Platinum microelectrodes on gadolinia doped ceria single crystals - bulk properties and electrode kinetics

To better understand the electrode kinetics of oxygen reduction and oxidation of gadolinia doped ceria (GDC), the electrochemical properties of platinum electrodes on GDC single crystals and polycrystalline samples were investigated with geometrically well-defined microelectrodes. For comparison measurements were also performed on polycrystalline samples using platinum interdigital electrodes in order to access the effect of the electrode geometry on the electrochemical properties. The transport properties were characterised using impedance spectroscopy, allowing to separate the transport processes of the electrode and the electrolyte. Evaluation of the temperature dependence shows activation energies of 0.77 eV for bulk transport and 1.03 eV for the electrode exchange. Oxygen partial pressure dependent measurements in a reducing atmosphere reveal a strong increase in activation energy due to electronic defect formation. A distinct chemical capacitance is observed in the electrode impedance for all sample types independent of the electrode geometry. While this chemical capacitance is only visible in the electrolyte contribution for the samples measured with interdigital electrodes, for the samples investigated with microelectrodes no chemical capacitance is observed in the electrolyte contribution of the impedance. As the chemical capacitance is related to stoichiometry changes in the electrolyte materials, the results confirm the non-uniform potential distribution occurring at a microelectrode, which results in a vanishing lateral potential gradient and therefore in a negligible stoichiometry gradient inside the electrolyte at a distance from the microelectrode.