CHARACTERIZATION OF THE ELECTRON TRANSFER REACTION MECHANISM OF TiOSO4 AND MnSO4 IN SULFURIC ACID SOLUTION

Abstract

An electrochemical characteristic evaluation of the Ti-Mn electrolyte system in sulfuric acid aqueous solution is studied in different acid concentrations with and without the mixing of TiOSO4 and MnSO4 electrolyte system for a redox flow battery application. The reaction mechanism of Mn in sulfuric acid electrolyte is studied using the cyclic voltammetry and the anodic voltage hold techniques. The overall anodic behavior of the Mn system can be described by the oxidation reaction of Mn2+ to MnO2 where Mn3+ is an intermediate species. As the acid concentration increases, less MnO2 is formed on the working electrode and more Mn3+ is formed in the solution; however, due to the instability of Mn3+ in an aqueous solution, Mn3+ undergoes a significant hydrolysis reaction to precipitate as MnOOH in the electrolyte solution. The addition of 1M TiOSO4 to the Mn electrolyte is found to lower the H+ concentration which is believed to increase the amount of MnOOH in the solution and lower the concentration of aqueous Mn3+. The formed MnOOH by the Mn3+ hydrolysis reaction is proven to be electrochemically active if it is deposited on the working electrode. The reaction mechanism of Ti in sulfuric acid electrolyte is studied using the cyclic voltammetry technique. Two electro-active species are found to be electrochemically active within the studied voltage window which are Ti4+, and TiSO42+. In 1M H2SO4, Ti4+ is found to be the dominant electro-active species, and in 3M H2SO4, TiSO42+ is found to be the dominant electro-active species. The addition of 1M MnSO4 to the 1M TiOSO4+3M H2SO4 solution results in a high concentration of undissociated MnSO4, and it is found that the dissociation of MnSO4 to SO42- improve the activity of the redox reaction of the Ti(IV)/Ti(III) couple. With the increase of H2SO4 and the addition of MnSO4 in the Ti electrolyte, the electrochemical behavior of the Ti(IV)/Ti(III) redox couple tends toward the reversibility.