Temperature Effects on the Kinetics of Ferrocene and Cobaltocenium in Methyltriphenylphosphonium Bromide Based Deep Eutectic Solvents

Abstract

The oxidation of ferrocene (Fc/Fc + ) and reduction of cobaltocenium (Cc + /Cc) under different temperatures has been studied by cyclic voltammetry and double potential step chronoamperometry in deep eutectic solvents (DESs) consisting of methyltriphenylphosphonium bromide salt with tri-ethylene glycol, glycerol or ethylene glycol as hydrogen bond donors. The temperature dependence of the measured physical properties of DESs (such as viscosity and conductivity) is discussed in detail. The kinetics of the redox couples are studied using cyclic voltammetry, and the standard heterogeneous electron transfer rate constant, k 0 is found to be of the order of 10 −5 to 10 −4 cms −1 at different temperatures. The diffusion coefficient, D, of Fc and Cc + is determined to lie between 8.28 × 10 −10 to 6.65 × 10 −9 cm 2 s −1 . These results show that both k 0 and D increase with temperature in the studied DESs. In addition, better kinetic parameters for the DES with ethylene glycol as hydrogen bond donor means that this could be evaluated favorably as both solvents and electrolytes for redox flow cells. In recent years, to overcome the toxicity and high price of ionic liquids (ILs), deep eutectic solvents (DESs) have been investigated as potential alternatives.1-3 DESs share many characteristics of conventional ILs (e.g., they have intrinsic ionic conductivity, low-volatility, biodegradability, high thermal and chemical stability, good electrochemical stability as well as non-flammability). In addition, they are simple to synthesize on a large scale.4-6 These properties have been explored for applications such as solvents for electrodeposition and electropolishing, 7-9 electrochemistry, 10 separation processes, 11 chemical and enzymatic reactions, 12 biochemistry 13 as well as organic and inorganic syntheses. 14,15 In its most common form, a 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, metal salts, alcohols, carboxylic acids and amines that may be used as complexing agents (typically an H-bond donor). 16,17 A DES has a melting point that is far below that of its individual constituents. This is because the complexing agent interacts with the anion and increases its effective size. This, in turn, decreases the anionic interaction with the cation thereby reducing the melting point of the mixture. 18Organometallic redox couples such as ferrocene/ferrocenium (Fc/Fc + ) or cobaltocenium/cobaltocene (Cc + /Cc) are frequently used as a standard reference for non-aqueous solvents in conjunction with a quasi-reference electrode (QRE) to control and evaluate electrode potentials. [19][20][21][22] Fc is often used as an ideal internal reference system for comparison of redox processes in different ILs, although Torriero et al. 23 indicate that the standard redox potential of Fc is dependent on the solvation effects of the solvent and supporting electrolyte used. Fc is often poorly soluble in some ILs, mostly in those with ...

Modelling of redox flow battery electrode processes at a range of length scales: a review

“…The application of deep eutectic solvents for this chemistry has also been reported. [108][109][110] Further optimisation of both aqueous and non-aqueous Fe/V systems including electrode geometry, electrode design and ow rate are needed to enhance performance. Mathematical models can assist in cell scale-up as well as the optimisation of RFBs by simulating the effects of various cell geometries, varied electrolyte compositions and membranes on cell performance.…”

“…As discussed above, aqueous RFBs have been more thoroughly investigated than non-aqueous RFBs; 24,108,320,321 therefore, a brief summary on how non-aqueous RFBs may be harnessed for practical and commercial applications 322 follows to conclude this section.…”

Investigation of Ammonium- and Phosphonium-Based Deep Eutectic Solvents as Electrolytes for a Non-Aqueous All-Vanadium Redox Cell

“…Results for other DESs are not shown here as our earlier experiments with ferrocene and cobaltocenium (Nernstian redox systems) couples displayed irreversible behavior. 11,12,27,30,33 These current peaks are attributed to the redox couples observed in Figure 3 to the following eactions (as represented in Figure A1 of the Appendix for acetonitrile): Figure 3 shows the cyclic voltammograms for the electrolyte containing 0.01 M V(acac) 3 and 0.5 M TEABF 4 in DES3, DES8, DES10 and DES14. Figure 4 compares different DESs at various scan rates in which, for the V(II)/V(III) redox couple in DES3, E p increased from 60 to 90 mV and the ratio of anodic to cathodic peak currents increased from 0.97 to 1.20 as the scan rate was raised from 0.05 to 0.50 V s −1 .…”

“…Electrochemical potential windows of DESs.-The limiting reduction and oxidation potentials of the DESs are analyzed by performing cyclic voltammetry using a GC working electrode at ambient temperature and a scan rate of 0.1 V s −1 as shown in Figure 2 (full details are available in our earlier publications 27,30,33 where the limiting current density reached 0.2 mA cm −2 ). It is discovered that some of the tested DESs have similar potential ranges to typical ILs.…”

Evaluation of a Non-Aqueous Vanadium Redox Flow Battery Using a Deep Eutectic Solvent and Graphene-Modified Carbon Electrodes via Electrophoretic Deposition

“…Cyclic voltammograms (CVs) of 0.1 M V(acac) 3 dissolved in the IL solution were measured in the potential range of 0.5 to −1.75 V at various scan rates (v = 50-300 mV s −1 ) and shown in Figure 4. CV data are available in Table 2 along with results of the DES from our previous investigation [15]. As the scan rate increases, two peaks are observed to move in the cathodic and anodic directions, respectively.…”