Impedance analysis of the anode and cathode impedances of a micro tubular solid oxide fuel cell (2nd report, separation of the anode and cathode overpotentials)

The present paper addresses a separate overpotential analysis of the anode and cathode of a practical solid oxide fuel cell (SOFC). Electrochemical impedance spectroscopy with two-electrode set-up is used for the analysis of a micro tubular SOFC having anode-supported electrolyte. This cell is composed of an Ni/(Zr0₂)_0.9(Y₂0₃)_0.2 cermet anode, an La_0.8, Sr_o.2Ga_0.8Mg _0.20_2.8electrolyte, and an La_0.6Sr _0.4(Co_0.2Fe_0.8O₃ cathode. Measurements are carried out for the cell operated at 700°C with varying flow rate and composition of H₂-N₂ mixture gas fed in the anode. In our previous study, the anode and cathode impedances have been found to appear in low and high frequency regions, respectively. Therein, the Ohmic resistance, and the charge and mass transfer resistances at the anode and cathode were evaluated. In the present study, the Ohmic, activation and concnetration overpotentials are then evaluated by integration of those resistances. The anode and cathode activation overpotentials are found to be separated using the Butler-Volmer equation. Thereby the concentration overpotential of the anode is also found to be separated by subtracting the anode activation overpotential from the anode overpotential, while the cathode concentration overpotential is found to be negligible. The separation of the overpotentials is successfully confirmed by the observation of the variation of the overpotentials in conjunction with the variation of the anode gas flow rate and composition, giving the exchange current densities. The analysis shows that decreases in the anode activation overpotential and concentration overpotential including the Nernst loss imporove the cell performance.