Carbonized tubular polypyrrole with a high activity for the Br₂/Br⁻ redox reaction in zinc-bromine flow batteries

Zinc-bromine flow batteries have been regarded as one of the most promising technologies for large-scale energy storage. However, the widespread application of this technology is still hindered by its low power density, primarily resulting from a large polarization in the Br₂/Br⁻ electrode. In this work, carbonized tubular polypyrrole with abundant nitrogen- and oxygen-containing functional groups is synthesized and, for the first time, applied to a zinc-bromine flow battery as a positive electrode material. Experimental results unravel that carbonized tubular polypyrrole exhibits a high activity toward the Br₂/Br⁻ redox reaction, and enables the zinc-bromine flow battery to be operated at a current density of 80 mA cm⁻² with a high energy efficiency of 76.0%. In contrast, the battery with the pristine graphite felt electrode can only output an energy efficiency of 69.4% at the same current density. In addition, no degradation is observed for the zinc-bromine flow battery equipped with carbonized tubular polypyrrole for 100 cycles. These superior results suggest that carbonized tubular polypyrrole is a promising positive electrode material for zinc-bromine flow batteries.