1 Introduction
Lignin is one of the most abundant renewable aromatic resources in plants.Lignin has high carbon content (55% ~ 65%), low price, easy to collect and high yield, so it has great potential to be used as industrial raw materials for large-scale utilization and become a potential alternative to traditional fossil energy
[1]. Industrial lignin mainly comes from cooking wastewater (mainly alkali lignin) from pulping and papermaking process, enzymatic hydrolysis residue (mainly enzymatic hydrolysis lignin) from bioethanol process and a small amount of organic solvent lignin. Industrial lignin is a kind of waste biomass resource, most of which is used in low-value combustion. Although the heat produced by industrial lignin is higher than that of ordinary biomass, it is still far from meeting the needs of the current industry, and greatly wastes aromatic resources. Therefore, it is of great significance for the sustainable development of society to develop high-value and diversified application ways of lignin and realize the efficient utilization of renewable aromatic resources.
At present, the high-value application research of industrial lignin mainly includes catalytic production of platform compounds, fast pyrolysis for bio-oil fuel, as a functional material for drug-loaded microcapsules and natural anti-ultraviolet materials
[2][3][4⇓~6][7,8]. Porous carbon materials have been widely studied as advanced energy storage materials for electrochemical energy storage devices in recent years due to their intrinsic properties such as good electrical conductivity, excellent chemical and mechanical stability, considerable specific surface area and rich pore structure. Lignin is one of the ideal raw materials for the preparation of porous carbon materials due to its high degree of aromatization and high carbon yield. In addition, a large number of hydroxyl groups on the aromatic ring provide a potential means to regulate the micro-nano characteristics of lignin porous carbon materials. In this paper, the research progress in the preparation of porous carbon materials from lignin at home and abroad is reviewed, and the application progress of lignin porous carbon materials (LPCM) in lithium-ion batteries, sodium-ion batteries and supercapacitors is emphatically summarized.It is proposed that the development of advanced carbonization technology to build a reasonable hierarchical pore structure, precise control of crystal structure and surface microenvironment is the main research direction in the future.
2 Preparation of Lignin-Based Porous Carbon Material
2.1 Activation method
2.1.1 Physical activation method
Physical activation, also known as gas activation, is generally divided into two processes: medium and high temperature carbonization (500 ~ 800 ℃) and high temperature activation (600 ~ 1000 ℃)
[9⇓~11]. In the process of carbonization, carbon atoms rearrange spontaneously to form graphite-like crystallite structure with the increase of temperature, and volatiles precipitate to form narrow, closed and blocked primary pores between crystallites. In the process of activation, the gasification agent reacts with amorphous carbon and heteroatoms to dredge the blocked and closed pores and expose the surface of microcrystals. Condly, the carbon atoms on the surface of the crystallite and the gasifying agent are subjected to heterogeneous reformation, and penetrating, deep and transversely expanding pore channels are gradually formed. Due to the different contact area between the gasifying agent and the crystallite and the different reactivity of carbon atoms, the reaction rates in different directions are different, thus forming an interconnected hierarchical porous network structure.
The gasification agents commonly used in industry are water vapor and carbon dioxide
[9]. Steam has low cost and endothermic characteristics, and carbon dioxide has relatively low high-temperature reaction activity, which is more conducive to controlling the reaction. Table 1 summarizes the pore structure parameters of lignin porous carbon materials (LPCM) prepared by direct carbonization and steam and carbon dioxide activation.
As shown in Table 1, in the direct carbonization process, the heating rate and temperature have significant effects on the structural parameters of alkali lignin hard carbon, such as specific surface area and pore volume. When the carbonization temperature is the same, the specific surface area of lignin hard carbon is the highest at low heating rate, and the pore size distribution is dominated by micropores. The specific surface area and micropore volume of lignin hard carbon were lower at high heating rate, which was related to the rapid release of volatiles and "expansion" to produce crack-like mesopores or macropores. When the carbonization temperature was below 350 ℃, the specific surface area of lignin hard char was low (<33 m2/g), which was less affected by the heating rate. With the increase of carbonization temperature, the specific surface area and micropore volume of lignin hard carbon increased first and then decreased. However, when the specific surface area and micropore volume of lignin hard char reach the maximum, the temperature at high heating rate is higher than that at low heating rate, which is attributed to the hysteresis effect.
The type of industrial lignin also significantly affects the structural parameters of lignin hard char. Compared with alkali-lignin hard carbon, the specific surface area and total pore volume of organic-solvent-lignin hard carbon prepared at high temperature are higher, which may be related to the more complete structure of organic-solvent-lignin.
As shown in Table 1, the specific surface area of LPCM increased with the increase of temperature during the activation of water vapor and carbon dioxide. The specific surface area of LPCM prepared by carbon dioxide is much higher than that of LPCM prepared by steam activation. In addition, the pore structure of LPCM prepared by water vapor activation is mainly micropore, and the pore structure of LPCM prepared by carbon dioxide activation is mainly mesopore.
