Effects of phosphate additives on the stability of positive electrolytes for vanadium flow batteries

• A series of phosphates is investigated as additives for vanadium flow battery.
• Superior V(V) thermal stability and improved electrochemical performance.
• Enhanced battery efficiency and slower capacity fading.
• Mechanism for the stabilization and performance improvement is put forward.
• NH4H2PO4 indicates a promising candidate for additive of the positive electrolyte.

A series of phosphates is investigated as additives to improve the stability of the electrolyte for vanadium flow battery (VFB). Two selected additives show positive effect on the stability of electrolytes under ex-situ stability tests and in situ flow cell experiments. The effects of additives on electrolyte are studied by Nuclear magnetic resonance (NMR), X-ray diffraction (XRD), Raman spectroscopy, Cyclic voltammetry (CV), Electrochemical impedance spectroscopy (EIS) and charge–discharge test. The results show that a VFB using the electrolyte with NH4H2PO4additive demonstrates significantly improved redox reaction reversibility and activity, and higher energy efficiency. In addition, the cell employing the electrolyte with NH4H2PO4 exhibits a charge capacity fading rate much slower than the cell without additives during the cycling at high temperature. These results indicate that the phosphate additives are highly beneficial to improving the stability and reliability of VFB.