Ion desolvation and confinement are key physical processes in porous-carbon-based Electrical Double-Layer supercapacitors (EDL) undergoing charging and discharging cycles. The term of electrical double layer (EDL) capacitor comes from the classical picture of non-porous electrodes that accumulate oppositely charged ions at their vicinity upon voltage polarization in a mean-field dielectric continuum description of the solvent populated with point charged ions at a given concentration. This picture although very popular obviously does not hold for porous electrodes with sub-nanometric pores. In particular, the mean-field Gouy–Chapman description of a EDL fails in high molar concentrations and for pore sizes smaller than a few nanometers (typically 4 nm for carbon substrates). EDL predictions usually rely on the Poisson-Boltzmann theory and modified versions such as Debye-Hückel. Presently, there is no theory capable of solving the electrostatics problem for ions confined in the nanopores of a disordered porous carbon material commonly used in electrochemical devices.

We propose a new approach to describe the charge and discharge process in sub-nanoporous carbon made electrodes (Pikunic 2003), in a supercapacitor setting, that we call chemically driven charge localization model (CDCL). Despite its mean-field character, the CDCL approach is an improvement of the current standard methods to simulate charged devices at the atomic scale, namely the constant charge and the constant voltage methods that are ineffective to correctly describe ionic docking in sub-nanopores (Dupuis 2022). Informed from DFT calculations, the CDCL method consists in localizing charges on defective non-sp2 carbon sites, including chemical or topological defects. By contrast to the standard methods, this allows simulating both adsorption on the electrodes external surfaces and in-sub-nanopore docking. Applied to a realistic texture of nanoporous carbon, we could unravel the fundamental processes underlying the capacitive effect of a sub-nanoporous carbon-based supercapacitor device in operation. We show in particular that subnanometric pores constitute about 20% of the capacitance of such a device using a standard aqueous electrolyte. In more details, we show that the docking of ions in pores is preceded by an assymetrical desolvation at the vicinity of the external pore surface. The desolvation process is actually different for sodium and for chloride ions as the hydration shell of sodium is tighter than that of chloride. Once ions are desolvated, they can access nanopores; sub-nanopores being mostly populated with bare ions in agreement with in-situ X-rays experiments (Prehal 2017).