Investigation of Self‐Discharge Behavior on Electrical Double‐Layer Capacitors by Hybrid Mechanism Simulation

Abstract

Electrical double-layer capacitors (EDLCs), one kind of energy storage device, have attracted a great deal of attention due to the characteristics of high-power density, long service life, good operation reliability, etc. [1][2][3][4][5][6][7][8] The outstanding performance is achieved by the activated carbon (AC), the key component of EDLCs with high specific surface area, which can provide convenient pathway for ions adsorption and desorption. [9][10][11][12][13] However, the rapid adsorption-desorption between the electrode-electrolyte interphase will cause the poor energy retention with the so-called self-discharge (SD) process. SD refers to the spontaneous voltage decay across the capacitor that occurs when the capacitor is left unconnected to either a charging circuit or an electrical load after being charged, which determines the energy storage time and has become one of the challenges of the applications of EDLCs. [14][15][16][17] It is the characteristic of EDLCs, which exists even after one charge process and is different from the long-time failure mode in the aging test.To date, most of the investigations are focused on the factors and the methods to suppress the SD process of EDLCs, including the research on the component, the type and the preparative technique of electrolyte, [18][19][20][21][22] separator, [23][24][25][26][27] and AC, [28][29][30][31][32] while the studies discussing the mechanism behind the SD process are limited. Conway et al. [33] proposed three kinds of SD models originally: activation-controlled faradaic process, ohmic leakage process, and diffusion control process. Ricketts and Ton-That [34] divided the SD process into two stages based on their experiment: first was the diffusion process in the first 8 h, and second was a slower current leakage process through the double layer after 8 h. Zhang et al. [35] found that the dual-mechanism model controlled the SD process in the tetraethylammonium tetrafluoroborate (TEABF 4 )/propylene carbonate (PC) system, relying on the competition between the diffusion-control model and the potential driving model. Kaus et al. [36] proposed an electrical model and simulated the SD behavior under almost all relevant conditions concerning charge duration, initial voltage, and temperature. However, the aforementioned literature only investigated the SD process of the entire capacitor while neglecting the roles of the positive and negative electrodes in the SD process. In addition, the corresponding SD control models in the aforementioned research are qualitative rather than quantitative. The contribution from each model is undisclosed. Moreover, the lack of comparison between the measured and fitted results makes it impossible to verify the reliability and accuracy of the model. This paper explores the SD profiles of EDLCs with three-electrode test cell (Ag/Ag þ as reference electrode [RE]) in 1 mol L À1 TEABF 4 /acetonitrile (AN). The potential of the positive and negative electrodes was monitored respectively with the aim to reveal their roles an...