Interface Reaction Process Of Fuel Electrode In Solid Oxide Cells

The efficient energy conversion application of solid oxide cells(SOCs)is divided into two working models.One is the solid oxide fuel cells,SOFCs,which converts the chemical energy in the fuel gas directly into electricity.The other is the solid oxide electrolysis cells,SOECs,which use clean energy such as solar and wind energy to produce carbon monoxide and hydrogen by electrolysis of water and carbon dioxide.In principle,SOEC is the inverse process of SOFC.Both SOFC and SOEC have the advantage of high energy conversion efficiency,free pollution and great fuel flexibility.In SOFC,when using hydrocarbons as the fuel,it is usually assumed that SOFCs must be operated on syngas,a mixture of CO and H2 produced by internal or external reforming.In SOEC,H2O and CO2 are electrochemcially redued at fuel electrode,producing H2 and CO.While,the oxidation/reduction reaction mechanism at the interface of fuel elctrode is not clear,and should be further studied.A right physical and chemical understanding of the reaction that happens at the interface of fuel electrode is significant for defining cell performance and finding lifetime limiting factors as well as for building new advanced electrodes.In this dissertation,we mainly discuss the electrochemical conversion reaction process of H2/H2O and CO/CO2 at the interface of fuel electrode,including electrochemical oxidation and electrochemical reduction processes.It includes three parts:the electrochemical oxidation of H2 and CO at Cu-SDC(samarium doped ceria)electrode interface;the electrochemical oxidation of H2 at Ni-SDC electrode interface;the electrochemical reduction of H2O and CO2 at Ni-YSZ(yttrium doped zirconia)fuel electrode interface.The first chapter is the introduction,mainly introducing the development history and working principle of SOC,and the electrolyte materials and electrode materials used in recent years.At the same time,the main research purpose of this dissertation is presented,and the characterization and testing methods of fuel electrode are introduced.In chapter two,the electrochemical oxidation process of H2 and CO on Cu-SDC electrode interface is studied.Firstly,dense SDC bars with Cu particles modified on the surface are prepared.The copper particles are evenly distributed,not connected to each other.The bar conductivity should not be affected since no connections are formed among these copper particles.The particle number per unit area increases with the sputtering time,and the average diameter of Cu particles is 55 nm.Statistics values of the length of Cu-SDC-gas boundaries per unit surface area and the surface coverage of the Cu particles with different Cu contents are thus obtained.Secondly,the surface oxidation reaction rate constant of H2 and CO is determined by the electrical conductivity relaxation method.We found the oxidation reaction rate of H2 and CO at Cu-SDC electrode interface is significantly enhanced by Cu particles.The surface exchange coefficient,i.e.the rate constant,for H2 oxidation increases from 1.42×10-5 cm s-1 for the bare SDC to 7.15×10-5 cm s-1 for Cu-SDC.And the rate constant for CO oxidation increases from 1.22×10-4 cm s-1 for the bare SDC to 3.49×10-4 cm s-1 for Cu-SDC.By analyzing the relationship of the rate constant and 3PB density,the oxidation reaction happens at both SDC-gas interface(2PB)and Cu-SDC-gas triple phase boundary(3PB).When the Cu content increases,the oxidation reaction rate increases,and the contribution of 3PB also increases.Thus the reaction happened at 3PB controls the whole reaction process.Lastly,by combing with the AC impedance analysis,the elementary reaction steps for H2 and CO oxidation at 3PB are proposed,and we discuss the rate limiting step for H2 and CO oxidation process.For H2 oxidation reaction,the rate limiting step is the formation of hydroxyl from oxygen vacancy and water vapor.And for CO oxidation reaction,the rate limiting step is the transformation of carbon monoxide to carbonate adsorbate.The ECR and AC impedance results show that the CO oxidation reaction rate is higher than H2.Thus,when hydrocarbons are used as the fuel,CO may be electrochemically oxidized first.In chapter three,the electrochemical oxidation process of H2 on Ni-SDC electrode interface is studied.The reaction rate constant of dense SDC bars with different Ni particle contents is determined by the electrical conductivity relaxation method.At 800 ?,the reaction rate constant of H2 oxidation reaction increases form 1.4210-5 cm s-1 for the bare SDC,i.e.the rate constant at SDC-gas interface,to 39.4×10-5 cm s-1 for SDC-Ni160,i.e.the rate constant at both SDC-gas interface and Ni-SDC-gas triple phase boundary.By combining the statistics values,including the length of Ni-SDC-gas boundary per unit surface area,the surface coverage of the Ni particles and the distance of Ni particles,the enhanced rate constant caused by Ni modifying,kNi,is obtained.The enhanced reaction rate constant linearly increases with 3PB,that is,the oxidation reaction is mainly controlled by Ni-SDC-gas boundary.While,kNi is not directly proportional to 3PB,suggesting the enhanced reaction rate caused by Ni is not only limited to 3PB,while extending to 2PB.The modifying of Ni not only increases the active reaction area,i.e.Ni-SDC-gas boundary 3PB,but also increases the reaction rate at SDC-gas interface by H spillover.Thus,there are three pathways for H2 oxidation at Ni-SDC electrode,the original reaction on bare SDC-gas interface(2PB),the reaction at Ni-SDC-gas boundary(3PB),and the reaction at 2PB with hydrogen spillover.The rate constant at 2PB with H spillover is 8.06×10-5 cm s-1,about six-fold of that for the original 2PB.The rate constant of per unit 3PB length is 1.05×10-5 cm ?m s-1.When the Ni particle distance is larger than 76 nm,the reaction is dominated by spillover,i.e.2PB controls the whole oxidation process.While,when the Ni particle distance is less than 76 nm,the reaction is dominated by 3PB,i.e.3PB controls the whole oxidation process,and the 3PB contribution increases with the Ni particles coverage.In chapter four,electrochemical reduction of CO2 and H2O at Ni-YSZ fuel electrode with BaCO3 modifying is studied.Firstly,single cells with Ni-YSZ cathodes,YSZ electrolytes and composite LSM-YSZ oxygen electrodes are fabricated by dry pressing and screen-printing method.BaCO3 particles are thus infiltrated into Ni-YSZ electrode surface.BaCO3 particles mainly distribute at Ni/Ni,Ni/YSZ and YSZ/YSZ interface.The average diameter of BaCO3 particles is 70 nm.Secondly,the single cell electrochemical performance at SOFC model is measured with 97%H2-3%H2O as the fuel.The peak power density at 800 ? is reduced from 0.47 W cm-2 to 0.37 W cm-2 after BaCO3 infiltrating.While the cell performance is improved by BaCO3 infiltrating when the cell working at SOEC model for H2O and CO2 electrolysis.The current density at 1.3 V and 800? is increased by 53%,103%and 72%for the electrolysis of H2O,CO2 and H2O-CO2,respectively.Besides,by analyzing the outlet gas component,we find that BaCO3 has a negative effect on RWGS reaction,suggesting that BaCO3 only has the positive effect on electrochemical reduction of CO2.Lastly,elementary reaction steps for H2O and CO2 electrochemical reduction at fuel electrode interface are studied by AC impedance method with three-electrode configuration.The positive effect of BaCO3 to electrolysis is due to the reduced interfacial polarization resistance of the charge transfer reaction,which is the rate limiting step for the H2O/CO2 reduction reaction.Specifically,the first electron transfer reaction of H2O and CO,electrochemical reduction is the rate limiting step.