Dense binary Fe–Cu sites promoting CO2utilization enable highly reversible hybrid Na–CO2batteries

High-performance and low-cost catalysts are particularly desirable for the exploitation of practical low-overpotential Na–CO2 batteries with protracted cyclability. Herein, a well-defined morphology of nitrogen-rich graphitic carbon frameworks with dense bimetallic active sites (Fe–Cu–N–C) was facilely prepared by introducing Fe3+ and Cu2+ to regulate in situ grown carbon nanotubes as an advanced catalyst toward hybrid Na–CO2 batteries. Through metal content tuning and carbon architecture altering, Fe–Cu–N–C proved to be dramatically more effective than Cu–N–C and Fe–N–C. As the cathodic catalyst of a hybrid Na–CO2 battery, Fe–Cu–N–C can facilitate the fast evolution and degradation of flocculent discharge products and achieve an excellent long-term cyclability with up to 1550 cycles (over 600 h), which makes it one of the greatest catalysts for hybrid Na–CO2/air batteries that have been reported to date. The observed outstanding battery performance is attributable to the cross-linked conductive framework affording a "highway" for accelerated electron transport and Na+/CO2 diffusion. Besides, the synergistic effects among defect-rich interfaces, Fe/Fe3C nanocrystals, and Fe–Nx and Cu–Nx sites derived from nitrogen atom doping enhance the catalytic activity. In addition, the possible growth and decomposition mechanisms of NaHCO3 products with different morphologies on Fe–N–C, Cu–N–C, and Fe–Cu–N–C electrodes were presented and discussed.