In the physical activation process, the gasification agent reacts with the longitudinal carbon atoms to form micropores, and reacts with the surface carbon atoms to expand the pore channels to form mesopores. The longitudinal reaction rate and the surface reaction rate determine the pore size distribution of LPCM. Therefore, water vapor tends to react with longitudinal carbon atoms to form deep micropores, while carbon dioxide not only reacts with longitudinal carbon atoms to form deep and through pores, but also reacts with carbon atoms on the surface of pores to form mesopores.
2.1.2 Chemical activation method
The chemical activation method is to use strong corrosive chemicals such as potassium hydroxide, ZnCl2 and H3PO4 to impregnate lignin raw materials. The activator plays a role of structural support and intercalation of carbon layer in the process of carbonization, and forms a porous structure after being removed. The LPCM prepared by KOH and ZnCl2 activation has high specific surface area and pore volume, and has been widely studied because of its wide applicability. Table 2 summarize that structural parameters of LPCM prepare by KOH and ZnCl2 activation in different research works.
The activation process of KOH can be divided into the following four steps: (1) KOH is dehydrated at 400 ~ 600 ℃ to produce K
2O and H
2O;(2) lignin, and the carbonaceous matrix reacts with H
2O to produce CO, CO
2 and H
2.And the K
2O and the CO
2 are further reacted at 400 deg C to form the K
2CO
3;(3)K
2CO
3 which is decomposed into the CO
2 and the K
2O at 700 to 800 deg C,At the same time, CO
2 is reduced to CO by the lignin carbonaceous matrix at high temperature, while K
2O and K
2CO
3 are reduced to metal K
[23,25⇓~27]; (4) The intercalation of metal K into the lattice of lignin carbonaceous framework leads to the separation and rearrangement of carbon layers, and the elastic recovery of carbon layers can not be achieved after deintercalation, resulting in branch-like micropores between carbon layers
[28].
As shown in Table 2, with the increase of activation temperature, the specific surface area and total pore volume of LPCM prepared by KOH activation increased first and then decreased, and the optimal preparation temperature was 650 ~ 800 ℃, which was related to the lignin/KOH ratio and heating rate.
At the same carbonization temperature (750 ℃) and high heating rate (10 ℃/min), the specific surface area and total pore volume of LPCM increased first and then decreased with the decrease of lignin/KOH ratio, and reached the highest at the ratio of 1:4. When the lignin/KOH ratio was lower than 1:3, the specific surface area and total pore volume of LPCM increased slightly. At the same lignin/KOH ratio, the specific surface area and total pore volume of LPCM increased first and then decreased with the increase of temperature, reaching the maximum at 650 ~ 750 ℃, which was closely related to the heating rate. The higher the heating rate is, the higher the temperature of LPCM with high specific surface area and pore volume is. In addition, Hayashi et al. Found that when the lignin/KOH ratio increased, the maximum specific surface area of the prepared LPCM was lower, and the corresponding activation temperature (800 ℃) was relatively higher
[22].
On the other hand, when the temperature is higher than 700 ℃, the K
2CO
3 plays an activating role in the latter half of the process. The K
2CO
3 and the K
2O generated by decomposition are reduced by carbon atoms to form metal K intercalated carbon skeleton to form a microporous network, and a large number of released CO
2 form deep, through pores and mesopores with the carbonaceous matrix of lignin
[29,30][31]. As shown in Table 2, the specific surface area of LPCM increases first and then decreases with the increase of temperature, which is consistent with the change trend of the specific surface area of LPCM prepared by KOH activation. The highest specific surface area of LPCM prepared by K
2CO
3 activation is higher than that of LPCM prepared by KOH activation, so the latter half activation is the key control step of KOH activation.
The activation mechanism of ZnCl
2 is mainly summarized as follows: :(1)ZnCl
2 has the characteristics of Lewis acid and catalyzes the gasification and dehydrogenation of lignin to form micropores at a lower activation temperature
[22,32,33]; With the increase of activation temperature, dehydrogenation inhibited the formation of aromatic compounds and promoted the increase of aromatization degree of lignin char, resulting in the decrease of micropore number and specific surface area. The (2)ZnCl
2 etches the internal structure of the lignin charcoal and plays a supporting role, and the structure cannot be restored after being removed, thereby forming micropores.
As shown in Table 2, the specific surface area and pore volume of LPCM prepared by ZnCl2 activation increased first and then decreased with the increase of activation temperature, and the optimal preparation temperature was 500 ℃. Compared with KOH activation, the highest specific surface area, pore volume and optimal preparation temperature of LPCM prepared by ZnCl2 activation are relatively low, which is attributed to its weak etching effect on carbon.
2.2 Template method
2.2.1 Hard template method
Hard template method, also known as nano-casting, is based on the principle of mold means, reverse replication of the topological structure of hard template agent, and its preparation process is shown in Figure 1
[34].
