Physicochemical and electrochemical characteristics of novel aqueous electrolytes for hybrid capacitors

Description

The application of aqueous electrolytes is advantageous in electrochemical technologies for their unique features like being eco-friendly, non-flammable and safe. Additionally, they have higher ionic conductivity than those of nonaqueous ones as well as low raw material and manufacturing cost [1, 2, 3]. However, one of the major shortcomings of aqueous electrolytes is their low operating voltage, which limits the energy density of device. Tuning the interactions among ions and solvents in electrolyte have been explored as one of the effective way for high voltage operation of electrochemical capacitors [4,5]. To this end, several water-in-salt, water-in-bisalt, bisalt-in-water, and hydrate melt, have attained a soaring attention very recently for electrochemical energy storage due to their enriched stability and much-enlarged electrochemical stability window [3]. However, the proper understanding of the fundamental ion-water interaction mechanisms and their correlation with supercapacitor performance has not been widely explored yet.
For that reason, we investigated several aqueous electrolytes for disclosing the fundamental ion-water interaction to correlate their physicochemical and electrochemical characteristics with supercapacitor performance. Firstly, electrolytes were prepared by varying the concentration of NaNO3, NaI, Na2MoO4⋅2H2O, combination (bisalt-in-water) of NaNO3 and 0.5M NaI, combination of Na2MoO4⋅2H2O and 1M NaNO3. Then, different fractions of ethylene glycol (EG) were added into 1M NaNO3 into water (W) to explore the interaction of ion and solvents. Water-in-salt electrolyte was prepared from choline chloride. The ionic conductivity of electrolytes is determined by electrochemical impedance spectroscopy and compared with values obtained from conductometer. For different concentration of NaNO3 the pH values were almost unchanged. At lower concentration both NaI and Na2MoO4⋅2H2O was almost neutral which gradually turned into alkaline with increasing concentration. For varying concentrations of NaNO3 into 0.5 M NaI the pH was almost unchanged. With increasing concentration of Na2MoO4⋅2H2O into 1 M NaNO3, the pH got higher. Interestingly, for water-in-salt electrolyte of choline chloride the pH level was also almost identical/neutral (pH of 0.5 mol/kg and 20 mol/kg solutions is 7.1 and 8.5, respectively). For all salt or/and bisalt-in-water electrolytes, conductivity was found to be increased with increasing concentration. With addition of EG fraction into 1 M NaNO3 the conductivity was linearly decreased. Interestingly, for water-in-salt electrolyte of choline chloride, with increasing concentration, the conductivity firstly increased and then decreased with a maximum at 5 mol/kg (97.6 mS/cm). Forthcoming study includes the determination of ionicity, and electrochemical stability window of the electrolytes and correlating those with the hybrid supercapacitor performance.

References
1.X. Zang, C. Shen, M. Sanghadasa, & L. Lin, ChemElectroChem 6 (2019) 976–988.
2.W. Qin, N. Zhou, C. Wu, M. Xie, H. Sun, Y. Guo, and L. Pan, ACS omega 5 (2020) 3801-3808.
3.M. Amiri, and D. Bélanger, ChemSusChem 12 (2021) 2487-2500.
4.L. M. Suo, O. Borodin, T. Gao, M. Olguin, J. Ho, X. L. Fan, C. Luo, C. S. Wang, K. Xu, Science 350 (2015) 938–943.
5.Q. Abbas, P. Nürnberg, R. Ricco, F. Carraro, B. Gollas, and M. Schönhoff, Advanced Energy and SustainabilityResearch 2 (2021) 2100115.

Period	11 Jul 2022 → 15 Jul 2022
Event title	8th Regional Symposium on Electrochemistry of South-East Europe together with the 9th Kurt Schwabe Symposium
Event type	Conference
Location	Graz, AustriaShow on map
Degree of Recognition	International