Unraveling the Interaction Mechanism between Amidoxime Groups and Vanadium Ions at Various pH Conditions
The all-vanadium redox flow battery (VRB) has attracted increasing attention because of the growing share of fluctuating renewables for power generation. However, the high crossover rate of vanadium ions through the separator is the major cause of capacity fading and efficiency loss in the VRB, limiting its use as an energy storage device. Ion exchange membranes (IEMs) with amidoxime groups, known for their ability to complex various metal ions, including vanadium, were prepared to address vanadium crossover and were identified as promising electrolyte membranes. In this article, an experimental and theoretical basis is provided to explain how amidoximated membranes act as an effective vanadium barrier in the VRB. A very low vanadium uptake was observed for amidoximated membranes at low pH around 0, suggesting that vanadium ions, instead of being complexed, are prevented from entering amidoximated membranes. By increasing the pH from 0 to 2, a significantly higher vanadium uptake resulted for membranes containing amidoximes. Experimental and in silico investigations using acetamide oximes as a model compound provided the most probable coordination motifs of acetamide oximes and vanadium ions at different pH conditions. They also revealed that for vanadium ions in all oxidation states the complexation by acetamide oximes is strongly discouraged in strong acidic conditions, whereas it is favored in a neutral environment. (Chemical Equation Presented).