Temperature gradients in microelectrode measurements: Relevance and solutions for studies of SOFC electrode materials

a r t i c l e i n f o Microelectrodes on solid electrolytes, contacted by current-collecting tips, have become increasingly important for analyzing electrochemical properties of electrode materials in solid state ionics. Samples are usually asymmetrically heated from the bottom side. It is shown experimentally, e.g. by thermovoltage measurements, impedance spectroscopy and infrared camera pictures, as well as by finite element modeling that substantial temperature gradients may arise in these experiments. Consequences in terms of data analysis are discussed and ways to alleviate the problem are described. For a complete avoidance of the temperature inhomogeneity problem, a novel symmetrically heated micro-contact setup was built. Electrochemical impedance spectroscopy (EIS) is a key method in solid state electrochemistry and frequently applied not only to separate electrode and electrolyte impedance contributions but also to obtain mechanistic details of electrochemical reactions, for example in solid oxide fuel cells (SOFCs), solid oxide electrolysis cells (SOECs), and gas sensors. Electrodes optimized in terms of electrochemical performance are usually porous. However, since electrode kinetics also depends on geometrical parameters such as particle size, porosity, exact morphology and phase distribution (in composite electrodes), measurements on porous electrodes are often difficult to interpret in terms of reaction pathways and rate limiting steps. Progress has recently been made by 3D reconstruction studies to clarify geometrical details of composite electrodes [1–6] but efforts to obtain such details of porous electrodes are considerable. For an effective comparison of different materials and mechanistic studies, model electrodes were therefore successfully introduced. Two types of model electrodes are employed: macroscopi-cally sized dense thin film electrodes and microelectrodes, which themselves may or may not consist of dense thin films. Thin film electrodes can be deposited with defined structure on single crystalline electrolyte substrates. Often symmetrical samples with two identical electrodes are prepared, which can be investigated in a homogeneously heated furnace without problems caused by temperature gradients. However, this approach is not very flexible in terms of electrode geometry, and analysis of three phase boundary effects requires additional micro-patterning [7–12]. Moreover, meaningful DC measurements can only be obtained when applying a reversible counter electrode or a three electrode arrangement. The latter is highly non-trivial in solid state electrochemistry due to difficulties in positioning the reference electrode [13,14]. Also microelectrodes are commonly used in the field of solid state electrochemistry and a main advantage is simply the fact that minimizing the size maximizes the polarization …