Simple And Low-cost Fabrication Of Electrode Materials For Supercapacitor

Energy issues have become a huge challenge in this recent year due to the over-expanding population all over the world.Traditional fossil fuels,e.g.coal,oil,and natural gas,are the primary sources for energy systems.However,the emissions of traditional fossil fuels have given negative impacts on the environment and human activities.A type of new energy sources and technology,such as solar cells,geothermal,sea wave,biomass,electrochemical sources,and wind,are invented and developed by researchers.In human daily life,electrochemical power sources provide incredible merits because of their vast application also clean.Supercapacitors reveal significant advantages for energy power storage because of the cycle time stability,highest power density,and rapid charge-discharge give considerable attention for potential application in memory backup,vehicles,and electronic devices.The key point to get high-performance supercapacitors is the electrode material.This thesis focuses on the preparation of good and stable electrochemical performance electrode materials with a simple and low-cost method;the following three works have been led out:（1）Pre-treatment process is an important task to do before using the biomass as the sources for activated carbon materials.Every kind of biomass has its specific structure and chemical bonding that one pre-treatment of one kind biomass could not be used for another biomass.Generally,the hydrothermal process is the most used pre-treatment for biomass wherein the hydrothermal process needs a high temperature for heating the biomass.We employed rice straw as a raw material for carbon sources and before converted it to activated carbon,the pre-treatment and activation processes were did.Cellulose,hemicellulose,and lignin that bonded each other by hydrogen bond would be cut off and elaborated into polysaccharide polymers and sugar monomers at which point a strong oxidative agent,hydrogen peroxide will oxidize the hydrogen bonds which construct the cell wall of rice straw.The addition of sulfuric acid will help to shorten the reaction,while for the activation process,potassium hydroxide has been used as an activation medium that contributed to the forming of rich micro-and/or mesopores on the surface of carbon materials.Besides as activation medium,potassium hydroxide also will represent the forming of slack layer morphology and expedited the thermal exfoliation to establish a graphene-like material.As a supercapacitor electrode material,not only the specific surface area,but also the morphology of the obtained activated carbon（ultra-thin layer which was rich in pores,micro-or meso-）give the effect into electrochemical capacitive properties.The existence of many surface active hetero elements of oxygen,sulphur and/or nitrogen-containing functional groups also enhanced the hydrophilic ability of the active carbon,further give effect onto its electrochemical performance.The capacitance of the obtained activated carbon from rice straw is up to255 F g^-1.（2）Spinel nickel cobaltite（NiCo₂O₄）,one kind of the popular TMOs attends a special interest for researchers as a promising bimetallic oxide electrode due to its outstanding electrochemical activity.Nevertheless,its relatively weak conductivity and small specific surface area make the capacity of NiCo₂O₄ significantly lower.To optimize the performance of NiCo₂O₄,innumerable efforts have been done by researchers such as the use of conductive carbon materials to increase its electroactive surface or control its microstructures,crystallinity,and electrical conductivity.In this work,a simple and efficient solvothermal method was extended to prepare nickel cobaltite precursors,which were annealed in the air to create flower-like NiCo₂O₄ materials.The typical characteristic of the synthesis method is the use of ethanol only as of the solvent,while nickel nitrate,cobalt nitrate,and human urine are used as other reagents for the fabrication of NiCo₂O₄ precursors.The organic compound in human urine was transformed into carbon.In the absence of ethanol,the most part of urine is water（95%）,the cobalt and nickel ions would aggregate together and stick on the organic compound.In the existence of ethanol,due to hydrogen bonds,the organic compound would be reacted with the lipophilic group within ethanol and become a carbon blanket for cobalt and nickel ions because the other group,hydroxide group has a binding with the water molecule.The H-bounding interactions contribute to the formation of the material topology.The functional groups guided to the isolation of the interpenetrated network of material topology.This result represents that human urine had a significant impact on the synthesis and it also confirmed that human urine could be a reagent（because of the mineral and substances contents）at once solvent（the presence of water）in the chemical process.The using of human urine in this work provides a new reagent-solvent to prepare TMOs.（3）VS₂,CoS₂,GeSe,GeSe₂,and SnSe are the kind of metal chalcogenides（LMCs）,and those kinds of metal have taken much attention by dint of their superior geometric structures,variable composition,and rich segment structures that offering a huge library of 2D materials for prospective applications in energy storage devices.However,the performance of those metals in SCs could not fulfill the increasing need for higher power density and the reason might be the less electrochemical activity of metal tin and germanium.Cobalt selenide is one kind of 2D layered LMCs and it has been confirmed and used to be an electrochemically metal cobalt composite for electrocatalytic applications,such as water splitting,dye-sensitized solar cells,oxygen reduction reaction（ORR）,and degradation of hydrazine hydrate.In this work,graphene oxide is used as a medium for cobalt selenides loadings,which can improve the electrochemical performance of cobalt selenides.By arranging the amount of graphene oxide solution in the preparation solution,the obtained cobalt selenides with different electrochemical performance could be acquired.It proved by cobalt selenides that containing graphene oxide has higher specific capacitance than cobalt selenides without graphene oxide content.As a supercapacitor electrode material,the specific capacitance of cobalt selenides/rGO composites is 325 F g^-1.