Analysis of the grain boundary conductivity of singly and doubly doped CeO₂ thin films at elevated temperature

Acceptor-doped cerium oxides are a promising solid electrolyte material for solid oxide fuel cells (SOFCs) due to their exceptionally high oxygen ion conductivity. Typical solid electrolytes are polycrystalline with a number of grain boundaries, and it is common for grain boundaries to block the motion of mobile oxygen ions as they migrate from one grain to the next. Accordingly, it is important precisely to measure the grain boundary conductivities of doped cerium oxides, particularly at the temperature of relevance (>500 °C). Here, we undertake a quantitative analysis of the transport properties of singly and doubly doped cerium oxides. So that we may precisely extract the grain boundary contribution out of the total conductivity at an elevated temperature (>500 °C), both epitaxial and polycrystalline thin films of the oxides are grown via pulsed layer deposition (PLD) on two different insulating, single-crystal substrates: Al₂O₃(0001) and SiO₂(001), and these are then characterized by a range of analysis tools, in this case TEM, SEM, XRD, ICP-MS and AFM. These samples with controlled microstructures, in combination with in-plane ionic conductivity measurements by impedance spectroscopy as a function of the thickness, temperature and pO₂, enable us precisely to measure the grain boundary conductivity of the doped ceria at temperatures ranging from 620 to 700 °C. The impact of the number and type of dopants on the transport properties are also explored.