Water-soluble titanium(IV) tris-catecholate metal coordination complex as a highly negative negolyte for alkaline aqueous redox flow battery

The titanium(IV) tris-catecholate metal coordination complex (Ti(cat) 3 ) is investigated as a negolyte candidate for alkaline aqueous flow batteries. The Ti(cat) 3 complex exhibits a low redox potential of −1.13 V versus the standard hydrogen electrode, which enables a high cell potential (>1.6 V) when paired with ferro-/ferricyanide. Electrochemical studies show that the redox potential of Ti(cat) 3 is pH-independent, but that highly alkaline conditions (pH > 12) significantly reduce its electrochemical behaviour. The apparent capacity fade of the battery is strongly correlated to parasitic hydrogen evolution that causes electrolyte imbalance and results in non-intrinsic capacity decay. The battery exhibits an intrinsic capacity fade of 2.51% per day at 50 mA/cm 2 . The influence of the charge and discharge cut-off voltage is investigated and evaluated by using in-situ measurements of the redox potential and pH. Here, it is found that the discharge cut-off voltage can minimize the degradation of the Ti(cat) 3 complex, and that tuning this parameter reduces the intrinsic capacity fade to 0.31% per day at 30 mA/cm 2 . These findings highlight both the opportunities and the challenges of the Ti(cat) 3 complex as a low-potential negolyte candidate for high-voltage aqueous batteries.