Investigation And Application Of Biomass-based Carbon Materials In Supercapacitors

Supercapacitors,also known as electrochemical capacitors?ECs?,area kind of energy conversion and storage devices for sustainable development and regeneration.They have attracted more and more attention due to their high power density,excellent cycle stability,good operational safety,high thermal stability,fast charge/discharge and long cycle life.According to the energy storage mechanism,supercapacitors can be divided into two types:double-layer capacitors?EDLC?and pseudocapacitors.The double layer is an ion layer with equal number of charges and opposite symbols on both sides of the solution,which forms two charge layers on the solution and the electrode,thus resulting in potential difference between the two phases,the electrode material is mainly carbon-based material.Pseudo-capacitance refers to the rapid and reversible chemical adsorption/desorption or oxidation/reduction reaction occurring at the interface between electrodes and electrolytes,metal oxides are the main materials of pseudo-capacitance capacitors.In this work,we first select biomass with large reserves,convenient sampling,rich in N,P and low cost as carbon precursors to prepare N,P co-doped porous carbon materials at different activation temperatures,and explore the effects of activation temperature and N,P elements on the morphology,structure and electrochemical properties of porous carbon materials.In order to further improve its electrochemical performance,nano-sized MnO₂ was grown in situ via simple hydrothermal reaction to produce composite electrode material.The electrochemical performance of the composite electrode material were studied in detail.The main research contents and conclusion are as follows:?1?A facile procedure for converting the duckweed biomass into N,P co-doped porous carbon material was proposed,a promising high-performance electrode material for use in supercapacitors.Under an optimum activation temperature?750oC?,the carbonized duckweed sample with enriched N,P contents of 3.39 at.%and 0.25 at.%,respectively,exhibits a hierarchical mesoporous structure with a high specific surface area of?1636 m² g^-1.The duckweed electrode displays a specific capacitance of 315.2 F g^-1 in the three-electrode electrochemical configuration at a current density of 1 A g^-1.In addition,the symmetric duckweed electrode offers a maximum capacitance of 225.0 F g^-1 and energy density of 25.3 Wh kg^-1 as well as a high stability(about 95%of capacitance retention after 10000 cycles at 5 A g^-1).?2?In this chapter,by applying duckweed biomass-derived porous carbon as martrix material,?-MnO₂ nanosheets were grown in situ at 140oC for 6 h by hydrothermal reaction.The electrodes prepared by the nanocomposites exhibit high specific capacitance,excellent rate performance and good cyclic stability.In addition,we assembled asymmetric supercapacitors using DWC-750@MnO₂ as cathode material,porous carbon?DWC-750?as negative material and PVA/LiCl is used as solid electrolyte.The range of potential window for constant current charging and discharging is 0² V.When the current density is 1 A g^-1,the energy density is as high as 40.1 Wh Kg^-1.