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

Abstract

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