A Low-Cost Zn-Ion Hybrid Supercapacitors with High Energy Density
Authors: Janes Alar , Eskusson Jaanus , Peensalu Paul-Egert , Thomberg Thomas , Lust Enn 
Published: December 2024

In recent years, hybrid supercapacitors with greater potential and improved energy density have been completed by combining some battery and supercapacitor type electrodes. According to some researchers’ research, “hybrid” supercapacitor systems are technically “asymmetric” supercapacitor systems since they are built on two separate supercapacitor type electrodes. The primary aim behind the development of a hybrid capacitor (HC) is to improve the energy density or specific energy of the device, which will significantly boost the storage property. Moreover, the use of an electrical double layer capacitor type electrode provides a large surface area, improving the electrode/electrolyte interaction region, and helps in the fast ion-adsorption process, which prevents the degradation of the battery-type electrode due to repeated charge-discharge cycles. A suitable combination of specific power (due to the EDLC electrode) and specific energy (due to the faradic electrode) could drastically enhance the stability and longevity of the hybrid supercapacitor device. The di/tri-valent metal ion (Mg2+, Zn2+, Ca2+, Al3+, etc.) hybrid capacitors have garnered considerable attention in recent years owing to their potential cost benefit and application in stationary storage systems, whose development is crucial for the mass-scale penetration of renewable energy technologies.
Zinc‐ion hybrid supercapacitors (ZIHSs) may be the most promising energy storage device alternatives for portable and large‐scale electronic devices, as they combine the benefits of both supercapacitors and zinc‐ion batteries. ZIHSs are mostly based on battery-type Zn metal as an anode and physical adsorption-based carbon materials as the cathode. Porous carbon materials with high surface area, good electrochemical stability, low cost, high electronic conductivity and tuneable surface structure are deemed as promising cathode materials for ZIHSs. It is generally recognized that the micropores facilitate electrochemical energy storage, and mesopores can effectively reduce the ion diffusion distance and transport resistance. Therefore, the adjustment and optimization of porous carbons electrodes are of great significance to ZIHSs.
This work provides some insight into the strategy for designing effective non-aqueous and aqueous Zn-ion hybrid supercapacitors. Electrochemical characteristics of Zn-ion hybrid supercapacitor cells based on non-aqueous 1 M acetonitrile (AN) or propylene carbonate (PC) electrolytes with addition Zn(BF4)2, zinc di[bis(trifluoromethylsulfonyl)imide] (Zn(TFSI)2) and zinc trifluoromethanesulfonate (Zn(OTf)2) have been studied. Very high energy and power densities (80 Wh kg−1 and 21.2 kW kg−1) have been measured for 1 M Zn(BF4)2/AN based Zn-ion based hybrid supercapacitors (Fig. 1a). Very good stability during 3,000 cycles of cells has been achieved demonstrating reasonably high energy efficiency value (66.8%) for Zn(TFSI)2/AN based ZIHS cell, decreasing in the order of electrolytes: Zn(TFSI)2/AN > Zn(BF4)2/PC > Zn(TFSI)2/PC > Zn(OTf)2/AN > Zn(BF4)2/AN. Some assembled ZIHSs had shown excellent cycling and energy stability over 20,000 cycles [1].
Electrochemical behaviour of Zn cation based salts in various aqueous electrolytes (ZnSO4, Zn(BF4)2, Zn(TFSI)2, and Zn(OTf)2) has been studied in thin ZIHS cell and compared with Zn(ClO4)2 aqueous electrolyte based cell electrochemical characteristics. At moderate specific power value (10 kW kg−
1) noticeable decrease of specific energy has been established in the order of aqueous electrolytes: Zn(ClO4)2 ⩾ Zn(BF4)2 > Zn(OTf)2 > Zn(TFSI)2 > ZnSO4 (Fig. 1b). The stability of Zn-ion hybrid supercapacitor cells under study in aqueous electrolyte solutions has been tested using the long lasting (up to 10,000 cycles) constant current charge/discharge method and very good stability for Zn(OTf)2, Zn(ClO4)2 and ZnSO4 has been observed [2,3].
Taking into account the cheap and environmental friendliness electrodes and electrolyte used, the results can be applied for assembling of the cheap high energy density hybrid supercapacitors for sustainable energy storage/recuperation complexes, combined with photovoltaic fields and/or wind electricity generating systems.

Acknowledgements

This work was supported by the Estonian Ministry of Education and Research (TK210, Centre of Excellence in Sustainable Green Hydrogen and Energy Technologies), Personal Research Grant PRG676 and R&D project EAG228. The experimental part of the research was partially co-funded by the Feasibility Fund of the University of Tartu Development Fund.

References

[1] K.-S. Põder, J. Eskusson, E. Lust, and A. Jänes, J. Electrochem. Soc.
170, 060501, 2023.
[2] J. Eskusson, T. Thomberg, E. Lust, and A. Jänes, J. Electrochem. Soc. 169,020512, 2022.
[3] J. Eskusson, T. Thomberg, T. Romann, K. Lust, E. Lust, and A. Jänes, J. Solid State Electrochem. 25, 2869, 2021.

Figure 1