Design And Synthesis Of Cobalt-based Oxide/Carbon Composites For Electrocatalysis

Electrochemical energy storage and conversion devices play a vital role in developing efficient and clean energy to meet the sustainable growing energy needs of our society.Among them,oxygen evolution reaction(OER)and oxygen reduction reaction(ORR)are important processes in electrochemical energy storage and conversion devices.Precious metal catalysts are generally recognized as efficient electrocatalysts for oxygen electrode reaction,but their scarcity and easy poisoning limit their application in fuel cell,water electrolyzer,metal air cell and other equipments.Cobalt-based oxides have been one of the hot areas of interests due to their long-term stability,high activity and low cost under alkaline conditions.However,due to the poor conductivity of cobalt-based oxides,its wide application is affected.In order to overcome this disadvantage,cobalt-based oxides can be combined with carbon materials to increase their conductivity and catalytic activity.Here,three kinds of electrocatalysts were designed and synthesized.They are needle like NiCo2O4(NiCo2O4-G-NCDs)composite modified by N-doped CDs supported on graphene,Co3O4/C-750 composite supported on graphene like carbon layer and Co3O4/C-350 catalyst with cluster structure.Their electrocatalysis properties were further studied.The research contents are as follows:1.In order to design an efficient and cheap bifunctional electrocatalyst,a needle like NiCo2O4(NiCo2O4-G-NCDs)composite modified by N-doped CDs on graphene was prepared by a simple one-step solvothermal method.The morphology of the catalyst was studied by means of SEM and TEM,the composition of the catalyst was determined by XRD,HRTEM and XPS,and other characterization of the catalyst was carried out by means of IR spectrometer,high pressure physical adsorption instrument,and UV-Vis spectrophotometer.The electrochemical test results show that the catalyst has both efficient ORR and OER properties.The ORR half wave potential of NiCo2O4-G-NCDs is 0.82 V.Compared with NiCo2O4-G and NiCo2O4-G-CDs,NiCo2O4-G-NCDs has higher electrocatalytic performance,smaller Tafel slope(63.94 mV dec-1)and lower OER over potential(308 mV).2.The Co3O4/C composite catalyst supported on graphene like carbon layer was synthesized by pyrolysis of cobalt phthalocyanine.The electrochemistry test shows that the catalyst is a good ORR electrocatalyst.In the preparation process,we adjusted different pyrolysis temperatures to optimize the performance of the catalyst.Among them,the Co3O4/C catalyst prepared at 750℃ shows the best ORR electrocatalytic performance in 0.1 M KOH.The ORR half wave potential of Co3O4/C-750 is 0.75 V.At the same time,it has better methanol tolerance ability and stability than 20%Pt/C.In addition,we also applied Co3O4/C-750 catalyst to zinc-air battery and discussed its performance.3.Co3O4/C-350 with cluster structure was successfully synthesized by a simple two-step synthesis method,and it was proved that the catalyst is a good electrocatalyst for oxygen evolution reaction.During the preparation process,we adjusted the carbonization temperature,and the Co3O4/C-350 catalyst prepared at 350℃ showed the best OER electrocatalytic performance in 1 M KOH,slightly lower than RuO2.The over potential of Co3O4/C-350 at 10 mA cm-2 is 410 mV,which is better than the catalyst synthesized at other temperatures.At the same time,the Tafel slope of Co3O4/C-350 is the lowest among the catalysts synthesized at several different temperatures,only 151.8 mV dec-1,higher than RuO2(91.4 mV dec-1),showing good catalytic kinetic performance.