Development of electrochemical capacitors focuses today on the improvement of their energy density (or specific energy) and cyclability. The energy density might be improved by capacitance enhancement and operating voltage increase. To some extent, the relation between capacitance and voltage and their impact on the energy output reflects an interfacial character of charge storage mechanisms in capacitors; capacitance is intimately linked with the electrode material whereas the operating voltage is governed by the electrolyte applied. Of course, they cannot be considered separately, since the final performance is always a combination of various factors (e.g., electrode porosity – electrolyte viscosity, wettability, etc.).

In case of capacitance-related issues, it appears that activated carbons with their well-developed surface area and suitable porosity, recently reached the limit of purely capacitive storage and further progress in this field require another insight into the electrostatic charge accumulation. That approach enforces intensive research on the novel architectures of the electrode materials.

On the other hand, the electrolyte-related issues create an interesting pathway for investigations aiming at maximum voltage increase and energy density enhancement. Undoubtedly, organic media (based on acetonitrile or propylene carbonate) and ionic liquids are the optimal electrolytic solutions in terms of electrochemical stability. However, their impact on the environment and user safety is quite often questioned. Water-based electrolytes seem to be an interesting alternative, but the major objection against their commercialization concerns their low electrochemical stability governed by water decomposition voltage. Although the neutral electrolytes might demonstrate a significant decomposition overpotential once applied to porous carbon electrodes and thus provide the operating voltage up to 1.8 V, the performance of water-based capacitors is still not satisfactory enough for a broad application.

Since the significant disadvantage of water-based solutions is attributed to the solvent decomposition, in our recent study, we decided to decrease the amount of water in aqueous electrolytes to the minimal level. Following the approach presented by the Belanger group, we have formulated the series of electrolytes with water: salt ratio up to 50% (either by mass, volume or molarity). Since the primary study has been done on LiTFSI salt, in our study, we have focused on the conventional inorganic salts, based on the well-known anions like NO₃^- or SO₄^2-. It appeared that these formulations demonstrated various correlations with concentration, conductivity and decomposition potentials. Moreover, their viscoelastic properties perfectly correlated with electrochemical performance, mostly with charge propagation. Surprisingly, the role of cation appeared to be crucial, essentially for capacitance values.

The paper will discuss the correlation between the pore size distribution of carbon electrodes, electrolyte viscosity, and capacitor performance. Elucidation of the energy/power characteristics as well as cyclability at various temperatures will give a novel insight into ‘water’-based electrolytes. Spectroelectrochemical measurements will support the physicochemical data (e.g., operando Raman spectroscopy), unraveling (or trying to unravel) the mysteries at carbon electrode/highly concentrated electrolyte interface.

Acknowledgements

European Research Council is acknowledged for financial support within the Starting Grant project (GA 759603) under European Unions’ Horizon 2020 research and innovation programme.