Electroanalysis of Neutral Precursors in Protic Ionic Liquids and Synthesis of High-Ionicity Ionic Liquids.

Daniel E Smith,Darren A Walsh,Joshua S Gibson,Robert G Jones,Sean E Goodwin

doi:10.1021/acs.langmuir.7b02294

Daniel E Smith, Darren A Walsh + Show 3 more

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https://doi.org/10.1021/acs.langmuir.7b02294

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Abstract

Protic ionic liquids (PILs) are ionic liquids that are formed by transferring protons from Brønsted acids to Brønsted bases. While they nominally consist entirely of ions, PILs can often behave as though they contain a significant amount of neutral species (either molecules or ion clusters), and there is currently a lot of interest in determining the degree of "ionicity" of PILs. In this contribution, we describe a simple electroanalytical method for detecting and quantifying residual excess acids in a series of ammonium-based PILs (diethylmethylammonium triflate [dema][TfO], dimethylethylammonium triflate [dmea][TfO], triethylammonium trifluoroacetate [tea][TfAc], and dimethylbutylammonium triflate [dmba][TfO]). Ultra-microelectrode voltammetry reveals that some of the accepted methods for synthesizing PILs can readily result in the formation of nonstoichiometric PILs containing up to 230 mM excess acid. In addition, vacuum purification of PILs is of limited use in cases where nonstoichiometric PILs are formed. Although excess bases can be readily removed from PILs under ambient conditions, excess acids cannot be removed, even under high vacuum. The effects of excess acid on the electrocatalytic oxygen reduction reaction (ORR) in PILs have been studied, and the onset potential of the ORR in [dema][TfO] increases by 0.8 V upon addition of acid to PIL. On the basis of the results of our analyses, we provide some recommendations for the synthesis of highly ionic PILs.

Highlights

Room temperature ionic liquids (RTILs) are defined as species consisting entirely of ions and which are liquid below 100 °C.1 Due to their low vapor pressures, wide electrochemical windows, and inherent conductivities, RTILs are being used in a wide range of applications, such as catalysis,[2] synthesis,[3] electrodeposition,[4] sensing,[5] and energy conversion and storage.[6,7] In each case, the physicochemical properties of the RTILs are the main drivers for their selection, and understanding the structure-property relationships of these media is an extremely active area of research
In all of our syntheses, we found that deep coloration of our protic ionic liquids (PILs) correlated with the presence of peak a during voltammetry
Angell and co-workers did observe these peaks previously during voltammetry of PILs but the PILs had been saturated with H2.31 They suggested that the peak resulted from reduction of the acid formed by H2 oxidation in H2-saturated PILs; when H2 is oxidised, the PIL anions are protonated to reform the parent acid (Equation 2), which is reduced at E < 0 V (Equation 3): H2 + 2A− " 2HA + 2e−

Summary

Introduction

Room temperature ionic liquids (RTILs) are defined as species consisting entirely of ions and which are liquid below 100 °C.1 Due to their low vapor pressures, wide electrochemical windows, and inherent conductivities, RTILs are being used in a wide range of applications, such as catalysis,[2] synthesis,[3] electrodeposition,[4] sensing,[5] and energy conversion and storage.[6,7] In each case, the physicochemical properties of the RTILs are the main drivers for their selection, and understanding the structure-property relationships of these media is an extremely active area of research.

Results

Conclusion