Authors: Junyi Tao | Gao Yao | Yosuke Komatsu | Anna Ściążko | Takao Okabe | Katsuhiko Nishimura | Naoki Shikazono
Source: ECS Meeting Abstracts (article, 2025-11-24)
Field: Materials Science — Materials Chemistry

Effect of Polarization on Ni Migration in Patterned Ni-YSZ Fuel Electrodes: Part 2, Phase Field Modeling Nickel-yttria-stabilized zirconia (Ni-YSZ) cermet is the most commonly used fuel electrode material in SOCs due to its excellent electrocatalytic activity [1]. However, fuel electrode degradation caused by Ni coarsening and migration remains one of the main challenges for SOCs commercialization [2]. Ni coarsening due to sintering is a thermally activated phenomenon [3], while Ni migration is known to be closely associated with the cell polarization which is influenced by the direction of current [4]. Several hypotheses have been proposed to explain the mechanism of Ni migration, e.g. Gibbs-Thomson effect [5], Ni-YSZ wettability enhancement [6], surface diffusion of Ni(OH) x [4], gaseous diffusion of Ni(OH) 2 [7], Ni + -ion migration [8], etc. However, the fundamental mechanism of Ni migration is not thoroughly understood. For example, dynamic Ni morphology oscillations have been reported in operando patterned Ni-YSZ fuel electrodes experiment [9]. Dynamic Ni spreading and splitting up occur periodically under fuel cell mode, while Ni does not move and tends to delaminate under open circuit and electrolysis operations. The results from phase field simulation with physics-informed neural networks suggest that Ni spreading and splitting are several orders of magnitude faster than pure Ni surface diffusion [10]. Overpotential is considered to be one of the key factors which govern Ni migration, where an existences of threshold value are reported to initiate this process [4,11,12]. In the electrolysis mode, reported overpotential threshold ranges from 160 mV to 300 mV, with variations depending on factors such as temperature, impurities and particle size. However, in the fuel cell mode, overpotential threshold for Ni migration remains unclear. Observation and quantification of the changes in TPB due to Ni migration in conventional porous Ni-YSZ composite fuel electrodes are challenging [6]. On the contrary, patterned Ni-YSZ fuel electrodes enable direct observation of TPB definition and Ni migration from top-view images [6,9,13]. To obtain overpotential threshold in SOFC mode, a series in ex-situ potentiostatic experiments were conducted at different terminal voltages using patterned Ni-YSZ fuel electrodes [14]. The patterned Ni-YSZ cells were operated in fuel cell mode with various voltages using a fuel gas with different humidities. Ni morphology changes were assessed by comparing top-view images of the patterned Ni-YSZ fuel electrode before and after operation. The results demonstrated an existence of threshold overpotential for Ni migration also for the fuel cell operation. In the present study, phase field model (PFM) is used to predict the experimental data of Ref. [14]. Several kinds of phase field models have been developed to simulate Ni coarsening and migration in SOC fuel electrode. The driving forces for Ni migration in the reported phase field models include gaseous Ni(OH) 2 formation and diffusion [7], Ni-YSZ wettability enhancement [15], wettability gradient [16], electromigration [17], electrochemical potential [18], etc. Equilibrium contact angles are usually given a priori in the reported PFMs [5,19,20]. The limitation of this approach is that Ni migration stops when the contact angle reaches the equilibrium value. However, operando experiments have revealed Ni spreads continuously as a very thin film [9,13]. Furthermore, it is difficult to reproduce the experimentally observed overpotential threshold required to trigger Ni migration [14]. Therefore, additional mechanism is necessary to simulate Ni migration and to reproduce the overpotential threshold. In the standard Cahn-Hilliard equation for phase field modeling, the driving force in the governing equation is the gradient of chemical potential. In the present study, the total driving force in the present PFM is modeled as [18]: F =-∇(µ ch + A*µ el ) Instead of considering only the chemical potential µ ch with an equilibrium contact angle, the model incorporates the gradient of electrostatic potential µ el to account for the effect of overpotential. Temperature, steam partial pressure, contaminants, etc. may affect µ el [17]. The prefactor A* is introduced to control the weight of electrostatic potential µ el in the total potential field. The results show that Ni spreading occurs under high polarization even when the equilibrium contact angle is larger than 90º. The prefactor A* can be further modified with more experimental data under different operation conditions, which are being conducted in the ongoing research. Acknowledgements This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO), JST SPRING, Grant Number JPMJSP2108, and by “Advanced Research Infrastructure for Materials and Nanotechnology in Japan (ARIM)” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. References [1] W.Z. Zhu, S.C. Deevi, Materials Science and Engineering: A , 362 (2003) 228–239. [2] M.B. Mogensen, Curr. Opin. Electrochem. , 21 (2020) 265–273. [3] J. Chang, P. Zhang, Y. Sun, Q. Wang, Z. Yang, H. Zeng, Int. J. Hydrogen Energy , 102 (2025) 1421–1431. [4] M.B. Mogensen, M. Chen, H.L. Frandsen, C. Graves, A. Hauch, P.V. Hendriksen, T. Jacobsen, S.H. Jensen, T.L. Skafte, X. Sun, Fuel Cells , 21 (2021) 415–429. [5] H.Y. Chen, H.C. Yu, J. Scott Cronin, J.R. Wilson, S.A. Barnett, K. Thornton, J. Power Sources , 196 (2011) 1333–1337. [6] Z. Jiao, N. Shikazono, Acta Mater. , 135 (2017) 124–131. [7] Y. Lei, T. Le Cheng, H. Abernathy, W. 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