# Lignin to Materials: A Focused Review on Recent Novel Lignin Applications
## 4 Electrochemical Energy Materials  
In the past decade, lignin has been increasingly investigated for its potential incorporation in the production of battery materials and supercapacitors for the main advantage of being environmentally friendly. With the increasing manufacturing of lignin in recent years, lignin has been available at a low cost. The relatively low cost of lignin makes it an attractive ingredient for the production of anodes for lithium batteries, gel electrolytes, binders, and sodium batteries [39]. Lignin is also valued for the production of energy materials due to its high carbon content that is greater than 60 wt %. Specific functional groups of lignin, such as benzyl and phenolic groups, act as active reaction sites for ions to be stored in applications to supercapacitors. Additionally, the abundant oxygen atoms in lignin are integral for facilitating electrolyte ion adsorption and redox reactions for supercapacitors. Lignin can be used for porous carbon structures in supercapacitors due to its favorable crosslinked structure [2]. While lignin types like lignosulfonate can act as a sulfur-doped agent in batteries or supercapacitors, alkali lignins have shown to be suitable for electrospinning processes to construct nanomaterials [39]. There are generally two processes commonly employed to derive carbon from lignin for energy material applications. One process utilizes precursor carbonization and carbon activation, while the other process uses chemical activators before carbonization and activation that happen at the same time [2]. Graphene, a promising carbon material for electrochemical energy materials, can be synthesized from lignin by catalytic graphitization, carbonization, or oxidative cleavage along with aromatic refusion [40; 41; 42]. The incorporation of lignin results in a high gravimetric capacitance and great cycling durability for energy materials. Table 2 shows the summary of recent studies on the electrochemical applications of technical lignins. In an experiment with electrospun carbon nanofibers produced from alkali lignin\(-\)PVA solutions, the capacitance of the supercapacitor reduced by 10% after 6000 cycles of discharging and charging [43]. Furthermore, a higher energy value of 42 W h kg\({}^{-1}\) and a power density of 91 kW kg\({}^{-1}\) were reported. The experiment suggested that an increasing amount of lignin in precursor nanofibers resulted in a decreased average pore size, increased pore volume, and increased specific surface area. In another study, electrodes constructed from 75 wt % alkali lignin and 25 wt % 0.5 M sodium sulfate electrolyte resulted in one of the highest specific capacitances for electrodes produced from biopolymers: 205 F g\({}^{-1}\)[44]. The mesopore range of carbon fibers had a wide pore distribution, which contributed to excellent electrochemical performance. Additionally, hierarchical porous carbons can be derived from steam explosion lignin through the carbonization\(-\)activation method. Lignin-based hierarchical porous carbons showed a high capacitance of 286.7 F g\({}^{-1}\) at 0.2 A g\({}^{-1}\)[45]. The structure of lignin provided advantages to the electrochemical performance, including accessible ion transportation pathways and a high surface area. Lignin-derived materials used to construct batteries have performed similarly with commercial graphite and other materials commonly used in the industry. Ball-milled hydrolysis lignin can be used for low-rate power sources. A study investigated the performance of a battery that used hydrolysis lignin as the lithium battery cathode material [46]. The cathode material had 76 wt % hydrolysis lignin, 13 wt % carbon black, and 11 wt % polytetrafluoroethylene (PTFE)-based binder. Lithium batteries using this hydrolysis lignin as a cathode material achieved a high discharge capacity of 450 m A h g\({}^{-1}\).