N-doped graphene nanoplatelets as a highly active catalyst for Br₂/Br⁻ redox reactions in zinc-bromine flow batteries

The low power density, due primarily to the sluggish reaction kinetic of Br₂/Br⁻, is one of the main barriers that hinder the widespread application of zinc-bromine flow batteries (ZBFBs). Here, N-doped graphene nanoplatelets are synthesized by a facile method and applied as a catalyst for the Br₂/Br⁻ redox reactions. Electrochemical characterizations reveal that N-doped graphene nanoplatelets exhibit a remarkable catalytic activity toward Br₂/Br⁻ reactions, thus enabling the ZBFB to achieve an energy efficiency of as high as 84.2% at 80 mA cm⁻², far surpassing those with the non-doped counterpart and pristine graphite-felt electrodes. More strikingly, even when the current density is raised up to 120 mA cm⁻², the battery can still maintain an energy efficiency of 78.8%, which represents the highest performance for the ZBFBs reported in the open literature. Additionally, the ZBFB with the N-doped graphene nanoplatelets catalyst shows no degradation after 100 cycles. These superior results demonstrate that N-doped graphene nanoplatelets are an efficient and promising catalyst for high-performance bromine-based flow batteries.