Ionic liquids are a new class of electrolytes with many potentially high impact properties such as wide electrochemical windows, low vapor pressure, and excellent thermal stability. These properties offer distinct advantages in electrochemical applications such as solar cells and supercapacitors. In these potential applications, the electrical double layers (EDLs) at the interfaces of electrodes and ionic liquids play a critical role in determining the system performance. However, the knowledge on these EDLs is very limited: many fundamental issues such as the capacitance potential correlation and the capacitance of EDLs in ultrasmall nanopores are poorly understood at present. Such a limited understanding renders the design and optimization of electro-chemical systems using ionic liquids as electrolyte difficult, and thus prevents the potential of ionic liquids from being fully exploited. Therefore, it is critical to improve our understanding of the EDLs in ionic liquids. The objective of this research is to investigate the electrical double layers (EDLs) in ionic liquids using molecular dynamics simulations. Specifically, the PI will first study the EDLs at the interfaces of ionic liquids and planar electrodes to delineate how their structure and capacitance are affected by the electrical/physicochemical properties of electrodes and ions. The EDLs in ionic liquids-filled sub-nanometer pores will then be studied to elucidate the synergistic effects of nano-confinement and surface curvature on the structure and capacitance of EDLs in these pores.
Intellectual Merit: In the study of planar EDLs, a new picture for the EDLs in ionic liquids, i.e., the Ion-ion and ion-electrode correlations play a key role in determining the EDL structure, is proposed. This picture represents a paradigm shift from the prevalent EDL models, and is supported by compelling preliminary data. Guided by this new idea, the PI?fs simulation design and data analysis differ distinctly from those in prior research and are expected to lead to new insights into the dependence of EDL structure on the nature of ions and on the polarization/chemistry of electrodes. With this, the PI will elucidate the mechanism of the diverse capacitance?]potential relations and surprising ion specificity of EDL observed in previous experiments, which defy existing EDL models. In the study of EDLs in sub-nanometer pores, by doing simulations in pores with precisely defined geometry and by simultaneously computing the microstructure and capacitance of the EDLs, the PI's group will, for the first time, self-consistently test the prior hypotheses on effects of confinement and surface curvature on EDL capacitance in sub-nanometer pores, and elucidate the underlying mechanisms of the test result. Together, these researches will greatly advance the fundamental understanding of the EDLs in ionic liquids and make a firm step towards building the knowledge base for the rational design of electrochemical systems using ionic liquids as working electrolytes.
Broader Impacts: The project will be tied intimately with the educational activities at Clemson University. Students participating in this project will be exposed to diverse fields such as physical chemistry, atomistic modeling and computational methods. Undergraduate students will be involved in the research through the Honors Research Program in the PI's home department. Various resources, e.g., the minority recruitment programs at Clemson University, will be utilized to recruit students from underrepresented groups to participate in this project. Research results will be developed into posters/movies to introduce electrical energy storage to K-12 students. Research will be disseminated through journal publications and presentations in professional conferences. A website centering on the fundamental physics of the EDLs in ionic liquids and their role in electrochemical systems will be developed and maintained. The website will be advertised to the target audience via formal and informal channels.
Room-temperature ionic liquids are as a new class of solvent-free electrolytes with many potentially high-impact properties such as low vapor pressure and wide electrochemical windows. These properties offer distinct advantages in applications such as supercapacitors. In many of these applications, the electrical double layers at the interface of electrode and ionic liquids, whose thickness rarely exceeds a few nanometers, play a key role in determining the system performance. For example, in supercapacitors, the capacitance and dynamics of the double layer directly control the energy and power densities of the supercapacitor. Because of the solvent-free nature of the room-temperature ionic liquids, the double layers in them cannot be understood using the classical double layer theories, which were developed for dilute electrolytes under low voltages. In this project, we used molecular dynamics simulations to study the double layers in ionic liquids at the molecular level. Some of the most important findings of this project include the followings. First, our study of the double layers near open electrodes suggested that ion size and surface curvature play an essential role in controlling the structure and capacitance of these double layers. We proposed a ‘counter-charge layer in generalized solvents’ model for double layers in ionic liquids based on this insight. Second, our study of the double layers confined in nanometer pores suggested that the capacitance of the double layer oscillates with the pore size. We also discovered that the mode of charge storage changes from ion swapping to ion removal and finally to ion insertion as the electrode voltage on the pore wall increase. These discoveries led to significant new knowledge of the double layers and enriched interfacial sciences. The insights gained in this project will help guide rational materials engineering to optimize the performance of electrochemical devices in which double layers plays an essential role. Most importantly, we identified that the key role of pairing of counter-ions and co-ions in controlling the structure and capacitance of the double layers. Controlling of such pairing, which can be achieved by molecular design of ion pairs, choosing appropriate nanopore size and shape or by tuning operating conditions such as electrode voltage, will help optimize the capacitance of double layers in supercapacitors. This will help improve the energy density of supercapacitor and thus can potentially benefit the society greatly. Four graduate students have been involved in this interdisciplinary project that spans electrochemistry, atomistic simulations and statistical physics. Two PhD students have successfully defended their dissertation and another PhD student is on track to defend his dissertation by December 2014. The research results have been documented in journal papers and presented in professional conferences. Some of the research results have been developed into seminars for undergraduate students at Clemson University and presentation materials for various visitors of the Mechanical Engineering department at Clemson University.
