To increase Solid Oxide Fuel Cells’ lifetime, lowering their working temperature is probably the most convenient solution, with a targeted range of 400-600°C, which is low enough to minimize the degradation kinetics while maintaining good electrochemical performances. Carbonate/oxide composites are promising materials as electrolyte in hybrid fuel cells, potentially operating at lower temperature than common Molten-Carbonate or Solid-Oxide Fuel Cells , with enhanced performances, but which are not yet fully understood.
A two-step modeling strategy, combined to experimental electrochemical measurements, has been considered to investigate these promising materials and to rationalize their peculiar performances .
First, density functional theory (DFT) calculations in a periodic framework have been performed to investigate the structural and electronic properties of the two components of the composite electrolyte: carbonate (LiKCO3 and LiNaCO3), and oxide (Yttria-stabilized zirconia, YSZ). Both bulk and various surface terminations have been taken into account[4-6], considering different surface reconstructions, when needed. Hybrid functionals were shown to be the most reliable in describing both components, and the most stable surface models obtained were YSZ-(111), LiKCO3-(001) and reconstructed LiNaCO3-(001). Li diffusion on the pure LiKCO3-(001) surface was also considered, by studying different Li-related? defects (neutral or charged) and their diffusion pathways along the surface. Low diffusion barriers (< 1eV) were obtained in some cases, supporting the possibility for Li to easily diffuse and play a key role in the cell operation. By combining the YSZ-(111) and LiKCO3-(001) surfaces obtained, a model of the composite LiKCO3/YSZ interface has been built in order to elucidate its structural and electronic properties, where Li-related ?defects (neutral or charged) and different diffusion pathways were studied, and compared to that of the pure carbonate.
In a second step, a force field has been derived from the DFT data, in order to perform classical molecular dynamics (MD) simulations of the composite, allowing to better identify diffusing species and paths at experimental operating temperatures, and to characterize related transport mechanisms.
In this contribution, we show how these two computational approaches can be successfully combined to experimental measurements to pinpoint some key points in the cell operation, in particular from a diffusion viewpoint, getting insights on their basic operating principles, and suggesting possible guidelines for improvement.
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