Reliability Of Solid Oxide Fuel Cells Research Articles

Creep constitutive law prediction via short-term evaluation using a small size testing method

To efficiently assess the mechanical reliability and long-term durability of solid oxide fuel cells (SOFCs) and solid oxide electrolyser cells (SOECs), a suitable creep test method and prediction of the material response is necessary. This study investigates the effectiveness of the small punch (SP) testing method for Ni–YSZ composites and in rapidly determining the constitutive creep law for SOFC materials. The creep properties of Ni–YSZ composites are evaluated under a 1% H2 environment at temperatures of 800–900 °C and stresses of 36–58 MPa. The finite element method is used to simulate the SP tests and generate numerical creep curves. These curves are fitted to the experimental curves to obtain the parameter set in the creep constitutive law of the strain hardening model. The numerical and experimental creep curves are well-fitted with only a 0.44% difference and convergent parameter sets equal to C1=1.0× 10−12, C2=1.03, C3 = -1.70, C4=96.78 in the strain hardening law, indicating the usefulness of the SP method and creep constitutive law for characterising and predicting the creep properties of SOFC constituent materials. This study highlights the importance of accurate characterisation of creep properties for predicting the long-term performance and reliability of SOFCs.

Fabrication and Characterization of High Performance Intermediate Temperature Alumina Substrate Supported Micro-Tubular SOFCs

The central challenge of solid oxide fuel cell (SOFC) technology lies in how to achieve sufficient robustness and reliability while significantly reducing system and operating cost. This paper studied a novel alumina substrate supported micro-tubular SOFC with high performance at intermediate temperatures. An in-house built extrusion system in combination with modified phase-inversion method was used to fabricate micro-tubular alumina substrate with radially well-aligned micro-channel array. The gas permeability of such an alumina substrate reached 69.2 × 105 Lm−2h−1, approximately ten times that of the substrate with conventional microstructures. The addition of NiO (10 wt%) sintering aid significantly improved the bending strength of α-Al2O3 substrate while forming NiAl2O4 spinel phase in the substrate. Built upon the fabrication of alumina substrate, anode electrode and electrolyte layers were fabricated through dip-coating and sintering process alternatively. A porous barrier layer strategy was employed to inhibit NiO in the anode layer from diffusing into the substrate during high temperature sintering process. The resulting dense electrolyte layer and anode functional layer with the thickness of ∼10 μm and ∼20 μm were obtained respectively. The alumina substrate supported micro-tubular SOFC with material system NiO-SDC/SDC/PrBaCo2O5+δ demonstrated the peak power density of ∼1420 mW cm−2 at 600°C, which is among the best performance of SOFCs with the same material system in open literature. The novel micro-tubular cell also demonstrated excellent thermal cycling stability. By replacing the high-cost rare-earth anode material with low-cost alumina, the novel design not only significantly reduced the cost but also improved the robustness and reliability of SOFCs.

Temperature Dependence of Carbon Deposition in Ni-YSZ Cermet with Various Steam/Carbon Ratio

