Molecular Dynamics Simulations of Metal/Molten Alkali Carbonate Interfaces

Neutral and charged interfaces between molten alkali carbonates M₂CO₃ (M = Li, Na, and K) and planar solid walls have been investigated by molecular dynamics based on a rigid-ions force field. Simulations cover the temperature range 1200 K ≤ <i>T</i> ≤ 1500 K at a moderate (∼15 kbar) overpressure to compensate for the slight overestimate of the system volume by the force field model. The results provide an intriguing view of the interplay among ion packing, oscillating screening, anisotropic correlations, and ion dynamics at the interface. The mass and charge density profiles display prominent peaks at contact, and tend to their constant bulk value through several oscillations, whose amplitude decays exponentially moving away from the interface. Oscillations in the charge density profile extend screening to longer distances and limit the capacitance of the interface. Ion–ion correlations are enhanced in proximity of the interface but retain the exponentially decaying oscillatory form of their bulk counterpart. Diffusion is slower in the molecularly thin layer of ions next to the interface than in the bulk. The analysis of interfaces is completed by the computation of structural properties of bulk phases, and by the estimate of transport coefficients such as self-diffusion, electrical conductivity, and especially thermal conductivity, which is seldom computed by simulation. All together, the results of our simulations for homogeneous and inhomogeneous molten carbonates provide crucial insight on systems and properties relevant for advanced devices such as fuel cells, that, in turn, might play a prominent role in future power generation strategies