Study On The Action Mechanism Of Defective Metal Oxides In Oxygen Evolution And Reduction Reactions

Oxygen evolution reaction(OER)and oxygen reduction reaction(ORR)are key steps in new energy conversion processes(such as water decomposition,fuel cells,etc.),which have attracted widespread attention.Recently,non-precious metal oxides have increasingly important applications in the field of OER and ORR,but their activity is still lower than that of precious metals.Defect engineering is an effective strategy to modify the microstructure and electronic structure of non-precious metal oxides,and reasonable regulation of its defects can significantly optimize the performance.In this paper,some defected non-precious metal oxides were designed and synthesized,including oxygen defected CeO2,nitrogen doped CeO2,cobalt defected Co3O4 and manganese defected Mn3O4.The action mechanism of defect structure in oxygen evolution and oxygen reduction reactions were studied.The oxygen defected CeO2 was synthesized by in-situ hydrothermal reduction.The results show that the presence of oxygen defects in CeO2 improve the light absorption capacity,facilitate the separation and migration of photo-generated carriers,and accelerate the surface reaction kinetics,and the photocatalytic oxygen evolution rate is increased from 78.9?mol·g-1·h-1 to 137.7?mol·g-1·h-1.Moreover,the mechanism of water oxidation especially the key O-O bond formation was identified.Furthermore,the surface and electronic structure of CeO2 is regulated by heteroatom doping and then investigate the factors that affect oxygen evolution.The optimal pathway of oxygen evolution is as follows:H2O*?OH*+H*,OH*+H*?O*+2H*,O*+2H*+H2O?OOH*+3H*,OOH*+3H*?O2*+4H*.The presence of nitrogen dopant effectively reduces the activation energy of the rate-limiting step(0.87 eV vs.0.69 eV)and accelerates the rate of oxygen evolution reaction.Cobalt defected Co3O4 and manganese defected Mn3O4 were synthesized by thermal oxidation of glycerolate metal compounds,respectively.The experimental and computational results show that cobalt defects contribute to the improvement of surface Co2+ concentration as the main active site to promote electrocatalytic OER activity,the overpotential at 10 mA/cm2 is much higher than that of IrO2(325 mV vs.440 mV).The presence of cobalt defects lead to the downshift of Fermi level,thereby enhancing the electron delocalization and helping the activation of H2O and faciliating the desorption of O2 and reducing the activation energy of rate-limiting step.For the manganese defected Mn3O4,the experimental and computational results reveal that manganese defects modify the electronic structure to improve conductivity and electronic delocalization with increasing the spin polarized electrons near the Fermi level,which helps to expose more surface Mn3+ as major active site,thereby facilitating O2 activation,OH*desorption and reducing the free energy in the rate-limiting step of ORR.