The Investigation Of Oxide Influence For Oxygen Reduction Kinetic In LSCF Cathode

Solid oxide fuel cells (SOFCs) are clean electrochemical energy conversion devices with high efficiency. Its efficiency is mainly restricted by the oxygen reduction reaction in cathode. In this thesis, typical cathode catalytic materials La1-xSrxCo1-yFeyO3-δ (LSCF) is studied to investigate the influence on catalytic activity or oxygen reduction kinetic by oxides, including doped ceria, alkaline earth metal oxides and transition metal oxides, to enchance oxygen reduction reaction rate and improve immediate-temperature SOFC power output.Chapter1brifely introduces the class and properity of SOFC key materials including electrolyte, anode and cathode. Cathode materials and structure especially the nano structure cathode made by infiltrating are introduced detailed. The long term stability of nano structure cathode is introduced as well as the fabrication process and the nano particles growth model. For different kinds of cathode materials, the electrochemical performance and stability of their nano structure are analysised.In chapter2, doped ceria (SDC, SmxCe1-xO2-δ) could greatly enhance the oxygen reduction kinetic in LSCF surface and SDC particles improve the oxygen surface exchange coefficient (Kchem) of LSCF by5times. Kchem value increase with SDC particles loading weight to the maximum and then decrease which suggests that the enhancing performance is related with the three phase boundary (TPB) where SDC, LSCF and gas phase count together. Kchem also relates with the SDC composition and under the same loading, Kchem increases with x reaching the maximum at x=0.2then goes decrease. Further research indicates that Kchem is related with the ionic conductivity (σ) of SDC which increase with σ value. The result suggests that it is the free oxygen vacancy in SDC promotes the oxygen reduction kinetic in LSCF surface. And the oxygen reduction reaction activation energy has no obvious change which imply that SDC doesn’t change the oxygen reduction mechanism in LSCF surface..In chapter3, the synergistic effect in oxygen reduction reaction by alkaline earth metal oxides is studied. Firstly, BaCO3nanoparticles could greatly improve the oxygen reduction rate in LSCF, LSM (La0.8Sr0.2MnO3-δ) and LSF (La0.8Sr0.2FeO3-δ(cathodes. After infiltrated with BaCO3, the Kchem value of LSF and LSCF has a10times increase. Further analysis reveals that the low frequency resistance of LSF electrode decreases by an order of magnitude. For example, at700℃it reduces form 2.54Ωcm2to0.26Ωcm2. The increase of Kchem and decrease of low frequency resistance indicate that BaCO3accelerate the oxygen surface process including adsorption and dissociation. And at800℃, the power output of single cell with LSF cathode, YSZ (Y0.15Zr0.85O2-δ) electrolyte and Ni-YSZ anode increase from0.3Wcm-2to0.53Wcm-2when LSF cathode is infiltrated BaCO3. BaCO3nanoparticles also could improve the cathode long term stability. At700℃, the resistance of bare LSF electrode has a90%increase which increase from2.96Ωcm2to5.48Ωcm2in380h long term test while the resistance of LSF infiltrated with BaCO3stay in0.84Ωcm2. And the microstructure analysis implys that BaCO3nanoparticle morphology has no change in thermal treatment.CaO particles have special property when used as synergistic catalytic in LSF electrode. It could increase the Kchem value of LSF by an order of magnitude, but the polarization resistance of LSF increased after infiltrated with CaO. AC impedance analysis indicates that the high frequency resistance associated with oxygen ionic transport increases and low frequency resistance regards oxygen surface process decreases. Oxygen ionic could transports along LSF bulk and surface, so it must be the surface path is blocked by CaO. The application of ionic conductor SDC in LSF to form LSF-SDC composite cathode resolves the problem. At750℃, the resistance of LSF-SDC electrode in YSZ electrolyte decrease from0.83Ωcm2to0.42Ωcm2after infiltrated with3.01wt%CaO. At800℃, in anode supported single cell with YSZ electrolyte using CaO infiltrated LSF, the power density increase from0.45Wcm-2to0.58Wcm-2.At last, SrO has different working mechanism with BaCO3and CaO. It could react with La0.8Sr0.2FeO3-δ (LSF113) electrode surface to form La0.8Sr1.2FeO4-δ (LSF214). LSF214is a Rullesden-Popper (RP) phase with An+1BnO3n+1structure that has high oxygen vacancy ratio and electrochemical catalytic activity due to its perovskite layer and rock salt layer displace. So the core-shell structure (LSF214shell and LSF113core) catalyst is fabricated after infiltration. The core-shell structure could improve the Kchem value by about15times and further to reduce the polarization resistance. XPS analysis reveals that LSF214nanoparticles only form in the out surface of LSF113electrode backbone. The LSF312-LSF113core-shell structure cathode formed by infiltrating provides a new way to fabricate heterostructured electrode.In chapter4, the catalytic activity effect of transition metal (Cr, Mn, Fe, Co, Ni, Cu and Zn) oxide in LSCF is studied. Electrochemcial conductivity relaxation results show that Cr oxide reduce the oxygen reduction kinetic in LSCF surface and cause cathode poisoned. While the other oxides all could accelerate the oxygen reduction reaction and CuO has the best enhacement which improve Kchem value by30times. In further research, CuO is deposited to LSCF surface by vapor sputter and the CuO surface coverage θCuO and per area TPB length LTPB in LSCF surface are indentified by SEM analysis to analysis the relationship between Kchem value and CuO loading as well as structure. The enhanced surface exchange coefficient by CuO, Kcuo doesn’t go through a liner relationship with neither θCuO nor LTPB.It suggests that the CuO enhanced oxygen reduction reaction maybe occur in CuO surface and the three phase boundary where CuO, LSCF and gas phase counted. In the test temperature range, the reaction contribution of CuO, λCuO increases with temperature. At800℃, λCuO is30%when LTPB is1.14cm-1and increase to80%when LTPB increase14.7cm-1. In CuO infiltrated LSCF electrode, the polarization resistance is reduced that at650℃, the polarization resistance is0.51Ωcm2with8.5w5%CuO compared to0.76Ωcm2for bare LSCF electrode. AC impedance analysis also indicates that it is the reduction associated with low frequency resistance. In single cell with SDC electrolyte, the power density increase from0.55W cm-2to0.75Wcm-2with CuO infiltrated LSCF electrode.