm).

Development of Methylene Bis-Benzotriazolyl Tetramethylbutylphenol-grafted lignin sub-microspheres loaded with TiO2 for sunscreen applications

Lignin serves as a promising Ultraviolet (UV) absorber within sunscreen industry. However, the commercial development of lignin-containing sunscreens faces challenges due to their low sun protection factor (SPF) and dark color in cosmetics industry. In this study, dual modifications on the chemical and physical structures of lignin were conducted to address these challenges. Initially, methylene bis-benzotriazolyl tetramethylbutylphenol (MBBT) was grafted onto alkali lignin (AL) through an atom transfer radical polymerization reaction, resulting in a polymer of AL-graft-MBBT3 (AL-g-MBBT3). The sunscreen prepared with 10 % AL-g-MBBT3 displays outstanding sun protection performance with a SPF of 42.93 and a light color with a color difference value (ΔE) of 45.6, in contrast to 10 % AL with a SPF of 4.74 and a ΔE value of 49.5. Subsequently, AL-g-MBBT3 was transformed into normal submicron spheres (AL-g-MBBT3 N) and TiO2-loading submicron spheres (AL-g-MBBT3/TiO2). The sun protection performances of 10 % AL-g-MBBT3 N@C and AL-g-MBBT3/TiO2@C sunscreens obviously surpass that of AL-g-MBBT3@C sunscreen, achieving SPFs of 60.38 and 66.20, respectively. Additionally, there is a considerable improvement in the color of these sunscreens, with ΔE values of 41.8 and 36.3, respectively. These results provide valuable insights into exploring lignin's high-value applications in sunscreen.

Lignin-Based Composite Film and Its Application for Agricultural Mulching.

Agricultural mulching is an important input for modern agricultural production and plays an important role in guaranteeing food security worldwide. At present, polyethylene (PE) mulching is still commonly used in agricultural production in most countries around the world, which is non-biodegradable, and years of mulching have caused serious agricultural white pollution. Lignin is one of the three major components of plant cell walls, and it is also the main renewable natural aromatic compounds in nature. Lignin-based composite film materials are green, biodegradable, and show good prospects for development in the field of agricultural mulch. This paper introduces the types, structure, and application status of lignin, summarizes the preparation of lignin-based composite film materials and its latest research progress, focuses on the types, preparation methods, and application examples of lignin-based agricultural mulching, and looks forward to the future development prospects of lignin-based agricultural mulching.

Effects of alkali-soluble hemicellulose separation on the structure of bamboo lignin and lignin-carbohydrate complexes

In this study, alkali treatment was used to preferentially separate hemicellulose, and the structural changes of lignin and lignin-carbohydrate complexes (LCC) in the residual bamboo solids were investigated. The chemical composition and structural characteristics of lignin and LCC were analyzed. After alkali treatment, the monosaccharide content in bamboo lignin decreased from 7.62 % to 2.90 % compared to the raw material. The obtained lignin exhibited higher purity and larger molecular weight, with the macromolecular lignin linked by β-O-4′ bonds remaining almost undegraded. After alkali treatment, the linkage bonds in the Björkman LCC were composed of phenyl glycoside (PhGlc), and the bond in LCC-AcOH consisted of benzyl ether (BE). The results showed that the preferential extraction of hemicellulose preserved the biopolymer structure of lignin in bamboo, thereby providing an important basis for the comprehensive separation and utilization of bamboo components.

Effect of acid-alcohol deep eutectic solvents-regulated lignin structure on subsequent pyrolysis products distribution

In lignin extraction and depolymerization, alcohol reagents are often used to stabilize carbon positive ions or to reduce lignin condensation. In this paper, the effects of different deep eutectic solvent (DES) on the distribution and pyrolysis properties of lignin subsequent pyrolysis products were investigated by treating softwood kraft lignin (SKL) with different acidic DES (acid-alcohol system). It was shown that the elemental composition of lignin changed considerably after acidic DES treatment, and the theoretical calorific value of DES-modified lignin increased. FTIR and XPS analysis showed a slight increase in the content of alcohol hydroxyl groups in DES-treated lignin, which also indicated that ethylene glycol might have accessed to the macromolecular structure of lignin. Pyrolysis experiments indicated that the yield of the optimal pyrolysis oil was increased by DES treatment up to 40.3 % (7.0 % increase compared to SKL). Both H-type phenols and C-type phenols increased in pyrolysis oil (of which, the C-type phenols produced by DES-treated lignin pyrolysis ranged from 15.1 % to 18.9 %). In-situ infrared analysis of noncondensable gases produced by lignin pyrolysis also provided an indirect insight into the pattern of change in the active functional groups of lignin. CO2 was the main gas in the pyrolysis gas, and CH4 yield increased with the temperature increasing from 300 °C to 500 °C and then decreased with the increase of temperature from to 500 °C to 600 °C.

