Activated carbon produced from hydrochars has been widely studied and used in applications such as energy storage, environmental remediation and resource recovery. In the fields of energy storage, the electrode materials are known as a key factor determining the performance of electrical double layer capacitor (EDLC) [1]. The present study examined the effect of zinc chloride, potassium hydroxide or a mixture of them to the energetic characteristics of EDLC.

Activated microporous carbon materials used for the preparation of electrodes have been synthesized from the hydrochars prepared via hydrothermal carbonization process (HTC) of D-(+)-glucose solution in H₂O, followed by activation with ZnCl₂, KOH or their mixture. Resulting 6 activated carbons were physically characterized by using scanning electron microscopy, Raman spectroscopy, X-ray diffraction and nitrogen and carbon dioxide sorption methods. 1 M triethylmethylammonium tetrafluoroborate solution in acetonitrile and activated carbon based electrodes were combined into test cells which electrochemical characteristics were established by cyclic voltammetry, constant current charge/discharge, electrochemical impedance spectroscopy and constant power discharge methods.

Activation of HTC only with ZnCl₂ or KOH substantially increases the number of micropores as well as creates some mesopores, thus, produces the carbon material with mixed micro- and mesoporous structure which enhances energy and power density of EDLC. Highest porosity and Brunauer-Emmett-Teller specific surface area (S_BET = 2150 m² g^-1), micropore surface area (S_micro = 2140 m² g^-1) and total pore volume (V_tot = 1.01 cm³ g^-1) have been achieved for hydrochar activated using KOH with a mass ratio of 1:4 at 700 ˚C. The S_BET, S_micro, V_tot and electrochemical characteristics were heavily influenced by the synthesis conditions of carbon materials (Fig. 1). Wide region of ideal polarizability (ΔV ≤ 3.0 V), very short characteristic relaxation time (0.66 s), and high specific series capacitance (134 F g^-1) have been calculated for the mentioned activated carbon material, demonstrating that this system can be used for completing the EDLC with high energy- and power densities [2].

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] 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.