Electrosorption-Induced Deformation of Nanoporous Carbons: Solvation Pressure from Molecular Dynamics and Continuum Theory

Nanoporous carbons play an important role in different electrochemical applications such as being utilized as electrodes in supercapacitors. Application of electric potential to a porous electrode in electrolyte solution stimulates adsorption or desorption of ions on the electrode surface. Electrosorption causes appearance of solvation pressure in the pores and results in electrode deformation. In this work, using molecular dynamics simulations and the continuum theory based on the modified Poisson-Boltzmann equation, we studied the structure of the electrical double layer in slit graphitic micropores filled with a NaCl aqueous solution, and solvation pressure in these pores. We focused on the behavior of the solvation pressure as a function of pore width and surface charge density. Within molecular dynamics simulations, two different water models were used -- an explicit model based on SPC/E water molecules and an implicit model, i.e., structureless background with fixed dielectric permittivity. The latter allows us to relate molecular dynamics simulation to the continuum theory. Simulations with explicit water show a qualitatively different behavior of the solvation pressure in the 1 and 2 nm pores as a function of the surface charge density. We demonstrated that the value of the solvation pressure is defined by a delicate balance between Van der Waals and electrostatic contributions. We demonstrated that the theory predicts the dependence of the solvation pressure on the pore width, which matches the results of simulations using the implicit water model. Finally, we adapted the continuum theory, developed for adsorption-induced deformation to estimate the deformation of a carbon electrode due to electrosorption. Our results can be used in the further development of nanoporous actuators working based on electrosorption-induced deformation.