Analysis of overpotential in discharge process associated with precipitation for vanadium-manganese flow battery

Abstract

The vanadium-manganese flow battery has a higher discharge voltage than the all-vanadium flow battery due to the higher standard potential of Mn(II)/Mn(III) redox couple. Coupling the 2-D transient model describing electrochemical behaviors with its polarization equations in simulations, the effects of electrode porosity, electrolyte concentration, electrolyte flow rate and discharge current density on the discharge performances in the vanadium-manganese flow battery are investigated, in which the activation, concentration and ohmic overpotentials rise with the discharge, and the specific energy density can be enlarged in the mode with higher electrode porosity, electrolyte concentration and electrolyte flow rate. The higher current density in the discharge will lead to larger attenuation of discharge capacity, and smaller MnO 2 precipitation in the vanadium-manganese flow battery. With electrolyte flow rate above a certain, less variations of MnO 2 precipitation amount occur. Near the cut-off voltage in the discharge, the MnO 2 precipitation closely relating to the side reaction of Mn(II)/MnO 2 influences the discharge voltage greatly. The 94.4% voltage efficiency can be obtained in the vanadium-manganese flow battery, and its voltage efficiency is more than that of all-vanadium flow battery under a large current density, thus the vanadium-manganese flow battery can be utilized to achieve the better discharge behaviors. • A 2-D transient model of vanadium-manganese flow battery is proposed. • The polarization is affected by the deposition and dissolution of precipitation. • Porosity, flow rate, current density and ionic concentration have great impacts.