Commonly used hard templates are silicon-based templates, including nano-silica spheres and zeolite molecular sieves, which need to be removed by strong corrosive reagents such as hydrofluoric acid, NaOH or KOH. Therefore, green hard templates including NaCl and metal oxides such as CaO and MgO were developed. The LPCM prepared by Xie et al. Using sodium lignosulfonate and NaCl as raw materials (1:10, w/w) and carbonized at 500 ℃ had a specific surface area of 438 m
2/g, and mesopore and micropore volumes of 0.73 and 0.12 cm
3/g, respectively
[35]. Song et al. developed a spherical nano-MgO hard template process to prepare mesoporous carbon, and prepared high mesoporous (68.93%) LPCM with alkali lignin/MgO ratio of 10:1 (w/w) at 1000 ℃, whose specific surface area (85 m
2/g) was four times that of LPCM prepared by direct carbonization (19 m
2/g)
[36]. In particular, the hard template particle size and its dispersion are important factors affecting the pore size distribution of LPCM. On the other hand, LPCMs with different morphologies can be prepared by using MgO with different shapes as hard templates. Zhang et al. prepared flower-like LPCM with specific surface area of 827 m
2/g and mesoporous structure at 600 ° C by using enzymatic hydrolysis lignin and sheet-like MgO as raw materials and hard templates, respectively (Fig. 2)
[37].
The LPCM prepared by the green hard template method has low specific surface area and single pore size, and is mainly mesoporous. In addition, the problems of poor dispersion of hard template in lignin molecules, time-consuming template removal and template residue restrict the industrial application of green hard template method.
2.2.2 Soft Template Method
The soft template method is also called organic template method. The soft template agent and lignin self-assemble to form composite nano-micelles through electrostatic or hydrogen bonding in the solvent environment. During the high-temperature carbonization process, the immiscible chain segments in the soft template agent decompose to form pores in the carbonaceous matrix. The preparation process is shown in Figure 3. Similar to the hard template method, the LPCM prepared by the soft template method has low specific surface area and is mainly mesoporous. F127, CTAB and P123 are commonly used as soft templates, among which F127 triblock copolymer is the most widely used.
The type of industrial lignin and its structural characteristics are the key factors affecting the structural parameters of LPCM prepared by F127 soft template. Saha et al. firstly used organic solvent lignin and F127 as raw materials to prepare LPCM with 65% mesopore and 205 m
2/g specific surface area by carbonization at 1000 ℃
[39,40]. Song et al. Used F127 as a soft template to study the structural characteristics of porous carbon prepared from sodium lignosulfonate and phloroglucinol respectively. The results showed that the specific surface area and mesopore ratio of LPCM derived from sodium lignosulfonate were 59 m
2/g and 80.5%, respectively.It is lower than the specific surface area (400 m
2/g) and mesopore ratio (91.2%) of phloroglucinol-derived LPCM, which may be caused by the poor self-assembly performance of lignin micelles due to its large molecular weight and severe hyperbranched structure
[36,41]. Qin et al. used F127 as a soft template to further study the structural characteristics of LPCM prepared from lignins with different molecular weights (1055,1476 and 2996 G/mol). The results showed that the pore structure of LPCM derived from low-molecular-weight lignin had an ordered two-dimensional hexagonal mesopore distribution, while the mesopore order of LPCM derived from high-molecular-weight lignin decreased, and its specific surface area decreased from 466.1 m
2/g to 236.4 m
2/g with the increase of lignin molecular weight
[42].
The soft and hard dual-template method can further improve the pore structure and connectivity of LPCM. The soft template not only plays the role of pore formation, but also has the role of dispersing the hard template in the lignin matrix. Song et al. Prepared LPCM with 712 m
2/g specific surface area and 0.90 cm
3/g total pore volume using alkali lignin, F127 and MgO as raw materials and templates, and studied the effect of the amount of templates on the pore structure characteristics of LPCM
[36]. The results show that the MgO hard template dose is the key factor to determine the pore abundance, while the increase of F127 soft template dose has little effect on the pore structure and mainly prevents the template agent from agglomerating (Fig. 4).
2.3 Hydrothermal Method
Hydrothermal method is a new biomass pretreatment technology, which can not only stabilize the initial microstructure, but also provide a mild self-assembly environment to regulate the microstructure, and has been widely used in the synthesis of carbon-based catalysts and other advanced materials
[43⇓~45]. Hydrothermal pretreatment generally uses water as the reaction medium, the temperature is less than 200 ℃, and the heat is transferred to the surface of the material through conduction, convection or radiation, and to the interior of the material through heat conduction
[46]. However, the low temperature of hydrothermal pretreatment and the low degree of carbonization lead to the low porosity of the prepared lignin hydrothermal carbon
[47,48]. Therefore, hydrothermal pretreatment is generally used as a pretreatment step of template method and activation method to broaden the preparation route of LPCM.
Garc García et al. prepared LPCMs with an average pore width of about 10 nm and a specific surface area up to 730 m
2/g from resorcinol, a lignin model compound, using a combination of hydrothermal (50 ° C) and soft template (F127, 600 ° C) methods
[49]. The results show that when the ratio of resorcinol/F127 is 1:1 (w/w), the pores of LPCM are in regular hexagonal distribution; With the decrease of resorcinol/F127 ratio, the pores of LPCM evolved into a disordered worm-like distribution, because the hydrogen bond self-assembly between phenolic hydroxyl groups and F127 was weakened due to the decrease of resorcinol content. Zhang et al. prepared LPCM from low molecular weight enzymatic hydrolysis lignin by a two-step method of hydrothermal pretreatment at 200 ℃ and KOH activation at 800 ℃. The specific surface area of LPCM was 2218 m
2/g, and the pore size distribution was about 0.72 and 1.4 nm micropore regions (67%), 2 ~ 7 nm and 25 ~ 75 nm meso/macropore regions
[50].
