Study On Cobalt-based Metal Oxide-reduced Graphene Oxide Composites: Preparation,Characterization And Properties

As a clean energy system,the fuel cell caused more and more attention,hydrogen storage is the key problem for the breakthrough of technology.Owing to the fast charge-discharge rates,reliable stabilities,and long cycle lives,rechargeable lithium-ion batteries?LIBs?have long been the predominant power source for portable devices.A wide variety of transitional metal oxides have been widely investigated by researchers based on their higher theoretical specific capacities,larger than that of the commercial carbonaceous anodes,combined with graphene with excellent physical and chemical properties,it will achieve satisfactory results for the application in catalysis and electrode materials.In this paper,graphene as the carrier,a series of two-dimensional materials based on graphene were synthesized for the hydrolysis of sodium borohydride,and the three-dimensional material coated by graphene was synthesized and as the anode material for lithium ion batteries.The morphology and structures were further characterized by ultraviolet spectroscopy?UV-vis?,X-ray diffractometer?XRD?,transmission electron microscopy?TEM?,surface area and pore size analyzer?BET?,X-ray photoelectron spectroscopy?XPS?.The catalytic performances and electrochemical properties were tested and discussed in details.A solvothermal method was adopted to synthetize Co₃O₄ nanocrystals with oil amine as a protective agent.As a result,Co₃O₄ nanocrystals of small size and well dispersed were synthesized and as a precursor.Combined with graphene of different oxidation degrees,CoO_x-PG and CoO_x-GCNFs were obtained after high temperature calcination process.In the process of calcination,a carbon thermal reaction between Co₃O₄ nanocrystals and graphene occurred.As the etching agent,Co₃O₄ nanocrystals were reduced and uniform etching holes left in the graphene.Owning to the catalytic action of Co₃O₄ nanocrystals,carbon fibers were grown on the surface of graphene,this is the innovation of this paper.When used to catalyze the hydrolysis of NaBH4 to produce hydrogen,the hydrogen generation specific rate reached to 1472mL·min-1·gCo-1 and 2696 mL·min-1·gCo-1 for CoO_x-PG and CoO_x-GCNFs at 30?,due to the synergistic reaction between CoO_x and PG/GCNFs.The CoO_x nanocrystals of little size had high catalytic activities.Graphene,as a two-dimensional substrate,which could prevent the agglomeration of nanocrystals and provided more catalytic sites.Besides,The magnetism produced in the catalytic process could make easier for separation and recycling of the catalyst,and we can control the occurrence of hydrogen production reaction by controlling the contact between catalyst and NaBH4 solution with the help of magnetic transmission.In conclusion,the catalysts can be used as cheap and efficient catalysts.The Co₃O₄-MS microsphere with a flower like structure was obtained by the solvothermal method with cobalt acetate as the precursor.Then coated by graphene that was reduced by hydrazine hydrate,the Co₃O₄ microsphere coated by nitrogen-doped graphene?Co₃O₄-MS@NG?was prepared.The Co₃O₄-MS,formed by the self-assembly of nanosheets,had a uniform size and a mesoporous structure.When Co₃O₄-MS and Co₃O₄-MS@NG were used as anode materials for LIBs,the capacities were up to 342.4 mA·h·g-1 and 407.5 mA·h·g-1 after 100 cycles under a current rate of 0.2 C,respectively.The coulombic efficiency was close to 100 %.The capacitiy still could return to a high value after a high current rate,and the electrochemical impedance was small.Above all,they could meet the performance requirements for the anode materials of LIBs.This superior performance was attributed to the unique three-dimensional structure with pores and the outer layer of nitrogen doped reduced graphene oxide,which provided a channel for the rapid transmission of electrons.