相场法模拟固体氧化物燃料电池阳极内 Ni–YSZ 界面处 YSZ 相变引发界面形貌演化

Introduction Solid oxide fuel cells (SOFCs) are considered as a leading candidate for clean energy conversion due to their high efficiency and flexibility. Yttria-stabilized zirconia (YSZ) serves as an electrolyte material in SOFCs due to its high ionic conductivity and mechanical stability at a high operating temperature. However, the long-term stability of YSZ can be impaired via the cubic-tetragonal (c–t) phase transformation. This transformation leads to a decrease in ionic conductivity and mechanical integrity, which contributes to an overall degradation of SOFC performance. Despite its importance, most of the previous investigations primarily focused on electrochemical degradation mechanisms, while less attention was paid to the material stability of YSZ, especially at the Ni–YSZ interface. For instance, the atomic diffusion of Ni into YSZ during sintering can accelerate the phase transformation, leading to a significant decrease in ionic conductivity within 100 hours. Despite these findings, there is a lack of comprehensive quantitative analysis on the kinetics and morphological impacts of the c–t phase transformation at the Ni–YSZ interface. Moreover, the existing models often oversimplify an interplay between phase transformation and mechanical stress, restricting their ability to predict a long-term degradation behavior accurately. In this work, the phase field method was applied to simulate the c-t phase transformation in YSZ at the Ni-YSZ interface for elucidating its effect on the interface morphology and gaining insights into the SOFC degradation mechanisms. Methods A 3D microelastic phase field model was proposed to investigate the c-t phase transformation in YSZ. The evolution of both conserved and nonconserved order parameters were governed by the Allen-Cahn and Cahn-Hilliard equations, respectively. The total free energy was constructed under multiphysics coupling. The elastic constants for cubic YSZ and tetragonal YSZ were defined based on their respective crystal structures, with the values assigned to reflect their distinct mechanical properties. The misfit strain was determined using the lattice parameters for the cubic and tetragonal phases. The simulations were performed on a 64×64×64 voxel grid representing a single-crystal YSZ. The governing equations could be solved by the Fourier spectral method and a semi-implicit time-stepping scheme. The shape of the nucleus distribution was restricted to the YSZ surface to ensure a realism under SOFC operating conditions. In this work, a dense YSZ disk with a diameter of 24 mm and a thickness of 0.5 mm was served as a substrate. After mechanical polishing and ultrasonic cleaning in acetone and ethanol, a stainless-steel shadow mask was employed to perform two perpendicular magnetron-sputtering depositions, producing a continuous Ni patterned film anode with a thickness of approximately 1 μm. For the cathode, a Pt paste was screen-printed onto the opposite side of the YSZ substrate. The discharge tests of the SOFC were carried out in a custom-designed alumina reaction chamber. Humidified 97% H₂ was supplied to the anode and pure O₂ to the cathode, both at a fixed flow rate of 50 mL·min^-1. The cell was polarized at 0.7 V for 100 h. After testing, the Ni film was mechanically peeled off, regions where Ni detached were selected and the underlying YSZ surface morphology was examined by scanning electron microscopy (SEM). Results and discussion The phase field simulations reveal that tetragonal variants preferentially nucleate at near the Ni–YSZ interface. High stresses appear at around the Ni–YSZ interfaces with the development of the t-variants. At certain iterations, high local stresses of exceeding 2 GPa can occur, leading to a radial growth of the variants into the bulk. A significant morphological evolution occurs, forming ridge structures along the YSZ surface and submicron pores in the interior at near the Ni–YSZ interface. The agreement between simulation data and experimental results confirms that the c-t phase transformation causes these morphological evolutions. The formation of pores is particularly detrimental as it disrupts the ion conduction pathways and weakens mechanical stability of YSZ, which can be correlated to the degradation of the SOFC performance. Conclusions In this work, a phase field modeling was used to simulate the c-t phase transformation in YSZ at the Ni–YSZ interface in SOFC anodes. The simulations accurately reproduced the morphological evolutions during long-term operation of SOFC, i.e., the formation of surface ridges and internal submicron pores, providing important insights into the SOFC degradation mechanisms. The preferential nucleation of tetragonal variants at the surface and stress-driven void formation could highlight some key factors affecting interface stability. Although the simulations were conducted based on a single crystal, this work could lay a foundation for future research on polycrystalline and porous YSZ substrates. This work could indicate strategies for improving SOFC durability, such as optimizing material compositions or designing stress-relieving structures via identifying stress generation as a primary driver of degradation. The findings of this work could contribute to advancing SOFC technology towards sustainable and efficient energy solutions.