Influence Of Composition Adjustment On The Performance Of Barium Lanthauanum Ferrite-based Cathode Materials

Solid oxide fuel cell(SOFC)is an efficient all-solid-state energy conversion device that has attracted widespread attention.At present,the development of intermediate and low temperature cathode materials is a research hotspot in this field.Fe-based perovskite oxides usually have suitable thermal expansion coefficients,higher stability and lower production costs,which are considered as potential cathode materials.In this work,the purpose of this paper is to improve the physical and electrochemical properties of the electrode materials.LaBaFe₂O_6-?and LaBa_0.5Sr_0.5Fe₂O_6-?were chosen as the parent materials for this study.By adopting non-stoichiometric and elemental doping methods at the A-site,B-site and O-site of the materials to realize the composition adjustment,which can make the improvement of the electrode performance.Meanwhile,the effects and the mechanism of the above optimization methods on the cathode properties were also studied.The vacancy defects of LaBaFe₂O_6-?were introduced through Ba non-stoichiometric method.LaBa_1-xFe₂O_6-?(x=0?0.03)series materials were synthesized by the sol-gel method.The study found that the formation of vacancy defects in the materials improved its oxygen adsorption ability.Compared with the parent material,the LaBa_0.98Fe₂O_6-?cathode showed excellent electrode performance.At 750^oC,the polarization resistance(R_p)value reduced from 0.226?cm²to 0.089?cm².In this study,it is also found that the catalytic activity of the electrode is closely related to the change of oxygen concentration,and it is further clarified that the oxygen reduction reaction of the electrode includes the processes of oxygen ion from TPB to the electrolyte,charge transfer and the adsorption of oxygen molecules on the electrode surface.The adsorption process of oxygen molecules on the electrode surface is the rate-limiting step.In order to investigate the influence of the substitution of the La element in LaBaFe₂O_6-?with the low-valent Ca²⁺on the physics and electrode performance,the series of La_1-xCa_xBaFe₂O_6-?(x=0?0.4)materials was prepared.The study found that after La replacement by the Ca element,the electrical conductivity of the material was increased while the thermal expansion coefficient was reduced.The increased oxygen vacancy concentration promoted the oxygen reduction reaction process of the electrode.In the La_1-xCa_xBaFe₂O_6-?system,La_0.7Ca_0.3BaFe₂O_6-?obtains the best electrode performance.At 800?,its R_pis reduced to about 3/8 of the parent material,and the maximum output power density(P_max)of the electrolyte-supported single cell reaches 452 mW cm^-2,and it still shows good electrode stability after 100h operation.In view of the B-site element is related to the catalytic activity of the material.The feasibility of high-valence Sn⁴⁺and Nb⁵⁺doping to improve the catalytic activity and electrode performance of LaBaFe₂O_6-?was compared and investigated.LaBaFe_2-xNb_xO_6-?(x=0?0.1)and LaBaFe_2-xSn_xO_6-?(x=0?0.2)series materials were prepared by the sol-gel method.The study found that the doping of high-valence ions enhanced the oxygen adsorption ability of the material,and this phenomenon was verified by first-principles calculations.Meanwhile,the increased average metal-oxygen bond energy in the material reduces the thermal expansion coefficient.The electrode and the electrolyte still showed good electrode interface stability after long-term operation.In the LaBaFe_2-xNb_xO_6-?system,the lowest R_pis obtained when the Nb doping amount x is 0.075.At 800?,the P_maxof a single cell with the LaBaFe_1.925Nb_0.075O_6-?|SDC|Ni O-SDC configuration can reach 553 mW cm^-2,and the electrode performance does not appear obvious degradation after 50 h operation.In the LaBaFe_2-xSn_xO_6-?system,the doping of Sn⁴⁺improves the electrode performance of the material.At the same overpotential,the LaBaFe_1.85Sn_0.15O_6-?cathode has a larger current density than the parent material,showing stronger electrocatalytic activity.At 800?,LaBaFe_1.85Sn_0.15O_6-?obtained the best electrode performance,and its R_pand P_maxwere 0.073?cm²and 514 mW cm^-2,respectively.In order to achieve further improvement of the electrode performance,the strategy of doping F anions at the O site was adopted,and the series of LaBa_0.5Sr_0.5Fe₂O_6-?-xF_x(x=0,0.1?0.15)materials was prepared.The research results indicate that doping with an appropriate amount of F ions can help to improve the mobility of lattice oxygen and accelerate the oxygen exchange process on the electrode surface.With the doping of F ions,R_pshows a decreasing trend.In the test temperature range,LaBa_0.5Sr_0.5Fe₂O_5.875-?F_0.125has the smallest R_pand exhibits the highest electrocatalytic activity.The oxygen reduction reaction kinetics and the corresponding distributions of relaxation times(DRT)research results indicated that the rate-limiting step was the charge transfer process.At 750?,the R_pof the LaBa_0.5Sr_0.5Fe₂O_5.875-?F_0.125cathode is 0.073?cm²and the electrode possesses a better stability after long-term operation,which is a potential electrode material.