Study On Preparation And Performance Of Ferrite Symmetrical Electrodes For Solid Oxide Fuel Cells

Solid oxide fuel cell（SOFC）is an effective device to convert chemical energy to electricity directly.The popular single SOFC is typically composed of dense electrolyte,porous cathode and anode.The anode and cathode of the symmetrical solid fuel cell（S-SOFC）have been prepared by the same material,which can optimize the preparation process,reduce the preparation cost,the poor compatibility,“carbon deposition”and“sulfur poisoning”also can be alleviated.Some ferrite perovskites have potential in the application of S-SOFC due to their good oxygen reduction reaction activity（ORR）,and stable structure in the reducing atmosphere.Based on the above reasons,we have focused on ferrite symmetrical electrodes for SOFC:Firstly,the preparation and performance of Ba_0.95La_0.05Fe_0.9Nb_0.1O_3-δwas studied.There are high ionic conductivity and good ORR activity for Ba Fe O_3-δwith a cubic structure.However,it cannot be used as an electrode due to the mismatched ionic radius of Ba²⁺and Fe^4+/3+causing various phases of Ba Fe O_3-δ.Besides,the poor stability of Ba Fe O_3-δin the reducing atmosphere makes it becoming an anode difficult.In this study,a novel Ba_0.95La_0.05Fe_0.9Nb_0.1O_3-δ（BLFN）ceramic electrode is developed by La and Nb co-doped.The X-ray diffraction（XRD）result reveals that the Ba_0.95La_0.05Fe_0.9Nb_0.1O_3-δhas a cubic perovskite structure and possesses stable structure in the hydrogen reduction.The maximum electrical conductivities of BLFN material are 4.91 S cm^-1 in air and 1.58S cm^-1 in 5%H₂/Ar,respectively.Polarization resistances of the symmetrical cell with BLFN electrode are as low as 0.055Ωcm² in air and 0.148Ωcm² in hydrogen at 750^oC,respectively.It shows outstanding catalytic activity toward oxygen reduction and fuel oxidation reactions.The electrochemical impedance spectra and the distribution of relaxation times（EIS-DRT）analysis indicate that the rate-limiting steps of BLFN electrode are determined by the hydrogen adsorption/dissociation process using as anode and the dissociation of adsorbed oxygen using as cathode,respectively.The single S-SOFC with LSGM electrolyte achieves a high peak power density(P_max)of581 m W cm^-2 and low polarization resistance of 0.096Ωcm² at 800 ^oC.Secondly,the preparation and performance of La_0.5Ca_xSr_0.5-xFe_0.9Mo_0.1O_3-δwas studied.La_1-xSr_xFe O_3-δ（LSF）series oxides have been used as the cathode of SOFC due to their high oxygen ion conductivity,good oxygen reduction reaction activity and low cost.However,it is difficult for LSF as anode due to their structures being easily decomposed in the reducing atmosphere.In order to make LSF as anode,La0.5Ca0.25Sr0.25Fe0.9Mo0.1O3-δ（LCSFM）and La0.5Ca0.5Fe0.9Mo0.1O3-δ（LCFM）perovskite oxides were prepared by Ca and Mo co-doped.The results show that LCSFM and LCFM keep the perovskite structure with tiny Fe has been exsolved in the humid hydrogen condition of 800 ^oC.The thermal expansion coefficient（TEC）values of LCSFM and LCFM in air are low(50-1000 ^oC:～14.2×10^-6 K^-1)and close to the electrolyte.The maximum electrical conductivities of LCSFM/LCFM materials are64.65/66.86 S cm^-1 in air and 1.27/1.29 S cm^-1 in 5%H₂/Ar,respectively.The EIS-DRT analysis shows that the gas adsorption and dissociation have been influenced by Ca content.The rate-limiting step of cathode reaction and anode reaction are charge transfer and the H₂ adsorption and dissociation,respectively.The P_max of LCSFM and LCFM single cell are 492 and 427 m W cm^-2 at 800 ^oC,respectively.The corresponding R_p are 0.238 and 0.302Ωcm²,respectively.Thirdly,the preparation and performance of NdBa(Fe_0.9Ti_0.1)_1.875O5+δwas studied.A-site layered perovskite NdBa Fe₂O5+δ（NBF）has drawn much attention due to its high oxygen ion conductivity and good oxygen reduction reaction activity.However,it is difficult for NBF to keep the original structure under a reducing atmosphere.In view of the strong metal-oxygen bond,Fe in NBF has been partially replaced by Ti.The B-site deficient NdBa(Fe_0.9Ti_0.1)_1.875O5+δ（NBFT02）was constructed due to a few impurities formed.XRD result reveals that NBFT02 has a stable structure under the reducing atmosphere.The maximum electrical conductivities of NBFT02 materials are 16.38 S cm^-1 in air and 0.64 S cm^-1 in 5%H₂/Ar,respectively.The R_p are 0.036 and 0.219Ωcm²for NBFT02 electrode in air and wet hydrogen atmosphere at 800 ^oC,respectively.The the dissociation of adsorbed oxygen is the main rate-limiting step of cathode reaction and charge transfer process is of anode.The P_max of the symmetrical cell based on NBFT02 at 800 ^oC is 725 m W cm^-2 and the R_p is 0.178Ωcm².The S-SOFC shows considerable long-term stability due to it is working last for 100 h at 700 ^oC without falling.