Preparation And Electrochemical Performance Research Of BaCoO_3-?-based Cathode Materials For Solid Oxide Fuel Cells

Solid oxide fuel cells(SOFCs)have attracted much worldwide attention due to their outstanding advantages such as high fuel utilization,modular design and wide fuel adaptation range.In order to realize the large-scale commercial application of SOFC,lowering the operating temperature of SOFC to the medium temperature range(600?800?)is a necessary way.But lowering the operating temperature will result a rapid increase of the cathode polarization resistance that significantly reduce the oxygen reduction reaction(ORR)activity of cathode materials.BaCoO_3-?-based perovskites have ionic and electronic conductivity which could extend the three-phase boundary(TPB)and enhance the electrochemical activity,which is a highly promising material for SOFC cathodes.Among them,the cubic phase BaCoO_3-?has the highest electron and oxygen ion conductivity.Due to the mismatch of ionic radii,which leads to an unstable octahedral structure,BaCoO_3-?usually exists as a non-cubic structure.Moreover,the higher thermal expansion coefficient(TEC,>20×10^-6 K^-1)is not compatible with the electrolyte material.These factors limit its further application of SOFC.BaCo_1-xNb_xO_3-? cathode materials using B-site doping are prepared by high temperature solid phase method.The optimal synthesis temperature is 1100?,and a stable cubic phase structure could be obtained when the Nb doping amount is greater than20 mol%.The electrical conductivity of BaCo_0.8Nb_0.2O_3-?is 14.9 S·cm^-1 at 800?,and the TEC is 23%lower than that of before doping sample.The polarization impedance is0.039?·cm² at 700°C,the activation energy is 140.44 KJ·mol^-1 and the maximum power density is1042 mW·cm^-2 at 800°C.BaCo_0.8Nb_0.2-xSc_xO_3-? cathode materials using Nb and Sc co-doping are prepared by high temperature solid phase method,and the substitution of Sc does not change the cubic phase of the materials.With the gradual increase of Sc doping,the cell volume and the oxygen vacancies concentration gradually increases,which provides a more favorable diffusion path for the diffusion of oxygen ions.The electrical conductivity of BaCo_0.8Nb_0.1Sc_0.1O_3-?at 800?is 19.9 S·cm^-1,and the TEC is 7%lower than that of BaCo_0.8Nb_0.2O_3-?.It is shown that the ORR rate control step of BaCo_0.8Nb_0.1Sc_0.1O_3-?cathode material is the adsorption and desorption process of oxygen.The polarization impedance is 0.029?·cm² at 700°C,the activation energy is 129.44 KJ·mol^-1.The maximum power density is 1070 mW·cm^-2 at 800°C.Porous cathode materials are prepared by the sacrificial template method.The appropriate amount of polyurethane foam as a template is beneficial to prepare the cathode material with a three-dimensional porous structure,and the pore size distribution of the cathode material is around 7?m when the template addition is 10 mass.%.The porous structure facilitates the gas transport and diffusion,enlarges the three-phase interface,reduces the effect of concentration polarization,and promotes the oxygen reduction reaction.The ORR rate control step of the porous cathode materials is charge transfer process and the oxygen adsorption-desorption process together.The polarization impedance is 0.025?·cm² at 700°C,the activation energy is 126.80 KJ·mol^-1.The maximum power density of the full cell is 1222 mW·cm^-2 at 800°C.BaCo_0.8Nb_0.1Sc_0.1O_3-? cathode materials with surface oxygen defects are prepared by a simple solution reduction method.The appropriate reduction time not only does not affect the cubic phase structure of materials,but also can improve the oxygen non stoichiometric ratio(?)of materials.Further studies show that the surface oxygen defect facilitates the adsorption and dissociation of oxygen,and also increases the oxygen vacancies concentration of materials,which weaken the materials capable of binding electrons and improved the electrical conductivity.Theoretical calculations show that the introduction of oxygen defects modulates the O-p orbital projection density distribution of materials,resulting in a higher O-p orbital center and enhanced the activity of ORR.The polarization is 0.023?·cm² at 700°C and the activation energy is 114.16 KJ·mol^-1.The maximum power density of the full cell is 1372 mW·cm^-2 at 800°C.