Room temperature ionic liquid (RTIL) is a salt, consisting of large asymmetric cation and complex anion, which is stable in liquid phase below 100°C. Properties of RTILs are solely dependent on their constituent cation and anion. In terms of electroc...
Room temperature ionic liquid (RTIL) is a salt, consisting of large asymmetric cation and complex anion, which is stable in liquid phase below 100°C. Properties of RTILs are solely dependent on their constituent cation and anion. In terms of electrochemistry, the most significant properties of RTILs that should be considered are viscosity, conductivity, and electrochemical stability window. Electrochemical stability window, which refers to a potential range where cation and anion are neither reduced nor oxidized, of RTILs is typically wider than aqueous electrolytes. This feature enables electrodeposition of metals with large negative reduction potential such as aluminum, zinc, magnesium, and lithium, which are otherwise unable to be recovered in aqueous solvents. In battery application, replacing conventional organic solvents with nonvolatile and nonflammable RTILs would help to improve battery safety.
This dissertation focuses on the electrochemical reactions of metal electrodes in RTILs, including electrodeposition and corrosion. Three different topics regarding the oxidation and reduction reactions of some selected metals in RTILs are discussed in this dissertation. The effect of RTIL structures on the electrodeposition/dissolution behavior of lithium is investigated in the first topic. The second topic is about the oxidation and reduction reversibility of zinc anode and the effect of water addition in RTIL-based zinc-air battery application. In the third topic, the corrosion susceptibility of some selected metals is evaluated in a protic RTIL containing available proton on cation, and compared with aprotic one with a similar structure.
RTIL structure is confirmed to affect the electrodeposition/dissolution potential of lithium. The largest portion of lithium metal in deposits is obtained from a sample electrodeposited in RTIL with the widest electrochemical stability window, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMPyr] [NTf2]). In regard to the application of RTILs as an electrolyte for zinc-air battery, zinc electrode shows reversible oxidation-reduction reactions in [BMPyr] [NTf2]. However, the presence of water gives a significant effect on the redox reactions of zinc electrode. Protic RTIL (1-butylpyrrolidinium bis(trifluoromethylsulfonyl)imide) is confirmed to show a narrower electrochemical stability window compared to that of aprotic RTIL ([BMPyr] [NTf2]) of similar structure and is more reactive towards metals. Lower corrosion potentials and higher corrosion current densities are observed in the protic RTIL.