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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.

Nanoscaled and nanoporous ͑La 0.5 Sr 0.5 ͒CoO 3−␦ ͑LSC͒ thin film cathodes ͑film thickness ranging from 200 to 300 nm, grain and pore size in the range of 50 nm͒ were electrochemically characterized to determine their potential for intermediate and lowtemperature solid oxide fuel cells ͑SOFCs͒. Chemically homogeneous, large area ͑25 cm 2 ͒, and nanoporous LSC thin films were derived from metallorganic precursors ͑metallorganic deposition͒ and deposited on yttria-doped zirconia ͓͑YSZ͒ "design 1"͔ and gadolinia-doped ceria ͓͑GCO͒ "design 2"͔. The area-specific polarization resistance ͑ASR pol ͒ of both designs was evaluated on symmetrical cells with special emphasis on constancy, depending on temperature ͑500-850°C͒ and time by means of electrochemical impedance spectroscopy. For both designs, we report the capability of low polarization resistances, e.g., at 600°C, 146 m⍀ cm 2 ͑LSC/YSZ͒, respectively, 130 m⍀ cm 2 ͑LSC/GCO͒. Oxygen reduction reaction was facilitated by a substantial inner surface area of the porous thin-film cathode, as suggested by the application of Adler's model. Nanoporous LSC thin-film cathodes from this study were compared to alternative design concepts for high-performance porous cathodes and with dense ͑La 0.52 Sr 0.48 ͒͑Co 0.18 Fe 0.82 ͒O 3−␦ thin-film cathodes. Furthermore, the aim of our study was ͑i͒ to find a temperature regime with a perspective for chemical durability of an LSC/YSZ interface. We could prove that at a temperature of 500°C and for 100 h, polarization resistance of the LSC/YSZ interface remains constant, and at unreported low values, which reopens LSC/YSZ for micro-SOFC application; ͑ii͒ to estimate the structural durability of nanoscaled and nanoporous LSC thin-film cathodes. We have been able to demonstrate stable and unreported low polarization resistance for LSC/GCO in the temperature range between 500 and 700°C, which is of technical interest for auxiliary-power-unit application.

Patterned Ni anodes on normalY2normalO3 -stabilized ZrO2 (YSZ) represent a promising approach to determine the kinetics of electrochemical reactions in solid oxide fuel cells. Contrary to technical Ni/YSZ cermet anodes, the reaction zone for the hydrogen oxidation has the potential to be well defined. This study is focused on the reproducibility of electrochemical characterization results of patterned Ni anodes, with a parameter variation of the partial pressures of normalH2 and normalH2O , temperature, and polarization voltage. Considerable (electro)chemical relaxation and degradation processes with time constants in the order of some hours were found and are discussed: (i) an initial decrease in the line specific resistance (LSR) during the first 20–25 h of temperature exposure (T=800°C) attributed to a restructuring process in the Ni thin film, (ii) reversible changes in LSR upon variations of the partial pressure of normalH2 and normalH2O in correlation to the initial gas composition, and (iii) rapid reversible changes in LSR upon anodic and cathodic polarization voltages followed by a slow relaxation. The corresponding preconditions for reliable measurement series were deduced and yield the data set of the LSR values. Furthermore, a detailed comparison of the obtained LSR values with literature data is given.

Abstract.A cyclic reduction and oxidation of Ni/YSZ-cermet anodes for Solid Oxide Fuel Cells (SOFC) resulted in an increase of the polarization resistance. Therefore, investigations concerning kinetics of oxidation/reduction and the impact of redox cycles on the mi-crostructure of Ni/YSZ bulk ceramics were made. The reaction process of the basic system Ni/NiO was compared with cermet bulk samples and the influence of NiO and YSZ particle sizes and sintering temperatures on kinetics and microstructure was studied using thermo-gravimetry and dilatometry. The investigations on bulk ceramics indicated that no length change occurred during reduction, whereas reoxidation led to an increase in the length of the samples which strongly depended on the microstructure. It was shown that bulk samples sintered at temperatures below 1300 ~ can withstand redox cycles much better than those sintered at higher temperatures. Furthermore, it was found that by decreasing the NiO particle size and using a NiO/YSZ particle size ratio of aproximately 3:2, a smaller length increase after reoxidation was achieved. An increase of the polarization resistance could be ascribed to the formation of cracks within the bulk sample which interrupt current paths and therefore reduce the amount of the active triple phase boundary.

Electrochemical impedance spectroscopy is a well suited method for studying the properties of electrochemical systems. In recent decades electrochemical systems were investigated at different scales, from small model electrodes to high power devices, such as fuel cells and batteries. In the latter case, the measurement of reliable spectra and their evaluation is challenging because (i) the impedance is usually very low (1 ³), (ii) there is more than one rate limiting, electrochemical process per electrode, (iii) their charge transfer and transport processes are coupled and (iv) cathodic and anodic contributions overlap in the frequency domain. The Distribution of Relaxation Times (DRT) is supportive when deconvoluting complex impedance spectra, and has therefore gained increased attention. In this paper we introduce selected results of advanced impedance analysis. We discuss the impact of impedance data quality, statistically distributed noise and single errors in the spectra. Furthermore, the applicability of DRT for establishing adequate equivalent circuit models for ceramic electrochemical devices, such as batteries and fuel cells will be discussed.