Lanthanum-Ferrite based cathode: Impedance data interpretation via complex nonlinear least-squares and distribution of relaxation times analyses

Lanthanum Strontium Cobalt Ferrite Oxide (LaSrCoFeO₃) has been widely used as cathode material for intermediate-temperature solid oxide fuel cells at the temperature of 500–800 °C. At this temperature range, understanding the electrochemical behavior which is commonly analyzed by complex nonlinear least-squares (CNLS) analysis is very crucial in improving the cathode's performance. However, this analysis shows some limitations in interpreting the electrochemical processes in detail, particularly at the electrode-electrolyte interface. Hence, this study is conducted to compare the electrochemical impedance data analyses by CNLS and the distribution of relaxation times (DRT) of a fabricated LaSrCoFe|BCZY|LaSrCoFe (LaSrCoFe[dbnd]La_0.6Sr_0.4Co_0.2Fe_0.8O₃ and BCZY=BaCe_0.54Zr_0.36Y_0.1O_2.95) symmetrical cell. Impedance data of the cell is collected at T = 800 °C at two different stabilization times and to further enhance the analysis process, the impedance data of the cell at measurement temperatures of 700 ^°C and 750 ^°C are also included. In a Nyquist plot, the cell exhibits depressed semi-circles that represent a few processes occurring at the interface. The DRT analysis is more precise and easily reveals the semi-circles consisting of four different sub-processes (represented by four peaks) than CNLS (represented by four impedance arcs). The extracted responses from both analyses correspond to the oxygen reduction reactions that follow the Adler-Lane-Steele model. The stabilized symmetrical cells for respective 34 h and 17 h show an area-specific resistance (ASR) of (i) 0.22 Ωcm² and 0.30 Ωcm² (by CNLS) and (ii) 0.20 Ωcm² and 0.26 Ωcm² (by DRT). In addition, the ASR of the cell at T = 700 ^°C and T = 750 ^°C after being stabilized for 34 h is (iii) 0.71 Ωcm² and 0.40 Ωcm² (by CNLS) and (iv) 0.70 Ωcm² and 0.44 Ωcm² (by DRT), accordingly. Conversely, the cell's microstructure is not affected by the applied stabilization periods as observed by a scanning electron microscope.