When the hydrothermal method is combined with the soft template method, the specific surface area of the prepared LPCM is significantly increased, and the average pore size of the prepared LPCM is lower than that of the LPCM prepared by the soft template method. When the hydrothermal method is combined with the chemical activation method, the prepared LPCM has a hierarchical pore structure with micropores as the main part and mesopores and macropores as the auxiliary part. Therefore, hydrothermal pretreatment combined with traditional pore-forming technology has a coupling effect on the microstructure regulation of LPCM.
3 Application of Lignin-Based Porous Carbon Materials in Energy Storage Materials
3.1 Cathode material of lithium ion battery
Lithium-ion battery is the most widely used new green electrochemical energy storage device in the world. Its specific energy, volumetric energy density and power density are higher than those of other metal ion batteries. It plays an important "bridge" role in the construction of efficient energy storage system and large-scale storage of intermittent clean energy
[51]. The porous carbon material is cheap and easy to obtain, has high conductivity, good chemical stability and excellent mechanical integrity, has high lithium ion storage capacity under the synergistic effect of a plurality of lithium storage mechanisms, has excellent lithium ion diffusion kinetics, and has the potential of replacing the traditional graphite anode material to meet the requirement of increasing energy density along with the rapid development of economy and technology. Table 3 summarizes the research progress of LPCM applied to anode materials for lithium-ion batteries.
The lignin hard carbon cathode material prepared by direct carbonization has low specific capacity and initial coulombic efficiency and poor cycling stability,It is mainly caused by the surface defects such as a large number of dangling bonds and oxygen-containing covalent bonds formed by condensation and re-polymerization of lignin-derived monomers or oligomers during carbonization, the formation of excessive solid electrolyte interface (SEI) and the lack of active lithium storage sites
[51,54,55][59,60].
Constructing hierarchical porous structure to optimize the reaction kinetics is one of the effective ways to solve the intrinsic defects of lignin hard carbon anode. Microporous defects provide abundant lithium storage sites, mesoporous structure shortens the charge transport distance and increases the contact area between electrolyte and electrode, and macroporous structure acts as electrolyte buffer reservoir
[61]. Zhang et al. prepared enzymatic hydrolysis lignin-derived LPCM with specific surface area of 907 m
2/g by KOH activation at 700 ℃. The LPCM anode material with a large number of micropores (microporosity of 79.3%) and a small number of mesopores/macropores had a reversible capacity of 470 mAh/G after 400 cycles at 0.2 A/G, which was much higher than that of enzymatic hydrolysis lignin hard carbon anode material (180 mAh/G)
[52]. The graphitization-like structure coupled with the hierarchical porous structure further improves the diffusion of lithium ions and the migration of electrons, thereby improving the lithium storage performance. Xi et al. prepared LPCM with high graphitization degree (I
D/I
G=0.7) and hierarchical porous structure (30% microporosity) at 900 ℃ by K
2CO
3 activation method using enzymatic hydrolysis lignin as raw material, and the specific capacity was 494 mAh/G after 200 cycles at 0.2 A/G current density
[53].
Surface chemical modification is one of the effective means to modify the surface defects of LPCM. Chang et al. modified the LPCM by removing oxygen atoms from the surface of the LPCM with a reducing hydrogen stream at 800 ℃ to induce the spontaneous rearrangement of carbon atoms, resulting in an increase in the graphitization degree and specific surface area (189 m
2/g) of the surface-modified LPCM. The specific capacity of the surface-modified LPCM was 222 mAh/G after 200 cycles at 2 C, which was 23.3% higher than that of the unmodified LPCM(108 m
2/g)
[54].
The lithium storage performance of LPCM derived from industrial lignin is obviously different due to the different types of lignin. Xi et al. prepared enzymatic lignin and alkali lignin-derived LPCM by K
2CO
3 activation and hydrogen flow surface modification. The specific surface area of enzymatic lignin-derived LPCM was 2300 m
2/g, which was higher than that of alkali lignin-derived LPCM(1618 m
2/g)
[55]. Secondly, the microporosity of enzymatic hydrolysis lignin-derived LPCM was only 30%, which was lower than that of alkali lignin-derived LPCM (44.3%), and the reversible capacities were 520 mAh/G and 186 mAh/G after 200 cycles at 0.2 A/G, respectively, indicating that the decrease of the proportion of microporous structure was beneficial to the improvement of the specific capacity of LPCM.
The LPCM prepared by KOH and K
2CO
3 activation has high specific surface area and large contact area with the electrolyte, but the excessive defects on the surface of LPCM intensify the side reactions at the interface, resulting in low initial coulombic efficiency. At the same time, the porous structure with high microporosity and low mesoporosity has many irreversible lithium storage sites and slow reaction kinetics, resulting in low lithium storage capacity and poor rate performance. A green and simple process was developed by our research group. The LPCM with high mesopore ratio of 66.7% and low specific surface area of 531 m
2/g was prepared by one-step control of ZnCO
3. The charge-discharge coulombic efficiency of the first cycle at 0.2 a/G was 54.9%, the specific capacity of 200 cycles was 550 mAh/G, and the rate performance was excellent (Fig. 5)
[56].