Use of Renewable Alcohols in Autocatalytic Production of Aspen Organosolv Lignins.

This study aimed to investigate the intrinsic efficiency of renewable alcohols, applied under autocatalytic conditions, for removing lignin from aspen and hot-water-extracted aspen while substantially preserving the lignin structure so as to facilitate various valorization strategies. Ethylene glycol (EG), propylene glycol (PG), 1,4-butanediol (BDO), ethanol (EtOH), and tetrahydrofurfuryl alcohol (THFA) were evaluated based on their lignin solubilization ability, expressed as the relative energy difference (RED) following the principles of the Hansen solubility theory. The findings indicate that alcohols with a higher lignin solubilization potential lead to increased delignification, almost 90%, and produce a lignin with a higher content of β-O-4 bonds, up to 68% of those found in aspen milled wood lignin, thereby indicating their potential for valorization through depolymerization. However, these alcohols also produce lignin with a higher content of β-β and β-5 bonds, resulting in a higher molecular weight and polydispersity, due to readily occurring homolytic reactions. Hot-water extraction (HWE) conducted prior to alcohol treatment reduced the delignification efficiency and resulted in a lignin with a lower β-O-4 bond content. The lignins produced in these experiments exhibited a superior UV-A absorption capacity compared with synthetic benzophenone, as well as a greater radical quenching ability than synthetic butylated hydroxytoluene, indicating their potential for use in the protection of polymers against degradation.

Unlocking the photothermal conversion capacity of lignin and lignin-derived materials

Lignin is the most prevalent polyphenolic organic solid waste generated by the pulping and biorefining industries. It confers upon woody biomass a remarkable combination of physical, chemical, and biological resilience, while simultaneously exhibiting considerable potential for photothermal conversion. In recent years, the utilization of technical lignin and lignin by-products to develop photothermal materials has emerged as a research hotspot. This review introduces the industrial separation processes and characterization of different types of lignin, highlighting the chemical structures of lignin and its derivatives linked to their photothermal conversion mechanisms. The evaluation pathways for photothermal conversion are summarized and critically assessed, as guided by mathematical descriptions of best practices in lignin-derived materials. This review covers and reflects on recent and emerging advances in materials and energy sciences that demonstrate how lignin can be utilized to maximize solar-to-thermal conversion and versatility in desalination, electricity generation, therapy, and intelligent and environmentally friendly building materials. Finally, the challenges and proposed future directions for the development of lignin-derived photothermal materials are identified with the objective of challenging the prevailing notion that lignin is merely an organic solid waste.

UV–vis spectroscopy as a rapid method for evaluation of total phenolic hydroxyl structures in lignin

Abstract Phenolic hydroxyl groups in lignin are crucial for understanding its structure, reactivity, and potential applications. Various methods have been developed for the determining phenolic groups in lignin. This study focuses on the comparison of a simple, cost-effective, and time-efficient UV–vis ionization difference technique with the highly accurate 31P NMR spectroscopy for analyzing lignin samples of different origins and isolation methods. The results were carefully evaluated, and the strengths and limitations of each method were discussed. Two eco-friendly UV–vis approaches were proposed for a rapid and comprehensive evaluation of the total phenolic-OH groups: one using a strong alkaline solution for analyzing common types of technical lignins, and another employing multipoint wavelength calculations, effective for analyzing softwood lignins regardless of the extraction method. Additionally, the research highlighted the importance of selecting appropriate model phenolic compounds to accurately assess the phenolic hydroxyl group content in lignins using the UV–vis method. Offering straightforward and rapid analysis, with results closely aligning with 31P NMR data, this method is a promising alternative for routine analysis.

Nanolignin-Facilitated Robust Hydrogels.

