Ionic Liquids of Widely Variable Proton Activity in Relation to BH Hydrogen Storage Technologies and Novel Low-Vapor-Pressure Superacids

Abstract

A common method of storing hydrogen gas for use in fuel cell applications is based on the solubility and stability of sodium borohydride in strongly basic (3 M NaOH) aqueous solutions. The release of hydrogen is accomplished by acidification of the solutions in the presence of catalysts or elevated temperatures. The importance of basicity control leads us to seek quantification of the basicity of the components important to these processes. We use available thermodynamic data to rank borohydrides relative to pure hydrides, and describe new alkali metal and ionic liquid compounds that could substitute for sodium hydroxide in the hydrogen storage technology. We then turn to the opposite, high proton activity, end of the basicity/acidity spectrum, and briefly review processes that produce protic ionic liquids of low vapor pressure but exceptional acidity. The total range of proton activities under discussion exceeds 50 orders of magnitude. Some consideration is given to possible electrochemical devices that exploit this domain in which the electrochemically generated voltage differences could exceed 3.5 V.