Therefore, establishing the structure-activity relationship between LPCM hierarchical porous structure and lithium storage performance, guiding the optimization and design of LPCM hierarchical porous structure construction, is expected to break the limitation of lignin species and lay the foundation for its universal application. At the same time, the development of green and simple LPCM preparation process has a certain reference value for promoting its industrial application.
Silicon-based materials have the highest theoretical specific capacity (4200 mAh/G) and lower lithium intercalation potential (0. 2 ~ 0.3 V), which are considered to be the most promising high specific energy anode materials for industrialization. However, in the process of lithiation/delithiation, silicon-based materials have some shortcomings, such as self-crushing failure caused by repeated volume expansion and contraction, repeated formation and crushing of SEI and electrolyte consumption, which lead to poor cycle stability and restrict their industrial application
[62,63]. One of the effective methods to solve this problem is to construct a rigid buffer matrix based on the design of lignin carbon coated core-shell structure to inhibit the volume expansion and shrinkage of silicon-based materials. Chen et al. used alkali lignin and SiO
2 as raw materials to prepare SiO
x@LPCM electrode materials in one step at 600 ℃ by ball milling combined with carbonization, and the specific capacity was 900 mAh/G after 250 cycles at 0.2 A/G
[57]. Huang et al. used quaternized modified alkali lignin as raw material to prepare honeycomb SiO
2@LPCM electrode material through electrostatic self-assembly strategy by soft and hard dual-template method. The reversible specific capacity was up to 1109 mAh/G after 100 cycles at 0.1 A/G, and it had excellent rate capability and cycle stability (Fig. 6)
[58].
3.2 Cathode material of sodium-ion battery
The low reserves, uneven geographical distribution and high price of lithium resources restrict the large-scale construction of energy storage systems and the further large-scale utilization of clean energy storage. Sodium resources are naturally abundant and cheap, making sodium-ion batteries one of the potential alternatives to lithium-ion batteries
[64]. However, the radius of sodium ion is 0. 102 nm, which is much larger than that of lithium ion (0. 076 nm), resulting in the slow diffusion rate of sodium ion, and the volume expansion and contraction of sodium ions in the process of intercalation and deintercalation from the electrode material, which leads to the collapse of the structure of the electrode material itself, resulting in poor cycle stability and diffusion kinetics
[65]. Lignin hard carbon prepared by direct carbonization (above 1000 ℃) has the characteristics of disordered graphite crystallite, large interlayer spacing and high electronic conductivity, which is conducive to the storage and rapid diffusion of sodium ions, and is considered to be the most promising anode material for industrial application. Table 4 mainly summarizes the research progress of lignin hard carbon used as anode materials for sodium-ion batteries. Carbonization temperature is the key factor affecting the sodium storage performance of lignin hard carbon anode materials. Navarro-Su Suárez et al. prepared lignin hard carbon by direct carbonization of alkali lignin at 700 ~ 900 ℃. With the increase of carbonization temperature, the specific capacity of lignin hard carbon increased from 159 mAh/G to 250 mAh/G after 5 cycles at the same current density
[66]. On the other hand, the sodium ion storage capacity of lignin hard char after long cycle also increased with the increase of carbonization temperature. Jia et al. prepared lignin hard carbon with specific surface area of 164 m
2/g by direct carbonization of alkali lignin at 1000 ℃, which had a reversible specific capacity of 180 mAh/G after 200 cycles at 0.05 A/G
[67]. Ghimbeu et al. prepared lignin hard carbon with specific surface area of 1.8 m
2/g by direct carbonization of alkali lignin at 1200 ℃, and the reversible specific capacity was 200 mAh/G after 50 cycles
[68].
The sodium storage properties of lignin hard carbon prepared from different kinds of industrial lignin were different, and the sodium storage capacity of lignin hard carbon showed the same trend with the increase of carbonization temperature. Ghimbeu et al. prepared lignosulfonate and alkali lignin derived hard carbons with specific surface areas of 180 m
2/g and 1.8 m
2/g, respectively, by direct carbonization at 1200 ℃, and studied their sodium storage properties
[68]. The results showed that the sodium storage capacity of lignosulfonate-derived hard carbon (120 mAh/G) was lower than that of alkali lignin-derived hard carbon (200 mAh/G), and the specific surface area of pre-ash-removed lignosulfonate-derived hard carbon decreased sharply to 10.1 m
2/g, while the sodium storage capacity increased to 270 mAh/G (Fig. 7). Yoon et al. prepared lignin hard carbon by direct carbonization at 900 ℃ and 1300 ℃ using low-ash acid-hydrolyzed lignin as raw material, and the specific capacities were 157 mAh/G and 297 mAh/G respectively after 100 cycles at 0.05 A/G (Fig. 8)
[69].
