Technological condition optimization and kinetic study on the electrochemical soluble manganese(III) production in H2SO4

Abstract

Soluble trivalent manganese solution can be used as an electrolyte for manganese redox flow battery (MRFB), due to its low cost, environmental friendliness and multiple states of charge. However, poor stability of soluble trivalent manganese solution and low trivalent manganese concentration are the disadvantages of electrolyte. To optimize the stability and conductivity of soluble trivalent manganese solution, the sulfuric acid concentration, one of the major influence factors, should be firstly improved. Two-compartment cell electrolysis technology was successfully used in this work. Moreover, to improve the concentration of Mn3+, some key factors, such as the concentration of MnSO4, current density, electrolysis time and temperature, should be further adjusted. The results demonstrated that the concentration of this produced trivalent manganese (≧ 0.3 M in 6 M H2SO4;≧ 0.4 M in 5 M H2SO4;≧ 0.5 M in 4 M H2SO4) is much higher than that have been reported. Solid product of manganese(III) sulfate was also firstly prepared. For the formation of soluble trivalent manganese, the 5 mol/L H2SO4 is the optimal acidity and 10.9 mA/cm2 is the best current density at 273 K for 3 h. Some electrochemical kinetics parameters had also been calculated (electrolyte is 5 M H2SO4 and 0.4 M MnSO4), e.g., diffusion coefficient of Mn2+ is 5.3057 × 10−6 cm2/s, transfer coefficient of electrode reaction is 0.3782, and exchange current density is 1.5662 × 10−4 A/cm2. The CV results demonstrated that the redox process of Mn(III)/Mn(II) is the quasi-reversible process.