Point defects and ion conduction in solid oxides: a first-principles case study of La2Zr2O7

Licentiate thesis, 2010

In the endeavor to attain a clean environment and sustainable energy consumption, the notion of a future hydrogen economy stands out as one of the grandest visions. Striving for this vision, a critical task lies in optimizing the performance of the fuel-cell devices responsible for extracting electric power from the energy stored in hydrogen molecules. Of particular interest in several applications are solid oxide fuel cells, in which a ceramic electrolyte material is used. In this class of electrolytes, enhancing the conductivity of protons and/or oxygen ions represents the greatest challenge. The aim of the project culminating in the present thesis was to elucidate some atomic-scale mechanisms which are important for the understanding and enhancement of protonic and oxygen-ionic conductivity in solid oxide electrolytes. More specifically, properties directly pertained to the mobility and concentration of protons in the pyrochlore-structured oxide La2Zr2O7 have been investigated by means of first-principles atomistic methods based on density-functional theory. Of particular interest has been the migration of protons and the effect of acceptor doping on the equilibrium concentration of oxygen vacancies. The latter is known to have direct implications on the concentration of protons via the hydration reaction. As an outcome of the calculations it has been concluded that under typical synthesis conditions an increased equilibrium concentration of oxygen vacancies in La2Zr2O7 can, via a charge-compensating mechanism, indeed be achieved by acceptor doping the material. More expressly the vacancy concentration can be significantly affected by dopant species, which implicates the possibility of optimizing proton concentration in the material by a careful choice of dopant. Furthermore, the energetically preferred proton positions in the material has been pinpointed, along with a continuous migration pathway which enables long-range proton transport. Finally, the choice of dopant species has been shown to affect the mobility of both protons and oxygen vacancies via a trapping effect due to pair interaction with the dopant.