For the study on capacitors the width of potential window is essentially important, since the energy storage in electric double-layer capacitors is proportional to the square of applied voltage 1. As a matter of fact the thermodynamic potential window of water is known to be 1.23 V. This is attributed to the narrow electrochemical potential windows of aqueous solutions. However, there exists serious disadvantage in aqueous electrolytes that water is easily electrolyzed to generate gases. It has been generally understood that aqueous electrolytes have many advantages compared with non-aqueous solvents with respect to electrochemical behaviour as well as economic and environmental impacts. Similar electrochemical behaviour was also confirmed in saturated aqueous solutions of other alkali and alkaline earth metal perchlorate salts.

Indeed, in an asymmetric capacitor containing MnO 2 and Fe 3O 4 mixtures in the positive and negative electrodes, respectively, the energy density enlarged to be 36.3 Whkg −1, which belongs to the largest value in capacitors. The SSPAS worked also as a functional electrolyte in the presence of an activated carbon and metal oxides in order to increase an energy density. Galvanostatic charge-discharge measurements showed that a graphite-based capacitor containing SSPAS as an electrolyte was stable within 5% deviation for the 10,000 times repetition at the operating voltage of 3.2 V without generating any gas.

In proof of this fact, the capability of SSPAS was examined for the electrolyte of capacitors. Such a wide potential window has never been reported in any aqueous solutions, and this finding would be of historical significance for aqueous electrolyte to overcome its weak point that the potential window is narrow. A saturated aqueous solution of sodium perchlorate (SSPAS) was found to be electrochemically superior, because the potential window is remarkably wide to be approximately 3.2 V in terms of a cyclic voltammetry.

Naclo4 xps peak sodium ion