The heteroatom-doped surface modification not only further improves the conductivity of lignin hard carbon, but also introduces active sodium storage sites, and the construction of developed pore structure can provide a shorter sodium ion diffusion path, thus improving the sodium storage performance of lignin hard carbon
[71][72]. Du et al. Prepared nitrogen-self-doped honeycomb LPCM at 700 ℃ by using alkali lignin and SiO
2 as raw materials and hard template agent respectively. The reversible specific capacity was 100 mAh/G after 1100 cycles at a high current density of 1 A/G, and the capacity fading rate was only 8%, showing excellent sodium storage performance
[70].
It is of great significance to explore and clarify the sodium storage mechanism of hard carbon for the preparation of high-performance lignin hard carbon. However, the natural disordered structure of hard carbon endows it with abundant sodium ion storage sites and complex sodium storage mechanisms. "Insertion-filling", "adsorption-filling" and "adsorption-insertion" sodium storage mechanisms were proposed based on the slope region above 0.1 V and the flat region below 0.1 V in the charge-discharge curve of hard carbon
[73⇓~75][76][77]. On this basis, Sun et al. proposed the "adsorption-insertion" sodium storage mechanism as shown in Fig. 9. The hard carbon prepared at a low carbonization temperature of 600 ~ 800 ℃ has a highly disordered structure.The wide carbon interlayer spacing acts as a special defect to provide surface-active sodium storage sites through a "pseudo-adsorption" mechanism with conventional defects such as pore surfaces, edges, and heteroatoms through an adsorption mechanism, and there is no phase change or structural change, showing a slope curve above 0.1 V
[78]; The carbon layers of the hard carbons prepared at 800 ~ 1 300 ℃ are rearranged and the distance between carbon layers decreases, and the mechanism of "pseudo adsorption" of sodium ions between carbon layers changes into the mechanism of "intercalation" of sodium ions into carbon layers, which increases the sodium storage capacity and presents a flat curve below 0. 1 V. The carbon layer rearrangement of the hard carbon prepared above 1 300 ℃ is strengthened and the interlayer spacing is less than 0. 36 nm, the sodium ions can not be inserted, and the capacity is sharply reduced. The adsorption and storage mechanism of sodium mainly caused by a small amount of residual defects and voids contributes to the low slope capacity of more than 0. 1 V. The structure of lignin is hyperbranched, complex and amorphous, which inhibits the rearrangement and ordered arrangement of carbon layers during carbonization, making it difficult for lignin to be highly graphitized even at 2700 C.When the sodium storage capacity of lignin hard carbon reaches the highest, the corresponding carbonization temperature is higher than that of other biomass hard carbons (1300 ℃), which indicates that lignin hard carbon has a wider modification potential in morphology, structure and surface microenvironment
[79][80].
3.3 Lectrode material for supercapacitor
Compared with lithium-ion and sodium-ion secondary batteries, supercapacitors are a class of power green electrochemical energy storage devices with high power density, short charging time and long service life, which play a vital role in military defense, rail transit and other fields
[81]. However, the low energy density restricts its further large-scale application
[82]. Carbon-based electrode materials are the core of supercapacitors, and the internal slow electron/ion transport kinetics is the main factor limiting the performance of supercapacitors
[83]. Designing and controlling the LPCM microstructure (such as specific surface area, morphology, pore size distribution, etc.) To optimize the charge transport kinetics is one of the feasible strategies to prepare high specific energy supercapacitors. The research progress of LPCM in supercapacitors is summarized in Table 5.
The pore structure of lignin hard carbon prepared by direct carbonization is mostly closed pores or narrow bottleneck pores, which can not be effectively infiltrated by electrolyte, and the migration rate of ions is severely limited at high current density, resulting in low electrochemical performance. The construction of hierarchical structures with reasonable distribution of micropores, mesopores, and macropores by the pyrolysis process can significantly improve the electron/ion transport kinetics. The uniform distribution of interconnected pore channels of micropores, mesopores and macropores shortens the diffusion distance of electrolyte ions and has enough ion storage space, and the three-dimensional porous carbon framework provides a fast electron transfer channel.
The specific surface area and hierarchical pore structure of LPCM were controlled by the two-step method of carbonization and activation, and the electrochemical performance of LPCM was excellent. Yu et al. prepared the hierarchical LPCM by the two-step method of carbonization at 600 ℃ and KOH activation at 800 ℃, and the specific capacitance at 1 A/G was 276 F/G, which was higher than that of LPCM prepared by the one-step method of carbonization and activation at 800 ℃ (210 F/G), due to the higher microporosity
[84]. Zhang et al. deeply studied the effects of pyrolysis process parameters, such as carbonization, alkali lignin char/KOH ratio and activation temperature, on the electrochemical performance of hierarchical LPCM prepared by two-step method of carbonization first and then KOH activation
[85]. The results showed that the specific capacitance of LPCM first increased and then decreased with the decrease of the ratio of lignin-carbon to potassium hydroxide. The specific capacitance of LPCM was the highest when the ratio of lignin-carbon to potassium hydroxide was 1:4. As for the activation temperature, the specific capacitance of LPCM increases first and then decreases with the increase of activation temperature from 700 ℃ to 900 ℃, and the specific capacitance of LPCM prepared at 800 ℃ is the highest. As for the carbonization temperature, the specific capacitance of LPCM decreases gradually with the increase of carbonization temperature from 500 ℃ to 800 ℃, which may be related to the collapse of pores caused by subsequent activation. The specific capacitance of LPCM prepared under the optimal process conditions is 286. 7 F/G at 0. 2 A/G, and the specific capacitance retention rate is 72. 2% at 8 A/G.
