Transition Metal Oxides As Oxygen Electrocatalysts

Since the first and second industrial revolutions,new inventions and technologies have brought great convenience to modern society.However,most advanced technologies rely on carbon-based fuels such as coal,natural gas,and petroleum.The world needs another industrial revolution in which our sources of energy are affordable,accessible and sustainable.Numerous innovative ideas have been proposed for achieving more efficient energy conversion or storage systems,among of which,fuel cells,and metal-air batteries are potential to be the next generation energy technology.But the activities of the ORR and OER catalysts in these two facilities are not high enough for their commercial applications.Recently,the best catalysts are Pt or Pt-based compounds for ORR and IrO2 or RuO2 for OER.However,the high cost and poor stability are hampering the commercial application of the Zn-air battery technology.So developing a low cost and high performance catalyst for both ORR and OER increasingly attract worldwide research attention.Transition metal oxides are potential to be applied oxygen catalysts in commercial application due to their special physical and electrochemical properties.In this paper,I was devoted to do research on the preparation,characterization of various transition metal oxides and revealed their oxygen catalytic mechanism from their electronic structure,energy band theory and crystal structure.This paper was devided into several parts as follows:1.Hollow porous ZnCo2O4 microspheres have been successfully prepared by a simple solution-based assembly followed by calcination under an air atmosphere using zinc acetylacetonate Zn(C5H7O2)2 and cobalt acetylacetonate Co(C5H7O2)3 as raw materials.Scanning electron microscopy(SEM)and Transmission electron microscope(TEM)are used to reveal the synthesis mechanism of hollow porous structure,meanwhile,BET is used to analyze the specific surface area and pore size distribution.In the oxygen reduction reaction(ORR)test,hollow porous ZnCo2O4 microspheres exhibit enhanced ORR performance than bulk ZnCo2O4,mainly owing to the hollow porous structure which has more catalytic sites and higher efficiency of reactant exchange.Moreover,such catalyst also exhibits superior methanol tolerance ability and durability over commercial Pt/C catalyst.2.In this part,we report,for the first time,the oxygen reduction activity of hollow porous spinel AB2O4 microspheres.Among various hollow porous microspheres crafted as catalysts,ZnMnCoO4(x = 1)microspheres was found to exhibit the best oxygen reduction activity with a half-wave-potential only 50 mV lower than that of the Pt/C counterpart and an excellent durability in the alkaline solution.Importantly,the electronic structure of Co3+ions was scrutinized to reveal the mechanism of the markedly improved oxygen reduction reaction(ORR)of ZnMnCoO4 microspheres.The electronic transition of Co"+ ions from low-spin state in commercial Co3O4 catalyst to a mixed high-spin and low-spin state in ZnMnCoO4 catalyst was found to weaken the Co3+-OH bond and facilitate the O2-/OH-displacemen t.More notably,the DFT calculation substantiated that the distances of both Co3+-O and O-O bonds were increased with the increased incorporation of Mn3+ ions.3.In this paper,we report a comparative study of the catalytic activities of two isostructural ACoO3(A?Ca,Sr)perovskites that have been synthesized under high pressure and characterized physically.Each perovskite is cubic,metallic,and exhibits long-range magnetic order,indicating a t4?*1 intermediate-spin state on a Co? formal valence state.The ?*-antibonding t4 electrons are localized in SrCoO3;they are at the crossover from localized to itinerant behavior in CaCoO3,which has a significantly shorter Co-O bond length than that of SrCoO3(1.87 A vs.1.92 A).The itinerant character of the ?*electrons is responsible for the metallic conductivity and lack of a cooperative Jahn-Teller distortion of the CoO6/2 sites that would be expected with localized e1 electrons.4.Therefore,we prepared a metallic layered-oxide Na0.67CoO2 with low-spin Co?/? ions(Co?:?*6?*°;Co?:?*5?*0)and a much shorter O-O separation than on CaCoO3 to study further the influence of zeta potential,surface oxygen separation,and the pH of the aqueous medium on the onset potential and OER activity.Metallic,low-spin Na0.67COO2 has a strong Co?/?-O interaction as a result of empty a-antibond eg orbitals and itinerant ?-bonding electrons,which reduces the charge transfer resistance during the OER;Na0.67CoO2 has a zero zeta potential at pH=4 and shows a pH-dependent onset potential for the OER consistent with the potential for the CoOH--e-+OH-=CoO-+H2O reaction.