Solid Oxide Electrochemical Cells (SOCs) may become key devices either in fuel or electrolysis mode in the future sustainable energy system. Nanoscale particles or fibres constitute the functional materials in the SOC electrodes that are responsible for the electrochemical energy conversion, and optimizing the performance and the lifetime of SOCs implies understanding the nanoscale morphology and the ion and electron conduction in the nanostructured electrode materials. Ion and electron conduction in granular materials is closely related to the grain boundary properties and as the dimensions approach the nanoscale regime, the grain boundary properties become the major limiting factor for conduction.
Often nanoscale gadolinium-doped ceria (CGO) is applied in SOC electrodes and many studies have shown segregation influencing the conduction properties. Further, it is expected that the crystalline mis-orientation between neighbouring grains will affect the conduction properties. For instance, a recent study [1] has suggested a linear relation between the crystalline mis-orientation angle and the grain boundary doping concentration.

We show here for the first time a study combining 3D Orientation Mapping in the Transmission Electron Microscope (3D-OMiTEM) [2] with Energy Filtered Transmission Electron Microscopy (EFTEM) performed to map crystal orientation and doping concentration with nm-precision to predict 3D-ion percolation paths in state-of-the-art electrospun CGO (Ce_0.9Gd_0.1O_1.95) nanofibres. These CGO nanofibers have shown potential as efficient and cost effective electrode material for SOCs.
The CGO nanofibers are electrospun with approximate diameter of 200 nm and they are polycrystalline, containing grains of mean size 30 nm. Various samples have been calcined under different conditions resulting in different conduction properties. We have imaged the nanofibers by 3D-OMiTEM and EFTEM and thereby analysed the segregation, i.e. change in gadolinium doping, throughout the individual CGO nano grains, and the crystalline mis-orientation among neighbouring nano grains.

These orientation maps contained full reconstructed information from the interior of nanofibers of 200 nm diameter and containing 400-700 grains per fibre showing that this method can be used for statistical analysis in combination with the acquired local crystalline and compositional information.

[1] Bowman, W. J.; Darbal, A.; Crozier, P. A. Linking Macroscopic and Nanoscopic Ionic Conductivity: A Semiempirical Framework for Characterizing Grain Boundary Conductivity in Polycrystalline Ceramics. ACS Applied Materials and Interfaces 2020, 12, 507–517.

[2] Liu, H. H.; Schmidt, S.; Poulsen, H. F.; Godfrey, A.; Liu, Z. Q.; Sharon, J. A.; Huang, X. Three-Dimensional Orientation Mapping in the Transmission Electron Microscope. Science 2011, 332, 833–834.