Powering a vanadium redox flow battery using spent vanadium catalyst: Extraction of direct-use V(IV)/V(III) vanadium precursors

Vanadium redox flow battery (VRFB) is an emerging energy storage system for large scale renewable energy storage. However, due to limited stock of primary sources of vanadium within the earth's crust, the sourcing of vanadium pentoxide for potential VRFB installations will warrant a steep price increment for vanadium commodity. To tackle this challenge, the present study focusses on powering a VRFB using secondary sources such as the spent vanadium catalyst. In this work, a novel and sustainable extraction technique is designed to extract vanadium from spent vanadium catalyst retired from sulfuric acid plants. The extracted vanadium precursors are pure phase V2O4 and V2O3 as confirmed by XRD and ICP-MS analysis. Furthermore, unlike V2O5, these can be directly used to synthesize the positive and negative VRFB electrolytes without any further treatment. Moreover, the electrolyte prepared from the extracted vanadium precursors deliver charge-discharge profiles and discharge capacity (Ah/L) fade trend similar to the commercial vanadium electrolytes for more than 300 cycles in a singlecell VRFB at 80 mA/cm2. Finally, an economic analysis model is developed for a 10 kW/120 kWh VRFB installation which suggests that the cost of synthesizing vanadium electrolyte using spent vanadium catalyst is 2.73 $/L. Moreover, the cost of extraction of V2O4 and V2O3 is 20.52 $/kg and 22.78 $/kg, respectively, which is significantly less than the commercial prices.