Self-Discharge Related to Iron Ions and its Effect on the Parameters of HES PbO[sub 2]∣H[sub 2]SO[sub 4]∣C Systems

Abstract

This work contains the results of research on the mechanism of the shuttle self-discharge determined by Fe2+ and Fe3+ ions in the electrolyte of a heterogeneous electrochemical supercapacitor (HES) of a PbO2∣normalH2SO4∣C system, as well as the results of the measurements of the effect of the shuttle self-discharge current on the energy, capacity, and operation parameters of HES capacitors. It is shown that the presence of the iron ions in the electrolyte of a HES capacitor brings about shuttle self-discharge, which continues until the voltage decreases to the value of 1.05V . The shuttle mechanism is related to oxidation and reduction of Fe2+ and Fe3+ ions in the positive and negative electrodes, respectively. An analytical expression is obtained for the calculations of the shuttle self-discharge current determined by different types of impurity ions in the electrolyte of HES capacitors. Analysis is made with respect to the obtained experimental and theoretical data of the self-discharge current, energy, and capacity parameters, as well as their dependence on the concentration of iron ions in the electrolyte, the separator’s thickness, and modes of the capacitors’ charge. It is established that the processes of oxidation and reduction of iron ions take place in the near-surface layers of the capacitor electrodes. It is shown that the linear increase of the separator thickness does not bring about any linear decrease of the shuttle self-discharge current determined by Fe2+ and Fe3+ ions.

Self-Discharge in Electrochemical Capacitors: A Perspective Article

“…However, threeelectrode measurements are blind to shuttle mechanisms, where the self-discharge product of one electrode diffuses to the other and undergoes a self-discharge reaction at that electrode. 19,[29][30][31] For instance, a Fe 3+/2+ couple in an aqueous EC can shuttle between the positive and negative electrodes, being oxidized and reduced, eventually discharging both electrodes. 17,29 Ideally, a combination of three electrode and full cell studies are required to provide a deep understanding of self-discharge.…”

“…19,[29][30][31] For instance, a Fe 3+/2+ couple in an aqueous EC can shuttle between the positive and negative electrodes, being oxidized and reduced, eventually discharging both electrodes. 17,29 Ideally, a combination of three electrode and full cell studies are required to provide a deep understanding of self-discharge.…”

A universal equivalent circuit for carbon-based supercapacitors

“…An alternative explanation is that ions might diffuse from regions of high electrochemical potential to regions of low electrochemical potential [106], but, again, there is little proof. A third suggestion is that the Fe(II)/ Fe(III) redox couple might act as a charge shuttle between the terminals of the device [107,108]. This, however, seems feasible only in aqueous solutions.…”

Performance evaluation and characterization of symmetric capacitors with carbon black, and asymmetric capacitors using a carbon foam supported nickel electrode

“…In order to increase the energy density, and at the same time maintain the long cycle life and fast charge capability, a hybrid system combining an EC of high power density to a battery of high energy density was introduced. 8 This is known as an asymmetric capacitor. Various combinations of anode and cathode have been reported, such as metal oxides/AC, conductive polymers/AC and Li 4 Ti 5 O 12 /AC.…”

“…Various combinations of anode and cathode have been reported, such as metal oxides/AC, conductive polymers/AC and Li 4 Ti 5 O 12 /AC. [8][9][10][11] Amorphous metal oxides (NiO and RuO 2 ) are also widely studied as potential electrode material candidates for higher energy redox capacitors. [12][13] Also, nickel hydroxide was introduced as a candidate for a positive electrode material.…”