Conductive polymer coatings control reaction selectivity in all-iron redox flow batteries

Aqueous all-iron redox flow batteries are an attractive and economic technology for grid-scale energy storage owing to their use of abundant and environmentally benign iron as redox active material and water as solvent. However, the battery operation is complicated by the plating/stripping reactions of iron and the competitive hydrogen evolution reaction on the negative electrode, which hampers battery performance and durability. Here we tailor the reaction selectivity of the negative electrode by introducing conductive polymer coatings onto porous carbonaceous electrodes. We conformally coated two conductive polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(pyrrole) (PPy) with the dopant poly(4-styrenesulfonate) (PSS), and studied the resulting electrochemistry on model electroanalytical platforms and redox flow batteries. Both polymers decrease the hydrogen evolution current on rotating disc electrodes with PPy/PSS strongly inhibiting the reaction at high overpotentials. In full all-iron redox flow cells, we find that PEDOT/PSS coated electrodes feature lower discharge overpotentials than uncoated electrodes and 150 mW cm-2 peak power density as revealed by polarization tests. Simultaneous measurement of evolved hydrogen gas during cycling experiments reveal that PPy/PSS coating extends cyclability and significantly reduces hydrogen evolution with a faradaic efficiency of ~97%, while PEDOT/PSS coating improves the round-trip efficiency, possibly acting as a redox shuttle for iron stripping reactions. These findings motivate the broader investigation and implementation of conductive polymers to engineer reaction selectivity for flow batteries and other electrochemical technologies.