Influence of Metal Impurities or Additives in the Electrolyte of a Vanadium Redox Flow Battery

As one of the most attractive large-capacity energy storage systems, vanadium redox flow batteries have many advantages such as a rapid response, long service life, environment-friendliness, and non-flammability. However, the high cost of the vanadium electrolyte has hindered the broad market penetration. The price of the vanadium electrolyte can be significantly reduced if a cost-competitive technology for producing the electrolyte from recycled V2O5 may be used. Accordingly, the effects of the impurities in the recycled V2O5 on the performance of vanadium redox flow batteries (VRFBs) must be understood. However, there have been very few published studies on the effects of these electrolyte impurities. In order to establish a reliable electrolyte specification for VRFB developers, we analyzed the influence of a wide range of potential metal impurities or additives on the electrochemical activity, diffusion of vanadium ions, and battery performance of these systems. The investigated impurities include Li+, Na+, K+, Mg2+, Cr3+, Mn2+, Fe2+/F3+, Co2+, Ni2+, Cu2+, Zn2+, and Mo6+ ions. Most of the metal ions affected the V2+/V3+ redox reaction rather than the VO2+/VO2+ redox reaction. On the basis of our findings, we suggest detailed specifications that may be used to manage the quality of the vanadium electrolyte.