The Influence of Conductive Salt Ion Selection on the Performance of High Voltage Edlcs

In state of the art electrochemical double layer capacitors (EDLCs), solutions of tetraethylammonium tetrafluroroborate (Et₄NBF₄) in propylene carbonate (PC) or acetonitrile (AN) are used as electrolytes, while activated carbons (AC) are used as electrode active materials. These combinations of materials enable the realization of EDLCs with high power (up to 10 kW kg^-1), extraordinary cycle life (>500.000 cycles) and energy in the order of 5Whkg^-1. The operative voltages of these devices are of 2.7 to 2.8 V [1].

It is known that an increase of the EDLCs energy would allow the introduction of these high power devices in a larger number of applications [1-2]. Therefore, in the last years many efforts have been made to increase the energy of EDLCs.

The energy E stored in an EDLC system is described by the equation E=1/2 CV², where C is the capacitance and Vthe operative voltage of the device. Considering this equation, it is evident that to increase the operative voltage in the most convenient strategy to increase the energy of these devices.

It has been shown that when the material-electrolyte combination reported above is used, the use of operative voltages higher than 2.8 V results in a significant decrease of the devices’ cycle life [1-2]. Therefore, the development of innovative electrolytes appears therefore a key aspect for the realization of high energy EDLCs: in the future new solvents, new conducting salts as well as new ionic liquids need to be considered [1-2].

Together with the electrode material, the ions of the conducting salts are forming the double layer, effectively influencing charge storage in in EDLCs. The size, electrochemical and chemical stability of these ions have a dramatic impact on the storage process. Nevertheless, it is interesting (and somehow surprising) to notice that in the past only a relatively low number of studies focused on the development of new conducting salt for high voltage EDLCs.

In this study we investigated the influence of conductive salts on the physical-chemical as well as electrochemical properties of PC based electrolytes in view of the realization of advanced high voltage EDLCs. We showed that the operative voltage of EDLCs containing PC-based electrolytes can be increased by a mere change in conductive salt. Nevertheless, anions and cations forming the conductive salt have to be carefully selected, as the choice of salt influences both electrochemical properties as well as ion transport properties of EDLC electrolytes, leading to an impact on energy and power storage in EDLC devices. While higher operative voltages are generally desirable, one has to consider and carefully balance the advantages and disadvantages of each electrolyte, as high values for viscosity or decreased ion mobilities can lead to decreased values for energy and power, even if operative voltages are increased [2].

Among the innovative electrolytes we investigated, those based on the salt N-methyl-N-butylpyrrolidinium tetrafluoroborate (Pyr₁₄BF₄) appear particularly promising. As a matter of fact, the electrolyte containing Pyr₁₄BF₄ as salt and in PC as solvent displays good conductivity and low viscosity and, thanks to its anion-cation combination, also allows the achievement of high operative voltages. As a consequence, its application enables the construction of EDLCs with higher energy and power compared to state-of-the-art devices (see Fig. 1) [3]. Furthermore, the use of these electrolytes makes also possible the realization of EDLCs exhibiting a high cycling stability [2-3].

References

[1] F. Béguin, V. Presser, A. Balducci, E. Frackowiak, Advanced Materials, 26 (2014) 2219-2251

[2] S. Pohlmann, C. Ramirez-Castro, A. Balducci, Journal of the Electrochemical Society 162 (5) A5020-A5030 (2015)

[3] S. Pohlmann, A. Balducci, Electrochimica Acta, 110, 221-227 (2013)