In this paper we report a novel redox flow battery using a titanium and manganese mixed solution as both positive and negative electrolytes. Ti(IV) ions existing in positive electrolyte suppress the Mn(III) disproportionation reaction, as well as particle growth of Mn dioxides. The energy density of 23.5 kWh m-3 was obtained in a single cell test, which is comparable to that of all-vanadium RF battery. Mn dioxide nanoparticles contribute to excellent flowablity and electro-active performance. In the cycling stability test, the performance is stable over more than 40 cycles. It is considered that the same electrolyte in both half-cells prevent mixing of electro-active species.

Surface treatments are often applied to carbon materials to impart specific functions to the surface. Surface oxidation is a typical treatment to form oxygen-containing surface functional groups on carbon fiber electrodes of redox flow batteries in order to enhance the performance, which has attracted much attention as a large-scale electric energy storage system. At present, however, little attention has been paid to the effect of the edge plane exposure. In this study, fine etching of the graphitized carbon fiber surface was attained by coating the surface with a metal-containing carbonaceous thin film and thermal oxidation. The etching was caused by the catalysis of the metal species; the mechanism and the effect of the carbonaceous film were demonstrated by in situ X-ray absorption fine structure measurements. The finely etched surface possessed substantially enriched edge planes and an enhanced activity for the positive and negative electrode reactions of the vanadium redox flow battery. The flow cell test with the carbon fiber electrodes after the tuned etching showed a significant decrease in the overpotential and increase in the efficiency as well as stable cycling performance.

The mixed electrolyte of titanium and manganese is one of the promising candidates as low-cost electrolytes for redox flow batteries. In this study, the effect of Ti4+ ion in the positive electrolyte was examined in detail. With an increase of Ti4+ ion concentration, the formation reaction of MnO2 particle was slowed down significantly, which means the stability of Mn3+ ion is enhanced. Moreover, the particle size of MnO2 precipitate became smaller in electrolytes containing certain amount of Ti4+ ion. These MnO2 particles were small enough to be flowed in the electrolyte and to be reduced to Mn2+ ion again. Ti4+ ion also affected the crystal structures of MnO2, limiting to gamma-MnO2 which is assumed to be reactive. In cyclic voltammetry study, the higher Ti4+ ion concentration was, the better reversibility of Mn redox reaction was observed. The ion interaction between Ti4+ and Mn3+ was confirmed by UV-visible spectroscopy.