Graphene Support Nanoparticles As Active Materials Of Electrodes And Quasi-solid Gel Electrolytes

This dissertation focuses on the synthesis of graphene-supported nano-metal active materials,aiming to increase the energy density of supercapacitors and reduce the amount of precious metal Pt in the anode catalyst of direct methanol fuel cells.In addition,cellulose/NaOH and "DMSO/H2O in salt" chitosan quasi-solid-state gel electrolytes were also developed successfully from the perspective of electrolyte design.In Chapter Two,an appropriate amount of CoFe2O4 nanoparticles were loaded onto graphene nanosheets by a mild hydrothermal method.By reducing the proportion of graphene,the CoFe2O4 nanoparticles became denser with an increase of their weight percentage.40%proportion of CoFe2O4 could effectively avoid particle agglomeration and make full use of active materials with a specific capacity of up to 289.7 C g-1.Even when the current density was expanded by 8 times,it could still maintain 94%specific capacity,showing excellent rate performance.Fe3O4/graphene hydrogel was also synthesized in an atmospheric water bath method.The material exhibited a specific capacitance of 278.5 F g-1 in the potential range of 0～-1.2 V(vs Ag/AgCl),making itself an ideal anode material for supercapacitors.The obtained asymmetric supercapacitor,with a wide working voltage of 1.7 V,could output an energy density of as high as 45.5 Wh kg-1.In Chapter Three,according to the Kirkendall effect,a simple and mild two-step hydrothermal method was adopted.Manganese cobalt hydroxide precursors were first prepared on the graphene support,and then in situ sulfurized to MnCo2S4.At the same time,graphene was also doped with N and S components,introducing additional pseudocapacitance.The usage of sulfur source(thiourea)could affect the particle size,the Co3+ content in sulfides,and the defect degree of graphene.The optimized electrode could exhibit the largest specific capacity of 529.7 C g-1,and 90.9%of its capacitance was maintained after 5000 cycles.The obtained asymmetric device exhibited a maximum energy density of 62.9 Wh kg-1.In Chapter Six,PtFeCo ternary alloy was in situ loaded on graphene nanosheets by a one-pot two-step reduction method.As a strong reducing agent,NaBH4 could first reduce PtCl62-with a higher reduction potential to Pt at 100℃.At 130℃,by adjusting the content of Fe3+ with the assistance of weak reducing agent ethylene glycol,PtFeCo with different morphology could be obtained.In this process,Fe3+ played an etching role.By adjusting the content of base metals in the alloys,the catalyst could exhibit an ideal surface strain value and a particular electronic structure.The optimized catalyst exhibited a mass current density of 1758.2 mA mg-1 in an acidic medium,which could be significantly increased to 9356.1 mA mg-1 at 60℃(close to the operating temperature of direct methanol fuel cells).In Chapter Seven,SnO2/graphene composites were firstly synthesized by hydrothermal and calcination methods.Then,by means of the semiconductor properties of SnO2,UV light could excite the hole-electron pairs,anchoring Pt exactly to the surface of SnO2 support.The UV-treated SnO2 possessed a higher vacancy oxygen content and better Pt-trapping ability.The synthesized catalyst not only had a large electrochemically active surface area(80.7 m2 g-1)and good mass activity(1833.6 mA mg-1),but also showed good long-term durability and poisoning resistance.If UV was simultaneously introduced in the testing process,the property of the catalyst could be further improved according to the photoelectric synergistic effect.In Chapter Four,the NaOH/urea system was used to dissolve 4%cellulose by freezing solution.The hydrogel electrode was pre-inserted with epichlorohydrin as the in-situ cross-linking agent at a low temperature to obtain the quasi-solid supercapacitor.Cellulose/NaOH hydrogel could act as both electrolytes and separators.A symmetric supercapacitor with two N-doped graphene hydrogels as electrodes could output a maximum energy density of 13.5 Wh kg-1 and exhibit excellent mechanical strength with no significant attenuation in a bending angle of 180°.In Chapter Five,chitosan was used as a polymer network and DMSO/H2O was applied as a mixed solvent.When 4-8 M lithium bis(trifluoromethanesulfonyl)imide(LiTFSI)was added,the system reached the standard of "salt in water" and the chitosan was crosslinked without crosslinkers.Interestingly,when the concentration of LiTFSI was lower than 4 M or higher than 8 M,all the chitosan failed to crosslink.By comparing the properties of different hydrogel electrolytes,4 M LiTFSI contributed to the highest utilization ratio.The voltage window of the double-layer capacitor could reach 2.1 V with a maximum energy density of 62.9 Wh kg-1.And 80.1%of its initial capacitance could be retained after 5000 cycles.