RESUMO
Optimizing electrosynthetic reactions requires fine tuning of a vast chemical space, including charge transfer at electrocatalyst/electrode surfaces, engineering of mass transport limitations, and complex interactions of reactants and products with their environment. Hybrid electrolytes, in which supporting salt ions and substrates are dissolved in a binary mixture of organic solvent and water, represent a new piece of this complex puzzle as they offer a unique opportunity to harness water as the oxygen or proton source in electrosynthesis. In this work, we demonstrate that modulating water-organic solvent interactions drastically impacts the solvation properties of hybrid electrolytes. Combining various spectroscopies with synchrotron small-angle X-ray scattering (SAXS) and force field-based molecular dynamics (MD) simulations, we show that the size and composition of aqueous domains forming in hybrid electrolytes can be controlled. We demonstrate that water is more reactive for the hydrogen evolution reaction (HER) in aqueous domains than when strongly interacting with solvent molecules, which originates from a change in reaction kinetics rather than a thermodynamic effect. We exemplify novel opportunities arising from this new knowledge for optimizing electrosynthetic reactions in hybrid electrolytes. For reactions proceeding first via the activation of water, fine tuning of aqueous domains impacts the kinetics and potentially the selectivity of the reaction. Instead, for organic substrates reacting prior to water, aqueous domains have no impact on the reaction kinetics, while selectivity may be affected. We believe that such a fine comprehension of solvation properties of hybrid electrolytes can be transposed to numerous electrosynthetic reactions.
RESUMO
Liquid phase separation using aqueous biphasic systems (ABS) is widely used in industrial processes for the extraction, separation and purification of macromolecules. Using water as the single solvent, a wide variety of solutes have been used to induce phase separation including polymers, ionic liquids or salts. For each system, polymer-polymer, polymer-ionic liquid, polymer-salt or salt-salt, different driving forces were proposed to induce phase separation. Specifically, for polymer-salt systems, a difference in solvation structure between the polymer-rich and the salt-rich was proposed, while other reports suggested that a large change in enthalpy and entropy accompanied the phase separation. Here, we reinvestigated the PEG/K2HPO4/H2O systems using a combination of liquid-phase nuclear magnetic resonance (NMR) and high-resolution Raman spectroscopies, coupled with injection microcalorimetry. Both NMR and Raman reveal a decreased water concentration in the PEG-rich phase, with nonetheless no significant differences observed for both 1H chemical shift or OH stretching vibrations. Hence, both PEG- and salt-rich phases exhibit similar water solvation properties, which is thus not the driving force for phase separation. Furthermore, NMR reveals that PEG interacts with salt ions in the PEG-rich solution, inducing a downfield shift with increasing salt concentration. Injection microcalorimetry measurements were carried out to investigate any effect due to enthalpy change during mixing. Nevertheless, these measurements indicate very small enthalpy changes when mixing PEG- and salt-rich solutions in comparison with that previously recorded for salt-salt systems or associated with mixing of two solvents. Hence, our study discards any large change of enthalpy as the origin for phase separation of PEG/K2HPO4 systems, in addition to large difference in solvation properties.