Investigation of Oxygen Vacancies in Sputtered GDC Thin Films Probed via Operando XAS

Abstract

In previous works, we reported that Solid Oxide Fuel Cells (SOFCs), having a room-temperature sputtered and then annealed GDC thin film as the cathode/electrolyte barrier layer, had showed a huge increase of the output current (up to +78%) and a decrease of the ohmic resistance (up to -42%) as compared to fully screen-printed industrial SOFCs. We correlated the performance improvement to grain size in the GDC layer as a function of annealing temperature. However, no information on the density and activity of oxygen vacancies in the thin film to correlate with functionality could be extrapolated in these studies.
Element and valence sensitive probes such as X-Ray Photoelectron Spectroscopy (XPS) and X-Ray Absorption Spectroscopy (XAS) enable atomic level characterization of nanostructured granular GDC layers deposited on polycrystalline anode/electrolyte bilayer substrates and the interplay between morphology and stoichiometry in determining the Ce³⁺/Ce⁴⁺ ratio, which in turns regulates their ionic and electronic conductivity.
Here we show the results obtained on three room-temperature RF-sputtered GDC thin films, annealed with the same annealing ramp but at different plateaux temperatures, making use of XPS measurements to study the unreacted surface and the operando XAS to monitor the changes in Ce³⁺/Ce⁴⁺ ratio in different reactive atmospheres (i.e. neutral, oxidizing and reducing). The latter were carried out using the ambient pressure cell available at APE-HE beamline (Elettra synchrotron in Trieste, Italy). Our measurements allowed to determine the role of the annealing parameters in the number of available oxygen vacancies available in the oxygen reduction reaction (ORR), highlighting the different modifications induced in the investigated samples by the annealing process.