On charge distribution and storage in porous conductive carbon structure

Abstract

The charge storage and distribution in a porous conductive carbon material, in the presence and absence of electrolyte, are studied using the Poisson-Nernst-Planck equations for the first time in the literature. By solving the Laplace equation, it is found that the charge distribution inside the pores is negligible in the absence of the electrolyte. In the presence of cations and anions of the electrolyte inside the pores, the charge distribution on the pore surfaces and ionic concentration gradients in the electrolyte are numerically evaluated by solving the Poisson-Nernst-Planck equations. The modelling results predict the classic electric double layer structure for meso‑ and larger pores. For smaller nano-level pores, however, the equations predict a structural shift from a classic electric double layer towards a near-uniform ion storage inside the pores, that is, a concentration of counter ions within pores. The accuracy level of the results is then analysed considering modified Poisson-Nernst-Planck equations, relative permittivity variations of the electrolyte, and the validity of the continuum assumption. The modelling predictions are experimentally tested by measuring the charge storage under different electrolyte concentrations in two activated carbon samples with different pore structure characteristics.