Abstract
Ionic liquids (ILs) are promising electrolytes for supercapacitors (SCs) aimed for high-temperature applications, where increased ionic conductivity results in superior capacitive performance compared to room temperature (RT) performance. However, an increased temperature also accelerates the self-discharge rate that adversely affects energy retention and restricts the usage of SCs in standalone applications. In this study, a detailed electrochemical investigation on the self-discharge behaviour of carbon-based SCs containing an IL, 1-Ethyl-3-methylimidazolium acetate (EMIM Ac), has been carried out in the temperature range RT - 60 °C, and the underlying self-discharge mechanisms are identified. The results reveal that at a high voltage of 1.5 V, the self-discharge is characterized by a combination of charge redistribution and diffusion at both RT and 60 °C. At 60 °C, the diffusion-controlled mechanism dominates at lower voltages over the charge redistribution effect, while at RT both mechanisms contribute to a similar extent. The observed difference in the self-discharge mechanism between RT and 60 °C is explained in terms of a decreased RC time constant (τRC) at elevated temperature, and the same conclusions are potentially applicable to other IL-containing SCs as well.
Highlights
Supercapacitors (SCs) are known to be efficient energy storage de vices with a very high power density and long cycle life owing to their intrinsic electrostatic charge storage mechanism
As a continu ation of the previous study, the current study aims at revealing the un derlying self-discharge mechanisms of SCs containing an Ionic liquids (ILs) electrolyte (EMIM Ac) at different temperatures, ranging from room temperature (RT) to 60 ◦C
If the voltage loss is plotted against log (t) and the self-discharge profile follows a linear drop after an initial plateau, the self-discharge rate-determining step can be related to an activation controlled mechanism [2,18]
Summary
Supercapacitors (SCs) are known to be efficient energy storage de vices with a very high power density and long cycle life owing to their intrinsic electrostatic charge storage mechanism (for electrical double layer type SCs). The self-discharge due to ohmic leakage is more relevant to the conventional dielectric capacitors, where the voltage loss is an electric field driven process occurring within the system with flat metal plates and does not involve any ions or complex porous structure that are part of SCs [21] This kind of self-discharge occurs due to an undesired faulty construction of devices that creates a conductive pathway between the electrodes [2]. ILs are more viscous compared to conventional organic or aqueous electrolytes due to the bulky nature of ions and absence of any solvent They tend to form aggregates due to high ionic association and exhibit quite high equivalent series resistance (ESR) that adversely affects the capacitive performance and power density. As a continu ation of the previous study, the current study aims at revealing the un derlying self-discharge mechanisms of SCs containing an IL electrolyte (EMIM Ac) at different temperatures, ranging from RT to 60 ◦C
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