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...

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It is known from literature that the capacitance performance of activated carbons (AC) can be enhanced by faradaic contributions owing to surface functionalities. Although many papers report on chemical oxidation of ACs to reach such objective, this method is not selective and leads to the formation of an important proportion of useless surface functional groups. Such disadvantage can be circumvented by using diazonium salts to graft molecules with the desired functionality on carbon, e.g., quinone-type. The redox activity of these molecules grafted on ACs has been demonstrated in sulfuric acid and potassium hydroxide aqueous media [1]. However, the materials display poor stability in KOH, and H2SO4 corrodes all non-noble metal current collectors used to build electrochemical capacitors (ECs). The objective of this presentation is to demonstrate the activity of anthraquinone-grafted carbons in non-corrosive aqueous media, such as neutral electrolytes [2], and to apply these materials as negative electrode in a hybrid capacitor where the positive electrode is from MnO2. Anthraquinone (AQ) moieties were chemically grafted on the surface of the activated carbon DLC Supra 30 from Norit (Supra), giving the Supra-AQ carbon. The surface oxygenated functionality of as-received and grafted carbons was characterized by temperature-programmed desorption (TPD), using a TG equipment coupled with a mass spectrometer (MS). The larger mass loss for Supra-AQ (13.6 wt.%), compared to the pristine Supra one (2.6 wt.%), mainly related to CO evolution, confirms surface functionalization by quinone. The electrochemical response of Supra-AQ has been studied in 0.4 mol L− 1 phosphate buffer (pH=7.2) in order to avoid the shift of the redox peaks related with pH change during the measurements. In addition to the capacitive current related with the EDL formation, the CVs of Supra-AQ display reversible redox peaks characteristic of the grafted AQ molecules. As a result of grafting AQ, the capacity increases from 47 mAh g− 1 for Supra to 72 mAh g− 1 for Supra-AQ. During floating at 1.8 V, the hybrid Supra-AQ/MnO2 cell demonstrated stable capacitance and resistance, with 70% higher energy density than a Supra/MnO2asymmetric capacitor. [1] G. Pognon, T. Brousse, L. Demarconnay, D. Bélanger, J. Power Sources196 (2011) 4117. [2] Q. Abbas, P. Ratajczak, P. Babuchowska, A. Le Comte, D. Bélanger, T. Brousse, F. Béguin, J. Electrochem. Soc. 162 (2015) A5148.