Concentration-Dependent Solvation Structure and Dynamics of Aqueous Sulfuric Acid Using Multinuclear NMR and DFT

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Sulfuric acid is a ubiquitous compound for industrial processes, and aqueous sulfate solutions also play a critical role as electrolytes for many prominent battery chemistries. While the thermodynamic literature on it is quite well-developed, comprehensive studies of the solvation structure, particularly molecular-scale dynamical and transport properties, are less available. This study applies a multinuclear nuclear magnetic resonance (NMR) approach to the elucidation of the solvation structure and dynamics over wide temperature (−10 to 50 °C) and concentration (0–18 M) ranges, combining the 17 O shift, line width, and T 1 relaxation measurements, 33 S shift and line width measurements, and 1 H pulsed-field gradient NMR measurements of proton self-diffusivity. In conjunction, these results indicate a crossover between two regimes of solvation structure and dynamics, occurring above the concentration associated with the deep eutectic point (∼4.5 M), with the high-concentration regime dominated by a strong water–sulfate correlation. This description was borne out in detail by the activation energy trends with increasing concentration derived from the relaxation of both the H 2 O/H 3 O + and H 2 SO 4 /HSO 4 – /SO 4 2– 17 O resonances and the 1 H self-diffusivity. However, the 17 O chemical shift difference between the H 2 O/H 3 O + and H 2 SO 4 /HSO 4 – /SO 4 2– resonances across the entire temperature range is nevertheless strikingly linear. A computational approach coupling molecular dynamics simulations and density functional theory NMR shift calculations to reproduce this trend is presented, which will be the subject of further development. This combination of multinuclear, dynamical NMR, and computational methods, and the results furnished by this study, will provide a platform for future studies on battery electrolytes where aqueous sulfate chemistry plays a central role in the solution structure.