In recent years, environmentally friendly energy storage devices have gained increasing attention due to the growing requirement of energy storage for sustainable energy applications. Among the various existing energy storage devices, supercapacitors (electrical double layer capacitors (EDLC) and hybrid capacitors (HC)) are considered as the most promising short-term energy storage devices [1]. EDLCs store energy in the electrical double layer, where the adsorption of ions is based mainly on the electrostatic interactions. The unique characteristics of EDLCs allow them to replace or combine with batteries and fuel cells in applications where the high-power pulses are important, such as the different energy recuperation systems and peak power sources, hybrid electric vehicles, wind turbines, digital communication devices, cameras and mobile phones, laptops, etc. [1].

Porous carbon materials are considered to be the most promising electrode materials for portable supercapacitors due to the high surface area, good electrical conductivity, good chemical stability, low gravimetric density and low cost [2]. The electrical charge accumulated in EDLC depends on the electrochemically active surface area and, thus, on the porosity and hierarchical porous structure of a carbon material. In addition, the presence of mesopores in porous carbon materials determines the power density of EDLC having a strong effect on the rate of mass transfer (diffusion and migration) and adsorption rate of charge carriers inside the hierarchical porous matrix. Therefore, the characteristics of micro- and mesoporous carbon materials (especially the ratio of micropore and mesopore surface areas and pore volumes) have to be optimized to improve further the specific energy and power density of EDLCs [1,2].

The objective of this study was to investigate the applicability limits of carbon material derived from granulated white sugar (GWS carbon) by hydrothermal carbonization (HTC) method combined with subsequent zinc chloride activation step of hydrochar, for supercapacitor electrodes. Synthesized GWS carbon material was used as an electrode material in the two-electrode single cells of EDLC filled with 1 M triethylmethylammonium tetrafluoroborate (TEMABF₄) solution in acetonitrile (AN) or 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid (EMImBF₄) as the electrolytes. Cyclic voltammetry (CV), constant current charge/discharge (CC), electrochemical impedance spectroscopy (EIS) and constant power discharge (CP) methods were used to study the electrochemical performance of EDLC.

The EIS, CV and CC measurement results show that the values of specific capacitance are somewhat higher for EMImBF₄ (135 F g^-1) electrolyte compared to the TEMABF₄ in AN (110 F g^-1). The CP test results show that at low constant power values the stored energy is higher for EDLC based on EMImBF₄ ionic liquid (48 W h kg^-1) compared with EDLC based on TEMABF₄ in AN electrolyte (39 W h kg^-1). However, the best capacitance retention and shortest relaxation time constant were established for EDLCs in 1 M TEMABF₄ solution in AN due to the lower viscosity and higher electrical conductivity compared to the ionic liquid based electrolytes and EDLC based on 1 M TEMABF₄ solution in AN electrolyte delivers substantially higher energy at higher constant power values. Thus, the GWS carbon material synthesized using cheap and abundant GWS as the starting material, show good electrochemical performance in TEMABF₄ in AN as well in EMImBF₄ ionic liquid and is promising carbon material for the high energy and power density supercapacitor application.

Acknowledgements

This research was supported by the EU through the European Regional Development Fund (Centers of Excellence, 2014-2020.4.01.15-0011 and 3.2.0101–0030, TeRa project SLOKT12026T. Higher education specialization stipends in smart specialization growth areas 2014-2020.4.02.16-0026) and Institutional Research Grant IUT20–13. This work was partially supported by Estonian Research Council grants PUT1033 and PUT55.

References

[1] R. Kötz, M. Carlen, Principles and applications of electrochemical capacitors, Electrochimica Acta. 45 (2000) 2483–2498.

[2] A. Burke, Ultracapacitors: why, how, and where is the technology, J. Power Sources. 91 (2000) 37–50.