Properties Of Y/Gd-doped And Equivalence Elements Substitution Of SOFC BaSrCoFeO Cathode Materials

The energy crisis and environmental development are becoming seriously, Solidoxide fuel cells （SOFCs） as a new kind of chemical device have attracted much attentiondue to their high-energy conversion efficiency and low pollution. With the developmentof practical application research in SOFC, lowering the operation temperature of SOFCsis the key point of the research now. Therefore, the development of new electrodes withhigh electro-catalytic activity for the oxygen-reduction reaction is one effective approachto lower down the operating temperature of SOFC.Complex oxides of (Ba_0.5Sr_0.5)1xLnxCo_0.8Fe_0.2O_3-δ（Ln=Y, Gd; noted as BSYCFxand BSGCFx） were synthesized via the modified sol-gel method, and their series ofperformances and electrochemical properties were investigated. The results demonstratedthat the rare earth-doped in A-site does not destroy the single phase cubic perovskitestructure of BSCF. With increasing the rare earth-doped content, the lattice constant andunit cell volume shrink. The ion radius of the Y³⁺is much smaller than that of the Gd³⁺,BSYCF have smaller cell volume and higher thermal expansion coefficient （TEC） thanBSGCF. The Y, Gd-doped greatly improve the electrical conductivity of BSCF. Themaximum conductivities of BSYCF20and BSGCF15are121.4and148S·cm-1.For BSGCF materials, the rcof BSGCF05is the smallest in all samples, whichmake that the starting temperature of weight loss, the temperature of the abnormalexpansion and the transition temperature of the electrical conductivity for the BSGCF05are higher than those of other samples, and the chemical diffusion coefficient （D^） ofoxygen and the diffusion active energy （Ea） of BSGCF05are the smallest in all samples.The results of thermograimetic measurements （TG） show the action of lattice oxygen,which is a major cause of abnormal expansion, the maximum conductivities at atemperature （350-500°C）, and the electrical conductivity relaxation for BSYCF andBSGCF materials.The oxygen nonstoichiometries of BSCF and BSGCF15tested at room temperatureby the iodometry technique. A larger oxygen nonstoichiometry δ for BSGCF is observeddue to the existence of Gd³⁺in the A-site. Partial substitution of Ba2+and Sr2+by Gd³⁺results in the reduction of the valence state in the B-site to maintain the electricalneutrality. The quantity of Co4+and Fe4+in BSGCF is therefore smaller than that ofBSCF. With the increase of temperature, the average valence state in the B-site willcontinue to decrease for the oxygen vacancy concentration increases, which is majorcause that the Gd-doping greatly improves the electrical conductivity and decreases thediffusion resistance of BSCF. The performance of an anode-supported single cell withBSGCF15cathode showed good properties in single-cell. At600°C, the power densitiesis551mW·cm-2, the polarization resistance of cathode is0.16·cm². Then, base on the results of Gd-doped BSCF, we systematically evaluated theeffects of A-site rare earth metal (Gd³⁺) doping and changing the ratio of Ba/Sr in A-siteon the performance of (Ba_xSr_1-x)0.95Gd_0.05Co_0.8Fe_0.2O_3-δ. The samples with x=0.1-0.7have a cubic ABO3perovskite-type structure, except x=0.9with a small amount ofBaCoO3impurity. The lattice parameter increases with doping content of Ba. The oxygennonstoichiometries_3-δ and the average valence state in the B-site were enhanced withthe increasing of Ba content from the results of iodometric titration. The crystal latticeenergy calculated results show that the cell expansion is due to the bigger ion radius ofBa2+than that of Sr2+. The Gd-doped and changing the ratio of Ba/Sr slightly reduce thethermal expansion character of BSCF. The thermal expansion coefficient （TEC） is around20×10-6K-1from50-800°C. The Gd-doped and changing the ratio of Ba/Sr greatlyimprove the electrical conductivity of BSCF. The maximum electrical conductivity ofBa10is298.8S·cm-1. the AC impedance results are clearly observed that the totalpolarization resistances （Rel） of (Ba_xSr_1-x)0.95Gd_0.05Co_0.8Fe_0.2O_3-δshow a first increasewith the increasing content of Ba2+(from x=0.1to_0.5), reaching a maximum when x=_0.5, and then a decrease with further increased in content of Ba2+（x=0.7）. The Relvaluesare0.043,0.061,0.065,0.14, and_0.059·cm2for x=0.1,_0.2,0.3,_0.5, and0.7at650°C,respectively.In order to ascertain the influence of Gd doping on the properties of the materialfurther, we systematically evaluated the effects of A-site rare earth metal (Gd³⁺) dopingand changing the ratio of Fe/Co in B-site on the performance of(Ba_0.5Sr_0.5)0.95Gd_0.05Co_1-xFexO_3-δ(_0.2≤x≤1.0). The results showed that, the(Ba_0.5Sr_0.5)0.95Gd_0.05Co_1-xFexO_3-δhave a cubic perovskite-type structure, and the latticeparameters firstly increase with increasing Fe-doping amount from x=_0.2 to0.4, andthen decrease with the further increase of x. The Gd-doped and changing the ratio ofFe/Co slightly reduce the thermal expansion character of BSCF. The thermal expansioncoefficient （TEC） is20.7-22.4×10-6K-1from50-900°C. The results of thermograimeticshowed that weight increase was more significant as Fe concentration increased, and theweight loss and TEC for Fe0.4are the smallest. The results of dTG curves and theaverage valence state in the B-site identify the oxidation temperature range of Fe/Co inB-site. The Gd-doped and changing the ratio of Fe/Co slightly changes the electricalconductivity of BSCF. The maximum electrical conductivities of(Ba_0.5Sr_0.5)0.95Gd_0.05Co_1-xFexO_3-δhave declined from64.54S·cm-1to21.4S·cm-1withincreasing Fe content. Based on the results of the AC impedance, the electrochemistryproperties of Fe0.4are better than others. For example, the impedance of Fe0.4is0.07·cm2at650°C. To distinguish the individual step of the electrode reaction, impedancespectra of Fe0.4symmetrical cell was measured with oxygen partial pressure from0.022 to0.86atm.