Effect of Carbonate Solvent Additives in Electrolyte on the Self-Discharge Phenomenon in Organic Electrical Double Layer Capacitors
Authors: KU Yun , KU Hao-Yu , Hu Chi-Chang 
Published: December 2024

As a result of industrialization, technological progress, and the growing global population, energy consumption worldwide is skyrocketing. Researchers are thriving to seek high power density storage device to meet these escalating energy demands. Electrical Double Layer Capacitors (EDLCs) also known as a supercapacitor, are energy storage devices that store energy by Coulombic attraction between ions, polarized solvent on the surface of the activated carbon electrodes to form an electric double layer without involving chemical reactions. EDLCs display some remarkable characteristics such as high-power density, ultrafast charge/discharge rates, and long cycle life, surpassing conventional lithium-ion batteries. These features make EDLCs ideal for a wide range of applications, including portable devices, energy backup systems, and electric vehicles.
However, the self-discharge effect in EDLCs is a deadly drawback that decrease the shelf life of EDLCs and impeding its practical applications. This phenomenon refers to the spontaneous voltage decay between the electrodes when the capacitor is under open-circuit potential after being charged, leading to a reduction in the stored charge without any external connection. This voltage declination is a complex process therefore, various mechanisms have been proposed to elucidate this phenomenon. There are primarily four mechanisms leading to the self-discharge. First, the internal short-circuit caused by imperfect seal of EDLCs leading to ohmic leakage current. Second, the uneven distribution of pore sizes in activated carbon electrodes leads to a higher resistance for ions to penetrate deeper into the material, resulting in ion accumulation on the surface during short charging times. This accumulation causes an uneven distribution of electron energy in the electrode material. After disconnecting, electrons rearrange to achieve overall energy consistency, while ions gradually redistribute and move into the smaller micropores within the activated carbon electrode. This phenomenon is referred to as charge redistribution effect. Third, Faradic current in EDLCs occurs due to redox reactions, stemming from the decomposition of the electrolyte or chemical reactions involving impurities within the electrolyte. Lastly, during the charging process, ions accumulate at the electrode surface, creating a concentration gradient between the electrodes and the electrolyte. As a result, ions gradually diffuse back into the bulk electrolyte until reaching equilibrium. Addressing self-discharge is paramount in ongoing research efforts.
Electrolyte solutions are critical components in EDLCs, providing a medium for ion transport between the electrodes. The electrolyte solution is typically composed of a supporting electrolyte dissolved in a solvent, and the choice of solvent can significantly impact the performance of the device. This study delves into the impact of electrolyte solvent additives in the electrical double layer of EDLCs, leading to varied self-discharge performances. In this report, five commonly-used organic carbonate solvents in EDLCs, were selected to investigate their effects on the self-discharge phenomenon. Cyclic carbonate solvents, such as EC and PC, are known to possess high dielectric constants, high viscosities and high flash points. In contrast, linear carbonates e.g., DEC, DMC, EMC have low viscosities, low dielectric constants and low flash point. By incorporating carbonate solvents into the 1M TEABF4/PC (Tetraethylammonium tetrafluoroborate/propylene carbonate) commercial electrolyte, the performance of five supercapacitors , minimal deviation in electrochemical analyses such as CV and GCD was observed. However, significant differences emerged in EIS, elucidating the varying degrees of self-discharge phenomena. These differences in self-discharge mechanisms across the five EDLCs stemmed from the distinct dielectric constants and molecule sizes of the five solvent additives (i) Low dielectric constant and large molecule size, as seen in DEC (239.4h) and EMC (165.1h), contributed to increased interfacial impedance which effectively prolonging self-discharge compared to commercial one (145.7h). Conversely, the DMC with smaller interfacial resistance initially exhibited lower voltage decline due to reduced charge redistribution effects. However, its rapid ion diffusion process from carbon pores to electrolyte resulted in the fastest self-discharge of DMC (124.7h). (ii) High dielectric EC competes for solvation coordination, reducing interfacial resistance and enhancing intermolecular interaction. This, in turn, leads to the formation of a denser electric double layer (EDL) with EC, therefore decelerating the self-discharge process (205.9h) in EDLCs. In summary, the study emphasizes the significance of solvent additive selection and demonstrates a simple, safe, and cost-effective method to prolonging the self-discharge duration of DEC to more than 1.5 times that of the commercial one, significantly inhibiting the critical self-discharge issue in supercapacitors.

Figure 1