Recently, certain challenges and accompanying drawbacks have emerged in the preparation of high-strength and tough polymer hydrogels. Insights from wood science highlight the role of the intertwined molecular structure of lignin and crystalline cellulose in contributing to wood's strength. Herein, we immersed prestretched poly(vinyl alcohol) (PVA) polymer hydrogels into a solution of nanosized lignosulfonate sodium (LS), a water-soluble anionic polyelectrolyte, to creatively reconstruct this similar structure at the molecular scale in hydrogels. The nanosized LS effectively fixed and bundled the prestretched PVA polymers while inducing the formation of dense crystalline domains within the polymer matrix. Consequently, the interwoven structure of crystalline PVA and LS conferred good strength to the composite hydrogels, exhibiting a tensile strength of up to ∼23 MPa, a fracture strain of ∼350%, Young's modulus of ∼17 MPa, toughness of ∼47 MJ/m3, and fracture energy of ∼42 kJ/m2. This hydrogel far outperformed previous hydrogels composed directly of lignin and PVA (tensile strength <1.5 MPa). Additionally, the composite hydrogels demonstrated excellent antifreezing properties (<-80 °C). Notably, the LS-assisted reconstruction technology offers opportunities for the secondary fixation of PVA hydrogel shapes and high-strength welding of hydrogel components. This work introduces an approach for the high-value utilization of LS, a green byproduct of pulp production. LS's profound biomimetic strategy will be applied in multifunctional hydrogel fields.

Hydrophobic, ultraviolet radiation-shielding, and antioxidant functionalities of TEMPO-oxidized cellulose nanofibril film coated with modified lignin nanoparticles

In this study, lignin nanoparticles (LN) and octadecylamine-modified LN (LN-ODA) were utilized as coating materials to enhance the hydrophobic, antioxidant, and ultraviolet radiation-shielding (UV-shielding) properties of a TEMPO-oxidized nanocellulose film (TOCNF). The water contact angle (WCA) of the TOCNF was approximately 53° and remained stable for 1 min, while the modified LN-ODA-coated TOCNF reached over 130° and maintained approximately 85° for an hour. Pure TOCNF exhibited low antioxidant properties (4.7 %), which were significantly enhanced in TOCNF-LN (81.6 %) and modified LN-ODA (10.3 % to 27.5 %). Modified LN-ODA-coated TOCNF exhibited antioxidant properties two to six times higher than those of pure TOCNF. Modified LN-ODA exhibited thermal degradation max (Tmax) at 421 °C, while pure LN showed the main degradation temperature at approximately Tmax 330 °C. The thermal stability of TOCNF-LN-ODA-coated materials remained consistent with that of pure TOCNF, while the crystallinity index of the sample showed a slight decrease due to the amorphous nature of the lignin structure. The tensile strength of TOCNF was approximately 114.1 MPa and decreased to 80.1, 51.3, and 30.3 MPa for LN-ODA coating at 5, 10, and 15 g/m2, respectively.

Supercritical CO2 as effective wheat straw pretreatment for subsequent mild fractionation strategies

The efficient utilization of lignocellulosic biomass in biorefineries is pivotal for the transition to a carbon–neutral society, emphasizing the need for environmentally friendly fractionation techniques. While the extensive use of chemicals in biorefinery operations can yield favorable quantities of specific biomass components, adopting less chemically intense conditions is crucial for holistic methodologies that preserve the inherent potential of all biomass constituents. This study comprehensively investigates the use of supercritical carbon dioxide (sc-CO2) as a green pretreatment to improve subsequent mild fractionation on wheat straw. Sc-CO2 at 300 bars, 100 °C and 70% moisture was found to have minimal impact on the chemical composition and the lignin structure, while there were significant morphological changes as heightened surface area and reduced density. Apart from increasing enzymatic saccharification efficiency, the treatment notably enhanced subsequent mild delignification through alkaline and flow-through organosolv extractions. The combination of the pretreatments enhances the lignin solubilization yields from 49 to 79% for alkaline and 74 to 91% for organosolv extractions, while retaining a high β-O-4 conservation of 49 and 59 linkages per 100 aromatic units, respectively. Additionally, the combined use of sc-CO2 with mild dilute acid pretreatment improved xylose solubilization from 59 to 76 % and enzymatic saccharification from 53 to 90%, albeit with increased lignin condensation. In summary, this study demonstrates the potential of sc-CO2 pretreatment as a versatile tool for biomass valorization within the evolving bioeconomy, by combining enhanced extraction yields with minimal lignin structural impact. Our work thereby highlights the promise of the use of sc-CO2 to contribute to the overall economic potential of improved biorefinery processes.

