The Electrode Processes Of Zinc Redox On Au/ionic Liquid Interface Studied By In-situ Surface Enhanced Infrared Absorption Spectroscopy

The electrodeposition of zinc is of extreme importance for Zn-air batteries, which are featured with high energy density and rich raw materials. However, in traditional alkaline electrolytes, the electrochemical deposition of zinc is always accompanied with severe problems such as dentrite formation, hydrogen evolution etc. Moreover, the volatilization issue of the aqueous electrolyte inhibits the development of rechargeable Zn-air batteries. Room temperature ionic liquids(RTILs), which are entirely composed of cations and anions, are promising alternatives to conventional alkaline electrolytes. Because ILs possess novel properties, such as extremely low volatility, good thermal stability, wide electrochemical windows and so on, the employment of ILs as the electrolyte for the Zn-air secondary batteries can effectively avoid the problems encountered so far. Researches show that the addition of water in different kinds of ionic liquids has a dramatic effect on the electrochemical behavior of zinc redox. It is well known that the redox process of zinc happens on the electrode surface which is closely associated with ions arrangement on the electrode/ionic liquid interface. However, so far the relevant interface structure is not clear. Therefore, it is important to study the electrode/ionic liquid interface structures during the zinc electrodeposition, which lay the theoretical foundation for the research of zinc deposition. In the present work, the Au / ionic liquid interface structures during the Zn2+/Zn redox process in several imidazolium-based ionic liquid as well as the effect of water addition have been investigated by means of in-situ surface enhanced infrared absorption spectroscopy(SEIRAS) as well as electrochemical method.Firstly, a series of cyclic voltammograms(CV) measurements have been carried out in three different ionic liquids(1-ethyl-3-methylimidazolium tetrafluoroborate [Emim BF4], 1-butyl-3-methylimidazolium tetrafluoroborate [Bmim BF4], 1-octyl-3-methylimidazolium tetrafluoroborate [Omim BF4]), containing 0.4M Zn(BF4)2 with and without 5v/v % H2 O respectively. The results show that the zinc deposition potentials in the ionic liquids mixtures(both with and without H2O) shift to negative values with the increase of alkyl chain length, and the peak current density of Zn redox is decreased as well. It has been ascribed to the increased viscosity. For each kind of ionic liquid, compared with water-free ILs mixed with 0.4 M Zn(BF4)2, the zinc deposition potential in IL + 5v/v% H2 O + 0.4 M Zn(BF4)2 shifts to less negative values, and the reversibility of Zn redox is improved. Moreover, the peak current density is increased. A series of SEIRAS measurements have been performed at different potentials in two different systems at Au electrode: Bmim BF4 + 0.4 M Zn(BF4)2 and Bmim BF4 + 5v/v% H2 O + 0.4 M Zn(BF4)2. According to SEIRAS results of Bmim BF4 + 5v/v% H2 O + 0.4 M Zn(BF4)2, water molecular preferentially coordinates with Zn2+ to form smaller size groups which adsorb on the electrode surface, forcing the complete desorption of [Bmim]+ cations and BF4- anions.Secondly, the zinc electrochemical behavior is also studied in the trifluoromethanesulfonate anion based ILs(1-ethyl-3-methylimidazolium trifluoromethanesulfonate [Emim OTf], 1-butyl-3-methylimidazolium trifluoromethanesulfonate [Bmim OTf], 1-octyl-3-methylimidazolium trifluoromethanesulfonate [Omim OTf]), containing 0.4M Zn(OTf)2 with and without 5v/v % H2 O respectively. The results show that in water added Emim OTf and Bmim OTf systems, the peak current density of Zn redox is increased compared to water-free system. But the peak current densities are decreased in water added Omim OTf systems, which may be related to the arrangement of IL anions and cations adjacent to the electrode in a compact layer. And, the interface structures of Bmim OTf + 0.4 M Zn(OTf)2, Bmim OTf + 5v/v% H2 O + 0.4 M Zn(OTf)2, Emim OTf + 0.4 M Zn(OTf)2 and Emim OTf + 5v/v% H2 O + 0.4 M Zn(OTf)2 during the deposition of zinc have been studied by SEIRAS. Due to the specific adsorption of [OTf]- on the Au electrode, in the water added ionic liquids systems, the electrodes have not been hydrated completely. During the deposition of zinc, water molecule and hydrated Zn2+ are absorbed on the electrode surface, while anions and cations are desorbed.Finally, the zinc electrochemical behavior has also been studied in two kinds of functionalized ionic liquids(1-cyanopropyl-3-methylimidazolium tetrafluoroborate [Cpmim BF4], 1-hydroxyethyl-3-methylimidazolium tetrafluoroborate [HOEtmim BF4]). The results show that Zn redox in [Cpmim BF4] and [HOEtmim BF4] shows better electrochemcal behavior than in [BMIM][BF4].In situ surface-enhanced infrared absorption spectroscopy(SEIRAS) has been employed to study electrode/RTIL interface structure during the redox process of zinc. The knowledge about electrode/RTIL interface structure explains in-depth the effect of water on zinc electrochemical behavior and lays theoretical foundation for zinc deposition in ioninc liquid systems.