The electrochemistry of elemental sulfur (S(8)) and the polysulfides Na(2)S(4) and Na(2)S(6) has been studied for the first time in nonchloroaluminate ionic liquids. The cyclic voltammetry of S(8) in the ionic liquids is different to the behavior reported in some organic solvents, with two reductions and one oxidation peak observed. Supported by in situ UV-vis spectro-electrochemical experiments, the main reduction products of S(8) in [C(4)mim][DCA] ([C(4)mim] = 1-butyl-3-methylimidazolium; DCA = dicyanamide) have been identified as S(6)(2-) and S(4)(2-), and plausible pathways for the formation of these species are proposed. Dissociation and/or disproportionation of the polyanions S(6)(2-) and S(4)(2-) appears to be slow in the ionic liquid, with only small amounts of the blue radical species S(3)(•-) formed in the solutions at r.t., in contrast with that observed in most molecular solvents.

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The electrochemical behaviour of ferrocene (Fc) is investigated in six different deep eutectic solvents (DESs) formed by means of hydrogen bonding between selected ammonium and phosphonium salts with glycerol and ethylene glycol. Combinations of cyclic voltammetry and chronoamperometry are employed to characterise the DESs. The reductive and oxidative potential limits are reported versus the Fc/Fc(+) couple. The diffusion coefficient, D, of ferrocene in all studied DESs is found to lie between 8.49 × 10(-10) and 4.22 × 10(-8) cm(2) s(-1) (these do not change significantly with concentration). The standard rate constant for heterogeneous electron transfer across the electrode/DES interface is determined to be between 1.68 × 10(-4) and 5.44 × 10(-4) cm s(-1) using cyclic voltammetry. These results are of the same order of magnitude as those reported for other ionic liquids in the literature.

The charge/discharge characteristics for vanadium acetylacetonate in deep eutectic solvents were evaluated using an H-cell with an anion-exchange membrane separator for the first time. Coulombic (CE) and energy efficiencies (EE) of the electrolyte containing V(acac) 3 /0.5 M TEABF 4 in DES3 (a hydrogen bonded eutectic between choline chloride and ethylene glycol) were obtained as 49-52% and 25-31%, respectively, when charging from 0 to 50% of theoretical maximum state-of-charge for 12 cycles. The low CE may be due to the crossover of the active species through the separator, or to the loss of active vanadium due to a parasitic reaction. However, the CE was similar to that for acetonitrile (CH 3 CN) indicating the promise of DESs as suitable electrolytes for future evaluation. Charge and discharge voltages are respectively higher and lower than the formal cell potential obtained by voltammetry. Ohmic drop in the DES results from the low conductivity of the electrolyte and the relatively large distance between the two electrodes in the H-cell. Further studies require investigation in a flow cell with analyses of polarization curves and impedance to determine the loss mechanisms in sufficient detail. Low energy density is often reported as a barrier in the commercialization of redox flow batteries using current aqueous electrolytes. Non-aqueous electrolytic solvents offer a wide potential window of operation and increase the energy capacity of the system.2-4 In contrast to organic systems which are either scarce or environmentally unfriendly, ionic liquids (ILs) have emerged as a relatively new class of non-aqueous electrolytes for energy storage applications. [5][6][7][8][9] ILs are salts that are liquid below 100• C. ILs have many favorable characteristics, e.g., low volatility, high intrinsic conductivity, large electrochemical window, etc. In addition, ILs can be tuned by combining different cations and anions. However, many reports point out the hazardous toxicity and the poor biodegradability of most ILs.7 ILs with high purity are also required since impurities, even in trace amounts, affect their physical properties. Additionally, their synthesis is not entirely environmentally friendly since it generally requires a large amount of salts and solvents in order to completely exchange the anions.10 These drawbacks together with the high price of common ILs unfortunately hamper their industrial applications. Such issues may be overcome by using deep eutectic solvents (DESs). 11,12A DES is a eutectic mixture of an organic salt (ammonium or phosphonium) and a hydrogen bond donor (HBD), that is made up of different components such as amides, metallic salts, alcohols, carboxylic acids and amines that may be used as complexing agents (typically an H-bond donor).13,14 DESs have a melting point that is far below that of either individual constituent. The mechanism is that the complexing agent interacts with the anion and increases its effective size. This, in turn, decreases the anionic interaction with the cation ther...