Technological condition optimization and kinetic study on the electrochemical soluble manganese(III) production in H 2 SO 4

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 Mn 3+ , some key factors, such as the concentration of MnSO 4 , 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 H 2 SO 4 ;≧ 0.4 M in 5 M H 2 SO 4 ;≧ 0.5 M in 4 M H 2 SO 4 ) 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 H 2 SO 4 is the optimal acidity and 10.9 mA/cm 2 is the best current density at 273 K for 3 h. Some electrochemical kinetics parameters had also been calculated (electrolyte is 5 M H 2 SO 4 and 0.4 M MnSO 4 ), e.g., diffusion coefficient of Mn 2+ is 5.3057 × 10 −6 cm 2 /s, transfer coefficient of electrode reaction is 0.3782, and exchange current density is 1.5662 × 10 −4 A/cm 2 . The CV results demonstrated that the redox process of Mn(III)/Mn(II) is the quasi-reversible process.