Measuring Diffusion Coefficients of the Ceria Phase in a Dual-Phase Oxygen Permeation Membrane Using a Combined Polarization - Kelvin Probe Force Microscopy Method

Dual-phase membranes with FeCo₂O₄ or its iron-rich pendant Fe₂CoO₄ and Gd-doped ceria as an oxygen ion conductor have already been successfully applied as oxygen permeation membranes with high permeability in the temperature range above 800 °C [1-3]. Apart from Gd-doped ceria, Sm-doped ceria also can be an interesting alternative in this kind of composite due to its high ionic conductivity [4].

As especially ceria or ceria-based composites have gained more and more interest for low-temperature applications, e.g. as catalyst materials, current research efforts are aiming to improve the composition and microstructure of such dual-phase membranes for application in a membrane reactor at considerably lower temperatures (below 600 °C) to perform partial oxidation reactions. For temperatures between 600-1200 °C the defect chemistry of these materials is well established, but experimental data for low temperature charge transport are limited and not fully understood, yet.

Kelvin Probe Force Microscopy (KPFM) is an Atomic Force Microscopy (AFM)-based measurement method which can measure the local surface potential (or from the physics view: the Volta potential) of the sample [5, 6]. The surface potential is a measure for local changes of the defect chemistry, as it is directly related to the local Fermi niveau [7]. The implications of local oxidation and reduction at low temperatures for charge carrier mobility in pure ceria, and especially in dual-phase materials, are not very well understood so far, but KPFM can be used to measure the surface potential difference directly at the polarized contact area as well as to map the time-dependent relaxation process of the introduced gradient, also giving information about the local surface potential distribution, lateral extent of the gradient, variations of the shape of the gradient etc. [8, 9].

In the present study, single ceria grains in a dual-phase Ce_0.8Sm_0.2O_2-δ + FeCo₂O₄ oxygen permeation membrane were polarized at room temperature in ambient air using a Pt-coated AFM tip and a large silver paste back contact. Subsequently, the introduced defect concentration gradient (which is visible as a gradient of the surface potential) and the relaxation process over time was monitored by KPFM.

By comparing the measurement results to previously acquired data of single phase ceria samples with different doping concentrations and crystallinity, we were able to show that by this technique, chemical diffusion coefficients of a single phase in a composite material are accessible.

Acknowledgements:

The work was funded by the German Research Foundation - project #387282673.

Literature:

1. Ramasamy, M.; Baumann, S.; Palisaitis, J.; Schulze‐Küppers, F.; Balaguer, M.; Kim, D.; Meulenberg, W. A.; Mayer, J.; Bhave, R.; Guillon, O. J Amer Ceram Soc 2016, 99 (1), 349-355.

2. Ramasamy, M.; Persoon, E. S.; Baumann, S.; Schroeder, M.; Schulze-Küppers, F.; Görtz, D.; Bhave, R.; Bram, M.; Meulenberg, W. A. Journal of membrane science 2017, 544, 278-286.

3. Lin, Y.; Fang, S.; Su, D.; Brinkman, K. S.; Chen, F. Nature Comm 2015, 6 (1), 1-9.

4. Mori, T.; Drennan, J.; Lee, J.-H.; Li, J.-G.; Ikegami, T. Solid State Ionics 2002, 154-155, 461-466.

5. Örnek, C.; Leygraf, C.; Pan, J. Corr Eng Sci Tech 2019, 54 (3), 185-198.

6. Melitz, W.; Shen, J.; Kummel, A. C.; Lee, S. Surf Sci Rep 2011, 66 (1), 1-27.

7. Zhang, Z.; Yates, J. T. Chem Rev 2012, 112 (10), 5520-5551.

8. Neuhaus, K.; Schulze-Küppers, F.; Baumann, S.; Ulbrich, G.; Lerch, M.; Wiemhöfer, H.-D. Solid State Ionics 2016, 288, 325-330.

9. Lee, W.; Lee, M.; Kim, Y.-B.; Prinz, F. B. Nanotechnology 2009, 20 (44), 445706.