Effect of Electrolyte pH during Pretreatment of Carbon Electrodes on the Subsequent Kinetics of the V II /V III and V IV /V V Reactions in Vanadium Flow Batteries

Growing environmental concerns and the depletion of fossil fuel reserves have prompted increased focus on renewable energy sources like wind and solar power. 1-3 However, integrating these renewables into power grids is challenging due to their intermittent nature. Large-scale energy storage systems are seen as a solution to improve the reliability and efficiency of renewable energy. Vanadium flow batteries (VFBs) are gaining attention for their potential in large-scale energy storage due to the advantage of utilizing the same active element on both sides of the battery, which eliminates cross-contamination issues and extends the lifetime of the battery. 4-6 A typical VFB consists of a cell stack and two external tanks containing electrolytes with redox active species, circulated through the cell stack via pumps. The negative half-cell contains V 2+ /V 3+ (V II /V III ) species and the positive half-cell contains VO 2+ /VO 2 + (V IV /V V ) species, separated by an ion exchange membrane. Carbon or graphite felts are commonly used as electrodes in VFBs due to their high electrical conductivity, chemical stability, and availability. 2,9 However, voltage inefficiencies arising from sluggish electrode kinetics can lead to increased overpotentials and reduced coulombic efficiency, impacting the overall energy efficiency of the battery. 5,6 Therefore, it is important to investigate the electrode kinetics to better understand and improve the performance of VFBs. Various electrode treatments, including chemical, electrochemical, and thermal treatments, have been reported to have significant impacts on the carbon electrode kinetics. 7-9 We have previously reported that cathodic treatment enhances the kinetics of the positive (V IV /V V ) electrode but inhibits the kinetics of the negative (V II /V III ) electrode, while anodic treatment inhibits the kinetics of the positive electrode but enhances the kinetics of the negative electrode. 9-18 Other factors affecting the electrode activity include ageing of the electrode, the final treatment potential, the redox rest potential, and the treatment potential window. 14,15 Recently we showed that the electrochemical activity of the V IV /V V and V II /V III redox reactions in highly acidic vanadium electrolytes at electrochemically treated glassy carbon electrodes is also affected by the pH of the treatment electrolyte. 19,20 The effect of pH of the treatment electrolyte was investigated for electrolytes from pH 0 to pH 5.