Structure Control of Redox-Reversible Perovskite Electrode Materials and Performance of Symmetrical Solid Oxide Fuel Cells

Author:Zhou Ning

Supervisor:ma zi feng yin yi mei


Degree Year:2018





Solid oxide fuel cell(SOFC)has attracted a great deal of attention in recent years because of its high efficiency,low emissions,flexible fuel,and solid-state structure.One material is used to as both cathode and anode of symmetrical SOFC(SSOFC),thus improve thermomechanical compatibility of the electrolyte and electrodes,reduced fabrication cost,and enhanced immunity to coking and sulfur poisoning by regeneration,greatly improving the cell life.However,most electrode materials of SSOFC have poor catalytic activity or poor stability;it is difficult to have excellent catalytic activity and high stability at the same time.Perovskite oxides can be used as an oxygen electrode to catalyze an oxygen reduction reaction(ORR)or an oxygen precipitation reaction(OER)in an oxidizing atmosphere,and can also form a composite fuel electrode by in situ exsoluion of mental nanoparticles in a reducing atmosphere.So they are potential SSOFC electrode materials.However,their activity and stability are significantly affected by exsoluion of cations under redox conditions.This thesis focused on understanding and effective control of exsoluion of A and B sites cations,in order to greatly improve the performance,stability,and regeneration of the electrodes.In this work,La0.5Sr0.5Fe0.8Cu0.2O3-δ(LSFCu)and La0.5Sr0.5Co0.2Fe0.8O3-δ(LSCoF)were used as precursors.High-valence cations doping in B sites,and the self-assembly of two phases by A-site nonstoichiometry were explored for cathode and anode performance,stability,resistance to carbon deposition,and fuel cell/electrolytic cell reversibility separately.Then,the catalytic activity and stability of cathode and anode have been effectively improved,the important influencing factors and effective control methods of cations segregation at A and B sites have been determined.A new way was developed to control the segregation behavior of cations by in situ formed self-assembled perovskite composites(single-perovskite(SP)/ruddlesden-popper(RP)).The details are as follows:Carbon tolerance and regeneration ability of LSFCuN was investigated as electrodes for SSOFCs.The LSFCuN exhibited excellent redox reversibility,and a LSFCuN based single cell can be regenerated from coking by treating the anode by reodx cycles.Then LSFCuN was evaluated as oxygen electrode for reversible solid oxide electrochemical cells(RSOCs).In fuel cell mode,the cell exhibits the maximum power density(MPD)of 1100 mW cm-2 at 800°C.In electrolysis mode,An electrolysis current of 850 mA cm-22 is achieved at 750°C with an applied voltage of 1.3V,and no degradation as well as delamination are observed for the cell after 50 h electrolys is under voltage of 1.60 V at 800°C.LSFCuN is a promising SSOFC electrode material which has good cathodic and anode catalytic properties.To investigate the effects of different B-substitution positions of high-valence cations,LSF0.80-xNxCu0.20 and La0.5Sr0.5Fe0.80Cu0.20-xNbxO3-δwere compared.Results show that,Reducibility of substituted cation is a key factor affecting structure and properties of materials.When substituting more reducible cations,the stability of the material will be significantly improved and decomposition after reduction will be reduced,surface Sr segregation is surpressed,but the concentration of oxygen vacancies will be reduced.To investigate the effects of B-substitution with different cations and amounts,high valency cations doped LSF0.80-xNbx Cu(x=0-0.15)and La0.5Sr0.5Fe0.8-xTixCu0.2O3-δ(x=0-0.3,LSF0.80-xTixCu)were designed and prepared.It was found that only the amount of doping reaches a certain threshold the stability will be significantly improved.The(La,Sr)FeO3-based perovskite oxides with high cation doping only partially decompose in the anode atmosphere,and decompose into SP,RP and corresponding B-site metal that can be exsoluted.Kind of exsoluted metal is depended on the doping amount,and tends to precipitate at the lower surface energy sites(grain boundary and defects)of the material.High cation doping reduces the ORR catalytic activity,but improves the anode catalytic activity.Moreover,catalytic performance of the material does not change monotonically with the increase of the doping amount,so it is particularly important to select an appropriate doping amount when doped with a high-valent cation.In addition,at 800°C,the SSOFC with LSF0.7Nb0.10Cu as the electrode has a MPD of 609 mW cm-2 in wet hydrogen,and 493 mW cm-2 in wet synthesis gas(3%H2O,CO:H2=1:1),with a reasonable stability under 0.7 V for 100 h;the SSOFC with LSF0.7Ti0.1Cu as the electrode has a MPD of 644 mW cm-2 in wet hydrogen,and has excellent resistance to sulfur poisoning and carbon deposition.A-site nonstoichiometry is another important factor affecting material structure and properties;it was found that structures of LSr0.5+x.5+x CoFN changed with it.Pure phase could not be formed,and oxide of B-site mental was observed at a high A-site deficiency;when A-site enriched dual-phase SP/RP composites can be in situ formed,and the content of RP phase enhances with the increase of richness,therefore,the same with reduced samples.Uniformly distributed nano-particles with shell-core structure were observed on the surface which growing on SP phase substrate,with RP phase as core and 2-4 nm thick Co3Fe7 alloy as shell.However,it was no relation with exsolution and A-site nonstoichiometry.Moreover,ORR catalytic activity of the materials were enhanced by A-site nonstoichiometry,among which LSr0.6CoFN has the highest ORR catalytic activity,the cathodic polarization impedance is 0.066Ωcm2,the MPD of the Ni-YSZ anode supported SOFC was 837 mW cm-2 at 800℃.A-site enrichness reduced anodic catalytic activity,no relation was obersved with A-site defection and anodic catalytic activity,and the existence of RP phase was the main reason of the increase in anodic polarization resistance.The results above show that performances of SSOFC were improved by A-site nonstoichiometry.The MPD of SSOFC with LSr0.6CoFN as electrode material at 800°C was 584 mW cm-2,and LSr0.6CoFN has good redox reversibility and anti-carbon deposition properties.