Design And Synthesis Of Transition Metal Compounds And Their Capacitance And Electrocatalytic Properties

Transition metals own variable oxidation values,the corresponding compounds are the main materials for redox reactions and electrocatalytic systems.The optimization of faradic activities and the interface catalytic activities for certain redox reactions through morphological,microstructural,surface polarity and oxidation value tuning are feasible approaches for improving the electrochemical performances of energy storage devices.This thesis mainly involves the design,synthesis and surface polarity,oxidation value and structural stability tuning of transition compounds so as to construct efficient and versatile catalytic electrode-redox electrolyte system,and the electrochemcial performances in supercapacitors,oxygen evolution and oxygen reduction reaction systems were investigated.The main content of this thesis includes the following aspects:?1?Ni foam was employed as current collector and catalytically active material to evaluate the capacitive performance in alkali Fe?CN?₆^3-/4-redox electrolyte.The capacitance of the 1.5*1.5cm² Ni foam electrode in 0.5 mmol Fe?CN?₆^3-/4-containing 2M KOH electrolyte is measured to be 1975 mF at 10mA.Besides,the capacitance increases linearly with the concentration of Fe?CN?₆^3-/4-in electrolyte,the concrete capacitance achieves 7000 mF at Fe?CN?₆^3-/4-concentration of 2.5 mmol,which is valuable for practical applications.Additionally,the capacitance can be well maintained within test temperature range of 0⁶0 ^oC,and 72%of its initial capacitance can be maintained undergoes 10000 successive charge-discharge cycles.The self discharge test reveals that of potential can still be retained to be 0.44 V after 12 h after being fully charged.The Ni foam-1.5 mmol ferrocyanid eelectrolyte//AC asymmetric supercapacitor can deliver C_cells of 3600,2432,1928,1664 and 1492 mF,respectively at 10,15,20,25 and 30 mA,which account for area capacitances of 1.60,1.08,0.86,0.74 and 0.66 F cm^-2,respectively,at 4.4,6.7,8.9,11.1 and 13.3 mA cm^-2,the values are superior to faradic metal oxides and sulfides based hybrid supercapacitors.This section of work provides a novel avenue to improve the capacitive performance of supercapacitor through efficient electrode-electrolyte interactions.?2?A simple method for preparing F^-anions enriched nickel hydroxide?F-NHO?mesocrystalline microspheres via NH₄F mediated hydrothermal hydrolysis of nickel precursors was proposed.The as-prepared porous mesocrystalline F-NHO and the high F^-anion doping level enables highly hydrophilic surface,and the high electrons withdrawing ability induces high oxidation values of Ni element.As an anodic electrocatalyst for alkaline oxygen evolution reaction,the F-NHO electrode can provide higher anodic current density and lower overpotential and Tafel slope relative to the F^-free NHN electrode.The potential of F-NHO electrode at current density is 10 mA cm^-2?1.51 V?is obviously lower than that of NHN electrode?1.57 V?,which manifests the higher catalytic activity of F-NHO electrode.In addition,its structure and composition are also favorable for high catalytic stability,the anodic current for OER decays steady with a retention ratio of 91.3%is achieved undergoes 24 hours of i-t operation,which is apparently superior to that of the NHN electrode?59.3%current maintaining ratio?.This section of work provides a feasible way to improve the catalytic efficiency for OER.?3?ZIF material was synthesized in ethanol solution at room temperature using KOH as strong deprotectant,cobalt as the central ion,pyromellitic acid,terephthalic acid and isophthalic acid as ligands.The formation of MOF was almost independent upon coordination ability of metal ions by forced deprotonization with potassium hydroxide.The yield of ZIF is higher over Co and Ni based ZIF materials by 6.3 and 130 folds,and a large number of Cu ZIF that is commonly difficult to synthesize at room temperature can be synthesized at large scale.Taking ZIF-67 as an example,its carbonized derivatives showed excellent catalytic performance for OER,HER and ORR.