Influence of porosity parameters and electrolyte chemical composition on the power densities of non-aqueous and ionic liquid based supercapacitors

Abstract

Influence of specific surface area (SDFT), total pore volume (Vtot) and other porosity characteristics on the electrochemical parameters and power density of two electrode electric double layer capacitors based on 1 M 3-ethyl-methylammonium tetrafluoroborate (Et3MeNBF4) solution in acetonitrile and on 1-ethyl-3-methylimidazolium tetrafluoroborate (EtMeImBF4) has been analysed. The pore size distribution data calculated from nitrogen, CO2 and Ar adsorption isotherms using mainly Carbon 2D non-local density functional theory for heterogeneous surface (2D-NLDFT-HS) model have been compared with crystallographic characteristics obtained by Raman, X-ray diffraction, photoelectron spectroscopy, etc. methods. It was shown that, chemical composition and crystallographic structure of precursor material, synthesis and activation conditions have decisive influence on the shape (spherical, cylindrical or slit-shape pores) and hierarchical porous structure and electrical conductivity of the carbon materials. Noticeable increase in series and parallel capacitances from 50 to 138 F g−1 in 1 M Et3MeNBF4 and up to 155 F g−1 in EtMeImBF4, gravimetric power density (up to 35 kW kg−1) and volumetric power density (up to 25 kW dm−3, both at discharge time 3.6 s in 1 M Et3MeNBF4 + AN electrolyte) for optimised EDLC has been demonstrated. For EtMeImBF4 based cells lower gravimetric (25 kW kg−1) and volumetric (10 kW dm−3) power densities have been achieved. For completing the EDLCs with high power and energy densities, highly micro-and mesoporous materials with optimum specific surface area (1200–1500 m2 g−1) but maximum (meso) pore volume (Vtot > 1.5 cm3 g−1) should be applied. Only for optimised EDLCs the very short characteristic charging/discharging times (lower than 0.3 s in Et3MeNBF4 + AN and 1.0 s in EtMeImBF4) can be achieved.