Toward enhanced supercapacitors by mutually adapting electrolytes and nanoporous carbons

Abstract

Electrochemical capacitors are energy storage systems able to be fully charged and discharged within seconds. Because of their high power density and long cyclability, they are interesting devices for implementing them in many modern applications including the electric vehicle. They are most commonly made of high surface area carbonaceous active materials. This work presents the use of microporous carbons derived from olive pits, an abundant natural waste material in Spain, in order to produce active materials for electric double layer capacitors (EDLCs) and lithium and sodium ion capacitors (LICs and NICs, respectively). The synthesis conditions of the chemical activation process (temperature and KOH amount) have been adapted to prepare activated carbons (ACs) with improved electrochemical performance. Therefore, in aqueous 6M KOH electrolyte, the maximum gravimetric and volumetric capacitance values achieved are 260 F g-1 and 140 F cm-3 and in organic 1.5M (C2H5)4NBF4 organic electrolyte, the values are 150 F g-1 and 65 F cm-3, respectively. Furthermore, the relationship between capacitance and the complex interplay of the accessible specific surface area, the accessible average pore size and the effective dielectric permittivity have been studied, suggesting that moderately solvated ions enhance the high-rate capability in pores that slightly exceed the solvated ion size. These ACs have also been used as active materials in more exploratory type of electrolytes, such as, aqueous neutral salts and ionic liquids (ILs). ACs working in 6M LiCl electrolyte have demonstrated to be able to provide a capacitance value of 160 F g-1 in a voltage window of 1.6 V with very low resistance thanks to the high conductivity of the electrolyte at high salt concentration values. Similarly, when these mesopore-free ACs operate in neat EMITFSI IL, they are able to give gravimetric and volumetric capacitance values as high as 180 F g-1 and 150 F cm-3 within a cell voltage of 3V, respectively, due to the adaptation of the pore size to the ion size. Finally, LIC and NIC devices have also been built using the hard carbon directly obtained from the pyrolysis of the olive pits in the negative electrode and previously mentioned ACs in the positive electrode. These LIC and NIC devices overcome their EDLC counterpart in terms of energy density throughout the studied power density range while keeping the cyclability exhibited by EDLCs, which opens up the range of possible applications for these devices.