Inlfuence Of Graphene And Its Composites Towards Dissolved Oxygen Reduction Reaction

Dissolved oxygen reduction reaction （ORR） plays an important role in metal marinecorrosion and new energy conversion technologies, and the mechanism study of ORR ondifferent materials is one of the hottest topics in the field of electrochemistry. Graphene, anew carbon nanomaterial, can decrease the overpotential of ORR and increase its current.However, the mechanism behind these phenomena has not been clear. In this thesis, intensivestudies have been done to investigate the ORR on graphene with the help of electrochemicalmethods, including cyclic, rotating disk electrode, and rotating ring-disk electrodevoltammetry, and the mechanism is proposed based on the kinetics calculation. Givenproposed mechanism, hetero-atoms doping and composites constructing are adopted toimprove the ORR properties to achieve4-electron reduction of O₂at lower overpotentials.The main results and conclusions are as follows:（1） The effect of preparation methods on the activities of graphene towards ORR hasbeen defined. In comparison with electrodeposited graphene, graphene prepared via chemicalreduction （G-CR for short） leads to higher beginning potential and larger current of ORR, andtherefore possesses better properties.（2） The mechanism of G-CR towards ORR in different electrolytes has been proposed.G-CR can decrease the overpotential, increase the current, and improve the kinetic currentdensity of ORR, but functions differently in different electrolytes. The ORR pathway hasbeen changed by G-CR in0.1M KOH solution, which leads to the reduction of O₂to HO₂–atlower overpotentials. While in3.5%solution,4-electron reduction of O₂at loweroverpotentials is achieved via catalytic decomposition of H2O₂by G-CR.（3） Electrolyte compositions and electrodeposition parameters have great influence onthe preparation of electrodeposited graphene and its ORR properties. When LiClO₄is utilizedas supporting electrolyte, graphene modified electrodes can not be obtained. Theelectrodeposition of graphene can be achieved by potentiostatic and cyclic voltammetricmethods with NaCl as supporting electrolyte, in which modified electrodes via the formermethod possess high ORR activity and low stability, and the latter gives electrodes with goodactivity and stability, and their properties are dependent on the number of scanning cycles. （4） The relationship between the nitrogen content and microstructure of nitrogen-dopedgraphene and ORR activity has been established. Nitrogen-doped graphene materials withdifferent nitrogen contents and microstructure have been synthesized via hydrothermalreaction of graphite oxide and urea with different mass ratios. Nitrogen-doped graphene witha nitrogen content of ca.7%and a moderate defect density （the intensity ratio of D band to Gband in Raman spectrum has a value of ca.1） gives the best ORR activity. If nitrogen contentis lower, active sites available are not enough, and higher nitrogen content brings moredefects to hamper the transport of current carriers.（5） The composite of G-CR with manganese oxide has been constructed, and the roles ofthese two components are fully played. The ORR properties of manganese oxide materials areclosely related to their crystallographic structures and morphologies, which follow an order ofβ-MnO₂miciroprism <amorphous MnOxnanoparticle <-MnO₂nanowire. G-CR decreasesthe overpotential, and-MnO₂nanowire catalyzes HO₂–decomposition, which results in theoccurrence of4-electron ORR at lower overpotentials on this composite.（6） ORR mechanism on the composite of G-CR with Co（OH）₂has been defined, andelectrochemical activation of G-CR is proposed. The microstructure of G-CR has beenchanged during the electrodeposition of Co（OH）₂with catalytic activity towards HO₂decomposition on G-CR surface from Co（NO₃）₂electrolyte, resulting in the activation ofG-CR. The activated G-CR brings a further decrease in overpotential and catalyticdecomposition of HO₂, and its combination with Co（OH）₂gives rise to4-electron reductionof O₂at lower overpotentials.