Atomistic Simulation of Interfaces: Proton transport across BaZrO3 grain boundaries

Licentiate thesis, 2013

Due to the negative environmental effects of fossil fuels it is necessary to develop technology that may reduce or eliminate the need for oil and coal. Fuel cells are highly important in this context as they provide an efficient way of converting chemical energy into electrical energy. However, the development is hampered by a lack of electrolyte materials able to function at temperatures high enough to enable use of hydrocarbon fuels, yet low enough to avoid the wear on component materials caused by high operating temperatures. Solid oxide proton conductors are found to have several of the characteristics of a good electrolyte material in this temperature range, but increasing the conductivity to the level needed in practical applications remains a challenge. The aim of this thesis is to elucidate microscale phenomena that affect the performance of proton-conducting oxides. The material under investigation is BaZrO3, which is regarded as a promising electrolyte material due to its chemical stability and high grain interior conductivity. However, the grain boundaries in the material are highly resistive and lower the total conductivity. The cause of this high grain boundary resistivity has been investigated using atomistic simulations and thermodynamic modelling. Particular attention is devoted to the role of defect segregregation to the grain boundaries. From atomistic simulations it has been found that positively charged defects such as oxygen vacancies and protons segregate to the grain boundaires of BaZrO3. The accumulation of positive charge in the grain boundaries creates a potential barrier and leads to depletion of positive mobile defects from the surrounding region, impeding transport across the boundary. Thermodynamic models have been used to determine the height of the potential barrier resulting from segregation of positive defects, and the results compare well with experimental findings.