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Understanding the sintering mechanism of Ni in a solid oxide fuel cell (SOFC) anode is one of the important issues to discuss the long-term durability of SOFC performance. The sintering behavior of Ni is affected not only by the operating temperature but also by the gas composition in the anode chamber. We analyzed the surface diffusion of a Ni adatom and Ni complexes on the Ni(111) surface under the operating temperature and gas compositions in the anode by using density functional theory. The Ni adatom, Ni-H and Ni-S complexes, which can be formed by H2 and H2S in anode gas, were considered as diffusion species on the Ni surface. It is theoretically confirmed that the formation of a NiS complex influences the sintering behavior of Ni depending on the temperature and the impurity H2S concentration in the fuel. Our calculated results are compared with the experimental observation on Ni sintering under various temperatures and H2S concentrations to find a good agreement. We clearly showed the important eff...

In this study, we propose a combination of molecular dynamics (MD) and the master sintering curve (MSC) approach to analyze the activation energy of sintering for porous materials. MD calculations ...

Microstructural change of porous electrodes in SOFC caused by sintering affected by materials and impurities is one of the main factors of the long-term degradation. Since it is difficult to clarify the effects of materials and impurities on sintering by experiments, atomistic scale simulation is important. In this study, we performed molecular dynamics (MD) simulation of porous structure of Ni and analyzed sintering properties of porous structure by employing master sintering curve theory. We have succeeded to obtain the sintering activation energy and its size dependency and to identify the dominant sintering mechanism by comparing sintering activation energy with activation energy of surface diffusion on each facet.

Employing reactive molecular dynamics simulation, we investigated diffusion pathways of H-atoms into the bulk from the Ni(100) surface. We found that surface pathway is dominant compared to bulk pathway for H atoms diffusion. Our simulation suggests that H atoms can diffuse into the bulk Ni. From 973 to 1273 K, H atoms migration into bulk is observed, but number of H atom is influenced by the temperature. At 1273 K, 30 % H atoms move into the bulk region. Diffusion barrier of H into bulk is calculated higher than the surface pathway. Diffusion coefficient of H between bulk and surface pathway is calculated to be one order of magnitude discrepancy.

The prediction of long-term durability of SOFC anode is important to understand the sintering phenomena in the anode, which are affected by microstructure, materials, and impurities. To understand the sintering phenomena considering the effects of materials and chemical species, molecular dynamics (MD) simulation is one of the most effective computational methods. In this study, we developed a theoretical methodology based on the master sintering curve to analyze the sintering property. We performed MD simulation for the densification of both Ni and YSZ porous structures. The sintering activation energy obtained from our proposed method was in reasonable agreement with the experimental result.

Solid oxide fuel cells (SOFCs) have been expected to be most efficient and versatile system for chemical to electrical energy conversion. One of advantages of SOFC is the possibility of direct utilization of various types of hydrocarbon fuels. It is however well known that fuel impurities in practical SOFC fuels such as sulfur compounds could cause SOFC poisoning or degradation. Therefore, understanding the effect of impurities in an atomic scale is essential to develop impurity-tolerant SOFCs with long-term durability. Here, we have studied the poisoning and degradation mechanisms of nickel anode by typical impurities, i.e. hydrogen sulfide, hydrogen chloride, etc., using density functional theory method.

A solid oxide fuel cell (SOFC) system anode material is usually composed of a mixture of Ni and other oxide ceramics, such as doped-CeO2, and Sc stabilized ZrO2, due to their high activity and structural stability. Among them the nickel/yttria-stabilized zirconia (Ni/YSZ) cermet is widely used today as an anode material of SOFC due to its good electrocatalytic property against low price. For SOFC performance, it is essential to improve the stability of the Ni/YSZ cermet anode. Combination of Ni and YSZ has been studied experimentally, but the interaction is not well understood in the boundary between Ni and YSZ. However, there is no evidence about the atomic-scale Ni/YSZ interface dynamics and chemical changes in the structure during the operation of SOFC. Unfortunately, most classical force fields are incapable of describing chemical reactions. To overcome this difficulty, reactive force field (ReaxFF) potential was developed by van Duin and co-workers. Recently, Ni/YSZ/H2 interactions are reported by ReaxFF. But in this interaction Ni/YSZ interface stability is not taken into account. As a first step we developed our own ReaxFF potential to investigate Ni/ZrO2 interface and for determining stable Ni/YSZ interface for modeling of the triple-phase-boundary in SOFCs. Big difference is found between our developed and reported ReaxFF and that is work of separation. Using our ReaxFF potential, work of separation is in excellent agreement with literature. It is found that O around Zr4+cation is reduced in case of bulk and interface regions. Although interface region exhibited less reduction compared to bulk region. More details and Ni/YSZ interface stability through O vacancy may present at conference.

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