First-Principles And Experimental Study On Surface Reactions Of Electrode Materials For RSOC

As an efficient,low-cost,solid-state,and flexible energy conversion device,Reversible Solid Oxide Cells(RSOCs)recently draw widespread attention.RSOCs can work in the fuel cell mode(Solid Oxide Fuel Cells,SOFCs),converting chemical energies of fuels into electricity;and they can also operate in electrolysis mode(Solid Oxide Electrolysis Cells,SOECs),to utilize electricity that generated by renewable energy resources(e.g.,wind and solar energy)to electrolyze CO2 or H2O etc.,converting electrical energy into chemical energy(e.g.,CO or H2 etc.)and to be stored.RSOCs can flexibly switch between their working modes,to meet the needs of different situations,and thus,be deemed as one of the potential technologies to solve energy and environment issues.A RSOC consists of three parts:the electrolyte,the air electrode and the fuel electrode.The electrolyte is used to apply sufficient ionic conductivity and to isolation the reducing and oxidizing gases;while the electrodes(air or fuel electrodes)act as electronic conductors and electrochemical catalysts for the gas involved reactions(e.g.,adsorption and dissociation reactions of gaseous molecules on electrode surfaces).To date,since researches on RSOC electrolyte materials have been quite mature,and the choice and use of electrolytes have been basically determined,the reaction rate on the electrode surface usually determines the overall performance of RSOCs.In addition to investigating the intrinsic properties of electrode materials,in-depth exploration of the reaction mechanism of various gas molecules on the electrode surface is crucial to understanding,designing and developing new electrode materials,which is the key to the development of RSOCs.With the help of the calculation method based on Density Functional Theory(DFT),combined with electrochemical experimental characterization methods,this thesis focuses on the key reactions that occur on the electrode surface,including oxygen reduction reaction(ORR),carbon dioxide reduction reaction(CO2RR)and oxygen evolution reaction(OER).On the basis of the above investigations,impacts of the structures and compositions of electrode materials on the surface catalytic performance are discussed,and relevant mechanisms are given.This thesis can be divided into six chapters,and the specific content of each chapter is as follows:The first chapter is a general introduction,which is the basis of the whole thesis.First,it specifically introduced the basic information of the working principle,polarization loss and key materials(air electrode,fuel electrode and electrolyte materials)of RSOC in different modes.Subsequently,the related contents of DFT were introduced,including the evolution of DFT,the advantages and disadvantages of each exchange correlation functional,and the applicable objects,as well as some commonly used software in the actual calculation and simulation process.Next,the electrochemical theory and principles used in this thesis were introduced,and examples of the practical applications of DFT in RSOCs were presented.The last is the basis and research content of this thesis.In the second chapter,La0.6Sr0.4Co0.2Fe0.8O3-?(LSCF)was selected as a base mixed conducting oxides,of which the impact of decoratation and segration of insulating phases on ORR performance were studied.This chapter is divided into two parts.The first part found impregnated by MgO insulator,LSCF demonstrated a much improved surface exchange coefficient,about 2.4 times that of that for the bare one at 750?.Resuls of electrochemical tests demonstrated that MgO could promote the charge transfer process on the LSCF surface.The second part investigated the oxygen surface exchange performance of single crystal LSCF thin films with three different orientations,i.e.,LSCF(100),LSCF(110)and LSCF(111),and the reaction processes on each surface were simulated using DFT calculations.Experimental results indicate there were normal and Sr-rich(SrCO3)regions on the surface of LSCF thin films,and thus two chemical oxygen surface exchange coefficients were characterized.Combined with DFT calculations,impacts of SrCO3 toward charge transfer process on each LSCF surface were studied in depth.It was suggested that construction of heterostructure on the surface,utilizing the difference in work function to control the band bending and charge transfer direction,was essential to improve the oxygen surface exchange activity of mixed conducing oxides.The third chapter introduced the potential applications of mixed conducting oxide electrodes in CO2RR.The electrochemical performance,surface interaction,and CO2RR mechanism of mixed conducting perovskite oxides were investigated as SOEC fuel electrodes for CO2 electrolysis.This chapter can be divided into two parts.The first part studied the electrochemical performance of La0.8Sr0.2Fe0.8O3-?(LSF)as a SOEC fuel electrode for CO2 electrolysis.The results of Raman spectroscopy investigation and DFT calculations jointly suggested that carbonate species was a key intermediate during CO2RR on the surface of LSF.The CO2RR mechanism on the surface of LSF was also proposed by DFT calculation,and the results manifested that the oxygen vacancy could reduce the energy barriers to be overcome,and thus be benefical to CO2RR.The second part explored impacts of Bi element on the properties of La0.75Sr0.25Cr0.5Fe0.5O3-?(LSCrF),including on electrical conductivity,surface exchange coefficient,CO2 adsorption and dissociation performance.Bi doping was suggested as an effective method to improve the performances of SOEC fuel electrodes for CO2 electrolysis.The fourth chapter investigated the CO2 activation and reduction mechanism on the surface of metal-oxide ceramics.For this investigation,surfaces of CeO2,Pt-CeO2,and Pt-Sm0.2Ce0.8O2.9(SDC)were theoretically constructed.DFT calculations were used to uncover the charge transfer process between Pt and CeO2-based surfaces,and to study the CO2 adsorption and activation properties on the three surfaces.It was found that the activity of the d electrons in metal was the key to the CO2 adsorption and activation.Subsequently,the possible CO2RR steps on each surface were simulated,and it was found that the most favorable CO2RR pathway was at the Ptupper site.The catalytic activity sequence of the three catalysts for CO2RR was determined,which indicated that Pt-SDC has the best catalytic activity toward CO2RR among the investigated catalytsts,agreeing well with ECR results.The fifth chapter studied performance and mechanis of mixed conducting oxide materials toward OER.This chapter also includes two parts.The first part took the stragegy of substituting K element into the A-site of perovskite oxides,aiming to increase the concentration of oxygen vacancy,to enhance the alkalinity,promote hydration,and to accelerate proton transfer of a material.For this study,K0.25Sr1.75Fe1.5M0.5O6-?(KSFM)was synthesized,which demonstrated that the above goals.DFT calculations suggested that the H2O dissociation was more favorable on KSFM surface,indicating that KSFM had better activity toward OER,and that K doping might be an effective strategy to improve the performance of P-SOC air electrodes.The second part studied the OER performance of F doped SrCoO3-? and hexagonal Sr2Co2O5 at room temperature.It was found that retaining the cubic structure of strontium cobaltate-based materials was of great significance for improve the OER activity.The sixth chapter is the summary and outlook,including the main research points and shortcomings of each part of this thesis,and prospects for future research directions.