Combining EIS with in Situ TEM in Characterizing Solid Oxide Cell Components

Abstract

Electrochemical Impedance Spectroscopy (EIS) was integrated with Transmission Electron Microscopy (TEM) to enable simultaneous electrochemical and structural characterization of functional materials. In this work, the EIS-TEM method was used to characterize an electrospun nanofiber of Ce0.9Gd0.1O1.95-δ (CGO), a widely used material for solid oxide fuel/electrolysis cells (SOC).EIS is a powerful method for studying the electrochemical behaviour of various materials and systems in different working environments. Solid-state EIS has been a critical tool in determining the electrochemical properties of SOC, including its electronic and ionic conductivity, double layer and chemical capacitance, and reaction rate. However, being exposed to reactive gases and elevated temperatures, SOC components undergo structural and compositional changes during operation which influence their electrochemical properties in a dynamic or permanent manner. It is therefore critical to determine the dynamic changes in structure and composition during operation and how they relate to the electrochemical activity of the cell components. For such a purpose, we developed a method that allows EIS measurements simultaneously with in situ TEM characterization.The CGO nanofiber used in this work was prepared by combining sol-gel synthesis and electrospinning. Using a micromanipulator inside a scanning electron microscope (SEM), a nanofiber was singled out and transferred to a MEMS chip with heater spirals and biasing electrodes. Using a MEMS chip-based heating and biasing holder, 2-probe EIS measurements and TEM characterizations were done simultaneously inside an Environmental TEM (ETEM) while exposed to 2.5 mbar pO2 gas at different temperatures from 28°C to 600°C.The simultaneous acquisition of EIS measurements and TEM images made it possible to determine the nanostructure-dependent conductivity of CGO. A low activation energy (Ea) of 0.44 eV was observed which matches the Ea associated with small polaron hopping in doped and pure ceria [1,2], suggesting dominant electronic conductivity. The approach of using the EIS-TEM method with electrospun nanofibers has also opened an opportunity of studying ceramic materials at very fine grain sizes, which with the CGO nanofiber in this work was ca. 10 nm.