Application Of Ruddlesden-popper Type Oxide La_1.5-xEu_xPr_0.5Ni_0.9Cu_0.1O_4+δ In The Cathode Of SOFC

The solid oxide fuel cell（SOFC）has attracted extensive attention due to many advantages,such as high efficiency,clean and wide fuel flexibility etc.Water produced at anode for oxygen ion-conducting solid oxide fuel cell（O-SOFC）or cathode for proton-conducting solid oxide fuel cell（H-SOFC）usually brings negative effects on the long-term stability of cells.In addition,the low oxygen reduction reaction activity of cathode materials at low operating temperature is also the main obstacle for the practicle application of high-performance SOFC.Based on the recently devoloped Ruddlesden-Popper（R-P）type La_1.5P_r0.5Ni_0.9Cu_0.1O_4+δcathode material,the novel La_1.5-xEu_xPr_0.5Ni_0.9Cu_0.1O_4+δ（LEPNC-x,x=0.1,0.2,0.3,0.4,0.6 and 0.8）was fabricated by partially substituting La with Eu.The effects of Eu content on the physical properties of LEPNC-x and and the application of LEPNC-x in both H-SOFC and electronic blocking O-SOFC are comparatively studied to obtain the optimal Eu content.In addition,a novel p-BZCY-SDC electrolyte structure composed of Ba Zr_0.1Ce_0.7Y_0.2O_3-δ（BZCY）proton conductor and Ce_0.8Sm_0.2O_2-δ（SDC）oxygen ion conductor in parallel connection is proposed.The corresponding p-SOFC is prepared,which is equivalent to the parallel connection of H-SOFC and electron blocking O-SOFC.It is expected that the long-term stability of SOFC can be further improved by adjusting the ratio of water amounts produced on the anode and cathode sides.XRD results show that a pure R-P type K₂NiF₄ structure can be obtained only for LEPNC-x with x=0.4 and lower while some reflections associated with the secondary phase of La₆Ni₅O₁₅ can be observed for x=0.6 and 0.8;furthermore,the lattice constant of LEPNC-x reduces with increasing x.A good chemical compatibility is found among LEPNC-x,BZCY and SDC.The thermal expansion coefficient（TEC）of dense LEPNC-x is almost constant and decreases from 13.35×10^-6 K^-1 for x=0 to 12.43×10^-6K^-1 for x=0.6 in 50～850℃,which is close to that of BZCY(11.2×10^-6 K^-1)and SDC(12.3×10^-6 K^-1).The electrical conductivity of dense LEPNC-x initially increases and then decreases with increasing temperature,which is caused by the transition from the semiconductor-like to metal-like behavior.With increasing x,the electrical conductivity of LEPNC-x decreases initially and then increases while the minimum electrical conductivity is 55.1 S·cm^-1 for x=0.2,which is still significantly higher than that(10 S·cm^-1)required for SOFC cathode.Using BZCY as the electrolyte and LEPNC-x（x=0,0.1,0.2,0.3,0.4 and 0.6）as the cathode,NiO-BZCY-supported H-SOFC single cells were prepared.The electrochemical performance for x=0.2 is the best among all H-SOFCs.The corresponding electrochemical impedance spectrum（EIS）results also confirm the result.The peak power density（PPD）and open circuit voltage（OCV）at 600～750℃are 87.6～544.0 mW·cm^-2 and 1.082～1.021 V,respectively.The corresponding polarization resistance（R_P）is 2.34～0.13Ω·cm²,and the activation energy E_a is 1.45eV.Using SDC as the electrolyte and LEPNC-x（x=0,0.2 and 0.4）as the cathode,NiO-BZCY supporting electron blocking O-SOFC single cells were fabricated.The electrochemical performance for x=0.2 is still the best among all single cells.The PPD and OCV at 550～750℃are 186.2～771.7 mW·cm^-2 and 1.084～0.974 V,respectively.The corresponding R_P is 2.20～0.18Ω·cm².In addition,the E_a is about 0.9 eV,which is some lower than that of general SOFC（above 1.0 eV）.For the same anode（NiO-BZCY）and cathode（LEPNC-0.2）,the effect of four kinds of electrolytes（BZCY,SDC,conventionally mixed m-BZCY-SDC and p-BZCY-SDC）on the performance of single cells were comparatively investigated.The results show that the electrochemical performance of electron-blocking O-SOFC（SDC）is the best while that of m-SOFC（m-BZCY-SDC）is the lowest.The performance of the p-SOFC is between that of H-SOFC and electron-blocking O-SOFC.The PPDs and OCVs at 550-750°C of p-SOFC are 92.7～641.8 mW·cm^-2 and 1.096～1.011 V,respectively.The corresponding R_P is 3.58～0.23Ω·cm²,and E_a is 0.99 eV.The best long-term stability is obtained for p-SOFC with p-BZCY-SDC electrolyte.After working at 600°C for 100 h,the corresponding degradation amounts for H-SOFC and O-SOFC are 45.2%and 50.0%,respectively.The degradation amount of current density for p-SOFC is 23.3%,which is only about half of that（45.0～50.0%）for H-SOFC or O-SOFC.It suggests that the present proposed p-SOFC can indeed improve the long-term stability of SOFC apparently,which shoud be caused by the decreasing negative effect resulted from the fact that the by-product water of p-SOFC can produce at both anode and cathode sides.In summary,LEPNC-0.2 shows the best performance in both H-SOFC and electron blocking O-SOFC.The present proposed p-SOFC can improve the long-term stability of SOFC markedly.It is of great significance to the development of single cell cathode materials and the improvement of single cell structure.