On the other hand, the introduction of pseudocapacitance by heteroatom-modified surface modification and the improvement of interfacial wettability between LPCM and electrolyte can further improve the capacitive performance
[92]. Wang et al. used aniline-modified organic solvent lignin as raw material to prepare hierarchical LPCM by two-step method of carbonization at 400 ℃ and KOH activation at 700 ℃. The specific capacitance was as high as 336 F/G at 1 A/G, which was mainly attributed to the pseudocapacitance contribution of pyridine N and pyrrole N groups on the surface
[86].
In the two-step method of carbonization followed by activation, the carbonization pretreatment temperature is generally higher than 400 ℃, even as high as 700 ~ 800 ℃, which leads to the decrease of the stability and connectivity of the hierarchical pore structure of LPCM prepared by subsequent activation. Compared with the carbonization pretreatment, the hydrothermal pretreatment has a lower temperature and retains the surface heteroatom functional groups to the maximum extent, which is conducive to the subsequent activation to construct excellent hierarchical structures.
Zhang et al. prepared N-self-doped (3.4%) hierarchical LPCM by two-step method of hydrothermal pretreatment followed by KOH activation using enzymatic hydrolysis lignin as raw material, with specific capacitance of 312 F/G at 1 A/G and specific capacitance retention of 81% at 80 A/G, and the rate capability in ionic liquid electrolyte system was better than that of silk-derived N-doped carbon nanosheets
[50][33]. In addition, Guo et al. compared the electrochemical performance of hierarchical LPCM prepared by two-step method of hydrothermal pretreatment and KOH activation with that prepared by one-step method of carbonization and activation. After hydrothermal pretreatment, the enzymatic hydrolysis lignin appeared intercommunicating pores from hundreds of nanometers to several microns, and then activated by KOH.A large number of 300 ~ 500 nm macropores, micropores and mesopores are formed, the specific capacitance is 420 F/G at 0.1 a/G and 284 F/G at 100 a/G, and the capacitance retention rate is 67%
[87]. However, the LPCM prepared by the one-step carbonization and activation method has random pore channels, narrow pore size distribution, low specific surface area, and relatively poor capacitance performance (Fig. 10).
The LPCM imitating natural topological porous network structure is prepared by direct carbonization and acid impregnation treatment with natural inorganic minerals in high-ash lignin raw materials as hard templates. Pang et al. prepared the sodium lignosulfonate-derived hierarchical LPCM by self-template method at 700 ℃, the specific capacitance was 247 F/G at 0. 05 A/G, but it was 104 F/G at 20 A/G, and the retention rate was only 42%.The rate capability increased with the increase of carbonization temperature, which was attributed to the increase of conductivity and the formation of 2 ~ 7 nm mesopores
[88]. Pang et al. Further preoxidized the morphology of sodium lignosulfonate at low temperature (≤ 200 ℃), and then prepared the hierarchical LPCM at 700 ℃ by self-template method. The capacitance performance of the hierarchical LPCM was improved. The specific capacitance was 276 F/G at 0. 1 A/G and 147 F/G at 20 A/G, and the specific capacitance retention was increased to 65. 3%
[89]. In addition, Liu et al. used alkali lignin as raw material to prepare O (9.88%), N (2.82%), S (1.73%) polyatomic co-self-doped hierarchical LPCM by self-template method at 700 ℃
[90]. Due to the enhancement of interfacial wettability and the introduction of pseudocapacitance, the LPCM has a specific capacitance of 244. 5 F/G at 0. 2 A/G and 200 F/G at 40 A/G, with a capacity retention of 81. 8%.
The specific surface area of the LPCM prepared by the self-template method is about 1200 m2/g, and the pore structure is dominated by micropores. Compared with the two-step method of carbonization followed by activation and hydrothermal pretreatment followed by activation, the LPCM prepared by the self-template method lacks sufficient ion storage space and fast migration path, and has low capacity and poor rate performance. The capacity and rate performance of LPCM were improved to some extent by the heteroatom surface self-modification coupling self-template method, but the capacity was still lower than that of LPCM prepared by the two-step method.
The capacity and rate performance of LPCM can be effectively improved by designing and constructing hierarchical porous structures with high mesoporosity to improve the rapid migration of ions and electrons. Saha et al. prepared 82% mesoporous hierarchical LPCM with alkali lignin as raw material and F127 as soft template. After CO
2 or KOH activation at 1000 ℃, the mesoporosity was reduced to 79% and 66%, respectively, and the specific capacitance was 77.1 F/G, 102.3 F/G and 91.7 F/G, respectively, under the same test conditions
[39]. In the process of constructing high mesoporosity hierarchical pores, the proportion of microporous structure is a secondary control index that can not be ignored. When the microporosity is too high, it is easy to cause the blockage of ion transmission under high current density. When the microporosity is too low, the ion storage space is reduced, resulting in low capacity. In response to the above problems, our group developed a green and novel strategy of gas phase exfoliation and in situ ZnO templating, and simply used ZnC
2O
4 to prepare LPCM with a reasonable hierarchical porous structure of 87.1% high mesoporosity and 12.9% microporosity, 1069 m
2/g high specific surface area and nanosheet morphology, which showed a high capacity of 320 F/G at 1 A/G and still maintained an initial capacity of 93.5% after 10 000 cycles at 5 A/G (Fig. 11)
[91].
