Structural Regulation And Electrochemical Performance Of The Transition Metal Compound Matrix Composites

With the rapid development of society,energy crisis with the wastage of fossil fuels and environmental pollution have become two serious challenges to human survival.Therefore,it is an effective way to develop renewable clean energy,which is limited by the conversion and storage of energy.Recently,various forms of energy conversion and storage systems have been proposed,including lithium-ion batteries,fuel cells and supercapacitors.The performance of fuel cells mainly depends on the activity of the oxygen reduction reaction?ORR?.Therefore,the development of catalysts for ORR has attracted worldwide attention to improve the efficiency of fuel cells.Supercapacitor as a new type of energy storage device has the characteristics of environmental friendliness,high charge-discharge efficiency and long cycle life.Electrode materials play an important role in the performance of the supercapacitor,therefore,developing an electrode material with high specific capacitance and good cycle stability is an important way to obtain high performance supercapacitors.Metal-organic framework materials?MOFs?have many advantages such as adjustable composition,large specific surface area,more active sites and adjustable pore structure,which have been used in various fields such as supercapacitors,solar cells,fuel cells and lithium ion batteries.However,its practical application in energy storage and conversion is limited by poor stability and low conductivity of MOFs.Materials with stable structure and excellent performance can be obtained by subsequent processes to preserve the large specific surface area and pore structure of MOFs.In this work,derivatives of NiCo-MOF-74 and composite materials are prepared by subsequent processes.The corresponding structure and electrochemical properties of its derivatives and composites have been characterized.The feasibility used as a catalyst for oxygen reduction reaction and a supercapacitor electrode material is evaluated.The main work is listed as follows:?1?NiCo-MOF-74 precursor was prepared by hydrothermal method with Ni?Ac?₂·4H₂O,Co?Ac?₂·4H₂O and 2,5-dihydroxyterephthalic acid.Derived oxide?NiCo-MOF-74-O?was prepared by heat treatment,subsequently phosphide?NiCo-MOF-74-P?and sulfide?NiCo-MOF-74-S?were obtained by heat treatment and solvothermal process.ORR performance and supercapacitor performance of NiCo-MOF-74-O,NiCo-MOF-74-P and NiCo-MOF-74-S were tested.According to the result of X-ray diffraction?XRD?,the NiCo-MOF-74 precursor was successfully synthesized,and derived oxide,phosphate and sulfide were obtained by different treatments.Hollow tubular structure for NiCo-MOF-74 precursor and its derivatives was observed by scanning electron microscopy?SEM?and transmission electron microscopy?TEM?.The surface of NiCo-MOF-74-O and NiCo-MOF-74-P is composed of particles,while the surface of the sulfide?NiCo-MOF-74-S?is composed of nanosheets,which can improve the electrochemical performance.NiCo-MOF-74-P exhibits excellent electrocatalytic activity for ORR,which is comparable to commercial Pt/C in ultimate diffusion current density,and superior to commercial Pt/C in stability and methanol resistance.For supercapacitance performance,NiCo-MOF-74-S exhibits high specific capacitance?930?4 F/g?,excellent rate capability and cycle stability.Hybrid supercapacitors were assembled with NiCo-MOF-74-S and commercial activated carbon as positive and negative,respectively,and the electrochemical performance was evaluated,which exhibited good power density and energy density.Therefore,NiCo-MOF-74-S can be used as an ideal electrode material for supercapacitor.?2?Supercapacitance properties of NiCo-MOF-74-S with different Ni/Co ratios are studied.NiCo-MOF-74 precursors with different ratios of Ni/Co including Ni-MOF-74,NiCo-MOF-74?Ni:Co=2:3?,NiCo-MOF-74?Ni:Co=1:1?,NiCo-MOF-74?Ni:Co=3:2?and Co-MOF-74,were prepared,and the corresponding sulfides were obtained by heat treatment and solvothermal method.The morphology and electrochemical properties of these sulfides were tested.SEM and TEM result showed that sulfides with different Ni/Co ratios maintain the hollow tubular structure,and the supercapacitance performance of sulfides was affected by Ni/Co ratio.When the ratio is 2:3,NiCo-MOF-74-S?Ni:Co=2:3?exhibits the best supercapacitor performance with a specific capacitance of 1910 F/g at a current density of 1 A/g and good rate performance and cycle stability.The capacitance retention of NiCo-MOF-74-S?Ni:Co=2:3?is 77%at a current density of 20 A/g for 1,000 cycles.?3?NiCo-MOF-74-S@GO composites were prepared by NiCo-MOF-74?Ni:Co=2:3?and graphene oxide,and the supercapacitance properties were investigated.To further improve the specific capacitance of NiCo-MOF-74-S at high current density,graphene oxide was added in the synthesis of NiCo-MOF-74?Ni:Co=2:3?precursor,and NiCo-MOF-74-S@GO was obtained with a similar method.The morphology of NiCo-MOF-74-S@GO is basically the same as that of NiCo-MOF-74.The capacitance retention of NiCo-MOF-74-S@GO at 10 A/g is 77%,while that of NiCo-MOF-74-S is73%,and the capacitance retention is 80%at 20 A/g for 1,000 cycles higher than that of NiCo-MOF-74-S?77%?,which suggests the graphene oxide can improve the rate capability and cycle stability.In summary,NiCo-MOF-74-S@GO be used as a supercapacitor electrode with great potential.