Solid oxide fuel cells (SOFCs) are one of promising power generation systems having high energy conversion efficiency. Although SOFCs have been already commercialized, there still exist technical challenges for their further widespread utilization, such as the improvement of durability and reliability. The carbon deposition on the anode is known as one of possible causes for the degradation of SOFCs. It may cause the deactivation of electrode catalysis, the obstruction of gas diffusion pass, the structural deformation, and, in the worst case, the structural destruction. The deformation or destruction of the anode can be a fatal problem for SOFCs because many of commercialized SOFC cells are mechanically supported by the anode. Thus, for improving the durability and reliability of SOFCs, carbon deposition behavior on the anode should be understood. In this work, the carbon deposition on the porous composite of nickel and yttria-stabilized zirconia (Ni-YSZ cermet), which is the most typical SOFC anode showing excellent electronic conductivity, catalytic activity, thermodynamic stability and mechanical property, was investigated. In our previous works [1], carbon deposition on the Ni-YSZ cermet was investigated under various kinds of hydrocarbon gases. In this study, the temperature dependence of the carbon deposition in the Ni-YSZ cermet was investigated under the exposure to methane-containing gas with various steam /carbon ratios, S/C. Ni-YSZ porous cermet was prepared by mixing NiO (NiO-FP, Sumitomo Metal Mining Co., LTD.) and 8 mol % YSZ (TZ8YS, Tosoh Corp.) with the ratio of Ni:8YSZ = 40:60 vol%. 30 vol% of polymethyl-methacrylate (PMMA, SSX-102, Sekisui Plastics Co., LTD.) was added to the NiO/YSZ mixture as a pore forming material. The mixed powder was molded into a disk shape, and pressed under 100 MPa of hydrostatic pressure. The disk was sintered in air at 1673 K for 3 hours. The pellet was cut into blocks of 2×2×2 mm. Specimens of Ni-YSZ cermet were obtained by reducing the blocks of NiO-YSZ in 100%H2at 1073 K for 5 hours. The porosity of the obtained Ni-YSZ cermet was about 35 %. Thermogravimetry measurements of Ni-YSZ cermet were carried out by TG-DTA (2000S，MAC-Science Co. Ltd.). The NiO-YSZ specimen was first heated at 1073 K in 5%H2–N2 for 1 hour for reduction, and was cooled down to room temperature. Then the atmosphere was switched to dry or moisturized 10%CH4-N2, and the weight change was measured when the temperature was heated up to 1273 K. After the measurement, the atmosphere was switched to pure N2, and temperature was decreased down to room temperature. Figure 1 shows the weight change and the weight change rate of the Ni-YSZ cermet during exposure to 10%CH4-N2 with S/C = 0.1 when the specimen was heated with the heating rate of 5 K‧min-1. The weight of the Ni-YSZ cermet started to change from low temperature, and slightly increased with temperature. The weight decreased once around 720 K. Above this temperature, the weight increased with temperature again, while the weight change rate showed a local maximum around 800 K. Above 873 K, the weight and the weight change rate monotonically increased with temperature. Thermogravimetry measurements were carried out also with various S/C, 0 (dry), 0.05, and 0.2. As results, the temperature showing the local maximum of the weight change rate increased with increasing S/C. This indicated that the local maximum of the weight change rate was related to reactions involving steam. According to the thermodynamic calculation of equilibrium composition of chemical species in the CH4-H2O atmosphere [2], carbon deposition become significant with increasing temperature. This means that the characteristic behavior of the weight change and the weight change rate observed around 720 and 800 K cannot be simply explained thermodynamically, and kinetic effects, such as catalytic activities of Ni and side reactions by water vapor, should be considered. In the presentation, further detailed results of thermogravimetry measurements as well as analysis of deposited carbon will be given. [1] T. Nakamura, N. Ohmura, K. Yashiro, K. Amezawa, Electrochem. Soc. Trans., 57(1)(2013) 1539. [2] The Thermodynamic Database MALT for Windows; Kagaku Gijutsu Sha, http://www.kagaku.com/malt. Figure 1

Surface-Related Effects Limiting the Performance of Solid Oxide Fuel Cell Cathodes

The oxygen exchange kinetics of the solid oxide fuel cell (SOFC) cathode materials La0.6Sr0.4CoO3-δ (LSC) and La0.58Sr0.4Co0.2Fe0.8O3-δ (LSCF) is measured by in-situ conductivity relaxation experiments at 600 °C. The impact of silicon and chromium on the chemical surface exchange coefficient kchem and the chemical diffusion coefficient of oxygen Dchem is assessed in dry and humid atmospheres in the 1,000 h time scale. X-ray photoelectron spectroscopy (XPS) shows that the degradation goes along with changes of the chemical composition of the surface. Poisoning with Cr and/or Si, and Sr-enrichment occurs in 10–100 nm thick layers, depending on the pre-treatment. Scanning electron microscopy (SEM) reveals changes in the surface topography of the degraded specimen. Based on experimental evidence as well as on thermodynamic data, it can be concluded that the surface poisoning is triggered by H2O(g). Both Si and Cr form volatile gas phase species under humid conditions, which react with LSC or LSCF. Even though the degradation is confined to a thin surface layer, a strong decrease in the oxygen exchange activity of the cathode materials occurs. Since neither Si nor Cr are transported to a significant extent via a dry, oxidising gas phase at 600– 700 °C, the use of dried air at the cathode side would be an important factor to improve the durability and reliability of SOFCs.

Modeling and Study of the Influence of Sealing on a Solid Oxide Fuel Cell

The properties of sealing materials are important for the performance and reliability of solid oxide fuel cells (SOFCs). Even if the properties of a sealing material can be studied separately, it remains difficult to quantify the effect of an imperfect seal on the repeat-element behavior. In this study, simulation is used to investigate the effects of an imperfect seal behavior on the performance and reliability of SOFCs. Diffusion through the sealing material and inherent local combustion of fuel are added to the computational fluid dynamics (CFD) repeat-element model, which also allows us to compute the flow field, the electrochemical reactions, and the energy equations. The results are in good agreement with experiments. The zones of parasitic combustion and local overheating are well reproduced. Furthermore, the model predicts a risk of reoxidation under polarization that is well observed. The model also shows the necessity to take into account the diffusion transport for the development of compressive seal materials, hence verifying the hypotheses made by other groups. The modeling approach presented here, which includes the imperfections of components, allows us to reproduce experiments with good accuracy and gives a better understanding of degradation processes. With its reasonable computational cost, it is a powerful tool for a design of SOFC based on reliability.