Quantitative and Structural Characterization of Native Lignin in Hardwood and Softwood Bark via Solid-State NMR Spectroscopy.

A major factor limiting bark's industrial use is its greater recalcitrance compared to wood. While lignin is widely recognized as a significant contributor, precise characterization of lignin in bark remains sparse, presenting a crucial gap that impedes understanding of its impact. In this study, we employed advanced solid-state nuclear magnetic resonance (NMR) spectroscopy to analyze bark samples from various species, including willow, poplar, and pine. We established and verified that lignin methoxy peak at 56 ppm serves as a reliable quantitative metric to assess lignin content, with which we calculated the lignin contents in bark are significantly reduced by more than 70% compared to those in wood. Furthermore, in situ characterization revealed significant reduction of β-ether linkage in bark lignin across species, revealing a more condensed and resistant structural configuration. Our results have substantially advanced our comprehension of the composition and structure of native lignin in tree bark.

Characterization of manganese(II)-coupled functional microorganisms in driving lignin degradation during straw composting

The intricate structure of lignin in straw makes it challenging to hydrolyze, making it a key focus of current research. However, there has been limited study on the effect of enzyme inducer (MnSO4) combined with functional microorganisms on lignin degradation during straw composting. Based on this, four composting treatment groups were set up in this study. Control (CK), functional microorganism addition treatment (F), Mn2+ enzyme inducer (Mn), and Mn2+ enzyme inducer coupled with functional microorganism addition treatment (FMn) were tested for composting. Manganese(II)-coupled microorganisms improved lignin degradation: FMn > Mn > F > CK. They increased the lignin loss rate from 25.54 % to 42.61 %. Laccase activity increased from 3.45 to 43.74 U/g and manganese peroxidase activity increased from 145.52 to 264.91 U/g. And gene abundance was increased. Microbial community structure and dominant genera changed. Structural equations support the idea that functional microorganisms coupled with manganese can modify physicochemical indices, thereby regulating gene expression and enhancing enzyme activity. Furthermore, the stimulation of fungal growth and increased extracellular laccase and manganese peroxidase activities can affect the degradation of lignin. This study provides new insights and theoretical support for efficient lignin degradation and efficient resource utilization of compost products.

Influences of 2-ethylanthraquinone on lignin degradation in alkaline cooking of Eucalyptus pellita wood

The impact of 2-ethylanthraquinone (2-EAQ) on alkaline cooking of Eucalyptus pellita wood was examined by analyzing pulp yield, kappa number, the structure and weight-averaged molecular weight (Mw) of dissolved lignin in black liquor and residual lignin in pulp (RL). During soda cooking under 23% of active alkaline (AA) condition, adding 0.06% 2-EAQ reduced the kappa number by 12 points compared to cooking without it. The addition of 0.06% 2-EAQ to kraft cooking increased pulp yield by about 4% at about 19 of kappa number compared to the no addition. The analysis of lignin structure by using semiquantitative heteronuclear single quantum correlation nuclear magnetic resonance (HSQC NMR) showed that the addition of 2-EAQ promoted β-ether cleavage during alkaline cooking and reduced peeling reaction of polysaccharides and prevention of the elimination of γ-OH of lignin to form enol ethers and stilbenes. Overall, the addition of 2-EAQ promoted the degradation of lignin to lower molecular weight compounds, with a more pronounced effect observed in soda cooking than in kraft cooking. Under optimal conditions, the delignification efficiency and pulp yield of soda-EAQ pulping were comparable to kraft cooking.

