Electrochemical and Transport Characteristics of V(II)/V(III) Redox Couple in a Nonaqueous Deep Eutectic Solvent: Temperature Effect

Abstract

Compared with conventional aqueous electrolytes, deep eutectic solvents (DES) have been used as the electrolytes of flow batteries for their unique merits, including a wide electrochemical window, ease in preparation, and low vapor pressure. However, the electrochemical and physical characteristics of DES are sensitive to the change of temperature, and there are few related studies. An urgent need exists for research into temperature adaptability of flow batteries with DES electrolytes before they can be utilized practically. This paper studies the electrochemical and transport characteristics of vanadium ions in a DES by varying the operating temperatures. The cyclic voltammetry curves indicate that vanadium ions show a quasi-reversible redox reaction in DES. With the rise of temperature from ambient temperature to 55°C, the gap between oxidation and reduction peaks decreases from 0.271 to 0.248 V at a scan rate of 100 mV/s. Both the oxidation and the reduction peak current densities of V(II)/V(III) have a similar incremental trend. Conductivity increases from 2.2 mS/cm at ambient temperature to 11.16 mS/cm at 55°C, whereas the viscosity of DES with vanadium ions decreases sharply. The results prove that operating temperature has a vital effect on the electrochemical reaction and transport characteristics of V(II)/V(III) redox couple in DES, and it deserves further study in cell performance.