Density functional theory (DFT) study of sulfur poisoning of metal/zirconia systems

• Main Author: Oleksandr I. Malyi.
• Other Authors: Chen Zhong
• Format: Theses and Dissertations
• Language: English
• Published: 2013
• Subjects: DRNTU::Engineering
• Online Access: http://hdl.handle.net/10356/52710
• Institution: Nanyang Technological University

The interest in the search of cheap and powerful energy sources increases with technology development. Solid oxide fuel cell (SOFC) is one of the most promising energy conversion devices producing electricity directly from oxidizing a fuel. However, cost of this technology is a significant limitation; therefore, cost reduction is an important direction of the modern SOFC research. The use of cheap fuels (biogas, methane, etc.) is one of the possible ways to solve this problem; however, most such fuels contain different concentrations of sulfur impurity affecting SOFC performance. Although the sulfur poisoning has been known for some time, the understanding of many experimental investigations is weak and requires a detailed theoretical analysis. Therefore, in this work, using first-principles calculations, we aim to understand the possible mechanisms of sulfur poisoning of Ni/zirconia systems explaining some experimental observations and to develop the friendly approaches to design new sulfur tolerant anode materials. We first analyzed possible reactions of sulfur impurity with Ni/zirconia systems by studying sulfur adsorption on Ni surfaces and sulfur poisoning of zirconia. The detailed analysis of the interaction of the impurity with Ni surfaces showed that sulfur might affect the adsorption and diffusion of active elements (oxygen and hydrogen), leading to the poisoning of catalytic properties of Ni surfaces and most likely providing the rapid initial degradation of SOFC performance. Interaction of sulfur impurity with zirconia is more complicated. High sulfur partial pressure may induce formation of new phase (zirconium oxysulfide (ZrOS)), changing the ionic conductivity of zirconia. The further analysis of formation conditions of the doped structure showed that existence of such structure is thermodynamically unfavorable under typical SOFC operating conditions. Nevertheless, the reaction of sulfur impurity with zirconia cannot be ignored because even partial substitution of oxygen atoms by sulfur might affect the stability of Ni/zirconia interfaces, leading to increased agglomeration rate of Ni particles. Since triple phase boundary (TPB) plays a key role in hydrogen oxidation, it is clear that increased agglomeration rate might contribute to long-term degradation of SOFCs. Because sulfur adsorption on Ni surfaces is the main reason for the rapid sulfur poisoning, to design new sulfur tolerant anode materials, we studied sulfur tolerance of Cu and Ni alloys. From this, it was shown that additions of low concentrations of Sn, Sb, and Bi to Ni can improve the sulfur tolerance of Ni/YSZ (yttria-stabilized zirconia) anode materials. Herewith, the alloying additions have a strong tendency to segregation at Ni surfaces and, consequently, do not change the melting temperature of the anode. Finally, since the agglomeration of Ni particles plays a key role in the long-term degradation of SOFC performance, the effect of alloying additions on the stability of Ni/zirconia interfaces was also investigated. Predicted results showed that Co, Fe, and V alloying additions can improve the stability of Ni/O and stoichiometric interfaces. Hence, such additions are attractive for designing new anode materials operating on hydrogen fuels. By contrast, Ag, Au, Cd, Cu, Sn, and Sb alloying additions have a tendency to reduce the work of separation. Nevertheless, since Ni/YSZ anode materials have significant porosity and the above alloying additions prefer to segregate at Ni surface, it is clear that at low concentrations the alloying additions have limited effect on the stability of Ni/zirconia interfaces. Based on all the above work, the strategy on the design of new sulfur tolerant anode materials is proposed.