Characterization of La₂Ce₂O₇ and Y₂Zr₂O₇ using polarization effects via Kelvin Probe Force Microscopy

P. Mowe¹, F. Pfeiffer¹, M. Winter^1,2, K. Neuhaus¹

¹ Forschungszentrum Jülich GmbH, Institute of Energy and Climate Research IEK-12: Helmholtz Institute Münster (HI MS), Corrensstr. 46, 48149 Münster, Germany

² University of Münster, MEET Battery Research Center, Institute of Physical Chemistry, Corrensstr. 46, 48149 Münster, Germany

Typically, fuel cells with high power density demand ion conductors with high ionic conductivity. However, materials with lower conductivities should not be ignored in research due to potential applications in Si-integrated devices such as electro-chemo-X devices (EC-X) or on-chip batteries and micro-fuel cells. These devices are intended to operate in a temperature range between 200 °C and room temperature. Promising proton conductors for these applications such as highly doped ceria have to be chemically inert with respect to Si and suitable for thin-film preparation [1].

During the last two decades, there have been a number of publications on noticeable high temperature proton conductivity combined with an oxygen ion conductivity in materials with the general formula A₂³⁺B₂⁴⁺O₇. However, depending on the ratio of the ionic radii of the cations and the sintering temperature, these materials may adopt different fluorite-derived structures, showing either a defective fluorite or pyrochlore structure or a partially disordered state with a mixture of fluorite and pyrochlore structure [2], which has a strong effect on the ionic transport properties. In addition, there are only limited information on proton conductivity of these materials below 400 °C.

In this study, ceramic pellets with the composition La₂Ce₂O₇ and Y₂Zr₂O₇ were prepared via the Pechini method. The received pellets were characterized by XRD, Raman spectroscopy and SEM for purity and (micro)structural information. However, investigation of electrical properties using classical electrochemical methods like impedance spectroscopy (EIS) at room temperature reaches its limits for the investigated ceramics. To enable measurements at room temperature even for materials with extremely low electrical conductivity and with a focus on the surface-near transport properties, a combination of Kelvin Probe Force Microscopy (KPFM) and local polarization-relaxation measurements have been used in this study [3]. With these techniques, the local contact potential difference between a Pt-coated AFM tip and the sample surface can be measured with very high lateral resolution [1].

Using KPFM-based polarization-relaxation measurements, diffusion coefficients were successfully determined in dry and wet environment by applying positive or negative polarization voltages. Additionally, KPFM measurements were used to analyze the local Volta potential distribution at the grain boundaries, indicating a strongly different behavior of the grain boundary potential barrier: By mapping the local surface potential in combination with topography information with an average calculated from 50-80 measurements per composition, a positive Volta potential difference between grain cores and the grain boundaries of 43 mV for La₂Ce₂O₇ could be determined, while Y₂Zr₂O₇ showed a negative grain boundaries potential barrier with a potential difference of -280 mV. The results are discussed in terms of a defect model.