Quasielastic neutron scattering and mathematical modeling with simulation of electrochemical supercapacitors

Abstract

Modern society faces energy challenges that demand the development of clean and efficient storage systems. Devices such as supercapacitors, or electric double-layer capacitors, stand out for their high efficiency, energy density, long lifespan, and fast charging speed. Optimizing the performance of these devices is intrinsically linked to the enhancement of component materials and the fine-tuning of the electrode-electrolyte interface.In this context, this doctoral work investigated the combination of activated carbon from the shell of the Mbokaja coconut species with the eutectic electrolyte choline chloride:ethylene glycol for application in supercapacitors. For comparison, a commercial activated carbon, reduced graphene oxide, and an aqueous potassium hydroxide solution were used as an alternative electrolyte. The coconut charcoal was synthesized in a water vapor atmosphere at 600ºC.A combination of advanced characterization techniques was employed to evaluate the electrode-electrolyte interface. The properties of the electrodes were analyzed by X-ray diffraction, scanning electron microscopy, and N2 adsorption. To understand the nanoscale mobility of the hydrogen atoms of the electrolytes when confined within the electrode pores, neutron spectroscopy and thermogravimetric analysis coupled with infrared spectroscopy and mass spectrometry were used. Neutron spectroscopy, applied before and after the electrodes were soaked with the electrolytes, allowed for the investigation of the dynamic behavior of the electrolyte molecules, revealing fast vibrational modes and slow diffusive motions. Electrochemical performance was evaluated by cyclic voltammetry and galvanostatic cycling.This work is divided into two parts. The first part was dedicated to understanding the mobility of hydrogen in the confined electrolytes using neutron spectroscopy. The second part proposes a mathematical model, based on a series capacitive-resistive circuit association, to analyze the behavior of capacitance and equivalent series resistance with increasing scan rate, using data from cyclic voltammetry. This model aims to elucidate the capacitance drop and resistance increase at faster scan rates.The results of the two parts that constitute this thesis are reported in two articles submitted to the Journal of Energy Storage (JES). At the date of publication of this thesis, Article 1 is under review, while Article 2 is currently published.