Impedance and Electric Modulus Spectroscopy of Polycrystalline La0.5Sr0.5Bi0.2Co0.4Fe0.4O3–d Cathode Ceramic for Intermediate Temperature SOFCs

Abstract

In the present research work, La_0.5Sr_0.5Bi_0.2Co₀.₄Fe_0.4O_3–d cathode ceramic powder is synthesized through cost-effective flash pyrolysis process and further conventional heat treatment for sintering for IT-SOFCs. The crystal structure, unit cell parameters and bond length are determined by using the Rietveld refinement program. The XRD result indicates existence of a pure single phase with R C space group symmetry detected from the sample which has been sintered at 700 ℃. SEM images of fracture surface of sample sintered at 700 ℃ showed a high porosity and nano grain sizes (50-100 nm). Impedance and electric modulus spectroscopic methods are used to identify the relaxation phenomena in La_0.5Sr_0.5Bi_0.2Co₀.₄Fe_0.4O_3–d ceramic over a broad range of temperature and frequency. A single relaxation peak is observed in the imaginary part of impedance and electric modulus spectra, which could be due to the contribution of grain boundary of La_0.5Sr_0.5Bi_0.2Co₀.₄Fe_0.4O_3–d ceramic. The imaginary part of modulus ( ) spectra is analyzed with help of non-exponential decay function or Kohlrausch–Williams–Watts (KWW) parameter (β). In the combined plot of the imaginary part of impedance and electric modulus spectra at 210 ℃, only a single peak of  and  is observed at the same frequency which indicates that the conduction process is a long-range motion of the charge carriers. The frequency-dependent conductivity is followed by the Jonscher’s double power law in the temperature range 30-210 ℃.