A new method for the direct fabrication of nitrogen-doped graphite felt has been developed, the study of which demonstrates that the felt is an excellent positive electrode for vanadium redox flow batteries (VRFBs). Nitrogen-doped graphite felt was synthesized by the controlled deposition of a thin layer of polydopamine on the surface of graphite felt followed by pyrolysis in an Ar atmosphere. Taking advantage of the versatile capabilities of the coating, as well as its high nitrogen content, dopamine was demonstrated to be an effective precursor for the preparation of nitrogen-doped graphite. The dopamine-derived graphite felt exhibited outstanding electrochemical performance when employed as a positive electrode in a VRFB. It exhibited 236% and 44% increased discharge capacity at a current density of 150 mA cm −2 compared to pristine graphite felt and thermally oxidized graphite felt, respectively. The enhanced performance of the VRFB could be caused by the improved catalytic activity of dopamine-derived graphite felt, resulting from the formation of nitrogen functional groups active in the VO 2+ /VO 2 + redox reaction and the increased specific surface area.

A new combined method of ozone exposure and thermal treatment is developed as a method of activating graphite felt electrodes for use in vanadium redox flow batteries (VRFB). The physical and electrochemical properties of ozone/heat-treated graphite felt was compared with the conventional air/heat treated graphite felts. The results demonstrated that the ozone/heat-treated graphite felt had higher specific surface area and oxygen content compared with the conventional air/heat-treated graphite felts. The ozone/heat treatment method is also very selective in the formation of carboxylic group which is active toward vanadium redox reaction. The performance improvement of the ozone/heat treated graphite felt was significant. It is notable that the ozone/heat treatment was performed at a temperature as low as 180°C for just 6 min. This temperature is a considerably milder condition compared with the conventional air/heat treatment process that requires long treatment time (5 hours) and high temperature of 500°C. This property indicates that the ozone/heat treatment is a cost-effective and practical method for activating graphite felt that is used in VRFB.

A new consecutive post-treatment for activating graphite felt electrodes is developed for use in vanadium redox flow batteries (VRFBs). Graphite felt is treated by thermal oxidation under air followed by thermal annealing with melamine as the nitrogen source. The physical and electrochemical analyses demonstrate that the oxygen-containing groups facilitates nitrogen doping in the carbon framework. A higher nitrogen content and pyridinic-N are observed when nitrogen is doped on the oxidized graphite surface using melamine compared to the content of pristine graphite felt. As a result, the treated graphite felt (GFO+N) exhibits outstanding electrochemical performance in VRFBs. It exhibits a 203% and 31% increased discharge capacity at a current density of 150 mA cm−2 comparted to pristine graphite felt and oxidized graphite felt, respectively. The improved performance is ascribed to the high nitrogen content and active nitrogen groups generated on the graphite felt, which improve the fast electrochemical kinetics of the vanadium redox reaction.

To increase the energy density of a vanadium redox flow battery (VRFB), the Mn(II)/Mn(III) system was used as a positive reaction and its effect on the performance and cycle life were investigated. The discharge voltage of the V/Mn system increased due to the higher redox potential of Mn(II)/Mn(III), which led to a 47% increase in initial energy density from 21 Wh L −1 to 31 Wh L −1 . However, Mn(III) ions in the positive electrolyte are converted to MnO 2 upon charging and remain in the precipitate without being reduced upon discharge, thus decreasing the energy density of the V/Mn system up to the 10 th cycle. As cycles progressed further, the number of vanadium ions permeating to the positive electrolyte increased, and the particle size of MnO 2 decreased. As a result, MnO 2 could participate in the reduction reaction without precipitating, resulting in increased energy density. These results show the possibility of using Mn ions for the positive reaction by appropriately controlling the particle size of MnO 2 .

A new aqueous redox flow battery with multiple redox couples is developed based on a vanadium redox flow battery by adding Ti and Mn ions to both negative and positive electrolytes to form a V-Mn-Ti/V-Mn-Ti system. In this system, V and Mn act as redox couples at the positive electrode and V and Ti at the negative electrode. Since there are multiple redox couples in both negative and positive electrolytes, the energy density of the V-Mn-Ti/V-Mn-Ti system reaches to 39.3 Wh L −1 , which is higher than single redox couple systems such as the V/V and V/Mn systems by 86% and 27%, respectively. In addition, since the negative and positive electrolyte compositions are the same, the problem of electrolyte contamination due to ion crossover can be solved through rebalancing. After rebalancing, the energy density is restored to 98% of the initial performance. Based on the experimental results, the new V-Mn-Ti/V-Mn-Ti redox flow battery is expected to be a promising candidate for large-capacity energy storage devices.