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 ≤ T ≤ 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.