Isotopic analysis (δ13C and δ2H) of lignin methoxy groups in forest soils to identify and quantify lignin sources

The relative apportion of above and below ground carbon sources is known to be an important factor in soil organic matter formation. Although lignin is the most abundant aromatic plant material in the terrestrial biosphere, our understanding of lignin source contributions to soil organic matter (SOM) is limited due to the complex molecular structure and analysis of lignin. In this study, we novelly apply the dual isotopic analysis (δ13C and δ2H values) of lignin methoxy groups (LMeO) with the Bayesian mixing model, MixSIAR, to apportion lignin sources in two contrasting soil types, a podzol and a stagnosol. Results of the isotopic analysis of LMeO demonstrate the ability of δ2H LMeO values to discriminate between above and below ground lignin sources, while δ13C LMeO values discriminated between photosynthesising and non-photosynthesising tissues. In the stagnosol subsurface horizons, a decreasing proportion of the leaf litter lignin was observed with increasing organic matter degradation, cumulating in the Ah horizon being dominated by lignin from roots. The podzol sites indicated a similar reduction in leaf litter lignin with an increase in organic matter degradation and depth. However, the Ah horizon was shown to accumulate lignin from the above ground woody material. Furthermore, given the significant abundance of LMeO groups in the terrestrial biosphere and the extremely depleted δ13C LMeO values in leaf litter, we employed a mass balance approach to determine the extent in which the 13C bulk enrichment generally associated with isotopic fractionation during organic matter decomposition can be attributed to the shift in lignin sources. Analysis reveals that 14 % and 11 % of bulk 13C enrichment can be attributed to the transition in LMeO sources from leaf litter to roots in the stagnosol and podzol, respectively. Thus, models relying on 13C enrichment with depth as an indicator of carbon turnover may be partially overestimating rates.

Lignin, extractives and structural carbohydrate characteristics of Thinopyrum intermedium biomass reveal additional valorization opportunities for dual-crop utilization.

Thinopyrum intermedium (Host) Barkworth & D.R. Dewey, or intermediate wheat grass (IWG), is being developed as the first widely-available perennial grain candidate. However, because the crop is still in development, grain yields are lower than those of traditional cereals. Utilization of its non-grain biomass (e.g. for biofuel production and as a source of fine chemicals) would increase the economic value of its cultivation. The present study provides a structural characterization of the lignin and cell wall carbohydrates in IWG biomass and qualitative profiling of biomass extractives and compares them to those of annual wheat (Triticum aestivum) biomass grown in the same location and growing season. The monosaccharide composition and ester-linked phenolic acid contents of vegetative biomass material from annual wheat and IWG were similar. IWG vegetative biomass is rich in feruloylated arabinoxylans (AX) with a very low substitution rate, whereas the AX from IWG bran have a slightly higher substitution rate. The structure of IWG lignin was investigated using both the quantitative derivatization followed by reductive cleavage method and 2D-NMR analysis, revealing an H:G:S lignin that incorporates tricin and is acylated with coumaric acid and smaller amounts of ferulates. IWG and wheat extractives contained fatty acids, various free phenolic compounds (tricin, monolignols and phenolic acids), phenolic conjugates and phytosterols. The present study provides firm support for the further exploration of T. intermedium biomass as a carbohydrate feedstock (e.g, abundant in lightly substituted AX and cellulose polymers) for biofuel production and source of high-value fine chemicals, such as tricin. © 2024 Society of Chemical Industry.

Biomass lignin as dispersion to substantially enhance carbon nanotubes thermoelectric converter for energy harvesting

Carbon nanotubes (CNTs) have attracted much attention as promising thermoelectric materials to harvest waste energy, however the agglomeration between nanotubes greatly limits their application. Despite various dispersants that have been devised to enhance the dispersion of CNTs, biomass lignin has been rarely employed to facilitate the dispersion of CNT for enhancing its TE properties. Herein, green lignin was incorporated with CNTs to synergistically improve the dispersion and TE properties of CNTs, in which π-π interactions between the aromatic ring structure of lignin and CNTs, as well as electrostatic charge brought by ionized groups of lignin promote the exfoliation of nanotubes and reduces CNTs entanglement. The well-dispersed structure of CNTs facilitates efficient charge transfer, and thereby enhances the thermoelectric properties. CNTs/lignin composite could reach a maximum power factor of 198.0 μW m−1K−2 at a lignin content of 11.8 wt%, which was then raised to 223.1 μW m−1K−2 with subsequent partial removal of lignin by alkali immersion treatment, higher than 98.6 μW m−1K−2 of pristine CNTs film. Furthermore, the flexible TE device with five pairs of CNTs/lignin and nickel wires generates a high power of 288 nW at ΔT = 30 K, demonstrating potential applications of lignin for sustainable energy harvesting.