Catalytic production of impurity-free $V^{3.5+}$ vanadium electrolyte for vanadium redox flow batteries

Abstract

Vanadium redox flow battery (VRFB) is considered as one of the most promising candidate for a large-scale energy storage system. However, commercialization of VRFB has been hindered due to high manufacturing cost of the vanadium electrolyte which is currently prepared by a costly electrolysis method with a limited productivity. To address the problem of high electrolyte production costs, we propose a simple chemical production of impurity-free $V^{3.5+}$ valence state electrolyte. The method features the use of catalyst and organic reducing agent (ORA) which enables reduction of $V^{4+}$ to $V^{3+}$ vanadium ion without releasing any impurity. Among various ORAs, formic acid showed fastest reaction kinetics in combination with Pt/C or PtRu/C catalysts. Activity and stability of catalysts were compared for a proper catalyst selection. In spite of a higher activity of PtRu/C, dissolution of Ru prohibits its use due to a severe gas evolution during cell operation by deposition of Ru onto the electrode. $V^{3.5+}$ electrolyte was successfully produced by combination of formic acid and Pt/C, and excellent VRFB performances were achieved with the electrolyte. A prototype of catalytic reactor employing Pt/C decorated carbon felts was designed and a high quality electrolyte production in continuous manner was demonstrated. We believe the proposed catalytic production of VRFB electrolyte is a breakthrough invention for the VRFB technology, and it would bring significant development of VRFB system in industry.