Accelerated reactive force field molecular dynamics simulation on the carbon diffusion process in Ni/YSZ anode of SOFCs with collective variable-driven hyperdynamics

Due to advantages such as high efficiency, low emissions, and fuel flexibility, solid oxide fuel cells (SOFCs) have gained attention as promising power devices for automotive applications. The nickel/yttria-stabilized zirconia (Ni/YSZ) material can catalyze chemical and electrochemical reactions of carbon-containing fuels and is one of the most widely utilized anode materials of the SOFC. However, the direct use of carbon-containing fuels can lead to carbon deposition on Ni/YSZ anode, affecting the performance and reliability of vehicle SOFC systems. The diffusion of carbon atoms in nickel influences the carbon deposition process and requires further investigation. In this study, a reactive force field molecular dynamics (ReaxFF-MD) model containing carbon atoms and Ni/YSZ crystal structure is established. The operating temperature of automotive SOFCs is 800-1200 K, which leads to a low frequency of diffusion events in the ReaxFF-MD model, resulting in high computational costs. Therefore, an acceleration technique called collective variable-driven hyperdynamics (CVHD) is applied to increase the time scale of simulation. The diffusion coefficient of carbon atoms in nickel at low temperatures is calculated. Combined with other high-temperature diffusion studies, the Arrhenius equation parameters for carbon diffusion in nickel are determined. Additionally, the spillover process of oxygen atoms in YSZ is considered, and diffusion coefficients of oxygen atoms in both Ni and YSZ are calculated. The escaping oxygen can oxidize the carbon in Ni. The formation of C-O bonds is observed, and reaction kinetics parameters are calculated, which form the basis for further studies on the mechanism of carbon deposition on Ni/YSZ anode materials.