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. 2017 Mar 21:7:45048.
doi: 10.1038/srep45048.

An aqueous electrolyte of the widest potential window and its superior capability for capacitors

Hiroshi Tomiyasu  1   2 , Hirokazu Shikata  1 , Koichiro Takao  2 , Noriko Asanuma  3 , Seiichi Taruta  4 , Yoon-Yul Park  1
Affiliations

An aqueous electrolyte of the widest potential window and its superior capability for capacitors

Hiroshi Tomiyasu et al. Sci Rep. .

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.

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Conflict of interest statement

The authors declare no competing financial interests.

Figures

Figure 1
Figure 1. Cyclic voltammogram of SSPAS and those of 1 M H2SO4 and 1 M NaOH aqueous solutions using a glassy carbon for a working electrode.
Measurements were made separately for the positive scan to 2.3 V and for the negative scan to −2.0 V at the scan rate from 30 mVs−1 to 200 mVs−1 at 25 °C.
Figure 2
Figure 2. Estimated structure of a capacitor using SSPAS as an electrolyte.
At least one water molecule should be strongly hydrated to NaClO4 and number of free water molecules would not be enough to form a hydrogen bond.
Figure 3
Figure 3. Galvanostatic charge-discharge cycles for various graphite-based symmetric capacitors.
(a) Charge and discharge cycles of a symmetric graphite-based capacitor with a CEM as the separator for10,000 times repetition; (b) Charge and discharge cycles of a symmetric graphite-based capacitor containing 20% of AC with a CEM, (c) with a MF and (d) with a FP as the separator (e). Charge and discharge cycles of a symmetric graphite capacitor containing 40% of AC with a MF as the separator. Other specific conditions are described in Table 1.
Figure 4
Figure 4
(1) Galvanostatic charge-discharge cycles plotted against time(s) by use of an Ag/AgCl reference electrode, where the electrode consisting of graphite mixture as a working electrode and the electrode containing 30% Fe2O3 as a counter electrode; (a) counter electrode, (b) working electrode, and (c) cell potential. (2) Charge-discharge cycles for various symmetric and asymmetric capacitors containing Fe2O3, Fe3O4, V2O3, V2O5 and MnO2. The capacitors are expressed as positive electrode/negative electrode. (a) GA/Fe2 O3 (30%), (b) Fe2O3 (30%)/Fe2O3 (30%). (c) GA/Fe3O4 (30%), (d) Fe3O4 (30%)/Fe3O4(30%), (e) GA/V2O3 (30%), (f) GA/V2O5 (30%), (g) MnO2(30%)/GA(20%AC), (h) MnO2(30%)/Fe3O4(30%).
Figure 5
Figure 5
Galvanostatic charge-discharge cycles of the symmetric GA-based capacitors using saturated aqueous solutions of (a) LiClO4, (b) Mg(ClO4)2, (c) Ca(ClO4)2, (d) Ba(ClO4)2 and (e)Al(ClO4)3 as the electrolyte. Other specific conditions are described in Table 2. (a) LiClO4, (b) Mg(ClO4)2, (c) Ca(ClO4)2, (d) Ba(ClO4)2, (e) Al(ClO4)3.
Figure 6
Figure 6
(1) XRD patterns in the negative (a) and positive (b) electrode for a capacitor consisting of 10% of V2O3 in the negative electrode and GA in the positive electrode. The samples were taken after the charge. (2) XPS spectra for carbon 1 s of graphite. The sample were taken in the same way as in Fig. 6(1) under the same condition. Because of the large 1 s signal of perchlorate oxygen, the S/N ratio of the sample signals are lower compared with the standard peak of original graphite (green). The red peak corresponds to the negative electrode and blue one positive electrode, respectively.
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