Electrochemical Strain Microscopy of Doped Ceria at Elevated Temperatures

Solid oxide fuel cells have emerged as a promising electrochemical conversion technology due to its high efficiency, stability, and low cost. A major component of these fuel cells is its electrolyte, a solid ion-conducting ceramic, which must be ionically conductive but electrically insulating. Nanocrystalline doped ceria has been explored as an electrolyte that can operate at lower temperatures, but the nature of the increased ionic conductivity is still under investigation. In this paper, electrochemical strain microscopy, a nanoscale probing technique in which an AC bias is applied to the surface of the electrolyte material, is used to evaluate induced strains on samarium doped ceria, and the corresponding electrochemistry on the surface is evaluated. Due to the typically high operating temperatures of solid oxide fuel cells, electrochemical strain microscopy was performed at varying temperatures, up to 250°C. We find that the induced strain increases proportionally with increased temperature, particularly at grain boundaries due to the space-charge effect. Furthermore, large DC biases were applied to the electrolyte at individual points, and the resulting strain relaxation after removal of the bias was measured at varying temperatures. We find here that the temperature and other environmental conditions can lead to dramatic differences in the relaxation behavior. Furthermore, the relaxation behavior along grain boundaries is also significantly different compared to the behavior on the bulk portion of the grain. Determination of the mechanisms behind such changes can lead to increased fundamental understanding of the chemical expansion behavior of ceramic electrolytes.