Fracture of Ni/YSZ Porous Material Under Water Vapor Environment in SOFC Anode: Reactive Molecular Dynamics Simulation

Abstract

In SOFC anodes, the concentration of water vapor in the hydrogen fuel increases when the hydrogen fuel supply is insufficient due to an accident. Here, when Ni/YSZ porous material is used for anodes, the high concentration of water vapor causes oxidation and volume expansion of Ni particles, leading to the destruction of the anode structure. To develop an anode material with high durability against such oxidation-induced fracture, it is necessary to elucidate the fracture process of Ni/YSZ porous materials due to the oxidation by water and the stress by the volume expansion of Ni particles. In this study, the fracture processes induced by the oxidation are investigated by reactive molecular dynamics method, which can handle chemical reactions. Especially, fatigue simulation by cyclic tension and compression is performed on the Ni/YSZ porous structures under water vapor environment to elucidate the effects of water-induced oxidation and fatigue on fracture.A Ni/YSZ porous model of about 2 million atoms is constructed with Ni and YSZ particles. A mass ratio of Ni particles to YSZ particles is 46:54, and a porosity is set to 40%. To consider water vapor environment, water molecules are placed in the vacancies (Fig. 1). Here, amount of water molecules is equivalent to 1500 times normal pressure to accelerate the oxidation by water. In fatigue processes, cyclic stress induces crack generation, crack propagation, and fracture of materials. Therefore, to investigate the fatigue fracture process, cyclic tension and compression by controlling the strain along the x-axis direction is applied to the Ni/YSZ porous model with a temperature of 1073 K, a pressure of 1.0 atm, and a strain rate of 1.0 x 10-9 s-1.The fatigue simulation was performed by changing the strain (i) from 0.00 to 1.50 for the 1st tension, (ii) from 1.50 to -1.50 for the 1st compression, and (iii) from -1.50 to 1.50 for the 2nd tension. The stress-strain curve during the fatigue simulation is shown in Fig. 2. To clarify the effect of oxidation by water on the fracture of Ni/YSZ porous structure, we first compared the stress-strain curves during the 1st tension of the Ni/YSZ porous models under vacuum and water vapor environment. Under water vapor environment, the tensile strength is 0.57 GPa at the strain of 0.10. This indicates that the water vapor environment leads to the fracture of the Ni/YSZ porous structure when the tensile stress is more than 0.57 GPa. On the other hand, under vacuum environment, the stress-strain curve raises and the tensile strength does not appear. Here, the tensile stress reaches 0.60 GPa at the strain of 1.50. Then, the fracture will ocuur by the tensile stress higher than 0.60 GPa under vacuum environment.Next, we compared the stress-strain curves during the 2nd tension after the 1st compression. The tensile strengths are 0.90 and 1.07 GPa under water vapor and vacuum environment, respectively. The lower tensile strength under water vapor environment indicates that fracture of the Ni/YSZ porous structure during fatigue is more likely to occur under water vapor environment than under vacuum environment.To discuss the reason why the water vapor environment induces the fracture of the Ni/YSZ porous structure, we investigated the structural change in the Ni/YSZ porous models in the fatigue simulations. Figure 3 shows the structural change around the interface between the Ni and YSZ particles during the 1st tension under water vapor and vacuum environment. Under water vapor environment, some cracks are generated at the interface between the Ni and YSZ particles (Fig. 3b). Around the cracks, H atoms and OH groups derived from water molecules are adsorbed on the Ni atoms and on the Zr and Y atoms of the YSZ particles. On the other hand, under vacuum environment, the Ni particles are stretched and no crack is generated by tension (Fig. 3c). These results suggest that the chemical reaction by water promotes the formation and growth of cracks at the Ni/YSZ interface during the tension of Ni/YSZ porous structures. Moreover, plastic deformation occurs within the Ni particles during tension under vacuum environment. On the other hand, there is less plastic deformation under water vapor environment, because the Ni particles are not stretched due to the crack generation at the interface between the Ni and YSZ particles. Therefore, the tensile strength under water vapor environment is lower than that under vacuum environment. Then, we suggest that water vapor environment induces crack generation and propagation at the interface between the Ni and YSZ particles and fracture of the Ni/YSZ porous material during fatigue. Figure 1