Investigation of degradation mechanisms by overpotential evaluation for protonic ceramic fuel cells

Abstract

Overpotential is frequently estimated from electrochemical impedance spectroscopy to evaluate the electrode performance of fuel cells. However, the overpotential of protonic ceramic fuel cells (PCFCs) is possibly underestimated because of hole conductivity in the electrolyte. In the present work, the overpotentials of PCFCs with BaZr0.8Yb0.2O3-δ (BZYb) and BaCe0.4Zr0.4Y0.1Yb0.1O3-δ (BCZYYb) electrolytes are accurately estimated using distribution of relaxation times analysis and an equivalent circuit model with considering a polarization resistance during durability tests. The activation overpotential in an open circuit state is not zero because leakage current flows in the PCFC electrolytes. The activation overpotential of the cell with the BCZYYb electrolyte is higher than that with the BZYb electrolyte because of the interdiffusion of nickel from the Ni-based anode to the cathode and cobalt from the La0.6Sr0.4Co0.2Fe0.8O3-δ cathode to the anode. The compressive internal stress in the electrolyte, determined by the cos α method using X-ray, decreases after discharging. Intergranular cracks formed at the grain boundary through nickel deposition, causing an increase in the ohmic overpotential with time for the both cells with BZYb and BCZYYb electrolytes. The interdiffusion of transition metals from the anode/cathode to the electrolyte in the manufacturing process should be prevented to improve the PCFC durability.