Engineering Ionic Liquid EDLCs: Influence of Cation Type, Carbon Structure and Increased Operation Temperature

Development of safe, robust and reliable electrochemical energy conversion systems with high energy and power densities can be a response to the universal demand for a clean transport industry free from any fossil fuels derivatives. Electrochemical Double Layer Capacitors (EDLCs) are potential candidates that not only provide short pulses of energy at high powers but also deliver stable charge-discharge cycles in excess of 106 cycles. Correspondingly when used in conjugation with a battery stack in an electric vehicle can assist the battery when power boosts are required and therefore extending the battery lifetime. In literature studies these devices are commonly referred to as Electrochemical Capacitors (ECs) and supercapacitors. Commercially available EDLCs are based on aqueous or organic electrolytes that can safely operate in limited potentials. However the room temperature ionic liquids (RTILs) are promising alternatives to replace the current electrolytes as they demonstrate significantly higher and safer operating potentials, thus improving specific energy density.
This study identified that the physiochemical properties, operating potential and the cation volume of the Ionic Liquids (ILs), as well as the pore size distribution of the carbon materials influencing the capacitance performance. Hence a systematic study of nine different ionic liquids with varying chain lengths and linkages from four classes of pyrrolidinium, sulfonium, ammonium and phosphonium RTILs was performed.
The utilized IL cations in this study are the following: 1-methyl-1- propylpyrrolidinium [Pyr13], 1-butyl-1-methylpyrrolidinium [Pyr14], diethyl- methylsulfonium [S221], triethylsulfonium [S222], butyltrimethylammonium [N1114], butyltriethylammonium [N2224], N,N-diethyl-N-methyl-N-(2methoxy- ethyl)ammonium [N122(2O1)], pentyltriethylphosphonium [P2225] and (2methoxy- ethyl)triethylphosphonium [P222(2O1)] that are combined with a bis(trifluoro- methane)sulfonimide [NTf2] anion.
The characterization of the utilized ILs was performed using Karl Fischer measurements, Differential Scanning Calorimetry, rheology, density and conductivity measurements and two/three electrode stability potential measurements.
The effect of pore size distribution was also investigated by combining each liquid with four different activated carbons produced in-situ where the pore characteristics of the produced carbons was controlled with varying the precursors quantities.
The temperature elevation approach was also used at 25°C, 40°C, 60°C and 80°C in order to study the effect of temperature on ILs physiochemical properties and capacitance response of the produced cells. The capacitance response was investigated with Galvanostatic cycling (GC) at a wide range of discharge densities. Electrochemical Impedance Spectroscopy (EIS) was also used to determine the capacitance performance at 0.01 Hz and monitor the solution, ionic and equivalent series resistances variation with pore size distribution and temperature.