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