Preparation And Electrochemical Performances Of Iron-based Metal Oxides/(Sulfides)

With ever-increasing energy demand and increasingly worsened environmental problems,there are a variety of new energy storage devices,such as lithium-ion batteries,lithium-sulfur batteries,fuel cells and supercapacitors,etc.Among them,supercapacitors show good promising application owing to their high power density,low cost and long cycle life.Although supercapacitors have high power density,their low energy density makes them impossible to meet the requirements of large-scale commercialization.Thus it is particularly important to look for a supercapacitor electrode material with high energy density.In this thesis,the iron-based metal oxides/sulfides were firstly prepared,followed by synthesizing the composite with graphene.Finally,asymmetric supercapacitors?ASCs?were fabricated with the other metal oxides/sulfides to further increase the energy density of supercapacitors.Firstly,graphene/Fe3O4 composite has been synthesized by a solvothermal method.According to the SEM and TEM images,it is found that the Fe3O4 nanoparticles are grown uniformly on the surfaces of graphene and in-between graphene layers.The specific capacitance of the GF-60%composite is 214.4 C·g^-1(268.0 F·g^-1)at 2 mV·s^-1,and the capacitance retention is 98.9%after 10000 cycles at a scan rate of 200 mV·s^-1,indicating excellent cycling stability.Additionally,graphene/MnO2 composite was prepared by an in-situ redox approach.It exhibits high specific capacitance of 311.4 F·g^-1 in 1 mol·L^-1 Na₂SO₄ aqueous electrolyte.Finally,an ASC was assembled with GF-60%and graphene/MnO2 as the negative and positive electrodes,respectively.Our fabricated ASC delivers a high energy density(87.6 Wh·kg^-1)and excellent cycling stability with the capacitance retention of 95.4%after 10000 cycles and 88.5%after20000 cycles.Secondly,FeS₂ was synthesized by a one-step hydrothermal method,followed by doping of Co and hybridization with graphene to prepare graphene/Co-doped FeS2 composite?GCFS-0.33?to further improve the electrochemical performance.The resultant composite shows good electrochemical performance with high specific capacitance of 310.2 C·g^-1(387.8 F·g^-1),which is higher than that of pristine FeS₂(249.4 C·g^-1)at the same scan rate of 2 mV·s^-1.Such excellent performances are mainly derived from its unique sandwiched and robust structure,which contributes to the rapid transport of electrolyte ions.Besides,G-LDH was synthesized by a solvothermal method followed by sulfuration treatment to prepare GCNAS.At the scan rate of2 mV·s^-1,the specific capacitance of GCNAS is 742.2 C·g^-1(1649.3 F·g^-1),higher than the unsulfidized G-LDH(582.6 C·g^-1).An ASC fabricated with GCFS-0.33 as the negative electrode and GCNAS as the positive electrode achieves a high energy density(66.8 Wh·kg^-1)and good cycling stability?102.2%capacitance retention after 1000 cycles?.Finally,FeS₂ was synthesized by a two-step method.In the first step,graphene/Fe₂O₃composite was prepared by thermal decomposition followed by Co-doped and sulfidization treatment to prepare graphene/Co-doped FeS₂ composite(graphene/Co_0.33Fe_0.67S₂).The composite shows high specific capacitance of 463.0 C·g^-1(578.8 F·g^-1)at 2 mV·s^-1,which is much higher than GCFS-0.33 composite prepared by one-pot hydrothermal method(310.2 F·g^-1).In addition,Binary NiMn-OOH was synthesized by a hydrothermal method,which exhibits high specific capacitance of 647.4 C·g^-1(1294.8 F·g^-1)at 1 A·g^-1 and remarkable rate performance(79.8%at 20 A·g^-1).An ASC was fabricated with the graphene/Co_0.33Fe_0.67S₂prepared as the anode and NiMn-OOH as the cathode.Our fabricated ASC could deliver a high energy density(67.4 Wh·kg^-1)and good electrochemical stability?105.4%capacitance retention after 11000 cycles?.