Strategies to Improve the Performance of Carbon/Carbon Capacitors in Salt Aqueous Electrolytes

Abstract

Strategies are presented to enhance operating potential and cycle life of AC/AC capacitors using salt aqueous electrolytes. Li 2 SO 4 (pH = 6.5) allows 99% efficiency to be exhibited at 1.6 V cell potential with low self-discharge, while in BeSO 4 (pH = 2.1) efficiency is low (81%). Li 2 SO 4 performs better due to high di-hydrogen over-potential at the negative electrode and related pH increase in AC porosity. When stainless steel current collectors are used in Li 2 SO 4 , the cell resistance suddenly increases after 12 hours floating at 1.6 V, due to corrosion of the positive collector. With nickel negative and stainless steel positive collectors, the electrode potentials are shifted by −105 mV at cell potential of 1.6 V, allowing stable cell parameters (capacitance, resistance) and reduction of corrosion products formation on positive steel collector after 120 hours floating. Phenanthrenequinone was grafted on activated carbon to get an additional faradaic contribution in buffer solutions (pH = 4.0 or 7.2). The three-electrode cell CVs show that the redox peaks of the phenanthrenequinone graft shift toward negative values when pH increases from 4 to 7. Electrical double-layer capacitors (EDLCs) based on activated carbons (AC) and traditional aqueous electrolytes, such as H 2 SO 4 and KOH, operate at low cell potential (up to ∼0.8 V), limiting their capability at industrial level. [1][2][3][4] In order to improve the energy (E = 1 2 CU 2 ), both capacitance (C) and cell potential (U) determined by the stability window of the electrolyte have to be optimized. In this context, it has been recently demonstrated that symmetric capacitors based on AC electrodes and salt aqueous electrolyte (0.5 mol L −1 Na 2 SO 4 ) exhibit excellent cyclability under galvanostatic charge/discharge up to 1.6 V. 5,6 . While using 1 mol L −1 Li 2 SO 4 and gold current collectors, excellent cycle life has been shown up to ∼1.9 V using galvanostatic charge/discharge. 7,8 Such high cell potential value is caused by a high over-potential for di-hydrogen evolution as a consequence of water reduction and OH − ions generation in the porosity of the negative AC electrode. 5,7 According to the Nernst equation (E red = −0.059 pH), the pH increase associated to OH − ions causes locally a shift of redox potential to lower values. Based on this fact, it has been recently demonstrated that the di-hydrogen evolution over-potential is higher in almost neutral electrolyte solutions (pH = 4-8) than in acidic ones, resulting in larger operating cell potential of asymmetric capacitors in the former media.9 From the foregoing, it makes sense to study in more details the influence of aqueous electrolyte pH on the electrochemical performance of AC/AC capacitors beyond only the cell potential.Besides, when shifting from gold to stainless steel current collectors in order to develop low cost AC/AC capacitors in 1 mol L −1 Li 2 SO 4 , constant capacitance and low cell resistance have been observed during potentiostatic floating at cell potential of 1.5 V, whil...

Self-discharge of AC/AC electrochemical capacitors in salt aqueous electrolyte

“…As already demonstrated in the literature, SD depends on parameters such as temperature, maximum cell potential reached and hold time at this value, and the charge/discharge history [20,23]. To date, the majority of studies were focused on SD dependence with the kind of aqueous electrolyte , in absence [6,[24][25][26][27] or presence of certain surfactants [26,27] or redox couple (redox-active electrolyte) [28], and type of separator [24,29].…”

“…The SD profiles reveal that the negative electrode is essentially responsible of SD whatever Ui value, and the difference between ∆E-and ∆E+ increases with Ui ( figure 6, table 3). Taking into account our previous data displaying that, at high cell potential, the lowest potential of the negative electrode is lower than the water reduction potential [6], it is obvious that higher SD of this electrode is related with water reduction, as it will be further demonstrated in figure 8 by its activation controlled characteristics at Ui ≤ 1.4 V. In addition, since SD measurements were realized with non-deoxygenated aqueous electrolyte, oxygen electrochemical reduction to hydrogen peroxide (at -0.3 V vs NHE at pH = 6.5) is surely another cause of negative electrode SD, as already demonstrated by Andreas in [12]. The plots of cell and electrodes potential variation vs t ½ and ln t are shown in figure 7 and 8, respectively.…”

“…Good cycle life was reported up to cell potential as high as 1.6 V and 1.9 V for activated carbon (AC)-based ECs in aqueous Na2SO4 (0.5 mol L -1 ) [3] and Li2SO4 (2 mol L -1 ) [4], respectively. Such high operating cell potential is owing firstly to the high over-potential for di-hydrogen evolution at the negative electrode [5,6], and secondly to the less corrosive character of neutral electrolyte as compared to, e.g.,…”

Grafting of Quinones on Carbons as Active Electrode Materials in Electrochemical Capacitors

“…To circumvent this difficulty, electrodes prepared with phenanthrenequinone-modified carbon were investigated because they displayed electroactivity at more positive potentials along with a significant increase of the stored charge relative to the unmodified carbon. 39,53 In addition to alkaline and acidic electrolyte, Norit carbon has been shown to be electroactive as well in neutral buffered electrolyte. A significant increase of the charge stored has been observed in biphthalate (pH 4) and phosphate (pH 7.2) buffers.…”

“…A significant increase of the charge stored has been observed in biphthalate (pH 4) and phosphate (pH 7.2) buffers. 53 The grafting of 9,10-phenanthrenequinone on Black Pearls has been shown to lead to a significant improvement of the charge storage performance of a carbon/Ni(OH) 2 hybrid electrochemical capacitor. 54 This is illustrated in Figure 4 by the increase of the voltammetric charge of the negative carbon electrode.…”