No abstract

The impedance of anode-supported single cells [ Ni∕8 yttria-stabilized zirconia (YSZ) anode; La0.58Sr0.4Co0.2Fe0.8normalO3−δ cathode; 8YSZ electrolyte; area 1cm2 ] was characterized in a broad measuring range of temperature and air/fuel gas composition. The data has been analyzed by calculating the distribution function of relaxation times (DRTs). DRT computations enabled us to separate five different loss mechanisms occurring inside the cathode and anode without the need of an equivalent circuit. Two processes exhibit a systematic dependency on changes in the oxygen partial pressure of the cathode gas and thus can be attributed to diffusional and electrochemical losses on the cathode side. The remaining three processes are very sensitive to changes in the fuel gas but are not affected by variations of the cathode gas. These resistances are classified as a gas diffusion polarization within the anode–substrate and as an electro-oxidation reaction at the triple-phase boundary, respectively.

A high-resolution impedance study of the hydrogen oxidation in Ni/8YSZ ͑yttria-stabilized zirconia͒ cermet anodes has been realized in consideration of a broad range of operating conditions ͑temperature and partial pressure of fuel gas components H 2 , H 2 O, N 2 , He͒. 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: ͑i͒ at first, by the deconvolution of a distribution function of relaxation times ͑DRT͒, four different processes and their characteristic relaxation times have been identified. Two processes at frequencies Ͻ1 kHz represent a gas-conversion process or, respectively, a gas diffusion, whereas two processes at higher frequencies ͑2-30 kHz͒ are associated with the electro-oxidation of hydrogen at active sites, including the charge transfer reaction and the ionic transport. ͑ii͒ Subsequently, the last mentioned processes were fitted to a "transmission line" model describing the electronic and ionic transport properties of the Ni/8YSZ cermet. The high resolution of the DRT combined with the numeric accuracy of the complex nonlinear least square ͑CNLS͒ fit enabled us to determine ͑i͒ the effective ionic conductivity of the Ni/8YSZ cermet, ͑ii͒ the spatial extension of the electrochemically active area adjacent to the electrolyte/electrode interface, and ͑iii͒ the charge transfer resistance and its thermal activation energy.

A zero-dimensional stationary model for the I-U characteristics of anode-supported solid oxide fuel cells (SOFC) is presented. The different kinds of electrode polarization resistances are separated from experimental impedance data by means of a detailed equivalent circuit model specified for anode-supported cells. This has the big advantage that partial pressure and temperature dependency of electrode exchange current densities could be determined by a fit of semi-empirical power law model equations. For the first time, the exponents a and b for the pH2- and pH2O-dependency of the anodic exchange current density are obtained independently. Equally, the exponent m for the pO2-dependency of the cathodic exchange current density is derived. The anodic and cathodic gas diffusion polarization is calculated without the estimation of parameters such as tortuosity and porosity. Our approach is advantageous to separate anodic and cathodic activation and diffusion polarization and precisely predicts I-U characteristics for a wide operating range.