Cage-Like Porous Carbon with Superhigh Activity and Br2-Complex-Entrapping Capability for Bromine-Based Flow Batteries

Bromine-based flow batteries receive wide attention in large-scale energy storage because of their attractive features, such as high energy density and low cost. However, the Br2 diffusion and relatively low activity of Br2/Br− hinder their further application. Herein, a cage-like porous carbon (CPC) with specific pore structure combining superhigh activity and Br2-complex-entrapping capability is designed and fabricated. According to the results of density functional theory (DFT) calculation, the pore size of the CPC (1.1 nm) is well designed between the size of Br− (4.83 A), MEP+ (9.25 A), and Br2 complex (MEPBr3 12.40 A), wherein Br− is oxidized to Br2, which forms a Br2 complex with the complexing agent immediately and is then entrapped in the cage via pore size exclusion. In addition, the active sites produced during the carbon dioxide activation process dramatically accelerate the reaction rate of Br2/Br−. In this way, combining a high Br2-entrapping-capability and high specific surface areas, the CPC shows very impressive performance. The zinc bromine flow battery assembled with the prepared CPC shows a Coulombic efficiency of 98% and an energy efficiency of 81% at the current density of 80 mA cm−2, which are among the highest values ever reported.