Diffusion rates of reactants and components in solid oxide cells

The electrochemical performance of solid oxide electrolysis cells (SOECs) is very dependent on diffusion rates of the gaseous reactants through the porous electrodes, and the degradation rate of SOEC Ni/YSZ electrodes can be dependent on Ni-migration. The Ni-migration is dependent on electrode polarization and diffusion rate of Ni-containing minority species impact the migration. The electrode polarization as well as the diffusion rate of minority Ni-species are dependent on the partial pressure and pressure gradient of H2O as well as on electrochemical potential gradients. Thus, the H2O diffusion gradient resulting from the electrical load of the cell is expected to affect the Ni migration.
Therefore, this contribution first evaluates and discusses the diffusion rates of H2/H2O and CO/CO2 in porous Ni-YSZ composites. A 10 kh durability test with electrochemical impedance spectroscopy recorded at the beginning and end of the test show significant signs of change in the diffusion resistance.
Next, the diffusion rate of Ni-species is evaluated based on observed migration of Ni in the electrochemical active Ni-YSZ layer with sub-micron Ni particles. This is compared with catalysis research literature and models for Ni particle diffusivity [1].
Based on the presented results and literature study a modified hypothesis for Ni-migration is provided.