Enhancing power and energy densities of supercapacitors with slit porous carbon electrodes by playing with pore width and ion shapes

Abstract

The energy and power performances of double-layer supercapacitors can be affected by pore size, the nature of the electrolyte, and electrode–electrolyte interaction. We performed molecular dynamics simulations of mixtures of ionic liquids confined into slit-shaped carbon electrodes with pores of 0.75 and 1.5nm under applied voltage to investigate the interplay of pore size, and ion shape to the performance of supercapacitor. Inside the pores of 1.5nm the ions are organized into two layers, and a single layer inside the pore of 0.75nm. The tetramethylammonium cations ([TMA]+) is steric hindered to fill the pores of 0.75nm that can be only filled by 1-ethyl-3-methyl-imidazolium cations ([EMIM]+) and bis(trifluoromethanesulfonyl)imide anions ([NTf2]−). The increase of [TMA][NTf2] in the mixtures results in small pores with a decreased density of ions or empty at the non-charged state of the electrode. Upon the application of voltage, these empty or partially filled pores have only the adsorption of counter ions, which are responsible for the best performance of mixtures with mole fraction of [TMA]+ equals to 0.25 and 0.75.