A Study of the Na/Na+ Redox Couple in Some Room Temperature Ionic Liquids

Cyclic voltammetry experiments were conducted using solutions of Na+ ions in 12 different ionic liquids (RTILs) on a nickel microelectrode in order to observe the deposition and stripping of metallic sodium. In most of the liquids the potential window was insufficient to observe the formation and removal of bulk sodium, despite the enlargement of some potential windows in the presence of sodium. However, in the ionic liquids N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)mide ([C(4)mpyrr][NTf2]), N-ethyldimethylpropylammonium bis(trifluoromethlsulfonyl)imide ([N2,1,1,3][NTf2] and N-hexyltriethylammonium bis(trifluoromethylsulfonyl)imide ethylsulfonyl)imide ([N-6,N-2,N-2,N-2][NTf2) the deposition and stripping peaks for the bulk sodium were observed, allowing simulation of the current-voltage curves and extraction of kinetic and thermodynamic data, notably the electrochemical rate constant, k(0), and the formal potential, E-r(0), for the Na/Na+ couple. Variable temperature measurements also gave quantitative information on the temperature dependence of E-r(0), dE(f)(0)/dT. Diffusion coefficients for Na+ ions in the three RTILs are also reported. All parameters are compared and contrasted with that of the Li/Li+ couple, demonstration that sodium possesses a more positive E-r(0) than lithium in the investigated RTILs, although the difference is markedly less than that in solvents such as water or ammonia.