Intermediate Temperature Electrochemical Performance Of BaFeO 3-δ Based Mixed Conductor Cathode Materials

Solid Oxide Fuel Cells（SOFCs）,as a kind of sustainable development of energy-conversion device,have attracted a great deal attention.In recent years,the problem of the ohmic losses has been solved due to the appearance of new electrolyte materials and the application of thin film technology.Improving the electrochemical performance of the cathode becomes the main key point.In this paper,we discuss the partially A-or B-site substituted based cobalt-free perovskite BaFeO_3-δ and composite cathode material,study the impact of crystal structure、microstructure and the change of Fe valence.Finding the method of improving cathode electrochemical performance according to analyzing the rate-limiting step for oxygen reduction reaction on cathode.The main research content includes:First,the BaFe1-xCuxO_3-δ（x = 0.05-0.15）samples was synthesized according to a conventional solid-state reaction.The research found that the partial replacement of B-site in BaFeO_3-δ with Cu can effectively reduce the thermal expansion coefficient of BaFeO_3-δ,and the cathode material has good chemical compatibility with CGO electrolyte.The polarization resistance（Rp）of BaFe1-xCuxO_3-δs cathodes decrease with the increase of the amount of Cu,the Rp value of BaFe0.85Cu0.i5O_3-δ is the smallest among the BaFe1-xCuxO_3-δ cathodes,only 0.35 Ω cm² at 700 0C.We choose the BaFe0.85Cu0.15O_3-δCGO as composite cathodes in order to further enhance the ionic conductivity and reduce the polarization resistance of the material.BFCO-40CGO has the smallest values of Rp,only 0.071 Ω cm² at 700 ℃,eighty percent has been reduced compare to BaFe0.85Cu0.15O_3-δ cathode.Cobalt-free perovskite BaFe1-xInxO_3-δ（x = 0.10-0.20）cathodes was synthesized according to a conventional solid-state reaction.The In dopant effectively stabilize the cubic perovskite structure.The values of 8 measured by iodometric titration were larger than other doped BaFeO_3-δ materials,which means the high electrocatalytia activity.The polarization resistances of BaFe0.9In0.1O_3-δ（BFIn10）was 0.25 Ω cm² at 700 ℃ at zero applied voltage.The high thermal expansion coefficients of BaFe1-xInxO_3-δ will be harmful to the fuel cells operational stability during cooling and heating process.There have been studies reported that partially Sr substituted Ba can effectively reduce the thermal expansion coefficients of BaFeO_3-δ.Therefore,the Ba0.6Sr0.4Fe0.9In0.1O_3-δsample was synthesized and studied as cathode material.BaFe1-xNixO_3-δ（x = 0.15-0.25）series cathodes were synthesized using the glycine-nitrate process.The Ni dopant improves the electrical conductivity of BaFeO_3-δ.The electrical conductivity of BaFe0.75Ni0.25O_3-δ cathode reaches 19 S cm-1,which is higher than BaCo0.7Fe0.2Ni0.1O_3-δ material,which is responsible for the high performance of cathodes.At 700 ℃,the polarization resistances of BaFe0.75Ni0.25O_3-δcathode was 0.095 Ω cm²,which is lower than that of many cobalt-based perovskite cathode.The cathode overpotential is-30 mV when the current density reaches-103 mA cm-2.Oxygen reduction kinetics proves that the rate-limiting step for electrode reaction on BaFe0.75Ni0.25O_3-δ cathode was the charge transfer reaction.