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.

We present a combined experimental and modeling study of the impedance of Ni/YSZ cermet anodes. Experiments are performed using symmetrical cells with 30 µm electrodes and 195 µm YSZ electrolyte. Impedance spectra are recorded in H 2 /H 2 O/N 2 /He atmospheres over a wide range of operating conditions (650-900°C, 4-46% H 2 O). The data are analyzed by both, an equivalent circuit model consisting of 4 RQ elements, and a two-dimensional detailed-chemistry kinetic model. The combined results allow a detailed insight into the coupled electrochemical and transport processes. This enables identification of the physical origin of the impedance features: 1) charge-transfer (hydrogen spillover reaction) and electrical double layer; 2) gas diffusion in the supply channels; 3) gas diffusion in the current collector mesh.

The impedance of anode supported single cells (Ni/8YSZ anode; La0.58Sr0.4Co0.2Fe0.8O3 cathode; 8YSZ electrolyte; Area=1 cm²) was characterized in a broad measuring range of temperature and air/fuel gas composition. The data has been analysed by calculating the distribution function of relaxation times (DRT). 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 polarisation within the anode-substrate and as an electrooxidation reaction at the triple-phase-boundary respectively.

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. Pre-Identification of Impedance DataIt is often a questionable and fraught task to use equivalent circuit models for the evaluation of EIS data when the number and nature of physical processes that contribute to the overall impedance are not known. This especially holds true for technical electrode structures that often do not allow a detailed modelling because of their complex interfacial geometries. Therefore we use a pre-identification method based on the "Distribution Function of Relaxation Times". This method separates polarisation processes with different time constants directly from impedance data (1). Distribution Function of Relaxation Times (DRT)In the simplest case a polarization process in an electrochemical system can be described by an equivalent circuit made of a parallel connection of an ohmic resistance R and a capacitance C. This process is characterised by its time constant ( )