Coordination Of Atomic Metal-Nonmetals On Oxygen Reduction/Evolution Electrocatalysis

The dramatically increasing global energy demand and overdependence on fossil fuels like coal and oil,and the excessive burning of fossil fuels has caused a series of problems,such as energy depletion,global warming.and respiratory diseases.For this reason,exploring clean and renewable energy is not only important for the reduction of greenhouse gases,but also an urgent need for the energy safety and economy development.The water circulation system composed of fuel cell and electrolysis device,which can realize mutual conversion between chemical energy and electricity,are considered as promising energy conversion systems in clean way.With devoting significant efforts for many years,it has been proved that the development of electrocatalysts with high activity,selectivity and stability is the key to achieve high performance of energy conversion systems,in which the catalysts help to reduce the energy barrier and promote the dynamics of electrochemical process.As the core and bottleneck of many energy storage or conversion devices,the oxygen reduction reaction(ORR)needs to explore low-cost and high-performance catalyst to accelerate the reaction for addressing its disadvantages that the cathode ORR is essentially 6 orders of magnitude slower than the anode hydrogen oxidation reaction.The precious metal platinum(Pt)and its alloys have been considered as the most active ORR catalysts.However,in practical applications,it is suffered from the high cost,scarce resources and the requirement of high purity of fuel over the effects of methanol crossover and CO deactivation.As a consequence,the searching for efficient,low-cost and resource-rich alternative catalysts has attracted tremendous interest.Herein,we synthesized a series of catalysts(Co-Nx-C-500,X=2,4,6)with atomically dispersed Co-Nx sites anchored on single layer graphene powder(G=Graphene).The obtained nitrogen-containing coordination structures of metal-nonmetals with defined metals Co and coordination numbers(2,4 and 6)are homogeneously dispersed on graphene by the bonding of?-?conjugation.Forming the catalysts with Co-Nx active sites under a low temperature of 500 ^oC in an Ar gas atmosphere.The low temperature treatment keeps the Co-Nx structure undestroyed but form strong interaction with the graphene for high conductivity.By exactly tailoring the coordination numbers of atomic metals coordinated with nitrogen to explore the structure-function correlation toward ORR.When applied to ORR performance tests(0.1 M KOH electrolyte)on prepared electrocatalysts,compared with the other two configurations of Co(Co-N₂ and Co-N₆),Co-N₄ coordination configuration shows a better onset(E _onset)and half-wave(E _1/2)potentials of 0.95 V and 0.84 V,respectively,which was closed to the commercial Pt/C.Accordingly,it is suggested that coordination environment significantly impact on the ORR activity.Thus,searching for optimized coordination structures is essential for design efficient catalysts for ORR,or more general,for electrochemical reactions.The slow reaction kinetics of Oxygen evolution reaction(OER)leads to high over-potential,which is one of the key obstacles to water electrolysis.The theoretical voltage of water electrolysis is 1.23 V,but in the actual electrolytic cell,the voltage of1.8-2.0 V is needed to promote the reaction.Therefore,it is necessary to develop an efficient electrocatalyst to accelerate the reaction kinetics.At present,Pt and Ir O₂/Ru O₂ are the best electrocatalysts for cathode and anode of industrial electrolytic cell respectively,but they are facing the problems of high price and shortage of resources.In this thesis,the Self-supporting Fe-O₆/NF with Fe-O₆ coordination configuration electrocatalyst was prepared by hydrothermal synthesis through direct growth in situ on nickel foam.When the current density is 10 m A cm^-2,only 1.548 V is applied to Fe-O₆/NF electrode in the two-electrode system.After 4000 CV cycles,the LSV curve almost coincides with the original LSV curve,showing the promising candidate for overall water splitting by electrolysis.