Compression of liquids in liophobic pores provides a viable mechanism for the conversion of mechanical work to interfacial free energy as a new form of energy storage. Reducing the hysteresis of the influx/expulsion cycle is imperative for energy recovery. We describe a variation of the Expanded Ensemble Grand Canonical method for studies of pressure-driven electrolyte uptake in a nanoporous medium. Simulations show more than doubled evacuation pressures for a concentrated electrolyte compared to neat water, improving the reversibility and the amount of the recovered work. We consider aqueous NaCl at several bulk concentrations permeating 1-2 nm wide pores at pressures of up to 3 kbar. A viable acceptance rate of Monte Carlo simulations is secured by relying on incremental changes of ion and water coupling during particle exchanges. Experimental pressure-induced changes in chemical potentials are added to the known ambient chemical potentials in selected model systems. The simulations provide a theoretical perspective into the mechanisms behind the improved reversibility of the process due to increased wall/solution interfacial free energy in narrower pores and concomitant infiltration <i>and</i> expulsion pressures. These changes reflect strong desalination of the solution in narrow confinements at all pressures and bulk salinities. Liquid spring behavior is indicated in highly soluble salts like LiCl and in sub nanometer pores.

Open ensemple sampling of nanoconfined electrolytes in surface energy storage

ACS Fall 2021

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