Viologen/Bromide Dual-Redox Electrochemical Capacitor with Two-Electron Reduction of Viologen.

Abstract

In this work, we report a novel dual-redox electrochemical capacitor (EC) using a modified viologen (V) as anolyte and bromide (Br) as catholyte. In general, modified viologens are dications. When they are used as anolyte in aqueous dual-redox ECs, only one-electron reduction reaction can occur, because most divalent and monovalent viologens are soluble but zerovalent viologens are insoluble. The insoluble and nonconductive zerovalent viologens will block the surface of the activated carbon electrode from subsequent reactions. The energy densities of the dual-redox ECs using viologens are expected to be greatly improved if those viologens can carry out multiple electron reduction reactions. In this work, 1,1'-bis[3-(trimethylammonio)propyl]-4,4'-bipyridinium (NV4+), a tetra-cationic viologen, has been used as anolyte for dual-redox EC. NV2+ produced by two-electron reduction of NV4+ is highly soluble in aqueous solution, so that two consecutive one-electron reductions of viologen can be utilized in dual-redox ECs. To further solve the cross-diffusion issue of the charging products, Br3- and NV cations, of the positive and the negative electrodes, we have used tetrapropyl ammonium cation (TPA+) to complex Br3-, and quaternized styrene ethylene butylene styrene (SEBS-QA) anion exchange membrane (M) to block the cross-diffusion of NV cation. The obtained NV/TPA/Br-M (NV4+/TPA+/Br- electrolyte with SEBS-QA membrane) dual-redox EC exhibits an average Coulombic efficiency over 99%. It also provides a high specific energy of 87 Wh/kgdry at 1 A/gdry and a peak power density of 4.8 kW/kgdry at 5 A/gdry. The functions of TPA+ and SEBS-QA membrane were characterized and are discussed in detail.