Sodium pyrophosphate tetrabasic (SPT) is employed as an inorganic additive in the positive electrolyte
of a vanadium redox flow battery (VRFB) to improve its long-term stability and electrochemical performance.
The results of precipitation tests show that the long-term stability of positive electrolytes
(2 MV(V) solution in 4 M total sulfates with 0.05 M SPT additive) is improved compared to the blank one.
UV-vis and cyclic voltammetry (CV) measurements also suggest that the addition of SPT can effectively
delay the formation of precipitation in positive electrolytes, and no new substances are formed in V(V)
electrolytes with SPT. The calcined precipitates extracted from the electrolytes with and without a SPT
additive are identified as V2O5 by X-ray diffraction (XRD) analysis. A VRFB single-unit cell employing
positive electrolytes with an additive exhibits the high energy efficiency of 74.6% at a current density of
40 mA cm2 at the 500th cycle at 20◦C, compared to 71.8% for the cell employing the electrolyte without an
additive. Moreover, the cell employing the electrolyte with an additive exhibits less discharge capacity
fading during cycling in comparison with the pristine one. The disassembled cell without an additive
shows a large number of V2O5 precipitation particles on the felt electrode after 500 cycles. Meanwhile,
the felt electrode of the cell with an additive has little precipitation. That precipitation gives rise to an
imbalance between the positive and negative half-cell electrolytes, which results in a significant capacity
loss. The additive has shown positive results under limited laboratory short-term and small-scale
conditions.