Abstract
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. 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. In proof of this fact, the capability of SSPAS was examined for the electrolyte of capacitors. 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. 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. Indeed, in an asymmetric capacitor containing MnO2 and Fe3O4 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. Similar electrochemical behaviour was also confirmed in saturated aqueous solutions of other alkali and alkaline earth metal perchlorate salts.
Highlights
Summarized on a variety of capacitors using various aqueous electrolytes and various electrodes
This leads to a conclusion that the O-H bond of water should be strengthened if a water molecule is isolated from others because of the weakened hydrogen bond
The addition of ions into water disturbs the hydrogen bond network, where the electronegative oxygen atom of water is attracted to positive ions and the electron poor hydrogen atom is attracted to negative ions
Summary
Summarized on a variety of capacitors using various aqueous electrolytes and various electrodes. In the case of saturated sodium perchlorate aqueous solution (SSPAS), since sodium perchlorate is extremely soluble in water (the solubility is 219.6 g in 100 g water at 25 °C), only 3.3 water molecules exist per one sodium perchlorate molecule Under such a condition the hydrogen bond could be destroyed almost completely due to the lack of neighboring water molecules and the strong hydration to Na+ and ClO4−. We do not know in detail, but it is true that the evaporation rate of SSPAS is extremely slow owing to the strong hydration This unusual phenomenon is known to be a theoretical base that water may exist in Mars[13]. It is not surprising that the potential windows of SSPAS is extended to the region of non-aqueous solvents because of the enhanced strength of O-H bond due to the loss of hydrogen bond
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