Improvements to the Coulombic Efficiency of the Iron Electrode for an All-Iron Redox-Flow Battery

Abstract

The all-iron redox flow battery is an attractive solution for large-scale energy storage because of the low cost and eco-friendliness of iron-based materials. A major challenge to realizing a continuously operable battery is the parasitic evolution of hydrogen at the iron electrode during battery charging. We found that the adsorption of ascorbic acid added to the electrolyte inhibited hydrogen evolution at pH = 0. Elevation of pH near the surface of the electrode during electrodeposition also raised the coulombic efficiency. Thus, electrolyte flow rates significantly influence the coulombic efficiency. Ascorbic acid also served to regulate the pH near the surface of the negative electrode by buffering action. We found that increasing the operating temperature enhanced the kinetics of iron deposition relative to the kinetics of hydrogen evolution, leading to a net rise of coulombic efficiency. Thus, by operating at 60°C and a pH of 3 with ascorbic acid and ammonium chloride, we achieved a coulombic efficiency of 97.9%. While this value of coulombic efficiency is among the highest values reported for the iron electrode in the context of the all-iron flow battery, further improvement in efficiency is needed for supporting repeated cycling. The results presented here provide insights for further improvements.