Electronic Structure Tuning Oxygen-Evolving Activity In The Transition Metal Oxides

Energy storage and conversion technology using water molecules as energy carriers is the most efficient and idea ways to achieve the hydrogen energy.However,the reaction involved the multi-step proton coupled electron transfer process leads to extremely slow reaction kinetics rate,thus requiring additional overpotential to drive the reaction.Although precious metal based catalysts can reduce water oxidation overpotential,out of earth's reserves and cost considerations,designing the low cost and highly efficient oxygen evolving catalyst is of great significance to promote the sustainable development of hydrogen energy.This dissertation selected the transition metal oxides with promising application in the electrocatalytic oxygen evolution reaction as the research object.The potential correlation between electronic structure and catalytic activity is explored to optimize reaction barrier via regulating the electronic structure.The following strategies,including particle size modulation,oxygen vacancies engineering,dimensionality control and coordinated regulation could be employed to tune electronic structure of catalysts,such as spin configuration,d band center,and the M d-O 2p covalency,effectively optimizing the adsorption of oxygen intermediates and thereby lowering the reaction barrier and promoting the intrinsic activity.The specific content of this paper includes the following points:1.According to Shao-Horn's principle,perovskite cobaltite LaCoO3 has been identified as a potential oxygen evolving catalyst due to the well-suitable electronic configuration of Co ions.However,its electrocatalytic activity is still far below what is expected,which suffers from the higher reaction barrier.Herein,we demonstrate a facile method to modify the electronic structure of Co ions for LaCoO3 by varying the particle size for the improvement in the oxygen evolution reaction activity.X-ray diffraction,magnetic measurements and EELS analysis reveal that the particle size induces spin-state transition of surface Co3+ions from low spin to high spin state.By reducing the particle size to about 80 nm,the spin filling of Co3+ ions is successfully tuned from unity to near the optimal value of 1.2 proposed by the Shao-Horn's principle,leading to enhanced intrinsic activity of active sites and lowered energy barrier of reaction kinetics,thereby improving the OER activity.2.Misfit-layered-structured Bi2Sr2Co2O8+? nanosheets are successfully prepared via liquid exfoliation strategy.The insight gained from magnetic measurement reveal the presence of the spin-state transition of cobalt ions at edge sites from the low spin to high spin state by reducing its thickness into atomic scale.The high spin state of cobalt ions significant enhances the charge transfer ability between electrochemical active sites and absorbed intermediates and hence improves water oxidation activity.In addition,increased electrochemical active surface area overcomes the deficiency of active sites and contributes to the promotion for oxygen evolving activity.3.Recently,many studies have reported that introducing oxygen vacancies into transition-metal oxides is an effective way to modify their OER catalytic properties.However,a full understanding of the principle of oxygen vacancies in the oxygen evolving process is still lacking.Take double perovskite PrBaCo2O6-? an example,with a large increase in oxygen vacancies the intrinsic OER activity for this material exhibits a significant reduction,which is quite different from those previously reported for cobalt oxides.The oxygen vacancies tend to form an ordered structure in PrO1-? layers with an alternative arrangement of pyramidal Co3+O5 and octahedral Co3+O6,leading to the spin-state transition of octahedral Co3+ions from high spin to low spin states.This spin-state transition is accompanied by a large reduction in e.filling of Co ions,an obvious increase in the electrical resistivity,and a remarkable weakening of the Co-O bond covalency,thereby greatly slowing the OER kinetics.Our work provides a deep understanding of the physical fundamentals of oxygen deficient OER catalysts.4.The weak hybridization between Mn 3d and O 2p orbitals and higher reaction barrier greatly limit the oxygen evolving activity for most of perovskite manganate.Herein,the Mn 3d-O 2p orbital covalency is significantly enhanced via adjusting Mn coordination number for improving water oxidation activity.The decrease of the coordination number by removing the apical oxygen atoms of the MnO6 octahedra significantly splits the eg orbitals of Mn3+ions.As a consequence,the 3dz2 orbital is lowered to close the 0 2p orbital,strengthening the Mn 3d-O 2p hybridization.The increased covalency of Mn-O bonds in CaMn7O12 enables the lattice-oxygen oxidation during the OER,which gives rise to the high activity.Specially,the perovskite oxide CaMn7O12with MnO4 square planes exhibits a small overpotential and Tafel slope under alkaline solution.Furthermore,the efficient water oxidation with high activity and desirable stability for this quadruple oxide still remains under neutral media,which is attributed to the enhanced stability of the active Mn3+ions in the deformed Mn-O framework due to the increased splitting of eg orbitals.5.Constructing complex structure oxides is proposed to optimize the durability in the acidic oxygen evolving process because most of the active catalysts are unstable under the harsh acidic operation conditions.Electronic structure studies and DFT calculations reveal that compared to that of RuO2,the adsorption of oxygen intermediates for ruthenate oxide CaCu3Ru4O12 is significantly weakened due to its lower Ru 4d-band center,which facilitates the sluggish kinetics of oxygen-evolving process and enhances its intrinsic activity.In addition,lower Ru 4d-band center generates a weaker Ru-O binding strength,resulting in the less participation of lattice oxygen redox and thereby reducing the Ru dissolution rate and improving its stability in acid.