Cell potential and free energy of electrolyte solutions of magnesium borohydride salt in mixtures of 2-methoxyethyl ether-propylene carbonate and 1,2-dimethoxyethane-propylene carbonate

Abstract

This study explores the outcome of mixed solvent systems of 2-Methoxyethyl ether (DGM)-Propylene carbonate (PC) and 1,2-dimethoxyethane (DME)-Propylene carbonate (PC) on the electrochemical performance of concentration cells in order to construe the behaviour of ions in the electrolyte medium. Organic electrolyte solutions of Magnesium borohydride salt were constituted with concentrations of 0.01 and 0.001 M in the binary mixtures of DGM-PC and DME-PC. The values of the cell potential measured at 298.15 K were found to increase over the composition range of 0 –50 % volume for DME and DGM; and then decrease over the range of 50 – 100 % volume for DME and DGM. The cell potential for the DME-PC system reached the maximum value at 0.448 mole fraction DME, while that for the DGM-PC system was 0.374 mole fraction DGM. The Gibbs free energy change ranged from -6.832 to -8.396 kJ/mol and -6.967 to -8.685 kJ/mol for DME-PC and DGM-PC systems respectively. The calculated values of Gibbs free energy change were more negative for DGM-PC than the DME-PC system signifying the spontaneity of the cell reactions, which is more pronounced in the case of DGM-PC system. The standard cell potential measured from 0.006 to 0.014 V for DME-PC, and 0.007 to 0.0015 V for DGM-PC within the tested range. The standard cell potential were higher in DGM-PC than DME-PC, suggesting better coordination with the three lone electron pairs on the oxygen atoms of DGM molecules, which afforded higher complex stability with Mg2+ and reaction rates for DGM-PC system.

Keywords: Cell potential, Gibbs free energy change, Concentration cell, Organic electrolyte, Solvated structure.