Temperature effects on the kinetics of ferrocene and cobaltocenium in methyltriphenylphosphonium bromide based deep eutectic solvents

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, κ⁰ 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 cm² s^-1. These results show that both κ⁰ 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.