Study On The Active Sites Of Fluorine-doped Transition Metal （Hydro） Oxide Electrocatalysts For Oxygen Evolution

Owing to the stipulation of carbon emission reduction,hydrogen-centered energy cycle is gradually established,that is,water electrolysis by renewable electric energy to produce hydrogen which can meet energy needs.Researchers have explored many basic OER（Oxygen evolution reaction）catalysts with high activity and excellent stability,such as oxides,（oxy）hydroxides,metal phosphides,metal chalcogenides and so on.However,dynamic processes such as element etching,metal dissolution and redeposition will occur during the reaction at high applied voltage,which changes the surface structure of catalysts,poses great challenge to the understanding of OER active sites/phase and makes it difficult to design highly efficient alkaline OER catalysts.Fluorine atom has the strongest electronegativity and the radius close to oxygen atom,which makes it is considered as an ideal N-type dopant for metal oxides.Besides,the weak metal-fluorine bond has strong ionicity and the F ion is easily exchanged by hydroxyl anion in alkaline electrolyte,which will speed up the surface reconstruction.In this thesis,the active sites/phase of OER will be explored by replacing oxygen with fluorine in oxide and hydroxide catalysts.1.Study on properties and active phases of fluorine-doped oxidesFluorine-doped spinel catalysts NiCo₂O₄-F_x were obtained by two-step method.NiCo₂O₄ phase and（NH₄）Ni_xCO_1-xF₃ new solid solution phase are included in its structure.At the current density of 100 m A cm^-2,the overpotential of NiCo₂O₄-F₁ is only 314 m V,which is much lower than 357 m V of NiCo₂O₄.And NiCo₂O₄-F₁ can run stably for over 100 h at 100 m A cm^-2.Synchrotron radiation tests show that the configuration of Ni and Co in NiCo₂O₄-F₁ are changed and the formation of tetrahedral Co(Co _Td)is appeared.In-situ Raman experiments show that the Ni_xCO_1-xOOH active phase appeares at a low potential of 1.2 V in NiCo₂O₄-F₁,which is much lower than the appearance potential of Ni OOH active phase in NiCo₂O₄precursor（1.4 V）.In-situ synchrotron radiation experiments also show that fluorine-doping activates the inert Co in NiCo₂O₄ to participate in the OER.At the same time,fluorine-doping also makes hydrophobic NiCo₂O₄ become more hydrophilic and increases the amount of oxygen vacancy（O_v）which is good for the adsorption and desorption of intermediate species.2.Study on properties and active phases of fluorine-doped hydroxidesFluorine-doped nickel-iron hydroxide catalysts Ni Fe（OH）_x-F-y growing on carbon paper were prepared by a simple in-situ electrodeposition method.With the increasing of F content,the structure of Ni Fe（OH）_x-F-y changes to LDH with layer structure and Fe OOH phase starts to emerge.The overpotential of Ni Fe（OH）_x-F-50 is only 320 m V at the high current density of 1 A cm^-2,which is much better than that of Ni Fe（OH）_x（400 m V）.What’s more,Ni Fe（OH）_x-F-50 can perform OER at 500 m A cm^-2 for over 120 h and the overpotential changes within 0.1 m V.In-itu Raman experiments show that the active phase on the Ni Fe（OH）_x surface is changed by fluorine-doping during OER.At the same time,fluorine-doping also changes the structure of Ni Fe（OH）_x and the interaction between Ni and Fe.These changes have a positive impact on the adsorption and desorption of oxygen-containing intermediates and the reaction free energy of intermediates at the active sites.Therefore,fluorine-doping not only causes the emergence of M-F bond with stronger ionicity,but also makes the OER active phase appear at a lower potential,which provides experimental and theoretical supports for the design and synthesis of efficient alkaline OER catalysts.