Effect of Additives on the Low‐Temperature Stability of Vanadium Redox Flow Battery Negative Half‐Cell Electrolyte

An extensive screening of additives was undertaken to identify precipitation inhibitors to reduce the lower temperature limit of the negative half‐cell electrolyte of the vanadium redox battery. In contrast to the 2 m V II blank solution that precipitated within 16 h at 5.0 °C, in a supporting electrolyte of sulfuric acid with 5 m total sulfates, the induction time for precipitation was more than 46 h in the presence of ammonium phosphate, ammonium sulfate, Flucon‐100, sodium pentapolyphosphate and glycerol. In the case of the 2 m V III solutions, considerable precipitation was observed after 4 days with 6.5 m total sulfates and the induction time increased to more than 40 days in the presence of ammonium phosphate, ammonium sulfate, sodium pentapolyphosphate and Orotan 1124. Cell cycling experiments confirmed that ammonium phosphate can dramatically extend the cell operation of a 2 m vanadium solution without precipitation over more than 250 cycles at 5 °C, compared with less than 15 cycles for a blank solution without additives.