Proton conductivity of lanthanum and barium zirconate: Microscale aspects on first-principles basis

Doctoral thesis, 2012

Fuel cells are devices which convert chemical energy into electrical energy cleanly and efficiently. Development of fuel cells compatible with hydrocarbon fuels would make more efficient use of present fossil and renewable fuels, and also enable progress towards a future hydrogen economy. One of the major hindrances to commercially viable fuel-cell technologies is the lack of materials with properties appropriate for devices operating at temperatures high enough to allow carbon-containing fuels, while low enough to suppress the negative side effects of high temperatures. Part in a direct route towards intermediate temperature fuel cell technologies is the optimization of the proton conductivity of solid, ceramic, oxide materials for use as electrolytes in so-called solid oxide fuel cells. This thesis puts forward theoretical investigations into atomic and microscopic mechanisms which directly influence the proton conductivity of solid oxide materials posing as candidates for proton-conducting electrolyte materials. The foundation of this work is the description of the atomic and electronic structure of materials offered by methods based on density-functional theory. Combined with thermodynamic and electrostatic theory, the pressing issue of grain boundary resistivity in the otherwise promising proton-conducting solid oxide material barium zirconate (BaZrO3), is addressed. Furthermore, fundamental aspects related to the optimization of proton conductivity by means of acceptor-doping are examined in the not as frequently studied material lanthanum zirconate (La2Zr2O7). Acceptor-doping is intended to increase proton concentration by causing vacant oxygen positions, which, by incorporation of water molecules, can be filled with hydroxide ions. The most important work and results can be summarized as follows: (i) By examining several different dopant species in La2Zr2O7 it is shown that a poor choice of dopant can not only lead to inefficient concentration improvement but also to trapping of both protons and oxygen vacancies. In consistency with experimental observations, Ca and Sr are pointed to as the most promising dopants out of the twelve investigated species. (ii) By calculating the energy of oxygen vacancies in the vicinity of different grain boundary structures of BaZrO3 it is demonstrated that accumulation of oxygen vacancies at the core of the grain boundary interfaces can significantly hamper the effective proton conductivity in the material. This accumulation leads to charged grain boundary cores and gives rise to a depletion of protons in the surrounding region. The magnitude of the effect corresponds well with experimental conductivity data.