Simulation of electric double layer capacitors with mesoporous electrodes: Effects of morphology and electrolyte permittivity

Abstract

Abstract This paper aims to numerically predict the capacitance of electric double layer capacitors (EDLCs) made of mesoporous electrodes consisting of closely packed monodispersed mesoporous carbon spheres in (C 2H 5) 4NBF 4/propylene carbonate electrolyte. The model faithfully accounts for the electrode packing morphology and the dependency of electrolyte dielectric permittivity on local electric field. Three sphere packing morphologies were investigated, namely, simple cubic (SC), body-centered cubic (BCC), and face-centered cubic (FCC). A cylindrical mesopore in a mesoporous carbon sphere was also simulated. This study demonstrates that the field-dependent electrolyte dielectric permittivity significantly affects the predicted capacitance of EDLCs. Moreover, the Stern layer needs to be accounted for in order to match predicted specific area capacitance with experimental data. This study also establishes that, for all packing structures, larger sphere diameter results in larger electric field at the electrode surface and thus larger diffuse layer specific area capacitance. For sphere diameter less than 40 nm, SC packing had the largest electrolyte volume fraction. This provided more space for the electric potential to decrease resulting in larger electric field at the electrode surface and diffuse layer specific area capacitance compared with BCC and FCC packings. On the contrary, FCC features the smallest volume fraction resulting in the lowest surface electric field and diffuse layer specific area capacitance. The packing morphologies of electrode spheres were found to have no significant effect on the diffuse layer specific area capacitance for sphere diameter larger than 40 nm.