Impedance studies on solid oxide fuel cells with Yttrium-substituted SrTiO3 ceramic anodes

Yttrium-substituted SrTiO3 is considered as a very promising anode material for solid oxide fuel cells. Impedance spectra of cells based on Sr0.895Y0.07TiO3 (SYT) ceramic anode substrates under different operating conditions have been studied in this work. The comparison of ohmic and polarization resistance between the cells based on SYT and conventional Ni-YSZ cermet anodes is described in detail. The different electrode polarization resistances for the SYT-based cells are separated from experimental impedance data by a complex nonlinear least-squares approximation combined with distribution function of relaxation times analysis. The anode kinetics plays a dominant role in the overall polarization losses which is related to four phases involved in the anode reaction and the extended surface diffusion and spill-over processes. According to the obtained data, the potential of performance increase for the SYT-based cells is evaluated.</jats:p

Anode-supported planar SOFC with high performance and redox stability

Solid oxide fuel cells with full ceramic anodes have recently attracted increasing attention, because the conventional Ni/YSZ cermet anodes may fail during practical operation due to their weak mechanical stability in the case of re-oxidation of the nickel. However, until now the reported fuel cells based on ceramic anodes have been fabricated only as small pellet-sized cells and electrochemical performance has been barely satisfactory, making it difficult to evaluate these attempts with respect to commercial feasibility. Herein, we report single cells based on Y-substituted SrTiO3 anode substrates. These planar cells have outer dimensions of 50 x 50 mm(2), which has not been reached for a ceramic anode-supported cell before. They show power densities of 0.7-1.0W cm(-2) at 0.7 V and 800 degrees C, which are sufficient for technical applications. The cells survived 200 anode-gas changes between fuel and air (redox cycles), providing a new direction for the development and commercialisation of anode-supported solid oxide fuel cells

Electrochemical performances of solid oxide fuel cells based on Y-substituted SrTiO3 ceramic anode materials

Yttrium-substituted SrTiO3 has been considered as anode material of solid oxide fuel cells (SOFCs) substituting of the state-of-the-art Ni cermet anodes. Sr0.895Y0.07TiO3-delta (SYT) shows good electrical conductivity, compatible thermal expansion with yttria-stabilized ZrO2 (YSZ) electrolyte and reliable stability during reduction and oxidation (redox) cycles. Single cells based on SYT anode substrates were fabricated in the dimension of 50 mm x 50 mm. The cell performances were over 1.0 A cm(-2) at 0.7 V and 800 degrees C, which already reached the practical application level. Although Ti diffusion from SYT substrates to YSZ electrolytes was observed, it did not show apparent disadvantage to the cell performance. The cells survived 200 redox cycles without obvious OCV decrease and macroscopic damage, but performance decreased due to the electronic properties of the SYT material. The influence of water partial pressure on cell performance and coking tolerance of the cells are also discussed in this study. (C) 2010 Elsevier B.V. All rights reserved

Degradation of anode supported cell (ASC) performance by Cr-poisoning

Performance and stability of solid oxide fuel cells (SOFC) have been continuously improved at the single-cell level. Connecting the individual cells by a metallic interconnector (MIC) in a stack, though, yields remarkable losses in performance and leads to an enhanced degradation. These effects are attributed to, inter alia, Cr evaporation from the MIC and, thus, Cr poisoning of the cathode.To determine the degradation rate caused by Cr poisoning, this paper focuses on the differences in single cell performance and short-term stability by using either an inert flowfielcl or a flowfield made of a chromia-forming alloy. To provide a homogeneous current collection and gas distribution over the complete cathode area and to avoid a direct contact between MIC made of Crofer22APU and LSM, a platinum mesh was used as current collector. The cell performance was evaluated by analyzing its current-voltage characteristics and using electrochemical impedance spectroscopy.A detailed analysis of impedance spectra by the distribution of relaxation times (DRT) and a subsequent Complex Nonlinear Least Squares (CLNS) fit facilitated the separation of anodic and cathodic polarization processes. In the presence of a chromia-forming alloy the polarization resistance of the cathode showed a significantly higher initial value (+64 m Omega.cm(2)) than without and a high degradation rate of 213 mu Omega.cm(2) h(-1) during 280 h of galvanostatic operation at 800 degrees C. (C) 2010 Elsevier B.V. All rights reserved

Time-dependent electrode performance changes in intermediate temperature solid oxide fuel cells

This study gives evidence that the time-dependent performance changes in anode supported cells for intermediate-temperature solid oxide fuel cells is essentially influenced by the mixed ionic-electronic conducting (MIEC) cathode. The impedance spectra recorded during 700 h of operation at 750 degrees C were interpreted using an appropriate equivalent circuit model by (i) a distribution of relaxation time analysis followed by (ii) a complex nonlinear least squares fit. Four electrode polarization processes were separated by selective experimental parameters. The cathodic part, initially the smallest, is only discovered among the anodic contributions by a change in fuel gas composition from H-2-H2O to CO-CO2 and increases by 310% (15 m cm(2) at 11 h, 62 m cm(2) at 700 h). A Sr (and Co) depletion of the MIEC cathode composition La0.58Sr0.4Co0.2Fe0.8O3-delta possibly caused this degradation. The anodic polarization has a proportion of 92% at the start and decreases to 73% (168 m cm(2) at 11 h, 173 m cm(2) at 700 h). The anode charge-transfer reaction initially causes 60% of the total polarization losses and 50% after 700 h. This is assigned to a change in the triple phase boundary and/or a degradation in ionic conductivity in the anode functional layer. The gas diffusion polarization remains constant at 58 m cm(2)