Theoretical and Experimental Approaches to Oxygen Reduction at Porous Composite Electrodes for Fuel Cells by Analyses of ac-Impedance Spectra and Potentiostatic Current Transients

This article covers the theoretical and experimental approaches to oxygen reduction at the porous composite electrodes for fuel cells by analyses of ac-impedance spectra and potentiostatic current transient (PCT). First, the analysis methods based upon the thin-film agglomerate model and the random packing model were introduced to theoretically calculate the ac-impedance spectra. From the results, it is suggested that the capacitance dispersion in the high frequency range is closely associated with oxygen ion migration through the electrode. The deconvoltion method by discrete Fourier transform and the PCT analysis method by inverse Laplace transform were also employed to simulate the distribution function of relaxation time and the PCTs, respectively. Finally, as an example of application, in the present work, we investigated the oxygen reduction mechanism at the porous (La0.85Sr0.15)(0.9)MnO3 (LSM.)-yittria-stabilized zirconia (YSZ) composite electrodes as a function of sintering temperature by means of the analysis methods proposed above. From the dependences of the constant phase element exponent beta for ion migration and the time to reach the steady-state current t(st) on the sintering temperature. the capacitance dispersion in the high frequency range was discussed in terms of the distribution of the relaxation times for ion migration, which was greatly affected by the YSZ grain size.