Development of quaternary ammonium based electrolytes for rechargeable batteries and fuel cells
Lang, Christopher M.
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In this work, electrolytes for secondary batteries and fuel cells were investigated. Ionic liquids (ILs), for use as battery electrolytes, were formed using quaternary ammonium salts (Quats) and aluminum chloride. The room temperature (RT) carbonate fuel cell was demonstrated by modifying a commercially available anion exchange membrane, utilizing positive quaternary ammonium fixed sites, to transport carbonate. The charge density on the nitrogen and the symmetry of the Quat were demonstrated to be the dominant factors in determining the IL melting point (MP). The introduction of a benzyl ring was found to lower the MP of the ILs by increasing the size of the Quat, while disrupting its symmetry. ILs formed from asymmetric quaternary ammonium salts having three distinct groups were found to have lower melting points than those formed using Quats with two groups. Replacement of an alkyl group with a rigid ether linkage can lower the IL melting point. Assymetric alkyl substituted Quats were found to form more electrochemically stable, less viscous ILs than their benzyl substituted counterparts. The increased electrochemical stability is due to the smaller butyl chain being a worse leaving group than the benzyl group. Similarly, the smaller size of the alkyl substituted Quats results in the lower viscosities. Lithium and sodium can be reversibly deposited from neutral ILs following the addition of an additive (such as SOCl2). The additive disrupts the strong coordination between Na+, or Li+, and AlCl4-. Chlorinated compounds, such as chloroform-D and carbon tetrachloride, were demonstrated to catalyze the reversible reduction of sodium. When neutralized with lithium and sodium, reversible Li-Na alloys were deposited. The Li-Na alloy appears to suppress dendrite formation and could potentially be used as a metal based anode in a rechargeable Li battery. A novel room temperature carbonate fuel cell was constructed. The alkaline environment could eliminate the need for water in the oxidation of methanol. Cells were operated on hydrogen, 1M methanol, and pure methanol fuels. CO2 was produced at the anode and O2 and CO2 were necessary at the cathode for operation, indicating that carbonate was the conducting ion.