Tetrapyridophenazine/graphene composites for aqueous hybrid flow battery anodes with long cycle life

Aqueous hybrid flow batteries (AHFBs) are considered as a cost-effective large-scale energy storage system for renewable energy applications. Herein, using pi-pi stacking directed self-assembly, we report an electrochemically active composite material comprising planar pi-conjugated molecules (tetrapyridophenazine, TPPHZ) and two-dimensional (2-D) graphene nanoplatelets for high-performance anodes of AHFBs. The TPPHZ/graphene composite shows highly negative redox potential, rapid electrochemical reaction kinetics and excellent electronic conductivity in aqueous alkaline electrolyte. Using TPPHZ/graphene composite as the anode and ferrocyanide salt as the catholyte, respectively, the AHFB cell yields a cell voltage of 1.3 V and realizes a specific power of 29.4 W g(-1) (at 100% state of charge) and an average energy efficiency of similar to 89.3% (at a current density of 5 A g(-1)). Furthermore, this AHFB cell shows extremely stable cycling performance, with over 81.4% capacity retention (99.9938% per cycle) after 3000 consecutive cycles. (C) 2020 Elsevier Ltd. All rights reserved.