Electrode Kinetics in Nonaqueous Aprotic Solvents

Abstract

The electrode kinetics of Na+ reduction are studied at dropping mercury electrode in four polar aprotic solvents, propylene carbonate (PC), dimethylformamide (DMF), dimethylsulfoxide (DMSO), and acetonitrile (AN), using the faradaic impedance method. The true rate constants measured for 1M NaClO4 in PC, DMF, DMSO, and AN are 0.38, 0.21, 0.05, and 0.35 cm‐s−1, respectively. The corresponding cathodic transfer coefficients are measured to be 0.62, 0.81, 0.43, and 0.42, respectively. Na+ reduction is found to be a simple charge‐transfer reaction, based on its conformance to the analysis using the classical Randles circuit. Using relative rate constants false(knormalshnormalt)normalrel , for Na+ with water as reference solvent, a standard potential scale for Na+/Na vs. NHE is developed for all four solvents and compared with standard potential scales for K+/K vs. NHE. The surface potentials of all four solvents are calculated to be positive and discussed in terms of solvent dipole orientation. Comparison of rate constants and free energy of Na+ solvation is made to determine the effect of solvation on the kinetics of Na+ reduction in PC, DME, DMSO, and AN. The trend of standard rate constants follows the trend of sodium ion solvation in the nonaqueous solvents PC≅AN<DMF<DMSO .