The increased global energy demand and pollution of environment due to the consumption of fossil fuels require utilization of energy generated from renewable energy sources such as wind, tidal and solar power. As the contribution of these energy resources grows the energy storage systems with high energy densities, efficiencies and long cycle life become increasingly important. Energy storage systems are needed at times of high energy demand by storing energy when in excess. In this regard supercapacitors (SCs) are considered as the most promising energy storage devices due to their high power density, short characteristic time constant, excellent coulombic reversibility (98% or higher), high energy efficiency (92 – 94%), long cycle life (over 10⁶ cycles) and wide operation temperature range [1-3].

Porous carbon materials are the most promising electrode materials for portable SCs due to the high surface area, good electrical conductivity, good chemical stability, low gravimetric density and low cost [4,5]. The electrical energy accumulated in SCs 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 SCs having a strong effect on the rate of mass transfer 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 [4,5].

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 SC electrodes. Synthesized GWS carbon material was used as an electrode material in SC cell 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, shortest relaxation time constant and substantially higher energy density delivered at higher constant power values were established for SC cell in 1 M TEMABF₄ solution in AN due to the lower viscosity and higher electrical conductivity compared to the ionic liquid based electrolyte [5].

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] B.E. Conway, Electrochemical supercapacitors: scientific fundamentals and technological applications, Springer, New York, 1999.

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

[3] J.R. Miller, A.F. Burke, Electrochemical capacitors: Challenges and opportunities for real-world applications, Electrochem. Soc. Interface. 17 (2008) 53–57.

[4] M. Härmas, T. Thomberg, H. Kurig, T. Romann, A. Jänes, E. Lust, Microporous–mesoporous carbons for energy storage synthesized by activation of carbonaceous material by zinc chloride, potassium hydroxide or mixture of them, J. Power Sources. 326 (2016) 624–634.

[5] M. Härmas, T. Thomberg, T. Romann, A. Jänes, E. Lust, Carbon for Energy Storage Derived from Granulated White Sugar by Hydrothermal Carbonization and Subsequent Zinc Chloride Activation, J. Electrochem. Soc. 164 (2017) A1866–A1872.