Preparation Of Lignin/Carbon Composites And Their Applications In Supercapacitors

In recent years,with the rapid growth of energy demand in modern life,fossil fuels have been over-developed and used,and energy crisis and environmental problems have become increasingly serious.In order to deal with these problems,it is urgent to develop efficient and environmentally friendly energy storage equipments.Supercapacitors（SCs）are a type of ideal energy storage devices with the advantages of high-power density,long cycle life and fast charge and discharge rate.Lignin is the only renewable natural aromatic biopolymer,which could be constructed into the electrodes of SCs by combining with conductive substrate materials,and the hydroquinone groups contained in lignin can directly participate in redox reactions to store and release electron and protons.However,the reported lignin electrodes demonstrated only a relatively low capacity.It was found that several critical parameters,such as content of effective groups of lignin,the internal structure and surface properties of conductive materials,the synergy between lignin and conductive materials etc could largely affect the electrochemical properties of materials.Based on the above considerations,a series of SCs composed of lignin and conductive materials were designed and prepared in this work.The active groups content in the lignin,the microstructure and surface properties of conductive materials on the properties of electrode materials were explored.The structure-function relationship of lignin/carbon composite supercapacitor electrode was studied.（1）Activated carbon（AC）has the advantages of excellent electrochemical performance,moderate cost,good stability and processability,which has been widely used as a conductive substrate to prepare the composite SCs electrodes.In addition,the surface modification of AC was carried out to enhance the uniform dispersion of kraft lignin（KL）on the surface of carbon substrates and improve the surface charge transfer rate of the composites.In this work,we modified AC surface by nitric acid and thus acquired TAC substrates grafted with copious oxygen-containing groups,and KL/AC and KL/TAC composites were synthesisd by KL combined with AC and TAC through a mild and simple ultrasonic assisted deposition method,and the synergistic effect of KL and TAC was also investigated.Physical characterizations show that the distribution of KL on the surface of TAC is greatly more uniform than that on AC.The oxygen-containing groups on the surface of TAC improve the compatibility,interaction,and charge transfer between TAC and KL.Compared with KL/AC,the charge transfer resistance and ion diffusion resistance of KL/TAC are reduced,making the mass transfer inside the electrode faster.The hydroquinone groups in KL and the oxygen-containing groups in TAC contribute pseudocapacitance jointly,which increase the capacitance of the composite material from 138 F g^-1（KL/AC）to 293 F g^-1（KL/TAC）at 1 A g^-1.After 1000 cycles,the capacitance retention rate increases from94.2%（KL/AC）to 98.1%（KL/TAC）.As the KL/TAC discharge rate increases from0.5 A g^-1 to 20 A g^-1,the capacitance remains 73.9%.Based on electrochemical analyses for KL/TAC with different KL loading contents,the capacitance of KL/TAC first increases and then decreases with the increase of KL content.And the maximum capacitance of 293 F g^-1 is obtanined at the KL content of 17 wt%,in which,the capacitive capacitance(254 g F^-1)occupies 86.8%,indicating this sample owns the optimal charge transfer and diffusion.The results show that the surface modification of the conductive material can greatly increase the the surface charge transfer rate of composite material,which should be further considered in the design of the electrode materials.（2）Although the modification of AC by nitric acid can contribute more pseudocapacitance to KL/TAC,its capacitance performance is still far from expectation.As hydroquinone groups in KL are the main species contributing the capacitance,creating more hydroquinone groups in KL may lead to improved performance.Based on this,we conducted an oxidation modification treatment to KL by using hydrogen peroxide and Fe₂O₃ as oxidant and catalyst,respectively.The degree of oxidation was controlled precisely,and the relations between the oxidization treatments and the electrochemical performances were determined.Physical characterizations show that within 4 h of oxidation treatment,β-o-4 bonds in KL molecules are broken and methoxyl groups are converted into phenolic hydroxyl groups with the increase of oxidation time.The content of phenolic hydroxyl groups in KL gradually increases from initial 1.56 mmol g^-1 to 1.96 mmol g^-1 at 4 h.After 4 h,with the increase of oxidation time,the content of phenolic hydroxyl groups decreases,and that of aldehydes and ketones groups increases,indicating that benzene ring is opened,and carboxylic acid is oxidized.After 7h the content of phenolic hydroxyl groups decreases to 1.52 mmol g^-1at 7 h.Electrochemical characterizations show that the change trend of the capacitances for the composites is consistent with that of phenolic hydroxyl contents in OKL.The capacitance reaches the maximum value of 364 F g^-1 after 4 h oxidation,and the contribution of capacitive capacitance to the total capacitance is up to 89.1%.The results show that the increased capacitance of KL/TAC composite comes from the increased hydroquinone groups from lignin by oxidation process,and the capacitance is positively correlated with the content of phenolic hydroxyl.Indicating that there is still room for improvement in the application of lignin supercapacitors.（3）The internal structures and properties of the conductive materials have important influences on the electrochemical properties of the lignin composite.In this work,nitrogen-doped carbon materials with hierarchical pores（NC）were synthesized by using SiO₂ as the hard template and dopamine as the nitrogen and carbon source.In comparison to NC electrode,the OKL/NC composite electrode demonstrates significantly improved performance.Physical characterizations show that NC has an evenly distributed macropore structure with the pore diameter of about 200 nm.Simultaneously 15 nm mesopores display uniform distribution around the macropores.These hierarchical pores are interconnected to form a structure with a specific surface area of 640.2 m² g^-1 and a total pore volume of 1.334 cm³ g^-1,which facilitates the mass diffusion and ions transfer.Dopamine provides nitrogen and carbon source at the same time,resulting in elemental oxygen and nitrogewn doping in the OKL,which also enhances the surface wettability.Electrochemical characterizations show the great capacitance increases from 154 F g^-1（NC）to 412 F g^-1（OKL/NC）at 1 A g^-1.The hydroquinone groups in OKL and the oxygen-containing groups in NC jointly contribute pseudocapacitances.When the discharge ratio increases from 1 A g^-1 to 20 A g^-1,the capacitance retention rates of NC and OKL/NC are 71%and 77%,respectively.The electrochemical analyses of OKL/NC with different OKL loading contents show that the capacitance of OKL/NC increases first and then decreases with the increase of KL content.When the OKL content is 21 wt%,the OKL/NC capacitance reaches the maximum value of 412 F g^-1.The results show that the synergistic effect of KL groups,heteroatom doping and hierarchical pore structure design of conductive materials can greatly increase the electrochemical properties of lignin composite electrodes,which provides an effective strategy for the design of electrode materials.