A Potential Approach to Ammonia-Hydrogen Synergy: Insights from Molecular Dynamics Simulations

Hydrogen nanobubbles dispersed in liquid ammonia represent a promising strategy for ammonia-hydrogen synergy, offering a simplified approach to hydrogen storage while enhancing the ammonia combustion efficiency for transportation propulsion. However, due to limited experimental data and simulations, the fundamental mechanisms governing this system are not yet fully understood. In this study, molecular dynamics (MD) simulations were employed to investigate the dynamic behaviors and evolution of hydrogen nanobubbles within liquid ammonia. The thermophysical properties of hydrogen nanobubbles were determined, and their stability mechanisms were identified across a range of supersaturation levels. Results show that stable hydrogen nanobubbles form only within a moderate supersaturation range. Low supersaturation fails to induce nucleation due to insufficient bulk free energy, whereas excessively high supersaturation leads to gas-liquid phase separation. The hydrogen nanobubbles exhibit high internal pressure and density, but their pressure-volume-temperature relationship deviates from predictions based on macroscopic experiments or conventional equations of state. The incorporation of hydrogen nanobubbles significantly reduces the viscosity of liquid ammonia but increases thermal conductivity and diffusion coefficients, with these effects well modeled by quadratic interpolation. Moreover, gas supersaturation and surface tension play crucial roles in maintaining the mechanical equilibrium of the stable hydrogen nanobubbles. As supersaturation increases, hydrogen's diffusion coefficient decreases, and its transport behavior increasingly reflects random thermal motion. Finally, by combining MD simulations with classical bubble theory, the maximum achievable hydrogen-to-ammonia ratio is estimated at 9.07%. This study offers valuable insights and foundational data for ammonia-hydrogen fuel preparation for combustion.