First-principles Study On Oxygen Reduction Reaction (ORR) Mechanism Of Transition Metal Oxides

Proton exchange membrane fuel cell (PEMFC) is an electrochemical device,which directly convert chemical energy into electrical energy through electrochemicalreaction. Due to its high energy efficiency, high power density, quick start and zeroemissions, fuel cell is considered as a promising candidate for mobile and stationarypower sources. However, the commercialized development of fuel cell is hindered bythe cost and catalytic activity of catalysts. Therefore, the key to the development offuel cell is the exploration of novel catalysts with low cost and high catalytic activity.Due to excellent acid corrosion-resistant performance, valve metal oxides with lowcost have attracted much attention for using as PEMFC catalysts. In this thesis, usingZrO2-based compounds as research subject, we systematically investigated theoxygen reduction reaction mechanism of transition metal oxides and the oxygenreduction reaction catalytic activity enhancement mechanism of N-doping transitionmetal oxides by first-principle study. According to the results of first principle, therelationship between the d-band electron occupation of active atoms in transitionmetal oxide catalysts and the oxygen reduction reaction catalytic activity ofhomologous catalysts is established, and a new catalytic activity indicator (i.e. d-bandelectron occupation) which is suitable for transition metal oxide catalysts is proposed.On the basis of proposed catalytic activity indicator, some new transition metal oxidecatalysts are designed. The main progress is summarized as follows.(1) Effect of monoclinic ZrO2surfaces on oxygen reduction reaction issystematically studied by first-principle method. The results showed that the“steric-hindrance effect” of superficial zirconium atoms could influence theadsorption process of oxygen atom, when it is adsorbed on monoclinic ZrO2surfaceswith different crystal indices. The oxygen molecule on monoclinic ZrO2surfaces isreduced to water molecule obeying the “associative mechanism”. The results alsorevealed that the rate-determining step for oxygen reduction reaction on monoclinicZrO2(-111) and (-101) surfaces is oxygen molecule adsorption step, and that on ZrO2(110) surface is the formation of OOH*step.(2) The oxygen reduction reaction on ZrO2catalyst surfaces with differentcrystal structure is systematically investigated by first-principle method. The resultsshowed that there is a “steric-hindrance effect” for active zirconium atoms on ZrO2catalyst surfaces with different crystal structure. But oxygen atom is trend to adsorbon the surface of zirconium atoms with weaker “steric-hindrance effect”. The oxygenmolecule on ZrO2catalyst surfaces with different crystal structure is reduced to watermolecule obeying “associative mechanism”. The results also revealed that the-ratedetermining step for oxygen reduction reaction on the (111) surface of monoclinicand cubic ZrO2is oxygen molecule adsorption step, and the-rate limiting step foroxygen reduction reaction on the (111) surface of tetragonal ZrO2is OOH*dissociation step.(3) The oxygen reduction reaction catalytic activity enhancement mechanism ofN-doping ZrO2is systematically studied by first-principle method. The resultsshowed that the adsorption strength of oxygen molecule on N-doping ZrO2surfaces isimproved after N-doping treatment, and the adsorption mode transferred fromsite-adsorption mode on monoclinic ZrO2to side-adsorption mode on N-doping ZrO2surfaces. It is beneficial for the oxygen molecule on N-doping ZrO2surfaces isreduced to water obeying “dissociative mechanism”. The results also revealed that therate-determining step for oxygen reduction reaction on the surface of N-dopingZrO2-based catalyst is OH*desorption step, and the rate-limiting step for oxygenreduction reaction on the surface of monoclinic ZrO2is OOH*formation step.(4) The electron density of state for slap models, involved in above-mentionedthree chapters, is studied using first-principle method. The results showed that the fulldegree of d-band electron of surficial zirconium atoms in slap models is positiveproportional to their oxygen reduction catalytic activity. That is, the oxygen reductioncatalytic activity is enhanced along with the increase of the full degree of d-bandelectron of surficial zirconium atoms. Based on this, we suggest using the full degreeof d-band electron of active center atoms in transition metal oxide catalysts as thecatalytic active indicator for this type of catalyst.(5) Based on the fourth chapter, IrO2catalysts with different morphologies are prepared by sol-gel and modified Adams method. The results showed that themorphology of IrO2prepared by sol-gel method is rod-like, while that of IrO2prepared by modified Adams method is ellipsoid granulous. In acidic electrolyte, forrod-like IrO2, the hydrogen evolution reaction was detected at the electrode potentiallower than0.3V, while the oxygen evolution reaction was observed at the electrodepotential higher than1.3V in acidic electrolyte. The rod-like IrO2catalysts isobserved to represent a typical pseudo-capacitance property in range from0.3~1.3V.The electrochemical stability of rod-like IrO2/C is superior to that of commercial Pt/Ccatalysts. The results also revealed that the oxygen reduction reaction catalyticactivity of ellipsoid granulous IrO2is superior to that of rod-like IrO2catalysts inalkaline electrolyte. The oxygen reduction reaction path is dominated by fourelectrons transferred process at high overpotential region, but is dominated by twoelectrons transferred process at low overpotential region.