Ex-Situ 3D Nano-Ptychography of Ni-YSZ SOFC Anode during Redox Cycling

Abstract

Solid oxide cells (SOC) are used for efficient and cost-effective conversion of a wide variety of fuels (SOFC) for power generation or for energy storage via electrolysis (SOEC). However, SOC lifetime under harsh operating conditions is limited by electrode microstructure durability. In particular, the state of the art anode electrodes are composed of a porous cermet of nickel and yttria stabilized zirconia (YSZ). Electrochemical reactions occur where the ion and electron conducting phases meet the pores at the triple phase boundaries (TPB). Thus, the electrochemical activity of the electrode is strongly dependent on the density of TPBs and the quality of the connecting pathways through each phase1. Despite nickel remaining stable under reducing environments, nickel oxidation can occur in the electrode due to air leakage in the fuel system or by mechanical failure of the electrolyte. During operation, redox cycles can impact the lifetime of the electrode thus tracking their effect on the microstructure in 3D provides valuable information. 3D characterization of a Ni-YSZ electrode was first achieved in 20062 using focused ion beam scanning electron microscope (FIB-SEM) nanotomography and it has been routinely performed for the last decade followed by the development of near edge X-ray nano-tomography3 and most recently holotomography4. Near edge X-ray nano-tomography and FIB-SEM has been used to study the effects of oxidation on Ni-YSZ electrodes3 X-ray Ptychography is a scanning based technique in which a set of several local far-field scattering patterns is obtained by scanning the sample with a coherent micrometer-size X-ray beam. When used in tomography mode, ptychography provides a quantitative measurement of the local electron density of materials in 3D. Over the years, ptychography has demonstrated high sensitivity to mass density changes and the highest resolution among the X-ray imaging techniques5. As a proof of concept, we report the first results of ptycho-nanotomography applied to an “ex-situ” redox experiment performed at the cSAXS beamline at the Swiss Light Source (PSI). In the experiment the same Ni-YSZ anode microstructure is first analyzed in its pristine state, then oxidized and finally reduced again at 850°C in air and in 4%H2-96%N2 respectively. A specimen containing both Ni-YSZ electrode and YSZ electrolyte was created using a high precision mechanical polishing procedure followed by FIB ion milling. The “ex-situ” treatments were carried out in a small custom made tube furnace between each ptychography scan at the beamline. Datasets typically 20x20x12 µm in size were quantified to have a 3D spatial resolution of 55 nm using the Fourier shell correlation method5. The high mass density sensitivity of ptychography, resulted in high phase contrast between Ni, YSZ, NiO and pore, enabled us to effectively resolve all the phases present in the electrode (figure 1a,b,c). Therefore, an accurate segmentation procedure could be applied minimizing the associated geometrical errors of calculated microstructural parameters. Here we describe the major microstructural changes which can be noticed in the microstructure showing the evolution of the nickel particle size distribution, connectivity and tortuosity in this recent experiment. Moreover, the morphological changes in the YSZ phase are also discussed with emphasis on the observed micro-cracks (figure 1d) and their effect on the overall connectivity.