Study On The Preparation And The Energy Storage Performance Of Pyrolyzed Lignin

Lignin as second most abundant natural polymer after cellulose is composed of a three-dimensional cross-linked structure between three kinds of phenyl propane monomers,which is of low cost,non-toxicity and renewability.The character of abundant aromatic bio-macromolecules makes lignin one of the most potential carbon precursors for energy storage.The paper investigates the applicability of lignin-based carbon materials in lithium ion battery and supercapacitors,respectively.Lignin as a by-product of fuel alcohol industry is used to prepare hard carbon materials by acetone extraction,stabilization in N2 at 300 oC,carbonization in N2 for 1 h and subsequent H2 reduction for 3 h at 800 oC or carbonization in N2 for 4 h.In comparison with samples in N2 atmosphere,hard carbon prepared in H2 atmosphere has a favorable impact,less surface oxygen-containing functional groups and relatively higher graphitization degree.Moreover,an obvious improvement of capacity performance is obtained with reduction treatment.The initial coulombic efficiency(CE)improves from 42.4% to 62.4%,a first discharge/ charge capacity of 882.2/550.5 mAh/g is achieved at 0.1 C,and even after 200 cycles at 2 C a charge capacity of 228.8 mAh/g(about 92.8% retention ratio)remains.Lignin-based activated carbon(LACs)is prepared by one-step direct activation with different KOH to lignin ratio(K/LG)for supercapacitors(SCs).It turns out LACs show type I isotherm adsorption curve.With improvement of K/LG,the BET surface area increases from 1440 m2/g to 3332 m2/g,micropores content decreases from 67% to 22% and the structure of LACs becomes more ordered,which indicates KOH activation plays dual roles of removal of non-carbon atom and rearrangement of carbon structure.A high Cg but poor rate performance can be obtained with low K/LG in aqueous solution while high K/LG is needed in organic solution.Overall,the capacitive performance balances with rate performance as K/LG equal to 3 no matter what kind of electrolyte is applied.Lignin-based activated carbon is prepared through a two-step pre-pyrolysis and activation method with an optimum K/LG of 3 for high-performance SCs.It is concluded LAC-P with pre-pyrolysis exhibits an outstanding improvement on both pore structures and electrochemical performances in comparison with directly-activated LAC-D.A gravimetric capacitance as high as 312 F/g and specific capacitance(14.0 μF/cm2)are achieved,and even after 10,000 cycles at 1 A/g a specific capacity of 261 F/g(about 95% retention ratio)remains.In a word,the pre-treatment process has strong operability and can be applied to other precursors to improve the electrochemical performance.