Lateral Structure of Electric Double Layers in Ionic Liquids Based Supercapacitor: A Molecular Dynamic Simulation Study


Student thesis: Doctoral Thesis

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Awarding Institution
Award date7 Sept 2018


Electric double layers (EDLs) play a decisive role in energy storage of supercapacitor. Understanding and manipulation of the lateral structure of EDLs emerge to be an important question, due to the correlation between the differential capacitance and voltage driven structural evolution in lateral direction. In the current work, this correlation was systematically investigated by all-atom molecular dynamics (MD) simulations. Existence of this correlation was examined, and the structural origin was elucidated. To apply this correlation, lateral structures of room temperature ionic liquids (RTILs) were manipulated via space confinement and molecular structure of ions.

The voltage driven lateral ordering of RTIL 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) on both cathode and anode were systematically characterized using 2D structure factor, radial distribution function and coordination number. It was found out that lateral structure evolutions of ions are different on two electrodes. On anode side, disorder to order transition of roundish ions PF6- starts when the number of first nearest neighboring ions increases and converges to six. What’s more, local maximum of differential capacitance profile not only appears at the disorder order transition point, but also occurs at the splitting point of radial distribution function peak and where the number of second & third nearest neighboring ions become converge and stable. On the cathode side, long range ordered phase of ions does not exist due to the multi adsorption states of BMIM+ on electrode. To understand the origin of correlation between lateral structure and differential capacitance, the correlation between structures in lateral and normal directions were investigated. Such a structural correlation is closely related with the three dimensional characteristic of EDL structure and over screening phenomenon.

Though the correlation between differential capacitance and lateral structural evolution was observed, the disorder order transition of ions PF6- occurs beyond the electrochemical window of RTIL [BMIM][PF6]. To apply this correlation in real experiment, disorder order transition of ions should be shifted to low voltage region. Space confinement is one of methods that can significantly influence long range ordering. In plane structures of imidazolium based RTILs [Cnmim][PF6] (n = 1, 2, 4, 6) immersed in sub nanometer slit pore were then investigated. In this simulation system, lateral ordering of ions and orientation of imidazole rings can be systematically characterized under influences of size of slit pore and length of alkyl tail linking to imidazole rings. Crystalline, partially ordered and disordered phases of ions were observed and quantitatively characterized. Ordering of ions can be manipulated by the formation of tail aggregations via tail length of imidazolium cations and pore size. With increasing of alkyl tail and pore size, number and size of tail aggregation increase, and ordering of ions in pores decreases. Based on the correlation between differential capacitance and lateral ordering evolution, manipulation of configurations by space confinement are expected to play an important role in differential capacitance of supercapacitors.