Modification Of Solid Oxide Fuel Cells Cathode La_1-xSr_xCo_1-yFe_yO_3-δ:a First-principles Study

Solid oxide fuel cells (SOFCs), which can directly converting chemical fuels into electricity, have been considered as one of the most promising new energy technologies in the21st century, due to its many advantages over the traditional engines, such as high energy conversion efficiency, zero emission of pollutants and etc. Unfortunately, the high operating temperatures of SOFCs greatly hinder their commercial applications. And lowering the operating temperatures down to intermediate temperatures (500-700℃) will unavoidably bring another problem that the catalytic activity and conductivity of SOFCs’electrode will be greatly depressed at the lowered temperature, which will lead to a big output loss. Hence, exploring new materials in order to get a high electrode performance at lower temperatures has become a big challenge to researchers.As the site for oxygen reduction, cathode can largely impact the electrochemical performance of SOFCs. Many researchers have put their heart on investigating new IT-SOFCs cathode materials. Recently, La1-xSrxCo1-yFeyO3-δ(LSCF) was reported to have higher catalytic activity and better oxygen ion conductivity than the traditional LSM cathode at low temperatures. Nevertheless, LSCF still can’t meet the commercial requirement especially at the intermediate temperatures. Typically, the cathode performance of LSCF can be intensively promoted through infiltration of noble metals and many other high electrical conductivity materials. Unfortunately, the enhancement mechanisms are still not clearly clarified. In this thesis, we have studied oxygen reduction on the Pt infiltrated LSCF surface based on the density functional theory, attempting to declare the cathode reaction mechanisms and to provide strategies for the modification of LSCF cathode.Chapter1:we briefly summarized the development of fuel cells. Firstly, we reviewed the SOFCs’typical electrolyte and electrode materials. And then we gave detailed introduction of cathode reaction mechanisms from both the experiment reports and the theoretical investigations, especially the oxygen reduction process on the LSM and LSCF cathode. Finally, we gave the theme of this thesis.Chapter2:we studied in details the oxygen reduction and migration processes on the Pt/LSCF surface, and the results can be summarized as follows:(1)The oxygen adsorption on the perfect LSCF (100) surface is not strong with an adsorption energy of only-0.30eV, and the dissociation of the adsorbed oxygen have to overcome a reaction barrier as high as1.42eV. These results suggest that the oxygen adsorption and dissociation on the perfect LSCF(100) surface is not active. While on the defect LSCF surface, the oxygen prefer to adsorb at the vicinity of the vacancy with an adsorption energy of-0.93eV, and especially, the dissociation energy barrier of the adsorbed oxygen is only0.57eV which is about1eV lower than that on the perfect surface, suggesting much better catalytic activity.(2)On the Pt/LSCF (100) surface, the adsorption energy of oxygen at the Pt/LSCF interfacial region is-1.16eV which is about4times stronger than that on the perfect surface. Besides, the oxygen dissociation proceeds with an energy barrier about0.38eV, which is only25%of that on the perfect surface. The whole adsorption and dissociation processes release0.83eV energy to the environment.(3) Interestingly, the oxygen migration along the Pt cluster is very difficult as it should overcome an overall2.58eV energy barrier. While the oxygen migration on the defect LSCF surface is rather fast which only need to overcome a small energy barrier about0.10eV. Based on all the results above, we concluded that Pt only promotes oxygen adsorption and dissociation while has little effect on its migration.(4)Finally, through the electricity conductivity relaxation experiment, we found much enhanced surface exchange reaction kinetics of Pt/LSCF sample which is about8times faster than that of the pure LSCF sample. And the chemical reaction coefficient of Pt/LSCF is about4.04×10-4cms-1which is about one magnitude higher than that of the pure LSCF. This experiment result agrees very well with our theoretical predictions.Chapter3:we summarized the main points of this dissertation. Meanwhile, we provided the potentials of this work for the future research.