Studies of using MoS2-based mixed transition metal sulfides (Iron sulfide,Cobalt sulfide,Nickel sulfide)as anodes of SOFCs

• Main Authors: Jui-Yi Zhuang, 莊君毅
• Other Authors: Kyan-Zong Fung
• Format: Others
• Language: zh-TW
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/91089279498463569581

碩士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 96 === Having high electrical conductivity and high fuel catalytic property is the key issue to select adequate electrode material for solid oxide fuel cell applications. Especially when small amount of hydrogen sulfide is present in common fuels like natural gas, the sulfidation of electrode material needs to be considered. Thus, searching for proper materials that are stable and functional in the H2S-containing environment is imperious and necessary. Most transition metal sulfides have high electrical conductivity, and provide with the ability of reforming hydrogen sulfide. Hence, they are the potential anode materials for SOFC applications. In this work, molybdenm disulfide, iron sulide, cobalt sulfide, and nickel sulfide was investigated as the anode material of SOFCs. Among them, molybdenm disulfide is an n-type semiconductor and have high hydrogen sulfide reforming capability. However, the electrical conductivity of molybdenum disulfide is only around 1 S/cm at 800℃ in reducing atmosphere. The transition metal sulfides, such as iron sulfide, cobalt sulfide, and nickel sulfide, have conductivity of 100~400 S/cm at 800℃ in reducing atmosphere. Therefore, MoS2 was mixed with transition metal sulfide individually to enhance its conductivity in this study. The structural variation, electrical conduction, and polarization were examined using XRD, AC-impedance spectroscopy and current-interrruption methods. The results show that most MoS2-transition metal sulfides mixtures remain unreacted two phases. Except for MoS2-FeS system, FeMo2S4 with defective NiAs structure was observed when the mixture of MoS2 and FeS was heated in reducing atmosphere at 800oC for 48h. Based on the results from ESCA analysis, the formation of FeMo2S4 is mainly due to the presence of divalent iron ions and trivalent molybdenum ions. The compatability of mixed sulfide with yttria stabilized zirconia(YSZ) electrolyte were examined using Dilamatic instrument. The thermal expansion coefficient of YSZ was found to be 10.26 ×10-6K-1 . Among sulfide mixtures, MoS2-FeS system exhibits very high thermal expansion coefficient of 22.09 ×10-6K-1 due to the contribution of FeS. For the systems of MoS2-Ni3S2 and MoS2-CoS, the thermal expansion coefficients were measured to be 10.96×10-6K-1 and 8.56×10-6K-1, respectively. Thus, MoS2-Ni3S2 and MoS2-CoS systems are thermally compatible with YSZ. The results of overpotential measurements indicate that the polarization of sulfide anode is significantly less than typical Ni-YSZ anode. The high polarization of Ni-YSZ is caused by the formation of nickel sulfide that suppress the path of gas transport. The reduced polarization in mixed sulfides is contributed by the presence of MoS2. According to H2S reforming experiments, it was observed that the decomposition of H2S started at temperature as low as 350oC when MoS2 is present. Without MoS2, the thermal decomposition of H2S began at 500oC. Based on the results in this study and comparing the phase stability, polarization behavior,and H2S catalytic properties of mixed sulfides versus Ni-YSZ, MoS2-Ni3S2 and MoS2-CoS may be potential anode materials for SOFC applications when the H2S-containing fuel is used.