Development of reliable, low cost, durable and ecofriendly energy storage systems for large scale applications is essential to support the intermittent generation from renewable energy sources. Among the different electrochemical energy storage technologies, redox-flow batteries are promising for large-scale applications due to its design, scalability and low cost.  In the quest for an inexpensive as well as eco-friendly energy storage system, we have been investigating a novel anion-exchange-membrane-based iron chloride redox flow rechargeable battery with the aqueous electrolytes composed of solution of iron chloride, ammonium chloride, and specific electrolyte additives.  Such an all-iron redox-flow battery based on iron chloride with the Fe3+
redox couple at the positive electrode and the Fe2+
redox couple at the negative electrode has a cell voltage of 1.21 V and a theoretical specific energy of 170 Wh/ kg.
Although we have shown the technical viability of the concept, the development of such a battery into a large-scale system has still remained a challenge due to the parasitic hydrogen evolution reaction at the negative electrode that lowers the efficiency of the charging process. Our efforts have aimed at increasing the efficiency of the charging process at higher charging current densities by minimizing the rate of the hydrogen evolution reaction.
This report, we present the effect of operating parameters on the rate of the parasitic hydrogen evolution reaction and the charging efficiency of the redox flow battery. By increasing the electrolyte pH to 3 in the presence of ascorbic as an electrolyte additive, we have achieved a charging efficiency of 94% at 80 mA cm-2 at the negative electrode (Fig.1a). Elevated plating bath temperature significantly increased the faradaic efficiency during both iron electrodeposition and electro-dissolution achieving 98% charging efficiency at 20 mA cm-2 at 333K at pH 3 (Fig.1b). We will also present results on the electrochemical kinetics for hydrogen evolution and iron deposition studied at iron-plated graphite electrode during charging process of the all iron redox flow cell.
Authors acknowledge US Army RDECOM CERDEC CP&I for financial support of this work.
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