Structural and Dynamical Characterization of Aqueous Hexahydrate Aluminum Nitrate Electrolyte Solution

An ever-increasing demand for nontoxic, high-performance electric batteries has motivated the exploration of novel aqueous electrolyte systems. One such system, hexahydrate aluminum nitrate [Al(H₂O)₆](NO₃)₃ solvated in water, is being investigated as an important candidate in this venture. While this system presents an interesting interplay of multiple interacting species, detailed theoretical studies of this complex aqueous system seem to be lacking. Here, we carry out such a theoretical study with the aim to understand the structure and dynamics for this system. We first perform a state-of-the-art quantum chemical calculation to develop the force field of this system. Subsequently, we employ molecular dynamics simulations to calculate various relevant quantities, such as the partial radial distribution functions among the species involved, and the transport properties like diffusion, of a 1 M solution. In order to validate our computational approach and the force field employed, we conduct NMR measurements to determine the diffusion coefficients of the different species involved. The computed diffusion coefficients show excellent agreement with NMR experimental values, reinforcing the accuracy of the developed force field. Furthermore, our results reveal that the diffusion of free water in this system is reduced by more than a factor of 2 compared to that in pure water, highlighting the effects of strong intermolecular correlations among hexahydrate aluminum nitrate and water molecules. Our calculated radial distribution functions further confirm the presence of pronounced structural correlations, while trajectory analyses provide valuable insights into the intricate dynamical behavior of the system where species move on different time scales.