External Current Dependence of Polarization Resistances for Reversible Solid Oxide and Protonic Ceramic Cells with Current Leakage

Gd-doped ceria (GDC) and Yb-doped barium zirconate (BZYb) are used as electrolytes for intermediate-temperature solid oxide cells (SOCs) and protonic ceramic cells (PCCs), respectively. However, the electron conductivity of GDC under low oxygen partial pressure and the hole conductivity of BZYb under high oxygen partial pressure result in current leakage, which decreases the open circuit voltage and conversion efficiency. In the present work, the effect of polarization resistances on current leakage is investigated through the distribution of relaxation times (DRT) analysis of electrochemical impedance spectra (EIS) for a reversible SOC with GDC electrolyte and PCC with BZYb electrolyte. The polarization resistances increase and decrease with increasing current density in the electrolysis and fuel cell modes, respectively, for the conventional SOC with scandia-stabilized zirconia (ScSZ). In contrast, the polarization resistances exhibit the opposite tendency for the SOC with the GDC electrolyte owing to current leakage. For the PCC with the BZYb electrolyte, the polarization resistances according to air electrode reaction processes indicate maximum values in the fuel cell mode. The effect of hole conductivity on the polarization resistance suggests elementary rate-determining processes of the PCC electrodes.