Abstract
Aqueous solutions have attracted considerable attention for use in aqueous-based energy-conversion devices such as aqueous lithium-ion batteries. While aqueous solutions have some desirable properties as electrolyte solutions, one drawback is the narrow potential window of water due to the electrolysis reactions of water (oxygen/hydrogen evolution reaction, OER/HER). Recently, it has been reported that the potential windows were expanded in concentrated electrolyte solutions. In this study, we investigated the potential windows in aqueous concentrated electrolyte solutions with various salts at different concentrations. A neutral pH solution with the lowest water concentration showed the largest potential window, and the potential windows were not affected by the electrolyte salts. In addition, we observed an asymmetric expansion of potential windows: the upper potential limits were shifted more than the lower limits. This can be explained by the local pH change in the vicinity of electrodes caused by OER/HER and the decrease in the rate of OER due to the reduced water concentrations.
Highlights
Aqueous solutions have attracted considerable attention for use in aqueous-based energy-conversion devices such as aqueous lithium-ion batteries
While aqueous solutions have some desirable properties as electrolyte solutions, one drawback is the narrow potential window of water due to the electrolysis reactions of water
We observed an asymmetric expansion of potential windows: the upper potential limits were shifted more than the lower limits. This can be explained by the local pH change in the vicinity of electrodes caused by oxygen evolution reaction (OER)/hydrogen evolution reaction (HER) and the decrease in the rate of OER due to the reduced water concentrations
Summary
Aqueous solutions have attracted considerable attention for use in aqueous-based energy-conversion devices such as aqueous lithium-ion batteries. Lead-acid rechargeable batteries have a working voltage of 2 V in a sulfuric acid solution This large voltage is due to the overpotentials of OER and HER on a positive electrode material (PbO2 ) and a negative electrode material (Pb), respectively.[1,2] Besides the catalytic activities of the electrode materials,[1,2] the properties of aqueous solutions such as hydration structures and salt concentrations affect the potential window. These results indicate that the electrolyte concentration affects the potential windows of water, but the underlying mechanism has not been sufficiently clarified
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