Vanadium Redox Flow Batteries Fabricated By 3D Printing and Employing Recycled Vanadium Collected from Ammonia Slag
Authors: Sun He , Takahashi Hirotaka , Sun Lina , Sato Kei , Matsushima Yuta , Khosla Ajit , Kawakami Masaru , Furukawa Hidemitsu , Yoshida Tsukasa 
Published: February 2020

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 (V2O5) 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 HNO3 solution in a sealed plastic bottle at 95°C for 48 h. After suction–filtrating the insoluble solid, KClO3 was added to the solution to fully oxidize the vanadium species to V5+. 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 H2SO4. 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 NaV6O15 and Na1.2V3O8 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 V2O5 (Aldrich) were dissolved to 2 M sulfuric acid to achieve clear yellow solutions of V5+. 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 and negative electrodes for the static battery tests. Charging (formation of V5+ and V2+ states) and discharging (V4+ and V3+ states) cycles were nicely achieved with no notable differences between the samples prepared from Precipitate C and reference V2O5 to prove the usefulness of the vanadium resource collected from the ammonia slag for the VFRB application.

In this study, we also demonstrated flow battery system, which is designed by CAD and fabricated by a 3D printer. This technique is so powerful that allows us to quickly test different designed parts in order to study optimum cell design using it as the method of device fabrication. Furthermore, our interests are not limited in printing different parts of the cell body, to develop conductive printable materials for micro structured electrode is also very important to maximize the mass transport for the redox media. We have tried to form composites of conductive materials and plastics for that use, and among those carbon fiber-PDMS (Polydimethylsiloxane) composite shows its potential to be the electrode material for RFB. Different flow channels are designed and 3D structured using this composite, which are also tested in the RFB system.