Role of Lattice Strain vs. Solid Solution Doping on Atomistic near Order and Oxygen Ionic Transport for Ceria-based Micro-Energy Conversion Membranes

Oxygen ion conducting membranes are the key for micro energy conversion devices, ie. electrolyte of micro solid oxide fuel cell (µSOFC). In particular, gadolinium-doped ceria thin films are currently intensively studied, as one of the most promising electrolyte candidates due to their good ionic conductivity at intermediate temperatures. For better electrochemical conversion efficiency, research is focused on the gaining fundamental new insights on electro-chemo-mechanic interactions of those thin films. In this sense, several studies have reported the study of the electro-mechanic link through doping in pellet or substrate-supported thin films. However, despite these investigations knowledge on near order-ionic transport-strain interaction for self-supported energy conversion membranes is to be pursued.

In this work, gadolinium-doped ceria thin film electrolytes are selected as model case. Doped ceria membranes were fabricated by Pulsed Laser Deposition and the dopant concentration was varied from 0 to 20 mol%. Strain was studied by both membrane buckling analysis and near order oxygen anionic-cationic bond vibrations via Raman spectroscopy. Electrochemical characterization was performed by directly depositing Pt microelectrodes on the free-standing membranes (Figure.1) and substrate-supported thin films (for comparison).

The compressively strained free-standing membranes reveal a generally increased activation energy of ~0.13 eV of ionic conductivity in-plane compressive strain when compared to the self-supported and flat films around typical bulk values of 0.83 eV. Interestingly, variation of the gadolinia doping concentration does not result in a significant variation of the activation energy of ionic transport for the ceria-based free-standing membranes. Viz, the strain alteration between free-standing membranes and the supported films affects the overall ionic transport activation energy more than the dopant concentration for the investigated range. We observe, that the net strain between electrodes on the free-standing membranes reveal an increase of compressive strain increased from 1.49% to 2.63% with the dopant concentration in ceria thin films ranging from 0 to 20 mol%.  Measuring via micro-Raman spectroscopy the cation-oxygen anion near order clear trends in shifts of the F_2g cationic-oxygen anionic vibration modes are measurable with respect to strain state: Firstly, we observe that all free-standing membranes reveal an increased compressive in-plane strain, when compared to the substrate supported and flat films. The atomistic near order structural trends match the strain change in the macro scale observed by buckling analysis. Secondly, the role of doping concentration of gradolinia on the near order structural changes with and without compressive lattice strain are discussed.

In conclusion, through this work we give first hands-on model experiments on the "electro-chemo-mechanics" in real energy conversion micro-devices exemplified on micro-solid oxide fuel cells. The competing role of doping strategies vs. mechanical stress management on ionic transport membranes are discussed for future designs of micro-energy conversion and storage devices.