A 10kW Class Ti/Mn Redox Flow Battery

Abstract

Large-scale energy storage systems play an important role in the effective use of renewable energy such as wind and solar power. Redox flow batteries (RFBs) have attracted a great deal of attention because of high-speed response, the independent design of power and energy, and advantages in safety. RFBs consist of tanks of electrolyte which store chemical energy and electrochemical cells which reversibly convert chemical energy to electricity. All-vanadium RFBs are widely used as the most mature technology while other various redox species can also be utilized as the electrolyte.Among redox species applicable in aqueous solution, we have reported an electrolyte of titanium and manganese. The discharged state consists of Ti (IV) and Mn (II), both of which are widely available from abundant and low-cost resources. An energy density of 23.5 kWh/m3 is achievable due to the two-electron reaction of manganese on the positive side [1]. Main issues of the Ti/Mn RFB are the precipitation of MnO2 over time in the positive electrolyte and its low power density. As for the precipitation, a systematic research on the effect of mixing Ti (IV) into the positive electrolyte revealed that Ti (IV) ions change the morphology and the crystal structure of MnO2 as well as the electrochemical property of the positive electrolyte [2]. The other issue of low power density can be overcome by thermal treatment of carbon electrodes [3]. Moreover, the low permeability of ions in the Ti/Mn electrolyte contributes to the high coulombic efficiency even when a thin membrane is used. We have reported that the combination of a heat-treated carbon electrode and a thin membrane led to the highest energy efficiency, which was 84.3% with a current density of 200 mA/cm2 [4].In this work, long-term cycle test was performed in a scaled-up system in order to further investigate the reliability of Ti/Mn RFBs. The evaluation system was designed to be approximately 10 kW×1h. In charge-discharge cycle tests, it was confirmed that the cell performance was stable over 100 cycles. A stable energy density over 14 kWh/m3 was successfully confirmed with a current density of 200 mA/cm2. The coulombic efficiency remained over 99%. The characteristics of this Ti/Mn RFB will be discussed in details in the presentation.