Study On Preparation And Performance Of Fe-based Layered Perovskite Cathodes For Intermediate-temperature Solid Oxide Fuel Cells

Solid oxide fuel cells（SOFCs）as novel energy-converted technology,could directly convert the chemical energy in fuel to electricity with high efficiency and low pollution.Therefore,SOFCs can help to mitigate energy crisis and environmental pollution,resultly receiving the global attention and support.Cobalt-based perovskite oxides are potential Intermediate-and Low-temperature SOFCs（ILT-SOFCs）cathode material due to excellent electrochemical performance.However,the high thermal expansion coefficient resulted in long-term thermal mismatch with electrolyte due to change of Co ion spin state.The cracking or peeling caused by thermal mismatch is easy to occur for long-term test,especially in the case of multi-cold and hot cycles,the service life is sharply shortened.Although cobalt-free perovskite oxides with low thermal expansion coefficient can form good thermal matching with electrolyte,their poor oxygen reduction catalytic activity hinders the commercialization process.Therefore,in this paper,based on Co-based cathode Pr Ba_0.5Sr_0.5Co₂O_5+δas the parent material and comparsion object,Fe-based layered perovskite cathode materials were studied.Firstly Pr Ba_0.5Sr_0.5Fe₂O_5+δ（PBSF）was obtained by substitution of A-site rare earth ions.Then B-site ion partial doping and A-site Ca ion substitution were used to optimize the performance of PBSF,respectively.Finally based on Pr Ba_0.1Sr_0.1Ca_0.8Fe₂O_5+δ（PBSCa08F）,a high entropy strategy was used to improve the performance and the IT-SOFCs cathode materials with high catalysis and good thermal matching performance was obtained.The main innovations in this paper are as follows:1)The novel layered perovskite Fe-based cathode materials Pr Ba_0.5Sr_0.5Fe₂O_5+δ,Pr Ba_0.5Sr_0.5Fe_1.5Zn_0.5O_5+δ,Pr Ba_0.1Sr_0.1Ca_0.8Fe₂O_5+δand Pr_0.4La_0.4Ba_0.4Sr_0.4Ca_0.4Fe₂O_5+δwith high performance were designed and prepared.2)The effects of A-site rare earth ions substitution,B-site ion partial doping,A-site Ca substitution and high entropy strategy on the properties of cathode materials were systemtically studied.3)The influence of A-site entropy on the electrical and electrochemical properties of cathode materials was discussed,as well as the mechanism of the effect of A-site high entropy on the conductivity.4)The relaxation time of distribution was used to analyze and clarify the electrochemical reaction process of cathode,respective contributions and rate-limited steps for ORR reaction process to reduce the cathodic polarization resistance.The main contents are as follows:1)The B-site doping strategy was used to make a trade-off between low thermal expansion coefficient（TEC）and high electrochemical performance for Pr Ba_0.5Sr_0.5Co₂O_5+δ（PBSC）cobalt-based cathode materials,which was the parent material and comparsion object.Pr Ba_0.5Sr_0.5Co_2-xFe_xO_5+δwas prepared and the influence of Fe-doping on the phase structure,thermal properties,ion valence state and electrochemical properties were studied.Furthermore,the complex electrochemical behavior of the single cell were investigated using equivalent circuit method（ECM）and distribution of relaxation time（DRT）.The composition and contribution sources of the cathode polarization impedance were analyzed.The results showed that the electrochemical performance can be improved when x=0.5,the polarization resistance at 750℃is only 0.037Ω·cm²,but the TEC is still as high as 19.59×10^-6 K^-1.Furthermore,further increasing the doping amount of Fe would seriously reduce the oxygen reduction catalytic activity of the cathode materials.2)Oxygen vacancies are mostly formed in the Ln-O plane layer.The Fe-based layered perovskite cathode materials Ln Ba_0.5Sr_0.5Fe₂O_5+δ（Ln=Pr,Sm,Eu,La）were developed by the strategy of A-site rare earth ions substitution.The effects of A-site rare earth ions on the structure,thermal and electrochemical properties of cobalt-free cathode materials were investigated.The results showed that the increase of Ln³⁺radius is conducive to improve the conductivity,but inhibit the formation of oxygen vacancy.Even if the comprehensive performance of Pr Ba_0.5Sr_0.5Fe₂O_5+δ（PBSF）was the best,its electrochemical properties were significantly lower than those of Pr Ba_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ（PBSCF05）with the best performance in the previous chapter.3)B-site doping strategy was used to further optimize the performance of Fe-based cathode materials for PBSF.Pr Ba_0.5Sr_0.5Fe_1.5T_0.5O_5+δ（PBSFT05,T=Ni,Zn,Ga,Mg）cathode materials were prepared,whose B-site was replaced by Ni,Zn,Mg and Ga,respectively.And the influence of B-site doping on the catalytic activity,conductivity and electrochemical properties were investigated.The results showed that the charge imbalance caused by Zn-doping is mainly compensated by the formation of oxygen vacancy,leading to the better electrochemical performance of cathode materials.The polarization resistance at 700℃,750℃and 800℃was 0.119Ω·cm²,0.064Ω·cm² and 0.042Ω·cm²,respectively,which was still higher than PBSCF05（750℃,0.037Ω·cm²）.4)Doping low-valence ions in A-site is conducive to form oxygen vacancy.Thus,A-site Ca substitution strategy was adoped to optimize the performance of cobalt-free cathode materials for PBSF.Pr(Ba_0.5Sr_0.5)_1-xCa_xFe₂O_5+δwas prepared and the effects of Ca-doping on crystal structure,elemental valence,conductivity,thermal properties,electrochemical properties and output performance of single-cells of cathode materials were investigated,and the influencing mechanism was explored.The results showed that when x=0.8,the process of oxygen ion transfer and oxygen gas adsorption and dissociation on the cathode surface is significantly accelerated.The low polarization resistance and excellent output performance of single cell with cathode of Pr Ba_0.1Sr_0.1Ca_0.8Fe₂O_5+δare also be obtained.The polarization resistance at 700℃,750℃and 800℃was 0.112Ω·cm²,0.056Ω·cm²and 0.038Ω·cm²,respectively.Moreover,the peak power density（PPD）of single-cell at800℃can reach 0.962 W·cm^-2,which is near to those of PBSCF05.Meanwhile the TEC of Pr Ba_0.1Sr_0.1Ca_0.8Fe₂O_5+δ（PBSCa08F）was significantly lower than that of PBSCF05.Moreover,the single cell with cathode of PBSCa08F-GDC exhibited a better durable output performance than that of PBSCF05-GDC.5)The A-site high-entropy strategy was adopted to develop a novel A-site high-entropy layered perovskite oxide with composition of Pr_0.4La_0.4Ba_0.4Sr_0.4Ca_0.4Fe₂O_5+δ（PLBSCF）.Moreover,Pr Ba Fe₂O_5+δ（PBF）and Pr Ba_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ（PBSCF）were used for reference to conveniently elaborate the influence of entropy on the phase structure,thermal properties,ion valence,and electrochemical properties of the cathode material.The results showed that the increase of configuration entropy value can increase the oxygen vacancy and the ratio of adsorbed oxygen to lattice oxygen(O_ads/O_lat),improving the oxygen adsorption capacity and oxygen reduction reaction activity.The polarization resistance of PLBSCF at 700℃,750℃and 800℃were 0.112Ω·cm²,0.052Ω·cm² and0.034Ω·cm²,respectively,which was slightly higher than that of PBSCF,but significantly lower than that of PBF,as well as that of most of the medium-and high-entropy cathode materials reported recently.