Abstract
The impact of cell voltage on the capacitance of practical electrochemical supercapacitors is a phenomenon observed experimentally, which lacks a solid theoretical explanation. Herein, we provide combined experimental and molecular dynamics investigation of the relation between voltage and capacitance. We have studied this relation in supercapacitor cells comprising of activated carbon material as the active electrode material, and neat ionic liquids (ILs), and a mixture of ILs as the electrolyte. It has been observed that the increase of accumulative charge impacts the conformation and packing of the cations in the anode, which determines its nonlinear behavior with increasing voltage. It has also been shown that for the mixture IL with two types of cations, the contribution of each type of cation to the overall capacitance is highly dependent on the different pore sizes in the system. The smaller tetramethylammonium (TMA + ) shows tendency for more efficient adsorption in the mesopores, while 1-Ethyl-3-methylimidazolium (EMIM + ) is found to be present almost exclusively in the micropores where TMA + is present in small quantities. Such microscopic insights from computer simulations of the molecular phenomena affecting the overall performance in supercapacitors can help to design more efficient electrolytes and devices.
Highlights
Development of new and more effective electrochemical energy storage, such as high energy and power electrochemical supercapacitors, is in high demand with the increase of renewable energy produced globally [1,2,3]
The results obtained from the molecular dynamics (MD) simulations and the experiments for the voltage/charge relation in the micropores are presented while the voltage/charge relation in the mesopores is presented
Ρ from the MD simulations has been calculated in C/cm3, where the average number of cations inside the pore per unit of volume has been calculated from the production run
Summary
Development of new and more effective electrochemical energy storage, such as high energy and power electrochemical supercapacitors, is in high demand with the increase of renewable energy produced globally [1,2,3]. Ionic liquids are promising electrolytes of choice for the electrochemical energy storage application due to their large operational voltage. Improving the capacitance of high-voltage supercapacitor in IL electrolyte is an efficient way to achieve higher energy and power density. The effect of the applied voltage on the increasing capacitance can be up to 50% for activated carbon (AC) electrodes and up to four times for singlewall carbon nanotube (SWCNT) [4, 5]. Both linear and nonlinear dependence of capacitance with the voltage has been observed in these studies
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