The High-Temperature Physical Properties And Electrochemistry Properties Of Rare-Earth Doped BaSrCoFeO Cathode Materials

Solid oxide fuel cell (SOFC) is one kind of chemical device, which can convert the chemical energy into electrical energy directly. Recently, the energy problem is becoming seriously, thus the SOFC is attracting more and more attentions because of its high-energy conversion efficiency, low pollution and fuel flexibility. How to lower down the operating temperature is the key point of the SOFC's application research. Finding out a cathode with good characteristics at intermediate temperature is one effective approach to lower down the operating temperature of SOFC while to retain the high performances. Many researchers have done mass works in developing the new cathode materials.The objective of this paper is around the development of the cathode material for intermediate temperature solid oxide fuel cell (IT-SOFC). Firstly, the Sm-doped material (Ba0.5Sr0.5)1-xSmxCo0.8Fe0.2O3-δ(BSSCF;0.05≤x≤0.20) was prepared by EDTA-citric acid method, and then the structure, the thermal character, the electrical character as well as the electrochemistry character were researched. The results demonstrated that the BSSCF have a cubic perovskite-type structure as BSCF, and the Sm-doping caused the lattice shrinkage because of the smaller radius of Sm3+ ion. The thermal expansion coefficients (TECs) of BSSCF are 19.1-20.3×10-6 K-1 from 30~800°C. The Sm-doping improved the electrical conductivity of BSCF effectively. While the conductivity is increasing with the increasing of Sm content, and the x=0.20 sample have aσ=150 S?cm-1 at 392°C. The improvement of electrical conductivity is mainly benefit from the enhanced concentration of electronic charge carriers, which is caused by the Sm-doping. Besides, at 350~500°C, there is an obviously electrical conductivity relaxation phenomenon for BSSCF. The results showed that, the chemical diffusion coefficient ( D~ ) of oxygen and the diffusion active energy (Ea) of oxygen were reducing with the increasing of Sm content. In the Thermograimetic measurements (TG) of BSSCF, a weigh addition at 200°C and a weigh loss at 350°C were observed, which is associated with the valence change of Co and Fe followed by the absorption/loss of oxygen.On the electrochemistry aspect, the sinter condition of cathode, which can influence the properties, was researched. The best sinter condition is, at 1050°C for 4h. Then, the electrochemistry properties of BSSCF were carried out on a half-cell. The results demonstrated that, the Sm-doping improved the electrochemistry properties of BSSCF effectively. Take the AC-impedance for example, the impedance of x=0.20 sample are 17.51,7.55,2.98,1.54 ??cm2 at 400, 450, 500, 550 oC respectively, while the impedance of BSCF are 41.06,18.59,4.57,1.82 ??cm2. The BSSCF cathode also showed good properties in anode-support, SDC film single-cell. For example, the maximum power densities are respectively 268,442,681 and 820 mW?cm-2 at 500, 550, 600, 650°C. Furthermore, the AC-impedance of BSSCF for half-cell and single-cell at the same temperature are different. The reasons are the different work-atmosphere of SDC electrolyte film in half-cell and single-cell, and the mixed conductivity character of SDC electrolyte film.Base on the results of Sm-doped BSCF, the La3+ was used to substitute the Ba2+ and Sr2+ ion in BSCF, and the (Ba0.5Sr0.5)1-xLaxCo0.8Fe0.2O3-δ(BSLCF;0.05≤x≤0.20) was compounded. An emphasis was made on the effect of La-doping on the material's properties, and the properties of BSLCF were compared with the Sm-doped materials. The results showed that, the BSLCF have a cubic perovskite-type structure. There is an obviously weight loss for BSLCF after 400°C, which is associated with the reduction of Co/Fe with quadrivalence. Furthermore, the La-doping increased the TECs slightly, and the TECs of BSLCF are about 18.7-25.8×10-6 K-1 from 30~800°C. The La-doping increased the electrical conductivity of BSLCF apparently. Compared with the Sm-doping sample, the improvement of BSLCF is more apparently e.g., the sample of x=0.2 demonstrated a conductivity ofσ=376 S ? cm-1 at 392°C. Furthermore, the chemical diffusion coefficient ( D~ ) and diffusion active energy (Ea) of BSLCF were researched by the electrical conductivity relaxation method. The results showed that D~ and Ea were reducing with the increasing of La content. The AC-impedance proved that the electrochemistry properties of BSLCF were better than that of the BSCF.In order to ascertain the influence of A-site rare-earth doping on the properties of the material further, we introduced the Nd into BSCF, and got the (Ba0.5Sr0.5) 1-xNaxCo0.8Fe0.2O3-δ(BSNCF; 0.05≤x≤0.20). The structure, thermal, electrical conductivity and electrochemical properties of BSNCF were researched. The results showed that, the BSNCF have a cubic perovskite-type structure. The behaviors of oxygen in the BSNCF are restricted by the diffusion and surface-exchange rate, and are also associated with the density and surface-measure of the sample. To a certain extent, the Nd-doping improved the TECs. The TECs of BSNCF are smaller than that of the BSCF, with the data of 18.7-19.7×10-6 K-1 from 30~800°C. The results of electrical conductivity showed that, the Nd-doping improved the conductivity of BSCF apparently, and the sample of x=0.20 has the highest conductivity. There is an obviously electrical conductivity relaxation phenomenon for BSNCF sample at 400~450°C. The diffusion coefficient and active energy of BSNCF are reducing with the increasing of Nd content. The electrochemistry properties of BSNCF are better than that of the BSCF, which are proved by the AC-impedance. For example, at 550°C, the impedance of BSCF is 1.82 ??cm2, while that of the BSNCF is 0.69 ??cm2.To reveal the intrinsic relation between the doping elements and the material's properties, moreover find out the regular, we analyzed the rare-earth elements (Ln= Sm, La, Nd) doped compounds'properties from the view of the doping rare-earth element's character and the experiment results. An emphasis was made on the difference in the structure, thermal, electrical character and the reasons. The analysis results revealed that, the rare-earth doping material still have a cubic perovskite-type structure. Although the Tolerance Factor changed a little with the varying of the doping element and content, while the value is still around 1. The results of titration experiment showed that, under the same doping content, the amount of Co3+/Co4+, Fe4+/Fe3+ and the oxygen vacancy are different, which caused the difference in the properties of different element doped material. The rare-earth doping have not reduce the TECs, moreover, under the same doping content, the TEC of the different element doped material are different. The A-site rare-earth doping increased the electrical conductivity of BSCF apparently. Under the same doping content, the conductivity of different rare- earth doped material is different.This paper is based on the important cathode material of IT-SOFC BSCF, the rare-earth element La, Nd, Sm were used to substitute the Ba2+ and Sr2+ ion in BSCF, which improved the electrical conductivity and electrochemistry properties. A new type of cathode materials (Ba0.5Sr0.5)1-xLnxCo0.8Fe0.2O3-δ(Ln=La,Nd,Sm) were developed successfully. The high-temperature physical properties and their principles associated with the high-temperature oxidation/reduction of these materials were also researched.