Redox Stable Anode Materials for SOFC Application

Abstract

The common material design for solid-oxide fuel cells (SOFCs) is based on the cermet (ceramic-metal composite) material group, for example anode compositions like nickel with yttria stabilized zirconia (Ni/YSZ). One of the main limitations of these material groups is that they may undergo unwanted thermal expansion during redox cycles, causing dimensional instability at the anode. The state-of-the-art Ni/YSZ anode experiences a linear expansion of up to 1% when oxidized, causing an irreversible microstructural change, decreasing the electrochemical activity. The problem cumulatively increases if the cell design is anode supported as this puts the electrolyte under tension, causing it to crack and leaking the fuel and oxidant gases. Currently researchers are focusing on the development of new SOFC anodes for internal reforming, lower temperature operation, and poison resistance, but there is a lack of study on the redox stability of these newly developed materials. In this thesis work, the fundamental development of redox stable anode materials has been undertaken. Materials from perovskite, double perovskite, scheelite and double rutile structures has been considered. The material compositions include CeNb1-xWxO4, CeNb1-xMoxO4, Sr2Mg1-xMo1+xO6 (x = 0, 0.1, 0.2), Nb2TiO7 and Nb1.33Ti0.67O4. The thermo mechanical and electrical redox stability of the various novel materials were tested by controlled-atmosphere dilatometry and four-point probe DC electrical conductivity respectively at SOFC operating temperature (800 o C). Based on the thermo mechanical and electrical redox results, several doping strategies were adopted in the material systems to improve their redox stability while maintaining the coefficient of Thermal Expansion (CTE) at a minimum. The qualified material system was subjected to symmetrical and fuel cell tests to analyze the polarization resistance of novel anode material as well as electrochemical activity via in situ EIS. It was found that in CeNbO4 material group, W and Mo doping seized the phase transition of CeNbO4 from monoclinic to tetragonal at elevated temperature. Also the CTE of CeNbO4 in air between 25-800 o C was considerably brought down from 20.77 to 13-14x10-6 K-1 by adopting this doping strategy. Regarding Nb2TiO7 and Nb1.33Ti0.67O4 materials, though these materials exhibit stable phase transformation, Nb2TiO7 as a starting SOFC composition is not ideal due to poor electrical properties (1.35 S/cm in reducing atmosphere at 800 o C). Whereas, though Nb1.33Ti0.67O4 recorded a conductivity of 85 S/cm in reducing condition, the material is mechanically not stable under redox condition at 800 o C. In Sr2Mg1-xMo1+xO6 (x = 0, 0.1, 0.2) material system, the material with Sr2Mg0.9Mo1.1O6-δ composition was made both mechanically and electrical redox stable at 800 o C, with a CTE of 14.5x10-6 K-1 in air between 25-800 o C and electrical conduction of 0.1 S/cm in air and 17.5 S/cm in reducing atmosphere. From symmetrical cell analysis, it was analyzed that Sr2Mg0.9Mo1.1O6-δ had a low polarizat...