Unraveling Cathode Reaction Mechanisms In Solid Oxide Fuel Cells:a First-principles Study

Solid oxide fuel cells(SOFCs) have garnered intensive attention as one of the most promising technologies for clean and efficient energy conversion, which have currently been playing a critical role in cracking the pressing issues, such as enviromental problems and energy shortage. In experiment, great performance loss of the fuel cells mainifests due to the large electrode polarization, especially the cathode polarization when lowering the temperature. Therefore, based on the first-princples calculations, we attempt to figure out the reaction mechanism on the microscopic level, bringing deep understanding of the experimental phenomena and designing novel and efficient cathode materials.To begin with, we summarized current research on the cathode reaction mechanisms. Two sorts of reactions on cathode of mixed oxygen ion and electron conductiong cathode and mixed proton and electron conducting cathode are simply introduced. Then, we reviewed these two kinds of cathode reactions from experimental and theoretical perspectives, respectively. Finally, the research goals and objectives were outlined.In chapter Two, the adsorption, dissociation, incorporation and successive diffusion of oxygen species on the La1-xSrxCo1-yFeyO3-δ(LSCF) cathode were systematically explored on the basis of the density functional theory calculations. O2molecule prefers to be adsorbed on the transition metal atoms at the B site(Co, Fe) than those at the A site(La, Sr). Superoxide(O-) or peroxide(02-2) species may be formed when O2adsorbed on the surface transition metal atoms with the isomerisation energy barrier less than0.14eV. Without surface oxygen vacancy, the oxygen dissociation on the Co site needs to overcome an energy barrier of0.30eV, while that on the Fe site is about0.14eV. Existence of surface oxygen vacancy can ameliorate the oxygen dissociation on the B-site atom, and no energy barrier is detected during the dissociation process. The smallest oxygen vacancy formation energy is determined on SrCo-terminated surface. Our calculations also indicate that the energy barrier of oxygen surface migration over LSCF is much higher than that in the bulk when containing oxygen vacancies. Moreover, increasing the concentration of Co will effectively lower the oxygen vacancy formation energy, implying an enhanced concentration of oxygen vacancy and therefore improving the oxygen bulk transport. Our study presents a comprehensive understanding of the mechanism of oxygen reduction and migration on the LSCF cathode.In the following chapter, mixed protonic/electronic conductor (MPEC) of BaZro.75Coo.2503(BZCO) was selected as a potential cathode for p-SOFCs, and its bulk proton transporting and oxygen reduction behaviours at the microscopic level were investigated using the first-principles approach. Two plausible proton migration pathways in BZCO were examined and the calculated highest proton migration barrier is0.63eV, which agrees remarkably well with experimental findings. Compared with the weak adsorption of oxygen on BaZrO3(100) surface, the BZCO(100) surface provides a relative large adsorption energy of-0.64eV, indicating that the Co doping enhances the oxygen adsorption on the surface. Furthermore, oxygen reduction reaction over the MPEC cathode surface was explored using a hydrogenated BZCO(100) surface model, where4protons are located to react with one O2molecule to generate two water molecules. For the formation and desorption of the first water molecule on the BZCO surface, four possible reaction pathways have been mapped out. The potential energy profiles indicate that the reaction with two protons simultaneously migrating to the adsorbed oxygen molecule to break the O-O bond (path-4) is the most feasible process for the formation of the first water molecule. Our study for the first time presents an atomistic level understanding of oxygen reduction and proton migration over or in the MPEC cathode.Finally, the main accomplishements that this disertation achieved are summarized. Meanwhile, potential research interests for the future work are pointed out as well.