Abstract
State of the art solid oxide fuel cell (SOFC) anodes are typically Ni based, one of the primary drawbacks of these electrodes is their significant dimensional change upon oxidation. As commercial SOFCs may typically be expected to undergo numerous redox cycles in their operating lifetime, it is important to understand the associated microstructural degradation process. Here we present a methodology for the use of synchrotron based X-ray nano-computed tomography to explore the step-wise oxidation of the Ni phase in a Ni–YSZ composite material. This non-destructive technique demonstrates the potential to track microstructural evolution on a grain-by-grain basis in three dimensions.
Highlights
In state of the art solid oxide fuel cells (SOFCs) the reactions are supported by porous composite electrodes, providing intimate contact of an electronic, ionic and pore phase at triple phase boundary points distributed normal to the electrode–electrolyte interface
Following the highest temperature (700 °C) oxidation step, significant evolution is observed as a result of Ni to NiO transition with observable densification and NiO film growth (Fig. 3). This sequence of 2D dimensional tomograms reveals important information on the processing of the original Ni–YSZ microstructure – as illustrated in Figs. 2 and 3, there is evidence of a NiO core at the centre of large Ni grains, suggesting that in spite of the rigorous reduction conditions employed during processing, full reduction of the NiO phase was not achieved. The authors believe this is because of the high operating temperature employed during the reduction step of electrode processing: at low temperatures the rate of NiO oxidation is dictated by the kinetics of the reaction at the Ni/NiO interface [3]; at higher temperatures, this process becomes distorted by Ni sintering which can restrict the access/egress of gas to the Ni/NiO interface [14]
Volume changes associated with redox cycling of Ni based electrodes for SOFCs can drive significant microstructural degradation
Summary
In state of the art solid oxide fuel cells (SOFCs) the reactions are supported by porous composite electrodes, providing intimate contact of an electronic, ionic and pore phase at triple phase boundary points distributed normal to the electrode–electrolyte interface. Nickel oxidation may occur under conditions of high fuel utilization and/or current density, or the anode may undergo oxidation due to failure of the fuel delivery system, or air leakage Under these conditions the dimensional changes associated with oxidation are not necessarily reversible. The most extensively adopted tomography technique utilizes the milling power of a focused ion beam (FIB) in conjunction with the imaging capabilities of high resolution FE-SEM, to provide a sequence of 2D images that can be effectively recombined in 3D space. Because this technique is destructive, studies of microstructural evolution are influenced by inherent sample variability. We present the application of high resolution, synchrotron nano-CT to study the dynamics of thermal oxidation of the Ni phase in a porous Ni–YSZ composite electrode exposed to high pO2 at elevated temperatures
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