Investigation on manganese (Mn ²⁺/Mn ³⁺) - Vanadium (V ²⁺/V ³⁺) redox flow battery

A novel Mn ²⁺/Mn ³⁺-V ²⁺/V ³⁺redox flow battery has been designed. The electrochemical response of high concentration Mn ²⁺/Mn ³⁺ couple in H2SO4 solution were investigated via cyclic voltammetry, steady polarization curve, electrochemical impedance spectroscopy, and charge-discharge experiments. The electrochemical and the kinetic parameters for anodic oxidation of Mn ²⁺ and cathodic reduction of Mn ³⁺ were measured. Performance of a RFB employing Mn ²⁺/Mn ³⁺ couple as anolyte active species and V ²⁺/V ³⁺ as catholyte ones was evaluated with constant-current charge-discharge tests. When the current density varies from 20 to 80 mA cm ^-2, the average coulombic efficiency varies from 81.5% to 92.5% and the voltage efficiency varies from 93.5% to 85.3%. Accordingly the whole energy efficiency varies from 76.2% to 81.2%. Energy efficiency is about 20% higher than that of the all-vanadium battery and average discharge voltage is 1.66 V, which is about 14% higher than that of the all-vanadium battery. The energy density is calculated to be 40.8 Wh/kg, which is 60% higher than that of the all-vanadium battery. The preliminary exploration shows that the Mn ²⁺/Mn ³⁺ couple is electrochemically promising for redox flow battery.