Molybdenum-assisted reduction of VO2+ in the presence of hydrazine monohydrate for low cost electrolytes of vanadium redox flow batteries

Efficient and affordable energy storage systems are indispensable to accomplish a successful energy transition from fossil fuels to renewable sources. Although all-vanadium redox flow batteries (VRFBs) possess many distinctive advantages and are at an early stage of commercialization, much improvement in the process for electrolyte preparation is needed to overcome low productivity and complexity of the current electrolysis process. Herein, we demonstrate a simple one-pot process for the preparation of V3.5+ (equimolar mixture of V3+ and VO2+) electrolytes from V2O5 by utilizing hydrazine monohydrate as a residue-free reducing agent and molybdenum as a homogeneous catalyst accelerating the reduction of VO2+. It is confirmed that the performance of the electrolytes prepared by the newly developed process is identical to that by electrolysis in terms of charge-discharge efficiency and capacity up to current density of 200 mA/cm2. This study can contribute to the wide spread of VRFBs as a large-scale and long duration stationary energy storage system by providing a scalable and highly reproducible process suitable for the mass production of V3.5+ electrolyte.