Influence of separator properties on electrochemical performance of electrical double-layer capacitors

Abstract

Influence of separator material characteristics on the high and low frequency series and parallel resistance, low and high frequency capacitance, characteristic time constant, energy and power densities of the electrical double-layer capacitors (EDLCs), based on the two identical ideally polarizable microporous titanium carbide-derived carbon electrodes in 1M (C2H5)3CH3NBF4 acetonitrile solution, has been tested by cyclic voltammetry and electrochemical impedance spectroscopy methods. Self-made separator materials have been prepared from 20% or 25% poly(vinylidene fluoride) solutions in N,N-dimethylacetamide or N,N-dimethylformamide and acetone mixture (8:2 mass ratio) by using the electrospinning method at different electric field strengths and polymer solution feed rates. Separator material total porosity, pore size distribution and specific surface area, calculated using mercury porosimetry data, noticeably depend on electrospinning conditions and mercury pressure applied, explained by the compressibility of materials under study. It was established that the separator material structure (total porosity, specific surface area), chemical composition and thickness determines the high frequency series and low frequency parallel resistance, characteristic time constant and power density values of the EDLC. However, the limits of ideal polarizability, low frequency capacitance and the specific energy only weakly depend on the separator material properties used in EDLC cells.