Investigation of Titanium Manganese Vanadium Electrolytes As Catholytes for Dual Circuit Redox Flow Batteries

Abstract

Dual circuit redox flow battery (RFB) serves as a viable means to bypass the low energy density of conventional flow batteries converting stored power to gas in the form of hydrogen and oxygen. It was previously studied by Amstutz et al for a vanadium - cerium RFB which suffered from degradation due to the high oxidizing properties of cerium electrolyte. The present master thesis focuses on utilizing manganese as a positive electrolyte thereby proposing an alternative to the V-Ce system. Mn(II)/Mn(III) redox potential is 1.51 V vs. SHE and could therefore be suitable for oxygen evolution reaction. However, Mn(III) is known for its disproportionation at high potentials (>1.6V vs. SHE), leading to the formation of MnO2 solid particles and Mn2+. Therefore, in order to prevent the disproportionation, TiO2+ and VO2+ were used as additives. As a perspective, a dual V-Mn-Ti system was investigated. The electrochemical behavior of Mn(II)/Mn(III) couple along with the addition of TiO2+ and VO2+ ions was thoroughly investigated using cyclic voltammetry, rotating disk and rotating-ring disk electrode. It was shown that the disproportionation reaction of Mn(III) cannot be totally restrained with the addition of TiO2+ or VO2+. The extent of MnO2 plating reduced on carbon felt electrodes was notably observed to quite an extent with the presence of Ti(IV) and V(V), which was further confirmed by SEM. It was shown that 1:1:1 of Ti/V/Mn in 5M H2SO4 was the best compromise to reduce the formation of MnO2(s). Further, a H-cell test to chemically discharge Mn(III) on IrO2 catalyst was performed for the purpose of oxygen evolution reaction. However, it resulted in low efficiency towards oxygen evolution owing to the disproportionation of Mn(III).