Abstract
Ni-YSZ anodes for Solid Oxide Fuel Cells are vulnerable to microstructural damage during redox cycling leading to a decrease in the electrochemical performance. This study quantifies the microstructural changes as a function of redox cycles at 800 °C and associates it to the deterioration of the mechanical properties and polarisation resistance. A physically-based model is used to estimate the triple-phase boundary (TPB) length from impedance spectra, and satisfactorily matches the TPB length quantified by FIB-SEM tomography: within 20 redox cycles, the TPB density decreases from 4.63 μm−2 to 1.06 μm−2. Although the polarisation resistance increases by an order of magnitude after 20 cycles, after each re-reduction the electrode polarisation improves consistently due to the transient generation of Ni nanoparticles around the TPBs. Nonetheless, the long-term degradation overshadows this transient improvement due to the nickel agglomeration. In addition, FIB-SEM tomography reveals fractures along YSZ grain boundaries, Ni-YSZ detachment and increased porosity in the composite that lead to irreversible mechanical damage: the elastic modulus diminishes from 36.4 GPa to 20.2 GPa and the hardness from 0.40 GPa to 0.15 GPa. These results suggest that microstructural, mechanical and electrochemical properties are strongly interdependent in determining the degradation caused by redox cycling.
Highlights
The solid oxide fuel cell (SOFC) is currently one of the most promising energy conversion devices, offering a high operating efficiency with minimal air pollution
As pointed out by Laurencin et al [28], the characteristic frequency associated to R2 remained relatively constant upon redox cycling, suggesting that the increase in R2 could be due to a mechanical damage along the electrochemical interfaces, that is, to a loss in effective triple phase boundary (TPB) length
The electrochemical and mechanical deterioration of Ni-YSZ anodes after up to 20 redox cycles was quantified by a combination of Focused ion beam (FIB)-SEM tomography, impedance spectroscopy, nanoindentation and modelling
Summary
The solid oxide fuel cell (SOFC) is currently one of the most promising energy conversion devices, offering a high operating efficiency with minimal air pollution. It is commonly agreed that under these conditions the accompanying performance loss is caused by an increase in average Ni particle size and a decrease in active triple phase boundary (TPB) length per unit volume. The size changes of Ni particles were suggested to be due to the agglomeration of metallic nickel [10] and the formation of porosity upon reduction [11] These parameters show a direct relationship with the change of the effective TPB length [7]. Focused ion beam (FIB)-SEM tomography [13,14] allows the microstructural change seen in Ni-YSZ electrodes after redox cycling to be quantified through ex-situ analysis, due to the good phase contrast between Ni and YSZ, and the relatively high resolution of the technique. The relationship between electrode microstructure properties and electrochemical impedance response has been recently quantified by Bertei et al [24], using a physically-based model [25,26]
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