Fabrication And Characterization Of La_1-xSr_xCo_yFe_1-yO₃ Cathode For IT-SOFC

Strontium and iron-doped lanthanum cobaltite materials (LSCF)are well known to have high catalytic activity towards the reduction of oxygen, high oxygen self-diffusion coefficients, and high electrical conductivity with mixed conduction that make them suitable as cathode materials. The properties of LSCF depend very much on the powder and cathode morphology besides their composition. In the sense, the synthesis method of the electrode powders plays a very important role in obtaining homogeneous and porous electrodes,which can produce significant improvements on the oxygen conducting properties and oxygen reduction activity of the cathode. In this dissertation, A novel synthesis method, microwave assisted sol-gel method, was used to synthesis the LSCF nanopowders. The properties of three different electrode like pure electrode, composite electrode and functionally graded electrode were studied and compared one another by polarization curves, electrochemical impedance spectroscopy and single cell tests. The kinetic mechanism of oxygen reduction reaction (ORR)was also studied. The major research results are as follows:(1)A novel route, microwave assisted sol-gel method (MWSG), was brought forward to synthesis LSCF naopowders; the cathode were also tried to fabricated by microwave.Microwave assisted sol-gel route was used for preparation the novel materials. By use of EDAX, X-ray diffraction and FT-IR at different temperatures, SEM and TEM, the properties and synthesized process were studied. The reaction process could be divided into some steps: at the beginning, (a)LSCF phase formation from nitrates, (b)decomposition of reactants into amorphous powders except for strontium nitrate, (c)formation of strontium carbonate from strontium nitrate and carbon bioxides from the decomposition of PVA, and at the medium and terminal stage, (d)LSCF phase formation mainly from strontium carbonate with other oxides.The effect of experiment conditions like starting concentration of mixed solution,PVA, microwave treated time and power was systematically studied by orthogonal experiment. LSCF nanopowder could be obtained at 700W for 35 min with a 38.9 m2/g for the BET surface areas and ~23 nm for the grain size. The synthesis period of 20 h usually observed for conventional heating mode is reduced to a few minutes. Thus, the MWSG method is proved to be a novel, extremely facile, time-saving and energy-efficient route to synthesize LSCF powders.(2)The properties of pure electrode, composite electrode and functionally graded electrode were studied and compared one another by polarization curves, electrochemical impedance spectroscopy.Results shows: the LSCF-GDC composite cathode is highest with a polarization resistance (Rp)of 0.03～0.06??cm2 at 800℃, followed the FGC with a Rp of 0.061～0.085??cm2, the lowest is pure cathode with a Rp of 0.13??cm2.The Rp for the LSM and LSCF cathodes at various temperatures is compared. The Rp for LSCF cathodes is as 10% lower as that of the LSM cathode. Mixed with GDC, the Rp for the LSCF composite cathode decreased to 20%~50% compared to pure LSCF cathode; for the LSM composite cathode, the Rp change to the 10% compared to pure LSCF cathode. The difference in cathode performance may be explained by the change of the varies of the TPB(three phase boundary). According to Adler et al. studies, the reactions can occur at internal surfaces away from the electrolyte for mixed conductor cathode and electrode reaction zone can extend up to 10μm from the electrolyte.The apparent activation energies for the corresponding cathode are 1.53eV(pure LSCF cathode ) ,1.54eV ( LSCF-GDC20 ) ,1.47eV ( LSCF-GDC40 ) , 1.53eV(LSCF-GDC60), 1.49eV(LSCF-GDC80)respectively. That is close to the results reported by P.Murray.The exchange current density for the corresponding cathode at 800℃are 69.14 mA/cm2 (pure LSCF cathode), 147.66 mA/cm2 (LSCF-GDC20), 77.90 mA/cm2 (LSCF-GDC40), 57.19mA/cm2 (LSCF-GDC60), 28.79mA/cm2 (LSCF-GDC80)respectively. Reslts also show that the exchange current density increase with the temperature increase. The exchange current density increase to 1~3 times When the temperature increase 10 K. The rule is similar to the Vant Hoff`s approximately law.(3)The effects of the GDC on the properties are studied. With the increase of the GDC content, the Rp firstly increases and then decreases. When added 20～40% GDC in the LSCF cathode, the Rp has a lowest value. The exchange current densities also has a highest value when added 20% GDC in LSCF composite cathode.(4)The effects of the DC bias on the properties are also studied from experiments and theory. The DC bias are in linear with the ln(Rp)under a large DC bias value, while at small value, the DC bias has no influence on the Rp. At the same time, the apparent activation energies decrease with the increase of the DC bias. So, the DC bias affects electric field of the double layer at electrode-electrolyte interfaces, and then affects the activation energy. That is, the DC bias can only affects the capacitance resistances, but have no effects on the ohm resistances.(5)The symmetry factors are near to 0.5, and the charge transfer numbers of the rate-determining step (RDS)are 2 to all cathodes. These phenomena can only be explained by the existence of intermediate species and multi-step reactions, and it is narrow down to the possible mechanisms of the oxygen reduction reaction to three mechanisms: (1)the direct exchange of oxygen vacancies at electrode-electrolyte interface;(2)reduction of intermediately adsorbed oxygen;(3)the adsorption and desorption step of oxygen. The mathematics models are used to explain the mechanisms. Compared to the experiment results, it is found that: the polarization curves simulated by the adsorption and desorption step of oxygen are in agreement with the experimental results at low overpotentials and a little lower at high overpotentials. so we can draw an conclusion that the ORR RDS may be the adsorption and desorption step of oxygen at the LSCF and LSCF-GDC cathode.