Electrochemical Study of Zn/Zn2+ Redox Behavior in Functionalized Ionic Liquids: Water Effect
Authors: Zhou Wei , Qi Chengzi , Yu Aoqiu , Wu Xinhu 
Published: February 2020

The redox behavior of Zn electrodeposition and stripping is essentially important for the development of Zn-air battery, which is featured with high energy density, rich raw materials and high safety. However, in alkaline electrolytes, which is usually employed in primary Zn-air battery, the electrochemical redox of Zn/Zn2+ is always accompanied with severe problems such as Zn metal dentrite formation, hydrogen evolution, poor reversibility, etc. Moreover, the volatilization issue of the aqueous electrolyte as well as the carbonation of the alkaline electrolyte by CO2 in air inhibits the development of rechargeable Zn-air battery. Room temperature ionic liquids (RTILs), which are entirely composed of cations and anions, are promising alternatives to conventional alkaline electrolytes used in Zn-air battery, due to their novel physical and chemical properties such as extremely low volatility, high thermal stability, wide electrochemical window and so on. The employment of ILs as the electrolyte for the Zn-air secondary batteries can hopefully enhance the redox of Zn/Zn2+, and avoid the problems encountered for the alkaline aqueous electrolyte as well. The investigation of Zn/Zn2+ redox in ionic liquids system is of significant importance for the development of new rechargeable Zn-air battery. On the other hand, since the Zn-air battery is an open system to air, the effect of trace water on the electrode processes of Zn in ionic liquids should also be considered.

Herein, we study the electrochemical behavior of zinc redox on Pt, Au and Zn electrode in several normal imidazolium based ionic liquids including 1-ethyl-3-methylimidazolium tetrafluoroborate [Emim]BF4, 1-butyl-3-methylimidazolium tetrafluoroborate [Bmim]BF4, 1-octyl-3-methylimidazolium tetrafluoroborate [Omim]BF4, as well as two functionalized-imidazolium based ionic liquids of 1-cyanopropyl-3-methylimidazolium tetrafluoroborate [Cpmim]BF4 and 1-hydroxyethyl-3-methylimidazolium tetrafluoroborate [HOEtmim]BF4. The effect of trace water addition to the electrochemical behavior of the Zn redox has also been investigated.

In three normal imidazolium based ILs, cyclic voltammogram (CV) measurements show that quasi-reversible Zn/Zn2+ redox has been observed. The zinc deposition potential shifts to negative value with the increase of alkyl chain length, and the peak current density of Zn redox is decreased as well. It could be possibly ascribed to the increased viscosity with the increase of alkyl chain length. Upon the introducing of trace water into the systems, the zinc deposition potential shifts positively, and the reversibility of Zn redox is largely improved. Moreover, the peak current density of the Zn redox has been enhanced. The improved behavior of the Zn redox could be due to that water molecular preferentially coordinates with Zn2+ to form smaller size of Zn2+ ion groups, which adsorb on the electrode surface to enhance the deposition of Zn.

To further investigate the effect of coordination state of Zn2+ ion to the electrochemical behavior of Zn redox, two functionalized ILs with a CN- group and an OH- group introduced into the imidazolium cations respectively have been employed. The results show that Zn redox in [Cpmim]BF4 and [HOEtmim]BF4 shows quite different electrochemical behavior comparing with the one in [Bmim]BF4. Better reversibility and higher redox current has been obtained. Water effect on the two systems has also been studied.

The knowledge about electrode/RTIL interface structure are of great importance to understand in-depth the effect of Zn2+ ion coordination as well as water molecule on the electrochemical behavior of Zn/Zn2+ redox, and then lays theoretical foundation for zinc deposition in ionic liquid systems. The study of the interfacial structure of Zn/Zn2+ redox on several metal electrodes by the employment of in situ surface-enhanced infrared absorption spectroscopy (SEIRAS) is still in progress in our lab.