Vanadium Redox Flow Batteries Fabricated By 3D Printing and Employing Recycled Vanadium Collected from Ammonia Slag

Vanadium redox flow battery (VRFB) is an energy storage system with separately designable power stacks and reservoir. It is one of the most established systems to convert electrical energy to chemical energy, or vice versa, due to its high reversibility, large storage capacity, and the high stability because of the fact that the redox species remain in solution at all times. Currently, the electrolyte for the VRFB is prepared by the electrolytic reduction of vanadium pentoxide (V 2 O 5 ) in sulfuric acid to produce a vanadium sulfate solution. However, the cost of the whole system exceeds $200/kWh, while the current cost of vanadium is around $ 23-25 Kg -1 , which represents around 30% of the overall capital cost of the battery. In order to reduce the cost of the electrolyte, we provide an environmentally–friendly wet process to extract vanadium from industry ammonia slags near the atmospheric conditions (< 100°C). The ammonia slag was treated in a 1.3 M HNO 3 solution in a sealed plastic bottle at 95°C for 48 h. After suction–filtrating the insoluble solid, KClO 3 was added to the solution to fully oxidize the vanadium species to V 5+ . Then the solution pH was raised to 6, at which Ca stays dissolved but the others precipitate, by adding a proper amount of a 15 M NaOH solution to obtain Precipitate A . After filtration and washing, Precipitate A was dried in an oven at 50°C overnight, then dissolved in a 1.8 M H 2 SO 4 . After that, the solution pH was adjusted to 14 to achieve Precipitate B , which contains the impurities of Mg, Al, Si, Ti, Ni and Fe. Precipitate B was then filtered out and a clear solution remained, in which V still stays. Finally, the solution pH was adjusted to 3 with sulfuric acid and a Precipitate C was formed in the solution after keeping it at 95°C for 12 h. Precipitate C shows a high content of V with a small amount of Na and S observed from EDS, and XRD of this sample indicated presence of NaV 6 O 15 and Na 1.2 V 3 O 8 as the main components. All of these treatments are done at low temperature (< 100°C) and successfully condensed V from the ammonia slag. Precipitate C and commercial V 2 O 5 (Aldrich) were dissolved to 2 M sulfuric acid to achieve clear yellow solutions of V 5+ . The vanadium concentration was adjusted to approximately 0.04 M by measuring UV-vis absorption spectra and concentrated to 0.4 M by distillation. A conventional H-cell separated by a Nafion® membrane was used, while two carbon plates served as positive