Towards Understanding the Impedance Response of Ni/YSZ Anodes

A comprehensive impedance study on the hydrogen oxidation in Ni/8YSZ cermet anodes has been realized in consideration of a broad range of operating conditions. A major problem in this respect concerns the origin and physical interpretation of empirical ‘equivalent circuits ’ used to fit the experimental data. We applied a two-stage approach for the evaluation of the impedance data: I) at first by the deconvolution of a distribution function of relaxation times (DRT) four different processes and their characteristic relaxation times have been identified. Two parts at frequencies <1 kHz represent a gas-conversion process or respectively a gas diffusion, whereas two parts at higher frequencies (2-30 kHz) are associated with the electro-oxidation of hydrogen at active sites. II) Subsequently the last mentioned processes were fitted to a “transmission line ” model describing the electronic and ionic transport properties of the Ni/8YSZ cermet. This approach resulted in the determination of 1) the total ionic conductivity of the Ni/8YSZ cermet and 2) the spatial extension of the electrochemical active area adjacent to the electrolyte/electrode interface