Investigations into the Dynamics and Reactivity of Excess Charges in Aliphatic Ionic Liquids

Abstract

Ionic liquids (ILs) are a unique class of materials that have gained popularity for their potential use in a wide variety of energy applications. In these applications, ILs are subjected to highly ionizing radiation and electrochemical stress, which can produce excess charges resulting in degradation and device failure. Understanding the relationship between the molecular structure of ILs and the sub-nanosecond dynamics of these excess charges is crucial to developing robust liquids tailored to specific applications. The first section of this dissertation characterizes the transient absorption and excess charge dynamics in 1-butyl-1-methyl-pyrrolidinium dicyanamide ([Pyr1,4][DCA]) following excitation at 4.66 eV. The results were compared to previous work from our group on an analogous system to determine the influence of the anion on the initial state of the excess electron and subsequent solvation dynamics. Next, the impact of cation structure is examined in the [Pyr1,x][DCA] series by in- creasing the length of the alkyl tail. The length was found to influence the absorption energy of the free electron, as well as the relaxation dynamics and competition between relaxation pathways. The third section addresses the hypothesis of the [NTf2 ] anion serving as an electron scavenger in the [Pyr1,4][DCA] liquid. The addition of the [NTf2 ] was found to have little impact on the excitation of the electron and subsequent relaxation dynamics. It was concluded that the excitation energy was selectively exciting electrons in [DCA]-rich domains, resulting in dynamics analogous to neat [Pyr1,4][DCA]. The final section of this dissertation explores the impact of excitation process on the solvation dynamics of excess electrons by comparing results from pulse radiolysis in [Pyr1,4][DCA] to the photolysis in the first section. The work was performed at Brookhaven National Lab in partial fulfillment of the research proposal funded by the U.S. Department of Energy’s Office of Science Graduate Student Research (SCGSR) Program. The pulse radiolysis technique was complimentary to the photolysis work performed at the University of Minnesota and allowed the relaxation dynamics to be examined in a larger time window.