4 Conclusion and prospect
Industrial lignin is a by-product from the process of high-value utilization of biomass, which has the advantages of low price, high carbon content and easy centralized collection, and is inexhaustible, and is expected to become the primary raw material for the preparation of porous carbon materials for large-scale industrial applications. In this paper, the effects of different pore-forming technologies on the structure and electrochemical properties of lignin-based porous carbon materials were summarized from the perspective of pyrolysis process, which provided a theoretical basis and guidance for the establishment of precise control methods for the preparation of functional lignin-based porous carbon materials and the clarification of key influencing factors. The main conclusions of this paper are summarized as follows:
(1) that lignin hard carbon prepare by direct carbonization has low specific surface area and almost no porous structure, and the porou structure needs to be constructed by means of activation method, template method, hydrothermal method and the like to broaden the application range.
(2) in physical activation, carbon dioxide is activated to form depth and run-through graded pores dominated by mesopores; Steam activation results in the formation of hierarchical pores, which are mainly deep micropores, and the yield of lignin porous carbon materials is low.
(3) In chemical activation, both KOH and ZnCl2 activation formed hierarchical pores with micropores as the main part, and the optimal specific surface area, pore volume and optimal preparation temperature of lignin porous carbon materials prepared by KOH activation were higher than those of lignin porous carbon materials prepared by ZnCl2 activation. However, KOH activation seriously corrodes the process equipment at high temperature, and the cost is high.
(4) that specific surface area of the lignin porou carbon material prepared by the template method is low, and the pore diameter is single and is mainly mesoporous. The mesopore size of lignin-based porous carbon materials is determined by the characteristics of hard templates, while the soft template method can only control the pore structure of low-molecular-weight lignin-based porous carbon materials.
And (5) that lithium storage capacity of the lignin porou carbon material can be increased by the synergistic effect between the reasonable hierarchical porou structure and the lithium ion intercalation/deintercalation graphite-like layer. Surface modification can improve the electrochemical performance of lignin porous carbon materials by reducing the occurrence of interfacial side reactions.
And (6) the lignin hard carbon prepared by direct carbonization has large carbon layer spacing, is gradually ordered along with the increase of the carbonization temperature, has continuously increased sodium storage capacity, and has wider shape, structure and surface microenvironment modification space.
And (7) construct a three-dimensional hierarchical interconnected porou structure through that combined regulation and control of the pyrolysis process to increase the accessible area of the electrolyte and the pore canal, effectively shorten the ion diffusion distance, have enough storage space, provide a rapid electron transfer channel, and greatly improve the capacitance performance of the lignin porous carbon material.
Based on the long-term strategic goal of green and sustainable development, the use of industrial lignin as raw material to prepare porous carbon materials for energy storage materials has great prospects for industrial development, but still faces many challenges. Future research should be carried out from the following perspectives:
(1) According to different electrode materials, a reasonable hierarchical interconnected pore structure is constructed to further optimize the ion/electron transport kinetics, increase the effective contact area between the electrolyte and the lignin porous carbon material, shorten the charge transfer path and provide a rapid transfer path.
And (2) that precise control of the carbon layer with proper lay spacing and highly ordered arrangement improves the rapid storage kinetic of lithium/sodium ions, increases the conductivity, and improves the rate capability and cycle stability of the lignin porous carbon material.
And (3) the targeted regulation of the functionalized surface microstructure provides additional anchoring ion active sites, increases the electron transmission rate, enhances the wettability of a surface interface and an electrolyte, and improves the capacity and the cycle performance of the lignin porous carbon material. In particular, the two-dimensional nanosheet morphology with hierarchical pores has a stronger functionalization effect.
(4) Research and develop advanced carbonization technologies to prepare lignin porous carbon materials with special crystal structure and morphology, such as ultra-high temperature thermal carbonization (> 300 ℃), laser carbonization, high-energy photothermal carbonization and dehydration carbonization, so as to further broaden the advanced application scope of lignin porous carbon materials. At the same time, the relationship between carbonization process parameters and pore structure, crystal structure and surface microenvironment was established.
And (5) combining the structure-activity relationship between the microstructure and the electrochemical performance of the functional lignin porous carbon material, developing a novel, green and simple preparation process route, and constructing a causal relationship between the preparation process factors and the electrochemical performance to guide the large-scale production of the functional lignin porous carbon material.
National Key Research and Development Program of China(2018YFB1501503)
National Natural Science Foundation of China(22038004)
National Natural Science Foundation of China(21878114)
Research and Development Program in Key Fields of Guangdong Province(2020B1111380002)
Natural Science Foundation of Guangdong Province(2017A030308012)
Natural Science Foundation of Guangdong Province(2018B030311052)
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