Effect of direct-current operation on the electrochemical performance and structural evolution of Ni-YSZ electrodes

The effect of electrolysis operations on Ni-YSZ fuel electrode stability was studied at different current densities and fuel mixtures during 1000 h life tests. For a typical electrolysis mixture of 50% H₂/50% H₂O and 0.6 A cm⁻² current density, cell ohmic resistance values were reasonably stable and no structural changes occurred. However, for more reducing conditions (97% H₂/3% H₂O), increasing the current density above 0.4 A cm⁻² increased the ohmic resistance accompanied by significant electrolyte degradation including fracture and void formation at grain boundaries. Numerical analysis was carried out to determine the effective oxygen partial pressure across the electrolyte. The results show that the oxygen partial pressure values at high current density and low steam content may be low enough to reduce zirconia to form a Ni-Zr alloy product, initiating the observed electrolyte structural degradation.