(682f) Energy Storage Mechanism of Porous Supercapacitors

Electrical double layer capacitors (EDLCs), also called supercapacitors, have attracted considerable attention in the electrical energy community. To address the limitation of supercapacitors related to their moderate energy density, many efforts have focused on the development of porous carbon-based materials as promising electrodes for supercapacitors to improve their energy density. Despite considerable work on porous carbon materials, the details of what happens under nano-confinement within the pores still require in-depth exploration.

Using molecular dynamics (MD) simulation, we investigated supercapacitors with porous carbon electrodes filled with two types of electrolytes: aqueous electrolytes and room-temperature ionic liquids (RTILs). For aqueous electrolytes, we analysed the ion and water distribution inside different-sized carbon pore and developed a sandwich capacitance model to describe the capacitance of EDLs formed inside carbon micropores. This model is capable of predicting the anomalous enhancement of capacitance experimentally observed at a specific pore size. We show that the free energy penalty for an ion getting into the pore with size of 0.82 nm is less than 14 kJ/mol for both sodium and chlorine ions, which provides strong impetus for further developing nanoporous electrodes featuring subnanometer pores. For RTIL electrolytes, we studied the pore-size dependence of capacitance over a broad range of pore size, by performing MD simulations of EDLs inside slit-shaped micropores with RTILs. We observed that the capacitance of porous supercapacitors exhibits oscillatory behavior as a function of electrode nanopore size and attributed this phenomenon to the interference of two electrical double layers originating from each wall of the carbon slit.